TEST METHODS TO ASSESS
BIODEGRADABILITY OF POLYMERS:
Enormous work is being carried out in the field of
biodegradable polymers. Hence the researchers must be aware of the Standard Test
methods available for the measurement of the degree of degradation. A range of International
Standards, and Test methods are developed specifically for biodegradability.
Laboratory test protocols are usually evaluation of environmental degradation
under simulated conditions to which a particular polymer will be exposed on
disposal. Correlation with real world exposure is more difficult for
biodegradation than photo degradation because the environment for
biodegradation widely differ in microbial composition, pH, temperature,
moisture, etc. hence cannot be readily reproduced. In early years, the only
tests to establish biodegradability were related to the microbial growth,
weight loss, tensile and other physical properties losses. These all are
indirect measurements of biodegradation often leads to results that are
difficult to reproduce from laboratory to laboratory, giving rise to confusion
on the susceptibility to biodegradation of a given polymer.
STANDARD ORGANISATIONS FOR TESTING:
There are number of International Standards, and Test
methods, developed specifically for biodegradability, product safety, and also
for compost derived products.
The main International Organizations that have established
standards or testing methods are:
à American
Society for Testing and Materials (ASTM) (www.astm.org); (ASTM D 638, 1980).
à European
Standardization Committee (CEN) (www.cenorm.be);
à International
Standards Organization (ISO) (www.iso.org);
à Institute
for Standards Research (ISR),
à German
Institute for Standardization (DIN); and
à Organic
Reclamation and Composting Association (ORCA) (Belgium).
American Society for Testing and Materials
(ASTM) methods
à ASTM
D5988-03: Standard Test Method for Determining Aerobic Biodegradation in
Soil of Plastic Materials or Residual Plastic Materials after Composting. This
test method determines the degree and rate of aerobic biodegradation of
synthetic plastic materials (including formulation additives that may be
biodegradable) in contact with soil, or a mixture of soil and mature compost,
under laboratory conditions.
à ASTM
D5526-94(2002): Standard Test Method for Determining Anaerobic
Biodegradation of Plastic Materials under Accelerated Landfill Conditions. This
test method determines the degree and rate of anaerobic biodegradation of
plastic materials and mixtures of household waste in an accelerated-landfill
test environment.
GROWTH
RATINGS G-21-70 AND GZZ-76:
In these test
the resistance of plastics to fungal and bacterial growth is assessed. Fungi
like aspergillus niger, pennicillium at 28-30°C temperature at 85% RH for 21
days is used. Bacteria like pseudomonas aeruginosa incubated at 35-57°C for
minimum 21 days are used. After suitable time the growth is assessed in terms
of % surface covered. The test gives quick results. It is easy to do and give
indication of biodegradation. But it is not conclusive for biodegradation of
polymer.
International
Standards Organization (ISO):
(
www.iso.org) European
biodegradable plastics are currently assessed by ISO 14855, which is a controlled
aerobic composting test and ISO 14851 and ISO 14852 are biodegradability tests
specifically designed for polymeric materials.
ISO 846 (1978):It specifies the use of
a mixture five strains of fungi over a period of at least 28 days at 30 ±2°C
and 95-100% RH. The results of attack are measured by visual examination for
growth. Method A is used to determine the ability of plastics to act as the
carbon and nitrogen source for the growth of microorganisms. Method B is used to
determine the fungi toxic properties of plastics.
Compost
Toxicity Tests: For a comprehensive assessment of toxicity
associated with compost applications, plastics can be tested on both plant and
animal species. A number of polyester types were tested including a plasticized
cellulose acetate, an aliphatic polyester,
polyhydroxybutyrate-co-hydroxyvalerate and polycaprolactone. Cell culture
medium with serum was used as the extraction medium.
Plant Phytotoxicity Testing: While a
product may not negatively impact plant growth in the short term, over time it
could become phytotoxic due to the build-up of inorganic materials, which could
potentially lead to a reduction in soil productivity. For this reason some
manufacturers use plant phytotoxicity testing on the finished compost that
contains degraded polymers. Phytotoxicity testing can be conducted on two
classes of flowering plants. These are monocots (plants with one seed leaf) and
dicots (plants having leaf with two seeds). Representatives from both of these
classes are typically used in toxicity testing - summer barley to represent
monocots and cress to represent dicots. Tests involve measuring the yield of
both of these plants obtained from the test compost and from control compost.
Animal Toxicity Test: Animal testing is generally carried out using earthworms
(as representative soil dwelling organisms) and Daphnia (as representative
aquatic organisms). Earthworms are very sensitive to toxicants. Since earthworm
feeds on soil, they are suitable for testing the toxicity of compost. In the
acute toxicity test, earthworms are exposed to high concentrations of the test
material for short periods of time. Earthworms are exposed to soil and compost
in varying amounts. Following 14 days of exposure, the number of surviving
earthworms is counted and weighed and the percent survival rate is calculated.
The earthworms are exposed to several mixture ratios of compost and soil
mixtures. Compost worms are used for testing the toxicity of biodegradable
plastic residues. These worms are very sensitive to metals such as tin, zinc,
and heavy metals and high acidity. For this test worms are cleaned and
accurately weighed at intervals over 28 days. The compost worm toxicity test is
considered to be an accurate method. The toxicity test can establish whether
degradation products present in liquids pose any problem to surface water
bodies. In the test, Daphnia are placed in test solutions for 24 hours. After
exposure the number of surviving organisms is counted and the percent mortality
is calculated.
SOIL BURIAL TEST BS 4618 SEC.4.5 1974: Soil
burial is a traditional way to test samples for degradation because of its
similarity to actual conditions of waste disposal. It lacks reproducibility
because of the difficulties in controlling climatic factors and the population
of various biological systems that are involved. Generally the samples are
buried in soil for periods of up to two years. At the end of the resting
period, changes in properties like loss in weight, mechanical strength, shape
etc. are studied. It provides qualitative indication of biodegradation.
Difference
Between Standards for Biodegradation: The main point of
differentiation between the various international standards is the percentage
of biodegradation required for compliance. This is an important issue that is
under discussion at ISO level.
Table Standards Compliance Requirements
|
Standard
|
Biodegradation
Requirement
|
|
DIN
|
60% 6 MONTHS
|
|
ASTM
|
60% 6 MONTHS
|
|
CEN
|
90% No specific time is mentioned
|
|
OECD
|
60% 28 days
|
