A New Additive to Eliminate Pinholes in Water-Based Paint Formulations: Surfynol® 118

Pinholes are a particular problem for formulators developing water-based coatings and paints that cure rapidly at elevated temperatures and are applied at high wet film thickness. Pinholes often form due to air and solvent vapors released from the coating film in situations where the coating has such high viscosity that it cannot flow back and repair the voids created by recoating. Pinholes can also occur in oven-dry coatings. Vapor trapped in the film layer, which softens under heat, regains its ability to move before cross-linking/hardening is complete, preventing the coating from filling or eliminating the voids formed. Hydrocarbon-based defoamers assist in eliminating foam and pinholes in these formulations; however, due to their limited compatibility, they cause reduced gloss in the coating film, spreading, and performance losses in surface appearance. This article provides information about Surfynol® 118, a new additive that can combine both micro-foam removal from the system and modified surface drying to eliminate pinholes. While this new additive delivers high performance in pinhole elimination comparable to hydrocarbon-based defoamers, it does not exhibit the poor surface appearance and compatibility problems caused by hydrocarbon-based defoamers.

Introduction

Pinhole formation is defined as "the instantaneous creation of small holes in the wet film during application and drying due to the bursting of air or gas bubbles in the wet film, resulting in small craters that cannot close or merge before the film dries." 1 These small defects, especially in gloss formulations, spoil the surface appearance and reduce the protective properties of the finished coating. Pinholes are a problem for both water-based and solvent-based coatings, although the cause of the defect may differ. Pinholes are also frequently seen in oven-dry systems. However, these defects may not appear until the film cures. The most common cause of pinholes is the release of volatile compounds or trapped air during drying after application. These gases form gas bubbles that move toward the coating-air interface due to buoyancy forces or surface tension-driven flow within the film. 2 The movement of these bubbles will slow with increasing viscosity of the paint film. This situation causes the void left behind by the bubble moving toward the surface to not be filled by the coating due to the difficulty of flow within the film, resulting in pinhole formation (Figure 1). Some bubbles may remain trapped in the dry film, but can regain mobility when the coating is heated and softens before cross-linking. As the coating cures, viscosity increases, which prevents air bubbles from flowing back into the voids formed. Gas pressure from the primer or top coat in a multi-layer application may also be sufficient for the bubble to advance to the surface. This can also create air bursts, craters, and other surface defects. This defect can also result from air or gases from the surface or absorbed and retained at the surface but released by heating. 3 Although bubble exit from the film is not a completely understood mechanism and continues to be researched, there are various methods to prevent pinhole formation or reduce their numbers: reducing film thickness and using slower-evaporating volatile solvents are some of these. Reduced film thickness reduces both the amount of gas and volatile matter present and the distance and time required for bubbles to escape. Using slower-evaporating volatile solvents can also delay film drying and increase viscosity. Thus providing more time for bubbles to escape. Reducing film thickness can negatively affect hide and surface protection performance. In addition, multiple applications may be required to achieve the necessary total film thickness, which can also lead to pinholes. Changing the volatile solvent content/combination can affect drying properties and conditions. Additionally, it may not be possible within the framework of VOC (Volatile Organic Compound) restrictions or local environmental regulations introduced by new regulations. Air release and defoaming additives are used to control pinholes. However, these additives can cause other problems such as craters, fisheyes, or weaker surface adhesion and/or reduced inter-coat adhesion. 4,5 Additionally, defoamers can affect properties that are critical in automotive coatings such as gloss, orange peel appearance, and image depth. 6 Therefore, in such coatings, defoamer selection should be made carefully when selecting additives for pinhole control.

