Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

The effects of the injection moulding temperature on the mechanical properties and morphology of polypropylene man-made cellulose fibre composites

Composites made of polypropylene and man-made cellulose fibres that are intended for injection moulding applications show potential for use in sustainable and light weight engineering with high energy absorption capacity. Due to the thermal sensitivity of the cellulose fibres, process parameters play an important role during the injection moulding process. A polypropylene and a man-made cellulose fibre were chosen for this investigation. Effective melt temperatures between 200 °C and 269 °C were used to process the compounds into test specimens. Tensile, impact and colorimetric tests, as well as an SEM analysis, and a measurement of the fibre length distribution were carried out in order to characterise the mechanical and optical properties of the composites. It was observed that the fibre length becomes shorter above 256 °C and elongation at break and Charpy strength (notched) of the composites already decrease at lower temperatures than tensile strength. A direct correlation between mechanical properties and discoloration was not observed. Therefore, melt temperatures up to 250 °C are suitable for these composites.     

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 5. Injection moulded long and short fibre PP

We present results of a step by step comparison of the mechanical performance of injection moulded ‘long’ (LF-PP) and ‘short’ (SF-PP) glass fibre-polypropylene compounds. The study allows direct comparison of the mechanical performance of long and short fibre systems in the same resin at the same fibre diameter, and the effect of fibre diameter in short fibre compounds. Furthermore, the comparison of these three systems has been made over the 0–40 wt% fibre content range. At the same fibre diameter and fibre content LF-PP gives significant improvements in room temperature tensile and flexural strength, notched and unnotched impact resistance. The improvement in impact resistance is higher still at lower test temperature. LF-PP also gives increasingly higher modulus over SF-PP as the strain is increased. The effect of lowering the fibre diameter in SF-PP has been shown to increase both strength and unnotched impact, but not to the levels obtained with LF-PP at higher fibre diameter. Notched impact and modulus of SF-PP were relatively unaffected by reduction of the fibre diameter. The relative mechanical data are shown to conform well to available models. The results are discussed in terms of the relevant micro-mechanical parameters of these materials.     

Effect of coupling agent on mechanical properties of hollow carbon microsphere/phenolic resin syntactic foam

Various hollow carbon microspheres/phenolic resin syntactic foams were prepared by varying the content of hollow carbon microspheres. To improve the compressive and flexural strength and fracture toughness of syntactic foam, the hollow carbon microspheres were oxidized in nitric acid and followed by the treatment with a coupling agent of glutaric dialdehyde. Compression, flexural, and fracture toughness tests were performed to observe the mechanical behavior of the foams. Results demonstrated that compressive and flexural strength decrease with increasing filler content. On the other hand, it is found that better interfacial adhesion could be induced from coupling agent treated hollow carbon microspheres, hence resulted in better mechanical properties. The fracture toughness results revealed that the use of coupling agent increases the fracture toughness and the maximum fracture toughness values occurred at 28.12 vol.% for both untreated and coupling agent treated hollow carbon microspheres. The toughening mechanisms, such as crack deflection, crack bowing, and debonding mechanism, were discussed.     

Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling

Recycling carbon fibres from waste composite materials would only be efficient if it were possible to separate the fibres and the matrix and to re-use the recycled fibres as new reinforcements. The challenge is to use non-continuous fibres to produce high-strength materials. The formation of defects in “semi-long” fibre composites has not yet been taken into account. In this paper the influence of fibre length and fibre alignment on the strength and the modulus of composite materials is illustrated. It is shown that the presence of defects may be modelled in order to understand what the quality of a second generation composite material would be.     

The influence of fibre properties of the performance of glass-fibre-reinforced polyamide 6,6

We discuss the effect of fibre strength and diameter on the balance of mechanical properties of glass-reinforced polyamide 6,6. The results show that the elastic properties of injection-moulded short-glass-fibre-reinforced polyamide 6,6 are not strongly influenced by fibre diameter in the 10–17 micron range. The ultimate properties of these composites (strength and Izod impact behaviour) showed a clear dependence on fibre diameter and were increased by the presence of high-strength S-2 glass fibres. The relationship between the observed mechanical properties and the length, diameter and orientation of the fibres is explored. We have measured fibre length as a function of diameter in composites containing a single glass-reinforcement product and blends of two glass products. The reduction in glass-fibre length from glass-fibre production to final composite moulding has been followed step by step. The final composite mechanical properties, the fibre length, strength, diameter and orientation are all inter-related.     

