Basics Basics of PHA by
Basics Basics of PHA by Hans-Josef Endres Andrea Siebert-Raths University of Applied Sciences Hannover, Germany Faculty of Mechanical Engineering Department of Bioprocess Engineering This article is an abridged extract - sections left out are marked (…) - from the new book ‘Engineering Biopolymers’ by H.-J. Endres and A. Siebert-Raths, Hanser Publishers, Germany [1]. The new book, as well as the German version of this book, is available via the bioplastics MAGAZINE bookstore (see p.9 or www.bioplasticsmagazine.com). Details about the material properties of commercially available PHA resins and other biopolymers can be found in the online Biopolymer-Database at http://biopolymer.materialdatacenter.com - MT When biopolymers are manufactured from genetically modified crops by direct fermentation, they polymerize during the fermentation process. Due to natural biosynthesis, no additional synthesizing step is required for polymerization. By contrast, the fermentative generation of monomers, such as PLA from lactic acid, requires man-made polymerization. Within the biopolymer group generated by direct biosynthesis, the best known and by far most important examples are the socalled polyhydroxy fatty acids and polyhydroxyalkanoates (PHA). Polyhydroxyalkanoates are polyesters that are intracellularly deposited by bacteria as energy storage or reserves. These polymers are formed mainly from saturated and unsaturated hydroxyalkanoic acids; thus the term polyhydroxyalkanoates. Their monomer building blocks can be branched or unbranched 3-hydroxyalkanoic acids or those with substituted side chains as well as 4- or 5-hydroxyalkanoic acids. PHAs are homo-, co- and terpolymers built from these various monomers. The variety of monomers, constitutional isomerism, wide range of molecular weights, as well as additional possibilities for manufacturing blends or chemically and/or physically modifying their microstructure create a potentially wide variety of biopolymers with different property profiles within this polymer family. In spite of the large number of theoretically possible PHAs, we can assume there will be a maximum of 10 industrially interesting different PHAs in the future [2, 3, 4]. From a chemist’s point of view, these PHAs are optically active, aliphatic polyesters with a structure illustrated in Fig. 1. For R = CH 3 , the result is so-called polyhydroxybutyrate, also called polyhydroxybutyric acid (PHB). For R = C 2 H 5 , the result is polyhydroxyvalerate (PHV), for R = C 3 H 7 , polyhydroxyhexanoate (PHH), and for R = C 4 H 9 , polyhydroxyoctanoate (PHO), etc. We also distinguish between homo- and copolymers in polyhydroxyalkanoates, see Fig. 2. The most prominent and best investigated representative of this biopolymer family is the homopolymer polyhydroxy butyrate. As a homopolymer, PHB from polyhydroxybutyric acid exhibits an absolutely linear isotactic structure and is highly crystalline (60−70%). Therefore, PHB is too brittle for many applications. If process parameters vary too widely, PHB’s relatively small difference between melting and decomposition temperature may also pose a problem. The small difference between these two temperatures can be attributed to the high melt temperature due to strong intermolecular interaction. Unfavorable conditions during PHB processing, e.g., humidity too high, temperature too high, or dwell time in the machine too long, can cause polymer degradation in the final products, such as films, coatings, or fibers. Another problem for PHB is the progressive decrease of its mechanical properties, such as tensile strength, because of secondary crystallization and gradual loss of plasticizers over time. In analogy with conventional polymers, these problems with pure PHBs can generally be eliminated by polymerization with comonomers. The longer the side chain of the polymerized functional group is, the less crystalline and more ductile is the material, and the lower is its melting temperature because of the reduction in intermolecular 42 bioplastics MAGAZINE [03/11] Vol. 6
Basics interaction caused by side chains. The first PHA used for, among other things, a shampoo bottle from Wella, was ICI’s PHB/PHV copolymer with the brand name Biopol (Fig. 3), which is no longer available. ICI has transferred the corresponding rights to Zeneca. From Zeneca, they passed first to Monsanto and now belong to Metabolix. PHAs can generally be processed well by injection molding, are insoluble in water, yet biologically degradable and biocompatible. Moreover, they exhibit good barrier properties against oxygen and, compared to other biopolymers, a slightly higher barrier effect against water vapor. Therefore, these PHAs are a promising group of materials for future development. Their molecular structure is variable, with the resulting range of property profiles, and there is a wide range of feedstock available for the production of these biopolymers. Beyond that, PHAs also represent an interesting source for smaller molecules or chemicals such as hydroxy acids or hydroxy alkanoles. Manufacturing Process In principle, three different approaches for the biotechnological production of PHA are known: • Bacterial fermentation • Synthesis in genetically modified plants • Enzymatic catalysis in cell-free systems Because the last two methods are (still) industrially irrelevant, they will be described only briefly in the following. With the aid of genetic engineering, PHA synthesis genes can be transmitted into useful crops. Transgenic crops yield PHA contents up to 10% of plant dry weight. However, to ensure economically viable and competitive PHA production, these PHA contents would have to be doubled and plant growth and yields would have to be significantly increased. Also, the plant preparation processes for PHA production and the monomer composition have to be further optimized [3]. In-vitro PHA synthesis can also be performed in cell-free systems by isolating the key enzymes. This method has the advantage that no by-products of cellular metabolism need to be removed. Pure polymers can be obtained, and monomers can be specifically polymerized that are not metabolized naturally. On the other hand, the disadvantages include limited stability, relatively high enzyme costs, as well as the use of relatively expensive substrates. Thus this approach is typically used for research purposes. On an industrially scale the much more important method to produce PHA is bacterial fermentation, which is discussed in more detail in the following. Various microorganisms can be used to produce PHAs (a comprehensive table of microorganisms can be found in the book [1]). Over all, more than 300 different microorganisms are known that generate PHAs as natural energy reserves [2, 5, 6]. O R CH Figure 1: General structure of polyhydroxyalkanoates (PHAs) a b c O O O CH 2 CH 3 O CH 3 O CH 3 O CH 3 O O O O C O O O O H 3 C O O O O O O O O CH 3 CH 3 CH 3 CH 3 a) poly(β-)hydroxybutyric acid (butanoic acid) b) copolyester from β hydroxybutyric acid and β-hydroxyvalerate acid (pentanoic acid) c) homopolyester from β hydroxyoctanoic acid Figure 2: Polyhydroxy β-alkanoate O Figure 3: PHBHV copolymer O CH 3 O CH 2 O CH 3 n O O n O bioplastics MAGAZINE [03/11] Vol. 6 43
