The Effect and Use of Filler Pigments in Paints and Plastics

Definition of Fillers

Filler pigments, or fillers for short, are generally used to reduce the cost of a paint formulation. However, they can also alter properties of the paint such as flow, viscosity, settling stability, and film durability. Fillers are paint additives that are insoluble in binders and solvents and have no opacifying or coloring effect on the film or very minimal covering power. They are used to modify the flow properties of liquid paint and also the mechanical permeability, gloss, and spreading properties of the paint film. Fillers are fundamentally used, as the name suggests, to fill a paint's pigmentation with an economical material. However, in doing so, fillers impart certain properties to the paint, and these contribute to the value and use of fillers. For example, they can be used to control the viscosity of paint, improve flow properties, and minimize the hard settling of heavier pigments. They can provide durability or matting properties and enable the production of high-quality primers that ensure maximum inter-coat adhesion. The extent to which a filler material can apply pigmenting properties is fundamentally related to its refractive index. Most polymers have a refractive index of approximately 1.5, and therefore, minerals with values lower than 1.7 do not contribute in any way to the opacity of the paint and polymer and have no visible covering power. However, when compared to TiO2, which has a refractive index of approximately 2.7, it can provide very little opacifying effect. If a filler's refractive index significantly exceeds 1.7, this filler can be considered a pigment, but fillers and minerals with a refractive index below 1.7 are normally classified as filler materials. Table 1.

Critical Properties of Filler Pigments Affecting Paint and Polymer Performance

The basic effects of fillers can be roughly listed as follows. • Control of rheological properties, for example, giving a gel-like structure to a certain degree to reduce the flow of the paint film after application, • Thickening the paint to provide better structure and at the same time helping to reduce settling in the container, • In primers, providing some degree of overpigmentation so that the film remains "open" and inter-coat adhesion is improved. In addition, filler properties affecting paint and polymer performance can be briefly explained as follows. Mineralogical Properties: Chemical composition, crystal structure, Mohs hardness. Physical Properties: Gloss, refractive index, pH, inertness, oil absorption, purity, and presence of soluble salts. Particle Properties: Particle size, shape, size distribution, and aspect ratio.

Figure 1. Mohs hardness of minerals

According to Figure 1, talc is a better filler material to improve sanding properties in a primer surfacer, while a silica-based filler such as quartz (SiO₂) will provide better scrub resistance in an architectural wall paint due to its hardness. The chemical composition of a filler can also play a major role in determining the overall effect on performance. For example, calcium carbonate in exterior latex paint can be degraded in the presence of acid rain, producing carbon dioxide and calcium bicarbonate, which are soluble in water. This causes the film to become porous and allows calcium bicarbonate to form a slight frosting of insoluble calcium carbonate on the surface of the paint film. Fillers with pH values below 7 can intensify corrosion when used in metal primers. Aluminum in a filler provides acidity, while calcium, potassium, barium, and sodium provide alkalinity. If a filler contains soluble salts, these salts can promote blistering when exposed to moisture. The oil absorption value of a filler is fundamentally a function of how easily the filler's surface area is wetted by linseed oil. Low oil absorption properties provide, particularly in solvent-borne paints, low binder requirements and low paint viscosity. The refractive index (R.I.) of a pigment measures how light is bent when it passes from one medium to another. The larger the R.I., the more light is refracted and the higher the opacity. Most fillers have a refractive index similar to the paint binder and therefore do not contribute to a high degree of opacity in the cured film. Some filler pigments, such as fine particle-size alkaline aluminum silicate (nepheline), contribute to opacity by entering inter-particle spaces of TiO₂ and thus provide improved opacity at a lower cost. Table 1 provides some common properties of several filler pigments.

Table 1. Physical properties of the most well-known filler pigments

*OA=Oil absorption

Particle size, size distribution, and shape can play an important role in determining what effect fillers may have on paint performance. The particles shown in Figure 2 may have the same "particle size" as indicated by a particle analyzer, but will provide different physical properties. The aspect ratio in a needle-like or fibrous particle is the ratio of average length to average diameter (a fibrous filler 50 microns long and 5 microns wide would have an aspect ratio of 10). The aspect ratio in a plate-like particle is the ratio of the average diameter of a circle in the same area as the face of the plate to the average thickness of the plate. Needle- or thread-like particles tend to increase inter-coat adhesion, especially when oriented perpendicular to the surface; whereas, plate-like fillers oriented parallel to the surface can minimize the oxygen and moisture permeability of the surface. Examples of the latter include mica and micaceous iron oxide.

