Timothy G. Rials

Review: Current international research into cellulosic fibres and composites

The following paper summarises a number of international research projects being undertaken to understand the mechanical properties of natural cellulose fibres and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the physical and micromechanical properties of cellulose fibres is reported. Current single fibre testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chemically modifying fibres to improve interfacial adhesion in composites are also reviewed, and the use of novel fibre sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibres in composites, with different chemical treatments, ought to be a focus of future research.

Interfacial contributions in lignocellulosic fiber-reinforced polyurethane composites

Whereas lignocellulosic fibers have received considerable attention as a reinforcing agent in thermoplastic composites, their applicability to reactive polymer systems remains of considerable interest. The hydroxyl-rich nature of natural lignocellulosic fibers suggests that they are particularly useful in thermosetting systems such as polyurethanes. To further this concept, urethane composites were prepared using both unused thermomechanical pulp and recycled newsprint fibers. In formulating the materials, the fibers were considered as a pseudo-reactant, contributing to the network formation. A di-functional and tri-functional poly(propylene oxide)-based polyol were investigated as the synthetic components with a polyol-miscible isocyanate resin serving as a crosslinking agent. The mechanical properties of the composites were found to depend most strongly on the type of fiber, and specifically the accessibility of hydroxy functionality on the fiber. Dynamic mechanical analysis, swelling behavior, and scanning electron micrographs of failure surfaces all provided evidence of a substantial interphase in the composites that directly impacted performance properties. The functionality of the synthetic polyol further distinguished the behavior of the composite materials. Tri-functional polyols generally increased strength and stiffness, regardless of fiber type. The data suggest that synthetic polyol functionality and relative accessibility of the internal polymer structure of the fiber wall are dominant factors in determining the extent of interphase development. Considerable opportunity exists to engineer the properties of this material system given the wide range of natural fibers and synthetic polyols available for formulation.

Characterization of the interface between cellulosic fibers and a thermoplastic matrix

The applicability of the microbond test to evaluate the interfacial properties between cellulosic fibers and thermoplastics was studied. Acetylation and heat treatment were applied to modify the surface of cellulosic fibers (rayon, cotton, and wood). The apparent diameters and surface free energies of the fibers were estimated by dynamic contact angle (DCA) analysis. Interfacial shear strengths between the cellulosic fibers and the polystyrene matrix were determined using the microbond test method. The test results indicate that acetylation increases the total surface free energy of the wood fibers, whereas heat treatment dramatically decreases the surface free energy of all cellulosic fibers tested. For heat treated and acetylated fibers, the greater the surface free energy, the greater the interfacial shear strength (ISS) regardless of fiber types. For control group fibers, a low ISS exists even though the fibers have high surface free energies because of the formation of a weak boundary layer. The high...

Evaluation of the cure kinetics of the wood/pMDI bondline

Abstract Micro-dielectric analysis (μDEA) and differential scanning calorimetry (DSC) were used to monitor cure of polymeric diphenylmethane diisocyanate (pMDI) resin with wood strands in a saturated steam environment. A first-order autocatalyzed kinetic model was employed to determine kinetic parameters. The kinetics were found to follow an Arrhenius relation. A single ramp DSC technique and μDEA produced models that predicted similar results at higher cure temperatures, but the μDEA-based model predicts a longer cure time at low temperatures. The isothermal μDEA method yields higher activation energies and Arrhenius frequency factors than models based on single DSC ramps. A modification to ASTM E698 was made to conform to the assumption of autocatalyzed kinetics. The modified ASTM E698 method predicted an earlier end of cure than the μDEA-based models and was in agreement with DSC results obtained by partial cure experiments. The activation energies and frequency factors for the different cure monitoring methods are sensitive to different stages of cure.

Chemistry of Pecan Tannins and Analysis of Cure of Pecan Tannin-Based Cold-Setting Adhesives with a DMA ‘Micro-Beam’ Test

Condensed tannins from pecan nut pith are the only vegetable tannins produced commercially in the United States. They are mixed procyanidin-prodelphinidin polymers in which the prodelphinidins predominate. Units with 2,3-cis stereochemistry occur more frequently than those with 2,3-trans stereochemistry. Although typical flavan-4-thiol and -phloroglucinol adducts are obtained by treatment of these polymers with dilute acetic acid and the appropriate nucleophile, yields are uncommonly low. The generation of phloroglucinol during these reactions suggested a novel side reaction that was shown to occur in degradation reactions of both procyanidins and prodelphinidins when at 105 °C for 1 to 2 days. Tannins obtained from pecan nut pith by extraction with 4-percent sodium sulphite and 0.4-percent sodium carbonate are especially rich (Stiasny no. 87). These polymers gel rapidly in the presence of phenol-resorcinol-formaldehyde resins and paraformaldehyde. A DMA ‘micro-beam’ test for analysis of the curing kinetics of these cold-setting phenolic resins provides information on the cure kinetics of these polymers.

The reaction of boric acid with wood in a polystyrene matrix

The reaction of boric acid with wood fibers in a polymer melt was examined using 13C-nuclear magnetic resonance (NMR), 11B-NMR, differential scanning calorimetry, dynamic mechanical analysis, and component extraction and by the determination of material properties. Samples were blended at 350and 380°F in a roll mill. The use of a plasticizer in the melt to facilitate the reaction of the acid with the wood fiber was studied. NMR data showed that no significant reaction occurred between the boric acid and the polystyrene. Experimental evidence supports the reaction of boric acid with wood components. The ultimate strength of the composites was either reduced or not significantly altered by the reaction, depending upon conditions. However, the stiffness increased significantly with boric acid additions for the 350°F reactions, but behaved differently for the same additions at 380°F. The glass transition temperature of the polystyrene was lowered by the addition of plasticizer, as expected, while boric acid addition had little effect. Extracted samples showed that some boric acid remained with the wood fraction. These preliminary data suggest that boric acid does react with wood fiber under the conditions of this study. These investigations illustrate the feasibility of performing chemical reactions on the wood phase of wood/polymer composites during the extrusion process. Further research is recommended.

Evaluating Cure of a pMDI-wood Bondline Using Spectroscopic, Calorimetric and Mechanical Methods

Abstract The cure of polymeric diphenylmethane diisocyanate (pMDI)/wood bondline in a controlled saturated steam environment was monitored using micro-dielectric analysis (μDEA). Saturated steam environments were produced between 110°C and 140°C. The degree of cure calculated from μDEA was a basis for further spectroscopic, calorimetric, and mechanical evaluation. Interpretation of calorimetric and spectroscopic analysis revealed large consumption of isocyanate early in cure. However, mechanical strength, as revealed by lap-shear tests, did not develop until late in cure. Low lap-shear strengths and a plateau in conversion rates were detected for samples pressed at 110° and 120°C. Several components of the analysis suggest that low temperature cure may result in crystal formation, leading to diffusion controlled cure.

Chemical Imaging of the Spatial Distribution and Interactions of Tannin Dispersal in Bioplastic Systems

IR chemical imaging has been used to characterise the dispersion of condensed tannin additives in a biodegradable plastic, poly(butylene succinate). By mapping key FTIR absorptions, acetylated tannin was found to remain in discrete aggregates, though the outer of these particles contained a mixed phase with PBS. In particles >100 μm, PBS does not appear to ingress the tannin core, only wetting the outer region. Analysis of a second tannin derivative indicated that there was a more uniform dispersion of the additive in the PBS matrix.