The meaning of the term “environmentally friendly” in relationship to coatings has evolved through the years. Initially, the term primarily referenced pollution prevention or reduction. This took the form of exempt solvents, high solids coatings, waterborne and 100% non-volatile coatings such as powder coatings and UV cured coatings.
Sustainability and coatings
Today, sustainability or accounting for the total effect of a product on the environment, society and the economy is becoming more important. When applied to coatings, this concept encompasses every stage of a coating’s life cycle from raw material manufacturing, formulation, application and disposal. Even the effects of an applied coating on the recyclability of the end use object is considered when determining its sustainability.
Sustainable products of all varieties are designed from the ground up to be in harmony with principles of sustainable development. With this in mind, sustainable coatings formulations should begin by focusing on raw material feedstocks and sourcing. Fossil based feedstocks such as oil, coal and natural gas are used in the majority of coatings raw materials. Exploitation of these resources exacts a huge environmental toll due to the release of greenhouse gases, pollution generated during extraction and refining, energy used for production and transportation, land use practices and accidental spills.
Feedstocks and sustainability
Feedstocks based on renewable materials such as plant biomass show promise in avoiding the drawbacks of fossil fuels. Just because a feedstock has a biomass source does not mean it is automatically sustainable. Growing plants for human use has huge impacts on global environments, societies and the economies. The history of the feedstock and its impacts are interrelated. Is the feedstock made from recycled materials or byproducts and/or waste from another process? Is the feedstock diverting biomass from the food chain, for example could it also be used for human or animal nutrition? The answers to these questions help determine the true sustainability of a material.
What is circularity?
A final consideration when designing a sustainable product is the potential for circularity. An interesting example of circularity is a snowboard which tries to maximize sustainable principles in its construction. The snowboard is made from plant-based epoxy resin with a unique, plant-based reversible cross-linker. After the useful life of the snow board is over, the consumer returns it to the manufacturer for recycling. Easily separated components are removed and recycled first. Next the body of the snowboard is ground up and the cross-linking reaction is reversed yielding a mixture of solid materials suspended in a resin/cross-linker solution. The solids are removed for recycling while the resin solution is used to make new snowboards. Nearly all the material is recovered and reused.
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Plant-based materials
Plant-based materials have been used for coatings going back approximately 17,0000 years but fell out of favor as synthetic alternatives became more available and offered higher performance. Oils such as linseed and soya are still used in coatings in limited amounts compared to historical volumes. Current developments target manufacturing replacements for accepted raw materials such as acrylic latexes and polyols for urethane coatings with alternatives based on renewable feedstocks.
Acrylic resins are the most widely used binder system for coatings today, largely as waterborne latex polymers for the architectural paints market. The common monomers used in acrylic polymerization are the various acrylic acids and co-monomers including styrene and (meth)acrylamide.
Approaches to plant-based monomers for paint
Two competing approaches are the most likely to produce commercially viable plant-based monomers for latex polymers for paint. The first approach is to make the commonly used monomers from bio-based chemicals such as glacial acrylic acid made from plants.
Another approach is to use alternate plant-based monomers copolymerized with acrylate monomers from non-renewable materials to make latexes with a significant proportion of bio-based content while maintaining the required coating performance. These same monomers can be used to make solvent borne polymers suitable for industrial and automotive uses.
Applications of plant-based polyols
Bio-based polyols are largely used in two part urethane coatings for industrial use. Various types of plant materials can be used to make polyols. Hydroxyl functional versions of the same bio-based monomers use to make acrylic polymers can be used to manufacture polyols for coatings. Another route to plant-based polyols is to modify naturally occurring vegetable oils with hydroxyl functionality.
These bio-based polyols can be cross-linked at room temperature with isocyanates or with amino resins at higher temperatures. Bio-based polyols can be formulated into coatings which meet the performance expectations of non-renewable content coatings after application and curing.
Another application of bio-based polyols is in the manufacturing of waterborne polyurethane dispersions, commonly referred to as PUDs. These serve as the primary binder for high-performance waterborne coatings without the need for an added cross-linker. PUDs often are used in coatings for furniture and building products.
Industry suppliers
Many coatings industry suppliers offer raw materials with renewable content to formulators. I would like to mention one supplier with a diverse line of products. DSM Coatings Resins offers the Decovery® series of resins with bio-based contents ranging from 27% to 49%. The products include binders suitable for coatings from wall paints to floor finishes and furniture to UV curable coatings.
Regulatory guidelines governing bio-based content of coatings
Both governmental and non-governmental agencies have developed regulations and guidelines to quantify bio-based content in coatings. Comparison of the ratio of Carbon 14 to Carbon 12 is the most cited method of certifying bio-based material content. Living (or recently living) biological matter has a higher ratio of Carbon 14 to Carbon 12 than fossil plants and animals, for example, coal and oil.
The importance of bio-based raw materials will only increase as the world moves toward a circular economy reducing carbon footprints and dependence on non-renewable resources.
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