Permeability describes the transmission of a medium, most likely of a gas, rather seldom of a liquid, through a test specimen. The permeability process can be divided into three separate steps:

• Entrance of the medium into the test specimen

• Diffusion of the medium through the test specimen

• Escape of the medium from the test specimen

The permeability is directly proportional to the temperature and inversely proportional to the wall thickness.

The following table shows the gas permeability of different gases at some important Desmopan – grades measured to DIN 53380, Part 1 at films of 100 µm thickness.

Thermoplastic polyurethanes are normally not used as barrier materials. But polyester – TPU tends to have a lower gas permeability compared to polyether – TPU. Hence they are more suitable as a barrier material.

For a further reduction in gas permeability, additives are known, and can be compounded into the material, which will reduce the gas permeability by about 30 %.

If high gas permeability is requested, either polyether – TPU or the special polycarbonate – TPU are more suitable. The later ones are especially interesting, if the higher gas permeability is required in combination with a good chemical resistance against a wider range of substances.

The following table shows the fuel permeability (FD) of Desmopan 786. The measurement was done according to DIN 53532 on a polyester fabric coated on both sides with a layer of 100 µm of Desmopan 786.

Fuel 1 = isooctane (2,2,4 – trimethylpentane)
Fuel 2 = isooctane / toluene (70 : 30 Vol.%)
Fuel FAM = pure toluene / isooctan / diisobutylen / pure ethanol (50:30:15:5 Vol.%)

Desmopan 786 is a polyether – carbonat – TPU especially suitable for the contact with fuel and water at the same time.

Due to the unique formulation of Desmopan 786, it has an equally high resistance against fuels and hydrolysis.