1、SSPC-TU 12May 4, 20151SSPC: The Society for Protective CoatingsTechnology Update No. 12Ambient-Curing Fluoropolymer Finish Coats Applied to Metal Substrates1. ScopeThis technology update provides a fundamental discussion of ambient-curing fluoropolymer finish coats applied to metal substrates, inclu
2、ding recent developments. It is intended as a resource for architects, specification writers, facility owners, and others charged with the selection and field application of coating systems requiring superior weathering resistance and appearance retention in both architectural and industrial applica
3、tions. It includes background information about the development of fluoropolymer materials, a basic description of fluoropolymer coating technology, the different types of fluo-ropolymers used in coatings today, application methods, and performance testing procedures.2. Description “Ambient-cured fl
4、uoropolymer finish coats” describes a class of coating materials based on resins (i.e., polymers) with a high fluorine content that have been formulated into liquid applied finishes that “air-dry” or cure under ambient conditions. Finish coats based upon fluoropolymer resin technology are designed t
5、o retain color and gloss after years of direct sunlight (UV) exposure. The two principal fluoropolymer resin types for finish coatings used at the time of publication are polyvinyli-dene fluoride (PVDF), and fluoroethylene-vinyl ether (FEVE). Fluoropolymer resin-based coatings are most frequently se
6、lected as finish coats to be applied over compatible primers or intermediate coats in service environments where long-term color and gloss retention and chalking resistance are required. Additionally, the barrier properties of fluoropolymer resin-based coatings provide corrosion resistance, making t
7、hem suitable for use as finish coats in marine and offshore environments. The chemical resistance of thermoset fluoropolymer resin coatings is similar to that of standard polyurethanes (such as SSPC-Paint 36)1, and their application in severely corrosive environments may be limited.3. Fluoropolymer
8、Technology3.1 HISTORY OF FLUOROPOLYMERS: Thermoplastic fluoropolymers are a family of hydrocarbon polymers having high fluorine content, first developed by the DuPont Company in the 1930s, that were initially limited to the production of extruded or molded plastic parts. As a family of materials, 1
9、SSPC-Paint 36 (latest revision), Two-Component Weatherable Aliphatic Polyurethane Topcoat, Performance-Based. these thermoplastic fluoropolymers offered extremely high performance in severe environments, primarily due to their resistance to a widely diverse range of harsh chemicals, resis-tance to e
10、levated temperature, low surface energy and low coefficient of friction (i.e., “non-stick” capability). 3.1.1 Original Development of Ambient-Cured Fluo-ropolymers: The first generation fluoropolymer-based coating materials were produced in the 1960s. They were solvent-borne liquid dispersions of po
11、lyvinylidene fluoride (PVDF) thermoplastic resin (see Figure 1). These PVDF dispersions were later formulated into coatings by blending with acrylic resins typically at a 70:30 PVDF:acrylic ratio by weight as finish topcoats for exterior metal substrates on commercial buildings. Formulations with le
12、ss than 70% PVDF in the resin are generally regarded as having reduced gloss retention and increased levels of chalking.CCCCCFFHHFFHHCFFHHFigure 1. Polyvinylidene Fluoride Chemical Structure.The need to bake these coatings at high temperatures limited their applicability to uses that could accommoda
13、te factory application and high-temperature bake processes. They were not an alternative to more conventional air-dry finish coats. The PVDF finish coatings were factory-applied to thin-gauge steel sheets coated with zinc or its alloys using a coil-coating process, or spray-applied to extruded alumi
14、num. The coatings were then baked in an oven at 230-250 C (approximately 450-500 F) to develop the final properties of the thermoplastic coating.A coating that offered comparable performance to the baked PVDF coatings that would air dry under ambient conditions and could be applied by conventional a
15、pplication methods was highly desirable. This led to the development of solvent-borne PVDF-acrylic coatings that formed a dry film by solvent evaporation without a high-temperature bake. These early air-dry coatings began to approach the performance of the baked coatings, but exhibited relatively lo
16、wer adhesion, lower tensile strength and limited scratch and mar resistance, SSPC-TU 12May 4, 20152when compared to the baked PVDF coatings. 3.1.2 Development of Second-Generation Ambient Cured Fluoropolymers: The second advancement in ambient-cured fluoropolymers occurred in Japan circa 1980. These
