Anhydride-cured Epoxy Research Articles

Reactive core/shell type hyperbranched blockcopolyethers as new liquid rubbers for epoxy toughening

Novel reactive core/shell-type hyperbranched blockcopolyethers were tailored as new class of liquid rubbers useful as flexibilizers and toughening agents of anhydride-cured epoxy resins. Anionic ring-opening polymerization of glycidol onto a six-arm star poly(propylene oxide-block-ethylene oxide) afforded hyperbranched polyether cores as macroinitiators for propylene oxide graft copolymerization. The hydroxy end groups of the resulting polyether–polyols have been modified in order to prepare stearate, hydroxy benzoate and epoxy derivatives. The modification afforded reactivity and polarity design which has been the key to improved blend performance with epoxy resins. In comparison to conventional hyperbranched epoxy-terminated polyesters, the influence of molecular architectures on thermal, mechanical and morphological properties of hexahydrophthalic acid anhydride-cured bisphenol-A diglycidylether was examined. As a function of polarity and reactivity design, it was possible to control phase separation and to vary mechanical properties from highly flexible to stiff and tough.

The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites

The change in fracture toughness and its dependence on the content of clay nanoplatelets and adhesion at the interface between clay nanoplatelets and anhydride-cured epoxy matrix are discussed. Three clay nanoplatelets with different chemical modifications were used in this investigation. To fabricate nanocomposites, the clay nanoplatelets were sonicated in acetone for 2 h. The role of the clay nanoplatelets in the mechanical/fracture properties was investigated by transmission electron microscopy (TEM). Bright-field TEM micrographs showed excellent dispersion of clay nanoplatelets in epoxy matrix. Both intercalation and exfoliation of clay nanoplatelets were observed depending on clay modification. Compact tension specimens were used for fracture testing. The fracture toughness increased with increasing clay content. The fracture toughness of clay/epoxy nanocomposites varied with the clay morphology in the epoxy matrix. Different morphologies of the fracture surfaces, highly dependent on the morphology of dispersed clay nanoplatelets, were observed using environmental scanning electron microscopy (ESEM). The fracture toughness was found to be correlated with the fracture surface roughness measured by confocal laser scanning microscopy (CLSM).

Interphase formation in model composites studied by micro-thermal analysis

Micro-thermal analysis was used for studying interphases formed in particle-filled composites based on an epoxy resin matrix. The subsurface distribution of the particles can be observed in the conductivity images, while spatial variations in thermal properties can be determined by local thermal analyses. For an anhydride-cured epoxy system filled with porous silica particles, an interphase is observed around the particles. In the ca. 60 μm wide interphase, the glass transition temperature gradually decreases from the bulk value to a value about 10 °C lower. The less densely cross-linked interphase can be detected in the thermal conductivity images and in the local thermal analyses. The interphase formation is attributed to the effect of adsorbed water on the polymerisation mechanism.

Study on the Flame-retardance and Thermal Stability of the Acid Anhydride-cured Epoxy Resin Flame-retarded by Triphenyl Phosphate and Hydrated Alumina

The effect of hydrated alumina and triphenyl phosphate on the flame-retardance and thermal stability of the acid anhydride-cured epoxy resin was studied by thermogravimetry and limiting oxygen index determination. The results show that both the hydrated alumina and triphenyl phosphate act as flame-retardants for epoxy resin in gas phase and display condensed phase flame-retardant synergism when they are used together. However, they have little influence on the cured resin’s thermal stability.

Hydro-thermal fatigue of polymer interfaces

We have developed an experimental procedure for measuring the crack growth along polymer interfaces under hydro-thermal (HT) fatigue conditions [relative humidity (RH) is held constant while the temperature is cycled]. The technique has been used to test an interface between an anhydride-cured epoxy and a polyimide (PMDA/ODA). Our sample is a trilayered cantilever beam with the epoxy bonded over a polyimide coated metallic beam. The displacement of the metallic beam at the lowest temperature in the thermal cycle is measured and is converted into the HT crack growth along the interface. The experimentally determined HT crack growth per unit temperature cycle (d a/d N) is correlated to the maximum difference in the thermal strain energy release rate at the crack tip during each temperature cycle (Δ G). While the Paris law relationship (where d a/d N increases as a power of Δ G) holds for thermal ( T) fatigue, it does not hold true for HT d a/d N measured under high RH. We have modeled the HT d a/d N as a summation of two components: (a) the d a/d N due to T-fatigue and (b) the d a/d N due to the stress-assisted water attack (SAWA) along the interface for a given T-cycle. While an empirical power law relationship describes the T-fatigue, the SAWA is described by a thermally activated kinetics model. Our model predicts the experimental observations and measurements well and provides an insight into the mechanisms behind the failure of the polyimide/epoxy interface during HT fatigue.

