Effect of ductility on the fatigue behavior of epoxy resins

The effect of ductility on fatigue behavior was studied using two DGEBA-based (diglycidyl ether of bisphenol A) epoxies: a ductile Epon 815/Versamid 140 and a brittle Epon 828/Epon Z. Failure modes were different although normalized stress-life relations were similar for both resins. Two competing failure mechanisms were identified: viscoelastic creep, and nucleation and coalescence into a main crack of microcracks. No signs of crazing or fibrillation were detected. The plastic elongation during fatigue was larger in Epon 815/Versamid 140. Fracture sources showed cracked material surrounded by a region of stable growth of the main crack. In the brittle Epon 828/Epon Z cracked material was scarce and the crack initiation region was clean, especially at high stress levels. Discontinuous crack growth bands and striations were seen in the stable crack growth regions. During unstable propagation the crack advanced at different levels joined by deep cleavage steps. Branching of the main crack occurred only in the brittle resin at the final stage of propagation.     

Dynamic mechanical analysis of rubber toughened epoxy resins

Dynamic mechanical analysis (DMA) was used to characterize cured epoxy resin formulations from ?150°C to temperatures above their α transitions. The resins were aromatic amine and aliphatic amine cured and were modified with carboxylterminated acrylonitrile-butadiene (CTBN) rubbers to improve their toughness, A DuPont 981 dynamic mechanical analyzer was used to measure the modulus and mechanical loss factor (tan δ) of the samples. Changes in the α and β transitions in the scan of tan δ as a function of temperature were related to changes in the formulation. Relations were also sought between changes in the DMA data and fracture and impact toughness of the cured formulations obtained using an instrumented impact test. Impact tests were performed at ?196°C and at room temperature. Results indicate that fracture toughness and the dynamic mechanical properties are affected by the amount of rubber, the compatibility of the rubber and epoxy, and changes in the curing agent stoichiometry.     

基体种类对CTBN改性环氧树脂结构和性能的影响

研究了ＣＴＢＮ对不同种类的环氧树脂（Ｅ－５１、Ｆ－５１）力学性能的影响,通过扫描电镜观察了２种增韧体系的微观结构形态,并探讨了２种增韧体系的微观结构形态与力学性能间的关系。     

橡胶弹性体增韧环氧树脂的研究进展

综述了环氧树脂增韧改性的研究现状,详细介绍了丁晴橡胶、丙烯酸酯橡胶和聚氨酯弹性体增韧环氧树脂的研究进展,展望了橡胶弹性体增韧环氧树脂的前号。     

On hybrid toughened DGEBA epoxy resins

Considerable improvements in the fracture resistance of epoxy resins have recently been achieved by adding either rubbery or rigid second phase dispersions, or both, to an epoxy matrix. These multiphase epoxy systems are particularly useful as high performance adhesives and as matrix materials in advanced composites. This paper describes the development of new toughened epoxy hybrids, which contain both rubbery and rigid dispersions. The latter dispersions were either zirconia particles, short alumina fibres or glassy-metal ribbons. Micromechanisms of toughening and failure processes in these new materials are identified and discussed in the light of microstructures.     

The influence of matrix modification on fracture mechanisms in rubber toughened polymethylmethacrylate

The fracture behavior of composite rubber particle-toughened polymethylmethacrylate has been investigated over a wide range of test speeds, encompassing impact conditions. When the entanglement density of the matrix was increased and its glass transition temperature reduced by copolymerization, there were significant increases in the crack initiation and propagation resistance of the particle-toughened materials at low to intermediate speeds. At impact speeds, on the other hand, where crazing became the dominant matrix microdeformation mechanism in all the materials investigated, the fracture response of the copolymer matrix was closer to that of the polymethylmethacrylate homopolymer, and the toughening effect of the rubber particles was no longer effective in either case. This is discussed in terms of the onset of the matrix β transition, associated with the transition from shear to crazing, and the α transition of the rubber domains, both of which occurred in the temperature range immediately below room temperature in low frequency dynamic torsion measurements.     

