Polyvinyl alcohol

Polyvinyl alcohol (PVOH, PVA, or PVAL) is a water-soluble synthetic polymer.

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Properties

Polyvinyl alcohol has excellent film forming, emulsifying, and adhesive properties. It is also resistant to oil, grease and solvent. It is odorless and nontoxic. 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.

PVA has a melting point of 230°C and 180–190°C for the fully hydrolysed and partially hydrolysed grades. It decomposes rapidly above 200°C as it can undergo pyrolysis at high temperatures.

PVA is an atactic material but exhibits crystallinity as the hydroxyl groups are small enough to fit into the lattice without disrupting it.

Uses

Some uses of polyvinyl alcohol include:

1. Adhesive and thickener material in latex paints, paper coatings, hairsprays, shampoos and glues.
2. Carbon dioxide barrier in polyethylene terephthalate (PET) bottles.
3. Carotid phantoms for use as synthetic vessels in doppler flow testing.
4. Children's play putty or slime when combined with borax.
5. Feminine hygiene and adult incontinence products as a biodegradable plastic backing sheet.
6. As a mold release because materials such as epoxy do not stick to it.
7. As a water-soluble film useful for packaging.
8. As fiber reinforcement in concrete
9. As a surfactant for the formation of polymer encapsulated nanobeads
10. Used with polyvinyl acetate to make Elmer's glue
11. Used in eye drops and hard contact lens solution as a lubricant.
12. Used in protective chemical-resistant gloves

Fishing

PVA is widely used in freshwater sport fishing. Small bags made from PVA are filled with dry- or oil-based bait and attached to the hook, or the baited hook is placed inside the bag and cast into the water. When the bag lands on the lake or river bottom it breaks down, leaving the hook bait surrounded by ground bait, pellets etc. This method helps attract fish to the hook bait, though it does result in dissolved plastic in the water.

Anglers also use string made of PVA for the purpose of making temporary attachments; for example holding in a coil a length of line that might otherwise tangle while the cast is made.

Production

There are over 22 producers worldwide with ~ 1 million tons on the open market in 2002. Larger producers include Kuraray (Japan) and Celanese (USA) but mainland China has installed a number of very large production facilities in the past decade.

Preparation

Unlike most vinyl polymers, PVA is not prepared by polymerization of the corresponding monomer. The monomer, vinyl alcohol, almost exclusively exists as the tautomeric form, acetaldehyde. PVA instead is prepared by partial or complete hydrolysis of polyvinyl acetate to remove acetate groups.