Cottonization: Making Hemp and Flax Fibers Into The Better Cotton [Hackaday]

June 20, 2023 Dashia Milden

These days it’s hard to imagine that fabrics were ever made out of anything other than cotton or synthetic fibers, yet it wasn’t too long ago that hemp and flax-based fabrics — linen — were the rule rather than the exception. Cotton production has for centuries had the major disadvantages of requiring a lot of water and pesticides, and harvesting the cotton was very labor-intensive, making cotton rather expensive. In order to make separating the cotton fibers from the seed easier, improved versions of the cotton gin (‘cotton engine’) were developed, with the 19th century’s industrial revolution enabling a fully automated version.

What makes cotton attractive is the ease of processing these fibers, which are part of the seed pod. These fibers are 25 mm – 60 mm long, 12 μm – 45 μm fine fibers that can be pulled off the seeds and spun into yarn or whatever else is needed for the final product, much like wool. Hemp and flax fibers, in contrast, are extracted from the plant stem in the form of bast fibers. Rather being pure cellulose, these fibers are mostly a mixture of cellulose, lignin, hemicellulose and pectin, which provide the plant with rigidity, but also makes these fibers coarse and stiff.

The main purpose of cottonization is to remove as much of these non-cellulosic components as possible, leaving mostly pure cellulose fibers that not only match the handleability of cotton fibers, but are also generally more durable. Yet cottonization used to be a long and tedious process, which made bast fiber-based textiles expensive. Fortunately, the steam explosion cottonization method that we’ll be looking at here may be one of the methods by which the market will be blown open for these green and durable fibers.

King Cotton?

Flax stem cross-section:
1. pith
2. protoxylem
3. xylem II
4. phloem I
5. Sclerenchyma (bast fibre)
6. cortex
7. epidermis

The stem of hemp, flax and similar plants consists of a number of structures, each possessing properties that combine to provide the plant with the ability to transport water and nutrients, as well as the rigidity to stay upright. This stability is a function of the sclerenchyma, the layer, marked as 5 in the image, that we’re interested in when we want to extract fibers for use in textiles and more. To get at the bast fiber, cellulose must be separated from the other plant constituents with as little extra effort as possible.

The traditional way to separate these bast fibers from the rest of the stem is using a process called retting. During this process the already cut stem is exposed to moisture and micro-organisms that swell up and degrade much of the internal structure. After drying, the now brittle stems are broken, followed by separation of the woody fragments (shives) and mostly intact fibers.

In a 2017 paper by Thibaud Sauvageon and colleagues in Textile Research Journal titled Toward the cottonization of hemp fibers by steam explosion: Defibration and morphological characterization, and a 2020 paper by Maria Moussa et al. in Industrial Crops and Products titled Toward the cottonization of hemp fibers by steam explosion. Flame-retardant fibers, steam explosion (SE) is presented as a viable route to large-scale commercialization, with SE also increasing the thermal stability of the resulting hemp fiber.

A 2023 review paper by K. Palanikumar et al. titled Targeted Pre-Treatment of Hemp Fibers and the Effect on Mechanical Properties of Polymer Composites in the journal Fibers compares the different bast fiber treatments. These include mechanical, chemical and thermal means, with SE providing the best balance between resulting fiber quality, economics and process complexity. Retting and mechanical separation is imprecise and can lead to significant defects in the produced fibers, while chemical treatments that remove the non-cellulosic components do work, but require the use of strong chemicals like alkali or sodium sulfate that produce undesirable waste products which require further processing before disposal.

Chemical composition of (A) hemp fiber raw material in wt.% and the chemical composition after wet oxidation and hydrothermal treatment, (B) steam-exploded hemp fibers (with and without impregnation) and untreated fibers in wt.%.

Steam explosion treatment of bast fibers requires as input only a source of power and high-pressure equipment: the material is first impregnated with water before being loaded into the SE vessel where it is heated to nearly 200 °C for a few minutes. Following this the pressure in the vessel is suddenly dropped, causing a steam explosion within the saturated fibers. The resulting separated fibers can then be separated and dried before further processing.

A big advantage of this approach is that it uses relatively little water, no harsh chemicals or expensive enzymes, produces high-quality fibers of lengths that are comparable to cotton fibers, and lends itself well to scaling up to industrial levels with modest equipment and consumable requirements. The current trend in the industry would seem to suggest that this is the approach that we’re likely to see pursued in the future, and which may make cottonized hemp and flax fibers serious competition for the cotton industry for the first time since the 19th century.

But There’s More

Although the sturdy nature of plants like hemp, their fast growing rate and their modest water demands should already make them favorites, another convenient feature of hemp is that it can grow on even on heavily contaminated soils, with no significant impact on growing rate, plant height or quality of the fibers, providing a valuable phytoremediation service for e.g. former mining sites, or arsenic-laced former cotton fields. Although this sounds like it may render the plant matter too contaminated for use, the metals do not become part of the fiber or seeds.

A recent study by Rabab Husain and colleagues (PLoS One, 2019) investigated the response of Cannabis sativa L. to being grown in soil obtained from heavily contaminated former coal mines in Pennsylvania. They found that the heavy metals absorbed by the plants collected in the leaves, with nickel, lead and cadmium being most prevalent in the samples examined, while arsenic and mercury in the soil didn’t get absorbed in significant amounts. This makes growing hemp on these soils an attractive option, along with other hyperaccumulators that can take care of arsenic, without compromising hemp grown in this soils for textile production.

Will we truly say farewell to cotton before long? That’s still very much an open question, but with these recent advances in processing bast fibers, it would seem at least reasonable to state that King Cotton’s reign at the very least is facing tumultuous seas ahead.

If you’re feeling adventurous and wish to experiment with steam explosion treatment yourself, you can give something like a popcorn cannon a try. Although these devices are mostly used for explosive food preparation, they could be convinced to ‘pop’ some bast fibers as well, if one were so inclined.

King Cotton?

But There’s More

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