Martin P. Ansell

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 effect of silane treatment on the mechanical and physical properties of sisal-epoxy composites

Abstract The surface of sisal fibres has been modified by mercerization and silane treatment to improve adhesion characteristics and moisture resistance. Silane treatment is most effective in reducing moisture uptake of fibres in humid environments. Sisal fibres with and without surface pretreatments have been combined with epoxy resin to form composite materials. The compressive strength of these composites is improved by pretreatment but the flexural strength and stiffness are not affected. However, in moist environments all mechanical properties are improved by silane treatment.

Jute-reinforced polyester composites

Raw jute fibre has been incorporated in a polyester resin matrix to form uniaxially reinforced composites containing up to 60 vol% fibre. The tensile strength and Youngs modulus, work of fracture determined by Charpy impact and inter-laminar shear strength have been measured as a function of fibre volume fraction. These properties all follow a Rule of Mixtures relationship with the volume fraction of jute. Derived fibre strength and Youngs modulus were calculated as 442 MN m−2 and 55.5 GN m−2 respectively. Polyester resin forms an intimate bond with jute fibres up to a volume fraction of 0.6, above which the quantity of resin is insufficient to wet fibres completely. At this volume fraction the Youngs modulus of the composite is approximately 35 GN m−2, the tensile strength is 250 MN m−2, the work of fracture is 22 kJ m−2 and the inter-laminar shear strength is 24 MN m−2. The properties of jute and glass fibres are compared, and on a weight and cost basis jute fibres are seen in many respects to be superior to glass fibres as a composite reinforcement.

The effect of chemical treatment on the properties of hemp, sisal, jute and kapok for composite reinforcement

Two chemical treatments were applied to hemp, sisal, jute and kapok natural fibres to create better fibre to resin bonding in natural composite materials. The natural fibres have been treated with varying concentrations of caustic soda with the objective of removing surface impurities and developing fine structure modifications in the process of alkalisation. The same fibres were also acetylated with and without an acid catalyst to graft acetyl groups onto the cellulose structure, in order to reduce the hydrophilic tendency of the fibres and enhance weather resistance. Four characterisation techniques, namely XRD, DSC, FT-IR and SEM, were used to elucidate the effect of the chemical treatment on the fibres. After treatment the surface topography of hemp, sisal and jute fibres is clean and rough. The surface of kapok fibres is apparently not affected by the chemical treatments. X-ray diffraction shows a slight initial improvement in the crystallinity index of the fibres at low sodium hydroxide concentration. However, high caustic soda concentrations lower the fibre crystallinity index. Thermal analysis of the fibres also indicates reductions in crystallinity index with increased caustic soda concentrations and that grafting of the acetyl groups is optimised at elevated temperatures. Alkalisation and acetylation have successfully modified the structure of natural fibres and these modifications will most likely improved the performance of natural fibre composites by promoting better fibre to resin bonding. Naturfasern aus Hanf, Sisal, Jute und Kapok wurden mit zwei chemischen Methoden behandelt, um eine bessere Faser-Matrix-Anbindung in naturfaserverstarkten Verbundmaterialien zu erreichen. Die Fasern wurden mit Natronlauge verschiedener Konzentration behandelt, um Oberflachenverunreinigungen zu entfernen und die Faserfeinstruktur zu modifizieren. Diese Fasern wurden dann mit und ohne sauren Katalysator acetyliert, um die Hydrophilie der Fasern zu reduzieren und deren Witterungsbestandigkeit zu verbessern. Die Auswirkung der chemischen Modifizierung wurde mit XRD, DSC, FT-IR und SEM untersucht. Nach der Behandlung ist die Oberflachentopographie der Hanf-, Sisal- und Jutefasern sauber und rauh. Die Oberflache der Kapokfasern blieb offensichtlich unverandert. Rontgenbeugungsmessungen zeigen einen leicht erhohten Kristallinitatsindex der mit niedrigen NaOH-Konzentrationen behandelten Fasern. Hohe NaOH-Konzentrationen erniedrigen jedoch den Kristallinitatsindex. Die thermische Analyse der Fasern weist ebenfalls auf eine Erniedrigung des Kristallinitatsindexes mit zunehmender NaOH-Konzentrationen hin und das die Acetylierung bei hoheren Temperaturen leichter ablauft. Mit der Alkalisierung und Acetylierung konnte die Struktur der Naturfasern erfolgreich modifiziert werden, was deren Leistungsvermogen durch verbesserte Faser-Harz-Bindung erhohen durfte.

