Page 167 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.37

FIGURE 3.35 Flexible curing agents for epoxy resins.

TABLE 3.32 Pot Life of Epoxy/Curing
Agent Systems
Aliphatic amines 1 hr
Amine-terminated polyamides 3 hr
Aromatic amines 18 hr
Acid anhydrides 84 hr
Lewis acids 6 mo

3.1.4.3 Other Formulating Ingredients. A number of classes of additives are often
used by individual formulators to modify or introduce new properties into the epoxy sys-
tem.

3.1.4.3.1 Diluents. When epoxy resins are too viscous or too exothermic for a partic-
ular process, they can be modified by addition of low-molecular-weight aliphatic ep-
oxides. Diepoxides can copolymerize directly into the curing process without reducing
cross-linking. Monoepoxides can also copolymerize but do reduce the degree of cross-
linking and thus soften properties. The literature also mentions nonreactive diluents such
as plasticizers, but these would raise serious questions about degradation of properties.
3.1.4.3.2 Polymer Blends. A number of polymers are mentioned as modifiers for ep-
oxy resins. Coal tar, phenol-formaldehyde, and polyurethane combine readily to produce
intermediate properties. Silicones can add more unique properties. Polyesters and
melamine-formaldehyde are also mentioned in the literature.
3.1.4.3.3 Flame Retardants. Flame retardance can be built into the epoxy resin by
use of tetrabromobisphenol A or anhydride curing agents containing phosphorus or halo-
gen. It can also be helped by nonreactive additives such as alumina trihydrate or organo-
halogens + antimony oxide.
3.1.4.3.4 Functional Fillers. A variety of fillers can be used to add specific proper-
ties. Metals, and beryllium and aluminum oxides, can be added to increase thermal con-
ductivity (Table 3.33). Metals can be added to increase electrical conductivity
(Table 3.34). Graphite increases lubricity and electrical conductivity. Mica increases elec-

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