Technical Specifications for Architects reprinted with kind permission of Fabric Architecture Magazine. www.fabricarchitecture.info
Fabric Basics
Guide to specifying architectural fabricsA fabric structure's material selection, proper design engineering, fabrication and installation all work interdependently to ensure a sound and architecturally pleasing structure. The fabric material's role in the structure's integrity and performance makes the selection process especially important. This is particularly true of tensile and air supported structures as their membranes act as their framework to carry the loads.
Membrane Materials
Most tensile structures utilise fabrics rather than meshes or films. The fabrics are typically coated and/or laminated with synthetic materials for greater strength and environmental resistance.
The most widely used materials are woven polyester cloths coated with polyvinyl chloride (PVC) and woven fibreglass coated with either polytetrafluoroethylene (PTFE) silicone.
PVC Coated Polyester Materials
These are the most frequently specified materials. The polyester base cloth is used because of its durability, strength and relatively low cost.
The polyester fabrics are woven or knitted to high specifications to lend the fabric strength, visual consistency and measurable properties of stretch and strength. These properties allow engineers and designers to accurately predict the behaviour and performance of the fabric before patterning and load analysis can proceed.
The polyester base cloths are then laminated or more usually coated with PVC to give the fabric colour, strength and waterproof properties. The PVC coating also allows adjoining panels of fabric to be seamed by high frequency welding which provides a consistent high integrity joint that is superior in strength to the fabric itself.
The PVC coatings are available in a wide range of standard colours. Custom colours are available at extra cost.
Topcoatings
Most architectural PVC polyesters have some sort of topcoating applied to their exterior or weathering surface. The topcoating improves the appearance of the material, extends its life and allows the material to be readily cleaned or washed by rainwater (self-cleaning).
The topcoats are applied in different ways depending on the nature of the topcoat and the required thickness. Lacquers are sprayed on whereas thicker coatings are "knife applied" or laminated to the PVC.
The thickness of top coatings has a direct relationship with the longevity of the PVC membrane. Topcoatings will degrade over time leaving the PVC surface exposed to airborne pollutants, UV degradation, wind and weather. The presence of a topcoat also tends to inhibit the migration of the the plasticisers which give PVC its elastic and flexible properties. Migration and degradation of these plasticisers cause the PVC to become brittle, to blister and delaminate. Different types of topcoatings include acrylic solutions, PVDF solutions and PVF film laminations.
Acrylic Topcoat
This commonly used finish is also the most economical and most widely available. It is a thin, spray-applied solution which gives a transparent glossy finish to the PVC. The acrylic coatings have a good resistance to UV degradation. The thinness of the coating application means that this material is easy to fabricate and repair by high frequency or hot air welding.
Acrylic topcoats are ideal for fabrics that are used for temporary structures and demountable structures such as marquees, circus tents, track side curtains, rock concert venues and warehouses.
100% PDF Topcoat
Polyvinylidene Fluoride (PVDF) is made up of fluoride, carbon and hydrogen. The compatibility of the carbon and fluoride is such that it offers a resistance to UV degradation and atmospheric chemical attack, which is far superior to the acrylic topcoat.
Controlled exposure tests in Florida show that colour differences and reduction in brilliance are significantly less with PVDF than with its acrylic counterpart over time.
PVDF topcoats also offer resistance to algae and fungal attack. They have good self-cleaning properties and therefore need less maintenance during their lives. These properties combine to give a membrane a life span of 15 to 20 or more years depending on site conditions and the initial quality of the membrane itself. Like acrylic topcoats 100% PVDF topcoats are highly flexible and resistant to cracking, making them easy to handle during installation.
The production procedures where the PVDF is chemically grafted to the PVC, as well as the polymers used, limits the choice of colours available. White is the only standard available colour. Any other colours desired with this coating are limited and subject to special manufacturing runs, making them less economical.
The chemical-resistant properties of the 100% PVDF are such that the finished topcoated material cannot be welded to itself in its raw state. To effect a weld on a 100% PVDF material the topcoating must either be abraded off or the material must be butt welded. This extra operation increases the price of the fabrication and carries risks associated with the grinding depth calibration of the abrasion machine, and the complete covering of the abraded seam strip by the overlap. Inaccuracies in both these areas can significantly weaken the welded seam.
Site repairs are also difficult to administer accurately as they usually require manual abrading of the membrane using sandpaper. This coating is marketed under the trade names Fluotop and Kynar.
PVDF/PVC Topcoating
This topcoating is effectively a dilution of the PVDF topcoat. This gives the advantages of being both more economical to produce and to fabricate. The saving in fabrication costs is derived from the finished fabric being weldable without need for abrasion. The diluted effect of the PVDF however means that environmental resistance is reduced along with longevity. this material is available in colours but is subject to minimum order manufacturing runs.
