Difunctional Epoxy Research Articles

Monitoring cure in epoxies containing carbon nanotubes with an optical‐fiber Fresnel refractometer

AbstractFresnel refractometry was used to follow the changes in the refractive index during the isothermal curing of a simple difunctional epoxy resin, a commercial formulated multifunctional epoxy blend, and the same two resin systems modified by the addition of multiwalled carbon nanotubes. The refractive index change, monitored via an embedded optical fiber, was correlated with the degree of cure established from isothermal differential scanning calorimetry measurements. A good correlation was established in all cases, indicating the potential of the technique to be used for online cure monitoring in thermosetting resin carbon nanocomposites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effect of the epoxy resin chemistry on the viscoelasticity of its cured product

AbstractA detailed study investigating the relation between the initial resin composition and mixing ratio on the viscoelastic behavior was performed. The resin system, containing a difunctional aromatic epoxy (DER 332), was mixed with three aliphatic amines containing functionalities of 2, 3, and 4 in different stoichiometric mixing ratios and was cured. For each of the cured mixtures, the viscoelastic master curve and corresponding shift factors were determined. Depending on the crosslink density, the viscoelastic curves shifted by about 12 decades with respect to the frequency scale. This shift in the viscoelastic curve was predicted by the combination of the Miller–Macosko theory for crosslink density with a parameterized Havriliak–Negami model for the master curve. A single set of 9 parameters turned out to be sufficient to describe all viscoelastic data. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Curing behavior of epoxy/POSS/DDS hybrid systems

AbstractThe curing behavior of a difunctional epoxy resin/octa(aminopropyl)silsesquioxanes hybrid resin system using 4,4′′‐diaminodiphenyl sulfone as curing agent was studied. Fourier transform infrared (FTIR) spectroscopy was used to evidence the formation of the hybrid resin and to monitor the curing process. Results showed that the reactivity of the hybrid resin system was slightly lower than that of the pristine epoxy resin. Differential scanning calorimetry was used to study the curing kinetics of the hybrid resin. The curing kinetics of the pristine and hybrid resins was analyzed by two methods: Kissinger and Flynn‐Wall‐Ozawa methods. The FTIR analysis accorded well with the results of the kinetics study. POLYM. COMPOS., 29:77–83, 2008. © 2007 Society of Plastics Engineers

Diglycidyl ether of bisphenol‐A epoxy resin modified using poly(ether ether ketone) with pendent tert‐butyl groups

AbstractBisphenol‐A‐based difunctional epoxy resin was modified with poly(ether ether ketone) with pendent tert‐butyl groups (PEEKT). PEEKT was synthesized by the nucleophilic substitution reaction of 4,4′‐difluoro benzophenone with tert‐butyl hydroquinone in N‐methyl‐2‐pyrrolidone. Blends with various amounts of PEEKT were prepared by melt‐mixing. All the blends were homogeneous in the uncured state. The glass transition temperature of the binary epoxy/PEEKT blends was predicted using several equations. Reaction‐induced phase separation was found to occur upon curing with a diamine 4,4′‐diaminodiphenyl sulfone. The phase morphology of the blends was studied using scanning electron microscopy. From the micrographs, it was found that PEEKT‐rich phase was dispersed in a continuous epoxy matrix. The domain size increased with the amount of PEEKT in the blends. The increase in domain size was due to the coalescence of the domains after phase separation. Dynamic mechanical analysis of the blends gave two peaks corresponding to epoxy‐rich phase and thermoplastic‐rich phase. The tensile strength and modulus of the blends remained close to that of the unmodified resin, while the flexural properties decreased with the addition of PEEKT to epoxy resin. The fracture toughness of the epoxy resin increased with the addition of PEEKT. Investigation of the fracture surfaces revealed evidences for local plastic deformation of the matrix, crack pinning, crack path deflection, and ductile tearing of PEEKT‐rich phase. Thermogravimetric analysis revealed that the initial decomposition temperature of the blends were close to that of the unmodified resin. Finally, the properties of the blends were compared with other modified PEEK/epoxy blends. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2481–2496, 2007

