Thermoplastic polyoxymethylene moulding compositions with high tenacity and their use.

The invention relates to thermoplastic polyoxymethylene molding compositions with high toughness and their use for the production of moldings.

Polyacetals have long been used as versatile materials, especially in the technical field. They are suitable because of their excellent mechanical properties such as high rigidity, hardness and strength, as well as the possibility of producing molded parts and moldings within narrow tolerance limits and the good resistance to many chemicals as a metal substitute. For a number of applications, however, they have an impact resistance which is too low, in particular a multiaxial impact resistance which is too low, owing to their relatively high glass transition temperature. For such applications, it is also desirable that the products retain their good impact properties even at low ambient temperatures.

A number of methods are known for improving the toughness of partially crystalline, thermoplastically processable polymers by incorporating crosslinked or uncrosslinked, and in some cases also grafted, elastomers. The following modifications have been described using polyoxymethylene: a polyoxymethylene modified with polyurethane (DE-B 1 193 240); a polyoxymethylene (DE-B 1 931 392) modified with a two-phase mixture of polybutadiene and styrene / acrylonitrile (ABS); a polyoxymethylene modified with a graft copolymer based on acrylic ester-butadiene (DE-C 1 964 156); a polyoxymethylene (DE-A 2 408 487) modified with a polydiene / polyalkylene oxide block polymer and finally a polyoxymethylene (DE-A 2 659 357) equipped with modified polysiloxanes or silicone rubbers. In addition, grafted diene rubbers which have a specific graft structure have been proposed for polyoxymethylene (EP-A 0 156 285). The mixtures mentioned generally have improved toughness parameters at room temperature, in particular the low-temperature impact strength is significantly increased in the mixtures of DE-A 2 659 357 and EP-A 0 156 285.

However, it has been shown that such molding compositions do not always have fully satisfactory property profiles for important applications in which large deformation forces have to be absorbed; especially when low temperatures down to minus 30 C or even down to minus 40 C are used.

EP-A 0 037 686 discloses impact-resistant mixtures of hard thermoplastic materials, a rubber-elastic elastomer and a component which favors the distribution of the elastomer in the plastic matrix. The elastomer component is graft copolymers with a core-shell structure, for example polybutadiene grafted with styrene and methyl methacrylate (MBS) or with styrene / acrylonitrile (ABS). Polyoxymethylene is not mentioned in this reference as a polymer matrix. The same also applies to EP-A 0 079 477, which relates to molding compositions with improved low-temperature impact strength made from polyesters and an impact-resistant component, inter alia with a core-shell structure. The shell of the particles of the impact-resistant component can be made up of several shells of different material composition. The impact-resistant component contains polycarbonate as an essential component.

The object of the present invention was therefore to provide impact-modified polyoxymethylene which can be used at temperatures up to approx. minus 40 C compared to previously known systems has significantly improved toughness parameters in the multi-axial puncture test, without the original, advantageous properties of polyoxymethylene, such as good weathering and aging resistance, high surface quality, advantageous thermoplastic processability are significantly impaired. It was also desirable to use low-cost, halogen-free impact components.

It has been found that this object can be achieved in that graft copolymers composed of a rubber-elastic, single-phase core based on polydiene with a bimodal particle size distribution and a hard graft shell (core-shell structure) are used as the impact-resistant component.

The invention therefore relates to thermoplastic molding compositions<ul id="ul0001" list-style="none"><li>A. 50 to 95% by weight polyoxymethylene</li><li>B. 5 to 50% by weight of a rubber-elastic graft copolymer,</li></ul>in each case based on the sum of components A) and B) and, if appropriate, customary additives, component B) consisting of a rubber-elastic, at least partially crosslinked core based on at least one polydiene with particle diameters of 0.05 to 1 μm and at least one hard graft shell at least one vinyl monomer from the series styrene, (meth) acrylonitrile and methylene methacrylate and wherein the graft product B) has a bimodal particle size distribution.

As mentioned, the primary particles of the graft copolymer B) are present in the polyoxymethylene matrix in a bimodal distribution. Combinations of styrene / acrylonitrile or styrene and methyl methacrylate are preferred for the graft shell. The particle diameter range of the polydiene core is generally 0.05 to 1 µm, preferably 0.06 to 0.8 µm.

