A fabric structure’s material selection, proper design, engineering,
fabrication and installation all work together to ensure a sound
structure. The material’s role in the structure’s performance makes the
selection process especially important. This is particularly true with
tensile and air-supported structures because their membranes, as well as
their frames, carry the loads.
Membrane materials
Most fabric structures use fabrics rather than meshes or films.
The fabrics typically are coated and laminated with synthetic materials
for greater strength and/or environmental resistance. Among the most
widely used materials are polyester laminated or coated with polyvinyl
chloride (PVC), woven fiberglass coated with polytetrafluoroethylene
(PTFE) or silicone. Meshes, films and other materials also have
appropriate applications.
Polyesters
Polyester is the most frequently used base material because of its strength, durability, cost and stretch. Polyesters laminated or coated with PVC films generally are the least expensive for longer-term fabrications.
Laminates usually consist of vinyl films over woven or knitted
polyester meshes (called scrims or substrates). Coated fabrics typically
use a high-count, high-tensile base fabric coated with a bondable
substance for extra strength. One fabric manufacturing method places
polyester fabric under tension before and during the coating processes.
The result is that yarns in both directions of the weave have identical
characteristics, giving the fabric increased dimensional stability.
Lighter fabrics (200 to 270g/m2) commonly are used as
acoustic and insulated liners suspended beneath a structure’s envelope.
For long-term exterior use, heavier materials are needed: 20- to 26-oz.
(680 – 880gm) fabrics with top coatings of polyvinyl fluoride (PVF, of
which Tedlar is an example) or polyvinylidene fluoride (PVDF, of which
Vidar, Fluorex® and Kynar® are examples). These top coatings provide a
protective finish to withstand environmental degradation.
Vinyl-coated polyester
Vinyl-coated polyester is the most common fabric for producing
flexible structures, such as custom-designed awnings, canopies,
walkways, tent halls, smaller air-supported structures and light
member-framed structures.
Vinyl-coated polyester is composed of a polyester scrim, a
bonding or adhesive agent, and exterior PVC coatings. The polyester
scrim supports the coating (applied initially in liquid form) and
provides the tensile strength, elongation, tear strength and dimensional
stability of the finished fabric. The scrim is made of high-tenacity,
continuous-filament yarns, which have high dimensional stability, and
can be bent thousands of times without losing any tensile properties.
The base fabric’s tensile strength is determined by the size (denier)
and strength (tenacity) of the yarns and the number of yarns per linear
inch or meter. The bigger the yarn and the more yarns per inch, the
greater the finished product’s tensile strength. For architectural
applications, base fabrics typically weigh between 2.5 and 10 oz/yd2,
with a tensile strength between 300 (2.662 N/5cm) and 650 lbs/in (5.60
N/5cm), although fabrics intended only for tent use may have lower
measurements.
The adhesive agent provides a chemical bond between the
polyester fibers and the exterior coatings and prevents wicking of
moisture into the fibers. Wicking is the capillary like action of fiber
to absorb water, which could result in freeze-thaw damage.
The PVC coating liquid (vinyl Organisol or Plastisol) contains
chemicals to achieve desired properties regarding color, water
resistance, mildew resistance and flame retardancy. The fabrics also can
be made with high levels of light transmission or complete opaqueness.
After the coating is applied to the scrim, the fabric goes through a
heating chamber to dry the liquid coating.
Vinyl-laminated polyesters
Vinyl-laminated polyesters are used for awnings, tents and
low-tension frame structures. Technically, a laminated fabric consists
of a reinforcing polyester scrim that is calendared between two layers
of unsupported PVC film. In general use, it refers to two or more layers
of fabric or film joined by heat, pressure and an adhesive to form a
single ply.
With an open-weave or mesh polyester scrim, the exterior vinyl
films bond to themselves through the openings in the fabric. Heavier
base fabrics, though, are too tightly constructed to permit this
lamination process, so an adhesive must bond the exterior films to the
base fabric.
