Blends based on vinyl-aromatic polymers having high tenacity and impact strength

Abstract: Blends based on vinyl-aromatic polymers having high tenacity and impact strength, comprising : - a vinyl-aromatic copolymer containing from 2 to 25% by weight of an ethylenically unsaturated nitrile and a rubber amount not higher than 15% by weight; and - a grafted polymer consisting of an elastomeric core hav­ ing a ----- transition temperature of the second order lower than 10°C containing, in the grafted form, chains of vinyl mon­ omers.

Description: The present invention relates to polymer blends based on vinyl-aromatic copolymers having high tenacity and impact strength. More particularly, the present invention relates to blends of vinyl-aromatic copolymers having high tenacity, suitable for producing shaped articles endowed with good impact strength properties. As known, the vinyl-aromatic copolymers in question are prepared by polymerization, either in mass or mass-suspension, of solutions of polybutadiene rubbers or styrene-butadiene rubbers, or saturated rubbers of the ethylene-propylene type, in a styrene-acrylonitrile mixture. The resulting polymer is suited for the production of molded or extruded articles having a fairly good tenacity, which is sufficient for normal uses but insufficient for those applications, in which a high impact strength is required. This is due to the fact that by said polymerization procedure (other than the polymerization in emulsion generally utilized for the production of the ABS resins) it is only possible to employ reduced amounts of rubber, owing to the too high viscosities of the polymerization mass. A way to overcome these disadvantages is that of preparing blends with other polymers which possess the lacking properties, in order to obtain a material exhibiting the desired combination of properties. However, this approach was successful only in a few cases: generally, in fact, the blending results in the combination of the worst characteristics of each component, so that a material with so poor properties is obtained, that it has no commercial or practical value. The reasons for this drawback is the fact that not all polymers are compatible with one another and therefore they do not perfectly adhere. This gives rise to interfaces between the components of the mixture, such interfaces representing the weak and break points. Furthermore, French patent 1 456 141 discloses a thermoplastic polymer composition comprising: from 1 to 99% of a graft copolymer A, obtained by emulsion polymerization of a mixture of vinyl and/or vinylidene monomers and an aqueous latex of a synthetic or natural elastomer; and from 99 to 1% by weight of a graft polymer B, obtained by the polymerisation of a mixture of vinyl and/or vinylidene monomers containing a synthetic or natural elastomer dissolved therein. Again, it is desirable to improve the physical properties, especially the impact strength and tenacity properties of corresponding shaped articles. The applicant has now found that it is possible to obtain a material which exhibits a high tenacity and a high impact strength and in which the components are compatible with each other and perfectly adhere to one another. Thus, subject of the present invention are polymer blends having a high tenacity and impact strength based on : (a) a vinyl-aromatic copolymer containing an ethylenically unsaturated nitrile and an elastomer, and (b) a grafted vinyl-aromatic polymer containing an ethylenically unsaturated nitrile and a synthetic elastomer, characterized in that the blends comprise (a) from 10 to 98% by weight of the vinyl-aromatic copolymer obtained by a polymerization process in suspension, mass-suspension or continuous mass and containing from 2 to 25% by weight of the ethylenically unsaturated nitrile and a rubber in an amount not exceeding 15% by weight and (b) from 90 to 2% by weight of the grafted vinyl-aromatic polymer consisting of a three-step polymer having as a first step a rubber core based on butadiene, a second step polymerized from styrene and a final step, or shell, polymerized from methylmethacrylate, the elastomer content of said grafted polymer being higher than 35% and up to 95% by weight. The proportions between the two components a) and b) can be varied over a wide range, although values from 35 to 85% by weight of a), and, from 65 to 15% by weight, of b) are preferred. The term "vinyl-aromatic copolymer", whenever used in the present description and in the claims, means any thermoplastic solid copolymer containing, chemically bound, at least 50% by weight, of one or more vinyl aromatic compounds having general formula : in which X is hydrogen or an alkyl radical having 1 to 4 carbon atoms; n is zero or an integer from 1 to 5 and Y is a halogen or an alkyl radical having 1 to 4 carbon atoms. Examples of vinyl-aromatic compounds having the above general formula are : styrene; methyl-styrene; mono-, di-, tri-, tetra and penta-chloro-styrene