Novel Short-Chain Fluorosurfactant Application in Water-based Industrial Coating

Jean Meng Ph.D. DuPont Company

Wilmington, Delaware USA jean.meng@usa.dupont.com

Charlene Guo

600 Cai Lun Road, Shanghai China

Abstract: The industrial coatings market is characterized by a diverse set of substrates and methods of application. Water-based industrial coating systems have been growing and expanding rapidly. However, the overall quality such as wetting and leveling performance of water-based coatings present challenges when compared to traditional solvent-based coatings due to the high surface tension of water. Formulators are continuing to make efforts to close the gap, and one approach is the use of specialty additives. Sustainable short-chain fluorosurfactant and flurororepellent (six or less fluorinated carbons) has been increasingly focused on in the coating industry. This new technology delivers superior performance and as well as providing an environmental-favorable image because it cannot break down into PFOA or PFOS in the environment. This study focuses on how short-chain fluorosurfactants can enhance the performance especially improving the surface defects of water-based industrial coating. It addresses the formulation design of a water-based epoxy primer and a two-component water-based polyurethane top-coat for general metal coating. Performance data includes wetting, leveling, stain- and corrosion-resistance, and durability. Impact on gloss, color and adhesion will also be evaluated. The result of this study revealed that the wetting, leveling, anti-corrosion and anti-tacky performance of epoxy primer were significantly improved with the addition of short-chain fluorosurfactants in the primer and topcoat and the use of short-chain fluorosurfactants do not have any negative effect on gloss, color and adhesion of topcoat. Key Words: Fluorosurfactant, Capstone®, water-based, industry coating, surface defects, wetting and leveling, surface tension, repellency, contact angle Fluorosurfactant (FS) Volatile organic compounds (VOC) Glass transition temperature (Tg) Perfluorooctane sulfonate(PFOS) Perfluorooctanoic acid (PFOA) Active ingredient (AI)

Introduction Generally, coating system can be divided into three major segments by method of application, functional goals of the coating and durability of the coating: consumer based, roll and brush applied architectural paints that last 3-5 years, professionally spray, brush or roll applied industrial protective coatings that last 10-20 years, and specialty coating (wood and plastic coatings) which is in between. The field of water-based systems for surface coating application is growing rapidly and expanding vigorously. This may be attributed to the requirement to meet the increasingly severe legal restrictions being applied to the amount of organic solvent emitted during coating applications. In the sector of architectural paints, waterborne system is widely used for wall paints and wood paints. In industrial coatings (controlled application), waterborne formulations have not been employed to a great extent to date because the quality like wetting, leveling performance of water-based industry coating still has gap with traditional solvent coating system due to the high surface tension of water, but chemists are continuing to put tremendous efforts in this fields. Waterborne coating formulations are a complex mixture of resin dispersions, pigments, solvents and additives such as surfactants and defoamers, which are often required to solve flow, leveling and foam problems that are exacerbated in waterborne technology because of the relatively high surface tension of the continuous water phase. In addition, surfactants are also necessary to stabilize resin and pigment dispersions. Commonly, these stabilizing surfactants are high HLB (hydrophile-lipophile balance) ethoxylate or ionic types that are prone to stabilize foam as well. Thus, the use of these surfactants necessitates the need to add defoamers to curtail the development and subsequent build-up of unwanted foam. Defoamers, however, often produce negative consequences such as surface defects (e. g., craters) and poor flow and leveling (e. g., orange peel). As a result, more surfactants are added to the system to eliminate the problems caused by the defoamers. In turn, these surfactants can re-aggravate the foam problem. Thereby, traditional defoamers and surfactants can perpetuate a negative feedback loop, which might ultimately compel the waterborne coatings formulator to make undesirable compromises in the final coating formulation. Because of this negative feedback cycle, it is beneficial for additives to perform multiple functions such as both wetting, low foaming, anti-blocking to minimize any potential detrimental effects. Preferably, the additives should also have higher efficiency. Recently the short-chain fluorosurfactant (six or less fluorinated carbons) has been increasingly focused on in the coating industry. This new technology delivers superior performance and as well as providing an environmental-favorable image because it cannot break down into PFOA or PFOS in the environment. Telomer-based short chain fluorosurfactants contain perfluoroalkyl chains and various functional groups. They have unique properties in significantly reducing surface tension in all aqueous, solvent, and neat systems. They are air-liquid interface active, providing wetting, re-wet and oil repellency while meeting surface tension needs in coatings. They have been used in solving low VOC formulation problems mentioned above. Until very recently, fluorosurfactants were used to address formulation problems near the end of the coating development cycle. Our recent research and practice have demonstrated that fluorosurfactants are most useful and cost effective due to the multiple functions they provide in formulating a low or VOC free architectural coating. The formulator

