INTRODUCTION
Traditional epoxy tank linings are limited in their applications due to minimum curing temperatures and the need for long cure times at low temperatures before being returned to service. Polycyclamine cured epoxies allow for the production of very high solid content, fast curing single coat tank linings systems that can be both applied at low substrate temperatures and cure rapidly at low temperatures to allow for a fast return to service which minimizes downtime for the asset owner. Furthermore, the use of this technology does not compromise chemical resistance or high service temperature resistance, unlike other low temperature curing systems. This paper discusses the chemistry and performance characteristics of these systems and demonstrates how the use of such systems can save an owner time and money.
Epoxy resin based coatings have been used for many years to line the inside of chemical storage tanks and process vessels. They are particularly suited for services that require resistance to hydrocarbons, water immersion, caustics and certain mineral acids. Generically the coatings are often identified by the terms “epoxy”, “epoxy phenolic (or phenolic epoxy)” and novolac epoxy phenolic”, although is there is no accepted definition of what this means1. The term “phenolic epoxy” is especially abused and has been used for coatings containing Bisphenol A, Bisphenol F and novolac epoxy resins, or combinations thereof. Apart from the inaccurate use of generic terms for epoxy resins, the use of this terminology misses out on a key part of an epoxy lining formulation - the curing agent. There is a relative handful of epoxy resins to choose from when formulating an epoxy coating, but there is a huge choice of curing agents. The experienced tank lining formulator knows that he or she can change the coating properties more through the choice of curing agent than by choice of epoxy resin. Curing agent selection plays a critical role in the chemical resistance, speed of curing, temperature of curing, ease of application and mechanical performance of the finished product.
There are three types of epoxy resin typically used in tank lining formulations - those based on Bisphenol A, Bisphenol F and epoxy phenol novolac resins. All of these resin types are available in a range of molecular weights (& therefore viscosities) and functionalities. Bisphenol F resins have a slightly higher functionality (typically 2.1) and a lower viscosity than equivalent Bisphenol A epoxy resins. Epoxy Phenol Novolac resins have a higher functionality - and therefore can give more highly crosslinked cured films - but also have higher viscosities which makes formulating high solids or solvent free paints more difficult. In principal, increasing the crosslink density will increase the chemical resistance (while reducing flexibility), at least to solvents, of the cured film, so that typically the chemical resistance imparted by these resins is as follows: epoxy phenol novolac resins > Bisphenol F resins > Bisphenol A. However, the reality of chemical resistance is much more complicated than this and cannot be simply defined according to generic epoxy resin types.
