Rubber-modified Epoxy Research Articles

The intrinsic fracture property of a rubber-modified epoxy adhesive: Geometrical transferability

This study presents recent achievements in understanding the fracture behaviour of rubber-modified epoxy adhesives, notably the determination of the fully developed fracture process zone (FPZ) and its associated intrinsic fracture energy, G0. The shape and size of the FPZ are identified by inspecting the fracture surfaces using SEM, and exploring subsurface damage using optical microscopy and TEM. The thickness and failure strain of the FPZ are found to be essentially the same for different fracture tests, i.e. tapered double cantilever beam and single edge notched bending tests. As a consequence, the results from different fracture tests are linked by using the geometrically transferable, true fracture properties, i.e. FPZ thickness and G0. The variation of total fracture energy observed in different fracture tests is attributed to varying plastic deformation energy dissipated in plastic deformation zone (outside FPZ). The fracture behaviour of the adhesive is then successfully predicted by a cohesive zone model using parameters extracted from experiments.

Fabrication and Characterization of Novel Liquid Rubber Modified Epoxies

In the present investigation, a DGEBA based epoxy has been modified by a novel liquid rubber (LR) (0-25 wt. %) obtained from the pyrolysis of scrap rubber. Mechanical characterizations viz. tensile, impact and fracture toughness were conducted on the prepared LR modified epoxies (LREs). A reduction in the Young’s modulus and tensile strength was observed with LR concentration. Highest increase in impact strength (∼ 73 %) and KIC (∼79 %) at 15 wt. % LR concentrations was observed in the LREs. FESEM revealed the formation of biphasic morphology and cavitation followed by shear yielding to be the prevalent toughening mechanism. Thermogravimetric analysis revealed that LR addition has negligible effect on the thermal stabilities of epoxy.

Mechanical and fracture properties of epoxy adhesives modified with graphene nanoplatelets and rubber particles

Graphene nanoplatelets (GNP) were introduced into a rubber-modified epoxy adhesive in order to simultaneously improve the bulk mechanical properties, fracture toughness and single joint lap shear strength of the adhesive. The Young’s modulus was observed to increase marginally from 2.46 GPa to 2.56 GPa due to the addition of 0.1 wt.% GNPs. No further increase in modulus was observed for GNP loading above 0.1 wt.%. A negligible effect on the measured tensile strength was observed. The fracture energy of the bulk adhesive increased by 21 % due to the addition of 0.1 wt.% GNPs. No further increase in measured fracture energy was observed as the GNP content was further increased to 0.5 wt.%. A systematic decrease in the lap shear strength was observed due to the addition of GNPs, i.e. the lap shear strength decreased from 21.7 MPa of the control adhesive gradually to 17.2 MPa of the adhesive modified by 0.5 wt.% GNPs. Imaging analysis of the failed adhesive joints reveal that the reduction in lap shear strength was attributed to the preferential alignment of the GNPs in the direction parallel to the adhesive bonding surface. This was further confirmed by comparing the electrical behaviour of the lap shear joints with that of the bulk adhesive samples.

Effect of Rubber Content on Fatigue Crack Growth Rate under Mode I Loading

Rubber-modified adhesives have attracted particular interest recently, owing to their property of dramatically increasing the fracture toughness of adhesively-bonded joints. In many cases, these joints are exposed to cyclic loads, and hence an understanding of the fatigue mechanisms is of paramount importance. This study was conducted to characterize the rubber content-dependent behaviour of rubber-modified epoxy adhesive under cyclic loading. Fatigue crack growth rates under mode I loading were measured using adhesively-bonded double cantilever beam specimens with rubber contents of 0, 5 and 14 wt.%. As a result, in the Paris region, an increase in rubber content reduced the fatigue crack growth rate. Similarly to the Paris region, the fatigue threshold for 5 wt.% was greater than that for 0 wt.%; however, the fatigue threshold of joints with 14 wt.% rubber content was nearly equal to those with 5 wt.%.

Mechanical Properties and Morphologies of Carboxyl-Terminated Butadiene Acrylonitrile Liquid Rubber/Epoxy Blends Compatibilized by Pre-Crosslinking.

