Characterisation and modelling of CNT–epoxy and CNT–fibre–epoxy composites

Abstract

This research presents an experimental and theoretical investigation on the effects of carbon nanotube (CNT) integration within neat epoxy resin (nanocomposites) and a carbon fabric–epoxy composite (multiscale composites). An approach is presented for the prediction of mechanical properties of multiscale composites. This approach combines woven fibre micromechanics (MESOTEX) with the Mori-Tanaka model which was used for the prediction of mechanical properties of nanocomposites in this research. Nanocomposite and multiscale composite samples were manufactured using cast moulding, resin infusion, and hand lay-up process. The CNT concentrations in the composite samples were from 0 to 5 wt-%. The samples were characterised using tensile, shear and flexural tests. The discrepancy between the theoretical predictions and the experimental observations was hypothesised to be due to dispersion and bonding issues and SEM images are presented in support of the hypothesis.     

Soy-based adhesive cross-linked by melamine–glyoxal and epoxy resin

To develop a soy-based adhesive with good water resistance, non-toxic melamine–glyoxal resin (MG) prepared in the laboratory was used as a cross-linker of soy-based adhesive. The FT-IR and ESI-MS results showed that there was a reaction between melamine and glyoxal. The resulted –CH–OH– groups could be the possible reactive groups for the cross-linking of soy-based adhesive. The wet shear strength of soy-based plywood indicated that the water resistance of soy adhesive cross-linked by MG improved with respect to that with no cross-linker, although it was not good enough to satisfy the relative standard. With the optimized preparation procedures for plywood, specifically, press temperature 180?°C, press time 3 min and resin loading 280 g m^?2, type I soy-based plywood could be prepared with a hybrid cross-linker, namely 12%MG + 2% epoxy resin (EPR). The DSC results showed that the reaction between soy-based adhesive and the hybrid cross-linker MG + EPR was very complex.     

Thermal and combustion behavior of phosphorus–nitrogen and phosphorus–silicon retarded epoxy

A novel trizine ring-based phosphorus–nitrogen flame retardant, 1,3,5-tris(3-(diphenylphosphoryl)propyl)-1,3,5-triazinane-2,4,6-trione (PN), was synthesized by the reaction of diphenylphosphine oxide and triallyl isocyanurate with triethylborane as catalyst. Chemical structure of the target compound was confirmed by Flourier transform infrared spectrum, nuclear magnetic resonances, matrix-assisted laser desorption/ionization time-of-flight mass spectrum measurements. The newly developed PN was used in the flame retardancy of o-cresol novolac epoxy/phenolic novolac hardener system. For comparison, another analogous phosphorus–silicon flame retardant, [(1,1,3,3-tetramethyl-1,3-disiloxanediyl)-di-2,1-ethanediyl]-bis(diphenylphosphine oxide) (PSi), was also applied in the same system. Experimental results revealed that PN showed superior flame retardant efficiency to that of PSi. In addition, the incorporation of flame retardants was in favor of the char formation during the thermal degradation process of epoxy thermosets. With the same flame retardant content, the char residue of epoxy thermosets with PSi was higher than that of epoxy thermosets with PN at 750 °C. Cone calorimeter results indicated that PN contributed to gas phase flame retardancy while PSi was more likely to take part in flame retardancy in the condense phase. X-ray photoelectron spectroscopy data revealed that the binding energies of phosphorus changed in different ways in PN and PSi after combustion. This implied that phosphorus exhibited different combustion behaviors when combined with nitrogen or silicon.     

Mechanical and thermal behavior of modified epoxy–novolak film adhesives

Modified epoxy-based film adhesives were developed for bonding structural joints. Film adhesives with different compositions were prepared by hot pressing the molten resins. Peel and shear tests were carried out to evaluate the adhesion properties. Dynamic mechanical thermal analyses were conducted to follow the changes in the adhesive structure and also the trend of impact strength. Incorporation of thermoplastic poly(vinyl butyral) (PVB) into an epoxy- novolak combination resulted in higher cohesive strength, better film-forming ability, enhanced adhesive shear and peel strengths, but decreased thermostability. However, due to the lower chemical functionality of PVB, a lower crosslink density was achieved. Incorporation of a small amount of ethylene glycol dimethacrylate (EGDM) as a flexibilizer led to improved mechanical properties, easy handling and facile application. Finally, good shear strength retention up to 200 °C for 1 h was observed in the case of EGDM-modified adhesives.     

