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Biodegradable Plastics - Developments and Environmental Impacts

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

Appendix A - Glossary

Abiotic disintegration
Disintegration of plastic materials by means other than by biological process such as dissolution (dissolving), oxidative embrittlement (heat ageing) or photolytic embrittlement (UV ageing).
Activated Sludge
Sludge with active, live degradation micro-organisms.
Aerobic degradation
Aerobic degradation is degradation in the presence of air (oxygen). Essentially aerobic degradation is composting. Aerobic degradation of plastics under controlled composting conditions is described in ASTM 5338-92.
Aliphatic-aromatic Copolyesters (AAC)
These copolymers combine the excellent material properties of aromatic polyesters (e.g. PET) and the biodegradability of aliphatic polyesters. They are soft, pliable and have good tactile properties. Melting points are high for a degradable plastic (around 200C).
Aliphatic polyesters (e.g. PCL)
Aliphatic polyesters are biodegradable but often lack in good thermal and mechanical properties. While, vice versa, aromatic polyesters (like PET) have excellent material properties, but are resistant to microbial attack. Typical aliphatic polyesters include polyhydroxy butyrate, polycaprolactone, polylactic acid and polybutylene succinate. Aliphatic polyesters degrade like starch or cellulose to produce non-humic substances such as CO2 and methane. They can be processed on conventional processing equipment at 140-260 C, in blown and extruded films, foams, and injection moulded products.
Amylose
A component of starch consisting of a chain polymer of linked D-glucopyranosyl structures. Thermoplastic starch polymers consist largely of amylose.
Anaerobic degradation
Degradation in the absence of air (oxygen) as in the case of landfills. Anaerobic degradation is also called biomethanization. Anaerobic degradation of plastics can be determined by measuring the amount of biogas released as described in ASTM 5210-91.
ASTM
The American Society of Testing and Materials
Bioassimilation
Chemical assimilation of a substance into the natural environment (see also mineralization)
Biodegradable
The ASTM defines biodegradable 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 condition." It is important to note that the definition of biodegradation should specify a time limit. If the biodegradation process is sufficient to mineralise organic matter into carbon dioxide or methane respectively, water and biomass, the material can be termed "biodegradable".
Bioerodable
Polymers that exhibit controlled degradation through the incorporation of prodegradant additive masterbatches or concentrates. Such polymers oxidize and embrittle in the environment and erode under the influence of weathering.
Biomass
The weight of all the organisms in a given population, trophic level or region.
Compostable
Compostable is defined as 'capable of undergoing biological decomposition in a compost site as part of an available program, such that the plastic is not visually distinguishable and breaks down to carbon dioxide, water, inorganic compounds, and biomass, at a rate consistent with known compostable materials (e.g. cellulose).' A polymer is "compostable" when it is biodegradable under composting conditions.
Compostable Plastics
A polymer is 'compostable' when it is biodegradable under composting conditions. The polymer must meet the following criteria:
  • Break down under the action of micro-organisms (bacteria, fungi, algae).
  • Total mineralisation is obtained (conversion into CO2, H2O, inorganic compounds and biomass under aerobic conditions).
  • The mineralisation rate is high and is compatible with the composting process.
  • The degree of degradability of the material shall be measured under controlled composting conditions as per ASTM D 5338-92. Those materials having a degree of biodegradation equivalent to that of cellulose (maximum permissible tolerance of 5%) will be considered to meet the compostability criteria.
Composting
Breaking down of plant and animal material using micro-organisms under aerobic conditions. For successful composting there must be sufficient water and air to allow the micro-organisms to break down the material. The U.S. EPA defines composting as 'the controlled decomposition of organic matter by microorganisms into a stable humus material.'
Copolyesters
Copolyesters combine aromatic esters with aliphatic esters or other polymer units (e.g. ethers and amides) and thereby provide the opportunity to adjust and control the degradation rates.
Decomposer organism
An organism, usually a bacterium or a fungus, that breaks down organic material into simple chemical components, thereby returning nutrients to the physical environment.
Degradability
Ability of materials to break down, by bacterial (biodegradable), thermal (oxidative) or ultraviolet (photodegradable) action. When degradation is caused by biological activity, especially by enzymatic action, it is called 'biodegradation'.
Degradable PET
Up to three aliphatic monomers can be incorporated into the PET structure to create weak spots in the polymeric chains that make them susceptible to degradation through hydrolysis.
Ecotoxicity
Ecotoxicity refers to the potential environmental toxicity of residues, leachate, or volatile gases produced by the plastics during biodegradation or composting
Foamed starch
Starch can be blown by environmentally friendly means into a foamed material using water steam. Foamed starch is antistatic, insulating and shock absorbing, therefore constituting a good replacement for polystyrene foam.
Functional Group
A particular grouping of elements in a molecule or compound which gives it particular properties, such as physical properties or the ability to undergo certain chemical reactions.
Humus
The organic substance that results from decay of plant or animal matter. Humus results from the degradation of lignin, carbohydrate, and proteins. Biodegradable plastics can form humus as they decompose. The addition of humus to soil is beneficial.
Hydrolysis
Hydrolysis refers to the cleavage or breakage of bonds by reaction with water or moisture. All polyesters degrade eventually, with hydrolysis being the dominant mechanism.
LCA
Life Cycle Assessment.
Masterbatch
A concentrate of additive in a polymer-based carrier resin. A masterbatch is generally added to a polymer in order to introduce various additives at a known level.
Mineralisation
Conversion of a biodegradable plastic to CO2, H2O, inorganic compounds and biomass. For instance the carbon atoms in a biodegradable plastic are transformed to CO2 which can then reenter the global carbon cycle.
Modified PET
Polyethylene Tetraphalate (PET) id a rigid polymer to which alphatic monomers can be added to enhance biodegradability, such as PBAT (polybutylene adipate/terephthalate) and PTMAT (polytetramethylene adipate/terephthalate).
Monomer
A molecule that can join with other molecules to form a large molecule called a polymer. A monomer is the smallest repeating unit in a polymer chain.
Organic Recycling
Organic recycling is defined as aerobic (ie. composting) or anaerobic (bio-methanisation) treatment of the biodegradable parts of plastic packaging under controlled conditions using micro-organisms to produce stabilised organic residues, methane and carbon dioxide.
Photo-biodegradation
Degradation of the polymer is triggered by UV light and assisted by the presence of UV sensitisers. In this process the polymer is converted to low molecular weight material (waxes) and in a second step converted to carbon dioxide and water by bacterial action.
Photodegradable
A process where ultraviolet radiation degrades the chemical bond or link in the polymer or chemical structure of a plastic.
Phytotoxicity
Phytotoxicity refers to toxic effects on plants. Plant phytotoxicity testing on the finished compost that contains degraded polymers can determine if the buildup of inorganic materials from the plastics are harmful to plants and crops and if they slow down soil productivity.
Plastified Starch
See Starch Composites (50 % Starch)
Polybutylene succinate (PBS) and polybutylene suuccinate adipate (PBSA)
Biodegradable synthetic aliphatic polyesters. Adipate co-polymers are added to the PBS polymer to make its use more economical.
Polycaprolactone (PCL)
Polycaprolactone is a biodegradable thermoplastic polymer derived from the chemical synthesis of crude oil. Although not produced from renewable raw materials, it is fully biodegradable.
Polyesters
Polymers with ester groups in their backbone chains. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET).
Polyhydroxyalkanoates (PHA)
PHAs are linear aliphatic polyesters produced in nature by bacterial fermentation of sugar or lipids. More than 100 different monomers can be combined within this family to give materials with extremely different properties. They can be either thermoplastic or elastomeric materials, with melting-points ranging from 40 to 180C. The most common type of PHA is PHB (polybeta-hydroxybutyrate).
Polyhydroxybutyrate (PHB)
PHB has properties similar to those of polypropylene, however it is stiffer and more brittle.
Polyhydroxybutyrate-valerate copolymer (PHBV)
Polyhydroxybutyrate-valerate is a PHB copolymer which is less stiff and tougher, and it is used as packaging material.
Polylactic Acid (PLA)
A biodegradable polymer derived from lactic acid. PLA resembles clear polystyrene, it provides good aesthetics (gloss and clarity), but it is stiff and brittle and needs modification for most practical applications (e.g. plasticisers increase its flexibility).
Polylactic acid aliphatic copolymer (CPLA)
Biodegradable CPLA is a mixture of polylactic acid and other aliphatic polyesters. It can be either a hard plastic (similar to PS) or a soft flexible one (similar to PP) depending on the amount of aliphatic polyester present in the mixture.
Polymer
A long molecule that is made up of a chain of many small repeated units (monomers).
Polyvinyl Alcohol (PVOH)
Polyvinyl alcohol is a synthetic, water-soluble and readily biodegradable polymer.
Prodegradant
An additive that can trigger and accelerate the degradation of a polymer. Typically prodegradants (or degradation promoters) are catalytic metal compounds based on iron, cobalt and manganese.
Recalcitrant Residues
Non-biodegradable residues that remain after partial or incomplete biodegradation of a 'biodegradable' plastic. The recalcitrant organics are the compounds that show resistance to biodegradation. Most of the synthetic polymers exhibit the phenomenon of recalcitrance because of dissimilar chemical structures to those of naturally occurring compounds
Starch Composites (10 % Starch)
Starch can be used as a biodegradable additive or replacement material in traditional oil-based commodity plastics. If starch is added to petroleum derived polymers (e.g. PE), it facilitates disintegration of the blend, but not necessarily biodegradation of the polyethylene component. Starch accelerates the disintegration or fragmentation of the synthetic polymer structure. Microbial action consumes the starch, thereby creating pores in the material which weaken it and enable it to break apart.
Starch Composites (50 % Starch)
Also called plastified starch materials. Such materials exhibit mechanical properties similar to conventional plastics such as PP, and are generally resistant to oils and alcohols, however, they degrade when exposed to hot water. Their basic content (40-80%) is corn starch, a renewable natural material. The balance is performance-enhancing additives and other biodegradable materials.
Starch Composites (90 % Starch)
Usually referred to as thermoplastic starch. They are stable in oils and fats, however depending on the type, they can vary from stable to unstable in hot/cold water. They can be processed by traditional techniques for plastics.
Thermoplastic Polymers
Becomes soft and 'plastic' upon heating and firm when cool, with this process able to repeated without the material becoming brittle.
Thermosetting Polymers
Sets firmly and cannot be heated plastic again.
Thermoplastic Starch
See Starch Composites (90 % Starch)