Blue agave fiber esterification for the reinforcement of thermoplastic composites

Abstract

Blue agave fiber esterification and its use in thermoplastic composite reinforcement was the purpose of this research. Spectroscopic techniques indicated the occurrence of the esterification reaction, since signals for carbonyl groups were found through FT-IR (1750 cm⁻¹) and ¹³C-CPMAS-NMR (172 ppm). Mechanical properties characterization showed that superficial modification was successfully achieved, since a change in the elastic modulus and improved impact resistance were observed when the modified fiber was used. In addition, electron microscopy studies indicated that the modified fiber produced an enhancement in fiber/HDPE interfacial interaction, since less fiber sliding evidence was observed on the composite surface. The Cox–Merz rule was used as an alternative tool in order to study composites rheometry along an expanded range of shear rate in the power law region.

Introduction

Lignocellulosics are biodegradable materials that nature recycles through biological, thermal, chemical and photochemical processes. Theses materials are hygroscopic by nature, since they are designed to perform in a wet environment. The chemical constitution of lignocellulosics varies from specie to specie but cellulose, hemicelluloses and lignin are the common constituents. Annually large amounts of lignocellulosics are produced as wastes, where wood and its derivatives constitute around 40% of the residual solid wastes from residential, commercial, industrial and institutional sources. Since lignocellulosic fibers can improve the toughness and strength of plastics, the production of thermoplastic composites is becoming important in applications for recovering and recycling these materials. However, as mentioned above, the hydrophilic nature of lignocellulosics difficult their interaction with most thermoplastic polymers and the resulting composites present poor mechanical properties. In order to improve the interfacial interaction lignocellulosic/thermoplastic, the modification of one of them should be achieved. In general, it is convenient to modify the fiber through physical or chemical methods. Physical methods include: corona or plasma treatments and mercerization, where fiber surface is modified by influencing its mechanical bonding with the polymeric matrix; whereas chemical methods consist in the reaction of some groups of the fiber with a proper reactive compound in order to form covalent bonds. The new component acts as an interface between both the fiber and the thermoplastic matrix, thus improving composite performance as a result of a better interfacial interaction (Bledzki & Gassan, 1999).

There are two main techniques for carrying out lignocellulosics chemical modification; the first technique consists of grafting polymeric chains on the fiber surface by forming free radicals using a redox-initiator and subsequently, propagate polymeric chains (Gupta and Sahoo, 2001, Gürdağ et al., 1997, Gürdağ et al., 2001, Huang et al., 1992, Mohanty et al., 2000, Román-Aguirre et al., 2004). The second method includes the reaction of the cellulose hydroxyl groups (OH) with certain kinds of reagents such as silanes (Coutinho et al., 1998, Redondo et al., 2002, Rong et al., 2001), acyl halogenures (Chuavelon et al., 1999, Nair et al., 2001, Sun et al., 2001, Thiebaud and Borredon, 1995, Thiebaud et al., 1997), isocianates (Ellis and O’Dell, 1999, Joseph et al., 2002, Vázquez et al., 1999), carboxylic acids (Rong et al., 2001, Sun et al., 2002) or anhydrides (Rowell et al., 1994, Sun et al., 2002, Vaca-García and Borredon, 1999, Vaca-García et al., 1998). Commonly, these reagents have reactionable functional groups where a polymer chain can be propagated or grafted (Joseph et al., 2002). The effect of lignocellulosics chemical modification is the reduction of hydrophilicity (if the new group lignocellulosic-modifier is hydrophobic), which in first instance should solve the problem related to polarity differences between the fiber and the thermoplastic.

As mentioned above, the chemical modification produces changes on the fiber hydrophilicity, which should improve the interfacial interaction fiber/thermoplastic and the composite mechanical performance. The present research was aimed to study the chemical modification of a lignocellulosic fiber; such modification consisted in the esterification reaction between the cellulosic fraction of the fiber and a nonsymmetric anhydride (in non-aqueous media). It is worthy to mention that solid ¹³C NMR was used as a basic tool in order to obtain strong evidence about the modified fiber structure. The fiber used was an agro-industrial waste derived from Weber blue agave fiber (agave tequilana weber blue variety or agave palmaris) which grows in Western Mexico (Jalisco) and through its fermentation tequila has been produced for long time. With tequila massive industrialization, large amounts of fiber are produced yearly as byproduct, and together with many other similar materials, its accumulation and disposal has become a critical environmental problem (Iñiguez-Covarrubias, Lange, & Rowell, 2001). The study was complemented with the evaluation of mechanical properties of fiber/polyethylene (HDPE) composites and the obtention of viscosity curves applying the Cox–Merz rule.