Extrusion and mechanical properties of highly filled cellulose fibre–polypropylene composites

This study focused on manufacturing of highly filled cellulose fibre–polypropylene composites and evaluation of the mechanical properties of the composites. Cellulose fibre reinforced polypropylene composites with up to 60 wt% of fibres with and without coupling agent were manufactured by extrusion. In order to achieve consistent feeding of the fibres into the extruder a pelletization technique was used where the fibres were pressed into pellets. Two commercial grades of cellulose fibres were used in the study, bleached sulfite and bleached kraft fibres. Fibre dimension measurements showed that the pelletization process and extrusion at high fibre loading caused the most severe fibre breakage. Flexural testing showed that increased fibre loading made the composites stiffer but reduced the toughness. Addition of maleic anhydride grafted coupling agent (MAPP) increased the stiffness and strength of the composites significantly. In general, there was no significant difference in the mechanical properties between the composites with kraft and sulfite fibres. An interesting finding was that the flexural modulus and strength of the MAPP modified cellulose fibre–polypropylene composites were not higher than what has previously been reported for wood flour–polyolefin composites. Scanning electron microscopy showed that addition of coupling agent improved the interfacial adhesion between the fibres and polypropylene matrix.     

Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from yarns perform better than composites made from random nonwoven mats. However, the twist structure of conventional ring spun yarns prevents the full utilization of fibre mechanical properties in the final composites. We produced flax/polypropylene commingled wrap yarns in which all flax fibres were twistless. Composites made from the wrap yarn demonstrated significant improvement of flexural modulus. Most currently available low cost natural fibres, such as decorticated hemp, cannot be efficiently made into yarns because of their lack of cohesion. Adding polypropylene fibres to decorticated hemp improved textile processing performance. The polypropylene fibres served as a carrier for the natural fibres during processing and became the polymer matrix in the final composites.     

Chemical modification of flax reinforced polypropylene composites

This paper presents an experimental study on the static and dynamic mechanical properties of nonwoven based flax fibre reinforced polypropylene composites. The effect of zein modification on flax fibres is also reported. Flax nonwovens were treated with zein coupling agent, which is a protein extracted from corn. Composites were prepared using nonwovens treated with zein solution. The tensile, flexural and impact properties of these composites were analysed and the reinforcing properties of the chemically treated composites were compared with that of untreated composites. Composites containing chemically modified flax fibres were found to possess improved mechanical properties. The viscoelastic properties of composites at different frequencies were investigated. The storage modulus of composites was found to increase with fibre content while damping properties registered a decrease. Zein coating was found to increase the storage modulus due to enhanced interfacial adhesion. The fracture mechanism of treated and untreated flax reinforced polypropylene composites was also investigated from scanning electron microscopic studies.     

Mechanical enhancement of cement-stabilized soil by flax fibre reinforcement and extrusion processing

Cement-based materials typically exhibit low tensile strength and their behaviour is generally brittle. Fibres can be added to make composites with enhanced tensile response and toughness. This study focuses on the effects of flax fibre content, mix design and processing on the hardened product properties (density, fibre orientation, surface quality, compressive and tensile strength). Effects of fibre addition on the mechanical performance of cast and extruded flax fibre reinforced composites are compared. Microstructure observations are used to study the influence of processing on fibre–matrix bond, fibre dispersion and fibre orientation. Flax fibre dispersion and orientation are also investigated to understand their effect on mechanical behaviour. In the case of cast materials, fibres do not significantly improve the mechanical behaviour. In contrast, improvement of fibre dispersion and fibre/matrix bond quality due to an extrusion process enhances mechanical performance.     

