Streaking is a term of art used to described a particular surfacial, cosmetic defect in a molded part. It is believed that this defect which appears as striations in molded pigmented articles results from poor dispersion of the pigment. Molding prepared from flame retardant, pigmented polycarbonates are susceptible to streaking particularly if the composition contains (polytetrafluoroethylene) PTFE.
Dispersion is defined as the breaking up of agglomerates of individual pigment particles which are held together by surface and/or electrostatic forces. After the breaking up of the agglomerations--by shearing action--the individual pigment particles must be wetted immediately if reagglomeration is to be prevented. A thorough dispersion of the pigment particles is essential in order to prevent pitting and streaking on the surfaces of extruded molded articles. Several solutions to the dispersion problem have been offered including adding dispersion aids--for instance, polyethylene glycol--and applying to the polycarbonate a low shear rate while applying a high shear rate to the pigment--U.S. Pat. No. 3,507,951. It is noted that this patent at col. 4, lines 20 et seq. regards iron oxide pigment to be the equivalent of carbon black. Also, U.S. Pat. No. 4,279,244 is noted to regard carbon black to be the equivalent of iron oxide pigments in a polycarbonate matrix--see col. 22, lines 24-25. Polymeric compounds containing olefinic unsaturation and epoxide groups were disclosed in U.S. Pat. No. 3,761,440 to be useful in reducing the deleterious effects caused by moisture in pigmented polycarbonates. Silica has been disclosed in U.S. Pat. No. 4,049,614 to aid in dispersing pigments in polycarbonate resins. U.S. Pat. No. 3,875,123 disclosed polymeric UV absorbers for stabilizing polycarbonates including pigmented polycarbonates. U.S. Pat. No. 4,017,457 which discloses ferrocene and ferrocene derivatives indicated that iron oxide can not be used as pigmentation agents in polycarbonates because such iron compounds cause color instability and are detrimental to the mechanical properties of the resin--see col. 3, lines 32 et seq. U.S. Pat. No. 4,289,745 disclosed acicular ferrite spinels said to be suitable as pigments in polycarbonates. Lastly, the art is noted to disclose the use of red and yellow iron oxide pigments in polycarbonate resins in Transparent Iron Oxide Pigments for Automotive Applications, Wolfgang E. Adams, NATEC '83 Society of Plastics Engineers, Brookfield Center Conn. p. 19-20. The suitability of iron oxide pigments in the context of the present invention is surprising in view of the art's teaching that black iron oxide starts to change color from black to brown (and finally red) at temperatures above 365.degree. F., which is considerably below the processing temperature of polycarbonate resins, i.e., 520.degree. F.