High density polyethylene film with high biaxial orientation

IS 2 195 166 T3

DESCRIPTION

High density polyethylene film with high biaxial orientation.

The invention relates to methods for preparing polymer films. Specifically, the invention relates to methods for biaxially orienting high density polyethylene films and to films prepared according to such methods.

An optional part of the film-making process is a procedure known as "orientation." The "orientation" of a polymer is a reference to its molecular organization, that is, the orientation of the molecules in relation to one another. Similarly, the "orientation" process is the process by which directionality (orientation) is imposed on the polymeric arrangements in the film. The orientation process is used to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is cast as a flat film or blown as a tubular film, the orientation process requires substantially different procedures. This was related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have higher stiffness, toughness, and barrier properties. In contrast, cast films typically have the advantages of greater film transparency and uniformity in thickness and smoothness, generally allowing the use of a greater range of polymers and producing a higher quality film.

Orientation is accomplished by heating a polymer to a temperature at or above its glass transition temperature (Tg) but below its crystalline melting point (Tf), and then rapidly stretching the film. On cooling, the molecular alignment imposed by stretching competes favorably with crystallization and the stretched polymer molecules condense into a crystal lattice with crystalline domains (crystallites) aligned in the direction of the stretching force. As a general rule, the degree of orientation is proportional to the amount of stretching, and is inversely related to the temperature at which stretching is carried out. For example, if a base material is stretched to twice its original length (2: 1) at a higher temperature, the orientation in the resulting film will tend to be less than it would be in another 2: 1 stretched film but at a higher temperature. a lower temperature. Furthermore, higher orientation is also generally correlated with higher modulus, that is, measurably higher stiffness and strength.

When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the stretch direction, but is weak in the other direction, that is, across the stretch, often breaking. or tearing into fibers (fibrillation) when flexed or pulled. To overcome this limitation, biaxial or two-way orientation is used to more uniformly distribute the strength qualities of the film in two directions, in which the crystallites are in the form of a lamina rather than fibrillar. These biaxially oriented films tend to be stiff and strong and also exhibit much better resistance to bending or folding forces, leading to their greater usefulness in packaging applications.

From a practical perspective, it is possible, but technically and mechanically quite difficult, to orient films biaxially by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use an apparatus that stretches the film sequentially, first in one direction and then in the other. Again, for practical reasons, the typical orientation apparatus stretches the film first in the direction of film travel, that is, in the "machine direction" (MD) or longitudinal, and then in the direction perpendicular to the direction of the machine, that is, in the "transverse direction" (DT) or lateral.

The degree to which a film can be oriented also depends on the polymer from which it is made. Polypropylene, as well as polyethylene terephthalate (PET) and nylon, are polymers that are highly crystalline and are easily heat stabilized to form dimensionally stable films. These films are well known for being able to be stretched to many times the dimensions in which they are originally cast (eg 5X by 8X or more for polypropylene).

High-density polyethylene (HDPE) exhibits even higher crystallinity (for example, about 80-95%) relative to polypropylene (for example, about 70%), and films containing

IS 2 195 166 T3

HDPE are generally more difficult to orient biaxially than polypropylene films. US patents nos. 4,870,122 and 4,916,025 describe unbalanced biaxially oriented HDPE-containing films that are oriented up to about two times in the machine direction and six or more times in the transverse direction. This method produces a film that tears relatively easily in the transverse direction.

British Patent No. 1,287,527 describes high-density polyethylene films that are balanced biaxially oriented to a degree greater than 6.5 times in both longitudinal dimension (i.e. MD) and lateral dimension (i.e. , DT). This method requires a specific range of orientation temperatures.

Each of the United States patents nos. 4,891,173 and 5,006,378 describe methods for preparing HDPE films that require crosslinking of the film, with optional biaxial orientation of the crosslinked film. The crosslinking process, which requires irradiation of the film, is reported to improve the physical properties of the film. Other crosslinking processes, such as chemically induced crosslinking, can have similar effects.

Blown HDPE films that have an ethylene vinyl acetate heat seal coating used for food packaging, but such films must be around 50.8 μm thick to meet the water vapor transmission rate (WVTR) requirements. , in its acronym in English) for the appropriate packaging for dry foods, such as cereals. Furthermore, blown HDPE films do not exhibit the desirable dead-fold properties in food packaging, particularly of the bag-within-a-box type.

In view of the above considerations, it was clear that existing methods for producing biaxially oriented HDPE films yield products that are deficient in desirable physical characteristics. Existing methods of making HDPE films generally require additional chemical components in the HDPE resin (eg crosslinking agents) and / or additional process steps (eg irradiation). Such limitations not only complicate production, but generally result in increased costs. Furthermore, crosslinking tends to reduce the crystallinity of the polymer, resulting in higher WVTR and lower stiffness.

Accordingly, one of the purposes of this invention is, among others, to overcome the above limitations in the production of biaxially oriented HDPE films by creating an inexpensive and relatively uncomplicated method of manufacturing biaxially oriented films that impart superior characteristics to films. , without the requirement of chemical additives such as crosslinking agents, and without the requirement for additional processing steps such as film irradiation.

The present invention is a biaxially oriented high density polyethylene (HDPE) film and a method for making the film. The film includes HDPE that has a density of at least about 0.940 (g / cm³), preferably at least about 0.950, and a melt ratio of from about 0.5 to about 10, which has been stretched in the solid state to a degree from about 5: 1 to about 8: 1 in the direction of the machine and up to a degree from about 6: 1 to about 15: 1 in the cross direction, and that includes an orientation imbalance that includes a greater degree of orientation in the transversal direction than in the machine direction. The film is made up of an HDPE layer or has an HDPE base layer that forms 70 to 95% of the film thickness.

Preferably, the biaxially oriented film is stretched to a degree of from about 6: 1 to about 7: 1 in the machine direction, most preferably the film is stretched to a degree of from about 6: 1 to less than about 6.5: 1 in the direction of the machine. It is also preferred that the biaxially oriented film is stretched to a degree from about 6: 1 to about 15: 1 in the transverse direction, preferably from about 9: 1 to about 13: 1. A highly preferred biaxially oriented film is stretched to a degree of from about 6: 1 to about 7: 1 in the machine direction and to a degree of from about 9: 1 to about 13: 1 in the transverse direction.

The biaxially oriented film preferably includes at least one layer of skin coextensively adhered thereto. Among the various types of skin layers known in the art, the film preferably includes a heat seal skin layer or an ink-receptive skin layer.

