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Free Radical Chain Polymerization                                            215


                 of synthetic polymers. In art, the term acrylic is used to describe a wide variety of polymers and
                 copolymers that can be considered as derivatives of acrylic acids. Most acrylics used in art binders
                 are poly(methyl methacrylate) (Equation 6.62), PVAc (Equation 6.65), poly(n-butyl methacrylate)
                 (6.66), and copolymers such as poly(ethyl acrylate-co-methacrylate).
                                                                   H C
                                                                    3

                                                  R
                                     R
                                                                              R
                                   H 3 C      O                R


                                                             H C
                                         O                    3
                                                                    O         O
                                        (6.65)                      (6.66)


                    Acrylic emulsions tend to flow nicely “leveling out” rather than giving a three-dimensional effect
                 sometimes offered in various oils. In the absence of pigment, acrylic emulsions give a milky white

                 appearance. As the water evaporates, the binder particles coalesce forming a tight film. When dried,
                 the film is clear and becomes water insoluble.

                    As noted above, acrylics are colorless when hardened so that the color comes from the addition
                 of the coloring agents, pigments. When thick, acrylics form plastics, but when applied to give thin
                 coatings, they give fl exible films. Unlike many of the natural coatings, such as oil-based paints that


                 crack and are not flexible, acrylic coatings are more flexible and do not crack as easily. Counter, the

                 permanence of the acrylic coatings has not been field tested since they are only about half a century

                 old, whereas oil-based paintings have remained for almost half a millennium. Even so, many artists
                 claim that acrylic paints are more permanent than the natural-based paints. Only the test of time
                 will truly give us the answer.
                    Acrylic paints have compositions similar to water-based house paints. Since they are synthetic,
                 manufactures can design paints that fulfi ll specific requirements and that are the same from tube to

                 tube. Because water is the main vehicle, such paints are largely odorless. They are not completely
                 odorless because while the overwhelming majority of the vehicle is water, there are often minute
                 amounts, generally 1%–10%, of organic liquid added to help the particles of polymer remain suffi -

                 ciently flexible so that they will “lay down” a thin smooth coating.
                    Unlike oils that take days and weeks to dry “to the touch,” acrylics dry in minutes to several
                 hours. They are also considered nonyellowing.
                    Acrylics are also the most used paint for baked polymer clay.


                 6.9   POLY(VINYL ALCOHOL) AND POLY(VINYL ACETATE)
                 Poly(vinyl alcohol) is produced by the hydrolysis of PVAc (Equation 6.44). Because of the ability
                 to hydrogen bond and small size of the hydroxyl grouping, PVA is a crystalline atactic material.
                 Because of the formation of strong internal hydrogen bonds, completely hydrolyzed PVAc is not
                 water soluble. Thus, hydrolysis of PVAc is stopped, giving a material that is about 88% hydrolyzed
                 and water soluble. PVA is resistant to most organic solvents such as gasoline. PVA fi bers (Kuralon)
                 are strong and insoluble in water because of a surface treatment with formaldehyde that reacts with
                 the surface hydroxyl groups, producing poly(vinyl formal) on the polymer surface.
                    Poly(vinyl alcohol) is used in the treatment of textiles and paper. PVA also acts as the starting
                 material for the synthesis of a number of poly(vinyl acetals) with the general structure 6.67. The







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