Retention of mechanical performance of polymer matrix composites above the glass transition temperature by vascular cooling

An actively cooled vascular polymer matrix composite containing 3.0% channel volume fraction retains greater than 90% flexural stiffness when exposed continuously to 325 °C environmental temperature. Non-cooled controls suffered complete structural failure through thermal degradation under the same conditions. Glass–epoxy composites (T_g = 152 °C) manufactured by vacuum assisted resin transfer molding contain microchannel networks of two different architectures optimized for thermal and mechanical performance. Microchannels are fabricated by vaporization of poly(lactide) fibers treated with tin(II) oxalate catalyst that are incorporated into the fiber preform prior to resin infiltration. Flexural modulus, material temperature, and heat removal rates are measured during four-point bending testing as a function of environmental temperature and coolant flow rate. Simulations validate experimental measurements and provide insight into the thermal behavior. Vascular specimens with only 1.5% channel volume fraction centered at the neutral bending axis also retained over 80% flexural stiffness at 325 °C environmental temperature.     

High thermal and mechanical properties enhancement obtained in highly filled polybenzoxazine nanocomposites with fumed silica

Highly filled polybenzoxazine nanocomposites filled with nano-SiO₂ particles were investigated for their mechanical and thermal properties as a function of filler loading. The nanocomposites were prepared by high shear mixing followed by compression molding. A very low A-stage viscosity of benzoxazine monomer gives it excellent processability having maximum nano-SiO₂ loading as high as 30 wt% (18.8 vol%) with negligible void content. Moreover, thermal analysis of the curing process of the compound of the PBA-a/nano-SiO₂ composites was found to be autocatalytic in nature with average activation energy of 79–92 kJ mol⁻¹. Microscopic analysis (SEM) performed on the PBA-a/nano-SiO₂ composite fracture surface indicated a nearly homogeneous distribution of the nano-scaled silica in the polybenzoxazine matrix. In addition, the enhancement in storage modulus of the nano-SiO₂ filled polybenzoxazine composites was found to be significantly higher than that of the recently reported nano-SiO₂ filled epoxy composites. The dependence of the nanocomposites’ modulus on the nano-SiO₂ particles content is well fitted by the generalized Kerner equation. Furthermore, the relatively high micro-hardness of the PBA-a/nano-SiO₂ composites up to about 600 MPa was achieved. Finally, the substantial enhancement in the glass transition temperature (T_g) of the PBA-a/nano-SiO₂ composites was also observed with the ΔT_g up to 16 °C at the nano-SiO₂ loading of 30 wt%. The resulting PBA-a/nano-SiO₂ composite is a highly attractive candidate as coating material in electronic packaging or other related applications.     

The potential of harakeke fibre as reinforcement in polymer matrix composites including modelling of long harakeke fibre composite strength

Mechanical properties of aligned long harakeke fibre reinforced epoxy with different fibre contents were evaluated. Addition of fibre was found to enhance tensile properties of epoxy; tensile strength and Young’s modulus increased with increasing content of harakeke fibre up to 223 MPa at a fibre content of 55 wt% and 17 GPa at a fibre content of 63 wt%, respectively. The flexural strength and flexural modulus increased to a maximum of 223 MPa and 14 GPa, respectively, as the fibre content increased up to 49 wt% with no further increase with increased fibre content. The Rule of Mixtures based model for estimating tensile strength of aligned long fibre composites was also developed assuming composite failure occurred as a consequence of the fracture of the lowest failure strain fibres taking account porosity of composites. The model was shown to have good accuracy for predicting the strength of aligned long natural fibre composites.     

Accelerated thermal ageing of epoxy resin and 3-D carbon fiber/epoxy braided composites

This paper reports the accelerated thermal ageing behaviors of pure epoxy resin and 3-D carbon fiber/epoxy braided composites. Specimens have been aged in air at 90 °C, 110 °C, 120 °C, 130 °C and 180 °C. Microscopy observations and attenuated total reflectance Fourier transform infrared spectrometry analyses revealed that the epoxy resin oxidative degradation only occurred within the surface regions. The surface oxidized layer protects inner resin from further oxidation. Both the resin degradation and resin stiffening caused by post-curing effects will influence the compression behaviors. For the braided composite, the matrix ageing is the main ageing mode at temperatures lower than glass transition temperatures (T_g) of the pure epoxy resin, while the fiber/matrix interface debonding could be observed at the temperatures higher than T_g, such as the temperature of 180 °C. The combination of matrix degradation and fiber/resin interface cracking leads to the continuous reduction of compressive behaviors.     

