Matting in Paints and Coatings - All You Need To Know

1. What is Matting Effect?
2. Optical Surface Modification to Obtain Matt Surface
3. Matting Agents – How Do They Work?
4. Raw Materials to Achieve Matting in Paints and Coatings
5. What are the Key Parameters that Influence the Quality of Matting?
6. Popular Test Methods to Assess Matt Coatings

What is Matting Effect?

Matting is a property of coatings that delivers a smooth, subtle look and shows little-to-no sheen. Matting lowers the gloss/sheen by scattering the light in many directions leading to surface appearance less glossy.

Matt coatings are used to obtain a less glossy surface depending on the application. These coatings are widely used to coat substrates, such as wood, plastics, VCT, metals, etc. to:

• Reduce glare & enhance appearance (aesthetic purpose), and
• Hide surface imperfections (functional purpose)

There are several options to the coatings formulator to create a matt coating. Matting agents, such as silica, waxes and fillers are mixed with coatings and paints in order to impart a matt finish to the substrate. These include:

Matting agents when used with paints and coatings scatter the incoming light thus giving a matt effect to the desired surface. Moreover, the higher the dosing of matting agent in the paint, the stronger is the matting effect.

Several matting agents are available in the market depending upon their particle size and chemical composition and the best choice can be made depending on the application.

Let's explore how to obtain matt surface with different optical effects using various types of matting agents.

Optical Surface Modification to Obtain Matt Surface

In many cases, the surface of a coating or ink has to be altered to a different optical effect like a lower gloss or a certain texture. Reducing the gloss of a system is achieved by "destroying" the smooth surface of the glossy film, i.e. by introducing a kind of "micro-roughness". Surface roughness reduces the amount of specular reflected light, thus resulting in low gloss surface.


Therefore, matting can be defined as a physiological-optical phenomenon produced by the surface of an object. It is subjective impressions that are identified by our senses.

Matting Agents – How Do They Work?

When a coating is applied to a substrate, the matting agent is uniformly distributed in the wet coating film. Upon the evaporation of solvents, they will extend beyond the film surface during the drying process due to film shrinkage, thus altering the film from a smooth and even - glossy - surface to a matt one.

This shrinkage is the main reason for creating (more or less) rough surface, or, in other words, a matted paint film. This shrinkage can vary from 30% to 60% of the wet film’s volume depending on the formulation’s volume solids.


Film surface micro-roughness is created by:

• Film shrinkage/volume reduction
• Size and number of matting agent particles
• Orientation of the particles
• State of dispersion
• Interactions with other components, e.g. surfactants, catalysts, etc.

It is important to note that conventional coatings are relatively easy to matt because the film shrinks upon hardening due to emissions, but solvent-free and 100% solid systems, such as radiation-curable coatings, powder coatings, etc. are difficult to matt.

Raw Materials to Achieve Matting in Paints and Coatings

Matting additives are used to achieve the desired matt surface depending on the application. Conventional matting agents include ultrafine silica, waxes, talc, aluminum stearate, calcium stearate, and low molecular weight thermoplastic resins.

Silica

Conventional silicas (precipitated and pyrogenic) are known to be highly effective in reducing the gloss of all kinds of systems including solvent as well as waterborne.


When added in a sufficient quantity to a coating or ink, they will extend beyond the film surface when the film shrinks during the drying process, thus altering the film from a smooth and even glossy surface to a matt one.

Use of silica-based matting has the following advantages / disadvantages:

Advantages	Disadvantages
Very efficient at low level of addition High wet film clarity  Economic	Sensitive to scratches  Imparts water sensitivity at high concentration  Causes “stress cracking” on flexible substrates  Very light and difficult to add during manufacturing  Can show hard settling in the coating after time  Limited matting, they are far less efficient in modern high solid- or solvent-free systems.

Silica Gel

Modern silica gel technology enables the production of highly pure, porous products. A silica gel is an amorphous form of silica composed of nearly 100% silicone dioxide (SiO₂) produced synthetically in a liquid process.


The physical properties of silica gel differ from other specialty silicas. The internal structure of silica gel is composed of a large network of interconnected microscopic pores that attract and hold water, hydrocarbons, and other chemicals by the mechanism of physical adsorption and capillary condensation. This huge pore volume and extensive surface area give the silica gel many of its unique properties.

Silica gels are widely used as matting agents in coatings.

• The rigidity of the silica gel particle is higher than that for precipitated silica and thus better resists the shear forces arising during the manufacture and application of the paint.
• Silica gels can influence rheological properties, although other agents are typically preferred for rheological control purposes.
• The application segments for silica gel matting agents include coil coatings, industrial wood coatings, general industrial and automotive coatings as well as printing inks, leather/textile coatings and decorative coatings.