Properties of epoxy nanoclay system based on diaminodiphenyl sulfone and diglycidyl ether of bisphenol F: influence of post cure and structure of amine and epoxy

AbstractMaterials were prepared with Cloisite 30B, diglycidyl ether of bisphenol F and diaminodiphenyl sulfone (DDS) to investigate the relationship between exfoliation and property enhancement. A higher level of exfoliation was found with 4,4′‐DDS than in the material cured with 3,3′‐DDS. The material with 4,4′‐DDS that exhibited a higher degree of exfoliation also resulted in larger improvements in physical properties with nanoclay. Increasing the ratio of trifunctional to difunctional epoxy led to a significant decrease in the level of clay dispersion. Increasing the ratio of trifunctional epoxy in the nanocomposite also caused a decrease in Tg with the addition of nanoclay. However, an increase in Tg with increasing clay loadings was detected in samples with a higher degree of exfoliation, with only difunctional epoxy. Similar behaviour was found for samples with and without a post cure. Excessive post cure led to a decrease in thermal stability in Cloisite 30B‐containing samples. Copyright © 2007 Society of Chemical Industry

E-beam curing of epoxy-based blends in order to produce high-performance composites

In this work, blends of a difunctional epoxy monomer and a thermoplastic toughening agent are E-beam irradiated at two different dose rates and two different total absorbed doses. The influence of the processing conditions on the thermal properties and on the morphology of the obtained matrices has been investigated. In particular, it is shown how the increase of the dose rate causes an increase of the temperature during irradiation, thus inducing a simultaneous thermal and radiation curing. On the contrary, at low-dose rate the system mainly undergoes to radiation curing, thus making the cured material very sensible to a post-irradiation thermal treatment with a significant improvement of the thermal properties.

Correlation between viscoelastic behavior and cooling stresses in a cured epoxy resin system

AbstractThis work deals with the study of the rheological behavior of an epoxy system subjected to three cooling processes referred to as the normal, air‐, and water‐cooling processes. The system was set up by a difunctional epoxy resin (DGEBA) mixed with a tetrafunctional diamine (mPDA) in stoichiometric proportions. Different samples of (1) neat and cured polymers and (2) alumina—epoxy composites were prepared. The first part of this work was devoted to optimizing the cure cycle of the neat samples using differential scanning calorimetry and rheometry. The best cure cycle is based on a first heat step at 110°C during 10 min followed by a crosslinking stage of 75 min at 180°C. In the second part of the study the influence of the cooling cycle on the thermomechanical behavior of polymer and composite samples was investigated. Measurement of loss modulus, G″, and loss factor, tan δ, versus temperature showed that an intermediate relaxation α′ peak arose between the main and secondary relaxations (resp. α and β). The position and amplitude of the α′ peak increased with the cooling rate. This rheological phenomenon is related to the presence of nonequilibrium stresses frozen in the sample during cooling. The effect of thermal aging on the α′ peak also is reported. Our work also showed the presence of an α′ peak for the composites. However, the amplitude of this peak was more pronounced in the composites because of additional stresses induced by the difference between the resin and the ceramic in thermal expansion coefficients. We showed that a calculation based on a thermoelastic model was useful for qualitative comparison of rheological data on the influence of the cooling process. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 679–690, 2006

Substitution effects of methyl and trifluoromethyl groups on the physicochemical properties of epoxy resins

AbstractWe studied the effects of functional groups on the physicochemical properties, such as rheological behavior, thermal stability, glass‐transition temperature, and electrical and mechanical properties, of 4,4′‐diaminodiphenyl methane (DDM) cured methyl‐substituted and trifluoromethyl‐substituted difunctional epoxy resins [i.e., 2‐diglycidylether of toluene (DGET) and 2‐diglycidylether of benzotrifluoride (DGEBTF)]. The DGEBTF/DDM system showed higher thermal stability and lower dielectric constant values than the DGET/DDM system. The impact strength of the DGEBTF/DDM system was significantly higher than that of the DGET/DDM system. The increase in intermolecular interactions because of the increasing polarity of the fluorine in the epoxy backbone was probably the key to these results. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1860–1864, 2005