Effect of molecular weight between crosslinks on the fracture behavior of rubber‐toughened epoxy adhesives

The effect of molecular weight between crosslinks, M_c, on the fracture behavior of rubber‐toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber‐toughened bulk system, fracture energy increased with increasing M_c. However, in the liquid rubber‐toughened adhesive system, with increasing M_c, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core‐shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with M_c. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 38–48, 2001     

橡胶增韧环氧灌封料的研究

以液体羧基丁腈橡胶、液体聚硫橡胶为增韧剂制成环氧树脂灌封料,通过冲击强度、弯曲强度和拉伸剪切强度评价增韧环氧树脂灌封料的效果。实验结果表明,加入增韧剂的灌封料韧性与对照组相比均有不同程度的提高;液体聚硫橡胶增韧的环氧树脂灌封料表现出较好的抗冲击性能和抗弯曲性能;液体羧基丁腈橡胶增韧的环氧树脂灌封料表现出较好的拉伸剪切性能。     

Effect of aluminum silicate on the impact and adhesive properties of toughened epoxy resins

Carboxyl randomized poly(2-ethyl hexyl acrylate) (CRPEHA) and epoxy randomized poly(2-ethylhexyl acrylate) (ERPEHA) have been used to toughen aluminum silicate filled epoxy resin cured with 4,4′-diaminodiphenyl methane. CRPEHA (A-1) and ERPEHA (B-1) were synthesized by solution polymerization technique in the form of liquid rubbers. The toughened epoxy networks were evaluated for their impact and adhesive properties. The epoxy/liquid rubber compositions were varied to study the effect of toughener concentration on the adhesive and impact properties for both filled and unfilled systems. Improved properties were obtained for epoxy resins toughened with (1 : 1) mixture of CRPEHA and ERPEHA. Lap shear strength of filled epoxy resins was higher than that of unfilled ones but the reverse was the case for impact strength. Analysis of adhesive failure surfaces by scanning electron microscopy (SEM) indicated the presence of a two-phase microstructure.     

Failure analysis of rubber toughened epoxy resin

Failure of the blends of epoxy cresol novolac resin (ECN) with varied proportions of carboxy terminated polybutadiene (CTPB) liquid functional rubber was studied. Addition of CTPB improves toughness as reflected in the improvement of tensile, flexural, and impact properties. However, 10 wt % of CTPB was the optimum concentration beyond which a rapid fall of properties, in all cases, was observed. Surface topography of the fractured surfaces, studied by scanning electron microscopy and atomic force microscopy, revealed marked changes in the phase morphology due to addition of rubber and also accounted for the variation of the strength properties. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 861–868, 2003     

Analysis of the fracture behavior of epoxy resins under impact conditions

In this work an analysis of the fracture behavior under impact of four epoxy resins was performed. The morphology of the fracture surfaces was analyzed by scanning electron microscopy and the topographic marks observed could be related to the thermal behavior of each epoxy system. The relevant properties that determine the thermal behavior were the thermal diffusivity, which was measured by using the open photoacoustic cell technique, and the glass transition temperature. As the thermal diffusivity of these materials is very low, and therefore also is their heat dissipation capacity, the impact test occurs under adiabatic conditions and a temperature increase occurs at the tip of the running cracks. Therefore, thermal blunting may occur at the crack tip and the energy absorption capacity of the resins is increased. The topographic marks observed at the fracture surface help to identify how efficient this mechanism is for each of the epoxy systems analyzed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2486–2492, 2000     

Microstructure,mechanical properties,and fracture behavior of liquid rubber toughened thermosets

Phenolic epoxy resin was toughened by carboxyl-randomized butadiene acrylonitrile copolymer (CRBN) for use as composite matrix. By adding different parts of butadiene acrylonitrile copolymer (BN-26, without carboxyl contained) to CRBN, different sizes of rubber domains and different numbers of chemical bondings between the resin matrix and the rubber phase were obtained. It is found that small rubber particles (less than 0.1 μm) are cavitated during the crack development. The interaction between secondary crack zones caused by the cavitation makes the fracture toughness K_IC of the materials high; by comparison, a local stress-whitened zone is produced in the material with large rubber particles (more than 0.1 μm) when it is subjected to tensile stress. In this case, the flexure strength σ_f of the material is great. Using ultrasection and TEM techniques, the stress-whitened zone was shown to be caused by the special multiple-phase structure of the material, in which many caves and “macrocrazes” coexist.     

