Chemistry Reference
In-Depth Information
1
Use of Rosin Acids as Rigid Building Blocks
in the Synthesis of Curing Agents for
Epoxies
Jinwen Zhang
1.1 Introduction
Epoxy is the most versatile family of polymers because a selected combination of epoxy
resin and curing agent is able to provide a broad spectrum of properties. The high
mechanical and physical properties of properly cured epoxies are mainly attributed
to the high content of the rigid aromatic or cycloaliphatic moiety in the epoxy
molecules, the curing agent molecule or in both. The choice of epoxies is generally
limited to the glycidyl ethers of the bisphenol A type which takes approximately 90%
of the market share. A few other epoxies are based on glycidyl ethers of novolacs,
polyglycols or bisphenol F. In addition, non-glycidyl ether epoxy resins, e.g., cyclic
aliphatic epoxy resins, have found some specialty applications. The properties of a
cured epoxy are greatly influenced by the structure of curing agent (also called the
curative or hardener). Commonly used curing agents for epoxies can be classified into
three general groups: tertiary amines, polyamines and acid anhydrides. Compared with
amine type curing agents, cyclic acid anhydrides are less toxic and usually result in low
curing exotherms and shrinkages of the cured resins. Various anhydrides are used as
curing agents. The majority of this type of curing agents are cyclic anhydrides (either
aromatic or cyclic aliphatic anhydrides) which impart high mechanical and physical
properties to the cured resins but low toughness, whereas aliphatic dicarboxylic acid
anhydrides bring improved toughness.
Figure 1.1
shows several cyclic anhydrides
including 1,2-benzenedicarboxylic anhydride (also called phthalic anhydride, PA),
1,2-cyclohexanedicarboxylic anhydride (CHDA), and 1,2,4-benzenetricarboxylic
anhydride (BTCA or trimellitic anhydride). These anhydrides alone or in combination
with other curing agents are used as important curing agents for epoxies. Some of
them can be further modified through an addition reaction with a polyamine or polyol
to achieve products which have better miscibility with epoxies, reduced softening
points and other improved properties. In addition, these anhydrides are widely used
in the syntheses of other chemicals and polymers. However, current hardeners are all
petroleum-based and are generally synthesised by complicated chemical processes.
