CONCLUSIONS
In this thesis, organoclays were prepared in supercritical carbon dioxide and their evaluation as nanofillers in polymer matrices was performed. The efficiency and versatility of scCO2 as a medium for the ionic exchange between natural clays and cationic surfactants was demonstrated, in particular through the preparation of thermally stable organoclays at the kilogram scale.
The organomodification of clays is commonly used to facilitate the delamination of individual nanolayers upon dispersion in a polymer matrix. Present industrial methods of clay modification are water-based processes. However, this medium limits the range of surfactants used to render the clay more organophilic. Therefore, we studied the use of scCO2 to modify natural clay with a large range of surfactants, in particular non water-soluble salts including ammonium, phosphonium and imidazolium salts. Reactions were typically carried out in high pressure reactors at 200 bar, 40°C and for 2 hours. We showed by X-ray analysis that intercalation of organic cation between the clay layers occurs when the salt is in its liquid state at the reaction temperature, with the formation of insoluble sodium halide as the driving force of ionic exchange. Moreover, the addition of a polar co-solvent (preferably ethanol) enabled the intercalation of high melting temperature salts, increasing the versatility of the process. Interestingly, the studied experimental conditions were successfully applied to the preparation of organoclays from two different phosphonium salts in a pilot scale reactor. Both salts (tetraoctylphosphonium or P8 and tetradecyltrihexylphosphonium or P14) are non water- soluble ionic liquids.
Secondly, the influence of the surfactant and clay type on the thermal stability of the organoclays prepared in scCO2 was analyzed by thermogravimetry. The difference of onset temperature of degradation between ammonium-modified clay (A-clay) and phosphonium- exchanged clay (P-clay) with the same surfactant structure can reach 90 K. It was also shown that a gain of thermal stability of 30 to 40 K can be obtained using hectorite clays (HCT) with phosphonium salts as surfactants instead of montmorillonite clays (MMT) modified with the same surfactant.
Thermally stable organoclays were then dispersed in PA-6, a polymer matrix that typically needs relatively high processing temperature (230°C). In a first study, the influence of onium stability on morphology and fire properties was evaluated. We showed by qualitative (XRD, TEM and rheology) and quantitative (NMR) analysis that a fairly homogenous distribution of clay stacks is obtained, with identical degree of dispersion for P8-clay compared to A8-clay (MMT or HCT). Surprisingly, the early degradation of the latter had no negative influence on the morphology of the material. We conclude that favorable clay-polymer interactions of this polar matrix play a major role compared to the organic modifier-polymer interactions. Fire properties were evaluated by cone calorimetry and showed that a decrease of 50 % of the maximum of heat release is obtained for all the organoclay compounds compared to PA-6 alone. The positive role of phosphonium core in nanocomposites is indicated by longer ignition times compared to ammonium-based compounds.
Additional study with this matrix was focused on the effect of clay modification on reinforcement (tensile strength). Therefore, MMT-P14 was compared with commercial, non- modified MMT (Cloisite Na+) and ammonium-based MMT (Cloisite 20A). Despite its lower surface coverage, MMT-P14 showed better dispersion and mechanical properties than the A-clay. We further illustrated that the reinforcing effect of the clays depends on two factors: contact surface and strength of interaction. The first increases with exfoliation but the latter decreases as an effect of organophilization. The calculation of extent of exfoliation based on specific surface area gave the same trend as quantitative NMR analysis: after melt blending with PA-6, clay particles subsist in stacks rather than as individual layers.
The same clays were used in a high performance semi-aromatic polyamide, PA mXD-6, processed at high temperature (250°C). Although MMT-P14-based compound presents a high degree of delamination (small clay stacks), no decrease in oxygen permeability was evidenced. This was explained by a high free volume created by the four alkyl chains of the organic modifier, impeding on one side heterogeneous nucleation as evidence by DSC and, on the other side, generating preferential locations for oxygen molecules to diffuse, up to totally counterbalance the awaited tortuosity effect arising from homogeneous dispersion of clay platelets.
Finally, the process was extended to the preparation of functional organoclays. Monomer- and
scCO2 with the addition of a co-solvent, as proved by X-ray diffraction. Moreover, PDMS- terminated ammonium cations were exchanged without the need of a co-solvent. These organoclays were tested for the in situ polymerization of methyl methacrylate (MMA) in scCO2. We showed that PDMS-clay platelets serve as effective stabilizer for the synthesis of PMMA in scCO2, leading to the formation of spherical microparticles with clay located at their surface. We also reported the first example of atom transfer radical polymerization (ATRP) in scCO2 from initiator-anchored clay surface. Nanocomposite microspheres were obtained with the use of a fluorinated macroligand. While the properties of these materials still need to be studied, we demonstrated the convenience of this environmentally friendly process: no organic solvent has to be eliminated and the nanocomposite is directly recovered as a fine powder.