Effect of Alkyl Groups of Latent Cationic Catalysts on Cure and Dynamic Mechanical Behaviors of Epoxy Resins

AbstractSummary: The latent thermal cationic initiators, benzyl‐2,5‐dimethylpyrazinium hexafluoroantimonate (BDPH) and benzyl‐2‐ethylpyrazinium hexafluoroantimonate (BEPH), were synthesized to investigate the effect of substituted alkyl groups on cure and dynamic mechanical behaviors of difunctional epoxy system. The cure temperature and activation energy of the diglycidyl ether of bisphenol A (DGEBA)/BDPH were higher than those of the DGEBA/BEPH, resulting from the steric hindrance of the substituted groups. The cross‐linking density of the DGEBA/BDPH was higher than that of the DGEBA/BEPH, whereas the Tg's of both specimens are similar. This may be explained by the free volume and the intermolecular hydrogen bonding induced by the hydrogen of the substituted methyl groups. Consequently, the position and number of the substituted groups of the latent thermal initiator were very important in the control of the latent thermal and dynamic mechanical behaviors of the epoxy resin.Dynamic DSC curves of DGEBA cured by each initiator.imageDynamic DSC curves of DGEBA cured by each initiator.

Influence of hydroxyl functionalized hyperbranched polymers on the thermomechanical and morphological properties of epoxy resins

AbstractRheological, thermomechanical, and morphological properties of blends of two different epoxy resins with hydroxyl functionalized hyperbranched polymers have been studied. The hydroxyl functionalized hyperbranched polymers used in this work had different generation number and different number of terminal groups. The difunctional epoxy resin has been also mixed with a linear aromatic thermoplastic in order to compare its effects with that of the hyperbranched polymers. All the blends have been characterized by linear elastic fracture mechanics (LEFM) testing to evaluate the efficiency of the two types of toughening modifiers. In addition, the water uptake has been evaluated for all the formulations. POLYM. ENG. SCI. 45:225–237, 2005. © 2005 Society of Plastics Engineers

Selective repartition ofin situ generated silica produced during the evolution of an epoxide network from a homogeneous precursors mixture and effects on properties

Phase separation of organic networks derived from mixtures of reactive monomers and/or oligomers can take place through the formation of IPNs or precipitation of particles. The latter systems are widely used as a means of increasing the fracture toughness of thermosetting resins, and particularly for products obtained from mixtures of epoxy resins and functionalized aliphatic oligomers. In the present work several mixtures comprised of difunctional epoxy resins, silane functionalized perfluoroether oligomers, prehydrolyzed tetraethoxysilane, and an aromatic amine hardener were examined in a variety of compositions and preparative procedures. The aims were to control the kinetics of phase separation and the repartition of the different components in the two phases, so that the silica domains could be preferentially located within the precipitated soft particles. It was found that the silane functionalization of the perfluoroether oligomer provided an effective mechanism for the localization of the siloxane networks within the precipitated particles. However, phase separation by the precipitation of particles would only take place for systems in which the perfluoroether oligomer could be reacted with an excess of epoxy resin prior to adding the alkoxysilane solution and the hardener. Moreover, it was difficult to achieve the total localization of the siloxane component into the perfluoroether network. The siloxane species remaining dissolved in the resin gave rise to severe embrittlement of the products through reactions with the epoxy chains. Using mixtures of suitably functionalized perfluoroether oligomers it was possible to produce the conditions by which the localization of the silica domains into the precipitated particles could be maximized. This has resulted also in the formation of graded interphase regions and to a global improvement in mechanical properties, manifested as a concomitant increase in modulus, strength, and toughness. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1279–1290, 2004

