Self-healing performance assessment of epoxy resin and amine hardener encapsulated polymethyl methacrylate microcapsules reinforced epoxy composite

In order to study the effect of healing materials viscosity on the self-healing performance of polymer composite, mostly available epoxy resin of viscosity 10–12 Pa.s and amine hardener of viscosity 0.01–0.02 Pa.s were chosen as two different healing materials and successfully encapsulated. Effect of core to shell(c/s) ratio on the synthesis of epoxy microcapsules was investigated and 1:1 c/s ratio is suggested as an ideal ratio to synthesize epoxy capsules. Chemical structure and thermal decomposition patterns of both microcapsules and capsules reinforced composite were analyzed. Tensile strength, impact strength and fracture toughness of capsules reinforced self-healing epoxy composite were evaluated. It was observed that the toughness of epoxy composite increased with the increase in microcapsules concentration. An optimum healing efficiency of 66% was observed with the addition of 7.5 wt% epoxy and hardener microcapsules at equal weight ratio. Stresses developed in the pure epoxy composite crack front were analyzed using Ansys V18.1.     

Preparation and assessment of a self‐healing material based on microcapsules filled with ethyl phenylacetate

A self‐healing material was developed on the basis of a biological system. The self‐healing epoxy resin, which incorporated microcapsules filled with ethyl phenylacetate (EPA), was investigated. The microcapsules were prepared by an in situ polymerization method. The microcapsule‐formation process was monitored by optical microscopy, and the surface morphology was observed by scanning electron microscopy (SEM). The self‐healing performance of the epoxy resin was assessed by manual and in situ healing experiments. We investigated the effects of the healing time, amount of EPA, and degree of curing of matrix on the healing performance by manually injecting EPA into the crack plane. The maximum healing efficiency was obtained within 24 h. The swelling curve was overlaid onto the healed load plot; this indicated that crack healing was achieved as a result of solvent diffusion. The healing load reached the maximum value when the amount of EPA was 0.5 μL and was capable of filling the crack volume. Moreover, the healing performance was related to the degree of swelling. The in situ healing efficiency was dependent on the microcapsule concentration. The fracture toughness could be fully restored when the microcapsule concentration was 10%. Finally, the crack interface was analyzed with SEM. The results show that the fracture line was difficult to detect, and this suggested complete crack healing by EPA. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43430.     

自修复聚合物材料用微胶囊

Microcapsules with dicyclopentadiene (DCPD) as core material and urea formaldehyde resin as wall material used for making self-healing polymer material were prepared with the in-situ polymerization method. The effect of microcapsules on the fracture toughness of epoxy resin was studied. The addition of microcapsules into epoxy resin results in the decrease of fracture toughness. When microcapsule content was kept constant, as the microcapsule size increased the fracture toughness of the epoxy resin decreased linearly and the percentage of decrease compared to the neat epoxy without microcapsules increased linearly. Moreover, the fracture toughness of the material decreases linearly with the increase of microcapsule content.     

High performance self‐healing bismaleimide/diallylbisphenol a/poly(phenylene oxide) microcapsules composites with low temperature processability

Novel high performance self‐healing 4,4′‐bismaleimidodiphenylmethane (BDM)/diallylbisphenol A(BA)/poly(phenylene oxide) microcapsules filled with epoxy resin (PPOMCs) systems with low temperature processability were developed. The effects of PPOMCs on the reactivity of BDM/BA resin system were investigated; the properties of cured BDM/BA/PPOMCs systems such as fracture toughness, dynamic mechanical property, dielectric property, and self‐healing ability were discussed. The morphologies of the cured resin systems were characterized using scanning electronic microscope and light microscopy. Results reveal that the addition of PPOMCs can catalyze the polymerization reaction of BDM/BA resins. BDM/BA systems with appropriate PPOMCs content cured at low temperature possess excellent fracture toughness, high glass transition temperature (T_g), and low dielectric property. The self‐healing ability of BDM/BA can be realized by the introduction of PPOMCs owing to the polymerization of the released core materials from PPOMCs. The self‐healing efficiency of healed BDM/BA/PPOMCs systems can be influenced by the size and content of PPOMCs and the contact areas between the crack surfaces. © 2013 Society of Plastics Engineers     

