Comprehensive Study on Using VTBN Reactive Oligomer for Rubber Toughening of Epoxy Resin and Composite

While vinyl-terminated butadiene acrylonitrile is frequently used for the toughening of vinylester and polyester, very limited research has been conducted on modification of epoxy with this oligomer. Herein, the effect of vinyl-terminated butadiene acrylonitrile addition to epoxy in bulk and glass reinforced composite is systematically investigated. Thermo-physical behavior and mechanical characteristics of the samples are determined. To interpret the test results, the void content of reinforced samples is measured and fracture surface of the specimens is investigated. It is found that vinyl-terminated butadiene acrylonitrile improves the toughness with slight negligible effects on other characteristics. Incorporation of 15 phr of vinyl-terminated butadiene acrylonitrile increases the K_IC of epoxy from 0.6 MPa to 2.3 MPam^0.5. Similarly, addition of 15 phr vinyl-terminated butadiene acrylonitrile leads to 67% enhancement in the interlaminar fracture toughness of composites. The toughening mechanisms and toughness transfer from bulk to composite are discussed.     

Fracture toughness of a hybrid rubber modified epoxy. II. Effect of loading rate

Effect of loading rate on toughness characteristics of hybrid rubber-modified epoxy was investigated. Epoxy was modified by amine-terminated butadiene acrylonitrile (ATBN) and recycled tire. Samples were tested at various loading rates of 1–1000 mm/min. Fracture toughness measurements revealed synergistic toughening in hybrid system at low loading rates (1–10 mm/min); hybrid system exhibited higher fracture toughness value in comparison with the ATBN-modified resin with same modifier content. However, synergistic toughening was eliminated by increasing the loading rate. At higher loading rates (10–1000), the fracture toughness of hybrid system decreased gradually to the level lower than that of ATBN-modified epoxy. Fractography of the damage zones showed the toughening mechanisms of ATBN-modified system was less affected by increasing the loading rate compared to that of hybrid system. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fracture behaviours of in situ silica nanoparticle-filled epoxy at different temperatures

Fracture behaviours of nanosilica filled bisphenol-F epoxy resin were systematically investigated at ambient and higher temperatures (23 °C and 80 °C). Formed by a special sol-gel technique, the silica nanoparticles dispersed almost homogenously in the epoxy resin up to 15 vol.%. Stiffness, strength and toughness of epoxy are improved simultaneously. Moreover, enhancement on fracture toughness was much remarkable than that of stiffness. The fracture surfaces taken from different test conditions were observed for exploring the fracture mechanisms. A strong particle-matrix adhesion was found by fractography analysis. The radius of the local plastic deformation zone calculated by Irwin model was relative to the increment in fracture energy at both test temperatures. This result suggested that the local plastic deformation likely played a key role in toughening of epoxy.     

Toughening of dicyandiamide-cured DGEBA-based epoxy resins by CTBN liquid rubber

Toughening of a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin with liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer has been investigated. For this purpose six blend samples were prepared by mixing DGEBA with different concentrations of CTBN from 0 to 25 phr with an increment of 5 phr. The samples were cured with dicyandiamide curing agent accelerated by Monuron. The reactions between oxirane groups of DGEBA and carboxyl groups of CTBN were followed by Fourier-transform infrared (FTIR) spectroscopy. Tensile, impact, fracture toughness and dynamic mechanical analysis of neat as well as the modified epoxies have been studied to observe the effect of CTBN modification. The tensile strength of the blend systems increased by 26 % when 5 phr CTBN was added, and it remained almost unchanged up to 15 phr of CTBN. The elongation-at-break and Izod notched impact strength increased significantly, whereas tensile modulus decreased gradually upon the addition of CTBN. The maximum toughness of the prepared samples was achieved at optimum concentration of 15 phr of CTBN, whereas the fracture toughness (K _IC) remained stable for all blend compositions of more than 10 phr of CTBN. The glass transition temperature (T _g) of the epoxy resin significantly increased (11.3 °C) upon the inclusion of 25 phr of CTBN. Fractured surfaces of tensile test samples have been studied by scanning electron microscopic analysis. This latter test showed a two-phase morphology where the rubber particles were distributed in the epoxy resin with a tendency towards co-continuous phase upon the inclusion of 25 phr of CTBN.     

