| Abstract: | An experimental study is reported on the effect of colorants on the warpage, 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 mixer or compounded with a single-screw extruder. Some blends also contained 0.077 wt% zinc stearate as surfactant. It was found that zinc stearate surfactant in the dry blend increases warpage by about a factor of 2 and shrinkage by ˜ 30%, but color quality and powder handling properties are improved. The dry-blended pigments concentrate along the fusion lines of the PE particles in rotomolded parts, which results 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 resistance can be improved by using compounded pigments or making two-layer parts with a pigmented outer and unpigmented inner layer. |
