1. Degumming and scouring and bleaching of hemp fiber

The hemp fibers used for clothing are mainly ramie, flax and jute, among which ramie and flax are of good quality. Ramie and flax can be purely spun into hemp fabrics. After the fabrics are made into garments, they will wear crisply, absorb moisture and dissipate heat quickly, and are good fabrics for summer clothing.

After the hemp fiber is harvested, hemp bark is stripped from the hemp stalk, and the epidermis is scraped from the hemp bark to obtain the bast of hemp. After drying, it becomes the raw material of the hemp textile factory, which is called raw hemp. Raw hemp contains a lot of impurities, of which polysaccharide gum-like substances are the main ones. Most of these gum-like substances surround the surface of the fibers, and the fibers are glued together to form a solid sheet. Before spinning, the gum in the bast must be removed, and the single fibers of the hemp must be separated from each other. This process is called degumming.

The gum in hemp fiber is basically an amorphous substance, which can be hydrolyzed under the action of alkali, inorganic acid and oxidant. The degumming method is generally "chemical degumming" or "biological enzymatic degumming". Ramie and flax have different specific methods for degumming because of the different gums contained in the fiber.

The process of chemical degumming of ramie is:

Tie the handle—→soak acid—→wash—→high-pressure secondary scouring—→wash—→fibrillate—→pickling—→wash (—→bleach—→refining)

Tie is to tie hemp bundles of similar quality into 0.5-1.0kg small handles to prepare for cooking.

Fibrillation, also known as ramming, is to use mechanical hammer and water spray to remove the colloid that has been destroyed by lye from the surface of the fiber, making the fiber loose and soft.

Pickling is to use 1-2g/L sulfuric acid to neutralize the residual glue on the fiber and other colored substances, so that the fiber is further loosened and white.

The process of chemical degumming of flax is:

Original flax stems—→selection of stems and bundles—→dipping—→drying—→crushing stems—→beating hemp—→beating into hemp

The impregnation in the process is commonly known as retting, or degumming. It uses caustic soda or microorganisms to destroy the gum in the flax fiber.

The scouring and bleaching of hemp fabrics is basically similar to that of cotton fabrics, but the process conditions are different, especially the mercerizing of ramie fabrics is semi-mercerized. Because ramie has a high degree of crystallinity and orientation, and its ability to absorb dyes is much lower than that of cotton, semi-mererization can significantly increase the ability of fiber to absorb dyes, thereby increasing the dye uptake rate. If conventional mercerizing is carried out, the permeability of ramie is greatly improved, and the dye can easily penetrate into the fiber, which reduces the apparent color of the ramie fabric, and the strength of the fabric decreases, and the hand feels rough and hard.

The waste water produced by the degumming of hemp fabric is seriously polluted, and the content of organic pollutants is relatively high.

2. Dyeing of linen fabric

Since hemp and cotton are both cellulose fibers, the dyeing process of hemp fabrics and the dyes and auxiliaries used are basically the same as those of cotton fabrics. However, due to the high crystallinity and orientation of the hemp fiber, the dye is difficult to penetrate into the fiber, and the coloring rate is poor, so the dyed products are mostly light-colored, and the color is slightly lower than that of cotton fabric dyeing wastewater.

Degumming wastewater and printing and dyeing wastewater are generated during the processing of hemp textiles. Among them, the degumming wastewater is high-concentration organic wastewater, which is relatively easy to biodegrade. Printing and dyeing wastewater is similar to cotton fabric.