Toughening of epoxy hybrid nanocomposites modified with silica nanoparticles and epoxidized natural rubber

Silica nanoparticles (SN) and epoxidized natural rubber (ENR) were used as binary component fillers in toughening diglycidyl ether of bisphenol A (DGEBA) cured cycloaliphatic polyamine. For a single component filler system, the addition of ENR resulted in significantly improved fracture toughness (K_IC) but reduction of glass transition temperature (T_g) and modulus of epoxy resins. On the other hand, the addition of SN resulted in a modest increase in toughness and T_g but significant improvement in modulus. Combining and balancing both fillers in hybrid ENR/SN/epoxy systems exhibited improvements in the Young’s modulus and T_g, and most importantly the K_IC, which can be explained by synergistic impact from the inherent characteristics associated with each filler. The highest K_IC was achieved with addition of small amounts of SN (5 wt.%) to the epoxy containing 5–7.5 wt.% ENR, where the K_IC was distinctly higher than with the epoxy containing ENR alone at the same total filler content. Evidence through scanning electron microscopy (SEM) and transmission optical microscopy (TOM) revealed that cavitation of rubber particles with matrix shear yielding and particle debonding with subsequent void growth of silica nanoparticles were the main toughening mechanisms for the toughness improvements for epoxy. The fracture toughness enhancement for hybrid nanocomposites involved an increase in damage zone size in epoxy matrix due to the presence of ENR and SN, which led to dissipating more energy near the crack-tip region.     

Preparation of binary and hybrid epoxy nanocomposites containing graphene oxide and rubber nanoparticles: Fracture toughness and mechanical properties

Binary and hybrid epoxy nanocomposites modified with graphene oxide (GO) and core–shell rubbers (CSR) were synthesized via the solvent-exchange method. X-ray diffraction analysis and scanning electron microscopy of the samples showed a homogeneous dispersion of GO and CSR in the epoxy matrix. The tensile modulus and tensile strength of the samples modified with CSR decreased continuously with increasing CSR content; however, with the addition of only 0.05 phr GO to the neat epoxy and rubber-modified epoxy, these properties significantly increased. The use of GO and CSR individually improved the fracture toughness, but the impact of GO was greater. The simultaneous use of GO and CSR improved both the fracture toughness and the mechanical properties. Our investigation of the toughening mechanism indicated that crack deflection–bifurcation, crack pinning, and particle debonding–pullout in the presence of GO nanosheets and limited rubber particle cavitation contributed to fracture toughness improvement in the hybrid systems. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46988.     

Toughening mechanisms of nanoparticle-modified epoxy polymers

An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles. The particles were introduced via a sol-gel technique which gave a very well-dispersed phase of nanosilica particles which were about 20 nm in diameter. Atomic force and electron microscopies showed that the nanoparticles were well-dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100 J/m² for the unmodified epoxy polymer to 460 J/m² for the epoxy polymer with 13 vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopies, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the presence of the nanoparticles.     

Effect of resin modifiers on the structural properties of epoxy resins

Thermomechanical, mechanical and fracture mechanical properties of modified epoxy resins with two different modifiers are investigated. Carboxyl‐terminated butadiene‐acrylonitrile (CTBN) is used as toughening agent and hexanediole diglycidyl ether (HDDGE) as reactive diluent. Both modifiers are admixed in contents from 0 up to 100 phr (parts per hundred resin) and exhibit flexibilizing and toughening qualities. The glass transition temperature is strongly depressed by the admixed reactive diluent, whereas the tensile modulus exhibits greater dependency on the toughening agent contents. The tensile strength and strain at break values are higher for the formulations with diluent compared to resins with toughening agent. Up to a content of 45 phr both modified systems exhibit comparable fracture toughness values. Only the toughened systems comprise increasing values for modifier amounts higher than 45 phr. For the formulation with both modifiers (toughening agent and diluent) a significantly higher toughness but a reduced glass transition temperature was obtained. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45348.     

