Process for preparing polyamide membrane filter media and products thereof.

MEMORY

P- 71,913

Sheet no. one

- There are in the market microporous membrane sheets that have an absolute capacity for particle separation comprised in the margin ds approximately 0,1 microns and greater. These sheets are made mostly of synthetic resins and cellulose derivatives, and are used as filtering media to separate particles and microorganisms suspended from fluids.

-v. Such membranes are manufactured using the so-called "dry-run" process of a solvent solution of the resin or cellulose derivative on a temporary substrate or substrate in the form of a thin film, after which the solvent is separated or changed under carefully controlled conditions, the separation and change of solvent are very slow, and although the procedure is adaptable for continuous operation, a very large supporting tape system is required as a substrate for the extension or casting of the film, and the drying equipment to carry out the separation of the solvent. This increases the size of the installation and the investment costs in the construction thereof, and ensures a high manufacturing cost.

Drunk to the very long length of the material (solution or film) that is being processed at any time, adjusting the operating conditions for a strict control of the characteristics of the product is difficult. Although the final product is separated and tested to check its characteristics, a very large volume of material is already being transformed into a membrane, and past the point where an adjustment of the parameters of the procedure to modify the characteristics of the product, pox

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Hojn ηίιιη. two

-Quick as it is, it could affect you. Thus, a considerable amount of membrane sheet is produced out of specification before the result of a correction can be seen at the end of the production line. This results in a large proportion of the membrane sheet being out of specification, and a wide range of product variation necessarily has to be accepted. . * * * keep rejections at a minimum. As a consequence of the high production cost and the high reject ratio, the price of such membrane sheets tends to be quite high. '

Another process for preparing sheets of membrane also starts from a solution of the resin c-derived from cellulose, by casting a film of the solution onto a support, and then forming the membrane by precipitation after immersion of the film solution. in ux non-solvent liquid. the resin. This method results in a skin-covered membrane, with portions of the surface having fewer pores or pores that are much smaller, or even completely devoid of pores, and an inner portion with larger pores, with the outer skin portions having a higher density apparent that \

the interior portions.

the skin-covered membranes are non-uniform with respect to particle separation; for example, the membranes used now for reverse osmosis are effective to perform functions such as salt rejection of 90% or better, thus giving good results in the range of 2 to 5 Angstroms (0.0002 to 0.0005 yw) , but are unable to provide sterility to the effluent, allowing

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Ptojn núm., Taking the passage of bacteria in the gauze of 2000 Angstroms (0.2yin). Such membranes are barely adequate when absolute separation of the material constituted by particles, such as bacteria, is needed.

Thus, for example, U.S. Pat. N2 3 · β15 · Ο24) granted to Mchaels on October 26, 1371, describes the formation of anisotropic membranes having pores between 1 and 1000 jnm from a diversity of synthetic resins by:.

(1) forming a dissolved melt mixture of a polymer in an organic solvent, (2) casting a film of said dissolved melt mixture, (3) preferably contacting one side of said film with a diluent characterized by a high degree of miscibility with said organic solvent and a sufficiently low degree of compatibility with said dissolved laundry mixture to effect a rapid precipitation of said polymer, and (4) keeping said diluent in contact with said membrane until substantially all of said solvent it has been replaced by said diluent.

The submicroscopically pink anisotropic membranes are composed of an integral film of macro-porous thickness of porous polymer, the thickness of which is usually greater than about 50 »3 microns and less than about 1.27 mm. One surface of this film is an excessively thin but relatively dense barrier layer or skin, with a thickness between about 0.1 and 5 microns, of microporous polymer in which the diameter

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Ρ1.^twenty . I Iojh no. / φ

-Pore size is included in the range of the millimicra, for example from 1.0 to 1000 millimicrons, that is, approximately one tenth to one hundredth of the thickness of the skin.

The rest of the structure of the integral film is a support layer constituted by a polymer structure of much thicker porosity through which the fluids with little hydraulic resistance can pass. P.orpe^k * * * Integral lens is a continuous phase, that is, a continuous polymer phase. When such a membrane is used as a molecular filter, with the side of the skin * in contact with a fluid under pressure, it is virtually encuei. All the resistance to the flow of the fluid through the membrane in the skin is retained, and the molecules or particles larger than the pores of the skin are selectively retained. Because the skin layer has such extraordinary thinness, and since the transition from the skin layer to the macroporous support structure is so abrupt, usually less than about half the thickness of the barrier layer or less than 1 miera, the overall hydraulic resistance to the flow of fluid through the membrane is very low; that is to say,

Michaels suggests that the formation of these anisotropic membrs seems to be related to certain phenomena of diffusion and osmotic change of solvent that are described below:

When a thin layer of polymer solution deposited on a suitable substrate (to ensure the preferential contact of the diluent with a surface) will. 5069

ΡHojn no. 5 contacts the diluent on one surface, the diluent and solvent diffuse to each other in the outermost layer almost instantaneously. In this way, the gelation or precipitation of the polymer occurs almost instantaneously. Due to the rapidity of this phenomenon, the upper layer of the casting film solidifies as an excessively thin membrane skin whose pore porosity and fineness are governed by the compatibility criteria set forth above. However, as soon as this membrane skin is formed, the speed of penetration of diluent into the underlying region of the casting film, as well as the speed of • •_% i extraction of the solvent component. (However, this should not be entirely canceled). Under these circumstances, the subsequent alteration in the composition of the solution within the film takes place very slowly. As a result, there is opportunity, when a suitable solvent is present, for a slow phase separation to occur with the formation of a grossly microporous substructure consisting of large interconnected voids occupied by solvent / diluent solution, and an interstitial polymer matrix comprising consolidated polymer, practically free of solvent. Therefore, the formation of a grossly microporous and highly permeable substructure is due in large part to the proper selection of a system.

So, Michaels membranes are all

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PHojn no. 6

-skin covers, and in addition, although the membranes are wettable by water while they remain wet, once dry they are all hydrophobic, and difficult to wet with water, except with the help of surfactants or other wetting aids.

US patent 2TS 4,032,309 granted to. Salemme on June 28, 1977, prepares membranes of polycarbonate resin described as hydrophobic, evidently of very small pore size, included in the range

.

of ultrafiltration. Salemme makes reference to U.S. Pat. 3.6δ 3,615,024 to Michaels and to the US patent, N2 3,709 * 774 to Kimura, and states that both Michaels and Kimura use the general procedure of preparing a polymer casting solution, casting a film of the same onto a smooth substrate and immersion of the substrate and the film in a suitable rapid cooling bath for the development of the asymmetric structural characteristics of the completed film.

These methods differ from each other in the manner in which some of the steps of the procedure are conducted. While the Michaels patent is particularly directed to the preparation of a membrane having a microporous support layer and an integral microporous skin, that of Kimura is primarily concerned with a film structure having a porous region adjacent to a non-porous layer. porous dense very thin. Kimura specifically teaches the preparation of a casting solution constituted by the polymer and two mutually miscible solvents in which the polymer is soluble in substantially different grades. Both the Michaels method co-30

5069 (njn núm. · * 7) or Kimura's, consider the immersion bath (or membrane former) as a bath that behaves as a solvent as far as the solvent system of the casting solution is concerned, therefore exclusively to separate solvent from the casting solution from the structure of the film.

Contrary to Kimura's procedure, Salemme does not use a three-component casting solution (-resin, satisfactory solvent, poor solvent) and, in contrast to Kimura and Michaels, Salemme uses an immersion bath (rapid cooling) to initiate the form of the film that has to provide a function neither described nor considered in the Kimura c Michaels patents; namely, causing swelling of the polycarbonate resin material at the same time that the casting solvent is separated from the film in this way.

The Salemme method for the preparation of porous membranes. Polycarbonate and other resins comprises the steps of:

(a) preparing a casting solution at room temperature constituted by polycarbonate resin material and a casting solvent composed of one or more satisfactory solvents, the casting solution being stable at room temperature;

(b) casting a layer of the casting solution thus formed on a clean and smooth surface, or support;

(o) allowing desolvation to occur during a predetermined time interval from said layer;

(d) immersing said layer and its support in a liquid sheet.

-do acting as a rapid cooling bath, being the liquid of the rapid cooling bath capable of dissolving the casting solvent and causing the swelling of the polycarbonate resin content of the layer at the same time as it is a non-solvent liquid of the polycarbonate resin, the immersion stage initiating the formation of a microporous membrane by entering the liquid of the quench bath in said layer and casting solvent outlet therefrom;

I (e) separating the microporous membrane from the rapid cooling bath; and (f) separating the casting solvent and the liquid from the remaining quench bath from the microporous membrane.

It is said that the microporous films produced by the Examples are at least as effective for filtration as those produced according to the prior art method of casting and maintenance in controlled atmosphere for extended periods of time. Generally, movies are said to exhibit better cash and are more easily soap than films of the prior art.

The response of these microporous films is measured in terms of the total foam formation point, which is the pressure required to cause foaming to develop on the surface of the film. This method is commonly used in this technique, and reference is made to it as the Bubble Point. In addition, the process for the manufacture of these membranes is not susceptible to adaptation for continuous production.

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I llojn no. ' 9

A number of polyamide resin insoluble alcohol membrane sheets have been described, but to the best of the knowledge of the authors of the present invention none of them has been commercialized. In cases where sufficient information has been provided to allow duplication of the production of these membranes, they have always been heavily covered with skin. Membranes of alcohol-soluble polyamides which lack skin have been manufactured, but these have to be used with means not containing alcohol or various other solvents in which they are soluble. Additionally, such membranes are not amenable to use after steam sterilization, a highly desirable quality for the media largely used for the production of bacterially sterile filtrates. Hollow fiber membranes made of polyamide resin are commercially available in equipment. Affordable, but these are sharply covered with skin, and serve to perform partial separations in the field of reverse osmosis.

The U.S. patent No. 2,733,894 issued to Lovell et al. * On March 5, 1957, and US Pat. US 3,408,315 issued to Paine on October 29, 1968, provide a process for producing polyamide membrane sheets soluble in alcohol using Nylon 4, poly-β-butyrolactam. A solution of nylon can colai. It is like a liquid film and then becomes a solid film that has a microporous structure when it dries. An alcohol-water solution containing nylon is prepared and adjusted to the point of incipient precipitation. The solution is brought to the incipient precipitation point,

Ρ10

I loja nítm. . * ¡Q

-te by addition to the solution of a non-solvent liquid miscible with the solvent that reduces the solubility of nylon. This point is reached when a small amount of the non-solvent liquid added to a sample of the solution causes an obvious precipitation of the nylon.

The nylon solution, adjusted to the point of incipient precipitation and containing the appropriate additives, is cast as a liquid film on an optically smooth surface of a solid base and is then converted to a solid film by exposure to an atmosphere-containing. a constantly maintained concentration of non-solvent interchangeable vapors, that is, vaporea of. a liquid in which the nylon is not soluble but which are · interchangeable with vapors of the nylon solvent. The resulting membranes are, of course, soluble in alcohol, as well as in a considerable number of other solvents, and can not be steam sterilized, which limits the extent of their utility. . .

U.S. Pat. N2 3,746,668, issued to Hiratsuka and Horiguchi on July 17, 1973, also prepares membranes-from alcoholic solutions of po · *

liamides which are soluble in alcohol, gelifying the solution by the addition of a cyclic ether as a gelling agent, and drying the film. Copolymers soluble in alcohol, of relatively low molecular weight, of Nylon are used.

and Nylon 66, and Nylon 6, Nylon 66 and Nylon 610.

U.S. Pat. No. 3,876,738, issued to Marinaccio and Khight on April 8, 1975, describes a process for producing microporous membrane sheets from polyamides soluble in alcohol and insoluble in

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I lojn num. . Alcohol 1 alcohol such as Nylon 6, poly-f-caprolactam, and Nylon 610, polyhexamethylene sebacamide, by casting a solution of the polymer onto a substrate and subsequent precipitation of the membrane, both steps being carried out sequentially or simultaneously in a bath rapid cooling of non-solvent liquid.

the nylon solution after its formation is diluted with a non-solvent liquid of nylon, and the non-solvent liquid used is miscible with the nylon solution. I ^ arinaccio and others discuss the aggregation of^one polymer molecules in solution, and state that the thickest or least porous polymer film is produced from a solution in which there is no formation of. aggregates

I agree with Marinaecio and others, · ... the robustness of the resulting film is determined mainly by the concentration of the polymer due to the greater number of chain entanglements, which are produced for higher levels of polymer. Additionally, for cast film from the ideal solution, the size; of pore would increase slightly with the concentration of polymer due to the increasing tendency to aggregation for higher concentrations, the aggregation in solution results in the porosity of the film, since it can be thought that the cast film as such is composed of aggregated spherical particles subjected to interactions. The older^- they are the spheres, so much more are the empty spaces in the film. Strictly speaking, it resembles a box of tennis balls or other non-spherical geometric bodies welded at their contact points.

