Members of WeaveTech had an interesting debate last year about dyeing bamboo. The following is a précis of the discussions of Laura Fry, Su Butler, Ruth Blau, Diane de Souza, Ingrid Boesel, Sara von Tresckow and others.
Bamboo yarn, according to the National Geographic, is made from an extremely fast growing species of bamboo, which makes it a viable renewable resource for fibre as compared to wood which takes years, not months, to reach harvest. The yarn is a viscose rayon fibre not manufactured by a retting process as is linen. Viscose can be made from any wood or cellulose source as that is what it is – regenerated cellulose fibre. Rayon has been around since the 1920’s. Instead of using cotton or wood pulp from trees, and instead of using chemicals to break down the cellulose into solution, industry is now looking at more sustainable crops (bamboo) and using enzymes which are then filtered out and re-used. Rayon has been considered by hand weavers as a
natural fibre because it is cellulose, not a petroleum product.
Like all living things, plants consist mostly of water and proteins, and are made up of units called cells. Each plant cell is surrounded by a cell wall containing cellulose, a complex carbohydrate. From a chemical point of view, carbohydrates can be thought of as being carbon + water: they contain C, H and O. Proteins, on the other hand, are made up of chains of amino acids which contain carbon, hydrogen, oxygen, and nitrogen. Proteins may also contain sulphur – hence the distinctive smell of the burn test.
The plant stem is made up of phloem and xylem. Phloem is largely made of protein. Phloem is the food-conducting tissue of a plant; mature phloem cells are alive (and located in a layer right under the bark of a woody plant, which is why you can kill a tree by ring barking it.) Protein is dyed best by acid dyes. The main components of phloem are sieve elements and companion cells.
Sieve elements are so-named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis – sugars and amino acids – from the place where they are manufactured (a “source”), eg leaves, to the places (“sinks”) where they are consumed or stored; such as roots, growing tips of stems and leaves, flowers, fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Companion cells move sugars and amino acids into and out of the sieve elements. In
source tissue, such as a leaf, the companion cells use trans-membrane proteins to take up – by active transport – sugars and amino acids from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements by diffusion through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes. In “sink” tissue, the sugars and amino acids leave the sieve tubes by diffusion through plasmadesmata connecting the sieve elements to the cells of their destination. Again, water follows by osmosis where it may leave the plant by transpiration, increase the volume of the cells or move into the xylem for recycling through the plant.
Xylem is made largely of cellulose and lignin. Xylem conducts water and dissolved minerals from the roots to all the other parts of the plant. Mature/functional xylem cells are dead – hollow tubes of cellulose, basically. Cellulose is dyed best by basic dyes.
In woody plants, the older xylem ceases to participate in water transport and simply serves to give strength to the trunk. Wood is xylem. When counting the annual rings of a tree, one is counting rings of xylem. Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lingnin. Perhaps the lignin, which is highly acidic, is why the bamboo breaks down over time. But perhaps that tells us something about the properties of bamboo as well.
If only the cellulose from bamboo is processed, then the bamboo will be cellulose. But if the lignin is processed the resulting crumb will contain proteins.
Lignin in neither protein nor cellulose. It is a polyphenolic compound and these are very active molecules. They could well bind to any number of other chemicals, including dyes. Who knows what the polyphenolics may be dyed with and how well. Cotton has no lignin. The seed fibres are largely cellulose.
Flax plant has both cellulose and fairly large amount of lignin making up the xylem of the stems. Retting rots the fibres so that the cellulose and the lignin are separated. Most of the lignin is washed away, leaving the cellulose fibres used in the manufacture of linen.
In the manufacture of bamboo fibre, if only the cellulose fibres remain as long fibres they could be all cellulose and stiff like linen. If the fibres are mashed to shorten them and treated to get rid of the phenolics they could be soft like cotton. But since we do not know what process they use, we cannot tell which components of the bamboo are removed and which remain. The actual yarn may well be 100% bamboo, but the amount of the different constituents native to the bamboo can give different end product.
The bamboo may be treated in such a way as to leave only protein fibres in the cellulose mash, or it may leave the phenolics in the mash, or both. We would have to do a chemical analysis of each manufacturer’s fibres. It may be more practical to do dye tests and look at type of dye, wash fastness, light fastness and depth of shade and record that with each type of bamboo yarn. This would still not tell what the properties of bamboo fibre are, but we would know how to dye the white yarn on our shelves.
Actually, as we discovered this summer, rayons can be dyed with either acid or fibre reactive dyes, as can the soy protein fibres. They don’t take the dyes in the same way, or to the same depth, but you can dye them with either.
The BambroTex web site about bamboo & dyeing, says:
It is better to use active dyestuff during dyeing process, and the alkali should not be over 20g/litre, the temperature should not over 100deg C. During drying process, low temperature and light tension are applied. This suggests that this company recommends dyeing with fibre reactive dyes in an alkali solution. But apparently acid dyes work, too (maybe all too well!)