Estimation of gel fraction of polyethylene cross-linked with silane by far-infrared absorption spectroscopy

Far-infrared absorption spectra were measured for low density polyethylene in a frequency range from 3.0 to 120 THz. When the polyethylene was cross-linked using vinyl silane, the absorption was found to increase in proportion to the cross-linking degree at 13.2, 30.9, and 32.7 THz, where absorption attributable to Si-O bonds appears. Therefore, the degree of cross-linking of polyethylene by vinyl silane can be estimated by far-infrared absorption spectroscopy.

1 Introduction

Low density polyethylene (LDPE) has been used widely for electrical insulation in the power cables. Its physical and electrical properties can be changed dramatically by introducing a cross-linked structure. For example, thermal durability can be improved [1]. Therefore, it is important to know the degree of cross-linking of polyethylene. Regarding this, the “gel fraction” or solvent extraction method has been used to estimate the cross-linking degree. This conventional method measures the fraction of gel insoluble in a solvent. Therefore, it is a completely destructive method [2]. Moreover, this method uses xylene, which is flammable and toxic, and it needs various time-consuming procedures. In that sense, spectroscopic methods such as Fourier-transform far-infrared (FT-FIR) spectroscopy and FT mid-infrared (FT-MIR) spectroscopy are much safer and easier-to-use.