Examination of crystallinity in polyethylene and polypropylene welds using Fourier transform infrared-microspectrometry

TWI Industrial Member Report Summary 423/1990

By S M Stevens

Background

Engineering thermoplastics are increasingly used to replace metals in a variety of applications, as they are light, flexible, tough and
corrosion-resistant. Thc properties of semicrystalline 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.