OUTLOOK
1 Scientific outlook
In this thesis, we demonstrated that the scCO2 process can be applied to the intercalation of a large variety of chloride and bromide oniums in montmorillonite or hectorite clay. Besides, our research enlightened the importance of selecting the right organoclay depending on the host polymer and the targeted application. In particular, for polar matrices and reinforcement effect, a smaller surface coverage of clay by organomodifier has to be favored. For optimum fire properties, phosphonium salts are to be privileged over ammonium salts. As far as barrier performances are concerned, organomodifiers with several long alkyl chains must be avoided.
This work can be continued in several ways: one is extending the range of organoclays, for example to other imidazolium salts. Only a few are commercially available but these oniums could be synthesized according to the procedure described by Awad et al.1. Their specific synthesis from precursors would allow the introduction of desired functionalities to improve clay dispersion, while providing enhanced thermally stability over commercial organoclays.
Another perspective is the testing of MMT-P14 and MMT-P8 in polyolefin matrices, with the aim of improving fire and mechanical properties. A third interesting study would be the use of other types of swellable clays, such as saponite or vermiculite, having different layer structure, size and cationic exchange capacities.
Furthermore, the processing conditions such as type of extruder, extrusion time and shear intensity are also important to favor a large extent of clay exfoliation2. These were not discussed in this thesis but one should keep it in mind while preparing polymer/clay nanocomposites by melt blending. The addition of compatibilizers can also be considered to increase chemical compatibility.
The in situ polymerization in scCO2 is an interesting alternative to reach a good exfoliation.
Efficient pre-exfoliated masterbatches of poly- -caprolactone were prepared via this route in a previous thesis3. Here, we successfully synthesized PMMA nanocomposites in scCO2. As both clay organomodification and polymerization occur in scCO2, a one-pot process can be envisaged. Moreover, the proposed ATRP process should be optimized to prepare polymers with well-defined molecular weight. After synthesis optimization, thermal and mechanical properties should be studied, to determine the potential applications of these materials.
2 Industrial outlook
The aim of this project was also to study the potential of commercialization of organoclays prepared with our patented scCO2 process. During the four years of research, we received clear demands for high temperature stable organoclays as well as for organoclays compatible with fluorinated matrices and with silicone-based matrices. Thermally stable organoclays could be proposed in the form of MMT-P14, HCT-P14, MMT-P8 and HCT-P8 up to the kilogram quantity.
Having used a small scale (50 ml) and a medium scale (50 l) high pressure reactor both with success for the clay modifications, we do not expect major issues in scaling up to a semiworks or even an industrial size reactor. We showed the importance of limiting the filling to ideally 25 g per liter (volume of reactor) and adapting the temperature to the used organomodifier.
The CO2 pressure, the reaction time and the quality of mixing are other important parameters that need additional studies.
The effectiveness of scCO2-prepared organoclays in preliminary tests depended largely on the polymer matrix and the property looked for, as illustrated here above. To propose other specialty organoclays, we were somewhat limited by the commercial availability of surfactants or their prohibitive price. We assumed that the latter, or an eventual multi-step
synthesis) can only be justified if excellent properties of final material are demonstrated, which requires further investigations. Finally, we think that the development and commercialization of new organoclays from the scCO2 process will only be possible through a strong partnership with industrial users.
Figure 1. Pilot high pressure reactor, CERM, Ulg
REFERENCES
1. Awad, W. H.; Gilman, J. W.; Nyden, M.; Harris, R. H.; Sutto, T. E.; Callahan, J.;
Trulove, P. C.; DeLong, H. C.; Fox, D. M. Thermochim. Acta 2004, 409, 3-11.
2. Dennis, H. R.; Hunter, D. L.; Chang, D.; Kim, S.; White, J. L.; Cho, J. W.; Paul, D. R.
Polymer 2001, 42, 9513-9522.
3. Urbanczyk, L.; “Preparation of fire-resistant polymer/clay nanocomposite foams with the supercritical fluid technology”, 2010, PhD thesis, University of Liège, Belgium.
