Page 81 - Machinery Component Maintenance

66 Machinery Component Maintenance and Repair

There are other liquid resins such as phenolics, polyesters, acrylics,
etc., which cure in similar fashion, but the epoxy resins possess a rather
unique combination of properties. The liquid resins and their curing
agents form low-viscosity, easy-to-modify systems. They can cure at
room temperatures, without the addition of external heat and they cure
without releasing by-products. They have low shrinkage compared to
other systems. They have unusually high bond strengths, excellent chem-
ical resistance, high abrasion resistance, and good electrical insulation
properties. The basic properties can be modified by blending resin types,
by selection of curing agents (hardeners), the addition of modifiers, and
by adding fillers.
Perhaps the most valuable single property of the epoxy resins is their
ability to cure, thus converting from liquids to tough, hard solids. This is
accomplished by the addition of a curing agent. Some agents promote
curing by catalytic action, while others participate directly in the reaction
and become part of the resin chain. Depending upon the particular agent,
curing may be accomplished at room temperature with heat produced by
exothermic reaction, or may require application of external heat. The
epoxies will react with over 50 different chemical groupings, but the ba-
sic curing agents employed in the epoxy resin technology are Lewis
bases, inorganic bases, primary and secondary amines, and amides.
An entire spectrum of properties can be obtained in a cured epoxy resin
system by careful selection of resins, careful selection of curing agents,
varying the ratio of resin to curing agent and by including additives or
fillers. The resins and curing agents, themselves, may even be blends.
As an illustration of the spectrum of obtainable properties, a cured epoxy
system may be as soft as a rubber ball or so hard that it will shatter when
dropped. Epoxies can be formulated to be either sticky or tack free. They
can be formulated to either melt or char when heated; to release tremen-
dous amounts of heat when curing or they may require heat for curing; to
bond tenaciously to sandblasted steel, even under cryogenic conditions,
or have relatively little bond; or to be either tough or friable.

Epoxy Grouts
Grout is a broad term covering all of those materials used in a wide
variety of applications which include chinking for cracks, fissures, or
cavities; a mortar for tile and other masonry; a support for column foot-
ings; a sealant for built-in vessels; or a mortar for setting heavy machin-
ery. This text, however, is concerned with those epoxy-based materials