Surface control additives are used in the coating industry to prevent surface defects during paint application and improve resistance and appearance of the dry film.
Possible surface defects during paint application are:
1. Orange peel
2. Pinholes
3. Craters
4. Fish eyes
5. Edge crawling
6. Air-draft sensitivity
7. Telegraphing
8. Floating (Bénard cells)
9. Silking

Orange peel: This typical appearance of the surface is only observed following spray applications. Three main factors influence this defect:

1. Viscosity of the sprayed liquid
2. Spraying conditions such as pressure, air/liquid ratio
3. Surface tension of the liquid

The first two factors depend strongly on the operator. The third factor can be controlled by the paint formulator. Lowering the surface tension of the liquid leads to smaller droplets under the same spray conditions and improves the flow of the single droplets after application.

Craters and pinholes:
Craters and pinholes are caused by liquid or solid contamination present on the substrate or in the paint, or from the surroundings, before the drying process starts. The reason for the formation of craters is the difference in surface tension between the liquid paint and the contaminant. Such defects increase, as surface tension differences grow. By lowering the surface tension of the liquid, such defects can be avoided. Pinholes are craters, where the liquid has not formed a homogeneous layer, thereby leaving a depression which penetrates to the substrate.

Fish eyes::
Fish eyes are caused by insufficient substrate wetting. This occurs when the surface tension of the liquid paint is higher than that of the substrate and no spreading takes place on its surface. Spreading is improved by lowering the surface tension of the liquid. See “Basic Principle of Wetting”.

Telegraphing:
Telegraphing (ghosting) occurs, when areas of different surface tension on the substrate are formed by wiping, by residual traces of a cleaning liquid or by finger prints. These marks appear on the surface of the applied paint film. This effect is clearly seen when the substrate/base coat contains interfacial active substances.

Air-draft sensitivity:
Local differences in surface tension of the paint by nonuniform evaporation of the solvent can cause air-draft sensitivity. Locally confined evaporation leads to an increased surface tension in the film. Rupture of the film can take place when this surface tension rises above the substrate’s surface tension, and dewetting takes place. This problem is avoided by lowering the surface tension of the liquid.

Edge crawling:
Edge crawling is found at the edges of the substrate, because the surface of the applied paint film is larger at the edges. Here faster evaporation of solvents takes place, leading to a stronger increase of surface tension than in the rest of the liquid film. A higher surface tension causes the surface to crawl, because it is trying to adopt a smaller overall size.This problem is avoided by lowering the surface tension of the liquid.

Floating (Bénard cells) and silking:
Floating and silking are related to processes taking place in the liquid paint during the drying phase. Differences in the density and surface tension give rise to turbulent flow of material from the lower to the upper part of the film. In pigmented systems the pigments settle in different areas depending on their mobility. On horizontal surfaces this floating is seen as hexagonal patterns, called Bénard cells. Silking has the same origin but is limited to vertical surfaces and shows as line-shaped patterns. These defects can be avoided by minimizing the surface tension differences occurring the drying process.

All these defects are caused by differences and changes in the surface tensions of the liquid paint, substrate and contaminants. In “Basic Principle of Wetting” it was shown that effective wetting of a solid takes place when the liquid has a lower surface tension than the solid. When the surface tension of the liquid paint is high, the system is more sensitive to wetting defects. In general, modern synthetic resins have a higher surface tension than those based on natural fatty acids. Aqueous systems have a high surface tension due to their polarity.

The main solution to these problems is to adjust the surface tension of the liquid to that of the solid. Therefore additives to improve levelling need a low intrinsic surface tension and a high mobility towards the interface they have to influence. The EFKA-range of levelling agents is based on the following chemical families:

1. Long chain polysiloxanes are the most common levelling agents in the coating industry. Pure polydimethylsiloxanes are not used in the EFKA-range due to their incompatibility with most resins. To improve this, the polydimethylsiloxane backbone is modified with alkyl or polyether side chains. In addition, reactive groups such as isocyanate, double bonds, hydroxyl groups and acid groups can be incorporated, leading to the advantage that the levelling agent can be crosslinked into the film. They are suitable for solvent-based systems, water-based systems or both, depending on the type of side-chain used.

2. Polyacrylates are produced from special monomers with a low surface tension. These structures move to the interface, equalising the surface tensions.They are known for not causing intercoat adhesion problems in the dry film. Depending on their modification, they can be used in solvent- or water-based systems.

3. Short chain polysiloxanes were developed to give silicone-based levelling agents which will not affect intercoat adhesion. The main benefit is found in water-based systems.An additional requirement for a coating is good slip. This so-called “slip” improves the aesthetic impression and helps to protect the film against scratches, metal marking and dirt. Slip properties are only achieved by using long chain polysiloxanes containing dimethylsiloxane groups. The friction at the surface caused by irregularities is reduced by the lubrication effect of the dimethylsiloxane groups concentrated at the surface, without producing a greasy layer.

The slip can easily be determined by slip angle measurement or by touch. Protection by slip is most important in the initial drying stage, where the surface is still vulnerable to mechanical actions. Only a small amount of silicone-levelling agent is needed for the optimum slip performance, whereas overdosage can lead to unwanted side-effects.

A common unwanted side-effect of some polyether-modified siloxanes is their surface-activity, sometimes giving rise to foam stabilisation. Often, therefore, the simultaneous use of a defoamer is recommended. For more information see “Defoaming Technology”.

Alternatively, an alkyl-modified siloxane or polyacrylate can be used as levelling agent to avoid foam formation during paint application. For more details on our range of slip and levelling agents for your system see our recommendations in the main menu choice “Application Field”.