Flow - the motion of the coating on a substrate during and after wetting.

Leveling - the surface aspect of the coating after wetting and before curing.

Dispersion stability - ability of a dispersant to wet out pigment and keep pigment particles from agglomeration and prevent settling.

Pseudoplasticity and Thixotropy - Pseudoplastic is shear-dependent viscoelastic flow. This is seen as a reduction in viscosity with increasing shear rate. A material is thixotropic when, in addition to shear strain rate, there is time-dependent response, i.e., an ink's viscosity changes with time even after shear forces are removed.4

Substrate wetting and flow are important for all coatings to ensure good protective functions (good adhesion, resistance to ingress of moisture and other environmental agents) and aesthetic appeal (smooth surface appearance). Typical problems encountered due to poor wetting are cratering or crawling and fisheye-like appearance on portions of the coatings (Figure 1).

Poor flow and leveling and high coating viscosity can occur due to sub-optimal pigment dispersion. Improper pigment stabilization can result in agglomeration, and settling can lower coating gloss. In filled topcoats, one encounters increased thixotropy, which creates problems during the application process and reduces surface appeal. Finally, poor flow and leveling can cause an "orange peel"-like effect (Figure1), produce surfaces that are harder to sand evenly and create poor surface appearance. This is especially true for fast-curing UV/EB coatings if curing line speeds are so high that the coating does not fully flow and level out prior to cure, after which time, the morphology of the coating is "frozen" or irreversible due to rapid crosslinking. Using four examples we evaluated how formulators tackle these issues in practice.

Experimental and Materials
Contact Angle

Contact angle measurements were done using a Ramie-Hart Goniometer at 25 °C. All measurements were made by placing a single drop of material on a Teflon substrate. The dynamic surface tension of the different UV acrylates was measured at 25 °C using a Krüss maximum bubble pressure tensiometer BP 2, in combination with a BPC2 capillary and BP23 software.

Viscosity

The viscosity of all monomers was measured using a Brookfield DVII Plus viscometer at 25 °C.

Rheology

The apparent viscosity of the inks was measured using a Bohlin-Rheometer RVO at 25 °C using an oscillatory shear strain rate range of 0.1 to 1000 s-1.

Curing Conditions

All coatings were cured as noted in the tables provided for the formulations in the results and discussion section.

Gloss

Gloss measurements were made using a BYK Gardner Micro-Tri-Gloss meter. All gloss readings were taken at 60°.

Materials

All starting materials unless otherwise noted were obtained from Cognis Corporation (Table 1).

Results and Discussion

Case I - Clear Coat on Tin Metal

This formulation refers to a coating on untreated or "oily" tin metal. The requirement in this case was a UV-curable coating of sprayable viscosity for tin metal without a pre-treat cleaning step which, after cure, had good adhesion to metal and excellent surface appearance. Optimum substrate wetting is very important for all industrial-coating processes. For best substrate wetting, the contact angle  made by the coating onto the substrate, should be as low as possible (Figure 2).

In other words, the surface tension of the liquid coating should be lower than the surface energy of the substrate to be coated. A typical way of achieving this is to treat the substrate with plasma, corona or flame (usually applied to plastics) or other chemical treatments (usually applied to metals) to clean and increase the surface energy of the substrate. This was not an option in our case. So we chose an alternative, i.e., to reduce the surface tension of the coating formulation.

Figure 3 shows the contact angle measurements for a series of UV-curable monomers on poly(tetrafluoroethylene) or Teflon®. This substrate, due to its extreme hydrophobicity, presents severe wetting challenges for any coating. So, the lower the contact angle of a monomer on Teflon, the better should be its wettability on other surfaces with higher surface energy than Teflon. From Figure 3, the three monomers with the lowest contact angles were acrylates of NPG(2PO)-OMe, TPG-OMe and DPG.

Similarly, a survey of viscosity and surface tension data of various monomers (Table 2), showed that the surface tension of monomers bearing propoxylate groups was consistently lower than those bearing ethoxylate groups.

Two monomers, NPG(2PO)DA and NPG(2PO)-OMe monoacrylate, were used in identical amounts in the sprayable formulation (Table 3). The surface aspect of the two formulations is shown in Figure 4. The formulation containing the monomer NPG(2PO)-OMe monoacrylate with the lowest contact angle and surface tension shows excellent coating coverage and surface smoothness. There is considerable "crawling" or withdrawal of coating of the formulation containing NPG(2PO)DA. The only structural difference between these monomers is a single methyl ether group.

An analysis of dynamic surface tension of these two materials reveals significant differences (Figure 5). The graph shows that the initial surface tension of NPG(2PO)-OMe acrylate is lower than that for NPG(2PO)DA by approximately 3.5 mN/m.

Further, the rise in dynamic surface tension over three orders of magnitude of bubble frequency is much smaller for NPG(2PO)-OMe acrylate versus NPG(2PO)DA. Bubble frequency is a proxy for rapidity of change of surfaces. This means that the methylether acrylate containing formulation is better able to maintain lower surface tension and wet out the "oily" metal surface as it undergoes very rapid and dynamic changes in its surface environment during the coating application process prior to UV curing of the coating.

Case II - Clear Topcoat on Waterborne Flexographic Ink (PET Film Substrate)

In this formulation (Table 4), the requirement was for a clearcoat with good reactivity and low blocking resistance. Two formulations were used. The only difference was that formulation B lacked an acrylated polysiloxane block copolymer additive. As a class, silicone-based surfactants are known to have among the lowest surface tension after fluorinated polyols. The no-overcoat requirement meant that we could use a silicone surfactant without worrying about intercoat adhesion issues encountered in multilayer UV coatings when the undercoat has silicone surface modifiers. Further, on curing, an acrylated silicone is incorporated randomly throughout the topcoat, allowing for more control of slip properties and reduced blocking as evidenced from a lower slide angle and lower block resistance for the cured topcoat.

Case III - UV-Curable Flexographic Ink on Polyester Film

The requirements for this application were: low and stable viscosity profile on the UV flexographic press, good pigment dispersion stability, good substrate wetting to PET and high gloss after cure. Non-homogenous systems like UV flexographic inks can present significant flow problems on a flexographic press. Typical symptoms such as high viscosity, poor ink flow across the roller train (the metering roll, fountain roll, the anilox and the impression cylinder), poor ink transfer to substrate and sub-par color and print quality. These can be minimized using: a) properly equipped roller train and b) UV flexographic inks with well-chosen dispersants that wet out and stabilize the pigment.

Table 5 shows a UV flexographic ink formulation for corona-treated PET film. A control formulation did not contain the polymeric dispersant. Figure 6 shows viscosity-shear profiles for these inks. From the figure it is clear that the formulation without the dispersant exhibits a stronger shear thinning (viscosity range of 8900-360 mPa.s at 1 s-1 and 1000 s-1) versus the ink with the dispersant (viscosity range of 1500-650 mPa.s at 1 s-1 and 1000 s-1). Both the "down-curve" (increasing shear) and the return "up-curve" (decreasing shear) for the dispersant containing UV ink exhibit manageable changes in viscosity all in the useful range for flexographic printing. No pigment settling was observed even after six months of storage.

In contrast to water-based flexographic inks, UV-curable inks are expected to have high gloss and possess better chemical resistance due to their highly crosslinked nature. A dramatic 25-point improvement in cured ink gloss (Figure 7) is also seen in the dispersant-stabilized UV ink, also implying the better pigment wet out by the binder in the dispersant-containing ink.

Case IV - Matte Finish UV Wood Coating

The requirement for this coating was a low-gloss, matte aspect for a wood finish. The coating was applied using a roll coater and needed excellent de-airing prior to cure to prevent pinholes and other defects that might produce reject parts due to a poor aesthetic surface. Table 6 shows the formulation used in this application. The control formulation did not contain the polyurethane polymer dispersant.

Figure 8 shows the rheology profile for the two formulations. The dispersant's role in this case is two-fold: a) to displace air at the interface between the binder and the matting agent to help improve wet out of the inorganic silica with the binder and b) to keep the matting agent from settling while maintaining stable viscosity at different coating line speeds (i.e., different shear rates). Without the dispersant, the formulation is clearly more thixotropic, and this then has the potential to cause poor flow and pinholes. Figure 9 shows the two formulations after they were shaken in a paint shaker (high shear) for two minutes and then left alone (zero shear) for five minutes. The formulation without the dispersant clearly shows much higher air entrainment and no observable settling of the matting agent. This shows the multifunctional role of the dispersant for pigment wet out; anti-settling also acted as a de-airing agent.

Conclusions

The best choices to solve issues relating to flow and leveling challenges depend on the intended goal. If the goal is to get good wet out of low-energy substrates, a combination of substrate surface treatment and the use of low-viscosity, UV-curable, low-surface-tension monomers (i.e., neopentylglycol (2PO) monomethyl ether acrylate) should help in coating wet out, flow and leveling. These monomers tend to have low substrate contact angles and are particularly useful in primers or undercoats. In contrast, for topcoats, either block copolymer silicones or acrylated polysiloxane copolymers can be used to reduce coating surface tension. Better control of slip and block is expected from the acrylated silicones. To get optimum in-storage, in-use and after-cure application performance in pigmented (organic or inorganic) systems, well-chosen polymeric (polyurethane) dispersants can produce dramatic changes in pigment wetting, dispersion stability and gloss control at low (3-5%) incorporation levels.

Acknowledgements

The authors thank Mr. Levi Scott of Cognis Corporation for his help in preparing this article.

For further information, please contact Dr. Anbu Natesh: anbu.natesh@cognis.com