Phase separation,cure kinetics,and morphology of epoxy/poly(vinyl acetate) blends

Epoxy based on diglycidyl ether of bisphenol A + 4,4′diaminodiphenylsulfone blended with poly(vinyl acetate) (PVAc) was investigated through differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and environmental scanning electron microscopy (ESEM). The influence of PVAc content on reaction induced phase separation, cure kinetics, morphology and dynamic‐mechanical properties of cured blends at 180°C is reported. Epoxy/PVAc blends (5, 10 and 15 wt % of PVAc content) are initially miscible but phase separate upon curing. DMTA α‐relaxations of cured blends agree with T_g results by DSC. The conversion‐time data revealed the cure reaction was slower in the blends than in the neat system, although the autocatalytic cure mechanism was not affected by the addition of PVAc. ESEM showed the cured epoxy/PVAc blends had different morphologies as a function of PVAc content: an inversion in morphology took place for blends containing 15 wt % PVAc. The changes in the blend morphology with PVAc content had a clear effect on the DMTA behavior. Inverted morphology blends had low storage modulus values and a high capability to dissipate energy at temperatures higher than the PVAc glass‐transition temperature, in contrast to the behavior of neat epoxy and blends with a low PVAc content. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1507–1516, 2007     

Poly(methyl methacrylate)‐modified vinyl ester thermosets: Morphology,volume shrinkage,and mechanical properties

Thermoset materials obtained from styrene/vinyl ester resins of different molecular weights modified with poly(methyl methacrylate) (PMMA) were prepared and studied. Scanning electron microscopy and transmission electron microscopy micrographs of the fracture surfaces allowed the determination of a two‐phase morphology of the modified networks. Depending on the molecular weight of the vinyl ester oligomer, the initial content of the PMMA additive, and the selected curing temperature, different morphologies were obtained, including the dispersion of thermoplastic‐rich particles in a thermoset‐rich matrix, cocontinuous structures, and the dispersion of thermoset‐rich particles in a thermoplastic‐rich matrix (phase‐inverted structure). Density measurements were performed to determine the effect of the PMMA‐modifier concentration and curing temperature on the volume shrinkage of the final materials. The development of cocontinuous or thermoplastic‐rich matrices was not too effective in controlling the volume shrinkage of the studied vinyl ester systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness showed that the addition of PMMA increased the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl ester networks. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

FTIR study of ageing epoxy resin reinforced by reactive graphitic nanofibers

We previously developed a novel modified epoxy matrix for macrocomposites using reactive graphitic nanofibers (r‐GNFs), which showed improved mechanical properties. This article is devoted to study the efficacy of the r‐GNFs against the damage due to exposition of the matrix to hygrothermal environment and UV radiation. Moisture sorption characteristics of the pure epoxy and nanoepoxy were investigated and all kinds of matrices showed similar absorption behavior. Both diffusion and relaxation processes in sorption were observed. DSC curves of nonaged specimens of all concentrations confirmed that samples were not fully cured and r‐GNFs lowered the curing degree. The effects of ageing were studied through the changes of molecular conformation by Fourier Transform infrared spectroscopy (FTIR). The results show that UV radiation assists in post‐curing of the matrices. Hydroxyl unit, carbonyl unit, and epoxide unit were chosen to study the degradation. It is observed that the degradation due to UV radiation is more severe compared with that due to hygrothermal expose. It also reveals that r‐GNFs resist the degradation of the epoxy resin; particularly, the nanoepoxy with 0.3 wt % of r‐GNFs has the highest ageing resistance, which is in agreement with our previously tested results of mechanical properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Ternary nanocomposites: Curing,morphology, and mechanical properties of epoxy/thermoplastic/organoclay systems

The influence of two organically modified montmorillonites on the curing, morphology and mechanical properties of epoxy/poly(vinyl acetate)/organoclay ternary nanocomposites was studied. The organoclays and poly(vinyl acetate) (PVAc) provoked contrary effects on the epoxy curing reaction. Ternary nanocomposites developed different morphologies depending on the PVAc content, that were similar to those observed in the epoxy/PVAc binary blends. The organoclays were only located in the epoxy phase independently of the morphology. All nanocomposites showed intercalated structures with similar clay interlayer distances. Both PVAc and organoclays lowered the T_g of the epoxy phase, the presence of clays did not influence the T_g of the PVAc phase. The addition of the organoclays to the epoxy improved stiffness but lowered ductility while the adition of PVAc improved toughness although reduced stiffness of epoxy thermoset. Ternary nanocomposites exhibited optimal properties that combine the favourable effects of the clay and the thermoplastic. POLYM. COMPOS., 37:2184–2195, 2016. © 2015 Society of Plastics Engineers     

