Polyvinyl Alcohol (PVOID) and Ethylene Vinyl Alcohol (EVOH)
Polyvinyl Alcohol
One might think that PVOH is an addition polymer of vinyl alcohol. However, it cannot be produced in that manner since the vinyl alcohol monomer is unstable. Consequently, PVOH is produced by hydrolysis of polyvinyl acetate, PVA. The resulting material is amorphous at first, since PVA is amorphous, but it tends to crystallize when oriented. PVOH is atactic, but since the -H and -OH are is amorphous groups, this does not interfere with crystallization. Very strong intermolecular forces are imparted by the OH groups with their ability to hydrogen bond. The forces are so strong that PVOH cannot be melt processed, since its decomposition temperature is below its melting temperature. Without stress, the molecules, though they have a strong tendency to crystallize, are not able to do so because they lack the mobility to rearrange into a crystalline structure.
PVOH has excellent barrier properties to oxygen and many other substances, because of its crystallinity and strong intermolecular forces. In its pure form, however, it is water soluble. Since the water solubility is imparted by the OH groups, the degree of solubility can be modified by controlling the amount of hydrolysis of the PVA. The more acetate groups that are retained, the lower is the water solubility of the PVOH,as well as the less effective the barrier properties.
Because of its extreme water sensitivity and difficulty in processing, PVOH has few packaging applications. Some important ma5kets include packaging for toxic chemicals, such as pesticides, which can be encapsulated in PVOH pouches and placed, package and all, in the mixing tank. The PVOH dissolves, freeing the chemical, which can then be sprayed as usual. PVOH biodegrades in the environment, and does not present problems such as clogging spray nozzles or otherwise interfering with the application of the chemicals. In a similar approach, PVOH is sometimes used for bags for soiled linen in hospitals. The bags and linen can be placed directly in the washer, where the bags dissolve, and thus hospital personnel are able to avoid handling contaminated items.
Ethylene Vinyl Alcohol
Ethylene vinyl alcohol (EVOH) is produced by a controlled hydrolysis of ethylene vinyl acetate copolymer. The hydrolytic process transforms the vinyl acetate group to vinyl alcohol in a manner analogous to production of polyvinyl alcohol. The highly polar OH groups increase the intermolecular forces, while the ethylene groups maintain molecular mobility Since, as discussed above, the OH group is isomorphous with H, the polymer can achieve a hi~ percent of crystallinity, even though ethylene and vinyl alcohol units are randomly distributed in the chain. The OH groups also make the polymer more compatible with water molecules, so it is much more hydrophilic than PE. The magnitude of all of these effects depends on the percentage of vinyl alcohol. The polymer will have properties similar to PE at a small amount of vinyl alcohol (VOH) content, or will be more like polyvinyl alcohol at a very high VOH content.
For packaging applications, the most important characteristic of EVOH is its outstanding O2 and odor barrier properties. Packaging structures with EVOH provide high retention of flavors, and prevent quality loss associated with reaction of oxygen with the product. EVOH also provides a very high resistance to oils and organic vapors. This resistance decreases somewhat as the polarity of the penetrating compound increases. For example, the resistance to linear and aromatic hydrocarbons is outstanding, but for ethanol and methanol it is low; it may absorb up to about 12% of ethanol.
EVOH resins were introduced commercially in 1970 in Japan, and their use has expanded rapidly, especially where excellent oxygen barrier is required Formulations currently in use typically contain 27 to 48 mole % ethylene. The lower the amount of ethylene, the better is the barrier - when the polymer is dry. The lower the percentage of ethylene, the greater is the moisture sensitivity, and the difficulty of processing. EVOH resins are melt processable, and exhibit high strength, toughness, and clarity For these reasons, EVOH resins are often the best choice for applications where excellent barrier is required, as long as exposure to moisture can be controlled.
As indicated above, the hydroxyl group, -OH, makes the polymer hydrophilic, attracting water molecules. The presence of water decreases the oxygen barrier properties of the material. Because of its moisture sensitivity, EVOH is usually incorporated into package structures as a buried inner layer in a coextrusion, surrounded by polyolefins or other good water vapor barrier polymers. These structures typically contain an adhesive, or tie layer, between the EVOH and the polyolefin to provide adequate adhesion between the polar EVOH and the nonpolar polyolefin. For retorted products, this level of moisture protection may not be enough to prevent unacceptable oxygen permeation during and shortly after retorting. In that case, it is possible t。incorporate a desiccant in the tie layer between the EVOH and the polyolefin. The desiccant absorbs moisture that penetrates the polyolefin during retorting, preserving the dryness, and hence the oxygen barrier, of the EVOH. EVOH properties are summarized.
EVOH can be extruded in films, blow molded, or injection molded. It can also be processed by coextrusion or lamination, in combination with PE, PP, PET, nylons, and other resins. Applications in packaging include flexible structures and rigid containers. Typical applications are ketchup and barbecue sauce bottles; jelly, preserves, vegetable juice, and mayonnaise containers; and meat packages. Non-food applications include of solvents and chemicals.