Component B) is based on the following rubbers:<ul id="ul0002" list-style="none"><li>a) 15-85, preferably 30-70, in particular 35-60% by weight (based on the sum a + b) of a rubber latex with an average particle diameter dso of 0.05-0.2, preferably 0.08-0 , 15 um, and</li><li>b) 85-15, preferably 70-30, in particular 65-35% by weight (based on the sum a + b) of a rubber latex with an average particle diameter dso of 0.2-1, preferably 0.25-0.8 , in particular 0.3 - 0.6 µm, with the proviso</li></ul><ul id="ul0003" list-style="none"><li>that dso of (b) is larger by a factor of 1.5 - 8, preferably 2.7-5, in particular 3 - 4.5, than dso of (a),</li><li>the quotient Q = (dso - dio) / d₅o of the individual rubbers is at most 2, preferably at most 1.8, in particular at most 0.8,</li><li>and the difference dio (b) minus dso (a) is 0.01 to minus 0.10, preferably zero to minus 0.08, in particular zero to minus 0.05.</li></ul>

The weights of the rubber latices a) and b) are based on solids.

To prepare the graft product B), either a rubber latex corresponding to a) and a rubber latex corresponding to b) can be mixed in the specified ratio and the vinyl monomers graft-polymerized onto this mixture, or the vinyl monomers separated onto a rubber latex according to a) and onto a rubber latex according to b) graft polymerize from each other and the two graft products (either as latices with subsequent co-precipitation or as mix existing solids).

The molding compositions according to the invention are more suitable for the above-mentioned applications than known molding compositions, and moreover they have no significant disadvantages.

It is essential to use at least partially cross-linked diene rubbers with rubber particle diameters dso of 0.05-1 µm, which have a "bimodal" particle diameter distribution. This means that by combining certain coarse-particle rubbers with certain fine-particle rubbers in the form of graft polymers B) molding compositions with significantly improved properties are obtained. To do this, the rubber particle size distribution used to produce component B) must be bimodal and should have two distinct maxima. However, in order to obtain the effect associated with these specific particle size distributions, it is necessary to start from rubbers, the particle size distribution of which obey certain parameters:<ul id="ul0004" list-style="none"><li>This includes a critical width of the distribution curve (expressed by the quotient Q), a critical distance of the distribution curve maxima (expressed by the difference di (b) - dso (a)).</li></ul>

The molding compositions according to the invention are characterized in that, depending on the modifier content, they retain the toughness behavior down to minus 20 C, sometimes down to minus 30 C, or in that the toughness properties decrease only below this temperature.

The effect of the mixture of different polydienes is surprising, since in general fine-particle graft rubbers only improve the impact strength parameters moderately under multiaxial loading of polyoxymethylene, and coarse-particle graft rubbers with falling temperatures lead to a steady decrease in the toughness parameters.

The synergistic effect that occurred was unpredictable because the known ABS plastics, which contain coarse-particle and fine-particle graft rubbers alongside one another (see, for example, DE-B 2 420 358), exhibit a continuous decrease in toughness with falling temperature and this was actually to be expected for polyoxymethylenes .

An essential feature of the mixtures according to the invention is the content of component B), ie the rubber-elastic graft copolymer, the amount of which has already been mentioned.

Component B) according to the invention consists of particles which predominantly, preferably more than 70%, have a core-shell (shell) structure. The core is formed by a rubber-elastic polymer phase onto which the hard shell, which can consist of several shells, is grafted. According to a further feature of the invention, the core should preferably be single-phase, which means that it consists predominantly, preferably completely, of the rubber-elastic soft phase and contains only to a small extent, preferably no inclusions of hard polymer components of the shell. The core generally makes up 40 to 95% by weight, preferably 60 to 90%, and in particular 70 to 80%, of the particles of component B); accordingly, the proportion of the shell (shells) is 5 to 60% by weight, preferably 10 to 40% by weight and in particular 20 to 30% by weight.

