Poly Vinyl Esters

Poly(vinyl alcohol) can be derived from the hydrolysis of a variety of poly(vinyl esters), such as poly(vinyl acetate), poly(vinyl formate), and poly(vinyl ben2oate), and of poly(vinyl ethers). However, all commercially produced poly(vinyl alcohol) is manufactured by the hydrolysis of poly(vinyl acetate). The manufacturing process can be viewed as one segment that deals with the polymeri2ation of vinyl acetate and another that handles the hydrolysis of poly(vinyl acetate) to poly(vinyl alcohol). 

Poly(vinyl alcohol) is thus prepared by alcoholysis of a poly(vinyl ester) and in practice poly(vinyl acetate) is used (Figure 14.5). 

As mentioned, vinyl ester polymers are used in every part of the world for a vast number of different applications. They can be found basically everywhere in everyday life, from housing to personal effects and in goods as well as in food. The environmental fate of poly(vinyl ester)s is therefore of great importance. 

PVAc, PVA and PVB homopolymers as well as the different copolymers mentioned above all have a similar chemical motif in common. They exhibit an all carbon-carbon single bond backbone, which needs to be broken at some point in a potential biodegradation mechanism. With respect to the backbone, poly(vinyl ester)s are closely related to poly(olefin)s, poly(styrene)s and poly(acrylate)s. These three are known not to be biodegradable. Instead, they usually decompose by the impact of UV radiation, oxidation and hydrolysis reactions, which are not considered to be biological degradation. 

Apart from the all-carbon backbone, poly(vinyl ester)s also exhibit a unique 1,3-diol structure (see Fig. 1). This structure is a common motif in many natural materials, e.g. carbohydrates. A number of oxidative or reductive electron transfer processes catalysed by natural redox systems are imaginable for this motif. The 1,3-diol structure is unique for a synthetic polymer and cannot be found in any other synthetic polymer class of significance. This explains the unusual biodegradation properties discussed below. 

Chigwada et al.36 have combined polyhedral oligosilsesquioxanes (POSS), which are cage-like hybrid molecules of silicon and oxygen, with TCP (tricresylphosphate) in poly(vinyl ester) resins (PVE). POSS molecule contains nonreactive organic functionalities allowing solubility and compatibility of the POSS with various polymers. POSS was incorporated alone (3-10 wt %) in PVE, and four compositions were made with TCP at 4wt % POSS + 4wt % TCP and 5wt % POSS + 5, 10, 15 wt % TCP. Fligh reductions in PHRR and THR were noticed. Nevertheless, the POSS/ TCP combination did not exhibit better performances than compositions with only 5 or 10 wt % of TCP alone. 

E. Kandare, G. Chigwada, D. Wang, C.A. Wilkie, and J.M. Hossenlopp, Probing synergism, antagonism, and additive effects in poly(vinyl ester)(PVE) composites with fire retardants, Polym. Degrad. Stabil., 2006,91 1209-1218. 

An HDS additive, zinc/copper hydroxy stearate, was melt-blended with low density poly(ethylene). X-ray diffraction analysis of the composite materials was similar to that found with copper hydroxy dodecyl sulfate combined with poly(vinyl ester), where nanocomposite formation was observed, but additional work is necessary for full characterization of the dispersion. The (nano) composites were found to have better thermal stability via TGA and improvement in PHRR in cone calorimetry. However, smoke production was observed to increase. The 5% loading had better overall performance than 10% in terms of thermal stability and most fire properties. 

Poly(vinyl alcohol) is typically obtained by alcoholysis of poly(vinyl esters), for example from polyfvinyl acetate) and methanol in the presence of NaOH. The process can be completed or only partially conducted. In this latter case a copolymer (alcohol/ester) is obtained. Other synthetic procedures are used, most of them also based on the hydrolysis of poly(vinyl esters). Poly(vinyl alcohol) is typically used in the atactic form, but isotactic or syndiotactic poly(vinyl alcohols) also are known. 

Table 6.5.8. Summary regarding literature information on thermal decomposition of several poly(vinyl esters).

