The effect of hydrogenation of epoxy resin on the dispersion of organo-clay in the epoxy resin matrix

The hydrogenated diglycidylether of Bisphenol A epoxy resin (HDGEBA)was successfully employed to prepare nanocomposites with a more homogeneous distribution of clay, compared to that of bisphenol A epoxy resin (DGEBA)/clay system. Nanocomposites, with amounts up to 7.5 wt% of Organo-clay, were synthetized by means of “slurry-compounding” method and followed by a curing process with cis-1,2-Cyclohexanedicarboxylic anhydride and Glutaric Anhydride as the curing agent. A combination of X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the dispersion behavior of organo-clay in epoxy/clay nanocomposites. It was found that, in HDGEBA/clay nanocomposites, organo-clay was uniformly dispersed and even partly exfoliated, whereas, in DGEBA/clay system, large particle aggregates were seen when examined by TEM under lower magnification. Accordingly, the rheology and compatibility experiments were carried out to investigate the interactions within each system. It turned out that, after hydrogenation, HDGEBA was endowed with stronger interactions with organo-clay, thus resulting in the enhanced dispersion behavior, which may generate more chances to be mechanically reinforced by adding inorganic clays.


Introduction
Epoxy resins are one of the most commercially successful materials known especially as composite matricx but also as coating materials and adhesives [1]. Among them, resins based on the diglycidyl ether of bisphenol A (DGEBA) are particularly well studied and most widely used.
After decades of successful production, this kind of epoxy resins now confrontsan intractable safety issue. Concerns have been growing that bisphenol A(BPA), which constitutes the backbone of DGEBA, may adversely affect humans [2]. BPA is an endocrinedisruptor that has been shown to be harmful in laboratory animal studies.Due to itswidespread human exposure and toxicity, Restrictions on the use of BPAin certain consumer products, ranging from manufacture, sale and distributions, have been suggested and implemented in many countries [3].Accordingly, a series of researches looking for alternatives have been carried out since then.
Many candidates alternatives have been proposed and studied, among them, Bisphenol S (BPS) and bisphenol F (BPF) are endowed with high hopes. Nonetheless, recent study shows that neither of them are safe replacements of BPA [4]. The bisphenol series of A, S and F are reported to be unsafe for human health, all of which contains aromatic rings in their backbones. These results remind us of the fact that the toxicity may derive from the aromatic rings that constitutes the backbone of the polymer. Considering this situation, we seek to hydrogenation of the BPA to transform the aromatic rings to cyclohexyl groups, thus eliminating the origins of toxicity.
The absence of aromatic rings will surely weaken the mechanical properties of the epoxy resinsthemselves, but, on the other hand, the hydrogenated diglycidylether of Bisphenol A epoxy resin (HDGEBA) turns out to be a good polymer matrix to well disperse stacked clays in, which will in return generate more chances to compensate the mechanical strength loss by adding inorganic clays.
The system of epoxy-claynanocomposites has been intensively investigated and commercialized for their unique physical and chemical properties, such as high modulus, high thermal stability, decreased flammability, and barrier properties [5][6][7][8][9][10]. The final properties of nanocompositesrely on the quality of clay dispersion in the polymer matrix.
A fully exfoliated structure is most desirable but the most difficult to get.An effective approach to improve the degree of clay exfoliation is to adjust polymer/clay interaction.In our previous work, the effect ofpolymer/clay interaction on the clay exfoliation, clay orientation and disorientationwas well elucidated by the rheology experiments, which has attracted considerable interest recently [6,[11][12][13][14][15][16] One frequently used strategy is to modify inorganic clay using organic additives in order toameliorate the compatibility between polymer and clay [17].The hydrogenated resin exhibits lower polarity than DGEBA for the presence of flexible cyclohexyl groups instead of rigid aromatic rings [18]. Due to the lower polarity of the hydrogenated resin, HDGEBA is expected to be more compatible with the organically modified clay.Il-Nyoung and coworkers found that nanomaterial reinforced HDGEBA/D230 adhesives were transparent [19]. María and co-workers reported that magnetite nanoparticles of about 10 nmin diameter modified with oleic acid exhibit a good dispersion in the matrix of HDGEBA [18].
In our study, we managed to adjust the polarity of the polymer to be more compatible with organo-clay by hydrogenating the unsaturated bonds of the aromatic rings in BPA.The hydrogenated diglycidylether of Bisphenol A epoxy resin (HDGEBA) wassuccessfully made and then employed to prepare nanocomposites with a more homogeneous distribution of clay, compared to that of bisphenolA epoxy resin (DGEBA)/clay system. Nanocomposites, with amounts up to 7.5 wt% of Organo-clay, were synthetized by means of "slurrycompounding" method and followed by a curing process with cis-1,2-Cyclohexanedicarboxylic anhydride and Glutaric Anhydride as the curing agent.The enhance dispersion of clay in HDGEBA was investigated with a series of experiments.
In this article, we focus our attention on the effect of hydrogenation of epoxy resin on the dispersion of organo-clay in the epoxy resin matrix. Organo-clay is found more easily to get exfoliated in the matrix of hydrogenated epoxy resin by a combined characterization of X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, we find there is stronger interaction between HDGEBA and organo-clay rather than DGEBA 5 and organo-claywith the help of compatibility and rheology experiment.

