TEXTILES.ORG
By Richard N. Seaman and Frank Bradenburg
Editor’s note: This two-part article is adapted from a paper presented at TechTextil North America, Atlanta, Ga., in March 2000. Part one of this article covers the performance properties of vinyl-coated polyester fabrics and their tensile strength, uniaxial and biaxial elongation, dimensional stability, tear strength, coating adhesion, and weldability and seam strength of architectural fabrics. This second part covers issues of wicking, ultraviolet light and weathering, fungus and mildew, flame resistance, cleaning and fabric aesthetics.
Non-wicking
The ability of a material to resist moisture from wicking into the polyester yarns is important for both structural and aesthetic reasons. Continuous filament polyester yarn can pull water into the space between the filaments by capillary action. If allowed to do so, this moisture can affect the adhesion properties of the material, causing seam problems or delamination of the coating compound. Even small amounts of moisture present in the base fabric can be a source of fungal growth, causing the material to discolor. This creates an aesthetic problem when viewed from the inside of the building.
Non-wicking properties are achieved by the selection of polyester yarns, the adhesive coat, and the coating procedure. In recent years, the use of anti-wick polyester yarns has greatly reduced the problems associated with wicking. The yarns are treated with a finish by the yarn producer to reduce wicking. In addition, the application of an adhesive coating compound that fully saturates the base fabric is another effective way to eliminate wicking.
A wicking test is performed by immersing a one-inch strip of PVC-coated polyester fabric into a dye-water solution. The sample is exposed on one end for a period of 24 hours, then removed from the solution, and examined for wicking.
Ultraviolet light and weathering resistance
The principle in extending the life of a structure is to maintain the tensile strength of the base fabric. To do this, it is necessary to protect the base fabric from UV light and other factors. With PVC-coated polyester fabric, it is the top exterior coating compound that provides protection from UV light. The PVC compound must be formulated to either reflect UV light or absorb the light, so that the UV light cannot affect the base fabric or the PVC compound itself. This is normally accomplished with the proper selection of pigments, the use of UV absorbers, or a combination of both. The formulating process gets further complicated when considering the desire for different color structures or light transmission into the structures.
Ultraviolet light testing of PVC-coated polyester fabrics can be performed by either ASTM G-26 Xenon-Arc testing or ASTM G-53 Fluorescent UV testing. These accelerated weathering machines combine high concentrations of UV light with water spray and high temperatures. These machines can simulate years of outdoor exposure in a matter of months, and have a very good correlation to actual field exposure.
Fungus and mildew resistance
Architectural fabric structures are frequently used in hot and humid environments, which are susceptible to fungus and mildew growth. Fungus growth on a PVC-coated polyester fabric can be not only an aesthetic problem but can lead to structural problems with the material. Frequently, fungus growth on a structure begins with a collection of dirt on the surface of the material.
To minimize the potential problems of a fungal attack on the material, manufacturers will incorporate a fungicide into the adhesive coat and the exterior coating compound. In addition, the use of a top-coating system to reduce dirt collection on the material will help reduce fungal attacks. While not a routine test, laboratory testing is done when a material is developed to assure that the material does not support the growth of fungus or mildew.
Flame resistance
Architectural fabric structures are frequently used in hot and humid environments, which are susceptible to fungus and mildew growth. Fungus growth on a PVC-coated polyester fabric can be not only an aesthetic problem but can lead to structural problems with the material. Frequently, fungus growth on a structure begins with a collection of dirt on the surface of the material.
The best way to describe the flame resistant characteristics of a PVC-coated polyester fabric is to refer to it as a “limited combustible” material. The material will burn when in the presence of a flame source, but will be self-extinguishing once the flame is removed. This property can actually be an advantage when considering what happens during a fire inside an architectural fabric building.
The fire-resistance properties of PVC-coated polyester fabric are related to the exterior-coating compound. The PVC compound must be formulated with the proper types and amounts of flame-retardant additives to impart the self-extinguishing properties that are required for a safe building material. Since these additives are incorporated into the PVC compound and are not extractable, the material will remain flame retardant for the life of the coated fabric.
There are a variety of flame resistance testing procedures that are used for building materials, but many of these do not apply to a PVC-coated polyester fabric. The primary test that is used in the United States for coated fabric is the NFPA 701 Vertical Flame Test. In this test, a sample of the PVC-coated polyester fabric is held in a vertical position and a flame is exposed to the bottom of the material for 12 seconds, then removed. The material must self-extinguish within 2 seconds after the flame is removed, and cannot have an excessive char length.
A second common flame test used with PVC coated polyester fabrics is the ASTM E-84 Tunnel Test. In this test, a 7.62m (25-ft.) sample of material is held in a horizontal position and ignited from one end. The test then rates flame spread and smoke development of the material as compared to a control material. PVC-coated materials have relatively low flame-spread ratings due to their self-extinguishing properties, and the smoke-development ratings are relatively low due to the material’s light-weight nature.
