Epoxy Phase Research Articles

Fracture Behavior of Polypropylene/Clay Nanocomposites

Polypropylene (PP)/clay nanocomposites have been prepared via a reactive compounding approach with an epoxy based masterbatch. Compared with PP and common PP/organoclay nanocomposites, the PP/clay nanocomposites based on epoxy/clay masterbatch have higher impact strength. The phenomenon can be attributed to the epoxy phase dispersed uniformly in the PP matrix, which may act as impact energy absorber and helps to form a large damage zone, thus a higher impact strength value is achieved.

Carbon black‐filled immiscible polypropylene/epoxy blends

AbstractCarbon black‐ (CB) filled immiscible thermoplastic/thermosetting polymer blends consisting of polypropylene (PP) and epoxy resin were reported in this paper. The PP/epoxy/CB blends with varied compositions and different processing sequences were prepared by melt‐mixing method. The CB distribution and the relationship between morphology and electrical properties of the PP/epoxy/CB blends were investigated. Scanning electron microscopy (SEM), optical microscopy, and extraction experimental results showed that in PP/epoxy blends CB particles preferentially localized in the epoxy phase, indicating that CB has a good affinity with epoxy resin. The incorporation of CB changed the spherical particles of the dispersed epoxy phase into elongated structure. With increasing epoxy content, the elongation deformation of epoxy phase became more obvious and eventually the blends developed into cocontinuous structure. When CB was initially blended with PP and followed by the addition of epoxy resin, the partial migration of CB from PP to the epoxy phase was observed. When the PP/epoxy ratio was 40/60, the percolation threshold was reduced to about 4 phr CB, which is half of the percolation threshold of the PP/CB composite. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 461–471, 2006

Cure kinetics and mechanical properties of the blend system of epoxy/diaminodiphenyl sulfone and amine terminated polyetherimide-carboxyl terminated poly(butadiene-co-acrylonitrile) block copolymer

The cure kinetics of blends of epoxy resin (4,4’-tetraglycidyl diaminodiphenyl methane; TGDDM)/curing agent (diaminodiphenyl sulfone; DDS) with ATPEI (amine terminated poly-etherimide) -CTBN (carboxyl terminated poly (butadiene-co-acrylonitrile)) block copolymer (AB type) were studied using differential scanning calorimetry under isothermal conditions to determine the reaction kinetic parameters such as activation energy and reaction constants. Final cure conversion decreased with increasing amount of AB in the blends. A diffusion controlled reaction was observed as the cure conversion increased, and the curing reaction was successfully analyzed by incorporating the diffusion control term in the rate equation for the epoxy/DDS/AB blends. The fracture toughness was improved to about 350% compared to that of the unmodified resin at 30% of AB block copolymer. This is attributed to the formation of co-continuous morphology between the epoxy phase and AB block copolymer phase. By increasing the amount of AB, the modulus of the cured blends decreased, which was due to the presence of CTBN rubbery phases.

Curing and Morphology Studies of Thermosets from Blends of a Main Chain Liquid Crystalline Polyester and Diepoxide/Diamine

AbstractSummary: Blends of the commercial liquid‐crystalline polyester Rodrun LC‐3000 (60–90 wt.‐%) with a bisphenol A‐diglycidyl ether based diepoxide (DOW D.E.R.330) and an aromatic diamine (MCDEA) prepared in a twin‐screw extruder have been compression‐moulded and cured either isothermally at 260 °C or in a temperature ramp between 160 and 230 °C. The blends were investigated with SEM and thermal analysis (DSC, DMTA). Blends with 80% Rodrun and less cured at 260 °C and the blend containing 60% Rodrun cured in a temperature ramp showed macro‐phase separation followed by reaction‐induced micro‐phase separation (RIPS) both in the Rodrun‐rich and in the epoxy‐rich macro‐separated phases. Blends containing 90, 80 and 70% Rodrun moulded at 160 °C and cured in the temperature ramp showed only RIPS and a morphology rather similar to that of the uncured blends that was most likely co‐continuous; the blend with 90% Rodrun cured at 260 °C showed RIPS and a dispersed epoxy phase in a Rodrun matrix. Phase composition has been determined by extraction of the soluble fraction and chemical analysis.SEM showing the reaction induced micro‐phase separation for sample REA90/10i.magnified imageSEM showing the reaction induced micro‐phase separation for sample REA90/10i.

