Chemical partners. Seite/Page: 34

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1 Chemical partners Synergy between fluorine and silicone enhances stain resistance of coatings. Stain resistance and easy-clean properties are increasingly demanded from protective coatings. The synergistic effect of combining fluorine and silicone within a single copolymer produces a coating with excellent chemical resistance, weatherability and thermal stability. In particular, the silicone component provides improved stain release due to its preferential orientation at the coating surface. Peter Hupfield, Eiji Kitaura, Tetsuya Masutani, Masaru Nagato. Low maintenance coatings that provide both stain-resistance and easy-clean properties are becoming increasingly popular in today's marketplace because of the consumer benefits they provide. The ability to provide these properties on a variety of substrates can reduce maintenance costs as well as protecting the natural appearance of the substrate. Fluorine-based materials are extensively used in surface protection due to the properties they can impart, such as water and oil repellency, weatherability, chemical resistance, stain repellency and durability. This paper reports about a novel protective fluoropolymer coating that also incorporates silicone, provides a low maintenance protective system for a wide variety of substrates and can be cured at room temperature. Fluoropolymer coatings may be difficult to apply A wide range of fluoropolymer coatings exist today that provide substantial performance benefits to the end consumer, but the processing conditions required to form a film on a substrate limit their usefulness. One of the commonest is polytetrafluoroethylene (PTFE), widely used as a coating for cookware and bakeware. It provides excellent stain resistance and durability but requires high-temperature processing after application, limiting its use to temperature-resistant substrates. Vinylidene fluoride (PVDF) and other common fluoropolymers such as copolymers of tetrafluoroethylene and perfluoroalkyl vinyl ether (PFA) or tetrafluoroethylene and ethylene (ETFE) also require high temperature processing. In addition, the high degree of crystallinity in these polymers makes their solubility in common organic solvents very poor and limits the options for use of these materials as coatings. Copolymers offer enhanced solvent solubility Monomers such as tetrafluoroethylene and chlorotrifluoroethylene can be copolymerised with a variety of different alkyl vinyl ether monomers to yield polymeric materials with an amorphous structure and excellent solubility in common organic solvents. Reactive functionality can also be incorporated into these copolymers through the introduction of suitably functionalised monomers during polymerisation. Polymers of this type have excellent room temperature processability and can be readily fabricated into coatings and films on a wide variety of substrates. Using TFE as the monomer allows some of the inherent characteristics of PTFE, such as excellent weatherability and dirt resistance, to be retained in the final tetrafluorethylene copolymer (TFEC) but with the added advantage of easier processing. Crosslinked systems can also be produced The introduction of reactive functional groups into TFE-derived copolymers enables the fluoropolymer to be crosslinked after application. Carbinol functionality, for example, can be introduced easily via the copolymerisation of hydroxyalkyl vinyl ether monomers, providing cure sites for both melamine and isocyanate crosslinkers, and also a mechanism by which adhesion can be selectively tailored for different substrate types. Non-reactive vinyl ether monomers bearing organic functionality allow modification of the copolymer characteristics to provide important properties such as solvent solubility, transparency and compatibility with organic or silicone resins and polymer additives. Figure 1 summarises the structural components of the TFEC copolymer and their role in providing performance properties in a finished coating. A wide variety of melamine, isocyanate and blocked isocyanate cross-linkers are commercially available and are easily combined with the TFEC copolymer using a suitable organic solvent. The main difference between the types of cross-linkers commercially available is the curing temperature. Isocyanates offer room temperature cure, but high temperatures are required for blocked isocyanate and melamine crosslinkers. Improvements in stain resistance are desirable The stain resistance and easy-clean properties of the TFEC coating were evaluated and compared with a 2K acrylic urethane system. Both systems were coated and cured with conventional isocyanate cross-linkers on aluminium panels and allowed to dry for three days at room temperature. A range of staining agents were then applied to the coatings and left for 24 hours prior to dry wipe removal from the coating surface. The degree of residual staining was then graded for both systems. As can be seen from Table 1, the cured TFEC coating provides excellent stain resistance and easy-clean properties across a range of staining agents, with improved performance in some areas when compared with the reference coating. However, the easy-clean and stain resistance against solvent-based marker pen were poor using the dry wipe method, although some improvement in the degree of residual staining was observed when using ethanol as a solvent. The poor performance of the TFEC coating against solvent based marker is thought to be due to the relatively low fluorine content of the TFEC coating, resulting in some compatibility with the solvent and components used in the marker pen. Because thin TFEC films can be applied and cured at room temperature, a number of methods were sought to improve the stain resistance to solvent based marker pen and the cleanability. Internal incompatibility leads to better performance The idea of developing a fluorine and silicone containing hybrid coating was investigated for a number of reasons. Silicones provide excellent release due to the flexible nature of the siloxane backbone (i.e. it has a low T g ), and their poor compatibility and miscibility with fluoropolymers may provide a mechanism for surface segregation and enrichment. The main area of focus was the use of organofunctional siloxanes as reactive surface modification agents to yield hybrid polymers. The effect of having no reactive functionality on the siloxane was also determined. Figure 2 shows the concept behind this siloxane/fluoropolymer hybrid. Incorporation of the reactive organofunctional siloxane

