PERS

 

Biodegradability

A biodegradable polymer does not generally sell simply because it is biodegradable, it must compete as a material on the basis of its own price/property characteristics, with biodegradability an added bonus. Given the above advantageous properties of the PVOH-based materials, potential markets fall into three main areas, depending upon the property to be exploited.
·         Applications which could make use of the controlled water solubility. These include hospital applications such as disposable infected laundry bags (which are already made from PVOH), biohazard bags, pathology sample bags, bedpans, urine bottle and vomit bowl inserts. The packaging of powders which are dust producing and are then further processed in the aqueous phase is another possibility. Examples are toxic herbicides, pesticides, dyes and pigments, where these materials are contained in a bag of cold water soluble PVOH, avoiding operator contact.
·         Applications making use of the barrier properties. PVOH materials can now act as a more effective replacement for the ethylene vinyl alcohol polymers currently used as oxygen barrier layers for food packaging. The resistance of PVOH to non polar organic solvents will also protect foodstuffs from secondary contamination by printing inks etc. Examples of potential food packaging uses are ketchup bottles (presently PP/EVOH/PP), and enhanced shelf life packaging.
·         Applications based on the biodegradability of the PVOH-based materials. A variety of applications are anticipated where the driving force is increasing pressures from environmental awareness issues, high landfill charges, legislation limiting use of landfills and non-degradable products, and the EEC Packaging and Landfill Directives. Such applications include film products made from hot water soluble PVOH, floating mulch films or transparent greenhouse/cloche films and silage wrap, pallet wrap, fertiliser bags, and general industrial and consumer packaging. Strict segregation would be necessary to prevent the biodegradable products being mixed with recyclable products such as polyethylene. A knowledge of the product origin, service life and end of life disposal method (for example dissolution in water, composting or incineration) is necessary for applications where the biodegradability of the PVOH is to be exploited

Physical Properties

The physical properties of PVOH, such as the strength, water solubility, gas permeability and thermal characteristics vary with the degree of crystallinity, which is heavily dependent on the degree of hydrolysis and the average molecular weight of the polymer. The actual amount of crystalline material present in compounded PVOH is further dependent on the amount of plasticiser and/or water incorporated into the compound, the production process (acid or base catalysed), the degree of hydrolysis and the molecular weight, figure 1. Partially hydrolysed grades contain residual acetate groups, which reduce the overall degree of crystallinity. This results in materials with lower strength and increased water solubility than the fully hydrolysed grades. The partially hydrolysed grades may be considered as copolymers, while those that are fully hydrolysed may be considered as homopolymers.


Figure 1. The effect of molecular weight and hydrolysis level on the physical properties of PVOH.

Primary author: Nigel Hodgkinson and Michael Taylor
Source: Materials World, vol. 8, pp. 24-25, April 2000.