The influence of silicate modification and compatibilizers on mechanical properties and morphology of anhydride-cured epoxy nanocomposites

Resin composition, silicate filler modification, and curing agents were varied systematically in order to improve the performance of anhydride-cured epoxy nanocomposites based upon organoclay. The filler component was fluorohectorite which was rendered organophilic by means of cation exchange of intergallery sodium cations for protonated alkylamines with chain lengths variable from butyl (C4), hexyl (C6), octyl (C8), decyl (C10), dodecyl (C12), hexadecyl (C16), octadecyl (C18) to 12-aminododecanoic acid (C12A). The alkyl chain length must exceed six C atoms to afford increased interlayer distances, accompanied by increased stiffness. Compatibilizer addition such as epoxidized and maleinated soy bean oil or dodecenylsuccinate afforded improved tensile strength without sacrificing stiffness and toughness. Morphology development, determined by means of transmission electron microscopy, and fracture behavior were examined.

Toughening of epoxy resins modified with polyetherester block copolymers: the influence of modifier molecular architecture on mechanical properties

Thermoplastic elastomers based on polyetheresters with polyoxytetramethylene soft segments and poly(hexamethyleneterephthalate) hard segments were used to toughen anhydride-cured epoxy resins. The ratio between hard and soft segments and the crystallinity of the hard segments prepared by incorporating poly(hexamethyleneisophthalate) in the block copolymer were varied in order to examine the effect of the modifier's molecular architecture on morphology and mechanical properties of the resin, such as toughness, strength, and stiffness. The experimental data show that segmented polyetheresters are suitable toughening agents for epoxies. The compatibility between resin and toughener and also the mechanical properties of the modified resin depend on the ratio between the hard and soft segments. Epoxy resins blended with 10 wt % of the polyetherester exhibit an increase in toughness by 50–150%, while strength and modulus decrease by 20% or less. An optimal phase adhesion at levels between 70 and 85 wt % of soft segments in the modifier results in a maximum of toughness enhancement (by about 150%) of the resin accompanied with only a slight drop in strength and stiffness (by about 15%). The glass transition temperature is only slightly affected. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 623–634, 2000

Analysis of the correlation between optical transmission loss and weight loss of thermally deteriorated epoxy resins using near infrared spectroscopy

The correlation between the optical transmission loss and the weight loss of thermally deteriorated epoxy resins was analyzed using near infrared (NIR) spectroscopy. Using three types of acid anhydride cured epoxy resins, the optical transmission losses at 850 nm of each resin and their weight losses were measured during the accelerated heating tests. At longer heating times the color of each resin darkens and its weight decreases, i.e., the values of the optical transmission loss and the weight loss are both increasing gradually. Both of tendencies are similar in magnitude, and the optical transmission loss of each resin has a good correlation with its weight loss. It is postulated that the cross-linking was increased in the resin by the thermal oxidation reaction, and that the simultaneous elimination reaction occurs from the surface. As the thermally oxidized structure is more conjugated, the optical transmission loss is increased in the shorter wavelength region, and thus theoretically, one can predict the weight loss of the thermally deteriorated epoxy resins non-destructively by measuring the optical transmission loss using NIR. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 14–19, 2000

A study of contact resistance of conductive adhesives based on anhydride-cured epoxy systems