Influence of the Physical Structure of Flax Fibres on the Mechanical Properties of Flax Fibre Reinforced Polypropylene Composites

This study investigates the influence of the physical structure of flax fibres on the mechanical properties of polypropylene (PP) composites. Due to their composite-like structure, flax fibres have relatively weak lateral bonds which are in particular present in flax fibres that are often used in natural fibre mat reinforced thermoplastics (NMT). These weak bonds can be partly removed by combing the fibres. In order to study the influence of the physical structure of flax fibres on NMT tensile and flexural properties, uncombed and combed flax fibre reinforced PP composites were manufactured via a wet laid process. The influence of improved fibre-matrix adhesion was studied using maleic-anhydride grafted PP. Results indicated that the flax physical structure has a significant effect on flax-PP composite properties and that the flax fibre reinforced PP properties are similar to values predicted with existing micromechanical models. The tensile modulus of flax-PP composites can fairly compete with commercial glass mat reinforced thermoplastic (GMT) modulus, the strength, however, both tensile and flexural, can not. In order to rise the strength of flax fibre reinforced PP composites to the level of GMT strength, the flax fibres have to be further isolated to elementary flax fibres.     

Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder

A novel robust non-woven sisal fibre preform was manufactured using a papermaking process utilising nanosized bacterial cellulose (BC) as binder for the sisal fibres. It was found that BC provides significant mechanical strength to the sisal fibre preforms. This can be attributed to the high stiffness and strength of the BC network. Truly green non-woven fibre preform reinforced hierarchical composites were prepared by infusing the fibre preforms with acrylated epoxidised soybean oil (AESO) using vacuum assisted resin infusion, followed by thermal curing. Both the tensile and flexural properties of the hierarchical composites showed significant improvements over polyAESO and neat sisal fibre preform reinforced polyAESO. These results were corroborated by the thermo-mechanical behaviour of the (hierarchical) composites, which showed an increased storage modulus and enhanced fibre–matrix stress transfer. Micromechanical modelling was also performed on the (hierarchical) composites. By using BC as binder for short sisal fibres, added benefits such as the high Young’s modulus of BC, enhanced fibre–fibre and fibre–matrix stress transfer can be utilised in the resulting hierarchical composites.     

Manufacturing sisal–polypropylene composites with minimum fibre degradation

Natural fibres, such as sisal, flax and jute, possess good reinforcing capability when properly compounded with polymers. These fibres are relatively inexpensive, originate from renewable resources and possess favourable values of specific strength and specific modulus. Thermoplastic polymers have a shorter cycle time as well as reprocessability despite problems with high viscosities and poor fibre wetting. The renewability of natural fibres and the recyclability of thermoplastic polymers provide an attractive eco-friendly quality to the resulting natural fibre-reinforced thermoplastic composite materials. Common methods for manufacturing natural fibre-reinforced thermoplastic composites, injection moulding and extrusion, tend to degrade the fibres during processing. Development of a simple manufacturing technique for sisal fibre-reinforced polypropylene composites, that minimises fibre degradation and can be used in developing countries, is the main objective of this study. Composite sheets with a fibre length greater than 10 mm and a fibre mass fraction in the range 15% to 35% exhibited good mechanical properties.     

Wheat flour thermoplastic matrix reinforced by waste cotton fibre: Agro-green-composites

New composites materials, 100% ecofriendly, having waste cotton fibre as reinforcement in wheat flour based thermoplastic matrix were prepared by extrusion method. The fibre content in the composite varied from 0 to 15% w/w. Using X-ray diffraction and scanning electron microscopy, the structure and morphology of the composites have been analysed. This investigation is focused on the effects of the fibre content on the mechanical and thermal properties of the composites. Addition of the waste cotton fibre to the matrix increased the tensile properties. For the composite with 10% w/w of fibre the values of the tensile stress are found maximum. We also show that thermal conductivity and resistivity are not affected by the fibre content. By thermogravimetry we show that the addition of fibres to the matrix has no significant influence on the thermal stability of the composites. Finally, to analyse the efficiency of the present system, a comparative study of the mechanical properties obtained with flax and cotton fibres is performed.     