Sheet no. . 1 £.

As a first step, therefore, Marinaccio and others control the porosity of the film by controlling the aggregation tendency in the casting solution. This is done ... by the addition of non-solvent liquid or other additives that change the solvent power of the solution, influencing and controlling therefore the tendency to aggregation of the polymer molecules, the interaction of these aggregates to determine the structure of the resulting film is further influenced by the i, various variables of the procedure previously maintained.^eleven.

This is the theory of Marinaccio and others, but it is not adequate to explain what is really happening, and in many respects it is not consistent with real observations. In addition-, • differs from others: more generally accepted theories proposed to explain the formation of polymer membranes, such as, for example, Synthetic Polymeric Membran.com, Késting (McGraw Hill 1971), p. 117 to 157 Kesting's theory is more credible for several reasons; for example, it explains the very high volume of holes in the membranes, which Marinaccio's tennis ball theory does not manage to do; It also explains why only relatively polar polymers are susceptible to membrane formation, which is also not the case with Marinaccio's theory. '

Marinaccio et al. Continue: The selection of a solvent for a selected film-forming polymer can be made on the basis of the foregoing information. The determination of the solvent systems: optimal as well as other variables of the procedure can

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Sheet no. '13 then be made on the basis of routine laboratory experimentation. However, the dilution of the solution by the addition of a non-solvent liquid has a limit:

the dilution with non-solvent liquid can be carried out up to the point of incipient precipitation of the nylon, but not further. The casting solutions are stable enough to be subjected to aging periods of as long as 5 to 8 days, and indefinitely in some cases, but not so much that the nylon di 'is separated;

loose./

The rapid cooling bath may or may not be comprised of the same non-solvent liquid selected for the preparation of the nylon solution, and may also contain small amounts of the solvent used in the nylon solution. However, the ratio of solvent to non-solvent is lower in the fast cooling bath than in the polymer solution, in order to obtain the desired result. The rapid cooling bath can also include other non-solvent liquids, eg water.

In all the Examples, the solvent used for the solutions is formic acid, but none of the quench baths contained even a small amount of formic acid.

It is said that the procedure of Marinaccio and others differs from conventional methods of preparation of microporous films in the use of more simplified casting solutions, but more importantly in the elimination of the slow equilibrium stage consisting of gelation in an atmosphere of high humidity. In conventional procedures this is a critical stage. · * J

-ca in the formation of the desired film structure.

In the procedure of Marinaccio and others, the film sneaks directly into the fast-cooling bath, and cools suddenly abruptly. By controlling the formulation of the casting solution as set forth above and by controlling the variables of the rapid cooling bath including composition and temperature, it is said that the structure is controlled. This technique forms the structure of the film catastrophically and is in direct contrast to the slow equilibrium technique required in conventional procedures.

In some cases, Marinaccio and others suggest that it may be desirable to pass the cast film through a short evaporation zone to the air located before the rapid cooling bath. The technique could be used, in those cases where a structure of gradual cross-section is desired in the film.

The product of the patent of Marinaccio and others has not been commercialized, and it is not available, the formation of a polymer film by direct immersion of the casting resin in a rapid cooling bath is difficult, and has not been economically feasible try to reproduce the procedure of Marinaccio and others in such a way that the characteristics of the product could be studied, given that such a study would require building a rather complex apparatus. It is also worth noting that none of the Examples, by Marinaccio and others include the formation of the film in a rapid cooling bath, but instead the films are manually cast in individual laboratory tests on glass plates.

ΡHoJh no. · Ί 5

Tests were carried out using the glass plate method described by Marinaccio et al., With waiting periods between the extension of the film and the immersion in the bath, ranging from less than 3 seconds to as much as 1 minute; there was no significant difference in the characteristics of the. product. Therefore, it can be assumed that the film resulting from the casting under the surface of the bath (which represents the extrapolation at zero time) will not be different. Taking this into account, the casting resins of their Examples were formed as thin films, and with a minimum waiting period, always less than 1 minute, in order not to allow any significant loss of solvent by evaporation, submerged in the baths described. ; in all cases, the films obtained were heavily covered with skin.

A number of polyamide resin membranes have been used for reverse osmosis and ultrafiltration, but all have pore sizes of less than 0.1 mire, and therefore provide flow rates below the useful range in the filtration of particulate matter and the bacteria filtration. Although the pores are small enough to separate microorganisms, such as bacteria, such membranes are not used for this purpose, but instead perform missions such as reverse osmosis and ultrafiltration, which are not quantitative, and which can tolerate the imperfections that characterize skin-covered nylon membranes.

The Steigelmann and Hughes patent, US 3,980,605 issued on September 14, 1976, provides semi-permeable membranes manufactured from

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Lo10 lojn num.

mixtures of polyamides, especially poly (N-alkoxyalkohyl) amides, and water soluble poly (vinyl alcohols). The membranes are preferably formed as hollow fibers. The membranes can be manufactured from compositions containing the polymer components and a di (lower alkyl) sulfoxide, eg, dimethyl sulfoxide. The. Membranes may contain metallic components that form complexes. Membranes are useful for separating chemicals from their mixtures by techniques utilizing an aqueous liquid barrier and complex forming metals, eg, for the separation of ethylenically unsaturated hydrocarbons such as ethylene from boiling hydrocarbons. next, but such membranes have pore sizes too small to provide useful flow rates in the filtration of particulate matter and filtration. bacteria

It is an unfortunate fact that most affordable membrane sheets are hydrophobic, that is, they are not easily wetted by water. The synthetic resin membrane sheet has almost invariably been made of hydrophobic synthetic resin, and retains the hydrophobic characteristic of the polymer from which it was made. The membranes of cellulose esters are also hydrophobic. The affordable membrane sheets useful in the champion of particle separation, only the regenerated cellulose sheet and the alcohol-soluble polyamide membrane sheet are hydrophilic, that is, mojabids by water.

U.S. Pat. N2 3.9O1.81O, granted to Brooks, G-aefke and Guilbault, proposed a procedure to overcome this problem by preparing ultrafilm membranes30

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I

I Ιο} η ηήιη. 1 7 tration made from segmented polymers having hydrophilic portions and hydrophobic portions. Brooks and others suggest that if the casting solvent is a better solvent-for the hydrophilic segments of the polymer than for the hydrophobic segments, the resulting film or membrane will exhibit a gross morphology in which the hydrophilic portion of the system exists as a phase. continuous while the hydrophobic portion is present as a dispersed phase. The membrane system will include domains "-" .. segregated from hydrophobic segments scattered in a background

of the hydrophilic segments of the polymer. For the same reason, if a casting solution is selected such that it is a better solvent for the hydrophobic segments of the polymer than for the hydrophilic segments, the phase relationships in the resulting films will be reversed and the film will not behave as a membrane for aqueous media, but will behave more like a hydrophobic film that has virtually no permeability to water.

However, this method uses merely combinations of hydrophilic and hydrophobic groups for cor. follow water permeability, and does not suggest any procedure to modify the normally hydrophobic groups in order to improve the water permeability of the hydrophobic polymers. polyamides are not cited in the Brool patent and others as acceptable membrane materials for their invention.

U.S. Pat. No. 4,073,733, Yamarichi et al., Discloses a hydrophilic hollow fiber membrane of polyvinyl alcohol with a relatively uniform distribution.

ΡIioj »no. * * | θ

-uniform of pore sizes in the range of 0.01: to 2 microns, but these pores are not interconnected, and the product serves for separation · in the field of dialysis (dissolved compounds of high molecular weight), rather than as a particle or bacterial filter.

As most filtration applications for membrane sheets correspond to the filtration of aqueous media, it is essential to obtain adequate wetting of the sheet to facilitate filtration, but this is not easy to achieve. Surfactants may be added to the filtering medium in order to make it possible for the medium to wet the sheet sufficiently to pass through it for filtration. However, the addition of foreign materials such as surfactants is not possible or desirable; in many applications, as for example, in the tests of bacteria, since some bacteria are destroyed by the surfactants. In other applications, the filtration media can not be adulterated by the addition of surfactants without harmful consequences.

Membrane sheets made of cellulose esters, which currently cover more than 95% of all the membrane sheet material sold, are inherently non-Mojabic by water; therefore, surfactants are added for water services. Additionally, these membranes tend to be brittle, and to counteract this, glycerin is added as a plasticizer, but this is also undesirable, since it will leach out in aqueous fluids, and result in a contamination problem that is unacceptable in many. applications.

In accordance with the invention, a

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Hojn no. * J polyamide resin membrane sheet insoluble in alcohol that is inherently hydrophilic. This is an extremely remarkable property, since the polyamide resin insoluble in alcohol from which the sheet is made is hydrophobic. The reason why the polyamide resin membrane sheet prepared according to the method of the invention is hydrophilic is not currently known exactly, but this seems to be due to a spatial orientation of the hydrophilic groups of the polymer chain that It is fixed on the surface of the solid polymer membrane as a result of the precipitation process. This may be related to the crystalline structure or structure of the solids, or to some spatial form of the HH and / or CO groups. on the surface of the membrane sheet, that facilitates that it is wet by water. The fact is that a drop of water placed on a dry polyamide resin membrane sheet of the invention will pass through the sheet and disappear in a few seconds. A sheet of dry membrane placed on the surface of a body of water will get completely wet and can even sink into the water in a few seconds. If the membrane is completely immersed in water, the membrane becomes completely wet in less than 1 second.

The ability of a membrane or substrate to be wetted by water is determined by placing a drop of water on the surface of the membrane or substrate. The contact angle provides a quantitative measure of wetting. A very high contact angle indicates poor wetting, while a zero contact angle defines complete or perfect wetting. the polyamide resins from

Hojn mím. twenty

-which the membranes of this invention are manufactured have a high contact angle, and are not wetted by water.

The wetting susceptibility of these membranes, i is not a function of the water retained. Samples of membranes dried at 176.7^SC for 72 hours in an inert atmosphere, in a vacuum, and in the air, remain unchanged in regard to the susceptibility to wetting by water. In 'change, if they are heated to a temperature immediately go. ferior at the softening temperature of the membrane i

(heating to a higher temperature would, of course, destroy the membrane, since it would melt) the membrane. it becomes again a hydrophobic material, which is already wetted by water. This suggests that hydrophilicity is a function of the structure of the solid, and is obtained by the method of membrane formation, probably during the precipitation of the membrane during the process. This may be associated with the structure of the crystal, or may be associated only with the structure of non-crystalline or amorphous solids, but it seems to be really related to a physical orientation of the hydrophilic groups in the polyamide chain, orientation that is lost when The membrane film is heated to a temperature high enough to allow reorientation to a normal configuration,

It follows, of course, that during processing and drying it is important not to heat the membrane above this temperature.

An additional important feature of the ho .5069 ί Iojή ηήιη. * 2 * |

The polyamide resin membrane sheets of the invention are their high flexibility. In the field of normal thicknesses in which they are useful, in the absence of a state of extreme dryness, they can be folded back and forth on themselves several times, without risk of deterioration, and without the addition of any plasticizer.

In the process of the invention, the polyamide resin is dissolved in a solvent of polyamide resins, such as formic acid; a liquid is added.

solvent under controlled conditions to achieve a nuovated solution, and the resulting solution is cast onto a substrate in the form of a film, and this solution film is contacted and diluted with a liquid which is a mixture of a solvent and a non-solvent of the polyamide resin. After that, the polyamide resin precipitates from the solution, forming a sheet of hydrophilic membrane without skin on the substrate, and the sheet can then be washed to remove the non-solvent liquid. The membrane can be detached from the substrate and dried, or if the sub-treatment is porous, it can be incorporated in the membrane; or fixed to the membrane to serve as permanent support, and can then be dried together with the membrane.

The conditions in which the polyamide resin precipitates determine the skinless nature of the membrane, as well as its physical characteristics, that is, the size, length and shape of the pores that traverse the membrane. Under certain conditions, a membrane is formed having continuous pores extending from surface to surface that are substantially uniform in shape and size. In other conditions, continuous pores

IIojh no. 22

They are of decreasing dryness, being wide on one surface, and narrowing towards the other surface of the membrane.

