Significant amounts of research is being conducted to develop environment-friendly and fully sustainable 'Green' polymers, resins and composites that do not use petroleum as the feedstock but are based on sustainable sources such as plants.
Fibers such as graphite, aramid, glass etc. are commonly used to fabricate high strength composites. These so called 'advanced composites' have revolutionized the materials world by providing very strong but light weight materials in place of metals for many years. Originally developed for defense applications such as fighter planes to make them lighter and faster, the composites technology has now become ubiquitous and can be found from circuit boards to sports gear and from automobile bodies to aerospace parts. Advanced composites are also being used increasingly in civil and structural engineering applications such as bridges, retrofitting old structures, etc. While the composites offer excellent mechanical properties and durability (no rust), most commercial fibers and resins (plastics/polymers) are derived from petroleum feedstock.
There are two major problems associated with this. First, petroleum is not a replenishable commodity and at the current rate of consumption, it is expected to last another 50 years or so. By one estimate the current consumption rate is about 100,000 times the rate at which the earth can generate it. And as the supplies dwindle, the prices are expected to soar. We are already seeing the effect of high demand on the oil prices. With the Chinese and Indian economies blazing ahead, the price of oil can only be expected to skyrocket in the future. Second, majority of the composites (and plastics) produced from petroleum are non-degradable under normal environmental conditions. Those made using thermoset resins such as epoxies, polyurethanes, etc. are also impossible to recycle or reuse. While a very small fraction of these composites are crushed into powder and used as filler or incinerated to obtain heat value, most of them end up in the landfills at the end of their life. In anaerobic conditions such as the landfills they last for several decades if not centuries, without degrading, making that land unusable. Whereas incineration produces large amounts of toxic gases that require expensive scrubbers. Both land filling and incineration, besides being expensive, are environmentally undesirable.
Things have been rapidly changing because of the rising concerns about the environment, pollution as well as the sustainability issues. Significant amounts of research is being conducted to develop environment-friendly and fully sustainable 'Green' polymers, resins and composites that do not use petroleum as the feedstock but are based on sustainable sources such as plants. Another welcome change is that more and more corporate leaders seem to believe that being good to the environment may also be good to their bottom line. In fact many major corporations now talk about the triple bottom line ”“ economical, environmental and social.
Such plant-based green materials can also be biodegradable so they can be easily disposed of or composted at the end of their life without harming the environment. Fibers such as jute, flax, linen, hemp, bamboo, etc. have been used for the past hundreds of centuries, mainly for apparel. They are not only sustainable but are annually renewable. Because of their moderate mechanical properties efforts are being directed towards using them to reinforce plastics and fabricate composites for various applications. These fibers may be used as fibers, yarns, fabrics or non-woven mats, individually or in combination as desired. Fully green composites fabricated using plant fibers (cellulose) and resins such as modified starches and proteins have already been demonstrated. Some products are commercially available as well. Since these fibers are weak compared to graphite, aramid, etc., the composites tend to have low mechanical properties, although comparable to or better than wood. As a result, these composites are suitable for applications that do not require high mechanical properties such as packaging, product casings, housing and automotive panels, furniture, etc. However, these are significantly large markets and if used in many applications should reduce the petroleum-based plastic/polymer consumption significantly.
