canopy

What Materials?

All the materials used in permanent external structures are known as “supported” fabrics. This simply means that they have a woven fibre basecloth, and in most cases this basecloth is coated to improve and preserve the properties of the base material.

The two materials most commonly used for permanent external fabric structures are PVC coated polyester, and PTFE coated fibreglass.

The two different base cloths share some similar properties, most importantly their tensile capacity. However, although weight for weight they have the same sort of breaking strain, the materials have different stretch characteristics.

We can see from manufacturers stress/strain data that the glass fibre material is actually stiffer than the polyester. We can also see that polyester typically requires less tension to eliminate the non-elastic stretch from the material than the glass.

If we installed two identical fabric structures side by side, one with a polyester basecloth, and one with a glassfibre basecloth, what would the differences be?

Firstly, we would find ourselves having to put more tension into the glassfibre cloth than into the polyester cloth to “prestress” the canopy, and the knock on effect of that could be larger boundary cable sizes, or larger steelwork sizes.

Secondly, we’d see that the polyester material would move more than the glassfibre in the same sort of wind conditions. The polyester can stretch more easily and quickly return to a prestress condition, whereas the glassfibre, stiffer and tighter in the first place, will remain more stable. Generally the predicted movement of the membrane under loads is not a concern, but it is a factor that we deal with during the analysis process, as excessive movement in high winds could cause premature wear or loosening of system components and could also lead to mild alarm in the public.

The most important thing we’d actually notice about the two materials is that the PTFE coated fibreglass structure will easily outlast the PVC coated polyester structure, providing the structures are maintained correctly.

The main reason for this is the coating. For all coated fabrics, the coating creates a waterproof material, protects the fibres from dirt and damage, and provides a surface that is able to be cleaned.

Polyester fabrics would quickly lose a lot of their strength if the raw basecloth was exposed to light. The coating protects the fibres from harmful UV radiation.

Glassfibre fabrics are not affected by UV light, but they weaken quite significantly when wet. The coating keeps the raw basecloth dry and protected.

So appropriate coatings have been developed for the basecloths - with polyester it’s PVC, and with glassfibre, it’s PTFE.


PVC

PVC  (polyvinyl chloride) is actually a rigid material. It can be made to be flexible by adding plasticisers to give it the required characteristics. The PVC is naturally clear, which doesn’t help prevent uv light from weakening the fibres. The addition of pigments to the PVC, typically white, hugely increases the uv resistance of the coating and provides full uv protection for the basecloth. Other additives can also enhance the properties of the PVC, such as anti fungal treatment and uv reflector/absorber agents. However, it’s well known that the plasticisers migrate towards the surface over time and then they attract and retain dirt, and this leaves the PVC brittle, discoloured and hardened. To improve the long term durability of the fabric, a top coat is added to the main pigmented coating.

The top coat is typically a very smooth and shiny application so that cleaning is facilitated, and so that most dirt gets washed off the surface by rainwater. Since it performs such a valuable job it’s important that it’s correctly maintained. Effectively this means treating it carefully during fabrication and installation, and keeping it clean after installation.

The least expensive, and also least effective top coating is a sprayed acrylic lacquer. The acrylic doesn’t have a very high resistance to uv light and will not offer long term protection to the PVC coating. Many suppliers provide architectural fabrics with acrylic finishes, but they are most suitable for marquee type structures which are regularly and frequently put up and taken down. They are likely to be cleaned quite often and so the top coating doesn’t have to withstand years of constant exposure and the build up of surface dirt.

Most usually, PVC coated fabrics now have a pvdf lacquer on the top surface. Pvdf has superb properties for top coating – it’s more durable, sheds dirt more easily and is more resistant to uv light. The downside is that in its pure form, it’s not weldable by normal PVC welding machinery without careful preparation. To overcome this, many manufacturers dilute the pvdf with PVC to create an excellent quality, durable finish that needs no special preparation for welding. Some fabrics have a pvdf top coat and an acrylic lacquer on the underside which still gives excellent durability but at a lower price.