The core of component B) consists of polymers based on polydienes, such as polybutadiene or polyisoprene. The core polymer may optionally contain up to 30% by weight, preferably up to 15% by weight, in particular up to 5% by weight, of comonomer units. The comonomers are preferably other ethylenically unsaturated monomers, for example styrene, acrylonitrile, esters of acrylic or methacrylic acid with monovalent C.₁-C₄Alcohols such as methyl acrylate, ethyl acrylate, butyl acrylate or the corresponding methacrylates. The core polymer is at least partially crosslinked, the gel fraction (in toluene) generally being ≧ 70% and preferably 80%, in particular 90%. The gel content of the diene rubbers is determined at 25 ° C. in toluene (M. Hoffmann, H. Krömer, R. Kuhn, Polymeranalytik I and II, Georg Thieme-Verlag, Stuttgart 1977). Divinylbenzene, for example, is suitable as a crosslinking agent.

The shell of the particles of component B) consists of so-called hard polymers which are grafted onto the core, ie the graft substrate. The shell can be single-shell or multi-shell, and in the case of multi-shell, the various shells generally consist of different polymers or copolymers.

Monomers which lead to suitable polymers of the particle shell are methacrylonitrile, acrylonitrile, methacrylates whose alcohol component has 1 to 4 carbon atoms, such as methyl methacrylate and acrylates which are derived from alcohols having 1 to 6 hydrocarbon atoms, such as ethyl acrylate, propyl acrylate or n- Butyl acrylate. Copolymers of these monomers or copolymers of one or more of these monomers with styrene, a-methylstyrene, vinyl toluene are also suitable for the construction of the shell. Mixtures of 20 to 80% by weight of acrylonitrile or methacrylonitrile with 80 to 20% by weight of the stated acrylates, methacrylates or vinyl compounds have proven to be particularly favorable. Preference is also given to those graft polymers as component B) in which the shell has a two-shell structure, the first shell consisting of polystyrene and the second (outer shell) made of poly (meth) acrylate, which can be uncrosslinked or - preferably - partially crosslinked. Multifunctional monomers such as, for example, ethylene glycol or butylene glycol dimethacrylate or triallyl cyanurate can serve as crosslinking monomers.

The glass transition temperature of the polydienes of component B) according to the invention described above should expediently be in the range from minus 40 C to minus 120 ° C, preferably below minus 60 C and in particular between minus 80 C and minus 120 ° C.

The type of preparation of the graft copolymers with core-shell structure used according to the invention as component B) is known and is carried out either by single-stage polymerization in the case of a single-shell shell or by multi-stage polymerization in the case of a multi-shell shell. The latter procedure is described, for example, in US Pat. No. 3,985,704. In both cases, the graft copolymerization is carried out by means of water-soluble initiators or by means of activated initiators or by means of activated initiator systems, one component (activator) of which is at least water-soluble (cf. CB Bucknall, "Toughened Plastics", page 98, Applied Science Publishers Ltd., 1977, London).

Both in the one-stage and in the multi-stage graft polymerization one starts from polydienes which are in the form of aqueous latices with an average particle size dso in the range from 0.05 to 1 μm and which are at least partially crosslinked.

The mean particle sizes indicated are in all cases the weight average of the particle size, as determined with the aid of an analytical ultracentrifuge according to the method of W. Scholtan and H. Lange, Kolloid-Z. and Z.-Polymer 250 (1972), pages 782 to 796. The ultracentrifuge measurement provides the integral mass distribution of the particle diameter of a sample. From this it can be seen what percentage by weight of the particles have a diameter equal to or smaller than a certain size. The mean particle diameter, which is also referred to as the dso value of the integral mass distribution, is defined here as the particle diameter at which 50 percent by weight of the particles have an equal or smaller diameter than the diameter which corresponds to the dso value. In addition to the dso value (average particle diameter), the dio and dso values ​​resulting from the integral mass distribution are used to characterize the width of the particle size distribution of the rubber particles. The dio or dso values ​​of the integral mass distribution are defined in accordance with the dso value with the difference that they are based on 10 or 90% by weight of the particles. The quotient<maths id="math0001" num=""><img file="EP0470618A2_D0001.tif" /></maths>represents a measure of the distribution width of the particle size.

The difference d₁₀ (b) - dso (a) then represents a measure of the overlap of the curves: If the difference is positive, there is no or only a slight overlap of the curves, if the difference is negative, there is a curve overlap, the extent of which is determined by the size of the negative value is described.