Electrical properties and morphology of polypropylene/epoxy/glass fiber composites filled with carbon black

AbstractThe volume resistivity and percolation thresholds of carbon black (CB) filled polypropylene (PP), PP/epoxy, and PP/epoxy/glass fiber (GF) composites were measured. The morphology of these conductive polymer composites was studied with scanning electron microscopy (SEM). The effects of the GF and epoxy contents on the volume resistivity were also investigated. The PP/epoxy/GF/CB composite exhibited a reduced percolation threshold, in comparison with that of the PP/CB and PP/epoxy/CB composites. At a given CB content, the PP/epoxy/GF/CB composite had a lower volume resistivity than the PP/CB and PP/epoxy/CB composites. SEM micrographs showed that CB aggregates formed chainlike structures and dispersed homogeneously within the PP matrix. The addition of the epoxy resin to PP resulted in the preferential location of CB in epoxy, whereas in the PP/epoxy/GF multiphase blends, because of the good affinity of CB to epoxy and of epoxy to GF, CB particles were located in the epoxy phase coated on GF. The decreased percolation threshold and volume resistivity indicated that conductive paths existed in the PP/epoxy/GF/CB composite. The conductive paths were probably formed through the interconnection of GF. Appropriate amounts of GF and epoxy should be used to decrease the volume resistivity and provide sufficient epoxy coating. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1142–1149, 2005

Cure Kinetics and Mechanical Properties of New Polyetherimide Toughened Epoxy Resin

The cure kinetics of blends of epoxy resin (4,4′-tetraglycidyl diaminodiphenyl methane; TGDDM)/curing agent (diaminodiphenyl sulfone; DDS) with ATPEI-CTBN-ATPEI triblock copolymer (ABA type) were studied using differential scanning calorimetry under isothermal conditions to determine the reaction parameters such as activation energy and reaction constants. By increasing the amount of ABA in the blends, the final cure conversion was decreased. Lower values of the final cure conversions in the epoxy/ABA blends indicated that ABA hinders the cure reaction between the epoxy and the curing agent. The value of the reaction order m for the initial autocatalytic reaction was not affected by blending ABA with epoxy resin, i.e., the value was approximately 1.0. The value of n for the n-th order component in the autocatalytic analysis was increased from 2.0 to 3.4 by increasing the amount of ABA in the blends. A diffusion controlled reaction was observed as the cure conversion increased and the curing reaction was successfully analyzed by incorporating the diffusion control term in the rate equation for the epoxy/DDS/ABA blends. The glass transition temperature of the cured blend was improved from 187 to 205°C and the value of the fracture toughness was also improved by about 400% compared to that of the unmodified resin at 30 wt% of ABA triblock copolymer. This is attributed to the formation of co-continuous morphology between the epoxy phase and ABA triblock copolymer phase. By increasing the amount of ABA, compressive strain, tensile stress and strain of the cured blends were increased, but the compressive stress was reduced due to the presence of CTBN rubbery phases.

Formation of a fibrillar morphology of crosslinked epoxy in a polystyrene continuous phase by reactive extrusion

AbstractAn immiscible polymer blend where the dispersed phase is fibrillar was prepared by in situ crosslinking of the minor phase. A model polystyrene/epoxy‐amine blend was selected on the basis of rheological (achievement of the fibrillar structure) and reactivity (fast crosslinking) criteria. The system was a polystyrene/diglycidyl ether of bisphenol A (DGEBA)‐aminoethyl piperazine (AEP) blend. At the temperature of extrusion, 180°C, the DGEBA is immiscible in PS and heterogeneous material is obtained. The elongational flow imposed by drawing the extrudate at the die exit permitted controlled generation of a fibrillar morphology of the dispersed epoxy phase, with a fiber diameter of 1 μm and an aspect ratio greater than 100. It was shown that when the amine comonomer was injected into the extruder, its reactivity with DGEBA at high temperature was high enough to ensure partial crosslinking of the epoxy. The fibrils were formed even though the gel point of the epoxy phase was exceeded. However, above a certain critical insoluble fraction that we estimated to be between 45% and 70%, a coarsening of the structure appeared, caused by the decreasing deformability of the domains and their coalescence. Finally, for our system, the crosslinking of the dispersed phase up to 90% of insoluble fraction did not totally stabilize the morphology after the second processing step (injection molding). Polym. Eng. Sci. 44:625–637, 2004. © 2004 Society of Plastics Engineers.