Multi-step microwave reduction of graphite oxide and its use in the formation of electrically conductive graphene/epoxy composites

The multi-step MW reduction technique was developed in this study to obtain reduced graphene oxides; EG, RGO-1, and RGO-2 with MW irradiation time of 1, 2, and 3 min, respectively. Results of TGA, IR, and elemental analysis demonstrated that the degree of reduction of GO increased with increasing the MW irradiation time. Overall, 3 min of MW irradiation of GO in 3 steps was sufficient to obtain highly reduced GO (C/O ratio 10.38 by elemental analysis). The electrical percolation threshold of composites was observed as 1 wt% and 0.3 wt% for RGO-1 and RGO-2, respectively. Even at 0.5 wt% loading of RGO-2 in epoxy, the T_g value of the composite increased by 10 °C, indicating a strong interfacial interaction between graphene and epoxy resin.     

Structure and properties of hybrid PLA nanocomposites with inorganic nanofillers and cellulose fibers

Polylactide reinforced with 3 wt% of organo-modified montmorillonite, 5 wt% of stearic acid-modified calcium carbonate nanoparticles, 15 wt% of cellulose fibers (PLA/MMT, PLA/NCC, PLA/CF) and hybrid composites containing 15 wt% of fibers in addition to montmorillonite (PLA/MMT/CF) or calcium carbonate (PLA/NCC/CF) were prepared and examined. The nanoparticles were dispersed in polylactide almost homogeneously; montmorillonite was exfoliated during processing. T_g of polylactide remained unaffected but its cold crystallization was enhanced; the cold-crystallization behavior of the hybrid composites was dominated by nanofillers nucleating ability. The fibers and calcium carbonate decreased whereas exfoliated montmorillonite improved the thermal stability of the materials. Polylactide, PLA/NCC and PLA/MMT exhibited ability to plastic deformation, although the latter the weakest. Tensile behavior of the hybrid composites was strongly influenced by the fibers and similar to that of PLA/CF. All the fillers increased the storage modulus below T_g; that of PLA/MMT/CF and PLA/NCC/CF was improved with respect to polylactide by 50% and 45%, respectively.     

Introducing cellulose nanocrystals in sheet molding compounds (SMC)

The mechanical properties of short glass fiber/epoxy composites containing cellulose nanocrystals (CNC) made using sheet molding compound (SMC) manufacturing method as well as the rheological and thermomechanical properties of the CNC-epoxy composites were investigated as a function of the CNC content. CNC up to 1.4 wt% were dispersed in the epoxy to produce the resin for SMC production. The addition of CNC in the resin increased its viscosity and slightly reduced the heat of reaction during the polymerization without altering the curing time and temperature and the effective pot life of the resin. The incorporation of 0.9 wt% CNC in the SMC composite resulted in increases in elastic modulus and tensile strength by ∼25% and ∼30% and in flexural modulus and strength by ∼44% and ∼33% respectively. Concentrations of CNC up to 0.9 wt% in the SMC composite did not alter the impact energy.     

Fabrication and characterization of 3-D Cf/ZrC composites by low-temperature liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composite, 3-D C_f/ZrC, were prepared by liquid metal infiltration process at 1200 °C using a Zr₂Cu intermetallic compound as infiltrator. The microstructure and properties of the composites were investigated. The results indicated that ZrC with a yield of 35.2 ± 1.8 vol.% was certified as the major phase of the composites. The formation of ZrC was controlled by a solution-precipitation mechanism. The obtained composites exhibited good mechanical properties, with a flexural strength of 293.0 ± 12.1 MPa, a flexural modulus of 82.7 ± 6.4 GPa and a fracture toughness of 9.8 ± 0.9 MPa m^1/2. The mass and linear ablation rates of the composites exposed to oxyacetylene torch were 0.0013 ± 0.0005 g s⁻¹ and −0.0009 ± 0.0003 mm s⁻¹, respectively. The formation of a dense ZrO₂ protective layer and the evaporation of residual Cu contributed mainly to the excellent ablation resistance.     

