Thermoplastic compositions based on vinyl-aromatic graft copolymer and polyamide.

The present invention relates to thermoplastic compositions based on vinyl-aromatic graft copolymer and polyamide.

More particularly, the present invention relates to thermoplastic compositions based on impact-resistant (preferably styrenic) graft copolymer and polyamide, endowed with improved mechanical properties, particularly an improved impact resistance especially when measured on test pieces of high thickness; these compositions are particularly suitable for injection molding and thermoforming.

As is known, impact-resistant vinyl-aromatic copolymers are usually obtained by bulk or bulk-suspension polymerization of olefinic rubber solutions, such as ethylene-propylene (EPM) or ethylene-propylene-non-conjugated diene (EPDM) rubber solutions, in a vinyl-aromatic monomer, either alone or mixed with other polymerizable ethylenically unsaturated monomers.

Styrene-acrylonitrile copolymers grafted onto olefinic rubbers (AES) show a very good impact resistance but their low thermo-resistance makes them unsuitable for applications where temperatures of, e.g., 120 to 160°C are used and for the manufacture of products which come into contact wiht parts heated at such temperatures.

It is known that a way of providing this missing property of AES copolymers is to prepare a blend with other polymers having a good thermoresistance such as, for example, polyamides. In fact, said resins show satisfactory elongation, high energy absorption, especially in ball drop tests, and a very good thermoresistance. On the other hand, the high notch sensitivity and water absorption thereof constitute significant limitations to the use of these resins. Moreover, polyamides are rarely compatible with impact-resistant vinyl-aromatic copolymers and therefore the interfaces between the domains of the blend components represent very weak areas along which the breaking of the material can easily take place.

In order to make polyamides compatible with impact-resistant vinyl-aromatic copolymers, DE-A-31 20 803 proposes to use a compatibilizer composed of a styrene copolymer containing from 2 to 50% by weight of carboxylic acid and/or reactive anhydride groups. The compatibilizing coplymer described in said document has a molecular weight between 500 and 20,000; in particular 500 and 7,000, and preferably between 800 and 2,000.

The so obtained mixtures have improved impact strength (IZOD) properties which, however, are not completely satisfactory for several applications.

According to EP-A-202214, the impact strength of the polyamide compositions can be improved by preparing a blend comprising: <ul id="ul0001" list-style="none"><li>a) 5 to 79.5% by weight of a composition grafted onto rubber, consisting of a graft copolymer of 15 to 85% by weight of at least one monomer selected from C₁-C₄ alkyl methacrylates, C₁-C₄ alkyl acrylates, methacrylonitrile and acrylonitrile and of 85 to 15% by weight of a vinyl-aromatic monomer, wherein the monomers are polymerized in the presence of and grafted onto a rubber substrate having a glass transition temperature below 0°C, the amount of rubber being from 5 to 80% by weight and the amount of graft copolymer being from 95 to 20% by weight;</li><li>b) 94.5 to 20% by weight of a polyamide; and</li><li>c) 0.5 to 60% by weight of a compatibilizing copolymer comprising a copolymerized, functionalized monomer capable of reacting with the polyamide, the concentration of the functionalized monomer being from 0.05 to 4% by moles, with respect to the copolymer.</li></ul>

However, the compositions so obtained do not show the optimum combination of properties for all of the applications in which they can be used.

In particular, an increase in impact strength can be obtained by using an excess of impact-resistant vinyl-aromatic polymer with respect to the polyamide resin; however, this increase is achieved at the expense of the thermoresistance. Therefore the obtained products cannot be processed at a temperature above about 120 to 140°C. On the other hand, by increasing the polyamide resin content, the thermoresistance of the composition increases, but at the expense of the impact strength.

It has now been found that the use of a high impact vinyl-aromatic graft copolymer containing in the chain units derived from an ethylenically unsaturated comonomer having in its molecule a carboxylic group or one of its derivatives, not only allows to avoid the use of a compatibilizing agent (due to the fact that it is compatible as such with the polyamide resin) but also imparts an optimum combination of properties to the corresponding compositions, making them applicable in all the fields where both high impact strength and high thermoresistance are required.

