How Fillers Impact System Properties?
How Fillers Impact System Properties?
Fillers are solid particles used in paints and coatings system to:
- Improve properties
- Lower costs
Hence, there are several aspects that should be taken into consideration when selecting fillers for coatings. Because key system properties are governed by the properties of the solid particles used in the system. These properties include:
-
Ease of production
- Application behavior
- Properties of the resulting film, and
- Storage stability
Now, let’s discuss a few important properties of systems and understand how those system properties are influenced by the fillers that are used. The paints and coatings systems are divided into three main categories:
- Paints, being the liquid system
- Coatings, being the systems that have formed a film on a substrate
- Primers, being the layers applied directly onto the substrate
- Topcoats, being in direct contact with the surroundings
Paints and Coatings (Primers & Topcoats)
Selecting Fillers for Paints
Selecting Fillers for Paints
Fillers have a significant role in the key properties of paints. Their influence is important during production, storage and application. Here are the main factors which are influenced by filler:
pH of the Paint
pH of water-based paints is important, for example with respect to colloidal stability. Some fillers can influence pH strongly while others have only a minor influence on pH. The surface composition, as well as chemical stability of the filler particles, govern the influence of the particles on pH.
Influence of Speed of Sedimentation in Fillers Selection
The speed of sedimentation in paints is mainly governed by the:
- Viscosity
- Density
- Size of the solid particles
Big particles of high density will sink faster than small particles of low density
When considering the size of the primary particles of fillers, most often the median particle diameter of individual primary particles is given, called d-50. Particle size can be given in micrometers (mm) or nanometers (nm).
Stability of Paints
Different types of stability of solid particles in liquid systems are important:
- Sedimentation/settling
- Dissolution
- Chemical stability
- Flocculation – It is the spontaneous gluing together of separated solid particles in liquid systems. This undesired process can take place during production, storage, application and film formation. Flocculation can be prevented by arranging repulsive forces between the particles by adsorbing specific additives, called dispersants, onto the surface of the solid particles, see figure below.
Solid particle-stabilized in a liquid system by means of a dispersant
Whether or not dispersant will be able to adsorb onto the surface of filler particles is governed by the:
- Chemical composition and morphology of the dispersant molecules, and
- Surface composition of the solid particles
Most fillers are hydrophilic, implying that polar groups are present on the surface of the particles. An exception is talc, being a filler having a relatively hydrophobic surface. Hydrophilic particles are easily stabilized because dispersant molecules can adsorb strongly onto the polar groups being present on the surface of hydrophilic particles.
Dissolution, as well as possible chemical reactions of filler, mainly take place during storage and it depends upon the chemical composition of the filler. It is important to select fillers having chemical resistance against the liquid medium: fillers must have sufficient fastness.
Some fillers are not inert, implying that the solid particles can, for example, be attacked by certain chemicals, solvents, acid, alkali or ultra-violet radiation.
Calcium carbonate (CaCO3), for example, is a filler dissolving slowly in an acidic environment. This implies that one should be careful when using calcium carbonate in water-based systems having low pH, or when the system will be applied on an acidic substrate.
Influence of Wear of Equipment
Equipment might be damaged both during production and during the application of paint. Fillers having both high hardness and sharp edges (shape) can damage equipment because of wear.
The hardness of a filler indicates how easy or how difficult it is to mechanically damage the solid. For pigments and fillers the Moh hardness scale is used to quantify hardness. The hardest mineral, being diamond, has Moh hardness being 10, whereas talc, being a soft filler, has Moh hardness of 1.
Hard fillers like quartz, can give wear to production equipment, especially when the particles have sharp edges.
Influence of Viscosity
Viscosity of liquid systems is influenced by fillers, depending on size, shape and surface composition of the filler particles.
- Particles having irregular shapes will raise the viscosity of a system
- Small or porous particles that adsorb high amounts of resin material result in high viscosity
Selecting Fillers for Primers
Selecting Fillers for Primers
A primer is a coating layer being in direct contact with the substrate. Let’s discuss how fillers affect main properties of primers:
Influence on Primer Adhesion
Adhesion of a coating is mainly governed by the binder being present in the system, possibly combined with specific adhesion promoting additives. Some fillers, having platelet shape, can improve adhesion because of two effects:
- Platelet particles form a barrier, thus enlarging the way molecules must travel through the coating to get to the substrate-primer interface
- Platelet particles give mechanical strength, called coherence, to the film, thus improving adhesion
Influence on Barrier Properties
The permeability of a coating refers to the ability of small molecules to diffuse through the coating. As most filler particles are closed solids, small molecules cannot go through the filler particles: the molecules have to travel around the particles. A coating having low permeability towards small molecules is said to have good barrier properties. A few factors govern the permeability of a film:
- Adsorption of the resin system onto the solid particles. Small molecules can diffuse along the filler particles when adsorption is weak or absent.