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The biaxially oriented film of the invention can be produced as a laminated HDPE film, including a core HDPE base layer and further including an outer cast promoter layer coextensively adhered to a surface of a base layer. Preferably, the film includes a layer of laundry promoter adhered to each of the major surfaces of the HDPE base layer, creating a three-layer structure.

The biaxially oriented film can also be modified by means known in the art, including, for example, coated (in-line or off-line), flame or corona treated, or metallized. In addition, the film may include an antioxidant, filler, particulate material, colorant, pigment, light stabilizer, heat stabilizer, antistatic agent, slip agent, release agent, abrasive, or other additive. In a preferred case, cavities have been created in the film during the film-making process.

The invention is also a method for manufacturing a biaxially oriented high-density polyethylene (HDPE) film, which includes:

biaxially orienting a solid state HDPE film, thereby creating a biaxially oriented HDPE film having an orientation imbalance that includes a greater degree of orientation in the transverse direction than in the machine direction.

The method may further include applying a skin material to the HDPE sheet so that the skin material is coextensively adhered as a skin layer to a surface of the HDPE sheet to give a multilayer HDPE sheet. For example, the method may include depositing a skin layer of a heat seal material on a surface of the HDPE sheet to give a multilayer HDPE sheet having heat seal properties. Furthermore, the method may include applying an ink receptor material to the surface of the HDPE sheet to give a multilayer HDPE sheet having increased ink retention properties. Other layers of skin can also be used. If a laundry promoter is used in the preparation of the film, the skin layer can be deposited on a surface of a laundry promoter layer. The method may include treating the biaxially oriented film to increase the wettability and adhesion of coatings, eg, inks.

The method may also include:

coextrude the HDPE together with a cast promoter to give an HDPE coextrudate, wherein the cast promoter includes a polyolefin having a lower crystallinity than HDPE, to give an HDPE coextrudate that includes an HDPE layer and at least one layer of laundry promoter; and casting the HDPE coextrudate to give an HDPE sheet for biaxial orientation.

In a highly preferred case, coextrusion involves coextruding the HDPE with the cast promoter to give a multilayer HDPE lamina having a core HDPE base layer and two outer cast promoter layers coextensive with and separated by the cast layer. HDPE.

The method may also include coating the HDPE film (or online or offline) by flame or corona discharge treating the film, metallizing the film, or otherwise treating the film to obtain a particular property as desired. In addition, HDPE may also include an antioxidant, filler, particulate material, colorant, pigment, light stabilizer, heat stabilizer, antistatic agent, slip agent, anti-adhesion agent, abrasive, or other additive.

In still another embodiment, the method relates to making a high density polyethylene (HDPE) film with cavities. Here, the method includes:

a) extruding HDPE having a density of at least about 0.940 and a melt index of from about 0.5 to about 10 and containing a cavity-forming agent therein to give an HDPE extrudate;

IS 2 195 166 T3

b) casting the HDPE extrudate to give an HDPE sheet;

c) biaxially orienting the HDPE sheet by stretching, thereby creating a biaxially oriented HDPE film with cavities having an orientation imbalance comprising a greater degree of orientation in the transverse direction than in the machine direction.

It has now been discovered that these and other ends can be achieved by the present invention, which creates a biaxially oriented high-density polyethylene film having low water vapor transmission rate (WVTR), excellent gauge profile, high resistance to impact (relative to monoaxially oriented films), high tensile properties, high stiffness, and other phasic properties that are remarkably better than blown HDPE films. The films also have dead-fold characteristics that make them highly suitable for food packaging in bag-in-box operations carried out on vertical form, fill and seal (VFFS) machinery. In other applications, the films of the invention have properties that make them useful for the manufacture of labels, for example, pressure sensitive labels, graphic arts materials, and, in general, substitutes for paper. The film can be beneficially provided with one or more layers of skin. For example, when provided with a heat seal layer by coextrusion or coating, the film of the invention is particularly suitable for use in packaging, especially dry food products. The method of producing the film employs conventional apparatus more efficiently, and does not require chemical crosslinking agents, irradiation, or other means that complicate production.

Figure 1A is a graph illustrating the thickness of a film having balanced orientation measured across its width and at three different points along its length; and Figure B is a graph illustrating the thickness of a film having unbalanced orientation also measured across its width at three different points along its length.

The present invention creates a biaxially oriented high density polyethylene (HDPE) film. As the term is used in this specification, "high density polyethylene" is defined to mean an ethylene-containing polymer having a density of 0.940 or greater. Generally, although HDPE having a density of 0.940 and above is acceptable for use, HDPE of higher density is preferred, with HDPE having a density of 0.950 or higher being more preferred. (As the density of HDPE increases from 0.940 to 0.960 and higher, the tensile strength increases substantially, and the WVTR decreases substantially. Toughness and impact resistance are much higher in the high molecular weight grades. KR Osborn and WA Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, PA (1992)). Although density is a useful parameter characterizing HDPEs, it is also recognized that HDPE suitable for use in the invention generally has a crystalline melting point in the range from about 130 ° C to about 137 ° C, and a crystallinity around 80-95%.

The melt index (MI) of the useful HDPE according to the invention is in the range of about 0.5 to about 10. Most preferably, HDPE has a melt index in the range of about 0.5 to about 10. 2.0. Generally, it is understood that melt index is inversely related to viscosity, and decreases as molecular weight increases. Accordingly, the higher molecular weight HDPE generally has a lower melt index. Methods for determining melt index are known in the art, for example, ASTM D-1238.

High density ethylene-containing polymers suitable for use in the invention include not only homopolymers of ethylene, but also include copolymers of ethylene with higher alpha-olefins. Appropriate high-density polyethylenes meeting the required criteria are commercially available. Series of HDPE resins having ranges of phasic properties are available from various manufacturers. A particularly preferred HDPE is the resin sold as M6211 by Lyondell Petrochemical Company, Houston, TX. Other suitable HDPE resins include, for example, BDM 94-25, available from Fina Oil and Chemical Co., Dallas, TX, and 19C and 19F available from Nova Corporation, Sarnia, Ontario, Canada.

The useful HDPE according to the invention may include a copolymer of ethylene with a lesser amount of another alpha-olefin. Preferred alpha olefins include C3-C8 alpha olefins. The most preferred are

ES 2 195 166 T3 copolymers of ethylene (eg about 50% or more) with a lesser amount of 1-propylene or 1-butylene. By selecting the appropriate comonomer, HDPE films can be manufactured having the particular desired physical characteristics. For example, the crystallinity and density of the resulting copolymer can be affected in a controlled way by the comonomer used with ethylene.

