Acrylic sheet and Polycarbonate sheet are two of the most widely used plastics for optical applications.
Acrylic sheet trade names include Plexiglas and Perspex. Polycarbonate sheet trade names include Lexan and Makrolon.
In Part 1 of this occasional series comparing Acrylic to Polycarbonate, we will look at five of the most obvious differences in the properties. Over the coming months we will come back and look at some of the lesser known differences between the materials.
It should be remembered that both materials have their advantages and it is not a case of which one is better. As always, it is important to select the right material for the application.
- Breakage
Polycarbonate is well known for its strength and resistance to impacts. When it is hit with an object it is almost impossible to break. This property makes it ideal for machine guards and protective screens. It is one reason why face shields and protective goggles are often made from Polycarbonate. Front headlights on cars are also often made from Polycarbonate as they can resist damage from stone chips.
Acrylic does not have the same strength and resistance to impacts as Polycarbonate. If it is hit with sufficient force it will shatter.
- Weathering
Acrylic has excellent resistance to weathering. UV light does very little damage to Acrylic over time and so Acrylic is often a good choice for outdoor applications. The rear tail-lights of a car are often made from Acrylic because the colors are very stable and resistant to UV and the potential damage from stone chips is low at the rear of the car. Acrylic has an almost unlimited resistance to weathering.
Polycarbonate weathers when exposed to UV light. This weathering often takes the form of yellowing and micro-cracking of the material. It is possible to reduce the effects of weathering by either adding a cap layer of UV absorbers or a coating loaded with UV absorbers. These solutions do however add to the cost of the Polycarbonate sheet and will only protect the product for 10 to 15 years. There are some advanced solutions to protect Polycarbonate for 25+ years from HighLine Polycarbonate but these are very expensive and are often cost prohibitive for most applications.
- Light Transmission
Acrylic has a light transmission of 92%. Polycarbonate has a light transmission of 88%. The reason for the difference is the refractive index of the two materials. Acrylic has a refractive index of 1.49 and Polycarbonate has a refractive index of 1.585.
If the higher light transmission of Acrylic is an important property for an application and some of the other properties of Polycarbonate are not required, Acrylic is a good choice of material.
However, as discussed in previous posts, it is possible to raise the light transmission of Polycarbonate to 90% with a simple abrasion resistant coating. It is also possible to use advanced anti-reflective coatings to raise the light transmission of Polycarbonate to 98.5%. The choice of which material to use for optical applications depends upon both cost and the other properties that are required.
- Heat stability
Acrylic has a heat distortion temperature under a load of 260 psi of 200 degrees F, whereas Polycarbonate has a heat distortion temperature of 264 degrees F. A full explanation of heat stability can be found under the previous blog post discussing the subject.
Polycarbonate is much more resistant to temperature than Acrylic. This means that if the application involves a higher temperature environment where the structural integrity of the material is required, Polycarbonate may be a better choice. The Heat Stability is also important in vapor deposition of coatings such as Indium Tin Oxide. It is possible to apply more conductive surfaces onto Polycarbonate than Acrylic.
- Scratch and Abrasion Resistance
Acrylic is more resistance to scratches and damage by abrasion than Polycarbonate. This is one of the well-known weaknesses of Polycarbonate. To overcome this problem there are a number of solutions including applying an abrasion or scratch resistant coating. The previous blog post on scratch and abrasion resistance discusses this subject in more detail. Adding a coating to Polycarbonate will also increase the cost of the material.
In conclusion, Polycarbonate sheet has significant advantages over Acrylic in terms of Strength and Heat Stability.
Acrylic has significant advantages over Polycarbonate in terms of weathering and scratch resistance. It is possible to improve both the weathering and scratch resistance of Polycarbonate by a number of methods, but these do add cost.
Acrylic does have a marginal advantage over Polycarbonate in terms of light transmission, but this can easily be overcome in high-tech applications with anti-reflective coatings.
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