Nonflammable injection-molding resins composed of a low-molecular-weight styrene resin and a low-polymerization-degree vinyl chloride resin are superior materials in view of mechanical properties and cost. Recently, these resins have been used in increased amounts as materials for housings of office automation (OA) apparatuses and the like. However, the requirements of molded articles are becoming more severe each year in OA apparatus applications. In particular, miniaturization of OA apparatuses, because it brings about an increase in internal heat generation in the apparatuses, necessitates a decrease in the thickness of the housing materials, and requires the use of highly heat-resistant resins. A general means for raising heat-resistance is to use a styrene resin which contains .alpha.-methylstyrene as a third component, or to use a post-chlorinated vinyl chloride resin.
These resins, however, are disadvantageously liable to cause jellowing and flashing of the molded articles and are not readily moldable. The disadvantages of the molded articles are considered to relate to the extremely high melt viscosity of the resins in injection molding, particularly in the case of a resin which contains a post-chlorinated vinyl chloride type resin. The use of such a resin results in large heat generation by the shearing action of metal parts such as a screw, a cylinder, a mold, etc. and causes heat decomposition of the resin. For improving the molding of such resins, methods such as the addition of a large amount of a stabilizer or a lubricant, and the addition of a moldability-improving agent comprising an alkyl acrylate type resin are generally used. The former method is not preferable, because the cost of the starting materials increases and, further, the impact strength and the heat distortion temperature are decreased adversely affecting the quality of the molded articles. The latter method, namely the addition of an alkyl acrylate type resin as a moldability-improving agent, is remarkably useful since the increase in the cost of the material is not so large and lowering of the heat distortion temperature is not remarkable. This method, however, causes a decrease of the impact strength and peeling of the molded articles. In particular, with an increase in the amount of the additive for increasing the moldability-improving effect, lowering of the impact strength and deterioration of the non-peeling property of the molded articles become remarkable. These are serious problems.
These problems are assumed to be caused by the alkyl acrylate type resin which is used as a moldability-improving agent and which is designed to improve moldability particularly of extrusion-molding vinyl chloride resins having an ordinary molecular weight. The inventors of the present invention considered the facts that the shear rate of a resin in injection molding differs greatly from that in extrusion molding, the shearing rate being in the range of from 10000 to 100000 sec.sup.-1 in injection molding and being in the range of from 100 to 1000 sec.sup.-1 in extrusion molding, and that the nonflammable resin to which the moldability-improving agent is added is not a resin of ordinary molecular weight but is a special nonflammable injection-molding resin which comprises a styrene type resin having a high content of low-molecular weight .alpha.-methylstyrene and containing a post-chlorinated vinyl chloride type resin of low polymerization degree. Based on the above consideration, the present inventors comprehensively investigated moldability-improving agents of the alkyl acrylate resin type to find modifier which does not lower the effect of the moldability-improving agent, decrease impact strength, nor cause peeling of molded articles. As a result, it has been found that an alkyl acrylate copolymer having a specific composition achieves the above objective.