环氧树脂微胶囊与潜伏性固化剂自修复体系断裂韧性的研究

微胶囊二元自修复系统对聚合物基复合材料在使用中产生的微小裂纹具有修复作用,但微胶囊和固化剂的加入会对基体材料的断裂韧性产生影响。本文研究了环氧树脂微胶囊和咪唑类潜伏性固化剂对聚合物基复合材料基体材料的断裂韧性的影响。采用环氧树脂E-51作为基体材料,三乙烯四胺为常温固化剂,咪唑类衍生物2MZ-Azine和实验室自制的包含环氧树脂芯材的微胶囊为材料制作断裂韧性拉伸试样。实验结果表明,当微胶囊的含量达到一定比例之前,基体材料的断裂韧性随着微胶囊含量的增加而增强,当微胶囊含量超过此比例后,基体材料的断裂韧性随着微胶囊含量的增加而减小,潜伏性固化剂的加入会增大基体材料的断裂韧性。这与环氧树脂材料增韧理论相符合。     

Influence of microcapsule shell material on the mechanical behavior of epoxy composites for self‐healing applications

In this article, we have studied the effect of microcapsule shell material on the mechanical behavior of self‐healing epoxy composites. Liquid epoxy healant was encapsulated in melamine‐formaldehyde (MF) and urea‐formaldehyde (UF), using emulsion polymerization technique to prepare microcapsules of different shell walls. The core content of the microcapsules, as determined by solvent extraction technique was found to be 65 ± 4%, irrespective of the shell wall of microcapsule. Morphological investigations reveal a rough texture of the spherical microcapsules, which was attributed to the presence of protruding polymer nanoparticles on the surface. Epoxy composites containing UF and MF microcapsules (3–15% w/w) were prepared by room temperature curing and their mechanical behaviour was studied under both quasi‐static and dynamic loadings. The tensile strength, modulus, and impact resistance of the matrix was found to decrease with increasing amount of microcapsule in the formulation, irrespective of the shell wall material used for encapsulation. Interestingly, substantial improvement in the fracture toughness of the base resin was observed. Morphological investigations on the cracked surface revealed features like crack pinning, crack bowing, microcracking and crack path deflection, which were used to explain the toughened nature of microcapsule containing epoxy composites. Our studies clearly indicate that the microcapsule shell wall material does not play any significant role in defining the mechanical properties of the composites. In addition, presence of secondary amine functionalities in UF and MF shell wall do not interfere with the reaction of epoxy with triethylene tetramine hardener during the curing process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40572.     

Fracture and fatigue response of a self-healing epoxy adhesive

A self-healing epoxy adhesive for bonding steel substrates is demonstrated using encapsulated dicyclopentadiene (DCPD) monomer and bis(tricyclohexylphosphine)benzylidine ruthenium (IV) dichloride (Grubbs’ first generation) catalyst particles dispersed in a thin epoxy matrix. Both quasi-static fracture and fatigue performance are evaluated using the width-tapered-double-cantilever-beam specimen geometry. Recovery of 56% of the original fracture toughness under quasi-static fracture conditions occurs after 24 h healing at room temperature conditions. Complete crack arrest is demonstrated for fatigue test conditions that render neat resin and control samples failed. Inspection of fracture surfaces by electron microscopy reveals evidence of polymerized DCPD after healing. These results are the first mechanical assessment of self-healing for thin (ca. 360 μm) films typical of adhesives applications.     

Influence of dual-component microcapsules on self-healing efficiency and performance of metal-epoxy composite-lap joints

Dual-component microcapsules were synthesized by solvent evaporation technique using epoxy resin and hardener as core materials and polymethyl methacrylate (PMMA) as shell wall materials. Morphology, core content, and size distribution of microcapsules were monitored by controlling the various processing parameters such as agitation speed, core–shell weight ratio, and concentration of emulsifiers. The molecular structure, morphologies, and thermal characteristics of the microcapsules were examined under Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and thermogravimetric analysis (TGA), respectively. Synthesized dual-component microcapsules were entrenched into the epoxy polymer to introduce the healing features in single lap shear epoxy adhesive joints. Healing efficiency as high as 89% was achieved when 10 wt% dual-component microcapsules were introduced in adhesives. Investigation of the fractured surfaces of the healing enabled adhesives reveals the presence of crack pinning and crack blunting sites represented by characteristic tails at the wake of microcapsules in cohesive zone. Such failure mechanisms responsibly influence the healing efficiency.     

