Structural Study of Photodegraded Acrylic-Coated Lime Wood Using Fourier Transform Infrared and Two-Dimensional Infrared Correlation Spectroscopy

Abstract

The weathering of acrylic films and acrylic-coated lime wood (Tillia cordata Mill.) were examined using Fourier transform infrared (FT-IR) and two-dimensional infrared correlation spectroscopy. The obtained results showed chemical changes induced by exposure to weathering conditions, in both films and coated wood. The observed spectral changes of the acrylic films refer to the absorption band assigned to the C–O stretching, which progressively decreases with increasing exposure time. In the spectra of treated wood samples the main signal indicating the advance of oxidation during the photodegradation exposure is the gradual increase and broadening of the band in the carbonyl region. This is due to the formation of the non-hydrogen bonded aliphatic carboxylic acids and γ-lactone structures in the acrylic resin and of the nonconjugated ketones, carboxyl groups, and lactones in wood. As a consequence, the increase of the 1734 cm⁻¹ band is due to the degradation of lignin from wood surface. These observations are also supported by the decreased intensities of the bands at 1598 and 1505 cm⁻¹, assigned to C=C of aromatic skeletal (lignin). The relative intensity of the characteristic aromatic lignin band at 1505 cm⁻¹ decreases up to 25% of its original value after weathering, being less than half of the value obtained for uncoated wood.

Two-dimensional infrared (2D IR) correlation spectroscopy was used to identify the sequence of the modifications of the different stretching vibrations bands under the weathering conditions, the method allowing the prediction of the order of degradation reactions. The acrylic resin degradation starts with the formation of radicals by abstraction of the tertiary hydrogen atoms of the methyl acrylate units and the α–CH₃ groups from the ethyl methacrylate units. The subsequent decomposition and oxidation led to the formation of alcohol groups, hydroperoxides, ketones, and/or carboxylic acid groups. The 2D IR correlation spectra of weathered impregnated wood also revealed the elimination of low molecular weight compounds following the degradation of lignin and carbohydrates components from wood.