In order to enhance the compatibilization and interfacial adhesion between epoxy and liquid carboxyl-terminated butadiene acrylonitrile (CTBN) rubber, an initiator was introduced into the mixture and heated to initiate the cross-linking reaction of CTBN. After the addition of curing agents, the CTBN/epoxy blends with a localized interpenetrating network structure were prepared. The mechanical properties and morphologies of pre-crosslinked and non-crosslinked CTBN/epoxy blends were investigated. The results show that the tensile strength, elongation at break and impact strength of pre-crosslinked CTBN/epoxy blends are significantly higher than those of non-crosslinked CTBN/epoxy blends, which is primarily due to the enhanced interfacial strength caused by the chemical bond between the two phases and the localized interpenetrating network structure. Both pre-crosslinked and non-crosslinked CTBN/epoxy blends show a bimodal distribution of micron- and nano-sized rubber particles. However, pre-crosslinked CTBN/epoxy blends have smaller micron-sized rubber particles and larger nano-sized rubber particles than non-crosslinked CTBN/epoxy blends. The dynamic mechanical analysis shows that the storage modulus of pre-crosslinked CTBN/epoxy blends is higher than that of non-crosslinked CTBN/epoxy blends. The glass transition temperature of the CTBN phase in pre-crosslinked CTBN/epoxy blends increases slightly compared with the CTBN/epoxy system. The pre-crosslinking of rubber is a promising method for compatibilization and controlling the morphology of rubber-modified epoxy materials.

Investigation of the Role of Rubber Particle Cavitation Resistance on the Toughening Mechanisms of Epoxy Resins

The occurrence of rubber particle cavitation in rubber modified epoxy resins has generated some controversy in the literature and role of cavitation resistance in toughening mechanisms has remained an enigma. In this study, hydroxyl terminated polybutadiene (HTPB) and synthesized epoxy terminated polybutadiene (ETPB) have been used to toughen an epoxy resin. FTIR and titration techniques were used to monitor the ETPB synthesis. Mean fracture toughness, KIC, of 0.65 MPa.m0.5 was measured for unmodified epoxy polymer. The measured toughness was increased by the addition of HTPB to 1.23 MPa.m0.5 and ETPB to 1.80 MPa.m0.5. The cavitation zone of the HTPB modified sample was bigger than that of ETPB modified sample which showed the higher cavitation resistance of ETPB particles. SEM studies showed volumetric expansion of rubbery phase in slow crack growth region of both samples which proved the plastic void growth. The results showed that the elevated cavitation resistance of ETPB did not suppress plastic void growth. The optical microscopy of crack tips showed shear banding which was more significant in ETPB modified sample. SEM micrographs of fracture surface of HTPB modified samples showed no tail formation while pinning tails were present in ETPB modified samples. The results showed that higher cavitation resistance activated crack pinning process and induced larger shear banding zone in ETPB modified samples. Deflection and forking of the main crack into secondary cracks was also observed in ETPB modified samples while these phenomena were not present in HTPB modified samples.

A Carboxyl-Terminated Polybutadiene Liquid Rubber Modified Epoxy Resin with Enhanced Toughness and Excellent Electrical Properties

The modification of epoxy (EP) resin with carboxyl-terminated polybutadiene (CTPB) liquid rubber was carried out in this work. The chemical reaction between the oxirane ring of EP and the carboxyl group of CTPB and kinetic parameters were investigated by Fourier transform infrared and differential scanning calorimetry. The resulting pre-polymers were cured with methyl hexahydrophthalic anhydride. Scanning electron microscopic observations indicate that the micro-sized CTPB particles dispersed uniformly in the EP matrix formed a two-phase morphology, mainly contributing to the improved toughness of the modified network. The best overall mechanical performance was achieved with 20 phr CTPB; above it, a fall in the strength and modulus was observed. The storage modulus and loss declined with the CTPB concentration due to its lower modulus and plasticizing effect from dynamic mechanical analysis measurements. Moreover, due to the weak polarity and excellent electrical insulation of CTPB, the CTPB-modified EP presented higher electrical resistivities and breakdown strength, and low dielectric permittivity and loss compared with neat EP.

