Naturally adhesive: New glues from renewable raw materials

Shoes, cars, airplanes, rotor blades for wind turbines, self-adhesive notes, plasters -- this is just a sample of the many products featuring adhesives. More than 820,000 tons of adhesive were produced in Germany in 2010, according to the German Adhesives Association -- Industrieverband Klebstoffe. To this day the majority of adhesives are manufactured from petroleum-based materials. Only gradually is the industry also offering adhesives made from renewable raw materials such as starch, cellulose, dextrins, and proteins. Pioneering products featuring these new adhesives include wallpaper pastes and glue sticks.

Adhesive based on polylactic acid

In two projects, researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT are working on further new adhesive formulas based on renewable raw materials. In cooperation with the Recklinghausen site of the Westfälische Hochschule, University of Applied Sciences, and the companies Jowat, Logo tape, and Novamelt, and with support from Germany's Federal Ministry of Food, Agriculture and Consumer Protection, researchers at UMSICHT in Oberhausen are developing a pressure-sensitive adhesive for industrial applications. Products using pressure-sensitive adhesives include adhesive bandages, self-adhesive labels, and adhesive tapes. They are subject to particularly demanding requirements: They have to remain permanently adhesive at room temperature. Gentle pressure should suffice for them to adhere to almost all substrates, and yet it must be possible to remove them without leaving behind any residue. To achieve this, the adhesive force must precisely match the respective use.

Pressure-sensitive adhesives are based on backbone polymers, which give the adhesives their inner strength (cohesion). The challenge for the UMSICHT researchers is to develop a backbone polymer from the raw material polylactic acid. What makes this biological material particularly attractive is its low production cost; since lactic acid is produced on an industrial scale, costs are in the region of prices for fossil-based backbone polymers. "However, the properties of polylactic acid are completely different from those of the polymers used to date, such as polyacrylates and styrene-based block copolymers," explains Dr. Stephan Kabasci, who heads the UMSICHT renewable resources business unit. This means that the researchers have to develop a completely new formula.

Packaging using compostable films

However, adhesives are also found in many types of packaging, for example where laminating films protect foodstuffs from dirt, moisture, and chemicals. This involves covering printed packaging and printed paper products on one or both sides with a transparent, shiny, matt, or embossed plastic film. In a collaborative project, UMSICHT scientists are working with the companies Achilles Papierveredelung Bielefeld, Jowat, and Deckert Management Consultants to develop innovative adhesive systems that meet the exacting quality requirements of laminated products as well as being compostable. In pursuit of this objective, the researchers are focusing primarily on water-based dispersion adhesives, in which the adhesive components are dispersed very finely in water. They are applied to one side of the product and joined while wet.

Nature shows us another path to developing biological adhesives. The buoy barnacle (Dosima fascicularis) produces a special adhesive which it uses to attach itself tightly to flotsam. This super-adhesive is so strong that it is almost impossible to break down into its constituent parts using ordinary solvents. Another special property it has is its ability to cure under water. Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Bremen are now trying to find out which amino acid components make up the relevant proteins. "Once we've done that, the next step will be to recreate the adhesive proteins in the laboratory," says Dr. Ingo Grunwald, expert for biological adhesives at the IFAM. Such bioadhesives are primarily of interest for medical applications, for example to close incisions or to replace or support the pins and screws used to treat bone fractures.