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Products Obtained by Size Reduction of Plant Material Wood Flour

Wood flour is the largest volume organic filler in commercial use today. The wood fibers in it are of such short length that it can be regarded as a particulate. Wood flour 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 it is still used in this type of material today.

Wood flour is generally regarded as being wood particles which are fine enough to pass through a 20 US standard mesh (about 850 pm). It is made from scrap wood sources and can thus be regarded as recycled as well as sustainable. The inherent properties of wood flour dictate that it cannot be regarded as a direct substitute for inorganic particulate fillers and must find applications of its own.

Production

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 particles contain significant amounts of porosity. When fully compressed, the specific gravity is in the range 1.4—1.5, depending on the type of wood and moisture content, but this is not always achieved in polymer composites. 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 together with an inorganic residue (ash). The exact ratio of these major components and the nature of the minor ones will vary with the wood type. Typical values are as follows:

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

The cellulose is a highly crystalline, high-molecular-weight, linear polymer and is present in the form of microfibrils. It gives most of the strength to the wood. Hemicellulose is a branched polymer of much lower molecular weight but still makes some structural contribution. Lignin is a cross-linked amorphous polymer of variable structure and acts as a glue to hold the fibers in place.

 
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