Fracture Behaviour of Epoxy Resins Modified with Liquid Rubber and Crosslinked Rubber Particles under Mode I Loading

Three-point bending tests were conducted with two kinds of rubber-modified epoxy resins: a liquid rubber-modified epoxy resin and a nano-elastomer particle-modified one. In most studies on rubber-toughened epoxy resins, fracture toughness has been evaluated by critical stress intensity factor or critical energy release rate, where the crack is assumed to propagate rapidly, and stable crack propagation has not been considered. However, stable crack propagation was observed for the present rubber-modified epoxy resins. To take into account crack propagation, fracture toughness was characterized using the crack-growth resistance curves (R-curve). In the present study, the evolution behaviour of the damage zone near the crack tip was observed using a video-microscope for the two kinds of rubber-modified resins. Furthermore, to investigate microscopic fracture mechanisms, the fracture surfaces were observed by scanning electron microscopy, and side views near the crack tip were also observed by a confocal laser scanning microscopy.

Erratum to: Mode I critical fracture energy of adhesively bonded joints between glass fiber reinforced thermoplastics

Critical fracture energies of adhesively bonded joints under mode I constant separation were experimentally investigated. Double cantilever beam (DCB) specimens comprising polyamide 6 (PA6) based fiber reinforced thermoplastics (GFRTP) were utilized for the experiments. The adherends of the joints were bonded with three different types of adhesives such as polyurethane and acrylates. A surface treatment method with a primer was applied to pre-bonded surface, matching with the different adhesives, which results in five combinations. Strongest combination, Plexus Primer PC120 and Plexus AO420, exhibited 2.95 kJ/m2 in mode I critical fracture energy, which is much higher than those of ordinary epoxy adhesive and similar to those of rubber-modified very-ductile epoxy adhesives. Therefore, it is confirmed that adhesive bonding can be applied to join PA6 based GFRTP even for structural use, although the material is thought too difficult to bond adhesively.

Mode I critical fracture energy of adhesively bonded joints between glass fiber reinforced thermoplastics

Abstract Critical fracture energies of adhesively bonded joints under mode I constant separation were experimentally investigated. Double cantilever beam (DCB) specimens comprising polyamide 6 (PA6) based fiber reinforced thermoplastics (GFRTP) were utilized for the experiments. The adherends of the joints were bonded with three different types of adhesives such as polyurethane and acrylates. A surface treatment method with a primer was applied to pre-bonded surface, matching with the different adhesives, which results in five combinations. Strongest combination, Plexus Primer PC120 and Plexus AO420, exhibited 2.95 kJ/m2 in mode I critical fracture energy, which is much higher than those of ordinary epoxy adhesive and similar to those of rubber-modified very-ductile epoxy adhesives. Therefore, it is confirmed that adhesive bonding can be applied to join PA6 based GFRTP even for structural use, although the material is thought too difficult to bond adhesively.

Analysis and Experimentation Research of the Static Compressive Mechanical Test of Foam Aluminum Composite

In order to achieve function-structure integration and improve mechanical properties and energy absorption capability of the foam aluminum composite,foam aluminum filled with silicone rubber-modified epoxy resin was prepared.Static compression test shows that the platform yield stage of foam aluminum filled with modified epoxy resin is uplifted,the mechanical properties and energy absorption capability of foam aluminum are improved.The results are helpful to expand the application of aluminum foam.

PEMFC용 탄성 탄소 복합재료 분리판의 기계적 강도 및 전기전도도에 미치는 탄소섬유 필라멘트와 흑연 섬유의 영향

Highly conductive bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) was prepared using phenol novolac-type epoxy/graphite powder (GP)/carbon fiber filament (CFF) composite, and a rubber-modified epoxy resin was introduced in order to give elasticity to the bipolar plate graphite fiber (GF) was incorporated in order to improve electrical conductivity. To find out the cure condition of the mixture of novolac-type and rubber-modified epoxies, differential scanning calorimetry (DSC) was carried out and their data were introduced to Kissinger equation. And tensile and flexural tests were carried out using universal testing machine (UTM) and the surface morphology of the fractured specimen and the interfacial bonding between epoxy matrix and CFF or GF were observed by a scanning electron microscopy (SEM).