Properties of Sisal-CNSL Composites

Cashew nut shell liquid (CNSL) is a natural monomer blend that has been condensation poylmerized with formaldehyde in the presence of an alkaline catalyst to produce a thermosetting resin. Plain woven mats of mercerized sisal fibre have been impregnated with CNSL-formaldehyde resin to produce plain and corrugated laminated composites that have a mean tensile strength of 24.5 MPa and Youngs modulus of 8.8 GPa. Bending tests have demonstrated that the corrugated composites have adequate strength for roofing applications. Dynamic mechanical thermal analysis has been used to assess the effect of simulated sunlight on composites as a function of time. After long irradiation times it has been deduced that the resin component of the composite undergoes further cross-linking whilst the reinforcing cellulosic sisal fibres suffer some degradation.

Hemp fibre reinforced cashew nut shell liquid composites

Hemp fibre bundles were alkalised at concentrations between 0.8 and 8% NaOH and the change in surface morphology was elucidated using scanning electron microscopy. Fibres were tested in tension and the 4 and 6% NaOH treatment resulted in the highest Youngs modulus and tensile strength of 65 GPa and 1064 MPa respectively. The Youngs modulus and tensile strength of untreated fibres were 38 GPa and 591 MPa respectively. The treated fibres with the highest strength were used as reinforcement for cashew nut shell liquid matrix. Nonwoven fibre mats and unidirectional fibre composites were manufactured by hand lay-up compression moulding. Tensile properties, porosity and fracture surface topography of the composites were analysed. The unidirectional fibre composites exhibited the best mechanical properties and composites with lowest porosity offered the highest mechanical properties. Increase in the moulding pressure resulted in a decrease in the mechanical properties except for untreated nonwoven hemp composites in which the presence of inherent binders in the fibres contributed to an enhanced fibre-matrix interface. The contribution of naturally occurring lignin-containing fibres with natural monomers containing similar phenolic compounds provides a compatible interaction on polymerisation hence improved mechanical properties.

Straw-reinforced polyester composites

Annual crop fibres are rich in cellulose and they are a cheap and rapidly renewable source of fibres with potential for polymer reinforcement. Straw fibres have been incorporated in a polyester resin matrix and the properties of the fibre and composite determined. The fibres have a Youngs modulus of approximately 8 GN m−2 and an effective density of 5.1 kN m−3 when combined with resin. Useful composites can be formulated with an optimum fibre volume fraction of about 0.61, resulting in a flexural stiffness of 7.3 GN m−2 and flexural strength of 56 MN m−2. The specific flexural stiffness is about 2.5 times greater than that of polyester resin and about half that of softwoods and GRP. The work of fracture measured in impact is about half that of softwoods. It is envisaged that alternative methods for processing the fibres and the use of a phenolic resin matrix will improve the composite properties further. Straw-based composites are suitable as core material for structural board products.

Jute and glass fibre hybrid laminates

Hybrid laminates have been fabricated from randomly oriented jute fibre mats and woven glass fabrics with a common polyster resin matrix. Hand lay up techniques were used to simulate practical production methods in the field. A variety of laminate constructions were mechanically tested and some laminates were in addition assessed for environmental stability. Modified rule of mixtures expressions successfully predicted the tensile properties of the laminates and the jute plies were seen to control the failure of hybrid laminates at about 0.8% strain. Fracture toughness measurements of GIC andKIC indicate that hybrid laminates have maximum toughness (GIC ≈ 12 kJ m−2 when jute plies are sandwiched between glass fabric facings. All the hybrid laminates were found to be tough in impact, although here fabric plies used as the laminate core maximize the work of fracture at a value of approximately 45 kJ m−2. Hybrid laminates with jute facings are, as expected, least able to withstand hot moist environments. However, significant moisture uptake by the polyester resin matrix was measured for all laminates. Optical and scanning electron microscopy have been used to explain the mechanical performance and environmental resistance of the hybrid laminates.

The determination of porosity and cellulose content of plant fibers by density methods

A combination of the Archimedes method and the technique of helium pycnometry was used to perform accurate measurements of porosity and cellulose content. For this purpose, the densities of hemp, sisal, jute and kapok fibers were measured. An equation was developed using the bulk and absolute density of the fiber and the density of pure cellulose to determine the amount of cellulose a particular fiber contains. As a result, true plant fiber porosity and cellulose content were accurately determined.

Bonded-in rod connections for timber structures—development of design methods and test observations

Abstract Bonded-in rods provide the potential for engineering design of connections in timber structures to be expanded beyond the common types of connection devices currently employed. The achievement of improved connections is the major challenge in timber engineering and has great potential in a wide scope of new build and renovation projects. The achievement of transfer of technologies and techniques from the realm of research to actual practice is reliant upon usability of codified design methods and compatibility with the wider scope of design codes. This paper provides an overview of the key aspects for address in relation to British Standard and Eurocode design of timber structures and draws upon examples of significant knowledge development to support code development achieved through full-scale structural testing of joints. This includes duration of load tests involving adhesive bonded rods with three types of adhesive, subject to high fractions of ultimate load in an outdoor climate, and fatigue studies.