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