By Donald C. Lawson III, Technical Service Chemist,and Robert Parker, Technical Service Chemist, AGC Chemicals Americas, Inc.
Fluoroethylene vinyl ether (FEVE) resins were developed in Japan in the late 1970s and entered the commercial market there in 1982. FEVE resins are amorphous A-B type copolymers with repeating units of fluoroethylene and substituted vinyl ether. Unlike pure fluoropolymers, FEVE resins are soluble in solvent due to the vinyl ether groups. Solvent solubility transforms FEVE resins from high-performance polymers into high-performance backbone resins for paints and coatings.
Fluoropolymers, like PVDF (polyvinylidene fluoride), were used in paints prior to the introduction of FEVE resins to the market. In fact, PVDF resins are still used widely today. In order to utilize these more traditional fluoropolymers like PVDF in liquid coatings, blending with other resins such as acrylics is needed. Special solvents are required to solubilize the blends, and ultimately heat is used to help the system flow and form a thermoplastic coating film. In contrast, the FEVE polymer was designed to have inherent solubility in conventional, widely used solvents via vinyl ether monomers. The chemistry of the FEVE polymer also is fully amorphous, unlike the PVDFacrylic systems that are semicrystalline. This amorphous morphology allows FEVE resins to form films without heat. The implication of this property is considerable. The introduction of FEVE fluoropolymers to the coatings industry brought extremely durable fluoropolymer coatings out of the factory and into the field.
The fluoroethylene groups are the strength of the FEVEresin. These groups are what make this class of polymersso resistant to UV degradation. The C-F bond is strong. Theenergy of this bond is ~486 kJ/mol, while the energy ofUV radiation at 300 nm is ~399 kJ/mol. The alternatingpattern, shown in Figure 1, is critical for the extreme UVresistance properties. The chemically stable and UV-resistantfluoroethylene unit sterically and chemically protectsthe neighboring vinyl ether unit.
The vinyl ether groups make FEVE polymers useable asresins for paint. Without the vinyl ether groups, FEVE resinswould not be soluble in solvent. This solubility is whatallows FEVE resins to be used in a wide array of coatingformulations that can be applied in factory or field setting. The vinyl ether groups also contribute to high glossand allow for functional groups, like hydroxyl groups, tobe incorporated into the structure. Table 1 shows typicalproperties of FEVE resins.
Weathering Performance of Typical Solvent- and Water-Based FEVE Coatings.
Weathering tests have shown that coatings based on FEVE resins have superior performance. This data has been discussed extensively in a previous study, and is shown in Figures(2-5).
Water-Based FEVE Resins for Highly Weatherable, Heavy-Duty Coatings.
Weathering testing has shown that 2K water-based coatings based on FEVE resins perform comparably with solvent-based2K FEVE coatings. As VOC regulations continue to tighten, thedemand for heavy-duty water-based coatings will rise. The resultsof South Florida testing illustrate the high level of performancethat can be achieved with FEVE 2K coatings (Figures 6-7).
Formulating 2K water-based coatings is a challenge because ofthe competing reactions of polyol with NCO and water with NCO.
The reaction between water and polyisocyanate can result inthe formation of polyurea instead of polyurethane. The followingequation illustrates the reaction mechanisms involved.
Another issue resulting from the reaction of water with polyisocyanateis the formation and evolution of carbon dioxide gas.If this occurs, it tends to happen throughout the pot life of thecoating. This results in initial applications that appear nice, butsubsequent applications may appear hazy. In severe cases, the gasbubbles can actually be seen with the naked eye.
Several approaches are available to overcome these challenges.One method is to use increased levels of polyisocyanate, often calledover-indexing. The theory is that enough NCO will be available toreact with water and the polyol. Another, less-common approachis to use catalysts specifically designed for water-based 2K systems.
Previous work showed the results of both of theseapproaches. The results yielded excellent weathering and corrosionresistance, but limited MEK double rubs. This indicatedthat the cure of these systems might not be optimal. A study (Figure 8a) was designed to better understand the factorsaffecting performance in MEK double rubs. The results suggestthat many factors impact performance in this test.