Electrically conductive adhesives (ECAs) are an environmentally friendly alternative to tin/lead (Sn/Pb) solders in electronics packaging applications. However, current conductive technology is still in its infancy and limitations do exist. One of the critical reliability issues is that contact resistance of silver flake-filled ECAs on nonnoble metals increases in elevated temperature and humidity environments. The main objective of this study is to investigate the contact resistance behaviors of a class of conductive adhesives, which are based on anhydride-cured epoxy systems. Curing profiles, moisture pickup, and shifts of contact resistance of the ECAs on a nonnoble metal, tin/lead (Sn/Pb), during aging are investigated. Also, two corrosion inhibitors are employed to stabilize the contact resistance. The effects of these corrosion inhibitors on contact resistance are compared. It is found that: (1) this class of ECAs shows low moisture absorption, (2) the contact resistance of the ECAs on Sn/Pb decreases first and then increases slowly during 85/spl deg/C/85% relative humidity (RH) aging, (3) one of corrosion inhibitors is very effective to stabilize contact resistance of these ECAs on Sn/Pb, and (4) the corrosion inhibitor stabilizes contact resistance through adsorption on Sn/Pb surfaces. From this study, it can be concluded that ECAs based on anhydride cured epoxy systems are promising formulations for electronics packaging applications.

The effects of promoter and curing process on exfoliation behavior of epoxy/clay nanocomposites

The effects of a catalyst and coupling agent as well as a curing process on exfoliation behavior of CH3(CH2)15NH3+–montmorillonite clay in an anhydride-cured epoxy–clay system have been investigated by XRD, DSC, and TEM. The results have shown that the organoclay is easily intercalated by the epoxy precursor during the mixing process, and the clay galleries continue to expand during the curing process, but the Na+–montmorillonite clay is not intercalated during either the mixing or the curing process. The results also suggest that in the cured system without any promoter although partial exfoliated clay layers have already formed, an amount of the intercalation structure still remains. Although addition of a promoter or coupling agent into the cured system significantly lowers the maximum reaction temperature, and during the curing process the layered organoclay can be gradually broken into nanoscale structures, in which no d001 diffraction peaks are observed, the complete exfoliation is achieved at gel time or before. The possible mechanism for the complete exfoliation is discussed on the thermodynamic and kinetic point of view. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 808–815, 2000

PROPERTIES OF CONDUCTIVE ADHESIVES BASED ON ANHYDRIDE-CURED EPOXY SYSTEMS

Recently, isotropic conductive adhesives (ICAs) have been identified as a potential alternative for lead-containing solders in surface mount technology (SMT) applications. However, current commercial ICAs have some reliability issues that seriously limit their universal acceptance in electronics packaging areas. One of the critical reliability issues is that contact resistance between the ICAs and non-noble metals increases dramatically particularly during elevated temperature and humidity aging. The objective of this study is to investigate the contact resistance behaviors of a class of conductive adhesives, which are based on anhydride-cured epoxy systems, in elevated temperature and humidity environments. Cure profiles, moisture pickup, and shifts of contact resistance of the ICAs on different non-noble metals, such as tin (Sn), tin/lead (Sn/Pb), and copper (Cu), during aging are investigated. It is found that (1) this class of ICAs shows low moisture absorption; (2) the contact resistance of the ICAs on Sn and Sn/Pb decreases first and then increases slowly during 85°C/85% relative humidity (RH) aging; and (3) the contact resistance of the ICAs on Cu remains almost constant after the initial decrease during aging. In addition, the reasons for the contact resistance decrease are studied. Experimental results indicate that the initial contact resistance decrease is probably due to the removal of the insulating organic lubricant layer on silver (Ag) flakes or/and the removal of metal oxide layer on the non-noble metal surfaces by the acid, which is formed after the moisture reacts with the anhydride. From this study, it can be concluded that ICAs based on anhydride cured epoxy systems are promising formulations for surface mount technology applications.

Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates

Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates

Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates

Basic correlations between polymer morphology, silicate superstructures, glass temperature, stiffness and toughness of thermoset nanocomposites were investigated as a function of layered silicate type and content. The nanocomposites were based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite and synthetic hectorite, all of which were rendered organophilic by means of ion-exchange with various mono- and difunctional alkyl ammonium ions. Enhanced toughness was associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates. Nanocomposite superstructures were imaged by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM).

