A biaxially stretched styrenic resin sheet is widely used in the field of food wrapping because of its transparency and nerve. However, it shows a strong blocking tendency, and formings obtained therefrom such as containers are hardly separated apart when stacked.
In order to overcome the blocking problem, it has been proposed to incorporate rubber particles into a styrenic resin as disclosed in JP-A-50-74649 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and to apply silicone oil on the surface of the stretched styrenic resin sheet as disclosed in JP-A-52-8080. The rubber particles added to the styrenic resin matrix are projected on the surface on biaxial stretching to improve blocking resistance of the stretched sheet. However, the rubber particles are flattened on biaxial stretching so that the proportion of the rubber projected on the surface is hardly constant. An increased amount of rubber incorporated to ensure satisfactory blocking resistance tends to cause reduction in transparency. In addition, with an increase of the rubber content, foreign matter may be incorporated into the resin composition because of scorching of rubber during long run operation or scorching of recycle rubber added, resulting in poor appearance of the sheet and causing a sheet break.
It was suggested to decrease the rubber content by increasing the size of the rubber particles to, e.g., about 6 to 10 .mu.m, but this method is accompanied with surface roughening or gloss reduction of the resulting biaxially stretched sheet.
It has also been proposed to incorporate synthetic silicon dioxide (JP-B-6-856, the term "JP-B" as used herein means an "examined published Japanese patent application") or a combination of silicon dioxide and styrene-grafted rubber (JP-B-6-855) to a styrenic resin so that the silicon dioxide may be projected on the surface of the sheet on biaxial stretching thereby to improve blocking resistance of the sheet. In these systems, however, the biaxially stretched sheet suffers from reductions in transparency and gloss because of a difference in a refractive index between silicon dioxide (refractive index: 1.3 to 1.45) and the styrenic resin matrix (refractive index: 1.55 to 1.59). Further, because silicon dioxide has poor wettability with the styrenic resin matrix, it falls off the matrix. Therefore, the resulting sheet has reduced physical properties and is of no practical use.
JP-B-47-2180 discloses a process of preparing a high molecular film (a polymer film) containing a fine particle having at least one functional group selected from an amino group, a hydroxyl group, a carboxyl group, an epoxy group and a maleic anhydride unit, but its application to a biaxially stretched styrenic resin sheet would not provide an article having an excellent transparency.
JP-B-55-7471 discloses a resin composition having a fine particle polymer dispersed therein which provides an opaque, translucent, or delustered sheet but does not provide a biaxially stretched transparent styrenic resin sheet as disclosed in the present invention.
USPTO Backgrounds
USPTO Backgrounds