the loss of performance of the base polymer due to diluents (other constituents) in the resin formulation is minimized. In addition, migration and/or bloom are completely eliminated while compounding costs are substantially reduced. Leaching of reactive flame retardants by solvents and corrosives is relatively more difficult than leaching additive retardants. Reactive flame retardants are used mainly with thermoset resins, particularly unsaturated polyesters, both reinforced and unreinforced. A technique for introducing flame retardants into unsaturated polyesters involves the use of compounds with residual unsaturation such as diallyl tetrabromophthalate, which is used as a cross-linking agent alone or with styrene. Intumescent flame retardance is based on the formation of a char on the surface of the resin on the application of heat, consequently insulating the substrate from further heat and flame. Phosphorous compounds such as inorganic or organic phosphates, nitrogen compounds such as melamine, and poly- hydroxy compounds — usually pentaerythritol — are used in intumescing formulations. High loadings, are usually required to achieve the required level of fire retardancy. This tends to degrade the physical properties of the base polymer. Intumescent flame retardant systems are generally limited to polymers with low processing temperature (e.g., PP) because they expand considerably on application of heat. Nonflame-retardant systems are polymeric systems that inherently have some level of flame retardance and therefore do not require additive or reactive flame retardants. Examples include PVC and its compounds, poly(vinylidene chloride) films and compounds, phenolic foams, amide-imides, polysul- fones, and poly(aryl sulfides).
VII. COLORANTS15,16
As we said earlier, very few polymers are used technologically in their chemically pure form; it is generally necessary to incorporate various additives and reinforcements to assist processing and achieve desired properties. Unfortunately, these components also often produce a significant amount of undesirable color and opacity in the resin. Each resin itself has its color that may vary from grade to grade or batch to batch. For example, polystyrene crystal is transparent, whereas high-impact polystyrene has a white, somewhat translucent, appearance while the common grade flame-retardant polystyrene is opaque. The color of general-purpose ABS is off-white and opaque. Glass fibers, the most common reinforcements added to nylons and polyesters, darken the color of these resins.17 The marketability of a polymer product quite frequently depends on its color; therefore the purpose of adding a colorant to a resin is to overcome or mask its undesirable color characteristics and enhance its aesthetic value without seriously compro- mising its properties and performance. Colorants are available either as organic pigments and dyes or inorganic pigments. They may be natural or synthetic. By convention, a dye is a colorant that is either applied by a solution process or is soluble in the medium in which it is used, while pigments are generally insoluble in water or in the medium of use. Dyes are generally stronger, brighter, and more transparent than pigments. As a result of the intrinsic solubility, dyes have poor migration fastness and this restricts their use as polymer colorants. Inorganic pigments are largely mixed metal oxides with generally good-to-excellent light fastness and heat stability but variable chemical resistance. Organic pigments and dyes are generally transparent and possess good brightness. The heat stability and light and migration fastness of organic pigments range from poor to very good. Table 9.11 shows some colorants, their characteristics, and their applications. Colorants are used in polymers either as raw pigments (and dyes), concentrates (solid and liquid), or precolored compounds. Precolored resins, solid and liquid concentrates, are all offsprings of the basic dry pigments. Colorants are available in a variety of forms, including pellets, cubes, granules, powder and liquid, and paste dispersions. Raw pigments are generally supplied as fine particles, which require dust control measures. To optimize color development when raw pigments are used, the size of pigment particles or agglomerates must be reduced and coated with appropriate resin. Most finished colors use multiple pigments. This requires a homogeneous mixing of all the pigments in the formula in high-shear mixing equipment to produce a uniform color. Precise metering into the processing machine is required to produce consistent colors since some components of the pigment system, though present in relatively small quantities, have strong color characteristics. Raw colorants or pigments generally cost less than other forms of colorants, but they can be more difficult to disperse and may result in inconsistent master batches.