Classification of Fillers by Production Method Natural Fillers

Natural filler materials are primarily silicates, carbonates, or oxides. As raw materials, fillers of natural origin are found either as solid rock layers in the form of sediments or as granular formations. Those of granular type are generally separated into different particle sizes using physical separation methods such as hydrocyclones. Because their surface areas range from 1 to 50 m2/g, natural fillers are fillers ranging in value from semi-active to inactive. Some Known Natural FillersSynthetic Fillers: Some Synthetic Fillers Classification by Activity: In elastomers, inorganic fillers are generally used, except in some special applications. Classification based on the activity of the filler material or related to the production method of the material is also possible. Other classification criteria are particle size and magnitude, pH value, and the relative whiteness of the color. Although the differences between the classes specified are not sharp, fillers can be classified as: • Inactive, • Semi-active, • Active. The definition is normally related to the effect of the filler in paint on viscosity and its effect on mechanical properties such as tensile strength, abrasion and tear resistance.

Table 2. Refractive indices

Classification Related to Activity and Production

Generally, it is assumed that the degree of activity is influenced by the interaction forces that arise between the polymer and the filler. A common method for measuring activity, particularly of white fillers, is nitrogen adsorption: calculated as BET surface area in m²/g (BET=Brunauer Emmett and Teller). Theoretically, it is possible to calculate a particle's surface area using the mathematical basis of a sphere related to particle size. However, calculated values are generally smaller than values obtained by adsorption methods. The adsorption method more realistically shows the true surface area. The difference between calculated and measured surface area is known as "internal surface area." It is also possible to classify white fillers according to BET surface, but in published literature there is often disagreement regarding appropriate ranges in all cases. In general, active fillers have more effect on the properties of a compound than inactive types. In most cases, optimum properties obtained from active fillers are precisely related to a minimum or maximum filler input, whereas in the case of inactive fillers, a change in properties develops proportionally with the input level. In general, active fillers have greater effect on the properties of a compound than inactive types. In most cases, optimum properties obtained from active fillers are precisely related to a minimum or maximum filler input, whereas in the case of inactive fillers, a change in properties develops in direct proportion to the input level. Comparison of filler activities in different polymers shows that filler classification may differ from classification based on BET surface. In this case, the effects of chemical composition, the pH value of the filler, polymer type, and other properties are important.

In general, classifying a filler according to BET surface really only shows theoretical guidance. For practical purposes, the effectiveness of a filler material for use in a specific polymer and specific application is determined only by testing with appropriate test methods.

Figure 2. Particle geometry, structure, and particle size forms

Particle structures of fillers can normally be classified into spherical, laminar, or cylindrical types. The primary sources of silicates used as filler material in polymers are kaolinites (Al₄(OH)8 [Si₄O₁₀] or A1₂O₃ 2SiO₂ 2H₂O). The structure of kaolinites is laminar and can be separated with hydrocarbons by additional classification if necessary. Their average particle sizes range from 1 to 5 microns. The role of filler pigments in paints and coatings; fillers are used in paints and coatings to provide or enhance the following properties. The absorbency of a pigment or filler is undoubtedly related to the porosity of the pigment particle. The binder of the pigment is absorbed into and adsorbed onto the pigment surface. This is why highly absorbent materials such as diatomaceous silica have high oil absorption values. These materials produced from the skeletal remains of primordial diatoms are also widely used as matting agents. However, as shown in the table, fillers are currently used by paint manufacturers for many purposes far beyond just fillers and matting agents.

Fillers with Nodular/Block Particle Shape

In a paint, if a pigment system absorbs more binder (and CPVC decreases), the solid content of the paint, proportionally consuming binder, inevitably increases viscosity in the system. When a pigment system is organized, more binder is absorbed (and CPVC decreases), the amount of solids and relatively binder-consuming system viscosity also inevitably increase. To remedy this, a solvent is added that reduces both the amount of solids and increases VOC. However, this is not a desirable outcome either in terms of cost or in increasingly restrictive regulations. Therefore, fillers with lower oil absorption values have real advantages. This has become a basic requirement in filler selection.