A good chemical bond is important to prevent delamination and is
critical in developing the proper seam strengths. The adhesive enables
the seam, created by welding vinyl-coated fabric to another piece of the
same material, to meet a structure’s shear forces and load requirements
at all temperatures. By preventing wicking of moisture into the scrim’s
fibers, the adhesive prevents fungal growth or freezing that can affect
the exterior coating’s adhesion to the scrim. In response to EPA
regulations, the adhesives are water-based.
Using an open-weave scrim such as mesh might make these fabrics
more economical, depending on the number and type of features required
in the vinyl. What weight is necessary to withstand abrasion and wear?
Is flame resistance needed? Is a particular color required? What width?
Virtually any color, plus UV resistance, abrasion resistance, and
colorfastness can be formulated into the vinyl, but the more of these
features incorporated, the higher the cost.
Fiberglass
Another widely used base material is woven fiberglass coated
with PTFE (also known as Teflon®) or silicone. The glass fibers are
drawn into continuous filaments, which are bundled into yarns. The yarns
are woven to form a substrate. The fiberglass has a high ultimate
tensile strength, behaves elastically and does not undergo significant
stress relaxation or creep. The PTFE coating is chemically inert,
withstands temperatures from minus 100F to 450F (minus 73C to 232C), is
immune to UV radiation and can be cleaned with water.
PTFE-coated fiberglass is available with as much as 25%
translucency, providing diffused interior light. Its ability to provide
natural daytime lighting and its highly reflective surface for efficient
nighttime interior lighting can reduce energy consumption.
For these and other reasons, fiberglass-based fabrics have been
the material of choice for stadium domes (both air- and cable-supported)
and many other permanent structures, particularly in the United States.
Another reason some industry experts cite for this is a perception
among code officials that its high melting temperature and lack of
creep, or long-term elongation, make it superior to polyester. Other
industry insiders note that polyester, like fiberglass, melts rather
than burns at high temperatures, and that properly constructed,
polyester structures may be equally durable.
Because of the differences in how polyester and fiberglass
perform in fire-resistance tests, PTFE-coated fiberglass is the only
membrane material that currently meets the U. S. model building codes
definitions of a noncombustible material. (The three U.S. model codes
are currently being reviewed and soon will be consolidated into one
code.) This is a more accurate reason for the PTFE-coated fiberglass
preference, but it raises questions about whether standards applied to
other building materials should be applied to membranes.
Woven PTFE
This material is constructed of PTFE fibers woven into a fabric.
As of now, only one such material is available. Woven PTFE combines the
environmentally-resistant advantages of the material with its ability
to withstand repeated flexing and folding, an advantage it has over
coated-fiberglass fabrics. Such flexibility makes it an especially good
option for convertible structures; however, it is a rather expensive
material and is not as strong as either polyester or glass.
ETFE foil
Perhaps the newest development in the fabric structures arena is
the introduction of ETFE (ethylene tetrafluoroethethylene), a
transparent membrane with fabric like qualities and the advantages of
PTFE, such as a self-cleaning capability. Resistant to atmospheric
pollution and UV light, ETFE has a very long expected lifespan of more
than 20 years. Effective thermal performance (average U value is 2.6W/m2K
for a two-layer system) and high light transmission (95% visible light,
85% UV light) enable a range of applications where traditional
materials, such as glass, would not be practical. It is more than 20
times lighter than glass (0.35kg/m2 for ETFE vs. 15kg/m2
for glass) and is ecologically friendly and energy efficient as its
constituent materials are fluorspar, hydrogen sulphate and
trichloromethane, all non petrochemical derivatives. It is 100%
recyclable.
Meshes, netting and film
These are the least-used materials for fabric structures. Mesh
is a broad term for any porous fabric with open spaces between its
yarns. It can be made from almost any fiber by a variety of methods,
including knitting, weaving and extrusion. In some cases it acts as a
substrate to beef up other fabrics or is coated to produce specific
characteristics.
For architectural use, meshes typically are
available as polyester weaves lightly coated with vinyl or as knitted
fabrics using high-density polyethylene (HDPE), polypropylene or acrylic
yarns. Polyester mesh dyes well, is strong, has a low water absorption
rate and can be economical. Nylon often is used in industrial
applications because of its strength and resistance to chemicals,
although it does have a high water absorption rate and may cost more
than polyester. Often used in agriculture, recreation and containment,
polypropylene and HDPE are inert, so they can’t be stained or dyed, and
are less expensive than polyester or nylon. Polypropylene, however, does
have a comparatively low melting rate, a factor in some industrial
applications.
