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Aspects of lignocellulosic-fibre reinforced "green" materials.

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Much research and industrial development focus on the development, processing and manufacturing, recycling and disposal of environmentally sound materials with increased recyclable and renewable content. This includes polymers, polymer blends, composites and other industrial products based on agricultural sources that are capable of competing with synthetic counterparts in terms of cost, impact, mechanical and thermal properties.

In the case of the automotive industry, stringent environmental legislation (EU Directive 2000/53/EC) has enforced demanding quotas for the recovery, reuse and recycling of end-of-life vehicle waste and has banned toxic substances from future passenger vehicles commercialized in the European Union (EU). The implications of such directives are manifold and affect original equipment manufacturers (OEMs) world wide.

Other countries, like Japan, also have similar legislation that requires that end-of-life vehicles (ELVs) are depolluted, recycled and disposed of in an environmentally sound manner. In Canada, the National Vehicle Scrappage Programme encourages drivers to recycle vehicles older than model year 1995 in order to reduce air pollution and green house emissions (under the premise that these vehicles do not comply with currently low emission requirements), prevent ELVs from being abandoned and toxic substances contained in specific vehicular applications (e.g. cadmium, mercury, hexavalent chromium, etc.) to be released into the environment.

Some statistics show that approximately five to six percent of the entire passenger vehicle fleet in Canada reaches the end of its useful life every year. This is between (approximately) one million to 1.2 million vehicles, of which (approximately) between 0.4 million to 0.5 million vehicles are generated only in Ontario. (1) However, the dimension of the issue worldwide is massive. To put it into perspective, approximately two million ELVs are generated every year in the United Kingdom, five million in Japan and more than 12 million in North America. (1,2,3) Even though more than 75 percent, by weight, of the vehicle can be recycled by traditional means, there is a relatively small amount which cannot be recycled or recovered; in the case of North America, this accounts or more than three million tons of waste per year. (1) This puts enormous pressure on the environment because ASR is generally landfilled.

On one hand, this opens the opportunity to produce greener automotive materials that are cost-effective to dismantle and dispose of (either by recycling, incineration, compostability, pyrolisys, or other means). On the other, it raises new challenges to increase the performance and cost competitiveness of current "green" materials, set up a wider network of infrastructure for the reception, de-polution, dismantling, sorting and disposal of ELVs, create competitive markets for the recovered/recycled materials from ELVs and enforce a common environmental legislation platform which assist OEMs to satisfy national and international legislation in terms of end-of-life waste disposal.

Lignocellulosic Fibre-Reinforced "Green" and "Truly Green" Composites

"Green" and "truly green" composites are being developed worldwide. It is generally accepted that "green" composites consist of natural fibres and biopolymers, with the latter usually produced from petrochemical or renewable sources. (4, 5) "Truly green" composites incorporate renewable sourced biopolymers produced from celluloseand soy-based plastics, starch, lactic acid, polybydroxyalkanoates, bacterial cellulose, soy-based plastics, among others (5), usually reinforced with plant fibres, specifically lignocellulosic fibres.

According to its origin, natural fibres can be classified as vegetable, animal or mineral. Vegetable fibres may be extracted from wood (e.g. softwood and hardwood), husk (e.g. maize, rice and wheat), fruit (e.g. colt, luffa), seed (e.g. cotton), leaf (e.g. henequen, sisal), stalk/bast (e.g. abaca, flax and hemp), cane or grass (e.g. bamboo). Lignocellulosic fibres, are composed of cellulose, hemicelluloses and lignin with small amounts of different free sugars, hollocelluloses, starch, pectins, proteins, several mineral salts and extractives, such as waxes, fatty alcohols, fatty acids and different esters.

The wide availability, low cost, renewable and thermally recyclable properties, low carbon foot print and sound damping properties of lignocellulosic fibres underpin its use in fibre-reinforced composite applications. Lignocellulosic fibres have lower mechanical properties than competing synthetic reinforcing fibres; however, their lower density and thus specific properties, are comparable to those of glass fibres. Lignocellulosic fibres have extraordinarily high potential as reinforcing elements in composite materials because the tensile strength and Young's modulus of the I-cellulose crystal that forms the crystalline regions of cellulose reaches values that are either similar or superior to those of glass fibres (~ 10 GPa (6) and between ~ 78 to 128 GPa (7, 8) respectively). According to some studies, the substitution of glass fibres for hemp fibres in automotive fibre-reinforced composites has the potential to save approximately 1.4 kilogram of carbon dioxide per each kilogram of glass fibres replaced, during the whole life cycle of the part until its disposal. (9) A number of potential applications for lignocellulosic-based materials are found in interior and exterior automotive applications where stiffness and low cost are among the required criteria, (10) e.g. thermo-acoustic insulation panels in undermats, door panels, tailgates and composite systems. New higher performance applications will be developed as "green" and "truly green" lignocellulosic fibre-reinforced materials improve several technical issues, mostly inherent to lignocellulosic fibres, including low fibre-matrix wettability and adhesion, hydrophilic behaviour and quality variability. Regular increases in the use of lignocellulosic fibres as fillers and reinforcements for thermoplastics and thermosets in the automotive industry can be expected for years to come. For example, the German automotive industry used more than 19,000 tons of hemp (Cannabis sativa) and flax (Linum usitatissimum) fibres in 2005, whereas in 1999 this amount accounted only for 9,600 tons. (9)