The most recent development in the fabrication of Advanced Green Composites using high strength liquid crystalline cellulose fibers and plant-based resins is expected to change the green composite landscape completely [1-3]. Until recently high strength cellulose fibers were almost impossible to be spun. The most common cellulose varieties, viscose rayon and lyocell were weaker than cotton. However, high strength cellulose fiber spinning by dissolving purified cellulose in highly concentrated phosphoric acid (H3PO4, that also is present in many soft drinks) to form liquid crystalline solution and using a modified wet spinning technique to spin into high strength filaments resulting from its high molecular orientation and high crystalline content [4]. The phosphoric acid can be reclaimed and recycled. This modified wet spinning process is identical to spinning aramid (e.g. Kevlar®) fibers. One difference is that Kevlar® fibers are spun in highly concentrated sulfuric acid (H2SO4). The high strength cellulose fibers were recently used to reinforce modified soy protein resin to obtain advanced green composites with high strength [1-3]. The highest strength of these experimental, hand-laid unidirectional composites was in excess of 600 MPa. This compares to about 350-400 MPa for soft steel. What is interesting is that these composites had a density of about 1.4 grams per cubic centimeter (1400 kg/m3) compared to above 7.8 grams per cubic centimeter (7900 kg/m3) for steel. While the experimental composites were reinforced using only 42% fibers by volume, with automation the fiber content of 60%, as in most advanced composites, could be easily achieved. In that case the strength could be in the range of 950 MPa making the advanced green composites more than 5.5 times stronger than some of the strongest varieties of steel on 'per weight' basis. These composites may be used in primary structural applications such as I-beams for construction. The mechanical and thermal properties of the composites may be further improved by adding nano- and micro-fibrillar-cellulose (NFC and MFC) and nano-clay to the resin. NFC and MFC are produced by mechanical and/or hydrostatic shearing plant fibers under high pressure. Plant fibers have fibrillar structure which breaks down to individual fibrils that contain highly oriented molecular structure and are highly crystalline giving them strength between 2 and 10 GPa (compared to Kevlar® fibers which have strengths of about 3.5 GPa) and tensile modulus of about 140 GPa, comparable to Kevlar® [5, 6]. Addition of NFC and MFC has been shown to increase both the strength and toughness of the soy based resins significantly and thus should improve the composite properties as well.
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As the spinning process for the liquid crystalline cellulose fibers is refined further, there is possibility to obtain even higher strengths and hence even higher mechanical properties of the advanced green composites. There is also significant research effort in developing spider-silk like fibers using genetic engineering to obtain high strength protein fibers. These fibers, when fully developed, would also provide excellent reinforcement for composites.
One of the problems that needs to be addressed with current green composites based on soy protein and cellulose fibers, is their moisture absorption. Both resin and fibers are hydrophilic and absorb moisture resulting in dimensional changes (swelling) and mechanical property reduction in very humid conditions. At present, cellulose/soy resin base advanced green composites can be used in interior applications. They may be easily protected from water by varnish, paint or other methods that are commonly used for wood for exterior applications. However, further research in resin formulations should solve this problem in the future.
In the future the advanced green composites should open up a variety of structural applications in the future that were not possible with the earlier green composites. What’s good about these composites is that at the end of their life they can be easily composted into beautiful organic soil ”“ not hurting but helping the nature - giving back to the nature!
Additional Reading
1 Netravali, A. N., “Towards Advanced Green Composites”, 3rd International Workshop on Green Composites (IWGC-3), Proceedings, pp., 11-15, March 16-17, 2005, Kyoto, JAPAN.
2 Netravali, A. N. and Huang, X. H., “Advanced Green Composites”, September, 4th International Workshop on Green Composites (IWGC-4), Proceedings, pp. 23-27, September 14-15, 2006, Tokyo, JAPAN.
3 Netravali, A. N., Huang, X. and Mizuta, K., Advanced Green Composites, Advanced Composite Materials, accepted, 2007.
4 H. Borstoel, Liquid crystalline solutions of cellulose in phosphoric acid, Ph. D. Thesis, Rijksuniversiteit, Groningen, The Netherlands, (1998).
5 W. Helbert, J. Y. Cavaille and A. Dufresne, Thermoplastic nanocomposites filled with wheat straw cellulose whiskers .1. Processing and mechanical behavior, Polym. Compos. 17, 604-611 (1996).
6 A. N. Nakagaito and H. Yano, Novel high-strength biocomposites based on microfibrillated cellulose having nano-order-unit web-like network structure, Appl. Phys. A 80, 155-159 (2003).
7 T. Nishino, K. Takano and K. Nakamae, Elastic-modulus of the crystalline regions of cellulose polymorphs, Journal of Polymer Science Part B-Polymer Physics 33, 1647-1651 (1995).
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