Under conditions that are outside the scope of the invention, yet another layer of the membrane is obtained, which has a dense skin traversed by pores of smaller diameter than the pores of the rest of the leaf. This skin

V 4 »

It is normally found on only one side of the membrane sheet, but can be formed on both sides of said membrane sheet. Such skin-covered membranes are conventional in the art, exhibit a relatively higher pressure drop and other deficient filtration characteristics *, and are undesirable. · Thus, by controlling the method by which nucleation of the casting resin occurs, and of the precipitation conditions, it is possible to obtain hydrophilic polyamide resin membranes with continuous pores of desired characteristics, which can be uniform in face of face, or of decreasing section, with larger pores on one face that are changing to finer pores on the other side.

The formation of a polyamide membrane having uniform pores or pores of decreasing section without skin on any of its surfaces is also remarkable. As demonstrated by the patent N2 3,615,024 by Michaels and N2 3,876,738 by Marinaccio et al., The precipitation of a polyamide resin membrane in a non-solvent liquor results in, as is known, a membrane covered with skin. . The formation of a hydrophilic and skinless polyamide resin membrane by this method has not been achieved before.

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PlcloM no. '2. 3

The method of the invention for preparing from hydrophobic polyamide resin a skinless microporous polyamide membrane having absolute particle separation capacities of 0.10 jim to 5 faces or greater in solid form which is hydrophilic and remains hydrophilic up to which is heated to a temperature immediately below its softening point, comprises preparing a solution of a polyamide resin insoluole in alcohol in a polyamide solvent; induce the nucleation by dilution of the solution with a non-solvent liquid. You under controlled conditions of concentration of dissolvent, non-solvent and resin, temperature, intensity of mixing,

no visible full redissolution of the precipitated polyamide resin; Separate any possible undissolved resin by filtration; extending the resulting solution onto a substrate to form a thin film of the substrate on the substrate; contacting the film with a mixture of non-solvent liquid containing a substantial proportion of solvent from the polyamide resin, thereby precipitating the polyamide resin in the form of a skinless hydrophilic membrane; and washing and drying the resulting membrane.

In a preferred embodiment of this process, the solvent for the polyamide resin solution is formic acid and the non-solvent is water, and the polyamide resin solution film is contacted with the non-solvent liquid by immersion of the sopor movie

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P-Hojn nú ni. '24

- placed on the substrate in a bath of non-solvent liquid comprising water containing a substantial proportion of formic acid.

The invention in another preferred embodiment provides a process for preparing hydrophilic, skinless, alcohol-insoluble polyamide membrane sheets having pores that are substantially uniform surface-to-surface, comprising preparing a solution of an alcohol-insoluble polyamide resin. in a polyamide solvent; induce nucleation by dilution of the solution while controlling the concentration of solvent, non-solvent and resin, temperature, JLa = mixing intensity, addition time and system geometry to obtain a visible precipitate of resin of polyamide during the addition of the diluent, with or. sin ·. · visually complete redissolution of the precipitated polyamide resin, thereby forming a casting solution;

Additionally, a continuous process for preparing inso30 polyamide membrane sheets is provided

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PHoja no. 2 ^?

alcoholic, hydrophilic and skinless liquids, comprising preparing a solution of a polyamide resin insoluble in alcohol in a polyamide solvent; induce nucleation by dilution of the solution with a non-dissolving liquid while controlling the concentration of solvent, non-solvent and resin, temperature, mixing intensity, addition time and system geometry to obtain precipitation of the polyamide resin during the addition of the non-solvent liquid, with or without visually complete redissolution of the precipitated polyamide resin, thereby forming a cast solution; separate ^ any possible undissolved resin by filtration; spreading the casting solution on a substrate which is non-porous and whose surface is wetted by the casting solution and preferably also by mixing non-solvent and solvent liquids to form a thin film of it on the substrate; contacting the film with a bath of non-solvent liquid containing a substantial proportion of the solvent of the polyamide resin, thereby precipitating polyamide resin in the form of a hydrophilic and skinless membrane; and continuously washing and drying the resulting membrane, while maintaining the relative proportion of solvent and non-solvent liquids in the bath constant. In a preferred embodiment, the rates of separation and addition of solvent and non-solvent liquids to and from the bath are kept substantially constant.

The invention further provides a process for preparing polyamide membrane sheets insoluble in alcohol, hydrophilic and without skin having layers

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Nú Sheet No. 26 of multiple membranes, comprising preparing at least two starting solutions of polyamide resin insoluble in alcohol in a polyamide solvent; induce the nueleación by dilution of the solutions with a non-solvent liquid while controlling the concentration of solvent, non-solvent and resin, temperature, mixing intensity, time of addition and the geometry of the system to obtain a precipitate Visible polyamide resin during the addition of the non-solvent liquid, · coi. i. , or without visibly complete dissolution of the precipitated polyamide resin; Separate any possible resin., not dissolved by filtration; extending the resulting solution onto a substrate that is non-porous and whose surface is wetted by the casting solution and preferably also by the mixture of non-solvent and solvent liquids to form a thin film of it on the substrate; contacting the film with a mixture of non-solvent liquid containing a substantial proportion of solvent from the polyamide resin, thereby precipitating the polyamide resin in the form of a skinless hydrophilic thin membrane; wash the two resulting membranes; assemble the two membranes thus formed as a double layer; and drying the double layer under conditions of restriction to prevent more than a small reduction in the length and width of the membrane;

the membranes thus bonded may have the same or different porosities, and the membrane layers may be selected from membranes having section pores

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Hojn rn'ini. Decreasing 2T and membranes that have uniform pores, in any combination, supported or not supported.

The two combined membranes can be obtained from a single roll of filtration media, and when combined with the matching faces in contact they form sheet sheet which is symmetrical, and which provides the same filtration characteristics regardless of which face is located upstream.

The invention also provides various types of polyamide resin membrane products. A preferred embodiment is. a hydrophilic microporous polyamide membrane comprising a polyamide resin normally hydrophobic in a solid structure which is hydrophilic, having absolute separation capacities ranging from about 0.1 yua to about 5 yom, and a thickness comprised within the range from about 0.025 mm to about 0.8 mm.

These hydrophilic microporous polyamide resin membranes may have pores extending from surface to surface in a relatively uniform structure, or in a pore structure of decreasing cross-section.

Hydrophilic polyamide resin membranes are also provided which are supported by the substrate on which the polyamide resin membrane is formed, either embedded therein, or in such a way that the substrate is bonded to one face of those.

Additionally, the invention provides microporous polyamide resin membrane compositions having a plurality of layers of polyamide resin membrane, formed by membranes prepared separately

ΡNo.

- by precipitation on separate substrates and then joined together by drying two or more layers maintained in close contact.

In all of these embodiments, the preferred polyamide resins are polyhexamethylene adipamide (Kylon 66.), poly-β-caprolactam (Nylon 6), and polyhexamethylene sebacamide (Iylylon 610).

Another purpose of the invention is to provide a method for the quantitative characterization of uniform pore membranes by their ability to provide a sterile effluent when subjected to the challenge of a given number of microorganisms of a given species. This / procedure is applicable to uniform distribution membranes manufactured from resins other than polyamides, and using other procedures.

In the drawings:

Figure 1 is a graph showing in a qualitative way the relationship between the degree of nucleation of the cast resin solution (abscissa) and the pore diameter of the resulting membrane (ordinate).

Figure 2 is a graph showing the relationship for a membrane of uniform pores between the reduction of. title, defined as the ratio of Pseudomonas diminutiae bacteria contained in the inlet liquid to those contained in the exit liquid, and the number of layers of uniform pore filtration media, represented on the abscissa, through the which is passed the liquid loaded with bacteria. In ordinates the logarithm of the reduction of the title is represented.

Figure 3 is a graph showing the relationship 'X

5069

HoJh numr.

-obtained when a wet membrane is subjected to pressure produced by a gas, and the -9 ratio.^toyóal de airé _andn air pressure axis of ordinates is represented as a function of the air pressure applied on the abscissa axis, the quantity is defined by the dashed line of Figure 3;

Figure 4 is a graph showing the relationship

25.4 between Τ_Ρ and E_, where Tr, = T- t (or, Log T "s = ££ íAlo; ^Λ1 n nj pt where '.

t is the thickness, in thousandths of mm (microns), of the uniform pore membrane showing a -reduction of titer, as defined above, equal to I "; Y · -......

Jx. Ip is the title reduction calculated for a^K1 membrane 25.4 micrometers thick and of equal pore size;

is the pressure, measured in kg / cm^two, for which the air flow through the membrane wet with agita increases very sharply (see Figure 3); and is the corrected value to correspond to that of a membrane of 127 microns thick, using the empirically determined correction factors listed in Table I. In the figure the values ​​are represented in abscissa in pounds / inch^two (psi).

psi = 0.07 kg / cm^two).

5069

11oj&gt; »níitn.

TABLE I

<td>Measured thickness Mieras</td><td>Correction factor</td>

<td>50.8</td><td>1.10</td>

<td>76.2</td><td>1,044 ^one</td>

<td>101.6</td><td>1,019</td>

<td>127.0</td><td>1,000</td>

<td>152.4</td><td>0.985. _Λ</td>

<td>203.2</td><td>0.962</td>

<td>254.0</td><td>0.946</td>

<td>381.0</td><td>0.920, / J</td>

<td>The curve of Figure 4</td><td>represents the results</td>

two of the measurement, of the values ​​and T ^ for 45 different samples manufactured by the method of this invention.

• ña Figure 5 is. an .electronic micrograph of. ' '

meticulous examination (SEM) performed with 1500 increments of a membrane with uniform pores, manufactured by the process of this invention, having a 3&gt; 30 kg / cm^two,

-i Q t = 94 microns, and an estimated 3 x 10 for the organism Pseudomonas' diminutiae. The central portion of this micrograph shows a section through the thickness of the membrane, in which it is seen that the pore sizes are uniform from surface to surface. The upper and lower micrographs show the upper and lower surfaces respectively adjacent to the section, it being possible to see that the pore size in each of these surfaces is also the same.

The Eigure 6 is an electron micrograph of meticulous examination made with 1000 increases of another membra

5069 l lojn ηι'ιιη. one

-na with uniform pores manufactured by the process of this invention, which has a K &amp; of 2, &amp; 1 kg / cm^two, t = 142.2 5 -i ς micras, and an estimated ox of 1.0 for the organism Pseudoaonas diminutiae ♦ Analogously to Figure 5, the central portion is a section through the membrane that shows the uniform size of the cells. pores of surface to its surface, and the upper and lower views show the upper and lower adjacent surfaces, again indistinguishable with respect to pore size.

Figure 7 is a minute scanning electron micrograph made with 1000 magnifications of a tapered pore membrane fabricated by the method of the invention. This membrane has a thickness of 81 ymn, and in the central part of the SEM it can be seen that the upper portion of the section has pore diameters considerably smaller than the adjacent material, the pore diameter gradually varying to reach the largest size. Comparing the top and bottom views the pore diameters in the upper surface are substantially smaller than those in the lower surface.

Figure 8 is an electron micrograph of meticulous examination performed with 1500 magnifications of a membrane slightly covered with skin, of the type obtained when using baths that are outside the scope of this invention.

• b Figure 9 is a similar micrograph of a membrane covered with skin more sharply.

Figure 10 is a graphical representation of the relationship between (a) K &amp; in ordinates in pounds / inch^two, A for

P.J. lojn no. two

-Measurement capacity meter of the membranes manufactured by the method of this invention, defined by this invention;

(b) the mixing intensity, expressed as revolutions per minute (rpm) of the in-line mixer used to treat a solution at 15.5% resin in 98.5% formic acid to obtain the casting solution used to produce the membranes; and (c) the formic acid concentration of the resulting casting solution.

The abscissa represents 0 HCOOH in laundry solution »

While the various polyamide resins are all copolymers of a diamine and a dicarboxylic acid, or homopolymers of a lactam of an amino acid, they vary widely in crystallinity or solid structure, melting point, and other physical properties. In accordance with the invention, it has been determined that the application of the process of the invention to copolymers of hexsmethylene diamine and adipic acid (Nylon 66), to copolymers of hexamethylene diamine and sebacic acid (Nylon 610), and to poly homopolymers -6-caprolactam (Nylon 6), easily produces polyamide resin membranes insoluble in hydrophilic alcohol and without skin. For reasons that are not understood, polyamide resins are fully susceptible to precipitation under the conditions of the process of the invention to form hydrophilic membrane sheets.

These polymers are available in a wide variety of degrees, which vary appreciably with respect to molecular weight, within the range of about

5069

PHoja nfttn.