Variability of kignocellulosic Fibres

One of the major drawbacks of lignocellulosic fibres is their quality variability, which is in part due to the presence of non-cellulose compounds. (11) The removal of non-cellulose compound from lignocellulosic fibres improves their compatibility with dyes, thermal resistance, chemical composition and mechanical properties. This is achieved by traditional mechanical, bacterial/enzymatic, physical or chemical processes, including fibre surface modification methods. Alternative methods are also being developed, for example, to reduce the amount of lignin using genetic manipulation. (12) During the growth and harvest of the lignocellulosic crop factors such as the fibre crop variety, soil conditions, climate, location, the section of the crop from which the fibres are extracted from and the harvest machinery used to harvest the crop also have an effect on the final properties of the lignocellulosic fibres. (5) The maturity of the crop at the time of harvest is also significant in terms of fibre quality and yield.

In the case of crops for fibre production, like hemp, the fibre yield is maximized three to four weeks after the crop has flowered. However, as the crop ripens and the mechanical strength of the fibres increases, a lignification effect occurs, which in time produces coarser fibres which adhere more strongly to each other and to the woody core. Lignification makes the breaking and separation of the fibres much more difficult,s In the case of crops for fibres and seed production, the optimum harvest time is about six weeks after flowering. Producing fibres that are stiffer, coarser and more brittle. The physical form of the fibres varies according to the fibre separation and extraction method used, including dew, stand, cold water, warm water, ultrasound, enzyme, chemical and surfactantretting. Decortication (breaking), scutching and hackling operations can produce lignocellulosic fibres that range from fibre bundles, technical fibres (50-100 [micro]m in diameter) to elementary fibres (20 [micro]m in diameter), or even smaller, micro fibrils (4-10 nm). (12)

The selection of the retting method is of the most economic importance as this has a direct impact on final fibre quality, chemical composition, physical appearance, mechanical properties, yield and the environment. Different parts of the same crop produce fibres with different chemical compositions. (13,14,15) Flax stalks contain approximately 54 percent cellulose, with smaller amounts of hemicelluloses (17 percent), lignin (23 percent) and ash (3.6 percent). After retting and decortication, flax fibres are separated from the woody core of the bast into what is known as flax shives. There are significant differences in the chemical composition of flax fibres and flax shives. Flax fibres are low in lignin (approximately) (five percent) and hemicellulose (six percent), and high in cellulose (78 percent) content, whereas flax shives contain more lignin (23-31 percent), more hemicellulose (13-26 percent) and less cellulose (34-53 percent) than flax stalk and flax fibres. In addition, because flax shives and fibres are lower in lignin and higher in hemicelluloses they can absorb moisture more easily.

Environmental and Economic Sustainability

Besides the intrinsic properties of lignocellulosic fibres, their overall environmental and economical sustainahility has to he analysed. Modern yield-orientated agriculture practices for the production of fibre crops require large amounts of water, pesticides, fungicides, herbicides and fertilizers, which may disrupt ecological balance in certain areas. Thus ,sustainability of lignocellulosic crops is of major importance from an environmental and economical perspective.

In the case of the EU, specifically the United Kingdom, changes in the subsidies policy for flax crop cultivation encouraged the development of new techniques for the production of high-quality flax fibreyarns for high volume manufacture of fine fabrics. (16) However, this development required the application of a translocating herbicide at different stages of plant maturity for optimum fine fibre production. The high water requirements and use of substantial amounts of fertilisers and pesticides for lignocellulosic and non-food crop production has prompted the evaluation of the environmental impact of two of the most used lignocellulosic crops, hemp and flax, using life cycle analysis (LCA). LCA is a method to assess the impact associated with a product by quantifying and evaluating the resources consumed and the emissions to the environment at all stages of the product's life cycle. The analysis (17) compared traditional hemp warm water retting to bio-retting (i.e. hemp green scutching followed by water retting) and dew retting of flax. The results, which did not account for fibre quality, suggest that traditional hemp warm water and dew retting of flax are similar in terms of environmental impact, except for pesticide use (i.e. higher for flax) and water use during processing (i.e. higher for hemp), whereas bio-retting had higher environmental impact than traditional hemp warm water retting because of higher energy requirements during fibre processing. (17)

Based on the findings, the authors suggested that any reduction in the environmental impact of these crops for high quality fibre yarn production should focus on a reduction in energy consumption during fibre processing and yarn production and on a reduction of the presence of macronutrients in the environment, in particular nitrogen and phosphorous.

