Basic The Development
Basic The Development Poly Hydroxy Fig.1 Microbial Cells containing PHA Fig.2 PHA powder Article contributed by Dr. Jim Lunt, V.P. Sales and Marketing, Tianan Biologic, Wayzata, Minnesota, USA As discussed in preceding issues of bioplastics MAGAZINE -Poly Hydroxy Alkanoates or PHA’s represent an emerging class of biopolymers which are presently produced through the fermentation of natural sugars, vegetable oils or fatty acids. These materials are unique in the field of renewable resource based biopolymers in that they represent the only class of polymers which are converted directly by microorganisms from feedstock to the polymeric form - no additional polymerization steps being required. The product in the form of microscopic granulates is extracted from the microbial cells (Fig. 1) and used either as the powder (Fig. 2) directly or converted to pellets for ease of shipping and handling. Also during the melt conversion to the pellets, additives such as an antioxidant and nucleating agent to accelerate crystallization, can be incorporated Although attracting recent pilot/commercial scale attention by companies such as Tianan Biologic, Telles, Meredian and others, interest in PHA’s has spanned many decades. Today there are actually over 300 known microorganisms capable of producing PHA’s [1] and over 10 monomer combinations that can produce PHA’s with widely different properties. In terms of commercial interest- poly 3 hydroxy butyrate-co-valerate (PHBV)-Tianan Biologic, poly 3 hydroxy -co-4 hydroxy butyrate, (PHB)-Telles, and poly 3 hydroxy butyrate-co-hydroxy hexanoate (PHBH) – Meredian, are probably the most well known polymers. The History of PHA The ability of micro-organisms to produce and store a PHA within their cells was first observed by Beijerinck in 1888. He observed inclusions within the bacterial cells but could not identify their structure. In 192 Lemoigne, using Bacillus megaterium, identified the polymer to be poly 3 hydroxy butyrate (PHB). It would appear little more was done in this area for another 30 years until in 198 McCrae and Wilkinson observed that bacteria stored PHB in their cells. When the carbon to nitrogen ratio in the fermentation medium was high and when the external carbon source was depleted [2], they consumed the PHB as a food and energy source. From this point, the fact that a biopolymer could be produced within a microbial cell, and become a source of intracellular reserve material, created significant interest among microbiologists and biochemists. The interest was still, however, essentially academic. The primary focus was directed on understanding the polymers significance on the functioning of the microorganisms and how external factors affected the rate of production and re-utilization by the microorganisms. Around 193, as oil prices climbed, this academic interest took on a more practical focus. In 19, ICI in the UK began to investigate if PHB could be commercially produced using glucose as the feedstock. They developed a practically viable process but the economics were so completely unattractive that this initiative was terminated and the technology was divested. PHA Technology The manufacture of PHA’S involves providing a microorganism a carbon feed source such as dextrose or glucose along with suitable nutrients, such as nitrogen, phosphorus or oxygen which encourage growth and multiplication of 3 bioplastics MAGAZINE [03/09] Vol. 4
Basic and Commercialization of Alkanoates (PHA) the microorganisms. Once the number of microorganisms reaches the required point, the nutrients are reduced to create an imbalance, which puts the microorganisms under stress. The microorganism then begins to convert the extracellular carbon source through a series of enzymatic pathways, to a reserve energy source in the form of polymeric inclusions within their cell. Under ideal conditions, typically, from 80% to 90% of the cell can comprise the polymeric form of the hydroxy esters conventionally referred to as the poly hydroxy alkanoates. The manufacturing process can be either a fed –batch or multi stage continuous process. When the mass of the polymer within the cell reaches the maximum level, the process is terminated. The polymeric material can be extracted from the cells by the use of solvents such as chloroform, methylene chloride, propylene chloride or dichloro methane. It is also possible to remove the polymer using only aqueous conditions [3]. Today it appears that only Tianan Biologic has successfully optimized the aqueous extraction route. Common PHA Structures The basic structure of the Commercial PHA’s is shown below: The R alky group at the C-3, can vary from one carbon (C1) to over 14 carbons (C14) in length. PHA’s are subdivided into three broad classes according to the chain length of the comprising monomers. PHA‘s containing up to C monomers are classified as short chain length PHA’s (scl-PHA). PHA’s with C–C14 and over C14 monomers are classified as medium chain length (mcl-PHA) and long chain length (lcl-PHA) PHA’s, respectively [4]. Today the short chain and medium chain length are the most common. The scl-PHA’s have properties close to conventional plastics while the mcl-PHA’s are regarded as elastomers and rubbers. PHB is the most common type of scl-PHA and has been studied most extensively. However, this polymer is extremely brittle and difficult to process without degradation. The common copolymers of PHA are formed containing 3-hydroxybutyrate (HB) with 3-hydroxyvalerate (HV), 3-hydroxyhexanoate (HH) or 4-hydroxybutyrate (4HB) monomers. These short to medium chain length PHA’s are typically more tough and ductile (PHBV) to elastomers or sticky materials which can be modified to product rubbers (PHBH). In addition they are easier to process due to their lower crystallinity and melting or softening point. Processing and Properties of Common PHA’s PHA’s are aliphatic polyesters. In common with petroleum based polyesters, these natural polymers are sensitive to hydrolytic breakdown. Before melt processing the products must be dried. Manufacturers, such as Tianan Biologic, typically recommend drying to approximately 20ppm moisture content before processing. Drying can be accomplished using desiccant or vacuum dryers. If the polymer is not dried to the recommended maximum moisture content and kept dry before melt processing, then hydrolytic degradation will occur leading to significant loss in molecular weight and reduced mechanical properties in the final product. Processors and compounders who run PET or Nylon and , are quite familiar with this issue and will have the correct drying equipment. Companies who only process polyolefins or polystyrene may not have dryers and so often this can cause problems for these companies in transitioning to the use of a PHA without incurring some capital investment. Short or medium chain length PHA’s with low commoner levels such as PHB ( 3 hydroxy-4hydroxy butyrate) or PHBV can be crystallized which allows drying at 80-100°C. Longer chain length more amorphous PHA’s must be dried at lower temperatures and this can also be an issue even for the polyester and polyamide processors if the drying temperature is too low. This problem may be even more problematic for the more elastomeric longer chain length PHA’s although their sensitivity to hydrolytic degradation during processing may not be as severe. Another concern with pure PHB is that the processing temperature and melting point are extremely close, which can readily cause thermal degradation of the polymer, producing crotonic acid. Many studies dedicated to thermal and thermo mechanical degradation of neat 3- PHB have revealed that the degradation occurs rapidly near the melting point according to mainly a random chain scission process (cis elimination of the ester group). The major by – products of this degradation are crotonic acid and its oligomers []. bioplastics MAGAZINE [03/09] Vol. 4 3