Cryogenic microcracking of rubber toughened composites

AbstractThis study investigated the influence of carboxyl‐terminated butadiene acrylonitrile (CTBN) liquid rubbers on the microcracking response of polymeric composite materials to cryogenic cycling. Matrices of carbon fiber/epoxy prepregs were modified with different concentrations of two CTBN liquid rubbers. The glass transition temperature and the interlaminar shear strength of the laminate systems were depressed as a result of the presence of CTBN in the epoxy phase. An increase in total rubber concentration with the continuous phase was found to decrease and in some cases eliminate microcracking in laminates exposed to cryogenic cycling.

Transient hygrothermal stresses in fiber reinforced composites: a heterogeneous characterization approach

Transient hygrothermal stresses induced in fiber-reinforced composites are studied in detail by adopting a novel heterogeneous characterization approach. This approach incorporates two distinct features: transient moisture absorption analysis of actual composite materials exposed to a humid environment, and highly detailed computational analyses that capture the actual heterogeneous microstructure of the composite. The latter feature is carried out by modeling a uniaxial laminate having more than one thousand individual carbon fibers that are randomly distributed within an epoxy matrix. Results indicate that these computational models are essential in capturing the accurate moisture absorption process of the actual specimen. In the analysis, the evolutions of thermal residual stresses and moisture-induced stresses within the humidity and thermal exposed composites have been analyzed. It was observed that high stress concentration develops in the epoxy phase where high fiber density or fiber clustering exists and its magnitude increases as the moisture content saturates. Large stresses can potentially initiate epoxy damage or delamination of epoxy and fibers. Furthermore, due to opposing effects of thermal and moisture exposure, lower stresses are found in the laminate when both are considered simultaneously.

Interpenetrating vinylester/epoxy resins modified with organophilic layered silicates

Organophilic layered silicates (OLS), modified by means of cation exchange, were added in amounts of 5–15 wt.% to vinylester (VE)/epoxy (EP) hybrid resins which can form interpenetrating networks (IPN) after curing at T=150 °C. The resulting phase structure was studied by atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was established that the OLS were intercalated by the hybrid resin. On the microscale silicate particles surrounded by a broad EP interphase were formed. This “encapsulation” was considered as the major cause for the reduction in both stiffness and glass transition temperature ( T g) of the resins. Irrespective to the coarse dispersion of the OLS, the fracture energy of the resins was doubled by adding 5 wt.% OLS. The fracture energy decreased, however, with increasing OLS content. This was explained by changes in the failure mode concluded from fractographic studies. The type of the organic cation modification of the OLS had no effect on either morphology or toughness. This was attributed to the coarse dispersion of the OLS and its encapsulation by the EP phase in the VE/EP hybrid resins.

Epoxy resins modified with poly(vinyl acetate)

Miscibility and curing behaviour have been studied in blends of poly(vinyl acetate) (PVAc) with diglycidyl ether of bisphenol A (DGEBA) resin and 4,4′-diaminodiphenylsulphone (DDS) used as curing agent. The PVAc component was used as a potential toughness modifier. Binary mixtures DGEBA+PVAc show complete miscibility over the whole composition range. Ternary mixture DGEBA+DDS in stoichiometric ratio +10% (w/w) PVAc behaves also as homogeneous phase. However PVAc was found to be inmmiscible with the highly amine-cross-linked epoxy resin. It was found that phase separation in DGEBA+DDS+PVAc blends occurred as the conversion reaches a value close to 40%. In the final cured material two phases are present: an epoxy phase and a PVAc phase. It was also found a slow down of the reaction when PVAc was present.

