Polyvinyl alcohol
Polyvinyl alcohol (PVOH, PVA, or PVAL) is a water-soluble synthetic polymer.
Preparation
Unlike most vinyl polymers, PVOH is not prepared by polymerization of the corresponding monomer. The monomer, vinyl alcohol, almost exclusively exists as the tautomeric form, acetaldehyde. PVOH instead is prepared by partial or complete hydrolysis of polyvinyl acetate to remove acetate groups.
Properties
Polyvinyl alcohol has excellent film forming, emulsifying, and adhesive properties. It is also resistant to oil, grease and solvent. PVOH is an atactic material but exhibits crystallinity as the hydroxyl groups are small enough to fit into the lattice without disrupting it. It has high tensile strength, flexibility, as well as high oxygen and aroma barrier. However these properties are dependent on humidity, in other words, with higher humidity more water is absorbed. The water which acts as a plasticiser will then reduce its tensile strength, but increase its elongation and tear strength. PVOH has a melting point of 230 degrees Celsius and 180 to 190 degrees Celsius for the fully hydrolysed and partially hydrolysed grades. PVOH also decomposes rapidly above 200 degrees Celsius.
Uses
The hydrophilics property of PVOH makes it permeable for both water and hydrated salt. Biodegradation of PVOH depends on the microbe population of the aqueous or soil environment, although decreasing the molecular mass of PVOH will increase the rate of biodegradation. Biodegradation of PVOH is also improved by adding natural components like starch, cane sugar or protein.
It is frequently used as an adhesive and thickener, and to form a barrier to carbon dioxide in polyethylene terephthalate (PET) bottles. It is also often used in biomedical application and food industry, and in hydrogels.
Sheets of PVOH, stretched during manufacture so as to align the polymer chains, can be doped with iodine to form Polaroid sheet polarizers.
It can be combined with borax to create a "slime" that is similar to other slimes sold in toy stores.
A new recent use (patented) for PVOH is in feminine hygiene and adult incontinence products. The PVOH is further treated and replaces the plastic backing sheet in these products making these products biodegradable. These new products are currently available in North America.
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