Callum A. S. Hill

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.

The preparation and characterisation of a series of chemically modified potato starches

A range of substituted starches has been prepared at moderate temperature (≤90°C) from gelatinised potato starch by treatment in lithium chloride/N,N-dimethylacetamide (DMAC) solution with acyl chlorides. Oleoyl, palmitoyl, lauroyl, capryloyl and butyryl modified starches have been synthesised with degrees of substitution (DS-values) ranging from 0.3 to 3. Characterisation by FT-IR spectroscopy and elemental analysis has confirmed reaction and degree of substitution obtained. DS-Values were controlled by metered addition of stoichiometric quantities of the required acyl chloride. Such structural modification of starch resulted in a significant change of physicochemical properties, and increased hydrophobicity with increasing acylation extent. It was found that the modified starches had improved thermal stabilities and enhanced solubilities in organic solvents.

Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites

The use of plant fibers as a reinforcement in polyester matrices requires the issue of compatibility between the two phases to be addressed. Because plant fibers present hydrophilic surfaces and polyesters are generally hydrophobic, poor fiber–matrix dispersion and wetting of the fibers by the matrix may result. As a consequence, the mechanical properties of the composite are severely reduced. This study considers the effect of fiber treatment by chemical modification of the fibers (acetylation) or the use of silane or titanate coupling agents on the mechanical properties of coir or oil palm reinforced polyester composites.

A study of the potential of acetylation to improve the properties of plant fibres

Abstract The chemical modification of Coir, Oil palm fibre, Flax, and Jute fibres using acetic anhydride has been investigated. The rate of acetylation is proportional to the lignin content of the fibres, with Coir and Oil palm empty fruit bunch (EFB) fibres exhibiting the fastest rate of reaction and ultimate substitution. The effect of acetylation at a reaction temperature of 120°C upon the tensile properties of EFB and Coir fibres has been found to damage the fibre structure resulting in poor mechanical properties, whereas at 100°C the modified fibres exhibit improved performance when compared with control samples. In microbiological decay tests, modified fibres showed a high degree of decay resistance in a variety of tests over a 5-month test period, while control samples failed in less than 1 month. No significant difference in decay resistance was found between fibres modified to a weight percent gain (WPG) of 8–10% and fibres modified to a WPG of 15–18%.

A critical discussion of the physics of wood–water interactions

This paper reviews recent findings on wood–water interaction and puts them into context of established knowledge in the field. Several new findings challenge prevalent theories and are critically discussed in an attempt to advance current knowledge and highlight gaps. The focus of this review is put on water in the broadest concept of wood products, that is, the living tree is not considered. Moreover, the review covers the basic wood–water relation, states and transitions. Secondary effects such as the ability of water to alter physical properties of wood are only discussed in cases where there is an influence on state and/or transition.

The water vapour sorption properties of Sitka spruce determined using a dynamic vapour sorption apparatus

The water vapour sorption properties of Sitka spruce (Picea abies) have been investigated over a range of temperatures (14.2–43.8°C) using a dynamic vapour sorption apparatus. The sorption kinetics behaviour was evaluated using the parallel exponential kinetics model which has been found to give very accurate fits to the data in studies of foodstuffs or plant fibres, but has not been previously applied to sorption studies with wood. Both the adsorption and desorption kinetics curve can be deconvoluted into a fast and slow exponential process. Under conditions of adsorption, the fast process appears to be associated with the formation of monolayer water (determined using the Hailwood Horrobin model) up to a relative humidity of 20%. Under desorption, there is no clear differentiation between fast and slow processes. The area bounded by the sorption hysteresis loop reduced as the temperature at which the isotherm was measured increased, due to movement of the desorption curve only, with the adsorption curve remaining the same at all temperatures. This behaviour is consistent with sorption processes taking place on nanoporous glassy solids below the glass transition temperature. The heat of wetting was determined from the temperature dependence of the desorption isotherms by using the Clausius–Clapeyron equation, yielding results that are consistent with literature values. However, doubts are raised in this paper as to the applicability of using the Clausius–Clapeyron equation for analyses of this type.

