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Fillers Selection for Paints and Coatings

Get familiar with the key factors to consider when selecting fillers depending on the final properties you want to reach for your paints and coatings. Also, you'll find out detailed information on types of fillers used and their influence on various properties such as abrasion resistance, gloss & matte effect, viscosity etc. when added in formulations whether it is a paint, primer or topcoat!

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 should to be taken into consideration when selecting fillers for coatings. Because, key system properties (below) are governed by the properties of the solid particles used in the system.


Impact of Fillers on Properties of Paints & Coatings

>> Check Out Several Filler Grades Available in the Market

Now, let’s discuss 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:

  1. Paints, being the liquid system
  2. Coatings, being the systems that have formed a film on a substrate

Paints and Coating Systems
Paints and Coatings (Primers & Topcoats)


Selecting Fillers for Paints


Fillers have a significant on key properties of paints. Their influence is important during production, storage and application. Here are the main factors which are influenced by filler:

Influence of Fillers on pH of Paints pH of water-based paints is important, for example with respect to colloidal stability.

Some fillers can influence pH strongly while others have only 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 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 micrometres (mm) or in nanometres (nm).


Fillers and Stability of PaintsStability of Paints - Different types of stability of solid particles in liquid systems are important:

  • Sedimentation/settling (Discussed Above)
  • 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
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 selecting 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 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 Influence of Wear of Equipment - Equipment might be damaged both during production and during 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 of production equipment, especially when the particles have sharp edges.


Influence of ViscosityInfluence of Viscosity - Viscosity of liquid systems is influenced by fillers, depending on size, shape and surface composition of the filler particles.

  • Particles having irregular shape will raise the viscosity of a system
  • Small or porous particles that adsorb high amounts of resin material result in high viscosity

Let’s see the impact of various fillers on key properties of liquid paints:

  pH Sedimentation Storage Stability Viscosity Wear of Equipment
Barium Sulfate black starblack star half star
Calcium Carbonate black star black star half star
Clay half star black star
Cristobalite black star black starblack star
Diatomaceous Earth half star black starblack star
Dolomite black star black star half star
Feldspar half star black star black starblack star
Glass powder black star
Kaolin black star half star half star black star
Mica half star half star black starblack star black star
Microspheres
Quartz black star black starblack star
Silica black star black star black starblack star black star
Talc half star black starblack star
- Positive Influence
half star - Average Influence
-Minor Influence
black starblack star - Negative Influence

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 of Fillers on Primer Properties

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

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.

Primers Help in Corrosion Protection 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.

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 high oil absorption value, for example, give 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 value 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.

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 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.

  Adhesion Barrier Properties Chemical Resistance Corrosion Resistance Gloss Pigment Volume Concentration Sandability
Barium Sulfate half star
Calcium Carbonate black star half star

Clay half star half star black star
Cristobalite

black starblack star
Diatomaceous Earth half star black starblack star
black starblack star
Dolomite black star half star
Feldspar half star half star
black starblack star
Glass powder black star
black starblack star
Kaolin half star half star half star black star
half star
Mica

half star
black starblack star
black star
Microspheres
black starblack star
Quartz


black starblack star
Silica black star black starblack star
black star
Talc

half star
black starblack star

-Positive Influence

half star- Average Influence

-Minor Influence

black starblack star- Negative Influence



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 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 shape.

  • 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 platelet shape can improve 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 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.


  Abrasion Resistance Barrier Properties Chemical Resistance Gloss Intercoat Adhesion Mechanical Strength Pigment Volume Concentration UV Resistance
Barium Sulfate black star




 half star

Calcium Carbonate black star

black star



 
Clay
half star

black star
half star
half star


Cristobalite





half star


Diatomaceous Earth


black star

half star


Dolomite black star

black star




Feldspar


half star





Glass powder








Kaolin black star
half star
half star
black star
half star
half star


Mica


half star
black starblack star





 half star
Microspheres







Quartz





half star


Silica


black starblack star half star
half star


Talc black starblack star


half star
black starblack star


half star



-Positive Influence

half star- Average Influence

-Minor Influence

black starblack star- Negative Influence

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