This research study discusses a variety of substrate-wetting additives for waterborne coatings and inks, giving special attention to a new class of silicone-free surfactants that combine dynamic wetting properties with foam inhibition.

This article covers different areas of application and the suitability of the different wetting agents in those areas. In addition, we consider the influence of the wetting additives' chemical composition on their performance in final application and give some guidance for the best choice of substrate-wetting additives.

How is Substrate Wetting Defined?

Substrate wetting is defined as the replacement of adherent air at the substrate surface by a liquid coating or ink material. This process is essential for successful coating application and printing.

Wetting is especially difficult on low-energy surfaces, i.e., substrates with low surface energy or substrates contaminated by dirty particles or liquid contaminants such as grease. In such cases, wetting defects can occur, such as crazing or crawling, or even poor adhesion of paint film (see Figure 1).

Even when the surface energy of the substrate is relatively high, wetting may become a critical factor if highly dynamic application processes are involved. On fast-running printing presses or roller-coater application, for example, wetting must be very fast to obtain good results. If the substrate wetting is too slow, uneven ink-lay, print defects or even poor ink transfer may result.

Under static conditions, the contact angle Q of a liquid drop placed on a given substrate determines whether wetting is good or poor (see Figure 2).

Good wetting occurs when the contact angle is relatively small.

In the formula, gs is the surface energy of the substrate, gI is the surface energy of the coating, and gsI is the interfacial energy between coating and substrate. The respective energies g are multiples of the energy differences between the energy of a molecule at the air interface - or in the liquid - and the molecule in the solid phase.

When the contact angle Q is greater than zero, incomplete wetting may occur with the application of low-thickness coating film. However, sufficiently high film thickness can still ensure a uniform coating layer.

An idealized coating might be thought of as having a contact angle Q = 0, with respect to the substrate. Liquids possess a spreading coefficient S, where S = 0; S > 0. Note that S must be positive in order for proper surface wetting to occur.

From the relationships expressed above, some general statements become possible.

A substrate with relatively high surface energy (gs) is easy to coat.

A liquid with relatively lower surface energy (gI) wets easily.

Particularly good wetting is obtained when the liquid has a substantially lower surface energy than the substrate (gI < gs).

The static surface tensions of commonly used solvents rank from ~ 14 mN/m for isopentane up to ~ 73 mN/m for water. Current waterborne coating systems, in particular, are likely to display wetting problems on substrates with low surface energy.

Measurement of Static Surface Tension

The best-known method for measuring the surface tension of liquids is the du Nouy Ring Method. A platinum-iridium ring is placed into the liquid, and then slowly pulled out so that a lamella is formed at the air interface. The force needed to pull this lamella is a direct measure for the surface tension of the liquid (see Figure 3).

This method is particularly suitable for comparing the surface tensions of aqueous solutions of various surfactants, or of clear solventborne coatings. Pigmented systems do not give reliable data because the presence of pigment hinders lamella stability.

Measurement of Dynamic Surface Tension

Under dynamic conditions, the mobility of a surfactant becomes increasingly important. A surfactant displaying excellent activity - either in the static surface tension measurement by du Nouy ring or in the contact angle measurement method - will, in fact, fail in dynamic application processes.

In both the du Nouy ring and contact angle measurement methods, the surfactants have plenty of time to orient at the interfaces and reduce interfacial tension there. Neither method requires fast orientation of the surfactants.

Therefore, a further test method is necessary to predict dynamic performance - that is, the ability of substrate-wetting additives to orientate quickly at newly created surfaces and reduce the surface tension there.

The maximum bubble-pressure technique measures the energy required to create an enlarged surface but, in contrast to the platinum ring (du NoAy) method, this technique uses a capillary to rapidly generate air bubbles, to which the surfactants must orientate very quickly.

The capillary is placed in the surfactant-containing test liquid. The pressure needed to develop a bubble rate is measured. This pressure is directly proportional to the dynamic surface tension of the liquid at this bubble rate.

It is important to note that the numeric reading should not be taken as an absolute value and cannot be directly related to any other measure of surface tension. However, when using this technique to compare the performance of different surfactants to each other, the measurement provides significant results.

The faster a surfactant can orient at newly created surfaces, the less the reading for dynamic surface tension will increase with increasing bubble speed (see Figure 4).

Figure 5 shows that under dynamic conditions product A - a silicone- and solvent-free surfactant - offers significant advantages over the other groups of surfactants. Its reduction of static surface tension, however, is not extremely strong.

With respect to dynamic surface tension, it is also interesting to note that silicone surfactants clearly outperform fluorosurfactants - in contrast to static surface tension performance.

Improved Wetting

There are two ways to improve wetting characteristics:

• Raising the surface energy of the substrate through cleaning - removal of oils and/or other contaminants - or through surface treatment (corona pre-treatment, flaming, acid or caustic wash).

• Lowering the surface tension of the coating by using additives made specifically for this purpose.

Further discussion of the different types of substrate pre-treatments would occupy too much space for the purposes of this article.

Tego Coating & Ink Additives, a business line of Degussa, manufactures a range of substrate-wetting agents for various applications. When used in relatively small amounts, they substantially lower the surface tension of coatings and printing inks. Depending on their composition, they can effectively reduce either static or dynamic surface tension.

