292 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS POLYVINYL ACETATE While polyvinyl acetates are new to the cosmetics field, they have a successful history in many applications where important properties such as product stability, lack of skin irritation, odor and color are prime require- ments. The single largest and oldest use of this versatile synthetic resin is as a packaging adhesive. A multitude of different packaging materials are effectively bonded and many of these uses require a product which is odor- less, colorless, and safe for food packages. Typical examples are milk cartons, paper cups and containers, paper straws, food cartons and bags, etc. In recent years po[yvinyl acetate has become popular for remoistening gums on envelopes. A second important category is pigment binding, exemplified in the popular emulsion based paints and paper coatings. As a raw material for paints, polyvinyl acetate is notable for its strong film which does not discolor when exposed to light and heat. In the latter case the resin binds clay and other low-cost pigments and adheres them to paper and board to provide a smooth surface suitable for printing and coating. CaO q- 3C --• CaC,• q- CO (gas) Lime Coke CaC2 + 2H,•O --,- Ca (OH)2 q- CeHe(gas) CH•CH q- CHaCOOH --,- CH.,:CH O--C:O CHa Vinyl acetate Monomer Figure 1.--Chemical derivation of polyvinyl acetate. Another major use is in the field of nonwoven fibe• binders, where textile fibers are adhered to each other rather than woven. Some of the many products made are table cloths and napkins, disposable diapers and sanitary napkins. The last major category to be mentioned, although there are many other important uses, is textile finishing. Polyvinyl acetate is widely used on many classes of textile goods. It is common to find clothing such as shirts, blouses, underwear, pajamas and accessories such as handkerchiefs given a lustrous and moderately soft finish through the use of a resin sizing. A noteworthy though minor and old-time use of polyvinyl acetate is as a base in chewing gum. CHEMISTRY OF PVAc Polyvinyl acetate is a synthetic •esin derived from abundantly available natural raw materials. The chain of events starts with lime and coke

VINYL COPOLYMER IN THE COSMETIC COSMOS 293 which are reacted in an electric furnace to yield calcium carbide. As shown in Fig. 1, when this material is mixed with water, acetylene is removed as a gas. Its reaction with acetic acid produces vinyl acetate monomer. You will note in Fig. 2 that the monomer units are joined when they are excited by an activator such as benzoyl peroxide. Under heat the peroxide splits into two benzoyl free radicals which are in a highly active state due to the presence of a free electron. The double bond of the vinyl group opens and forms a covalent bond with the benzoyl radical. At the same time the energy is transferred to the free end of the monomer which in turn reacts with a second monomer unit. This step repeats itself many times until the toohomer is used up and the active ends of the chains join with each other. I Benzoyl Peroxide .... | o_=o [ Heat CH• L CH• n Vinyl acetate Polyvinyl Monomer Acetate O O O Benzoyl Peroxide Active Benzoyl Radical o o OCOCHa '•/ OCOCHa Figure 2.--Polymerization of polyvinyl acetate. Polymerization reactions are usually carried out in solution, emulsion or suspension form to insure a low viscosity for uniform agitation and removal of the heat of polymerization. When emulsion or suspension polymeriza- tion is used, suitable emulsifiers and/or suspending agents must be used. The solvent used for solution polymerization must be chosen carefully, since it plays an important role in reaching the desired molecular weight level. The polyvinyl acetate copolymer being highlighted is prepared by such polymerization methods and dried to yield a translucent bead. Although we are all familiar with the terms polymer and copolymer, it is important to call attention here to some of the differences between them. A conventional polymer, or homopolymer as it is sometimes called, can only be varied to a certain extent, for example its moleuclar weight and degree of chain branching. Its film properties will have essentially the same physical and chemical characteristics. Thus, by introducing special