Preparation of epoxy/C18-claynanocomposites
The nanocomposites of resin/clay were synthetized by the means of "slurry-compounding" according to the method reported by Miyuki and co-workers [21].
The dried C18-clay was added to dimethylacetamide (DMAC, Sinopharm Chemical Reagent Co., Ltd (China))to prepare the slurry-clay. The weight ratio of C18-clay and DMAC was kept constant at 1:3. The mixture was stirred at 70 o C and 200 rpm for 3 h.
The mixture of the slurry-clay and epoxy resin was stirred at 70 o C and 300 rpm for 10 h. However, the XRD pattern of HDGEBA/C18-clay displays no obvious (001) basal reflection peak, indicating possible exfoliation of the clay. The absence of the (001) peak is not due to the low quantity of clay in the nanocomposite, as the (060) in-plane reflection peak of the single-clay sheet could still be detected (the inset of Fig. 2a).
When we increase the clay content to 7.5 wt%, both the XRD patterns of the DGEBA/C18clay-7.5 wt% and HDGEBA/C18-clay-7.5 wt% show a weak and broad hump at around 2.0 o (2θ), indicating the clay was intercalated rather than exfoliated. The hump on the XRD pattern of HDGEBA/C18-clay-7.5 wt% is weaker and appears at a smaller angle, indicating the clay in the nanocomposite of HDGEBA/C18-clay-7.5 wt% is closer to the state of exfoliation, compared to that of the DGEBA/C18-clay-7.5 wt%. Therefore, these experiments clearly indicate that the hydrogenation of the epoxy resin has a significant 8 effect on the exfoliation of the organo-clay.
As XRD generally fails to distinguish the disordered intercalated and exfoliated morphology, TEM measurements have been further performed to observe the dispersion morphologies of clay directly. The TEM micrographs of the HDGEBA/C18-clay (7.5 wt%) and DGEBA/C18-clay (7.5 wt%)nanocomposites are shown in Fig. 3. Figure 3a is a low magnification version, in which the disorderly exfoliated small cluster structure of the HDGEBA/C18-clay (7.5 wt%) was observed. Moreover, both stacked (intercalated) and isolated (exfoliated) silicate layers in the sample of HDGEBA/C18-clay (7.5 wt%) can be observed at high magnification versions as shown in Fig. 3b and Fig. 3c. The large clay blocks in DGEBA/C18-clay (7.5 wt%) were observed in Fig. 3d, which is a low magnification version.
We can see that in the sectioned samples of the C18-clay epoxy nanocomposites, the clay aggregates are fewer and well dispersed in the HDGEBA/C18-clay matrix than in the DGEBA/C18-clay matrix. These TEM observations are consistent with the XRD results above. Thus, one can get the conclusion that the clay dispersed in the nanocomposite of HDGEBA/C18-clay is easier to get exfoliated, compared to that of the nanocomposite of DGEBA/C18-clay.

Discussion of polymer/clay interaction
It is well-known that the polymer/clay interaction is a key factor in controlling the clay exfoliation. The epoxy/C18-clay interactionis studied by compatibility experiments and rheological measurements in our work.
The results of compatibility experiments carried out between epoxy resin and C18-clay are shown in Fig. 4. It can be observed that C18-clay can uniformly disperse in the matrixes of HDGEBA even at 150 o C, while the mixture of DGEBA/C18-clay exhibit a separation state since the temperature of 100 o C. From the results of compatibility experiments, one can infer that there is a stronger interaction between HDGEBA and organo-clay rather than DGEBA and organo-clay.
The viscosities of two epoxy resin and their C18-clay nanocomposites (with 5 wt% and 10 wt% of C18-clay) were measured and the results are listed in Table 1. The viscosities of DGEBA/C18-clay (5 wt%) and DGEBA/C18-clay (10 wt%) increased only 2.72 and 3.73 times, respectively, if compared with that of the pure DGEBA, indicating the absence of strong polymer/clay interaction in these two composites [6,14]. While the viscosities of HDGEBA/C18-clay (5 wt%) and HDGEBA/C18-clay (10 wt%) increased 14.58 and 3.41 × 10 4 times, respectively, if compared with that of the pure HDGEBA, indicating the presence of strong polymer/clay interaction [6,14].Then, it is not surprising that the C18-clay can get a homogeneous dispersion and partly exfoliation in the matrix of HDGEBA because of the existence of strong interaction between HDGEBA and C18-clay due to the hydrogenation effect.

Conclusion
The hydrogenated diglycidylether of Bisphenol A epoxy resin (HDGEBA) was found to effectively exfoliate organo-clay in a clay-polymer nanocomposite. Due to the lower polarity of HDGEBA, the interaction between HDGEBA and organo-clay is stronger than that of DGEBA/organo-clay, which is confirmed by rheology and compatibility experiments.
This result confirms that the match up between the polarity of the clay surface and that of the polymer matrix is an effective strategy to achieve well dispersion..