Water Soluble Films (PVOH)

Properties

Polyvinyl alcohol or PVOH is a crystal clear, water soluble thermoplastic derived from polyvinyl acetate through partial or complete hydroxylation. The degree of hydroxylation determines its physical, chemical, and mechanical properties. The lower the number of residual acetate groups, the lower the water solubility and crystallinity but the higher the glass transition temperature. The degree of hydroxylation also affects the maximal moisture uptake which acts as a plasticizer and thus affects the mechanical and physical properties of the polymer. With increasing water uptake, the tensile strength, modulus of elasticity, and hardness are reduced, while impact resistance and elongation at break are greatly increased. In other words, dry PVOH is a hard and crystalline thermoplastic with high Tg of about 375 K and high tensile strength whereas moist PVOH is rather soft and flexible and has noticeably lower tensile strength depending on the amount of water absorbed.

PVOH is extremely hydrophilic which explains its good solubility in water and its high resistance to hydrocarbons, mineral oils, and many organic solvents such as ethers, esters, and ketones. Films made from polyvinyl alcohol have outstanding heat-sealing properties and exceptionally good adhesion to cellulose and other hydrophilic surfaces. They also have good barrier properties to gases such as oxygen, nitrogen, and carbon dioxide. PVOH films are fully biocompatible, biodegradable and nontoxic, making the film especially suited for packaging of food, drugs, and cosmetics.

Due to its water solubility, PVA needs to be crosslinked when structural stability and water insolubility is needed or required. The degree of crosslinking determines the amount of water uptake, permeability, and biodegradation rate as well as the mechanical and physical properties of the polymer. The mechanical and physical properties can also be changed by blending with other compounds and/or by copolymerization with other monomers which also greatly change the water uptake/swelling and decomposition rate. One example is a blend of PVOH with ethylene-vinyl alcohol copolymer or EVOH.

Commercial PVOH Films

Water-soluble films made from polyvinyl alcohol are available in a wide range of grades, offering different solubility in cold and hot water to meet various requirements. Important manufacturers of PVOH resins and films include Aicello (Solublon^®), Sckisui (Selvol^™), Kuraray (Poval^™), Nippon Gohsei (Hi-Selon), and Soltec.

Applications

PVOH has received FDA approval for close contact with food products; in fact, PVA films are fully biocompatible, nontoxic and PVOH waste does not produce any toxic or harmful by-products when incinerated or digested.¹ These properties and its high resistance to many solvents and oils make PVOH suitable for many packaging applications under FDA regulations. It is widely used as a cold and hot water soluble film for diverse packaging applications including food products, detergents, pharmaceuticals and agricultural chemicals. Some noteworthy applications include fully soluble laundry sacks for hospitals, automatic dishwasher soap tablets, window washing concentrate capsules, toilet blocks, mold releasing films, as well as water soluble bags, capsules, pouches, and films for toxic herbicides, pesticides, cement additives, dyes and pigments, and food products.

¹M.I. Baker, S.P. Walsh, Z. Schwartz, and B.D. Boyan, J. Biomed. Mater. Res., Part B, 100B, 1451–1457 (2012)