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PAINT AND COATING FORMULATIONS OFTEN incorporate high boiling fluids as plasticizers where rigid or brittle resins fail to meet toughness and flexibility requirements. The primary function of the plasticizer is to impart flexibility to the resin, thus minimizing film cracking. Depending on resin and other ingredients used in the system, plasticizer choice may affect compatibility, toughness, flammability, smoke generation, heat and light stability, and other aging or permanence-related performances. Plasticizers are primarily employed in heavy gage coatings and/or when improved toughness is required for industrial, automotive, and appliance applications. Plasticizers function by reducing the glass transition temperature of the resin to a point below its application temperature. The chemical mechanism of plasticization involves a strong polar association of polymer-plasticizer molecules, but not a chemical reaction between them. Plasticizers fundamentally reduce van der Waals forces between polymer-polymer molecules in the amorphous regions and do not penetrate crystallites [1–3]. The plasticized morphological phase is then of a different nature than that of the neat polymer and has unique mechanical properties. The absence of a chemical bond between the plasticizer and the polymer impairs permanence; plasticizer molecules are free to leave the polymeric coating by means of extraction and volatility. Plasticizer loss, however, is minimal in most applications except for very low molecular weight plasticizers and/or exposure to very severe thermal conditions. Thus, plasticized coatings products have high durability and long service life in most applications. Plasticizers are liquids of molecular weight greater than that of solvents—to limit volatility—but are not solids, such as alloying polymers, etc. It should be noted that cross-linked resinous coatings significantly reduce plasticizer loss due to diffusibility and volatility. Several thousand high boiling fluids are potential plasticizers for coatings applications. The choice of plasticizer is dependent on compatibility with the resin in use, cost, and other desired attributes. Plasticizers may be classified by both chemical structure and performance characteristics, as shown in Table 1 . Typical plasticizers are liquid esters of molecular weight between about 200 to 800, with specific gravities between 0.75 and 1.35 at 20/20°C, viscosities between 50 to 450 cSt, vapor pressure of less than 3.0 mm of mercury at 200°C, and flash points greater than 120°C (248°F). They are generally stable and innocuous and should not be considered a significant threat to humans or the environment [5,6]. Plasticizer extenders are commonly used in extruded or molded flexible plastic shapes. Extenders are low-cost organic oils that may be subdivided as groups of aliphatic, aromatic, or chlorinated hydrocarbons. They are seldom used in coatings due to their relatively high volatility and limited compatibility in polar resins. This chapter lists the basic properties of plasticizers and methods for their determination. Methods for the isolation, identification, and quantitative determinations of these plasticizers are also included.
Manager, Exxon Chemical Asia PTE LTD,
Krauskopf, Leonard G.
Research associate, Exxon Chemical Company, Baton Rouge, LA