Home > Knowledge > How Easy is it to See Through the Optical Properties of Thermoplastics?

How Easy is it to See Through the Optical Properties of Thermoplastics?

Transparency in thermoplastics is a property that differentiates them from many other manufacturing materials. This includes metals, ceramics and wood, and, in some cases 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. This thereby reduces the amount 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.

transparency composite

Material suppliers and processors can influence levels of transparency in the finished products.   For semi-crystalline thermoplastics, the level of transmitted light goes up as the section thickness decreases. This is not just a factor of thickness but also because of more rapid cooling from the melt (quenching) which suppresses the amount of crystallisation.

There is another trick that film processors can employ. This is to orientate the crystallites in one plane. Stretching the film in two directions as it cools, makes the material more optically homogeneous and reduces 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.

transparency composite

This article was written by Dr.Charlie Geddes for Hardie Polymers.

For more of Charlie’s blogs why not read more of our knowledge base articles?

For information on all the specified grades that we can offer please contact us today or try our polymer search.

Posted in Plastics,

Find the exact grade you require

Improve on your current price

Get the material you need faster

Our no risk Polymer Search

  • We’ll conduct a no obligation search so you don’t have to
  • We can supply the exact grade you need so you can meet your spec
  • Minimum quantity is just 25kg so you can get just what you need
  • If we can’t find your grade of plastic there is no cost to you
  • Products can be dispatched by express shipping if required

Chat to us first as we’re happy to answer any of your questions.


Request a Quote

  • Our team are available from 8am-5pm and we always aim to get back to you the same day. If you would rather contact us immediately please join us on live chat
    or give us a call on 0141 952 1900.
  • This field is for validation purposes and should be left unchanged.