Film resists punctures and leaks

Tom Shelley reports on a few of the more surprising products that depend on thermoplastic polyurethane film

Blown thermoplastic polyurethane film is finding uses that range from hospital mattresses, bonding dissimilar materials, and making fibre-reinforced laminates for bullet proof vests to producing airships.

The material is a lot tougher and more tear-resistant than other film-blown polymer sheet material and is also weather resistant and elastic. Applications for it include use in fuel cells, the lining of fuel tanks, sealings for gaskets and in the manufacture of impact-resistant glass.

The only company we are aware of in the UK that blows PU as film is Permali Gloucester, which market it under the brand name, 'Tuftane'. Managing director Gus King told us that, "It is an unusual material to film blow", possibly because PU is produced by reacting pairs of components – isocyanates and polyols, rather than the single component resins that are normally film blown. Manufacturers in the USA, where use of TPU film seems to be more common, include Argotec in Massachusetts and American Polyfilm in Connecticut.

Film blowing is the process normally used to make plastic bags, but these seem to be about the only type of thin sheet plastic products that are not recommended applications for Tuftane, which is produced in a range of thicknesses from 35µm to 0.6mm. Permali only produces the film. Laminating it to other substances is undertaken by companies such as Holden Laminates, which is part of a long-established Lancashire textile company, John Holden, that as well as making a huge range of industrial textiles, waddings and non-wovens, can laminate just about anything fibrous to almost anything thermoplastic.

To create a laminate or bond different materials together, the first stage is to use infra red or flame heating to melt TPU laid onto the fibre or other material to be bonded, and then pass this and the material to be bonded to through a laminating machine. In the case of the plastic bullet proof vests, the laminated layers are then placed in a mould and pressed together. Julie Thornhill, the sales manager at Holden Laminates told us that they only go as far as producing laminates and there are yet other companies who specialise in pressing laminates together to form shaped products with thickness.

The hospital mattress application is produced, by among others, Huntleigh Healthcare, which has devised mattresses with individual inflated cells that can be cycled round, so as to vary which parts of the mattress are in most forceful contact with the anatomy of really sick patients so as to reduce the risk of pressure sores.

For this application and many others, it is essential to minimise the possibility of gas leakage. For this reason, TPU film is finding favour with makers of escape hoods and suits for emergency services, and with makers of small airships popular for advertising promotions and as camera platforms, especially in the USA, where PU is displacing PVC.

While airship manufacture is clearly a niche application, it is a very demanding one that gives some idea of the material's properties. One manufacturer, Airship Solutions, which uses PU film that is only 3.5 MIL or 0.09mm thick, states: "Polyurethane is an incredibly resilient material that is ideal for use in blimps due to its properties for containing helium", and "In the event the hull gets punctured, it is very simple to fix." The alternative balloon and airship material is currently Nylon Ripstop, which is favoured for larger lighter than air vessels, but suffers from the problem that it does not stretch. Hence if used for the hull, pressure has to be regulated, either by a pressure release valve, an expansion panel, or internal bladders called ballonets.

Author
Tom Shelley

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Similar polyurethane films can be made by calendering.
This method of manufacture is more expensive than blowing but allows the production of films with a number of technical advantages, eg the potential to have an embossed (3D) surface. An embossed surface can reduce surface area of contact and facilitate air flow under negative pressure (important characteristics for certain medical inflatables).
In addition calendering allows the manufacture of thicker films than are feasible by blowing. Films from 60microns to 2mm thick film are readily available.
This naturally broadens the range of applications.
Some examples of applicatins of thicker films are conveyor belting, flexible containers, medical strapping and inflatable drain/pipe stoppers.
More detailed information can be found at: www.chiorinocoatedfabrics.co.uk


Comment Ian Shaw, 27/03/2012
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