and the corresponding alpha-methyl-styrenes, nucleus-alkylated styrenes and the corresponding alpha-methyl-styrenes such as ortho- and para-methyl-styrenes, ortho- and para-ethyl-styrenes, ortho- and para-methyl-alpha-methyl-styrenes, etc. These monomers are utilizable either alone or in admixture with one another or with other copolymerizable comonomers such as for example maleic anhydride. The rubbers are employed to render the vinyl-aromatic polymers impact resistant and, according to the present invention, the rubber content shall not exceed 15% by weight. Amounts from 2 to 12% by weight are preferred. The rubbers utilized to this purpose are : polybutadiene, polyisoprene, the butadiene and/or isoprene copolymers with styrene or with other monomers, or the saturated rubbers of the ethylene-propylene type, the ethylene-propylene-diene terpolymers, silicone rubbers with unsaturated groups, and the like. Ethylenically unsaturated nitrile, copolymerized with the vinyl-aromatic monomer, means, first of all and preferably, acrylonitrile; other ethylenically unsaturated nitrile monomers such as methacrylonitrile can be advantageously utilized. The modified vinyl-aromatic copolymers can be prepared according to any conventional polymerization process, either in suspension, or mass-suspension or continuous mass, provided the above-mentioned compounds are utilized as starting monomers. Said modified vinyl-aromatic copolymers differ from the known acrylotrile-butadiene-styrene resins, which are known as ABS resins, regarding morphology, structure and size of the rubber particles dispersed in the rigid polymeric matrix. As a consequence of such morphological differences, the blends of the present invention exhibit a resilience or impact strength exceeding that of the ABS resin, when the rubber content is lower than 15% and preferably lower than 10% by weight. The grafted polymer utilized to increase the impact strength of the vinyl-aromatic copolymer consists of a special grafted vinyl-aromatic polymer having an elastomeric core and containing, in the grafted form, chains of vinyl monomers. The elastomeric core (backbone) having a second order transition temperature lower than 10°C is based on polybutadiene, and can be polybutadiene copolymers of butadiene with styrene and/or with acrylonitrile, in which the content of butadiene is higher than 50% by moles. The function of the grafting monomer is to provide a residual content of unsaturation in the elastomeric step, particularly in the last polymerization steps and, by consequence, at, or in proximity of, the surfaces of the elastomer particles. These copolymers can be prepared by any known method such as bulk polymerization, polymerization in suspension, bulk-suspension polymerization, polymerization in solution or polymerization in emulsion. The grafted polymers cited hereinbefore are well known commercial products and are available from a great number of manufacturers such as, for example, Rohm and Haas Company, Philadelphia, USA, under the trade-name Acryloid KM 653 and KM 323, or from Kanegafuchi, under the trake-name KaneACE B 28, etc. To the blends object of the present invention it is possible to add other polymers compatible therewith, such as polycarbonate, polyesters, thermoplastic polyurethanes, polymethacrylates, styrene-methylmethacrylate copolymers, acrylic polymers, ABS, styrene-maleic anhydride copolymers, SAN and other engineering polymers, vinyl chloride polymers, etc. Such engineering polymers or polymers can be added in any ratio, such as for example from 1 to 90% by weight referred to the blend. Furthermore, the blends of the present invention can be co-extruded with said polymers or engineering polymers to provide composites having specific characteristics for the individual applications. The blends forming the subject of this invention are preparable by hot mixing, in any known mixing unit, such as single--screw and two-screw extruders, Banbury mixers, mixing rollers and the like, at temperatures ranging from 180 to 260°C. The compositions may contain stabilizers or other intimately incorporated additives, such as plasticisers, lubricants, antiflame agents, antistatic agents, dyestuffs, pigments, glass fibres or other inorganic fillers etc., in order to impart particular characteristics to the material. The blends, subject of the present invention, are easy to be processed and exhibit a complex of properties which make them suited to be utilized for preparing articles having a high tenacity along with a high impact strength, Said mixes are therefore used in the field of the household electrical apparatus, in the field of electronics and technical articles in general, in the form of films, sheets, strips, tapes, rods, boxes, cups, containers and the like. The blends are utilizable for producing foamed articles, using the conventional techniques of the art. For a better understanding of the present invention and to facilitate the embodiment