Trelic’s picture of the week 05/2018

What happened to this material?

 

The material shown in the picture is polyethylene terephthalate (PET), so it’s the same material we wrote in our last week’s blog text. As we mentioned in the blog, PET is widely used in many applications due to its good properties and decent price.

In the picture a piece of PET sheet has been bent over a metallic form and then exposed to organic solvent. In this case isopropanol (IPA) solvent was used. It is often used as a cleaning product. The exposure was done because we wanted to study the environmental stress cracking (ESC) behavior of PET. Lots of small cracks can be seen on top of the PET sheet making it look stripy. These cracks are formed because of ESC.

ESC is a very common reason for catastrophic failures in plastic components. In some publications it has been called the “plastic killer”. In everyday use ESC can sometimes be seen for example as a cracking of plastic cups due to exposure to chemicals and water. When this happens, its mostly a nuisance. However, in industrial applications ESC can be a serious reliability problem. For example a casing of your product could crack due to use of cleaning products.

So what happens in ESC and why does it happen? In ESC polymer or plastic is simultaneously exposed to a chemical agent and tensile stress. The tensile stress may be due to use, but also an internal stress formed during the processing of a plastic part. For example the plastic cup most likely cracks due to internal stresses formed in processing. In industrial use both are possible, making ESC both common and important to consider.

As mentioned above, in ESC solvent and mechanical stress affect a polymer simultaneously. In some cases solvents may broke the polymer chains of plastics and cause permanent damage. This is what happens in hydrolysis discussed in our last week’s blog text. However, in ESC the solvent does not cause direct molecular degradation of the plastic. Instead, the solvent penetrates into the plastic and interferes with the intermolecular forces between the polymer chains. This leads to accelerated molecular disentanglement. In other worlds, the polymer chains in the plastic can move much more easily. This leads to formation of cracks, to their growth, and eventually fracture. The basic principle of the process is shown below.

 

 

What makes ESC so dangerous is, that the ESC failures are caused by brittle fractures, even with materials which could be assumed to have ductile behaviour. This means that such cracking can be very sudden and cause totally unexpected failures.

This week’s picture shows one way to study the probability of ESC happening. There are also other standardized methods. If you are dealing with conditions and materials where ESC may be critical,it is useful to test how easily the ESC happens. If you are interested in ESC testing and analysis, we are happy to help!