Toughness Of Epoxy Resin Research Articles

Tough and Conductive Nacre‐inspired MXene/Epoxy Layered Bulk Nanocomposites

AbstractA long‐standing quest in materials science has been the development of tough epoxy resin nanocomposites for use in numerous applications. Inspired by nacre, here we report tough and conductive MXene/epoxy layered bulk nanocomposites. The orientation of MXene lamellar scaffolds is enhanced by annealing treatment. The improved interfacial interactions between MXene lamellar scaffold and epoxy through surface chemical modification resulted in a synergistic effect. Tailoring the interlayer spacing of MXene nanosheets to a critical distance resulted in a fracture toughness about eight times higher than that of pure epoxy, surpassing other epoxy nanocomposites. Our nacre‐inspired MXene/epoxy layered bulk nanocomposites also show high electrical conductivity that provides self‐monitoring capability for structural integrity and exhibits an excellent electromagnetic interference shielding efficiency. Our proposed strategy provides an avenue for fabricating high‐performance epoxy nanocomposites.

Durable flame-retardant, strong and tough epoxy resins with well-preserved thermal and optical properties via introducing a bio-based, phosphorus-phosphorus, hyperbranched oligomer

The manufacture of transparent, durable fire-safe, strong yet tough epoxy resins (EPs) remains a great challenge. Herein, a novel hyperbranched, P-P synergetic, bio-based flame retardant (DIT) was synthesized, which was composed by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), bio-derived itaconic anhydride (ITA) and trihydroxymethylphosphine oxide (THPO) moieties. Adding 2 wt% DIT enabled the EP/2DIT sample to achieve a UL-94 V-0 rating and a limiting oxygen index (LOI) of 36.6%. The peak heat release rate and total smoke production of EP/4DIT sample with 4 wt% DIT were 23.8% and 19.9% lower than those of virgin EP sample. Notably, the EP/DIT sample featured durable flame retardancy because of the oligomeric structure of DIT. The high flame-retardant efficiency of DIT was mainly due to the synergism between DOPO and THPO. DIT simultaneously strengthened and toughened EP thermoset at a low addition (≤2 wt%), and 2 wt% of DIT increased the elongation at break, tensile strength, flexural strength and impact strength of EP/2DIT sample by 35.8%, 40.5%, 24.2% and 21.2%. Moreover, the glass transition temperature (Tg) of EP/2DIT sample was equal to that of EP sample, and the high transparent was maintained. Hence, this work offers an all-rounded design for the creation of durable flame-retardant EPs with superior mechanical properties, thermal resistance and transparency by introducing hyperbranched, P-P, bio-based flame retardants, which shows huge potential and broad prospect.

Effect of the Mixing Technique of Graphene Nanoplatelets and Graphene Nanofibers on Fracture Toughness of Epoxy Based Nanocomposites and Composites.

In this work, the effect of different mixing techniques on thermal and mechanical properties of graphene nanoplatelets (GNPs) and graphene nanofibers (GANFs) loaded epoxy nanocomposites was investigated. Three dispersion methods were employed: a high shear rate (HSR), ultrasonication (US) and the fluidized bed method (FBM). The optical microscopy has revealed that the most suitable dispersion, in terms of homogeneity and cluster size, is achieved by implementing the US and FBM techniques, leading to nanocomposites with the largest increase of glass transition temperature, as supported by the DMA analysis data. The fracture toughness results show a general increase of both the critical stress intensity factor (KIC) and the critical strain energy release rate (GIC), likely due to the homogeneity and the low scale dispersion of the carbonaceous nanostructures. Based on the nanocomposite fracture toughness improvements and also assuming a potential large scale up production of the nanocomposite matrix, a single mixing technique, namely the FBM, was employed to manufacture the carbon fiber reinforced composite (CFRC). This method has resulted in being less time-consuming and is potentially most suitable for the high volume industrial production. The CFRCs were characterized in terms of tensile, flexural and interlaminar fracture toughness properties and the results were analyzed and discussed.