Conclusions

The development of "green" and "truly green" lignocellulosic fibre-reinforced composites, biopolymers and any other environmentally friendly material should not only focus on maximising technical performance, cost effectiveness, dismantling and final feasibility, but also should encourage economic, environmental and social sustainability.

References

(1.) 1877 End-of-life Vehicles, Green Vehicle Disposal. Website link available in: http://1877endoflifevehicles.com/eol.cfm.

(2.) UK Environmental Agency, End of Life Vehicles Directive. Website link in: http:// www.environment-agency.gov.uk/busines s/444217/444663/591015/?version = l&lan g=_e.

(3.) Japan for Sustainability, The Recycling of End-of-Life Vehicles in Japan. Website link available in: http://www.japanfs. org/en/mailmagazine/newsletter/pages/ 027816.html.

(4.) A. K. Mohanty, M. Misra and L. T. Drzal, "Sustainable Bio-Composites from Renewable Resources: Opportunities and Challenges in the Green Materials World," Journal of Polymers and the Environment 10, 1 (2002), pp. 19-26.

(5.) A. Bismarck, S. Mishra and T. Lampke, Plant Fibres as Reinforcement for Green Composites, (Boca Raton: CRC Press, 2005), pp. 37-108.

(6.) M. A. Said-Azizi-Samir, F. Alloin, M. Paillet and A. Dufresne, "Tangling Effect in Fibrillated Cellulose Reinforced Nanocomposites," Macromolecules 37, 11 (2004), pp. 4313-4316.

(7.) G. Guhados, W. K. Wan and J. L. Hutter, "Measurement of the Elastic Modulus of Single Bacterial Cellulose Fibres Using Atomic Force Microscopy," Langmuir 21, 14 (2005), pp. 6642-6646.

(8.) 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, 11 (1995), pp. 1647-1651.

(9.) M. Karus and M. Kaup, "Natural fibres in the European automotive industry," Journal of Industrial Hemp 7, 1 (2002), pp. 119-131.

(10.) R. Kozlowski and M. Wladyka-Przybylak, Uses of natural fibre reinforced plastics, (USA: Kluwer Academic Publishers, 2004), p. 249-274.

(11.) R.B. Dodd and D.E. Akin, Recent Developments in Retting and Measurement of Fibre Quality in Natural Fibres: Pro and Cons, (Boca Raton: CRC Press, 2005), p.141-157.

(12.) H. Bos, M. Van Den Oever and O. Peters, "Tensile and compressive properties of flax fibres for natural fibre reinforced composites," Journal of Materials Science 37, 8 (2002), pp. 1683-1692.

(13.) M. Sain and D. Fortier, "Flax shives refining, chemical modification and hydrophobisation for paper production," Industrial Crops and Products 15, 1 (2002), pp. 1-13.

(14.) J. Young, "Canadian nonwood pulp mill to start up in early 1993," Pulp Paper 66, 9 (1992), pp. 144-145.

(15.) A.U. Buranov and G. Mazza, "Lignin in straw of herbaceous crops," Industrial Crops and Products 28, 3 (2008), pp. 237-259.

(16.) J. Harwood, P. McCormick, D. Waldron and R. Bonadei, "Evaluation of flax accessions for high value textile end uses," Industrial Crops and Products 27, 1 (2008), pp. 22-28.

(17.) H. M. G. van der Weft and L. Turunen, "The environmental impacts of the production of hemp and flax textile yarn," Industrial Crops and Products 27, 1 (2008), pp. 1-10.

Mohini Sain is a professor and director of the Centre for Biocomposites and Biomaterials Processing at the University of Toronto. He is currently working on the research and development of bio-plastics, cellulose-based micro- and nano-composite technology, industrial biomaterials and biocomposites manufacturing, and biomass technology.

Alexis Baltazar-y-Jimenez is a post-doctoral fellow at the Centre for Biocomposites and Biomaterials Processing at the University of Toronto. He is working on the development, characterisation and processing optimisation of hybrid (nano)fibre-reinforced composites produced from renewable sources for high performance applications.
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Author:Baltazar-y-Jimenez, Alexis; Sain, Mohini
Publication:Canadian Chemical News
Geographic Code:1CANA
Date:Jun 1, 2009
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