How to improve gloss in paints and coatings?

The glossy aspect is an optical property of paints. The gloss is due to the high reflectivity of the surface of the paint film. It is strongly related to the application area (car body, kitchen, doors, etc.) it is intended to be used.

Initially when the coating is applied there is some level of glossiness. But this gloss starts to degrade affecting the appearance of the coating due to the:

• environmental factors and
• aging process

Gloss retention is the ability of the coatings or finishes to retain their glossiness. This becomes an important characteristic to have long-lasting performance and visual appeal.

In cases where there is no possibility to retain the gloss anymore, gloss enhancement could be the way to go. Here, the level of gloss and shine is increased. This is done by using specific products or techniques to bring back the shine that is lost due to several degradation factors.

Get an in-depth understanding of the gloss in paints and coatings:

1. What is gloss?
2. What are the types of paint finishes of gloss and matt paints?
3. What factors influence gloss enhancement and retention in coatings?
4. How is the gloss of paints and coatings measured?

What is gloss?

Gloss is a measure of the paint finish when light reflects off it. It increases the ability of the painted surface to reflect light. The more light a surface can reflect, the glossier it appears. It can be said that the smoother the coating surface, the glossier it appears.

• If the coating surface has few microscopic irregularities, it reflects light more easily. This gives a shiny or glossier finish.
• In the same way, if there are many irregularities, the surface appears dull.

This optical property is a critical acceptance factor. Achieving the right gloss plays a crucial role in altering the visual appearance of the product.

Composition of gloss coatings

Gloss coatings consist of the resins that will become the final coating, pigments, and other additives. They are dissolved in some compatible solvent blend.

• Resins – They may be very viscous liquids or even solids by themselves.
• Solvents – They reduce the viscosity and aid the surface leveling of the coating.
• Additives – They are added to impart durability, and moisture resistance and some of them impart a shine or dull finish.

A gloss finish, as it dries, leaves a surface that is smooth within a small fraction of a wavelength of light. Paints with glossier finishes, like satin paint, have higher levels of resin and lower levels of pigmentation. High resin quantities allow the paint to flow more smoothly and evenly than low resin paints. High resin quantities keep pigment particles low and embedded within the resin. This gives the painted surface a strong glossiness.

Key properties

• They have a highly reflective sheen.
• They are very durable and stable, easy to clean and maintain.
• They exaggerate imperfections on surfaces.
• They are a good choice for surfaces that need frequent cleaning.

On the opposite end of the scale are flat (matt) paints that reduce glare and hide imperfections in the surface and finish.

Is your gloss too low? Here is a selection of resources that you can refer to solve your gloss problem.

  

What are the types of paint finishes of gloss and matt paints?

Paint finish (or sheen) refers to the amount of light that the paint reflects from its surface. The most common categories of paint finish are matt and gloss. Each of these gloss finishes offers unique characteristics and applications. Both gloss and matt finishes have a wide range of variants, ranging from:

1. High gloss finish — These coatings provide the highest shine and reflectivity. They offer a mirror-like appearance and are very durable. This makes them suitable for surfaces that require maximum protection and a glossy finish. High gloss finishes are commonly used on automotive surfaces, metal fixtures, and architectural accents.


2. Gloss finish — They offer high shine and reflectivity with excellent durability. They are highly resistant to moisture, stains, and wear. Gloss finishes are often used where a dramatic and lustrous appearance is needed. For example, metal surfaces, exterior doors, window frames, furniture accents, and decorative elements.


3. Semi-gloss and gloss paints: — They reflect light, giving them a bright, shiny finish. They deliver exceptionally smooth surfaces with few microscopic irregularities. They are suitable for surfaces that require frequent cleaning like trim, cabinets, doors, and furniture.


4. Satin finish — In satin finishes the sheen falls between matte and semi-gloss. They provide a smooth, silky appearance along with a higher level of durability and washability. Satin finishes are commonly used in hallways, kitchens, bathrooms, etc.


5. Eggshell finish — They offer a subtle glow and a smooth, velvety appearance with a slight sheen. Eggshell finishes are more durable than matte finishes. They are suitable for walls in living rooms, dining rooms, and bedrooms.


6. Flat or matt paints — They absorb more light than they reflect due to rough surfaces, so they have no sheen. They provide a smooth appearance and work well at hiding surface imperfections. Matte finishes are commonly used in interior walls, ceilings, and low-traffic areas.

Different types of finishes along with the percentage of light reflectance are mentioned in the table below.

