Biofillers for coating applications

Microcrystalline cellulose is another name for very fine wood pulp. It comes in very small particle sizes, and the shape and form can vary depending on the supplier. It is an ideal filler for biobased coatings because it can be added at relatively high concentrations without significantly affecting the viscosity. Consequently, incorporating up to 15% microcrystalline cellulose into water-based coatings is feasible. A major advantage for decorative coatings is its white colour, which allows you to achieve any colour you want. Particle sizes of up to 15 µm are available, meaning these fillers can be used in most coating applications.

Image: microcrystalline cellulose is pre-sieved by the manufacturer up to 15µm. The fine white powder is therefore suitable for many coating applications.

Cork is a well-known natural product that is used in many applications where sealing (e.g. bottle stoppers) or lightness (e.g. buoyancy aids) is desired. From a chemical perspective, cork is quite special. It consists of suberin, which is mostly derived from fatty acids. It is therefore a highly hydrophobic material. Its shape is unique too. Under a microscope, cork appears as large cells with very thin walls. Cork can therefore be regarded as a kind of natural closed-cell foam. It protects trees and offers a water barrier. Due to its elasticity, cork cannot easily be ground into a fine powder. However, a major source of fine cork dust is the polishing of bottle stoppers. It is offered by different suppliers as a cheap material and is usually just combusted because there is no other use for it. One disadvantage of cork powder is its relatively large particle size; it therefore needs to be sieved before use.

SEM (electron microscopy) picture reveals that the exceptional lightness of cork is due to the large open cells with very thin walls

Polyhydroxyalkanoates are a group of polymers produced by bacteria. When certain bacterial groups have an abundant supply of carbon sources, such as sugars or fats, but insufficient nutrients to grow, they store this carbon in their vacuoles in the form of polyhydroxyalkanoates. These thermoplastic granules degrade quickly to produce compost. The polyhydroxyalkanoate family is very diverse, with different types (e.g. PHB, PHBV and PHBH) available on the market, all of which have different properties. The PHBV type, in particular, is sold as a matting additive or filler due to its fine powder form.

Image: These especially small spherical particles are made within bacteria. PHBV is also commercially available in a micronized form, but can also be used in thermoplastic processing.

Starches are an obvious choice of biobased filler. Many plants produce it as a storage polymer. As a chain of glucose, it is a polysaccharide; however, unlike many other polysaccharides, it does not dissolve in cold water or many polymers. It is a white powder, which allows colours to be added. However, starch can decompose, giving rise to brown or burnt colours. There are many differences between starches from different sources (e.g. maize, wheat, rice, potatoes and tapioca) and therefore their use as fillers in coatings depends on their dispersability, particle size and compatibility with the coating chemistry. Clearly, starch is easily biodegradable and prone to hydrolysis, so it is better not to use it in high quantities if durable coatings are required.

Image: potato starch particles

Humic acids are 100% natural molecules that occur in natural bodies of water, such as streams, lakes and seas, as a result of the breakdown of leaves, twigs and decaying wood. In certain drinking water production processes, humic acids are removed because they give the water a harmless yellow colour. Humic acids can be obtained as a brown liquid or dried into a powder. It dissolves in water and forms a precipitate with certain compounds, such as metals. Therefore, the compatibility of humic acid must be tested when using it as a filler in biobased coatings. Humic acid is also brown when used in higher concentrations. However, as it is a by-product of certain purification steps, it may be appropriate to use it as a biobased filler that may also provide UV protection or antimicrobial properties.

Image: hollow spherical particles formed during spray drying of humic acid concentrates

Wood dust is probably the most obvious choice of biobased filler. It is becoming increasingly popular in "WPC" (wood plastic composites), but it can also be used for other purposes. They come in different particle sizes and can be obtained from local woodworking companies, bearing in mind that a sieving step is required. In chemical terms, wood consists of three polymers: cellulose (50%), hemicellulose (25%) and lignin (20%). One annoying feature of wood dust is that it absorbs a lot of water or solvent, so only limited amounts can be added to coatings. Nevertheless, it can give coatings an attractive visual effect.

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is a mineral consisting of calcium and phosphate, and is a by-product of gelatin production. Due to its high phosphorus content, it is often used as a fertiliser or an addition to animal feed. Hydroxyapatite occurs naturally in our bones, where it forms part of a complex structure with collagen. It is supplied as a coarse powder that can be sieved to isolate smaller particles that can be used as fillers. It should be noted that the product is obviously not suitable for 'vegetarian/vegan' products and is sometimes not halal either.

Lignin is one of the three main chemical components of trees. Along with cellulose and hemicellulose, wood contains around 20% lignin. This hydrophobic substance waterproofs the plant fibres and allows water to be transported within the tree. Lignin can be considered a kind of waterproof glue within the tree and is a substance that cannot be easily extracted from wood. During the pulping of paper, it degrades in heavy alkalis or acids, forming soapy fragments or lignosulfonates. Therefore, the term 'filler' is not used for lignin, but rather for chemical derivatives derived from lignin, which are available on the market in various forms (tars, refined products, processed products, liquids, powders, etc.). It is used as a filler in asphalt, improving its quality due to its sticky behaviour. It can also be used as a tackifying filler in waterproofing membranes, for example. Academic literature describes it as a promising flame retardant filler, although this is somewhat doubtful. It is brown to black in colour and has a distinctive odour. Nevertheless, as it is biobased, biodegradable and abundant, it can be considered a filler for coatings.

Struvite is a mineral product obtained during the purification of waste waters containing lots of phosphate and ammonia. The phosphates and ammonia are precipitated from the waste water into crystals called struvite having the chemical formula NH4MgPO4·6H2O. Common sources of struvite are waste water treatment plants of e.g. potato peals or manure. The mineral product forms relatively large particles and are therefore not so easy to just process in a thin coating. 

Image: this very large magnification of a struvite particle shows cracks and mineral forms of struvite