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.
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.
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 aromatic Desmopan - grades are between 1.52 and 1.57, depending on their formulation.
The use of aliphatic isocyanates are lowering the refractive indexes. Thus aliphatic Desmopan - grades based on HDI are showing refractive indexes between 1.49 and 1.50, while the refractive indexes of aliphatic Texin - grades based on Desmodur W are around 1.51.
The measurement methods referred to above, and also further measurement methods, are available to our customers in the Thermoplastics Testing Center (TTC).
Dispersion and the Abbe number
The Abbe number (formula character: V) is a dimensionless measure used in optics for the dispersion of a transparent medium. The lower the Abbe number, the greater the dispersion.
It is defined as:
A low-dispersion material has a high Abbe number. The reciprocal value of the Abbe number is often referred to as the relative dispersion.
Optical dispersion means that the refraction of the light is a function of the wavelength (and thus of the color). This dispersion is the reason that a ray of white light striking the edge of prism is separated into a color spectrum. Dispersion occurs any time light is defracted, however. Dispersion also occurs whenever a ray comprising multiple wavelengths of light is supposed to be depicted at a single point. The depiction at a single point is a crucial prerequisite for the projection of sharp images by an optical system, such as a lens. In lens systems, dispersion causes to a different degree color fringing around images of bright objects This imaging error is called chromatic aberration.
To completely describe the disperion of a material (e.g. of a type of glass), one must indicate how the defraction index n of the material changes with the wavelength λ of the light. The complete functionn(λ) must therefore be indicated. For simple calculations, it is often sufficient to describe the dispersion in the visible light range with just the Abbe number.
The Abbe number defined for wavelengths in the visible light range is unsuitable In the infrared and ultraviolet ranges.
The following table shows the refractive index measured at 589.3 nm and the associated Abbe number of several Desmopan grades that are potentially suitable for optical applications.