Characterization of acetylated eucalyptus wood fibers and its effect on the interface of eucalyptus wood/polypropylene composites

Abstract

The acetylation of eucalyptus wood fibers (WFs) was conducted in 50%/50% (v/v) of acetic acid and acetic anhydride catalyzed by sulfuric acid at 60 °C for 2 h. Twenty one percent weight percent gain (WPG) level was achieved. Then the contact angles of three probe liquids on the surface of unacetylated and acetylated eucalyptus WFs were tested by the capillary rise method, and the surface free energies of unacetylated and acetylated eucalyptus WFs and their dispersion and polar components were calculated according to the method suggested by Owens–Wendt–Kaelble. The results show that the surface free energy of unacetylated eucalyptus WFs is 24.7 mJ/m², and its corresponding dispersion and polar components are 14.8 mJ/m² and 9.9 mJ/m², respectively. However, the surface free energy of acetylated eucalyptus WFs is 38.3 mJ/m² which is higher than the surface free energy of PP (29.4 mJ/m²), and its corresponding dispersion and polar components are 35.6 mJ/m² and 2.7 mJ/m², respectively. So it is possible for the spreading of PP on the surface of acetylated eucalyptus WFs and forming a good interfacial adhesion between acetylated eucalyptus WFs and PP. The better interfacial compatibility between acetylated eucalyptus WFs and PP was corroborated by the scanning electron microscopy (SEM) micrographs.

Introduction

In recent years wood polymer composites (WPCs) based on polyolefins (polyethylene, PE; polypropylene, PP) have gained increasing interest because of environmental concerns and the depletion of fossil fuels [1]. WPCs have been used in a variety of applications, including construction, auto parts, storage, furniture, packaging, decorating [2].

The most important drawback in WPCs, and the main obstacle to overcome when their material properties should be improved, is the lack of compatibility between wood fibers (WFs) and polymers [3], because WFs are hydrophilic, while thermoplastic polymers are hydrophobic.

Numerous strategies to improve the compatibility at the interface have been developed and published [4], [5], [6], [7], [8], [9]. The methods for surface modification of wood fibers can be physical or chemical according to the way they modify the fiber surface. The main chemical method used in the surface modification of wood fibers is chemical coupling, where a coupling agent is used to form chemical bonds between the cellulose chains in the WFs and the polymer [10]. There has been a lot of research over recent decades on different types of coupling agents for improving the adhesion between WFs and polymers. The most commonly used coupling agents are maleic anhydride grafted polyolefins [4], [11], [12] and silanes [13], [14], [15]. Also, some studies on other coupling agents such as isocyanates [16], [17], vinyl acetate [18], chitin and chitosan [19] have been conducted.

Another important chemical method is the acetylation of WFs. Acetylation as a method for solid wood modification is known since 1946 [3]. At that time this treatment is for improving wood durability and dimensional stability. Of course, for reasons of increased surface/volume ratio, this modification can be applied to wood fibers or particles even more easily. Nowadays, many researchers applied this method to improve the interfacial adhesion between WFs and polymers. Ozmen et al. [20] have investigated the effect of wood acetylation with vinyl acetate and acetic anhydride on the properties of wood–plastic composites. They report an improvement in tensile strength and modulus, as well as flexural strength and modulus. The composites reinforced with acetylated wood flour present higher thermal stability than the composites reinforced with untreated wood flour. Bledzki et al. [21] have prepared injection-molded PP composites with 30% flax fibers. They report tensile and flexural moduli increased by 20% and 5%, respectively, as a result of fiber acetylation. The respective strengths were found increased by 15% and 10%, while notched Charpy IS was reduced by about 10%. Hung et al. [22] have studied the natural weathering properties of bamboo plastic composites reinforced with bamboo fibers acetylated to different weight gains. The results showed that the retention ratios of mechanical properties of acetylated bamboo plastic composites, especially those containing fibers with a high weight gain, were significantly improved as compared to the unmodified composite during natural weathering. Moreover, the mildew resistance of the acetylated bamboo plastic composites was higher than that of unmodified composites. Lisperguer et al. [23] have investigated the influence of wood acetylation on thermal behavior of wood-polystyrene composites. The results showed that the wood-plastic composites produced with acetylated wood flour have better thermal stability than those with non-acetylated wood flour. However, these studies mainly focused on the qualitative characterization of acetylated wood fiber and its effect on the properties of wood–plastic composites. Very few studies focused on the quantitative characterization of acetylated WFs and the compatibility between acetylated WFs and polymers [24], [25].

In this study, I try to develop a quantitative method to characterize the acetylated and non-acetylated eucalyptus WFs and investigate its effect on the interface of eucalyptus wood/PP composites.