HDPE can be composed exclusively of a single HDPE resin, a blend (blend or alloy) of HDPE resins, or HDPE containing a lower proportion of other resource polymers (polyblend). For example, HDPE can contain up to about 10% by weight of microcrystalline wax to improve processability. These HDPEs typically have melt indexes in the range of from about 0.5 to about 10, and are normally selected to result in a blend having the desired melt index, for example, from about 0.7 to about of 2. A blend of HDPE resins generally results in better extruder processing characteristics by reducing extruder torque.

The HDPE blends can include two or more HDPEs, each of which preferably has a density of 0.940 or greater. HDPE polymer blends advantageously include a higher proportion (ie, 50% by weight or more) of an HDPE having a melt index of 0.5 to 2, and polymeric units having different melt indexes. For example, HDPE blends have been found to be suitable for use according to the invention. Suitable mixes may include, for example, about 50 to about 98% by weight, preferably about 84 to about 96% by weight, of HDPE having a density of 0.940 or greater and a melt index greater than 0. .5 to about 2.0; 1 to 25% by weight, preferably 3 to 8% by weight of HDPE having a density of 0.940 or higher and a melt index of 0.1 to 0.5; and 1 to 25% by weight, preferably 3 to 8% by weight, of HDPE having a density of 0.940 or greater and a melt index greater than 2 to about 8. Preferably, the second and third HDPE polymers, which they are minor components, tin present in approximately equal amounts. Other HDPE blends and termixtures can also be used.

Blends (alloys, polyblends) of HDPE with a lesser amount of one or more different polymers are also useful in particular situations. For example, the physical properties of the resulting film can be selected by including polymers of varying crystallinity. Thus, high crystallinity polymers such as polypropylene can be included. On the other hand, lower crystallinity or amorphous polymers can be included, such as polystyrene, styrene-butadiene copolymer, or polyvinyl acetate. US Patent No. 4,191,719, for example, describes HDPE materials that are blends that include five different components. In this embodiment, the basic HDPE material includes at least about 50% by weight HDPE, and preferably at least about 90% by weight HDPE.

In another alternative, the film may include a base material that is a blend of HDPE and another polyethylene such as a low-density PE (LDPE), ultra-low-density PE (ULDPE), or a linear low-density PE (LLDPE). . The skilled artisan will understand that these other types of polyethylene can be used in lesser amounts to adjust the physical properties of the resulting films for particular purposes. In this embodiment, the base material includes at least about 50% by weight HDPE, and preferably at least about 90% by weight HDPE.

The film of the invention is biaxially oriented to a relatively greater degree than has previously been possible. The high degree of biaxial orientation of HDPE film is an important aspect of this invention in view of the fact that the proper degree of orientation has been found to impart desirable physical properties to the resulting film. Specifically, the orientation method of the invention introduces into the film such desirable properties as excellent WVTR characteristics, resistance to flexing cracking, elongation, tensile strength, impact resistance and cold resistance, all of which properties can be measured. using standard techniques known in the art. See, for example, the 1994 Annual Book of ASTM Standards, American Society for Testing and Material, Philadelphia, PA (1994); or TAPPI Test Methods 1994-1995, TAPPI Press, Atlanta, GA (1994). The film of the invention also possesses better dead-fold properties. Dead fold was generally evaluated by qualitative means, but can be indicated by fold retention as determined by ASTM D-920-49 (% fold retention after 30 s). These improved physical properties make the film of the invention ideally suitable for packaging, including for packaging food and other materials containing liquids. These physical properties also make the films very suitable for use in labels and other similar applications.

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The skilled artisan will recognize that an HDPE film can be prepared such that the biaxial orientation is balanced, that is, stretched to a substantially equal degree in both DT and MD. However, it has been unexpectedly found that a highly biaxially oriented HDPE film benefits from an unbalanced orientation, that is, biaxial orientation in which the orientation in the DT and the orientation in the MD are unequal. Mine specifically, the film benefits from an imbalance in orientation because the film has been stretched to a greater degree in the transverse direction than in the machine direction.

The properties of the film can be selectively controlled by adjusting the stretch ratio of the film, which is defined as the ratio of the degree of stretch in the transverse direction (DTX) to the degree of stretch in the machine direction (DMX), that is ie the DTX / DMX ratio. Thus, a film that stretches in the DM to a degree of about 6: 1 and in the DT to a degree of about 9: 1 will have a stretch ratio of about DTX / DMX = 9/6, or about of 1.5. (A balanced movie would have a stretch ratio of about 1).

Accordingly, it is preferred that the film of the invention has an orientation imbalance, that is, it is oriented to a greater degree in the transverse direction and to a lesser degree in the machine direction. Therefore, the film preferably has a DTX / DMX ratio greater than 1. For example, a film of the invention can be stretched in the machine direction to a degree of up to 6.5: 1, and stretched in the transverse direction to a degree of about 10.5: 1, to give a ratio. DTX / DMX of about 1.6.

The skilled craftsman will appreciate from the description given in this specification that the film of the invention is not only of unbalanced orientation, but rather that the unbalanced film is oriented to a greater degree in both directions. Therefore, the film of the invention is said to have "high biaxial orientation". In a more descriptive way, it can be said that the film of the invention has "high unbalanced biaxial orientation".

Accordingly, the high biaxial orientation of a film prepared according to the invention implies that the film has been dimensionally altered to a high degree. This dimensional alteration manifests itself as a relatively large increase in the surface area of ​​the film. The resulting increase in surface area is substantially equal to the product of the stretch factors. Take, for example, an HDPE film that has been oriented according to the invention by stretching it by a factor of 6: 1 (i.e. 500% increase) in MD and stretching it by a factor of 9: 1 (i.e. 800 % increase) in DT. In this example, the surface area of ​​the film is 6 x 9 = 54 times the surface area of ​​the original sheet (5,400% of the original).