Effect of coupling agent on natural fibre in natural fibre/polypropylene composites on mechanical and thermal behaviour

To enhance the adhesion between the natural fibre and the thermoplastic matrix, a coupling agent of maleic anhydride grafted polypropylene MAPP is applied. In literature, there are different guidelines of the optimum percentage required of MAPP. Therefore, a systematic work is carried out to optimise the MAPP percent with respect to the type of the natural fibre. Different parameters are investigated namely; Coupling agent ratio to the fibre (0%, 6.67%, 10%, 13.3%, 16.67%), coupling agent source, fibre type (flax, hemp, sisal), and fibre content (30%, 50%). Composite is produced using a kneader and the resulting material is assessed mechanically, thermally, microscopically and for water absorption. For different MAPP source and the natural fibre type, optimum MAPP to fibre ratio is found in average to range between 10% and 13.3% according to the investigated property (stiffness, strength and impact). Increase of MAPP is found to decrease the melting temperature. The thermal behaviour is also linked to the copolymer molecular weight.     

Regenerating the strength of thermally recycled glass fibres using hot sodium hydroxide

Results are presented from the ReCoVeR project on the regeneration of the strength of thermally conditioned glass fibres. Thermal recycling of end-of-life glass fibre reinforced composites or composite manufacturing waste delivers fibres with virtually no residual strength or value. Composites produced from such fibres also have extremely poor mechanical performance. Data is presented showing that a short hot sodium hydroxide solution treatment of such recycled fibres can more than triple their strength and restore their ability to act as an effective reinforcement in second life composite materials. The implications of these results for real materials reuse of recycled glass fibres as replacement for pristine reinforcement fibres are discussed.     

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.     

Pull-through performance of carbon fibre epoxy composites

An investigation of the through-thickness properties of carbon fibre prepreg laminates, Non-Crimp Fabric laminates and non-crimp 3D orthogonal woven composites by pull-through testing was performed. Influence of matrix system and curing temperature on the performance of the 3D woven composites was investigated.     

Effect of removing polypropylene fibre surface finishes on mechanical performance of kenaf/polypropylene composites

Natural fibre/polypropylene thermoplastic composites are often produced by compression moulding of a blended preform of polypropylene fibre and natural fibre treated by chemicals or enzymes. Two preform processing routes may be adopted: (1) treating the natural fibre first and then blending it with the polypropylene fibre (the pre-treatment route), and (2) forming a blended preform of the natural fibre and polypropylene fibre first and then carrying out the chemical/enzyme treatment on the blended preform (the post-treatment route). The kenaf/polypropylene composites produced according to the post-treatment route show up to 36% higher flexural strength and up to 63% higher flexural modulus than the composites produced according to the corresponding pre-treatment route. These differences were attributed to the chemical surface finishes of the polypropylene fibre, which have been removed in the post-treatment processing route, but persisted into the final composites in the pre-treatment processing route.     

Growth of carbon nanotubes on carbon fibre substrates to produce hybrid/phenolic composites with improved mechanical properties

Carbon nanotubes were grown by chemical vapor deposition (CVD) on different carbon fibre substrates namely, unidirectional (UD) carbon fibre tows, bi-directional (2D) carbon fibre cloth and three dimensional (3D) carbon fibre felt. These substrates were used as the reinforcement in phenolic resin matrix to develop hybrid CF–CNT composites. The growth morphology and other characteristics of the as grown tubes were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetry (TGA) which confirmed a copious growth of multiwalled carbon nanotubes (MWNTs) on these substrates. The mechanical properties of the hybrid composites was found to increase with the increasing amount of deposited carbon nanotubes. The flexural strength (FS) improved by 20% for UD, 75% for 2D and 66% for 3D hybrid composites as compared to that prepared by neat reinforcements (without CNT growth) under identical conditions. Flexural modulus (FM) of these composites also improved by 28%, 54% and 46%, respectively.     

Interfacial properties of phosphate glass fibres/PLA composites: Effect of the end functionalities of oligomeric PLA coupling agents

Polylactic acid (PLA) oligomers as coupling agents have shown higher interfacial shear strength in phosphate based glass fibres/PLA composites. To influence bonding on the glass fibre surface, short chain PLA oligomers with different end groups were used as coupling agents. The low molecular weight PLA with a sodium salt terminal group, a carboxylic acid end group and also with one, two and five hydroxyl groups were produced and applied on the fibre surface through a condensation reaction. Mechanical properties of the sized fibres/PLA composites were found to be increased. XPS and TG analyses showed the presence of the coupling agents on the fibres surface. SEM analysis further confirms the presence of the agents.     