Transparent multiphasic polystyrene/epoxy blends

AbstractBlends of polystyrene (PS) with an epoxy monomer (DGEBA) and a tertiary amine (BDMA), were initially miscible at 120°C but phase‐separated at very low conversions in the course of polymerization. Although there was a significant difference between the refractive indices of polystyrene and the DGEBA/BDMA solution, the refractive index of the epoxy network increased in the course of polymerization, attaining a value close to that of PS at complete conversion. A sharp decrease of the light transmittance was observed at the cloud‐point, observed at very low conversions. However, the continuous increase of the refractive index of the epoxy phase with conversion produced an approximate matching of both refractive indices, leading to transparent materials at complete conversion. Morphologies generated by reaction‐induced phase separation depended on the molar mass distribution of polystyrene and its mass fraction in the blend. For a PS with a high value of the mass‐average molar mass (Mw), it was possible to generate a dispersion of PS particles in the epoxy matrix (blends containing 5 wt% PS), phase‐inverted morphologies (blends containing 15 wt% PS) and double‐phase morphologies (blends with 10 wt% PS). Therefore, PS/DGEBA/BDMA blends could be used to obtain transparent epoxy coatings toughened by polystyrene particles or transparent polystyrene parts reinforced by a dispersion of epoxy particles.

Piezoelectric properties of 3-Xperiodic Pb(ZrxTi1−x)O3–polymer composites

The piezoelectric properties of lead zirconate titanate (PZT)–polymer composites were studied as a function of composition and phase connectivity. PZT skeletal structures were fabricated by robotic deposition, densified at 1275 °C, and subsequently infiltrated with epoxy to produce the desired PZT–polymer composites. These 3-X structures consisted of a three-dimensional lattice of PZT rods (3–3) embedded in a polymer matrix, a PZT lattice/polymer matrix capped with PZT face plates (3–2), or PZT lattice/polymer matrix capped with PZT face plates and encircled by a solid PZT ring (3–1). The PZT:polymer ratio was varied systematically by changing the lattice (rod) spacing in each composite architecture. The concentration of PZT pillars, which formed along the poling direction at the intersections between PZT rods, varied as the PZT volume fraction squared. These 3-X composites displayed enhanced hydrostatic figures of merit relative to monolithic PZT due to stress concentration in the PZT pillars and their dramatically reduced dielectric constant, with the highest values found for the 3–2 and 3–1 composites. Our experimental observations were compared to theoretical predictions based on an isostrain, unit cell model modified to account for the partial support of stress in the stiff epoxy phase.

Rubber-Modified Glassy Amorphous Polymers Prepared via Chemically Induced Phase Separation. 2. Mode of Microscopic Deformation Studied by in-Situ Small-Angle X-ray Scattering during Tensile Deformation

The mode of microscopic deformation in rubber-modified amorphous polymers has been investigated by small-angle X-ray scattering during tensile deformation. Synchrotron experiments were performed for blends consisting of poly(methyl methacrylate) (PMMA) with a finely dispersed rubbery epoxy phase. These blends were prepared via chemically induced phase separation, as shown in the first paper of this series. On macroscopic deformation these blends show that the toughness of brittle amorphous polymers can be significantly enhanced by the introduction of submicron size rubber particles. The objective of the present study is to establish the relationship between the morphology and the macroscopic mechanical properties of the blends. As observed for neat PMMA, crazing is found to occur for the macroscopically brittle PMMA/epoxy 90/10 blend. In contrast, the ductile blend with 20 wt % epoxy deforms via shear yielding which is preceded by cavitation. Shear yielding also occurs for blends having even higher epoxy contents, although it is not accompanied by the occurrence of dilatation processes. The changes in the scattering patterns during deformation are attributed to morphological changes like orientation. Cross-linking of the epoxy phase appears to have an important influence on the mode of microscopic deformation. A blend with 20 wt % un-cross-linked epoxy appears to deform via crazing instead of cavitation. The change in deformation mechanism is associated with the plasticization of crazes on a local level. The local strain is defined as the local deformation of the sample exposed to the incident beam as measured by recording the beam intensity in front of, and after, the sample during the drawing process. Thus, the local strain in the beam can accurately be measured and related to the corresponding scattering patterns. The local strain values obtained are in agreement with those from macroscopic tensile tests.