Dimensional alteration manifested by an increase in surface area is generally accompanied by a concomitant decrease in the gauge (thickness) of the film. The decrease in gauge is directly proportional to the product of the stretch factors, and is normally substantially equal to that number. Accordingly, in the example above, the final gauge of the film following the biaxial orientation procedure is generally about 6 x 9 = 54 times smaller than the gauge of the original HDPE sheet. Therefore, to produce a film having a final thickness of 25.4 µm, oriented 6 DM x 9 DT, the stock material immediately before the orientation should be approximately 1,371.6 µm thick. Due to the high degree of biaxial orientation, therefore, the sheet to be oriented must typically be heavy gauge. The high biaxial orientation process of the invention allows dimensional alterations from about 30 to about 120. Therefore, as a general rule of thumb, a heavy gauge HDPE sheet is a sheet that is about 30 to about 120 times thicker than the intended HDPE film, depending on the stretch factors in the MD. and in the DT to be used.

The film of the invention is produced (in preparation for orientation) using conventional casting apparatus. For example, cast extrusion is generally carried out using a multi-roll stacking system or a casting roll with an air capsule (high speed air applied to the outside of the film). Other casting apparatus is also useful, such as a casting roll and water bath system, although this type of system can affect the transparency of the film, generally giving a rough and more opaque film.

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Following casting, the cast material, typically a coarse film, is oriented using conventional orienting apparatus. Preferably, the lamin is sequentially oriented, most preferably being oriented first in the machine direction and then being stretched in the transverse direction. Thus, the cast material is typically heated (optionally including a preheating step) to its orientation temperature and is subjected to orientation in the MD between two groups of rollers, the second group rotating at a speed greater than the first by an amount. effective to obtain the desired stretch ratio. The monoaxially oriented sheet is then oriented in the DT by heating (again optionally including preheating) the sheet as it is fed through an oven and subjected to transverse stretching in a branch. Alternative stretching approaches are also possible, including employing apparatus capable of simultaneous stretching, or stretching sequentially first in the transverse direction and then in the machine direction, but these approaches are less preferred as they often suffer from serious technical limitations that currently exist. make them either impractical or too expensive.

For the present invention, high biaxial orientation processes, including any preheating stage as well as the stretching stage, are carried out using equipment temperatures in the range from the glass transition temperature (Tg) of HDPE to above the crystalline melting point (Tf) of HDPE. Specifically, orientation in MD is carried out at a temperature of from about 60 C to about 160 C, more preferably from about 110 ° C to about 146 ° C. Orientation in DT is carried out at a temperature of from about 110 ° C to about 160 ° C, more preferably from about 124 ° C to about 146^°C. The skilled craftsman will understand that the orientation temperature employed in a particular situation will generally depend on the residence time of the sheet and the size of the rollers. An apparatus temperature higher than the Tm of the HDPE film may be appropriate if the residence time is short. The skilled craftsman also understands that the temperatures involved in these processes are related to the measured or set temperatures of the equipment better than the temperature of the HDPE itself, which generally cannot be measured directly.

The overall thickness of the biaxially oriented film is not critical, and can range from about 6.35 µm to about 254 µm. However, another advantage of the process of the invention is that the resulting film has an excellent gauge profile, even on films with thicknesses less than about 25.4 µm. Films around 17.8 µm have been produced that have excellent gauge profile, in addition to other superior properties. For example, films of this invention having a thickness of from about 6.35 µm to about 50.8 µm have been determined to have excellent WVTR (g-25.4 µm / 64516 mm²-24 h-1 atm) less than about 0.2 / 25.4 μm, while HDPE blown film requires a slightly heavier gauge (1.5 times thicker or more) to achieve a WVTR comparable. Although a higher density HDPE resin having a density of 0.957 or higher can be formed directly into thin films by cast extrusion, the problems of crimp, uniformity, smoothness and high WVTR have remained as obstacles. Accordingly, HDPE thin films of around 20 to 38 µm having the best balance of properties, particularly for VFFS applications, are obtained in the highly biaxially oriented films of this invention when prepared from extrudates having a thickness from about 381 to 5080 µm, which is reduced to the desired film gauge by biaxial orientation according to the invention. Generally, regardless of actual thickness, HDPE material at any stage between casting and completion of the required biaxial orientation is referred to as a sheet in this specification, while HDPE material is referred to as a film after biaxial orientation. .

The biaxially oriented HDPE film of this invention can be beneficially provided with at least one skin layer coextensively adhered to a base material including HDPE. Processes are known for making films having multiple layers, including up to five or more such layers. Skin layers can be used to obtain desirable properties in these films. In particular embodiments, for example for packaging applications, it may be preferred that the film includes a heat seal layer. For printing applications or label applications, a skin layer that improves printability (eg, ink receptivity) may be desirable. For example, an acrylate layer may be desirable to improve receptivity to acrylic-based inks. Other polymer layers can be made that have desirable barrier properties for gases such as oxygen. Methods for applying such skin layers to olephene films are known in the art, as are the appropriate skin layer materials to adapt such

ES 2 195 166 T3 films for different purposes.

Skin layers can be applied to HDPE film in a number of ways. The skin layer materials can be applied to the base HDPE material during extrusion, after extrusion but prior to orientation, between sequential orientation steps, or even following completion of the required biaxial orientation. Methods of manufacturing multilayer film structures include, for example, coextrusion, in which two or more polymer melts are extruded together without the melts mixing to a significant degree. The resulting coextrudate has a layered structure. Another method is extrusion lamination, in which a coating layer is extruded over a previously formed base film. Another method is co-stratification, in which a base layer and a skin layer are held in close contact and then subjected to orientation together. This type of method can use base layers and off-line prepared skin layers. On the other hand, a base layer can be oriented in one direction, then a skin layer is applied, and the composite material is then oriented in the other direction. In adhesive laminating, an adhesive or intermediate bonding layer is provided between a base film and the desired coating layer. On the other hand, when additional layer (s) are added to a previously extruded film, the surface of the film can be prepared to receive the added layer (s) by pretreatment according to methods. known, including chemical oxidation, flame treatment, corona discharge, and the like. Lamination methods that can be adapted for use with the films of the invention are described, for example, in US Patent Nos. 4,916,025, 5,223,346, 5,302,442, 5,527,608, and 5,500,283.

In a multilayer film, where HDPE constitutes a base or core layer, single layers or skins can be applied to one or both surfaces of the film. In such cases, the HDPE base layer will typically represent from about 70 to about 95% of the overall film thickness, or even a higher percentage thereof. Commonly mine, such other layers are applied by being coextruded on it, for example, coextruded from a conventional extruder through a flat sheet die, the melt streams being combined in an adapter and / or a multi-cavity die prior to extrusion. After extrusion from the die, the structure is rapidly cooled and quenched, and then subjected to the high biaxial orientation process. Finally, the edges of the film can be trimmed and then rolled onto a roller.