Effects of rubber‐rich domains and the rubber‐plasticized matrix on the fracture behavior of liquid rubber‐modified araldite‐F epoxies

The fracture behavior of a bisphenol A diglycidylether (DGEBA) epoxy, Araldite F, modified using carboxyl‐terminated copolymer of butadiene and acrylonitrile (CTBN) rubber up to 30 wt%, is studied at various crosshead rates. Fracture toughness, K_IC, measured using compact tension (CT) specimens, is significantly improved by adding rubber to the pure epoxy. Dynamic mechanical analysis (DMA) was applied to analyze dissolution behavior of the epoxy resin and rubber, and their effects on the fracture toughness and toughening mechanisms of the modified epoxies were investigated. Scanning electron microscopy (SEM) observation and DMA results show that epoxy resides in rubber‐rich domains and the structure of the rubber‐rich domains changes with variation of the rubber content. Existence of an optimum rubber content for toughening the epoxy resin is ascribed to coherent contributions from the epoxy‐residing dispersed rubber phase and the rubber‐dissolved epoxy continuous phase. No rubber cavitation in the fracture process is found, the absence of which is explained as a result of dissolution of the epoxy resin into the rubber phase domains, which has a negative effect on the improvement of fracture toughness of the materials. Plastic deformation banding at the front of precrack tip, formed as a result of stable crack propagation, is identified as the major toughening process.     

Self-healing thermoset using encapsulated epoxy-amine healing chemistry

Self-healing was achieved with a dual-microcapsule epoxy-amine chemistry in thermoset epoxy. One capsule contained a modified aliphatic polyamine (EPIKURE 3274) while the second capsule contained a diluted epoxy monomer (EPON 815C). Amine microcapsules were prepared by vacuum infiltration of EPIKURE 3274 into hollow polymeric microcapsules. Epoxy microcapsules were prepared by an in situ polymerization method. Both types of capsules were incorporated into an epoxy matrix (EPON 828:DETA) and recovery of mode-I fracture toughness was measured using tapered-double-cantilever-beam (TDCB) specimens. The optimal mass ratio of amine: epoxy capsules was 4: 6 and an average healing efficiency of 91% was achieved with 7 wt% amine capsules and 10.5 wt% epoxy capsules. Long-term stability of the healing system was demonstrated for six months at ambient conditions. Thermal stability was investigated by post curing samples at 121 °C and assessing healing performance.     

Hydroxyl terminated poly(ether ether ketone) with pendent methyl group toughened epoxy resin: miscibility, morphology and mechanical properties

The synthesis, processing, thermal and mechanical properties and fracture toughness of epoxy resin formulated with hydroxyl terminated poly(ether ether ketone) with pendent methyl group are reported. Hydroxyl terminated poly(ether ether ketone) oligomers based on methyl hydroquinone (PEEKMOH) were synthesised from methylhydroquinone and 4,4′-difluorobenzophenone in N-methyl-2-pyrrolidone. PEEKMOH oligomers with different molecular weights were synthesised and characterised. Blends of diglycidyl ether of bisphenol-A epoxy resin with PEEKMOH were prepared by melt mixing. The uncured blends were homogeneous and the T_g-composition behaviour was predicted using Fox, Gordon–Taylor and Kelley–Bueche equations. Reaction induced phase separation occurred in the blends on curing with 4,4′-diaminodiphenyl sulfone. Scanning electron microscopy studies revealed the two-phase morphology of the blends. Domain size of the blends increased with increase in PEEKMOH8 in the blends. Phase separation in the blends occurred by nucleation and growth mechanism. Infrared spectroscopic studies revealed that some of the epoxy groups were opened up by hydroxyl group of PEEKMOH. The tensile and flexural properties of the blends were comparable to that of neat epoxy resin and the properties were dependent on the composition of the blend and molecular weight of PEEKMOH used. Dynamic mechanical analysis revealed two glass transition temperatures corresponding to epoxy rich and thermoplastic rich phases. The crosslink density of epoxy resin decreased with the addition of PEEKMOH to epoxy resin. The blends exhibited superior fracture toughness compared to unmodified epoxy resin. The increase in fracture toughness was due to local plastic deformation of the matrix, crack path deflection and crack pinning. The thermal stability of amine cured epoxy resin was not affected by the incorporation of PEEKMOH into the epoxy resin.     