New development of self-damping MWCNT composites

Epoxy resin modified with nanofillers cannot be used alone for high performance structural applications due to their low-mechanical properties. Therefore, the objective of this work is to hybridize unidirectional and quasi-isotropic glass fiber composite laminates with 1.0 wt% multi-walled carbon nanotubes (MWCNTs). Results from flexural and damping characterizations showed that the flexural strength and modulus, storage modulus, and damping ratio of MWCNT/E nanocomposite are improved by about 7% ± 1.5% compared to neat epoxy. The enhancement in the flexural strength of quasi-isotropic laminate (20.7%) is about ten times higher than that for unidirectional laminate (2.1%). The flexural moduli of the nano-hybridized laminates are reduced by about 7.5–10.8%. Accordingly, the ultimate failure strain and damping properties are evidently improved. The improvement in damping ratio in some cases is about 100%. The high correlation coefficient (0.9995) between flexural and storage moduli suggests using the dynamic nondestructive tests for evaluation the elastic properties of composites.     

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.     

Ambient cure POSS–epoxy matrices for marine composites

A room-temperature cure epoxy consisting of diglycidyl ether of bisphenol-F (DGEBF) and diethylene triamine (DETA) was modified with 26.5 wt.% and 63.5 wt.% octaglycidyl polyhedral oligomeric silsesquioxane (POSS) to investigate elevated temperature thermomechanical performance. Composites fabricated using vacuum assist resin transfer molding (VARTM) were compared to vinylester and epoxy standards. POSS modified matrices were low viscosity of 0.25–0.40 Pa s. Although T_g was 20% lower than vinylester, we observed an increase of >300% in 150 °C storage modulus, >50% in tensile modulus, >35% in flexural modulus, and the complete elimination of a heat distortion temperature (HDT) up to 200 °C. The matrices demonstrated an excellent balance of flow, wetting, and pot-life behaviors making them attractive alternatives for ambient cure marine applications.     

Highly mechanical strength and thermally conductive bismaleimide–triazine composites reinforced by Al2O3@polyimide hybrid fiber

Bismaleimide–triazine (BT) resins have received a great deal of attention in microelectronics due to its excellent thermal stability and good retention of mechanical properties. Thereafter, developing BT based composites with high mechanical strength, thermal conductivity and dielectric property simultaneously are highly desirable. In this study, one hybrid fiber of Al₂O₃ nanoparticle (200 nm) supported on polyimide fiber (Al₂O₃@PI) with core–shell structure was introduced into BT resin to prepare promising Al₂O₃@PI–BT composite. The results indicated that the resultant composites possessed high Young’s modulus of 4.06 GPa, low dielectric constant (3.38–3.50, 100 kHz) and dielectric loss (0.0102–0.0107, 100 kHz). The Al₂O₃@PI hybrid film was also conductive to improve thermal stability (T_d5% up to 371 °C), in-plane thermal conductivity (increased by 295% compared to that of the pure BT resin). Furthermore, the Al₂O₃@PI–BT composite were employed to fabricate a printed circuit substrate, on which a frequency “flasher” circuit and electrical components worked well.     

Grafting of nano-TiO2 onto flax fibers and the enhancement of the mechanical properties of the flax fiber and flax fiber/epoxy composite

In this article, a flax fiber yarn was grafted with nanometer sized TiO₂, and the effects on the tensile and bonding properties of the single fibers and unidirectional fiber reinforced epoxy plates were studied. The flax fiber yarn was grafted with nanometer sized TiO₂ through immersion in nano-TiO₂/KH560 suspensions under sonification. The measured grafting content of the nano-TiO₂ ranged from 0.89 wt.% to 7.14 wt.%, dependent on the suspension concentration. With the optimized nano-TiO₂ grafting content (∼2.34 wt.%), the tensile strength of the flax fibers and the interfacial shear strength to an epoxy resin were enhanced by 23.1% and 40.5%, respectively. The formation of Si–O–Ti and C–O–Si bonds and the presence of the nano-TiO₂ particles on the fiber surfaces contributed to the property enhancements. Unidirectional flax fiber reinforced epoxy composite (V_f = 35.4%) plates prepared manually showed significantly enhanced flexural properties with the grafting of nano-TiO₂.     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

The effect of graphene presence in flame retarded epoxy resin matrix on the mechanical and flammability properties of glass fiber-reinforced composites

The effect of adding graphene in epoxy containing either an additive (MP) or reactive-type (DOPO) flame retardant on the thermal, mechanical and flammability properties of glass fiber-reinforced epoxy composites was investigated using thermal analysis; flexural, impact, tensile tests; cone calorimetry and UL-94 techniques. The addition of MP or DOPO to epoxy had a thermal destabilization effect below 400 °C, but led to higher char yield at higher temperatures. The inclusion of 10 wt% flame retardants slightly decreased the mechanical behavior, which was attributed to the poor interfacial interactions in case of MP or the decreased cross-linking density in case of DOPO flame retarded resin. The additional graphene presence increased flexural and impact properties, but slightly decreased tensile performance. Adding graphene further decreased the PHRR, THR and burning rate due to its good barrier effect. The improved fire retardancy was mainly attributed to the reduced release of the combustible gas products.     