Surface morphology,thermomechanical and barrier properties of poly(ether sulfone)‐toughened epoxy clay ternary nanocomposites

Poly(ether sulfone) (PES)‐toughened epoxy clay ternary nanocomposites were prepared by melt blending of PES with diglycidyl ether of bisphenol A epoxy resin along with Cloisite 30B followed by curing with 4,4′‐diaminodiphenylsulfone. The effect of organoclay and thermoplastic on the fracture toughness, permeability, viscoelasticity and thermomechanical properties of the epoxy system was investigated. A significant improvement in fracture toughness and modulus with reduced coefficient of thermal expansion (CTE) and gas permeability were observed with the addition of thermoplastic and clay to the epoxy system. Scanning electron microscopy of fracture‐failed specimens revealed crack path deflection and ductile fracture without phase separation. Oxygen gas permeability was reduced by 57% and fracture toughness was increased by 66% with the incorporation of 5 phr clay and 5 phr thermoplastic into the epoxy system. Optical transparency was retained even with high clay content. The addition of thermoplastic and organoclay to the epoxy system had a synergic effect on fracture toughness, modulus, CTE and barrier properties. Planetary ball‐milled samples gave exfoliated morphology with better thermomechanical properties compared to ultrasonicated samples with intercalated morphology. Copyright © 2010 Society of Chemical Industry     

Enhancement of Interlaminar Fracture Toughness of Carbon Fiber/Epoxy Composites Using Silk Fibroin Electrospun Nanofibers

In this article, the effect of silk fibroin nanofibers as a toughening agent of carbon fiber/fabric-reinforced epoxy composites is experimentally investigated. The composites showed up to 30% improvement in Mode II fracture toughness at 0.1 wt% of silk fibroin nanofibers content. The scanning electron microscopy observation revealed that the fracture surface of silk fibroin nanofibers modified carbon fiber/fabric-reinforced epoxy composites appearance of the broken fiber and the ductile-like matrix cracks showed a good adhesion between matrix resin and carbon fibers, which are reasons for the enhanced mode II interlaminar fracture toughness.     

Influence of Submicron TiO2 Particles on the Mechanical Properties and Fracture Characteristics of Cured Epoxy Resin

Submicron titanium dioxide (TiO₂) was used in different weight fractions as a toughening agent for amine-cured epoxy resin. After the use of X-ray photoelectron spectroscopy (XPS), which confirmed that the TiO₂ particles were evenly distributed in the cross-linked epoxy resin matrix, the composites were characterized by tensile and impact testing, followed by scanning electron microscopy of the fracture surfaces. The results indicated that the submicron TiO₂ toughening particles markedly improved the mechanical properties of the cured epoxy resin compared to the untoughened epoxy resin. The optimal properties were achieved at a TiO₂ concentration of 4 wt. %, at which point the toughness and the impact resistance values increased by 65% and 60%, respectively. The results also indicated that an increase in the amount of TiO₂ causes a decrease in toughness. Stress whitening, out-of-plane flaking, and thumbnail markings were the major visible features of the toughening mechanisms.

It is suggested that, at 4 wt. % of the submicron TiO₂ particles, microvoids are developed in the epoxy matrix. These microvoids are able to absorb some of the deformation work applied to the material, and thus enhance the toughness of the material. On increasing the TiO₂ content in the matrix (> 4 wt. %), the submicron particles got closer to each other and the microvoids were converted to macrovoids, which may act as stress concentrating flaws, leading to the deterioration of the mechanical properties of the epoxy resin.     

Fracture and toughening behavior of aramid fiber/epoxy composites

The effect of short Aramid fibers on the fracture and toughening behavior of epoxy with high glass transition temperature has been studied. Fine dispersion of the fibers throughout the matrix is evidenced by optical microscopy. Compared with neat epoxy resin, the fracture toughness (K_IC) of the composites steadily increases with increasing fiber loading, indicating that addition of Aramid fibers has an effective toughening effect to the intrinsically brittle epoxy matrix. Scanning electron microscopy (SEM) indicates that formation of numerous step structures for fiber‐filled epoxy systems is responsible for the significant toughness improvement. SEM and transmitted optical microscopy show that fiber pullout and fiber breakage are the main toughening mechanisms for the Aramid fiber/epoxy composites. POLYM. COMPOS. 26:333–342, 2005. © 2005 Society of Plastics Engineers.     