Formulations were made and MEK double rubs were tested overthe course of the pot life of each system. Analysis of the impact ofNCO:OH index and the presence of catalyst were performed first.Figure 8b shows clearly that the addition of catalyst significantlyimproves MEK double rubs. An increase in index makes a difference in combination with catalyst but not alone. This was seen in a studydone previously with the FEVE dispersion, shown in Figure 9.
When testing the FEVE dispersion with a different isocyanate,the impact of NCO:OH index seems more critical than theuse of catalyst (Figure 10).
Similar tests were done with the hydroxyl functional FEVEemulsion (Figure 11). The emulsion in combination with EasaquaTM XD401 had slightly better initial MEK double rubsbut followed a similar trend as the FEVE dispersion with EasaquaXD401 over the course of the four-hour pot life study.
The FEVE emulsion fared much better with the Bayhydur®302 isocyanate, even surpassing the FEVE dispersion withBayhydur 302 (Figure 12).
The previous analysis of the data looked at the impact ofindex and catalyst. In Figure 13, the impact of resin chemistry isreviewed at a constant index without catalyst.
The results indicate that all systems, save the FEVE dispersionwith Bayhydur 302, have respectable initial MEK double rubs.However, over the pot life, the FEVE emulsion with Bayhydur 302clearly outperforms the other systems.
The previous analysis showed the results of MEK doublerubs over the course of the pot life. The results of the FEVEdispersion with Bayhydur 302 raise the question of whetheran appreciable “sweat in” time is needed in the uncatalyzedsystems because the highest MEK double rubs were not alwaysseen initially, but after 30 min to an hour of pot life. This wasnot the case with the FEVE dispersion with Easaqua XD401 orwith any of the emulsion systems.
It is important to note that the FEVE dispersion formulationsdid not require cosolvents, while the FE emulsion formulations didrequire cosolvents due to the higher Tg of the emulsion. It is alsoworth noting that the Easaqua XD401 is 85% solids in solvent,while the Bayhydur 302 is 100% solids. It is possible that the systemswith cosolvent are very stable and well blended immediatelyafter mixing, while the FEVE dispersions without cosolvent needtime to blend well post-mixing. This needs to be studied further.
Common practice is to study degree of cure using MEK doublerub testing right after mixing. The following graphs show theperformance of all the systems at time zero. In Figure 12 the cleartrend is that increasing index and the use of catalyst significantlyimproves initial MEK double rubs with the FEVE dispersion. InFigures 14 and 15 the trend of increasing index improving performancefollows with the FEVE emulsion systems.
Table 2 shows the formulation used for the FEVE dispersion followedby the properties of that formulation.
Additional Testing in Progress
The aforementioned study shows that crosslinker, index, catalystand possibly cosolvent level impact performance in MEKdouble rubs. All systems tested in this study have been putinto QUV-A and Xenon Arc accelerated testing. Heavy-dutycoatings used on metal substrates must also prevent corrosion;therefore, the formulations tested in this study are also beinganalyzed by EIS spectroscopy to determine barrier propertiesthat are indicative of likely performance in corrosion testing. Aphysical property testing is also in progress.
FEVE fluoropolymer resins offer ultra-weatherability to solventandwater-based coatings. As VOC regulations continue to increase,the need for heavy-duty water-based coatings also increases. However,formulating water-based 2K coatings presents many challenges.Though the results of accelerated and real-world, SouthFlorida exposure testing were excellent, early analysis of degree ofcure as measured by MEK double rubs was concerning.
Unlike the high levels that typical, high-performing solventbased2K coatings exhibit, preliminary testing of water-based 2K FEVE coatings were lower. Heavy-dutycoatings for metal substrates need more than weatherability; they also need corrosionresistance. A better understanding of the factors affecting the cure in waterbased2K coatings was needed.
This study reviewed analysis of several formulations based on an FEVE emulsion and an FEVE dispersion. Results indicated that both the use of excess isocyanate (overindexing)as well as specially designed catalysts for water-based 2K polyurethane coatings improve cure as measured by MEK double rubs. Formulations are currently being evaluated for weathering,corrosion resistance and physical properties.