Natural composite materials

Materials World magazine
,
1 Feb 2014

Rachel Lawler looks at natural materials finding use in high-performance composites.

Thanks to increasing public awareness of the finite nature of Earth’s resources, demand for alternatives to petroleum and other non-renewable materials is growing. Natural fibres can offer a significant advantage over synthetic materials in terms of biodegradability, cost and reducing your carbon footprint.

From natural fibres woven into synthetic fabrics or used as a replacement for glass and plastic in insulation materials, the possibilities for creating composites using natural materials are almost limitless. However, ensuring that these materials perform as well as their man-made counterparts requires precise specification.

One popular material for natural composites is found in abundance in poor rural areas of India and the Philippines. Often used as lining in flower baskets and garden beds, the bristly fibres of coconut shell, known as coir, and its husk, known as pith, are coming under increasing demand for use in aggregates. Tom de Vesci, Managing Director at UK-based Horticultural Coir Ltd, explains, ‘Coir fibre has extraordinary air porosity and a great ability to absorb water. It has been used extensively for centuries for mattress and upholstery stuffing, ropes, bristles, brushes, carpets and doormats. These uses take advantage of its stability, its high tensile strength and its springiness.’ Its properties also make the material ideal for use in a range of composites. Coir can be used in flooring tiles and reconstituted wood panels that are well suited to use in construction, thanks to its strength. Its breathability also makes it ideal for housing insulation. Coir production is sustainable, but some natural materials have been criticised for causing more environmental damage than they prevent. De Vesci says, ‘Palm oil production has been cited as being responsible for destruction of jungle peatlands in Asia. It is produced on large-scale monoculture plantations that inevitably result in habitat destruction. In contrast, coir is produced in vast quantities every year as a by-product of the more commercially important edible coconut produce sector. Most coconut production takes place in a small-scale operation and complements other small-scale farming activities. For example, you will often see other crops growing under coconut trees as well as farm animals, such as hens.’

Another natural material experiencing increasing demand from the construction industry is hemp. Concrete-like blocks can be created from a mixture of hemp fibre and lime. With a compressive strength of approximately 1MPa, the material has about 1/20 of concrete’s strength so must be supported by brick or steel frames, but lacks the brittleness of concrete and does not require expansion joints. Hemp can also be used, along with flax fibres, in plastic composites. The strength of their fibres makes these materials ideal for use in panels for vehicles.

As with other plants, hemp and coconut trees absorb CO2 as they grow. This gives the materials a negative carbon footprint, but their abundance in areas of Asia and India mean that most commercial uses require long-distance travel to the point of use.

Moving away from construction, natural materials have also found use in medical applications. The natural antibacterial properties of silver have led to its use in bandages, medicinal socks and clothing. While controversy has followed the use of nano-silver in some of these products, materials that use full-scale silver fibres are a popular way of taking advantage of its properties. Combining silver and cotton in textiles can create clothing suitable for use in hospitals preand post-surgery. These garments are also suitable for those suffering with psoriasis and dermatitis.

Medical need has also led to the development of a new natural composite in the form of Qmilk – a milk-based plastic that can be used in textiles. Anke Domaske, founder of Germany based company Qmilk, was looking for clothing for her stepfather who was suffering with cancer. She was looking for something that hadn’t been chemically treated, as chemotherapy can make skin more sensitive, but couldn't find anything on the market. Working in her kitchen with just a jam thermometer, she began experimenting with milk proteins – which were used in early plastics in the 1930s – and Qmilk was created.

‘Qmilk is based on casein, a milk protein. We add water to create a dough that is then pressed through a spinneret. Next, we add other materials to make it waterproof,’ says Domaske. The casein used to make Qmilk is sourced from raw milk that is no longer tradable and cannot be used for food. In Germany alone, 1.9 million tonnes of milk are disposed of each year. Domaske adds, ‘Qmilk is compostable, while many traditional plastics aren’t. It is also created using 100% natural and renewable materials. We produce absolutely no waste, process at 80ºC and need a maximum of two litres of water to create each kilogramme of product, so the manufacturing process is very eco-friendly.’

But you’d be mistaken if you thought this product was just about eco-credentials. Fabrics made using Qmilk are silky soft and make for comfortable wearing. The material also inhibits bacterial growth and is particularly effective against bacterial strains Staphylococcus aureus and Pseudomonas aeruginosa, making it an ideal choice for hospital bedding or surgical gowns. The absence of chemical additives in the manufacturing process can also be beneficial to those who suffer with allergies. Not bad for a product essentially made from spoiled milk.

Unsurprisingly, Domaske is keen to take her revolutionary concept further. She says, ‘Qmilk has got great potential in a range of fields. We’re now working on cosmetics as well as bio-polymer options for toys and packaging.’ These fabrics could soon be just one of many applications of Qmilk.

As each of these examples demonstrates, natural materials are rapidly becoming real alternatives to their synthetic equivalents. As 2020 and its associated carbon targets loom heavy on manufacturers’ minds, these could become increasingly attractive options for a variety of industries. But as is true of all materials, the key to gaining the most from their use lies in careful specification and selection.