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Supramolecular design of high performance Benzoxazine/Kaolin composites Why? Benzoxazine resins (BZ) are getting more and more interest as potential substitutes of bisphenol A-based A epoxy resins due to their outstanding thermal, mechanical, barrier, solvent resistance and fire properties [1]. A convenient and smart route to the design of BZ-based based composites relies on filler-to-BZ filler interactions, as already reported for carbon nanotubes [2] or clays [3]. Due to its high intrinsic hydrogen bonding, kaolin aolin is expected to be an ideal support for the assembly of BZ molecules on its outer -OH platelets.
What and How?
a)
BZ monomer was prepared by the reaction of phenol, formaldehyde and ethylene diamine. The monomer was fully characterized by FTIR (Fig 1a) MALDI-MS, 1H NMR (data not shown).
b)
Fig. 1. a) FTIR spectra of kaolin (red line), ), BZ (black ( line) and BZ/kaolin (blue line). b) Zoom in between 4000 and 3200 cm-1. Below-depicted is the the chemical structure of BZ monomer. ↓ O
c)
N N O
The incorporation of 10 wt% kaolin was performed by solventless method: method mixtures ixtures were introduced in a mold constituted of 2 Teflon-coated coated plates separated by a 1mm thick Teflon joint. A thermal treatment of 1h at 150°C 150 followed by a post-cure at 170°C during 1h was applied.
Main Results The platelets structure of kaolin is strongly maintained through hydrogen bonding. Similarly, it is well known that the structure of BZ results not only from crosslinking but also thanks to the self-assembly assembly of molecules through th inter and intramolecular hydrogen bonding. It is reasonable to expect similar interactions when kaolin and BZ monomers are mixed together. These interactions are evidenced by FTIR/ATR (Fig. 1b), with a clear shift of the absorption peak attributed to the outer OH of kaolin from 3685 to 3692cm-1. Kaolin inner -OH groups at 3619cm-1 are not affected, indicating that BZ does not intercalate within kaolin platelets, further confirmed by XRD measurements. Strong hydrogen bonding between matrix and fillers are a clear asset for the final properties of composites and is generally accompanied by an increase of the polymer chains hindrance and the material rigidity. A shift of the Tg from 150°C to 170°C was observed by DMA analyses (Fig. 2) with an increase of E' from 3.6 GPa to 5.1 GPa when kaolin was used. Interestingly, the addition of other fillers like SiO2 or sepiolite leads to a clear decrease of the Tg of the materials, assimilated to a steric hindrance towards BZ self-assembly self during crosslinking. ← Fig. 2. DMA analyses of neat BZ (green solid line) or BZ filled with 10%wt SiO2 (dark blue dashed line), 10wt% sepiolite (light blue dotted line) or 10wt% kaolin (green dashed line).
References [1] Ghosh, N. N. et al.. Progress in Polymer Science 2007, 32, 11, 1344. [2] Zuniga, C et al., Journal of Materials Chemistry A 2014, 2, 6814. [3] Takeichi, T et al., Polymer 2002 43, 1, 45. 45 Contact :
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