Compatibilization of a poly(butylene terephthalate)/poly(ethylene octene) copolymer blends with different amounts of an epoxy resin

AbstractNew toughened poly(butylene terephthalate) (PBT) materials were obtained by melt blending with 20 wt % poly(ethylene octene) (PEO) copolymer and different levels of a difunctional epoxy resin in a twin‐screw extruder followed by injection‐molding. The presence of neither PEO or epoxy influenced either the phase nature of the two amorphous phases of the blends or the crystallization process of PBT, despite the slight reaction of epoxy with PBT as stated by the observed torque increases. The addition of epoxy led to a decrease in the particle size that stopped due to the concomitant viscosity increase. Supertough PBT‐based blends with an impact strength more than 18‐fold that of PBT were obtained without previous chemical modification of any of the blend components at 1.0 wt % epoxy contents. The interparticle distance was the parameter that controlled notched toughness in these PBT/PEO blends. The adhesion at the interphase was the parameter on which the critical interparticle distance appeared to depend. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 260–269, 2004

Liquid crystalline thermosets from dimeric LC epoxy resins cured with primary and tertiary amines

AbstractLiquid crystalline thermosets (LCTs) were prepared by curing difunctional LC dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different length. Primary diamines or tertiary amines were used as curing agents obtaining materials with different characteristics. The results obtained were related to the mesogen structure, since dipolar moments in the mesogenic cores affect the ability to form ordered networks.

Block copolymer modified novolac epoxy resin

AbstractThe phase behavior of uncured and cured mixtures containing stoichiometric amounts of Epon164 novolac epoxy resin and 4,4′‐methylenedianiline combined with a nearly symmetric poly(methyl acrylate‐co‐glycidyl methacrylate‐b‐polyisoprene) diblock copolymer was investigated with small‐angle X‐ray scattering and transmission electron microscopy. A series of morphologies were documented as a function of the copolymer concentration, which ranged from pure diblock to 2.5 wt % in the thermoset resin. Ordered lamellae bordered a wide multiphase region followed by disordered wormlike micelles that transformed continuously into vesicles at the lowest compositions. The thermal curing of this pentafunctional epoxy system to complete conversion had little impact on the phase behavior of the mixtures, and this was consistent with previous experiments with difunctional epoxy and the same hardening agent. However, changing the epoxy component led to gross changes in the phase behavior that were interpreted with the concept of a wet‐to‐dry brush transition. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1994–2003, 2003

Thermal properties and fracture toughness of epoxy resins cured by phosphonium and pyrazinium salts as latent cationic initiators

AbstractIn this work, the latent thermal cationic initiators triphenyl benzyl phosphonium hexafluoroantimonate (TBPH) and benzyl‐2‐methylpyrazinium hexafluoroantimonate (BMPH) were newly synthesized and characterized with IR, 1H NMR, and P NMR spectroscopy. The thermal and mechanical properties of difunctional epoxy [diglycidyl ether of bisphenol A (DGEBA)] resins cured by 1 phr of either TBPH or BMPH were investigated. The DGEBA/TBPH system showed a higher curing temperature and a higher critical stress intensity factor than the epoxy/BMPH system. This could be interpreted in terms of the slow thermal diffusion rate and bulk structure of the four phenyl groups in TBPH. However, the decomposition activation energy derived from the Coats–Redfern method was lower for epoxy/TBPH. This result was probably due to the fact that a broken short‐chain structure was developed by the steric hindrance of TBPH in the difunctional epoxy resin. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2393–2403, 2003