2 (ROFS) at low levels into the TFEC/isocyanate coating formulation resulted in a dramatic improvement in the stain resistance and easy-clean properties of the TFEC coating with excellent beading of the solvent-based marker pen on the surface and full removal on dry wiping, with the same curing and evaluation conditions as described earlier. Surprisingly, the non-reactive organofunctional siloxane (NROS) did not provide the same performance benefit, although some visual improvement was observed over the standard TFEC coating. An increase in water and oil repellency was observed with the hybrid material, indicating some synergistic effect which can also be seen from the water and hexadecane contact angles (Table 2). Surface siloxane enrichment explains the improvements In order to understand the role of silicone in the hybrid coating system, surface analysis experiments were conducted using ESCA (electron spectroscopy for chemical analysis) on both the uncured and the isocyanate cured films to determine what surface changes might be taking place in the presence of the ROFS. From the surface elemental composition data (Figure 3) it appears that in both the uncured film and the cured coating the ROFS segregates towards the surface, providing localised siloxane enrichment. Interestingly, a significant increase in siloxane surface enrichment is observed in the cured coating compared with the uncured film (Figure 3a). This behaviour is attributed to the cross-linking of the fluoropolymer being significantly faster than the reaction of the ROFS in the coating network. This results in migration to the surface as the network of the coating is formed, driven by incompatibility of the siloxane with the developing network. This surface migration effect is further confirmed when ESCA surface analysis is carried out on both the front and back (substrate) sides of the cured coating. Figure 3b shows the results of this study which indicate that the substrate side of the coating has a significantly higher fluorine content than the topside, and thus a lower siloxane content. This indicates that there is a siloxane concentration gradient through the coating with the highest concentration observed at the coating surface. Surface saturation occurs at low silicon levels The relationship between ROFS concentration and maximum surface saturation during curing has been investigated along with the relationship of ROFS concentration to the stain resistance and easy-clean properties of the coating, with respect to solvent based marker. Figure 4 clearly shows that maximum ROFS enrichment of the surface occurs at a level of approximately 0.2% w/w in the coating formulation, which corresponds directly to the level required to provide the stain resistant and easy-clean properties for solvent-based marker pen. Above this level, no difference is observed in the elemental composition of the coating surface with respect to silicon and fluorine or the stain resistance. Due to the pronounced enrichment of the surface with the ROFS on cross-linking, studies were conducted to ensure that the ROFS was fully cured into the coating network so that it would provide a durable release mechanism. After 15 cleaning cycles with an organic solvent in which ROFS is soluble the coating surface still showed the same level of beading to solvent based marker as the uncleaned coating surface. ESCA surface analysis of the cleaned and uncleaned coating surfaces (Figure 5) gave very similar results, indicating that the ROFS is fully cured into the coating network. Any unreacted ROFS would have been extracted from the surface during the solvent cleaning cycle. The stain resistance and easy-clean properties of the TFEC/ROFS coating system were evaluated at a level of approximately 0.2% w/w ROFS and compared with the TFEC and the 2K acrylic urethane reference systems using the conditions already described. In this study, a solvent-based lacquer paint and a urethane sealant were also included as staining agents. The results of this study are presented in Table 3, from which it can be seen that the performance of the TFEC/ROFS coating system is an improvement over the TFEC