Epoxy resins
Chemically, an epoxy resin contains more than one α-epoxy group situated terminally,
cyclically, or internally in a molecule that can be converted to a solid through a thermoset-
ting reaction. The conversion of epoxy resins from the thermoplastic state to tough, hard,
thermoset solids can occur via a variety of crosslinking mechanisms. Epoxies can catalyti-
cally homopolymerize or form a heteropolymer by coreacting through their functional ep-
oxide groups with different curatives. In epoxide technology, curatives are most frequently
called curing agents. Often, the terms hardener, activator, or catalyst are applied to specific
types of curing agents. For most commercial products, the curing agents' chemical struc-
ture is kept proprietary, or the amount of reactive functional group is ambiguous. Epoxy
curing agents can be divided into two major classes: alkaline and acidic. The alkaline class
includes Lewis bases (tertiary amines), primary and secondary aminesand amides, and other
nitrogen-containing compounds. The acidic class of epoxy curing agents includes Lewis
acids (metal halides such as zinc, aluminum, and ferric), phenols, organic acids, carboxylic
acid anhydrides, and thiols.
The properties of epoxy resins can vary over a wide range, depending on the selection of
a formulation's ingredients, their relative proportions, the processing of the formula, and
the configuration and environment of the final product. Some generalization about epoxy
resin properties are possible. Epoxy resins, toppings, and patching materials may be used
for the repair of cracks, spalls, joints, and other problem areas. Epoxies generally cure
within 8 to 12 hours at ambient conditions of 21C (70F). Liquid resins and curatives can
form low-viscosity, easily modified systems. They can cure at temperatures from 40C
(40F) to 200C (392F), depending on the curing agents used. They exhibit very low
shrinkage and do not evolve volatile by-products during cure. Commercially, various kinds
of modified low-temperature formulations are available, some of which can be quite appli-
cable for cold-weather applications.
Furan polymers
The term furan polymer or resin is a loosely defined term. It can be used to denote
polymers based on furfural, furfuryl alcohol, or furan. Fulfural is the starting material for
all of these compounds. The chemical resistance of furan resins has been used for many
years to advantage in chemical cements. Ureaformaldehydefurfuryl alcohol resins for
foundry usage constitute the largest market for furans. This cement system has very good
potential for low-temperature applications.
Furan resins potentially provide the following important advantages: 1) low cost; the
fact that they do not require petroleum-based feedstock should enhance the cost/availabil-
ity outlook for these resins; 2) rapid cure and low-temperature cure; 3) extended shelf life.
The uncatalyzed furan resins have virtually unlimited shelf life. Furan resins (mainly fur-
fural-acetone resin) are widely used in the Soviet Union (now Commonwealth of Indepen-
dent States) and eastern Europe. The furan resin systems are often acid-catalyzed; because
of this, sometimes the bonding with alkaline portland cement systems has been found to be
a problem. The most popular resin used in European countries is formed using a ratio of
1.5:1 furfural to acetone, the main components of which are monofurfurylidenacetone,
difurfurylidenacetone, and furfural. Different percentages of these components in the vari-
ous resins greatly influence the polymerization mechanism, causing noticeable variances
in the properties of the material. The most popular hardener for furfural acetone resins is
benzosulfoacid; n-toluosulfochloride and toluosulfoacid are also commonly used. Chlo-
ride compounds of iron, concentrated sulfuric acid, and amine hardeners are also some-
times used.
A furfuryl-alcohol-based polymer has been developed for achieving high early strength
at ambient temperatures of 52 to 32C (125 to 25F). The polymerization of the furfu-
ryl alcohol was controlled by using a unique combination of α, α, α,-tricholorotoluene
(TCT) catalyst, and zinc chloride promoter in conjunction with a pyridine retarder. The
working time for the polymer slurry can be controlled at >15 minutes over the entire tem-
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