-15,000 to approximately 42,000, and in other features. The formation of a hydrophilic membrane seems to be a function not of these characteristics, but of the chemical composition of the polymer, that is, of the arrangement of the |

units that make up the polymer chain. the preferred chemical species of the units that make up the polymer chain is polyhexamethylene adipamide, and molecular weights in the range of greater than about 30,000 are preferred. Polymers free of additives are generally preferred, but the addition of antioxidants or similar additives may have favorable effects in some^oneterms;' for example, it has been shown that the addition of the antioxidant Ethyl 330 (1, 3,5-trimethyl-2,4,6-trisβ "3,5-di-tert.butyl-4-hydroxybenzyl-7-benzene) prolongs the shelf life of polyamide membranes exposed to extreme oxidant hydrolytic conditions.

the solution of polyamide resin from 1ε.

i 20 i

What precipitates the polyamide membrane film can be a solution in any solvent of the polymer. These solvents are well known, and do not themselves form part of the invention. A preferred solvent is formic acid, at any temperature from the solidification point to the boiling point, but hot acetic acid can also be used, as well as phenols such as phenol and cresol, as well as trifluoroethanol and various other polyfluorinated solvents, as well as acids inorganic, such as phosphoric acid and sulfuric acid.

the polyamide resin solution, hereinafter referred to as the starting resin solution, is prepared by dissolving the resin of .5069 loj «no. ij.

-polyamide to be used in the membrane in the desired solvent. The resin can be dissolved in the solvent at room temperature, but a higher temperature can be used to accelerate dissolution.

If the starting resin solution has to be stored for more than a few hours, no more than about 1 to 2% should be present, otherwise a slow hydrolysis of the polyamide resin will take place. resulting in an undesirable reduction in the molecular weight of the polyamide. In general, the amount of water in this case should be less than 2%, and preferably the solution is free of water. If water or formic acid-water mixture is added to produce nucleation,. the addition can be done immediately before casting &lt;preferably between about 5 and 60 minutes before the casting operation.

The resin-casting solution is prepared from the starting resin solution by dilution thereof with a non-solvent liquid, or with a mixture of solvent and non-solvent. The de-nucleation state of the resulting casting solution is strongly affected by the following factors;

1) Concentration, temperature and molecular weight of the starting resin solution;

2) Composition and temperature, of the non-solvent liquid, or of the mixture of solvent and non-solvent liquids.

3) The speed at which the non-solvent liquid is added, or the mixture of solvent and non-solvent liquids.

PHojn oúm. 35

4) The mixing intensity during the addition.

5) It gives geometry of the apparatus in which the mixing is carried out.

6) the temperature of the resulting casting solution,

The cast resin solution thus prepared is then transformed into a thin film by pouring it on a suitable substrate, and the film is immersed with a minimum delay in a bath containing a non-solvent liquid of the polyamide resin, together • a substantial proportion of solvent for the resin • If the non-solvent liquid contained in the bath is water, and if the solvent is formic acid, the presence is desirable, of at least about 200 and -usually at least 30 to 4% of formic acid to prevent the formation of a skin-covered membrane, which occurs at lower concentrations of formic acid.

The stability of the casting solution varies considerably depending on the method used to prepare it. For example, the cast resin solution prepared under small-scale loading conditions tends to be relatively unstable; for example, the characteristics of the membranes that it produces will be completely different if it is cast in a period of time between 5 and 10 minutes after it has been prepared, or it can be transformed into a non-colloquial semi-solid gel in a period of 10 minutes or less . In contrast, a cast resin solution prepared using a continuous on-line mixer, which can produce a membrane of equal characteristics, tends to be stable for a period of time.

5059

Ρ Hojn nftm. 36

-period of one hour or more. Casting resin solutions prepared in this way, however, must be used within one hour or less, in particular if kept at elevated temperature, to prevent a substantial reduction in the molecular weight of the polyamide resin, which would occur otherwise due to the presence of water in the acid solution, with the consequent hydrolysis.

Any of the above methods can be used to produce cast resin solutions that behave equally when they are cast as membranes, and regardless of which one is used, the addition of the non-solvent liquid is accompanied by the appearance of a visible precipitate of polyamide resin, in order to produce a useful and suitably nucleated pouring resin solution. Casting solutions prepared by other methods, for example, by dissolving the resin granules in a formic acid solution and water, or by adding the non-solvent liquid in such a way as not to produce such a precipitate, do not produce useful membranes.

Useful membranes are those that have uniform pore structures or of decreasing section, skin free, and with air and water permeabilities such that can be made, pass through substantial amounts of fluids with low pressure differences, at the same time they provide a required degree of filtration.

A convenient index of utility can be obtained considering the air and water permeabilities of the membranes of cellulose esters of uniform pores existing on the market, manufactured by the so-called pr &lt;_

5069 lojn no.

dry yield (by evaporation). These are shown in Table-II below, together with typical permeabilities of media of similar rank manufactured by the process of this invention.

TABLE II

Speeds of. typical flow of useful membranes

Capacity abMembrenás of separation solute,

...... mieras

0.1

0.22

0.45 polyamide of this invention ^one

Flow per cm^twoand per kg / cm^twoof differential pressure ~

Commercial membranes of ceiloseous esters

<td>liters / min H? 0</td><td>0.0023</td><td>0.0058</td>

<td>liters / min air</td><td>1.04</td><td>1.08</td>

<td>liters / min H_?0</td><td>0.022</td><td>0.033</td>

<td>liters / min air</td><td>3.47</td><td>3.64</td>

<td>liters / min H_?0</td><td>0.058</td><td>0.058</td>

<td>liters / min air</td><td>7.37</td><td>7.37</td>

Membranes having remarkably lower flow capacities for equal separation characteristics, when compared to the membranes currently on the market, are not very commercially acceptable, and have been defined, for the purposes of this description, as being located outside the useful range.

It is an important feature of this invention that the conditions for achieving a casting solution with a controlled degree of nucleation are described. to manufacture membranes with useful characteristics of pressure drop.

In this report, the terms nucleation and nucleation state are used to explain the discovery10

I lojii ηύηι. 38

- ation that (a) casting resin solutions can be prepared with a wide variation of composition with respect to resin, solvent and non-solvent concentrations, yielding identical or nearly identical membranes; yl

(b) co-resin solutions can be prepared having equal concentrations of resin, solvent and non-solvent, which are then cast at equal temperatures in the same bath, however producing very different membranes; in fact, the resulting membranes can span the gamut since they are not useful in the sense. 'I have very significantly lower flows -with factors of 2 to 5 or more- compared with those in Table I,' through the range of 0.1 absolute or thicker jum, producing membranes in all those ranges with capacities of flow rates, for example, equal to those indicated in Table II.

Since it has been observed that the preparation of cast resin solutions capable of producing membranes with flow properties in the useful range is invariably accompanied by local precipitation and at least partial redissolution of solid resin, and since it is perfectly known by experts in chemical techniques that the characteristics of a solid precipitated from a solution can be markedly influenced by the presence or absence of submieroscopic nuclei, the authors of the present invention have chosen the use of the term nucleation state to differentiate solutions of casting that have the same composition, but different results, as described in

5069

Sheet number 3S paragraph (b) above, and to also explain the observation of paragraph (a).

The assumption that the nucleation explains the differences in behavior of manufactured membranes, from pouring resin solutions of the same compositions, is confirmed by the results of an experiment in which a resin resin solution was prepared. stable, with an uncontrolled degree of control to give an absolute membrane of 0.4 ^ a. A portion of the cast resin solution was subjected to fine filtration to determine if the nucleation performance would be affected, and the properties of cast membranes were compared from the two batches of cast resin solution.

Examples 5θ and 59 show the results of this experiment; the characteristics of the product will be remarkably enhanced by fine filtration; the finely filtered cast resin solution produces a membrane with a very poor ratio of flowability to separation capacity; the Δρ of the sample US 59 is more than three times greater than that of a similar membrane made using a properly nuled casting resin solution of this invention.

This result supports the theory that resin cores develop during dilution under controlled conditions used to prepare the pouring resin solution, or number, size, or other characteristics have a marked influence on the characteristics of the membrane generated by said solution. casting resin, and that at least a portion of these cores were separated by fine filtration.

Pllojn núm. 4-0 i

/5

It should be understood, however, that it has not been established unquestionably that nucleation is the only explanation of the observed results, and that these could be due to other phenomena other than nucleation. •

The viscosity of the casting solution is preferably adjusted so as to be between about 500 centipoise and 5000 centipoise at the temperature at the time it is cast as a film. The lower viscosities. Approximately 500 cp allow some of the cast film to

It floats like a liquid on the surface of the bath, so that it forms a film precipitate, adversely affecting in this way the properties of the membrane.

ϊ i

i!

ñas coladas and muddy the bathroom. Viscosities well above 5000 cp, for example, 100,000 cp, are not necessary to obtain a smooth and coherent cast film, but are useful in casting membranes in which no substrate is used, eg, hollow fibers, or film not supported

Solutions that have a viscosity much higher than 5000 op to the pouring temperature can be cast without difficulty; however, the preferred viscosity limit is approximately 5000-cp, since at higher viscosities the energy input to the mixture when a non-solvent liquid is mixed with the polyamide resin solution is very high, with the result that the solution can reach an excessively high temperature, with subsequent operating problems. In addition, the pumping of the starting polyamide resin solution to the casting operation becomes progressively more difficult, as.

5069 toja no. 4 1 increases the viscosity. Also, the manipulation of the casting resin solution inside the container from which the resin is cast as a film on the substrate becomes cumbersome if the viscosity is very high. When a porous substrate is used, with the intention of impregnating it completely with cast resin solution, viscosities much higher than about 3000 cp may cause inadequate penetration, and the resulting product has undesirable voids.

The temperature of the cast resin solution is not critical, and useful membranes have been made, throughout the range from '85^andC down. In some circumstances, somewhat higher flow rates are obtained in relation to the separation capacity by reducing the temperature of the resin to a lower value before glue; the movies.

After the. liquid film enters the bathroom, there is a phenomenon of precipitation, whose mechanism is not completely known. The non-solvent mixture of the bath diffuses in. the cast film, and the solvent mixture contained in the solution of the casting resin diffuses out of the film into the bath, but it is not understood why this results in a uniform pore size throughout the thickness of the film. film when the solvent-non-solvent ratio of the bath is maintained within certain limits.

If the bath contains only non-solvent liquid (such as water, alcohols or organic esters), or non-solvent with a small proportion of solvent (eg water with less than 15 to 20% formic acid), precipitin is produced. ,two

-take very quickly, and the solid membrane forms in a few seconds, typically in less than 1 to 10 seconds. Membranes manufactured in this manner are markedly covered with skin, whatever the mode of preparation of the casting solution, and are undesirable.

If the bath contains about 43 to 47 $ of formic acid in aqueous solution, and the casting resin solution is nucleated suitably as described in this specification, the resulting membrane will be uniform in the "pore structure" facing the face, with the only condition that · if it is cast on a solid substrate, the surface of said substrate is wetted by the cast resin solution and by the solution of the bath. The time required for the film to be formed in these circumstances is a function of the following:

(a) Cast resin solutions that produce membranes that have high values ​​(eg, greater than 7.03 lg / cm^two) solidify very quickly, eg in less than 10 seconds. The solutions of casting resin less intensely nucleated, which produce membranes with values ​​£ &amp; from approximately 2.81 to 3.52 kg / cm^two they will solidify typically in the range of 10 to 20 seconds, and the solidification time continues to increase as Κβ decreases, such that membranes approximately 152 microns thick with values ​​less than 1.40 kg / cm^two they require approximately 5 minutes or more for their solidification, and even lower K + values ​​require even longer periods.

(b) The thickness of the cast film is a paraΣ⁵10

J lojít núm. 7J.3

- important meter, with shorter solidification times for thin films.

fe) The use of lower temperatures in the cast resin solution results in faster solidification.

i (d) Solidification is faster at the lower end of the recommended range of $ 43 to $ 47, and may be further accelerated by the use of bath concentrations below $ 43 of formic acid, at the expense of only

i-, .. a slight deviation of the pore uniformity.

As the concentration of the bath decreases to and below the range of 40 to 43 $&gt; the membranes · become progressively more asymmetric, varying progressively from uniforms as shown in the Figures.

and 6, to pores of decreasing section as shown in. FIG. 7, to skin-covered membranes as shown in FIG. 8, and to membranes markedly covered with skin as shown in FIG. 9 "Operation with formic acid concentrations much lower than those that produce section pores. decreasing as those illustrated in Figure 7 is undesirable.

The formation of the membrane from a cast resin solution can be carried out as an intermittent or batch operation or as a continuous or semi-continuous process. A small-scale operation can be performed very conveniently as a load operation, while for high production rates a continuous or semi-continuous operation is more convenient. In all types of procedure, it is important to carefully control all operating parameters.