Engineering Double Load-Sharing Network in Thermosetting: Much More than Just Toughening

Current synthetic epoxy thermosets exhibit a trade-off between stiffness and toughness. Despite the urgent demands for highly stiff and tough epoxy thermosetting resins, existing technologies have struggled to attain resins that satisfy this balance. To circumvent this inherent trade-off, a new strategy to obtain a double load-sharing epoxy thermoset network via hierarchical curing process without adding any other filler is proposed. In the epoxy resin bearing acetyl ester side groups cured by an aromatic amine, the “in situ” step-growth phase-separated morphologies and hydrogen-bonding interactions between hydroxyl groups and the ester groups, which endow the resin with high strength and toughness, can be easily controlled by simply tuning the feed ratio of the amine curing agent. Notably, previously unachievable control of mechanical properties is attained, the processing performance is improved, and the product resists hydrothermal aging. The successful application of this strategy provides promising perspectives for construction versatile and high-performance thermosetting.

Synthesis, Curing Behaviors and Properties of a Bio-Based Trifunctional Epoxy Silicone Modified Epoxy Thermosets.

Tremendous effort has been focused on improving the toughness of epoxy, but the common approaches diminish the mechanical properties. In this work, a new silicone-modified trifunctional epoxy monomer SITEUP is synthesized from the hydrosilylation transformation of eugenol epoxy (EPEU) and tris-(dimethylsiloxy)phenylsilane. The chemical structures and curing kinetics of SITEUP are investigated based on 1H-NMR, 13C-NMR, MADLI-TOF-MS, and DSC analyses. SITEUP is introduced into DGEBA/IPDA systems as a functional modifier in varied loadings for toughening the resulting epoxy thermosets. The impact strength of the modified epoxy thermosets containing 20% SITEUP is 84% higher than that of the pristine epoxy thermoset and also maintains high flexural strength. Further morphology study reveals that the plastic deformation caused by siloxane segments is the key factor accounting for the enhanced toughness of the finalized epoxy thermosets. Si-O-Si segments incorporated into the thermosetting network could absorb more energy by increasing the mobility of polymer chains under external stress and led to improved thermal stability and damping characteristics. In addition, SITEUP is able to decrease the surface tension and increase the hydrophobic properties of the resultant epoxy materials.

Investigation of polyurethane toughened epoxy resins for composite cryotank applications. Part II: Theoretical analysis of toughening mechanisms

Based on the phase separation phenomenon, cryogenic mechanical behaviors and toughening effects for polyurethane toughened epoxy (PU/EP) systems demonstrated in our previous work (Part I), as a followed investigation, this study aims to conduct theoretical analysis of temperature-dependent toughening mechanisms. First, 3D finite element unit cells of PU/EP systems with random particle distributions are established to evaluate the maximum stress concentration factor for the theoretical fracture energy calculation. Then, the temperature-dependent fracture energies of PU/EP systems are investigated quantitatively for the first time, and the theoretical results are shown to be in good agreement with the experimental results. From 10 phr to 40 phr, the particle bridging contribution rises from 47.3% to 65.5%, which elucidates that the particle bridging plays a major role in determining the fracture energy at room temperature. As the temperature decreases from RT to −183°C, taking PU10/EP as an example, the untoughened matrix proportion goes up from 30.9% to 65.2%; the particle bridging contribution decreases from 51.8% to 27.8%; the shear banding contribution drops from 17.3% to 7%. This work explores temperature-dependent toughening mechanisms in depth and offers an accurate quantitative evaluation of fracture energies for toughened epoxy matrix for composites cryotank design.

Unstructured Direct Ink Write 3D Printing of Functional Structures with Ambient Temperature Curing Dual‐Network Thermoset Ink

