Chickens and particularly their eggs are suffering from bad publicity at the moment. There is no denying it: as soon as one food scandal has died down, the next one starts. A US research project is now highlighting a completely different characteristic of our egg-laying, feathered friends that has nothing to do with the debate about fraudulently labelled food products and contamination. Instead of using them to fill duvets, ingenious scientists have found a way to make bioplastic from chicken feathers. k-online has taken a closer look at this ...
It is a well-known fact that rubber ducks cannot fly. They are at home in the bathroom, where children (and German comedian Loriot’s pug-nosed cartoon characters) play with them in the bathtub. Rubber chickens can’t fly either – and they are not even made of rubber. The term is used primarily for roast chicken of dubious quality, the meat of which requires lengthy chewing and tastes rather bland. There is, however, a connection between these birds and plastic that is intriguing enough to write a story about. What is involved here is an innovative new bioplastic made from chicken feathers, most of which are discarded unused at present. Believe it or not, 1.5 million tonnes of chicken feathers have to be disposed of every year in the USA – roast chicken land – alone. While they are not the material of choice as down filling for pillows or cushions, they may well prove to be the basic feedstock for bioplastic soon.
That chicken feathers have what it takes is attributable to the fact that they contain keratin. This is the name given to a fibrous protein that occurs in the hooves and horns of animals. In human beings, keratin gives stability to finger- and toenails and strength to hair. More than 80 per cent of our hair consists of keratin. So it is no surprise that the cells which form hair (in the scalp, for example) are called keratinocytes. The structure of hair keratin is determined by the chemical elements carbon (C, content: 50%), oxygen (O, 23%), nitrogen (N, 17%), hydrogen (H, 6%) and sulphur (S, 4%). They bond together as amino acids, which create what is known as prekeratin as a result of molecular chain formation (polymerisation). This polypeptide has the shape of a helix, in other words it resembles a screw-shaped spiral. In the hair formation process, cross-linking occurs between the individual peptide spirals (hornification process). The most stable bonds here are between the sulphur atoms, that are known as disulfide bridges. The hydrogen bridges are what give hair its elasticity, on the other hand. They separate in the presence of water, as a result of which curls form for a short time. The salt bridges (ion bonds) are water-soluble to some extent too; when the hair dries, they develop again.
William Thomas Astbury (1898-1961) and Henry John Woods (1903-1984) carried out in-depth research into keratin as long ago as the 1930s. They found out “that keratin normally contains polypeptide chains with a statistical sequence of the individual amino acids, adjacent chains being connected to form net-like structures via ‘side chain bonds’”. The British chemists also “induced stretching and contraction [...] by various forms of treatment”, as the Austrian physical chemist Otto Kratky (1902-1995), Departmental Manager at the Kaiser Wilhelm Institute in Berlin, wrote in his essay “The radiographic investigation of fibrous materials” that appeared in 1939. So although there were already indications at an early stage that keratin is a natural polymer and although the first bioplastics were invented as early as the end of the 19th century (see “Bioplastic: historical predecessors”), there has been an almost complete lack of interest in keratin within the plastic industry. Polymer research scientists from the University of Nebraska / USA are now putting the spotlight on it again: they have succeeded in isolating keratin from chicken feathers. In a second stage, the research scientists have treated it in such a way that it remains water-resistant and mechanically stable. In other words: they have managed to make plastic from chicken feathers.
Chicken feathers normally react to moisture in a similar way to horsehair – they expand. This is why classic hygrometers have been made out of horsehair: advantage is taken of the stretching effect to measure the change in air humidity. “Although attempts were already made in the past to use feathers for composite plastics, problems were encountered in contact with water”, explains Yiqi Yang, Professor at the Institute for Material Sciences and Nanotechnology at the University of Nebraska. Problems that appear to have been overcome, because Professor Yang’s team has come up with a special cocktail, has added methyl acrylate to the chicken feathers. Long molecular chains form as a result that interact in different ways and thus form a solid mass. The chicken plastic created as a result now has properties that have been found lacking in it up to now, particularly water resistance, durability and a longer useful life. It also weighs less than comparable compounds made from polyethylene or polypropylene. And last but not least: it is biologically degradable.
Although the research scientists are keen to avoid excessive optimism, since a modest start is all that has been made so far, a patent application has already been filed for the process in the USA. Tests to produce a bioplastic from keratin have already been carried out in Germany too. Horn to which caustic soda solution or acetic acid has been added has been used for this purpose rather than feathers: “The final product itself was a solid, film-like substance. It is possible that short peptide fragments have been created as a result of the separation of the peptide bond by the solution / acetic acid. They could perhaps be repolymerised by adding a catalyst. The model for this could be the polymerisation of caprolactam, a feedstock used in the chemical industry for polyamides such as Perlon.”
This raises the issue of “natural polymer fibres”. The German Wool Research Institute (DWI) at RWTH Aachen University intends to link the proteins that chicken feathers contain together to create innovative new fibres. One conceivable application here is as a cotton substitute in the textile industry, but what are of even greater interest are technical fibres, from which insulation materials and geotextiles, e.g. reinforcement for motorway embankments, are made. For this purpose, chicken feathers have been boiled in water at high pressure, so that the proteins in them are split up into even smaller components known as oligopeptides. They consist of amino acids that form crystals and are considered to be the basic material for new polymers. This feather protein is now being added to conventional plastic fibres, in order to improve their physical properties.
Apart from feather keratin, the wheat protein gluten is also considered to be a suitable feedstock for new fibres. In further experiments, the substance known as lignin is added to polymers; in its natural context, lignin bonds the cellulose fibres together in wood. Rayon (“artificial silk”) must be considered the historical model here: “An evening dress made from wood” is how the authors of the book “The Magicians’ Industry” that appeared in 1952 put it triumphantly. The fascination of the “magical transformation of stiff, inflexible wood into soft, supple spun fibres from which clothes can be made” is still apparent today in the following excerpt:
“The wood from which we intend to conjure your evening dress comes from a beech or spruce tree [...]. The bark is removed before the wood is sawn, washed, chipped and boiled in huge vats at enormous temperatures. The spices for this type of soup are chemicals that ‘digest’ the basic wood material. [...] (Then, editor’s note) this ‘soup’ is dewatered, dried and pressed into an endless pulp web, which in turn is cut into panels. [...] The panels [...] are the raw materials for rayon factories. The pulp fibres are separated, bleached and then transformed into a thick spinning fluid by all sorts of chemical additives. [...] It is then clarified, filtered, finished and, finally, forced through fine dies. The result of this process is already the rayon fibre. [...] It is superior to natural fibres in that the fibre fineness is absolutely consistent, while any desired fibre thickness and length can be produced.”
The reasons for starting the bioplastic age today are prosaic rather than magical. Although conventional plastic manufacturing “only” consumes about four per cent of crude oil production, this natural resource is finite, so that use of it should be reserved for purposes that are dependent on crude oil due to the lack of alternatives. “The objective is for plastic made from renewable resources to replace those made from oil one day”, says Professor Yang. Sounds good, but there is a downside: environmental associations and scientists criticise that bioplastic does not degrade readily, can overfertilise water and can damage the ozone layer. Only time will tell whether chicken plastic of all things will lead to any changes to these conclusions. GD