Bio Resins, Raw Materials and Products

Biodegradable polymers may be defined as a group of materials that respond to the action of enzymes or from chemical degradation associated with interaction with living organisms.  Biodegradation may also occur through chemical reactions that are initiated by photochemical processes, oxidation and hydrolysis that result from the action of environmental factors.  It should be noted that biodegradation of polymers is not restricted to naturally-occurring materials but includes some synthetic polymers that possess chemical functionalities also found in natural compounds.
A summary of some of the main biodegradable polymer products that are now commercially available is given below:

Starch
Starch is a complex polymer comprising a mixture of amylose and amylo-pectin polysaccharides; the exact structure is as yet uncharacterized.  The properties of starch will vary according to the amylose/ amylo-pectin ratio and hence according to the plant source.  A major source of starch is corn but it can also be extracted from potato, wheat and rice.  The polymer is crystalline due to the presence of the amylo-pectin component.  This is a hydrophilic polymer with the latter property resulting in some water solubility in an unmodified form.  Starch is not itself a thermoplastic but can be made thermoplastic through the addition of a plasticizer such as water, glycerine or sorbitol which acts by de-structuring the complex molecular structure.  Plasticization is usually achieved at elevated C) and under shear conditions during an extrusion process. °temperatures (90-180

The two main disadvantages of starch are its water-solubility and poor mechanical properties.  Hence, this polymer is suited to applications where long term durability is not needed and where rapid degradation is advantageous.  There are also strategies for improving the properties of the starch.  For example, the water resistance can be increased by mixing with synthetic polymers, by adding cross linkers (e.g. Ca, Zr salts) or lignin, or through the addition of natural fibres.

Starch is used in variety of ways: as filler in synthetic-based composites; as a copolymer blended with synthetic polymers or, most recently, as the primary material base in a fully biodegradable plastic.  It is often processed as foam where it provides an alternative to polystyrene for use in the manufacture in food trays, moulded shaped parts or as loose packing filler.

Polyesters
This group includes polyhydroxyalkanoates and poly(alkylene dicarboxylates) and are produced synthetically by condensation reactions between dicarboxylic acids and diols.

Poly(alpha-hydroxy acid) examples include PGA (polyglycolic acid) and PLA (polylactic acid).  PLA in particular shows potential as a structural material since it can be polymerized to a high molecular weight and is hydrophobic.  The latter property renders the polymer sufficient lifetime to maintain mechanical properties without rapid hydrolysis but whilst maintaining good composting properties.  Current uses for this polymer group centre on medical applications such as implants, sutures, drug delivery systems and grafts.

Polyester amides are thermoplastics products that have been developed to exhibit similar properties to polyethylene with high toughness and tensile strength demonstrated.  Synthesis is achieved through reaction between a diol, -amino acids including glycine, alanine andadi-acid and amino acid.  Various  phenylalanine have been used for this purpose.  Being a thermoplastic, most melt-processible processing routes are available to biodegradable polyester amides including extrusion, blow moulding, thermo-forming and injection moulding.  Applications are varied and encompass film, bags and containers.

Cellulose acetate
Cellulose acetate is a modified polysaccharide which can be prepared from a reaction between acid anhydride and cellulosic products derived from cotton linters, wood pulp, recycled paper or sugar cane.  Biodegradation occurs through microbial attack.  The manufacturing process for cellulose acetate was first patented at the end of the nineteenth century and the polymer found use in filaments, films and lacquers since that time.  This biodegradable polymer exhibits good toughness and a high degree of transparency.

Strictly speaking, cellulose acetate is not a thermoplastic since the decomposition temperature is below the melt temperature.  However, it is possible to induce melt-processible properties through the addition of a plasticizer.  Commercially today, cellulose acetate is used for film, fabric and coating applications.  Examples include adhesive tape, spectacle frames and textiles.  Textiles made from cellulose acetate fibres are valued for their absorbency, “breathability” and suitability for dyeing.  The superior absorbency of cellulose acetate fibres is also of application in the manufacture of wound dressings, personal hygiene products and cleaning cloths.

Polyurethanes
Polyurethanes as a generic polymer type are not generally biodegradable unless chemically modified.  Such modified biodegradable polyurethanes are now being synthesised for use in regenerative medicine.  Examples include the fabrication of porous scaffolds for use in soft tissue engineering and cartilage repair [1].  Other medical applications include bone graft substitutes and wound dressings. These biodegradable polyurethanes are more elastic and pliable than the harder and more brittle materials that characterise other biodegradable polymers.  A PU typical synthesis is based a two-step condensation involving a di-isocyanate, a diol based on natural products like PCL or PEO and an amino acid chain extender like phenylalanine [2].   Polyols derived from soya beans are also being used as the diol starting material.

Other non-medical uses for biodegradable polyurethanes include as a ‘shock absorber’ materials in shoes soles and heels and as a carpet backing using PU derived from soy.

Soy plastic
Plastic derived from soybeans is of limited take-up at present (<0.5% used for industrial products).   In terms of composition, soybeans typically contain ~50% as protein and 20% as soy oil. 

The soy proteins are polypeptides comprised of a mixture of non-polar and polar amino acids.  Modifying the polar/non-polar ratio can be used to control the water solubility and reactivity of the derived plastic material.  Water sensitivity is a particular issue with soy-derived plastic since it limits the scope of applications to dry and non-structural uses.  These biomaterials are thermoplastic and amenable to most melt-processible processing techniques including extrusion and injection moulding.

Furfural alcohol and furan resins
The pre-cursor to furfural alcohol and furan-based resins is furfural, a compound which is extracted from naturally occurring agricultural residues.  Residues may derive from sugar cane bargasse as well as corn cobs, wood products or cereal by-products.  These hemicellulosic agricultural wastes are converted to furfural via an acidization, dehydration and steam distillation process [3].  Furfuryl alcohol and related furan resins find widespread application in the foundry industry as a foundry sand binder.  The exceptional mechanical properties of these resins are also attracting applications in other fields such as fibre-reinforced plastics and cements when a high degree of corrosion resistance is required.  Furfuryl alcohol can also provide a useful additive to other naturally occurring polymers to form biocomposites with improved mechanical properties [4].

Other Bio-Resins
Functionalised triglycerides, epoxidized vegetable oils, polyoles and aminated fats have all been identified as potentially suitable plant-oil derived materials for the synthesis of biodegradable thermosets.   As with all thermosets, cross-linking can be achieved through the action of chemical cross-linkers, radiation or heat.   Synthetic chemical cross-linkers are still needed with various isocyanates, amines, polyoles and polycarboxylic acids being used, although current research is concentrating on finding isocyanates from a biological source.  Chemical modifications to the plant oil prior to polymerization include epoxidation, maleinization, amidation, hydroxylation, acrylation or glycerolysis.   Fillers can be standard inorganic materials or natural products such as starch or natural fibre.  The use of natural fibre with maleinated resins can be advantageous from the standpoint of fibre-matrix adhesion since the carboxyl groups from a maleinated resin can bond with hydroxyl groups on the natural fibres thereby improving fibre-matrix bonding.