Composition of Substrate-Wetting Additives

Like all surfactants, a substrate-wetting additive is a molecule having both a hydrophilic and a hydrophobic part. The additive's molecular structure determines that the orientation will drastically lower the surface tension of the liquid. The non-polar area of the molecule rests airward, and the polar section of the molecule rests in the water phase.

Most polar molecular parts contain either ionic substances or polyether segments. The hydrophobic part is usually a specially selected polyhydrocarbon. Using fluorinated groups or polysiloxane chains, can endow special properties.

Comparison of Different Substrate-Wetting Additives

The effectiveness of an additive is often judged by its ability to lower a liquid's static surface tension at the lowest possible concentration. Equally important, the additive should not cause undesirable side effects, such as interference with intercoat adhesion, increased tendency to foam, or increased water sensitivity. From the range of silicone surfactants, certain molecules known as hydrophilic silicone polyethers, have been developed (see Figure 6).

The most effective molecules we have found for reducing static surface tension of waterborne coatings - without affecting other properties - are low-molecular-weight silicone surfactants such as tri- penta- or hexasiloxanes. These surfactants lower the static surface tension of waterborne coatings significantly better than hydrocarbon-based surfactants, and they are demonstrably more effective than higher molecular weight silicone-based surfactants.

Low-molecular-weight silicone surfactants combine the strong reduction of static surface tension with excellent spreading ability. Their dynamic performance is acceptable. This makes them the ideal substrate-wetting additives for coatings on difficult-to-wet substrates, like plastics and metals. They have become very popular because of their outstanding wetting of pores for wooden substrates.

Fluorocarbon surfactants reduce static surface tension to an even greater degree than silicone surfactants. However, they do not provide good spreading performance, nor could they be considered dynamic surfactants.

In highly dynamic application processes, such as printing, static surface tension is of only minor significance. Highly mobile surfactants are able to orient rapidly at newly created surfaces during application. They can reduce surface tension even under highly dynamic conditions and provide still other benefits that will be discussed.

Criteria for the Selection of Substrate-Wetting Additives

There are always several criteria to consider when selecting a substrate-wetting additive for a particular coating or printing ink system. Which criteria are most important and which additive is most suitable depend on the specific application or problem.

Fluorosurfactants, for example, reduce static surface tension to an extremely high degree. Their ability to prevent and eliminate craters caused by contamination proves to be highly useful in brushing and spray applications.

Silicone surfactants are appropriate for a range of applications. Their ability to improve the penetration properties of coatings formulations makes them especially suitable for difficult-to-wet substrates.

Special organic surfactants improve wetting and flow properties both in general and in highly dynamic application processes. Depending on their composition, they can inhibit foaming and promote deaeration, which makes them ideal for a variety of waterborne inks and coatings - and not solely for high-speed application.

Properties of a New Class of Silicone-Free Surfactants

This new class of wetting additives provides such a variety of different beneficial effects - without negative side effects - that the term 'substrate-wetting additive' does not describe them completely enough. They:

• Reduce (primarily dynamic) surface tension
  
• Help wetting a variety of substrate materials
  
• Prevent surface defects
  
• Promote flow and leveling
  
• Are foam neutral to defoaming
  
• Avoid air entrainment in baking applications
  
• Help the film formation of polymer emulsions
  
• Promote pigment wetting and reduce viscosity during pigment grinding step
  
• Maintain recoatability and water-resistance

Selecting the Correct Substrate-Wetting Additive

Formulators of waterborne coatings or inks should first determine which properties are most important for their target applications. The more pronounced the effect that the formulator is looking for, the higher the risk of undesired side effects, such as foam stabilization.

Whenever general wetting improvement is required, we recommend the new silicone-free surfactants as a starting point. They are basically risk-free to use. Because of their dynamic properties, these products are always the first choice for printing processes. As a universal surfactant for pigment grind, the most hydrophobic type of silicone-free surfactant deserves special mention. But, in general, these silicone-free surfactants are universally compatible and safe to use in all types of coating applications.

Serious crater problems that occur with very low energy substrates require more intense reduction of static surface tension. In such applications, the silicone surfactants show their true strengths. They ensure wetting and spreading. They also ensure penetration and wetting of wood pores. When craters result from contamination, the highest level of static surface tension reduction is necessary and it then may make sense to test a fluorosurfactant too.

Naturally, the beneficial properties of the new silicone-free surfactants and silicone surfactants can be combined when using these additives together.

Conclusion

Not only static surface tension, but also dynamic surface tension and several other positive or negative properties play an important role in the overall performance of a substrate-wetting additive.

When choosing a substrate-wetting additive the formulator should consider the overall product characteristics of silicone-free surfactants. A variety of customized additives are now commercially available. These include the very versatile new silicone-free surfactants and the highly active silicone-based substrate additives. These additives fulfill practically every possible formulation need. The performance properties desired will determine which is the best choice for you.

For more information on substrate-wetting additives, contact Nicholas P. Wood, Technical Service Mgr., Degussa, Tego Coating & Ink Additives 710 South 6th Ave., PO Box 1111, Hopewell, VA 23860-1111 phone 800/446.1809; fax 804/ 541.6290; or e-mail nick.wood@degussa.com.