Publications archive - Waste and recycling
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Nolan-ITU Pty Ltd
Prepared in association with ExcelPlas Australia
Polyesters play a predominant role as biodegradable plastics due to their potentially hydrolysable ester bonds. As shown in Figure 3.1 below, the polyester family is made of two major groups - aliphatic (linear) polyesters and aromatic (aromatic rings) polyesters. Biodegradable polyesters which have been developed commercially and are in commercial development are as follows:
|PHA - polyhydroxyalkanoates||PHB - polyhydroxybutyrate|
|PHH - polyhydroxyhexanoate||PHV - polyhydroxyvalerate|
|PLA - polylactic acid||PCL - polycaprolactone|
|PBS - polybutylene succinate||PBSA - polybutylene succinate adipate|
|AAC - Aliphatic-Aromatic copolyesters||PET - polyethylene terephthalate|
|PBAT - polybutylene adipate/terephthalate||PTMAT- polymethylene adipate/terephthalate|
Figure 3.1 - Biodegradable Polyester Family
While aromatic polyesters such as PET exhibit excellent material properties, they prove to be almost totally resistant to microbial attack. Aliphatic polyesters on the other hand are readily biodegradable, but lack good mechanical properties that are critical for most applications. All polyesters degrade eventually, with hydrolysis (degradation induced by water) being the dominant mechanism.
Synthetic aliphatic polyesters are synthesised from diols and dicarboxylic acids via condensation polymerisation, and are known to be completely biodegradable in soil and water. These aliphatic polyesters are, however, much more expensive and lack mechanical strength compared with conventional plastics such as polyethylene.
Many of these polyesters are blended with starch based polymers for cost competitive biodegradable plastics applications. Aliphatic polyesters have better moisture resistance than starches, which have many hydroxyl groups.
The rate of soil degradation of various biodegradable plastics has been measured by Hoshino (2001). Poly-(3-hydroxy-butyrate-valerate) (PHB/PHV), PCL, PBS, PBSA, and PLA were evaluated in soil burial for 12 months and samples were collected every 3 months for the measurement of weight loss. The rate of degradation of PBSA, PHB/PHV and PCL was found to be similar; with the rate of PBS and PLA respectively slower.
Polyhydroxyalkanoates (PHAs) are aliphatic polyesters naturally produced via a microbial process on sugar-based medium, where they act as carbon and energy storage material in bacteria. They were the first biodegradable polyesters to be utilised in plastics. The two main members of the PHA family are polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV).
Aliphatic polyesters such as PHAs, and more specifically homopolymers and copolymers of hydroxybutyric acid and hydroxyvaleric acid, have been proven to be readily biodegradable. Such polymers are actually synthesised by microbes, with the polymer accumulating in the microbes' cells during growth.
The most common commercial PHA consists of a copolymer PHB/PHV together with a plasticiser/softener (e.g. triacetine or estaflex) and inorganic additives such as titanium dioxide and calcium carbonate.
A major factor in the competition between PHAs and petroleum based plastics is in production costs. Opportunities exist however for obtaining cheaper raw materials that could reduce PHA production costs. Such raw materials include corn-steeped liquor, molasses and even activated sludge. These materials are relatively inexpensive nutrient sources for the bacteria that synthesise PHAs (Purushothaman, 2001).
The PHB homopolymer is a stiff and rather brittle polymer of high crystallinity, whose mechanical properties are not unlike those of polystyrene, though it is less brittle. PHB copolymers are preferred for general purposes as the degradation rate of PHB homopolymer is high at its normal melt processing temperature. PHB and its copolymers with PHV are meltprocessable semi-crystalline thermoplastics made by biological fermentation from renewable carbohydrate feedstocks. They represent the first example of a true biodegradeable thermoplastic produced via a biotechnology process. No toxic by-products are known to result from PHB or PHV.
The applications of PHA are blow and injection-moulded bottles and plastic films. Such films are available in Australia under the BiopolTM trademark.
PHAs are biodegradable via composting. Optimum conditions for the commercially available BiopolTM (PHA) degradation during a 10-week composting period were 60°C, 55% moisture, and C:N ratio of 18:1. BiopolTM reached close to a 100% degradation rate under these composting conditions. The aliphatic polyesters function like starch or cellulose to produce non-humic substances such as CO2 and methane. These aliphatic polymers are suited to applications with short usage and high degradation rate requirements (Gallagher, 2001).