Paint Finish	Description
	Type: Full gloss
	Light Reflectance: 70% - 90%
	Description: Shows clear specular reflection at all angles of view
	Type: Semi-gloss
	Light Reflectance: 41% - 69%
	Description: Shows specular reflection when viewed at low angles to the surface but only a hazy reflection at higher angles
	Type: Sheen & satin
	Light Reflectance: 15% - 40%
	Description: Shows partial reflection of light
	Type: Eggshell
	Light Reflectance: 10% - 15%
	Description: Shows hazy reflection for all angles of view with clear specular reflection near grazing incidence
	Type: Matt
	Light Reflectance: <10%
	Description: Shows no specular reflection even at grazing incidence

What factors influence gloss enhancement and retention in coatings?

The role of surface roughness & additives

Gloss is a function of roughness and can be used to characterize surface roughness. The smaller the roughness of the surface of the object, the more light is reflected and the higher the gloss. On the contrary, if the surface of the object is uneven, it induces diffused light reflection. This reduces the amount of light reflected, resulting in a decrease in gloss.

The surface modifying additives help to enhance gloss by:

• helping the paint to level out after application and
• increasing pigment dispersion

There are specialty surfactants and leveling agents designed to enhance paint leveling and thus, act as a gloss enhancer. Pigments are also used to vary the surface roughness in an attempt to increase or decrease gloss. The protrusion of pigments causes the reflected light to scatter resulting in low gloss.

Concentration of pigment in the formulation

There are two main concepts governing the quantity of pigments during the coating formulation process. They are:

• Pigment volume concentration (PVC) — PVC is of fundamental concern when formulating paints that are needed to have an optimum performance with respect to durability.
• Pigment-to-binder ratio (P:B) — P:B ratio, by weight or occasionally by volume, is often used for balancing a formulation for gloss and opacity.

For high gloss coating systems, low PVC is required, while primers and undercoats can have much higher PVC. This can be up to 90% because they need to be more durable and flexible.

Dispersibility of pigment

The dispersibility of pigment or the solid particles in a formulation directly affects the gloss of the coating. Developing a high-performing coating with the right characteristics requires:

• proper dispersion
• correct raw material and equipment, and
• precise final particle size

The solid particles need to be dispersed evenly to have a higher gloss value. The undispersed solid particles cause irregularities in the film that form after the coating dries. This in turn can lower the gloss values considerably.

The solid particles tend to glue together forming clumps or agglomerates if not dispersed properly. Selection of the right dispersant can avoid flocculation and stabilize the solid particles. A compatible dispersant can help achieve the right final particle size as needed for the coating.

Particle size and distribution

The pigment particle size and particle size distribution are crucial factors affecting the gloss of the coating. The final particle size determines the uniformity and in turn the glossiness of the applied coatings. The smaller the particle size, the glossier the coatings. A uniform particle size of the pigment allows the light to reflect better. This in turn makes the applied coating appear smoother and glossier.

The ability of a given pigment to absorb light (tinctorial power) increases when the diameter of the particles is reduced. This is done by increasing the specific surface until the size at which the particles become transparent to incident light is attained. For example, in the case of titanium oxide (TiO₂), the optimal size required is close to 1 µm.

How is the gloss of paints and coatings measured?

Gloss is determined by measuring the percent reflectance using a simple photodetector arranged. This ensures that illumination and detection occur at equal and opposite angles. It is measured using a glossmeter on a scale from 0 to 100, with:

• 0 being no gloss and
• 100 being perfectly mirror-like

These gloss levels are measured at different angles i.e. 20°, 60°, or 85° depending on the gloss level to be achieved. This is based on the angle of sight that these paints would normally be seen. For example, flat paints are measured at 85° as they are often used on the ceiling. Whereas, gloss paints are measured at 20°, as they are normally seen straight on (e.g. doors).

Paint Type	Gloss Units (GU)	Degree of Gloss Measurement
Ultra-flat	≤1	85°
Flat	1-2	85°
Matt	3-7	60°
Low sheen	8-12	60°
Satin	20-30	60°
Gloss	40-80	20°
High gloss	> 80	20°

Standards test methods to evaluate gloss

• ISO 2813 2000 (International) — It determines the specular gloss of non-metallic paint films at 20°, 60°, and 85°.


• ASTM D523 - 14 (2018) — It determines the specular gloss at roughly corresponding angles.

The following are technically similar to above standards but specific for the mentioned countries:

• DIN 67530 1982 (Germany) — It determines the specular gloss of smooth painted and plastic surfaces using a reflectometer.


• NFT 30-064 1999 (France) — It determines the specular gloss of paints at 20°, 60°, and 85°.


• AS 1580 MTD 602.2 1996 (Australia) — It is a test method for paints and related materials.


• JIS Z8741 1997 (Japan) — It determines the specular glossiness.


• SS 18 41 84 1982 (Sweden) — It determines the specular gloss of non-metallic paint films at 20°, 60°, & 85°.


• BS 3900: Part D5 1995 (UK) — It determines the specular gloss of non-metallic paint films at 20°, 60°, and 85° (withdrawn).