If a heat seal layer is desired, the layer can be made from any of the conventional materials used for this purpose together with polyolefin films, particularly polyethylene. For example, ethylene-vinyl acetate copolymers or ethylene-methacrylic acid salt ionomers (eg, available from DuPont under the trademark SURLYN) can be used. Films in which the heat seal layer is an ethylene methacrylic acid salt ionoimer have been found to be particularly useful in preparing films that are suitable for VFFS applications. The heat seal layer may include the heat seal resin as such or it may include small amounts of other materials. For example, the relatively expensive SURLYN ionomer can be mixed with small amounts of less expensive materials such as low-density polyethylene.

To meet the requirements for other applications, various layers of skin can be applied. For example, methods are known for making films adapted for use in printing, eg films having increased printability useful for making labels and paper substitutes. Skin layers that improve the receptivity to or retention of inks, including water-based inks, and that are suitable for use to prepare labels, whether opaque or transparent, are described in South African Patent Application Serial No. SA 94/9712, the disclosure of which is incorporated herein by reference in its entirety. Acrylate skin layers can be used to improve the retention of acrylic based inks.

The film of the invention can also be metallized according to methods known in the art. Such metallized films can replace metal foils in many applications.

Additives can also be incorporated into the polymeric film materials, either in the HDPE base material or in a skin layer, if any. Numerous such materials are known and numerous methods for their incorporation into films are also known. Suitable additives include, without limitation, antioxidants, fillers, particulate materials, colorants, pigments, light stabilizers, heat stabilizers, antistatic agents, slip agents,

ES 2 195 166 T3 contact release agents, abrasives, and the like.

Opacifying agents may be included in the biaxially oriented HDPE film of the invention. Such agents are typically included in an amount of up to about 10% by weight, preferably at least about 1% by weight. Such agents can be included in the HDPE resin prior to extrusion. Suitable opacifying agents include, for example, iron oxides, carbon black, aluminum, aluminum oxide, titanium dioxide, and talc.

In an especially preferred embodiment, cavity-forming agents or void-initiating particles are included in the film in amounts up to about 25% by weight. Such agents are typically added to the HDPE melt prior to extrusion and are capable of generating voids (cavities) in the film structure during the film-making process. Small inhomogeneities introduced into HDPE by the cavity-forming agent are believed to result in points of weakness in the sheet. The biaxial orienting process then induces small tears in the HDPE, causing cavities in the processed film. Suitable cavity-forming agents include, for example, finely ground inorganic materials. A preferred cavity-forming agent is calcium carbonate (CaCO3). Organic cavity-forming agents are also known, but are generally less preferred due to their limited operating temperature range. However, such organic cavity-forming agents can be useful if they divide extremely finely and are either resistant to melting at operating temperatures or produce appropriate inhomogeneity in the HDPE material. Cavity-forming agents can be included using methods known in the art, such as those described in application WO 94/14606, all of which are incorporated herein by reference. Accordingly, in the method of the invention in which a cavity-forming agent is employed, CaCO3, polystyrene or other cavity-former can be included in an amount from about 5% by weight to about 25% by weight. , on a HDPE core. Applicants are currently unaware of any commercially practicable process for making biaxially oriented, cavity HDPE films. Therefore, another advantage of the invention is that the method is useful for making cavity films that exhibit physical properties (eg, improved gauge control) that are substantially better than previously possible.

The film can be treated to improve its wettability and adhesion to coatings such as, for example, inks. Such treatments are conventional and known in the art, for example, exposing the film to corona discharge, flame treatment, and the like.

As indicated above in this, the high biaxial orientation employed in the invention requires the use of cast HDPE lines having thicknesses substantially greater than those currently used for monoaxial orientation. For example, for a film that stretches in MD to a degree of 6: 1 and stretches in DT to a degree of 10: 1, the film is thinned by a dimensional factor proportional to the product of stretch at the MD and the stretch in the DT, or --- 60. In this example, the cast lamin should have a thickness of at least about 60 times the desired thickness of the biaxially oriented film. Accordingly, in the case where a film that is about 25.4 µm thick is desired, the cast sheet should be about 1524 µm thick. In contrast, for a 10X monoaxially oriented film having a thickness of about 25.4 µm, the cast sheet would only have to be about 10 times as thick or about 254 µm.

Such high-gauge HDPE laminates pose specific handling problems in their preparation. In particular, heavy gauge HDPE lamines are so thick that quenching on the casting rolls is difficult to carry out effectively. Using a relatively low cast roll temperature, for example ~ 60-71 ° C, which should otherwise be acceptable to handle thinner HDPE sheeting, tends to result in crimp of the sheeting. coarse corn out of the pouring roller. Also, such lower temperatures often produce wrinkled and uneven edges, leading to tear problems. Despite such problems, it has unexpectedly been found that the casting process can be beneficially modified to give sheets having the characteristics desired for biaxial orientation processes.

Applicants have made a surprising observation that a much higher cast roll temperature can be used, for example -93^°C or greater, to keep the sheet adhered to the cast roll so that curling is avoided and proper edges are formed, thereby making subsequent orienting procedure possible. However, although this approach produces high gauge lamines suitable for use in the biaxial orientation process, the high casting temperatures

ES 2 195 166 T3 introduce practical difficulties into the casting process itself. For example, a high cast roll temperature lowers the net temperature difference between the cast roll and the film, thereby reducing the rate of heat transfer out of the film. In addition, higher cast roll temperatures impose disadvantages in using a water bath to cool the film, since the water removes a large amount of heat from the roll, making it difficult to maintain the temperature of the cast roll. Also, higher temperatures produce significant increases in mineral deposits left on the machinery (and potentially transferable to the film) by evaporated bath water.

On the other hand, it has been unexpectedly found that curling of the HDPE sheet can be prevented, even when using a lower casting roll temperature, if the HDPE to be cast is provided with an outer layer of a promoter material. of the laundry. A casting promoter is a material that promotes the casting process by substantially improving the casting characteristics of the HDPE material to avoid some or all of the problems described in this specification. For example, the use of a cast promoter, among other things, reduces or eliminates sheet curl and improves edge uniformity during casting while simultaneously allowing the use of substantially higher cast roll temperatures. Such casting promoting materials improve the processability for the lamina apparently allowing optimization of the rate of heat transfer from the lamina without engendering defects such as curling, etc. Accordingly, any cast promoting material of a high gauge HDPE sheet is suitable for use as a cast promoter.

