From Science & Research
From Science & Research Protein-Based Plastics and The adoption of protein-based plastic by industry has been slow for several reasons; principal among them are costs of raw materials, limited thermalmechanical strength, and solvent sensitivity. There are, however, strong reasons for using protein-based plastics, including the reduction of petrochemical dependence as well as the reduction of greenhouse gases and other environmental impacts, such as landfill usage. In addition, the poor properties can be significantly enhanced with appropriate additives and through careful processing techniques. For these reasons there is a renewed push for industry to seriously consider the adoption of proteinbased polymers for many existing applications which use petroleum-derived plastics. Article contributed by David Grewell, Dpt. of Agricultural and Biosystems Engineering; Michael Kessler, Dpt. of Materials Science and Engineering; William Graves, Dpt. of Horticulture; all from Iowa State University, Ames, Iowa, USA www.egr.msu.edu/cmsc/biomaterials Fig 2: Plant pots formulated from Zein (left) and petrochemical plastic pot Additives and Processing Proteins in their native state can be resilient, strong, and low density macromolecules, as evident by several biological protein structures, such as bone and hair. However, to utilize the proteins, their structure must fist be partially broken down during processing and reformed to a new structure. As such, the thermal-mechanical and water absorption properties of protein-based plastics depend heavily on several factors, such as the plasticizers used (e.g., glycerol, ethylene glycerol, butanediol, sorghum wax, ethanol and sorbitol), the addition of cross-linking agents (e.g., zinc sulfate, acedic anhydride, formaldehyde), and processing parameters (e.g., extrusion pressure and temperature and initial moisture content). Reduce Moisture Susceptibility Work at Iowa State University (ISU) is under way to characterize these protein derived polymers and evaluate various treatments and formulations to enhance their properties. For example, it is now understood that the addition of selective salts, such as zinc stearate and zinc sulfate, can reduce the water sensitivity of plastics derived from soybean proteins. In addition, it has been demonstrated that thermal treatments (ranging from 80 to 120 °C) can reduce the susceptibility of these plastics to water. Soy plastics were also co-blended 50:50 with polycaprolactone (PCL), a biodegradable polyester, in order to reduce water absorption. Figure 1 shows that the control sample, untreated soy plastics, absorbs over 150% water by weight within a few hours. It is also seen that by heat treatment the water absorption decreased significantly after 24 hrs. However, the largest improvement was seen when the soy plastic was blended with biodegradable PCL, where the water absorption was less than 20% even after 24 hrs. of exposure. It is believed that the relatively water insoluble PCL formed a continuous phase within the blend and shrouded the soy plastic from the water. This allows the PCL to reduce water exposure of the soy plastic thus reducing the overall water sensitivity. While the addition of 2 parts of zinc stearate and zinc sulfate did reduce the water absorption, the reduction is limited compared to heat treatment and co-blending with PCL. 34 bioplastics MAGAZINE [02/07] Vol. 2
From Science & Research Applications Nano-Clays and Foamed Structures Other mechanical, thermal, and physicochemical property enhancement techniques being investigated at ISU include the use of high powered ultrasonics to promote exfoliation of nano-clays. These polymer-layered silicate nanocomposites can have enhanced vapor barrier properties, further reducing water absorption, while simultaneously increasing tensile modulus and strength and heat distortion temperature. Fig 3: Natural fiber reinforcement 200 In order to overcome the issues related to costs of raw materials, researchers at ISU are working with Trexel Corp. (Woburn, Massachusetts) to characterize foamed substrates. It is expected that these materials will reduce the raw material requirements while maintaining mechanical strength. Potential Applications Resulting data from this work were used to formulate selected grades and processing conditions for application studies. In two initial application examples, Creative Composites (Brooklyn, Iowa), and Vermeer Manufacturing Co. (Pella, Iowa) are supplying materials to evaluate their use for selected products, including hay bale sealing films and locomotive grease applicators. Some studies have shown that water resistant films formulated from Zein, a protein derived from corn, can be formed as thin as 100 μm. The films are flexible and relatively strong. Currently, processing and characterization tests are being performed with these films. Other applications include bio-degradable pots for plants. The photograph (Fig. 2) shows a pot formulated from Zein (left) next to a conventional petrochemical plastic pot (right). Other formulations include the reinforcement of the Zein polymer with natural fibers (Fig 3). While this research is in its early stages, the preliminary results are promising. It has already been shown that the water sensitivity of soy protein plastics can be decreased through simple blending or heat treatment. Similar enhancement in mechanical properties and cost with the incorporation of exfoliated clay platelets and Zein-based proteins is expected. The authors would like to gratefully acknowledge the Grow Iowa Value Funds for supporting this work, as well as Zein Corporation, Trexel Corporation, Creative Composites, and Vermeer Manufacturing Co. Moisture absorbition (%wt) 150 100 50 0 0 5 10 15 20 25 30 Fig 1: Moisture absorption over time Time (hrs) Fig 4: Reinforcing rib structure Control ZnS=4 Zinc Steartate 80 °C 100 °C 120 °C PCL Coblend bioplastics MAGAZINE [02/07] Vol. 2 35