can reduce other ingredients and create higher performing paints by minimizing the use of other coating additives, such as defoamers, wetting agents, leveling aids, anti-block additives, and open-time extenders. Oil repellency and other functions that fluorosurfactants provide have also made them attractive to formulators in developing dirt pickup resistance exterior paints. In this paper, we will discuss fluorosurfactant’s role in waterborne metal coating system which is a major part of industry maintenance coating. The goal of the study is to investigate if new short-chian fluorosurfactant (FS) can enhance the various performances of the coating. Hydrocarbon surfactant (HCS) and silicone-based surfactant are commonly used in water-based industrial coating as wetting and leveling agent. These two not in-kind competitive products were selected in the study in order to compare their performances with FS. For formulation design, the water-based epoxy primer and two component water-based polyurethane topcoats for general metal coating were selected as model formulations. The wetting, leveling, surface water and oil repellency, corrosion resistance, anti-tacky and durability performances were evaluated. Selected FS were added to the primer and topcoat formulations with fluorosurfactant. The gloss, color retention and adhesion performances were also tested to check adverse effects. From the data collected in the study, significant performance improvement was observed in wetting, leveling, corrosion resistance, anti-tacky and durability of the coating with the addition of fluorosurfactants. The study also reviewed the use of FS did not have any negative effect on gloss, color and adhesion of topcoat. 1 Experimental 1.1. Coating formulation and application Water-based epoxy primer and 2K polyurethane topcoat formulations are design as table 1 and table 2 for general metal coating. The metal panels are steel and the application is by air spray. Dry film thickness of the primer is 45-55micro and topcoat is 40-50micro.

Table 1 Composition of coating formulations-Primers

Epoxy Primer #1 Epoxy Primer #2 Epoxy Primer #3 Water 7 7 7 BYK-190 4 4 4 BASO4 5 5 5 TALC 3 3 3 ZP-10 7 7 7 Tipure 902+ 16 16 16 OK412 0.25 0.25 0.25 Duroxyn VAX 6127w 54 54 54 Butyl Carbitol 2 2 2 FS 0.172 0 0 HCS 0 0.2 0 SiS 0 0 0.3 Additol VXW 4940N 1.2 1.2 1.2 BYK 024 0.3 0.3 0.3

Total 100 100 100

Table 2 Composition of coating formulations-Topcoat part 1

Topcoat 1 Topcoat 2 Topcoat 3 Topcoat 4

Bayhydrol® A 2470 65 65 65 65

DI Water 6 6 6 6

FS 0.172 0 0 0

HCS 0 0 0.2

SiS 0 0 0 0.3

White Colorant 28.5 28.5 28.5 28.5

Black Colorant 0.2 0.2 0.2 0.2

BYK® 024 0.3 0.3 0.3 0.3

Total 100 100 100 100

Part 2

Bayhydur XP 2655 21 Methoxy Propyl Acetate 10

1.2 Surface tension measurement Surface tension is measured with KRUSS tensiometer using plate method based on modified ASTM-D1331. A thin plate is lowered to the surface of a liquid and the downward force to the plate is measured to calculate surface tension. 1.3 Contact angle measurement The test method of contact angle is based on modified ASTM-D7334-08.The water and hexadecane contact angle were measured by goniometer with high speed CCD camera for image capturing liquids were dropped onto the surface. Computer aided goniometer measures a droplet’s contact angle by assuming the droplet fits the geometry of a sphere when the drop is stable after 1 min. High value of oil contact angle indicates good oleophobic nature of the coating, while the low value indicates that the oil wets the surface. 1.4 Inter-coating adhesion Measurement Inter-coating adhesion of two coats was reflected by adhesion test. The test method of adhesion is referred to ASTM D3359 crosshatch tape testing. Before the testing, the paint has to be air dried for 7 days. The classification of adhesion result follows as:

Classification Percent area removed

5B 0% 4B Less than 5% 3B 5-15% 2B 15-35% 1B 35-65% 0B Greater than 65%

1.5 Anti-corrosion measurement The test method for anti-corrosion performance in this study is ASTM B117-11. The salt solution shall be prepared by dissolving 4-6 parts by mass of sodium chloride in 95 parts of water. The apparatus required for salt spray (fog) exposure consists of a fog chamber, a salt solution reservoir, a supply of suitably conditioned compressed air, one or more atomizing nozzles, specimen supports, provision for heating the chamber, and necessary means of control. Apply the paint in the standard steel panel and dry for at least 7 days. Then cut cross in the middle of panel. In the cut edges of plated, coated, or duplex materials and areas shall be protected with a suitable coating or sealing stable under the conditions of the practice. The panels shall be supported or suspended between 15 and 30° from the vertical and preferably parallel to the principal direction of flow of fog through the chamber, based upon the dominant surface being tested. 2 Results and discussion 2.1 Surface tension reduction The surface tension data of various classes of surfactants such as HCS, SiS and FS in DI water and commercial resins are listed in Figure 1, 2, and 3. The dosage of FS is 400pmm and 600ppm A.I for weight in total formulations. HCS and SiS are dosed by 1000ppm and 1500ppm A.I. The data indicated that FS are more effective to reduce surface tension at much lower loading rate in both DI water and selected resin systems than SiS and HCS. Among FS products, anionic FS is most effective product to reduce surface tension especially in resin systems such as the figure.

Figure 1 Surface tension data in DI water

Figure 2 Surface tension data in primer epoxy resin

Figure 3 Surface tension data in topcoat resin

2.2 Superior substrate wetting In waterborne industrial coatings, the inherent high surface tension of water often leads to substrate wetting problems over contaminated areas that has low surface energy. This is common cause for coating defects such as cratering and fisheyes. In some serious conditions, for metal substrates, residual oil and organic contaminates can cause severe surface defects, such as seeding, channeling. In addition, high speed application of water-based coatings over metal is a particularly difficult challenge for formulator to obtain desired spreading and coverage. In these cases, the surfactants with best surface tension reduction should provide best performances. The epoxy primer with three competitive surfactants was spray applied in the artificially contaminated metal panels. Our observation is shown in figure 4. The coating with FS provided best coating quality without any surface defects. The coating with HCS and silicon surfactants both have surface defect of channeling. 2.3 Improved flow and leveling Leveling performance of formulations with various surfactants was evaluated in topcoat 2K PU formulations and listed in Figure 5. The dosage of FS is 400pmm and 600ppm A.I for weight in total formulations. HCS and SiS are dosed by 1000ppm and 1500ppm A.I. The visual result indicated that the formulation with FS showed better leveling and gloss compared with HCS. The formulation using SiS also has better leveling but cratering were observed in the film surface. FS migrates rapidly within an applied coating to eliminate surface tension gradients, one cause of flow and leveling problems. FS has a positive influence on flow and leveling which will lead to improved coating appearance and better gloss shown in figure 6.

Figure 4 Substrate wetting of epoxy primer with different additives

Figure 5 Flow and leveling of the formulations with different surfactants

With FS With HCS With SiS

Figure 6 Wet films leveling of the topcoat formulation with 600ppm FS

2.4 Low foam The surfactants were diluted with water to the same dosage in the paint formulations, and foam was generated by hand shaking. The volume of foam is shown in Figure 7, which was taken 15 sec after shaking. The figure shows that FS and HCS generated less foam than the silicone surfactant SiS. FS is low-foaming products, as there is almost no foam after shaking.

Figure 7 Foaming performances of surfactants

600ppm A.I 1500ppm A.I 1500ppm FS HCS SiS

2.5 Improve surface repellency The contact angle of the primer and topcoat formulation with various surfactant are measured in figure 8 and 9. The data indicated that the addition of FS dramatically improved oil repellency of the paint film.

Figure 8 Contact angle of epoxy primer

Figure 9 Contact angle of topcoat

2.6 Inter-coating adhesion Since FS provides high surface repellency to the coating film, formulators always have concern about inter coating adhesion when incorperating FS in their formulations. Inter-coating adhesion was reflected by adhesion test between the primer and the topcoat including the same FS. The result is listed in table 3. From the result below, the using of FS will not have negative effect on inter-coating adhesion.

