Polyvinyl alcohol

Polyvinyl alcohol (PVA) is a colorless, water-soluble synthetic organic compound composed of a viscous liquid substance (also called a resin) that is used primarily in the textile and paper industries. Among the many available polymers, or chemical compounds composed of multiunit molecules, PVA is unique in its formation. Typical polymers undergo a polymerization process, where multiple monomers chemically combine to produce a chainlike network, or the polymer. Conversely, PVA is created from the dissolution of polyvinyl acetate (PVAc) in the presence of an alcohol (such as methanol) and an alkaline catalyst (such as sodium hydroxide). This hydrolysis or alcoholysis reaction, which describes the breakage of a molecular bond through the addition of water or alcohol, respectively, eliminates acetate groups from the PVAc molecules while maintaining the primary long-chain structure of the molecule. Partial abstraction of the acetate groups produces a less water-soluble product that also solubilizes in some organic liquids. The produced vinyl alcohol has a CH₂CHOH repeating unit. Polyvinyl alcohol is soluble in water and insoluble in most organic solvents.

Background

In 1924, PVA was first synthesized by German scientists through the hydrolysis of PVAc in the presence of ethanol and potassium hydroxide. In industrial processes, PVA is produced from PVAc in a continuous process, whereby the acetate groups proceed with hydrolysis or alcoholysis, particularly ester interchange with methanol, in the presence of anhydrous sodium methylate or aqueous sodium hydroxide. The functionality of PVA depends strongly on the degrees of polymerization and hydrolysis of the compound. For example, partially hydrolyzed PVA can be used as moisture barriers for food supplements and dry food, while fully hydrolyzed PVA is used in other plastic industrial processes, such as in the automobile and technology industries. Partially hydrolyzed PVA, used for food, is an odorless, translucent, white- or cream-colored powder that is highly soluble in water, partially soluble in ethanol, and insoluble in all other organic liquids. The 5 percent solutions of PVC normally have a pH value within the range of 5 to 6.5, with a melting point temperature between 180(C to 190(C.

A vinyl acetate monomer is used as the principal raw material in the manufacturing of PVA. Vinyl acetate is first polymerized and is subsequently hydrolyzed by partially replacing an ester group in the vinyl acetate compound with a hydroxyl group. The process occurs in the presence of aqueous sodium hydroxide. An aqueous saponification agent is gradually added to aid the reaction of the base (sodium hydroxide) and the ester group of the vinyl acetate, allowing the production of a soap-like product. The saponification reaction and the amount of saponification agent added strongly determines the degree of hydrolysis. Following the completion of the necessary reactions, PVA is precipitated, washed, and dried.

Sodium acetate, methanol, and methyl acetate are a few of the impurities and degradation products that are produced or withheld during the manufacturing process. Sodium acetate levels, a reaction by-product, can be supervised based on the remaining residue from ignition tests. On the other hand, methanol and methyl acetate levels can be monitored continuously during the PVA manufacturing process.

Impact

Polyvinyl alcohol is an important component in most sizing agents that require a gelatinous coating substance to increase the strength and stiffness, while reducing the absorbency, of a material. PVA is used as a protective, water-soluble coating for adhesives and emulsifiers. It is also employed as an additive in the preparation for other resins, specifically polyvinyl butyral and polyvinyl formal. Polyvinyl alcohol can be reacted with either butyraldehyde (CH₃CH₂CH₂CHO) or formaldehyde (CH₂O) to form these resins. Polyvinyl butyral is a colorless, adhesive, water-resistant plastic film extensively employed in the automobile industry for laminated safety glass, while polyvinyl formal is employed for wire insulation.

In the food industries, partially hydrolyzed PVA serves as a food additive for film-coating agents covering food supplements. PVA has a low acute oral toxicity level and has minimal absorption following oral administration. The results of in vitro (in glass) and in vivo (within the body) genotoxicity assays as part of a ninety-day oral toxicity and a two-generation reproductive toxicity study performed on rats showed no significant evidence of toxicity. Even at the highest dosage levels of 5000 mg/kg bw (body weight)/day, both studies showed no evidence of toxicity. Polyvinyl alcohol is considered nonmutagenic, nongenotoxic, and has no carcinogenic activity. That is to say that the consumption of PVA does not change the body’s genetic material, is not toxic, and does not cause cancer. In Europe, Japan, and the United States, PVA formulations for film coatings are prevalent in pharmaceutical tablets and capsules.

Polyvinyl alcohol intake via pharmaceutical products can be compared to the no-observed-adverse-effect-level (NOAEL) to determine the PVC intake value that increases the user’s risk to toxicity. Human exposures to PVA are typically low, and experimental animals have at most 1.8 mg/kg bw/day that is exceeding the 2,780-fold below the NOAEL standards. Given the possibility for additional exposure via pharmaceutical tablets and capsules, the PVA intake is projected to be 4.8 mg/kg bw, which is a value that is 1,040-fold below the NOAEL regulatory value. These values indicate and reiterate the nongenotoxicity of PVA. However, it is important to remember that these values are gross overestimates and cannot be used to generalize the entirety of all food supplements and pharmaceutical products. It is difficult to measure the exact measurements of PVA intake through the daily consumption of PVA-containing film coatings. Scientific panels have requested petitioners to provide specifications for PVA consumption. The overall use of PVA for food supplement and pharmaceutical tablets does not hold significant concern. PVA has been previously evaluated by the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives at the sixty-first meeting in 2003 and suggested an acceptable daily intake of 50 mg/kg bw.

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"Polyvinyl alcohol (PVA)." Encyclopedia Britannica, 25 Nov. 2014. Web. 4 January 2016.