Chemistry Reference
In-Depth Information
content of ESO is much higher, at higher EVO concentrations the HDTs and
strengths of the epoxidized lard oil blends decreased more rapidly than
those of the ESO blends.
38
EPO is richer in saturated fatty acids and thus possesses lower oxirane
content and has a greater plasticizing effect than ESO. EPO-DEGBA blends
showed significantly reduced T
g
with an increase in EPO concentration.
Polymer blends also showed higher coecients of thermal expansion and
tan d due to increased free volume and chain flexibility in the cross-linked
networks.
136
To retain thermal and mechanical performance, a low EPO
concentration,
r
10 wt%, is necessary.
131
ELO shows the highest oxirane content among common EVOs. Miyagawa
et al.
137
observed that cross-link densities, T
g
s, and storage moduli of
MHHPA cured diglycidyl ether of bisphenol F (DGEBF) systems remained
relatively constant or were slightly decreased at up to 70 wt% ELO loading
and then started increasing again upon further increase in the ELO content.
This abnormal phenomenon was ascribed to higher oxirane content of ELO
such that more curing agent was required for proper formulation. In add-
ition, ELO is rich in linolenic acid content that facilitates dense cross-linked
structures. Thus, it was found possible to replace petroleum-based epoxy
with ELO while still maintaining high performance.
However, amine-cured ELO-DGEBF systems showed a completely
different trend.
138
The cross-link densities, T
g
s, and storage moduli of
blends decreased continuously with an increase of ELO concentration. The
reduction of storage modulus was especially significant and T
g
was close to
room temperature for ELO concentrations of greater than 20 wt%. The trend
is due to the much lower reactivity of ELO with amine curing agent and the
unreacted ELO can plasticize the rigid DGEBF matrix. A decrease of reaction
exotherm, thermal stability and mechanical strength of IPD cured DGEBA
with increase in epoxidized rapeseed oils or ESO concentration was also
reported by Czub
139,140
where polymer blends of high EVO contents were
highly flexible and properties were dominated by the EVO content.
Though cationic cross-linking of EVO generally shows higher reactivity
than curing with amine or anhydride, it results in rubbery polymers since all
the networks are composed of flexible fatty acid components. The addition
of stiffer petroleum-based epoxies, cycloaliphatic or DGEBA can increase the
EVO hardness and modulus.
141,142
Adding EVO into DGEBA as a diluent not
only reduces the viscosity but can shift polymerization temperature lower
since the cationic reactivity of EVO is higher than that of DGEBA. Decker
et al.
91
found that the addition of 20 wt% ESO accelerated the photo-initiated
cationic curing process of DGEBA and formed a relatively tight polymer
network of better chemical resistance. Park
100
found polymerization of
ECO-DGEBA blends initiated using BPH as catalyst had a maximum onset
decomposition temperature at 10 wt% ECO content due to an optimum
network structure. However, further increases in the EVO content still led
to decreased T
g
, thermal stability, and cross-link density and increased
coecient of thermal expansion.
143
