AROMATIC TRIESTER PLASTICIZED COPOLYMERS OF ACRYLONITRILE AND STYRENE

Abstract: Styrene/acrylonitrile copolymers containing a plasticizing quantity of an aromatic triester.

Description: AROMATIC TRIESTER PLASTICIZED COPOLYMERS OF ACRYLONITRILE AND STYRENE This invention relates to new, resinous compositions having a high degree of transparency, good color retention properties and unusual adaptability to injection molding techniques. More specifically the invention relates to copolymers of styrene and acrylonitrile suitably plasticized to result in compositions having good melt flow without sacrificing any of the desirable properties of the copolymer. Copolymers of styrene and acrylonitrile, and methods for their preparation are well known in the art. The copolymers possess good resistance to organic solvents and are highly transparent. However, in some applications it is desirable to provide improved, i.e., increased melt flow properties in order to produce faster cycle times and other molding improvements. This is accomplished by use of a plasticizer. For example, it is well known in the art to use epoxidized soybean oil as a suitable plasticizer for styrene/acrylonitrile copolymers. Disadvantageously, however, resins containing epoxidized soybean oil have a tendency to discolor, especially when exposed to elevated temperatures for extended time periods. In addition, molders of complex and intricately shaped objects of styrene/acrylonitrile resins often must employ very high molding temperatures in order to provide improved resin melt flow. Previously known plasticizers such as alkyl phthalates have failed in such severe environments due to "sweating" or deposition of the plasticizer onto the mold surfaces. The primary purpose of this invention is to provide methods of improving the melt flow rate and other properties of copolymers of acrylonitrile and styrene. A further purpose of this invention is to provide a means of plasticizing copolymers of acrylonitrile and styrene without sacrifice of the desirable transparency or color properties of the copolymer. A further purpose of the invention is to prepare a new and more useful composition of matter that may be utilized at extremes of temperature without encountering problems of "sweat out", i.e., loss of plasticizer and condensation thereof on cool surfaces of molding equipment or injection molded parts. It has been found that triesters of aromatic tricarboxylic acids, especially trimellitic acid, wherein the ester group contains an alkyl or aryl radical of up to 20 carbon atoms, are unusually compatible with copolymers of styrene and acrylonitrile, and effectively improve the melt flow properties of the molten polymer without loss of crystalline polymer properties. In particular such polymers demonstrate little discoloration or plasticizer sweat out. The invention is preferably used in copolymers of 65 to 80 weight percent styrene and 20 to 35 weight percent acrylonitrile. An elastomeric impact modifier such as a butadiene based rubber, copolymers of ethylene and propylene and optionally a nonconjugated diene, and alkyl acrylate rubbers may be present in the copolymers as well. Suitable amounts of impact modifier are from 1-20, preferably 5-15 percent by weight. The copolymer may be prepared by emulsion, mass, solution or suspension polymerization methods. The preferred plasticizers for the practice of this invention are the acid triesters having the structural formula: Ar(C(0)0R) wherein Ar is an aromatic radical of up to 20 carbons, and R is an alkyl or aryl radical having up to 20 carbon atoms. Preferred triesters are linear Cg_2o alkyl triesters of trimellitic acid. Particularly preferred esters are tridecyl trimellitate, tridodecyl trimellitate, tri-n-octyl trimellitate, mixed C 810 linear alkyl trimellitates, etc. The straight chain or linear alkyl triesters are particularly preferred due to the improved color stability. Additive discoloration upon prolonged heating is believed to be due to the greater instability of tertiary hydrogen proton compared to a secondary hydrogen moiety. The plasticized copolymers are prepared by mixing the desired plasticizer and copolymer in any of the conventional mixing machines, for example, roll mills, Banbury type mixers, extruders, etc., which may be heated to soften the copolymers. The plasticizer may also be coated onto the surface of polymer granules to be later melted and incorporated into the polymer or incorporated into the polymerization mixture and thus into the polymer prior to devolitalization and granulation. The quantity of plasticizer used will depend upon the ultimate use of the composition. A plasticizing amount of the aromatic acid triester, i.e., sufficient amount to improve the polymer's properties, is employed. Preferred amounts of plasticizer are from 0.1 to 5 percent, more preferable 0.5 to 2.5 percent by weight. The optimum proportion of triester may readily be determined by preparing test samples and measuring the melt flow rate enhancement, rheology, and other desired physical properties of the resulting resin. The new compositions are unusually valuable for making injection molded articles which are of high clarity with improved molding properties. Compared to the use of epoxidized soybean oil (ESBO) plasticizers, the present invention results in improved melt flow rates and less discoloration of the resulting copolymer when exposed to high temperatures. When compared to phthalate or terephthalate diester plasticizers, mold sweating is substantially improved and a modest improvement in flexural strength is noticed. These results are not expected or predicted from known teachings. Additional additives such as colorants, UV stabilizers, reinforcing aids, blowing agents, fibers, fillers, etc. may be included in the copolymeric compositions of the present invention. Further details of the preparation and evaluation of the new products are set forth with respect to the following specific examples: Example 1 A recirculated coil reactor was employed to prepare catalytic styrene acrylonitrile containing approximately 25 percent by weight acrylonitrile and molecular weight (Mw) of about 150,000. (Actual molecular weights of samples tested are provided in Table I.) A 0.8 inch (20 mm) Welding Engineers extruder was used to compound plasticizers into the styrene/acrylonitrile feedstock previously described. Various plasticizers were added to provide a final plasticizer content of 1.0 percent by weight. Results of the physical property testing are contained in Table I. Table I Polymer Flex Strength 3 Plasticizer (Percent) MwdO 3) MFR 1 Vicat 2 psi (kPax IO 3 ) melt flow rate g/10 min/ASTM D1238 condition I Vicat heat softening temperature, °C flexural strength ASTM D790 epoxidized soybean oil di-isodecyl phthalate n-octyl, n-decyl phthalate tri-(2-ethyl hexyl) trimellitate n-octyl, n-decyl trimellitate, triester formed by reaction of trimellitic acid with a mixture of linear C 810 alcohols Comparative As may be seen by examination of Table I the trimellitate esters gave slightly improved melt flow rate compared to the use of epoxidized soybean oil. Additionally, the decrease in flexural strength for the trimellitate esters was not as great as with the phthalate diester plasticizers. When compared with the phthalate diesters for mold sweating problems, the trimellitate esters were shown to possess improved properties.

Claims: -1- CLAIMS 1. A composition of matter comprising a copolymer of from 65 to 80 weight percent styrene and from 35 to 20 weight percent acrylonitrile and a plasticizing quantity of a plasticizer characterized in that the plasticizer is an aromatic triester corresponding to the formula: Ar(C(0)0R) 3 wherein Ar is an aromatic radical of up to 20 carbons and R is an alkyl or aryl group of up to 20 carbons. 2. A composition according to Claim 1 wherein the aromatic triester is a trimellitic acid triester. 3. A composition according to Claim 1 comprising from 0.1 to 5 percent by weight aromatic triester. 4. A composition according to Claim 1 wherein R is linear C.g_2o alkyl. 5. A composition according to Claim 2 wherein the trimellitic acid ester is tri-n-octyl trimellitate, tridecyl trimellitate or a mixed C gl0 linear alkyl trimellitate.

Inventors: MAURER BRIAN R, PERRY JAMES D, ROUKEMA MENNO, MAURER, BRIAN, R., PERRY, JAMES, D., ROUKEMA, MENNO
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1990
Main classification: C08K5
IPC codes: C08K5, C08L25