Studies of epoxy resin systems: Part D: Fracture toughness of an epoxy resin: A study of the effect of crosslinking and sub-Tg aging

The fracture toughness of an epoxy resin system, diglycidyl ether of butanediol, DGEB, cured with 4-4′ diaminodiphenyl sulphone, DDS, has been studied by varying the crosslinking density and state of aging. A stable, but rough, crack propagation was observed with specimens that were 99 percent cured and quenched. When the extent of curing was less than 99 percent or the material was aged for more than 20 min at 62°C, crack propagation was of the unstable stick-slip nature. Aging was found to decrease the initiation fracture toughness dramatically, but the arrest fracture toughness was almost unchanged. This result was associated with a change of relaxation strength of the primary, a, transition with aging. An increase of crosslinking density was found initially to reduce the fracture toughness of this epoxy resin, but the fracture toughness increased after 87 percent of curing. The initial decrease of the fracture toughness was attributed to a decrease of relaxation strength of the primary transition (i.e., the area under the α-relaxation peak), while the increase of the fracture toughness after 87 percent curing was explained by the onset of the stablerough crack propagation, Micrographs taken by scanning electron microscopy-showed possible existence of blunting during crack propagation and a decrease of blunting with the extent of aging.     

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     

Microencapsulated amino-functional polydimethylsiloxane as autonomous external self-healing agent for epoxy systems

The self-healing material under investigation is a diglycidyl ether of bisphenol A (DGEBA) epoxy matrix, which incorporates microcapsules filled with amine-functionalized polydimethylsiloxane (PDMS-a), as a new healing agent, combined with microcapsules filled with triethylenetetramine (TETA). The fracture toughness (K_IC) and healing efficiency of these systems were measured using tapered double-cantilever beam specimens. Fractographic analysis shows the rupture of microcapsules and release of the healing agent, inducing a change in the mirror-like fracture plane morphology of neat epoxy and healing of cracks. It was also observed that PDMS-a reduces the stiffness of the epoxy matrix, resulting in greater energy release rate (U_IC) values. The values of self-healing efficiency for systems healed for 48 h at room temperature varied from 0.82 to 1.0 (η, from K_IC values) and from 1.0 to 1.4 (η’, considering U_IC values). Specimens healed at 80°C containing 2.5 wt% of microcapsules filled with PDMS-a and TETA achieved η = 1.1 and η’ = 2.0, indicating a highly efficient self-healing process. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47627.     

Rapid crack propagation and arrest in polymers

Published results on dynamic fracture toughness vs crack velocity relations of polyester resin (Homalite-100), epoxy resin Araldite-B, modified epoxy resins and polycarbonate are reviewed. Commonality between the seemingly diversified experimental results as well as the existences of minimum dynamic fracture toughness, K_Im, and crack arrest stress intensity factor, K_Ia, as inherent material properties are discussed.     

Synergistic Toughening in Ternary Silica/Hollow Glass Spheres/Epoxy Nanocomposites

Herein, the fracture toughness of ternary epoxy systems containing nanosilica and hollow glass microspheres (HGMS) is investigated. The experimental measurements reveal synergistic fracture toughness in some hybrid compositions: The incorporation of 10 phr of HGMS and nanosilica alone modify the fracture toughness of epoxy by 39% and 91%, respectively. However, use of 10 phr hybrid modifier can enhance the fracture toughness of the resin up to 120%. Observations reveal different toughening mechanisms for the blends i.e., plastic deformation for silica nanoparticles and crack bifurcation for HGMS. Both of these toughening mechanisms additively contribute to the synergism in ternary epoxies.     

