Status of investigations for improving weatherability of linear polyethylene and copolymers



In an attempt to extend the outdoor serviceability of items molded from linear polyethylene and copolymers, three ultraviolet absorbers, namely, Cyasorb UV-9 (2-hydroxy-4-methoxybenzophenone), Cyasorb UV-314 (2,2′-dihydroxy-4-n-octoxybenzophenone), and TBS (4-tert-butyl phenyl salicylate), sixteen different pigments, and combinations of both ultraviolet absorber and pigment were investigated. Injection-molded test specimens were employed in this study. The laboratory source of radiation to which these samples were exposed was an Atlas XW Weatherometer. Similar samples also were exposed at various outdoor locations including Phoenix, Arizona, Summit, New Jersey, and Newark, New Jersey, to utilize practical conditions of outdoor weathering. During these weathering studies surface cracking was observed as occurring at approximately the same time as a significant decrease in tensile strength, elongation, and melt index. Such observations became a method of rapidly screening a large number of formulations for resistance to ultraviolet radiation. For optimal outdoor serviceability with respect to retention of elongation, tensile strength, and melt index properties of molded items, the following steps are necessary. A high-density resin having as low a melt index as possible is selected. The molded item is designed to minimize the formation of molded-in stresses and strains. Proper selection of UV absorbers, pigments, and combinations of both are made. Pigments such as green, orange, peacock blue, monastral purple, red, and black, in high concentrations, will prolong the outdoor weatherability of a given product.