Epoxy coatings are highly durable, corrosion resistant, and are commonly used as floor and metal coatings or as protective primers. They are typically 2-part coating systems and consist of an epoxy resin combined with a reactive hardener (such as amines, phenols, anhydrides or thiols). Traditional epoxy resin molecules are based on a rather rigid chemical structure, which, combined with high levels of crosslinking from the hardener, gives good mechanical properties to the final coating. The rigidity can however be a disadvantage, as high levels of crosslinking can make the coating brittle and inflexible. They are also sensitive to UV light, which causes cracking and flaking, hence they are only used in indoor applications, or as primers which are protected by a UV-resistant topcoat.
Many commercial epoxy materials employ bisphenol A diglycidyl ether, which is both petroleum-based and has significant health concerns. However, there are several up-and-coming bio-based products that can be readily converted to epoxy chemistry, or used directly as hardeners, to address some of the disadvantages of traditional materials.
- Vegetable oil fatty acids (such as linseed, castor and soybean oils) can be readily epoxidized using straightforward chemical reactions and are commercially available. They are inherently flexible molecules so are excellent for flexible coatings and the lack of aromatic content should decrease their UV sensitivity
- Terpenes (from citrus skin and coniferous trees) similarly contain epoxidisable groups, as does natural rubber latex
- Plant phenols and polyphenols can be used directly as hardeners or undergo further reactions for epoxy resin preparation, depending on their chemical structure. Small molecules include cardanol extracted from cashew nutshell liquid (CNSL) and polyphenols include lignin from woody waste products.
- Biobased curing agents with nitrogen groups are harder to source, but can be produced from bio-based alcohols which can be used directly or converted into polyamides in combination with bio-based diacids from fermentation processes
A past sticking point for development of high bio-based content epoxy resins was the availability of bio-based epichlorhydrin, which is used to add epoxy functionality to a wide range of base chemicals. With the commercialisation of routes to glycidol and epichlorhydrin from glycerol, a by-product of biofuel production, the options for bio-based epoxy ingredients have increased dramatically.
A recent project at PRA investigated the preparation of coatings using epoxidized linseed oil and other bio-based ingredients. As with all new technologies, there is a learning curve to finding the right combination of ingredients for your application and getting the best out of them. At PRA, we can help to accelerate that development process. Contact [email protected] for assistance with your transition to bio-based coating technology.
