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Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.

Biodegradable Plastics - Developments and Environmental Impacts

Nolan-ITU Pty Ltd
Prepared in association with ExcelPlas Australia
October, 2002

Biodegradable Starch-based Polymers

Starch is a linear polymer (polysaccaride) made up of repeating glucose groups linked by glucosidic linkages in the 1-4 carbon positions. The length of the starch chains will vary with plant source but in general the average length is between 500 and 2 000 glucose units. There are two major molecules in starch - amylose and amylopectin. The alpha linkage of amylose starch allows it to be flexible and digestible.

Starch-based biodegradable plastics may have starch contents ranging from 10% to greater than 90%. Starch based polymers can be based on crops such as corn (maize), wheat or potatoes. Starch content needs to exceed 60% before significant material breakdown occurs. As the starch content is increased, the polymer composites become more biodegradable and leave less recalcitrant residues. Often, starch-based polymers are blended with high-performance polymers (e.g. aliphatic polyesters and polyvinyl alcohols) to achieve the necessary performance properties for different applications.

Biodegradation of starch based polymers is a result of enzymatic attack at the glucosidic linkages between the sugar groups leading to a reduction in chain length and the splitting off of sugar units (monosaccharides, disaccharides and oligosaccharides) that are readily utilised in biochemical pathways.

At lower starch contents (less than 60%) the starch particles act as weak links in the plastic matrix and are sites for biological attack. This allows the polymer matrix to disintegrate into small fragments, but not for the entire polymer structure to actually bio-degrade.

There are several categories of biodegradable starch-based polymers including:

2.1 Thermoplastic Starch Products

Thermoplastic starch biodegradable plastics (TPS) have a starch (amylose) content greater than 70% and are based on gelatinised vegetable starch, and with the use of specific plasticising solvents, can produce thermoplastic materials with good performance properties and inherent biodegradability. Starch is typically plasticised, destructured, and/or blended with other materials to form useful mechanical properties. Importantly, such TPS compounds can be processed on excisting plastics fabrication equipment.

High starch content plastics are highly hydrophilic and readily disintegrate on contact with water. This can be overcome through blending, as the starch has free hydroxyl groups which readily undergo a number of reactions such as acetylation, esterification and etherification.

Developments

The CRC for International Food Manufacture and Packaging Science Australia has developed its own version of TPS biodegradable plastics. These natural vegetable starch polymers have a amylose content greater than 70%.

Trials have been successfully performed using maize starch polymers as mulch film, and the material was found to perform as well as polyethylene film, with the added advantage that after harvest, the film can be simply ploughed into the soil. These natural starch polymers are now being commercialised through a new company called Plantic Technologies Ltd. based in Melbourne.

Applications

The applications of thermoplastic starch polymers are generally film, such as shopping bags, bread bags, bait bags, over wrap, 'flushable' sanitary product backing material, and mulch film.

Foam loose fill packaging and injected moulded products such as take-away containers are also potential applications. Foamed polystyrene can be substituted by starch foams that are readily biodegradable in some loose-fill packaging and foam tray applications.

Foamed starch loose-fills are rather easy products to produce and this area has become an early market for biodegradable plastics. During its preparation, raw starch is premixed with 25 to 50 weight percent water and fed into an extruder capable of imparting intensive shear and operating at high temperature (higher than the boiling point of water, i.e., 150-180C). Under these conditions of shear and temperature, starch breaks down, loses its crystallinity, and gets plasticised with water, resulting in a homogenous amorphous mass. When this gelatinized starch/water mixture exits the extruder, the water that is present in the mass at a temperature higher than its boiling point expands into steam due to a sudden drop in pressure, and the foam is formed. Generally a plasticiser (such as glycerol) and another polymer (such as polyvinyl alcohol) impart more reproducible properties to starch foam.

Degradation Mechanisms and Properties

Along with the biodegradation of the polymers by sugar molecules, certain TPS grades are also fully water soluble.

2.2 Starch Synthetic Aliphatic Polyester Blends

Blends of biodegradable synthetic aliphatic polyesters and starch are often used to produce highquality sheets and films for packaging by flat-film extrusion using chill-roll casting or by blown film methods since it is difficult to cast films from 100% starch in a melted state. Approximately 50% of the synthetic polyester (at approximately $4.00/kg) can be replaced with natural polymers such as starch (at approximately $1.50/kg), leading to a significant reduction in cost. Furthermore, the polyesters can be modified by incorporating a functional group capable of reacting with natural starch polymers.

