*3.6. Sorption and Solubility*

*3.6. Sorption and Solubility*  The water sorption for neat resin and its composites prepared with either MMT or PLG at different contents are shown in Figure 6. As expected, results showed that clays induce water sorption in the composites (although this parameter seems not to be affected by the nanoclay type) as it is well The water sorption for neat resin and its composites prepared with either MMT or PLG at different contents are shown in Figure 6. As expected, results showed that clays induce water sorption in the composites (although this parameter seems not to be affected by the nanoclay type) as it is well known that natural clays have a hydrophilic character and are naturally prone to absorbing water [23].

known that natural clays have a hydrophilic character and are naturally prone to absorbing water

8 2.99 + 0.03 2.99 + 0.02 10 2.99 + 0.04 2.99 + 0.05 concentrations.

restorative materials.

**Figure 6.** Water sorption of dental composites prepared with either MMT or PLG at several **Figure 6.** Water sorption of dental composites prepared with either MMT or PLG at several concentrations.

The hydrophilic behavior of composites depends on characteristics of constituents, i.e., organic matrix and inorganic filler. In this regard, unfilled resin exhibited a water absorption slightly below 40 mg/mm3 (which is the maximum value for dental restorative materials stated by ISO 4049), The hydrophilic behavior of composites depends on characteristics of constituents, i.e., organic matrix and inorganic filler. In this regard, unfilled resin exhibited a water absorption slightly below 40 mg/mm<sup>3</sup> (which is the maximum value for dental restorative materials stated by ISO 4049), whereas all nanocomposites exhibited water sorption values slightly higher.

whereas all nanocomposites exhibited water sorption values slightly higher. Interestingly, composites containing PLG exhibited a similar water sorption behavior to that displayed by materials prepared with MMT, although the latter clay was modified organically by a cation exchange reaction between the silicate and methyl, tallow, bis-2-hydroxyethyl and quaternary ammonium chloride in order to reduce the clay hydrophilicity. This could be attributed to the Interestingly, composites containing PLG exhibited a similar water sorption behavior to that displayed by materials prepared with MMT, although the latter clay was modified organically by a cation exchange reaction between the silicate and methyl, tallow, bis-2-hydroxyethyl and quaternary ammonium chloride in order to reduce the clay hydrophilicity. This could be attributed to the presence of two hydroxyethyl groups in the organoclay as suggested by Mucci et al. [8].

presence of two hydroxyethyl groups in the organoclay as suggested by Mucci et al. [8]. Water uptake in dental resin composites occurs by diffusion of water molecules within a polymeric matrix and may cause hydrolytic degradation of the matrix and/or filler matrix interface Water uptake in dental resin composites occurs by diffusion of water molecules within a polymeric matrix and may cause hydrolytic degradation of the matrix and/or filler matrix interface [23], yielding leachable substances, which could be quantified in a solubility test.

[23], yielding leachable substances, which could be quantified in a solubility test. Figure 7 shows the solubility results obtained from composites prepared with either MMT or PLG at several concentrations. It is clear that materials containing palygorskite exhibit a different behavior than that displayed by MMT composites. For instance, when PLG was added to dental resin, solubility decreased at lower nanoclay contents and then practically returned to the initial value at higher PLG concentrations; in contrast, solubility of composites containing MMT decreased monotonically with increasing nanoclay content. Regardless of the above fact, a statistically significant difference was only detected between nanocomposites containing 10 wt.% of clay. Further, it is interesting to note that solubility measurements for all composites (including an unfilled sample) Figure 7 shows the solubility results obtained from composites prepared with either MMT or PLG at several concentrations. It is clear that materials containing palygorskite exhibit a different behavior than that displayed by MMT composites. For instance, when PLG was added to dental resin, solubility decreased at lower nanoclay contents and then practically returned to the initial value at higher PLG concentrations; in contrast, solubility of composites containing MMT decreased monotonically with increasing nanoclay content. Regardless of the above fact, a statistically significant difference was only detected between nanocomposites containing 10 wt.% of clay. Further, it is interesting to note that solubility measurements for all composites (including an unfilled sample) remained below 7.5 mg/mm<sup>3</sup> as suggested by the ISO 4049 [17] standard for dentistry-polymer-based restorative materials.

remained below 7.5 mg/mm3 as suggested by the ISO 4049 [17] standard for dentistry-polymer-based

**Figure 7.** Solubility of dental composites prepared with either MMT or PLG at several concentrations. **Figure 7.** Solubility of dental composites prepared with either MMT or PLG at several concentrations.