PTFE

Ingrained dirt on PTFEPTFE (poly tetra flouro ethylene) is an amazing material – it has the second lowest co-efficient of friction of all materials known to man which means it’s very slippery, and that in general dirt just can’t stick to it. It is also totally inert to almost all chemicals and solvents, and, other than bleaching white once exposed to sunlight, it is unaffected by uv radiation.

Although the PTFE can also be pigmented, the range of colours is very limited and production run quantities would normally have to be ordered. However, the beauty of the coating is that it is virtually maintenance free, and is expected to give well in excess of twenty years life. It seems likely that some of the earliest installed PTFE canopies will actually exceed thirty years. Unsuprisingly, the PTFE coated glass is a lot more expensive than PVC on polyester. It’s slipperiness makes it quite difficult to handle during fabrication and installation, and the welding process is a lot more time consuming as well.

On a microscopic scale, the surface of the PTFE coating contains tiny pores. Although the fabric still remains totally waterproof, airborne pollutants and dust can build up on the surface and in time, clog the pores so that a routine clean will leave them visible as tiny dark pin pricks. It’s not considered to be a problem in the long term, but it can be avoided if the canopy is given a regular quick clean to prevent the build up of dirt occurring.


Differences between PFTE/Glass and PVC/Polyester

PTFE bleached and unbleachedThe biggest difference between PVC/polyester and PTFE/glass is the cost. The PTFE fabric costs at least five times the price of the equivalent PVC. It’s also more difficult to handle, slower to weld, and requires more care and time during installation on site.

This might be a good time to mention that PTFE coated cloth is a light brown colour when it is first manufactured. Only on exposure to uv radiation (eg natural daylight) does the colour fade, ending up as white as the whitest PVC. This bleaching process is likely to take several weeks, or even months if parts of the canopy don’t receive much daylight, but any manufacturing pigmentation always fades completely, generally in an unnoticeable sort of way.

When choosing the fabric for your structure, you need to decide on how long you want the structure to last. Other factors to consider are local environmental conditions, and likelihood of damage requiring fabric replacement. In some cases, it may be a consideration to use PVC and then replace the whole membrane at the end of its life, if the structure is still required.

It’s typical to recommend PTFE on glass for large, long term installations, such as stadium roofs for example. The durability and low maintenance requirements make it an ideal choice for such a situation. In these types of installations, there’s a good chance that the fabric will never be cleaned, so the “self cleaning” properties of PTFE, and it’s resistance to all forms of staining or pollution make it the clear choice.

However, on smaller structures, which fabric is best?
In most cases, a good quality PVC on polyester fabric offers the ideal balance of durability and cost. However, there are cases where you may decide to specify PTFE on glass. For example, if the canopy is not easily accessible for cleaning, then PTFE may provide a better solution for the client. Also, if the membrane itself is of relatively simple construction, the extra fabrication costs may be acceptable in return for the benefits of a much higher life span.

An important factor to remember is that the actual cost of the fabric is generally only a small part of the overall project cost, so the overall project costs won’t necessarily increase significantly despite specifying a much higher quality fabric.

The other approach, as mentioned earlier, is to specify PVC, but to expect to replace the canopy at the end of its useful life with a new membrane. Providing all the original calculations and patterning details are still available, the production and installation of a new canopy onto the existing supporting structure is a relatively straightforward operation.

In conclusion, it should be remembered that all architectural grade membranes, whether PVC/PE or PTFE/GF are manufactured to an extremely high quality. The production processes are highly specialised to ensure both flawless weaving of the basecloth and the subsequent applications of the fabric coatings. They really are remarkable materials, in that they diffuse light, create shade, are waterproof, can span huge distances unsupported, can self clean, block uv light, and provide a fascinating visual focus. The existence of materials such as these give us opportunities for providing permanent outdoor covered spaces in an almost unlimited number of exciting ways.  

We have tried to pass on as much useful information to you as we can in this article, and we hope that this has given you a real insight into the properties of the two main materials in common use for permanent external structures.

If you would like some samples of the different fabric types we normally use, we have lots of offcuts, so please just ask!

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