In this section, the main techniques used in the extraction of the natural fibers from plants will be discussed.
Retting
Retting is a controlled rotting biological process used to extract fibers. In this process, enzymes and bacteria are used for the degradation of gummy (pectineus) substances surrounding fiber bundles as shown in Figure 1. These substances that glue fibers together are found in the woody stem of the plant. They are removed to breakdown the chemical bonds holding fibers. Then, the bundles can be separated from the non-fibrous matter. Exposing the harvested stalks to alternate periods of high humidity and dryness can make the process works better. This enhances the enzymatic activity with minimal stalk decomposition.
The Under-retting which means that it is difficult to separate the fiber bundle from the wood and the Over-retting which results in stalk decomposition by microorganisms, reduced fiber yield and weakened fiber. The Over-retting can be caused by the excessive rainfall.
Dew or Field Retting
It is the most common microbiological retting process, in which the cut or pulled up stems are left on the field as shown in Figure 2 to rot without deterioration in the quality of the fiber. The biological process takes place in the field under the action of sun, air and dew present in both, soil and plant. The microbial enzymes break down the adhesive substance that binds the fibers and woody cores together. Moisture is needed for the microbial breakdown to occur. The duration of this process depends on the weather conditions. This action also may not be uniform which can cause the inconsistency in the fiber quality. The process can take 3–6 weeks and the straw is often turned 2–3 times during this period. The changes in environmental conditions can speed up this process: high humidity and low temperatures during the night and higher temperatures and drier conditions during the day. In General, dew-retted fibers have darker color and poorer quality than fibers which are water-retted.
Water or Microbial Retting
It is an alternative microbiological process, where the bundles of stalks are left in ponds, bogs, tanks, rivers or slow streams and acted upon by bacteria as shown in Figure 3. The bacteria digest the gummy material around the bark’s fiber strands (Bast fibers). Slow-moving water penetrates to the central stalk portion, swells the inner cell and bursts the outermost layer.
Water retting has two variants: tank retting and double retting. Tank retting employs concrete vats and allows great control with uniform quality. It takes 4–6 days and can be implemented in any season. It starts with the leaching period in 6–8 hrs., a lot of dirt and coloring matter is removed by water which is usually changed to keep a clean fiber. The wasted water by retting contains harmful and toxic elements and requires treatment, but it can be used as liquid fertilizer since it is rich with chemicals. Whereas double retting produces excellent fiber but retting is done twice. During the first retting, stalks are removed earlier from water. They are dried 7 months and are retted again.
Chemical or Enzymatic Retting
In this process, chemicals are used to dissolve bonds between fibers. The harvested straws are boiled by placing them in temperature-controlled tanks. They are treated with chemicals or enzymes that have ability to break the pectin very quickly, but deterioration must be prevented by careful control. Chemical methods are expensive, difficult to control and tend to damage the fibers.