Page 423 - Handbook of Adhesives and Sealants

366 Chapter Ten

dency toward lower peel strengths. Anhydride cured epoxy adhesives
when cured at elevated temperatures have lower cure shrinkage than
most other epoxy systems. The anhydride component must be kept
covered when not in use because it will react with the moisture in the
air and result in poor physical properties. Common anhydrides are
phthalic anhydride, nadic methyl anhydride, and chloroendic anhy-
dride.
Epoxy can also homopolymerize using strong acids or strong bases.
Certain Lewis acids, such as boron trifluoride monoethanolamine
(BF3), can polymerize epoxy resins, resulting in high temperature re-
sistance and gel times of less than 60 sec. However, low peel properties
5
and poor impact resistance result. The cured products have high tem-
perature and chemical resistance, but are brittle without modification
to the formulation. The fast rate of cure is often considered a liability
because of high exotherm, inconsistency in mixing, and application
problems.
Single component epoxy systems usually work in one of two ways.
In one method, a curing agent is used which is mostly insoluble in the
resin and, therefore, inactive at room temperature. The curing agent
liquefies and diffuses into the resin at the cure temperature, thereby
activating the cure mechanism. An example of this type of curing
agent is dicyandiamide. It can cure in 40 mins at 250 F and has a 6
month shelf life at room temperature. Dicyandiamide reacts with the
epoxy only on heating, and the reaction stops on removal of the heat.
There are several popular single component epoxy adhesives that use
dicyandiamide as the curing agent. Another mechanism for achieving
a one part system is to have the curing agent chemically blocked.
An example of this is cyanourea compounds such as toluene bis-
dimethylurea that cures epoxy formulation in 60 mins at 250 F. These
single component epoxies normally have better environmental resis-
tance than do two component, elevated temperature curing adhesives.
Imidazoles are also curing agents that are appropriate for single
component epoxy systems. Like dicyandiamide, the imidazoles react
with epoxide groups and provide catalytic activity resulting in high
heat resistance.

10.3.2 Epoxy hybrids
A variety of polymers can be blended and co-reacted with epoxy resins
to provide certain desired properties. The most common of these are
nitrile, phenolic, nylon, and polysulfide resins. These blends can take
a number of different forms. The added resin may be reacted with the
epoxy resin, or it may be included as an unreacted modifier. The mod-
ifier may be blended into a continuous phase with the epoxy resin or
precipitated-out as a discrete phase within the epoxy resin matrix.