5069

Hojn no.

- tion to ensure a uniform product, including operating temperatures, and relative proportions? of resin solution and non-solvent liquid. The control of the conditions of addition of the non-solvent liquid is particularly important, including the geometry of the apparatus, the flow rates, and the duration and intensity of mixing; likewise, the interval between the addition of the non-solvent liquid and the casting of the repine film should be controlled. Such controls may be established by trial and error experimentation without undue difficulty by those skilled in the art, taking into account the following considerations: ......... '.......; ·; -. .- ···. ····

It is important that the solution of the casting resin be clear, and be free of suspended material, before it is spread over the substrate to form a film. If suspended material is present, such as undissolved resin particles, these are separated by sieving or filtering prior to casting.

Any type of substrate or support can be used as the surface on which the solution of the casting resin is cast to form the solution film. If the desired product is an unsupported membrane film, then the substrate must have a surface to which the membrane does not adhere, and from which the membrane film can be easily peeled off at the conclusion of the drying operation, the susceptibility of detachment usually requires that the surface of the substrate be smooth, and not porous. When the solvent is one that has a relatively high surface tension, such as formic acid, and the non-solvent liquid has

PHojn no. Four. Five

-also a relatively high surface tension (such as, for example, water), it is important that the non-porous surface on which the film is cast is wettable, that is, has a zero or almost zero contact angle, when

it is brought into contact with the cast resin solution, and preferably also when it is brought into contact with the bath in the same way. If this condition is not met, a skin will form on the membrane on the side of the substrate, with an undesirable effect on the properties of the substrate.

the membrane. Such temporary surfaces of substrate or

I.

Support can be of a suitable material, such as glass, metal or ceramic. Plastics, such as polyexylene, polypropylene, polyester, natural and synthetic rubber, polytetrafluoroethylene, polyvinyl chloride, and similar materials are not inherently suitable, since they are not wetted by the casting resin and the non-solvent liquid liquor. , but can be made suitable by application of an appropriate surface treatment, oxidant or the like. For example, a corona discharge may be used to treat the Mylar film (polyester), and polypropylene. The substrate may be entirely made of such materials, or merely surface coated therewith.

If the substrate has to be part of the final membrane film, as a permanent support layer, then it must be of a non-porous material that is preferably wetted by the cast resin solution, such that the resin solution of casting cross the same during the casting of the solution on the substra30

5069

ΡNo.

-to, and becomes firmly attached to it during the precipitation of the polyamide membrane film. It is not essential, however, that the substrate get wet; If it does not get wet, the polyamide resin film will remain confi. I swim mainly to the surface of the support, but nevertheless is adherent to it. Such substrates may, for example, be non-woven or woven fibrous material, such as non-woven mats and upholstery felts, and woven textiles and fabrics, as well as networks of various types, including de-pia filament networks.

Exotic materials, papers, and similar materials.

Oomo permanent supports that are not wet; by the cast resin solution, fine pored woven webs, made from fibers with poor wetting characteristics, such as, for example, polypropylene or polyethylene, can be used. The resin solution is cast as a film on the non-woven web, and as it does not wet the fibers of the web, it is transported on its surface. The substrate carrying the casting solution film on its bottom surface is immersed in a bath of non-solvent liquid or allowed to float on the surface of the bath, and the membrane film is precipitated on the substrate, the film. resulting has a satisfactory adhesion to the substrate,

In the case of permanent supports that are wetted by the cast resin solution, the fibers from which the substrate is made must have a relatively high critical surface tension, so that the film

5069

Lo1 lojn num. 47

The cast resin solution molecule is completely infiltrated in the support band, and the resulting membrane precipitates in and around the fibrous material, and is permanently supported by it, since the material of the support is embedded in the membrane. The resulting membrane has a somewhat higher pressure drop when tested with fluid in ovulation, but the increase compared with that. the unsupported membrane is small, if the supporting web has an open structure.

Suitable wet substrates that can serve as permanent supports for the membrane include polyesters, such as a non-woven fibrous web or as a 'han · i

í í

It is woven, using monofilament or multifilament yarn, monofilaments being preferable in terms of open structure and lower pressure drop; likewise woven webs of polyimide fibers, woven and nonwoven webs of aromatic polyamides or Nomex, and other relatively polar fibrous products such as cellulose, regenerated cellulose, cellulose esters, cellulose ethers, glass fiber, and similar materials.

Cellulose and synthetic fiber filter papers, as well as perforated plastic sheets, and expanded open-mesh plastics such as Delnet or similar extruded and then expanded networks can be used. If the substrate is relatively coarse or has a very open fabric structure, even if the fibers are not well wetted by the resin solution, the substrate may nevertheless be embedded or surrounded by the membrane material in the membrane product. end supported; such relatively poorly wet materials like poly5069

1'10

Hojfi no. 48

-propylene and polyethylene can behave as embedded substrates if they have a sufficiently open structure. If a polyolefin substrate has a relatively smaller pore size, for example about 30 microns, the casting solution will not penetrate into it, but will instead form a membrane external to the polyolefin substrate, but adhered the same.

In a continuous process, the substrate may be in the form of an endless belt, which circulates throughout the entire length of the film forming operation, from casting the cast resin solution film into and through a bath -of precipitation of the non-solvent liquid, and then, through the separation stage of the bath liquid. A metal drum, corrosion resistant, or endless metal tape may be used, but the surfaces on which the film is cast should be. treat or dress in such a way that they become mojabi.

The nucleated cast resin solution can be cast or spread on the substrate with the desired film thickness using a conventional doctor blade or roller, soft contact rollers or squeezing rollers, or other conventional devices, and then put in contact with the bath liquid with the least possible delay.

The choice of non-solvent liquid depends on the solvent used. Water is one of the preferred non-solvent liquids. Other non-solvents include formamides and acetamides, dimethyl sulfoxide, and similar polar solvents, as well as polyalcohols such as glycerol.

5069

PHojn no. 49

-na, glycols, polyglycols, and ethers and esters thereof, as well as mixtures of such compounds. Salts can also be added.

After precipitation, the membrane film is washed to remove the solvent. The water is adequate, but any liquid can be used as a washing liquid.

ii

ί Volatile liquid in which the solvent is soluble and which can be separated during drying.

, One or more washes or baths can be used

I; as required to reduce the solvent content to below the desired minimum. In the continuous process, the flow of washing liquid circulates countercurrently with the membrane, which can, of course, be passed through a series of shallow washing liquid baths in the washing step.

The amount of washing required depends on the desired residual solvent content in the membrane. 31 / the solvent is an acid such as formic acid, the residual formic acid can cause hydrolysis during storage of the polyamide from which the membrane is composed, with a consequent reduction in molecular weight; for this reason, the washing should be continued until the level of formic acid is low enough to avoid any significant hydrolysis during the anticipated storage period.

The drying of the washed membrane film requires a technique that takes into account the tendency of the membrane to contract linearly when it is dried without support, with the result that the dried membrane film is warped. Oon object to obtain a uniform film pla30

5069

Hojn níim. '^) 0

-na, the membrane has to be protected against shrinkage during drying. A convenient way of doing this is to wind a continuous band over a plastic or metal core, with a high degree of tension to obtain a tight roll, then firmly wrap it with a rigid but porous outer core, and dry finally the whole. Other methods of preventing shrinkage, such as tensioning, or drying in felt drums, are also satisfactory.

I Individual membrane sheets / of a selected size can be dried to produce flat, free sheets. of warping by holding the leaves in a frame that prevents the contraction of the sheet on all four sides, and subsequent heating of the membrane placed in the frame at: high temperature until it has dried. "It has been found that two or more membrane sheets of equal size can be contacted and dried together in a same frame to avoid shrinkage. When this is done, the layers in contact adhere to each other, and after that they can behave as if they were a single sheet. When the individual starting sheets are relatively thin, eg of thickness less than 127 microns, and are of the unsupported type (free of "substrate"),

The membranes may be dried by any of the methods described above, and subsequently crimped, joined to provide a closed cylinder, and

Hoju nftm. 51 closed at the ends. It has been found that this process can be greatly simplified, while a superior product is obtained, by undulation of the filter medium while it is still wet, together with layers of dry porous material located above and below, this material being selected in such a way that it is relatively rigid, and not subject to more than a small shrinkage during the drying operation. The corrugated package thus formed is compressed slightly, so that the corrugations are in tight firm contact, while being held in a fixing rig, preferably one perforated to allow free access for heating and steam escape, and put on. in a stove for drying. 31 resulting wavy and dry set shows only slight contraction, and the corrugated polyamide membrane thus obtained is free from warping, with well-formed smooth undulating ridges, and flat faces between them. When they are transformed into a filter element by joining the sides and closing the ends, the porous support layers provide flow spaces for the access of the upstream (dirty) fluid and its passage through the element to become the fluid downstream (clean;

If the filter cartridge is made using two or more thin layers of the polyamide membrane, these will be firmly adhered to each other at the termination of the drying operation, and will behave mechanically as if it were a single layer.

Control of precipitation in order to achieve the formation of a hydrophilic polyamide membrane sheet of desired pore size and flow characteristics

ΡI Ioj «no. '52

- it requires that the solution of the casting resin be controlled with respect to a characteristic referred to herein as nucleation. The variables that have to be controlled include the choice of resin and solvent and non-solvent, the concentration of the resin in the starting polyamide resin solution, the temperatures of all the components, the amount and mode of addition of the liquid not solvent, including the rate of addition, and the geometry of the apparatus, including the latter especially the size and location of the nozzle through which the non-solvent liquid is added.^JFor a given resin, and a given solvent and non-solvent, the effect of these variables on the degree of nucleation is set qualitatively in Table III.

TABLE III

Variables that affect the degree of nucleation

Birection of change to obtain a degree

Variable type _Variable of major nucleation

Conditions of . _t mixed Temperature Decrease

Speed ​​of addition of non-solvent liquid Increase!

<td>Size of the inlet opening through which the non-solvent liquid is fed</td><td>Increase</td>

<td>Distance from the ab</td><td></td>

<td>entrance area from the mixing area</td><td></td>

<td>real</td><td>Increase</td>

<td>Mixing intensity</td><td>Decrease</td>

5069

ΡHoJh nfun. 53

TABLE III (continued)

Type, of variable

Variable

Direction of change to obtain a greater degree of nucleation

Concentration of the components in the non-solvent liquid resin casting solution

Increase

Increase

Degree of non-solvent power of the non-solvent liquid Increase

In Table III, the solvent concentration is not included, since it is defined by the concentration of the resin and the non-solvent liquid.

It will be appreciated that the mixing intensity in a given system is a function of a large number of variables. Sii However, for a given system, the relative intensity of mixing may be expressed in terms of the rotation speed of the agitator, or of the cutting blades of a homogenizer, etc. For a continuous production system (as opposed to a batch operation) an in-line mixer is required, and in a suitably designed multi-blade mixer approximately 1 / 4- to 2 hp is required to produce approximately 30 kg per hour of solution of casting resin of 2000 centipoise - at a speed of rotation comprised between approximately 200 and 2000 revolutions per minute. Such equipment can take various forms,

5069

PI Iojh níun. 54

-two to similar results.

Since mixing intensity is difficult to quantify, the transfer of manufacturing technology from discontinuous systems to continuous systems requires trial and error experimentation, modifying the parameters of the operating conditions until the desired membrane sheet is obtained, all This is within the capacity of experts in this technique, since it involves the manipulation of variables that will be customarily adjusted in the manufacturing processes of the industry. the chemical processes. - Γ

There can not be too much emphasis on importing, mixing intensity and other mixing conditions. For example, a series of cast resin solutions with the same concentrations of the same resin, solvent, and non-solvent, and the same temperature and viscosity can be produced by simply changing the revolutions per minute of the mixer. The most highly nucleated of these cast resin solutions, produced using the minimum speed of the mixer, will then produce a membrane having an absolute pore evaluation of 0.1 ^ im, the stirred casting solution cor. Immediately higher intensity, cast in the same bath, will produce, if the mixing speed is correctly chosen, a membrane of 0.2 pm absolute, and analogous. tea,

The diameter of the nozzle through which the non-solvent liquid is supplied during the preparation

5069

<img file="ES480577A1_D0002.tif" />

llojn no.

of the cast resin solution is also very important. It is in this nozzle where the precipitate is formed, which is subsequently redissolved at least in part, and the complete and partial formation and redissolution of the precipitate seems to play an essential role in the preparation of the casting resin solutions of this invention. . When all the other parameters are kept the same, a cast resin solution of completely different characteristics will be obtained, in terms of the pore size

- - t of the resulting membrane, simply varying the diameter of the nozzle. Nozzle diameters varying from 330 microns to 3.18 mm in diameter were used, but smaller or larger nozzles could be used cor. 'satisfactory results.