Carbon fibre reinforced poly(vinylidene fluoride): Impact of matrix modification on fibre/polymer adhesion

The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibres and the composition of the matrix. The composition of poly(vinylidene fluoride) (PVDF) was modified by the addition of maleic anhydride grafted PVDF. The surface properties of the various matrix formulations were characterised by contact angle and electrokinetic measurements. Carbon fibres were modified by industrial electrochemical oxidation and oxidation in nitric acid, or the use of a traditional epoxy-sizing of industrially oxidised fibres. The surface composition, morphology and wetting behaviour of the carbon fibres was characterised. The interaction between modified PVDF and the carbon fibres was studied by direct contact angle measurements between PVDF melt on single carbon fibres and by single fibre pull-out tests. The best wetting and adhesion behaviour was achieved between PVDF containing 5 ppm grafted maleic anhydride (MAH) and epoxy-sized carbon fibres. The addition of MAH-grafted PVDF to the unmodified PVDF caused the apparent interfacial shear strength to increase by 184%. The apparent interfacial shear strength of this fibre–matrix combination allowed for the utilisation of 100% of the yield tensile strength of PVDF.     

Mechanical studies of single glass fibres recycled from hydrolysis process using sub-critical water

The study reports mechanical performance of the recycled glass fibres produced from a water-based solvolysis technology, known as the hydrolysis process. The chemical reaction was carried out using sub-critical water to dissolve polyester resin and recover the glass fibres from composites. The effect of temperatures, times, catalyst and water amount on mechanical properties of the recovered glass fibres were investigated. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and time-of flight secondary ion mass spectroscopy (ToF-SIMS) analyses were also employed to examine the fibre surface associated with the polyester resin eliminated level after the hydrolysis reaction. The results revealed that by carefully adjusting the hydrolysis parameters the tensile strength and failure strain of the recycled fibres decrease by approximately 40-70% in comparison with virgin fibres while Young’s moduli remain similar. The relationship between the hydrolysis conditions, recovered fibres and mechanical performance was discussed in this study.     

Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol

Three different PAN based carbon fibres (Toray T600S, T700S and Tenax STS5631) were recycled from epoxy resin/carbon fibre composites using supercritical n-propanol. The recycled carbon fibres were characterised using single fibre tensile tests, SEM, XPS and micro-droplet test. The tensile strength and modulus of the recycled carbon fibre was very similar to the corresponding as-received carbon fibres. However, the surface oxygen concentration decreased significantly, which caused a reduction of the interfacial shear strength with epoxy resin.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.     

Influence of titanium carbide on the interlaminar shear strength of carbon fibre laminate composites

The potential use of carbon fibre laminate composites is limited by the weak out-of-plane properties, especially delamination resistance. The effect of incorporating titanium carbide to the mesophase pitch matrix precursor of carbon fibre laminate composites on interlaminar shear strength is studied both on carbonised and graphitised composites. The presence of titanium carbide modifies the optical texture of the matrix from domains to mosaics in those parts with higher concentrations and it contributes to an increase of fibre/matrix bonding. This fact produces an increase of the interlaminar shear strength of the material and changes the fracture mode.     

Effect of crystallite size of zinc oxide on the mechanical,thermal and flow properties of polypropylene/zinc oxide nanocomposites

ZnO nanoparticles were prepared using zinc chloride and sodium hydroxide in chitosan medium. Prepared ZnO (NZO) and commercial ZnO (CZO) was characterized by scanning electron microscopic and X-ray diffraction studies. PP/ZnO nanocomposites were prepared using 0–5 wt% of zinc oxide by melt mixing. It was then compression moulded into films. Transparency of the composite films were improved by reducing the crystallite size of ZnO. Melt flow index studies revealed that NZO increased the flow characteristics of PP while CZO decreased. X-ray diffraction studies indicated α-form of isotactic polypropylene. An increase in mechanical properties, dynamic mechanical properties and thermal stability of the composites were observed by the addition of ZnO. Uniform dispersion of the ZnO was observed in the scanning electron micrographs of the tensile fractured surface of composites.