Developments

Lim et al. (1999) studied the properties of an aliphatic polyester blended with wheat starch. The polyester was synthesized from the poly-condensation of 1,4-butanediol and a mixture of adipic and succinic acids. The wheat starch-polyester blends were found to have melting points near that of the polyester alone. A plasticiser was added to the starch, making the blends more flexible and processable than the polyester itself. Plasticised blends were found to retain a high tensile strength and elongation at the break point, even at high concentrations of starch.

Blending starch with degradable synthetic aliphatic polyesters such as PLA and PCL has recently become a focus of biodegradable plastic development. Biodegradable plastics can be prepared by blending up to 45% starch with degradable PCL . This new material is not strong enough for most applications, as the melting temperature is only 60C and it gets soft at temperatures above 40C. These drawbacks greatly limit the applications of the starch-PCL blends. Table 2.1 details some starch-PCL polymers that are commercially available.

Table 2.1 - PCL Polymers (Commercially Available)
Polymer Type Trade-name Supplier Origin
Starch-polycaprolactone (PCL) blends Mater-BiTM Novamont Italy
BioflexTM Biotech Germany

Applications

The applications for starch-synthetic aliphatic polyester blends include high-quality sheets and films for packaging and other film applications.

Several starch-based plastics are currently available on the Australian market. One of these is the 'BioBag', which is produced from the Novamont resin that has been around since 1994, and is made from corn starches in combination with fully biodegradable plastics or polylactic acid.

Degradation Mechanisms and Properties

The wheat starch-aliphatic polyester blend studied by Lim et al. (1999) demonstrated excellent biodegradability. Soil burial tests revealed complete biodegradation within eight weeks. The excellent properties exhibited by these blends make them ideal as commodity biodegradable plastics.

2.3 Starch and PBS/PBSA Polyester Blends

Other polyesters that are blended with starch to improve material mechanical properties are polybutylene succinate (PBS) or polybutylene succinate adipate (PBSA). A small amount (5% by weight) of compatibiliser (maleic anhydride functionalised polyester) can be added to impart phase stability to these starch based polymer blends. At higher starch content (>60%), such sheets can become brittle. For this reason, plasticisers are often added to reduce the brittleness and improve flexibility.

Developments

Ratto et al.(1999) investigated the properties of PBSA and corn starch blends of varied compositions. PBSA is biodegradable, and exhibits excellent thermoplastic properties. The objective of the study was to obtain a mixture that maximised these properties while minimizing cost. Corn starch is an inexpensive polysaccharide that was blended with PBSA at concentrations of 5-30% by weight for analysis. Tensile strength of the blends was lower than that of the polyester alone, but there was not a significant drop in strength with increasing starch content. In addition, melt temperature and processing properties were not appreciably affected by the starch content.

Applications

Starch and PBS or PBSA blends are used to produce biodegradable plastic sheet which can be thermoformed into products such as biscuit trays or film products.

Degradation Mechanisms and Properties

Ratto et al.(1999) investigated the blends the biodegradability properties of PBSA and corn starch blends by measuring CO2 production in a soil burial test. Addition of only 5% starch showed a large reduction in half-life from that of the pure polyester. The half-life was found to decline with increasing starch content until a minimum 20% starch content was reached.

2.4 Starch-PVOH Blends

Polyvinyl alcohol (PVOH) is blended with starch to produce readily biodegradable plastics.

Developments

Propak, a Sydney-based company, produce loose-fill peanuts from a starch-PVOH blend (a grade of Mater-biTM) that exhibits water solubility in approximately 3 minutes. These expanded products have a closed cell pore structure and densities ranging from 0.01 to 0.1 g/cm3. Table 2.2 details some of the starch-PVOH blends commercially available.

Table 2.2 - Starch-PVOH Blends Commercially Available
Polymer Type Trade-name Supplier Origin
Starch-Polyvinyl Alcohol (PVOH) blends NovonTM Chisso Corp Japan
NovonTM Warner Lambert USA
Mater-biTM Novamont Italy

Degradation Mechanisms and Properties

PVOH is readily water soluble, and the starch-PVOH blends are therefore degraded via hydrolysis and biodegradation of the sugar molecules.