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Permeability

Permeability

Permeability

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.

Gas Permeability

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.

Gas Permeability G in cm3 / m2 x d x bar

Desmopan 385 Desmopan 385 Desmopan 786 Desmopan 786 Desmopan 9385 Desmopan 9385
Gases 25°C 60°C 25°C 60°C 25°C 60°C
Air 450 1000 600 3500
N2 330 1800 500 3600 500 2800
O2 840 4500 1500 8000 2000 9400
CO2 6500 19000 8400 27000 6000 28000
He 2600 9700 4500 16000 4600 16500
Propane 2400 4000 18000
Natural Gas 1000
N2O 12400 14000

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.

Fuel Permeability

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 Fuel 2 Fuel 3
FD (g / m² x d) FD (g / m² x d) FD (g / m² x d)
8 280 2600

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.

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