Therefore the present invention provides thermplastic compositions having an optimum balance of mechanical, thermal and rheological properties, particularly high impact resistance (IZOD) characteristics, especially on test pieces of high thickness, comprising: <ul id="ul0002" list-style="none"><li>(A) from 10 to 90% by weight of high impact vinyl-aromatic graft copolymer (preferably composed of a vinyl-aromatic monomer/acrylic monomer-copolymer and an olefinic elastomer, said copolymer being at least partially grafted onto said olefinic elastomer) and containing (preferably in turn grafted onto said copolymer) from 0.1 to 2% by weight of units of an ethylenically unsaturated monomer having a carboxylic group or one of its derivatives such as anhydride, amide or ester in its molecule;</li><li>(B) from 90 to 10% by weight of polyamide resin.</li></ul>

The weight ratio of high impact vinyl-aromatic graft copolymer and polyamide resin (A:B) preferably ranges from 70:30 and 30:70.

Component (A) of the thermoplastic compositions of the present invention is a high impact vinyl-aroamtic graft copolymer which preferably comprises: <ul id="ul0003" list-style="dash"><li>100 parts by weight of a high impact vinyl-aromatic copolymer composed of from 90 to 30% by weight of a vinyl-aromatic monomer/ethylenically unsaturated acrylic monomer-copolymer which is at least partially grafted onto 10 to 70% by weight of an olefinic elastomer, and</li><li>from about 0.1 to about 2% parts by weight of units of an ethylenically unsaturated monomer containing in its molecule a carboxylic group or one of the derivatives thereof, such as anhydride, amide or ester groups, said monomer being preferably grafted onto said high impact vinyl-aromatic copolymer.</li></ul>

In the vinyl-aromatic monomer/acrylic monomer-copolymer, the weight ratio of the two types of monomers preferably ranges from 90:10 to 50:50, particularly from 80:20 to 60:40.

The amount of ethylenically unsaturated monomer(s) containing in the molecule a carboxylic group or one of its derivatives is preferably from 0.3 to 1% by weight, with respect to the high impact vinyl-aromatic copolymer.

The high impact vinyl-aromatic graft copolymers (A) used in the compositions of the present invention can be prepared, e.g., by means of a direct polymerization of the constituting monomers or by grafting the ethylenically unsaturated monomer containing a carboxylic group (derivative) onto the previously prepared high impact graft copolymer of vinyl-aromatic monomer/acrylic monomer/olefinic elastomer, in an extruder in the presence of a (peroxidic) catalyst and at a temperature which corresponds to at least the melting point of the components.

The polymerization is preferably carried out continuously, using a solution of a mixture of monomers including at least one vinyl-aromatic monomer, at least one acrylic monomer and at least one ethylenically unsaturated monomer with a carboxylic group (derivative) in the presence of an olefinic elastomer, a polymerization initiator and an inert solvent.

Also, the polymerization is preferably carried out in one or more reaction steps (arranged in series) and in each step the reaction mass is gently and homogeneously stirred to ensure a perfect thermal exchange.

For instance, the olefinic elastomer and the initiator are homogeneously dissolved in the monomers and in the solvent, then the solution is fed to the first reaction step; the polymerization is then continued in the subsequent steps at a temperature of, e.g., about 70 to about 150°C and at a pressure which is higher than the vapor pressure of the fed monomers and the solvent.

When the polymerization process is finished, the reaction mass is caused to flow through a thin-film evaporator heated at a temperature of, e.g., about 150 to 300°C and equipped with rotating blades which render the polymer sufficiently shear stress resistant to obtain the required morphological structure. The unreacted monomers and the solvent nay then be recycled from the evaporator and fed to the first step.

This procedure makes it possible to polymerize the monomers with gentle stirring, allowing, at the same time, an excellent thermal control of the reaction without taking into consideration the size and structure of the particles which will be carried out in the final step, in the thin-film evaporator in which the unreacted monomers and solvent are eliminated. At the same time, the particle size is reduced to values below 1, preferably of from 0.1 to 0.4 µm by means of a dynamic action which causes a variation in the structure of the elastomeric phase.

