Heat-resistant high-impact polystyrene compositions

Abstract: A heat-resistant, high-impact polystyrene composition comprises by weight: (A) 100 parts of high-impact polystyrene having a polybutadiene content of from 4.5 to 8.0% and an average rubber particle diameter of 0.5 to 2.0 µm and containing no plasticizer; (B) 1.5 to 5 parts of glass fibers of average length of 0.5 to 2 mm and an average aspect ratio of 20 or more and having undergone surface treatment (e. g. spraying) with silane coupling agent (e. g. an aminosilane); and (C) 2.5 to 10 parts of a styrene-butadiene-styrene copolymer having a butadiene unit content of 40 to 80%, and (D) 0.05 to 5 parts of a metal salt of a (C12-22) fatty acid (e.g. calcium or magnesium stearate). The components are blended, possibly pelletised and injection molded to make moldings, e.g. for office machines or interior parts of automobiles, of good appearance, high heat resistance and high impact strength.

Description: HEAT-RESISTANT HIGH-IMPACT POLYSTYRENE COMPOSITION The present invention relates to a polystyrene composition which has good appearance, high heat resistance, and high impact strength and which can be used extensively for making, for example, housings of copying machines or office automation (OA) apparatus and interior automotive trim materials. Materials for housings of copying machines, OA apparatus, etc. and for interior automotive trims are required to have good appearance and high heat resistance and high impact resistance. As such materials, ABS resins and modified polyphenylene ether (hereinafter abbreviated as modified PPE) resins are being generally used. ABS resins and modified PPE resins are expensive as compared with high-impact polystyrene (hereinafter abbreviated as HIPS) and, hence, there is a desire, in the industries, for a less expensive resin material in order to attain cost reduction. Incorporation of glass fibers into HIPS which is less expensive than ABS and modified PPE has conventionally been practiced for the purpose of improving heat resistance. However, since the incorporation of glass fibers generally impairs the appearance and impact strength of the HIPS, the glass fiber-incorporated HIPS cannot be used as housings of copying machines or OA apparatus, or as interior automotive trim materials, to which the composition of the present invention can be advantageously applied. We have therefore developed a composition containing HIPS as a main component and having improved appearance, heat resistance, and impact resistance. Our prior EP-A-0272114 describes HIPS compositions, useful for molding to make office machines, consisting of polystyrene and blended with a styrene polymer, e.g. a styrene-diene elastomer, reinforced with glass fibres of average fibre length 3-15 µm and surface treated with an aminosilane; the addition of unspecified stearates is suggested. There is no disclosure of the addition of an SBS copolymer. DE-A-2505701 discloses blends of a HIPS with an SBS copolymer and a metal (Zn) stearate, but no reinforcing fibres. The polystyrene composition of the present invention is a heat-resistant, high-impact polystyrene composition comprising: (A) 100 parts by weight of high-impact polystyrene, (B) from 1.5 to 5 parts by weight of glass fibers, (C) from 2.5 to 10 parts by weight of a styrenebutadiene-styrene copolymer, and (D) from 0.05 to 5 parts by weight of a calcium, magnesium, zinc, sodium or aluminum salt of a saturated fatty acid having from 12 to 22 carbon atoms, the high-impact polystyrene (A) having a polybutadiene content of from 4.5 to 8.0% by weight and an average rubber particle diameter of from 0.5 to 2.0 µm and containing no plasticizer (e.g., mineral oils); the glass fibers (B) having an average fiber length of from 0.5 to 2 mm and an average aspect ratio of 20 or more and having undergone surface treatment with a silane coupling agent; and the styrene-butadiene-styrene copolymer (C) having a butadiene unit content of from 40 to 80% by weight. All of the above requirements are essential for attaining the object of the present invention. The HIPS to be employed in the composition of the present invention can be prepared by dissolving polybutadiene in a styrene monomer and subjecting the solution to bulk polymerization or bulk-suspension two-stage polymerization. Either high-cis polybutadiene having a high cis form content or low-cis polybutadiene having a low cis form content can be used as the polybutadiene. However, the amount of the polybutadiene contained in the resulting HIPS should be from 4.5% to 8.0% by weight, in terms of the content of polybutadiene in the styrene monomer solution thereof to be polymerized. If the amount of polybutadiene contained in the HIPS