21 March 1997

Stronger plastic with straw

STRAW could find its way into many plastic products if a new process proves successful (Arable, Jan 31).

Warks-based engineering consultant John Wood says the cellulose content of straw allows it to homogenise with certain synthetic raw materials widely used in the plastics industry.

Used as a reinforcing agent, he claims it brings added strength to products made of polyethylene and polypropylene. Working closely with plastics compounders Johnson Brothers of Honeybourne, near Evesham, he has produced a straw-based filler for these plastics. It could consume up to 0.5m tonnes of surplus straw a year.

The straw addition is said to raise the impact strength of polymers to a level where they could compete with specialised – and far more costly – glass-reinforced materials in many electrical and automotive applications.

"Industry can help the farmer and the farmer can help industry," Mr Wood explains. He has been working on this project for seven years, in co-operation with plastics processors. He claims that trials show polymer compounds containing wheat straw filler, which he has named Stemform, can be compounded, extruded and injection-moulded, using standard plastics processing conditions. Items already produced with Stemform include mouldboards for ploughs. Trials have shown they match the performance of traditional steel mouldboards but with just a third of the weight. The compound has also been used successfully in extruding land drainage pipe and injection-moulding small electrical parts.

Producing the Stemform compound is relatively straight- forward. The straw is reduced to 2mm particles in a hammer mill. The moisture content has to be carefully controlled – a maximum of 2% provides the plastics processors with a highly consistent raw material.

The straw particles are then fed into a mixer with the polymer granules and subsequently into the hopper of the compounding extruder, with the resulting extrudate taken off by means of a die-face cutter.

A patent for Stemform has been applied for and Mr Wood has formed a company – Stemform Products – to promote its use.

Johnson Brothers, a farm business, which set up a plastics compounding subsidiary some years ago, will be licensed by Stemform to produce the compound. "With our current capacity, we can produce about 10t or more a week," says director Stephen Johnson. "As the market grows, we shall add extra compounding lines." &#42

Plastics reinforced with cereal straw offer lower weight and improved strength, says consultant John Wood. Lightweight mouldboards for farm ploughs could be one potential use. Patents are pending.


&#8226 Grind to 2mm and dry to 2% moisture.

&#8226 Add to polyethylene and polypropylene for strength.

&#8226 As strong as glass-reinforced plastics.

&#8226 Far cheaper.

&#8226 Stemform patent pending.