Method for preparing maleic anhydride terpolymers

Abstract: A terpolymer of a monovinylic aromatic monomer, an acrylic monomer and maleic anhydride is formed according to a polymerization method in which the maleic anhydride is added to the reactor in a molten state, free of solvents and diluents, and the polymerization reaction is conducted in bulk in the absence of solvents and diluents. The terpolymer is useful in various applications including blending with ABS graft polymers.

Description: FIELD OF THE INVENTION The present invention relates to methods of preparing maleic anhydride terpolymers and, more specifically, to methods for preparing monovinylic aromatic-acrylonitrile-maleic anhydride terpolymers wherein the polymerization reaction is conducted in bulk in the absence of solvents and diluents. BACKGROUND OF THE INVENTION Various terpolymers of a monovinylic aromatic compound, an acrylic compound and maleic anhydride are known in the art and have been prepared according to various processes. For example, the Wingler et al U.S. Patent No. 4,141,934 discloses polymers of styrene and/or α-methylstyrene, a monoolefinically unsaturated monomer containing nitrile groups such as acrylonitrile and at least one additional monoolefinicially unsaturated monomer such as maleic anhydride which are formed using bulk polymerization methods. The bulk polymerization methods employ one or more initiators and pressures of from 1 to 20 bars. The Dufour U.S. Patent No. 4,298,716 discloses a tetrapolymer of α-methylstyrene, styrene, maleic anhydride and acrylonitrile which is prepared by mass/suspension batch processes wherein the α-methylstyrene acts as a chain transfer agent. The Lee et al U.S. Patents Nos. 4,223,096 and 4,262,096 disclose styrene-acrylonitrile-maleic anhydride terpolymers which are formed by bulk polymerization methods in the presence of a rubber such as polybutadiene. The Lee et al processes employ a catalytic initiator and a chain transfer agent and are conducted in the presence of one or more solvents. The Zimmerman et al U.S. Patent No. 3,336,267 discloses a process for the solution polymerization of styrene, maleic anhydride and a methacrylic or acrylic acid ester using from 5 to 50 weight percent of one or more solvents. The Liebig et al U.S. Patent No. 4,167,543 discloses styrene-acrylonitrile-maleic anhydride terpolymers which are formed using a polymerization initiator and which are suitable for use in thermoplastic molding compositions with an ABS type copolymer. Various processes for preparing styrene-acrylonitrile-maleic anhydride terpolymers which are known in the art are disadvantageous in one or more respects including, for example, they are relatively expensive, they require increased process control and/or they result in inferior terpolymer products. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a new process for the polymerization of a terpolymer from a monovinylic aromatic monomer, acrylonitrile, and maleic anhydride. It is an additional object of the present invention to provide such a method which is both economical and efficient in providing a homogeneous terpolymer product. It is a further object of the invention to provide a method for producing such a terpolymer having improved properties and which are superior for use in forming blends with ABS graft polymers. These and additional objects are provided by the method according to the present invention which provides a method for polymerizing a terpolymer, comprising adding monomers consisting essentially of a monovinylic aromatic monomer, acrylonitrile and maleic anhydride to a reactor, the maleic anhydride being added to the reactor in a molten state, with the molten maleic anhydride being free of solvents and diluents, and the feed of the monomers to the reactor being controlled to provide a terpolymer consisting essentially of from 60 to 95 weight percent of the monovinylic aromatic monomer, from 5 to 35 weight percent acrylonitrile and up to 30 weight percent maleic anhydride and conducting the polymerization reaction in bulk in the absence of solvents and diluents, the polymerization reaction being initiated thermally in the absence of a chemical initiator. The present methods allow superior reaction control and are both economic and efficient in monomer conversion. Additionally, the resulting terpolymers are useful in various applications including blending with ABS graft copolymers. These and additional objects and advantages will be more apparent in view of the following detailed description. BRIEF DESCRIPTION OF THE DRAWING The following detailed description will be more fully understood in view of the accompanying drawing which comprises a sole figure setting forth a schematic diagram of a reaction system for use in the present methods. DETAILED DESCRIPTION The present invention relates to a method for the polymerization of a terpolymer from a monovinylic aromatic monomer, acrylonitrile and maleic anhydride. As will be set forth below, important features of the present method include adding the maleic anhydride to the reactor in a molten state, with the molten maleic anhydride being free of solvents and diluents, and conducting the polymerization reaction in bulk in the absence of solvents and diluents, thermally in the absence of a chemical initiator. The monovinylaromatic monomers which may be utilized in preparing the terpolymer are generically described by the following formula: wherein R is selected from the group consisting of hydrogen, halogens, alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy and aryloxy. Specific vinylaromatic compounds include styrene, para-methylstyrene, α-methylstyrene, 3,5-diethylstyrene, 4-n-propylstyrene, α-methyl vinyltoluene, α-chlorostyrene, α-bromostyrene, dichlorostyrene, dibromostyrene, tetrachlorostyrene and mixtures thereof. The preferred monovinylaromatic monomer used is styrene. The method according to the present invention produces homogeneous terpolymers of the monovinylic aromatic monomer, acrylonitrile and maleic anhydride by continuously feeding the monomers to a polymerization reactor and continuously removing a polymer/monomer solution from the reactor. While in the past it has been customary to add maleic anhydride in solution with solvents, it is an important feature of the present method that the maleic anhydride is added to the reactor in a molten state which is free of solvents and diluents. The molten maleic anhydride may be premixed with the other monomers prior to addition to the reactor. It is also an important feature of the present method that the polymerization reaction is conducted in bulk in the absence of solvents and diluents. While in the past it has been customary to polymerize styrene-acrylonitrile-maleic anhydride polymers in reactors maintained at pressures of from 1 to 20 bars, the present methods may be conducted at reactor pressures of from 0.5 to 1.5 bars (absolute), and preferably at reactor pressures of from 0.5 to 0.9 bar (absolute), in order to obtain better reactor control of the polymerization reaction. In the present method, the polymerization reaction is initiated thermally in the absence of chemical initiators such as the peroxide initiators which are commonly employed in such polymerization reactions. Suitably, the reactor may be operated at a temperature of from 104°C (220°F) to 127°C (260°F), whereby a monomer conversion of 30 to 75 mole percent may be achieved. As noted above, polymer/monomer solution may be removed from the reactor and then directed to a devolatization system wherein the unreacted monomer is separated from the polymer. Various devolatization apparatus which are known in the art may be employed to remove the unreacted monomers from the polymer. For example, common devices such as flash chambers or vented extruders may be employed. The majority of the heat of polymerization may be removed from the reactor via refluxing of the unreacted monomers. The unreacted monomers are then recycled to the reactor for further reaction. Reaction rates of 15 to 30 percent per hour have typically been achieved. As noted above, the reaction takes place totally in bulk with no solvents or diluents present. The Figure discloses a schematic diagram of a reaction system suitable for use in the present method. With reference to the Figure, a single stir tank reactor 10 operating at a pressure of 0.5 to 1.5 bars (absolute), preferably from 0.5 to 0.9 bar (absolute), and at a temperature of 104°C (220°F) to 127°C (260°F) may be employed. Molten maleic anhydride is supplied to the reactor, free of any solvents or diluents. Styrene and acrylonitrile may be added to the reactor separately or together in a usual manner. A polymer/monomer solution is continuously removed from the reactor via outlet line 14 and is directed to a devolatization system 16 where unreacted monomer is separated from the polymer product. Unreacted monomer is directed via line 18 to a refluxing apparatus 20 for heat removal and is then recycled via line 22 back to the reactor for further reaction. As shown in the Figure, the recycled monomer may be combined with the styrene and acrylonitrile feed lines prior to introduction into the reactor. The resulting rigid terpolymer is particularly suitable for use in blends with ABS type graft copolymers to produce improved ABS blend compositions. Since the rigid terpolymer is formed in the absence of a rubber substrate, the reactor may be operated under less viscous conditions than those in which a bulk ABS type product is produced. This allows for higher reactor operating conversions and easier operation while producing a rigid terpolymer which is advantageous for blending with ABS type graft copolymers. The preferred embodiments set forth above are to illustrate the invention. Additional embodiments and advantages within the scope of the claimed invention will be apparent to one of ordinary skill in the art.