The solvent used, if any, is preferably an aromatic hydrocarbon such as benzene, toluene, ethyl-benzene, xylene, isopropyl-benzene, etc. The amount of solvent used generally is from 50 to 300, and preferably from 60 to 150 parts by weight per 100 parts by weight of monomers plus elastomer.

The polymerization reaction, as generally all the solution graft-polymerization reactions, is carried out in the presence of a free-radical polymerization initiator. Any known free-radical initiator can be used, even if the known organic peroxides normally used in graft-polymerizations are preferred. Examples of suitable organic peroxides are aromatic diacyl peroxides such as dibenzoyl peroxide; peroxy-esters such as tert-butyl peroxy-isobutyrate, tert-butyl peroxy-laurate, tert-butyl peroxy-benzoate, etc.; perketals such as 1,1-di-tert-butyl-peroxy-3,3,5-trimethyl-cyclohexane, 2,2-di-tert-butyl-peroxy butane, etc.; peroxycarbonates such as tert-butyl-peroxy-isopropyl-carbonate; and peroxy-ketones such as 1,1-bis-tert-butyl-peroxy-3,3,5-trimethylcyclohexanone,etc.. It is also possible to use mixtures of two or more of the above peroxides. The amount of peroxide usually is such as to allow a conversion in the first polymerization reactor of between 40 to 80% by weight; normally the amount used is from 0.1 to 2.0% by weight with resepct to the total amount of monomers and elastomer.

The above polymerization process allows to achieve a high degree of grafting onto the elastomeric chain of the resin of the vinyl-aromatic monomer(s), the ethylenically unsaturated acrylic monomer(s) and the ethylenically unsaturated monomer(s) containing a carboxylic group (derivative). This grafting takes place mainly during the first polymerization step which occurs isothermally, preferably at a temperature which corresponds to the half-life of about 1 hour of the peroxide initiator used. In practice, the temperatures used are usually from about 100 to about 250°C.

The thin-film evaporator used in the above process for the final operation and for obtaining the desired particle size, can be of any known type. In particular, the thin-film evaporator described in EP-A-267025 is preferred.

The above continuous solution polymerization process is already known and described, e.g., in EP-A-286071, whose contents are an integral part of the present specification.

The high-impact vinyl-aromatic graft copolymers (A) used in the compositions of the present invention can also be prepared, by extrusion or by means of any other equipment suitable for heat mixing, by direct reaction of the previously prepared high impact copolymer of vinyl-aromatic monomer, ethylenically unsaturated acrylic monomer and olefinic elastomer with the ethylenically unsaturated monomer containing a carboxylic group (derivative), preferably in the presence of a peroxidic initiator having a half-life of, e.g., from about 2 to about 20 seconds at about 200°C.

It is preferred to carry out the reaction at temperatures higher than 150°C and, more preferably, from about 170 to about 250°C with a residence time of more than about 15 seconds.

Specific examples of suitable peroxidic initiators are tert-butyl-cumyl peroxide, bis(tert-butyl-peroxy-isopropyl) benzene, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di-tert-butyl peroxy-hexane, dicumyl peroxide etc. All of the above peroxides are commercially available, e.g., under the trade names Trigonox® and Perkadox® (AKZO).

Concentrations of the peroxidic initiator of from 0.01 to 1 and preferably from 0.1 to 0.5 parts by weight per 100 parts by weight of the high-impact copolymer are normally used for this purpose.

