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 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. The second part covers issues of wicking, ultraviolet light and weathering, fungus and mildew, flame resistance, cleaning and fabric aesthetics.
For thousands of years, waterproof fabrics have been utilized for many different types of protective applications. Other than protective outerwear, most of these applications have been referred to as industrial uses of fabrics.
Although strong waterproof fabrics have been utilized for centuries, it has only been in the last 50 years that research and development efforts have resulted in significant achievements in fabric design and coatings.
One of the most dynamic applications in the last 30 years has been high performance fabrics for architectural structures. For example, who would have thought 50 years ago that millions of dollars worth of inventory could be reliably protected from inclement weather by a building manufactured from a vinyl-coated fabric that weighs less than 1,084.9g /sq. m (2 lbs per sq. yd)? Or that 80,000 fans would watch a football game in a stadium covered with an air-supported Teflon®-coated fiberglass fabric?
Two basic types of building systems have evolved utilizing high performance architectural fabrics—the air-supported structure and the tension membrane structure. These structures also compete with conventional building systems, which have a proven long, useful life. However, today’s PVC-coated polyester fabrics for architectural applications will provide quality performance properties for a period of 15 to 20 years.
Performance properties for architectural fabrics
Before architects and engineers can utilize PVC-coated polyester fabrics as a building material, it is necessary to understand the performance properties of these architectural fabrics. Some of these properties are similar to those of conventional building materials, but many of the properties are unique to the flexible material.
But, first it is important to understand how these products are designed and manufactured. Architectural Fabrics are made up of four components: base fabric (greige goods), adhesive or primer coat, exterior coatings (plasticized PVC), top coating systems.
Each of these components contributes to the different performance properties, with some of the components having an effect on several properties. Our review of the different critical performance properties of the architectural fabrics will continue to refer to the four different components that make up the coated fabric.
The first and most important performance property that needs to be considered is the tensile strength of the material. Because the tensile strength of the architectural fabric depends on the base fabric and the polyester yarns, the useful life of a structure is then dependent on keeping the yarns from deteriorating. If the yarns start to break down, then the structural integrity of the entire building system is in question. Protecting the yarns from damage is one of the main functions of the exterior coating compounds.
Testing the tensile strength of a material can be done by either the “Cut Strip Test Method” or the “Grab Test Method” as outlined in ASTM D-751. Samples of a material are tested in both the warp and fill directions and three to five samples are taken across the width of the material.
Uniaxial and biaxial elongation
As a load is applied to PVC-coated polyester fabric, the material will stretch and ultimately break at its breaking strength. This property is similar to conventional building materials such as steel or glass. However, the length of elongation will be significantly greater for PVC coated materials. Typical elongation at break values for PVC coated materials will range from 20 percent to 50 percent.
From a design and engineering viewpoint, the ultimate elongation at break is not as important as the elongation at the design working loads on the structure. Again, PVC coated polyester fabric exhibits much higher elongation at the working loads as compared to other building materials. It is very important that the engineer understand what the elongation values are for a given material and that the elongation values are consistent from roll to roll.
The elongation properties are related to the polyester base fabric; more accurately these properties are dependent on polyester yarn selection, weave pattern, and coating methods. Different types of polyester yarns can be used to produce the base fabric; some yarns have relatively low elongation under stress, and some yarns have high elongation under stress. It is not unusual to use one type of yarn in the warp direction and a different type of yarn in the weft direction, but different types should not be used in the same direction.
The second factor affecting the elongation properties is the weave pattern that is used to make the base fabric. If the warp and fill yarns are combined in a conventional plain woven construction, the yarns are crimped as they are placed in the material. When this fabric is stressed the yarns begin to straighten, reducing the crimp. This results in a material that has relatively high elongation at low loads.
If the warp and fill yarns are combined using a warp-knit weft-insertion machine, the warp and fill yarns are laid into the material in a straight and flat position, and held in place with a stitching yarn. As a load is applied to this type of material, the load goes directly on the polyester fibers. A warp-knit weft-inserted material typically will have a lower elongation at a given load than a woven fabric of a similar strength.
The third factor effecting the elongation properties is related to the coating method used to apply the PVC coating compound. In most coating procedures, the base fabric is run through coating ovens and can be stretched under heat. These processes can cause the material to be drawn or can cause the material to shrink; in either case the elongation properties will be affected.
Testing the uniaxial elongation properties of a material can be done per ASTM D-751 Cut Strip Test Method, or testing under a static load can be done by ASTM D-4851. Biaxial testing is done by various test methods as developed by the material manufacturers or structure fabricators.
The dimensional stability properties of any building material are important. If a material changes in size due to change in temperature or humidity, these changes need to be considered when engineering the building. This is very important when designing a tension membrane structure since patterns are cut to a given size to allow for a given pre-tension on the building.