New Pinhole Eliminating Additive Surfynol® 118

Surfynol® 118, a new product, was developed with a combination of traditional and molecular defoaming chemistries 7 to control foam, enhance surface activity, and extend application time (open time). This product was tested in several different formulations to demonstrate how it can reduce pinholes in sensitive formulations. To evaluate additives that prevent pinhole formation, a common and effective test is used that involves either applying the coating with a spray gun in a manner that increases film thickness or applying it with a "Wedge" type applicator with increasing film thickness (Figure 2). The coating layer can then be visually evaluated for pinholes. If the coating is applied to a transparent film or glass, backlighting can be used for better observation of defects. This test also simulates variations in different film thicknesses that may occur when painting vertical or shaped parts. Measuring the lowest film thickness at which pinholes are first observed is an effective way to distinguish additive performance. [caption id="attachment_126626" align="aligncenter"] Figure 2. "Wedge" type applicator application test[/caption]   The new pinhole prevention additive was tested in a white automotive base coat formulation formulated with Bayhydrol® UA2856XP from Covestro. The paint was sprayed in a "Wedge" type applicator pattern onto electrostatically coated steel panels at room temperature (20°C, 48% RH). Flash-off was then performed for four minutes before curing at 120°C for 30 minutes. Panels were subsequently visually evaluated and minimum film thickness was measured for pinholes. (Figure 3) The new pinhole formation prevention additive, Surfynol® 118, tested at 2% in the base paint formulation, showed similar pinhole performance when compared with mineral oil-based defoamer (Benchmark A) and polymeric defoamer (Benchmark I). While foam breaking performance measured by testing at high mixing speed also appears similar, much better performance is observed with Surfynol® 118 in criteria such as surface appearance (gloss, haze, spreading, and craters). (Figure 4) The mineral oil product created significant orange peel appearance and DOI loss, while the polymeric defoamer caused craters in the film. Perfect compatibility in the top coat can be achieved with Surfynol® 118 without any negative impact on spreading or craters. Similar results were observed in a water-based, metallic top coat formulation formulated with Daotan TW6466 from Allnex. Surfynol® 118 provided excellent foam control in this formulation, and no pinholes were detected in the top coat with or without clear base application. Additionally, when compared to coatings without additives and those containing other defoamers, Surfynol® 118 showed no negative effect on surface adhesion and inter-coat adhesion. Gloss and flop index were also not affected by additive selection. Slight improvement was seen in orange peel appearance and spreading.   Similar results have been seen in other water-based coatings. Figure 5 shows the performance of the new additive Surfynol® 118 in plastic coatings by pulling samples mixed at 2000 rpm for 15 minutes onto glass panels with an applicator (bird bar) at 90 μ wet film thickness. Surfynol® 118 and polymeric defoamer showed significantly fewer pinholes after application, and Surfynol® 118 also had better color stability after storage at 50°C for 14 days. Surfynol® 118 is also quite effective in reducing pinholes in water-based epoxy floor coatings and primers even when compared to some siloxane-based defoamers. It is as effective in foam control in water-based packaging inks as other organic polymer-based defoamers, while also demonstrating improved compatibility when compared to oil and siloxane-based defoamers.

Conclusion

Surfynol® 118 is a new additive free of alkyl phenol ethoxylates (APEO), silica, and siloxanes, with improved resistance to solvent evaporation (solvent pops) and pinholes that may occur during application in water-based systems, and which maintains film surface appearance in all these respects. The coating film thickness can be increased for application without surface defects. In sensitive water-based formulations, it provides good macro and micro foam control. It is especially suitable for oven-dry coatings.

References 1.Resene Datasheets, 1 (ix) Paint Terminology and Definitions, May 1996 (https://www.resene.co.nz/archspec/datasheets/Section1-Paint-Definitions.pdf) 2. Bubble Entrapment and Escape from Sprayed Paint Films, A. Dalili, S. Chandra, J. Mostaghimi, C. Fan and J.C. Simmer, Prog. Org. Coatings, 97 (2016), 153 – 165. 3. https://www.paint.org/article/automotive-coatingsapplication-defects/ 4. Novel Zero VOC Deaerators for Waterborne Coatings, C. J. Reader, C. Hegedus, C. Louis, K. T. G. Lai, S. el Ajaji and W. Chaigneau, Paper 16.4, 2012 American Coatings Show 2012. 5. Predicting Defoamer Performance in Waterborne Coatings, C. J. Reader and K. T. G. Lai, PCI Magazine, March 2013, 50 – 59. 6. Orange Peel Formation due to Surface Tension Flows within Drying Paint Films, N. Saranjam, S. Chandra, J. Mostaghimi, H. Fan and J. Simmer, J. Coat. Technol. Res., 13 (3) (2016), 413 – 426. 7. New Additives for Water Based Coatings: A New Class of Defoamers Is Born!, S.Y. Chan and C. Louis, EUROCOAT 2003, Lyon, France, September 23-25, 2003.