Experimental and theoretical assessment of flexural properties of hybrid natural fibre composites

The concept of hybridization of natural fibre composites with synthetic fibres is attracting increasing scientific attention. The present study addresses the flexural properties of hybrid flax/glass/epoxy composites to demonstrate the potential benefits of hybridization. The study covers both experimental and theoretical assessments. Composite laminates with different hybrid fibre mixing ratios and different layer configurations were manufactured, and their volumetric composition and flexural properties were measured. The relationship between volume fractions in the composites is shown to be well predicted as a function of the hybrid fibre mixing ratio. The flexural modulus of the composites is theoretically assessed by using micromechanical models and laminate theory. The model predictions are compared with the experimentally determined flexural properties. Both approaches show that the flexural modulus of the composites is consistently increased when the flax fibre fabrics are replaced by glass fibre fabrics from the inner layers to the outer layers. The observed deviations between the experimental and theoretical values are explained by the simplifying model assumptions applied for the configuration of the composites, in particular the flax fibre composites. This needs to be addressed in further work.     

The effect of reprocessing on the mechanical properties of polypropylene reinforced with wood pulp,flax or glass fibre

Composites of polypropylene, substitutable for a given application and reinforced with: Medium Density Fibreboard fibre (MDF) (40 wt%); flax (30 wt%); and glass fibre (20 wt%), were evaluated after 6 injection moulding and extrusion reprocessing cycles. Of the range of tensile, flexural and impact properties examined, MDF composites showed the best mean property retention after reprocessing (87%) compared to flax (72%) and glass (59%). After 1 reprocessing cycle the glass composite had higher tensile strength (56.2 MPa) compared to the MDF composite (44.4) but after 6 cycles the MDF was stronger (35.0 compared to 29.6 MPa for the glass composite). Property reductions were attributed to reduced fibre length. MDF fibres showed the lowest reduction in fibre length between 1 and 6 cycles (39%), compared to glass (51%) and flax (62%). Flax fibres showed greater increases in damage (cell wall dislocations) with reprocessing than was shown by MDF fibres.     

Fabrication and testing of natural fibre composites: Vakka, sisal, bamboo and banana

A study has been carried out to investigate the tensile, flexural and dielectric properties of composites made by reinforcing vakka as a new natural fibre into a polyester resin matrix. The fibres extracted by retting and manual processes have been used to fabricate the composites. These composites are tested for tensile, flexural and dielectric properties and compared with those of established composites like sisal, bamboo and banana made under the same laboratory conditions. The composites are fabricated up to a maximum volume fraction of fibre of 0.37 in the case of tensile testing, and 0.39 for flexural and dielectric testing. It has been observed that the tensile properties increase with respect to volume fraction of fibre for vakka fibre composite and are also more than those of sisal and banana composites and comparable to those of bamboo composites. The flexural strength of vakka fibre composite is more than that of banana composite and is closer to sisal fibre composite with respect to the volume fraction of fibre, where as the flexural modulus is much higher than those of banana and sisal fibre composites and also very much closer to bamboo fibre composites. The dielectric strength of vakka fibre composite increases with increase in volume fraction of fibre in the composite unlike the case of sisal, bamboo and banana composites. The dielectric strength being a unique feature of vakka fibre composite, can be suggested for electrical insulation applications.     

The manufacture and mechanical testing of thermosetting natural fibre composites

High volume fraction hemp and flax fibre composites were manufactured using low viscosity epoxy and phenolic resins. Using 80% volume fraction of flax fibres in epoxy resin, composites with a mean stiffness of 26 GPa and a mean strength of 378 MPa were produced. By reducing processing damage of the plant fibres mechanical properties could be increased by 40%. Strips of retted fibre tissue were found to be just as effective for reinforcement as fibre bundles and individual fibres. Phenolic resin and decorticated flax fibres produced very poor composites. Using 40% volume fraction of fibres the mean stiffness was 3.7 GPa and the mean strength was 27 MPa. Two fibre pre-treatments were devised to improve adhesion with resins. The first, 6 M urea was used only in natural fibre-epoxy composites where it increased the stiffness but not the strength. The second pre-treatment was a 50% PVA solution, which was cured prior to the addition of space filling resin. The PVA treatment improved the stiffness and strength of both natural fibre-epoxy composites and natural fibre-phenolic composites.     

Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization

The effect of fibre treatments and matrix modification on mechanical properties of flax fibre bundle/polypropylene composites was investigated. Treatments using chemicals such as maleic anhydride, vinyltrimethoxy silane, maleic anhydride-polypropylene copolymer and also fibre alkalization were carried out in order to modify the interfacial bonding between fibre bundles and polymeric matrix. Composites were produced by employing two compounding ways: internal mixing and extrusion. Mechanical behaviour of both flax fibre bundle and hybrid glass/flax fibre bundle composites was studied. Fracture surfaces were investigated by scanning electron microscopy. Results suggest that matrix modification led to better mechanical performance than fibre surface modification. A relevant fact is that silanes or MA grafted onto PP matrix lead to mechanical properties of composites even better than those for MAPP modification, and close to those for glass fibre/PP.     

Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement

“Hairy” bacterial cellulose coated sisal fibres were created using a simple slurry dipping process. Neat sisal fibres were coated with BC to create (i) a dense BC coating around the fibres or (ii) “hairy” fibres with BC oriented perpendicular to the fibre surface. These fibres were used to produce hierarchical sisal fibre reinforced BC polylactide (PLLA) nanocomposites. The specific surface area of the BC coated fibres increased when compared to neat sisal. Single fibre tensile tests revealed no significant difference in the tensile modulus and tensile strength of “hairy fibres”. However, when sisal fibres were coated with a dense BC layer, the mechanical fibre properties decreased. The tensile, flexural and visco-elastic properties of the hierarchical PLLA nanocomposites reinforced by both types of BC coated sisal fibres showed significant improvements over neat PLLA.     

Developing and characterizing new materials based on waste plastic and agro-fibre

This study optimizes flexural properties of composites made from waste materials, corn stalk, seed flax and Agave americana fibres along with waste linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) matrices. The surface morphology of fibres and composites was characterized with light and scanning electron microscopes. Thermal behaviour of the corn stalk outer rings and pith parenchyma was shown using thermogravimetric analysis. There was no significant difference between the LLDPE composites made with corn stalk outer rings versus whole stalks, possibly due to an insignificant role played by parenchymatous part of the pith. Water retted seed flax fibres in LLDPE matrix optimized the composite flexural strengths at 6–8 days of retting, above which the properties declined. Field retted seed flax varieties in LLDPE matrix showed no significant differences in their flexural strengths. With additional fibre loading, flexural strength of A. americana HDPE composites first dropped before improving after extrusion, and only improved when using the layer method.     

Effects of organic peroxide and polymer chain structure on morphology and thermal properties of sisal fibre reinforced polyethylene composites

The effect of polymer chain structure, addition of dicumyl peroxide (DCP) to initiate grafting onto the fibre, and different fibre loadings on the morphology and thermal properties of polyethylene/sisal fibre composites was investigated. The gel content results suggest both crosslinking between the polyethylene chains and grafting onto the sisal fibres. There were significant differences in gel contents between the composites because of the differences in the polyethylene molecular structures. The SEM micrographs of the samples show clear evidence of grafting, particularly in the case of the LDPE and LLDPE composites. The presence of the sisal fibres gave rise to thermally less stable composites compared to the neat matrices, whereas marginal differences in stability were observed between the untreated and peroxide treated composites. The DSC results show interesting trends in terms of the influence of fibre content and dicumyl peroxide treatment on the crystallisation behaviour of the composites.     

Evaluation of mechanical and thermal properties of banana–flax based natural fibre composite

Hybrid materials of any kind are the keynote for today’s demands. This paper deals with one of such hybrid composite made of natural fibres namely, banana and flax fibres. The structural build-up is such that one layer of banana fibre is sandwiched between two layers of flax fibres by hand layup method with a volume fraction of 40% using Epoxy resin and HY951 hardener. Glass fibre reinforcement polymer (GFRP) is used for lamination on both sides. This lamination also increases the overall mechanical properties along with better surface properties. The properties of this hybrid composite are determined by testing its tensile, impact, and flexural loads using a Universal testing machine. Thermal properties are analysed and hybrid composites of flax and banana with GFRP have better thermal stability and flame resistance over flax, banana with GFRP single fibre hybrid composites. Morphological analysis is done using Scanning Electron Microscope (SEM). The result of test shows that hybrid composite has far better properties than single fibre glass reinforced composite under impact and flexural loads. However it is found that the hybrid composite have better strength as compared to single fibre composites.     

