Material of the month – hemp

Materials World magazine
,
2 Aug 2016

From nanotechnology to automotive components and textiles – hemp has a number of diverse applications. This month, Anna Ploszajski discovers the history behind the fibre. 

Hemp is a member of the Cannabis Sativa plant family, and is one of the earliest known plants grown for agricultural purposes by humans. Its cultivation for the fibrous stem dates back to the end of the Stone Age, in China. The ancient Chinese separated the long bast fibres from the outer portion of the tall stem to make ropes, and ground down the short woody ones on the inside, called hurds, to make pulp and an early form of paper. They would weave the fibres into textiles to make clothes – a practice that did not develop in the West until the Iron Age.

It is well known as a hardy plant and can be grown on a wide variety of soil types and in different climates. Archaeological evidence of hemp products has been found across the globe. By the Middle Ages in Europe, the development of trade meant that hemp textiles remained in the rural setting, whereas those in the towns and cities had access to higher quality textiles. 

Hemp remained an integral material in the following centuries, allowing international trade across the seas – many sailing ships prior to the mid-1800s were rigged with hemp sails and ropes. Although the bast fibres are strong and tough, the ropes had to be protected from the elements by tarring, since they had a propensity to rot. The capillary effect of the woven fibres in hemp rope tends to hold liquid at the interior, yet the rope appears dry on the outside so, without tarring, rotting ropes could go undetected. Tarring could be a messy business, and earned sailors the nickname Jack Tar – they were known to use it to waterproof their clothes, too. Hemp ropes were largely phased out by the introduction of manila, a fibre from the abacá tree, a relative of the banana, which didn’t require tarring, though there is still a niche market for hemp ropes. 

Hemp for victory! 

As a cheap, quick-to-produce material with a similar texture to linen, textiles made from hemp fibre were used extensively during the Second World War to make anything from uniforms to canvases – the word ‘canvas’ likely derives from the name of the parent plant, cannabis. In 1942, the US Department of Agriculture released a video called “Hemp for Victory!” which incentivised farmers to grow hemp and help the war effort, most of which was done in the American states of Kentucky and Missouri. 

After the war, because of the explosion of the synthetic textiles industry and legislation against the growth of cannabis for recreational drug use, hemp materials fell out of fashion. The global production of up to 350,000 tonnes a year in the early 1940s decreased to 75,000 tonnes by the 1990s, and has remained at roughly this level ever since. Hemp’s unfortunate reputation is perhaps unfair and unfounded. Its cousin, the marijuana plant, also comes from the Cannabis Sativa family, but contains much higher levels of tetrahydrocannabinol (THC), the psychoactive substance enjoyed by recreational drug users. Hemp plants used for industrial applications have been selectively bred to typically contain less than 0.2% THC.  

France is the largest producer of hemp, accounting for 60% of global output in 2014. China follows with 18%, and countries, such as Chile, Ukraine and Russia largely make up the remainder. The strict legislation in many parts of the world against growing certain cannabis plants, together with renewed interest in applications for industrial hemp, has led to renewed research to develop new varieties with increased fibre quality, yields and low THC content. 

Industrial hemp is planted densely to encourage tall, slender growth for longer fibres. Its fast, competitive, crowded growth means that weeds can be effectively eliminated in hemp fields, and hemp has long been used as a method of killing tough weeds in farming. Hemp is considered as a useful tool to organic farmers, since they can avoid the use of herbicides, and although it can be vulnerable to various pathogens, diseases don’t have a major impact on yield, so it can be grown without using pesticides. 

Greedy for water and nutrients, hemp also can be used as a mop crop to clear up impurities of wastewater and unwanted chemicals, a process called phytoremediation. Hemp plants have even been experimented with to clean contaminants from the ground at Chernobyl, Russia.

A versatile material 

Just as the ancient Chinese used different parts of the plant for different applications, almost all of the plants, today, can be put to use for various purposes. The long bast fibres are the ones used for textiles, and they can be blended with other organic fibres such as flax, cotton or silk for different texture fabrics. The woody hurds have found use in some novel and exciting composite materials. Hempcrete is a mixture of hemp and lime, which has been developed as an insulating material and moisture regulator for buildings. While it is unsuitable for structural elements, hempcrete is considered an environmentally friendly building material since it partially offsets the carbon footprint of concrete by removing CO2 from the atmosphere before use. Hempcrete has already been used in multiple properties in France, the UK and the USA, and hemp hurds can also be made into panels and plaster to insulate both heat and sound in the built environment.

The automotive industry is always on the lookout for lightweight, natural materials. Audi, BMW, Ford, GM, Chrysler, Honda, Iveco, Lotus, Mercedes, Mitsubishi, Porsche, Saturn, VW and Volvo have all used hemp in their cars by incorporating it into composites similar to fibreglass. Hemp offers a welcome weight reduction in components – BMW shed 10% of the weight from the model i3’s door panels by using a hemp-plastic composite instead of traditional materials. The Lotus Eco Elise contains hemp in the body panels, rear spoiler, and seats, and the Mercedes-Benz C-class contains 20 components made from natural fibres, including hemp. 

It’s not just the stem of the hemp plant that can be used. The seeds are a high-protein food, rich in amino acids, essential fatty acids and dietary minerals such as magnesium, zinc and iron, and are consumed as a health food. They are used to make cereals, dairy-free milk, tofu, protein powder and nut butters. The Hemp Industries Association estimates that the value of hemp-based food supplements and cosmetics in the US totalled US$184 million in 2013. 

Oil from the seeds is used to make oil-based paints, cosmetics and plastics. Hemp could also aid the transition from fossil- to bio-fuels. The combination of the oil from the seeds, the stalks and ethanol from the fermentation from the whole plant can be used to make biodiesel, sometimes referred to as hempoline. Although Henry Ford’s early engines were designed to run on hemp-derived and other biofuels, hempoline hasn’t since been commercialised as a mainstream biofuel.

Bio and nano applications 

Hemp can also be used to make innovative bioplastics. A start-up company called Kanèsis, in Sicily, has developed a material derived from hemp waste which can be 3D-printed, and could be rolled out across other thermoplastic applications. As well as its green roots and recyclability, Kanèsis claims the material is 20% lighter and 30% stronger than PLA – a standard plastic used in 3D printing.

Hemp has even found its way into nanotechnology. In 2013, researchers led by Dr David Mitlin from Clarkson University, USA, published results in ACS Nano describing a capacitor with high surface area electrodes made from a graphene-like carbon nanosheet derived from hemp bast fibre with similar or even better performance than graphene at a fraction of the cost. Hemp can also reduce the cost of cellulose nanoparticle production, which could find uses as diverse as biodegradable plastics, batteries and drug delivery systems. 

There are also potential applications for hemp in the medical sector. Hemp fabrics have been found to be antibacterial and could replace cotton or polyester fabrics in hospitals to help fight infection. 

With apparently no end to the possibilities for this versatile plant, if only hemp hadn’t been tainted by the same brush as its relative marijuana, we could already be seeing many of these exciting applications coming into the mainstream. Attitudes are changing slowly and new pressures surrounding stability and sustainability could see hemp restored to its former glory.