coating system and a significant improvement over the 2K acrylic urethane reference system. In terms of general coating properties such as flexibility, weatherability, chemical and solvent resistance, the TFEC/ROFS coating shows excellent performance comparable to that of PTFE coatings. Thus it retains many of the performance benefits of PTFE without any negative effects from the presence of the ROFS in the hybrid polymer. Fluorosilicones: a versatile stain-resistant technology In conclusion, protective coatings based upon hybrid materials derived from fluoropolymers and reactive organofunctional siloxanes have been developed which can be applied at room temperature. Curing can be carried out across a wide temperature range (~20 C to 150 C) using different crosslinkers. This coating technology can be applied to a wide variety of substrates such as stone, metals, glass, plastics, leather and paper and provides a durable stain resistant and easy-clean finish that retains the natural appearance of the substrate. The incorporation of reactive organofunctional siloxanes extends the stain repellency and easy-clean properties of the fluoropolymer to a wider variety of staining agents. This synergistic effect is due to surface enrichment of the coating with the organofunctional siloxane, made possible by the flexibility of the siloxane backbone and the low barrier to rotation of the Si-O-Si bond. Results at a glance - Fluoropolymers provide excellent stain resistance and easy-clean properties but their application requires high temperatures as their solvent solubility is extremely low. - Polymerisation of tetrafluoroethylene (TFE) with hydrocarbon-based monomers has produced new curable fluorocarbon resins having high compatibility and solubility with organic solvents, organic resins and silicones. - By combining functionalised fluorocarbon resins, organofunctional siloxanes and organic crosslinkers, coatings can be produced that provide excellent and durable stain resistance and easy-clean properties. - The combination of fluorine and silicone has synergistic effects, since the silicone component migrates preferentially to the top surface, improving stain release properties. This paper was presented at the 8th Nürnberg Congress, Creative Advances in Coatings Technology, Nuremberg, Germany, 25/26 April 2005 The authors: -> Dr. Peter Hupfield is a senior research and development specialist at Dow Corning, currently researching the development of new organofunctional siloxanes. He holds a Ph.D. in the free radical chemistry of silyl hydrides from the University of Wales College, Cardiff. -> Eiji Kitaura is a research chemist at Dow Corning Toray, in the Specialty Chemicals business. He graduated from Kyusyu University with a masters degree in the study of 'in vivo' oxygen carriers. He is currently researching the

3 development of new antifoams, coating additives and paint systems. -> Dr. Tetsuya Masutani graduated from the University of Osaka prefecture with a Ph.D. in agricultural and natural product chemistry. He has extensive experience in the development of novel fluorinated specialty chemicals and is an executive researcher currently responsible for new technology and business development at Daikin. -> Masaru Nagato joined Daikin Industries in A graduate of the University of Osaka prefecture with a masters degree in UV degradation mechanisms in polymer systems, he is currently researching the development of new fluoropolymer coating systems.

4 Figure 1: Structural components and their roles in tetrafluoroe thylene-copolymer (TFEC)-based coatings. Figure 2: Concept behind the siloxane and fluoropolymer hybrid showing the basis of the chemical reaction.. Figure 3: ESCA surface analysis of TFEC/ROFS coatings: a) ESCA analyses of cured and uncured coatings; b) ESCA analyses of cured coatings (front and reverse sides). Figure 4: Relationship between ROFS content and surface composition.

5 Figure 5: Effect of solvent cleaning on surface composition of TFEC film: ESCA analysis shows that the solvent has failed to remove any significant amount of silicone from the surface.

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