Hemp is actually a strain of the Cannabis Sativa plant. But unlike its close cousin, it is nowhere near as high in THC (tetrahydrocannabinol) and so it does not have the same psychoactive effects. Hemp fabric is often described as one of the most sustainable of textile fibres and does have a range of eco-friendly credentials. As you will discover in this article, it has a number of beneficial properties and a wide range of uses. There is much to recommend it for use in a range of textiles and in other applications. Not only can hemp be used to make textiles and clothes, but it can also be used to make biodegradable plastics, paint, insulation, food, animal feed, and sustainable buildings (1). So, let’s learn a little more about this fascinating fibre.
How and Why Hemp Fabric is Used in Weighted Blankets
Hemp can be used in either the outer or inner, insulative layer of weighted blankets. It is sometimes blended with other materials to create an outer cover with a softer and more pleasing skin feel – lyocell is sometimes blended with hemp for this purpose. It can also serve as an inner layer which will help keep in the heat while allowing for good breathability, and when used in this way, it can also be useful for its moisture absorbing properties.
Strong and tough, hemp is a material that can last, which can make it a sustainable choice. What is more, once it finally reaches the end of its useful life, it is biodegradable, and so will not contribute to the global waste problem. As you will learn below, hemp also has a range of beneficial, sustainable characteristics with regard to how it is grown and processed, and so can be an excellent choice for those who want to limit their negative impact on people and planet.
The Hemp Plant
Cannabis sativa is an annual, herbaceous flowering plant. Originally indigenous to eastern Asia, it is now widely cultivated in various regions across the world. The species was first classified by Carl Linnaeus in 1753. Hemp, used for industrial fibre, belongs to one of three main cultivar groups of Cannabis Sativa that are grown today. Hemp, Cannabis sativa cultivated primarily for its fibre, is characterised by long stems, and little branching. It differs from the cultivar groups which are grown for seed, or for medicinal or recreational purposes.
One of the ways in which hemp differs from other cultivar groups of this plant is that it has been specifically bred to produce minimal levels of THC and higher levels of cannabidiol (CBD) and to maximise the fibre production. Cannabis sativa L. subsp. Sativa var. sativa is the variety grown for industrial use. Fibre and seeds of this plant generally have less than 0.3% THC. The legality of industrial hemp varies from country to country and some governments will only allow the agricultural production of this plant with an especially low THC content (2).
Hemp is usually planted between March and May in the northern hemisphere and between September and November in the southern hemisphere and will be ready for harvesting in approximately three to four months (3). It is very fast growing and is planted closely, and so hemp can be a sustainable choice with regard to land use and the maximisation of yield over a given area.
The Structure of Hemp Fibre
A stem of hemp grown for fibre (at higher sowing densities) does not branch and has about 10–13 mm diameter. At low sowing densities, on seed plantations and especially in the case of single plants, the stem strongly branches. The height of plants varies from 150–200 cm to 400 cm and sometimes more.
The anatomy of hemp stems is characteristic of this species. It consists of a cortex, collenchymas, a ring of primary bast and rings of secondary bast and wood. A layer of bast in hemp, which becomes fibre after proper processing, lies directly under the cortex. It is arranged in bundles of tidily adhering cells of bast glued with pectin and joint with anastomoses. The fibre is not distributed equally along the length of the stem and the highest amount of fibre is found in the stem’s middle portion.
Hemp fibres are made up of primary and secondary single fibres and usually take a pericyclic form. The primary fibres are formed during the early growth stage. They are about 20 mm long and have cell wall thickness of 7–13 mm. The secondary fibres are smaller in dimension. They are about 2 mm long and have a cell wall thickness of 3–6 mm (4).
The quality and precise structure of the hemp fibre is dependent on a number of environmental and growth-related factors, as well as on the individual cultivar that is used.
The Physical Properties of Hemp Fibre
Understanding the physical properties of fibre can help us to understand its benefits in a range of different applications. In this section of this article, therefore, we will take a brief look at some of the key properties and characteristics of this bast fibre.
