(26d) Surface Infusion of Colorants and Nanoparticles into Processed Thermoplastics

Colorants, UV stabilizers, antioxidants, and nano-fillers are examples of additives that are traditionally blended with thermoplastic polymer resins prior to conducting melt-state shaping operations such as die forming or injection molding. This presentation concerns a novel approach to plastics modification whereby colorants or other compounds are infused into a molded, cast, or extruded plastic article after melt-state processing using an aqueous plasticizer solution (AURA® Color Infusion Technology, Bayer MaterialScience). Infusion of colorants or other compounds into the plastics is achieved at a temperature well below the melt-processing temperature by immersing the articles in a mixture of water and organic surfactants, which plasticizes a thin surface layer and allows entry of the infusant. The infusant is able to penetrate 30 micrometers or more into the surface of the article, achieving coloration or other surface modifications within a few minutes, usually without degradation of surface gloss. Because the infused compounds are not subjected to melt-state processing conditions, the infusion technique provides a novel approach to introducing additives that ordinarily present problems with thermal degradation, microphase separation, or agglomeration. In addition, because the infusion affects a thin surface layer, it is especially applicable to processes in which the desired outcome is modification of surface characteristics with minimal change to bulk properties.

Earlier work successfully demonstrated rapid infusion of standard organic dyes into commercially important plastics including polycarbonates, thermoplastic polyurethanes, acrylics, and others. Recent work has explored infusion of metal-containing compounds and infusion of metal nanoparticles using a two-step redox process. To infuse metal nanoparticles, the plastic article is first immersed in a solution of a transition metal salt in the plasticizing solvents, which allows the salt to enter the plastic surface. The article is then treated with a second solution of an oxidizing or reducing agent, generally in water. The resulting reaction-diffusion process produces metal or metal oxide particles within the topmost layer of the plastic. Precipitation of the nanoparticles is arrested by the high viscosity of the polymer matrix, producing a stable dispersion of particles (< 100 nm typical diameter). Thus, it is possible to create polymer-metal nanocomposites that could not be processed in the melt state without precipitation of the nanoparticles. We demonstrate the effectiveness of the technique by examining infusion of gold (Au0) nanoparticles into thermoplastic polyurethanes and a polyester-coated acrylic sheet. Manipulation of processing conditions permits infusion of particles having controlled size and good dispersion in these materials, though poor solubility of the metal precursor compounds is found to limit the infusion in other plastics.