The Use of Kynar® as a Top Finish
in Architectural Fabrics
Susan L. Uhler, Technical Marketing, Industrial Fabrics Division, Seaman Corporation, Wooster, Ohio USA
Felon R. Wilson, P.E., Business Director, Industrial Fabrics Division, Seaman Corporation, Wooster, Ohio USA
Overview
The introduction of Kynar as a premium top finish in the architectural fabrics industry represents a major technological advancement. When used in conjunction with a high performance membrane, Kynar provides designers, fabricators and users a superior option for a long-life, low-maintenance, and high-aesthetic shell in an Architectural Fabric structure. Many designers and users are familiar with Kynar from actual experience and performance in other building applications. This paper provides a brief history and further insight into the chemistry, performance, and sustainability of Kynar finished fabrics.
History of Kynar Use
The technology of protective coatings was first introduced in 1948, when DuPont™ developed the fluoropolymer, polyvinyl fluoride (PVF). Arkema, previously known as Pennsalt, based in Colombes, France continued the development of fluoropolymers with the introduction of Kynar® in 1960, which found initial applications in chemical handling due to its exceptional chemical resistance. Subsequent applications included other protective uses such as wire insulation, jacketing and ultimately building protection due to its superior environmental durability. Arkema reports that the growth of the use of Kynar has been quite widespread with Kynar 500 (most common Kynar resin) with the worldwide architectural community now considering it to be the resin-based coating of choice for superior long-term performance in an array of building applications.
Corrosion in the building envelope is most pronounced in exposed metals. Kynar-based coatings quickly found extensive use to protect various metal components including aluminum, and galvanized and aluminized steels, in various applications such as standing seam roofs. These coatings have been proven to be effective both on test, and full-scale, applications. One of the most dramatic test examples of the durability of the Kynar-based coatings was a study conducted in Japan by Arkema in 1981 which compared the weatherability of Kynar-coated metal to other coatings including Acrylic, Polyester, and Silicone Polyester. After 14 years of exposure in warm, humid conditions, the Kynar color retention was far superior to the other materials as indicated in Figure 1.
Figure 1
Kynar-based Coating Weathering Results
Full scale demonstration of the durability exists along the Las Vegas Strip where multiple buildings such as the MGM Grand, Rio Las Vegas, and Mandalay Bay Hotels have extensive exterior protection with Kynar-based coatings. These buildings have been in service for over 20 years and maintain vivid color retention, weatherability, and maintenance-free reliability. With the long-term performance of Kynar established in the metal building industry, the transition into the flexible membrane market has been forthcoming.
Finishes Used in Coated Fabrics
Industrial polymer coated fabrics, like metal, utilize surface coatings to protect the aesthetic appearance and improve the cleanability. Flexible coated fabrics used in this market utilize synthetic resins that contain modifiers in the base polymeric matrix to impart elastic properties to the coatings. Over time, these modifiers will migrate and adhere to the surface resulting in dirt accumulation. A membrane that collects dirt loses its aesthetic qualities and provides a venue that promotes fungal growth on the surface. Acrylic, polyvinylidene fluoride (PVDF), Tedar® (PVF) and now Kynar film, are surface treatments (top finishes) that are used in this industry to negate this phenomenon. Based on the level of protection required, each has their own distinct properties and performance characteristics.
Acrylics: Acrylics are a good choice as a top finish on architectural membrane structures. The acrylic resin systems are typically comprised of a blend of polymethyl methacrylate (PMMA), polyvinyl chloride polymers, modifiers, and a carrier. These translucent acrylic systems, typically 0.1 to 0.3 mil in thickness, help to block the liquid modifiers from reaching the surface and inhibit dirt collection. Acrylic resins contain esters and incorporate other functional groups which are susceptible to photochemical degradation and hydrolysis, both of which are found in environmental exposure. These conditions contribute to the ultimate breakdown of the protective Acrylic layer.
