A preliminary survey of the interfacial bonding of some tropical hardwoods towards succinic anhydride and 2-octen-1-yl succinic anhydride molecules: Impact of lignin and carbohydrate polymers structure on the chemical reactivity

Abstract

From the perspective of the utilization of the wood waste from African timber industry in the field of wood/polymer composites, the reactivity of four tropical hardwood species from Gabon, Tesula gabonensis Pellegr (T. gabonensis), Holoptelea grandis Hutch (H. grandis), Aucoumea klaineana Pierre (A. klaineana Pierre) and Tieghemella africana Pierre (T. africana) towards succinic anhydride (SA) and 2-octen-1-yl succinic anhydride (OSA) was studied on the basis of their carbohydrate complex and lignin polymers structure and composition as well as on the basis of the capability of their wood cell walls to absorb the dimethylformamide (DMF) and pyridine (Py) solution. T. gabonensis, H. grandis and T. africana wood sawdust are more reactive with SA than A. klaineana. However, T. gabonensis is the most reactive with SA as well as with OSA. The better reactivity of T. gabonensis should be connected to the abundance of the reactive hydroxyl functions in its native lignin. Furthermore, T. gabonensis, H. grandis and T. africana are rich in long polysaccharide fibers; their cell walls displayed a strong capability for swelling after DMF/Py absorption than A. klaineana. The latter displays short polysaccharide fibers assumed to agglomerate within the cell walls, limiting like this the contribution of the polysaccharide chains located within A. klaineana cell walls on its chemical reactivity; protecting thereby its cellulose matrix from a decrystallization as pointed out by X-ray diffraction (XRD) analysis. The crystalline lattice of T. gabonensis, H. grandis and T. africana for which the cell walls are more accessible to DMF/Py exhibit the most important decrease of the crystallinity index (CrI) and the crystallite sizes (D_0 0 2).

Introduction

Timber processing operations in Africa, from logging to end products, produce waste materials of every sort, size and shape. Its is estimated that when no waste recovery takes place, overall yields from logging to end products amount to no more than 15%. In the processing chain alone from the sawmill to the end product and without including felled timbers in forests, yields are rarely above 25%. These figures, viewed in the light of sustainable development and equitable natural resource management, clearly illustrate the importance of recovering and recycling timber industry waste, trimmings and residues. Any technological or commercial solution that gains a few percentages in yield without affecting the industry's competitiveness can help to keep resource use under control while ensuring the continuity of raw material supplies (Sales, 2003). From this perspective, four tropical hardwoods from Gabon, Tesula gabonensis, Holoptelea grandis, Aucoumea klaineana and Tieghemella africana, collected from a local sawmill (Moungengui, 2002) and commonly named Izombé, Béli, Okoumé and Douka were selected for their potential utilization as fillers in wood polymer composites (WPC).

The utilization of wood wastes in composite materials has several benefits such as its low density, its flexibility during the process, its low cost per unit of volume and its biodegradability (Oksman and Clemons, 1998). One of the promising wood wastes utilization way is their incorporation as fillers in WPC materials witch associate hydrophilic wood powders/fibers matrix with hydrophobic homopolymers such as polypropylene or polyethylene (Qingxiu and Matuana, 2003). The poor interfacial bonding and the weak compatibility between wood based materials and the mostly used polypropylene and polyethylene polymers are the major defiles of WPC (Oksman and Clemons, 1998) and explain their weak mechanical properties. Several authors studied the improvement of the interfacial bonding between wood flour/fiber and organic polymers (Ichazo et al., 2001, Pickering et al., 2003, Sun-Yong et al., 2004, Matuana et al., 1999); they found that coupling agents bearing reactive chemical groups such as silanes or cyclic anhydrides functions have been probed to be useful for the interfacial strength transfer of wood polymer composites. It is noteworthy that maleic anhydride grafted polypropylene or polyethylene (MAPE and MAPP, respectively) are mostly used as coupling agents in extruded WPC or in lignocellulosic based composite materials performed in liquid phase (Kazayawoko et al., 1999). Their reactivity is governed by acid–base interactions and ester bonds formation between the anhydride carbonyls of coupling agent and the most abundant hydroxyl groups of wood matrix, known to be single site for reactivity in lignocellulosic polymers (Rowell, 1984).

The aim of this study was to investigate the chemical reactivity of the selected tropical hardwood species towards succinic anhydride (SA) and 2-octen-1-yl succinic anhydride (OSA) molecules for their potential use as fillers in wood polymer composites. The choice of SA was justified by its better reactivity and photostability than analogous such as maleic anhydride (Chang and Chang, 2001, Mishra and Patil, 2003), by its 100% add-on of the skeleton reagent and by the fact that it does not liberate by-product compared with acetic anhydride (Rowell, 1984). On the other hand, the similarity between the succinic anhydride ring and the reactive unit of MAPP or MAPE coupling agents account for advantages. OSA was used to simulate the reactivity of the wood samples with coupling agents like MAPP or MAPE. The chemical reactivity of the selected wood samples through SA and OSA molecules was studied by the weight percent gain (WPG) measurement. For a better understanding of the reactivity of the selected woods, the contribution of their respective cellulose-hemicellulose or carbohydrate complex was discussed. According to the good reactivity of lignin (Rowell, 1982, Yuang-Zong and Masamitu, 1993), a particular attention was paid to their lignin content, but also to the hydroxyl (OH) groups content of this polymer. The modified wood samples were characterized by Fourier transformed infrared (FTIR) and CP/MAS ¹³C NMR spectroscopies. The supramolecular stability of the crystalline lattice of the wood sawdust modified with succinic anhydride was investigated by X-ray diffraction (XRD).