Waxes

Waxes are typical additives that significantly influence the surface properties of any coatings by modifying the surface free energy. Waxes also may function as matting agents and they are incorporated in paints and coatings in different form, such as:

• Micronized
• Micro powder
• Emulsified, and
• Dispersions

Waxes should be present at the surface of the system to function as a matting agent depending on the type of wax used and the particle size of the wax. Various wax grades fall in two groups:

Natural Waxes	Synthetic Waxes
Carnauba wax Beeswax Shellac Ozokerite wax, etc.	Polyethylene wax Polypropylene wax PTFE wax Polyamide wax Montana wax, etc.

Wax emulsions and dispersions are formulated additives made of fine and stabilized wax particles, homogeneously distributed in the water. Wax dispersions aid in imparting a matting effect to most aqueous coatings and varnishes, while enhancing other properties such as abrasion resistance and water repellency.

As a coating film dries, wax particles act by virtue of their capacity to float to the surface of the film. In wax dispersions, particles are transported to the film surface because of the turbulence flow induced by the water evaporation. The protruding wax particles create a certain degree of micro-roughness, induced as the film shrinks. As a result, the incident light is scattered instead of being reflected, conveying the impression of low gloss or matt appearance.

The degree of the micro-roughness is determined by the number of particles present at the surface, which directly depends on wax properties such as:

• Particle size and particle size distribution
• Particle density
• Amount of matting agent incorporated

Silica vs Waxes

In "conventional" systems, waxes are approximately 50% less effective when compared to silica. In other words, if an addition of 2% silica is needed to accomplish the desired gloss, approximately 4% of a good matting wax is required to arrive at the same matting.

However, in the case of high-solid or solvent-free systems, especially for deep matt coatings, the amount of silica needed is significantly high which causes a strong increase in viscosity and add additional thixotropy. Therefore, in such systems, the right selected wax shows better matting compared to silica. This is due to the ability of the wax to float to the surface. This means that with low additions of wax, good matting can be achieved while keeping the viscosity of the system at acceptable levels.

The most difficult property to reach in the case of matt coatings is to preserve or obtain good resistance against scratching. The lower the gloss, the more difficult it will be to prevent a scratch sensitive surface. Once more, silica and waxes behave differently in this characteristic.

• Scratching the surface of a coating matted with silica only, will often result in a white marking.
• Doing the same with a Wax matted surface will show less sensitivity to scratching but, depending on the wax used, it may result in a "glossy scratch".

The explanation is obvious: Scratching a silica matted surface will break-down the protruding silica particles, resulting in white marking. As waxes are not as brittle as silica - but are prone to temperature changes - the protruding wax particle will not break but melts under the high pressure and "smears" over the coatings surface thereby producing a glossy scratch.

Fillers

Functional fillers are fine materials added to paints and coatings to obtain low-gloss surfaces, such as:

• Diatomaceous earth
• Kaolin
• Fumed silica
• Metallic stearates, etc.

The addition of fillers causes a significant decrease in gloss. When the coating film is dried, their fine particles form a micro-rough surface on the surface of the coating film, reducing the reflection of light to obtain a matt appearance.

The matting effect of such matting agents is subject to many factors. For example, the particle size of the filler has a major influence on the gloss of the coating film. The larger the particle size, the lower the gloss. It is to be noted that increasing the amount of the filler causes a decrease in the flatness and mechanical strength of the coating film.

In powder coatings, matt finish majorly depends on the coating thickness and the loading level of the filler(s). Apart from the loading level of the filler(s), the level of gloss is strongly affected by:

• Particle shape
• Particle size, and
• Particle size distribution (PSD)

Nanomaterials

In recent years, new strategies for using inorganic & organic nanomaterials in matt coatings have emerged at the forefront especially for 100% solid coatings.

The challenge: Most matting agents increase viscosity, a problem that is usually addressed by the addition of reactive diluents. Some monomers may be added as well, but their addition significantly affects the properties of coatings.

One strategy includes the dispersion of amorphous nanoparticles in monomer components to produce additives for the formulation of matt coatings with nearly zero increase in viscosity.

Get detailed information about the various types of matting agents used in coatings and inks to make the right selection for your end-use applications.

What are the Key Parameters that Influence the Quality of Matting?

Several factors that influence the quality of matting in paints and coatings. Some of them are discussed below.

Chemical and Physical Properties of Matting Agents

The chemical and physical properties of matting agents, such as pore volume, average particle size and particle size distribution, and whether the particle surface is treated or not, etc., will affect its matting effect.

