FACTORS AFFECTING DEGRADATION

Synthetic polymers are inherently resistant to biological attack. But susceptibility to biodegradation varies and is affected by:

ADDITIVES

Various compounding ingredients may have nutritive value for microorganisms and hence may invite microbial attack. Most of the plasticizers, lubricants, thickening agents, starch and cellulose fillers are susceptible to microbial attack.

PLASTICIZERS

Plasticizer tends to force the chain apart, giving them greater freedom of movement and also reducing van der Waals’ forces between the chains. All plasticizers are affected by fungal or bacterial attacks. Susceptibility of microbial attack increases as the plasticizer level increases.

THE TYPE OF CHEMICAL BOND

Various chemically degradable polymer bonds are: polycyanoacrylates, polyanhydrides, polyketals, polyorthoesers, polyacetals, poly (2-hydroxy-esters), poly (E-caprolactone), polyphosphazenes, polyB-Hydroxyesters, polyamino carbonates, polypeptides, polycarbonates, polyphosphate esters.

WATER UPTAKE

The hydrolysis of the polymer backbone requires water. Degradation rates increase when the hydrophilic component contents are increased.

CRYSTALLINITY AND MOLECULAR WEIGHT

Crystalline polymers degrade slower than amorphous polymers. High molecular weight leads to slower degradation.

PH

pH changes can modify hydrolysis rates by orders of magnitude.

COPOLYMER COMPOSITION

The presence of variety of functional groups having different hydrolysis rates affects degradability.

ENZYMATIC DEGRADATION

Biodegradable polymers can be hydrolyzed passively or actively via enzymatic catalysis.

MEASURING BIODEGRADABILITY OF POLYMERS

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

ENVIRONMENTAL EFFECTS OF BIODEGRADABLE POLYMERS

Plastic shopping bags have advantages and disadvantages when compared to alternatives such as paper bags. All disposable bags are problematic from an energy use and disposal perspective.

Advantages

The durability, strength, low cost, water and chemicals resistance, welding properties, lesser energy and heavy chemicals requirements in manufacture, fewer atmosphere emissions and light weight are advantages of plastic bags. Many studies comparing plastic versus paper for shopping bags show that plastic bags have less net environmental effect than paper bags, requiring less energy to produce, transport and recycle; however these studies also note that recycling rates for plastic are significantly lower than for paper. Plastic bags can be incinerated in appropriate facilities for waste-to-energy. Plastic bags are stable and benign in sanitary landfills. Plastic carrier bags can be reused as trash bags or bin bags.

Disadvantages

Our main concerns with plastic shopping bags:

1.                  Plastic bag littering and associated indiscriminate waste disposal and consumer behavior.

2.                  Resource consumption issues, including reduction, reuse and recycling.

3.                  Plastic degradability issues relating to littering and resource use.

4.                  Social issues, community education and awareness, and consumer perceptions.

5.                  Plastic bags are made of petrochemicals, a nonrenewable resource.

6.                  Plastic bags are flimsy and often do not stand up as well as paper or cloth.

7.                  When disposed of improperly, they are unsightly and represent a hazard to wildlife.

8.                  Conventional plastic bags are not readily biodegradable under any normal circumstance.

9.                  Plastic bags can cause unsupervised infants to suffocate.

DISADVANTAGES OF BIODEGRADABLE POLYMERS

1.        Most of the biodegradable polymers are not commercially viable due to their higher cost.

2.        Major applications of biodegradable polymers are in medical field.

3.        Biodegradable polymers are too unstable for long term industrial use.

4.        There is a public opposition against genetically modified foods.

5.       Manufacture of biodegradable polymers consumes more energy than the conventional polymers.

6.        Mode of the degradation is mainly by hydrolysis which in turn pollutes the water.

7.     Migration of plastic degradation by-products such as residual pigments, catalyst residues and isocyanate via run-off and leachate from landfills and composting facilities to groundwater and surface water bodies is resulted.

8.   Trauma and death of marine species resulting from only partial or slow degradation of biodegradable plastic products in marine environments is caused.

9.        They increase the littering as the people believe that biodegradable plastics will disappear quickly.

10.      It increase the soil and crop degradation resulting from the use of compost that may have unacceptably high organic and or metal contaminants derived from biodegradable plastic residuals, additives and modifiers such as coupling agents, plasticizers, fillers, catalysts, dyes and pigments.

11.      They do not disappear completely and leave visible trace and toxic residues.

12.       They do not possess the strength and storage comparable to that of the conventional polymers.