The types of materials flexible and strong enough for architectural tensile membrane purposes are woven fabrics and foils. Woven fabrics consist of two set of yarns (warp and weft) woven together in a loom. Foils, on the other hand, are made of thinly rolled or extruded homogeneous material. The most common woven fabrics suitable for architectural applications are fiberglass, the aramids, olefins, nylon, acrylic, cotton, and polyester.
Woven Fabrics for Architectural Applications
Fiberglass is commonly used as a base fabric for architectural membranes. As the name implies, fiberglass is glass that has been heated up and spun into fibers. Fiberglass itself is inert, ages well, and does not emit toxic gases. It is stable (has very little shrink and stretch), has a high tensile strength (pound for pound it is as strong as steel), and is non-combustible. Although the silica used to spin fiberglass fiber is plentiful, fiberglass takes a tremendous amount of energy to produce. Its drawbacks are its low tear strength, its low elastic strain, its inherent brittleness (it can be damaged when folded), and its abrupt failure once its limits are reached. For these reasons, in tensile applications, typically only very heavy fiberglass fabrics are used and require great care in installation and fabrication.
The aramid fibers, such as Kevlar, are extremely strong (three times the strength of steel). They have good abrasion resistance and little chemical and thermal degradation. However, they are subject to rapid UV deterioration. Although they are extremely expensive they have found a few rare high-strength architectural applications.
The olefins are the family of polymers consisting mainly of polyethylene, polypropylene or combinations of the two. Along with Teflon (PTFE see below) they are called “non-stick” polymers. This is because their inactive molecular structure is described as a “low energy” state. This non-stick quality is what makes olefins such common and useful plastics. Impressive architectural membrane materials are starting to be developed using olefins for both the base fabric and coating. High tensile and tear strengths along with flame-resistance can be achieved in materials that have a life span equal to PVC-coated polyester with a fraction of the embodied energy. Another advantage is that olefins can be woven and coated into widths of up to 160″.
NYLON, ACRYLIC, COTTON AND POLYESTER
The other common industrial base fabrics are nylon, cotton, and polyester. Since nylon (polymide 6.6) and acrylics stretch considerably, swell in length when they get wet and break down from UV exposure, they are rarely used beyond awnings and camping tents. Cotton, on the other hand, is very UV-resistant. However, cotton has relatively low strength and is much more susceptible to mildew than synthetics. Polyester has better UV and heat-resistance than other synthetics, good tensile and tear strength, and is mildew-resistant. These advantages, coupled with polyester’s reasonable cost, make it the most commonly used base fabric. Often cotton and polyester are combined, the cotton providing heat and UV-resistance and the polyester providing strength and mildew-resistance.
Unlike cotton, which is one of the most heavily sprayed crops in agriculture, there are other natural fiber plants that require little or no pesticides. Some of these alternatives include hemp and flax (linen), nettles and kudzu. These yield a tremendous amount of fiber per acre and their production is very sustainable causing less pollution and petrochemical inputs. They are stronger than cotton and have almost as much UV-resistance. These fibers should not be ignored when it comes to light-duty architectural applications.
Architectural Fabric Coatings
In order to be suitable for architectural purposes, a woven fabric must be coated or laminated to protect the fabric itself from UV radiation and moisture, to keep out the elements, and in a few cases, to add increased strength.
PVC (vinyl) is by far the most commonly used coating for architectural purposes, generally being coated or laminated onto polyester. One reason that PVC is universally used for a tent and architectural fabric is that it is inexpensive and can be easily made flexible and flame retardant when coated on polyester by adding chemical compounds. However, unlike other polymers, PVC does not chemically bond to these additives, and as a result the additives routinely leach out. Furthermore, PVC suffers from a limited life span. By adding a topcoat, the life of PVC can be increased and its tendency to attract dirt can be reduced. Typical PVC top-coatings are acrylic lacquers, PVDF coatings and PVF laminates. However, PVC is a very harmful substance and really shouldn’t be as inexpensive as it is for the true cost of its environmental impact is not reflected in its price.
URETHANE AND ACRYLIC
Urethane and acrylic coatings can be made flame-resistant and don’t have nearly the adverse environmental impact of PVC. Unfortunately, they have a rather limited amount of UV-resistance. A new generation of aliphatic urethane is being developed that promises to overcome this shortcoming; however, the cost will be high.
Polyolefins are unique in that the same polymer can be used both for the base fabric and the coating as well. This, coupled with the fact that olefins are easily and commonly recycled, makes olefin-coated olefin fabric one of the only truly recyclable architectural materials (along with Tenera below). UV stabilizers (including some carbon black in the pigment) are added to produce a material that will last in the sun, but flame-resistance is also possible to achieve. Furthermore, olefins can be coated on a base fabric of polyester. Because polyester is more heat resistant than olefin fabric, heavier amounts of olefin coating (which is cured by heat) can be applied. This yields a fabric that, although not recyclable and flame-resistant will last longer in the sun because of it’s heavier coating.