Fillers with Laminar/Flat Plate Particle Shape

Flat, plate-type fillers such as mica and some talc and clay are quite absorbent due to their very high surface area per unit weight compared to nodular fillers such as silica, barite, or calcium carbonate. These fillers tend to improve the mechanical properties of the film through lateral reinforcement. At the same time, it has been found that these types of flat plate fillers not only strengthen the film but also minimize internal stress buildup by providing better stress distribution across the pigment plane. Films containing mica have been found to have stress distribution properties. Mica and other flat plate-type fillers have shown some important values in reducing the transmission properties of the film. They provide a barrier through overlapping plates, affecting the passage of water, oxygen, and ionic solutions in a way that minimizes it.

Fillers with Rod-Shaped Particle Form

Wollastonite, with its rod crystal structure, is a unique property that contributes to increased film strength. Therefore, this filler is used in the reinforcement of gel coat coating polyester layers used in places such as bathtubs in tanks made of fiberglass. Film reinforcement with flat, plate-shaped filler pigments - especially rod-shaped fillers such as wollastonite, is recommended in many paints and coatings that require reinforcement to prevent cracking during service. This loss of adhesion is especially dangerous when it occurs as a result of the film becoming brittle from aging or exposure to UV or other destructive radiation. Oxidative systems that tend toward cross-linking on aging cause cracking unless properly reinforced. Rods have a lower surface area than plates or fibrous forms, and therefore, it is not surprising that reinforcement with rod-shaped wollastonite crystals has less adverse effect on viscosity reduction and allows for lower VOC values compared to the use of plate-shaped fillers.

Fillers with Fibrous (Fiber) Particle Shape

Fibrous fillers, particularly asbestos, have long been used as a reinforcing filler material. In this regard, they functioned in the same way as rod-shaped fillers. Fibrous asbestos was widely used as a filler in light-sensitive (asphalt) top coats, particularly in roof coatings where fibers held the paint or coating together, preventing cracking and alligator skin-like appearance on the surface of the cured film when exposed to UV light. After asbestos was abandoned in industry due to its toxic status, non-mineral-based fillers were used. Fibrous polyolefin slurries, Kevlar®, acrylics, and cellulosics are examples of such fillers. Applications of fibrous fillers include block fillers, joint compounds, textured and special effect paints, and roof coatings. Combinations of cellulosic materials and slurries such as microfiber extensions with high aspect ratios (wollastonite) have been used as reinforcing materials in place of asbestos in asbestos roof coatings. Mica, besides its reinforcing properties, also has sealant properties and is widely used in drywall joint compounds.

Hardness in Fillers

Another important property of fillers is the hardness of the mineral. The hardest fillers are predominantly silicas, especially quartz-type crystalline silicas. These materials are used to increase the hardness and abrasion resistance of paints and coatings, provide optimum brushing and abrasion resistance in interior emulsion paints, and provide abrasion resistance in floor coatings and maintenance paints. Their coarser types, visible on the film surface, will give excellent "tooth" to primers that can be applied coat over coat. "Amorphous" silicas, which are micronized versions of the same crystalline materials used in floor coatings, are used in this way. In situations where slipping is likely, additional coarse crystalline silicas can be spread on the wet film before drying, excess "unbonded" material is swept from the floor after the floor coat dries. However, the toxicological hazards of free silica have begun in recent years to limit the use of such materials in coatings. Slightly less hard than silica are nepheline syenite and wollastonite. These materials can be considered as suitable alternatives to silica, and wollastonite has recently begun to gain importance in traffic paints.

Chemical Effects in Fillers

The physical aspects of fillers are particularly important for their use in paints, but their chemical composition is as important as their physical aspects. Fillers were once classified as inert pigments. Today we know this is not true. Some fillers such as calcium carbonate or chalk are easily affected by acids. Therefore, they should be avoided in the pigmentation of alkali-sensitive systems such as carboxylated vinyls and water-based alkyds that could react with them, or in the pigmentation of systems that will be in acidic environments. Acid-catalyzed systems will require some adjustment at the catalyst level if such alkaline pigments are used. Care should also be taken not to use alkaline fillers with alkali-sensitive color pigments such as iron blue. Films containing alkaline pigments are also susceptible to staining by soluble iron and copper compounds. The dissolution of fillers and the subsequent separation of their salts from the film in acidic environments will tend to weaken the film. This is also the case where calcium sulfate salts tend to accumulate as white crystalline residue on dark-colored films in places where soluble salts cannot be dissolved by rain, and can also lead to other defects such as freezing. Wollastonite is again more useful as a corrosion-resistant auxiliary filler pigment. This filler, although quite basic, is neither soluble nor acid-sensitive like carbonates.