#### **4. Conclusions**

**4. Conclusions**  Bis-GMA/TTEGDMA-based resin composites with two different types of nanoclays were successfully prepared and characterized. Results indicate that Tg, Td, depth of cure and water absorption were not greatly affected by the type of nanoclay, while the mechanical properties of dental resin composites depended on nanoclay type and concentration of inorganic filler. In general, MMT composites displayed higher mechanical strength than those shown by resins prepared with PLG, due to dispersion problems as revealed by SEM. Solubility of the composites was also dependent on nanoclay type and the mineral concentration. In general, dental composites prepared in this study fulfilled the minimum depth cure and solubility criteria set by the ISO 4049 standard. In contrast, the minimum bending strength (50 MPa) established by the international standard was only satisfied by dental resins containing MMT. Based on these results, composites containing either MMT or PLG (at low filler contents) are potentially suitable for use in dental restorative resins, although Bis-GMA/TTEGDMA-based resin composites with two different types of nanoclays were successfully prepared and characterized. Results indicate that Tg, Td, depth of cure and water absorption were not greatly affected by the type of nanoclay, while the mechanical properties of dental resin composites depended on nanoclay type and concentration of inorganic filler. In general, MMT composites displayed higher mechanical strength than those shown by resins prepared with PLG, due to dispersion problems as revealed by SEM. Solubility of the composites was also dependent on nanoclay type and the mineral concentration. In general, dental composites prepared in this study fulfilled the minimum depth cure and solubility criteria set by the ISO 4049 standard. In contrast, the minimum bending strength (50 MPa) established by the international standard was only satisfied by dental resins containing MMT. Based on these results, composites containing either MMT or PLG (at low filler contents) are potentially suitable for use in dental restorative resins, although those prepared with MMT displayed better results.

those prepared with MMT displayed better results. **Author Contributions:** Conceptualization, J.M.C-U., Y.V-P. and J.J.E.-A.; Methodology, J.J.E.-A., Y.V-P. and J.M.C-U.; Validation, J.M.C-U. and Y.V-P.; Formal Analysis, J.J.E-A. and J.M.C-U; Investigation, J.J.E-A.; Resources, J.M.C-U., Y.V-P., J.A.U.-C. and J.V.C.-R.; Writing-Original Draft Preparation, J.M.C-U.; Writing-Review & Editing, J.J.E.-A., Y.V-P., J.A.U.-C., J.V.C.-R.; Visualization, J.J.E-A. and J.M.C-U; Supervision, J.M.C-**Author Contributions:** Conceptualization, J.M.C.-U., Y.V.-P. and J.J.E.-A.; Methodology, J.J.E.-A., Y.V.-P. and J.M.C.-U.; Validation, J.M.C.-U. and Y.V.-P.; Formal Analysis, J.J.E.-A. and J.M.C.-U; Investigation, J.J.E.-A.; Resources, J.M.C.-U., Y.V.-P., J.A.U.-C. and J.V.C.-R.; Writing-Original Draft Preparation, J.M.C.-U.; Writing-Review & Editing, J.J.E.-A., Y.V.-P., J.A.U.-C., J.V.C.-R.; Visualization, J.J.E.-A. and J.M.C.-U; Supervision, J.M.C.-U. and Y.V.-P.; Project Administration, J.M.C.-U.; Funding Acquisition, J.M.C.-U. All authors have read and agreed to the published version of the manuscript.

U. and Y.V-P.; Project Administration, J.M.C-U.; Funding Acquisition, J.M.C-U. All authors have read and **Funding:** This work was supported by the Consejo Nacional de Ciencia y Tecnologia (CONACYT).

agreed to the published version of the manuscript. **Funding:** This work was supported by the Consejo Nacional de Ciencia y Tecnologia (CONACYT). **Acknowledgments:** The authors thank Q.I. Santiago Duarte for the SEM micrographs, and W.H.K. for the physicochemical characterization.

**Acknowledgments:** The authors thank Q.I. Santiago Duarte for the SEM micrographs, and W.H.K. for the physicochemical characterization. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
