This month, polymer expert and technical blog author, Dr Charlie Geddes, discusses the optical properties of different types of thermoplastics.
Transparency in thermoplastics is a property that differentiates them from many other manufacturing materials…metals, ceramics and wood, and, in some cases, even surpasses glass. But not all thermoplastics offer good transparency.
In semi-crystalline thermoplastics, because of the different densities of the amorphous and crystalline regions, and hence different refractive indices, transmitted light gets diffracted and scattered as it passes from one region to the next, thereby reducing the amounted of light transmitted in the normal direction. In highly crystalline thermoplastics, (POM and PTFE) scattering is so high that the material looks almost opaque. Fillers and other additives will generally have different refractive indices and will vastly reduce transparency even in amorphous thermoplastics.
Material suppliers and processors can influence levels of transparency in a finished product. For semi-crystalline thermoplastics, the level of transmitted light goes up as the section thickness decreases, not just a factor of thickness but also because of more rapid cooling from the melt (quenching), which suppresses the amount of crystallisation.
Another trick that film processors can employ is to orientate the crystallites in one plane by stretching the film in two directions as it cools, making the material more optically homogeneous and reducing scattering, for example, biaxially oriented polypropylene film (BOPP). Controlled biaxial stretching during the blowing stage ensures high transparency bottles made from PET, a semi-crystalline thermoplastic that normally cools to give poor transparency.
Impressive improvement in the transparency of polypropylene can be achieved by limiting the dimensions of the crystalline regions to below the wavelength of light, using small quantities of nucleating agents, which create more crystallites but smaller ones.
Occasionally chance plays a part. Polymethylpentene (TPX) is a semi-crystalline thermoplastic, in which the packing of the crystalline and amorphous regions result in the same density and refractive index. Hence TPX has high transparency.