To date, poly(vinyl alcohol) (PVA) is the only water-soluble polymer with exclusively carbon atoms in main chains in possession of good biodegradability. It is currently used in textiles, paper, and packaging industries as paper coatings, adhesives, and films [6]. PVA is a water-soluble polymer with 1,3-diol units or 1,2-diol units, depending on the hydrolysis of poly(vinyl acetate). In addition, the content of 1,2-diol units is reduced when the temperature of vinyl acetate polymerization decreases. The chemical structures of PVA and vinyl alcohol are shown in Fig. 14.5. PVA has been synthesized by several routes such as the polyaldol condensation of acetaldehyde despite little success until now [27]. In addition, the polymerization of metal compounds, followed by the saponification process, produces low-molecular-weight PVA. However, the synthesis of PVA by the polymerization of vinyl ester or ethers, further subjected to the saponification or esterification, is regarded as the most popular method [26–28]. In this technique, vinyl ester or other derivatives such as vinyl mono- and dichloroacetate are usually used as base materials to synthesize PVA. Moreover, PVA characteristics like molecular weight and residual content of acetyl groups can be controlled by the optimization of processing parameters such as polymerization temperature, vinyl acetate/methanol ratio, and polymerization conversion [29]. It has been reported that the decrease in polymerization temperature with a suitable proportion of methanol and conversion leads to an increase in the molecular weight of final PVA [29]. In addition, PVA should be free from monomeric vinyl acetate to avoid cosaponification [27]. The conversion process of PVAc to PVA is usually carried out in the solution, suspension, or emulsion in the presence of alkaline or acidic catalysts. As such, the most recommended process is the transesterification in methanol using a proper amount of sodium methoxide as a typical catalyst. The residual content of acetyl groups can be controlled by manipulating the catalyst content, reaction temperature, and reaction time. Moreover, the types of catalysts and solvents used are selected based on the distribution nature of acetyl groups in the partial saponification of PVA. For example, acetyl groups are distributed in a blockwise manner in the case of alkaline saponification, while they are distributed statistically for acidic saponification [29]. PVA properties are determined from the molecular weight and degree of hydrolysis, as depicted in Fig. 14.6. When these two parameters are enhanced, water resistance, tensile strength, block resistance, and solvent resistance increase accordingly, while solubility, flexibility, and water sensitivity decline instead [31]. Remarkable PVA properties, such as good resistance against organic solvents, water solubility (even in cold water), low material and processing costs, good mechanical properties, and excellent barrier properties [6], make it an excellent biopolymer candidate for various applications ranging from food packaging, textile products, paper coating, finishing adhesives to medical devices [26,28,31]. The biocompatibility of PVA has been widely investigated with its apparent nontoxic characteristic. For example, it was reported that subcutaneous and intramuscular implantation of PVA hydrogel into rabbits did not induce any adverse effects on the surrounding tissues [26]. Similarly, the injection of PVA hydrogels into the eyes of crab-eating macaques did not show any evidence, even after a 3-month tissue loss, as compared to the change in opthalmoscopic finding or increase in intraocular pressure [6]. In addition, after the investigation of PVA biocompatibility by the food industries, PVA is recommended as a safe biopolymer product [26,28,31].

Ethylene Diamine Tetraacetate Acid Tetrasodium Salt (EDTA-4Na)

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Figure 14.5. Chemical structures: (A) vinyl alcohol and (B) PVA [26].

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Figure 14.6. Effect of degree of hydrolysis and molecular weight on PVA properties [30].