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Applications of PHA as Bioplastic

Applications:

  1. Packaging films (for food packages), bags, containers, paper coatings.
  2. Biodegradable carrier for long-term dosage of drugs, medicines, insecticides, herbicides, insecticides or fertilizers.
  3. Disposable items such as razors, utensils, diapers, feminine hygiene products, cosmetics containers, shampoo bottles, cups etc.
  4. Starting material for chiral compounds.
  5. Medical applications – Surgical pins, sutures, staples, swabs, wound dressings, bone replacements & plates and blood vessel replacements, Stimulation of bone growth by piezoelectric properties.

Monsanto’s resulting Biopol resins can be converted into various types of plastic products, depending on the physical properties of the resin used. The first major product, a degradable shampoo bottle, was developed about 5 years ago. However, because Biopol resin prices ranging from $4 USD to $6 USD per pound (somewhat higher than prices for other degradable resins) the number of markets for Biopol may be limited. According to Monsanto, major target products are likely to be plastic films and coatings. With environmental regulations in several European countries, particularly Germany, favoring degradable products, the principal markets for Biopol are in Europe and to a limited extent, Japan. Current sole production capacity is in Europe and is estimated at 300 metric tons annually.

For food packaging applications such as those used for produce and meats, PHAs have moisture vapor barrier properties comparable to existing food-packaging materials such as polyethylene terephthalate and polypropylene. PHA materials also have a high surface energy making the material receptive to printing inks and dyes. PHAs are also UV resistant and have oxygen barrier comparable to that of ethyl vinyl alcohol (EVOH).

PHA is currently being considered for flexible packaging applications used for cheese, peanuts and other high oil content foods, frozen foods and natural foods.

Possible applications of PHAs -

Early investigations of PHA granules by electron microscopy after freeze-etching showed that the polymer in the granule underwent a cold drawing process indicating the plastic nature of polyester and suggesting that it can be processed as a conventional thermoplastic. In addition to its potential as plastic material PHA is useful source of stereoregular compounds which can serve as chiral precursors for the chemical synthesis of optically active compounds, particularly in synthesis of some drugs or insect pheromones. These substances are biologically active only in the correct stereochemical configeration.

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 b-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 b-hydroxy acid esters and related derivatives are likely to find growing use as green solvents similar to lactic acid esters. b-hydroxy acids are 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, (i) is 100% biodegradable, (ii) can be processed like thermoplastic and (iii) is 100% water resistant, so that it could be used for similar applications as conventional commodity plastics.

Processing of PHA

PHAs can be incorporated into packaging components such as coatings, laminations and biodegradable printing inks. Additionally, PHA structures can include rigid thermoplastics, thermoplastic elastomers and grades useful in waxes, adhesives and binders. Properties range from elastomeric to resins as stiff as nylon 6 or polycarbonate.

For food packaging applications such as those used for produce and meats, PHAs have moisture vapor barrier properties comparable to existing food-packaging materials such as polyethylene terephthalate and polypropylene. PHA materials also have a high surface energy making the material receptive to printing inks and dyes. PHAs are also UV resistant and have oxygen barrier comparable to that of ethyl vinyl alcohol (EVOH). PHBV can be heat-formed into a flexible plastic suitable for many applications where biodegradable plastics are desirable, such as packaging.

PHA is currently being considered for flexible packaging applications used for cheese, peanuts and other high oil content foods, frozen foods and natural foods.

Metabolix makes about 20,000 lb/month of PHA for market development. Its Natural Plastic is aimed at opaque sheet, paper coatings, and semi-transparent film.

The first two commercial PHA products for extrusion coating and cast film are in the market. Potential applications are stretch film, bags, soluble detergent sachets, and mulch films. Injection molding grades are also being tested.

PHA is hydrophobic and resists both water and oils, even when hot, so it can be used in applications up to 230 to 240 F. It can coat hot-drink cups. PHA has a narrow molecular-weight distribution (which can be fine-tuned) and good printability.

PHA is semicrystalline (up to 60%) with density of 1.20 to 1.25 g/cc, melting points of about 50 to 180 degree C, and Tg from -20 to +5 degree C. Melt temperatures are from 176 F to 320 F, not far from its thermal degradation temperature of 338 F. So extruder barrel temperatures are reversed, starting at 365 F and dropping to 329 F at the die.

Cast film and downstream handling use heated rather than cooled rolls at 140 to 149 F, so the plastic has time to crystallize before cooling. Pellets of PHA should contain less than 0.1% moisture, which requires drying at 176 F for about 1 hr.

Unoriented cast film has tensile strength of 1600 psi at less than 1 mil thick, up to 3600 psi at 15 mils. Tensile modulus ranges from 58,000 to 145,000 psi over that thickness range, and elongation at break is 100% to 1000%.

Co-polymers with Improved Properties

PLA however, is limited from introduction into many current fiber and textile applications due to its unfavorably placed glass transition temperature (Tg), loss of mechanical properties at high processing temperatures, and its hydrolytic instability.

PHAs have been widely used in packaging and biomedical applications due to their biodegradability, and more specifically, their anaerobic biodegradable properties. However, their wide-spread use in commodity applications is limited due to the poor thermal and mechanical properties compared to other petroleum derived plastics such as polyethylene.

Co-polymerization of lactide with PHA in the melt as well as in solution (toluene) can be done. The resulting polyester copolymers of PHA and PLA have better processing and blend properties. Solution cast PHA-co-PLA polymer gives flexible and tough films with variable crystallinity (transparency) depending on thermal conditions. Fabrication of a nonwoven mesh of electro-spun fibers and solution cast films of the new copolymer showed its use for fabric and tissue engineering studies.

Perfluoropolyethers have high thermal and chemical stabilities and segmented polyesters containing them should provide new elastomeric fibers with unique moisture management properties. Through incorporation of these fluorinated macromers into the ROP of lactide and insertion into PLA via transesterification routes, a variety of copolymers have been introduced with enhanced properties. The new copolymers exhibit improved melt processability, greater hydrophobicity, increased ductility and elongation, and controlled environmental stability over the homopolymer of PLA alone.

PHA_PLA

Lactide was successfully copolymerized with commercial PHA polymer (NodaxTM). Nodax is a family of bacterially produced polyhydroxyalkanoate (PHA) copolymers comprising 3-hydroxybutyrate (3HB) and other 3-hydroxyalkanoate (3HA) units with side groups greater than or equal to three carbon units. The PLA- co -PHA polymer shows unique crystal morphology as compared to PHA and PLA alone, whereas the blend shows both phases of PHA and PLA. The loss modulus and storage modulus of the PLA- co -PHA polymer remains relatively constant as compared to PLA alone, becoming more applicable to flexible film applications. Electro-spun fibers and solution-cast films show utility of this new random copolymer for fiber and film applications. Lactide has been copolymerized with novel depsipeptide for film and fiber applications. The incorporation of 3HA units with medium-chain-length (mcl) side groups effectively lowers the crystallinity and the melt temperature, Tm, of this class of PHA copolymers, in a manner similar to that of alpha olefins controlling the properties of linear low density polyethylene. The lower Tm makes the material easier to process, as the thermal decomposition temperature of PHAs is then relatively low. The reduced crystallinity provides the ductility and toughness required for many plastics applications. When a small amount of ductile PHA is blended with poly(lactic acid) (PLA), a new type of polymer alloy with much improved properties is created. The toughness of PLA is substantially increased without a reduction in the optical clarity of the blend.

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