218 / 2018-09-21 12:50:13

Origins of Chemiluminescence in Polymeric Insulating Materials

Polymer,degradation,oxidation,aging,polyethylene

Industrially synthesized polymers such as polyethylene are important materials for our daily life in many fields. One of inevitable weak points of organic polymers is oxidative degradation. That is, if organic polymers are exposed to high temperatures or high energy radiations for long periods, they are oxidized and their various properties such as electrical and mechanical ones are deteriorated.
For detecting oxidative degradation induced in polymeric insulating materials, a spectroscopic analysis method called chemiluminescence (CL) is known to be adequate. It has widely been believed that singlet molecular oxygen and electronically excited triplet carbonyl products are two luminous sources. To the authors’ knowledge, almost all papers dealing with CL in organic polymers, the above two sources are assumed a priori. To confirm the validity of this assumption, CL was measured for epoxy resin (EP), silicone rubber (SiR), and a variety of thermoplastic polymers such as polyethylene. Note that SiR is an intermediate between organic and inorganic. Several kinds of instrumental analyses including Fourier-transform mid-infrared absorption spectroscopy (FT-IR) were also conducted on these materials, before and after the CL measurement. As a result, the following two important results were obtained.
1) The CL appears in all the materials such as low density polyethylene (LDPE), cross-linked polyethylene (XLPE), flame-retardant ethylene propylene diene copolymer (FR-EPDM), EP, and SiR as shown in Fig. 1a.
2) It has become clear that carbonyl groups appear after the completion of CL measurement in each material except for EP and SiR as shown in Fig. 1b.
These results indicate strongly that CL is also induced by other reactions than oxidation of carbon atoms that induces carbonyl groups abundantly. In our previous research, we confirmed that cross-linked structures are formed through the formation of Si-O-Si chemical bonds between two adjacent siloxane backbones. Although the formation of the Si-O-Si bonds is a kind of oxidation, it is quite different from what we imagine in relation to the oxidation of organic polymers. The above-mentioned result indicates that the formation process of the cross-linked structure in SiR by bridging two neighboring siloxane backbones via oxygen and the curing process of EP as well induce the CL.