Silicone surfactants as performance enhancers in waterborne coatings.

Silicones have long been used in the coatings industry to enhance performance, but with the increasing use of waterborne formulations, new solutions are required to achieve the same level of performance. Dr Thomas Easton and Denise Stephens examine one route-chemical modification of the silicone

Silicones have been used as versatile performance enhancing components of solvent-borne coatings for many years. Improved wetting of substrates and improved mar resistance are just two of the benefits they have introduced. However, in recent years great emphasis has been put on the development of waterborne formulations which must match or better the performance of their solvent-borne counterparts.

For solvent-based formulations, polydimethylsiloxane (PDMS) fluids are most commonly used. This is a class of polymer consisting of linear chains of alternating silicon and oxygen atoms, as shown in Fig 1. The viscosity of the material is related to the length of the polymer chain. Low molecular weight polymers have viscosities as low as 0.65 cSt whereas high molecular weight materials have viscosities in the millions of cSt range.

The benefits of silicones in coatings

Surface activity is the driving force for migration of the polymer chains to interfaces in solvent-borne formulations. They aid wetting of substrates and reduce the surface tension gradients which can produce Benard cells and orange peel during drying. Mar resistance is also improved. Migration to the air/liquid interface results in a layer of PDMS on the coating surface which, because of the low inter-molecular forces between chains, confers lubrication or 'slip'.

Several factors explain these kind of benefits. First, the highly flexible inorganic polymer backbone allows the methyl groups to be presented readily at interfaces, giving a very low surface energy[1]. Secondly, since the methyl groups effectively shield the polar nature of the inorganic backbone, PDMS has very low inter-molecular forces; the chains are able to move past each other with little hindrance.

The new challenge: water solubility

Although water may be regarded as an extremely 'friendly' solvent or carrier, it does bring some inherent difficulties. Water has a surface tension against air of approximately 72mN/m. This is much higher than traditional coating solvents and results in a lesser ability to wet low energy surfaces such as polyethylene.

Since silicones perform this function in solvent-borne coatings, it seems reasonable to extend their use into aqueous systems. But this cannot be done directly via the PDMS materials mentioned previously since they are virtually insoluble in water. There are, however, several ways to render silicone polymers compatible with waterborne formulations. One method is to emulsify PDMS so that it can be readily dispersed. The other option is chemical modification.

Chemical modification

Silicone polyether copolymers are the most widely used class of silicone surfactants for waterborne coatings [ILLUSTRATION FOR FIGURE 2 OMITTED]. One of the most important advantages of this type of material is the versatility associated with the large number of variables that can be altered to suit a particular application. Water solubility is achieved with the introduction of a high level of polyethylene oxide into the copolymer. Between the two extremes, the full range of solubility/dispersibility is available.

Manufacture

There are two commonly used synthetic routes to silicone polyether copolymers. One involves a condensation reaction between Si-OH on the silicone intermediate and C-OH on the polyether, with the aid of a suitable catalyst (see Equation 1). Copolymers prepared by this route have limited use in aqueous systems because of the hydrolytic stability of the Si-O-C linkage. This can cleave, particularly at pH values removed from neutrality, to give free polyether and silicone. In many circumstances, this is undesirable. The rate of cleavage is related to the steric environment around the Si-O-C linkage so that this class of polymer is not totally excluded from use in aqueous formulations.

A more stable Si-C linkage is formed in the second route (see Equation 2), via a reaction usually referred to as 'hydrosilylation' and using a precious metal catalyst. This type of material is more widely used than the Si-O-C analogues. All of the silicone polyether copolymers discussed in this article have Si-C linkages.

The effects demonstrated by silicone polyether copolymers in waterborne coatings are governed by their composition. This includes not only the ratios of silicone to polyether, or polyethylene oxide to polypropylene oxide, but also the 'architecture' of the molecule ie the spatial arrangement of the various groups. There are two general classes-the pendant and the ABA copolymers.

In 'pendant' copolymers, polyether groups can be imagined as dangling from a linear silicone backbone similar in appearance to the head of a garden rake; in 'ABA' copolymers, the polyether groups are attached to the ends of the linear silicone chain. Copolymers with both types of structural arrangement are also available.

Improved slip

Fig 3 shows how the miscibility of silicone polyether copolymers in water is related to the observed benefits. 'Miscibility' is used here as a general term covering a range from 'poorly dispersible' to 'water soluble'. It is used instead of hydrophilelipophile balance (HLB) values since the latter were developed primarily for organic surfactants of the linear hydrophillic head/hydrophobic tail variety. In a surfactant with a linear silicone backbone and pendant hydrophillic polyether groups, the relationship of calculated HLB value to behaviour is tenuous. The relationship depicted in Fig 3 is based on practical experience gained by this supplier and coating formulators. It is important to note that the named effects can overlap so that, for example, a single silicone surfactant may give some degree of slip, wetting and levelling.

Clearly, this class of material is capable of a wide range of effects. Poorly miscible copolymers exert a foam controlling influence (low miscibility being one of the requirements of an anti-foam[2]). These silicone derived materials also feature the other anti-foam requirements of lower surface tension than the foaming medium, dispersibility and chemical inertness.

In aqueous systems, the surfactant nature of these copolymers results in preferential adsorption at the air/liquid interface. In addition, slip is improved on the surface of the cured film. This is thanks to the silicone portion with its low intermolecular forces producing a reduced coefficient of friction. This is analogous to the behaviour of PDMS in solvent-borne coatings.

Table 1: Wetting of aqueous flexographic ink on polyethylene
film(*).

% Dynamic wetter      % Non-wetted
                          area

             Magenta

0                        10
0.1                       5
0.2                       0.5

            Cyan Blue

0                        30
0.2                      30
0.3                       1

* High slip PE - contains fatty amide slip additive.

Furthermore, the greater miscibility of the silicone polyether in water compared to PDMS results in the absence of the 'fish eyes' which are sometimes associated with PDMS when added to aqueous systems. Silicone surfactants can be designed for controlled compatibility with the coating resin during and after drying, as well as with the aqueous environment of the liquid coating.

Improved wetting for low energy substrates

Wetting is another phenomenon influenced by the surface active nature of silicone polyether copolymers. These materials, chosen carefully, can greatly reduce the surface tension of aqueous coatings, allowing them to wet low energy substrates. Fig. 4 shows equilibrium surface tension values for a selection of silicone surfactants as a function of the log concentration in water.

Putting different architectures to the test

The three surfactants used in this study may be described as follows:

* A - Linear silicone backbone with pendant polyethylene oxide groups, of intermediate total molecular weight.

* B - Linear silicone backbone with terminally attached polyethylene oxide groups (ABA type), of intermediate total molecular weight.

* C - Linear silicone backbone with pendant polyethylene oxide groups, of low total molecular weight.

Surfactants A and B have almost identical silicone and polyether chains, but they differ in the molecular "architecture" mentioned previously. Surfactant C is a low molecular weight material with short silicone and polyether chains. It can be seen from Fig 4 that A and B achieve very similar lowest surface tension values (about 33mN/m). In contrast, C achieves a lowest value of about 21mN/m, very close to that of neat PDMS. This is attributed to the ability of C to pack very efficiently at the air/liquid interface, presenting a surface rich in methyl groups.

Equilibrium surface tension vs dynamic values

Equilibrium surface tension values have been extensively used to compare the surface tension reducing behaviour of surfactants. Recently, however, the value of such measurements in coatings related fields has been called into question.[3] In reality, the conditions during application of many coatings are far from an equilibrium state - during spraying or roller coating for instance. Under these circumstances it is important to know how quickly a surfactant can migrate to newly formed interfaces. A surfactant which achieves a low equilibrium value but migrates slowly to new surfaces will be of limited use in a dynamic environment such as on a high speed printing press. Comparison of the dynamic surface tension behaviour of surfactants is often a far more valuable measure.

Fig 5 shows dynamic surface tension results for surfactants A, B and C at 0.1%w/w in water. The measurements were made on a Kruss BP1 instrument which operates by the maximum bubble pressure technique. Values of surface tension are measured as the rate of bubble formation is increased from one to 10 bubbles/second. This represents a steadily increasing rate of air/liquid interface formation, and assesses the ability of surfactants to migrate to the newly formed interface. The picture in Fig 5 is very different from that in Fig 4. Under dynamic conditions the intermediate molecular weight materials A and B demonstrate a limited ability to migrate to the new interface. Even at the lowest bubble rate of one/second, the surface tensions are around 60mN/m, compared to equilibrium values close to 35mN/m at 0.1%.