PTFE AND SILICONE
PTFE is a fluoro-polymer commonly known as Teflon. It has an extremely long life span. There are 25+ year-old PTFE-coated fabric structures that are still like new. It is well known that nothing sticks to Teflon, allowing it to stay clean throughout its life span. But PTFE as a fabric coating does have its disadvantages. For one, it is very expensive. Secondly, it is difficult to fabricate, hard to handle, and rather brittle. Although it takes extremely high temperatures to burn PTFE, once ignited, PTFE emits a highly toxic gas. For the past few decades PTFE-coated fiberglass was the only choice for high-end architectural applications where a long life span was desired. Fortunately, a few alternative materials are starting to emerge..
Silicone as a fabric coating offers a lot of promise. It is less expensive than PTFE although still more expensive than PVC. Like PTFE, it is flame and UV-resistant and has a long life span. Furthermore, silicone blocks out the short wave UV-B and UV-C part of the ultraviolet spectrum, the rays that are responsible for causing damage to the fibers and harm to humans and plants, while transmitting UV-A light which is essential for plant growth. Being a type of rubber, silicone is very flexible, and when coated on fiberglass, can reduce damage from folding. Silicone when burned (at high temperature, given its high heat resistance) leaves merely an ash of silica dioxide. With all these advantages, it is a bit of a mystery why it is not being embraced by the small but growing architectural fabric industry.
ETFE is the primary foil polymer suitable as a tensile membrane. Being a fluoro-polymer similar to PTFE, it is very UV-resistant, has excellent flame-resistance and doesn’t attract dirt. Furthermore, it can be made very transparent, allowing 5% more light to enter than glass. Since it has considerably less tear strength than woven materials and a very different behavior under stress, it is commonly used for air-supported pillows and short span frame-supported architectural applications.
One of the most exciting recent architectural membrane offerings is an ePTFE product made by Gore Inc called Tenara. After several years of development, Gore has come up with a material that solves most of the shortcomings suffered by other membrane options. First they start with a foil of ePTFE and slit it into fibers. These fibers are specially woven into a fabric and then both sides are coated with ePTFE. The result is a very long-lasting material with excellent fire resistance, high strength and flexibility. Unlike earlier ePTFE products, this material it is waterproof and can be heat welded with a special adhesive tape developed by Gore. Its translucency is equal to that of silicone-coated materials, allowing up to 40% of the light to enter. Furthermore, like olefins, the base fabric and the coating are of the same polymer and can hence be recycled. The only catch is the price. Although Tenara is competitive with PTFE-coated fiberglass, it is still several times the cost of most other membrane materials.
This is a manufactured fiber, spun from a liquid crystal polymer (LCP) and used in high strength applications. Chemically it is an aromatic polyester. This fiber has resistance to both moisture and ultraviolet degradation. Often the yarn is made by wrapping polyester around a Vectron core.
PowerFilm Inc. has developed ultra thin flexible silicon photo-voltaic modules that are bonded to a polymer substrate. Both sides of this solar cell material are then laminated to an ETFE foil. As this technology develops PowerFilm may well find applications as an architectural membrane that can provide both shelter and electrical energy.
Konarka Technologies has invented a photo-voltaic nanotechnology to make an ink type material 1,000 times smaller than a human hair. This material can convert light into electricity very efficiently because the minute size of the particles can expose more surface area to collect sunlight and utilize a wider range of the sun’s spectrum. This material can be printed in any color or pattern onto a range of fabrics. Furthermore, Konarka is developing “Power Fiber” which is a method where a photo-active material and it’s transparent electrode are made in tubular strands of fiber that can be woven directly into a fabric.
Both of these photo voltaic technologies rely on alternatives to polysilicon. The silicon is extracted from silane gas rather than cultured silicon crystalline wafers. Polysilicon is a major component in conventional PV panels; however, it also is a key ingredient for the computer industry and the current demand for it is greater than the supply.
Sources for non-PVC Architectural Materials
There are plenty of sources for PVC fabrics suitable for architectural purposes; PVC alternatives are harder to find. Below is a list of sources and contact information for various flame-resistant non-PVC fabrics. Be aware that as knowledge of the hazards of PVC grows, there is a tendency for material manufacturers to conveniently neglect to list PVC as one of the ingredients in their fabric.
-Sheerfill by Saint-Gobain Performance Plastics Corp.
-Solus by Taconic International Ltd.
-Duraskin by Verseidag US Inc.
-Atex by PD Interglas
-Weathermax FR by Safety Components Fabric Technologies Inc.
-Fireset HUV by Glen Raven Custom Fabrics LLC
-Nova-Sheild by Engineered Coated Products
-Monotec by Value Vinyls
OLEFIN OPEN WEAVE MESH
-SaFRshade by GALE Pacific Fabrics
-Comtex & Polytex by Polyfab
ACRYLIC COATED POLYESTER
-Main Street by TriVantage
-Top Gun FR (Holiday) by Marlen Textiles
-Firemaster Plus by Sattler
LCP FIBER MEMBRANE
-Vectron by Kurarary Co. LTD
-Tefzel by Chemours
PHOTO-VOLTAIC MEMBRANE TECHNOLOGY
-Power-Film by PowerFilm Inc.
-Power-Plastics and Power-Fiber by Konarka Technologies
-Wave Sol by Ascent Solar
-Solar Cloth by The Solar Cloth Company