Internal fracture of notched epoxy resins

The brittle fracture of round-notched epoxy resin bars subjected to plane strain bending has been studied at varying strain rates. Observations of fracture processes and surface morphologies revealed that the internal crack was nucleated at the plastic-elastic boundary when the plastic deformation zone at the notch root reached a certain size. A slip-line field theory allows calculation of the stress components at the plastic-elastic boundary from a knowledge of the location of the internal crack. An analysis of the data concluded that the triaxial stress level ahead of the plastic zone was raised by plastic constraints to an ideal fracture stress which is considerably larger than that of glassy thermoplastics.     

核-壳粒子和液体橡胶增韧环氧胶粘剂的研究

采用丙烯酸酯核-壳粒子和液体橡胶分别增韧改性环氧树脂胶粘剂,探讨了2种增韧体系对冲击强度和粘接强度的影响。研究发现,核-壳粒子在用量较少的情况下可获得比液体橡胶更优异的冲击强度和粘接强度,剥离强度达到10 kN/m以上。通过SEM观察改性环氧树脂断裂表面的微观形貌,并通过TEM进一步观察核-壳粒子增韧环氧树脂的分散状态,探讨微观形态与冲击强度和粘接强度之间的关系。     

Morphological,mechanical properties,and fracture behavior of bulk‐made ABS resins toughened by high‐cis polybutadiene rubber

Acrylonitrile‐butadiene‐styrene (ABS) resins with various rubber contents were prepared by applying nickel catalyzed high‐cis polybutadiene rubber (BR9004) as toughening agent via bulk polymerization. The influence of rubber content and its characteristics on the morphology, mechanical properties, and fracture mechanisms of ABS resins were investigated. The relevant performance parameters were also evaluated and compared with a commercial injection grade resin (ABS‐8434). The results show that each synthesized resin generally contains some irregular microsized particles with a certain amount of subinclusions besides the analogous “sea‐island” morphology to ABS‐8434. The subinclusions considerably enhance the volume fraction of rubber phase; this leads to an increasing maximum loss tangent (tan δ) value, a decreasing storage modulus and glass transition temperature (T_g) of rubber phase. Besides, the higher grafting degree can not only produce a higher T_g of grafted copolymer but also improve the compatibility of rubber phase with matrix. Based on the performance measurements andfractography, the final product with a rubber content of 9.3% reveals ductile fracture behavior and excellent comprehensive properties far superior to ABS‐8434 due to combined shear yielding and massive crazing. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Study of epoxy toughened by in situ formed rubber nanoparticles

The effect of rubber nanoparticles on mechanical properties and fracture toughness was investigated. Rubber nanoparticles of 2–3 nm were in situ synthesized in epoxy taking advantage of the reaction of an oligomer diamine with epoxy. The chemical reaction was verified by gel permeation chromatography (GPC) and ¹HNMR, and the microstructure was characterized by transmission electron microscope. The rubber nanoparticles caused much less Young's modulus deterioration but toughened epoxy to a similar degree in comparison with their peer liquid rubber that formed microscale particles during curing. Fifteen wt % of rubber nanoparticles increased fracture energy from 140 to 840 J/m² with Young's modulus loss from 2.85 to 2.49 GPa. The toughening mechanism might be the stress relaxation of the matrix epoxy leading to larger plastic work absorbed at the crack tip; there is no particle cavitation or deformation; neither crack deflection nor particle bridging were observed. The compound containing rubber nanoparticles demonstrates Newtonian liquid behavior with increasing shear rate; it shows lower initial viscosity at low shear rate than neat epoxy; this provides supplementary evidence to NMR and GPC result. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal and mechanical characterization of epoxy resins toughened using preformed particles

