The impact resistance development of
cured epoxy by addition of poly(ethylene oxide)
Sixun
et al. (1994) conducted a study to blend epoxy resin (diglycidyl ether of
bisphenol A or DGEBA) and poly(ethylene oxide) or PEO and cure them with an aromatic
amine curing agent. The epoxy resin had epoxide equivalent weight 185 – 210 and
PEO terminated with hydroxyl group had molecular weight of 20,000; moreover,
the aromatic amine curing agent was 4,4’-diaminodiphenylmethane (DDM). DGEBA
and PEO were first blended at 80oC (above the melting point of PEO)
by continuous stirring. The curing agent DDM was added to the mixture by
continuous stirring until a homogeneous ternary mixture was formulated. The
samples of mixture were cured completely by 80oC for 2 h plus 150oC
for 2 h. The glass transition temperature (Tg) of all samples was
investigated with thermal analysis by differential scanning calorimeter (DSC)
and dynamic mechanical analysis (DMA) and the Fox equation was used to compare
with the Tg – composition behaviors. The results showed that
addition of PEO to the system can decrease Tg of all compositions.
However, the cure reaction was incomplete and there was larger negative
deviation between the experimental and Fox Tgs because of the
dilution effect of PEO as miscible inactive cure reaction, when there was more
PEO content in the blends. The researchers suggested that PEO might make
toughen epoxy resins with low glass transition temperature because of
plasticization effect of PEO.
This study provides investigating
toughen epoxy resins with low Tg by blending PEO in epoxy resin. However, there
are some limitations.
1) The
researchers did not study the crystallization and melting point of the mixture
which could be effect from PEO or thermoplastic polymer blend. The
crystallization could be formed when there is more PEO content in system and it
could decrease and hinder the cure reaction (Guo et al., 2001). In addition,
the melting point may reduce the thermal stability if the cured sample is applied
in an extensively non-isothermal situation.
2) The study
did not inspect the change of functional group by Fourier-transform infrared
(FTIR) in order to determine hydrogen-bonding interaction between the hydroxyl
groups of cured epoxy and the ether oxygen of PEO in the blends. The
hydrogen-bonding interaction may be an important driving force for the
miscibility of the epoxy-PEO blends. This can confirm that cured epoxy-PEO
blends are completely miscible (Guo et al., 2001).
3) This study
used only DSC and DMA to confirm the homogeneous ternary mixture. It should
study the morphology of the mixture by scanning electron microscope (SEM)
(Horng and Woo, 1997) or real-time SAXS measurement (Guo et al., 2001) to
support and confirm the result from DSC and DMA.
The strength of this study is that
the addition of PEO in epoxy resins can improve the main disadvantage of epoxy
resins which is low impact resistance with highly crosslinked structures.
Although there are several modifiers, such as carboxyl- terminated
butadiene-acrylonitrile rubber, amine- terminated butadiene-acrylonitrile
rubber and silicone to improve the problems, these modifiers may make to
toughen epoxy resins with high modulus and Tg. Therefore, the
addition of PEO in epoxy resins may be a good way to improve impact resistance.
References
Guo,
Q., Harrats, C., Groeninckx, G., & Koch, M. H. J. (2001). Miscibility,
crystallization kinetics and
real-time small-angle X-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends of
epoxy resin and poly(ethylene oxide).
Polymer, 42(9), 4127-4140.
Horng,
T. J., & Woo, E. M. (1998). Effects of network segment structure on the
phase homogeneity of crosslinked poly
(ethylene oxide)/epoxy networks. Polymer, 39(17), 4115-4122.
Sixun,
Z., Naibin, Z., Xiaolie, L., & Dezhu, M. (1995). Epoxy resin/poly(ethylene
oxide) blends cured with aromatic amine.
Polymer, 36(18), 3609-3613.