From the fact that the rubber particles in the molding compositions are in the grafted state, one could assume that the grafting changed the rubber particle size compared to the rubber particles present in the rubber latices (for the production of B). However, it has been shown that the grafting and the amount of graft sheaths according to the characteristics described here have practically no influence on the size of the rubber particles, so that distribution curves determined on molding compositions can be compared and equated with distribution curves determined on latex.

Rubbers with the particle diameters listed can be obtained by emulsion polymerization of suitable monomers. In the emulsion polymerization processes known for this purpose, the latex particle diameters can be set by selecting the process conditions, for example by the type and concentration of the emulsifier, particle agglomeration, electrolytes, temperature, monomer / polymer concentration.

For the preparation of component B), the monomers or monomer mixtures are polymerized in the presence of the polydiene latices, a predominant part of the monomers being grafted onto the polydiene particles. The amount of polydiene is generally 40 to 95 wt .-% and the amount of the monomer (mixture) 5 to 60 wt .-%, each based on the total amount. The graft yield achieved is between 60 and 95%, preferably between 80 and 90%. The graft polymerization is carried out in solution or emulsion, preferably in an aqueous dispersion. For this purpose, the finely divided polydiene latices are added with the addition of the usual polymerization auxiliaries such as emulsifying or suspending aids, radical initiators, regulators, etc., the monomers or the monomer mixture are added and the mixture is polymerized at temperatures between 30 and 95 ° C., preferably 50 to 80 ° C. In a one-step reaction, it is expedient to use a water-soluble initiator, for example water-soluble peroxides, percarbonates or perborates. In the case of a multi-component initiator system (redox system), at least one component must be water-soluble. Examples of emulsifiers (dispersants) are aliphatic and aromatic sulfates, sulfonates, and salts of aromatic or hydroaromatic carboxylic acids.

In the case of the multistage reaction, which is preferred according to the invention, the graft polymerization and the working up take place as described in US Pat. No. 3,985,704. To form a multi-layer shell, a monomer, for example styrene, is first grafted onto the core polymer, for example a butadiene-styrene copolymer and then another monomer or monomer mixture, if appropriate in the presence of a crosslinking agent.

The "bimodal" graft products B) can be produced by various processes. So you can graft a dispersion of finely divided rubber particles (corresponding to a) with monomers, then mix this graft polymer emulsion with a separately produced graft polymer dispersion of coarse-particle rubber particles (corresponding to b) and then work up this mixture.

It is also possible to mix the rubber particle dispersion (latices) and graft-polymerize the graft monomers onto this mixture, in particular in an aqueous emulsion using systems which form free radicals, and then work up in a known manner.

Both in the processes of separate grafting and joint processing of the graft product and in the separate production of finely divided and coarsely divided graft product, it is possible to obtain graft products with different structures of the finely divided and the coarsely divided component.

According to a preferred method, the mixtures of rubber dispersions of the bimodal particle distribution according to the invention are grafted with graft monomers, in particular in an aqueous emulsion using systems which form free radicals, and then worked up in the known manner, for example by spray drying.

However, fine-particle graft rubbers and coarse-particle graft rubbers can also be prepared separately and mixed in amounts corresponding to the composition a / b according to the invention with the thermoplastic polyoxymethylenes for the production of the molding compositions according to the invention.

According to the invention, the known polyoxymethylenes, as described, for example, in DE-A 2 947 490, are used as base polymers (component A). These are essentially unbranched linear polymers which generally contain at least 80%, preferably 90% oxymethylene units (- CH₂0 -) contain. The term polyoxymethylene includes both homopolymers of formaldehyde or its cyclic oligomers such as trioxane or tetroxane and corresponding copolymers.

Homopolymers of formaldehyde or trioxane are those polymers whose hydroxyl end groups are chemically stabilized against degradation in a known manner, for example by esterification or etherification.

Copolymers are polymers of formaldehyde or its cyclic oligomers, in particular trioxane, and cyclic ethers, cyclic acetals and / or linear polyacetals.