The Effects of Alkalized and Silanized Woven Sisal Fibers on Mechanical Properties of Natural Rubber Modified Epoxy Resin

Bisphenol-A based epoxy resin was modified by blending with 1% by weight (wt%) of methyl methacrylate (MMA)/glycidyl methacrylate (GMA) grafted depolymerized natural rubber (GDNR). The blend showed the impact strength of 163% higher than that of neat epoxy resin. In order to study the effect of alkalized and silanized woven sisal fiber on the properties of 1 wt% GDNR/epoxy resin blend, alkalized woven sisal fiber was prepared using 2 wt% NaOH solution and silanized fiber was prepared by treated alkalized fiber with γ-glycidoxypropyltrimethoxysilane (A-187). Composites of the blend with alkalized and silanized woven sisal fiber were manufactured by hand-lay up process. Amounts of woven sisal fiber in the composite were 3, 5 and 7 wt%. The flexural modulus of all composites was higher than that of the blend and increased with increasing the amount of fiber. In addition, when silanized sisal fiber was used, the composites showed further improvement in flexural modulus. Flexural strength of all composites was lower than that of the blend but it was improved after silanized sisal fiber was employed. Compared to GDNR/epoxy resin blend, the impact strength of the composite containing 7 wt% silanized sisal fiber was significantly enhanced to 230%.

Compressive properties of nanoparticle modified epoxy resin at different strain rates

Epoxy resins often exhibit high strength yet are often brittle, especially at high strain rates. Block copolymer modified epoxy resins have generated significant interest since it was demonstrated that the combination could lead to nanostructured thermosets through the self-assembly of the block copolymer. Such nanostructured epoxies exhibit increased ductility without the significant loss in yield strength exhibited by traditional rubber-modified epoxies. In this study, the effect of different nanoscale additives on the compressive yield strength of a model epoxy resin has been studied. In the first case, a block copolymer styrene-b-butadiene-b-polymethylmethacrylate (SBM) was added to the model epoxy resin. In the second case, carbon nanotubes (CNTs) were added. In the final case, both additives were mixed simultaneously with the epoxy resin. The compressive mechanical behavior of these materials has been investigated over a wide range of strain rates (0.001–3500s−1). The yield behavior was found to fit the cooperative yield model proposed by Fotheringham and Cherry.

Composite Materials Based on Natural Rubber Modified Epoxy Resin and Sisal Fiber

In the present work, bisphenol-A based epoxy resin was blended with methyl methacrylate (MMA)/glycidyl methacrylate (GMA) grafted depolymerized natural rubber (GDNR). GDNR/epoxy resin blend was composite with woven sisal fiber. GDNR was prepared by solution grafting MMA/GMA (90/10 w/w%) onto depolymerized natural rubber (DNR). The occurrence of GDNR was confirmed by proton nuclear magnetic resonance spectroscopy (1H-NMR). Amount of GDNR in the blend was 1 part per hundred resins. Impact strength of epoxy resin was increased by 62% when GDNR was added. Composites of GDNR/epoxy resin and woven sisal fiber were prepared by hand-lay up process. Amounts of woven sisal fiber in the composite were 3, 5 and 7% by weight (wt%). The flexural modulus of the composites was higher than that of neat epoxy resin and increased with increasing amount of woven sisal fiber. Nevertheless, flexural strength of all composites was lower than those of neat epoxy resin and the blend. Compared to neat epoxy resin, the impact strength of the composite containing 7 wt% woven sisal fiber was further increased to 114%.

Characterization of nanostructured epoxy networks modified with isocyanate-terminated liquid polybutadiene

Polybutadiene-block-epoxy prepolymer (DGEBA-b-PBNCO) copolymers with multi-branched topological structure were prepared by reacting isocyanate-multifunctionalized liquid polybutadiene (PBNCO) with DGEBA prepolymer and used to develop nanostructured rubber-modified epoxy thermosets cured with triethylene-tetramine (TETA) as the aliphatic amine. The nanoscopic structure was obtained with the addition of as high as 20 phr of rubber component and successfully demonstrated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The glass transition temperature of the rubber-modified epoxy networks was slightly higher than the neat epoxy system. In addition, a unique combination of outstanding toughness and increased modulus and T g was achieved in these modified systems, which was attributed to the peculiar morphology associated with a strong interfacial adhesion imparted by the reaction between the isocyanate and hydroxyl groups present in the PBNCO and epoxy resin, respectively. The effect of the PBNCO on the gelation time of the epoxy/TETA system was investigated by rheological techniques. The NCO-functionalized polybutadiene decreased the gelation time, indicating an accelerating effect on the curing process, probably because of the urethane groups formed by the reaction between PBNCO and the epoxy resin during the PB-b-ER block copolymer preparation.