is below 4.5% by weight, the HIPS cannot impart sufficient impact strength to the final composition. If it exceeds 8.0% by weight, the final composition not only shows insufficient heat resistance but has poor appearance. The polybutadiene rubber present in the HIPS must have an average particle diameter of from 0.5 to 2.0 µm. If the average rubber particle diameter of the HIPS used is below 0.5 µm, the final composition has insufficient impact strength although it has good appearance. If it exceeds 2.0 µm, the final composition has too poor appearance to put to practical use. The HIPS should not contain plasticizers such as mineral oils and liquid paraffins. Use of HIPS containing a plasticizer is not preferable in that the final composition has impaired heat resistance. The amount of glass fibers incorporated in the composition must be in the range of from 1.5 to 5 parts by weight per 100 parts by weight of the HIPS. If the amount of glass fibers incorporated is below 1.5 parts by weight, insufficient heat resistance results. If it exceeds 5 parts by weight, poor appearance results. The glass fibers must have an average fiber length of 0.5 to 2 mm and an average aspect ratio of 20 or more. If glass fibers with an average fiber length below 0.5 mm are used to produce a composition, sufficient heat resistance cannot be imparted to the final composition. Use of glass fibers having an average fiber length exceeding 2 mm results in poor appearance. If glass fibers having an average aspect ratio below 20 are used, sufficient heat resistance cannot be obtained. The aspect ratio as referred to herein means a ratio defined by the equation: Aspect Ratio = ℓ i /d i wherein d i is the diameter of glass fiber, and ℓ i is the length of glass fiber. Both average fiber length and average aspect ratio are determined by measuring, with an optical microscope or by other means, the diameters and lengths of about 100 glass fibers arbitrarily selected, and then calculating the respective arithmetic means. The glass fibers to be used must further be ones which have been surface-treated with a silane coupling agent such as an aminosilane. Untreated glass fibers have poor adhesion to HIPS and cannot impart sufficient heat-resistance and impact-resistance to the final composition. Usually, the surface treatment of glass fibers is carried out in such a manner that the glass fibers are dipped in an aqueous solution of a silane coupling agent such as an aminosilane or applied with a silane coupling agent or an aqueous solution thereof by spraying or other means, to thereby attach the silane coupling agent onto the glass fiber surfaces. The amount of the styrene-butadiene-styrene copolymer (hereinafter abbreviated as SBS copolymer) incorporated in the composition must be from 2.5 to 10 parts by weight per 100 parts by weight of the HIPS. If the amount of the SBS copolymer incorporated is below 2.5 parts by weight, sufficient impact resistance cannot be imparted to the final composition. If it exceeds 10 parts by weight, sufficient heat resistance cannot be obtained. The SBS copolymer to be employed in the composition of the present invention must have a butadiene unit content of from 40% to 80% by weight. If an SBS copolymer having a butadiene unit content below 40% by weight is used to prepare a composition, sufficient impact resistance cannot be imparted to the composition. On the other hand, if the butadiene unit content of the SBS copolymer exceeds 80% by weight, the final composition has impaired heat resistance. The metal salt of the C₁₂-C₂₂ fatty acid is incorporated in the composition in an amount of from 0.05 to 5 parts by weight per 100 parts by weight of the HIPS. Calcium stearate, magnesium stearate and zinc stearate are most preferably used. These fatty acid metal salts may be used alone or in combination of two or more thereof. The incorporated amount thereof should be in the range of from 0.05 to 5 parts by weight per 100 parts by weight of the HIPS. If the incorporated amount of the fatty acid metal salt is below 0.05 part by weight, the resulting composition has reduced tensile elongation and shows poor mold-release properties in injection molding processes. If it exceeds 5 parts by weight, the resulting composition has impaired heat resistance. The fatty acid metal salt is essential to the composition of the present invention, and the object of the invention cannot be accomplished if this essential component is replaced by any other lubricant such as stearic acid amide or polyethylene wax, because other lubricants impair the tensile elongation of the composition. Generally employed antistatic agents, antioxidants, pigments, fillers, etc. may be added to the polystyrene composition of the