The laundry promoter is preferably a polyolefinic material, that is, a homopolymer, copolymer or terpolymer of an alpha-olefin, or a mixture of polymeric materials comprising a greater proportion of one or more polyolefins. It is believed that the cast promoter can act to hold the lamina on the cast roll by reducing the shrinkage of the lamina during the cooling process. The rate of shrinkage is believed to be related to the rate of crystallization and the degree of crystallinity in the casting promoting polymer. Therefore, the laundry promoter is preferably a polyolefinic material that exhibits less shrinkage than HDPE. Accordingly, the casting promoter should have a crystallization rate and a degree of crystallinity that are lower than that of HDPE. Since the crystallinity of a polyolefin is generally correlated with its density, the density of the laundry promoter is typically less than that of HDPE. Preferably, the density of the casting promoter material is below about 0.945. For example, medium density polyethylene material (eg, Dowlex 2027 (d = 0.942) from Dow Chemical Co., Midland, MI) can be used as a casting promoter according to the invention. On the other hand, a blend of low density polyethylene and HDPE can be used. For example, LDPE / HDPE blends containing from about 2% by weight to about 50% by weight HDPE, preferably from about 5% by weight to about 25% by weight HDPE, have been shown to be capable of to function as laundry promoting materials.

Preferred laundry promoting materials include copolymers or terpolymers of a higher proportion of propylene with a lower proportion of at least one other alpha-olefin. Most preferably, the use of a propylene and ethylene copolymer, or a propylene, ethylene and butylene terpolymer, will give a high quality film according to the invention. An ethylene-propylene copolymer containing at least about 80% propylene and up to about 20% ethylene, preferably from about 2% to about 2.5% ethylene, and most preferably about 2% ethylene , has excellent laundry promoting qualities. A preferred copolymer contains about 98% propylene and about 2% ethylene. This material has also been found to substantially improve the transparency (lower haze) and gloss properties of the film. Ethylene-propylene-butylene terpolymers containing at least about 80% propylene, preferably containing from about 2% to about 7% ethylene, and mine preferably 3% ethylene, and containing from about 2% % to about 7% butylene, preferably about 4% butylene, are also excellent laundry promoters. A preferred terpolymer contains about 3% ethylene, about 93% propylene, and about 4% butylene. These materials have also been found to be useful in imparting beneficial improvements in optical properties.

Propylene copolymers and terpolymers as described can also be blended to give a material that improves the heat sealability of the film. Such materials include, for example, LDPE or ethylene vinyl acetate (EVA), as well as other equivalent materials. Such dual function blends acted not only as laundry promoters but to provide the resulting films with heat sealability, thereby avoiding the need to separately apply a heat seal skin layer. For example, applicants have obtained beneficial results using blends of a propylene copolymer.

ES 2 195 166 T3 leno / ethylene and up to about 35% by weight LDPE or up to about 20% by weight EVA.

The casting promoting material is generally provided as a layer on at least one surface of the HDPE material prior to casting by means of coextrusion. Preferably, the coextrudate includes at least about 80% by weight of the HDPE base material and up to about 20% by weight of the casting promoter layer (s). In the resulting cast sheet, the HDPE material (optionally including one or more tie layers) constitutes at least about 80% of the sheet thickness, with the remainder being the cast promoter layer (s). . Furthermore, in order to optimize the beneficial effect observed for the laundry promoter, the laundry promoter is preferably provided as a layer on either side of the HDPE material, providing a lamina with outer layers of the laundry promoter. casting and an inner core layer of HDPE (eg an ABA structure, where A is the casting promoter and B is the HDPE base material). In a highly preferred embodiment, each outer layer of the cast promoter constitutes from about 1% to about 10% of the thickness of the sheet. The skilled craftsman will appreciate that different laundry promoting materials and / or different amounts of laundry promoting materials can be employed in the same application if different properties are required for each of the faces of the lamina.

It can be seen that, in the method used to produce a multilayer film having one or more laundry promoter layer (s), the outermost layer can be either a skin layer or the laundry promoter layer. For example, if a sheet is provided with a skin layer after casting, the outermost layer would be a skin layer applied to the outer surface of the laundry promoter layer. This approach can be used to prepare a film having a CABAC structure, where C is the skin layer, A is a cast promoting material, and B is the base HDPE material. On the other hand, a film can be produced by coextrusion in which the outer layers are layers of the laundry promoter, while an intermediate layer, for example a binding layer, contains a pigment. This approach can be used to create a film having an ACBCA structure, where A is the cast promoter, C is the bonding layer, and B, again, is the base HDPE material. The skilled craftsman will appreciate that other permutations are possible to formulate multilayer films for particular purposes.

Example 1

A series of filmmaking attempts were made to determine the ability of HDPE films to be biaxially oriented. HDPE which had a density of 0.958 and a melt index of 1.1 (Lyondell M6211) was used to prepare cast material using a conventional process, except that a high temperature (96 ° C) cast roll (i.e. the temperature of the cooling water in the roll) to allow the handling of relatively thick casting materials (<1106 μm). Combinations of orientation conditions were experimented to examine the effects of unbalanced biaxial orientation. Specifically, the orientation was carried out by stretching the casting material to a degree from 4.0: 1 to 7.0: 1 in the machine direction, at temperatures from 124^°C up to 135^°C, and up to a degree from 7.0: 1 to 9.5: 1 in the transverse direction, at temperatures in the range from 121^°C up to 160^°C. Various tests are described in Table I.

TABLE I

<td>Show</td><td>DMX</td><td>Tdmo^{1 2}(° C)</td><td>DTX</td><td>rp 2 Tdto (° C)</td><td>Comments</td>

<td>TO</td><td> 4,0</td><td> 127/124</td><td> 7,0</td><td> 160/127</td><td>Very bad gauge profile</td>

<td>B</td><td> 5,7</td><td> 135/135</td><td> 9,5</td><td> 146/121</td><td>Some bands in DT</td>

<td>C</td><td> 6,6</td><td> 135/135</td><td> 9,5</td><td> 146/121</td><td>Uniform gauge</td>

<td>D</td><td> 7,0</td><td> 135/135</td><td> 9,5</td><td> 146/121</td><td>Not operable</td>

¹ Preheat temperature / stretch temperature ² Preheat Zone Temperature / Stretch Zone Temperature

IS 2 195 166 T3

The sample A film was made using orientation conditions which will generally be very good for making opaque oriented polypropylene (OPP) films, ie, stretching 4 DMX and 7 DTX. However, the resulting film was of very poor quality, including having a very poor gauge profile. Specifically, the sheet stretched on the MD had strips of thick material aligned on the DT (bands on the DT), and the film at the exit of the orientator on the DT had these strips along with a broad strip of very thick material below the DT. its center (band in the DM). No temperature conditions were found to be capable of imparting a uniform gauge to the films at this degree of biaxial orientation.

