by Max Greiner, Lara Herrmann & Sophie Kikowatz
Description of the material
Chitosan is a substance obtained by deacetylation of chitin (the second most abundant natural biopolymer after cellulose). Natural occurrences of chitin are shellfish, exoskeletons of insects and fungi. The shells or insect carapaces are available in large quantities, e.g. in the production of seafood (in Europe alone approx. 280,000 tonnes annually, worldwide six to ten million tonnes) or are a by-product of protein production.
Properties of the material
Chitosan is biodegradable, biocompatible, antimicrobial, haemostatic, antiviral, anti-allergenic, fat-binding, water-binding, promotes wound healing/cell growth, colourless, tasteless, odourless, antifungal, structure-forming.
The production process
The first step in the production of chitosan is the extraction of chitin. This is usually extracted from crab shells by treating them alternately with 3-5% hydrochloric acid or sodium hydroxide solution to remove lime, colour and residual protein. Depending on the protein content, these processes are shorter or longer. In order not to mix lye and acid, the shells are washed in a water bath in between. To avoid harmful waste, the acid and lye waste are then diluted and combined to produce a salt water solution that is harmless to nature.The resulting protein and lime are pressed into tablets and used as animal feed. The pure chitin is then dried. The next step is the deacetylation, or splitting off of the N-acetyl group. For this purpose, a 50% sodium hydroxide solution is used, which is often heated to a high temperature. As soon as more than 50% of the chitin has been deacetylated, it is called chitosan.
Depending on the derivatives used, degradation in the body takes one week to three months. Thin, pure chitosan films are decomposed in 14 days according to OECD Guideline 301.
Areas of application
As a flocculant for water purification/sludge removal, as it can bind biomass, mineral substances and heavy metals (e.g. iron and uranium). An example is the Spreewald, where water polluted by iron from open-cast mines can be purified using chitosan, which binds and „flocculates“ the metal particles, causing them to sink due to their greater mass and making it easier to filter them out of the water.
Agriculture: as a biostimulant in plants, contributing to plant health and yield enhancement, or as a seed coating.
Food: coatings to delay spoilage. Used in disinfectants, it can prolong the normally short duration of action of alcohol-containing agents.
In cleaning agents, the antistatic properties create a lotus effect.
Textiles: improves tensile strength, as a sizeing agent ensures less thread breakage; no washing out of the agent necessary. Chitosan spun into staple fibres has a high skin compatibility, especially with neurodermatitis sufferers and in the care of the elderly.
Cosmetics: Used in hair gels, creams, toothpaste and for the targeted, prolonged release of fragrances in perfumes.
Medicine: used in wound dressings or as a fibre material in „growing“ implants in paediatric plastic surgery. It is also very successful in the treatment of burns. Due to its strong positive charge, it can activate the body‘s own antibodies, which are also positive. By binding blood fat, it would even be able to prevent strokes and heart attacks. Even suture material and bandages for artificial skin are already made of chitosan.
Motivation for the development of the material
The large natural occurrences of chitin and the high amount of chitin waste produced in diverse industries open up a great opportunity to use the material in promising chitosan plastics with different mechanical qualities. In addition, there are numerous biochemical properties that make the material interesting for special areas of application, e.g. in medicine and cosmetics.
Variants of the material
In nature, chitin occurs in different qualities. By transforming it into chitosan, powders/granulates with different concentrations are produced according to requirements. In addition to versatile additives, chitosan can be processed in acid solutions into liquids that can be cast, sprayed or printed and, after drying, have different properties depending on their composition.
Advantages / disadvantages
The biggest advantage is the natural occurrence of chitin. The natural positive charge opens up a whole range of exciting properties that would otherwise only be achievable through the use of chemicals. Different charge densities make it easy to modify and cross-link chitosan with other substances. Unfortunately, however, it is still very expensive to produce. Alternative plastics must not be more than 20-30% more expensive on the market to have a realistic chance against fossil-based plastics.
Chitin, as the source material for chitosan, is the second most abundant material in nature and it is, along with cellulose, starch and protein, is a naturally occurring polymer. It therefore fits into the category of bioplastics in three respects: it is biodegradable, biobased and even biocompatible.
Possibilities and obstacles of recycling
Mechanical recycling is not possible due to the lack of thermoplastic properties.
Chemically, it would be possible to break down chitosan plastics again and separate them enzymatically. However, this process would be unreasonably more costly (energy, chemicals, chemical waste, transport routes) than the alternative: returning it as a nutrient to the best recycling system already in existence; nature.
In agriculture, insects are bred as an alternative source of feed, fed on crop waste. The chitin that falls off can be used either as fertiliser or to obtain chitosan. This, in turn, can be processed in many ways and later returned to the biosphere to serve as a nutrient.
Exciting tactile material properties encourage experimentation. Visually, one can play with the translucency and give free rein to the naturalness of the material. Another aspect could be to play with Western culture‘s low acceptance of entomophagy, and thus draw attention to the usefulness of insects as an alternative feedd/food and plastic source.
Regional importance of the material
The special properties of chitin and chitosan are still largely unexploited, but are nevertheless represented as an important component in various regional companies. Biolog Heppe, for example, produces a wide range of chitosan derivatives for various applications. MadeByMade, on the other hand, produces insect proteins in which chitin-containing insect carapaces fall off as a by-product.