thereof, a few illustrative but not limitative examples are given hereinafter. In the examples, unless otherwise specified, all parts and percentage are by weight. Examples 1 - 3 By means of a single-screw extruder Bandera TR45, with a length/diameter ratio of 25, there were extruded, under degassing and at a temperature of 210°C, blends consisting of : a) a vinyl-aromatic copolymer consisting of 72% by weight of styrene, of 12% by weight of alpha-methyl-styrene and of 8% by weight of butadiene rubber and of 8% by weight of acrylonitrile, and b) a grafted polymer of the type indicated in Table 1. The weight ratios between the two components of the mix are reported in Table 1. By cutting the strands leaving the extruder, granules were obtained, which were dried for 2 - 4 hours at 80-90°C. To determine the characteristics, the granules were injection molded at a temperature of 210°C in a press NEGRI & BOSSI V-17-110 FA in order to obtain samples having dimensions in compliance with the standards. The properties measured on the samples so obtained are reported in the following Table 1. For measuring the characteristics of the blends, subject of the present invention, the following methods were employed : A. Mechanical properties There were measured the tensile strengths and elastic modulus, according to ASTM D 638, and IZOD resilience with notch at 23°C, according to ASTM D 256, on 3.2 mm thick samples. B. Thermal properties Softening temperature VICAT B (5 kg in oil) was determined according to standard ISO 306. C. Rheological properties The melt index was determined according to standard ASTM D 1238 at 220°C and 10 kg. Examples 4 - 8 By operating under the same conditions as in examples 1 to 3, granules were prepared starting from blends composed by : a) a vinyl-aromatic copolymer consisting of 72% by weight of styrene, of 20% by weight of acrylonitrile and of 8% by weight of polybutadiene rubber; b) a grafted polymer of the type reported in Table II, and optionally c) an aromatic polycarbonate type "SINVET"® sold by ANIC, San Donato Milanese - MILANO. The weight ratios of the blends are reported in Table II as well as the properties measured on the samples obtained from said blends.

Claims: Polymer blends having a high tenacity and impact strength based on (a) a vinyl-aromatic copolymer containing an ethylenically unsaturated nitrile and an elastomer, and (b) a grafted vinyl-aromatic polymer containing an ethylenically unsaturated nitrile and a synthetic elastomer, characterized in that the blends comprise (a) from 10 to 98% by weight of the vinyl-aromatic copolymer obtained by a polymerization process in suspension, mass-suspension or continuous mass and containing from 2 to 25% by weight of the ethylenically unsaturated nitrile and a rubber in an amount not exceeding 15% by weight and (b) from 90 to 2% by weight of the grafted vinyl-aromatic polymer consisting of a three-step polymer having as a first step a rubber core based on butadiene, a second step polymerized from styrene and a final step, or shell, polymerized from methylmethacrylate, the elastomer content of said grafted polymer being higher than 35% and up to 95% by weight. The blends according to claim 1, in which the vinyl-aromatic copolymer (A) ranges from 35 to 85% by weight, and the grafted polymer (B) ranges from 65 to 15% by weight, based on the blend. The blends according to any of the preceding claims, in which the vinyl-aromatic copolymer (A) contains at least 50% by weight of one or more vinyl-aromatic compounds of formula: in which X is hydrogen or an alkyl radical containing 1 to 4 carbon atoms; n is zero or an integer ranging from 1 to 5 and Y is a halogen or an alkyl radical having 1 to 4 carbon atoms. The blends according to any of the preceding claims, in which the ethylenically unsaturated nitrile copolymerized with the vinyl-aromatic monomer is acrylonitrile. The blends according to any of the preceding claims containing, in addition, other compatible polymers such as polycarbonate, polyesters, thermoplastic polyurethanes, polymethacrylates, styrene-methylmethacrylate copolymers, acrylic polymers, ABS, styrene-maleic anhydride copolymers, SAN and vinyl chloride polymers. The blends according to claim 1 comprising the vinyl-aromatic copolymer, the grafted vinyl-aromatic polymer and a polycarbonate. The blends according to any of the preceding claims, in which said other compatible polymers are comprised between 1 and 90% by weight. The blends according to any of the preceding claims containing stabilisers, plasticizers, lubricants, antiflame agents, antistatic agents and/or glass fibers or other inorganic fillers. Composites prepared by co-extruding the blends of the preceding claims with other polymers or engineering polymers.

Inventors: BIGLIONE GIANFRANCO DR, BIGLIONE, GIANFRANCO, DR.
Applicants: ENICHEM POLIMERI, ECP ENICHEM POLIMERI S.R.L.
Application year: 1986
Main classification: C08L55
IPC codes: C08L51, C08L55, C08L25