BIODEGRADATION

SK ANSARI AND SEEMA ANSARI
(feedback@pgeconomist.com)

Nov 1 - 7, 2010

In the most general sense and/or good judgment biodegradable means that a substance is able to be broken down into other substances with a significant change of chemical structure by the activities of living organisms and is therefore unlikely to persist in the environment. All polymers are subject to some type of biodegradation.

Discarded conventional plastic remains in the environment for decades. It blocks sewers and drains, disfigures the streets, beaches and countryside, and kills wildlife on land, in rivers and oceans.

To overcome these problems increasing attention has been paid to the development of degradable plastic.

1. Starch-based biodegradable
2. Aliphatic polyester biodegradable
3. Photodegradable, and
4. Oxo-biodegradable

The starch-based plastic does not degrade totally and only the starch constituent is consumed by microbial activity, and the plastic residues can be harmful to the soil and to birds and insect.

Aliphatic polyesters are relatively expensive. In the same way as starch, it relies on microbial activity in compost or soil to degrade. Both these products degrade by a process of hydro degradation.

Photodegradable plastics degrade after prolonged exposure to sunlight, so they will not degrade if buried in a landfill, a compost heap, or other dark environment, or if heavily overprinted.

Oxo-biodegradable plastics are a new development. The plastic degrade by a process of oxo-degradation. The technology is based on a small amount (typically 3 per cent) of prodegradant additive (proprietary) being introduced into the conventional manufacturing process, thereby making the plastic degradable. The degradation, which does not rely on microbes, starts immediately after manufacturing and accelerates when exposed to heat, light, or stress. The process is irreversible and continuous until the whole material reduces to CO2 and water, leaving no fragments in the soil.

The oxo-biodegradable plastics (OBPs) are also consumed by bacteria and fungi after the additives have broken down the molecular structure sufficiently, allowing access to living microorganisms. So organic wastes in home, restaurants, hospitals etc. can be put into oxo-biodegradable plastic sacks for disposing straight into the composting plant without emptying the sacks. Even OBPs were eaten by cow, deer, turtle or other animal. They would degrade even faster due to the temperature and bacteria present in the gut without causing blockage, unlike conventional plastic bags which could kill the animal.

OBPs have an advantage over plastics produced from starch or other agricultural products. They biodegrade and can be composed but do not need to be buried in a compost heap or landfill in order to degrade. The fact they can degrade in a normal environment is a significant factor in relation to litter, because a large amount of plastic waste on land and at sea cannot be collected and buried. OBPs can be made transparent and can be used for direct food contact. The length of time OBP takes to degrade totally can be "programmed" at the time of manufacture by varying the amount of additive and can be as little as a few months or as much as a few years.

BIODEGRADABLE POLYMERS

Although there certainly has never been a great incentive for making unstable polymers, the idea of making degradable polymers has long existed due to its environmental significance, and quite a bit of effort has gone into research along these lines. There are two important ways of making a degradable polymeric material. One is to make the polymer sensitive to sunlight, which fractures its chemical bonds and breaks it down by photo degradation; the other is to make a polymer out of a material that is biodegradable.

To make a photodegradable, a material can be implanted that will absorb sunlight and becomes sufficiently reactive to attack the polymer molecules from which the material is composed. One other means of photo degradation is to incorporate in the polymer backbone suitable groups, e.g. carbonyl groups that absorb the ultraviolet (UV) component of the sunlight to form excited states energetic enough to undergo bond cleavage.

The main problem in making a photodegradable polymer is that it is hardly possible to combine rapidly degradation upon exposure to light in a landfill after use with a good light stability of the polymer during service. This contradiction is probably the reason why this method was never really caught on.

The difference between photo degradation and biodegradation lies in the possibility to create an environment (as in landfill) completely different from that encountered under normal storage conditions; e.g. microorganisms that can destroy organic polymers may be added to the landfill. In spite of the fact that substantial research time was spent on studies in this field, it is claimed that surprisingly little is understood about the molecular level interaction between polymer and microorganisms. For polyester, however, a number of interesting data are available. Ester-hydrolysing enzymes and some microorganisms are known to biodegrade polyester at a reaction rate depending upon polyester structure.

While many aliphatic polyesters, specifically poly (hydroxyl fatty acids), are suitable for biodegradation, aromatic polyester (e.g., PET) does not possess this property.

Poly is a thermoplastic used extensively in laminates for food containers due to its excellent film forming and oxygen barrier properties. Whether or not the copolymer is biodegradable is apparently related to the size and distribution of the ethylene blocks.

Biodegradable plastics have an expanding range of potential applications, and are driven by the growing use of plastics in packaging.

Advanced technology in petrochemical polymers has brought many benefits to humankind. However, it becomes more evident that the ecosystem is considerably disturbed and damaged as a result of the non-degradable materials for disposable items.

Our whole world seems to be wrapped in plastic. Almost every product we buy, most of the food we eat and many of the liquids we drink come encased in plastic. The environmental impact of persistent plastic wastes is evoking more global concern as alternative disposal methods are limited.

Incineration may generate toxic air pollution, and satisfactory landfill sites are limited. Also, the petroleum resources are finite and are becoming limited. It becomes important to find durable plastic substitutes, especially in short-term packaging and disposable applications. Recently, the continuously growing concern of the public for the problem has stimulated research interests in biodegradable polymers as alternatives to conventional non-degradable polymers such as polyethylene and polystyrene etc.

Biodegradable plastics made with plant-based materials have been available for many years. Their high cost, however, has meant they have never replaced traditional non-degradable plastics in the mass market. The area of degradable polymers, products, and definitions has evolved considerably over the last 20 years.

With this definition, neither a time limit nor environmental conditions are prescribed and in this sagacity most materials could be classified as biodegradable. However, many materials will remain non-degraded in typical refuse conditions, such as a landfill, or will degrade to products with greater toxicity than the original material. Other terms those are of relevance here include photodegradable, where degradation results from the action of natural sunlight and disintegration, which is the falling apart into very small fragments of material caused by degradation processes.

Now-a-days a biodegradable plastic would typically be defined as one in which degradation results from the action of naturally occurring micro-organisms such as bacteria, fungi and algae. There are ranges of standards for biodegradable plastics. The requirements vary from 60 to 90% decomposition of the material within 60 to 180 days of being placed in a standard environment - this may be either a composting situation or a landfill.

A material that simply breaks up into smaller and tiny portions is no longer regarded as being biodegradable. Naturally occurring polymers include: polysaccharides e.g., starch from potatoes and corn, their derivatives, cellulose from marine crustaceans; proteins such as gelatin (collagen), casein (from milk), keratin (from silk and wool) and zein (from corn); polyesters such as poly hydroxyl alkanoates formed by bacteria as food storage; lignin; shellac and natural rubber poly lactic acid, jute, flux, silk, cotton can fall into the category of natural polymers where the monomer is produced by fermentation. The rate of degradation of each of these depends very much on their structural complexity, as well as the environmental conditions.

While there are a number of biodegradable synthetic resins including polyalkylene esters, polylactic acid polyamide esters, polyvinyl esters, polyvinyl acetate, polyvinyl alcohol, polyanhydrides. The materials mentioned here are those that exhibit degradation promoted by microorganisms. This has often been coupled to a chemical or mechanical degradation step.

There are five different kinds of degradable plastic:

Biodegradable,
Compostable,
Hydro-biodegradable,
Photo-degradable

These can be either organically based from renewable resources or synthetic with a petroleum base.

Biodegradable plastic is a degradable plastic in which the degradation process must be resulted from the action of naturally occurring microorganisms over a period (up to 2-3 years in a landfill).

The degradation process is triggered only when material is exposed to specific environmental conditions (such as UV, heat and moisture). Residues are not food matter for microorganisms and are not biodegradable or compostable.

Biodegradable plastics are a new generation of polymers emerging in the market. Biodegradable plastics have an expanding range of potential applications, and are driven by the growing use of plastics in packaging and the perception that biodegradable plastics are 'environmentally friendly', their use is predicted to increase. However, issues are also emerging regarding the use of biodegradable plastics and their potential impacts on the environment and effects on established recycling systems and technologies.

There is an extensive range of potential applications. Some of these include: film including over wrap, shopping bags, waste and bin liner bags, composting bags, mulch film, silage wrap, landfill covers, packaging, bait bags and cling wrap, flushable sanitary products, sheet and non woven packaging, bottles, planter boxes and fishing nets, food service cups, cutlery, trays, and straws.

The properties of the copolymer can be tailored to make it suitable either for mold articles such as bottles, or thin films for plastic envelopes or carrier bags. However, the polymer is costly, a container made of biopol being about seven times more expensive than polyethylene. This polymer is now in production and used for packaging, agriculture products, and disposable items of personal hygiene.

Another approach to making biodegradable plastics for packaging consists of mixing small amounts of biodegradable polymers (e.g. starch) with a regular polymer (e.g. polyolefin), in order to make the end product destroyable as well. Starch is of interesting as a biodegradable material because of its low cost, its availability as agricultural surplus raw material, and the thermo process ability of the blend using conventional plastics processing equipment.

One blend of polyethylene and starch incorporates an auto-oxidant that reacts with metal salts in soils or other environments to form peroxide radicals. The radicals degrade the polyethylene polymer chains into smaller oligomers susceptible to mineralisation. The starch is treated with a silane coupling agent for compatibility with polyethylene, and unsaturated ester such as soya or corn oil is used as the auto-oxidant. Later technology has considered temperature triggers based on a composting regime.

HOW LONG DOES IT TAKE?

