Surface Modification of Organic Pigments for the Suppression of Mold Shrinkage in Colored Plastics

Abstract

Suppression of the mold shrinkage in colored HDPE (high-density polyethylene) systems was investigated from the standpoint of pigment-surface modification using pigment analogues. Phthalimidomethyl derivatives were found to be quite effective in suppressing the mold shrinkage. However they were found soluble in HDPE, which brings poor heat stability and undesirable bleeding. On the other hand, insoluble derivatives such as tetrachlorophthalimidomethyl- or RCOOCH₂-substituted phthalocyanines exhibited an excellent suppression effect when they were introduced in to pigments by acid-pasting. On the contrary, the effect was quite small when they were mixed with pigments in powdery form. On the basis of the above results, the surface modification in molecular level by acid-pasting was concluded to be the determinant factor for the suppression of the mold shrinkage in colored HDPE.