Comonomers are a) cyclic ethers with 3, 4 or 5, preferably 3 ring members, β) cyclic acetals other than trioxane with 5 to 11, preferably 5, 6, 7 or 8 ring members and y) linear polyacetals, each in amounts of 0 , 1 to 20, preferably 0.5 to 10 wt .-%, in question. Copolymers of 95 to 99.5% by weight of trioxane and 0.5 to 5% by weight of one of the aforementioned co-components are best suited.

The values ​​for the reduced specific viscosity (RSV values) of polyoxymethylene are generally 0.3 to 2.0 dl.g-¹, preferably 0.5 to 1.5 dl.g-¹ (Measured in butyrolactone, stabilized with 2% by weight diphenylamine at 140 ° C in a concentration of 0.5 g / 100 ml). The melt index values ​​MFI 190 / 2.16 are mostly between 0.02 and 50 g.min-¹ (DIN 53 735). The crystal melting point of the polyoxymethylene is in the range from 140 to 180 ° C, preferably 150 to 170 ° C; its density is 1.38 to 1.45 g.m1-¹, preferably 1.40 to 1.43 g.ml^-1 (DIN 53 479).

The mixture according to the invention can optionally contain additives, such as stabilizers, nucleating agents, antistatic agents, light and flame retardants, lubricants and lubricants, plasticizers, pigments, dyes, optical brighteners, processing aids and the like, the amount of which is up to 50% by weight, based on the total mix can be.

Suitable stabilizers of the polyacetal phase are, in particular, polyamides, amides of polybasic carboxylic acids, amidines, for example dicyandiamide, hydrazines, ureas, poly- (N-vinyllactams) and alkaline earth metal salts of aliphatic, preferably hydroxyl-containing, mono- to tri-based carboxylic acids with 2 to 3 against the influence of heat 20 carbon atoms, e.g. calcium stearate, calcium ricinoleate, calcium lactate and calcium citrate. Above all, biphenol compounds are used as oxidation stabilizers, preferably diesters of monobasic 4-hydroxyphenylalkanoic acids which contain 7 to 13, preferably 7, 8 or 9, carbon atoms. In any case, it is advantageous to add sulfur-containing costabilizers, such as β, β'-thio-di (propionic acid lauryl ester), in addition to the usual diphenol compounds, for increased protection against oxidative attack by the graft copolymers (component B). Systems based on diphenylamine or naphthylamine have also proven to be excellent oxidation stabilizers. Phenolic antioxidants (BE-A 853 844 or EP-A 0 044 159) are also suitable for stabilizing component B).

Suitable light stabilizers are, for example, a-hydroxybenzophenone derivatives and benzotriazole derivatives.

The stabilizers are generally used in a total amount of 0.1 to 5, preferably 0.5 to 3 wt .-% (based on the total molding composition).

Furthermore, the mixture according to the invention can also contain conventional fillers, for example fibrous reinforcing materials such as glass fibers, carbon fibers; non-fibrous fillers such as glass powder, graphite, carbon black, metal powder, metal oxides, silicates, carbonates and molybdenum (IV) sulfide. These fillers can be equipped with an adhesion promoter or adhesion promoter system. The amount of the filler is up to 50% by weight, preferably 5 to 40% by weight, based on the total mixture. However, the mixture according to the invention preferably contains no fillers.

The mixtures according to the invention are prepared by intensive mixing of the constituents at elevated temperature, ie generally at temperatures above the melting point of component A, that is to say at about 160 to 250 ° C., preferably between 180 and 220 ° C. in well-mixing units such as kneaders, extruders, preferably twin screw extruders, or on mixing rollers. Usually, the powdery components are first mixed mechanically at room temperature and then melted for complete homogenization. The molding compositions according to the invention are notable for excellent toughness properties. An outstanding feature is the ability to absorb high multiaxial deformation forces even at low temperatures.

The mixtures according to the invention can be comminuted mechanically, for example by chopping or grinding to give granules, chips, flakes or powders. They are thermoplastic and therefore accessible to all processing methods typical of thermoplastic materials. They can therefore be processed to any shape by injection molding, extrusion, melt spinning or deep drawing and are suitable as a technical material for the production of semi-finished and finished parts, for example Bands, rods, plates, films, tubes and hoses, as well as machine parts, for example housings, gear wheels, bearing parts and control elements, as well as car accessories, such as spoilers and the like.