Meshes can provide shade as well as shelter from wind. Since
they’re porous, meshes are little good against rain. Still, they are
inexpensive and have been used for some low-cost membrane structure
applications. For obvious reasons, meshes are not used in traditional
air-supported structure design.
Netting consists of a nylon, polyester or polypropylene with extruded or spun yarns that is knotted or raschel knitted to form the material. Each material has its advantages and appropriate applications. Polyester holds dye better than nylon but is more expensive; nylon is easier to coat, but has a higher water absorption rate and doesn’t hold dye as well. Polypropylene floats on water, is durable and chemically resistant, but can’t be dyed. Raschel knitting is a newer, faster manufacturing method than knotting. One drawback is that the knitted material can unravel, which can be thwarted by heat-setting the netting to shrink and stabilize the fibers.
Films are transparent polymers extruded in sheet form without a supporting substrate. They are not laminated or coated. Examples include clear vinyl, polyester or polyethylene. These films are cheaper than textiles, but they are neither as strong nor as durable.
Films are much weaker in tension, though more elastic, than scrim-based fabrics. Films sometimes have application in air-inflated structures. Air-inflated structures are composed of fabric tubes in which the air is pressurized, but the structure’s interior itself is not. Some air-inflated roofs or building envelopes have been made using two or three layers of films to form air pillows. The film layers are thermally welded and sealed, and the resulting pillows are inflated by small fans. The inflation increases the internal pressure to prestress the surface, creating load resistance. Such film pillows are framed by an aluminum extrusion perimeter, which must accommodate some structural movement.
Films range in thickness from 30 to 200 microns and can be produced with levels of translucency varying from 25% to 95% light transmission. Films are low weight, have a life expectancy of 20 to 25 years and highly resist dirt. The inflated pillows exhibit good thermal insulation values. More research needs to be done to develop a range of standard reliable, economical details, for instance, to improve the water seals and reduce wicking.
Blackout fabric
Blackout material, sometimes called blockout material, is an
opaque or nontranslucent fabric. Primarily used for tents, the fabric is
a laminate that sandwiches a dark opaque layer between two white
exterior layers. Because no light transmits through the tent top or
walls made with blackout material, lighting and heat can be controlled.
In addition, any stains, dirt, repairs or slightly mismatched panels on
the tent’s exterior will not be noticeable from the inside.
Blackout fabric also has its disadvantages. Heating may be
necessary, as the tent’s interior may be colder than using nonopaque
fabric. Heating and lighting, of course, will increase the operating
costs. The multiple layers make the fabric heavy and harder to handle,
and increase the possibility of delamination over time. The fabric is
more expensive that other tent materials, pinholes or snags in it are
apparent to occupants, and often, the interior color does not exactly
match the exterior color.
Other materials
Shade cloth, often a knitted fabric such as
monofilament polyethylene, originally found use as agricultural crop
protection. It has been adapted for tension structures whose purpose is
solar shading. Shade cloth can be manufactured in a variety of colors,
offers stretch and resiliency and remains flexible without
tensile-strength loss under a range of conditions. Light transmission
can vary from 20% to 90% shade factor; its UV filter construction can
range from 30% to 70%.
Nylon, which may be laminated with vinyl films,
is stronger and more durable than polyester, but has a higher cost and
more stretch. It may be a good choice for some small jobs, but it
stretches too much for use in large buildings.
Spandex is difficult to use in long-term
exterior applications because of the wind’s effect on its seams and
construction, but its stretch and splash of colors can contribute to
interior spaces or temporary exterior use, for instance, at festivals
and special events.
Solution-dyed acrylic and modacrylic
have gained use, particularly for small shading structures. Their wide
range of colors and modacrylic’s flame retardancy make the materials
attractive to designers (see the Awnings & Canopies section for more
information).
Kevlar® is an excellent lightweight fabric for
construction, but it is very expensive, it is rarely used for large
structures, such as dome roofs.