Synergistic toughening of epoxy resin by CTBN and CM-β-CD

Carboxymethyl-β-cyclodextrin (CM-β-CD) and carboxyl terminated liquid nitrile rubber (CTBN) were used as binary component fillers in toughening the epoxy resin (E-54). For a single component filler system, the addition of CTBN resulted in significantly improved fracture toughness but reduction of glass transition temperature (T_g) and modulus of epoxy resin. On the other hand, the addition of CM-β-CD resulted in a modest increase in modulus and T_g, and significant improvement in toughness. This work provides a promising route of nanocomposites with excellent toughness. Besides the mechanism of synergistic toughening in this project was explained, and the major toughening mechanisms were attributed to interfacial micro-cracks, energy dissipation of CM-β- CD. This work gives us a further understanding of the modification effect of β- CD.     

Matrix–particle interactions in catalyzed and uncatalyzed copper-filled epoxy matrix composites

Epoxy composites filled with copper particles with sizes of the order of 100 μm are studied with the aim of analyzing the particle–matrix interphase. Two matrixes are used: diglycidyl ether of bisphenol A resin (DGEBA)–anhydride catalyzed using a tertiary amine, and uncatalyzed DGEBA–anhydride. The surface of both types of composites was analyzed using scanning electron microscopy, X-ray photoelectron spectroscopy, and instrumented nanoindentation. The formation of Cu(I) and Cu(II) complexes is revealed using X-ray photoelectron spectroscopy, while instrumented nanoindentation measurements allow us to determine regions with different mechanical properties in the uncatalyzed composite. The influence of anhydride and the type of curing reaction on the formation of copper complexes is analyzed. The main results point out that copper particles can interact strongly with the epoxy, depending on the chemistry and kinetics of the curing reaction, to modify the composite. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47511.     

Hydrogen and oxygen evolution on graphite fiber—epoxy matrix composite electrodes

The electrochemical behavior of three graphite fiber—epoxy matrix composite materials containing various fiber orientations and fiber loadings was studied. Cyclic voltammetry was used to detect surface functionalities and to determine the electrochemically active surface areas of each material in 1 N H₂SO₄ and 30 weight percent (w/o) KOH.Hydrogen and oxygen evolution were studied on each electrode in 1 N H₂SO₄ and 30 w/o KOH, respectively. Tafel slopes for the hydrogen evolution reaction on the composite electrodes ranged from 0.14 to 0.18 V decade⁻¹ while exchange current densities ranged from 4 to 11 × 10⁻⁷ A cm⁻². Tafel slopes for the oxygen evolution reaction on the composite materials were high, ranging from 0.25 to 0.28 V decade⁻¹. Exchange current densities for the oxygen evolution reaction ranged from 2 to 5 × 10⁻⁷ A cm⁻². In general, fiber orientation and volume percent loading in the limited range studies had no noticeable effect on the kinetic behavior for the gas evolution reactions studied.     

High performance HTLNR/epoxy blend—Phase morphology and thermo-mechanical properties

An attempt was made to toughen diglycidyl ether of bisphenol A (DGEBA) type epoxy resin with liquid natural rubber possessing hydroxyl functionality (HTLNR). Epon 250 epoxy monomer is cured using nadic methyl anhydride as hardener in presence of N, N dimethyl benzyl amine as accelerator. HTLNR of different concentrations up to 20 wt % is used as modifier for epoxy resin. The addition HTLNR to an anhydride hardener/epoxy monomer mixture has given rise to the formation of phase-separated structure, consisting of small spherical liquid natural rubber particles bonded to the surrounding epoxy matrix. The particle size increased with increase in rubber content. The viscoelastic properties of the blends were analyzed using dynamic mechanical thermal analysis. The T_g corresponding to epoxy rich phase was evident from the dynamic mechanical spectrum, while the T_g of the rubber phase was overlapped by the β relaxation of epoxy phase. Glass transition of the epoxy phase decreased linearly as a function of the amount of rubber. The mechanical properties such as impact and fracture toughness were also carefully examined. The impact and fracture toughness increase with HTLNR content. A threefold increase in impact strength was observed with 15 wt % HTLNR/epoxy blend. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrical properties of a composite comprising epoxy resin and α-hematite nanorods

