Processing of flax fibre

  Extraction of fibres

The process of extraction is of great importance since the quality as well as quantity of extracted fibres is strongly influenced by the method of extraction employed. Extraction of fibre means separation of fibre from the cementing substances such as pectin or lignin, wax, resin, fats, and other carbohydrates. Fibres from flax plants are extracted by any one or in combination of mechanical and chemical retting processes. The choice of the extraction method depends largely upon the quality of fibre to be regained. For centuries, the use of hand scrapers, blunt and crescent shaped knives, wooden beater and hand comber has been common for fibre extraction. Nowadays, fibres are extracted mechanically. The flax stalk passes through various processes of extraction, viz, retting and scutching after harvesting. Flax is harvested just before the seed is ripe, tied in bundles and after a few days drying, seed and leaves are removed by a process called rippling. The plants after rippling are spread out on grass and left to ferment for some weeks by the action of weather. The fibre is extracted from the stalks by different retting processes. Water retting is commonly used in India. Dew retting is practiced in European continent.

Retting

Retting is defined as the process of separating the embedded fibre from the flax stem through partial rotting by immersion in water; this rotting is brought about by a complex enzyme action of microbes naturally present in retting water. Retting ranks as the single most important factor governing the quality of fibre. Precise retting helps to enhance the genetic quality of a variety as well as the crop management skill, while faulty retting can completely spoil the positive contributions of a variety, balanced fertilizer, proper spacing etc. The muck that accumulates at the bottom of the retting tank is rich in organic matter content and plant nutrients. It can be returned to crop fields thereby desilting the retting tank.

Retting is best done in clear, slowly flowing water. Hence canal and even rivulets are best suited for retting. The depth of water should be sufficient to allow the stem bundles to float. Alternative tanks, ponds or ditches with at least a depth of 1.8m are used. When retting is done in stagnant water, the minimum ratio of plant material to water should be 1:20 by volume to ensure good results.

The important conditions to observe are:  

1. The water should be clear and non-saline;
2. The volume of water should be enough to allow the stem bundles to float;
3. When the bundles are immersed, they should not touch the bottom;
4. The same retting tank or ditch may not be used repeatedly, particularly if the water becomes too foul and dirty. 

Various methods employed for retting are

Stream or water retting

This method produces the best quality of flax fibre, as process of fermentation can be retarded and easily controlled. Bundles of flax straw are weighted down in streams of cool soft running water for five to fifteen days. Slowly running water is preferable as swiftly running water carries away the bacteria and thus retards fermentation. The straws are submerged a few inches below the surface of water, anchored in place with an extra weight placed on them. When sufficiently retted, the straw is soft and covered with a green slime, and when bent, the woody portion will spring out of the stalk. The fibre produces strong superior linen of pale yellow colour. Quality of water is important; it must be clean and free from minerals especially iron, which would stain or discolour the fibre.

Dew retting

The flax straw is spread on the grass in a thin layer and is exposed to the action of weather for several weeks, usually from 4 to 8. Moisture, which is essential to the process of fermentation, may have to be supplied by artificial means. The swath must be turned over during the period of retting. After retting is complete, the straw is raked up and again stacked to dry. Dew retting involves the production of enzyme by fungi and bacteria, thereby causing decomposition of pectins by enzymatic hydrolysis. This method produces strong fibre grey in colour. Dew retting is common in Europe, due to its low cost and objection against pollution caused by other ways of retting.

Tank retting

Tank retting is water retting carried out in specially built tanks. Colour from stems and dirt is usually leached out and drained off in the first few hours, resulting in light coloured, non stained fibre. Some of the water is drained off each day and fresh water added so that fermentation can be more carefully controlled. This method requires less time than dew retting, from 2 to 3 days. Stagnant pools of water are used in this method, which sometimes causes over retting. Over retting is responsible for brittle and weak flax fibre. 

Chemical retting

Chemical retting shortens the retting process, but chemicals will affect the strength and colour of flax fibre. Soda ash, oxalic soda and caustic soda in warm water or boiling in dilute H_2­ SO₄ solution are the chemicals used. The bark of the stem is first scraped out and cut into pieces containing different concentrations of NaOH and HCl solution in such a manner, so that they are completely immersed in the solution. The beakers are covered with watch glasses and kept for ten hours. After this each of them are boiled for 30 minutes. The stems are washed with cold water to separate the fibres. Drying is necessary to prevent further fermentation. In chemical retting process, it becomes important to establish the uniformity of the fibre quality from the base to the tip of the plant. 

Double retting

This consists of removing the fibre from the tank when the ret is about half complete, drawing straw in sun and air, beginning the ret again in a few months, and continuing the ret until completed. This results in a very gentle retting action on fibre and hence good quality.

Enzyme retting

In this method, the stalks are immersed in enzyme mixture, all of which have cellulase, pectinase and hemicellulase activities. Though individual enzyme activities vary, all enzymes separate bast fibres from the lignified core and partially from the cuticle near the cut surface of the stem sections. Lignin remains in middle lamellae after enzyme retting and would likely prevent separation of the fibre bundles.

Microbial retting

Certain microbes, which are residents of retting water, penetrate into the cortex and phloem regions of the bark through openings in the epidermis and cut ends of the stems, and attack the interconnecting tissues between the filaments. As the retting proceeds, the undesired tissues and intercellular binding substances are gradually degraded into water soluble organic compounds and eventually consumed by the microbes. The reeds that loosen up during this period are mechanically peeled off, washed with water and finally dried in the sun. The chemical constituents that are removed from the bark during the retting process are free acids, small carbohydrate appendages, and biopolymers like pectins, proteins and gummy substances.      

Flash hydrolysis

Flash hydrolysis process maintains and even refines the fibre quality. This process comes from a technique used in the paper industry and it produces a finer fibre with a lower content of non cellulosic material, a better fineness and good crystalline shade. The extraction process consists in subjecting the non-retted flax to a water vapour treatment at about 200⁰C for a specified time, followed up with a sudden expansion at atmospheric pressure. This treatment aims at hydrolyzing the cements of joint lamella which combines the elementary fibres in technical or vascular bundles. A washing at high temperature (60⁰C) allows to remove the hydrolyzed compounds. The material is then thoroughly washed, squeezed, opened and dried. After drying the material is opened mechanically to obtain a presentation adapted to the blow equipment. The fibre produced in this method has a better fastness, a greater cellulosic purity and a greater productivity of spinning.

Other methods of retting are chemical desiccation of standing crop, bacteriological, chemical enzymatic and hydro-thermo chemical retting processes.     

Breaking, scutching and hackling

The fibre is separated from retted stalks either by hand or by breaking and scutching with machinery. The scutching is defined as beating and scraping of the dried stems to separate the fibres. Scutching is done with a classical scutching machine which gives separately scutched flax and tows. Scutching process frees the bast bundles by removing the bark and woody portions of the stalk, sometimes termed shive or boon. Method and equipment for scutching vary, depending upon customs and mechanization available in an area. It consists of feeding the dried, retted stems to pairs of fluted rolls, which partially crush and break the woody core or boon of the stem. In the subsequent beating treatment, this fragmented centre is shaken loose from the bark. Subsequent to scutching the first stage of fibre preparation is a combing process known as hackling. The hackling machine takes lengths of scutched flax and combs them between hank's successively finer pins. The product of the hackling process is called line fibre. Short fibres and fragments of fibres separated during the scutching and hackling process are recovered and marketed as scutch tow and machine tow respectively. The scutched long fibre is about 20-30 inches with an average diameter of 0.009 inches. The average length of the cleaned fibre is sorted out into grades according to quality, length of fibre and colour.