Coatings World - April 2009

Business Corner

STRATEGIES & ANALYSIS BY PHIL PHILLIPS

CONTRIBUTING EDITOR PHILLIPS@CHEMARKCONSULTING.NET

High heat resistant coating systems
A look at the
global market
for high heat
resistant
coating
systems.
The first of
a two-part
series.

Certain organic binders, notably phenolics and epoxies, are tolerant of relatively high temperatures without modification, but coatings designed to provide protection against high service temperatures generally incorporate silicon in some form or another. Since the silicon bond requires much higher energy for its disruption than the corresponding carbon bonds in analogous molecules, it is much more resistant to thermal degradation.

Silicone is so effective in this respect that some degree of thermal resistance can be achieved simply by cold blending ten percent or more of a silicone resin with a conventional binder. The temperatures that such a coating will resist are limited to approximately 220°C. Copolymerization, even with modest levels of silicone resins, is more efficient, and can be achieved with, for example, alkyds, phenolics, epoxies, acrylics and saturated polyesters.

Silicone may also be utilized in the form of inorganic silicate coatings, which form a glassy layer on curing, and will react with both masonry and steel substrates to form a tight bond.

At the top end of the performance spectrum, silicone resins which will withstand temperatures above 800°C have been commercially available for more than 50 years. Systems which will withstand more than 1,000°C have been developed, originally for space vehicles, but now with applications in the chemicals industry. However, these pure silicone resins are expensive and their curing via condensation of the silanol groups requires high temperatures. Thus alkyl silicones or the copolymers discussed above are more widely used and are capable of meeting the majority of domestic and industrial requirements.

A means to avoid the time and expense of high-temperature stoving is to formulate “burn-off” coatings. These incorporate a binder system which cures at low or ambient temperatures, bonding the coating to the substrate. The organic binder decomposes when parts are exposed to service temperatures high enough to cure the silicone binder. The system is not ideal, in that its effectiveness will depend upon the conditions under which this secondary curing occurs, and air pollution during this phase is inevitable.