Electrical properties of pure epoxy and epoxy-hematite nanorod composites have been investigated. The nanorods were synthesized by the forced hydrolysis method and further mixed with epoxy to obtain the nanocomposite. TEM analysis revealed that they have an average diameter of about 8 nm, with an average aspect ratio of 25. DC-conductivity and DC-current relaxation measurements showed a significant influence of Fe₂O₃ nanorods on the DC-electrical properties of the epoxy matrix. However, the observed effects of the filler below and above the glass transition are different. Because of their high specific surfaces, nanorods affected segmental mobility of epoxy molecules to a large extent, which resulted in an increase in the glass transition temperature (T_g) and a decrease in the real part of dielectric permittivity in high frequency/low temperature region. It is further observed that at elevated temperatures (above T_g) and low frequencies the real part of dielectric permittivity of the nanocomposite exceeds that of the pure matrix, i.e. there is a transition towards microcomposite-like dielectric behaviour.     

Cure kinetics of several epoxy–amine systems at ambient and high temperatures

Epoxy-crosslinker curing reactions and the extent of the reactions are critical parameters that influence the performance of each epoxy system. The curing of an epoxy prepolymer with an amine functional group may be accompanied by side reactions such as etherification. Commercial epoxy prepolymers were cured with different commercial amines at ambient as well as at elevated temperatures. Singularly, only epoxy–amine reactions were observed with diglycidyl ether of bisphenol-A (DGEBA)-based epoxides in our research even upon post-curing at 200°C. Etherification side reaction was found to occur at a cure temperature of 200°C in epoxides possessing a tertiary amine moiety. A combined goal of our research was to understand the effect of tougheners on the cure of epoxy–amine blend. To discern the effect of tougheners on the cure, core–shell rubber (CSR) particles were incorporated into the epoxy–amine blend. It was observed that CSR particles did not restrict the system from proceeding to complete reaction of epoxy moieties. Besides, CSR particles were found to accelerate the epoxy-amine reaction at a lower level of epoxy conversion. The lower activation energy of epoxy–amine reaction of CSR incorporated system compared to control supported the catalytic effect of CSR particles on the epoxy-amine reaction of epoxy prepolymer and amine blends.     

Microstructure and thermal properties of silyl-terminated polycaprolactone–polysiloxane modified epoxy resin composites

With a direct nucleophilic addition between  OH groups of polydiol and  NCO of a silane, a blend of silyl-terminated polycaprolactone PCL-Si and silyl-terminated polydimethylsiloxane PDMS-Si oligomer, PCS-2Si, were firstly prepared, and then blended with a commercial epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) to form a ternary composite. The formed ternary composites of different content of DGEBA were cured using a polyamidoamine as a curing agent and a sol–gel process at ambient temperature. The microstructures and properties of the cured composites were investigated by SEM, TGA, and energy dispersive spectroscopy. The results showed the compatibility between DGEBA and PDMS increased with increasing content of PCS-2Si, but higher content of PCS-2Si resulted in a slight enrichment of silicon in the surface of the cured film. TGA showed that incorporating PCS-2Si into epoxy resin altered the composites' thermal stability and degradation characteristics. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and modification of silica-based epoxy nanocomposites with different sol–gel process enhanced thermal and mechanical properties

This research article describes the results of nano-silica composites filled with different epoxy contents containing nano-SiO₂ particles from (5–25 wt%). Reinforcing hybrid composites enhance thermal and mechanical properties to achieve vital and sustainable products. Silica-based nanocomposites with high purity were prepared and used for the surface modification of nanosized silica particles. The surface structure's composition and physical properties of modified nano-SiO₂ particles were characterized through Fourier transferred infrared spectrometer, X-ray photoelectron spectroscopy, thermogravimetric analyzer, and scanning electron microscopic. Silica-based nanocomposites were prepared by incorporating of modified nano-SiO₂ as an enhancing filler. The morphology of fracture surface and dynamic mechanical properties were investigated. Results showed that the silica-based epoxy nanocomposites are bearing a long chain structure that could improve the compatibility of silica nanocomposites with epoxy resin and contribute to a better dispersion state in the matrix, which enhanced the overall performance of epoxy-cured products.     