Hemp fibre is composed of cellulose, pectin, and waxes. Pectin is found in the middle lamella of the cells. It glues the elementary fibres to form bundles. Lignin encrusts the cellulose and contributes to the hardness and breakability of the fibres. The content of additional substances (other than cellulose) in hemp fibre is much higher in hemp than in cotton. This means that hemp requires different processing. The higher the cellulose content of hemp fibre, the better quality the fibre will be.
The Fibre Density of Hemp
Hemp fibres range from 2-90mm in length, with an average length of 15mm. The diameter of the fibre is around 10-30 μm – the thickness of the fibre is, on average, 3460 nm as a metrical number (5). The average density of hemp fibre has been found to be in the region of 0.86 g/cm3. While the density of the fibre can vary significantly, this property can contribute some of the beneficial properties that make hemp a good choice for a range of textiles applications.
The Thermal Properties of Hemp
The thermal properties of hemp will depend on whether we are talking about non-woven or woven uses for the fibre. When used as a woven fabric, hemp shares characteristics with other bast fibres, with relatively good thermal retention but also excellent breathability.
It is when we are talking about non-woven applications, however, that hemp fibre’s beneficial thermal properties really come to the fore. Raw hemp fibre is coming to the fore as a sustainable insulative material for green construction. It can also have application as insulation in bedding or clothing. Its excellence as an insulation material is due to the low thermal conductivity coefficient of the fibre.
Water Absorption and Retention in Hemp
While it has a lower moisture absorbance than wool, hemp’s recommended allowance or commercial regain is higher than that of cotton, at 12%. This reflects the fibre’s ability to absorb water. Its absorption regains at 65% relative humidity at 20 degrees C; it is comparable to that of cotton. As with other natural fibres, hemp’s ability to retain moisture, along with its breathability, can make it useful in bedding and clothing, as it will naturally wick moisture away from the skin, which can be beneficial for human health.
The Mechanical Properties of Hemp
Throughout history, hemp has been prized for its strength and hard-wearing properties. Naturally, the fibre is strong and sturdy, without much stretch.
In all natural cellulose fibres, the molecules are highly oriented parallel to one another in fibrils, but they spiral around the fibre, thus reducing the degree of orientation parallel to the fibre axis. In hemp and other bast fibres, the spiral angle is small, less than 6°, so that these fibres are highly oriented and give high strength and low extensibility (6). The tenacity (N/Tex) of hemp fibre is 0.47.
The tensile strength of the single hemp fibre is in the range of 340–527 Mpa (7). However, hemp fibres can vary considerably in the tensile strength and other mechanical properties due to the variability in growing conditions and other factors. Different varieties of hemp also differ considerably in these regards. Retting period length and other processing factors also play a role in determining the average fibre strength and characteristic strength of hemp fibres.
Biodegradability
As a natural fibre, hemp will biodegrade in the natural environment. Raw hemp fibre will naturally break down in a compost heap, and so will not contribute to the global waste crisis at the end of its useful life. This is one of the reasons why hemp is viewed as a sustainable choice for textiles as well as for a wide range of other applications.
One other interesting thing about hemp is that it can be used not only in woven and non-woven textiles but also to make bioplastics. Due to their high cellulose content, these fibres can be used to create plastics that will fully compost and break down over time. This, along with hemp’s other sustainable characteristics – which you will find out more about below – means that hemp has important applications within the bioplastics industry and an important place within the sustainable materials world.
The History of Hemp
Hemp has been cultivated for food and fibre for over 10,000 years. Earliest references to its cultivation date back to ancient China and Mesopotamia and it may have been one of the earliest plants to be cultivated on planet earth. Hemp was certainly used as far back as the Neolithic Age in China, where hemp fibre imprints can be seen on Yangshao culture pottery which dates from the 5th Millennium BCE.
Interestingly, though hemp cultivation for fibre was clearly well-known in Neolithic East Asia, it did not spread west into Europe and the West until relatively late – in the Iron Age. That was in spite of the fact that the plant grew and most certainly was known across northern latitudes of Eurasia. Likely, the reason why hemp rose to prominence as a cultigen and spread west rapidly in the 1st millennium BCE was the habit of pot-smoking that originated in south-central Asia, where the drug-bearing variety of the plant occurred (8).
