Chemistry Reference
In-Depth Information
reacted with amine less than 2 h showed opacity, i.e., phase separation. More
transparent blends were obtained by synthesizing EPO pre-polymers and
reacting for longer than 2 h at 120 1C. Frischinger and Dirlikov
132
prepared
liquid rubber pre-polymers of EVO with amines, producing rubbery particles
of 15 to 30 wt% rubber, randomly distributed in a rigid DGEBA matrix. Phase
inversion was observed at higher, intermediate rubber contents of 30 to 35
wt%. However, homogeneous morphologies were also observed at either
lower or higher rubber contents, i.e., 470 wt%. The authors concluded that
the particle size and concentration of phase inversion depended on the
miscibility between the rubbery and DGEBA phases, which was regulated by
the nature of the EVO pre-polymer.
9.6.2 Thermal and Mechanical Properties
Although a polymer blend is a simple idea, combination of the advantages of
EVOs and petroleum-based epoxies is not always successful. As with earlier
discussions, lower reactivity and oxirane content generally shift the onset and
peak reaction temperatures higher. At the same time, a decrease of reaction
heat or an increase in E
a
has been observed.
128
Unreactive EVO monomers
and/or an inherently flexible structure plasticize rigid epoxy matrices such as
DGEBA.
133
At high EVO concentrations, the polymerization may occur in two
stages to form heterogeneous structures. EVO components not only
reduce the cross-link density, but also behave as weak points or flaws where
fracture is prematurely initiated by stress concentrations obtained when
mechanically straining non-uniform materials' structures. Adding EVOs
into petroleum-based epoxies has been frequently observed to decrease
mechanical strength, T
g
, thermal stability, and chemical resistance.
134
The concentration of EVO in polymer blends, cost, and acceptable property
loss are important considerations during epoxy formulation. To retain
optimum properties, the concentration of ESO was limited. An addition of
o
40 wt% ESO into DGEBA produced storage moduli and T
g
values comparable
to neat DGEBA polymers, but with a 38% increase in impact strength and
without loss of transparency.
126
An abrupt decrease in T
g
and flexural strength/
modulus at high ESO concentrations, e.g., 450 wt%, was observed by Wang
and Schuman.
57
Degradation of physical and thermal properties appear to be
predominately effected by the incompatibility of ESO, i.e., to form hetero-
geneous structure of less synergy between ESO and DGEBA. A non-linear
transition of properties and lack of synergy between epoxy monomers was
observed in an anhydride-cured ESO-DGEBA system. The Gordon-Taylor
equation was applied to account for the T
g
-composition relationship of this
system. The interaction between ESO and DGEBA was of only medium
strength.
135
The optimum EVO concentration was also related to the structure of the
EVO, especially the oxirane content. The heat distortion temperature (HDT)
and tensile strength of EVO-DGEBA polymer blends were almost identical for
either ESO or epoxidized lard oil at up to the 20 wt% level. Since the oxirane
