Comparative determination of the grafting distribution and viscoelastic properties of wood blocks acetylated by vinyl acetate or acetic anhydride

Abstract

Extracted maritime pine sapwood blocks were acetylated to different weight gains with vinyl acetate (VA) and acetic anhydride (AA) and the samples were characterized with regard to their grafting distribution and dynamic viscoelastic properties. We found that the reaction with the VA/DMF/K2CO3 system was controlled by the diffusion of VA and/or K2CO3 (catalyst) within the solid wood blocks, and that this diffusion was somehow more difficult than the diffusion of AA. Fourier transform infrared (FTIR) spectroscopy was used to confirm the grafting and investigate the distribution of the acetyl moieties within the wood blocks. Whatever the acetylating agent used and the reaction time applied, we observed that the surface of the wood blocks was always more acetylated than the bulk. Some swelling of the cell walls also occurred, but the AA-acetylated blocks were always more swollen than the VA-acetylated ones at a given WPG. The viscoelastic properties of the samples were investigated by dynamic mechanical analysis (DMA). The temperature profiles of the storage modulus (E′) and loss modulus (LogE″) indicated that some plasticization occurred after acetylation and also confirmed that the cellulose sites in wood were more attacked by VA than by AA. Results also revealed that the softening, induced by the acetyl groups introduced in the substance matrix, was counterbalanced by some stiffening effect at high WPG, more particularly when VA was used. We hypothesized that some VA-acetylated material may have expanded into the cell wall micropores or lumen during reaction, leading to the limited swelling and increased stiffness observed.