Wollastonite has been successfully used as a filler in epoxy systems for acid resistance, but is sufficiently soluble to provide the degree of pH control required in metal primers for improved passivity.

In this context, wollastonite has been successfully used as an auxiliary pigment in anticorrosive systems used to reduce the usage levels of more expensive corrosion-inhibiting pigments to provide the required passivation. In addition, wollastonite combined with flat, plate-shaped passive pigments such as aluminum flake is used in barrier systems. Mica is also used as a filler for aluminum barrier systems. Aluminum flake given up to 25% by weight has been replaced by mica without adverse effects on corrosion resistance. Because mica has better chemical resistance than aluminum, this method becomes particularly important in cases where the film reaches extreme pH levels. In barrier systems, the hydrophilic degree must be controlled, as "water-loving" fillers such as china clay will tend to promote water entry into the film. Therefore, mica and plate-shaped talc and chlorites are used much more in barrier systems than china clay. On the other hand, china clays can be used more effectively in anticorrosive systems. Plate-shaped china clays such as talc have also been found to have a tendency to reinforce the film and improve the sanding of industrial primers. In the past, some materials used in paints had quite high water solubility. Anhydrous calcium sulfate falls into this category. Its use has now declined significantly, but it was used for many years as a complex with TiO2. Barium sulfate is an inert nodular pigment that provides excellent enamel retention properties. Barium sulfate is found in two forms. These are naturally ground barium sulfates and the less commonly used but whiter precipitated-type barium sulfates. Precipitated barites generally have higher oil absorption values than natural barium sulfates and are of finer particle size. "Blanc fixe," a softer precipitated-type barium sulfate, disperses easily and is widely used in auto repair primers and primers for gloss systems. Barium sulfate is the heaviest filler with the lowest oil absorption properties. It is widely used in low-VOC systems, but additional support may be needed to control settling.

Effect of Fillers on Opacity

Fillers do not increase the true opacity or color of the paint film. Opacity, or covering power, is the physical effect of light beams refracted and scattered within the film at the interface between pigment and binder. The more light is refracted, the lower the light transmission through the film and the higher the opacity. Light is refracted to a greater extent when there is a large difference in refractive index between the pigment and binder on both sides of this interface. A more complex way to achieve the highest opacity efficiency is the use of delaminated or layered calcined kaolin plates to separate adjacent TiO₂ particles. For this to occur, each adjacent TiO₂ particle must stop the light wavelength at the same point at the specified wavelength. Because the wavelength of light is on average somewhat larger (0.4-0.7m) than the diameter of an optimally dispersed TiO₂ particle (0.25m), it is necessary to use intermediate plates to increase the distance between two adjacent particles to the center. TiO₂ thus becomes equal to one complete wavelength. Although the system is complex, it works effectively and is used in many architectural paints.

Other Uses of Fillers

Besides paints and coatings, fillers are widely used in other industries. Examples include plastics, ceramics, rubbers, adhesives and sealants, mining, cosmetics, and asphalt production. The use of fillers in papermaking is particularly important. Here, acidic papermaking processes lead to alkaline systems, so materials such as china clay and calcium carbonate are used. Mehmet Namık Kayaalp Chemical Engineer Ecelak Paint Chemistry Ltd. Co.         References 1. Paint Formulation, by J.Boxall and J.A.von Fraunhofer. Gorge Godvin Limited, London 2. Jocca, Oil, Colours Chemists'Association-London 1966 3. Hoffmann Minerals 4. Payne, H.F. "Organic Coatings Technology," Vol II, p.773-804, Wiley, NY. 1961. 5. Extenders, By Clive Hare / Coating System Design Inc., Lakeville, MA and Roland Beck / Manager, Marketing, NYCO Minerals Inc., Calgary, Alberta, Canada 6. Prospector Knowledge Center, Ronald J. Lewarchik, President and CEO of Chemical Dynamics, LLC,