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Lignin

A very recent patent to Goodyear Tire and Rubber (US 8664305) indicates that they may be following a new path in what might eventually lead to a new tire filler technology. This patent describes the functionalization of the surface of lignin, especially using esterification or silylation and proceeding through lignin sulfonate. It is claimed that the so modified lignin can be used as a reinforcing filler for rubber, where it can replace precipitated silica. Lignin is also being studied as filler for use in thermoplastics (Mainka et al. 2015).

Wood Flour

This is the most widely used particulate filler of plant origin used today. It is discussed in detail in ? Chap. 16, “Fillers from Organic Sources” and only treated in outline here (Clemons 2010).

General

Wood flour is a fairly fine particulate powder and was one of the first fillers to be used commercially in polymer applications, being an important component of Bakelite, a phenol-formaldehyde composite, first produced over 100 years ago and still in use today. While made from a fibrous material, the fibers in wood flour are relatively short (aspect ratio below 5).

Wood flour is usually made from scrap wood sources and can thus be regarded as recycled as well as being sustainable. Its properties dictate that it cannot be regarded as a substitute for common mineral particulate fillers and must find applications of its own.

Wood flour is a very variable material, with a variety of wood sources and types being used to make it. The main steps in production are purification, size reduction, and classification. Size reduction can be accomplished in a variety of ways including hammer mills, chippers, rollers, or attritors. The methods used will depend on the source material, especially its initial size. Classification can be achieved by standard methods such as screens or by using air classification. Some drying is also often necessary, although, for practical reasons, wood flour is usually shipped with some moisture.

The chemical composition of wood flour is complex. The main components are cellulose, hemi-cellulose, and lignin. There will also be some extractables such as oils and inorganic ash. The exact ratio of these major components and the nature of the minor ones will vary with the wood type. Typical values are:

Cellulose

37-45% w/w

Hemicellulose

19-30% w/w

Lignin

21-34% w/w

Extractables

2-5% w/w

Ash

<0.7% w/w

 
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