Expanded polystyrene, commonly used in coffee cups, coolers and packaging, will dissolve if it comes in contact with most vegetable oils, including biobased phase change material. The phenomenon is proving effective as a “green” solution to reducing EPS waste. But it demonstrates that care must be taken in both the containment and shipment of organic phase change material.
By William R. Sutterlin, Ph.D.
The term “phase change material” (PCM) is used to describe materials that use phase changes (e.g., melting or freezing) to absorb or release relatively large amounts of latent heat at relatively constant temperature. The material can be used as a natural thermostat in a variety of applications, from cooling vests and warming blankets to large-scale thermal energy storage tanks.
PCMs operate on the principle that when the temperature becomes warmer than the freeze point of the PCM it will liquefy and absorb and store heat. Conversely, when the temperature drops, the material will solidify and give off heat, warming the material coated or impregnated with PCM.
PureTemp phase change materials were developed under five years of research sponsored by the USDA and the National Science Foundation. These PCMs are made from underutilized bio-based products – beef tallow, palm oil, coconut oil and soybean oil. These products are non-toxic and provide a new value-added non-food product for American farmers. They are capable of tens of thousands of melting and freezing cycles without performance degradation. There is no concern for oxidation or other reactions because these bio-based PCMs do not have chemical sites for oxidation and other such reactions to occur.
PureTemp PCMs are often shipped in expanded polystyrene foam containers. Expanded polystyrene (EPS) is made from polystyrene, one of the most widely used plastics in the world. EPS foam has relatively good insulation properties and is used widely in coffee cups, coolers, packaging and other materials. EPS is produced from a mixture of about 90-95 percent polystyrene and a 5-10 percent gaseous blowing agent, most commonly inert gases. These blowing agents form bubbles and expand the foam.
One of the benefits of EPS and these foam containers are that they are very resistant to dissolving in water. However, they are not resistant to dissolving in oils, such as some vegetable oils derived from various plants and fruits. You may recall from your high school chemistry class the old saying, “Like Dissolves Like.” This means that polar compounds such as water will dissolve other polar compounds like salt, sugar and vinegar. Water is polar, oil is nonpolar. This is why the oil and vinegar used in salad dressing tend to stay separated. The vinegar is polar and the vegetable oil is nonpolar. In a similar manner, polystyrene is made from petroleum oil sources and PureTemp PCMs are made from vegetable oil sources. These materials are both nonpolar and will dissolve in each other.
EPS is soluble in most oils to some extent, depending upon the particular oil, the temperature and contact time. For example, if you have a foam cup and drop a piece of lemon skin into a hot cup of tea you will see the polystyrene begin to disintegrate due to the lemon oil. This is a physical and not a chemical action. It is an “action,” not a “reaction.” It is similar in nature to water dissolving sugar. It is not like strong acids or bases burning a finger or eating through paper. There is no danger to you if you drink from the cup. However, it is not recommended as the integrity of the cup breaks down and may no longer hold the hot liquid.
The physical effect of certain oils acting as a solvent for polystyrene has long been observed, studied and reviewed by the plastics industry and by the U.S. Food and Drug Administration. In fact, this well-known effect is being used as a “green” method to reduce the volume of polystyrene waste. Many companies and individuals are using citrus oil derivatives and oil extracts such as limonene to dissolve recycled EPS. This method can reduce the volume of EPS to 10 percent of its original volume. Sony Corp. is using this method on an industrial scale to manage the huge quantities of foam packaging used in appliance shipments. After dissolving the EPS bricks and pellets in limonene, a vegetable oil, the plastic foam is separated from the vegetable oil and reconstituted as new EPS, while the vegetable oil solvent can be reused for the next batch of foam to be dissolved.
In a similar manner, some PureTemp PCMs will dissolve EPS. One might even notice gas or bubbles generated during the process of the biobased PCM dissolving EPS. This is not a chemical reaction but the release of the trapped inert blowing gas that was used during the blowing process to convert polystyrene to EPS. The result is not a chemical reaction or the chemical breakdown of polystyrene but simple a dissolving effect. The phenomenon is generally harmless but does underscore the importance of proper containment and packaging of phase change material.
About the author
Dr. William R. Sutterlin, Entropy Solutions’ chief chemical officer, has received organizational, local, state and federal recognition including being a recipient of the prestigious Presidential Green Chemistry Challenge Award and is part of the United States and Brazil research collaboration in biomass conversion. His research interests include the conversion of agricultural products and other biomass materials into higher value added chemicals.