Because of their excellent moldability, aromatic monovinyl resins are widely used as materials for electric articles, various industrial materials, materials of miscellaneous goods and containers for foods, packaging materials, etc.
However, as to aromatic vinyl resins, for example, polystyrenes, the resulting products sometimes suffer from the problem of development of odor if amounts of styrene monomers contained in the resins are large. Furthermore, if dimers or trimers of styrene monomers are contained in large amounts in the resins, they cause unfavorable phenomena such as deterioration of heat resistance, and volatilization and retention of them in the mold at injection molding, which are transferred to the resulting molded products or they cause problems such as reduction of productivity due to increase in the number of cleaning of the mold.
As for the odor of products, JP-A-7-149817 and JP-A-7-149818 propose to reduce the odor with use of phenolic heat deterioration inhibitors, but this technique has a defect of inferior color tone of the resulting products. Moreover, for inhibiting the production of dimers and trimers, JP-5-170825 proposes to add phenolic heat deterioration inhibitors at polymerization step or devolatilization step in production of resin, but this method also has the defect of inferior color tone of the resulting products as above.
On the other hand, U.S. Pat. Nos. 4,325,863, 4,338,244 and 5,175,312 propose to stabilize polymers with addition of 3-arylbenzofuranone, but they make no mention of relation between concentration of the aromatic monovinyl monomer and color tone of the resulting products.
Furthermore, if the resins contain large amounts of the aromatic monovinyl monomer, or dimers and trimers thereof, the following problems may be caused in production of non-foamed sheets and foamed sheets.
That is, if the monomer is present in a large amount in production of non-foamed sheets (molding), removal of gas from dice is not smoothly carried out and the gas is locally present, which sometimes causes formation of die lines on the surface of sheets (a phenomenon of occurrence of lines on the surface of the sheets). If the dimer and the trimer are present in large amounts, they condense and stay at the dice part, and are deposited on the surface of the sheet to form contaminants. These phenomena sometimes bring about serious problems such as bad appearance of the products obtained after forming and unsatisfactory printing on the products.
Moreover, if amount of the monomer is large in production of foamed sheets (molding), the monomer is condensed in the mandrel of extruder, especially, in a mass production machine, and the condensed liquid deposits on the surface of the foamed sheet (sagging phenomenon) to cause serious problems such as bad appearance of the products obtained after forming and unsatisfactory printing on the products. Furthermore, if amounts of the dimer and the trimer are large, extension viscosity of the resin lowers, sometimes resulting in narrow molding width due to drawdown at the time of forming and reduction of productivity.
In order to solve these problems, it is desired that amounts of residual low molecular components such as styrene monomer and dimer and trimer thereof are small in the production of resin, and further desired that these low molecular components are not produced owing to the heat decomposition of polystyrene in producing the sheets and forming. The low molecular components can be diminished to some extent by controlling the conditions at the polymerizing step or devolatilizing step in the production of resin. However, for the inhibition of heat decomposition of the resulting products, it is necessary to stabilize the resin by adding a heat deterioration inhibitor, etc.