Polyacetal is a well known engineering resin which exhibits excellent chemical resistance properties, as well as balanced mechanical properties. Polyacetal resin has therefore been used in a variety of end-use applications. However, since polyacetal resins also tend to exhibit high coefficients of shrinkage and linear expansion during molding due to the high crystallinity of the resin, it is known to have fairly poor dimensional accuracy. In addition, the weather resistance and acid resistance properties of polyacetal resin are known to need improvements.
These disadvantageous properties of polyacetal thus make it difficult to use the resin in certain end-use applications, such as the material from which exterior components of automobiles and the like (which are exposed directly to atmospheric conditions) may be made. Although some attempts have been made to improve the weather resistance properties of polyacetal resin by blending an acrylic resin, such as polymethylmethacrylate (PMMA), with the polyacetal as a base resin, merely blending the acrylic resin with polyacetal has its own problems. That is, a blend of polyacetal resin and acrylic resin typically cannot be utilized as an exterior component (especially components that are large or complicated in shape) because the characteristic mechanical properties of the polyacetal resin (such as its impact resistance and tensile elongation properties) are impaired by the presence of the polymethylmethacrylate. Although it is not known precisely why such polyacetal property deterioration occurs, it is surmised that it probably occurs due to the poor dispersion of the polymethylmethacrylate in the polyacetal base resin. Moreover, blending polymethylmethacrylate resin with polyacetal base resins does not improve the dimensional accuracy, such as distortion and shrinkage during molding, to sufficient degrees. It is therefore towards solving these problems that the present invention is directed.
The present invention is broadly directed to polyacetal resin compositions and processes for making the same whereby a polyacetal base resin, a particulate filler material and an acrylic resin may be blended with one another under melt conditions and in certain specified amounts so as to improve the dispersion of the acrylic resin and filler throughout the polyacetal base resin. The present invention more specifically relates to polyacetal compositions whereby an interconnected network consisting essentially of a continuous phase of the acrylic resin is formed within the polyacetal base resin (which therefore forms a matrix for the continuous acrylic resin phase). The particulate filler material is dispersed homogeneously throughout the polyacetal base resin but is encapsulated within the continuous acrylic resin phase. In this connection, it is most preferred that the filler material exhibits a greater surface tension at melt-blending temperatures as compared to the surface tension of the acrylic resin forming the continuous phase in the polyacetal matrix base resin.
Further aspects and advantages of this invention will become more clear from the following detailed description of the preferred exemplary embodiments thereof.