Hydrothermal effects on the thermomechanical properties of high performance epoxy/clay nanocomposites

The effects of hydrothermal ageing on the thermomechanical properties of high performance epoxy and its nanocomposite were studied. The epoxy–clay nanocomposite was prepared through a recently developed “slurry‐compounding” approach. The cured samples were immersed in distilled water at 60°C for different periods of time before subjecting to characterization. The storage modulus, relaxation behavior, fracture toughness, and tensile properties were investigated. It was found that the storage modulus and α‐relaxation were strongly affected by water uptake, while the fracture toughness and Young's modulus were less influenced. Dependence of tensile strength and strain at break on water uptake was found to be different in neat epoxy and epoxy–clay systems. POLYM. ENG. SCI., 46:215–221, 2006. © 2005 Society of Plastics Engineers     

Control of morphologies and mechanical properties of thermoplastic‐modified epoxy matrices by addition of a second thermoplastic

Ternary mixtures based on stoichiometric mixtures of the diglycidyl ether of bisphenol‐A (DGEBA) and 4,4′‐diaminodiphenyl sulfone (DDS) and two miscible thermoplastics, poly(methyl methacrylate) (PMMA) and the poly(hydroxy ether of bisphenol‐A) (phenoxy), were investigated by optical microscopy (OM), atomic force microscopy (AFM) and dynamic mechanical analysis (DMA). Mechanical testing was used to study the ultimate behavior. All the modified epoxy mixtures were heterogeneous. DMA has been shown to be an excellent technique for detecting the morphologies generated after curing when the loss modulus is used for analysis. Morphology varied with the thermoplastic content on the mixtures. The addition of a second thermoplastic in small amounts changed the morphological features from particulated to co‐continuous and from that to phase‐inverted morphologies. A significant increase in fracture toughness was observed above all for the mixtures with some level of co‐continuity within the epoxy‐rich matrix. Phase inversion led to poor strength and also fracture toughness. Copyright © 2003 Society of Chemical Industry     

Development of an epoxy system characterized by low water absorption and high thermomechanical performances

Water absorption and thermomechanical properties of epoxy systems based on multifunctional dicyclopentadiene epoxy novolac resin Tactix556 cured with 4,4′ diaminodiphenilsulfone (4,4′DDS) as curing agent has been studied. The base system was modified by the addition of a novel 40 : 60 PES : PEES (Polyethersulphone : Polyetheretheresulphone) amine‐ended copolymer to improve toughness properties. The effect of thermoplastic addition on water adsorption was studied by gravimetric experiments. The viscoelastic properties of the resulting blend were analyzed by means of dynamic mechanical thermal analysis. The formulated systems were compared with a system based on tetraglycidyl‐4,4′diaminodiphenylmethane resin (MY721) cured with 4,4′ diaminodiphenilsulfone. The use of Tactix556 resin showed that water uptake values were minimized while retaining high glass transition temperatures, and toughness values were found in the same range of standard toughened matrices used for aerospace composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4880–4887, 2006     

Poly(vinyl acetate)/polyacrylate semi‐interpenetrating polymer networks. II. Thermal,mechanical, and morphological characterization

The thermal, dynamic mechanical, and mechanical properties and morphology of two series of semi‐interpenetrating polymer networks (s‐IPNs) based on linear poly(vinyl acetate) (PVAc) and a crosslinked n‐butyl acrylate/1,6‐hexanediol diacrylate copolymer were investigated. The s‐IPN composition was varied with different monoacrylate/diacrylate monomer ratios and PVAc concentrations. The crosslinking density deeply affected the thermal behavior. The results showed that a more densely crosslinked acrylate network promoted phase mixing and a more homogeneous structure. The variation in the linear polymer concentration influenced both the morphology and mechanical properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical properties of carbon woven reinforced epoxy matrix composites. A study on the influence of matrix modification with polysulfone

An experimental investigation has been carried out to study the influence of thermoplastic addition on the mechanical properties of woven carbon fiber/epoxy matrix composites. As toughening agent bisphenol‐A polysulfone, PSu, has been added to the epoxy matrix. Flexural tests haved been performed to characterize the mechanical behavior of unmodified and PSu‐modified bulk tetra‐ and bifunctional epoxy matrices and also for the corresponding woven carbon fiber, CF, composite materials. Three‐point notched flexural tests been used to investigate the influence of polysulfone addition in the mode‐I fracture properties of the bulk epoxy matrices, relating them to their microstructural features investigated by atomic force microscopy (AFM). The double‐cantilever bea (DCB) and the end‐notched flexural (ENF) tests have been applied to characterize the interlaminar fracture toughness of the corresponding composites. For composites, the flexural properties were simmilar independent of the funcetionality of the epoxy matrix and of the thermoplastic content. Nevertheless, PSu addition to the epoxy matrix celarly enhanced the ode‐I and II interlaminar fracture toughness of the corresponding composites, the immprovement being higher for the composites manufactured with the bifunctional epoxy matrix at every thermoplastic content because of the lower crosslink density of the epoxy matrix.     