Fabrication of structures in unstructured environments is a promising field to expand the application spaces of additive manufacturing (AM). One potential application is to add new components directly onto existing structures. Herein, a versatile, reconfigurable direct ink writing (DIW) manufacturing method is developed in tandem with a two‐stage hybrid ink designed to fabricate high‐strength, self‐supporting parts in unconventional printing spaces such as underneath a build surface or horizontally. This two‐stage hybrid DIW ink combines a photopolymer and a tough epoxy resin. The photopolymer can cure rapidly to enable layer‐by‐layer printing of complex structures. It also possesses adequate adhesion to allow the fabrication of large volume structures on a diversity of substrates including acrylic, wood, glass, aluminum, and concrete. The epoxy component can cure after 72 h in ambient conditions with further increased adhesion strengths. The capabilities of the reconfigurable DIW extrusion nozzle method to print complex structures in inverted and horizontal environments are demonstrated. Finally, via addition of DIW‐deposited conductive paths, a functional 3D‐printed structure capable of in situ deformation monitoring is created. This work has the potential to be used for applications such as appending new parts to existing structures for increasing functionality, repair, and structure health monitoring.

Evaluation ofbisphenol‐Aepoxy cured with diethylenetriamine by using phosphorus containing liquid poly(1,2‐butadiene) as multifunctional additives

AbstractPhosphorus containing liquid epoxidate polybutadiene (P‐LEPB) is synthesized and used as the additive to improve the flame retardancy and toughness of epoxy resin (EP). The structure of P‐LEPB is confirmed by Fourier transform infrared (FTIR),1H and13C nuclear magnetic resonance (NMR). With the addition of P‐LEPB, EP composites show earlier degradation; and P‐LEPB plays flame retardancy functions as reflected by the increase of limiting oxygen index (LOI) and its promotion of carbonization as seen from the char scanning electron microscopy images after LOI tests. The addition of P‐LEPB improves the water proof performance of EP composites, playing positive effect on resisting water spreading on the surface and immersing in the EP composites as supported by the water absorption rates and contact angle measurements. The elongation at break increases with addition of P‐LEPB and reaches nearly three times higher than the neat epoxy at P‐LEPB addition of 10%, the toughening effect of P‐LEPB is attributed to the diverting function by the P‐LEPB microspheres upon stress loading, which is related to the size and distribution of P‐LEPB additions as seen from their cross‐sectional images of the composites.

Simultaneously improving the flame retardancy and toughness of epoxy composites with hyperbranched phosphorus-containing polysiloxane functionalized halloysite nanotubes

Development of high-performance thermosets with simultaneously improved flame retardancy, toughness and strength but unaffected glass transition temperature (Tg) has been a great challenge. In this work, hyperbranched phosphorus-containing polysiloxane functionalized halloysite nanotubes (HBPPSi-HNTs) were synthesized to prepare flame-retarding, mechanically-strong and tough epoxy composites. The tensile and impact strengths of the composites with 5.0phr HBPPSi-HNTs were improved by 23.7 % and 48.8 % respectively compared with the neat epoxy resin. The epoxy composites also exhibited excellent flame-retardancy due to the instantaneous gas-phase and condensed-phase flame retardant behavior of HBPPSi-HNTs. The composite with 10.0phr HBPPSi-HNTs passed UL-94V-0 rating, with an LOI value as high as 42.6 %. And compared with neat epoxy resin, the peak of heat release rate (PHRR), total heat release (THR), total smoke production (TSP) and CO generated per second (COY) were reduced by 50.1 %, 32.6 %, 30.1 % and 39.5 %, respectively. This work presented a general approach to prepare epoxy composites with simultaneously improved flame retardancy and mechanical properties without sacrificing the glass transition temperature.

A P/Si-containing polyethylenimine curing agent towards transparent, durable fire-safe, mechanically-robust and tough epoxy resins

Epoxy resin (EP) features superior comprehensive performances, but inherent flammability seriously limits its industrial applications. Current flame-retardant strategies often bring about enhanced flame retardancy but deteriorated mechanical, thermal and optical properties and neglect flame retardant durability. Herein, a phosphorus/silicon-containing polyethylenimine (PES) is designed as a multifunctional curing agent for EP. Replacing polyethylenimine (PEI) with PES maintains high visible light transmittance of EPs and improves the UV-blocking properties. The EP cured by 30 wt% PES (EP/30PES) exhibits enhanced mechanical properties compared with the EP cured by 15 wt% PEI (EP/15PEI). EP/30PES achieves a limiting oxygen index (LOI) of 35.0% and a UL-94 V-0 rating, and its peak smoke release rate (PSPR) reduces by 63.8% relative to that of EP/15PEI. Hence, the great flame retardancy, smoke suppression and mechanical performances of EP/30PES enable it to outperform previous flame-retardant EP/aliphatic amine counterparts. Meanwhile, EP/30PES shows durable flame retardancy, which still achieves a UL-94 V-0 classification after acid/alkali/aging resistance tests. This work provides an advanced design of multifunctional curing agents to create durable fire-safe EPs with great UV-shielding and mechanical performances.