The vinyl-aromatic monomer used in the production of the high-impact graft copolymer is preferably selected from those of general formula (I): <chemistry id="chem0001" num="0001"><img file="EP0471973A2_D0001.tif" /></chemistry> wherein: <dl id="dl0001"><dt>X</dt><dd>is hydrogen or C₁-C₄ alkyl (e.g. methyl or ethyl);</dd><dt>Y</dt><dd>represents halogen (F, Cl, Br, I, preferably F and Cl) or C₁-C₄ alkyl (e.g. methyl or ethyl); and</dd><dt>n</dt><dd>is zero or an integer of from 1 to 5.</dd></dl>

Specific examples of vinyl-aromatic monomers of general formula (I) are styrene; methyl-styrene; mono-, di-, tri-, tetra-and penta-chlorostyrene and the corresponding alpha-methyl-styrenes; styrenes alkylated in the nucleus and the corresponding alpha-methyl-styrenes such as ortho- and para-methyl-styrenes; ortho- and para-methyl-alpha-methylstyrenes, 3,5-dimethyl-styrene and tert-butyl-styrene, etc.

These monomers can be used alone or as mixture of two or more thereof.

The ethylenically unsaturated acrylic monomer preferably has the general formula (II): <chemistry id="chem0002" num="0002"><img file="EP0471973A2_D0002.tif" /></chemistry> wherein R is hydrogen or a C₁-C₅ alkyl group (e.g. methyl or ethyl). Acrylonitrile is particularly preferred.

Olefinic elastomers suitable for use in the copolymers (A) include both those with a high degree of unsaturation and those with a low degree of unsaturation. Along the elastomers with low unsaturation degree EPM and EPDM rubbers in which the ethylene/propylene weight ratio preferably ranges from 90:10 to 20:80 may be mentioned. The content thereof in unconjugated diene is preferably from 4 to 50 in terms of the iodine number. The unconjugated diene can be, e.g., a norbornene, a cyclic diene such as di-cyclo-pentadiene and cyclo-octadiene-1,5, an aliphatic diene such as pentadiene-1,4, hexadiene-1,4, hexadiene-1,5, etc..

The preferred EPDM rubbers are the terpolymers of ethylene/propylene/5-methyl-tetrahydroindene, ethylene/propylene/6-ethylidene-2-norbornene, ethylene/propylene/6-methylene-2-norbornene and ethylene/ propylene/5-ethylidene-2-norbornene.

Examples of elastomers with a high degree of unsaturation are polybutadiene, polyisoprene, copolymers of butadiene and/or isoprene with styrene or with other monomers, polychloroprene etc..

The amount of elastomer is preferably such as to have a concentration of rubber of from 10 to 70, and particularly from 15 to 40% by weight in the final copolymer.

Any ethylenically unsaturated monomer containing a carboxylic group or a derivative thereof in its molecule can be used for the production of the high impact vinyl-aromatic graft copolymers (A). Specific examples of such monomers are maleic acid, fumaric acid, maleic anhydride, maleic amide, unsaturated monocarboxylic acids (preferably having 3 to 30 carbon atoms) such as acrylic acid, crotonic acid, vinyl-acetic acid, 2-, 3- or 4-pentenoic acid, alpha-ethyl-acrylic acid, beta-methyl-crotonic acid, 2-methyl-2-pentenoic acid, alpha-ethyl-crotonic acid, 2,2-dimethyl-3-butenoic acid, heptenoic acid, 2-octenoic acid, 9-tetradecenoic acid, 9-hexadecenoic acid, 2-octadecenoic acid, 9-octadecenoic acid, decosenoic acid, erucic acid, 2,4-pentadienoic acid, diallylacetic acid, linoleic acid, linolenic acid, etc. and esters, amides and anhydrides of the above unsaturated acids.

The preferred ethylenically unsaturated monomers are maleic anhydride, maleic imides and fumaric acid, particularly the first one because of its high reactivity.

It is understood that in the graft-polymerization not all of the unsaturated monomers are grafted onto the rubber substrate; some of them may form free copolymers which are present in physical admixture with the graft-copolymer.

The amount of monomers grafted onto the rubber substrate can be determined by the extraction of the copolymer with a solvent of the ungrafted resin.

The polyamide resins (B) useful for the compositions of the present invention are the conventional ones, injection moldable, generally known as nylon resins, including aliphatic polylactams obtained by opening of the lactam rings and the polyamides produced by polycondensation of aliphatic diamines (e.g. containing from 4 to 14 carbon atoms) with aliphatic dicarboxylic acids (e.g. containing from 4 to 12 carbon atoms).