聚硅氧烷型增容剂改性环氧体系的研究

本文用环氧树脂、瑞羟基聚二甲基硅氧烷等为原料,合成了增容剂,其可使聚二甲基硅氧烷有效地分散在环氧体系中而使环氧增韧。通过红外光谱测定、扫描电子显微镜观察,热失重分析及其他力学性能测试,对该体系的增容和增韧效果进行了研究。     

Chemically stable polyphenylene ether microcapsules prepared using a facile method for the self‐healing of polymers

Chemically stable polyphenylene ether (PPO) microcapsules (MCs) filled with epoxy resins (PPO‐EP MCs) were prepared using low‐molecular‐weight PPO with vinyl end‐groups as shell wall and epoxy resins as core material using an oil‐in‐water emulsion solvent evaporation method. This method for synthesizing MCs with PPO shell walls is simple, convenient and novel, which can avoid the influence of processing parameters on the chemical stability of the epoxy resin core material. The resulting PPO‐EP MCs exhibit good chemical stability below 255 °C mainly owing to the absence of a polymerization catalyst of the epoxy resins. The initial thermal decomposition temperature of the MCs is about 275 °C. The MCs were embedded in a 4,4′‐bismaleimidodiphenylmethane/O,O′‐diallylbisphenol A (BMI/BA) thermosetting resin system. When processed at high temperature (up to 220 °C), the microencapsulated epoxy resins could be released from the fractured MCs to matrix crack surfaces and bond the crack surfaces. An amount of 8 wt% MCs restored 91 and 112% of the original fracture toughness of the BMI/BA matrix when heated at 220 °C/2 h and 80 °C/1 h + 220 °C/2 h, respectively. The MCs only slightly decreased the thermal property of the matrix. © 2016 Society of Chemical Industry     

Fracture properties of epoxy/poly(styrene‐co‐allylalcohol) blends

The epoxy/polystyrene system is characterized by a poor adhesion between the constituent phases, which determines its mechanical properties. The adhesion can be improved via blends based on epoxy resin and random copolymers, poly(styrene‐co‐allylalcohol) (PS‐co‐PA). In this work, the influence of PS‐co‐PA content and the good adhesion between the phases on the tensile properties and the fracture toughness achieved through instrumented Charpy tests have been investigated. The tensile strength and the deformation at break showed an increase in the PS‐co‐PA content while the Young's modulus remained the same. The tensile fracture surfaces revealed that the improvement of these magnitudes was mainly due to a crack deflection mechanism. Also, the fracture toughness of the blends was superior to that of the pure epoxy resin. The main operating toughening mechanism was crack deflection. The fractographic analysis showed that ～ 80% of the particles were broken, and the crack tended to divert from its original path through the broken PS‐co‐PA particles. The remaining particles were detached from the epoxy resin, and the holes left suffered plastic deformation. Analytical models were used to predict successfully the toughness due to these mechanisms. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Delamination toughness of fiber-reinforced composites containing a carbon nanotube/polyamide-12 epoxy thin film interlayer

We present a simple, out-of-autoclave approach to improve the delamination toughness of fiber-reinforced composites using epoxy interlayers containing 20 wt.% polyamide-12 (PA) particles and 1 wt.% multi-walled carbon nanotubes (MWCNTs). Composites were prepared by integrating partially cured thin films at the laminate mid-plane using vacuum-assisted resin transfer molding. The introduction of epoxy/PA interlayers increased fracture toughness due to the ductile deformation and crack bridging of PA particles within an interlaminar damage zone with uniform thickness of about 20 μm. Composites interlayered with epoxy/PA/MWCNT exhibited nearly 2.5 and 1.5 times higher fracture toughness than composites containing neat epoxy and epoxy/PA interlayers, respectively, without an observable increase in interlaminar thickness. The fracture surface was analyzed to identify failure modes responsible for the fracture toughness improvement. The MWCNTs are proposed to inhibit critical loading of defects by minimizing stress concentration within the interlaminar region, thereby enabling greater deformation of the PA particles during fracture.