Polyol and polyurethane containing bisphenol‐Z: Synthesis and application for toughening epoxy

AbstractDespite their excellent properties, epoxy resins have the disadvantage of low toughness. With the aim to improve the toughness of epoxy resin, polyol and polyurethane are synthesized using bisphenol‐Z (BPZ). The synthesized material is dispersed in the epoxy resin and used as a toughening agent. Polyol (modified bisphenol‐Z [MBPZ]‐OH) is synthesized by a ring‐opening polymerization of propylene oxide and caprolactone, and polyurethane (MBPZ‐PU) is synthesized by a one‐shot method using MBPZ‐OH and hexamethylene diisocyanate (HMDI). The effect of MBPZ‐PU on the mechanical strength of the epoxy resin is analyzed by monitoring the flexural and impact characteristics. The addition of the synthesized MBPZ‐PU improves the tensile strength, flexural strength, flexibility, and impact strength of the cured epoxy composite. Field‐emission scanning electron microscopy measurements indicate that the fracture surface had a specific pattern. Thermal properties such as the curing temperature, glass transition temperature (Tg), and modulus are investigated through differential scanning calorimetry, dynamic mechanical analysis, and thermomechanical analysis. Based on the results, MBPZ‐PU is expected to be applied to a wider field for increasing the toughness of epoxy.

Investigation of polyurethane toughened epoxy resins for composite cryotank applications. Part I: Phase separation phenomenon and cryogenic mechanical behaviors

For composite cryotanks used in aerospace engineering, polyurethane toughened epoxy (PU/EP) resins are promising as matrices by introducing soft segments. There is a special phase separation phenomenon during preparation process, and the microscale morphology is of great significance for mechanical behaviors. In this work, the relationship between phase separation phenomenon and room temperature (RT)/cryogenic mechanical behaviors is explored in-depth for PU/EP systems. As the second phase particle nucleation and growth are demonstrated by the numerical method, scanning electron microscopy, transmission electron microscope, energy dispersive spectroscopy, and viscosity test are executed to reveal particle size distributions for different PU contents. Notably, it is shown that the phase separation starts at a relatively high rate in the initial curing stage, and particle mergences and absorptions play significant roles in enlarging particle diameter subsequently. Afterwards, the RT and cryogenic tensile and fracture behaviors of PU/EP systems are systematically investigated. The addition of PU would reduce tensile strength and modulus at RT while the reducing effect vanishes at 90 K. Fracture toughness results reach the maximum values at 40 phr which are 137.3% and 60.3% higher than those of the untoughened epoxy at RT and 90 K, respectively. This work provides an effective guidance for composite cryotank designs and also offers the precise acknowledge of PU/EP phase separation structures and mechanical properties for theoretical toughening mechanism analysis in our following Part-II investigation.

Simultaneously stiffening and toughening epoxy by urea treated hydroxylated halloysite nanotubes

Epoxy and its composites have been extensively applied in widespread applications, such as encapsulation of electronics, automobile parts, aircraft components, and so on. The stiffening and toughening of epoxy via the incorporation of nanofillers, such as halloysite nanotubes (HNTs), a naturally occurring and low-cost aluminosilicate, have attracted increasing research attention. Herein, we introduce an effective and facile method that substantially enhances the interface of hydroxylated HNTs (h-HNTs) and epoxy matrix by the introduction of urea molecules. As confirmed by the characterization results, urea formed hydrogen bonds with the external surface of the h-HNTs and covalent bonds with the epoxy matrix. The resultant epoxy/h-HNT-urea nanocomposites exhibited significant enhancements simultaneously in tensile strength, Young's modulus, and fracture toughness by 23.4%, 31.4%, and 61.4%, respectively, as compared with the neat epoxy. Through systematic investigation, it was determined that micro-cracking, nanotube bridging, and crack pinning were responsible for the toughening of the epoxy/h-HNT-urea nanocomposites.

