No abstract

Paint films used to protect meta lic surfaces are commonly polymeric in nature. The extent of protection offered by tic electrical resistance behaviou, and its effect on impeding local electrochemical processes. In the present work a range of polymeric coatings have been produced with systematically varied crosslinked density using an ultra-violet light curing technique. Their electrical resistance behaviour in an environment of varying concentrations of K C I electrolyte has been examined. It is demonstrated that there are signs of the beginnings of a mechanism changeover from "D-type" t o "I-type" behaviour a t higher levels of crosslink density thus giving some tenuous support t o previously unproven hypotheses in this area.film depends on many factors including the characteris- INTRODUCI'IONThere has been some interest over the last decade as to ihe role of crosslink density in determining the ability of a coating t o protect metallic surfaces from corrosion. When coatings are detached from their substrate and their electrical resistance measured in varying concentrations of surrounding electrolyte, two types of characteristic behaviour are observed. If the electrical resistance decreases with increasing molar concentration of electrolyte the behaviour has been labelled 'D-type'.' If the electrical resistance increased the behaviour has been labelled 'I-type'. Evidence has been pro-ferred2 that predominantly 'I-type' areas within a given coating may be considered as capable of imparting good corrosion resistance to a mild steel substrate on which it may be situated. In this present work a series of ultra-violet light curable coatings cured t o different cross-linked densities have been used as model compounds t o examine these effects in more detail even though such coatings are not widely used in a metal protection sense at the current time. EXPERIMENTAL WORK MaterialsThe ultraviolet coating was designated RL22/35 and supplied as a proprietary product by Donald MacPherson & Co. The formulation is based on (a) (b) (c) (d) (e) solvent -12% w/w acrylated epoxy resin -30% w/w acrylated epoxidised soya bean oil -30% w/w conventional resin (40% phthalic anhydride, 40% dehydrated castor oil, 20% glycerol) photo initiator (benzoin isobutyl ether) -8% w/w Coatings of varying thickness were applied t o ultrasonically solvent cleaned aluminium and later detached for electrical resistance measurements. A spin coating technique was used to produce the films in each case. Crosslinking ProcedureThe coating samples were exposed t o ultra-vioiet light using a Wallace Kliight 'lab-cure' unit. The radiation peaks were produced at 254, 366 and 300 (smal1)nm. Dosage variations were obtained by varying the conveyor belt speed transporting the sample panels beneath a medium pressure mercury arc lamp operating at 80 W/cm. Crosslinking DeterminationCured, detached films of thickness 1 1 , 2 1,30 and 38 pm were examined by transmission infra-red spectrophotumetry using a Unicam SP200G instrument. Three represelitative selected areas ...