Foam A B CO 2 CO 2
Foam A B CO 2 CO 2 Photosynthesis/ carbon fixation Photosynthesis/ carbon fixation Figure 2: (a) Synthesis of PHBV by bacterial fermentation process; (b) Direct synthesis of PHBV in crop plants. Graphic according to Y. Poirier, Nature Biotechnology, Vol. 17, p. 960, 1999 Propionic acid Starch Glucose PHBV Harvest & processing Fermentation Harvest & processing Threonine 2-ketobutyrate isoleucine Propionyl-CoA Acetyl-CoA Fatty acids PHBV PHBV Harvest & processing PHBV foams and its By Alireza Javadi 1,2 , Srikanth Pilla 2 , Lih-Sheng Turng 2,3 , Shaoqin Gong 1,2 1 Department of Biomedical Engineering, University of Wisconsin–Madison, WI, USA 2 Wisconsin Institute for Discovery, Madison, WI, USA 3 Department of Mechanical Engineering, University of Wisconsin–Madison, WI, USA 3HB PHBV 3HV Figure 2: Schematic chemical structure of Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Introduction In the past few years, extensive research on biobased and biodegradable polymers has led to a better understanding of their properties and morphologies, as well as their structure–property relationship. Poly(hydroxyalkanoates) (PHAs), a family of linear polyesters produced in nature by bacterial fermentation of various renewable sources such as sugars, lipids, and alkanoic acids, are among the most promising biobased and biodegradable materials currently being investigated [1]. Among PHAs, poly(3-hydroxybutyrate) (PHB) and its copolymers Poly(3-hydroxybutyrate-co-3- hydroxyvalerate) (PHBV) have attracted a lot of attention in the past two decades due to their unique properties. PHBV is either produced directly from plants or synthesized by microorganisms by consuming sugars in the presence of propionic acid (Figure 1) [2]. PHBV (Figure 2) is available commercially under various names including Tianan Biologic’s ENMAT Y1000P, Biomer’s Biomer L, and Metabolix’s Mirel. In spite of improved mechanical (e.g., toughness) and thermal properties compared to PHB, PHBV still exhibits some disadvantages including low strain-at-break, narrow processing window, slow crystallization rate, and higher cost as compared to petroleum-based synthetic polymers [3]. In order to tailor its properties and decrease its total cost, several approaches have been proposed such as forming blends or composites with biodegradable polymers, natural fillers, or inorganic fillers. PHBV-based polymer blends and composites have been extensively studied in order to reduce their material cost, improve their processability, and engineer their 28 bioplastics MAGAZINE [01/12] Vol. 7
Foam Figure 3: Representative scanning electron microscopy (SEM) image of the tensile fractured surface of a component processed by microcellular injection molding. engineered composites mechanical (e.g., toughness) and thermal properties (e.g., degree of crystallinity) [4]. In order to fully utilize PHBV in diverse applications, improving its thermal and mechanical properties (such as brittleness and low strain-at-break) and employing economic processing techniques (such as microcellular injection molding [5]) is important. Processing Similar to other thermoplastics, PHBV processing can also be done using conventional polymer processing equipment such as twin-screw extruder, injection-molding machine, etc. However, due to its sensitivity to thermal degradation, it is critical that lower processing temperatures are employed. Since this is practically difficult to implement with conventional processing equipment, a special fabrication technology has been implemented by the authors in all of their work on PHBV. This unique processing method, called microcellular processing technology, is an environmental-friendly polymer processing method capable of mass-producing components with minimally compromised material properties while consuming less energy and materials, as compared to components produced by the conventional processes [6]. The microcellular process uses a supercritical fluid (either CO 2 or N 2 ) which acts as a plasticizing agent thereby reducing the processing temperature of PHBV. Some of the most common types of microcellular processes available today are microcellular extrusion, injection molding, and blow molding. The microcellular process encompasses three major steps: gas dissolution, cell nucleation, and cell growth. Due to their unique properties, microcellular components (Figure 3) are particularly attractive for applications such as food packaging, automotive industry, sporting equipments, roof sheet insulators, microelectronic circuit board insulators, electronic wire insulation, and molecular-grade filters [37]. Properties One of the major drawbacks of PHBV is its poor thermal stability [7]. This co-polyester, similar to other types of polyesters, undergoes thermal degradation and hydrolysis which can lead to a reduction in molecular weight at temperatures above 170°C. Several methods such as incorporation of supercritical fluids (discussed above) [8], natural fibers (including kenaf fiber [9], pineapple fiber [10], and bamboo fiber [11]), and inorganic nanofillers [7] (e.g. organically modified nanoclay) into the PHBV matrix have been shown to improve the thermal stability of PHBV. Another significant drawback of PHBV is its brittleness which can be attributed to: (1) low nucleation density and a slow crystallization rate which leads to the formation of large spherulites [12]; (2) a logarithmic increase in the degree of PHBV crystallinity during storage time when more amorphous regions integrate into the crystalline regions, which will result in physical aging and a significant reduction in the impact strength [13]; and (3) circular and radial cracks inside the large spherulites which can act as stress concentration spots and promote the brittleness of PHBV [14]. To improve the mechanical properties of PHBV, several approaches such as blending with tough polymers (including poly(propylene carbonate) (PPC) [4] and poly(butylene adipate-co-terephthalate) (PBAT)) [5], and organic/inorganic nanofillers [7, 15] (including hyperbranched polymers and nanoclay) have been utilized to improve the PHBV’s strainat-break and toughness [15]. bioplastics MAGAZINE [01/12] Vol. 7 29