Shin et al. (1997) found that bacterial PHB/PHV (92/8 w/w) degraded nearly to completion within 20 days of cultivation by anaerobic digested sludge, while synthetic aliphatic polyesters such as PLA, PBS and PBSA did not degrade at all in 100 days. Cellophane, which was used as a control material, exhibited a similar degradation behavior to PHB/PHV. Under simulated landfill conditions, PHB/PHV degraded within 6 months. Synthetic aliphatic polyesters also showed significant weight losses through 1 year of cultivation. The acidic environment generated by the degradation of biodegradable food wastes which comprises approximately 34% of municipal solid waste seems to cause the weight loss of synthetic aliphatic polyesters.
Poly-hydroxybutyrate-co-polyhydroxyhexanoates (PHBHs) resins are one of the newest type of naturally produced biodegradable polyesters. The PHBH resin is derived from carbon sources such as sucrose, fatty acids or molasses via a fermentation process.
These are 'aliphatic-aliphatic' copolyesters, as distinct from 'aliphatic-aromatic' copolyesters. Besides being completely biodegradable, they also exhibit barrier properties similar to those exhibited by ethylene vinyl alcohol (see Section 3.1.2). Procter & Gamble Co. researched the blending of these polymers to obtain the appropriate stiffness or flexibility.
They have been developed by Kaneka Corp. (a Japanese manufacturer) and marketed by Procter & Gamble Co. under the NodaxTM tradename.
The applications of the PHBH polymer are film, manufactured via casting or blowing methods. Potential applications are mono/multilayer film and non-woven paper packaging at costs comparable to traditional materials such as EVOH.
PHBH resins biodegrade under aerobic as well as anaerobic conditions, and are digestible in hot water under alkaline conditions.
Polylactic acid (PLA) is a linear aliphatic polyester produced by poly-condensation of naturally produced lactic acid or by the catalytic ring opening of the lactide group. Lactic acid is produced (via starch fermentation) as a co-product of corn wet milling. The ester linkages in PLA are sensitive to both chemical hydrolysis and enzymatic chain cleavage.
PLA is often blended with starch to increase biodegradability and reduce costs. However, the brittleness of the starch-PLA blend is a major drawback in many applications. To remedy this limitation, a number of low molecular weight plasticisers such as glycerol, sorbitol and triethyl citrate are used.
PLA does not have full food contact approval due to its fermentation manufacturing method.
A number of companies produce PLA, such as Cargill Dow LLC. PLA produced by Cargill Dow was originally sold under the name EcoPLA, but now is known as NatureWorks PLA, which is actually a family of PLA polymers that can be used alone or blended with other natural-based polymers.
Table 3.1 details some of the other PLA biodegradable plastics that are commercially available.
The applications for PLA are thermoformed products such as drink cups, take-away food trays, containers and planter boxes. The material has good rigidity characteristics, allowing it to replace polystryene and PET in some applications.
PLA is fully biodegradable when composted in a large-scale operation with temperatures of 60°C and above. The first stage of degradation of PLA (two weeks) is via hydrolysis to water soluble compounds and lactic acid. Rapid metabolisation of these products into CO2, water and biomass by a variety of micro-organisms occurs after hydrolysis.
PLA does not biodegrade readily at temperatures less than 60°C due to its 'glass transition' temperature being close to 60°C.
Polycaprolactone (PCL) is a biodegradable synthetic aliphatic polyester made by the ring-opening polymerization of caprolactone. PCL has a low melting-point, between 58-60°C, low viscosity and is easy to process.
Until recently, PCL was not widely used in significant quantities for biodegradable polymer applications due to cost reasons. Recently however, cost barriers have been overcome by blending the PCL with corn-starch.
Table 3.2 details some of the various PCL biodegradable plastics that are commercially available.
|Tone||Union Carbide (UCC)||USA|
|Placeel||Daicel Chemical Indus.||Japan|
PCL is suited for use as food-contact foam trays, loose fill and film bags.
Although not produced from renewable raw materials, PCL is fully biodegradable when composted. The low melting point of PCL makes the material suited for composting as a means of disposal, due to the temperatures obtained during composting routinely exceeding 60°C.
Rutkowska et. al. (2000) studied the influence of different processing additives on the biodegradation of PCL film in the compost with plant treatment active sludge. It was found that PCL without additives, completely degraded after six weeks in compost with activated sludge. The introduction of processing additives gave better tensile strength of the materials but made them less vulnerable to micro-organism attack.