PVDF: Weldable PVDF systems are considered by many to provide improved performance, while still allowing thermal bonding to other synthetic resins. As PVDF is inert, acrylic resins are blended with PVDF to provide a composite that will adhere to other resin systems. In fact, the percent of PVDF resin found in this system is typically less than 30% by resin weight content, with the remainder being primarily acrylic. The overall thickness is slightly higher than acrylic at 0.3 to 0.6 mil. Weldable PVDF, generally non-pigmented, is considered to be an improvement over the acrylic systems as the PVDF resin does offer some unique characteristics. The low amount of PVDF provides for some hydrolysis resistance and allows ultraviolet wavelengths to pass through, increasing the weatherability of the finish. Because the acrylic resin is the major polymer in the system, the properties inherent with acrylic polymers result in the degradation of the surface coating system. In addition, studies have shown that at the low PVDF content, mechanical properties and toughness are sacrificed due to disruption of the crystallinity of the molecular structure.
PVF: PVF is the predecessor to PVDF, and has performed in the architectural market for over 30 years. Known primarily under the trade name Tedlar, this film has been used extensively in the construction market on vinyl siding, architectural shingles, and membrane structures. Low water permeation, high UV resistance, color stability, and good chemical resistivity are properties of this 1 mil film that has provided for a high-performance, long-term durable surface in harsh environments. Until recently, this product was the best solution for long-term performance in this market. With the rise in usage of Tedlar in photovoltaic cells, Kynar film has emerged in the architectural market to provide yet another superior alternative to the acrylic and weldable PVDF systems.
Kynar: Kynar film, licensed only by Arkema, is based on over 40 years of outdoor exposure of the Kynar PVDF resins. Arkema produces and controls the proprietary formulation of the 1 mil Kynar film to provide a consistent, high-performance finish for the end product. Unlike weldable PVDF, this film carries the inert properties of the Kynar resin and is not weldable to synthetic resins. The percent by weight of the resin content found on the surface is a minimum 70% PVDF. This allows the crystalline structure to form that provides superior barrier properties. Proprietary welding techniques have been developed to allow the Kynar to adhere to synthetic resins for ease of fabrication with standard welding equipment common to the industry.
Kynar Chemistry
Kynar’s molecular structure, as well as the percent weight of resin content, determines the overall performance characteristics of the film. Kynar contains carbon-fluorine(C-F) bonds, one of the strongest and most stable chemical bonds known. Tedlar provides one C-F, three carbon-hydrogen (C-H) bonds, whereas the C-F and C-H bonds are alternating in Kynar. Figure 2 illustrates the molecular structure comparison of Kynar vs. PVF (Tedlar).
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Figure 2
Kynar vs. PVF (Tedlar) Molecular structure
The alternating C-F bond results in a higher polarity on the surface to resist environmental degradation and dirt accumulation. The very low surface energy and low coefficient of friction of the Kynar film resists soiling as compared to weldable PVDF finishes. The lack of dirt accumulation and the nonnutrient value of Kynar to support fungal growth further inhibits mildew development. The photos in Figure 3 compare two coated fabrics in the southern US after 3 years of exposure. One has a standard PVDF top finish and the other has Kynar.
Coated Fabric with PVDF vs. Kynar Top Finish
Figure 3
Alternating 2 C-F bonds, combined with a Fluorine content of over 59%, results in excellent chemical resistance and stain repellency of the Kynar film. The amount of Fluorine present and the high electronegativity of the Fluorine atoms, in addition to the small atomic radius, are the bases for the Fluoropolymer effectiveness. The two Fluorine bonds found in Kynar contribute to the superior resistance against chemical, thermal, and UV degradation.
Kynar Performance
The Kynar polymer is transparent to ultraviolet rays resulting in a high UV stable surface coating. Further, the molecular structure inhibits moisture and oxygen penetration, reducing hydrolysis and oxidation of the underlying coating. Microscopic evaluation of the surface after 5000 hours QUV B exposure illustrates enhanced resistance of the Kynar film in comparison to other finishes. QUV B weathering provides the shortest wavelengths found at the earth’s surface. Figure 4 shows magnification of parallel weathered surfaces of Kynar and two types of PVF Film.