The study of rubber-modified plastics with higher heat resistance and higher toughness and its application

For rubber-modified plastics, toughness enhancement is generally at the cost of heat resistance. Of significance, this paper reported a finding that rubber-modified plastics with a special morphology could enhance toughness and heat resistance cooperatively. The special morphology was such that an interface in situ formed between plastic matrix and rubber particle had higher hardness than plastic matrix. As a result, the hard interface not only helped rubber soft component integrate with plastic matrix by covalent bonds to impart plastic matrix high toughness, but also covered rubber nanoparticles as hard shells to protect them from deforming at high temperature. The special morphology had been achieved in rubber-modified epoxies and phenolic molding material. The forming mechanism of the hard interface was studied in detail with AFM, DSC and in situFTIR, by using rubber-modified epoxy resin as an example. The finding could be applied to any rubber-modified plastics as long as the special morphology could be realized.

A Probe on the Failure Mechanism in Rubber-Modified Epoxy Blends: Morphological and Acoustic Emission Analysis

In this work, diglycidyl ether of bisphenol A based epoxy resin (DGEBA) was modified with varying amounts of two liquid rubbers: carboxyl terminated copolymer of butadiene and acrylonitrile (CTBN); and a hydroxyl terminated polybutadiene (HTPB), using an anhydride hardener. The ultimate aim of this study was to investigate the failure mechanism operating in the rubber-modified epoxies and to evaluate this by correlating these results with the miscibility and interfacial adhesion between the components and the morphology of the cured network. Some of the mechanical and fracture properties, which are associated with the two-phase particulate morphology, were investigated. The visoelastic behavior of modified epoxies was also analyzed and variations in the shift of T g values in toughened epoxies were explained. The samples were carefully analyzed by an acoustic emission technique to investigate the failure mechanism operating in them. From the response of force and number of acoustic events as well as from the amplitude of acoustic events, we were able to explain the failure mechanisms in the elastomer incorporated epoxy resins supplemented by morphological evidence.

Loading Rate-Induced Transition in Toughening Mechanism of Rubber-Modified Epoxy

The effect of loading rate on toughening mechanisms of rubber-modified epoxy resin was investigated using amine-terminated butadiene acrylonitrile (ATBN) rubber. Rubber-modified samples were tested at various loading rates in the range of 1–1000 mm/min. At low loading rates, modified resin exhibited a high fracture toughness, which decreased by increasing the rate of loading. At higher loading rate a transition region was observed, where the fracture toughness did not change noticeably with increasing the loading rate. Beyond the transition region, the resin fracture toughness dropped dramatically to the level of unmodified epoxy. Fractography of the damage zone showed different toughening mechanisms for the various loading rate regions, i.e., shear yielding and crack branching. It is believed that the interaction between these two toughening mechanisms, each dominating in one extreme loading rate condition, is responsible for the intermediate transition region, where toughness is virtually independent from loading rate.

R-curve characteristics of adhesives modified with high rubber content under mode I loading

For rubber-modified epoxy adhesives with high rubber content, stable crack propagation is observed under mode I loading. In such a situation, a further increase in rubber content not only extends the plastic zone in front of the crack tip, but also promotes the formation and growth of voids in the adhesive layer. To clarify the effect of a further increase in rubber content on crack initiation and propagation behavior, fracture toughness tests were conducted under mode I loading using adhesively bonded double cantilever beam (DCB) and compact tension (CT) specimens: these specimens were used for evaluating the crack growth resistance and fracture toughness in the initial stage of crack propagation, respectively. Stable behavior in crack growth was observed for adhesives with rubber contents of both 10 and 14 wt%. The crack growth resistance of the adhesive with 14 wt% rubber was higher than that with 10 wt%, whereas in the initial stage of crack propagation the energy release rate for 10 wt% rubber was higher than that for 14 wt%. The crack growth behavior of the DCB specimen was discussed on the basis of Gurson’s model. The estimated R-curve agreed well with the experimental result in the initial stage of crack propagation, although the difference between estimated and experimental plots increased with crack growth.