When a surface must be protected against corrosion, it is typically coated with paints or varnishes. The proportion of bio-based, environmentally friendly coating solutions, however, is extremely small. In Germany alone, 100,000 metric tons of coating materials are produced every year for protection against corrosion, but paints and varnishes with bio-based binders or film formers usually have been too expensive or could not meet the requirements.
To close this gap, researchers at the Fraunhofer Institute for Applied Polymer Research IAP (Potsdam-Golm, Germany) in cooperation with the Fraunhofer Institute for Manufacturing Engineering and Automation IPA, are developing cost-effective coatings based on renewable raw materials, with a current focus on potato starch.
By using modified starch, scientists at the Fraunhofer IAP have found a way to make sustainable and cost-effective solutions possible. Christina Gabriel, a scientist at Fraunhofer IAP comments that in the field of paints and varnishes, starch is typically used as an additive. “With starch as the main component of a water-based dispersion, we now have very promising adhesion results,” she says. At the center of their research are metal coatings for indoor use on aluminum items such as fire doors, computer housings, and window frames.
Using starch as the main component of paints and varnishes poses various challenges for the Fraunhofer researchers. Film formers must fulfill several tasks, Gabriel explains. They must form a continuous film that adheres well to the substrate material, be compatible with additional layers and additives, and be able to embed pigments and fillers as well. In its natural form, starch exhibits several properties that stand in the way of its use as a film former.
The solution by the scientists involves an initial degradation step of the starch to improve its solubility in water as well as its film-forming ability. To produce a starch-based coating material, however, the film former should initially be soluble or dispersible in water, and the coating must subsequently no longer dissolve in water. The starch is then modified further through a chemical process known as esterification. The resulting starch esters are dispersible in water, form continuous films, and have very good adhesive properties on glass and aluminum surfaces.
Tests to check the resulting coating’s long-term stability, the researchers exposed coated materials to rapidly changing temperature cycles in an environmental test set-up. The test objects also were exposed to electrolyte-enriched water to determine how the coating reacts to water and its endurance under extreme conditions. Additionally, the researchers examined the adhesion and corrosion resistance of the modified starch coating on different metal substrates. New formulations are being tested to optimize the properties of the coating even further.
“Our investigations show that with its good film-forming and very good adhesion properties on various substrates, starch esters have the potential to be future alternatives to petroleum-based film formers in the coatings industry,” states Gabriel.
Source: Fraunhofer,www.fraunhofer.de/en.html.