Qualitative and quantitative assessment of water sorption in natural fibres using ATR-FTIR spectroscopy

Highlights

• Effect of moisture sorption on plant fibres was investigated by FTIR spectroscopy.

• Hydroxyl and carboxyl moieties are specific sites for water sorption.

• Partial least squares regression quantified the water content of natural fibres.

• Models fitted the sorption isotherms and diffusion kinetics.

Abstract

In the field of composite materials, natural fibres appear to be a viable replacement for glass fibres. However, in humid conditions, strong hydrophilic behaviour of such materials can lead to their structural modification. Then, understanding moisture sorption mechanisms in these materials is an important issue for their efficient use. In this work, the water sorption on three natural fibres (flax, hemp and sisal) was studied using Fourier transformed infrared spectroscopy. The spectral information allowed both qualitative and quantitative analyses of the moisture absorption mechanisms. The main chemical functions involved in the water sorption phenomenon were identified. The absolute water content of the fibres was also determined by using a partial least square regression (PLS-R) approach. Moreover, typical sorption isotherm curves described by Park model were fitted as well as water diffusion kinetics. These last applications confirmed the validity of the FTIR spectra based predictive models.

Introduction

Nowadays, natural fibres are considered to be a good alternative for glass fibres replacement in the purpose of composite material reinforcement (particularly in automotive manufacturing or sport and leisure sector) (Bledzki and Gassan, 1999, Suddell and Evans, 2005). These fibres present a better environmental impact than glass fibres (recyclability and biodegradability) as well as higher specific mechanical properties because of their low density. However, their pronounced hydrophilic behaviour – due to their particular structure – leads to high level of moisture absorption in wet environments (Célino, Fréour, Jacquemin, & Casari, 2013). This results in the structural modification of the fibres and an evolution of their mechanical properties together with the composites in which they are fitted in (Dhakal et al., 2007, Placet et al., 2012, Symington et al., 2009). Thereby, the understanding of these moisture absorption mechanisms is of great interest to get a better control of such new biomaterials.

Generally, one of the most important factors controlling the water diffusion phenomenon in polymeric materials is the molecular interaction occurring between the diffusing compound and the substrate. The diffusion phenomenon is subjected to the ability of the polymer molecular sites to establish hydrogen bonds with the water molecules. Spectroscopic techniques such as nuclear magnetic resonance (NMR), dielectric or Fourier transform infra-red spectroscopy (FTIR) have been proved to be well adapted to study this phenomenon since they allow to characterize molecular interactions involving potential sorption sites for water (Mijovic and Zhang, 2003, Popineau et al., 2005). Among these approaches, FTIR spectroscopy has been widely used to study water transport in polymer and particularly to study the water sorbed into epoxy resins (Cotugno et al., 2001, Feng et al., 2004, Fieldson and Barbari, 1993, Musto et al., 2000). Indeed, this technique provides attractive features, i.e. the very high sampling rate, the sensitivity, the accuracy of the quantitative analysis and the information at the molecular level contained in the vibrational spectra. Moreover, the development of attenuated total reflectance FTIR spectroscopy (ATR-FTIR) encouraged and facilitated the use of this non-invasive technique directly onto solid materials (Chalmers & Dent, 1997).

On pure cellulosic polymers, the potential sorption sites for water were determined to be hydroxyl and carboxyl groups which are particularly easily detected in FTIR spectroscopy (Berthold, Olsson, & Salmén, 1998). However, few studies have been performed to characterize water sorption by FTIR directly on raw lignocellulosic fibres. Laity and Hay (2000) demonstrated that it was possible to reproduce water sorption kinetics by recording infrared spectra in reflexion mode on cellophane. More recently, Olsson and Salmén (2004) examined the association of water on pulp paper using FTIR spectra acquired in transmission mode. Their results indicated the existence of characteristic bands affected by water. Moreover they determined a linear relationship between the absorbance of those bands and the water content, using a univariate approach. This last work was of particular importance since it highlighted major features helping the understanding of chemical groups involved in water diffusion in pulp paper. However both approaches, based on univariate analysis of FTIR signal failed to properly reproduce the sorption isotherms.

The aim of this work is the use of FTIR as an experimental tool to investigate the water sorption onto raw plant fibres, known to be good candidates for the reinforcement of composite materials (i.e. flax, hemp and sisal). First FTIR spectral signature of each fibre was investigated in order to describe the molecular effect of the water sorption mechanisms. Secondly, a multivariate model linking water sorption and the whole FTIR spectra was developed using partial least square regression (PLS-R). Finally this model was applied for the accurate monitoring of the water diffusion in the tested biomaterials.