Optical properties

Transparency

Thermoplastic polyurethanes are generally moderately to strongly opaque, or even milky and cloudy. A number of Desmopan® grades are still slightly transparent up to a wall thickness of approximately 2 mm. This is not constant, however, and can vary a great deal depending on the processing conditions. Polyether TPUs tend to be more transparent than polyester TPUs.

To overcome this drawback, special aromatic series of Desmopan® grades have been developed with a constant, guaranteed transparency of > 90 % up to a wall thickness of 6 mm. These UV-stabilized series of grades have the designation DP 36xx (polyester) and DP 96xx (polyether) in line with the Desmopan nomenclature.

We also supply aliphatic TPU with a particularly high transparency from our US Texin range. These grades, based on Desmodur® W, still offer outstanding transparency in a wall thickness of 10 mm as well as having all the advantages of aliphatic TPU.

Yellowing

Aromatic thermoplastic polyurethanes experience yellowing under the influence of UV light, triggering the formation of compounds with conjugated double bonds in the rigid segment which are similar to coloring agents in terms of their chemical structure.

The Yellowness Index is measured in order to quantify the extent of yellowing. The addition of UV stabilizers will only delay the yellowing effect and not prevent it. This yellowing constitutes a drawback for parts subject to stringent optical requirements. Aliphatic thermoplastic polyurethanes were thus developed which do not display this effect.

The following diagram shows the difference between aliphatic and aromatic TPU both with and without UV stabilizer.

Refractive index

The refractive index is a measure of the refraction, i.e. the change in direction of a beam of light at the transition from one medium to another. It is formed from the phase velocity of the light in a vacuum expressed in terms of the phase velocity of the light in the medium concerned.

The refractive index is of practical importance in the case of Desmopan® in so far as, when compounded in blends, TPU will only produce a transparent compound if the refractive indexes are fairly close together. The refractive indexes of Desmopan® are between 1.52 and 1.56.

The measurement methods referred to above, and also further measurement methods, are available to our customers in the Thermoplastics Testing Center (TTC).