Cotton rags 1-5 months
Paper 2-5 months
Rope 3-14 months
Orange peels 6 months
Wool socks 1 to 5 years
Cigarette butts 1 to 12 years
Plastic coated paper milk cartons 5 years
Plastic bags 10 to 20 years
Nylon fabric 30 to 40 years
Aluminum cans 80 to 100 years
Plastic 6-pack holder rings 450 years
Glass bottles 1 million years
Plastic bottles May be never

BIODEGRADABLE BOTTLES

It is estimated that more than 100 billion plastic bottles are added to landfills each year. When a traditional plastic bottle enters a landfill, it can take thousands of years to break down - if ever. As you can imagine, this can have a profound impact on our environment.

The rate of decay in landfills could be even slower than previously thought. This is because some landfills are so tightly packed the conditions are not optimal for product breakdown.

In order for plastics to properly break down, tiny microscopic organisms must find the discarded products an irresistible morsel, and begin consuming it bite by bite. Only then, can the item be broken down entirely. Traditional plastics are unattractive to microbes and therefore inedible.

Many products on the market claim to be biodegradable but are in fact only compostable and are unable to degrade in a landfill environment, or the product simply breaks down into smaller pieces (plastic flakes).

The first thing to keep in mind when answering this question is that everything on the planet is made from atomic particles. Even things that are considered manmade utilise atomic particles that were and will be here on the planet. Plastics are no different and in fact most plastics are hydro-carbons meaning they are made mostly from hydrogen and carbon atoms. Plastics have been designed for their properties to keep out oxygen so that the food product inside is preserved from naturally biodegrading/rotting.

Oxygen is an extremely permeable atom and can make its way into just about any type of barrier (including plastics). Plastics used in the beverage industry is poly PET which has been designed to have an extremely tight chemical bond.

The other aspect to keep in mind is that everything on the planet will decompose and biodegrade over time. Microbes are found all over the planet in every aspect of our lives and are constantly breaking things back into their atomic parts. This is also true for plastics although plastics have been engineered to be very strong which is why it takes hundreds of years for microbes to break plastic back into biogases and/or biomass.

The technology behind the degradation bottles is an additive, which is added into the PET resin during the manufacturing process. The degradation additive adds organic compounds which hydrostatically bonds to the PET molecules. Because we are not changing the PET bonds we are not changing the chemical structure, which allows the bottles to maintain the same beneficial physical properties of the PET used.

Bottles containing biodegradable material do not begin until the plastic is placed into a highly microbial environment i.e. landfill. Once placed in a microbial environment the biodegradation additive has a microbial attractant to help facilitate microbial colonisation on the plastic and a swelling agent, which opens the PET bond to allow completed biodegradation of the plastic polymer. Once microbes have colonised on the plastic they begin to break down the PET bond through atomic reorganisation to use some of the atoms as energy and leaves behind either methane (anaerobic) or CO2 (aerobic) and inert humus based on the environment the bottle is placed into.

Having the plastic biodegrade from microbial digestion is the natural process of everything and does not leave behind any polymer residue or toxic materials.

PHASES OF BIODEGRADATION IN A LANDFILL ENVIRONMENT

Aerobic Phase (first few days) - This is the phase when aerobic microbes are becoming established and moisture is building up in the refuse. While standard plastic absorption capability is relatively small, the additive causes further swelling, weakening the polymer bonds and creating molecular spaces where moisture and microbial growth can rapidly begin the aerobic degradation process. Oxygen is replaced with CO2.

Non-methanogenic Phase (roughly 2 weeks to 6 months) - After oxygen concentrations have declined sufficiently the anaerobic processes begin. During the initial stage (hydrolysis), the microbe colonies eat the particulates, and through an enzymatic process, solubilise large polymers down into simpler monomers. The secreted monomers mix with the organic additive, causing additional swelling and opening of the polymer chain and increased quorum sensing. This further excites the microbes to increase their colonisation and consumption of the polymer chain. As time progresses, acid genesis occurs where the simple monomers are converted into fatty acids. CO2 production occurs rapidly at this stage.

Methanogenic Unsteady Phase (6 to 18 months) - The microbe colonies continue to grow eating away at the polymer chain and creating increasingly larger molecular spaces. During this phase, acetogenesis occurs where fatty acids are converted into acetic acid, carbon dioxide, and hydrogen. As this process continues, CO2 rates decline and H2 production eventually ceases.

Methanogenic Steady Phase (1 year to 5 years) - The final stage of decomposition involves methanogensis. As colonies of microbes continue to eat away at the remaining surface of the polymer, acetates are converted into methane and carbon dioxide, while hydrogen is consumed. The process continues until the only remaining element is humus. This highly nutritional soil creates and improved environment for the microbes and enhances the final stage of decomposition.

The terephthalic acid which is made of the benzene ring with the carbon double bond to an oxygen atom will break down from microbial digestion in either an aerobic or anaerobic environment.

Starch Based Biodegradable Packaging
Biodegradable and water-soluble starch-based packaging
Cellulose Based Biodegradable packaging