Examples

The graft copolymers (component B) used in the examples were obtained by aqueous emulsion polymerization of the monomers or monomer mixtures mentioned in Table 1 in the presence of the dispersed polybutadienes. The graft copolymers prepared in this way, which according to the invention (examples 1 to 12) have a core-shell structure, were spray-dried and mixed in a twin-screw extruder in different weight ratios with polyoxymethylenes of different melt viscosity. The melt temperature of the mix was around 210 to 230 C (case temperature approx. 190 ° C). The dried granules were sprayed in the usual way to test specimens on which the values ​​given in Table 2 below (Examples 13 to 25) and Comparative Examples 1 to 3 were determined.

The following materials were used:

Component A

<ul id="ul0005" list-style="none"><li>I: copolymer of trioxane and approx. 2% by weight ethylene oxide with a melt viscosity MFI 190 / 2.16 of approx. 9.0 g / 10 min according to DIN 53 735</li><li>II: Homopolymer of formaldehyde with a melt viscosity MFI 190 / 2.16 of approx. 10.0 g / 10 min</li></ul>

Component B

<ul id="ul0006" list-style="none"><li>III graft rubbers<ul id="ul0007" list-style="none"><li>111.1 Production of Styrene / Acrylonitrile Grafted Polybutadiene (ABS) In a glass reactor, 1000 parts by weight of a mixture of the polybutadiene latices (gel content approx. 85%) with the mean particle sizes dso = 0.12 (a) and dso = 0.45 were added under an inert gas (b) and the ratio given in Table 1. The solids content was 30% by weight and the pH was 10.0. After the mixture had been heated to 70 ° C., 0.6 part by weight of potassium persulfate, dissolved in 20 parts by weight of water, was added. A mixture of the monomers 90 g of styrene and 38.5 g of acrylonitrile and a solution of 5 parts by weight of an emulsifier (^(R)Dresinate 731, manufacturer: Hercules Inc., Wilmington DE, USA) was added dropwise evenly in 80 parts by weight of water. The pH was monitored and kept constant at 10.0 with 1N NaOH. After the graft monomers had been fed in, the temperature was raised to 80 ° C. and the mixture was polymerized out over a period of 3 hours. The graft copolymers obtained were obtained by spray drying as a finely divided powder.</li><li>111.2 Production of Styrene and Methyl Methacrylate Grafted Polybutadiene (MBS) In accordance with the instructions given under 111.1, 1000 parts by weight of the finely dispersed polybutadiene latices a) and b) were initially introduced into a reactor under inert conditions. The pH was adjusted to 9.0 with dilute acetic acid and 10% by weight of the required amount of styrene were added. The mixture was heated to 70 ° C. and, after the addition of 0.6 part by weight of potassium persulfate, was kept at 70 ° C. for 15 minutes. After the polymerization had started, the remaining styrene was added dropwise within 90 minutes and the mixture was brought to 80 ° C. after about one hour. Methyl methacrylate was metered in over the course of 60 minutes, to which 1% by weight of ethylene glycol dimethacrylate had been added and the mixture was then kept at 80 ° C. for about 90 minutes. The graft copolymer was obtained as in Example 111.1 by spray drying as a fine powder.</li></ul></li><li>IV. Production and Testing of the Molding Compounds The components according to Table 2 were melted, homogenized and stabilized in a continuously operating twin-screw extruder. The cylinder temperatures were set so that the melt temperatures were between 210 and 230 ° C. The emerging melt strand was cooled, granulated and dried at about 100 to 110 ° C under an inert gas.</li></ul>

The granules were injection molded into test specimens using an injection molding machine and the damage work was tested in the puncture test with electronic measurement value recording in accordance with DIN 53 453 at the temperatures given in Table 2.

The test specimens made from the molding compositions according to the invention showed up to test temperatures of minus 40 C compared to previously known systems significantly improved toughness parameters without the originally advantageous properties of the polyoxymethylenes, for example good weather and aging resistance, high surface quality, excellent sliding and Abrasion behavior or favorable thermoplastic processability, were significantly impaired.<tables id="tabl0001" num="0001"><img file="EP0470618A2_D0002.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0470618A2_D0003.tif" /></tables>