The Science of Biodegradable Plastics: The Reality Behind Biodegradable Plastic Packaging Material
Interest in biodegradable disposable plastic items has steadily grown over the last decade. Disposable packaging materials used to ship and protect purchased items as well as disposable containers used for food and drink are of special interest. The idea that one time use items can be disposed of with the peace of mind, that they will not remain for centuries in a landfill, or as litter, is one of the tenets driving the recent interest in “green” technologies and lifestyles. With packaging materials, the reduction in usage of raw materials, re-use and recycling is of course the best route to sustainable lifestyle. However, for various reasons, in practice, much of the material ends up being discarded to a landfill or accidentally shows up as litter. For these instances, it is advantageous to have a plastic material that would biodegrade when exposed to environments where other biodegradable materials are undergoing decay.
What is Biodegradable?
Biodegradation is degradation caused by biological activity, particularly by enzyme action leading to significant changes in the material’s chemical structure. In essence, biodegradable plastics should breakdown cleanly, in a defined time period, to simple molecules found in the environment such as carbon dioxide and water. The American Society of Testing and Materials (ASTM) defines ‘biodegradability’ as:
“capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms, that can be measured by standardized tests in a specified period of time, reflecting available disposal conditions.”
Aerobic and Anaerobic Biodegradation
Aerobic biodegradation is the breakdown of an organic substance by microorganisms in the presence of oxygen. Almost all organic materials can be metabolized in an oxidative environment by aerobic organisms. The organism has secreted enzymes that breakdown substances into smaller organic molecules which are then absorbed into the cells of the microbes and used for cellular respiration. During the respiration process, the organic molecules absorbed into the cells are broken down in steps, where a molecule known as adenosine-5’- triphosphate (ATP) is used to store and transport energy for cells, for life processes such motility and cell division. In biochemistry this chemical reaction sequence is known as Electron Chain Transfer. In the case of aerobic metabolism, oxygen is used at the end of the chain as the final electron acceptor, producing the main byproducts of carbon dioxide and water.
Composting is a well known and common use of aerobic biodegradation, during which the volume of organic material is typically reduced by about 50%, where the remaining, slow-decaying humus material left over can be used as a rich planting medium. The ASTM defines a compostable plastic material as being:
“capable of biological decomposition in a compost site as part of an available program, such that the plastic is not visually distinguishable and break down to carbon dioxide, water inorganic compounds and biomass at a rate consistent with known compostable materials (e.g. cellulose).”
The biomass material referred to here is humus. The bioactivity in active compost will generate heat that further enhances the rate of microbial growth and metabolism. However, for the purpose of the ASTM definition, the available program is an industrial compost facility where heat and moisture are artificially added to the mass to maximize the degradation rate. As we will see, this artificial environment becomes critical for degradation of some biodegradable plastic materials.
Anaerobic biodegradation occurs in the absence of oxygen where anaerobic microbes are dominant. In the absence of oxygen the organism must use some other atom as the final electron acceptor. Hydrogen, methane, nitrogen and sulfur are common along with oxidizing minerals. Thus, the effluent from anaerobic digestion is biogas, consisting of mostly methane and carbon dioxide, with trace gasses such as ammonia and hydrogen sulfide. Often, the complete digestion will require several different types of bacteria where one type partially processes the waste to a point where another bacterium strain takes over (4). Most biodegradation of solid waste in landfill occurs under anaerobic conditions by design because it is typically much slower than aerobic degradation.
Most biodegradable substances come from plant and animal matter, or from artificial materials that are very similar in molecular structure to these naturally occurring substances. As the naturally occurring substances evolved, microorganisms also evolved to use the substances as a food source: the carbon in particular, used as a building block for life-sustaining compounds. Simple sugars are readily absorbed into the cell to be metabolized. However, larger and more complex molecules such as starches, proteins and cellulose, require enzymes and acids to reduce their size enough to be absorbed. Living organisms have developed the ability to secrete specific digestive compounds so as to best utilize the available food supply. For example, the enzyme amylase, found in human saliva, is used to breakdown long-chain starch molecules into and smaller simple sugars.
