Civil Engineering Reference
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characteristics, the interior membrane uses a lighter version of the same exterior
coated fabric. However, if the outer layer is porous the inner membrane becomes the
thermal barrier and must account for condensation. For improved acoustics, the inner
membrane is generally a very porous fabric that allows for sound decay. This fabric
could have similar cutting patterns to the outer fabric or become a separate and
unique element using completely different fibers.
3.3 Coatings
There are several coatings for architectural fabric structures and generally the coating
and the fibers cannot be interchanged as the two elements form a powerful composite
material. The most common are described below:
Polyvinylchloride (PVC)
PVC is relatively soft and pliable which enables it to work well with tensile
structures. It is typically applied to polyester fibers. It is somewhat resistant to UV
light, however generally after about 15 years the plasticizers migrate and the coating
becomes brittle at which time it is necessary to replace the fabric. It can be sealed
easily using heat or radio-frequency (RF) welding machines. PVC coatings are
available in a large variety of colors and can have a blackout layer inserted into the
coating for non-translucent applications. The PVC can be printed on or painted using
vinyl inks. Outdoor billboards use digital printing techniques on PVC mesh fabrics
which are the same material as those used for tensile structures, only a lower strength
fabric.
PVC coatings by themselves attract dirt and under high temperatures can seal in dirt
to look visually unattractive. In order to avoid this, a series of top surface coatings
have been developed. This top-coating, besides improving the fabrics self-cleaning
properties, also helps protect the coating from UV light. One of the materials used is
Tedlar which is a polyvinylflouride (PVF). Other top coatings for polyester are
polyvinyldeneflouride (PVDF) lacquers or urethane. These top coatings provide a
similar non stick, self cleaning surface as other fluoroplastics.
PTFE (Teflon)
The resistance of fabrics to dirt is an important element in fabric selection as it affects
both appearance and the cost of cleaning. PTFE is chemically inert and used as a
coating is a non-stick surface, resistant to moisture and containing a low deterioration
with age. The coating is either combined with a fiberglass fabric or as a coating on
expanded PTFE woven cloth. PTFE has excellent flame resistive characteristics,
moderate translucencies, a high tensile strength and modulus of elasticity.
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