High-quality pigments and UV additives are formulated into the film to reflect and absorb these harmful rays. UV rays found below 300nm have been determined to cause the most damage to synthetic polymers used in the base coating of flexible architectural fabrics. UV radiation is the major source of environmental degradation of surface coatings. The UV blocking capability of the film, shown in Figure 5, provides excellent protection of the coated fabric as opposed to clear finishes which do not offer this feature.
Surface Magnification after Laboratory Weathering: Kynar Film vs. PVF Film
Figure 4
UV Blocking (Transmittance) for Kynar Film
Figure 5
The weathering characteristics are critical when evaluating surface coating for long-term performance in the architectural market. The Kynar film provides resistance to UV degradation, moisture penetration, oxidation, and dirt accumulation, all of which can be found in environmental exposure. With the use of accelerated weathering equipment, exposure of Kynar coated fabric samples has been performed that correlates to over 10 years in south Florida. The results illustrate a lower change in color shift of the Kynar film in combination with a flexible coated fabric used in the membrane market. Figures 6 and 7 illustrate laboratory data confirming the stability of the Kynar film use in conjunction with Shelter Rite architectural fabric. The weathering characteristics are critical when evaluating surface coating for long-term performance in the architectural market.
Color Retention vs. Exposure Time: Kynar vs. PVF Coated Fabric
Figure 6
Color Retention vs. Exposure Time: Kynar vs. PVF Coated Fabric
Figure 7
The stretch features of a finished membrane are directly related to the base cloth. Kynar film on coated fabrics has little to no effect on the stretch characteristics. Biaxial stretch tests, stretch properties at a given load on a given fabric, are industry-typical for Kynar top finish systems as illustrated in conjunction with a Shelter Rite Style 8028, shown in Figure 8.
Kynar (Brite) vs Tedlar Biaxial Stretch Test
Figure 8
The superior weathering, chemical and dirt resistance characteristics, in addition to sustainability aspects of the Kynar film positions this as the benchmark for surface finishes in the industry. Pairing of the Kynar with high-performance flexible coated fabrics provides additional capabilities to designers giving them alternatives to standard building materials. Further development in Kynar technology with architectural fabrics may prove to offer designers even broader capabilities.
Sustainability and Design with Shelter Rite Brite® with Kynar
In addition to Kynar’s excellent weatherability in outdoor exposure, the Kynar film further promotes sustainability qualities into the architectural membrane. Natural daylight can be designed into the structure by utilizing the film over more transparent coatings to block the damaging UV rays. The film used over white coating increases the solar reflectance index (SRI) reducing solar heat gain over the life of the structure. Further, Kynar film provides a surface that is low maintenance. The low surface energy of the Kynar provides for minimal dirt collection and natural cleansing, reducing the need to for cleaning chemicals. The stain repellency of the film helps to preserve the aesthetically pleasing finish by allowing easy removal of markers, food stains, and graffiti. Flexible coated fabrics provide high versatility which allows an architect or engineer the option to design energy savings into the building.
The strength and stretch characteristics of a fabric are key properties that will influence the final decision of which material will be used in the project. There is a range of materials based on tensile strength that can be selected with the Kynar finish.
References
Arkema, Inc., Personal Communication and General Literature, 2011.
“Creating Coatings for Better Buildings”, Iezzi, Robert A. Ph.D., Elf Atochem North America Inc., Research Center, King of Prussia, PA.
“70% PVDF Coatings for Highly Weatherable Architectural Coatings”, Wood, Kurt, et al, Atofina Chemicals, Inc., King of Prussia PA.
Kynar® is a registered trademark of the Arkema Corporation.
Tedlar® is a registered trademark of the DuPont Company.
Brite® is a registered trademark of Seaman Corporation.
