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Cellulose

More recently, other polysaccharides in abundant supply have become of interest, notably cellulose. Micro- and more recently nanocrystalline cellulose has been rumored as being studied for tire applications, but few details are available. The most interest is in nanocrystalline cellulose, and this is covered in ? Chap. 23, “Nanofillers.” One of the problems with plant-based cellulose technologies is the need to disrupt cell walls to access the cellulose and then to separate this from the rest of the plant matter. In what may prove to be a breakout development, the University of Austin, in Texas, USA (2013), has recently announced the development of a route that starts from a blue-green algae (a genetically engineered cyanobacteria). This offers the prospect of a much more efficient and environmentally sound process than those starting from cellulose-containing plants such as wood.

Lignin

Lignin is the second most common organic material on earth (after cellulose), with about 20 billion tons being formed each year by photosynthesis. It makes plant cell walls fibrous and hard and has great credentials for making sustainable products. Chemically, lignin is a cross-linked phenolic polymer, the exact structure of which varies with plant source. Its specific gravity also varies with source but is in the range 1.3—1.5.

Lignin is responsible for the yellowing of low-grade paper and is thus removed from wood during higher-quality paper production, a process which generates about 60 million tons of lignin a year as a waste product. There are various chemical methods used for removal of the lignin; often these result in an aqueous solution of lignosulfonates. This lignin stream can be used on site as low-grade fuel.

Lignin or lignosulfonate powders of various sizes can also be produced from such waste streams and have a number of established uses. Recently they have begun to receive attention for the production of particulate fillers for polymers.

While still in its infancy, there have been some interesting developments. The carmaker Volkswagen, working with the TU Dresden and the Faserinstitut Bremen, has investigated the effect of lignin powder as filler in polypropylene (Mainka et al. 2015). They claim that the use of lignin as a thermoplastic filler offers “an enormous lightweight potential” for large volume auto-production and that “a (polypropylene) compound filled with up to 30% lignin powder offers a 20% weight reduction compared to traditional filled PP compounds and provides the same mechanical performance.” They further claim that costs would be reduced as much as 30% through replacement of fillers such as glass, talc, carbon black, aluminum oxides, and silicates. The use of maleated polypropylene coupling agent is claimed to enhance the adhesion between the lignin and polymer and improved some of the properties. Improvements in lignin extraction methods are also being commercialized (e.g., see http://renmatix.com).

The tire industry is under pressure to lightweight their products and to improve the sustainability of the raw materials used. A recent patent to Goodyear (US 8,664,305, 2014) describes the use of lignosulfonate as a filler for rubber, which is claimed to be able to replace precipitated silica in energy-efficient tire applications. A key aspect of the invention is the functionalization of the lignin surface to improve interaction with the polymer. The preferred functionalizing reagent is a silane coupling agent (e.g., (trimethoxysilyl)propyl methacrylate). An even more recent patent to the same group (US 9,102,801, 2015) describes a method for manufacture of lignin nanoparticles suitable for use as a polymer filler. These are very early days and it will be interesting to see how this develops.

 
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