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Executive Summary
Introduction
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The UK Forest Resource
Secondary Metabolites From Trees
Non-Timber Markets For Trees
Extraction Technologies For Tree Metabolites
Adding Value To Tree Metabolites

Adding Value to Cellulose
References

Further Research
Modelling Tools
Adding Value To Tree Metabolites: Cellulose extraction and modification
The biosynthesis and physical properties of cellulose are reasonably well understood. Separation of cellulose from wood is routinely carried out in paper manufacture, where the major contaminants are lignin and hemicellulose. Secondary metabolites are also a source of contamination, but as demonstrated in the previous sections using clean extraction technology secondary metabolites could become a source of revenue to complement cellulose extraction.
Global competition means that there is intense pressure on the UK pulp industry to remain competitive. Adding value to UK forestry will therefore rely increasingly on value-added products that can command premium markets. This could be achieved through adding value to cellulose itself for some markets or improving extraction of co-metabolites and product with commercial value.
Cellulose is a polymer of β-bonded D-glucose units with over 3500 repeat units in a single chain. The chemical and mechanical separation of cellulose generally involves quite harsh conditions (e.g. the Kraft pulp process), but a rigid structure of cellulose makes it resistant to damage. Moreover, its chemical resistance, thermal stability and insolubility of cellulose in aqueous media make it an interesting material from which a number of novel products can be derived.
Those industries exploiting cellulose as raw material are experiencing a 'renaissance' in existing and a surge in the development of new cellulosic derivatives with many different applications (Harms 1998). This section reviews the current status and future potential of these tree metabolite derivatives.
Adding value to cellulose
Cellulose can now be deemed as a generally applicable polymer for use in a variety of materials and chemical reactions. In addition to paper production, cellulose is currently utilised in the production of:
  1. Viscose filaments (applications: fibres, sheeting, spunbondeds, membranes, composites and sponges etc.)
  2. Cellulose acetate (applications: textile fibres, artificial silk, surgical implants, glasses frames, various coating materials, micro- and ultra-filtration membranes, contact lens coatings, controlled release matrices for drugs, adhesives etc.)
  3. Cellulose esters (applications: most commonly uses polysaccharide polymer in food and pharmaceutical industries, drilling aids, detergents, coatings and adhesive additives, film, sheeting, wallpaper, leather production, cosmetics, animal feed etc
This range cellulosic derivatives and potential applications is now set to expand as a result of the many recent advances made in cellulose chemistry.
Creating high surface area cellulose
The natural small surface area of cellulose limits the rate and extent of chemical modification (i.e. during processes to create materials with novel properties). In previous work in this area, cellulose with a surface area of up to 200m2g-1 has been reported from a method involving swelling of cellulose in ethanol and then replacing the ethanol with supercritical CO2 (Weatherwax & Caulfield 1971). In a second method, a cellulose material was prepared with a surface area of over 380m2g-1 through supercritical drying of cross-linked cellulose (Tan et al. 2001). The problem with such methods is the expense of the supercritical equipment required for drying the expanded materials.
More recently, research at the University of York has resulted in the production of high surface area cellulose materials that avoids the need for a supercritical drying method. The method involves the preparation of expanded surface area cellulose materials (Clark et al. 2002). The process is shown schematically below.
Figure 11. Preparation of high surface area of cellulose
Using a method such as this, under controlled conditions, cellulose with surface areas of > 50mP2PgP-1P can be prepared.
Modification of cellulose and expanded cellulose
There are numerous modifications of cellulose that have been carried out to make a wide variety of saleable products. Examples of such products are methylcellulose (Hirrien, Desbrieres, & Rinaudo 1996), hydroxyethylated and hydroxypropylated cellulose, cellulose acetate and other cellulose esters (Edgar et al. 2003) and nitrocellulose.
More recent developments have been aimed at adding new functions to cellulose products: these include a role as selective adsorbents and traps, catalyst support materials, controlled release materials and chromatographic separation media. There are numerous chemical modifications that can be used to introduce new properties to cellulose. For the most part, the new expanded cellulose materials are more reactive than unmodified cellulose. Several potential chemical modifications to make useful cellulose based materials are shown below.
Figure12. Chemical groups that can be incorporated into expanded cellulose (EC)
There are numerous functional groups that could be chemically bonded to a cellulose support, which would give a diverse range of functional properties. It is possible that multi-functional materials could be prepared by chemically bonding two different species to the surface. For instance, in theory a material with two different, complementary, catalytic sites could be synthesised. However, this area of research (especially relating to expanded cellulose materials) is immature and work needs to be carried out in order to assess the potential of modified cellulose materials.
The market potential of such modified cellulose products is immense. The anticipated industrial uses of such modified cellulose derivatives are expected to include replacement for polystyrene in packaging, absorbents in air conditioning systems, polymer blends, selective adsorbents and traps, catalyst support materials and controlled release matrices for pharmaceuticals. Significant further work and investment will be required to realise the market potential of these novel interesting materials.
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References
Clark,J.H., Doi, S., Millowski, K., & Macquarrie, D. J. (2002) New materials based on renewable resources: Chemically modified expanded corn starches as catalysts for liquid phase organic reactions. Chemical Communications pp. 2632-2633
Edgar,K.J., Buchanan, C. M., Debenham, J. S., Rundquist, P. A., & Seiler, B. D. (2003) Advances in cellulose ester performance and application. Progress In Polymer Science 26, pp. 1605-1688
Gandini,A. & Belgacem, M. N. (1997) Furans in polymer chemistry. Progress In Polymer Science 22, pp. 1203-1379
Hirrien,M., Desbrieres, J., & Rinaudo, M. (1996) Physical properties of methylcelluloses in relation with the conditions for cellulose modification. Carbohydrate Polymers 31, pp. 243-252
Tan,C., Fung, B. M., Newman, J. K., & Vu, C. (2001) Organic aerogels with very high impact strength. Advances in Materials 13, pp. 644-646
Weatherwax,R.C. & Caulfield, D. F. (1971) Cellulose aeorgels: An improved method for preparing a highly expanded form of dry cellulose. Tappi 54, pp. 9