Could biopolymers reinforced by randomly scattered flax fibre be used in structural applications?

Thermoplastics reinforced by natural fibres are mainly used for fitting-up products in the automotive industry. The aim of this work is to study the tensile properties of natural fibre-biopolymer composites in order to determine whether or not, biocomposites may replace glass fibre reinforced unsaturated polyester resins. The materials used are flax fibre, polylactic acid (PLA), l-polylactide acid (PLLA), poly(3-hydroxylbutyrate) (PHB), polycaprolactone and starch thermoplastic (MaterBi® Z), poly(butylene succianate) (PBS) and poly(butylene adipate-co-terephtalate) (PBAT). The tensile properties of the flax fibres have already been determined [C. Baley, Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Comp Part A 2002;33:939–948]. The composites are manufactured using a film stacking technique. After studying the processing parameters, these are then adapted to each thermoplastic composites. Test samples are cut out from the composites to test their mechanical properties under tensile loading conditions. These tensile properties are then compared to those of similar polypropylene flax composites. Preliminary results show that the tensile properties are improved with the fibre volume fraction. The tensile strength and Young’s modulus of PLLA and PLA flax composites are greater than those of similar PP/flax fibre composites. The specific tensile strength and modulus of flax fibre/PLLA composite have proved to be very close to those of glass fibre polyester composites.     

Mechanical properties of natural fibre reinforced polyester composites: Jowar,sisal and bamboo

In this paper, the experiments of tensile and flexural tests were carried out on composites made by reinforcing jowar as a new natural fibre into polyester resin matrix. The samples were prepared up to a maximum volume fraction of approximately 0.40 from the fibres extracted by retting and manual process, and compared with established composites like sisal and bamboo developed under similar laboratory conditions. Jowar fibre has a tensile strength of 302 MPa, modulus of 6.99 GPa and an effective density of 922 kg/m³. It was observed that the tensile strength of jowar fibre composite is almost equal to that of bamboo composite, 1.89 times to that of sisal composite and the tensile modulus is 11% and 45% greater than those of bamboo and sisal composites, respectively at 0.40 volume fraction of fibre. The flexural strength of jowar composite is 4%, 35% and the flexural modulus is 1.12 times, 2.16 times greater than those of bamboo and sisal composites, respectively. The results of this study indicate that using jowar fibres as reinforcement in polyester matrix could successfully develop a composite material in terms of high strength and rigidity for light weight applications compared to conventional sisal and bamboo composites.     

Micro and macro analysis of sisal fibre composites hollow core sandwich panels

In the view of the growing environmental concerns, hollow cores from recyclable natural fibre composites were manufactured to reduce the undesirable impact on the environment. To evaluate the feasibility of using short sisal fibres as reinforcements in the composites, existing micromechanical models have been used to predict properties starting from the intrinsic properties of its constituents. The stress relaxation behaviour of the composites was examined experimentally by performing tensile stress relaxation tests and to understand the process, it was modelled using variations of Maxwell’s model. A steady-state finite element analysis in the linear range was performed in ANSYS environment to examine flexural properties of the panels, and the shear strength of the hollow cores was experimentally determined by subjecting them to flexural loads in a four-point bending scheme. The micromechanics models indicated that the fibres had failed to provide effective reinforcements with their existing lengths, acting as fillers rather than reinforcements. The stress relaxation models indicated that the formed part needs to be cooled to room temperature within the die under suitable forming loads to avoid local deformations due to warping. The mid-span deflections of the sandwich panels predicted by the FE model agree well with the experimental results, the analysis predicted facing buckling as a mode of failure when wood veneers facings of modulus 4.5 GPa and thickness 1.7 mm were used. The specific shear strengths of the reinforced core are more than twice than those of the unreinforced polypropylene cores, increasing the scope of such panels as structural members in various engineering facets.