It was found unexpectedly, however, that increasing the DMX served to reduce the severity of the bands in DT. (The DMO temperature was increased to reduce the slip associated with increased stress on the base material). Accordingly, a series of tests will be carried out in which the DMX is varied while the orientation temperature in the DM was kept constant, which was 135 ° C, the stretch ratio in the DT (DTX = 9 , 5) and the orientation temperature in the DT which was 146/121 ° C. From these tests, the BD samples of Table I were obtained. It is seen that at a DMX of 5.7, the bands in the DT (sample B) were reduced, while a uniform gauge was obtained using a DMX of 6.6 (sample C). However, by further increasing the DMX towards 7.0 or above, the orientation process became inoperable as the film was too split to allow stretching in the DT (sample D).

The oriented film of Sample C not only exhibited a superior gauge profile, but also possessed excellent tensile properties and associated stiffness as well as will be tested by conventional methods known to the skilled artisan. Specifically, it was determined that the traction modulus in the MD was 3 x 10⁶ kPa, the traction modulus in DT was determined to be 5.9 x 10⁶ kPa and the WVTR was determined to be 0.14 g / 64.516 mm² - 24 h at 38 ° C and 90% relative humidity. The dead fold of the film of sample C was also superior according to the subjective evaluation.

From these results it is clear that a high biaxial orientation of an HDPE film requires specific conditions in order to obtain a film with desirable properties. Specifically, if the machine direction (DMX) stretch is either too high or too low, films with undesirable properties are created, and completely unusable films can even result. However, although the properties of the film of the invention are highly sensitive to DMX orientation, the film is substantially less sensitive to DTX, and maintains its useful characteristics at higher DTX, for example, substantially above 9: 1 ( data not revealed). Examples 2-7

Table II summarizes a series of experiments illustrating the features and advantages of the present invention. In each case, sheets were made using conventional extrusion and casting equipment, and orientation will be carried out using conventional orientation equipment. In all experiments, the HDPE polymer was Lyondell M6211. When a copolymer (CP) was used as a casting promoter, the copolymer was a 6573XHC ethylene-propylene copolymer (2% ethylene and 98% propylene), obtained from Fina Oil and Chemical Co., Dallas, TX; When using a terpolymer (TP), the terpolymer was a 7510 ethylene-propylene-butylene terpolymer (3% ethylene, 93% propylene and 4% butylene), obtained from Chisso Corp., Tokyo, Japan.

IS 2 195 166 T3

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IS 2 195 166 T3

Example 2

Samples E and F in Table II illustrate a significant prior art problem that has been solved by the present invention. Specifically, sample E was manufactured using a cast roll set to below 88^°C, a temperature similar to that used to prepare thin HDPE sheets suitable for use in the low biaxial or monoaxial orientation processes of the prior art. However, the resulting sheet was impractical, that is, unable to be used in the high biaxial orientation process of the invention since it curled off the casting roll and had ragged edges. Sample F, by contrast, was placed using casting rollers set at a temperature of 97^°C. The resulting lamina was fully usable in the orientation process of the invention, insofar as it adhered to the casting roll and produced a good quality lamina with regular edges. The lamine was stretched to a degree of 6.6: 1 in the MD, and to a degree of 9.4: 1 in the TD. The resulting film exhibited good WVTR properties and gauge profile (20.3 µm). Clearly, the use of a higher temperature casting roll is an effective solution to the problem of obtaining a sheet of appropriate dimensions and quality for use in the high biaxial orientation process of the invention.

Example 3

Samples F, G and H together illustrate the specific unexpected benefits obtained by employing a cast promoting material when casting the HDPE sheet. Sample F, mentioned above in Example 2, did not include a cast promoter and required the use of a cast roll at high temperature, i.e. 97^°C. Samples G and H were cast as tri-layered lamines, each including a cast promoter layer on each side of the core HDPE material. Each layer of laundry promoter constituted 2% of the total weight of the lamin. Specifically, Sample G included layers of a terpolymer cast promoter, and Sample H included layers of a copolymer cast promoter. In each case, the cast promoter layers allowed the cast of the HDPE lamine at 60^°C, a temperature reduction of more than 15^°C.

Both casting promoting materials gave a sheet capable of high biaxial orientation according to the invention. Furthermore, adding any of the cast promoting materials to the sheet imparted more unexpected beneficial properties to the final oriented films, including improved haze and gloss. The terpolymeric cast promoter employed in sample G allowed the production of a film that had only 13% haze, compared to 79% haze in sample F; and 64% gloss, compared to only 6% gloss of sample F. Haze was evaluated according to ASTM D-1003-92, and gloss was evaluated according to ASTM D-2457-90, at an angle of 45^°. The copolymer casting promoter performed even better with regard to the film's optic properties, giving a film (sample H) in which the haze was reduced to only 5%, and in which the gloss was increased to 90%. .

Example 4

Three additional films, analogous to samples F, G and H described above in Example 3, were produced using another apparatus having different physical dimensions. Sample I is comparable to Sample F, again illustrating that a higher casting temperature allows the production of an HDPE film that has good WVTR properties and gauge profile. Samples J and K are comparable to samples G and H, respectively, except that 3% of the casting promoter was used in each layer of mine rather than 2% as in the first samples. In this apparatus, the casting promoter allowed the use of a casting temperature as low as 49^°C, in contrast to a casting temperature of 102^°C for sample E, that is, a temperature reduction of 35^°C.

Again, the casting promoters also improved the ioptic properties of the films. Significantly, the terpolymeric cast promoter reduced the haze to as low as 26% in sample J, compared to 40% in sample I; and increased the brightness to 32%, compared to 27% in sample I. (These benefits were probably compromised to some extent by the use of a water bath to cool the cast film, a procedure that generally tends to degrade the optic properties). The copolymer cast promoter used in Sample K further improved haze and gloss, reducing haze to only 7% and increasing gloss to 85%.