RTM Hemp Fibre-Reinforced Polyester Composites

Hemp fibre-reinforced polyester composites were prepared using a Resin Transfer Moulding (RTM) technique and the flexural and impact behaviour investigated. Flexural stress at break and flexural modulus showed an increasing trend with fibre content. Impact strength was found to decrease at low fibre content, then gradually increase with further addition of fibres.A strong interfacial adhesion between hemp and polyester was obtained using chemically modified hemp. This modification consisted in introducing reactive vinylic groups at the surface of the fibres, via esterification of hemp hydroxyl groups, using methacrylic anhydride. Increased bonding between fibres and matrix did not affect the flexural stress at break of the composite but was detrimental to toughness. This behaviour was ascribed to a change in the mode of failure, from fibre pull-out to fibre fracture, resulting in a marked reduction in the energy involved in the failure of the composite, leading to a more brittle material.

The biological effectiveness of wood modified with linear chain carboxylic acid anhydrides against Coniophora puteana

Pinus nigra Schneid) sapwood. A brown rot fungus [Coniophora puteana (Schum.:Fr)] was selected in order to determine and compare the effectiveness (threshold value) of the linear chain anhydrides. The work described in this paper has demonstrated that chemically modified Corsican pine sapwood afforded substantial bioprotection against Coniophora puteana. With all anhydrides studied, a weight gain of 18% following reaction ensured complete protection. The results indicate that degree of cell wall bulking by the bonded adduct, rather than extent of hydroxyl substitution is the primary factor controlling decay resistance.P. nigra Schneid.). Geprüft wurden die Anhydride von Essig-, Propion-, Butter-, Valerian- und Capronsäure. Als Testpilz diente der Braunfäulepilz Caniophora puteana (Schum. : Fr.). Die Ergebnisse zeigen eine erhebliche Verbesserung der biologische Resistenz gegen diesen Pilz nach der chemischen Modifizierung des korsischen Kiefernholzes. Mit allen Anhydriden konnte mit einem Massenzusatz von 18% ein vollständiger Schutz erreicht werden. Die Ergebnisse deuten darauf hin, dass das Ausmaß der Zellwandbelegung und nicht so sehr die Substitution der Hydroxylgruppen der ausschlaggebende Faktor zum Schutz vor biologischem Abbau ist.

The effect of environmental exposure upon the mechanical properties of coir or oil palm fiber reinforced composites

Polyester matrix composites reinforced using nonwoven coir or oil palm empty fruit bunch fiber mats were manufactured. Fibers were used unmodified, chemically modified by acetylation, or treated with silane or titanate coupling agents. Composite test pieces were exposed to decay fungi in unsterile soil for up to 12 months, along with samples made of unreinforced, or glass fiber reinforced, resin. Water exposure tests were also performed. The effect of such exposure on the mass loss, tensile and flexural properties of the samples was evaluated. Mechanical properties deteriorated as a result of exposure. However, acetylation of fibers, or treatment with silane coupling agent was found to afford a significant degree of protection.

The fracture toughness of bast fibre reinforced polyester composites Part 1 Evaluation and analysis

Hemp and jute fibre reinforced polyester composites were fabricated to various fibre volume fractions (Vf) up to 0.45. Laminates reinforced with a chopped strand mat (CSM) glass fibre were also manufactured. The tensile properties of these materials were evaluated. Fracture toughness was assessed, using linear elastic fracture mechanics (LEFM) principles, under quasi-static loading conditions. At equivalent Vf (0.2) it was found that the fracture toughness (KIc) of the CSM glass fibre reinforced material was approximately 3 times greater than that of the natural fibre reinforced laminates and an order of magnitude greater than the unreinforced polymer alone. Critical strain energy release rates (Gc) and plastic zone radii were computed. The Gc of the natural fibre reinforced laminates was approximately an order of magnitude lower than that of the CSM reinforced material at the same Vf. It was hypothesised that the size of the crack-tip plastic zone influences the energy absorbing capacity of the material. By comparing the relative volumes of the plastic zones, implications regarding the toughening mechanisms operative in natural fibre reinforced composites have been made. The applicability of LEFM to characterise toughness in these materials is discussed.