Epoxy/acrylonitrile-butadiene-styrene copolymer/clay ternary nanocomposite as impact toughened epoxy

Epoxy resins have low impact strength and poor resistance to crack propagation, which limit their many end use applications. The main objective of this work is to incorporate both acrylonitrile-butadiene-styrene copolymer (ABS) and organically modified clay (Cloisite 30B) into epoxy matrix with the aim of obtaining improved material with the impact strength higher than neat epoxy, epoxy/clay and epoxy/ABS hybrids without compromising the other desired mechanical properties such as tensile strength and modulus. Impact and tensile properties of binary and ternary systems were investigated. Tensile strength, elongation at break and impact strength were increased significantly with incorporation of only 4 phr ABS to epoxy matrix. For epoxy/clay nanocomposite with 2.5% clay content, tensile modulus and strength, and impact strength were improved compared to neat epoxy. With incorporation of 2.5% clay and 4 phr ABS into epoxy matrix, 133% increase was observed for impact strength. Ternary nanocomposite had impact and tensile strengths greater than values of the binary systems. Morphological properties of epoxy/ABS, epoxy/clay and epoxy/ABS/clay ternary nanocomposite were studied using atomic force microscopy (AFM) phase imaging, scanning electron microscopy (SEM) and wide angle X-ray diffraction (WAXD). New morphologies were achieved for epoxy/ABS and epoxy/ABS/clay hybrid materials. Exfoliated clay structure was obtained for epoxy/clay and epoxy/ABS/clay nanocomposite.     

The toughening mechanism in hybrid epoxy-silica-rubber nanocomposites (HESRNs)

Two different size nanosilica (NS) particles, nominally 20 nm and 80 nm in diameter, and carboxyl terminated butadiene acrylonitrile (CTBN) were blended into a lightly crosslinked, DGEBA/piperidine epoxy system to investigate the toughening mechanisms in hybrid epoxy-silica-rubber nanocomposites (HESRNs). Adding small amount of NS particles into CTBN toughened epoxies further improved the fracture toughness to a level that could not be achieved by increasing CTBN content alone. Interestingly, this toughening effect is diminished when NS particles clustered at high CTBN contents. In addition, the effect of NS particle size on toughening behavior was not considerable, except the case when NS clustering is observed. According to the SEM and TOM investigations, the plastic zone, which consists of shear banding and matrix dilation, is further enlarged in front of the crack tip in HESRNs. Irwin’s model is used to evaluate the process zone concept and the result indicates that zone shielding is credited for the toughening mechanism in these HESRNs.     

Mechanical properties and fracture behavior of high-performance epoxy nanocomposites modified with block polymer and core–shell rubber particles

The mechanical properties, thermomechanical properties, and fracture mechanic properties of block-copolymer (BCP), core–shell rubber (CSR) particles, and their hybrids in bulk epoxy/anhydride system were investigated at 23 °C. The results show that fracture toughness was increased by more than 268% for 10 wt % BCP, 200% for 12 wt % of CSR particles, and 100% for hybrid systems containing 3 wt % of each, BCP and CSR. The volume content of nanoparticles influences the final morphology and thus influences the tensile properties and fracture toughness of the modified systems. The toughening mechanisms induced by the BCP and CSR particles were identified as (1) localized plastic shear-band yielding around the particles and (2) cavitation of the particles followed by plastic void growth in the epoxy polymer. These mechanisms were modeled using the Hsieh et al. approach and the values of G_Ic of the different modified systems were calculated. Excellent agreement was found between the predicted and the experimentally measured fracture energies. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48471.     

Influence of processing conditions and physicochemical interactions on morphology and fracture behavior of a clay/thermoplastic/thermosetting ternary blend

This study provides information on the mechanical behavior of epoxy‐poly(methyl methacrylate) (PMMA)‐clay ternary composites, which have been prepared using the phase separation phenomenon of PMMA and the introduction of organophilic‐modified montmorillonites (MMTs), the continuous matrix being the epoxy network. Two dispersion processing methods are used: a melt processing without any solvent and an ultrasonic technique with solvent and a high‐speed stirrer. TEM analysis shows that phase separation between PMMA and the epoxy network was obtained in the shape of spherical nodules in the presence of the clay in both process methods used. Nanoclay particles were finely dispersed inside thermosetting matrix predominantly delaminated when ultrasonic blending was used; whereas micrometer‐sized aggregates were formed when melt blending was used. The mechanical behavior of the ternary nanocomposites was characterized using three‐point bending test, dynamic mechanical analysis (DMA), and linear elastic fracture mechanics. The corresponding fracture surfaces were examined by scanning electron microscopy to identify the relevant fracture mechanisms involved. It was evidenced that the better dispersion does not give the highest toughness because ternary nanocomposites obtained by melt blending present the highest fracture parameters (K_Ic). Some remaining disordered clay tactoids seem necessary to promote some specific toughening mechanisms. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synergistic toughening and electrical functionalization of an epoxy using MWCNTs and silane- /plasma-activated basalt fibers