IS 2 195 166 T3

Example 5

Sample L described in Table II is exemplary of a cavitated film prepared according to the present invention. A multi-layered HDPE sheet having five layers was coextruded. The core layer was HDPE having 9% calcium carbonate (CaCO3) as a cavity-forming agent. The outer layers on each side of the film were layers at 2% terpolymeric cast promoter. Between the center layer and the two outer layers of laundry promoter were bonding layers (18% each) of the HDPE that included 4% TiO2 as a bleach.

The multilayered sheet was then biaxially oriented high according to the invention, stretching to a degree of 6.0 DMX and 11 DTX. Accordingly, the process parameters for preparing this film are comparable to those for sample J described above. The resulting cavitated film gave a yield of 3.7 x 10⁷ mm²/ kg and exhibited 18% light transmission. The brightness of this film was high, that is, 50%, while the WVTR was somewhat higher than for the film without cavities.

Example 6

Sample M is exemplary of a pigmented film prepared according to the invention. A multilayered film was prepared by coextrusion to have five layers having the ACBCA structure. The core or base layer (B = 83%) and the two outer layers (A = 2.5% each) were of the same HDPE. The bonding layers (C = 6% each) between the center and outer layers on each surface were HDPE containing 10% by weight of a 50% TiO2 blend in low-density polyethylene (density = 0.914, index of fusioen = 7), (product number 11171 obtained from Ampacet Corp., Tarrytown, NY). The pigmented leamine was biaxially oriented stretching to 6.6 DMX and 9.5 DTX to give a highly biaxially oriented film according to the invention. The resulting film had excellent WVTR characteristics, and exhibited 82% light transmission.

Example 7

Samples N and O illustrate the advantages obtained by creating an orientation imbalance in the film of the invention. Sample N was a film set to have balanced orientation, ie 6.5 DMX and 6.5 DTX. Sample O was a film made from a foil identical to that used for Sample N, but prepared to have an orientation imbalance, ie 6.0 DMX and 9.5 DTX. (The process conditions for casting and orienting the two films were substantially identical). The balanced film (sample N) exhibited a gauge band in the DM below its center, producing a wide fluctuation in gauge and preventing practical utility as a film. It has been observed that the gauge in DT cannot be made uniform when using typical branching conditions to make a film having balanced orientation. Sample O, by contrast, shows that gauge uniformity can be obtained by changing the mechanical orientation to provide unbalanced orientation. Sample O provided a film that was 25.4 μm thick and had a highly uniform gauge profile.

The differences in the gauge profile of samples N and O are shown through the comparison of Figures 1A (sample N) and 1B (sample O). The gauge profile in cross-sectional dimension was measured by means of a micrometer by taking groups of 25 measurements spaced 25.4 mm apart along a 609 mm section in the center of the film. Figures 1A and 1B show groups of gauge measurements taken at three different positions along the length of the films. The film from sample N was 914 mm wide, while the film from sample O was 1397 mm wide. Clearly, the film that had balanced orientation (sample N, figure 1A), exhibited wide variability in gauge, while the film that had unbalanced orientation (sample O; figure 1B) exhibited substantially uniform gauge, demonstrating another benefit obtained by the method. of the invention.

Example 8

A biaxially oriented film (sample H from Example 3) was produced by the method of the invention, as follows: HDPE (Lyondell M6211) was extruded through a 63.5 mm diameter main extruder at 254 ° C at 130 rpm . Two 38.1 mm diameter satellite extruders operating at 238 ° C and 22 rpm, extruded a propylene and ethylene copolymer (Fina 6573XHC). The extrudates were fed through a three-layer melt adapter to a die, both set at 249 ° C to give a three-layer coextrudate (ie, an ABA structure). The coextrudate was cast later

ES 2 195 166 T3 on a chill roll operating at 60 ° C without a water bath. The cast sheet was then fed to a DM orientation apparatus consisting of preheating, drawing and annealing rolls. Orientation in the MD was carried out using a preheat temperature of 143^°C and a stretching temperature also 143^°C. Post-orientation annealing in the MD was carried out at 110^°C. Orientation in DT in a rame apparatus was carried out using a preheat temperature of 153^°C and a stretching temperature of 127^°C. The traction on the DT was running at 20 m / min. Immediately after stretching in DT, annealing was carried out at 116^°C. Following orientation, the thick edges of the film, which had been gripped in the forceps of the rame, were trimmed. For convenience, the web was trimmed from a width of 1,460.5mm to a width of 663mm. The film was then treated by corona discharge on one side to a wetting tension of 36 dynes / cm.² and rolled into a roller.

Example 9

Biaxially oriented films were prepared according to the invention using various materials containing LDPE as casting promoters. HDPE (Lyondell M6211) was coextruded with various materials containing LDPE and the resulting lamines were oriented by stretching to a degree of 6.6 in the MD and to a degree of 9.4 in the DT. The cast promoting material was included as layers on each face of the HDPE core, each layer constituting 3% of the total thickness of the cast lamina. The physical data obtained from these experiments are summarized in Table III.

TABLE III

<td>Show</td><td>Laundry promoter</td><td>^TWash (° C)</td><td>Tdmo * (° C)</td><td>Tdto * (° C)</td>

<td>P</td><td>25% HDPE</td><td> 66</td><td> 118/118</td><td> 152/127</td>

<td>Q</td><td>10% HDPE</td><td> 66</td><td> 118/118</td><td> 152/127</td>

<td>R</td><td>5% HDPE</td><td> 66</td><td> 118/118</td><td> 152/127</td>

<td>S</td><td>100% LDPE</td><td> 66</td><td> 104/114</td><td> 150/127</td>

* Preheat / stretch temperatures

The formulations of the casting promoter materials used LDPE (Chevron 1017: density = 0.918; melt index = 6.9; obtained from Chevron Chemical Co., Houston, TX) mixed with different proportions (0-25% by weight) HDPE (Lyondell M6211). In addition, the PR samples contained 16% polystyrene (BASF 1800, obtained from BASF Corp., Mt. Olive, NJ) in the HDPE core as a cavity-forming agent. The presence of the polystyrene had only a negligible effect on casting performance and stretch uniformity.

The PS samples show that a lamine can be cast using low density polyethylene materials as casting promoters. The PR samples, in particular, demonstrate that the addition of a small amount of HDPE to LDPE produces excellent casting promotion, allowing self-casting 66^°C. The films obtained in the PR samples had excellent gauge profile and good physical characteristics. It was found that, when using 100% LDPE by weight (sample S), the DMO temperatures had to be kept somewhat lower to prevent sticking of the sheet to the DMO rolls. At these lower temperatures, it is difficult for me to stretch the film evenly.