Double modified amino phenolic resin particles: A novel modifier to thermally stable, mechanically-strong and tough epoxy

We proposed a method to prepare double modified amino phenolic resin (D-APS) particles, which includes a hydrothermal synthesis and subsequent two-step modification. The prepared D-APS particle has a uniform spherical morphology. There were abundant and evenly dispersed amino hydrogen and phosphaphenanthrene groups on its surface. The formed D-APS/epoxy composite possesses improved mechanical properties at room and cryogenic temperature. At room temperature, the tensile strength of epoxy composite firstly increased and then decreased with the increase of D-APS loading. A maximum tensile strength of 98.4 MPa was obtained when the D-APS loading was 3.0 wt%. The fracture toughness of D-APS/epoxy composite containing 3 wt% D-APS particle was 1.68 MPa m1/2. At 90 K, a maximum tensile strength of 208.4 MPa was obtained when the D-APS loading was 3.0 wt%. The corresponding KIC is 3.23 MPa m1/2. A 49 °C increase in 50 wt% mass loss temperature for epoxy composite containing 3 wt% D-APS particle is observed. DMA tests show that D-APS was also helpful to increase the glass transition temperature of epoxy composites. Possible modification mechanism of D-APS/epoxy composites was also discussed.

Toughening epoxy resin with liquid rubber and its hybrid composites: A systematic review

Toughening epoxy resin with liquid rubber and its hybrid composites: A systematic review

Cyclotriphosphazene hyperbranched P/N/Si prepared flame retardants improve mechanical properties and flame retardancy of epoxy resins

AbstractIn this study, a cyclotriphosphazene hyperbranched P/N/Si prepared flame retardant (HBPPSi) was synthesized, and the effects of different loadings of HBPPSi on the mechanical properties, thermal stability and flame retardancy of epoxy resin were investigated. The mechanical property studies showed that the addition of HBPPSi could effectively improve the strength and toughness of epoxy resin, in which the strength and toughness of 5% HBPPSi/EP composites were improved by 43% and 59%, respectively. Meanwhile, thermogravimetric analysis and combustion performance test analysis indicated that the loading of HBPPSi could significantly improve the carbon formation rate, flame retardancy, and smoke suppression performance of EP composites. 5% HBPPSi/EP composites achieved a V‐1 grade in the UL‐94 test, and the LOI value increased to 28.3%. Compared with the pure EP material, the maximum heat release rate (PHRR) and total smoke release (TSP) of the 5% HBPPSi/EP composite were reduced by 47% and 32%, respectively. In addition, the release of toxic smoke CO was also significantly reduced. More importantly, the mode of action of the effect of HBPPSi on the mechanical properties, flame retardant properties, and smoke suppression properties of EP materials was investigated.

Hybridization effect of functionalized microcrystalline cellulose and liquid acrylonitrile butadiene rubber on epoxy

Toughening epoxy resins by adding different agents has been employed as a way to reduce brittleness in composites. Some hybridization strategies combining liquid rubbers and rigid fillers can be found in the literature, but the pair cellulose-based reinforcement, such as microcrystalline cellulose (MCC), and liquid acrylonitrile butadiene rubber (NBR) is hardly studied. The aim of this work is to investigate the effect of hybridizing MCC and amino-functionalized MCC (MCCSi) with NBR on the thermal, mechanical, and dynamic-mechanical behavior of epoxy. X-ray microtomography showed a good dispersion of MCCSi fillers in epoxy/NBR compared to the non-treated MCC filler. NBR with MCCSi was found to slightly increase the thermal stability of epoxy. The addition of MCC, regardless of the functionalization, decreased tensile strength and elastic modulus, but improved impact strength and toughness properties (KIC). Also, MCCSi composites displayed better dynamic-mechanical behavior, attributed to the enhanced chemical interaction, with a small effect on glass transition temperature.