In sharp contrast, the low molecular weight surfactant C gives much lower dynamic values. The values rise with increasing bubble rate but even at higher rates they are lower than those for A and B. This is not surprising since it is expected that a small molecule such as surfactant C will be considerably more mobile than its higher molecular weight relatives. It can therefore be predicted that C will produce more effective wetting of waterborne coatings on low energy surfaces under dynamic conditions. The term "dynamic wetter" is a fitting description of members of this family of surfactants.

Measuring a dynamic wetter

A simple demonstration of the wetting behaviour of surfactant C is given in Fig 6. Aqueous solutions of C and a commercially available fluorosurfactant were placed on polyethylene film and the radius of the droplets measured after 30 seconds. The difference is dramatic. Surfactant C at 0.1% increases the radius five-fold compared to water alone, and three times compared to the fluorosurfactant. This increase in wetting behaviour has been found to be generally applicable to low energy hydrocarbon surfaces.

From the lab to the workplace

Behaviour in water is interesting but must be extended to fully formulated coatings to be of practical use. Table 1 shows some results for surfactant C in two waterborne flexographic inks applied to the kind of polyethylene film used in packaging.[4] The film used contained a fatty amide slip additive to assist processing during manufacture, a practice which is common but can have a negative impact on printability.

The improvement in wetting compared to the inks without surfactant C is substantial. In this test the amount of surfactant C used to get near complete coverage of the substrate is considerably higher than the critical micelle concentration in water (0.01%). This is attributed to the ability of the surfactant to wet and spread on the surface of the pigment and other particulate components of the ink. A higher concentration than the cmc is therefore required to obtain full substrate wetting.

The ability of surfactant C and variants to improve wetting of waterborne coatings on a variety of substrates has been further demonstrated in the field. Apart from low surface energy plastics, improvements have been seen on leather, metal foil and ceramics. What is more, improvements in wetting are not restricted to substrates to be coated; it has been observed that pigment wetting and hence dispersion can also be improved through the use of these surfactants.

Table 2: Water reducible stoving paint.

                  Single coat           Recoatability
                  Slip angle (deg)

Control                  32             Some craters
Surfactant A             14             Some craters
Surfactant B             10             Retraction
Surfactant C             30             No craters

The overall result of these improvements is that the ink formulator can reduce the amount of co-solvents such as alcohols or glycol ethers used in his formulation.

Different architectures, different properties

Despite the impressive performance of surfactant C, the intermediate molecular weight surfactants such as A and B also have roles to play. In certain applications, such as improved slip, they offer benefits that C cannot match.

Slip

Table 2 gives results for surfactants A, B and C in a water reducible polyester stoving paint of the type used on metal drums. In a single coat of this paint sprayed onto phosphated steel panels, all of the surfactants gave improved wetting compared to the control. This is to be expected since phosphated steel has a relatively high surface energy. The slip angles of the same panels, however, were quite different. The greatest reduction was obtained with B, then A and C last. This is interesting since the main difference between these materials is the 'architecture' as described previously. Apparently ABA surfactants produce greater lubrication at the surface than the equivalent pendant surfactants.

Interestingly, the dynamic wetter, C, has almost no impact on surface slip. This is believed to be due to the very short nature of its silicone chains. A minimum amount of dimethyl siloxy units are required to give observable changes in slip. So clearly, in circumstances where a combination of wetting and improved slip is desirable, type A and B surfactants are superior to the dynamic wetter C.

Levelling

An improvement in levelling was observed with all three surfactants A, B and C; compared to the control, there was a reduction in orange peel and an increase in the sharpness of reflected images.

Recoatability

The presence of silicone species at the surface of a coating has to be considered when recoatability is required. Table 2 contains results of applying a second coat with the same surfactants to the panels discussed above. With surfactant B, which gives the greatest increase in slip of the first coat, there were serious wetting problems, to such an extent that during spray application of the second coat the droplets of paint rebounded from the panels! In this case the substrate is a low energy silicone rich surface, and surfactant B in the droplets of the second coat does not have sufficient time to migrate to the air/liquid interface to effect wetting when the droplets impact the surface. This low migration rate was seen in Fig 5.