Effects of nanosilica and titanium oxide on the performance of epoxy–amine nanocoatings

Different types of composite coatings were prepared by the blending of colloidal nanosilica (SiO₂) and titanium dioxide (TiO₂) in epoxy resin to investigate their coating performances. A fixed amount of silica nanoparticles (20 wt %) and different amounts (5, 10, and 15 wt %) of microsized TiO₂ particles were used in the coatings. The functional groups of the formulated coatings were confirmed by Fourier transform infrared spectroscopy. These results indicate that the SiO₂–TiO₂ particles interacted well with epoxy. Scanning electron microscopy images of the composite coatings revealed a good dispersion of TiO₂ particles at a lower amount of loading; this improved the adhesiveness, glass-transition temperature, thermal stability, and chemical resistance properties. At higher loadings, the performances decreased. The composite coatings were also characterized by their UV radiation-absorption properties with an ultraviolet–visible spectrophotometer. Interestingly, this property was found to be enhanced at higher loadings. An impressive result was noticed in the nanocomposites in terms of oxygen transmission rate performance compared to that of the neat epoxy. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47901.     

Epoxidized soybean oil modified using fatty acids as tougheners for thermosetting epoxy resins: Part 2—Effect of curing agent and epoxy molecular weight

A series of bio-rubber (BR) tougheners for thermosetting epoxy resins was prepared by grafting renewable fatty acids with different chain lengths onto epoxidized soybean oil at varying molar ratios. BR-toughened samples were prepared by blending BRs with diglycidyl ether of bisphenol A resins, Epon 828 and Epon 1001F, at different weight fractions and stoichiometrically cured using an amine curing agent, 4, 4′-methylene biscyclohexanamine (PACM). Fracture toughness properties of the unmodified and BR toughened polymer samples—including critical strain energy release rate (G_Ic), and critical stress intensity factor (K_Ic)—were measured to investigate the toughening effect of prepared BRs. It was found that the degree of phase separation and toughening were more controllable relative to similar polymers cured using the aromatic curing agent Epikure W, and the use of higher molecular epoxy resins produces a synergistic effect increasing the toughness much more than similar polymers made with lower molecular weight epoxy resins. Average BR domain sizes ranging from 200 to 900 nm were observed, and formulations with G_Ic, values K_Ic as high as 1.0 kJ/m² and 1.4 MPa m^1/2 were attained respectively for epoxy systems with T_g greater than 130°C.     

Investigation of pre-reaction and cure temperature on multiscale dispersion in POSS–epoxy nanocomposites

Dispersion of monoamine-functionalized polyhedral oligomeric silsesquioxane (POSS) in an epoxy network was improved by pre-reacting the POSS with excess epoxide and employing a high-temperature cure. DGEBA/DDS networks were formulated with 2.5 and 10 wt% POSS. In some samples, POSS was pre-reacted with DGEBA. The hybrid materials were characterized via SEM, TEM, and DMA. The microscopy and DMA results evinced a multiscale morphology with POSS-rich glassy domains, nano- and microcrystallites, and crystallite agglomerations. For a loading level of 2.5 wt% POSS, the sample with unmodified POSS cured at 125 °C had inorganic crystallites on the order of 1–5 μm and agglomerations on the order of 10–20 μm, whereas the sample with pre-reacted POSS cured at 180 °C had near-perfect dispersion with no agglomerations and very few POSS crystallites. The 10 wt% POSS epoxies also showed improved dispersion with pre-reaction and increasing cure temperature. The dispersion improvements were attributed to the enhanced miscibility of the pre-reacted POSS and the increased rate of POSS reaction into the growing epoxy network at a higher cure temperature.     