Interdigital pair bonding for high frequency (20-50 MHz) ultrasonic composite transducers.

Interdigital pair bonding is a novel methodology that enables the fabrication of high frequency piezoelectric composites with high volume fractions of the ceramic phase. This enhancement in ceramic volume fraction significantly reduces the dimensional scale of the epoxy phase and increases the related effective physical parameters of the composite, such as dielectric constant and the longitudinal sound velocity, which are major concerns in the development of high frequency piezoelectric composites. In this paper, a method called interdigital pair bonding (IPB) is used to prepare 1-3 piezoelectric composite with a pitch of 40 microns, a kerf of 4 microns, and a ceramic volume fraction of 81%. The composites prepared in this fashion exhibited a very pure thickness-mode resonance up to a frequency of 50 MHz. Unlike the 2-2 piezoelectric composites with the same ceramic and epoxy scales developed earlier, the anticipated lateral modes between 50 to 100 MHz were not observed in the current 1-3 composites. The mechanisms for the elimination of the lateral modes at high frequency are discussed. The effective electromechanical coupling coefficient of the composite was 0.72 at a frequency of 50 MHz. The composites showed a high longitudinal sound velocity of 4300 m/s and a high clamped dielectric constant of 1111 epsilon 0, which will benefit the development of high frequency ultrasonic transducers and especially high frequency transducer arrays for medical imaging.

Emulsion polymerization of an epoxy-acrylate emulsion stabilized with polyacrylate. II. Using the results of statistically designed experiments to deduce a possible polymerization mechanism

Epoxy-acrylate composite emulsions such as the one in this study can be used as metal coatings, etc. Many factors can influence the final quality of the product, and it is the aim of this study to highlight some of these factors. Statistically designed experiments were performed to investigate the influence of monomer level, the polymeric dispersant level, epoxy level, and the initiator level on particle size (light scattering), particle size distribution, and fractional conversion. It was found that the monomer level significantly influenced particle size and particle size distribution. The epoxy level and the monomer level influenced fractional conversion significantly. A qualitative model based on these observations is put forward to describe the mechanism of particle formation and polymerization. This model states that the high internal viscosity of the initial dispersed epoxy phase inhibits the formation of smaller particles and accelerates the polymerization rate during the first few minutes of polymerization by inducing a gel effect that inhibits termination and chain transfers of radical species. The addition of a monomer lowers the internal viscosity of the particles and causes them to break up into droplets containing dissolved epoxy, polyacrylate, and monomer. At the same time, radical species inside the smaller droplets can now undergo termination and chain transfer reactions. To confirm this model, polyacrylates of varying molecular masses were synthesized. Variation of the molecular masses of these polyacrylates resulted in variation in the viscosity of the dispersed phase. Polymerizations conducted with the polyacrylates confirmed the model observations. A dispersed phase with a high viscosity results in an increased polymerization rate, larger overall particle size, and a higher mass average molecular mass copolymer compared to lower molecular mass polyacrylates.

CTBN rubber phase precipitation in model epoxy resins

This study investigated the precipitation of three types of end-functionalised liquid rubber (two CTBN and one ATBN) from two homologous series of amine-cured epoxy polymers with variable cross-link density. The CTBN carboxyl endgroups strongly enhanced the curing rate through the impurity catalysis mechanism, however, the amino-terminated ATBN only retarded the reaction, possibly by dilutional effects or owing to changes in the dielectric constant of the reacting medium. The glass transition temperature (Tg) of the epoxy phase was depressed by CTBN rubber dissolved in the epoxy matrix and the Tg of the CTBN rubber was also lowered. The height of the DMTA rubber peak was a linear function of the level of CTBN and suggested that ca. 3% of the rubber was dissolved in the cured epoxy matrix. The microstructure of the rubber particles was studied by SEM and related to the kinetics and thermodynamics of phase separation.

