Page 416 - Handbook of Adhesives and Sealants

Adhesives Families 361

Cycloaliphatic resins, epoxidized cresol novolacs, and other resins
derived from the epoxy backbone are used in specialty applications.
These are often used to take advantage of specific properties such as
low viscosity, increased flexibility, clarity, improved color stability, or
2
increased reactivity. New high temperature fluorine epoxy resins may
extend the useful temperature range of these adhesives. Glass tran-
sition temperatures of up to 554 F have been reported for some poly-
mers made from these resins.

10.3.1.2 Curing agents. Epoxy resins are the most versatile of struc-
tural adhesives because they can be cured and co-reacted with many
different resins to provide widely varying properties. Table 10.7 de-
scribes the influence of curing agents on the bond strength of epoxy
adhesives to various adherends. Curing agents affect cohesive
strength, hardness, and durability more than adhesion (controlled pri-
marily by surface energies). Thus, differences in shear strength, peel
strength, and environmental resistance may be attributable to the
choice of curing agent. DGEBA resin can be cured with amines or
polyamides for room-temperature setting systems; anhydrides for el-
evated temperature cure; or latent curing agents, such as boron tri-
fluoride complexes, for use in single component, heat-curing adhe-
sives. The characteristics of curing agents used with epoxy resins in
adhesive formulations are summarized in Table 10.8. Epoxies can un-
dergo various reactions. Some of the more important reactions are
shown in Fig. 10.3.
Polyamide curing agents are used in most ‘‘general-purpose’’ epoxy
adhesives. They provide a room-temperature cure and bond well to
many substrates including plastics, glass, and elastomers. The poly-
amide-cured epoxy also offers a relatively flexible adhesive with fair
peel strength, moisture resistance, and thermal-cycling properties.
Mixing ratios are generally not critical. Within limits the greater the
amount of polyamide in an epoxy formulation the greater the flexibil-
ity, impact strength and peel strength. However, glass transition tem-
perature is lowered as are the shear strength and temperature resis-
tance. There are several polyamides with varying viscosity. The
reaction with conventional DGEBA epoxy resins produces a relatively
low degree of exotherm.
Aliphatic primary amines provide a faster room temperature cure
but have poor elevated temperature resistance. They produce adhesive
systems that have a high exotherm. Primary amines are generally
considered to be a strong irritant and proper ventilation and personal
safety equipment must be employed. The amine/epoxy reaction occurs
at room temperature and it can be accelerated with an elevated tem-