17、 newer resins were designated fluoroethylene vinyl ethers, or FEVE (see Figure 2). They were also formulated with solvents, but could undergo thermosetting reactions during ambient temperature cure. Early FEVE thermosetting coatings show marked improvement in many physical proper-ties (such as highe
18、r gloss capabilities) compared to the earlier PVDF thermoplastic solution coatings.Today, most FEVE products are cross-linked with poly-isocyanates, yielding coatings with superior transparency, gloss, and hardness, with many FEVE systems now demon-strating outdoor weatherability comparable to the 7
19、0% PVDF technology.3.1.3 Third-Generation Water-borne Ambient-Cured Fluoropolymer Systems: Environmental concerns have led to severe restrictions in the amount of volatile organic compounds (VOCs) that can be released to the atmosphere as solvent-borne coatings cure. Considerable work has been done
20、to adapt both the FEVE and the PVDF resin technolo-gies for use in water-borne fluoropolymer coatings to reduce the amounts of solvent released, and hence the VOCs emitted to the environment. The FEVE technology can be adapted to water, either in an FEVE emulsion form (higher molecular weight), or i
21、n water dispersion form (lower molecular weight). The PVDF technology can be adapted to water by making PVDF-acrylic hybrid dispersions (latex), which incorporate a miscible blend of fluoropolymer and acrylic polymer in each latex particle. Either resin chemistry can be adapted to make either thermo
22、plastic (non-crosslinking) or thermoset (cross-linking) coating systems. Generally speaking, higher molecular weight fluoropolymers are used for thermoplastic systems, and lower molecular weight fluoropolymers are preferred for ther-mosetting systems. 3.2 CURING MECHANISM OF ARCHITECTURAL AND INDUST
23、RIAL FLUOROPOLYMERS: Most fluoropolymer products for thermosetting systems are hydroxy functional. This means that they can be cross-linked with the same prod-ucts used with standard acrylic or polyester polyols. Examples of cross-linkers include oligomers of isophorone diisocyanate (IPDI), hexameth
24、ylene diisocyanate (HDI), and hydrogenated methylene diphenyl diisocyanate (HMDI). Ambient-curing coatings of this type are generally two-component or “2-k.” For water-borne thermosetting systems, a number of special-ized crosslinker products have been developed, which are adapted to better incorpor
25、ate the hydrophobic crosslinker into an aqueous medium.Thermoplastic systems do not “cure” via crosslinking, but rather develop their properties by means of increasing polymer chain entanglements during and after a physical drying, a process known as “coalescence.” Solution coatings and dispersions
26、have different mechanisms for film formation. Thermoplastic systems are generally single-component or “1-k.” 3.3 FACTORS AFFECTING THE FINAL COATING PERFORMANCE3.3.1 Resin Structure and Fluorine Content: Fluoropolymer resins derive their weatherability from their chemical structure. The unique prope
27、rties of fluorine atoms not only make the resin much more hydrophobic (important for both corrosion resistance and weathering), but also impart increased chemical and UV resistance to the resin, including nearby non-fluorinated segments on the polymer chain. For instance, the regularly alternating s
28、tructure of FEVE resins allows the fluorinated segments of the molecule to protect Figure 2. Structure of Fluorolymer Resin Polymer.The fluoroethylene segments (en-closed by boxes in figure above)provide weatherability, durability and chemical resistance. Each of the vinyl ether segments (not enclos
29、ed by boxes) contributes different attributes: clarity; gloss and hardness; flexibility; and crosslinking.LUMIFLON FLUOROPOLYMER RESINS Long Term Performance Vs. PVDF LUMIFLON resins are known generically as fluoroethylene vinyl ether (FEVE) resins. The name derives from the reactants used to form t
30、he polymer. These materials form the regularly alternating structure shown below. The alternating structure is critical to the weatherability of coatings made with LUMIFLON resins. Weatherability Solubility Flexibility Crosslinking Sites Durability Transparency Adhesion Gloss Hardness Physical prope
31、rties of the LUMIFLON resin can be modified by changes in the nature of the vinyl ether units. Ultra-weatherability, durability, and chemical resistance are derived from the alternating fluorinated units. LUMIFLON resins are usually reacted with aliphatic isocyanates to form crosslinked fluorouretha
32、ne coatings. Recently, claims have been made that FEVE resins do not offer the same weatherability as PVDF coatings because they are lower in fluorine content. These claims are misleading, as explained below. The fluorine content of FEVE based coatings is not directly related to their durability. Ra
33、ther, it is the distinctive alternating polymer structure of LUMIFLON resins that enables the development of ultra-weatherable fluorourethane coatings. The chemical bond between carbon atoms and fluorine atoms is too strong to be broken by sunlight. This means that the polymer is not degraded by ult
