Analysis of polyethylene-based cold end coatings on glass containers with contact angle and IR spectroscopy

The aim of cold end coating is to protect the glass surface from abrasive w ear with the help of a suitably anchored film of organic material which dec reases friction, by preventing direct glass to glass contacts. At present, routine controls on cold end coatings are limited to a lubricity test, i.e, the determination of the slide angle. A laboratory procedure based on the determination of the contact angle with water is described in this work. It allows one to estimate the coverage with cold end coating locally from the water-repellence caused by the organic deposit. This method can be used to investigate a range of problems associated with glass wettability, includi ng label adhesion, acid etching and washing residues. The contact angle cor relates well with the slide angle. Qualitative information on the nature of cold end coatings is obtained by extracting the glass with volatile organi c solvents such as methylene chloride, evaporating them and analyzing the r esidue with infrared spectroscopy. In the case of a coating with RP 40 LT t he IR curves show the presence of polyethylene partially oxidized in order to improve adhesion to the glass surface, besides polyethylene glycol added as an emulsifying agent. If the cold end coated container is rinsed with w ater, the glycol dissolves, but the polyethylene derivative remains strongl y anchored. Such bond withstands even drastic washings with water and deter gents, alkali or oxidizing acids. Even when the glass is extracted with org anic solvents, only a fraction of the deposit is dissolved. Therefore, even a quantitative analysis of the IR spectrum of the extract provides values smaller than the true coverage due to incomplete extraction.