For microorganisms, this adaptive process can be applied to other, more complex carbon containing compounds in crude oil. This type of microbial biodegradation has been demonstrated for hydrocarbons derived from petroleum (10)
Biodegradable Plastic Materials
Currently available degradable plastic materials can be broken down into two main groups:
Polyester Polymers Synergistic and Hybrid Polymers
When one thinks of polyesters in general, the polymers that come to mind are very durable with good physical and mechanical properties. A good example is polyethylene terephthalate (PET). This polymer is strong, abrasion and stain resistant, so it can be a good choice for carpeting and clothing. It also has good gas barrier properties which make it ideal for soda bottles. These polymers, which are also resistant to biodegradation, typically contain a large number of six-carbon rings in their molecular structure. In chemistry, compounds containing these rings are known as aromatic compounds.
Biodegradable polyesters which do not contain six-carbon rings are known as aliphatic polyesters. They will typically react with moisture at elevated temperatures to breakdown the long polymer chains. This process, called chemical hydrolysis, reduces the higher molecular weight polymer to much smaller hydrocarbon compounds. The resulting molecules can then be absorbed by microorganisms and metabolized for energy. Since it is a chemical reaction, the hydrolysis occurs at a much higher rate than one would expect for a purely biological process, and as a result, relatively quick degradation is observed.
Aliphatic polyesters have attracted interest as biodegradable plastic materials; however they typically have poor physical and mechanical properties (3) like strength, flexibility, heat resistance, etc. Some common biodegradable polyester polymers in commercial use include poly(caprolactone) (PCL), poly(glycolic acid) (PGA) and poly(butylene succinate) (PBS). These are synthetic polymers, made from petroleum-based, raw materials, and like most biodegradable polyesters have inferior mechanical properties e.g. low heat deflection temperature and low elongation failure (brittle). They will also begin to hydrolyze at modest temperatures in the presence of moisture, rapidly losing molecular weight and further decreasing mechanical properties. Although expensive to make, these biodegradable polymers are ideal for use in specialized, high margin applications such as medical devices (e.g. dissolving, drug delivery systems, tissue engineering scaffolds and bone repair etc.).(2)
Another well known aliphatic polyester is poly(lactic acid). PLA is a synthetic polymer made from fermented sugars extracted primarily from food crops such as corn, beets or sugarcane. The resulting lactic acid monomer is chemically processed and then polymerized, in the presence of a metal catalyst, to form the high molecular weight plastic material. Like the petroleum-based biodegradable polyesters, PLA has many of the same undesirable mechanical properties, such as low heat deflection temperature. The polymer is also very brittle and has a low-melt strength leading to difficulty in processing. Consequently, most commercial applications using PLA require a synthetic rubber and/or acrylic additive to compensate for these deficiencies.
Degradation of PLA occurs quickly through a multistep process (4) of chemical depolymerization, followed by dissolution of the intermediate lactic acid in the presence of moisture, and the absorption into the cells of microorganisms with subsequent metabolization. Initiation of this chain of events typically occurs at elevated temperatures (above heat deflection temperatures), such as conditions existing in an industrial compost operation. The relatively fast chemical reaction at the beginning of the chain of events explains the surprisingly quick degradation of polymer in an industrial compost environment. This mechanism of chemical attack followed by cell metabolism does not meet the true definition of a biodegradable material inasmuch as biological activity is not required for the initial breakup of the material. In low temperature aerobic or anaerobic environments where initial hydrolysis occurs slowly, biodegradation of PLA also proceeds very slowly if at all.
Another family of biodegradable polyesters, which could in a way be viewed as more complex extensions of the molecular structure of PLA, is known as polyhydroxy alkanoates (PHA’s). Intriguingly, PHA’s are natural polymers also derived from plant sugars but are synthesized within the bacteria themselves. The PHA’s are manufactured and used as carbon storage in the cells(6), similar to the way the human body stores fat to be used as an emergency food source.
It has been shown that bacterially produced PHB/PHV (92/8 w/w) deteriorated nearly to completion within 20 days of cultivation by anaerobic digested sludge, while synthetic aliphatic polymers such as PLA, PBS, and poly(butylene succinate adipiate) (PBSA) did not degrade at all in 100 days (1).
For degradable polyesters, the best improvement in physical properties is obtained by synthetically creating a polyester copolymer using both aliphatic and aromatic groups. These are typically derived from oil-based raw materials such as 1,4-butanediol, adipic acid, and terephthalic acid (7). Using this technique, the polymer can be tailored to balance the excellent physical and mechanical characteristics of the aromatic polyester groups with the degradation and subsequent mineralization of the aliphatic groups. These polymers are also readily mixable with pure aliphatic polyesters like PLA, or natural polymer like cellulose, to form a hybrid, degradable polymer with improved performance.