Synthesis of aralkyl novolac epoxy resins and their modification with polysiloxane thermoplastic polyurethane for semiconductor encapsulation

A series of phenol‐based and naphthol‐based aralkyl epoxy resins were synthesized by the condensation of p‐xylylene glycol with phenol, o‐cresol, p‐cresol, or 2‐naphthol, respectively, followed by the epoxidation of the resulting aralkyl novolacs with epichlorohydrin. The incorporation of stable dispersed polysiloxane thermoplastic polyurethane particles in the synthesized epoxy resin's matrix was achieved via epoxy ring‐opening with the isocyanate groups of urethane prepolymer to form an oxazolidone. The mechanical and dynamic viscoelastic properties of cured aralkyl novolac epoxy resins were investigated. A sea‐island structure was observed in all cured rubber‐modified epoxy networks via SEM. The results indicate that a naphthalene containing aralkyl epoxy resin has a low coefficient of thermal expansion, heat resistance, and low moisture absorption, whereas phenol aralkyl type epoxy resins are capable of imparting low elastic modulus result in a low stress matrix for encapsulation applications. Modification of the synthesized aralkyl epoxy resins with polysiloxane thermoplastic polyurethane have effectively reduced the stress of cured epoxy resins, whereas the glass transition temperature was increased because of the formation of the rigid oxazolidone structure. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1905–1916, 1999     

Epoxy modification with poly(vinyl acetate) and poly(vinyl butyral). I. Structure,thermal, and mechanical characteristics

Efficiency of the application of high strength heat resistant thermoplastics for improving fracture toughness and impact properties of epoxy resins motivated authors to try large‐scale production thermoplastics for the same purpose. Epoxy/anhydride systems were modified by up to 8 wt % poly(vinyl acetate) (PVAc) and up to 6 wt % poly(vinyl butyral) (PVB). In epoxy–PVAc blends it was possible to obtain morphologies with continuous thermoplastic phase. However, only sea‐island morphologies with a very small size of PVB‐rich phase were observed in epoxy–PVB matrices. The former type of morphology allowed a notable 2.4‐fold increase in the fracture toughness of epoxy resin and simultaneous up to 30% decrease in its' impact strength. The latter type of morphology caused a notably lower (45%) enhancement of the epoxy fracture toughness combined with a 50% increase in its' impact strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44081.     

Silica aerogel/epoxy nanocomposites: Mechanical,vibrational, and morphological properties

The present study was an attempt to examine the effects that adding silica aerogel (SA) nanoparticles to epoxy would exert on its mechanical, vibrational, and morphological properties. Neat epoxy was consecutively combined with 1, 2, and 4 wt% of SA nanoparticles. A number of tests of mechanical properties were then performed on the samples, including tests of tensile, bending, compressive, dynamic mechanical thermal, hardness, and Izod impact. Vibration and water uptake tests were also conducted on the samples. The highest modulus and strength values were found in the nanocomposite sample with 4 wt% of SA, and the highest toughness and elongation values were detected in the sample with 1 wt% of SA. Furthermore, adding the SA nanoparticles to the epoxy improved the energy absorption and hardness of the epoxy matrix. The findings from the tests of dynamic mechanical thermal and vibration properties demonstrated that, with an increase in the nanoparticles content in the samples, the values of storage modulus and natural frequency increased while the values of tan δ and damping ratios decreased. A comparison between the values of natural frequency from the vibration test and the values from the Euler–Bernoulli beam theory showed a good agreement between the theoretical and experimental results.     

