Making buildings, cars and planes from materials based on plant fibres

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USING carrots to create concrete, turning wood into plastic, or even compressing it into a “super wood” that is as light and strong as titanium might sound like a series of almost Frankensteinish experiments. Yet all three are among the latest examples of employing natural fibres from plants as eco-friendly additives or alternatives to man-made materials.

Materials-science researchers are finding that plant fibres can add durability and strength to substances already used in the construction of buildings and in goods that range from toys and furniture to cars and aircraft. A big bonus is that, because plants lock up carbon in their structure, using their fibres to make things should mean less carbon dioxide is emitted. The production of concrete alone represents some 5% of man-made global CO{-2} emissions, and making 1kg of plastic from oil produces 6kg of the greenhouse gas.

Start with the carrots. These are being investigated by Mohamed Saafi at Lancaster University, in England. Dr Saafi and his colleagues do not use whole carrots, but rather what they call “nanoplatelets” that have been extracted from carrots discarded by supermarkets or as waste from food-processing factories. Sugar-beet peelings are also a useful source of nanoplatelets. The researchers are working with CelluComp, a British firm that produces such platelets for industrial applications, including as an additive that helps toughen the surface of paint as it dries.

Each platelet is only a few millionths of a metre across. It consists of a sheet of stiff cellulose fibres. Although the fibres are minute, they are strong. By combining platelets with other materials a powerful composite can be produced. Dr Saafi is mixing the platelets into cement, which is made by burning limestone and clay together at high temperature. (The chemical reaction between them releases carbon dioxide from the limestone.) To turn cement into concrete it is mixed with aggregates such as sand, stones and crushed rocks, which act as reinforcement, and with water, which reacts with the chemicals in the cement to form a substance called calcium silicate hydrate. This starts off as a thick gel, but then hardens into a solid matrix that binds the aggregates together.

Carrot soup

By adding vegetable platelets to the mix, Dr Saafi and his colleagues can make concrete stronger. This is useful in itself, but it also permits a reduction in the ratio of cement to aggregates that is required to achieve a given level of strength. Reducing the amount of cement in this way consequently reduces CO{-2} emissions.

The group is still exploring exactly how strong it can make concrete by adding platelets, but initial studies suggest that the impact could be considerable. Just 500 grams of platelets can reduce the amount of cement needed to make a cubic metre of concrete by about 40kg—a saving of 10%. Dr Saafi and his team have now embarked on a two-year study to investigate the process in more detail and to perfect the best mix for use by the construction industry.

Unlike cement, wood is already a composite material. It is made of cellulose fibres embedded in a matrix of lignin, an organic polymer that serves a number of purposes, including providing woody plants with their rigidity. In May Stora Enso, a Finnish forestry-products company, launched a wood-derived alternative to oil-based plastics. This material, called DuraSense, looks a bit like popcorn. It consists of wood fibres, including lignin, obtained from pulping and other operations. The fibres are mixed with oil-based polymers and other additives, such as colouring agents. The resulting granules can be melted and moulded in the same way as plastic is in factory processes. Adding wood fibres, the company says, can reduce the amount of plastic needed to make goods with plastic parts by 60%.

Stora Enso has also found a use for pure lignin, which is often a waste product of papermaking, since most paper is made of pulp with the lignin removed. Stora Enso’s engineers have worked out how to use lignin as a substitute for the oil-based resins and adhesives employed in the manufacture of engineered timbers, such as plywoods. Nor are they alone in looking for structural applications for lignin. Along with others they are seeking ways to use it to replace oil-based materials in carbon-fibre composites, which are used to make lightweight parts for cars and aircraft.

By contrast, Hu Liangbing and Li Teng of the University of Maryland are trying to make a better material by removing, rather than adding, lignin. Their aim is to create a “super wood” that is stronger than most metals. Their approach is to treat blocks of wood with sodium hydroxide and sodium sulphate in a chemical process similar to that used to remove lignin from papermaking pulp. The difference is that they remove only enough lignin to make the wooden blocks easier to compress. They do that by squeezing the treated wood at around 100°C, which causes most of the pores and tubelike fibres within the wood to collapse. This increases its density threefold and its strength elevenfold.

That puts super wood on a par with some of the lightweight titanium alloys used in high-strength aerospace components. It is also bulletproof. In one test Dr Hu and Dr Li made a laminated sample by placing five sheets of the stuff on top of one another, each sheet having its fibres aligned at right-angles to those of the sheet below. When shot at, this material splintered but was still able to trap a steel projectile that passed effortlessly through a similarly sized sample of natural wood.

The team are now trying to commercialise their process, which Dr Hu thinks will be cheap to scale up. It works on both hardwoods and softwoods, so a range of materials can be created. Dr Hu reckons that, one day, houses, cars, furniture and many other things will be made mainly or partly from densified wood.

from Hacker News https://ift.tt/W7tzurN