The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Structure property relation of hybrid biocomposites based on jute,viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties

In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (K_IC) and the fracture energy (G_IC) of the PP–J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP–J30–V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa m^1/2 and 28.9 to 31.2 kJ/m², respectively. Similarly, the fatigue life increased 51% compared with the PP–J30–V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.     

Mechanical and morphological properties of fly ash/epoxy composites using conventional thermal and microwave curing methods

Conventional thermal and microwave curing methods were utilized to cure fly ash/epoxy composites, and the mechanical and morphological properties of the composites were evaluated. The conventional thermal curing was performed at 70 °C for 80 min while microwave curing was carried out at 240 W for 18 min in order to achieve the optimum cure of the composites, determined using Differential Scanning Calorimeter. The results suggested that the tensile and flexural moduli of the composites increased with increasing fly ash content while the effect became opposite for tensile, flexural and impact strengths, and tensile strain at break. Improved mechanical properties of the composite could be obtained by addition of N-2(aminoethyl)-3-aminopropyltrimethoxysilane coupling agent, the contents of 0.5 wt% being recommended for the optimum mechanical properties. Beyond these recommended contents, the mechanical properties greatly reduced, except for the flexural modulus. The comparative results indicated that the composites by the microwave cure consumed shorter cure time and had higher ultimate strengths (especially impact strength), and strain at break than those by the conventional thermal cure. The composites with higher tensile and flexural moduli could be obtained by the conventional thermal cure.     

Mechanical reinforcement while remaining electrical insulation of glass fibre/polymer composites using core–shell CNT@SiO2 hybrids as fillers

Carbon nanotubes (CNTs) have been widely used as mechanical reinforcement agents of composites. However, their aggregations, weak interfacial interaction with polymer, as well as high electrical conductivity limit their use in some especial applications. In this paper, the silicon oxide (SiO₂)-coated (CNT@SiO₂) core–shell hybrids with different SiO₂ thickness were prepared and employed to reinforce glass fibre-reinforced bismaleimide–triazine (BT) resin (GFRBT) composites. The results indicated the mechanical properties, including tensile strength and Young’s modulus increased with the increase of SiO₂ thickness and CNT@SiO₂ loading. Such enhanced mechanical properties were mainly attributed to the intrinsically nature of CNTs, homogeneous dispersion of the hybrids, as well as improved interfacial interaction. Meanwhile, the composites remained high electrical insulation (9.63 × 10¹² Ω cm) due to the existence of SiO₂ layer on CNT surface. This study will guide the design of functionalized CNTs and the construction of high-performance composites.     

The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites

In this work, kenaf fibers were pre-treated in a NaOH solution (6% in weight) at room temperature for two different periods (48 and 144 h). The chemical treatment of kenaf fibers for 48 h allowed to clean their surface removing each impurity whereas 144 h of immersion time had detrimental effect on the fibers surface and, consequently, on their mechanical properties.Untreated and NaOH treated kenaf fibers (i.e. for 48 h) were also used as reinforcing agent of epoxy resin composites. The effect of the stacking sequence (i.e. using unidirectional long fibers or randomly oriented short fibers) and the chemical treatment on the static mechanical properties was evaluated showing that the composites exhibit higher moduli in comparison to the neat resin. As regards the strength properties, only the composites reinforced with unidirectional layers show higher strength than the neat resin. Moreover, the alkali treatment increased the mechanical properties of the composites, due to the improvement of fiber–matrix compatibility.The dynamic mechanical analysis showed that the storage and the loss moduli are mainly influenced by the alkali treatment above the glass transition temperature. Moreover, the alkali treatment led to a notable reduction of tan δ peaks in addition to significant shifts of tan δ peaks to higher temperatures whereas the stacking sequence did not influence the trends of storage modulus, loss modulus and damping of the composites.