Although the lifespan of a steel framed building depends on environmental conditions and the potential corrosiveness of materials stored inside, many Rubb architectural membrane buildings worldwide are still in useful existence twenty-five years or more after being erected
When considering the fabric cladding, the longevity issue becomes even more complex. There are so many different types of fabric and fabric coatings to choose from and care must be taken to ensure that comparisons are being made on a like for like basis.
Rubb uses only the highest grades of PVC coated, flame retardant, high tenacity polyester which features a rip-stop structure weave to prevent tears. For structures up to 12m span, a 700g/m2 (21 oz/yd²) material is used while larger structures use 900g/m2 (28 oz/yd²) material. Special coatings protect against UV radiation to extend design life.
The fabric cladding on Rubb steel-framed structures has been proven to last at least 25 years before replacement becomes even a consideration. This, however, depends on a number of factors such as environmental conditions and the corrosiveness of the materials stored inside.
An excellent example of a long-lived Rubb building is a structure built in 1975 for Peck & Hale in New York State. An outside laboratory tested a sample of fabric taken from the building that confirmed that the fabric's self-extinguishing and flame retardant characteristics were still present and the tensile strength had been maintained -- after 25 years the only deterioration was a slight reduction in the fabric's tear strength.
Fabric Basics
A Guide to specifying architectural fabrics.
A fabric structure's material selection, proper design, engineering, fabrication and installation all work interdependently to ensure a sound architecturally pleasing structure. The 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 since their membranes, as well as their framework, carry the loads.
Membrane Materials
Most tensile structures utilize 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 fiberglass coated with either polytetrafluoroethylene (PTFE) or 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 highly controlled specifications to give the fabric strength, visual consistency and measurable properties of stretch and strength. This allows engineers and designers to accurately predict the behavior and performance of the fabric before patterning and load analysis can proceed.
The most commonly used weaves are plain weave, panama weave, precontraint and Malimou.
The polyester base cloths are then laminated or more usually coated with PVC to give the fabric color, strength and waterproof properties. The PVC coating also allows adjoining panels of fabric to be seamed by high frequency welding. By this process, plasticizers, found in the PVC, are exited by the application of microwaves along the seam overlap.
The exited plasticizers then bond to each other to form a consistent high-integrity joint that is stronger than the fabric itself.
The PVC coatings are available in a wide range of colors. However specifying a non-standard color for small amounts of fabric may be uneconomical as they are frequently subject to minimum order runs of 2000+ linear meters. This is to allow the coating machine to purge itself of all traces of the previous color.
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, whereas thicker coatings are "knife applied" or laminated to the PVC.
The thickness of topcoatings has a direct relationship to 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 inhibits the migration of the plasticizers that give PVC its elastic and flexible properties. Migration and degradation of these plasticizers 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 that 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 give the material a ten-year lifespan depending on the ambient climatic conditions and air quality at the site where it is installed.
Acrylic topcoats are ideal for fabrics that are used for temporary structures and demountable structures such as marquees, circus tents, lorry sidecurtains, rock concert venues and warehouses.
100% PVDF Topcoat
Polyvinylidene fluoride (PVDF) is made of 59% fluorine, 38% carbon and 3% hydrogen. The compatibility of the carbon and fluorine 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 indicate that color difference and brilliance diminishment are significantly less, over time, than its acrylic counterpart.
PVDF topcoats also offer resistance to algae and fungal attack. They have good self-cleaning properties and therefore need less maintenance. These properties combine to give a membrane a lifespan of 15 to 20 years depending on site conditions. Like acrylic topcoats, they 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, limit the choice of colors available. White is the only standard color. Other colors are limited and subject to special manufacturing runs, making them less economical.
The chemical-resistant properties of PVDF are such that the finished, topcoated material cannot be welded to itself in its raw state. To effect a weld, the topcoating must be abraded off to expose the PVC below it.
This extra operation increases the cost of 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 require manual abrading of the membrane using sandpaper.
This coating is marketed under the trade names, Fluotop T and Kynar.
PVDF/PVC Topcoating
This topcoating is effectively a dilution of the PVDF topcoat. This gives the finished fabric the advantages of being less expensive to produce and to fabricate. The saving in fabrication costs is derived from the finished fabric being weldable without recourse to abrasion. The diluted effect of the PVDF however means that environmental resistance is reduced along with longevity. Fabrics with this coating have a life expectancy of 10 to 15 years, depending on prevailing conditions. This material is available in colors but is subject to minimum order manufacturing runs.
Tedlar Topcoating
Tedlar is the trade name for polyvinyl fluoride (PVF). This is not a liquid topcoat, but a film-layer that is laminated to the PVC fabric during manufacture. This results in a thicker finished fabric that is more resistant than its competitors to weather and chemical attack. It has superior self-cleaning capabilities than 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 industrialized areas, high saline coastal zones and desert environments.
Having a thicker coating, it 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 eliminates the migration of plasticizers from the base PVC coating.
The Tedlar topcoating is flexible, allowing a consistent and strong bond to the PVC.
Tedlar topcoated membranes are available in a wide range of colors that are subject to minimum manufacturing runs.
Like PVDF 100% topcoating, the Tedlar film renders the PVC sheet non-weldable. This problem is overcome, not by abrasion, but by the application of butt welds. This means that the seams are not over- lapped but butted up to each other. An extra welded seam is then applied to the underside of the membrane that does not have a topcoating. This eliminates the risks associated with abrasion.
Tedlar topcoated material is comparable in price to PVDF but substantially less costly than PTFE coated glass fiber.
Comparative Performances of Commonly Used Topcoats
According to testing done by Du Pont, there are three different indicators of degradation for each topcoating type over a period of time.
The difference in thickness of the topcoats when submitted to over nine years of Florida outdoor exposure. Note that the acrylic coating and thinner or diluted PVDF coatings are degraded after five years. The heavier PVDF materials perform better but the Tedlar coatings, although degraded, still offer protection due to the superior thickness.
Color change using the same compounds over a similar period. Note that the change in color would reflect the accumulation of dirt rather than changes in pigment.
Models variations of gloss in the same conditions. Note that the gloss retention is associated with the self-cleaning properties of the topcoating.
PTFE Coated Glass Fabric
PTFE glass fabric is a frequently specified material due to its life expectancy of 30 years, depending on conditions.
Its base fabric is made up of glass fibers that are drawn into continuous filaments and then bundled together in yarns. The yarns are then woven to form a substrate. The woven fiberglass has a high tensile strength, behaves elastically and does not undergo significant stress relaxation or creep. The glass fiber is also completely incombustible.
The flexural properties of the woven glass substrate is, however, inherently poor. This leads to cracking, difficulty in handling and self-abrasion within the coating.
The PTFE or Teflon coating is also non-combustible. These coatings are very inert and have a low coefficient of adhesion that gives PTFE 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 color when it leaves the mill or fabrication plant. This discoloration 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 as it involves the material being cooked in an industrial oven for long periods and 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 a PTFE membrane requires slow and specialized 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 of time on site. This factor also contributes to its higher cost.
Trust Rubb Building Systems to choose the best roofing material for your structure. Contact us to learn more about the quality engineering and support that is the foundation of all Rubb Fabric Structures.