Only a solution of casting resin of composition and temperature can be manufactured by varying the intensity of mixing and thus the degree of nucleation to produce very different membranes, but the opposite is also true, namely that membranes can be made equal or nearly equal characteristics using a wide variety of concentrations of solvent resin, and non-solvent in the cast resin solution; for example, an increase in the water content will increase the degree of nucleation, 'but if the mixing intensity is also increased, a casting solution with the degree of nucleation will be obtained unaltered,

Ρ1

Sheet ηήιη.

The relationship between the degree of nucleation and the absolute capacity of particle separation of the resulting membrane is represented graphically in Figure 1, which shows an inverse relationship between the pore diameter of the membrane sheet and the degree of nucleation, ie ,

I that to obtain a small pore diameter is required i

a high degree of nucleation.

The observation of the graph of Figure 1 shows that in Region A, in which the degree of nucleation is very small, the pore size tends to become non-reproducible. In addition, the pressure drop for a given pore diameter is high. The membranes fabricated assuming that l 's'. concentrations of the components are the controlling factors, and without nucleation, for example, by the procedure of. Marinaccio, fall within this field, and tend to be of relatively poor quality. In Region B, the pore size decreases in a regular way, although not necessarily linearly, as the degree of nucleation increases. In Region 0, the cast resin solution becomes increasingly populated by resin particles that have not been redissolved, but still produces membrane? of satisfactory quality if these are separated by filtering before the. wash; and in Region B, the resin solution from which these lumps have been separated by filtration becomes unstable, and prone to local or total early gelling before the film can be squeezed.

The very high degree of nucleation in Region D is sometimes manifested by an opalescent appearance, suggesting that the nucleation procedure has resulted in excessive number and / or un. exceedingly large core size .5069 · ν

Ρ10 llojii nftm. * 5 * 7

Since the methods of achieving a required mixing intensity vary so much among the various tiί.

For equipment used in the mixing technique, it is not possible to quantify this characteristic. Consequently, any given apparatus has to be used initially on a base of scores to produce casting solutions of the desired characteristics, applying the principles tabulated in Table III. Once the mixer speed parameters have been set, set it. tions, / temperatures, flow rates, etc., casting resin solutions can be produced which have fully reproducible characteristics in Region B and 0 of Figure 1 during successive days or weeks of operation. ·

A favorable condition for producing membranes having low pressure drops and particle separation capabilities covering a wide field uses, / a starting resin containing 15.5% Eylcn 66 resin of molecular weight · 42,000, 83.23 $ of formic acid, and $ 1.27 of water. When this starting resin solution is diluted using the conditions of'Examples 1 to 39, the results shown in Figure 10 are obtained.

The field of K_T obtained is such that membranes are obtained 5.

with absolute separation capacities of particles ranging from approximately 0.1 miera (for example, a membrane of 305 micrometers thick with K_T · = 7.03 kg / cm^two a ^ 5 approximately. 1 miera (for example, a membrane of 25.4

Λ microns thick with El = 1.90 kg / cm).

The curves of Figure 10 were obtained using a specific in-line mixer configuration, in which the rotor had a diameter of 6.25 cm. They can

5069

Ρlíoj »núm (

i ι

j

5.069

-to obtain the same results using other mixers, and the revolutions per minute necessary to produce these results may vary; however, it is within the possibilities of a person familiar with the art to empirically determine the conditions required with his apparatus to accurately reproduce the mixing intensity represented, for example, by the conditions of 1950 · rpm and 400 rpm of Figure 10. , and once this has been achieved, the conditions for the manufacture of membranes will be clear for said person

I cover the entire field of Figure 10. ' ₍

This same correlation of the mixing conditions would then be applicable equally to the other example of this invention, in which a mixer was used, in line.

The cast resin solution can be extruded above or below the surface of the liquor bath. non-solvent, especially if it is used to manufacture hollow fibers; this process is carried out more easily in practice by use of relatively high resin viscosities (eg 100,000 cp) and by rapidly solidifying the casting solution in relatively low formic acid concentration baths, e.g. in the range of 20 to 40 $.

As previously described, three types of substrates are used:

(a) non-porous, for example, polypropylene or other commercial plastic film, glass, etc .;

• (b) porous, not wetted by the resin solution, casting; Y

Pllojn núm, 59!

Item

i (e) porous, wetted by the cast resin solution.

The non-solvent liquid precipitation baths used in this invention contain a mixture of solvent and non-solvent of the resin. The characteristic . i of the bath that has a significant effect on the properties of the resulting membrane is the relative concentration of solvent and non-solvent in the bath. If the concentration of solvent is zero, or is at a low level, for example, less than $ 20, a membrane markedly covered with skin will be obtained. If the concentration is adjusted to one of the preferred ranges of this invention (about 43 to 47 $ of formic acid, in the case of a bath containing only water and formic acid) the resulting membrane will have uniform one-sided pores to the other. '·

If the concentration of the bath is 43 to 47 $, and the substrates used are of the types (b) or (c) described above, the pores will always be uniform throughout the thickness of the polyamide membrane. However, if the film is cast on a non-porous substrate of type (a), it is important that the surface of the substrate be wettable by the casting resin, and by the. bath fluid.

Glass, and similar surfaces, are naturally wetted in this way; in contrast, synthetic plastic film materials, such as polyethylene, polypropylene, polyvinyl chloride, and polyesters are not, and if the casting solution is spread on such a substrate, and immersed in a bath that It contains 45 $ of formic acid and 55 $ of water, and will forge a film with open pores on the face that is in contact with the bathroom.

5069

Ρ10

I Iojí&gt; no. 60

- Uniform pores through most of the body of the film, but with a dense skin on the substrate side. However, it has been found that if such films, plastic films become more wettable, for example by oxidative processes such as chromic acid treatment or corona discharge treatment, the resulting membrane lacks skin on both sides, and it has a uniform pore size throughout its thickness .... In such a membrane, it is difficult, if not impossible, to determine by any type of /, 7 1.

appreciation of which side was in contact with the^jsubstrate

.&gt; ·

- To obtain such sheets, of · membrane without piei, · a wide range of surfaces can serve as a substrate, provided that the critical surface tension is maintained at a high enough value. This will vary somewhat. depending on the concentration of formic acid in the resin solution and in the bath, and the temperature, and the best way to determine it is by treatment of scores of the surface of the substrate for a given system. The critical surface tensions required are generally in the range from about 45 to about 60 dynes / cm, and most often in the range of 50 to 56 dynes / cm.

If a given cast resin solution is immersed as a film in a series of baths, each with a slightly increasing water content, the characteristics of the membrane on the face facing the bath will gradually come through, producing films that have finer pores on and near this face, compared to the thickness of the membrane. These pores more

5069

ΡHojn no.

-fines show a gradual transition to the uniform pores of the rest of the membrane. Membrane salts are described herein as tapered pore membranes, and are useful for the fact that, when some suspensions are filtered, with flow from the coarsest face to the thinnest, a longer service life is obtained. ga (greater dirt holding capacity), with equal separation. Figure 7 shows electronic micrographs of meticulous examination of a membrane of. pores of decreasing section. The concentration of solvent in the bath required to obtain any pore membrane of desired decreasing section varies considerably, depending, for example, on the nucleation state of the membrane resin solution, and must be determined for a series of conditions given by trial and error. ;: However,

As the concentration of water in the bath increases, the membranes begin to be covered with increasingly dense skins, and are characterized by a high pressure drop, and poor pore size distribution characteristics.

The uniform pore membranes manufactured by the method of this invention, such as those shown in the minute-scanning electron micrographs of Figures 5 and 6, are characterized by liquid displacement curves such as those shown in the figure. 3 «When the membrane is immersed in water, its pores fill with water, forming within the membrane a film of immobilized water, which remains in its

5069

PHojn no. 62

-place when the membrane is removed from the immersion bath. When air pressure is then applied through the membrane, a very small air flow is observed. When this air flow is divided by the applied air pressure, the quotient remains constant as the pressure increases, which is represented as in Figure 3. A «•« "π, -,.-.- . , starting from the thickness of the film, and from the known diffusion constant of air in water, can be calculated using Fick's law, that this flow is due to the diffusion of air through the water film, and it does not indicate flow through the pores of the filter media. At a sufficiently high pressure, as shown in Fig. 3, it is seen that the flow increases sharply, which reflects the displacement of water from the larger pores,

The rapid transition from zero air flow (except due to diffusion) to an abruptly increasing flow regime for small changes in applied pressure characterizes uniform pore media, which have clearly defined separation characteristics; such means will separate, for example, quantitatively a bacterium, but will allow the passage of an organism only slightly smaller. Such membranes also generally have a favorable low pressure drop, for a given separation.

5069

OIIojh no.

ί

I j

ί j

i

The skin-covered membranes behave in a very different way; when they are wetted with water and their air flow to pressure drop ratio is determined, the curve is not flat initially, but is inclined upward, indicating the presence of large pores; the transition to a line closer to the vertical is slow, with a large radius, and in the vertical area, instead of the sharp rise in Figure 3, a sloping line is obtained, which reflects a wide range of poí-:

ro. Such membranes are poorly suited for obtaining sterile filtrates when exposed to bacteria; either a non-sterile fluid is obtained, or if sterility is achieved, it is at the cost of a very high pressure drop to achieve a low production capacity.

It is evident from the foregoing discussion that a control within narrow limits of the concentration of formic acid in the non-solvent liquid contained in the bath is desirable to obtain a uniform product. In a continuous process, this control is obtained by an appropriate feed of non-solvent liquid to the bath, while simultaneously removing some liquid from the bath to maintain a constant total bath volume. A relatively higher concentration of formic acid enters the bath from the casting solution, and the concentration of formic acid in the bath tends therefore to increase. For this reason water is constantly added to the bath to compensate. I agree with it,

5069

Sheet no. 54

-Formic acid in the solution, within the limits that give a membrane of 'desired characteristics.

Thus, Example 47 shows that to obtain a skinless membrane sheet having a uniform pore distribution, with sufficiently fine pores to quantitatively remove all bacteria and particles larger than 0.2 nnn, a cast resin solution Relatively, very nucleated is like a film and the membrane is precipitated in an aqueous solution of acid, formic at 46.4 $ as a bathing liquid.

To produce a membrane with thin pores of decreasing section, a solution of mono cast resin! intensively nucleated, an aqueous solution of formic acid precipitated as a bath in the form of a membrane, as I ate in Example 50.

It is instructive to indicate that in the range of 0.2 yum and below, the face-to-face uniformity of the regenerated cellulose membranes and commercially available cellulose esters becomes very decreasing. Within the same range, the membranes of the invention remain uniform; or they can be of decreasing section, as desired.

Thus, in the continuous production of membrane sheets according to the invention, in order to obtain uniform characteristics in the membrane, the casting resin solution has to be prepared under carefully controlled conditions and the composition of the bath liquid must remain constant. Reference is made to such a liquid as a bath in equilibrium, that is, a bath in which the concentration of ingredients remains constant, regardless of the additions and separations.

PHojn no. 65

As an illustration, consider a solution of casting solution containing 13% resin and 69% formic acid, the rest being water, which is continuously cast in the form of a film on a substrate, and which is then submerged. i go in a bath of non-solvent aqueous liquid containing 46 $ of formic acid. As the resin membrane precipitates, a proportion of the solvent from the cast resin solution film (containing 69 parts of formic acid per 18 parts of water, or 79.3 $ of 7!

formic acid) diffuses into the bath, thus altering its composition. In order to counteract this, water is continuously added to the bath at a controlled rate, for example, by means of a device that uses density measurements to express the formic acid concentration, at the level of 46 $, and liquid is continuously removed. of the bathroom to keep the total volume of the bathroom constant. Maintaining this bath in equilibrium makes it possible to continuously produce a membrane sheet having uniform pore characteristics.

When used continuously, with high production rates, the temperature of the bath will rise gradually; Cooling can be used by a heat exchanger to maintain constant conditions.

From the above-mentioned pouring resin and bath solution, unsupported membrane sheets can be manufactured by casting the resin solution on an endless belt, or on a plastic sheet unwound from a coil, as a substrate for Support the cast film.

5069

PHoja no. 66

The membrane sheet has a tendency to adhere to the surface of the substrate when it is dried, and for this reason it is important to separate the membrane sheet from the surface while it is still wet, and before it has dried and developed adhesion. The unsupported membrane sheets obtained by the process of the invention are very robust, with tensile strength when wetted with water comprised in the range of 28.1 to 42.2 kg / cm.^two, and alarms that generally exceed $ 40.