Tedlar Topcoating
Tedlar is the DuPont registered trade name for polyvinyl fluoride (PVF). This is not a liquid topcoat. It is a film layer which is laminated to the PVC fabric during manufacture. This results in a thicker finished fabric which is more resistant than its competitors to weather and chemical attack. It has superior long term self-cleaning capabilities when compared to the PVDF range of topcoatings and will resist attack from graffiti, acid rain and bird droppings.
For this reason it is frequently specified for use in highly industrialised areas, marine coastal zones and desert environments.
Having a thicker coating, Tedlar erodes at a much slower rate giving it a life expectancy of about 25 years depending on conditions. The Tedlar film topcoating not only resists environmental degradation but also lessens the migration of plasticisers from the base PVC coating.
The Tedlar topcoating is flexible allowing a consistent and strong bond to the PVC.
However, Tedlar's modulus of elasticity is such that it can be creased by folding and these lines can be noticeable up close.
Tedlar film is available in a wide range of colours which are subject to minimum manufacturing runs.
Like PVDF 100% topcoating, the Tedlar film renders the PVC sheet unweldable. This problem is addressed either with abrasion or by the application of butt welds. Tedlar topcoated material is comparable in price to the PVDF equivalent but substantially more economical than PTFE coated glass fibre.
PTFE Coated Glass Fabric
PTFE glass fabric is a frequently specified material due to its longevity. It has a life expectancy of approximately 30 years depending on conditions. Its base fabric is made up of glass fibres which are drawn into continuous filaments then bundled together in yarns. These yarns are then woven to form a substrate. The woven fibreglass has a high tensile strength, behaves elastically and does not undergo significant stress relaxation or creep. The glass fibre is also completely incombustible.
The flexing behaviour of the glass is however inherently poor. This leads to cracking, poor handling ability and self-abrasion within the coating.
The PTFE or Teflon coating is also noncombustible. These coatings, being very inert, have a low coefficient of adhesion. This quality means that the coating itself has good self-cleaning ability. In the finished fabric however, the self-cleaning ability is slightly diminished by the grainy surface of the membrane under tension, providing small indentations in which airborne solids can accumulate.
Most PTFE membrane is an off-white/brown colour when it leaves the mill or fabrication plant. This discolouration bleaches to white in the presence of UV light. For this reason PTFE fabric should not be used indoors or in permanent shade without being pre-bleached. Pre-bleaching can be undertaken, but it is expensive because it involves the material being cooked in an industrial oven for long periods at temperatures exceeding 250 degrees C. Although most PTFE cloths are supplied off-white, it is possible to pigment the Teflon prior to coating. The glass scrim combined with the bleached coating results in a fabric with good light transmittance.
Fabrication of PTFE membrane requires specialised welding techniques under controlled environmental conditions. It also requires extra care in handling and packaging due to susceptibility to cracking and self abrasion. These properties contribute to its high cost and to the need for additional tensioning hardware for the finished fabric structure.
The tensioning of PTFE glass fabric is a slow process, as it requires incremental adjustment over long periods on site. This factor also contributes to its cost.
PTFE glass cloth is often specified for high profile projects, for example, stadium roofs where longevity is required and where budget is not an issue. It is also suitable for desert and marine conditions found in places like the United Arab Emirates.
Other PTFE products include PTFE coated glass open mesh and PTFE coated glass mesh with a Teflon film laminated to it. These materials are used for translucent barriers and screens and for cladding purposes where the material is not contributing structurally to the finished structure.
Other Frequently Specified Fabrics
Modified AcrylicThis is a woven acrylic material suitable for smaller structures with a limited life. Its degree of fire retardancy means that this fabric is only suitable for outdoor applications. It is available in a wide range of colours and is frequently specified for awnings and shade structures. It has a good resistance to discolouration and biological attack. Adjacent panels are joined by sewing.
Silicone Coated Glass Cloth
This material has a similar base cloth to PTFE glass membrane. However, the silicone coating gives the finished fabric a more flexible rubbery form. It therefore does not suffer from stress cracking in the same way.
The silicone coating is more economical and 100% fire proof. This finished fabric is available in a range of colours, widths and thicknesses.
The fabric is seamed by sewing or via an adhesive bond. Its high translucency and fire rating make it suitable for indoor tensioned ceilings and atrium shades.
Only the most advanced silicone coated fabrics have any self-cleaning properties, so it is not often specified for outdoor use.
Cottons
These materials are environmentally friendly and are easily dyed to any colour. They can be treated to make them non-combustible and therefore suitable for indoor ceilings and exhibition structures. These materials quickly discolour in outdoor conditions, as they are vulnerable to biological attack. Unlike most other fabrics however, they can be laundered.
Coated Nylons
Usually used for yacht sails and hot-air balloons, nylon has a high strength/low weight ratio making it suitable for very lightweight structures, drapes and exhibition work. It can be packed and folded without creasing and is available in a wide range of colours and coatings. Nylon can be fire treated, UV treated and waterproofed.