The dimensional stabilityof an architectural fabric is directly related to the base fabric and the polyester yarns. Early architectural fabrics were made from nylon fibers, but these materials were not dimensionally stable and were quickly replaced with polyester yarns. The dimensional stability of a base fabric made from polyester yarns is so good that this performance property is generally not specified or tested, other than to require a polyester base fabric.
The tear strength of an architectural fabric is an important performance property. The ability ofa material to resist a tear or tear propagation may be critical to the structural integrity of the building. This can be particularly true in an air-supported structure where the loss of air pressure inside the building can lead to a catastrophic failure.
Tear strength properties are related to a combination of factors involving the base fabric, weave construction, and adhesion values. To obtain the highest possible tear properties, the yarns need to be able to slide within the PVC coated fabric. If the yarns are locked into place, a tearing force is applied to individual yarns one at a time, resulting in lower tear values. In general, a warp-knit weft-inserted material will have a higher tear strength than a conventional plain woven fabric, since the yarns are not inter-woven.
The adhesive coat and adhesion values between the base fabric and the coating compound will also greatly influence the tear strength properties. Higher coating adhesions will limit the ability of the polyester yarns to slide and rope-up within the PVC coated fabric, thereby reducing the tear strength. While low coating adhesion may yield higher tear strength, it will introduce other significant problemss.
Testing for tear strength of a material can be done by either ASTM D-751 Tongue Tear Method or ASTM D-4533 Trapezoid Tear Method. In many cases both methods are used to better characterize the tear properties. In addition, tear testing is performed on material that has been aged, either naturally or by accelerated weathering, to determine if there is a loss in tear strength over time.
Coating adhesion is the ability of the exterior coating compound to be adhered to the polyester base fabric. Having the strongest base fabric and the best-formulated PVC compound is of no value if the two cannot be properly bonded together. Good coating adhesion is required to allow the material to be handled and welded. It is also important in preventing the exterior coating compound from delaminating when the material is exposed to the environment.
Developing good coating adhesion is the primary function of the adhesive coat. The adhesive coating compound is formulated as a PVC plastisol with an adhesion promoter added to the compound. When this compound is applied to the base fabric, a chemical bond forms between the polyester yarns and the adhesive coat. This process is carefully monitored to develop the right level of adhesion. Too little adhesion will cause problems with seam strength or coating delamination, and too-high adhesion will adversely affect tear strength.
Coating adhesion is tested per ASTM D-751 Peel Adhesion test. Samples are prepared by either welding or gluing two pieces of material together, then peeling the samples apart in a constant-rate-of-separation testing machine. Results are reported as pounds-force per inch.
Weldability and seam strength
One of the most advantageous performance properties of PVC coated polyester fabrics is its ability to be efficiently welded into large panels that can be incorporated into a structure. Unlike conventional building materials such as wood, steel or bricks that require assembly at the job site, PVC coated polyester fabrics can be pre-fabricated into large panels and then brought to the job site for final assembly.
The PVC-coated polyester fabric uses a plasticized PVC exterior coating compound on both the top and bottom of the material. This PVC compound is a thermoplastic material, meaning that it can be heat bonded to itself. The heat bonding process can be accomplished with a radio frequency welder or a hot air or hot wedge welder. Seams can be produced at speeds of up to 6.1m (20 ft.) per minute.
Since the base fabric carries the loads on a building, the seams must be able to transfer these loads from one piece of coated fabric to another. This creates a shear force on the seam. As a result, it is important that the tensile performance properties of the finished seam be equal to the strength of the fabric itself to ensure the integrity of the entire structure. Each seam must be able to handle all of the load requirements on the building under the full range of environmental conditions.
The strength of the seam is a function of the adhesive coat,exterior coat, and the welding process. The adhesive coat must form a bond between the polyester base fabric and the exterior coating compound such that it can handle the shear forces that are created under loads. The exterior coating compound must be formulated and applied properly such that it can be welded to itself and handle the shear forces. The welding process must be designed to give the proper amount of overlap and the necessary amount of heat and time to form a good weld. Typically, high tensile strength materials require a greater overlap at the seam to carry the shear forces.
Seam strength testing involves a series of tests that include weld adhesion, seam shear strength, and dead (static) load testing. The weld adhesion is done with the same ASTM D-751Peel Adhesion Test previously described. This is a quick check to determine that the PVC coating compound has been heat bonded to itself.
The seam shear test is a modification of ASTM D-751 Cut Strip Tensile test. In this test, a 2.54cm (1-in.) sample is cut perpendicular to the seam and a tensile test is performed across the seam. The coated fabric should always break outside the seam area, with results equivalent to the tensile strength of the PVC coated fabric, assuring that the fabricated seam is at least as strong as the fabric itself.
While no current ASTM procedure exists for a dead load or static load test on a seam, this is the most important test that can be performed. The test involves applying a load across the seam on a 2.54cm (1-in.) sample for a period of four hours. The test is performed at both room temperature and at high temperature, usually 71C (160 F). This test most closely simulates actual field conditions in that there is a constant load on the seam when the building is in service.
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.
Architectural fabric structures will continueto 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.