Reinforcement of crosslinked rubbery epoxies by in-situ formed silica

Rubbery crosslinked epoxide was reinforced with silica–siloxane structures formed in situ by sol–gel process from tetraethoxysilane. The increase in modulus by two orders of magnitudes was achieved at a low silica content (<10 vol%). Various polymerization procedures including simultaneous or sequential formation of the epoxide network and silica resulted in different structures of the microphase-separated organic–inorganic hybrid composite. Structure and morphology of the heterogeneous system were analyzed by electron microscopy and small-angle X-ray scattering. Temperature dependences of storage modulus and loss factor were used to evaluate phase structure of the hybrids and the interaction between epoxy and silica phases. Efficiency of the reinforcement depends on the reaction conditions; a crucial effect of an interphase formation is shown. Acid catalysis of the sol–gel process probably promotes grafting between epoxide and silica phases and leads to a more uniform and finer structure with smaller silica domains. Comparison of the mechanical data with the theoretical models reveals that studied composites correspond to the morphological model of the hybrid consisting of co-continuous epoxy and silica structures.

Rubber-Modified Glassy Amorphous Polymers Prepared via Chemically Induced Phase Separation. 4. Comparison of Properties of Semi- and Full-IPNs, and Copolymers of Acrylate−Aliphatic Epoxy Systems

The introduction of rubbery particles can be applied to enhance shear yielding and, consequently, the toughness of brittle amorphous polymers. The critical transition in these polymers from crazing to shear yielding requires a submicrometer- or even nanometer-sized rubbery phase. These can be obtained via coalescence suppression in processes involving chemically induced phase separation but are also obtained in interpenetrating polymer networks (IPN) where cross-linking or gelation is responsible for the morphology control. In all cases, the formation of a nanometer-sized morphology is accompanied by an enhanced interphase mixing, i.e., incomplete demixing resulting in a broad interface in which the composition gradually changes from one phase to the other. In this study, the influence of interphase mixing on the mechanical properties has been investigated. Besides the standard semi-IPN system based on poly(methyl methacrylate) and aliphatic epoxy resins, two additional systems being composed of the same constituents but with an increased degree of interfacial mixing have been investigated: a full-IPN prepared by cross-linking the acrylate phase and a copolymer system in which the acrylate phase is chemically bonded to the epoxy phase. In situ small-angle X-ray scattering experiments during tensile deformation demonstrated that the microscopic deformation mechanism is clearly influenced by the degree of demixing. Despite this, the macroscopic toughness is found to be rather system independent since for all three systems, a comparable synergistic toughening effect is observed in both tensile and impact deformation.

Processing of (in)tractable polymers using reactive solvents: Part 5: morphology control during phase separation

Processing of intractable polymers using reactive solvents (monomers) has been studied extensively in our laboratory, notably the system poly(phenylene ether) (PPE)/epoxy (resin). PPE can be dissolved at elevated temperatures in epoxy resin and the solution can be easily transferred into a mould or into a fabric. Upon curing the epoxy resin, phase separation and phase inversion occurs and the originally dissolved PPE becomes the continuous matrix phase. The dispersed (cured thermoset) epoxy particles become an integrated part of the system and could act as fillers or as toughening agents, depending on the type of epoxy resin used. An important parameter for the (ultimate) physical and mechanical properties is the size of the dispersed particles. The aim of the present study is to control the morphology development in order to produce a dispersed phase in the sub-micron to nanometre range. The size of the dispersed phase will be determined by the competition between the coarsening rate, e.g. by the coalescence of dispersed droplets, and the vitrification and/or gelation rate induced by curing. For the coarsening process, the viscosity of the system plays an important role which is usually mainly determined by the temperature. However, in the case of PPE/epoxy, the viscosity can be controlled at a chosen curing temperature by adding polystyrene. The ternary phase diagram shows that the miscibility of PPE–polystyrene (PS) is retained upon the addition of epoxy at relatively low concentrations. However, thermally induced phase separation upon cooling occurs for solutions with an epoxy content of 30 wt% and more. Upon curing, a two phase morphology is obtained in which the PPE–PS phase acts, as expected, as one single phase. The size of the dispersed phase can be decreased by one order of magnitude if curing is performed at the glass transition temperature, T g, of the initial solution, attributed to the high viscosity at T g that slows down coalescence. During the additional post-curing steps, necessary to reach a maximum epoxy conversion, these original morphologies are maintained. In conclusion, by controlling the polymerisation temperature, relative with reference to the T g of the original solution, the final morphology of the chemically induced phase separated systems can be tuned.