Electrodeposition and characterization of Zn–Ni alloys as sublayers for epoxy coating deposition

The chemical composition and phase structure of Zn–Ni alloys obtained by electrodeposition under various conditions were investigated. The influence of the deposition solution and deposition current density on the composition, phase structure, current efficiency and corrosion properties of Zn–Ni alloys were examined. It was shown that the chemical composition and phase structure affect the anticorrosive properties of Zn–Ni alloys. A Zn–Ni alloy electrodeposited from a chloride solution at 20 mA cm^–2 exhibited the best corrosion properties, so this alloy was chosen for further examination. Epoxy coatings were formed by cathodic electrodeposition of an epoxy resin on steel and steel modified with a Zn–Ni alloy. From the time dependence of the pore resistance, coating capacitance and relative permittivity of the epoxy coating, the diffusion coefficient of water through the epoxy coating, D(H₂O), and its thermal stability, it was shown that the Zn–Ni sublayer significantly affects the electrochemical and transport properties, as well as the thermal stability of epoxy coatings. On the basis of the experimental results it can be concluded that modification of a steel surface by a Zn–Ni alloy improves the corrosion protection of epoxy coatings.     

Thermally and mechanically superior hybrid epoxy–silica polymer films via sol–gel method

Hybrid organic–inorganic polymer films composed of an epoxy resin crosslinked with a flexible diamine hardener, and a silica reinforcing phase were produced and their thermo-mechanical properties were determined. Two types of hybrid epoxy–silica polymer films, named EAS-1 and EAS-2, were obtained by hydrolysis and condensation of various amounts of tetraethoxysilane within epoxy network matrix. In EAS-2 hybrids, minor amounts of an amine silane coupling agent were added to enhance interfacial compatibility. FTIR spectroscopy confirmed the formation of organic and inorganic networks. The grafting of amine silane on to the epoxy resin influenced the size and distribution of hyper-branched clusters of silica as indicated by transmission electron microscopy (TEM). The dynamic mechanical and thermal analysis (DMTA) and thermo-gravimetric analysis (TGA) results showed an increase in the storage modulus, the glass-transition temperature, and the thermal stability of hybrid polymer films as compared to the neat matrix. The integration of amine silane coupling agent produced smaller, effectively dispersed silica nanoparticles and consequently improved the ultimate properties of polymer films.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

Preparation and characterization of chemically modified Jute–Coir hybrid fiber reinforced epoxy novolac composites

In this study, the effects of fiber surface modification and hybrid fiber composition on the properties of the composites is presented. Jute fibers are cellulose rich (>65%) modified by alkali treatment, while the lignin rich (>40%) coconut coir fibers consist in creating quinones by oxidation with sodium chlorite in the lignin portions of fiber and react them with furfuryl alcohol (FA) to create a coating around the fiber more compatible with the epoxy resins used to prepare polymer composites. The maximum improvement on the properties was achieved for the hybrid composite containing the jute–coir content of 50 : 50. The tensile and flexural strength are recorded as 25 and 63 MPa at modified coir fiber content of 50 vol %, respectively, which are 78% and 61% higher than those obtained for unmodified fiber reinforced composites, i.e., tensile and flexural strength are 14 and 39 MPa, respectively. The reinforcement of the modified fiber was significantly enhanced the thermal stability of the composites. SEM features correlated satisfactorily with the mechanical properties of modified fiber reinforced hybrid composites. SEM analysis and water absorption measurements have confirmed the FA-grafting and shown a better compatibility at the interface between chemically modified fiber bundles and epoxy novolac resin. Hailwood–Horrobin model was used to predict the moisture sorption behavior of the hybrid composite systems. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation and properties of bisphenol A epoxy resin microcapsules coated with melamine–formaldehyde resin

Microcapsules containing epoxy resins have potential applications, such as in adhesive, electronic packaging, and self-healing polymeric composites. A series of microcapsules were prepared by in situ polymerization with poly(melamine–formaldehyde) as the shell materials and a mixture of diglycidyl ether of bisphenol A and epoxy diluent as the core substances. Morphology, chemical structure, mean particle size, and thermal properties of the microcapsules were studied by means of optical microscope, Fourier transform infrared spectroscopy, laser particle size analyzer, and microcomputer differential thermal balance, respectively. Effects of kind of epoxy diluent, surfactant type, emulsifier concentration, and emulsifying rate on the physical properties of microcapsules were investigated. Results indicate that the formation of microcapsules is affected by the epoxy diluent type and surfactant type. The highest core content of the resultant microcapsules is about 88 wt% and average diameters of the capsules range from 67 to 201 μm, which can be adjusted by changing the emulsifier concentration and emulsifying rate. Thermo gravimetric analysis indicated that the prepared microcapsules experienced excellent stability up to 235 °C.