34、raviolet radiation from sunlight. The alternating structure of the LUMIFLON resin also increases the strength of other chemical bonds in the polymer, which protects the entire coating against degradation. The result is a fluoropolymer that is as weatherable as PVDF. The protective mechanism is illus
35、trated in the figure below. FCCF8888XFHCCH8888OH8R1FCCF8888XFHCCH8888OH8R2FCCF8888XFHCCH8888OH8R3OH88888888888888888888888888888SSPC-TU 12May 4, 20153the vinyl ether segment from degrading due to exposure to UV radiation and corrosives. This protective mechanism continues to work even in the cross-l
36、inked coating. Because the ability of fluorine atoms to protect nearby groups depends on details of the molecular structure, fluorine content alone is not sufficient to predict how weatherable a coating system will be. In some fluoropolymer coating systems, including nearly all PVDF coating systems,
37、 non-fluorinated blend resins such as thermoplastic acrylics or acrylic polyols are incorporated in order to modify the balance of properties In weathering studies, it was found that the lower the level of fluorinated resin in the system, the more the performance tends toward that of a non-fluorinat
38、ed system. However, the relationship is not always proportional. For example, paints based on air-dry PVDF solution coatings, with 30% added acrylic, maintain color retention and chalk resistance for decades, as long as weatherable pigments are used (Figure 3). Nevertheless, for any given fluoropoly
39、mer resin type, the total resin fluorine content will often give a general idea of the weathering performance.3.3.2 Examples of Effect of Formulation Variables on Coating Performance: By using the proper components in a coating formulation, coating manufacturers can maximize the performance and form
40、ulate to a balance of properties for a given application. The color and gloss stability of the coating is highly dependent on the light-fastness of the pigment chosen. It is important to use pigments that will not change color over time. The pigment must withstand UV light and humidity. As a general
41、 rule, inorganic oxide pigments are more durable (UV- resistant) than organic pigments, but the organics give a wider range of bright colors. In addition to highly durable pigments, UV stabilizers can also be used in the formulations to improve both color and gloss retention.3.4 EXPOSURE STUDIES OF
42、FLUOROPOLYMER: Fluoropolymer resin-based coatings have been evaluated in a range of accelerated exposure tests, as well as outdoor expo-sures. In these exposure tests, gloss retention, color retention, and chalking resistance can serve as the measure of coating durability, particularly for coatings
43、that have a decorative function. 3.4.1 Fresnel-type Exposure (ASTM G90)2: This method of “natural accelerated weathering” involves concentrating 2 ASTM G 90, “Standard Practice for Performing Accelerated Outdoor Weathering of Nonme-tallic Materials Using Concentrated Natural Sunlight,” (latest editi
44、on), ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. Standards available online from http:/www.astm.org. Figure 3. Baked (top row) and air-dry (2nd and 3rd row) PVDF-based paints with 30% added acrylic, after 25-30 years South Florida exposure. Topcoat formulations, 1-mi
45、l (25 microns) dry film thickness, on chromated aluminum. Exposure at 45 south facing, unbacked. The top quarter of each panel is the original panel color. Source: Arkema, Inc.Figure 4. Fresnel-type (ASTM G90) Testing of Fluoropolymer Resin-Based Coatings. Percent gloss retention is plotted vs. the
46、total UV dose received by the coating. Source: Asahi Glass Chemicals Co.SSPC-TU 12May 4, 20154sunlight via reflective mirrors onto the coating specimen, with intensity approximately equal to that of 8 suns (5 suns in the UV range of the spectrum). The unit mirrors track the solar image as it moves a
47、cross the sky, exposing the test specimen to the full spectrum of radiation found in natural sunlight. An oscillating nozzle sprays the specimens with deionized water on a preset schedule. Results are reported in units of energy to which the coating is exposed. Figure 4 shows the results of ASTM G90
48、 testing on a typical FEVE fluoropolymer resin-based coating, along with results for a PVDF-based coil coating.3.4.2 UV-Fluorescent Cabinet and “Weatherometer” Testing: Accelerated methods to test fluoropolymer resin coatings often make use of xenon arc weatherometers and UV fluorescent test cabinet
49、s such as those described in ASTM G154 and G155.3In one commonly used test cycle, 3 ASTM G 154, “Standard Practice for Operating Fluorescent Light Apparatus for UV Exposure of Nonmetallic Materials,” (latest edition); ASTM G155 “Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Nonmetallic Materials” ASTM Interna-tional, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. Standards available online from http:/www.astm.org.the coating is exposed to