This work studied the effects of adding short basalt fibers (BFs) and multi-walled carbon nanotubes (MWCNTs), both separately and in combination, on the mechanical properties, fracture toughness, and electrical conductivity of an epoxy polymer. The surfaces of the short BFs were either treated using a silane coupling agent or further functionalized by atmospheric plasma to enhance the adhesion between the BFs and the epoxy. The results of a single fiber fragmentation test demonstrated a significantly improved BF/epoxy adhesion upon applying the plasma treatment to the BFs. This resulted in better mechanical properties and fracture toughness of the composites containing the plasma-activated BFs. The improved BF/epoxy adhesion also affected the hybrid toughening performance of the BFs and MWCNTs. In particular, synergistic toughening effects were observed when the plasma-activated BFs/MWCNTs hybrid modifiers were used, while only additive toughening effects occurred for the silane-sized BFs/MWCNTs hybrid modifiers. This work demonstrated a potential to develop strong, tough, and electrically conductive epoxy composites by adding hybrid BF/MWCNT modifiers.     

Preparation and properties of MWCNTs/poly(acrylonitrile‐ styrene‐butadiene)/epoxy hybrid composites

Poly(acrylonitrile‐styrene‐butadiene) (ABS) was used to modify diglycidyl ether of bisphenol‐A (DGEBA) type epoxy resin, and the modified epoxy resin was used as the matrix for making multiwaled carbon tubes (MWCNTs) reinforced composites and were cured with diamino diphenyl sulfone (DDS) for better mechanical and thermal properties. The samples were characterized by using infrared spectroscopy, pressure volume temperature analyzer (PVT), thermogravimetric analyzer (TGA), dynamic mechanical analyzer (DMA), thermo mechanical analyzer (TMA), universal testing machine (UTM), and scanning electron microscopy (SEM). Infrared spectroscopy was employed to follow the curing progress in epoxy blend and hybrid composites by determining the decrease of the band intensity due to the epoxide groups. Thermal and dimensional stability was not much affected by the addition of MWCNTs. The hybrid composite induces a significant increase in both impact strength (45%) and fracture toughness (56%) of the epoxy matrix. Field emission scanning electron micrographs (FESEM) of fractured surfaces were examined to understand the toughening mechanism. FESEM micrographs reveal a synergetic effect of both ABS and MWCNTs on the toughness of brittle epoxy matrix. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Deformation and fracture of glass bead/ CTBN-rubber/epoxy composites

Carboxyl-terminated butadiene-acrylonitrile-rubber decreases modulus and yield stress of the studied epoxy but increases fracture toughness. The addition of glass bead compensates for the loss in modulus but has little effect on yield stress. However, it significantly contributes to the fracture toughness by providing additional mechanisms for toughening of both the unmodified and rubber-modified epoxy. For the toughened epoxies studied, fracture surfaces gave only limited information on fracture mechanisms since significant energy absorption also occurs in the material below the fracture surface. Suggestions for suitable material compositions for fiber composite matrices are given.     

Hybrid silane-treated glass fabric/epoxy composites: tensile properties by micromechanical approach

The effect of interface modification on the interfacial adhesion and tensile properties of glass fabric/epoxy composites was evaluated in two directions of 0° and +45°. Herein, the glass fabric surface was modified by colloidal nanosilica particles and by a new blend of silane-coupling agents including both reactive and non-reactive silanes. Composite samples with high strength and toughness were obtained. A simultaneous improvement of tensile strength and toughness was observed for an epoxy composite reinforced with a hybrid-sized glass fabric including silane mixture and nanosilica. In fact, the incorporation of colloidal silica into the hybrid sizing dramatically modified the fiber surface texture and created mechanical interlocking between the glass fabric and resin. The results were analyzed by the rule of mixtures (ROM), Halpin–Tsai (H–T), and Chamis equations. It was found that the ROM equations provided approximate upper bound values for all investigated composite samples. In the samples containing nanosilica, the shear and elastic moduli values calculated by the Chamis and ROM equations showed good agreement with those obtained from experiments. However, in other samples, the values calculated by the H–T equation showed a better agreement with the experimental data. The analysis of fracture surfaces indicated that both silane and nanosilica particles had influence on the mode of failures at the interface.