Thermoplastic polyurethane (TPU) is well known and specified in the medical industry for advanced medical and healthcare products, due to its high performance characteristics. Because of its excellent mechanical properties, durability, and resistance against oils and chemicals, TPU is very desirable for medical applications. Since TPU does not contain plasticizers, it also offers the medical industry an environmentally friendly replacement to PVC without sacrificing flexibility. Ether TPU types also meet the requirements of National Sanitation Foundation (NSF) Standard 61, U.S. Food and Drug Administration (FDA) 21 CFR for certain food contact applications, and USP Class VI classification.
Examples of medical TPU applications include devices used for diagnostic, anesthesia and artificial respiration, as well as healthcare mattresses, dental materials, compression stockings, medical instrument cables, gel shoe orthotics, and wound dressings.
TPU belongs to the family of materials known as thermoplastic elastomers (TPE). It attains virtually the same level of elasticity as cross-linked elastomers (rubber) while simultaneously offering the advantages that it can be processed like a thermoplastic and is recyclable. (See Figure 1)
TPU is made up of block copolymer molecules with alternating rigid and flexible segments. It is this combination of flexible, elastic segments—with a high extensibility and low glass transition temperature, with rigid crystallizing segments, with a high melting point, that gives this material its elastomeric nature. (See Figure 2)
By altering the ratio of the rigid phase, it is possible to vary properties such as hardness, strength, rigidity, extensibility, and low-temperature flexibility over a broad range. A distinction is drawn between polyether and polyester polyurethane as a function of the polyols employed. The cable industry uses predominantly polyether TPU on account of its good resistance against microbes and considerably better resistance against hydrolysis.
Mechanical properties: Thermoplastic polyurethane is supplied with a surface hardness of 35 Shore A (with plasticizer), 70 Shore A (without plasticizer), up to 74 Shore D. Further distinction is made between grades by using flameretardant additives, matting agents (for low-adhesion surfaces), or plasticizer (for even higher flexibility, especially decreased bending modulus and partly flame retardance). However, most of these grades containing additives are not suitable for healthcare applications. (See Table 1)
Flexibility at low temperatures: Dynamic Mechanical Analysis (DMA) is a method used to study and characterize materials. It is used for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing to determine the storage modulus G′ and to locate the glass transition temperature TG.
The glass transition temperature TG for the unfilled Elastollan® 1185A based on DMA-analysis is -27.7°C (-17°F). However, feedback from the market shows that, under real environmental conditions, the 1185A performs in flexible installations at even lower temperatures.
Abrasion: While the abrasion values of TPU as per ISO 4649-A are hardly dependent on the hardness, harder versions withstand the typical abrasion test for insulation for an extremely long period of time. In ABS brake cable applications for example, car manufacturers frequently specify at least 150 or 200 double strokes until insulation wears through.
Thermal properties/Long term heat aging: Long-term ther mal behavior is determined according to ISO 2578. The test sample is permanently conditioned at a selected temperature until a defined criterion is no longer achieved. The achieved temperature resistance is therefore dependent from the end criterion and the test time.
For TPU the end criterion is usually an elongation at break of 300 percent, which would be 50 percent of initial values, but it is also usual to test to an elongation at break of 50 percent. The electrical engineering sector frequently specifies a 20,000 hour test (2.3 years) for estimating the service life time. TPU can be aged at 105°C (221°F) over this period of time still having an elongation at break of 300 percent. If an elongation at break of 50 percent is sufficient the temperature resistance is increased to 110°C (230°F) over a period of 20,000 hours.
Resistance against hydrolysis: For outdoor applications, the resistance against hydrolysis is absolutely necessary. In general, the rate of hydrolytic degradation of TPU increases with increased temperatures. A standard polyetherpolyol based TPU has very good hydrolytic resistance. Polyester-polyol based TPU in comparison have lower hydrolytic resistance.
Weathering and ozone resistance: Weathering performance can be measured according to ISO 4892-2, part A, using injection molded test plates with a thickness of 2 mm. Due to the chemical structure, TPU in general shows very good resistance against ozone. Selected grades have been stored for 72 hours at 40°C (104°F) with an ozone concentration of 200 pphm and a strain of 20 percent. All grades remained free of cracks.
Regulations for Medical Applications
In the following chapters, information is given concerning basic regulations regarding materials intended for use in contact with foodstuffs or as medical devices. It is the responsibility of manufacturers and traders to ensure that intermediate and finished articles are in compliance with all relevant legal requirements and standards for the particular application area.