Though its spread was likely through the use of the drug-bearing variety, in Europe, cultivation for fibre predominated during the Middle Ages and the early modern period. While it was also used in cooking in Germany and Italy, for example, its main use was as a textile fibre. It was especially used to make ropes for ships. The ropes had to be tarred, and this led to the nickname ‘Jack Tar’ given to mariners. Hemp was used to make the ropes for the ships of Christopher Columbus, and many other explorers, traders, and Imperialists. Hemp was also commonly used to make the canvas for sails. The word ‘canvas’ is actually derived from the word cannabis.
Plain hemp, a line-like fabric, was also used a domestic cloth. Though its use was largely in more rural areas in the countryside, as those who lived in cities and towns often had access to finer fabrics for clothing and bedding and other domestic textile uses.
The Spanish brought hemp to the Americas and cultivated it in Chile from around 1545 (9). They also attempted to grow it in Peru, Colombia, and Mexico, though these attempts were not successful. In the 16th Century, European settlers reported the use of hemp being cultivated and used as clothing by native Americans in the Cape Cod, Plymouth Bay and Richmond, Virginia areas. Wild hemp, ‘better than that in England, was perceived to grow along the banks of the Upper Potomac River. As early as 1619, the House of Burgesses in Virginia passed an act that required all planters in Virginia to sow both English and Indian hemp on their plantations. Puritans are first known to have cultivated hemp in New England in 1645.
George Washington pushed for the growth of hemp as a cash crop to make rope and fabric. He noted details of the sowing and harvest of hemp in his diary in 1765. Several other early Presidents also farmed hemp. Hemp became an important cash crop in North America, especially in Kentucky, where it made up a significant proportion of the states antebellum economy. Before the American Civil War, many slaves worked on plantations producing hemp (10).
The hemp industry in the United States was destroyed by the passing of the Marihuana Tax Act of 1937. This is rumoured to have involved businessmen Andrew Mellon, Randolph Hearst and the Du Pont family. One interesting (though unconfirmed) claim is that Hearst believed that his extensive timber holdings were threatened by the invention of the decorticator, which he feared would lead to hemp being used as a cheaper substitute for the paper pulp used for the newspaper. However, if he did fear this, his fears were unfounded, as these machines (which separated hemp fibres from hemp stems) could not actually make hemp a cheaper substitute than other fibre sources, and, what is more, decorticators were shown not to work well in commercial production (11).
Another possibility is that Mellon, the wealthiest man in America at that time, having invested heavily in Du Pont’s new synthetic fibre – nylon – viewed it essential to the product’s success that it replace the traditional resource of hemp fibre. However, Du Pont company, many historians, dispute a link between the two fibres.
Though the tax act had only recently been signed into law, the tax on hemp was lifted during WWII. The trade route the jute and manila used to make cordage for US ships was cut off, and so the United States had to turn inwards and revitalise the cultivation of hemp domestically for this purpose. Hemp was used extensively during the war, not only to make rope but also to make canvas and uniforms for the military. So important was hemp during this period that a short film ‘Hemp for Victory’ was made in 1942. US farmers succeeded in growing 20 times more hemp than they had before the war effort (12).
Though in Europe, hemp cultivation was not banned, by the 1930s, production here had also fallen dramatically. This was largely due to the increasing popularity of synthetic fibres. In the early 1940s, Russia was the biggest producer. Over the rest of the 20th Century, world hemp production continued to fall. It fell from over 300,000 metric tons in 1961 to around 75,000 metric tons in the 1990s and has after than remained stable at that level (13). In recent years, because of the search for alternative renewable resources in Europe, hemp is again of interest and enjoying something of a resurgence as interest in sustainable textile fibres grows.
How Hemp is Grown Today
China is the world’s leading hemp producer. They produce more than 70% of the global output of this fibre. France is responsible for around 25% of world production. Smaller hemp industries also survive in the rest of Europe (including the UK and Germany, which resumed commercial hemp production in the 1990s), Chile and North Korea. Over 30 countries produce this fibre on a commercial basis.
The Food and Agriculture Organisation of the United Nations argue that an optimum yield of hemp fibre is more than 2 tonnes per hectare, though average yields are around 650kg/ hectare (14).