Influence of new thermoplastic sizing agents on the mechanical behavior of poly(ether ketone ketone)/carbon fiber composites

In this study, unidirectional poly(ether ether ketone)/carbon fiber (CF) composite sheets were elaborated with unsized, epoxy‐sized, and thermoplastic‐sized CFs by hot‐press molding. The thermoplastic sizings that we used were poly(ether imide) (PEI) and poly(ether ketone ketone) oligomer aqueous dispersions. Scanning electron microscopy observation of the composites freeze fractures showed that unlike unsized or epoxy‐sized CFs, the thermoplastic sizings improved the interaction between the fibers and the matrix. A comparative study of the mechanical relaxations by dynamic mechanical analysis was carried out on the different composites before and after immersion in kerosene. At low temperature, the PEI sizing had a significant influence on the β relaxation, particularly after kerosene immersion. The thermoplastic sizings did not modify the glass‐transition temperature but improved the kerosene resistance on the composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42550.     

Adhesion of glass/epoxy composites influenced by thermal and cryogenic environments

Little information regarding the effects of prior thermal and cryogenic conditionings on hydrothermal and mechanical behavior, for varied volume fractions of constituent phases in polymer matrix fiber composites, has been published to date. The present experimental investigation uses flexural test to assess the effects of thermal and cryogenic treatments, and concurrently followed hydrothermal aging on quality of adhesion of multilayered laminates for 55, 60, and 65 wt % of E-glass fiber-reinforced epoxy composites. The specimens were conditioned at 80°C and −80°C temperatures for different time durations, and thereafter they were immediately immersed in boiling water for an hour. Water absorption rates were evaluated for those conditioned specimens in such environment. Absorption study in hydrothermal aging showed a remarkable variation for the two cases of prior conditionings. The shear strength values were compared with the test value of as-cured samples. Degradation of mechanical property was found to be less prevalent during hydrothermal aging, with a prior conditioning at 80°C temperature compared to −80°C treated glass/epoxy composites. Improved shear strength for almost all conditions of thermal conditioning in the initial stages has highlighted better adhesion influenced by postcuring phenomena during thermal or cryogenic conditioning. It was also observed from water absorption data that high temperature conditioning contributed more strengthening effect and better adhesion at the interfaces. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1943–1949, 2006     

The influence of structural order of anthracite fillers on the curing behavior,morphology, and dynamic mechanical thermal properties of epoxy composites

This article concerns the study of polymer composites with anthracite fillers of various structural order. Raw Svierdlovski anthracite of turbostratic structure and the anthracite thermally treated at 2,000°C of graphite‐like structure were used as fillers of low‐molecular‐weight diglycidyl ether of bisphenol A cross‐linked with aliphatic amine. Two anthracites of extremely different structures were compared to natural graphite that is composed of well‐ordered graphene sheets. Systematic studies of the influence of the structure of anthracite filler on the curing behavior, morphology, dynamic mechanical thermal properties, and thermal stability of epoxy composite were performed. It was found that the structure of anthracite filler affects the cross‐linking reactions of the epoxy matrix as well as the morphology of the composites and their viscoelastic properties. Raw anthracite added to epoxy matrix had a visible effect on the activation energy and differential scanning calorimeter parameters of the curing process, in contrast to the epoxy matrix modified with anthracite heated at 2,000°C. On the contrary, the effect of anthracite on dynamic mechanical behavior of composites is more evident when the anthracite prepared at 2,000°C was used as a filler. POLYM. COMPOS., 36:336–347, 2015. © 2014 Society of Plastics Engineers     

An investigation of epoxy/thermoplastic blends based on addition of a novel copoly(aryl ether nitrile) containing phthalazinone and biphenyl moieties

In this paper, a novel soluble copoly(aryl ether nitrile) containing phthalazinone and biphenyl moieties (PPBEN) was synthesized for the first time to improve the impact resistance of tetraglycidyl 4,4'‐diaminodiphenylmethane epoxy resin cured with 4,4‐diaminodiphenylsulfone. Then a series of blends were prepared via solution blending with different contents of PPBEN. The thermal and mechanical properties and the micromorphology of the cured blends were investigated by differential scanning calorimetry, dynamic mechanical analysis (DMA), parallel plate rheometry, mechanical property tests and SEM analysis, respectively. The results indicated that the incorporation of thermoplastic PPBEN delayed the epoxy curing reaction, and the crosslinking density of epoxies was also reduced. The no‐notch impact strength of the cured blend with 15% PPBEN was up to 16.7 kJ m^?2, higher by about 104% than that of pure epoxy resin without sacrificing the modulus due to a specific sea‐island structure. All the blends showed two‐phase morphology characterized by DMA and SEM. The size of the thermoplastic morphology was only 70?80 nm, much less than that of commonly used thermoplastics, due to the special segment structure of PPBEN. © 2015 Society of Chemical Industry     

Durability of Aluminium Adhesive Joints Bonded with a Homopolymerised Epoxy Resin

The durability of epoxy-aluminium joints that use a homopolymerised epoxy resin was studied, and the effects of relative humidity, temperature, and salt concentration were analysed. The adhesive properties were measured by lap-shear tests, and the water uptake of the epoxy resin was determined by gravimetric measurements. Ageing and degradation effects on the epoxy resin and on the aluminium substrates were also analysed.