For some applications, even higher tensile strengths may be desired. Additionally. , The unsupported membrane sheet requires special care for its handling within the typical thickness range from 50.8 to 254 microns in which it is normally manufactured. In such cases, a membrane sheet on the cover is desired. Such a membrane sheet is prepared by forming the film of resin solution on a substrate that adheres to the membrane sheet after it has been pre-precipitated thereon. Either of the two types of substrates can be used; those that are not wetted by the resin solution, and those that are.

The unsupported filter membrane obtained at the conclusion of the membrane formation process is wetted with water, and also contains a small amount of residual formic acid. This product can be dried in various ways.

For example, it can be collected in a coil on a suitable core in lengths of 15.2 to 30.5 meters and introduced in an oven until it dries. During the se30

5069

ΡHojn no.

-cared, a certain contraction occurs, but an acceptable product is obtained.

It is also possible to fix a certain length of membrane in a frame that keeps all sides protected against shrinkage, and then to dry the membrane by exposure to heat, such as by infrared radiation, or in an air oven. The resulting sheet is very flat, and when disks are cut from it, they are suitable for use in apparatus designed to accommodate disc filter membranes. The membrane discs are completely robust and flexible, and can be mounted easily and reliably on such devices. ..

A similar product can be obtained with less labor use by passing the sheet of. wet membrane on a hot drum, against which it is held firmly by a band of taut felt or other porous sheet, and the dry band is collected in the form of a coil.

If two or more layers of wet unsupported membrane sheet are dried in contact with one another, using any of the drying methods described above, they adhere to each other, forming a multi-layer structure. No binding agent or other adhesion technique is required.

The resulting multilayer membranes are useful in the same way as a single layer filter membrane. Since a small proportion of undetected faults can be produced in manufacturing, caused, for example, by air bubbles included in the casting solution, the use of two layers instead of

5069

Pliojn no. 68

- one neutralizes such areas, covering them above with a second layer of filter membrane that is also capable of providing the required separation capacity; in this way an extremely high degree of reliability is obtained.

ii

$

5069

A very satisfactory adhesion of the adjacent layers is also obtained if a supported and an unsupported resin membrane layer is dried in contact, using the same procedures. In this way, filter media can be manufactured in which a supported layer of uniform pore size is attached to an unsupported tapered pore membrane layer, which provides efficient prefiltration, the thin face of the pore layer of decreasing section would have approximately the same pore size or somewhat larger than the pore size of the supported layer, and this face would be adjacent to the unsupported layer.

The filter membranes supported in accordance with the invention are particularly suitable for use in press filters, where self-sealing characteristics are needed, and the filters are subjected to high stresses. They are also useful for manufacturing smooth or corrugated filter cartridges for use at high differential pressures, or for pulsating type services.

The filter membranes of the invention are perfectly suitable for use as filter media in filter cartridges. Two filter cartridges are self-contained filter elements, provided with a filtering sheet in tubular form closed by closing capsules at each end. One or both closing capsules can be stored in a no. G9

5069

-To provide a direct opening for the flow of fluid through the filter sheet in any direction, the filter cartridges are designed to be installed in and easily detachable from the mounting housings!

of the filter when its replacement is necessary.

A good filter cartridge has a filter sheet that is defective, and with separation characteristics that are relatively uniform with stable standards. The filter cartridges take many forms, including the inclusion of simple cylinders, corrugated cylinders, stacked discs, etc. '_i

Of these configurations, a preferred form for the filter membranes of the invention is a corrugated cylinder.^:Such a cylinder is manufactured by crimping one or more layers of wet membrane supported or unsupported, (two layers are preferred) sandwiched between two porous or perforated open sheets that allow fluid flow, water up and downstream of the surfaces of contact of the filtration medium inside the corrugations, the resulting corrugated structure can dry out while being slightly repressed, in the course of which process the membrane layers in contact are joined together, thus forming a more rigid structure and more Robust, and then closed by joining along the ends in contact, using heat-sealing techniques similar to those used to seal conventional thermoplastic filter materials. Are fixed. then the end capsules in a sealed manner at both ends of the resulting cylinder. The preferred method is in accordance with U.S. Pat. Hs 3 «457 * 339» granted on December 8

LoI lojn 'no. I

-from 1965 to Pall and others. The material of the end capsule can be any of a wide range of thermoplastic synthetic resin materials, particularly polypropylene, polyamides, polyesters and polyethylene. Extreme polyester capsules, particularly those made of poly (ethylene terephthalate) and poly (butylene terephthalate), make a very good seal with I03 membrane materials made of polyamides, and have the advantage that the mounted cartridge is quickly wetted by water , which allows an assay using the standardized methods of the invention to check the integrity of the mounted filter cartridge.

In the manufacture of cylindrical cylindrical filter cartridges, a joint has to be made joining the ends of the corrugated structures. Since the polyamides used to make the membranes of this invention are thermoplastic, the heat sealing technique can be used to close the gasket, and for many or most purposes it is an acceptable method. Sealing by heat has some disadvantages, however:

(a) In order to make the seal (sealing), it is practically necessary to fold the last sheet of. each outer ripple at an angle of 902, which is sometimes difficult to perform without weakening or other deterioration of the filter medium at the bend;

(b) the temperature used and the duration of the closing operation need to be changed to accommodate changes in the thickness of the layers of the filter media used; and (c) there is a weakening of the structure,

5069

Leaf níitn, ΎΊ

-because of the introduction of a stress concentration at the edge of the closure area; if the effort is high, the filter will fail at this edge, preferably any other part of the set.

All these disadvantages are solved by a new joining technique. It has been discovered that a solution of trifluoroethanol containing 3 to 7% of Nylon 66 in solution can be applied to the outermost face of each extreme corrugation, and the two surfaces can then be fixed slightly to each other, allowing the evaporation to evaporate. Fluoroethanol Other solutions, for example, a 33% solution of Iylylon 66 in formic acid, as well as solutions of polyamide resins in hexafluoroisopropanol or hexafluoroacetone sesquihydrate can be used. An excellent seal is obtained as a result, free of all the disadvantages listed above; in fact, the sealing area is now stronger than the remaining undulations. '

The amount and concentration of the resin solution are not critical at all, and closure has been obtained »! Successful tobacconists have as little as zero percent or as much as $ 9 of ITylon 66 resin in the trifluoroethanol solution, but in this solvent solutions close to 5% are preferred, which are stable, and have a convenient viscosity if a resin is used of high molecular weight to prepare the solution. Formic acid solutions have also been used successfully.

The exact determination of effective pore size for membrane filter media that is significant in its representation of expected effectiveness as a filter is difficult. Cusndo a pore filter medium uni5069

ΡHojn no.

fl $

J

- reports of this invention, or any of the uniform pore membranes currently on the market are examined using a scanning electron microscope, e.g. ex. As shown in Figure 5, and the apparent pore openings are measured as seen in the micrograph, a pore size is determined which is approximately 3 to 5 times the diameter of the largest particle that can pass through the filter, what is determined, for example, by exposure to bacteria. Similarly, an attempt has been made to mine the pore diameter from the value as .. 'is determined by the method of applying air pressure a. a wet element, obtain the value in the manner shown in Figure 3, and introduce the pressure thus determined in the well-known capillary rise equation;

Such methods, considered meditatively, seem to have little relation to the capacity of the membrane as a filter. What the user needs to know is not the pore size; instead, what needs to be known is the ability of the filter to separate materials made up of particles, bacteria, yeast, or other contaminants.

Contrary to established thinking, it has been determined experimentally that the effectiveness of membranes similar in structure to those of this invention as filtering means depends not only on the pore size, but also on the thickness. In the development of the present invention it has been shown, for example, that of two membra ···························· '

5069

Ρlíojn núm. Jonas, one of the enales has small pores and is very thin, the other having relatively larger pores and being much thicker, the membrane that has larger pores but also greater thickness can be more effective as a filter. '

Accordingly, the effectiveness of the membrane sheets according to the invention as filter media is not evaluated in terms of pore size, but in terms of effectiveness in separating a contaminant of known dimensions. One of the main applications of this type of filter membrane is to provide a filtrate free of all incidental bacteria, and therefore bacterially sterile. A technique commonly used in the industry to determine the ability of a filter to deliver a bacterially sterile effluent is to cope with a suspension of Pseudoaonas diminutiae, which is a relatively non-pathogenic small diameter bacterium referred to in shorthand as . It is generally accepted in the industry that filters that successfully overcome such a confrontation have a filtering capacity of 0.22 absolute microns, and in any case Pseudomonas diminutiae is a bacterium that represents the lower limit of the bacterial dimensions. If no combination of exposure conditions allowing the passage of even a single organism of Pseudomonas diminutiae can be found, the filter can be considered as capable of quantitatively separating all bacteria.

This invention employs a standardized assay based on the separation of Pseudoaonas diminutiae that binds it30

5069

I loja no. Ύ4 ·

- such separation with the measures of air flow through the wet membrane and. the thickness of the membrane, and is able to provide a very complete characterization of the separation characteristics of the membrane filter sheet being tested.

The separation of Pseudomonas diminutiae is funt * .... &lt;. I. I «tion not only of the pore size but also of the thickness, and is expressed by the exponential relationship:

/ /

^TR " ^TÍL or log t log T_R^R1 where T is the reduction of the titer for the membrane and is. the relation of the content of Pseudomonas diminutiae in the input fluid to the content thereof in the output fluid;

I_ is the reduction of the title obtained by ^R1 a membrane of unit thickness; and t is the thickness of the membrane.

As an example of the application of this formula, if a given membrane has a titer reduction of 1Q "two layers of the membrane will have a reduction of titer of 10%.¹⁰, three layers of 10 ^, etc.

Since the incident test bacterium is monodisperse (ie, of uniform dimensions), the applicability of this formula is self-evident. Your mail has also been confirmed experimentally, determining reductions of title for 1, 2, 3, 4 and 5 layers of the same membranes. As shown in Figure 2, the resulting graph of log Tp as a function of the number of layers

5069

Ρ Hoj "no. Υρ

-is linear, as predicted by the formula.

It is known in the industry the measurement of air flows through a membrane that has been wetted with a liquid; such measurements provide useful information about the pore size characteristics of the membrane. A parameter designated as a form of abbreviation for the location of the elbow of the curve of Figure 3 has been used in the course of this invention. When the ratio air flow / unit pressure applied through a wet membrane the function of increasing applied pressure is represented, as in Figure 3, the initial air flow is very small and the flow per unit of applied pressure remains practically constant, until a point is reached at which a very small increase in pressure causes a very -brusque increase in the flow, so that the curve becomes almost vertical.

It has been measured for a group of 45 membranes manufactured by the process of this invention from poly (hexsmethylene adipamide); these membranes were selected in such a way as to cover a range of thickness comprised between 38.1 and 305 microns, and with a wide range of pore diameters. These same membranes were then confronted with a suspension of Ps bacteria, and the number of entry bacteria was then divided by the number of exit bacteria, thus determining the value for each of the membranes. The thickness of each membrane was then measured, in thousandths of mm (microns), and using the log shape log l'p = - 2.7. log was then calculated log T

Ri

5069

I lojn no.

-for each membrane, being T_p the theoretical title reduction for a membrane of 25.4 microns.

The K · values ​​were measured for both relatively rough and relatively thin membranes for varying thin membranes. These same membranes were superposed by then forging 2, 3 and more layers, and the values ​​for the multiple layers were again measured. In this way, a relationship between the thickness and the value of membranes of equal pore size was determined; this relationship is summarized / -; In Table I. Using Table I, the values ​​of the membranes were corrected to the value that could be applied to a membrane of equal pore size and of .127 microns in thickness; these values ​​are designated as El. : · O

It was then represented graphically log T_fí para- ·· each membrane in function of E ~ for said membrane. All ^ 5 the results fell next to the same line, which is represented in Figure 4 »

Using Figure 4, the degree reduction can be calculated (T_R) which can be expected to be obtained with any membrane made from hexamethylene adipamide by the process of this invention, using the measured values ​​of E ^ and thickness- (t) for the sample concerned. The procedure is as follows :

(1) measure the thickness and thickness corresponding to the sample;

(2) use Table I and determine El;

^h5 (3) use Κ_γ to determine T_p from ^ 5 1 Figure 4; yt / 1

25.4 (4) calculate T_fí from the equation T_p = T

There is an upper limit for the number of bactere

Hojn no. 77 .20 that can be collected on a membrane; when they have collected approximately. 10^one^ Ps per square meter of filtration medium, the flow through the. filter has dropped to less than 0.01% of a normal initial flow of 21.5 to

Λ

53.8 liters / minute per meter. It has been verified by real tests that this value is valid for the membranes of the present invention, as well as for commercially available membranes, for the entire range of from 10 to&gt; 10 ^ θ. Thus, the figure of 10 ^ / meter ~ can be taken as 'a practical upper limit of invasion of Pseudomonas di Λ' minutiae.