Specific examples of suitable polyamide resins are nylon 4; nylon 6; nylon 7; nylon 11; nylon 12; nylon 6,6; nylon 6,9; nylon 6,10 and nylon 6,12.

Partially aromatic nylons can also be used as component (B) in the compositions of the present invention.

The term "partially aromatic nylons" denotes the nylons obtained by partial or complete substitution of an aliphatic residue of an aliphatic nylon by an aromatic residue.

For example, the residues of adipic acid in nylon 6,6 can be replaced by terephthalic or isophthalic acid residues or by mixtures thereof; similarly some amine residues can be of aromatic nature.

Preferred polyamide resins are nylon 6, nylon 6,6 and random copolymers of nylon 6 and nylon 6,6.

The average molecular weight of the polyamide resins (B) is advantageously at least 10,000 and preferably at least 15,000 and the melting point thereof is preferably at least 200°C.

The compositions of the present invention can be prepared by hot mixing in any known mixing unit endowed with medium or high mixing power, such as single- and double-screw extruders, Banbury mixers, mixing rollers etc., e.g., at a temperature of from about 220 to about 260°C.

The compositions of the present invention may additionally contain stabilizing agents, UV stabilizers and/or other additives such as plasticizers, lubricants, flame retardant agents, flow agents, antistatics, dyes, pigments, glass fibres or other inorganic fillers etc. which are to impart particular characteristics to said compositions.

The compositions of the present invention are easily processable and show a host of properties which make them suitable for use in the manufacture of articles having both a high impact strength and a high thermal resistance and a reduced water sensitivity.

These compositions find application in the field of motor transportation, for the production of pieces that can be painted in an oven, parts that come into contact with the motor, electric household appliances, electronic and generally technical articles having different forms such as cups, boxes, containers, panels, sheets, rods, etc.

The compositions can also be used for the production of foamed articles, using known techniques.

The following examples are merely illustrative but not limitative of embodiments of the present invention.

In said examples all parts and percentages are expressed by weight, unless otherwise indicated.

EXAMPLES 1 TO 6

By means of a Baker-Perkins MPC V30 double-screw extruder there were extruded, with degassing and at a temperature of 240 to 250°C, compositions consisting of: <ul id="ul0004" list-style="none"><li>(A) styrenic high impact graft copolymer, consisting of 35% by weight of EPDM rubber with a Mooney viscosity at 100°C of 62-72 ML-4 and an iodine number of 18 and 65% by weight of a styrene/acrylonitrile (SAN) copolymer (weight ratio 70/30) containing grafted thereon maleic anhydride in the amount reported in the following table I. This copolymer was prepared according to the process described in example 2 of copending European patent application No. 91,107,794.9 (claiming the priority of May 15,1990) in the name of the applicant;</li><li>(B) Nylon 6 sold under the trade name TERNIL® B 27 (Montedipe) and having an average molecular weight of 18,000.</li></ul>

The weight ratios of the components of the composition are reported in table I.

By cutting the strands coming out of the extruder granules were obtained which were dried for 2 to 4 hours at 80 to 90°C.

To evaluate their mechanical and thermal properties, the granules were injection molded at a temperature of 220 to 240°C on a Negri &amp; Bossi V17 press to obtain test pieces having the dimensions required by the ASTM standards.

The determined characteristics and the used methods were as follows:

Mechanical properties

The elongation at break in tensile tests, the ultimate tensile stress, the yield strength and the elastic modulus were determined according to ASTM D638, and the IZOD notched impact strength at 23°C was determined according to ASTM D256, on test pieces having thicknesses of 3.2 mm and 12.7 mm.

Thermal properties

The VICAT A softening temperature (1 kg in oil) was determined according to ISO 306.

The measured properties are reported in the following table I. <tables id="tabl0001" num="0001"><img file="EP0471973A2_D0003.tif" /></tables>