Synergistic or Hybrid Polymers
Synergistic polymers are typically intimate mixtures of oil-based and naturally occurring polymers where the two have some chemical affinity for each other. When mixed, there is intimate contact between the two polymer chains so as to create a homogenous single phase. In other words, once mixed they could not be mechanically separated. This is somewhat akin to mixing gelatin powder with hot water to form a single uniform substance, once cooled.
The intimate mixing of the natural and synthetic polymers can be taken one step further: where the attraction of the synthetic and natural polymers is enhanced by grafting other chemically compatible groups along the chains of the natural and/or synthetic polymers. As with the PVOH, this technique enhances biodegradation through generational adaptation which can be initiated with relatively small additions of natural polymers. To illustrate how this could be possible, it has been shown that polyethylene will biodegrade via a monooxygense enzyme pathway (9). Initiation of the process begins with the formation of a biofilm on the surface of the polymer, which is facilitated by the inclusion of the compatible natural polymers. These films of microorganisms have been shown to efficiently biodegrade petroleum based polymers (8).
Low-level synergistic enhancement does not materially impact the physical and mechanical properties of the original synthetic polymer. Therefore, the product applications are not restricted beyond what would normally be expected for the un-amended polymer. Since the additive itself will not degrade the polymer or affect processing, the ability for recycling or reuse of the plastic article will be unaffected. Unintended degradation will not occur since the initial colonization requires an environment where existing biodegradation is occurring or would normally be expected to occur, either aerobic or anaerobic. Additional heat is not required, and no chemical, polymer-chain weight reduction process is needed beyond the enzymatic action of the microorganisms.
For the choice of materials to be used in the manufacture of a more environmentally friendly packaging material, the criteria needs to take into account business considerations and strategies, while addressing environmental concerns related to the life cycle of the packaging. The primary purpose of the packaging material is to protect the items being shipped from damage via impact or abrasion, and therefore protection should be the first consideration. The material will also need to perform in largely uncontrolled, ambient conditions of heat and humidity; thus, the next consideration should be given to the products’ possible end-of-life scenarios. The scenarios include disposal in landfills, litter, recycle, etc. Finally, material costs need to meet market criteria.
Conventional polymer technologies have been able to tailor materials that can meet the market need of both cost and performance. There is infrastructure in place for recycling and/or re-use of many of these materials, which is the most desirable destination in the life cycle of the packaging product. With inclusion of a synergistic additive, such as that used by FP International, the materials would also be well-suited for the less desirable destinations, such as landfills.
The other biodegradable polymer options have no recycle infrastructure, and could possibly be viewed as having been designed to be thrown out. However, the fact that many of these polymers, like PLA, are limited to biodegradation in only commercial compost facilities, further decreases the potential for a desirable end-of-life scenario. Moreover, while the bacterially produced polyesters (PHB/PHV) would biodegrade in a more general disposal scenario, they are particularly cost-prohibitive for most packaging applications.
In addition to sustainable choices in materials for FP International’s products, FP has ongoing programs for reduction of raw material and energy usage, recycling, increased production efficiencies, efficient product design and increased recycled, raw material usage.
Rod Alire is Chief Scientist for FP International. Mr. Alire carries more than 20 years experience in polymer processing with particular expertise in polymeric foam and film extrusion processes. His projects have emphasized environmental impact mitigation and sustainability through new product design and manufacturing technologies. By Mr. Alire’s development of a theoretical model for mass transport phenomena and foam expansion behavior in polystyrene packaging material production, FP International was able to reduce the usage of raw materials and density of the products being produced.
In order to replace the use of CFC’s – chlorofluorocarbon, (known to be harmful to the ozone and environment), Mr. Alire designed and built a high pressure, foaming agent metering and delivery system for polymer foam extrusion. Also, he developed polyolefin polymer blends and a blown film process for production of PMOS (products manufactured on-site) air bag materials. These airbags are 99% air, since they can be manually deflated or popped, they also reduce the size of materials produced for recycling. Also, the size of the film air cushion materials takes less space to ship, thus less cost and lowers the use of trucks (lowering gas use and truck emissions). Currently, Mr. Alire is working to increase the strength and mechanical property of the film itself, in order to produce film which uses less material.