Electrostatic spray coating of large pipes - produces highly fluid coat giving firm bond and long life

摘要

A method of producing a plastics coating on a metal object, by pre-heating of the metal and applying the plastics in powder form and electrostatically charges, is carried out in such a way that the quantity of powdered plastics applied per unit area of metal surface and the degree of heating are so related to each other that the resultant melted coating is sufficiently fluid to entirely wet the surface before it commences to thicken and set. The preferred method esp. for coating large diameter metal pipes, consists of moving the powder-spraying unit and pipe raltive to each other, and so adjusting the feed speed to the degree of heating and the powder application rate that the desired fluid spread of the plastics is obtained. Pref. epoxy resin powder is used, at an application rate such as to produce a coating thickness of approx. 100 micron. The process is used esp. for coating large dia. metal pipes. It has been found that the effectiveness of the coating against long term corrosion is a function not of its thickness but of the effectiveness of the bond between coating and metal. A prerequisite for this satisfactory bond is complete wetting of the metal surface by the plastics. This is achieved by having the plastics in a highly fluid state.