- Shape of the particles. Platelet-shaped filler particles, like talc or mica, act as 'roof tiles', lowering permeability, when they orient themselves during film formation
Platelet-shaped solid particles improving the barrier properties of a coating
Influence on Chemical Fastness of Primer Coatings
Chemical resistance of a filler is governed by the chemical composition of the filler. Most fillers have good resistance (= fastness) against chemicals and solvents. An exception is calcium carbonate (CaCO3) having poor resistance against acids. Fillers of high chemical fastness should be used in primers for metal.
Influence on Corrosion Resistance
Fillers having platelet shape can improve corrosion resistance by forming a barrier, thus retarding diffusion of molecules that can worsen corrosion, like oxygen, salt and acids.
Also, fillers having a high pH, being basic, can retard corrosion because corrosion is faster in acidic surroundings. Basic fillers, like dolomite and feldspar, are often used in anti-corrosive primers because corrosion of metal proceeds slower at high pH. This beneficial influence of basic fillers on corrosion resistance is called passivation.
Influence on Gloss
Primers preferably have a low gloss implying that the surface of the primer is rough. A rough primer surface improves intercoat adhesion between topcoat and primer. Fillers having high oil absorption value and high specific surface area lower the gloss of coatings effectively.
Related Read: Learn how to improve the gloss of your paints here »
The oil absorption value of a filler is defined as the amount of oil, in grams, needed to just make a flowing paste out of 100 grams of the powdery filler. For non-porous particles, the oil absorption value is directly related to the specific surface area of the material. The oil absorption value of the solids to be used in a liquid system is important with respect to the key properties of the resulting system.
- Solid particles having a high oil absorption value, for example, give a stronger viscosity increase of a liquid system than solids of low oil absorption value.
- And, fillers of low oil absorption value should be used in systems that should have a low viscosity, for example, solvent-free systems or high-solids systems.
The oil absorption value is governed by the key properties of the filler particles:
- Smaller particles have higher oil absorption values than bigger particles
- Particles being spherical and smooth and having a closed surface (being non-porous) have a low oil absorption value
- Particles of irregular shape have a high oil absorption value, especially when the particles are porous
The specific surface area of a filler is governed by particle size, particle shape and surface morphology of the particles: fillers consisting of big spherical particles having a smooth and closed surface (being non-porous) have a low specific surface area.
Watch Tutorial: How to reduce the fineness of grind »
Impact of Pigment Volume Concentration (PVC)
PVC is defined as the volume percentage of solid particles, being pigments and fillers, of systems that have formed a film. This implies that the PVC of a system must be calculated without the volatile components like water and/or solvents.
Formulations are most often given in weight parts of the components. As PVC is about the volume percentage, all weights of the components have to be transferred into volumes by using the density of each of the components. A filler of high density will have a low influence on the PVC of a system.
Influence on Sandability of Primer Coatings
- Primers are often sanded before a topcoat is applied. Good sandability can be obtained by using fillers of low Moh hardness (see Wear of equipment), like talc, in the primer.
Selecting Fillers for Topcoats
Selecting Fillers for Topcoats
A topcoat is a coating layer being in direct contact with the surroundings. Let’s discuss how fillers influence key properties of topcoats:
- Abrasion and Scratch Resistance in Topcoats - Resistance against mechanical damaging, like abrasion or scratching, of a coating can be improved by using fillers of high Moh hardness, see Wear of equipment. Hard fillers, like quartz and feldspar, improve the resistance of coatings against mechanical wear like abrasion and scratching.
- Barrier Properties / Permeability - Barrier properties of a topcoat can be improved by using fillers having platelet shapes.
- Chemical Resistance of Topcoat - Chemical resistance of a topcoat is influenced by the chemical composition of the fillers used and can go down when the fillers are used that have insufficient chemical fastness.
- Gloss & Matt Effect - Fillers having high oil absorption value and having high specific surface area will effectively lower gloss of a coating.
- Intercoat Adhesion - Intercoat adhesion refers to the adhesion of the topcoat onto the primer. Specific fillers, having platelet shape, can improve intercoat adhesion.
- Mechanical Strength - Filler particles having a platelet shape can improve the mechanical strength of a topcoat because platelets improve the coherence of a film.
- Pigment Volume Concentration - Density of a filler property influences the PVC of a topcoat, a filler of high density will have a low influence on the PVC of a system.
- UV Resistance - The influence solid particles, like fillers, can have on the UV resistance of a topcoat depends upon the chemical composition of the particles. Most fillers are not sensitive towards UV-radiation in sunlight. In general, the UV-resistance of topcoats will therefore not be influenced by the choice of the fillers used in the system. An exception is mica, being able to absorb UV radiation, thus being able to improve the UV resistance of a topcoat.
Fillers/Fibers for Paints and Coatings
View a wide range of filler/fiber grades available in the market today, analyze technical data of each product, get technical assistance or request samples.