There are many new monomers that have novel structural elements

Which bio-based raw materials do you believe are currently important for the paints and coatings industry?

Dr. Tobias Robert: The coatings industry has traditionally used a large number of bio-based raw materials. These include cellulose, starch and vegetable oils. But the goal, of course, is to extend the variety of the bio-based raw materials in use. This is why many new (partially) bio-based raw materials, such as polyols and isocyanates, have recently been launched onto the market. These are mainly fatty acids serving as starting materials, as well as new monomers that can now be produced biotechnologically from renewable raw materials.

Among these monomers are 1,3-propanediol, sebacic acid, itaconic acid and succinic acid which serve as building blocks for polyesters, alkyds and polyurethanes. However, there are also binders, such as polyacrylates, for which hardly any bio-based production solutions exist.

And which raw materials will become important in the future?

Robert: There are synthetic building blocks such as furan dicarboxylic acid and isosorbide, which I believe are on the cusp of application. Admittedly, though, there are still a few problems to overcome before these can be used in standard applications.

Dr Tobias Robert is working at the Fraunhofer Institute for Wood Research (WKI) in Braunschweig, Germany. He is also a tutor for the European Coatings Seminar on bio-based raw materials for coatings in November 2018. (Picture: Fraunhofer WKI)

What sort of problems?

Robert: Studies are currently being conducted into the potential of furan dicarboxylic acid for use especially in the plastics sector. It’s conceivable that polyethylene furanoate (PEF) could prove to be a bio-based alternative for PET bottles. However, availability is still very limited. At the moment, furan dicarboxylic acid can only be obtained at very high prices from suppliers of fine chemicals.

Why is that?

Robert: There is not enough production capacity at the moment.

That sounds like a solvable problem, at any rate.

Robert: It is. Hopes are running high that large quantities of furan dicarboxylic acid will eventually become commercially available at competitive prices. Hardly any cyclic building blocks based on renewable raw materials are available for the coatings industry in particular, but they are essential for certain properties, such as high hardness. Furan dicarboxylic acid’s tendency towards yellowing would limit its applicability in some areas.

Isosorbide could possibly serve as an alternative monomer. It is currently the subject of much research. Unfortunately, there are not enough applications for it. The problem here is that the resin producers would have to switch chemistries – away from a chemistry they are already familiar with – because isosorbide, for example, is somewhat less reactive and that makes it more difficult to incorporate this starting material into the binders.

The industry is probably balking at this because it doesn’t particularly want to develop completely new formulations?

Robert: Exactly. Consider, if you will, the case of succinic acid. Some companies are now going to great lengths to bring large quantities of succinic acid to market. A great many studies have been conducted to demonstrate that this building block can be used successfully.

Do you think bio-based raw materials are more likely to be deployed as direct one-for-one replacements for fossil raw materials or will the focus be placed more on their oft-vaunted new properties?

Robert: That depends very much on the raw materials. For example, drop-in applications exist in which the petrochemical feedstock is produced in a bio-based manner, i.e. from renewable raw materials. The advantage of this approach is that the industry doesn’t have to modify its processes or products, provided that the bio-based raw materials are of good enough quality. Bio-based acrylic acid would be an example of this, even though production costs are still higher than for petrochemically produced acrylic acid. It will therefore probably take several years for the bio-based version to become commercially competitive.

Robert: However, there are also many new monomers which offer the benefit of having novel structural elements that can also be used to produce new products or properties. I think there is great potential here for bio-based raw materials. This potential is also driving our research and development work here at the Fraunhofer WKI.

Can you give an example of such new properties?

Robert: Well, if you incorporate itaconic acid into polyester, the double bonds are distributed along the polymer backbone. In conventional polyester acrylics for UV-curing applications, on the other hand, the reactive bond is only found at the end of the chain. Although the level of reactivity is different, it is possible to obtain coatings that possess totally different mechanical and physical properties. For example, greater hardness or improved weathering.

Do you see any negative effects of bio-based coating raw materials?

Robert: There is a recurring discussion to the effect that renewable raw materials are displacing the agricultural land that is reserved for growing food. However, it must be considered that the area required for bio-based raw materials lies in the low single-digit percentage range when compared with the areas currently used for animal feed or bio-fuels. Against this background, I therefore believe that bio-based coatings and plastics are a good alternative.

In the long run, it will also be necessary to take a closer look at the processes by which bio-based monomers are produced. Here we need more life cycle analyses – which are not yet available for all products. Many monomers are touted as being sustainable alternatives, but this is not always a proven fact.

Lignin is a bio-based raw material that could be sourced from secondary streams. What do you think of its potential?

Robert: This is indeed a substance which has long been treated as a bio-based raw material and we also see it as having great potential. However, it must be borne in mind that lignin is a solid which has a high glass transition temperature and is insoluble in common solvents. This means that some chemical tricks are required to convert lignin into a useful form. Another characteristic of lignin is its brown colour. For decorative coatings, the achievable colour palette would therefore be very limited. It would be more realistic to use it in adhesives or plastics. In this connection, both industry and research institutes have come up with good ideas for using lignin as a raw material in recent years.

Are there other hurdles to the use of bio-based coating raw materials?

Robert: There are several things that need to be mentioned here. First, the costs of bio-based raw materials are often higher than those of their conventional counterparts. Second, security of supply is lacking in some cases and the number of suppliers is small. Although some bio-based raw materials are already available in large quantities at competitive prices, the question is whether this will remain so in the long term, especially if demand increases.

You will also be holding a seminar on bio-based raw materials for coatings shortly. What can participants expect?

Robert: It will be about bio-based raw materials and monomers for the coatings industry. In other words, which raw materials are already being used, which will be important in the future and what the advantages are of switching to them. For example, a higher bio-based content in the products, something which is also a growing concern for consumers. But it will also cover the new properties that can be obtained with new monomers and the applications this can lead to.