On the other hand, since poly(vinyl ester) radicals are reported to be electron-rich104, they are electron donors with respect to solvent. The poly(vinyl acetate) radicals is more likely to form a complex than the poly(vinyl benzoate) radical, since the methyl group increases the electron density of the propagating radical as compared to the phenyl group. If the complexed radical is assumed to be less reactive than the free radical or inactive, the concept of the 7r-complex along with the experimental evidence of similar methyl affinity47 of both monomers explains the fact that kp for vinyl acetate is smaller than that for vinyl benzoate. 

TABLE IV Structiural Parameters of PoIy(vinyl alcohols) Derived from Various Poly(vinyl esters) [50]... 

Poly(vinyl ester) Polymerization temp. ro Intrinsic viscosity, [)/] of derived poly(vinyl alcohol) (dl/gm) 1.2 Diol content, mol% in poly( vinyl alcohol)" Water soluble (%) Swelling index"... 

Nonchain scission refers to reactions involving pendant groups that do not break the backbone. Typical of such reactions are dehydrochlorination of poly( vinyl chloride) (Equation 1.57), elimination of acid from poly(vinyl esters)—for example, poly(vinyl acetate) (Equation 1.58)—and elimination of alkene from poly(alkyl acrylate)s (Equation 1.59). 

This section deals with paints based on vinyl resins (including vinyl copolymers) which are synthesized by polymerization of monomers containing terminal CH2 = CH groups. Polyolefins, poly(vinyl halides) and vinyl halide copolymers, poly(vinyl esters), poly(vinyl alcohol), poly(vinyl acetals), poly(vinyl ethers), and polystyrene are discussed. Polyacrylates (acrylic resins) are treated in Section 2.5. 

Poly(vinyl esters) used in paints and adhesives are available as homopolymers and copolymers in the form of solid resins, solutions, and dispersions. 

Poly(vinyl ester) dispersions are quantitatively more important than solid resins. Homopolymer and copolymer dispersions are used for binders in emulsion (dispersion) paints, plastic-bonded plasters, and water-thinnable adhesives. Poly(vinyl acetate) dispersions are less important than vinyl acetate copolymer dispersions. The most important comonomers of vinyl acetate are vinyl laurate, dibutyl maleate, Versatic Acid esters (VeoVa, Shell), ethylene, vinyl chloride, and butyl acrylate. Poly(vinyl propionate) and copolymers of vinyl propionate with butyl acrylate, styrene, or vinyl chloride are also marketed and used as dispersions. 

Poly(vinyl ester) dispersions are important binders for indoor (conventional, solvent-free) and outdoor paints, special coatings, and textured finishes. Special types are used for wood paints and for coating paper and cardboard. Poly(vinyl ester) dispersions are also important in the adhesives and textile finishing industries. 

Poly(vinyl esters) and Poly(vinyl alcohol)... 

Vinyl alcohol monomer does not exist because its keto tautomer is much more stable. Poly(vinyl alcohol) can be prepared from either poly(vinyl ester)s or from poly(vinyl ether)s. Commercially, however, it is prepared exclusively from poly(vinyl acetate). The preferred procedure is through a transesterification reaction using methyl or ethyl alcohols. Alkaline catalysts yield rapid alcoholyses. A typical reaction employs about 1% sodium methoxide and can be carried to completion in one hour at 60 °C. The product is contaminated with sodium acetate that must be removed. The reaction of transesterification can be illustrated as follows ... 

In monomers with two or more polymerizable sites the structure of the resulting polymer depends on the initiator. Vinyl isocyanate, CH2=CHNCO, polymerizes via the vinyl group free radically, but via the nitrogen/ oxygen double bond in anionic polymerization. Diketenes polymerize to polyesters, polyketones, or poly(vinyl esters) according to what initiator is used ... 

Poly(vinyl acetate) is used for adhesives and as a wood glue (40% solution), as a raw material in lacquers and varnishes (dispersions), and as a concrete additive (in the form of a fine, dispersible powder obtained by spray drying). Poly(vinyl acetate) grades that are more resistant to hydrolysis are obtained by copolymerization with vinyl stearate or vinyl pivalate, since the saponification rate is reduced by the bulkier side groups. Pure poly(vinyl pivalate) has too high a glass transition temperature, 78 C, for most poly(vinyl ester) applications. Other copolymers of vinyl acetate are produced with ethylene (see Section 25.2.1) or vinyl chloride (see Section 25.7.5.3). 

---