Profitable hemp farming requires particularly deep, humus-rich, fertile soil with controlled water flow. It is a crop which can withstand cold temperatures, and which is hardy until around -5 degrees C. However, it can fail to thrive on waterlogged, compressed or extremely light and free-draining soils, which can affect the early development of the plant; hemp is best not grown above 400m above sea level. While hemp can withstand frost in the winter, warm summer temperatures are required for the plants to come to maturation.
One of the benefits of hemp as a sustainable textile fibre is that it takes far less water to grow than conventional cotton, around 1/14th of the amount. It is also a low maintenance crop requiring low inputs, including agro-chemicals, during the growing season. Hemp grows rapidly, faster than weeds, and it has to date not been plagued by pests.
Interestingly, hemp can also benefit crops that are grown after it, as it is good at suppressing weeds and loosening the soil with its large root system. It has a beneficial effect on soil tilth thereby reducing the disruption of the soil ecosystem through extensive tilling. Often, it is grown before winter cereal crops. Not only will it improve the soil in which it is grown, but hemp will also return 60-70% of the nutrients it takes from the soil, thereby further benefiting plants grown thereafter in the same location.
What is more, since it is grown densely spaced, hemp requires a relatively small amount of land to cultivate. It can produce up to double the fibre yield per hectare of cotton. It can also absorb more carbon dioxide than trees whilst in growth and produces more fibre per acre than trees.
Another interesting and beneficial feature of hemp is that it can be grown on land polluted with heavy metals, and can extract and accumulate substantial amounts of elements such as copper, lead, zinc, and cadmium with no detrimental effect on the quantity and quality of the crop (15).
The Hemp Harvest
Small-scale hemp plantations are usually harvested by hand, though mechanical harvesting is now common, especially on larger farms, and makes use of adapted cutter-binders or simpler mechanical cutters. Hand-harvesting does, of course, reduce the carbon cost of hemp. But this human-labor intensive process is a barrier to the widespread proliferation of this useful fabric and is likely to be one of the reasons why the use of hemp in the textiles industry is not more prevalent.
While it can be labour intensive, however, it is worth noting that hemp farmers can often make a lot more from growing hemp than they could do from growing grain or cotton or other crops. This can mean that growing and harvesting hemp can provide benefits not just for the environment, but also for people in a more immediate way.
Hemp is harvested after flowering but before the seeds set (the fibre content is reduced and becomes coarser toward seed formation). After it is taken from the field, hemp is then retted. Retting is the process whereby naturally occurring bacteria and fungi, or chemicals, break down the pectins that bind the hemp fibres to be released. Retting usually involves immersing the stems in water or leaving them to be wet by dew for 3-6 weeks. The stems are then broken and beaten in a process known as scotching, which allows the fibre to be separated from the woody core. After this, fibres are ‘hackled’ (combed) to remove any woody particles that remain and to align the fibres. Once they are hackled, fibres are now roved, or wound onto bobbins – this twisting and drawing out process improves strength. Finally, the fibres are spun. Wet spinning results in a finer yarn, while dry spinning creates a coarser one (16).
How eco-friendly the process of creating the yarn will largely depend on where in the world the hemp is being processed. China, where, as mentioned above, most of the world’s hemp is produced uses chemical methods for processing. These chemicals can sometimes pose a threat to workers, or leach into the surrounding environment, polluting land and waterways. Producers in Europe, however, have mostly moved to a clear, greener processing method which uses biologically-based enzyme technology.
Hemp Fabric Manufacture
As mentioned above, hemp has a huge range of different applications – even within the textiles industry. It can be used, either alone or in combination with other natural or semi-natural materials, to create a wide range of different woven and non-woven textiles. Hemp/silk, hemp/cotton, and hemp/lyocell options are all now becoming more prevalent.
Of course, the processes involved in the creation of hemp fabrics will depend on which type of fabric is being created, and for what purpose. Hemp is naturally coarser than cotton and other natural fibres like silk, and so is often blended with materials such as these, which have somewhat less pleasing sustainability records. The good news is that modern innovations utilising enzyme processes have led to the creation of softer, whiter hemp fabrics such as a ‘Canadian cotton’ product known as ‘Crailar’.