present invention as long as such additives used do not inhibit the effects of the present invention. The composition of the present invention may be produced by weighing predetermined amounts of the constitutional ingredients, i.e., the HIPS, glass fibers, SBS copolymer, and higher fatty acid metal salt, and granulating them in an ordinary extruder into pellets which are then provided for the use. Alternatively, the composition may be produced by simultaneously dry-blending predetermined amounts of the constituent ingredients and directly feeding the blend to a hopper of an injection molding machine. Thus, molded articles having good appearance, high heat resistance, and high impact-resistance, which the present invention is intended to provide, can be obtained. The resin composition of the present invention has good appearance, high heat resistance, and high impact strength and, hence, is suited for use as a molding material for producing housings of copying machines and OA apparatus and interior automotive trim parts. The present invention will be explained below in more detail by reference to the following Examples and Comparative Examples. EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLES 1 TO 16 (1) Raw Materials Used (A) HIPS: The HIPS used is one produced by the ordinary bulk-suspension polymerization process using a 20-liter autoclave. (B) Glass fiber (GF): Four kinds of glass fibers available from two manufacturers given below were used. (C) SBS copolymer: The SBS copolymer used is one produced from styrene and butadiene by a living polymerization process using an alkyl-lithium as a catalyst. (D) Higher fatty acid metal salt: Commercially available calcium stearate (Ca-st) and zinc stearate (Zn-st) were used. (2) Method of Measurement of Physical Properties (A) Preparation of Specimen: In accordance with ASTM D638, Type I test specimens were prepared by injection molding, and the specimens were subjected to measurement after being conditioned for 24 hours in a conditioning room having a temperature of 23°C and a relative humidity of 50%. (B) Heat resistance: Heat resistance was evaluated at a load of 18.6 kg/cm² in accordance with JIS K7207. (C) Izod impact strength: Measurements were made on 6.35 mm (1/4-inch) notched specimens at 23°C in accordance with ASTM D256. (D) Appearance of molded article: The surfaces of molded articles produced by injection molding were visually evaluated, in which articles having a markedly roughened surface due to GF were judged to be poor, while articles with no such conspicuously roughened surface were judged to be good. (E) Tensile elongation: Tensile test was conducted in accordance with JIS K7113, and the tensile elongation of a 50-mm section marked in the middle of the specimen was calculated using the following equation: Tensile Elongation (%) = [(ℓ₁ - ℓ₀)/ℓ₀] x 100 wherein ℓ₁ is the elongation at break (mm), and ℓ₀ is the length of the marked section (50 mm). (3) Preparation of Polystyrene Compositions According to the formulations shown in Tables 1 to 3 for the respective Examples and Comparative Examples, the ingredients for each composition were dry-blended and granulated in a 40-mm single-screw extruder into pellets which were then formed into test specimens for measurement of physical properties by means of a 150-ton injection molding machine. The results obtained are summarized in Tables 1 to 3.

Claims: A heat-resistant, high-impact polystyrene composition comprising: (A) 100 parts by weight of high-impact polystyrene, (B) from 1.5 to 5 parts of weight of glass fibers, (C) from 2.5 to 10 parts by weight of a styrenebutadiene-styrene copolymer, and (D) from 0.05 to 5 parts of weight of a calcium, magnesium, zinc, sodium or aluminum salt of a saturated fatty acid having from 12 to 22 carbon atoms, the high-impact polystyrene (A) having a polybutadiene content of from 4.5 to 8.0% by weight and an average rubber particle diameter of from 0.5 to 2.0 µm and containing no plasticizer; the glass fibers (B) having an average fiber length of from 0.5 to 2 mm, an average aspect ratio of 20 or more and having undergone surface treatment with a silane coupling agent; and the styrene-butadiene-styrene copolymer (C) having a butadiene unit content of from 40 to 80% by weight. A composition as claimed in Claim 1, wherein the glass fibers (B) have been surface treated with an aminosilane. A composition as claimed in Claim 1 or 2, wherein said metal salt (D) is calcium stearate, magnesium stearate or zinc stearate.

Inventors: OKAMOTO YASUSHI, UNO TETSUO, OKAMOTO, YASUSHI, UNO, TETSUO
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1990
Main classification: C08K13
IPC codes: C08L25, C08L55, C08K13, C08L53, C08K7, C08K5, C08L51