Films & Biodegradable Plastic: Summary
In a world increasingly conscious of problems with non-degradable but disposable plastics, there is a growing demand for more environmentally friendly products. Limited laboratory testing indicates that levan has the potential to be a part of the solution.
ADVANTAGES OF LEVAN AS A NATURAL PLASTIC COMPONENT
- Rapid mineralization in sea water
- Excellent oxygen barrier
- Moderate water vapor barrier
- Solvent resistance
- Lack of antigenicity
ENVIRONMENTALLY FRIENDLY: Levan is made by fermentation of sugar from either sugar cane or sugar beets. Sugar beets grow on marginal land and take less energy to process than many crops. Sugar cane has been touted as a good CO2 sink. As an exopolymer, levan production does not require extraction or separation from the biomass, minimizing processing costs.
BIODEGRADABLE: 60% of levan is mineralized in 10 days in a marine environment. This is a faster degradation rate than many other bio-based plastics have reached.
EXTRUSION: In the laboratory, levan ribbons have been formed on a single screw extruder (Carbohydrate Polymers paper in reference section below) and strands have been formed by a twin screw extruder. Early research suggests levan may be most useful when used in conjunction with more elastomeric materials.
One problem encountered by bio-feedstocks for degradable plastics has been that the higher force developed by the production equipment breaks up bio-materials. Levan spheres at only 50-200 nm in diameter are much smaller than many other particles, especially starch. The finer grind actually seems to work better for levan which may give levan an edge over some competing natural materials.
CAST FILMS: Cast films of 98% levan and 2% Laponite® (a synthetic clay) were very flexible, withstanding repeated bending and stretching.
Levan films consisting of 10% montmorillonite and 5% PEG were tested as a barrier to oxygen in a MOCON Ox-Tran permeation unit at 0% relative humidity and 23°C. Prior to testing, the films were conditioned at 25% relative humidity for 3 weeks. Initial results on 110 micron films show an oxygen permeability of 0 using the L-type module of the MOCON testing unit. The lower limit of this module using masked film samples is 0.05 cc/m2-day. Therefore, it is expected that the oxygen permeability of this film sample at these testing conditions is at or below 0.05 cc/m2-day.
The same film formulation allowed passage of water vapor at 123 g/m2-day. The following table shows how this compares with other plastic materials.
Water vapor barrier. (Table adapted from Michael.)
SPRAY-ON FOOD WRAP: The 50-200 nm diameter levan spheres permit application of a very thin coating without clogging problems. Adhesive properties of levan allow bonding to various surfaces. One possible use of these properties is to formulate levan into a spray-on food wrap. Rather than struggling to cover a turkey with several pieces of plastic wrap, the consumer would spray on a light coating of levan. It may be possible to peel off the coating. Alternatively, since levan has no taste and once regulatory requirements have been met, it is possible that the levan coating could be eaten. Spray on levan could possibly find use as a sealant for bread. Sandwiches made too far ahead become soggy as moisture in the filling is absorbed by the bread. Bread coated with a very thin layer of levan applied as a spray would appear no different to the consumer, but the levan would form a barrier, blocking the entry of much of the water into the bread.
Michael D. Bioplastics Supply Chains. Rural Industries Research and Development Corp. Publication No. 04/044 (2004).