Plastic packaging materials represent a significant portion of the world's plastics consumption. Although these materials are low-cost and effective for many Department of Defense applications, the logistics of managing packaging waste in remote areas and foreign countries is becoming increasingly problematic and costly for the military. Polyhydroxyalkanoates (PHA) are a family of biobased, biodegradable natural plastics that can functionally replace over 50% of the plastics used today. They range in properties from strong, moldable thermoplastics to highly elastic materials to soft, sticky compositions, and can be made as resins or as latex with excellent film-forming characteristics. PHAs are also biodegradable in aquatic, soil, and municipal waste treatment environments, and can be composted.
The objective of this project was to demonstrate the use of biodegradable PHA natural plastic for foamed packaging and stretch/shrink film applications. PHA packaging foams and stretch/shrink films were produced and tested for functional performance and biodegradability.
Specific PHA formulations were designed for each application. For foamed PHAs, a suitable blowing agent and a foam cell nucleant were selected, and a foam extrusion process was identified. For stretch and shrink films, film orientation introduced during processing was a necessary requirement for achieving our target properties. Once the formulation and processing parameters were identified, prototype PHA foams and stretch/shrink films were produced and converted to useful packaging products. Mechanical performance and composting and biodegradability testing was then performed.
The research team successfully demonstrated PHA polymer branching, thus enabling production of long-chain length PHA polymers. They also produced high melt-strength and melt-elasticity PHA polymers by blending these branched, long-chain length PHA polymers with linear PHA polymers. As a result of these accomplishments, PHA biodegradable plastic was successfully formulated into both foam and film grades.
The implications of these findings are important to note. Different foam expansion ratios enable different applications. Low expansions use more material but are much stronger. Very high expansions are most cost efficient, but don’t have sufficient strength for some applications. PHA foams with expansion ratios of 15X were successfully made. One of the goals of this project was to develop PHA foams with higher (30X) expansion ratios (for use in packaging peanuts). While this goal was not achieved, the medium expansion ratios achieved are ideal for semi-structural applications, such as foam food plates.
PHA films produced under this project were used to make blown and cast films. These films did not exhibit any shrinking properties, which the team attributed to the slow crystallization kinetics of PHA and the consequently low levels of orientation frozen-in during film fabrication. Through additional experimentation, however, the team was able to determine the approximate temperature and stretch rate conditions to produce a properly oriented PHA film. It was also not possible to stretch the PHA film at high speed commercial rates. Stretching was achieved at a slow, hand stretching rate.
The biodegradable natural plastic products developed in this project can improve logistics and reduce the costs of transporting and disposing of packaging waste that are burdening military operations worldwide through a substantial reduction in nondegradable plastic waste. PHA plastics also benefit the environment by reducing greenhouse gas emissions and lowering energy utilization for resin production relative to conventional fossil fuel-based thermoplastics. In addition, PHAs offer the only opportunity for marine degradable packaging while being water resistant and having good barrier properties.