Kinetic Study and New Applications of UV Radiation Curing

AbstractHighly crosslinked polymers can be readily synthesized by photoinitiated polymerization of multifunctional monomers or functionalized polymers. The reaction can be followed in situ by real‐time infrared (RT‐IR) spectroscopy, a technique that records conversion versus time curves in photosensitive resins undergoing ultrafast polymerization upon UV exposure. For acrylate‐based resins, UV‐curing proceeds with long kinetic chains (7700 mol/radical) in spite of the high initiation rate. RT‐IR spectroscopy proved very valuable in assessing the influence of various parameters, such as initiation efficiency, chemical structure of the telechelic oligomer, light intensity, inhibitory effect of oxygen, on polymerization kinetics. Interpenetrating polymer networks can be rapidly synthesized by means of UV irradiation of a mixture of difunctional acrylate and epoxy monomers in the presence of both radical and cationic‐type photoinitiators. The same UV technology can be applied to crosslink solid polymers at ambient temperature, which bear different types of reactive groups (acrylate and vinyl double bonds, epoxy ring). UV radiation curing has been successfully used to produce within seconds weathering resistant protective coatings, high‐resolution relief images, glass laminates and nanocomposites materials.Photoinitiated crosslinking polymerization.magnified imagePhotoinitiated crosslinking polymerization.

Physical miscibility and chemical reactions between diglycidylether of bisphenol-A epoxy and poly(4-vinyl phenol)

Physical miscibility at ambient and chemical interactions at elevated temperatures between a difunctional epoxy of diglycidylether of bisphenol-A (DGEBA) and a linear phenylhydroxyl-containing polymer, poly(4-vinyl phenol) (PVPh), were investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy, and solid-state 13C nuclear magnetic resonance (NMR). The DGEBA/PVPh mixture was proven to be homogeneous and miscible. At elevated temperatures, the epoxy/PVPh mixture developed a network-like homogeneous solid, which was a result of reactions between the epoxide of DGEBA epoxy and phenylhydroxyl of the linear PVPh polymer. By using FT-IR and solid-state 13C NMR techniques, mechanisms of reactions leading to a homogeneous network between the DGEBA epoxy resin and PVPh polymer have been probed.

Toughness enhancement of poly(ethylene terephthalate) with functionalized ultrahigh molecular weight polyethylene

AbstractUltrahigh molecular weight polyethylene (UHMWPE) is available commercially in the form of powder, consisting of fine primary particles, 1–5 μm in diameter, agglomerated into secondary “free‐flowing” particles with overall dimensions in the region of 50 to 150 μm. These are normally sufficiently coherent and retain their conglomerated particulate structure when blended with other polymers because of the extremely high viscosity of UHMWPE. In this study the surface of the agglomerated primary particles was acid functionalized by reactions with aqueous solutions of acrylic acid, after being irradiated with γ‐radiation at 15–45 kGy. The acid groups were used to introduce a glycidoxyl functionality through reactions with a difunctional cycloaliphatic epoxy resin and also to a “partial” metal carboxylate functionality through reactions with zinc acetyl acetonate. When blended with polyethylene terephthalate (PET) in either a small‐batch mixer or in a twin‐screw extruder all the treated powders, except those functionalized with acrylic acid, were broken down to their primary size and were uniformly dispersed and strongly bonded to the surrounding matrix. The blends containing the deglomerated particles were found to have much greater ductility and toughness than those produced from both untreated and acid functionalized powder. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2972–2986, 2001

Roles of difunctional epoxy in the tetrafunctional epoxy matrix system

Roles of difunctional epoxy in the tetrafunctional epoxy matrix system

Thermal and toughness property studies on a polybenzimidazole-modified epoxy resin system

The effect of polybenzimidazole (PBI) on a silica-filled epoxy resin matrix has been investigated. Polybenzimidazole (PBI) was incorporated into a difunctional epoxy resin matrix to the extent of 10 %, before being cured with an anhydride hardener. The effects of PBI on the curing reaction and glass transition temperature (Tg) and on the toughness of the cured epoxy matrix have been studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and a universal testing machine (Instron). The results indicate that the PBI modifier enhanced not only the glass transition temperature of the difunctional epoxy matrix but also its toughness, by its catalytic action. Further investigations have been carried out on the fractured specimens, using scanning electron microscopy (SEM) to support the enhanced toughness property of the epoxy matrix. © 2000 Society of Chemical Industry