Thermoplastic polyurethane (TPU) was discovered in the late 1930s as part of Germany’s World War II research. It is a thermoplastic elastomer (TPE), which combines the mechanical and physical properties of rubber with the advantages of thermoplasticity and processability. Other examples of TPEs include polyethylene and polypropylene.
Noted for its high performance and general overall toughness, urethane rapidly became the material of choice for a wide range of critical and ‘can’t fail’ applications. Urethane’s unique characteristics make it an extremely versatile material that outperforms many other thermoplastics. For example, it retains its flexibility even at low temperatures, where polyvinyl chloride (PVC) becomes brittle.
On a molecular level, urethane is comprised of the four most common elements in the world: carbon, hydrogen, nitrogen, and oxygen. To be a urethane, it must contain the molecular urethane linkage (NHCO2). The actual chemistry consists of a series of block copolymers with alternating hard and soft phases. A block copolymer is a string of chemically different molecules in repeated sequences. The ratio and molecular structure of these segments determines the specific characteristics of the resin, which can be varied by modifying its chemical backbone.
Depending upon its chemical makeup, the urethane soft phase is generally either Polyether or Polyester. These two categories are further segmented into Aliphatic or Aromatic hard segments.
Polyether or polyester
Polyether urethane provides a softer feel or drape than polyester, with better moisture vapour transmission rates and superior low temperature properties. It is also inherently stable when exposed to high humidity, and polyethers are naturally more fungus resistant.
Polyester urethane offers greater toughness (ie abrasion resistance, tensile/tear strength) at a given durometer than polyether. It is more resistant to fuels, and offers better aging (oxidation) resistance. However, polyester urethanes will eventually break down when exposed to conditions of high humidity.
Aliphatic urethane is inherently light stable, resistant to ultraviolet light degradation, and provides excellent optical clarity. It is used as an optical interlayer, providing strength as a lamination adhesive in encapsulated glass and security glazing, for example. It is often a more expensive product.
Aromatic urethane is a strong, general-purpose resin originally developed as synthetic rubber. It is generally less expensive than aliphatic urethanes, but it is susceptible to UV light degradation, which tends to yellow the polymer without affecting physical properties.
At Stevens Urethane, part of JPS Elastomerics, extruded products and extruded blown products are made. Extruded urethane products are sheet, tubing, cord and profiles; blown urethane just comes in film.
Why TPU film?
Polyurethane combines the best properties of rubber and plastic, without the weaknesses inherent in plasticised vinyl films. Because it contains no plasticisers, it is not subject to the brittleness and other problems caused when they leach to the surface. Urethane is easy to work with, and is readily modified to suit a particular application through the addition of fillers, colours, stabilisers, and lubricants, as well as other additives. Urethane compounds possess a combination of properties which are not available in any other thermoplastic material, offering significant design flexibility.
Urethane film and sheet can be produced in a range of durometers (Shore A 75 to Shore D 55) from a very stiff material to a very thin film having an extremely soft, non-plastic feel. The latter is ideal for use in products that come into contact with the skin. Urethanes are still flexible over a wide durometer range, even at temperatures as low as -600F.
A tensile strength range from 4,000 to 10,000psi assures reliability and durability over the lifespan of the end product. And because urethanes are tough, they can be used in thinner gauges than vinyl.
And it’s tough
Urethane film and sheet can elongate as much as 800% and return to its original dimension without significant loss of memory. Urethane provides excellent abrasion resistance. For example, it is used in profile form as a conveyor drive belt, and to protect elevator cables and pulleys from excessive wear while cushioning the ride.
The material has excellent resistance to hydrocarbons, chemicals, ozone, bacteria, fungus, and moisture, as well as skin oils. Polyether or polyester urethanes offer specific characteristics that provide durability and long life to products that must survive and perform in harsh industrial environments.
The number of different ways in which urethane can be fabricated enhances its design versatility. It can be cut, sewn, adhesive bonded and laminated to a wide selection of substrates. It can be vacuum formed, radio frequency sealed, and thermally bonded to itself or other materials. Sheet can be fabricated with urethane tubing to provide the benefits of urethane to an entire product system, such as in linked bladder applications. Urethane may also be printed or silkscreened.
Urethane is more environmentally friendly than vinyl and other films. Recently, manufacturers of disposable medical products have begun to use urethane because of the toxic byproducts resulting from incinerating contaminated medical disposables fabricated from PVC. Urethane is also readily recycled, and trim, can be reground and reused if kept free from contamination.
Where to use TPU
Because of its toughness and versatility, as well as a higher price tag than other TPES, urethane is typically used in applications where a high level of performance and a challenging combination of properties is required. The medical industry uses it in compression dressings, pressure infuser bags, ergonomic cold packs, transdermal patches, and flotation mattresses. Industrial uses include drive belts, hoses and bellows. TPU film is used for keyboard covers, flotation devices, headphone ear cushions, and packaging of delicate items.
A recent development in TPU was introduced at the National Design Engineering Show in Chicago by Stevens Urethane. It is a flame resistant TPU tested to 2mm thickness suitable for applications such as fire fighting gear and life jackets.
JPS Elastomerics Tel: +1 413 533 8100