Preformed, multilayer particles have been used to toughen an epoxy resin. The particles were formed by emulsion polymerization and consist of alternate glassy and rubbery layers, the outer layer having glycidyl groups to give the possibility of chemical bonding of the particles in the cured resin. Two variants of this type of particle were used, termed GM(47/15) and GM(47/37); both types have an overall diameter of 0.5 µm, but the former have a thicker rubbery layer. For comparison, acrylic toughening particles (ATP) with no surface functionality and a liquid carboxyl‐terminated butadiene–acrylonitrile (CTBN) rubber were used as toughening agents. The epoxy resin system consisted of a commercial diglycidyl ether of bisphenol A (Shell Epon 828) with diamino‐3,5‐diethyl toluene as hardener, two commercial sources of which were used, namely Ethacure‐100 (Albemarle SA) and DX6509 (Shell Chemicals). These hardeners contain a mixture of two isomers, namely 2,6‐diamino‐3,5‐diethyltoluene and 2,4‐diamino‐3,5‐diethyltoluene Thermogravimetry in nitrogen shows that the preformed toughening particles begin to degrade at 230 °C, whereas the cured resin begins to degrade rapidly at 350 °C. Thus, even though the particles are less thermally stable than the cured resin, their degradation temperature is well above the glass transition temperature of the resin, and their use does not affect the thermal stability of the toughened materials at normal use temperatures. The performance of the toughening agents was compared using Ethacure‐100 as the hardener. The GM(47/15) and GM(47/37) toughening particles gave rise to a greater toughening effect than the ATP and the CTBN. For example, the fracture energies were: 0.26 kJ m^?2 for the unmodified resin; 0.60 kJ m^?2 for the resin toughened with CTBN; and 0.69 kJ m^?2 for the resin toughened with the GM(47/15) particles. The ultimate tensile stress of the unmodified epoxy resin was 43 MPa, which increased to 55 MPa when 20 wt% of GM(47/15) toughening particles were added. The toughness of resins cured with the DX6509 hardener were superior to those obtained with the Ethacure‐100 hardener, most probably due to DX6509 producing a less‐highly‐crosslinked network. This highlights the sensitivity of the toughening process to the hardener used, even for hardeners of a similar nature. © 2001 Society of Chemical Industry     

The effect of urea bond on structure and properties of toughened epoxy resins

In this article, modified poly(oxypropylene) diamines were synthesized and used as a new flexible curing agent for epoxy resins. The purpose of modification is to introduce urea group into epoxy resins. The reaction rate, mechanical properties, glass transition temperature (T_g), and fracture surface morphology of these toughened epoxy resins were investigated. Because of urea groups, the reactivity between poly(oxypropylene) diamines and epoxy resins was significantly enhanced. At the same time, the urea groups resulted in strong intersegmental hydrogen bonding between modified poly(oxypropylene) chain, which reduced the compatibility of poly(oxypropylene) with epoxy resins and resulted in higher T_g of toughened epoxy. The modified sample had tensile strength of 15.8 MPa and ultimate elongation of 118% at room temperature, whereas the unmodified sample only had 6.2 MPa and 70%. The scanning electron microscope analysis showed that the modified system displayed tough fracture feature, whereas the unmodified system showed typical brittle fracture. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of cloisite 30B clay and sisal fiber on dynamic mechanical and fracture behavior of unsaturated polyester toughened epoxy network

This study examined the dynamic mechanical properties of sisal fiber reinforced unsaturated polyester (UP) toughened epoxy nanocomposites. The chemical structures changes in Epoxy, UP and UP toughened epoxy (Epoxy/UP) systems were characterized by Proton Nuclear magnetic resonance (¹HNMR) spectroscopy. The morphological alterations of the nanocomposites were analyzed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The untreated, chemically treated fibers, nanoclays, and the fiber reinforced Epoxy/UP nanocomposites were confirmed by FTIR spectrometer. The obtained mechanical results showed that alkali‐silane treated fibers improve the tensile strength (96%) and flexural strength (60%) of the Epoxy/UP nanocomposite than that of Epoxy/UP blend due to the strong interfacial bonding between the sisal fiber and matrix. The fracture toughness (K_IC) and fracture energy (G_IC) of treated sisal fiber reinforced DGEBA/UP/C30B nanocomposites found to be higher than that of untreated sisal fiber nanocomposites. The dynamic mechanical analysis (DMA) reveals that the fiber reinforced Epoxy/UP nanocomposites contains 30 wt% treated fiber and 1 wt% nanoclays, exhibits the highest storage modulus and better glass transition temperature (T_g) among the other kind of systems. The surface morphology of the fibers, fractured surface of the resins and composites were confirmed by scanning electron microscope (SEM). POLYM. COMPOS., 37:2832–2846, 2016. © 2015 Society of Plastics Engineers