Eco-friendly fabrics which no only utilise hemp can also often involve the use of low-impact, closed-loop lyocell, or organic cotton, alongside hemp, to create fabrics that are kinder to the planet.
Standards & Certifications in the Hemp Industry
Hemp accounts for only a very small proportion of the textiles industry. Yet it offers disproportionate benefits in terms of the environment and sustainability. For this reason, choosing hemp textiles could be an excellent choice for those looking to go greener. Nonetheless, it is important to recognise that even this sustainable material does not come without its problems.
As mentioned above, hemp made in China is often processed with the use of chemicals during the retting stage. This can have all sorts of negative effects on people and the environment. For this reason, if you are considering hemp products, it is important to know where the hemp has come from. Looking for certifications and standards such as those of Oekotex can also help you determine whether any harmful chemicals have been used during the life-cycle of the product.
Since hemp is largely unbothered by pests, most hemp production is already organic in nature. However, if the hemp is used alongside other natural materials such as cotton, it is a good idea to make sure that the cotton in the product is 100% organic. The Global organic standard is a mark that will tell you that the product is organic.
Another thing to bear in mind when considering the sustainability profile of a given textile is how far it has had to travel to reach you. In certain areas, hemp may be a local product. However, where it is not grown close by, the carbon cost of global distribution may largely outweigh the environmental benefits. So this is something else to take into account.
Overall, however, hemp has great promise as a product for the textiles industry, and other industries moving forward and could be a good choice for a range of domestic and commercial products.
References
(1)Johnson, Renée (22 March 2019). Defining Hemp: A Fact Sheet (PDF). Washington, DC: Congressional Research Service.
(2)Crime, United Nations Office on Drugs and (2009).Recommended Methods for the Identification and Analysis of Cannabis and Cannabis Products: Manual for Use by National Drug Testing Laboratories. United Nations Publications. p.12.
(3)Hall, J.; Bhattarai, S. P.; Midmore, D. J. (2013). “The effects of different sowing times on maturity rates, biomass, and plant growth of industrial fiber hemp”.Journal of Natural Fibers. 10: 40–50.
(4)Sankari H (2000) Towards bast fibre production in finland: stem and fibre yields and mechanical fibre properties of selected fibre hemp and linseed genotypes, Academic Dissertation, Faculty of Agriculture and Forestry of the University of Helsinki.
(5)’Biodegradable and Sustainable Fibres’, Textiles Institute, Woodhead Publishing.
(6)’Physical Properties of Textile Fibres’, Textiles Institute, Woodhead Publishing.
(7)Lilholt H, Lawther J M (2000) ‘Nature of organic fibres,’ in Kelly A and Zweben C, Comprehensive Composites Materials,Elsevier Inc, Amsterdam.
(8) Barber, E. J. W. (1992). Prehistoric Textiles: The Development of Cloth in the Neolithic and Bronze Ages with Special Reference to the Aegean. Princeton University Press
(9)Daryl T. Ehrensing (May 1998).“Feasibility of Industrial Hemp Production in the United States Pacific Northwest, SB681”.
(10)James F. Hopkins, “Slavery in the Hemp Industry”
(11)Dewey LH (1943). Fiber production in the western hemisphere. United States Printing Office, Washington. p. 67
(12)USDA (1942). Hemp for Victory
(13) Lynn Robins; et al. (July 2013).“Economic Considerations for Growing Industrial Hemp:Implications for Kentucky’s Farmers and Agricultural Economy”(PDF). Department of Agricultural Economics, University of Kentucky.
(14)Graeme Thomas. “Natural Fibers: Hemp Food and Agriculture Organization of the United Nations, 2009
(15)Grzebisz, W., Chudzinski, B., Diatta J.B. and Barlóg, P. Phytoremediation of Soils Contaminated by Copper Smelter Activity. Part II. Usefulness of Non-Consumable Crops. Natural Fibres-Wlókna Naturalne, Special Edition, 118–122, Flax and Other Bast Plants Symposium 30.09–01.10.1997. Poznan.
(16)’Ecological Footprint and Water Analysis of Cotton, Hemp & Polyester’, SEI Report, 2005
