“Preparation of biodegradable polymers and resins from proteins”
Using nitrous acid or nitrous oxide, a protein is converted to a hydroxyl carboxyl polyamide protomer. Protomers are condensed through hydroxyl and carboxyl groups to make inter-protomer ester bonds, resulting in a polyester polyamide polymer, resin or plastic.
Due to environmental considerations, the disposal of solid waste is a major industrial concern. One of the most difficult to dispose types of waste is non-biodegradable plastics used, for example, in agricultural applications and packaging. Concerns over the effect of non-biodegradable plastics have resulted in a worldwide prohibition of plastic disposal at sea, increasingly severe regulatory limits on use of non-biodegradable plastics and incentives to reduce the amounts of non-biodegradable plastics used. These trends have resulted in an increased demand for biodegradable plastics
Currently available commercially important biodegradable plastics are:
[A] Polylactide polymers (PLA)
These are produced by polymerization of lactic acid. Lactic acid is first oligomerized with the loss of water to yield linear chains of polylactic acid. The polylactic acid is de-polymerized to yield lactide, a cyclic dimer The six-member ring of the lactide is purified and subjected to ring-opening polymerization to produce PLA resin. Cargill Dow Polymers LLC (Minneapolis, Minn., USA) is the largest producer of PLA resins. PLAs can be processed by most melt fabrication techniques including thermoforming, sheet and film extrusion, blown film processing, fiber spinning and injection molding. Applications include coatings for paper plates.
[B] Starch-based plastics
The main starch-based plastics are starch/ethylene-vinyl alcohol copolymer blends. These blends are prepared by blending a starch-based component and an ethylene-vinyl copolymer in an extruder in the presence of water or other plasticizer. The temperature and pressure conditions destructurize the starch and the resulting extruded composition forms a thermoplastic melt. Such plastics have physical properties similar to polystyrene or polyethylene but are moisture-sensitive. Novamont SpA (Novara, Italy) produces such resins containing 60% starch that are marketed under the name Mater-Bi.RTM. and can be shaped by film blowing, extrusion, injection molding or thermoforming. Novon International (Tonawanda, N.Y., USA) produces a resin containing up to 95% starch and marketed under the name Novon.RTM.. It is important to note that such materials are not truly biodegradable as the non-starch component of the plastic remains as minute particles.
[C] Polyhydroxyalkanoates (PHA)
Polyhydroxyalkanoates (PHA) plastics are linear polyesters produced by bacterial fermentation of sugar or lipids. The production of PHA plastics is described in Doi. Y. Microbial Polyesters, VCH Publishers, New York, N.Y., 1990. More than 100 different monomers can be combined within this family to give materials with a wide-range of properties. PHAs can be either thermoplastic or elastomeric materials, with melting-points ranging from 40.degree. to 180.degree. C and are suitable for blow molding, injection molding and extrusion. The leading producer of PHAs is the Monsanto Company (St. Louis, Mo., USA) manufacturing a polyhydroxybutyrate under the name Biopol.RTM.. PHAs are entirely biodegradable.
PHAs can be easily depolymerised to a rich source of optically pure bifunctional hydroxy acids. PHB, for example can be readily hydrolysed to R-3-hydroxybutyric acid and is used in the synthesis of Merck's anti-glaucoma drug Truspot. Along with R-1,3-butanediol, it is also used to synthesise-lactams. PHBV received European approval for food contact use in 1996. This opens opportunities in food service and packaging industry. Plant-derived PHAs may in future be depolymerised and used for bulk chemical manufacture. Western Europe, for example produces several million tonnes of butanols , half of which are used directly, or following esterification, as solvents. Besides replacing existing solvents -hydroxy acid esters and related derivatives are likely to find growing use as green solvents similar to lactic acid esters.-hydroxy acids a re more resistant to hydrolysis and are therefore better suited to certain applications. Hydroxy acids may also be converted into crotonic acids, 1,3-butanediol, lactones etc. all of which have existing markets of thousands of tonnes.
PHB, • is 100% biodegradable,
•can be processed like thermoplastic and
•is 100% water resistant, so that it could be used for similar applications as conventional commodity
Practical Applications of PHA -
• Packaging films (for food packages), bags, containers, paper coatings.
• Biodegradable carrier for long term dosage of drugs, medicines, insecticides, herbicides, insecticides or fertilizers.
• Disposable items such as razors, utensils, diapers, feminine hygiene products, cosmetics containers, shampoo bottles, cups etc.
• Starting material for chiral compounds.
• Medical applications - Surgical pins, sutures, staples, swabs, wound dressings, bone replacements & plates and blood vessel replacements, Stimulation of bone growth by piezoelectric properties.
There is a widely recognized need for, and it would be highly advantageous to have a general method for producing plastics from inexpensive raw materials. There is also a widely recognized need for, and it would be highly advantageous to have a general method for producing biodegradable plastics. There is also a widely recognized need for and it would be highly advantageous to have a general method for producing biodegradable plastics having many and varied physical properties.
Various common possible methods degradation
Disintegration of plastic materials by means other than by biological process such as dissolution (dissolving), oxidative embrittlement (heat ageing) or photolytic embrittlement (UV ageing).
Sludge with active, live degradation micro-organisms.
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. (contd.)
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 200°C).
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.
A component of starch consisting of a chain polymer of linked D-glucopyranosyl structures. Thermoplastic starch polymers consist largely of amylose.
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.