Table 3 Adhesion testing

E1+T1(FS-63) 4B

E1+T2(FS-65) 4B

E2+T3(104) 3B

E3+T4(BYK346) 4B

Figure 10 Inter-adhesion testing of primer and topcoat

Another frequently asked question from formulators is whether topcoat will have the recoating issue when Capstone is only used in primer. The inter-adhesion testing of the coating package including primer with FS and topcoat with SiS was conducted to compare with the coating package including primer and topcoat both with SiS. The figure 11 showed that the adhesion of coating package of primer with FS and topcoat with SiS is 4B and the adhesion of the coating package of primer and topcoat both with SiS is 3B. The result indicates that even FS is added in the primer, the topcoat without FS will not have recoating issue.

Figure 11 Inter-adhesion testing

Primer with FS Primer and topcoat and topcoat with SiS both with SiS

2.7 Improve Tackiness Epoxy primer is very tacky and easily to attack stains and dirts before the topcoat is applied. The improved tackiness will lead to good stain resistance of the paint to get high quality paint films. After the primer paint is dry for 7 days, place the paint surfaces face-to-face in 50 degree C oven. At the same time, the top of the testing panels were covered by10pcs steel as weight. After 30 mins, cool the panel to room temperature and peel off to observe the appearance. The less damage of the panel indicated the better anti-blocking performance. The blocking testing result is listed in figure 17. The primer formulation with Capstone® shows better anti-blocking performance.

FS HCS SiS

2.8 Color check 9 readings of L, a, b value in the top, middle and bottom area of the panel are measured by color-meter to check the uniformity of the color in the panels and listed in table 5. For all the formulations, the standard deviations of L, a, b in different readings is quite low. The use of FS will not have any negative effect on color of the paint.

Table 5 color check for topcoat

L a b

78.36 0.6 2.48

78.35 0.69 2.47

78.42 0.61 2.48

78.35 0.6 2.46

78.32 0.6 2.45

78.33 0.6 2.45

78.35 0.6 2.46

78.35 0.6 2.46

78.37 0.6 2.47

Average 78.36 0.61 2.46

standard deviation 0.027 0.028 0.011

Topcoat 2(FS-65)

L a b

78.37 0.6 2.46

78.35 0.61 2.47

78.36 0.61 2.46

78.35 0.63 2.46

78.35 0.62 2.47

78.39 0.6 2.46

78.35 0.61 2.5

78.35 0.61 2.45

78.33 0.6 2.47

Average 78.36 0.61 2.47

standard deviation 0.016 0.009 0.013

Topcoat 1(FS-61)

3 Conclusions Extensive evaluations of FS in model formulations indicate that these unique products provide multiple benefits in waterborne industry coatings. These benefits include: • Reduce surface tension of primer to improve substrate wetting and eliminate cratering even when the substrate is contaminated with residual or greasy.

•Reducing surface tension to allow rapid coverage of waterborne topcoats over primer • Improving flow and leveling after spray application • Improving inter-coat adhesion • Minimize foam during production and application • Improve oil repellency

References [1] Jean Meng, Novel applications for fluorosurfactants in low VOC Coatings, Journal of Paints &Coatings Industry, April 2007, P84-88. [2] Jean Meng, Charlene Guo, Novel application of fluorosurfactants in waterborne wood coatings, Paint &Coatings Industry, Oct.2011, P42-46.

L a b

78.2 0.66 2.11

78.28 0.62 2.33

78.24 0.61 2.35

78.23 0.62 2.3

78.29 0.62 2.35

78.29 0.61 2.36

78.17 0.61 2.35

78.24 0.61 2.34

78.27 0.61 2.36

Average 78.25 0.62 2.32

standard deviation 0.039 0.015 0.075

Topcoat3(104H)

L a b

78.62 0.62 2.38

78.61 0.62 2.4

78.61 0.62 2.41

78.61 0.62 2.38

78.58 0.63 2.41

78.61 0.61 2.42

78.62 0.62 2.38

78.6 0.61 2.42

78.57 0.62 2.42

Average 78.60 0.62 2.40

standard deviation 0.016 0.006 0.017

Topcoat 4(BYK 346)