Cleanability and aesthetics
On any building the general appearance of the structure is a major concern, not only when the structure is new, but also as the structure ages. Many structures that are designed using PVC-coated polyester fabrics are high-profile buildings that have unique patterns and shapes. These structures need to maintain their appearance for a long time, and architects and engineers are typically very concerned with color changes or excessive dirt pickup that might affect the visual appeal of the structure.
Color change in PVC-coated polyester material is usually related to the exterior coating compound, but the top-coating system can also have an effect on the color. As a result, when formulating the PVC compound, care must be taken to use colorfast pigments. To help accomplish this, PVC formulas can be tested in an accelerated test machine to determine any significant levels of color change.
A more difficult aesthetic problem is related to dirt pickup on the architectural material. Because PVC-coated polyester fabrics are produced using soft and flexible PVC compounds that contain plasticizers, these materials are susceptible to dirt sticking to the surface. This problem is usually addressed by using a top-finish system on the exterior of the building. The principle behind the top-finish system is to create a thin, hard film on the surface to block dirt from adhering to the PVC compound and to allow the dirt to be washed off by normal rainfall.
Several popular top-finish systems are available commercially. The first system uses an acrylic-solution top finish applied to the material. The acrylic solution is a formulation of acrylic resin and possibly other resins such as PVC or polyurethane, which is dissolved in solvents. A very thin layer of this solution is applied to the surface of the material, with the resulting thickness of 508 to 1016μm (0.2 to 0.4 mils). One of the problems with this system is that it will erode over time, allowing the structure to become dirty after several years. This top-finish system is used on many industrial and recreational buildings.
A second top-finish system is similar to the acrylic system, but uses a PVDF (polyvinylidene fluoride) resin in the solution formulation. This PVDF top finish may actually blend PVDF resin and acrylic resin in the formula. The solution is applied to the surface of the PVC-coated material in the same manner as an acrylic topcoat, but it is usually applied at a thickness of 762 to 1524μm (0.3 to 0.6 mils). While some of the PVDF top finish may last longer than acrylic top finishes, these materials still erode over time, and the structure will gradually become dirty.
A third top-finish system involves the application of a Tedlar® PVF film to the PVC coated material. The Tedlar PVF film is chemically similar to Teflon® fluoropolymer material, and therefore is a very chemically inert and durable material.* Tedlar PVF film is available in pigmented films ranging in thickness from 0.0254 to 0.0381mm (1.0 to 1.5 mils). The pigmentation provides the color to the exterior of the building, and enhances the UV protection of the PVC-coated fabric. In addition to a variety of colors, Tedlar PVF films are available in several different white formulas, providing light transmission of 3 to 60 percent.
It is important to note that Tedlar PVF is only available in a film form, as opposed to the solution-applied acrylic and PVDF top finishes. Tedlar PVF films are inherently flexible and do not contain any plasticizers. These films must be applied to the PVC-coated material with the use of a specially formulated adhesive. When properly manufactured, these products provide the best dirt resistance available with any system. A Tedlar PVF-film top-finish system is typically specified for high-profile structures or those that require ease of cleaning.
A recently completed testing program by DuPont researchers compared the durability and effectiveness of the three different top-finish systems. The testing program involved the use of an accelerated weathering apparatus and the measurement of the top-finish thickness using optical microscopy. Results indicate that the acrylic top-finish systems last between three and five years. PVDF top finishes last from four to seven years. Tedlar PVF systems will last over 10 years. In fact, after an equivalent of 10 years of accelerated testing, the Tedlar PVF film maintained 70 percent of its original thickness. While accelerated testing is a useful comparative tool, the best method to determine dirt resistance is to perform long-term outdoor exposures.
Summary
High performance properties in an architectural fabric are achieved by the proper selection of the base fiber, the selected fabric weave, the appropriate formulated coating compounds and the coating processes utilized to produce the fabric.
The type of yarn selected and the weave design of the base fabric will provide the following performance properties:
- High tensile strength
- High tear strength
- Uniaxial and biaxial stretch characteristics
- Resistance to tear propagation
- Puncture resistance
- Dimensional stability of base fabric under changes in temperature and humidity
- Resistance to chemical attack
- Resistance to UV light degradation
- Retention of these properties in years of outdoor exposure
The proper compounding of the vinyl coating and the appropriate coating processes will impart the following characteristics to the architectural fabric:
- Protection of the base fabric
- Quality adhesion to the base fabric
- High-temperature, dead-load performance
- Non-wicking
- Abrasion resistance
- Flame resistance
- Color capability
- Non-fading colors
- Flexibility in cold weather
- Flexibility in years of outdoor exposure
- Weldability
- Repairability in the field
- Chemical resistance
- Maintenance of these properties after years of outdoor exposure
A number of proven test methods are available for specifying and evaluating the performance quality of fabric building materials for architectural fabric structures. The test characteristics and test methods have proven through the years to be excellent indicators of the key performance properties of fabrics.
The number of architectural fabric structures will continue to grow and expand in the future. The successful long-term performance of this building concept will be dependent on the quality of the fabric selected for the structure. Architects, engineers and building owners must understand these performance requirements and include proper testing qualifications as a primary part of their procurement process.