TWI Technology Briefing 423 - August 1990
S M Stevens
Scope
Engineering thermoplastics are increasingly used to replace metals in a variety of applications, as they are light, flexible, tough and corrosion-resistant. The properties of semi-crystalline polymers such as polyethylene (PE) and polypropylene (PP) are determined by a number of factors, including the crystallinity and molecular conformation. Crystallinity is dependent on the thermal history of the polymer and would therefore be expected to be influenced by welding.
However, few data exist on the crystallinity developed in welds, and it is difficult to determine this by established techniques. This report describes an evaluation of Fourier transform infrared (FTIR)-microspectrometry for the quantitative determination of crystallinity in PE and PP parent materials, and the subsequent application of the method to PE and PP hot plate and vibration welds.
Main conclusions
- Comparisons with measurements on parent materials using density (for PE and PP) and differential scanning calorimetry and X-ray diffraction (for PE only), have shown that FTIR-microspectrometry is suitably accurate for thedetermination of crystallinity and helical content in small sample areas of PE and PP, respectively.
- Following definition of appropriate sample dimensions, crystallinity variations were shown to exist across the welds studied, the total ranges being some 55-65% for the medium density PE (MDPE), 73-78% for high density PE (HDPE),and 50-65% for the PP. Overall, reproducibility of measurement using FTIR-microspectrometry was between 0.2 and 0.7%, while agreement between consecutive measurements was 0.1% for PE and 0.2 to 0.3% for PP.
Recommendations
FTIR-microspectrometry should be used for the investigation of chemical changes in weld areas in PE and PP, in order that these may be correlated with welding parameters and weld performance.