Claims: A method for polymerizing monomers to form a terpolymer, comprising adding monomers consisting essentially of a monovinylic aromatic monomer, acrylonitrile and maleic anhydride to a reactor, the maleic anhydride being added to the reactor in a molten state, with the molten maleic anhydride being free of solvents and diluents, and the feed of the monomers to the reactor being controlled to provide a terpolymer consisting essentially of from 60 to 95 weight percent of the monovinylic aromatic monomer, from 5 to 35 weight percent acrylonitrile and up to 30 weight percent maleic anhydride and conducting the polymerization reaction in bulk in the absence of solvents and diluents, the polymerization reaction being initiated thermally in the absence of a chemical initiator. A method as defined by Claim 1, wherein the reactor is operated at a pressure of from 0.5 to 1.5 bars (absolute). A method as defined by Claim 2, wherein the reactor is operated at a pressure of from 0.5 to 0.9 bar (absolute). A method as defined by Claim 1, wherein the reactor is operated at a temperature of from 104 to 127°C (220 to 260°F). A method as defined by Claim 1, wherein polymer and unreacted monomer are removed from the reactor and directed to a devolatization system where the unreacted monomer is separated from the polymer. A method as defined by Claim 5, wherein the separated monomer is recycled to the reactor. A method as defined by Claim 1, wherein the molten maleic anhydride is added separately to the reactor. A method as defined by Claim 1, wherein the molten maleic anhydride is premixed with at least one additional monomer prior to addition to the reactor. A method as defined by Claim 1, wherein the monovinylic aromatic monomer comprises styrene.

Inventors: HAYNES JAMES M, PACE JOHN E, ROBINSON JOHN C, HAYNES, JAMES M., PACE, JOHN E., ROBINSON, JOHN C.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1990
Main classification: C08F212
IPC codes: C08F222