Electrostatic phenomena in polyethylene
Measurement of the electrostatic nature of materials is an essential step towards understanding the underlying processes and to the eventual modification of charge behaviour. Indirect monitoring was achieved by resistivity measurements, however, this approach is limited to studying the material and not the electrostatic charge and is, in general, not a satisfactory method. Electrostatic field meters allowed the electrical behaviour of polyethylene to be monitored; its charge accumulation and decay processes were accurately monitored, thus giving a good information base for material modification later in the programme. The use of a data-logging system (based on the Apple//e computer) allowed the manipulation and long term storage of information, whilst giving improved presentation of results. High Density Polyethylene is an intrinsically insulating polymer whose electrostatic properties can be greatly modified by the use of internal antistats. Such additives migrate to a surtace and become effective [at reducing surtace resistivity] over a period of time depending on the diffusivity of the antistat species, and on the external atmospheric conditions, particularly humidity. The effusion of antistat onto a surface was monitored by surtace analysis techniques including X-ray photoelectron scattering, goniometry and infra-red spectroscopy, however, whilst such techniques offered a direct indication of the surtace state and the effect of any surtace modification, electrostatic testing was found to be the most sensitive technique for following antistat action. It is possible to tailor individual antistats to specific applications, however such usage provides only specific protection (fast acting but short lived, or slow acting but long lived). Two single antistat species were combined, and a synergistic effect was obtained for fast acting and long lived systems. It was found that antistats operate by welting (coating) an insulating surtace, and then absorbing atmospheric water vapour, thus lowering the surtace resistivity. Complete surtace coverage is not possible over an HDPE surtace due to its low surtace energy (28mNm'1), however, more complete surtace coverage was achieved by surtace oxidation [raising the polar surlace energy). Natural HDPE is not readily susceptible to surface oxidation; therefore oxidation treatment had only a limited effect. An addition of the fully compatible polymer LLDPE raised the oxidation level and allowed more complete antistat coverage, thereby improving antistatic activity and raising the level of protection gained by using a standard level of antistat. A patent is being taken out covering the use of LLDPE in conjunction with mixed antistat systems.