Significantly enhancing fracture toughness of epoxy composite with promising γ-FeOOH@Fe2O3 hybrid nanoparticles by magnetic field assistance

The toughening of epoxy resin (EP) and the interlaminar toughening of carbon fiber reinforced composite (CF/EP) laminates have been widely concerned. In this work, the needle-like γ-FeOOH nanoparticles were prepared by liquid phase deposition-air oxidation method, and then were calcined under different conditions to obtain γ-FeOOH and γ-Fe2O3 hybrid nanoparticles (γ[email protected]2O3). Effect of calcination condition of γ[email protected]2O3 and magnetic field assistance on fracture toughness (KIC) of EP was systematically investigated. Then the selected γ[email protected]2O3 with the best toughening effect were used to improve the mode I interlaminar fracture toughness (GIC) of CF/EP laminate. The resulting γ[email protected]2O3 have a length of around 1 ​μm, a diameter of around 100 ​nm and the Ms of 8.99–45.96 emu/g. After calcinated at 250 ​°C for 1 ​h, the γ[email protected]2O3 containing 24 ​wt% FeOOH and 76 ​wt% Fe2O3 achieved the best toughening effect. Under a magnetic field of 0.09 ​T, the KIC of the γ[email protected]2O3/EP composite (2.45 ​MPa ​m1/2) is 81.7% and 66.7% higher than that of neat epoxy and the composite without magnetic field induction, respectively. Furthermore, the GIC of the γ[email protected]2O3/CF/EP composite (0.914 ​kJ/m2) is also significantly increased by 88.8% and 51.8% compared to that of CF/EP and the corresponding composite without magnetic field induction, respectively.

Performance study of a new epoxy resin IR-3 in HTS-based high-field magnet application

REBCO (Rare-earth-based barium copper oxide) conductors are appropriate materials for high-field magnet applications. Vacuum impregnation using epoxy resin is a technique widely used for stable operation of superconducting coils. However, epoxy-impregnated REBCO coils often experience critical current degradation problems. Finding a suitable impregnating material for REBCO coils is important for their application in high-field superconducting magnets. A new toughness epoxy, IR-3, was developed recently. An in-depth understanding of IR-3 on the performance of REBCO coils is critically necessary for its application. Thus, this paper explores the effects of IR-3 impregnation on the performance of REBCO coils at 77 K and 4.2 K. The test results are compared to similar coils impregnated with CTD-101 K and MY750. Meanwhile, the radial stresses at 77 K in self-field and 4.2 K under 10 T were simulated. All epoxy impregnated REBCO coils showed no decay in critical current after thermal cycles at 77 K. When charged at 4.2 K in external fields of 5 T and 10 T, the IR-3 impregnated REBCO coils avoided performance degradation problems and had superior electrical stabilities. Combing the excellent performance at low temperatures, IR-3 is a promising candidate material for impregnating high-field REBCO coils.

Mechanical, wear and fatigue behavior of silane‐treated corncob biosilica toughened epoxy resin composite reinforced with prickly pear short fibers

AbstractThis research focused on investigating the mechanical, wear, and fatigue behavior of an epoxy composite toughened with corncob biosilica (CCB) and prickly pear fiber (PPF). The primary aim of this research was to improve the load bearing effect of high‐brittle matrix by adding eco‐friendly fiber and particle. The surface treatment on the fiber and particle was done using an amino silane, 3‐Aminopropyltrimethoxysilane (APTMS). The natural fiber composites are prepared by hand layup process and characterized in accordance with ASTM standards. The experimental results indicated that the addition of fiber and particles increased the tensile and flexural strength up to 61% and 50%, respectively. Similarly, it is noted that the biosilica particles improved the bonding and reduced the rate of wear of composites, as well as lowered the stress intensity factor. A lowest wear rate of 0.037 and COF of 0.38 were noted for EFC2 composite designation. The morphological study of the fractured composites revealed improved fiber adhesion to the matrix. This reduced intensity of stress, increased wear resistance and higher fatigue life ensures the composite's application in structural and industrial applications.