Basics PHA Bioplastics
Basics PHA Bioplastics and how they’re made Article contributed by Daniel Gilliland, Business Development Director of Telles, the joint venture between Metabolix Figure 3, Mirel is formed into numerous items in a variety of conversion processes and Archer Daniels Midland, Cambridge, MA, USA Years ago, scientists noticed that micro-organisms utilized a different “nutrient buffer” system than humans did. Instead of storing fat in their cells like we humans do, they stored a naturally occurring plastic in their cells, polyhydroxyalkanoate (PHA). This interesting material was discovered in the 1920s and has been vigorously investigated for the past 30 to 40 years in attempts to understand it and to commercially exploit its potential. Most recently, companies have begun using this material as a substitute for traditional plastic derived from petroleum or fossil fuel. Clearly, much has changed in the past 80 years and this paper will try to explain, in layman’s terms, how PHA like Mirel is made today and the environmental impact it can make on the world. PHA is really a family of polymers. The polymers differ from one another by the nature of the pendant groups or side chains attached to the polymer. Large pendant groups tend to break up crystalinity and form more rubber like properties with lower melting points and low glass transition temperatures (Tg). Polyhydroxyoctanoate (PHO) is one such material. Short chain pendant groups such as polyhydroxybutyrate (PHB) are more highly crystalline with higher melting points and higher Tg. This results in higher melting points, higher levels of stiffness and higher heat distortion temperatures. Methods for making these various types of PHAs are becoming well understood due to the intense effort by scientists at assessing metabolic pathways. Scientists can use different micro-organisms and different feed stocks to create a cellular factory that efficiently produces the right polymer. A variety of naturally occurring, renewable feed stocks ranging from glucose, dextrose, fatty acids, and vegetable oils can be used, depending upon the type of PHA desired. Figure 1 shows a microphotograph of PHA accumulating in cells of a microbe. The PHA is the large white nodules. This particular microbe grows to over 80% plastic in just a few hours! For those wishing a detailed understanding of the cellular biology and enzyme pathways to the various PHAs, please see Oliver People’s article in Chemtec 1 . Now that the biology discussion is over, we can talk about why these PHAs are good for us. To understand the impact of PHA on us, we need to understand society’s needs for plastics. First, society has come to rely upon plastic for its many advantages over more traditional materials like paper, steel, aluminum: keeping food safe, protecting products in shipping, replacing heavy materials, etc. Secondly, responsible consumers want the plastic to be easy to dispose of at the end of its usefulness or to not persist in the environment. Third we would like plastic that does not create harm- 34 bioplastics MAGAZINE [03/07] Vol. 2
Basics Figure 1, microscopic thin section of microbes with white nodules of PHA. The microbe is 80% plastic, just prior to recovery. Figure 2, parts made of Mirel before and after 60 days submersion in the ocean. 1: Chemtec, Biodegradable Plastics from plants, 1996, 38-44, Oliver Peoples et al 2: American Chemical Society, ACS Symposium Series 939 June 2006, Ramani Narayan ful emissions, such as greenhouse gases like carbon dioxide, during its manufacturing and disposal. Finally, we want plastic that minimizes the use of “non-renewable” resources like fossil fuels. Before we discuss the functionality of PHA, we should summarize the environmental aspects: • They can reduce greenhouse gases: since PHAs are made from renewable resources, they can be produced and used in ways that can actually remove greenhouse gases from the atmosphere, not just reduce emissions! In most end of life scenarios, use of the right PHA instead of a fossil based plastic will reduce greenhouse gas emissions by 80% to 100%. For a more complete discussion of the carbon cycle, please read Ramani Narayan’s treatise 2 on the subject. It is important to understand the life cycle assessment of both the process used to make PHA and the usage of the material to understand its true impact on greenhouse gases. Early processes used to make PHA were energy intensive and released significant amounts of greenhouse gases, but new processes have superseded them, resulting in breakthroughs that make PHA economically and environmentally viable. • PHAs will quickly return to nature at the end of their usefulness: since PHAs are made by the “cousins” of naturally occurring microbes found broadly in nature, and since these cousins already have the enzymes required to digest PHA, they will be digested and returned to nature in virtually any environment supporting a healthy microbial population such as soil, lakes, rivers, oceans, home and industrial composting systems. Figure 2 shows samples of Mirel bioplastic, made from PHA, before and after 60 days submersion in the ocean. Though these Mirel bioplastics quickly return to nature, they are durable in use. • PHAs can considerably reduce fossil energy usage. Depending upon how they are manufactured, PHAs can significantly reduce the amount of fossil energy used to produce them compared to the traditional plastic they replace. Mirel Bioplastics reduce fossil energy usage by over 90% in some applications. The future seems even brighter, since this remaining fossil energy is used to harvest the feed stocks, and much of this fossil energy can and probably will be converted to renewable fuels in the future. Mirel bioplastic is a family of PHA resins that can replace fossil fuel based plastics in a growing variety of applications. There are various grades of Mirel being developed. Some have “film like” properties with the look and feel of low density polyethylene. Other grades perform more like polystyrene or polypropylene in injection molded applications such as soil stabilization stakes, caps and closures, food containers or cosmetic cases. Grades have been developed for coating paper board to replace polyethylene in cups and food containers and still other grades for sheet used in thermoforming applications such as storage containers, lids, and other food service items. Future grades are being developed for foam and fiber applications replacing polystyrene and polyester. Figure 3 depicts some common applications under development. Beyond the production of PHA in microbial bio-factories, research is continuing to find ways to make PHA commercially viable using waste products as feed stock or by growing the plastic in sugar cane or in non food crops such as switch grass. Although these potential pathways are most likely years from commercialization, they demonstrate the variety and environmental potential some of the production methods for this new family of plastics. bioplastics MAGAZINE [03/07] Vol. 2 35
