THE EFFECT OF COLORANTS ON THE PROPERTIES OF ROTOMOLDED POLYETHYLENE PARTS

An experimental study is reported on the effect of colorants on the wa rpage, shrinkage, and mechanical properties of rotomolded polyethylene (PE) parts. Five pigments were investigated (titanium dioxide white, cadmium oxide yellow, iron oxide red, carbon black, and phthalocyanine blue). Their concentration was in the range of 0.11 to 0.36 wt%. The pigments were added to the Linear low-density polyethylene (LLDPE) by various techniques, dry blended in low intensity or high intensity mix er or compounded with a single-screw extruder. Some blends also contai ned 0.077 wt% zinc stearate as surfactant. It was found that zinc stea rate surfactant in the dry blend increases warpage by about a factor o f 2 and shrinkage by similar to 30%, but color quality and powder hand ling properties are improved. The dry-blended pigments concentrate alo ng the fusion lines of the PE particles in rotomolded parts, which res ults in >50% decrease of impact strength with some pigment-surfactant combinations. Mold pressurization minimizes warpage and shrinkage, but causes no significant improvement in impact resistance. Impact resist ance can be improved by using compounded pigments or making two-layer parts with a pigmented outer and unpigmented inner layer.