When selecting a plastic for a specific application, the material’s resistance to the media that it can be expected to come into contact with and its resistance to environmental conditions are of major importance. Due to their widely varying chemical structure, Desmopan® grades can behave in very different ways towards chemicals. Polyether-based products display fundamentally different behavior from those based on polyesters, and hardness can also play a role. In addition to this, external conditions, such as temperature, concentration, and the duration of exposure to the media, will naturally also have a major impact. The table on the “Chemical resistance of DESMOPAN® grades” sets out guide values for this.
If the resistance of Desmopan® to mixtures of substances, in particular, is not clear, it is recommended that special tests be conducted, where the mechanical properties of test specimens are, for example, determined subsequent to aging in the medium in question. When Desmopan® comes into contact with chemicals, this generally results in swelling and/or chemical reactions, coupled with degradation of the material. The swelling involves the absorption of what are generally low-molecular substances by the TPU – a process which is reversible, since the molecule chains are not damaged. Dimensional changes can, however, occur, together with a deterioration of the mechanical properties. In extreme cases, the TPU will dissolve.
Acids and alkaline
Many of the lubricants employed in practice, however, are preparations with additives that damage TPU. Immersion tests are essential here. Metallic soaps, in particular, are critical.
The Thermoplastics Testing Center (TTC) can carry out the following chemical resistance tests for you:
Under unfavorable conditions (hot climate, high humidity, soil contact), flexible polyester-based TPU can be attacked and decomposed by fungi and bacteria. The ester bonds are split by enzymes in the microorganisms, and low-molecular building blocks result. Attacks of this type are frequently visible in the form of discoloration and, subsequently, the formation of cracks in areas of the parts subjected to mechanical load. A certain amount of protection can be achieved through the addition of microbicides. Thermoplastic polyether-based polyurethanes and rigid polyester TPU are inherently resistant to microbe attacks.
At high temperatures and with a high humidity, flexible TPUs based on polyesters display a certain susceptibility to hydrolysis. Water molecules then split the ester groups, and the molecular weight decreases considerably, thus leading to reduced mechanical properties. Flexible polyester TPUs rarely fail in practice. All the general-purpose Desmopan® grades contain effective stabilizers, which are not necessary for ether grades.
The extent of yellowing and the impairment of the mechanical properties can be slowed down considerably through the addition of appropriate light stabilizers. Desmopan® grades that already contain these agents are available, as are masterbatches which can be added as and when required.
During weathering tests that run for prolonged periods, degradation phenomena due to UV radiation and hydrolysis can be superimposed on each other.
The ozone resistance of a material plays a key role in the rubber industry, since a large number of elastomers contain double bonds, which can be split by ozone with its strong oxidizing effect.
Desmopan® grades display very good resistance to ozone. No embrittlement is observed.
The Thermoplastics Testing Center (TTC) can carry out the following weathering tests for you: