Mechanical properties of aligned multi-walled carbon nanotube/epoxy composites processed using a hot-melt prepreg method

This study examined the mechanical properties of aligned multi-walled carbon nanotube (CNT)/epoxy composites processed using a hot-melt prepreg method. Vertically aligned ultra-long CNT arrays (forest) were synthesized using chemical vapor deposition, and were converted to horizontally aligned CNT sheets by pulling them out. An aligned CNT/epoxy prepreg was fabricated using hot-melting with B-stage cured epoxy resin film. The resin content in prepreg was well controlled. The prepreg sheets showed good drapability and tackiness. Composite film specimens of 24-33 μm thickness were produced, and tensile tests were conducted to evaluate the mechanical properties. The resultant composites exhibit higher Young’s modulus and tensile strength than those of composites produced using conventional CNT/epoxy mixing methods. For example, the maximum elastic modulus and ultimate tensile strength (UTS) of a CNT (21.4 vol.%)/epoxy composite were 50.6 GPa and 183 MPa. These values were, respectively, 19 and 2.9 times those of the epoxy resin.     

Multi-walled carbon nanotube-reinforced silicon carbide fibers prepared by polymer-derived ceramic route

Multi-walled carbon nanotube (MWNT)-reinforced silicon carbide (SiC) ceramic fibers were successfully prepared by blending MWNTs (0-0.5 wt.%) with polycarbosilane, followed by melt spinning, curing, and pyrolysis. The MWNTs used in this study were modified with a chemical treatment. It was found that the MWNTs were well-dispersed in the matrix and aligned with the axis of the fibers after ultrasonic dispersion combined with melt spinning. Mechanical measurements revealed that significant improvement in Young’s modulus and tensile strength was achieved by incorporating MWNTs into the ceramic fibers. The addition of 0.5 wt.% MWNTs led to a 93.6% increase in the Young’s modulus and a 38.5% increase in the tensile strength.     

Determination of Young’s modulus in polyester-Al2O3 and epoxy-Al2O3 nanocomposites using the Digital Image Correlation method

Young’s modulus of nano-composite systems composed of unsaturated polyester and epoxy resins with alumina nanoparticles of different sizes has been experimentally estimated. The nanoparticles used were spherical alpha-Al₂O₃ having 30-40 and 200 nm in diameter. Young’s modulus was estimated using an inverse problem that is solved by means of the classical Levenberg-Marquardt technique. A cantilever beam under bending was used in the experiments and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. Experimental results indicate that Young’s modulus increases with increasing nanoparticle volume fraction. Finally, the estimated Young’s moduli were compared with classical theoretical models, showing that the experimental results are in agreement with literature data.     

A composite material used as a membrane for ophthalmology applications

New biomaterials for intracorneal ophthalmologic implants were designed, manufactured and characterized. A composite material in the form of a membrane was manufactured in a two-stage process. The first stage of the process depended on preparation of multidimensional (MD-type) fibrous polymer composite. A stable terpolymer polytetraflouroethylene-co-polyvinylene fluoride-co-polypropylene (PTFE–PVDF–PP) was used as a composite matrix, and sodium alginate-based biopolymer (NA) in the form of short fibres and/or powder were used as porogenic constituents. The composite materials were subjected to physicochemical treatment in order to remove a water soluble biopolymer. The treatment led to about 50% of open porosity within the polymer matrix. Depending on the membrane type the mean pore size determined with SEM microphotographs was 15–25 μm. Permeability and durability of the membranes in simulated eye fluid (culture medium enriched with albumin) was tested. The size and shape of the pores before and after the permeability test were compared (SEM), and they depend on the porogen form. Mechanical parameters of the composite materials such as; tensile strength, Young’s modulus, and strain to failure were measured. A membrane derived from fibres and particles showed better mechanical properties than a membrane derived from porogen particles. Microstructure and mechanical properties make the membranes a good candidate for ophthalmological implants.     

Carbohydrate derived copoly(lactide) as the compatibilizer for bacterial cellulose reinforced polylactide nanocomposites

A novel, entirely bio-derived polylactide carbohydrate copolymer (RP1) is used as a compatibilizer, to produce bacterial cellulose (BC) poly(l-lactide) (PLLA) nanocomposites with improved mechanical properties. Contact angle measurements of RP1 droplets on single BC nanofibres proved that it has a higher affinity towards BC than PLLA. RP1 has a comparable Young’s modulus, but lower tensile strength, than PLLA. When RP1 was blended with PLLA at a concentration of 5 wt%, the tensile modulus and strength of the resulting polymer blend decreased from 4.08 GPa and 63.1, respectively, for PLLA to 3.75 GPa and 56.1 MPa. A composite of BC and PLLA (with 5 wt% RP1 and 5 wt% BC) has a higher Young’s modulus and tensile strength, compared to either pure PLLA or PLLA–BC nanocomposites.     

Mechanical properties of multi-walled carbon nanotube/epoxy composites

Untreated and acid-treated multi-walled carbon nanotubes (MWNT) were used to fabricate MWNT/epoxy composite samples by sonication technique. The effect of MWNT addition and their surface modification on the mechanical properties were investigated. Modified Halpin–Tasi equation was used to evaluate the Young’s modulus and tensile strength of the MWNT/epoxy composite samples by the incorporation of an orientation as well as an exponential shape factor in the equation. There was a good correlation between the experimentally obtained Young’s modulus and tensile strength values and the modified Halpin–Tsai theory. The fracture surfaces of MWNT/epoxy composite samples were analyzed by scanning electron microscope.     

Design of a textile composite bone plate using 3D-finite element method

Bone plates are the most common devices used for long bone fracture fixations. Metallic bone plates are conventionally used for load bearing regions suffer the disadvantages that they usually needs to be removed 1–2 years after surgery due to stress shielding and ion releasing effects. One solution to overcome these problems is to use bone plates made of composite materials with desirable mechanical properties as substitutes for the metallic types. In this research, a partially resorbable composite bone plate consisting of a poly L-Lactic acid matrix and textile bioglass fibers used as reinforcement was modeled and analyzed using the ANSYS software V. 9.0. Micromechanical study of a representative volume element (REV) was carried out using the 3D-finite element method to optimized volume fraction of the reinforcement. In this stage, ultimate tensile strength of the composite was determined. In the macromechanical analysis, a three dimensional, quarter symmetric finite element model was developed for a plate with five holes. Bending analysis was performed to determine the bending strength and the bending modulus of the plate. Results showed that for a volume fraction equal to 45%, the longitudinal modulus of elasticity and the ultimate tensile strength would be 23 GPa and 230 MPa, respectively. The bending strength and bending modulus of the plate were calculated to be about 55 MPa and 16.6 GPa, respectively. Compared to the data available on forearm bones in which the longitudinal modulus of elasticity is about 18 GPa, the tensile and bending strength are about 150 MPa and 40 MPa and the bending modulus is 7 GPa, it is concluded that the composite plate system is suitable for forearm region and it is capable of reducing stress shielding effects at the fracture site.     

Thermo-mechanical characterisation of AA 6056-T4 and estimation of its material properties using Genetic Algorithm

This paper presents an experimental investigation of thermo-mechanical material properties of AA 6056-T4, which is used extensively in aeronautic applications. Monotonic tensile tests have been carried out on the dog-bone type specimens at temperatures ranging from room temperature (16 °C) to high temperature (450 °C) with two different strain rates; viz. high strain rate (∼0.002 s⁻¹) and low strain rate (∼0.0002 s⁻¹). Specimens were heated with the help of Joule heating system using Gleeble® 3500 machine at a temperature rate of 25 °C/s. Material properties which were investigated include the Young’s modulus, yield strength at 0.1% plastic strain and hardening modulus.     

High performance polyurethane/functionalized graphene nanocomposites with improved mechanical and thermal properties

This communication reported the substantial improvement in the mechanical and thermal properties of a polyurethane (PU) resulting from the incorporation of well-dispersed graphene oxide (GO). The stress transfer benefited from the covalent interface formed between the PU and GO. The Young’s modulus of the PU was improved by ∼7 times with the incorporation of 4 wt% GO, and the improvement of ∼50% in toughness was achieved at 1 wt% loading of GO without losing elasticity. Significant improvements were also demonstrated in the hardness and scratch resistance measured by nano-indentation. Thermogravimetric analysis revealed that the decomposition temperature was increased by ∼50 °C with the addition of 4 wt% GO.     

Covalent functionalization of graphene with organosilane and its use as a reinforcement in epoxy composites

Functionalized graphene nanosheets (f-GNSs) produced by chemically grafting organosilane were synthesized by a simple covalent functionalization with 3-aminopropyl triethoxysilane. The f-GNSs showed a larger thickness, but smaller width and than the un-treated graphene. The covalent functionalization of graphene with silane was favorable for their homogeneous dispersion in the polymer matrix even at a high nanofiller loading (1 wt.%). The initial thermal degradation temperature of epoxy composite was increased from 314 °C to 334 °C, at a f-GNS content of 1 wt.%. Meanwhile, the addition of 1 wt.% f-GNSs increased the tensile strength and elongation to failure of epoxy resins by 45% and 133%, respectively. This is believed to be attributed to the strong interfacial interactions between f-GNSs and the epoxy resins by covalent functionalization. The experimentally determined Young’s modulus corresponded well with theoretical simulation under the hypothesis that the graphene sheets randomly dispersed in the polymer matrix.     

Influence of carbon nanotube-polymeric compatibilizer masterbatches on morphological, thermal, mechanical, and tribological properties of polyethylene

Study was made of the effect of multiwall carbon nanotubes (MWCNTs) and polymeric compatibilizer on thermal, mechanical, and tribological properties of high density polyethylene (HDPE). The composites were prepared by melt mixing in two steps. Carbon nanotubes (CNTs) were melt mixed with maleic anhydride grafted polyethylene (PEgMA) as polymeric compatibilizer to produce a PEgMA-CNT masterbatch containing 20 wt% of CNTs. The masterbatch was then added to HDPE to prepare HDPE nanocomposites with CNT content of 2 or 6 wt%. The unmodified and modified (hydroxyl or amine groups) CNTs had similar effects on the properties of HDPE-PEgMA indicating that only non-covalent interactions were achieved between CNTs and matrix. According to SEM studies, single nanotubes and CNT agglomerates (size up to 1 μm) were present in all nanocomposites regardless of content or modification of CNTs. Addition of CNTs to HDPE-PEgMA increased decomposition temperature, but only slight changes were observed in crystallization temperature, crystallinity, melting temperature, and coefficient of linear thermal expansion (CLTE). Young’s modulus and tensile strength of matrix clearly increased, while elongation at break decreased. Measured values of Young’s moduli of HDPE-PEgMA-CNT composites were between the values of Young’s moduli for longitudinal (E₁₁) and transverse (E₂₂) direction predicted by Mori-Tanaka and Halpin-Tsai composite theories. Addition of CNTs to HDPE-PEgMA did not change the tribological properties of the matrix. Because of its higher crystallinity, PEgMA possessed significantly different properties from HDPE matrix: better mechanical properties, lower friction and wear, and lower CLTE in normal direction. Interestingly, the mechanical and tribological properties and CLTEs of HDPE-PEgMA-CNT composites lie between those of PEgMA and HDPE.     

Improving post irradiation stability of high density polyethylene by multi walled carbon nanotubes

Sterilization of implants and other clinical accessories is an integral part of any medical application. Although many materials are used as implants, polyethylene stands unique owing to its versatility. Carbon nanotubes are being used as a filler material to enhance the properties of polyethylene. However, the role of multi walled carbon nanotubes (MWCNTs) as an effective antioxidant and radical scavenger in resisting the deteriorating effects of sterilization is yet to be studied in detail. The present work is aimed to investigate the mechanical properties and oxidation stability of irradiated high density polyethylene (HDPE) reinforced by MWCNTs with various concentrations such as 0.25%, 0.50%, 0.75% and 1.00 wt.%. The composites were exposed to ⁶⁰Co source in air and irradiated at different dosage level starting from 25 to 100 kGy and then shelf aged for a period of 120 days prior to investigation. The loss in toughness, Young’s modulus and ultimate strength at 100 kGy for 1 wt.% MWCNTs composite were found to be 21.5%, 20.3% and 19.2%, respectively compared to that of unirradiated composite. FTIR and ESR studies confirmed the antioxidant and radical scavenging potentialities of MWCNTs with increased concentration and irradiation dosage. It was found that by the addition of 1 wt.% MWCNTs into virgin HDPE, the oxidation index of the composite at 100 kGy was decreased by 56.2%. It is concluded that the addition of MWCNTs into polyethylene not only limits the loss of mechanical properties but also improves its post irradiation oxidative stability.     

Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone)

Cryogenic mechanical properties are important parameters for epoxy resins used in cryogenic engineering areas. In this study, multi-walled carbon nanotubes (MWCNTs) were employed to reinforce diglycidyl ether of bisphenol F (DGBEF)/diethyl toluene diamine (DETD) epoxy system modified by poly(ethersulfone) (PES) for enhancing the cryogenic mechanical properties. The epoxy system was properly modified by PES in our previous work and the optimized formulation of the epoxy system was reinforced by MWCNTs in the present work. The results show that the tensile strength and Young’s modulus at 77 K were enhanced by 57.9% and 10.1%, respectively. The reported decrease in the previous work of the Young’s modulus of the modified epoxy system due to the introduction of flexible PES is offset by the increase of the modulus due to the introduction of MWCNTs. Meanwhile, the fracture toughness (K_IC) at 77 K was improved by about 13.5% compared to that of the PES modified epoxy matrix when the 0.5 wt.% MWCNT content was introduced. These interesting results imply that the simultaneous usage of PES and MWCNTs in a brittle epoxy resin is a promising approach for efficiently modifying and reinforcing epoxy resins for cryogenic engineering applications.     

Strain rate dependency of mechanical properties of TCP/PLLA composites after immersion in simulated body environments

In order to clarify strain rate dependency of mechanical properties of β-tricalcium phosphate (TCP)/poly(l-lactic acid) (PLLA) composites after immersion in simulated body environment, tensile tests at various loading rate were conducted on the TCP/PLLA specimens with and without immersion. TCP contents in the composite were 5, 10 and 15 wt%. Phosphate buffered solution was selected as simulated body environment and immersion periods were 8, 16 and 24 weeks. Young’s modulus and tensile strength increased with increasing strain rates. However, the strain rate dependencies decreased with immersion. Swelling and cracks around TCP agglomerations were observed in the cross-section of 15 wt% specimen after 24 weeks immersion. From the fracture surface observation, voids existed only in the ductile fracture surface of the specimen without immersion, whereas they existed in both ductile and brittle surface of the specimen with immersion. These results indicated that diffused water through the interfaces between TCP and PLLA hydrolyzed and weakened the interfaces and/or matrix near the interfaces.     

Development of aligned-hemp yarn-reinforced green composites with soy protein resin: Effect of pH on mechanical and interfacial properties

Unidirectional hemp yarn-reinforced green composites were fabricated with soy protein concentrate (SPC) resin processed at various pH values. To preserve the yarn alignment during the fabrication of green composites, hemp yarn was wound onto a metal frame with slight tension and precured SPC resin was applied to the yarns. Effects of pH values on the tensile properties of the SPC resin and hemp yarn/SPC resin interfacial shear strength (IFSS) were investigated. Increasing pH of the SPC resin from 7 to 12 decreased its fracture stress and Young’s modulus from 13.1 MPa and 357.5 MPa to 8.1 MPa and 156.2 MPa, respectively. At the same time fracture strain and moisture content increased from 31.5% and 15.65% to 53·4% and 19.30%, respectively, indicating resin plasticization. However, hemp yarn/SPC resin IFSS increased from 17.7 MPa at pH 7 up to 28.0 MPa at pH 10, after which it decreased. The fracture toughness of the composites increased up to pH of 10 but further increase in pH reduced the toughness. SEM photomicrographs showed fracture surfaces of hemp yarn-reinforced green composites that indicated better resin/fiber interaction at pH of 10 than 7 or 12.     

Hygromechanical properties of composites of crosslinked allylglycidyl-ether modified starch reinforced by wood fibres

In a previous work a new family of thermoset composites of allylglycidyl ether modified starch as matrix, an ethylene glycol dimethacrylate as cross-linker and a wood fibre as reinforcement were prepared. The aim of the present work was to study the hygromechanical properties of the new composites including density, dimensional stability in water, water uptake, stiffness, and ultimate strength in three-point bending. It was shown that the samples with a starch matrix of a high degree of substitution (DS = 2.3), HDS, absorbed less water, were more stable in water and had also higher stiffness and strength than corresponding composite samples with a starch matrix of low degree of substitution (DS = 1.3), LDS. Overall, the fibre addition improved water stability. An increased fibre content from 40 to 70% by weight had a negligible impact on the water uptake. An increase in fibre content did, however, improve the mechanical properties. The HDS-sample with highest fibre content, 70% by weight showed the highest Young’s modulus (3700 MPa) and strength (130 MPa), which are markedly higher compared with the samples based on the pure HDS matrix (Young’s modulus of 360 MPa and strength of 15 MPa). The measured Young’s modulus and tensile strength values were roughly one order of magnitude higher than earlier reported cellulosic fibre reinforced natural polymer composites.     

Nanoindentation of laser micromachined 3C-SiC thin film micro-cantilevers

Single crystalline thin films of 3C-SiC with a thickness of 1.7 ± 0.2 μm were deposited on Si (100) substrate using atmospheric chemical vapor deposition technique. A Q-switched Nd:YAG laser in the fundamental wavelength with a pulse duration of 100 ns and average power of 1 W was then used to pattern 50 μm wide and 150 μm long cantilever beams in direct-writing mode. Following laser patterning, wet chemical etching using KOH anisotropic etchant was carried out to remove the underlying Si and form free-standing 3C-SiC cantilever beams. The cantilevers were subjected to nanoindentation test to obtain deflection versus load curves. The average Young’s modulus and fracture strength were determined to be 423 GPa and 1.5 GPa respectively which are comparable to those obtained by the reactive ion etching. Laser patterning thus offers nearly identical properties as that of ion etching with the added benefit of much higher etch rates.     

Reinforcing potential of micro- and nano-sized fibers in the starch-based biocomposites

Starch-based biocomposites reinforced with jute (micro-sized fiber) and bacterial cellulose (BC) (nano-sized fiber) were prepared by film casting. Reinforcement in the composites is essentially influenced by fiber nature, and amount of loading. The optimum amount of fiber loading for jute and bacterial cellulose in each composite system are 60 wt% and 50 wt% (of starch weight), respectively. Mechanical properties are largely improved due to the strong hydrogen interaction between the starch matrix and cellulose fiber together with good fiber dispersion and impregnation in these composites revealed by SEM. The composites reinforced with 40 wt% or higher bacterial cellulose contents have markedly superior mechanical properties than those reinforced with jute. Young’s modulus and tensile strength of the optimum 50 wt% bacterial cellulose reinforced composite averaged 2.6 GPa and 58 MPa, respectively. These values are 106-fold and 20-fold more than the pure starch/glycerol film. DMTA revealed that the presence of bacterial cellulose (with optimum loading) significantly enhanced the storage modulus and glass transition temperature of the composite, with a 35 °C increment. Thermal degradation of the bacterial cellulose component occurred at higher temperatures implying improved thermal stability. The composites reinforced with bacterial cellulose also had much better water resistance than those associated with jute. In addition, even at high fiber loading, the composites reinforced by bacterial cellulose clearly retain an exceptional level of optical transparency owing to the effect of the nano-sized fibers and also good interfacial bonding between the matrix and bacterial cellulose.     

Rigidity of diamond reinforced metals featuring high particle contents

Diamond/metal composites with diamond contents between 57 and 72 vol% have been produced by gas pressure assisted liquid metal infiltration using Ag–3 wt% Si and Al–2 wt% Cu as matrix. The experimental data cover a range of Young’s moduli from 300 to 425 GPa and 245 to 370 GPa for the Ag–3Si and the Al–2Cu-based composites, respectively. Experimental Young’s moduli are compared to the Mori–Tanaka mean field scheme (MTM), the generalized self-consistent scheme (GSCS), the bimodal hard sphere model (TBHS), and the differential effective medium scheme (DEM). At the lower end of volume fractions investigated, the predictions by the GSCS, the TBHS, and the DEM are very close to each other and to the experimental results while the MTM is clearly lower. With increasing volume fraction the differences between the models accentuate and the data up to 72 vol% of diamond are best described by the DEM.     

Novel bacterial cellulose–acrylic resin nanocomposites

The preparation and characterization of new nanocomposite films based on two acrylic emulsions, composed of random copolymers of butyl acrylate and methyl methacrylate, and bacterial cellulose is reported. The new composite materials were obtained through a simple and green approach by casting water-based suspensions of the acrylic emulsions and bacterial cellulose nanofibrils. The excellent compatibility between these matrices and the natural reinforcing fibers, observed by scanning electron microscopy (SEM), was reflected in the enhanced thermal and mechanical properties of the ensuing composites. Thus, an increase of around 30 °C in the maximum degradation temperature was observed for a 10% content of bacterial cellulose. The new composites showed glass–rubber transition temperature profiles comparable to those of the pristine matrices, as shown by DMA, and increasing elastic moduli with increasing the bacterial cellulose content. The tensile tests revealed a substantial increase in Young’s modulus and tensile strength and a corresponding decrease in elongation at break with increasing bacterial cellulose load.