(HDPE) Outdoors Application
Note: the following analysis and its write-up are property of The Madison Group and cannot be copied and/or distributed in anyway without prior permission from The Madison Group. This analysis in no way suggests that any or all plastic failures occur in the manner described. Each plastic failure is unique and should be treated as such.
Polyethylene without additives is a grade that can easily degrade when exposed to the environment. UV light generated by the sun can quickly affect the molecular structure of the part as the light penetrates into the material and starts breaking the molecular chains. As the molecular chains break the part weakens to the point where the material can turn into dust. A typical additive used to protect outdoor applications is carbon black. In most applications, carbon black in concentrations of 3% or more are known to protect HDPE from the sun.
An analytical test that can be used to detect degradation is the FTIR (Fourier Transform Infrared Spectroscopy). FTIR spectroscopy was performed at the surface and core of the HDPE sample. The FTIR shows the typical spectral results expected for polyethylene. However, the spectral shows two additional absorption bands (one between 1750 cm-1 and 1700 cm-1, and another between 1300 cm-1 and 1100 cm-1) that indicate the formation of carbonyls and byproducts associated with oxidation. The FTIR performed at the surface of the part shows stronger absorption bands compared to the FTIR at the core. Therefore, the level of oxidation at the surface is much higher than the oxidation at the core of the part. The large difference between oxidation at the surface and core is typical of degradation caused by ultraviolet light, as opposed to thermal degradation, which tends to degrade a material uniformly through the thickness.
Further, a melt flow rate (MFR) test was performed to get a qualitative idea of the molecular weight of the part. The melt flow rate for the failed part was higher than 150 grams/10 minutes. The material specified for this application was an HDPE with MFR of 15. This large difference between specified and tested MFR is due to the molecules breaking because of material degradation. As the molecules break, the molecular weight drops dramatically, thus the melt flow rate increases. The MFR test was further evidence of material degradation, confirmed to be consequence of UV light by the FTIR.
The corrective action recommended for this application was to improve the UV stabilization package.