This upper limit is taken in combination with the calculated value to obtain the assurance that a membrane will achieve sterility under all conditions of use. For example, a membrane can be selected with an estimated T_ of '10^two8 · statistically,

Λ if faced with 10^one^ Pseudomonas dlminutiae, such a membrane would have to be exposed to such conditions 10 ™ (ie 10,000 million) times, in order to produce a simple effluent with a single bacterium, and such a high ratio can be taken as an adequate guarantee of sterility , so it can be considered that the filter has a capacity, absolute separation of 0.2 yum. In practice, it is. difficult to consistently produce a membrane with an estimate of exactly 10^two^_F but it is feasible to establish an allowable interval, for example 10 ^ 3 a.10^two^, with 10^two8 as the lower limit, and thus obtain the safety of consistently achieving sterile sterile bacteria filtered.

In a similar way, and the thickness can be

5069

I iojn no. '78

-correlated with the reduction of title.for larger bacteria, yeasts of known size, and other materials constituted by particles, the latter being taught by methods of particle detection, within a range of sizes smaller than 0.1 m or greater.

The curve of Figure 4 is applicable to the membranes manufactured by the process of this invention.

The procedure by dial developed this curve can be applied to membranes manufactured by other procedures. The location / location of the curve for other membranes may be somewhat displaced, but sufficient tests have been carried out using membranes of dry procedure with uniform pores existing on the market to determine that the same principles are applicable.

The horizontal portion of the curve in Figure 3 is truly horizontal only if the pore size is completely uniform. The means-uniform pores will-go. they are further characterized by a sudden change in the slope until reaching an almost vertical path for the value K ^. If the filter medium is relatively nonuniform in pore sizes, it will tend to have a different slope in the horizontal portion of the curve, and exhibits a relatively large radius for the change of slope to the more vertical portion of the curve, followed by a tilted portion rather than an almost vertical portion.

The lower or horizontal portion of the curve is a measure of the diffusion of air through the immobilized liquid film that fills the pores of the membrane.

The wetting liquid can be water, in which case a relatively low air flow is obtained in the horizontal part

5069

Ρ10

Hojn no. '79 such of the curve, or alcohol, in which case the diffusion air flow is greater. At the change of slope, the wetting liquid begins to be expelled from the pores, and in the vertical portion of the curve begin to pass air a large number of pores of almost equal size.

When the data of Figure 3 is plotted graphically for a tapered pore membrane, that is, a membrane with larger pores on one side, which, narrow to give smaller pores on the other side of the membrane, the curves obtained by inverting the direction of the pressure do not coincide. Instead, two different curves are obtained, one of them lying&gt; and Xa another higher and inclined upwards, of which the · inclined curve with higher flow values ​​is obtained when the more open side is located waters above, and reflects the penetration of air partially into the coarser face of the membrane, thereby effectively decreasing the thickness of the liquid film, and increasing the velocity of diffusion of the air.

Thus, by application of air pressure-and measurement of the flow through a membrane successively in both directions, it is possible to determine whether it is a membrane of uniform pores or of decreasing section. If the flow-pressure curves are equal, or approximately equal, in both directions, the pores are uniform, and the method described in this report to relate and the thickness with the reduction of title for any given organism, or for a material constituted by monodisperse particles, can be applied to said membrane.

The following examples represent, in the opinion

5069 *

<img file="ES480577A1_D0003.tif" />

-v

LoI lojn no. 'QQ

-of the inventor, preferred embodiments of the invention:

EXAMPLES 1 to 5

Nylon 66 resin granules of molecular weight approximately 42,000 were dissolved in 98.5% formic acid, to give a 35% solution.^twoC containing 15.5 $ of the resin. Without delay, this solution was supplied, at a flow rate of 250 g / minute, to an in-line mixer. Simultaneously, it supplied the mixer with a controlled water flow rate at 31 ° C.^twoC, the amount being such as to produce a pouring resin solution containing&gt; 70.2 $ of formic acid and 13.1% of the resin. The cast resin solution was filtered through an iO 'filter to separate the visible resin particles, and. it was then formed as a thin film by a scraper roller with a separation of 216 microns on a mobile polyester sheet surface, which had been previously treated by corona discharges to improve its wettability; and in less than 3 seconds it was immersed in a bath containing · 46.5 $ of formic acid, and the rest of water, for about 3 minutes. The concentration of the bath was kept constant by continuous addition of water, in the required amount. The nylon membrane thus formed was washed with flowing water for 1 hour.

The speed of rotation of the in-line mixer was varied from 400 to 1600 revolutions. Per minute du30

5069

LoI Loja No.-81^-

-refer this operation. The Wise IV shows the caracterís10 i

-E. 20 products of the obtained products. In this wise unified pores it means that the pore size was equal, as determined by SEM examination along the entire width of the membrane. Examples 1 and 2 represent conditions in region A of figure 1, in which the degree of nucleation is too low to produce a satisfactory product; in this zone the pressure drops are high, and the characteristics of the product tend to be irre- • producible. Example 5, in which the speed of the mixer was 400 rpm, falls within the region D of Figure 1, and resulted in an unstable condition, generating in the interior of the mixer so much precipitated resin that it began to clog , so that casting resin solution could not be supplied.

It should be noted the wide variation in behavior and characteristics of the products for the same cast resin solution defined by the concentration of its components. .........

ELEMENTS 6, 7 and 8

The casting resin solution was prepared and treated as for Example 4, except that it was heated by means of an in-line heat exchanger to respectively 53, 61 and 682c before casting. The characteristics of the products were not noticeably different from those of Example 4 · This result confirmed data from previous tests that indicated that the temperature of the cast resin solution is not a very important parameter, except to the extent that the vis30

5069 lojn nftm.

-cosity to the point (less than approximately 500 cp) in which casting problems can be experienced.

EXAMPLES-9 to 13

The membranes were prepared in the same manner as in Examples 1 to 5, except that the amount of water added was such that a casting resin containing 69.80 of formic acid and 13.00 of resin was produced. results, are shown in lane V. the cast resin solution manufactured at a mixer speed of 1950 rpm was insufficiently nulored, resulting in a deficient product with a high pressure drop;

EXAMPLES 14 to 18 ·

These membranes were prepared in the same manner as in Examples ia 5, except that the amount of water added was such that a cast resin solution containing 69.00 of formic acid and 12.850 of resine was produced. show in Table VI ... EXAMPLES 19 to 39

These membranes were prepared in the same manner as in Examples 1 to 5, except that the amounts of water added were such that casting solutions containing 71.40, 67.50 and 66.00 of formic acid were produced, and 13.30, 12.550 and 12.410 respectively of resin.

The results are shown in graphical form, together with the data of Examples 1 to 19, in Figure 10.

Figure 10 includes only those membranes that fall in the P and 0 regions of Figure 1, and therefore tieΡHoja núm.

They have a favorable low pressure drop in proportion to their thickness and capacity, of particle separation, and

<img file="ES480577A1_D0004.tif" />

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Ilojn no. • 87 j- EXAMPLES 0 to 46

These membranes were prepared in the same manner as in Examples 1 to 5, except that (A) A starting resin containing 14.5% Lylylon 66 was supplied to the mixer at a rate of 400 g / minute. i (B) Water was added in various amounts to obtain the indicated concentrations of formic acid and resin.

i - I (0) The scraper roller was adjusted to 559 microns.

<img file="ES480577A1_D0005.tif" />

5069

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EXAMPLES 47 to 50

Nylon 66 resin granules of molecular weight approximately 42 -000 were dissolved in 98.5% formic acid, to produce a 35% solution.^twoC containing 15.5 $ of the resin. Without delay, this solution was supplied at a flow rate of 250 g / minute to an in-line mixer rotating at 1200 rpm. Simultaneously, a controlled water flow rate was supplied to the mixer, also at 3020, the amount being such as to be effected as effluent. one so &lt;&gt;^{J Λ}casting resin containing 69.0 $ of formic acid and 12.9 $ of resin. The temperature of the resulting casting solution was 57^twoC. The casting resin solution was filtered without delay through a 10 ml filter to separate the visible resin particles, and was then formed into thin films by means of a doctor blade with a separation of 254 microns on plates. glass, and in less than 10 seconds it was immersed in a bath containing formic acid and water, for about 5 to 10 minutes. The nylon membranes thus formed were washed with flowing water for 1 hour. The layers of the membrane were dried in a furnace in mutual contact, while they were subjected to restriction to prevent shrinkage of their length and width during drying.

The HIV Table shows the characteristics of the products obtained, for different conc.enttions of bath

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EXAMPLES 51 to 57

Membranes were prepared exactly as in Examples 4-7 to 50, except as follows:

The speed of the mixer was 1600 rpm

The temperature of the co-resin solution was 64^twoC ·

Table IX shows the characteristics of the products.

Examples 55 and 56 are not within the scope of this invention; they are included to illustrate the effect of the use of bath concentrates less than about 20 $ of formic acid. '--- ··

This group of examples also illustrates the advantages of baths in the field of about 4o, 5 $ to produce membranes with a minimum pressure drop for pa'1.

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^one - EXAMPLES 58 and 59

These membranes were prepared using the same procedure as in Examples 47 to 50, except as follows:

(a) The starting resin concentration was

17 $.

2 ° (b) The cast resin solution was prepared from 344.7 grams per minute of starting resin solution using as a non-solvent diluent a solution containing 32.8% formic acid in water, supplied to the mixer at a rate of 132.1 grams / minute.

(c) The speed of the mixer was 1900 rpm

(d) The composition of the cast resin solution was: 12.1% resin, and 67.8% formic acid. · (E) After filtering through a filter of 10 yam, half of the solution was further filtered through a filter with a separation capacity of approximately 0.05 to 0.10 jwl. The two portions were then cast as films in a 46.5% formic acid bath, as Examples 58 (filtered through the 10pm filter only) and 59 (filtered through the 10 jm filter and subsequently through the filter). from about 0.05 to 0.10 yum). The data for both Examples, measured in a single thickness

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5069

PI lojn no. 94

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The pressure drop of 29 cm of Example 59, resulting from the fine filtration step, should be compared with that of a normal product of this invention with the same values ​​of thickness and £, which would be approximately ^υ5 í

8.8 cm of mercury.

EXAMPLES 60 a, 64

In these Examples, polyhexamethylene adipamide was transformed into membrane sheets using a small filler process. A 20% starting resin solution was prepared by dissolving molecular weight resin = 34000 in 98.5 $ formic acid. An amount of 500 grams of this solution was heated to 652c in a glass-lined vessel of approximately 10 cm in internal diameter by 20 cm in height, provided with a 5 cm diameter propeller-type stirrer, and an automatic valve in its fund, externally driven.

A solution of non-solvent liquid containing 12.77 $ of formic acid and the remainder of water was prepared.

With the agitator rotating at 300-500 rpm, 241 g of this non-solvent solution was pumped into the apparatus, at a constant speed, for a period of 2 minutes, having the nozzle inlet. 2.mm of. inner diameter, and being located at 6.35 mm of the arc described by the rotating propeller. During the last portion of the 2 minute period, the resin was seen to precipitate in the inlet nozzle, all of which subsequently redissolved exceeded a small amount of resin lumps of about 3.18 mm in diameter.

Approximately 20 grams of the solution of resi30 • 5069

Hojn no. The casting thus formed was removed through the bottom valve, passed through a 354 micron sieve to separate lumps, and spread without delay on a glass plate in the form of a thin film, using a scraper of 254 microns, and the film was quickly immersed in a bath containing formic acid and water, at 25 ° C.^SC.

The membranes were allowed to solidify for several minutes, then separated from the glass plate, washed in water and dried by exposure to infrared heat. The properties of the resulting membranes are shown in Table XI.

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PHojn no. 98

Examples 60, 61 and 62 illustrate the effect of the degree of nucleation on the characteristics of the products. Examples 60 and 61 are suitably nucleated and lead to products with a favorably low pressure drop, for their separation capacities. In Example 62, the higher rotation speed resulted in a casting solution with too low a nucleation degree, and as a consequence, a relatively high pressure drop.

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5069

P- 71,913. Nftm sheet '

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