Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-07-22
2004-04-06
Zalukaeva, Tatyana (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S074000, C526S319000, C526S317100, C526S324000, C526S329200, C526S346000
Reexamination Certificate
active
06716951
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a heat-resistant methacrylic copolymer, a process of producing the same, and an optical element.
BACKGROUND ART
Generally, methacrylic resins containing methyl methacrylate as the main component are used in various fields such as illumination covers, automobile parts, sign boards, ornaments, general merchandises and the like, due to their heat resistance and remarkable transparency. However, the heat resistance of methacrylic resins is as insufficient as about 100° C., and application development thereof is restricted in considerable fields, consequently, improve in heat resistance is desired. Therefore, studies are conducted widely to improve the heat resistance of methacrylic resins, and there are some reports and suggestions.
For example, there are suggested a copolymer of methyl methacrylate, &agr;-methylstyrene and maleic anhydride (JP-A No. 4-300907), a terpolymer of methyl methacrylate, styrene and maleic anhydride (JP-A No. 4-227613), a quaternary polymer of methyl methacrylate, &agr;-methylstyrene, styrene and maleic anhydride (JP-A No. 61-271313), and the like.
As the method of polymerizing these ternary and quaternary polymers, a bulk polymerization method, solution polymerization method, suspension polymerization method and emulsion polymerization method are exemplified.
The emulsion polymerization method has a problem that since an emulsifier and a salting agent are used in the production process, these substances remain in a copolymer, and the resulted copolymer has lowered transparency and color tone. In the emulsion polymerization method and suspension polymerization method, since maleic anhydride is polymerized in water, maleic anhydride turns into maleic acid and production of a copolymer containing a maleic anhydride unit becomes difficult. Further, the solution polymerization method requires a solvent removal process, namely, is disadvantageous in cost.
On the other hand, the bulk polymerization method is advantageous in cost as compared with the solution polymerization method. As this bulk polymerization method, a continuous bulk polymerization method and a casting polymerization method are general. However, in the continuous bulk polymerization method, a large scale apparatus is necessary causing an enormous cost for the facility, and production of multiple products in small amounts is difficult. In the case of the casting polymerization method, general is a cell casting method in which raw materials such as monomers and the like are poured into glass cells or metal cells such as stainless and the like and the raw materials are heated in an aqueous phase or gas phase, however, when maleic anhydride is used, there are problems that peeling of a polymerized product from cells made of glass, metal and the like after polymerization is difficult, meaning poor workability, and the like. Further, the heating mode in an aqueous phase generates large cost necessary for the facility. Though the heating mode in a gas phase is advantageous in cost, when the thickness of a cell increases, heat removal in polymerization is small due to low heat conductivity of air, and heat generation in polymerization cannot be suppressed to cause a tendency of burst of a polymerization reaction, decrease in transparency of a polymerized product due to temperature disproportionation occurs easily, and other problems are caused.
For the purpose of improving heat removal from raw materials in this polymerization, there is also a method, for example, in which a monomer mixture is cast into a polymerizing vessel made of a thin material such as a nylon polymer film and the like and heated in a gas phase, to suppress burst of a polymerization reaction and temperature disproportionation in a polymerizing vessel. However, in this method, yellowing of a polymerized product of raw materials containing maleic anhydride is often remarkable. The copolymer showing remarkable yellowing is not easily utilized in optical elements and the like.
DISCLOSURE OF THE INVENTION
The present invention has been accomplished in view of the above-mentioned problems in conventional technologies and the object thereof is to provide a methacrylic copolymer which is polymerized under heating in a gas phase, reduced in yellowness, and excellent in transparency and heat resistance, a process of producing the same, and an optical element composed of a methacrylic copolymer.
The present inventors have intensively studied to attain the above-mentioned object, and resultantly found that yellowing after polymerization of a copolymer containing maleic anhydride can be improved by using a material having oxygen permeability of not more than specific value as a material of polymerizing vessel.
Namely, the present invention is a process for production of methacrylic copolymers, characterized by charging a monomer mixture comprising from 70 to 95% by weight of methyl methacrylate, from 0 to 15% by weight of &agr;-methylstyrene, from 0 to 20% by weight of styrene and from 2 to 15% by weight of maleic anhydride at a molar ratio of the sum of &agr;-methylstyrene and styrene to maleic anhydride laying within the range of 1.0 to 2.5 into a polymerizing vessel containing at least one polymeric membrane layer having an oxygen permeability of 1.5×10
−13
(mol m
−2
·s
−1
·Pa
−1
) or below, sealing the polymerizing vessel, and polymerizing the monomer mixture under heating in a gas phase.
BEST MODES FOR CARRYING OUT THE INVENTION
In the present invention, the monomer mixture subjected to copolymerization comprises from 70 to 95% by weight of methyl methacrylate, from 0 to 15% by weight of &agr;-methylstyrene, from 0 to 20% by weight of styrene and from 2 to 15% by weight of maleic anhydride, and has a molar ratio of the sum of &agr;-methylstyrene and styrene to maleic anhydride laying within the range of 1.0 to 2.5.
In the present invention, the methyl methacrylate (hereinafter, described as “MMA”) which is the main component constituting a methacrylic copolymer excellent in heat resistance is a component necessary for maintaining optical property, weather resistance and mechanical property inherent to a methacrylic resin, and used in an amount laying within the range of 70 to 95% by weight. Further, this amount is preferably 80% by weight or more. When this amount is less than 70% by weight, the above-mentioned properties are lost, and when over 95% by weight, heat resistance lowers.
The &agr;-methylstyrene (hereinafter, described as “&agr;MS”) constituting a copolymer is a component which improves the copolymerization property of individual components in polymerizing a monomer mixture, and is one of components improving the heat resistance of the resulting copolymer, being used in an amount laying within the range of 0 to 15% by weight. Further, this amount is preferably 3% by weight or more, more preferably 6% by weight or more, and preferably 12% by weight or less. The content of &agr;MS is preferably controlled by the content of other monomer. By increase in this content, the heat resistance of the resulting copolymer can be increased. When this content is over 15% by weight, mechanical property decreases, and the amount of remaining monomers in the resulted copolymer increases because of lowering of polymerization speed and deterioration of heat-resistant decomposing property, further, the copolymer tends to be colored easily or a coloring tendency is observed in molding a copolymer.
The styrene (hereinafter, described as “St”) constituting a copolymer is a component which promotes copolymerization of MMA, &agr;MS and maleic anhydride component. Though copolymerization property of MMA and maleic anhydride is poor, copolymerization property is improved by adding St and copolymerizing them, and a copolymer containing a small amount of unreacted monomers is obtained. By addition of St, the heat resistant decomposing property of the resulting copolymer can be improved, and the molding property of the copolymer can also be improved. However, when the addition
Hatakeyama Hiroki
Marutani Takao
Sano Yoshiko
Mitsubishi Rayon Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Zalukaeva Tatyana
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