• Concentration of the Matting Agent – The gloss of the system is reduced with an increased concentration of matting agents.
• Particle Size – The bigger the particles of the matting agent, the more effective is the matting. Especially true for precipitated silica and silica gels. However, particle size should be taken into consideration w.r.t paint film thickness while selecting matting agent. Coarse matting agents can negatively affect other properties in a thin coating film.
• Pore Volume (Density) – The porosity of this highly porous material is expressed by the pore volume, which indicates the number of internal voids (e.g. silica). High pore volume of matting agent can influence rheology of the system.

Type of Coating System

As discussed above, creation of matt surface is directly related to surface roughness. Paints with higher solvent and low solids content are easy to matt. However, in the low shrinkage systems, the mechanism of matting does not function any longer because of the lack of sufficient solvent as there is no or very little shrinkage of the film. Such systems are:

• Powder coatings
• Radiation-curing systems
• High-solids systems
• Waterborne system

This means that the percentage of silica that has to be added to break down the gloss must be increased to much higher levels. But then, the viscosity and rheology properties of the system rises beyond acceptable levels. In other words, it is virtually impossible to reach sufficient matting and good application properties at the same time.

WHAT IS THE SOLUTION TO THIS PROBLEM?  The only solution to this problem is to use additives that are not depending on film shrinkage for breaking the smooth surface of the film. They should be added in relatively low percentages to maintain the application properties but still be effective at the surface of the film.

Additives in the Formulation

Coating additives can have a crucial effect on the effectiveness of matting agent in the coating system. Additives, such as leveling agents influence the wetting behavior and surface tension of the wet paint and so can strongly influence the gloss of the system.

Application Method and Film Thickness

Different methods such as curtain coating, spraying, brushing, or roller coating have an influence on the orientation of the matting agent particles and the evenness of the film thickness. Thicker coatings generally require more matting agent.

Curing of the Coating

The curing characteristics have a significant impact on the matting properties of the coating. Conventional coating systems, i.e. water and solvent-based coating contain volatile components. They are easy to matt because during curing, film shrinkage happens by evaporation of volatile compounds. The speed of solvent evaporation plays a key role in the degree of roughening of a film surface.

However, in the case of high-solid systems, volatile solvents are absent resulting in achieving matt surface a challenge. For example, the speed of cure in UV-curing coatings directly affects how easily a low gloss can be achieved on the film surface. Generally, slow-curing systems are relatively easy to matt, while the difficulty in reaching low gloss levels increases with increasing reactivity and cure speed.

Humidity

Relative humidity is an important parameter that can have a crucial influence on the matting of coatings during the solvent evaporation phase.

Dispersion and Suspension Behavior

Powder-based matting agents are often difficult to handle and only show their performance, if they are well-dispersed in the formulation. This is not always guaranteed, especially in aqueous coating systems. A variety of water-based matting agent concentrates come with optimum and controlled dispersing fineness and allow easy, dust-free handling and dosing to obtain desired matt surface finish.

Besides these factors, the rheology of the coating and type of substrate (e.g. wood has large pore size) also influence the matting appearance

Note: For targeted gloss level adjustment, matting agents should be selected to ensure they have a negative effect on chemical resistance, mechanical resistance or viscosity. Different matting agents can be used in combination to achieve semi-matt to matt gloss level range.

Popular Test Methods to Assess Matt Coatings

It is now clear that matt is a function of light reflection – the more light is scattered, the matt-ire the surface appears to be. The measure of the ability of the surface of the object to reflect light is called gloss. The higher the gloss of the surface of the object, the stronger its ability to reflect light and the higher the brightness.

The size of the gloss was measured by a photoelectric gloss meter. Glossmeter (also gloss meter) determined gloss percentage by projecting a beam of light at a fixed intensity and angle onto a surface and measuring the amount of reflected light at an equal but opposite angle.

There are a few different geometries available for gloss measurement each being dependent on the type of surface to be measured. For coatings, the amount of reflected light increases with an increase in the angle of illumination as some of the light penetrates the surface material and is absorbed into it or diffusely scattered from it depending on its color.

ASTM D523 - 14(2018) is the commonly used standard Test Method for Specular Gloss measurement. This test method covers the measurement of the specular gloss of most coating applications at three measurement angles (20°, 60°, and 85°). The gloss of hardwood flooring is typically measured at 60°.

For high gloss surfaces the 20° angle is used, for medium gloss ones the 60° angle and for very low gloss surfaces, the 85° angle (also known as “sheen”) is the most important. Gloss is measured in GU (gloss units) and is usually between 100 and 0.

Other standards that determine specular gloss of coatings at these measurement angles include:

• ISO 2813:2014 Paints and varnishes — Determination of gloss value at 20°, 60° and 85°
• ASTM D2457 - 13 Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics
• DIN 67530 - Reflectometer as a means for gloss assessment of plane surfaces of paint coatings and plastics

(Source: SPEKTROCHEM Paint R&D and Testing Laboratory)

Commercially Available Matting Agents Suitable for Paints and Coatings