Using PHA as matting agent

Sirris and Centexbel joined forces to investigate, research and facilitate the use of biobased additives in coatings. In the COOCK project "AddBIO" wedinvestigated biobased functional additives, including flame retardants, reinforcing fibers, water repellency, dyes and pigments, matting agents and more in coatings. We explore the possibility to use biobased materials as additives with high added value. This presentations shows the possibility to use PHA as matting agent.

PHA is a naturally occurring polymer. It is produced by bacteria as an energy storage polymer. In a sense, it performs a similar function to the fat layer in animals or starch in plants. We store energy in different forms in order to use it later. What is special about this PHA is that the bacteria store it in granules called carbonosomes. These granules have a hard texture and a melting point. This means they can be used as a type of plastic that is not only biobased, but also easily biodegradable. PHA can be produced using various raw materials, including oils and sugars, as well as waste products such as brewer's spent grain, sewage, cooking oil, and even household vegetable waste. Therefore, PHA is a promising new ingredient, not only as a bioplastic but also as an additive for coatings.

Matting agents are added to coatings to prevent shiny reflections. Glossy surfaces can give consumers the impression that products look cheap and plastic-like. There is also a fashion trend for matt-looking surfaces. Matting agents therefore make items look more luxurious. This can be achieved by using a matt-look coating or paint. What we perceive as shine is actually the reflection of light around us. Matting agents are simply small perturbations on the surface that cause light to scatter rather than creating a glossy impression. To achieve this effect, small, colourless, abrasion-resistant particles are added to the object's coating or finish.

Matting is measured using a reflectometer. This instrument first measures the reflectance of a black mirror surface in total darkness at an angle of 60°. It then compares the reflectance of the coating at the same angle and compares it to the black mirror.

A value of 100% means that the surface is extremely reflective, 0% has no reflection at all and is matte.

Some matting agents are more useful than others. For example, some matting agents must perform well under abrasion, others must retain perfect colours. In the case of PHA, which comes as a white powder, we can see that we can make the matting agents completely transparent by simply heating the coating just above the melting point of the PHA. The matting effect is slightly reduced, but the colours are perfectly preserved. In the picture below we can see that the matting agent becomes transparent when heated, but the matting effect is retained quite well (the coating without matting agent had a reflection of .65%).

We used scanning electron microscopy to look at the surface to see what happens when the matting agent is heated above its melting point. 

As you can see in the pictures, PHA often occurs in large agglomerates. These are made up of clusters of small PHA particles. These look white when they are not fused together. However, after heat treatment, no individual PHA particles can be seen and the aggregates look transparent.

There are many types of PHA. We have found that not all commercial PHA powders are equally efficient. What makes a grade of PHA valuable as a matting agent depends on how well it disperses in coating pastes, but also on the size of the particle and how it behaves in coatings.

In general, PHA can be used as a high performance matting agent in a variety of coating systems. Attention must be paid to the grade of PHA used to obtain the best performance. There is also an added value to be gained from PHA as it can give excellent colour retention to coatings when heat treated. 

Image: exellently performing PHA for matting, it shows the structures of matting agent that scatter light