The results for surfactant A show a better level of wetting of the second coating on the first, even though the dynamic behaviour of A is slightly poorer than B. Again this may be attributed to the different architectures of A and B, and perhaps a lesser influence of silicone in the surface of the first coating containing A, suggested by the higher slip angle.

The second coating with the dynamic wetter C wets the surface completely, indicating rapid migration of C to the air/liquid interface as the droplets of paint approach the panels. There is also the possibility that the surfactant on the surface of the first coating rapidly migrates into the wet film of the second coating, further assisting effective wetting.

These effects are illustrated in Fig 7. The ability to lower the surface tension of the liquid paint droplet formed at the spray gun tip before it reaches the substrate is truly dynamic.

Wetting is a necessary but not sufficient condition for good adhesion between subsequent layers of a coating. Testing of the double coatings containing surfactants A and C by scratching through to the metal and then attempting to remove the second layer from the first at the exposed edges showed no reduction in intercoat adhesion compared to the control. This method was used in preference to the well known cross hatch adhesion test since the presence of silicone in the surface of the top coating can result in a release effect on the adhesive tape and give misleading results.

Stability

A surfactant which changes in some way during storage or application of a coating can lead to reduction in performance, or in severe cases, to coating defects. It is therefore important to assess the stability of surfactants under conditions likely to be encountered during use.

Fig 8 shows equilibrium surface tension values for a type C surfactant at 0.1%w/w in aqueous solutions buffered at pH 6 and 8 over a period of 40 days. As can be seen, there are only relatively small changes in the surface tension reducing ability of this material on this time scale. There may be a slightly greater tendency to change in acidic as opposed to alkaline media with the surface tension rising from 1 to 5 mN/m at pH 6. Even with the increase, the value is still significantly lower than those exhibited by aqueous solutions of type A and B silicone surfactants, and also by many organic surfactants. At pH8, there was no change. Most of the coatings in which the use of dynamic wetters is envisaged are slightly alkaline, with pH in the range 7.5 to 8.5.

Fig 9 shows a thermogravimetric analysis (TGA) of a dynamic wetter (type C surfactant) in air. This material shows no measurable weight loss below approximately 180 [degrees] C. Above this temperature there is a smooth volatilisation. So although dynamic:wetters of this type are described as low molecular weight surfactants, the volatility and thermal stability are suitable for use in cure conditions commonly encountered with waterborne coatings.

Conclusions

It has been shown that silicone surfactants are an extremely versatile class of performance enhancing additives for waterborne coatings. They offer benefits such as levelling, mar resistance and improved wetting of difficult, or low energy, substrates. In dynamic coating application conditions, such as high speed coating lines, the use of mobile, low molecular weight silicone surfactants is an effective means of achieving good wetting. The improvements in wetting can lead to reductions in the amount of co-solvents used.

The choice of silicone surfactant to give a particular effect in a coating formulation needs to be based on a full knowledge of the type of coating and the conditions under which it will be applied. Factors such as method of application, high or low dynamic conditions, single or multiple layers, and single effect or combinations of effects need to be considered. In this respect, the coating formulator and additive supplier need to work together to meet the challenges presented in the rapidly growing area of waterborne coatings.

Acknowledgements

The authors are indebted to Mr D Tapscott of Dow Corning Ltd for some of the surface tension measurements.

References

1. Owen, M J, "The Surface Activity of Silicones: A Short Review", I&EC Prod. Res. Dev., 19 (1980), 97.

2. Owen, M J, "Defoamers", Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Kroschwitz, I, and Howe-Grant, M, Eds New York: John Wiley & Sons, 1993, Vol 7, pp. 9"8 - 945.

3. Berger, P D, and Berger, C, "The Dynamic Surface Properties of Surfactants", Surface Phenomena and Fine Particles in Water-Based Coatings and Printing Technology, Sharma, M.K., and Micale, F J, Eds New York: Plenum Press, 1991, pp 283 - 298.

4. US-Pat 4,765,243.

Dr Thomas Easton, Denise Stephens are with Dow Corning Ltd, Barry, based in the UK