Article made of propylene-ethylene copolymer

#1. Field of the Invention

#The present invention relates to articles made of a highly crystalline olefin polymer which have improved clarity and impact strength. More particularly, the present invention relates to articles such as containers obtained by the biaxial orientation blow molding of said olefin polymer.

#2. Description of the Prior Art

#According to the present invention, the two properties of transparency and impact strength are improved from the standpoint of the polymer material used. The first embodiment of the present invention relates to articles such as hollow containers obtained by using a statistical random copolymer of propylene and ethylene having a melt index of from 0.6 to 3.0 g/10 min. and an ethylene content of from 0.3 to 5.0% by weight, cooling and solidifying a tubular body formed by extrusion molding of said copolymer and then extending the body in biaxial directions at an orientation temperature below the melting point in such a manner so as to attain an extension magnification ratio of at least 1.5 in each of longitudinal and lateral directions. The second embodiment of the present invention is characterized in that a mixture of from 0.03 to 0.20 part by weight of aluminum p-tert-butyl benzoate or sodium p-tert-butyl benzoate and 100 parts by weight of the statistical random copolymer of the first embodiment is used as the raw material.

#In the past, random type propylene-ethylene copolymers have been used generally for biaxially oriented films in order to increase heat-sealabiliy as well as tear strength, and for injection molded products or the like for the purpose of increasing impact strength. However, little or no improvement in transparency has been obtained in either case. Therefore, it is apparent that the improvements in transparency and impact strength, and especially the remarkable increase in transparency obtained according to the present invention, are new and unexpected. While some increase in one or both of said properties, when compared to other blow molded products, can be obtained when such propylene ethylene copolymers are subjected to conventional blow molding, the increases are not of any significant amount. Noticeable improvements in both properties can be achieved only when these copolymers are employed in the biaxial orientation blow molding of a tubular body formed by extrusion as set forth in the present invention.

#The copolymer used according to the present invention is a statistical random copolymer of propeylene and ethylene having a melt index (MI) of from 0.6 to 3.0 g/10 min. and an ethylene content of from 0.3 to 5.0% by weight. If the MI value is lower than 0.6, transparency and also extrudability will be decreased. When the value exceeds 3.0, impact strength decreases and the consistency of dimensions obtained during extrusion of the tubular body is reduced so that irregularities in wall thickness tend to occur. When the ethylene content is less than 0.3%, no improvement in transparency or in impact strength will be apparent whereas when the ethylene content is above 5.0%, no further increase in transparency over that of 5.0% content can be seen and the obtained article is impracticable as a packaging container because of marked decreases in rigidity and buckling strength.