Flax Straw: Processing

The goal of flax straw processing is to separate the useful fibres from the other parts of the stem.

In a flax stem, bundles of fibres are embedded in what's called bast tissue (which is why they're usually called bast fibres). This lies just under the waxy skin of the stem, and is bound to a woody core by a layer of pectins.

The first step to freeing the fibres is retting. The cheapest and simplest way to accomplish this is dew retting: the straw is simply spread over the ground so soil microorganisms can decompose the pectins, loosening the fibres' connection to the woody core. During this process the straw will lose a considerable amount of weight: 740 kilograms of straw will be reduced to 580 kilograms during retting, with most of the weight change being due to the loss of various water-soluble compounds, the rest to the loss of dust. The process takes anywhere from three to seven weeks, depending on the weather. (Bos., 2004)

Another form of retting that used to be common in Europe is water retting, in which bundles of flax stem were immersed in running water or standing water in ponds or specially prepared pits. As the stems fermented, anaerobic bacteria degraded the pectins. The resulting fermentation products polluted the water, however, and the process also produced foul odors, so water retting is no longer used in Western Europe.

Research is continuing into enzyme retting, which would use selected pectin-degrading enzymes to accomplish the retting. This has the potential of producing cleaner and more consistent fiber, but is currently very expensive. It's not exactly a new method, either: as long ago as 1916 experiments were conducted in retting flax by placing it in water and adding yeast.

Studies have indicated that water-retted fibres have the highest quality: the fibres are finer, stronger, and freer of non-cellulosic compounds. The quality of dew-retted fibre is highly variable due to the uncontrolled conditions under which the retting takes place, and in general dew-retted fibres are only about 70 percent as strong as water-retted fibres. Enzyme-retted fibres are as fine as water-retted fibres, but only half as strong. (Bos., 2004)

Some processors also use unretted, or green, flax fibres, using mechanical methods only to free the fibres from the stem. These are very coarse, but may be better suited for use in various composite materials. Their strength is comparable to that of dew-retted fibres.

Once the fibres have been loosened from the stem by some form of retting, the stem is broken, typically by threading it through fluted rollers. The broken stem parts then go into the scutching turbine, which scrapes the broken stem parts, collectively known as shives, from the fibres, a process called decortication. (Bos., 2004)

The resulting ribbon-shaped fibre bundles are still relatively coarse and thick, which for some applications may be exactly what's desired. For other applications, however, the "ultimate fibres" – the tiny fibres that make up the fibre bundles in the stem - are needed.

Flax ultimate fibres are roughly the diameter of cotton fibres but are longer and need to be cut to a cotton-like length. This makes them valuable because more than 90 percent of the world's spinning and weaving equipment is designed to use fibres that have approximately the same diameter and length as cotton fibres. For that reason, flax fibre that has been cleaned and cut in this manner is generally said to have been "cottonized."

The mechanical systems that have traditionally cottonized flax also produce large amounts of dust and waste fibres, and could only be used with fibre-flax fibres that had been well-retted. This made cottonized flax much more expensive than cotton, which in turn limited the demand for it to various small markets.

Researchers are now looking at alternative ways to produce cottonized flax that will minimize the waste and produce more consistent, lower-cost fibre. Enzyme retting may be part of this: steam-explosion (in which fibres are heated to a high temperature in a steam reactor, then cooled suddenly by the explosive release of the steam) and ultrasound are also being explored. If cottonized flax can be produced at a price competitive with that of cotton fibre, the potential market for it is huge. The demand for cotton worldwide in 2000 was 20 million tonnes, and it's growing by about 200,000 tonnes annually. (Flax Council of Canada, 2008)