The rate of marine biodegradation of PCL has been studied by Janik et. al. (1988) by measuring the tensile strength and percent weight loss over time in both seawater and a buffered salt solution. It was found that the weight loss, as a percent of total weight, decreased more rapidly in seawater than in the buffered salt solution. After eight weeks, the PCL in seawater was completely decomposed, whereas that in salt solution had lost only 20% of its weight. The same trend was seen for the tensile strength, where after eight weeks, the PCL in seawater was destroyed and that in buffered salt solution had decreased to roughly one-sixth its original value. It is therefore apparent that enzymes in the seawater solution assist to accelerate the biodegradation of PCL and other biodegradable plastics.
Polybutylene succinate (PBS) is a biodegradable synthetic aliphatic polyester with similar properties to PET. PBS is generally blended with other compounds, such as starch (TPS) and adipate copolymers (to form PBS-A), to make its use economical.
Table 3.3 shows some PBS and PBS-A biodegradable plastics which are commercially available.
|SkyGreen BDP||SK Polymers||Korea|
PBS has excellent mechanical properties and can be applied to a range of end applications via conventional melt processing techniques. Applications include mulch film, packaging film, bags and 'flushable' hygiene products.
PBS is hydro-biodegradable and begins to biodegrade via a hydrolysis mechanism. Hydrolysis occurs at the ester linkages and this results in a lowering of the polymer's molecular weight, allowing for further degradation by micro-organisms. Data from SK Chemicals (Korea), a leading manufacturer of PBS polymers, quotes a degradation rate of 1 month for 50% degradation for 40 micron thick film in garden soil.
Aliphatic-aromatic (AAC) copolyesters combine the biodegradable properties of aliphatic polyesters with the strength and performance properties of aromatic polyesters. This class of biodegradable plastics is seen by many to be the answer to making fully biodegradable plastics with property profiles similar to those of commodity polymers such as polyethylene. To reduce cost AACs are often blended with TPS.
Although AACs have obvious benefits, their market potential may be affected by legislation, such as that in Germany, which distinguishes between biodegradable plastics made from renewable resources and those, like AAC, which use basically the same raw materials as commodity plastics and petrochemicals. Currently in Germany, biodegradable plastics must contain greater than 50% renewable resources to be accepted.
The two main types of commercial AAC plastics are EcoflexTM produced by BASF and Eastar BioTM produced by Eastman. Under each trade name are a number of specific grades. Each grade of polymer has been designed with controlled branching and chain lengthening to match its particular application.
AACs come closer than any other biodegradable plastics to equalling the properties of lowdensity polyethylene, especially for blown film extrusion. AACs also can meet all the functional requirements for cling film such as transparency, flexibility and anti-fogging performance, and therefore this material has great promise for use in commercial food wrap for fruit and vegetables, with the added advantage of being compostable.
Whilst being fossil fuel-based, AACs are biodegradable and compostable.
ACCs fully biodegrade to carbon dioxide, water and biomass. Typically, in an active microbial environment the polymer becomes invisible to the naked eye within 12 weeks. The extent and rate of biodegradation, apart from the inherent biodegradability of the polymer itself, depends on several environmental factors such as:
Modified PET (polyethylene tetraphalate) is PET which contains co-monomers, such as ether, amide or aliphatic monomers, that provide 'weak' linkages that are susceptible to biodegradation through hydrolysis. Depending on the application, up to three aliphatic monomers are incorporated into the PET structure. Typical modified PET materials include PBAT (polybutylene adipate/terephthalate) and PTMAT (polytetramethylene adipate/terephthalate).
DuPont have commercialised BiomaxTM which is a hydro-biodegradable modified PET polyester. Certain BiomaxTM grades also contain degradation promoters to provide tailored combinations of performance properties and degradation rates.
The options available for modified PET provide the opportunity to produce polymers which specifically match a range of application physical properties whilst maintaining the ability to adjust the degradation rate by the use of copolyesters.
Modified PET is hydro-biodegradable, with a biodegradation steps following an initial hydrolysis stage. It contains weak linkages which create sites for microbial attack. The mechanism involves a combination of hydrolysis of the ester linkages and enzymatic attack on ether and amide bonds. With modified PET it is possible to adjust and control degradation rates by varying the comonomers used.