Three-components organic-inorganic hybrid materials as protective coatings for wood: Optimisation, synthesis, and characterisation

The present work explores new solutions for the development of functional flame-resistant hybrid coatings for wood, by using oxocluster-reinforced hybrid materials. Hybrid coatings and bulk materials were produced by photopolymerisation of 3-methacryloxypropyltrimethoxysilane (MAPTMS) with methylmethacrylate (MMA) in the presence of the dimeric oxocluster (Zr6O4(OH)4(OOCCH2CHCH2)12(n-PrOH)]2·4(CH2CHCH2COOH), (Zr12), characterised by the presence of 12 vinylacetate groups for each molecules. The molar ratios among silane, MMA monomer and oxocluster were changed to optimise the best performing formulation. The final molar ratio chosen for the spray deposition of the coatings under inert atmosphere and for the preparation of the bulk specimens was MAPTMS:MMA:Zr12 = 1:3:0.008. Attenuated Total Reflectance-Fourier Transform Infra Red (ATR-FTIR) spectroscopy and Differential Scannig Calorimetry (DSC) were used in time-resolved fashion to optimise the photopolymerisation time, resulting to be 20 min. The polymerisation of the organic part and the condensation of the siloxane groups were investigated by the combined use of DSC, Fourier Transform Infra-Red (FTIR) and solid state Nuclear Magnetic Resonance (NMR) spectroscopies, showing that, whereas a complete organic polymerisation degree was reached, the condensation of the silica component was not completed. Dynamical Mechanical Spectroscopy (DMS) evidenced that: (1) the copolymerisation of the silane with the Zr12 oxocluster without MMA yields materials with very poor mechanical thermo-properties; (2) the Zr12 oxocluster copolymerised with MMA gives very stiff but fragile hybrids; (3) the ternary system yields instead flexible materials, which are endowed with outstanding thermo-mechanical properties. The optimised formulation was used for the deposition of coatings on wood (Larex), which were analysed by Scanning Electron Microscopy (SEM), contact angles measurements and tested toward flame-resistance.