The homopolymerised epoxy resin absorbs little water (1.5 wt%) because of its nonpolar network structure. The water uptake is enhanced by increasing relative humidity and temperature; however, the joint strength remains constant because of epoxy plasticization. A saline environment is damaging to the adhesive joints, because of metal corrosion, but was not significantly harmful to the epoxy resin, because of a lower diffusion coefficient of salt water. The T_g decrease of the epoxy adhesive due to water absorption depends only on the amount of absorbed water and is independent of the hydrothermal ageing conditions.     

Mechanical and physical properties of polymer‐based nanocomposites containing different types of clay

Epoxy‐clay nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) epoxy reinforced with 2 wt% of four different types of clay were prepared by high shear mixing (HSM) technique. The resultant nanocomposites were investigated to determine the effects of clay addition and clay types on their mechanical, thermal, and physical properties. The XRD and TEM analyses revealed that good dispersions of nanoclay within the epoxy matrix have been achieved especially for the samples prepared with I.30E clay where a combination of disordered intercalated and exfoliated morphology was observed. The structure of samples synthesized with other types of clay was dominated by intercalated morphologies. The tensile results illustrated that the nanocomposite containing I.30E clay has the best mechanical properties as compared to other nanocomposites. This is mainly due to better dispersion of I.30E nanoclay in the epoxy matrix for this nanocomposite. The increase or decrease in the glass transition temperatures of nanocomposites were found to be dependent on the type of clay used. The effect of clay addition on the barrier properties was examined using water exposure test which demonstrated that the addition of 2% of I.30E and C10A clays resulted in 60% reduction in diffusivity. Noticeable reduction in maximum water uptake was also observed for all nanocomposites. The improvement in these physical properties was attributed to the tortuosity effect, where water molecules have to move around clay layers during diffusion in nanocomposites. POLYM. COMPOS., 36:1998–2007, 2015. © 2014 Society of Plastics Engineer     

Study on epoxy/thermoplastic blends based on the addition of a novel aromatic block copolymer

In this article we present a study of the properties of some epoxy/thermoplastic blends based on the addition of a novel block copolymer of an ABA type. The block A was a commercial poly(2,6‐dimethyl‐1,4‐phenylene ether or oxide) (PPE or PPO) while block B was a random copolyethersulphone synthesized in our laboratory. The comparison with the epoxy blends modified only with the homopolymers A or B is also reported. The homopolymers were also mixed via solution blending and the resulting physical mixture was used as modifier for comparison purposes. The blends have been thoroughly characterized by parallel plate rheometry, dynamic mechanical thermal analysis (DMTA), and scanning electron microscopy. Upon curing, the epoxy blends containing the physical mixture underwent phase separation showing a morphology which resembled the combination of those found for the blends modified with each of the homopolymers. On the contrary, the addition of the block copolymer resulted in a unique morphology. The viscoelastic properties of the blends both in the unreacted and cured state varied significantly when the block copolymer was added rather than the homopolymers or their physical mixture. POLYM. ENG. SCI., 47:2027–2033, 2007. © 2007 Society of Plastics Engineers     

Toughening of epoxy/polycaprolactone composites via reaction induced phase separation

A series of melt processable thermoset/thermoplastic blends were prepared by mixing bisphenol‐A diglycidyl ether (Epon‐828)/diaminodiphenyl sulfone (DGEBA/DDS) system with two grades of polycaprolactone resin. Phase separation behavior of the blends was investigated by means of optical microscopy, microstructure by scanning electron microscopy (SEM), and thermo‐mechanical properties. The toughness of polycaprolactone modified epoxies was measured by instrumented falling weight impact (IFWI) testing. Various blend morphologies were observed depending upon the cured epoxy network/thermoplastic composition. Spinodal decomposition as characterized by modulated structure of unique periodicity and phase connectivity was found to be the probable mechanism of phase separation. SEM examination of fracture surfaces indicated a strong adhesion between the epoxy‐rich and polycaprolactone‐rich phases. Optimum improvement in failure energy was obtained for the compositions containing 10‐20% polycaprolactone without significantly compromising the elastic modulus and the thermo‐mechanical stability of the epoxy. In light of morphological evidences, a possible toughening effect was postulated in terms of tearing of the thermoplastic component and induced plastic deformation of the epoxy matrix.