Major Project (1st draft)

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 80^oC (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 80^oC for 2 h plus 150^oC for 2 h. The glass transition temperature (T_g) 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 T_g – composition behaviors. The results showed that addition of PEO to the system can decrease T_g of all composition. However, the cure reaction was incomplete and there was larger negative deviation between the experimental and Fox T_gs 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 T_g. 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.