Photooxidation of anhydride-cured epoxies: FTIR study of the modifications of the chemical structure

Anhydride-cured epoxies are used in electrical engineering to manufacture insulators. The purpose of this study is to determine the modifications undergone by these resins when exposed to solar-type radiation (radiation with a wavelength of more than 300 nm). Two epoxy–anhydride systems have been studied. Using transmission Fourier transform infrared (FTIR) and attenuated total reflection (ATR), we have determined the structural modifications and the groups formed during photooxidation. The degradation is very heterogeneous, more significant on the surface than in the bulk of the material, and a considerable decrease in almost all the initial functional groups is observed. The flexibilized system, richer in ester groups but containing fewer phenyl groups, degrades more rapidly during the first hours of exposure than the nonflexibilized system. Two explanations accounting for this faster degradation in the first hours of exposure are drawn from ultraviolet absorbance analysis and degradation mechanisms considerations. After 40 h of irradiation, the disappearance rate of the aromatic groups depends only on their initial concentration, whereas the formation kinetics for the hydroxyl groups is limited by the diffusion of oxygen in the material. Chemical treatments using SF4, NH3, and NaCl have revealed the formation of hydrophilic products, acids, and alcohols. Their presence and particularly high concentration at the surface could very well be responsible for the degradation of the insulating properties of the electrical insulators. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1221–1237, 1998

Modification of epoxy resin with α,ω‐oligo(butylmethacrylate)diols

Novel liquid rubbers based on bishydroxy-terminated oligo(butylmethacrylate) were used to toughen anhydride-cured epoxy resins. Concentration and molecular weight of the toughening agents were varied in order to examine the effects on important mechanical properties, such as toughness, strength, stiffness, and glass transition temperature. Experimental data show that telechelic methacrylates are suitable toughening agents for epoxies. The compatibility between resin and toughener can be adjusted by varying the molecular weight of the rubber. The best results are obtained by modifying an epoxy resin with 10 wt% of bishydroxy-terminated oligo(butylmethacrylate) of a molecular weight of 5000 g/mol. Fracture toughness Klc increases by 150% accompanied by a decrease in modulus of only 11% and in strength of 16%, as compared to the corresponding properties of the neat resin. Due to an almost complete phase separation of the rubber upon curing, the glass transition temperature is scarcely affected. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 785–796, 1998

Modification of epoxy resin with ?,?-oligo(butylmethacrylate)diols

Novel liquid rubbers based on bishydroxy-terminated oligo(butylmethacrylate) were used to toughen anhydride-cured epoxy resins. Concentration and molecular weight of the toughening agents were varied in order to examine the effects on important mechanical properties, such as toughness, strength, stiffness, and glass transition temperature. Experimental data show that telechelic methacrylates are suitable toughening agents for epoxies. The compatibility between resin and toughener can be adjusted by varying the molecular weight of the rubber. The best results are obtained by modifying an epoxy resin with 10 wt% of bishydroxy-terminated oligo(butylmethacrylate) of a molecular weight of 5000 g/mol. Fracture toughness Klc increases by 150% accompanied by a decrease in modulus of only 11% and in strength of 16%, as compared to the corresponding properties of the neat resin. Due to an almost complete phase separation of the rubber upon curing, the glass transition temperature is scarcely affected. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 785–796, 1998

Reworkable Epoxies: Thermosets with Thermally Cleavable Groups for Controlled Network Breakdown

A series of new cycloaliphatic epoxies were synthesized which have either secondary or tertiary ester bonds between cross-link sites. Thermal analysis data shows that the anhydride-cured epoxies with tertiary ester bonds can decompose at ∼220 °C, while those with primary ester links decompose at ∼340 °C. Dynamic mechanical analysis reveals that these new thermosets retain moduli comparable to the non-reworkable commercial thermosets with similar structure.

Photo-oxidation and electrical aging of anhydride-cured epoxy resins

Multifactor surface aging of anhydride cured epoxides was studied. Three systems based on diglycidyl ether of bisphenol A (DGEBA) and cycloaliphatic epoxy were investigated. They were submitted either to a solar irradiation alone, or to a tangential ac electric field, or to both stresses in a laboratory setup or outdoors. The ac voltage was applied between two electrodes inserted into the sample surface. Before aging, the applied ac voltage was lower than the inception voltage of discharges. The main structural changes of the surface of the three epoxy networks were determined by Fourier transform infrared (FTIR), attenuated total reflection (ATR) and X-ray microanalysis. The evolution of the surface conductivity was investigated by current-voltage measurements and by the method of the surface potential decay. Solar irradiation of the different resins led to the buildup of IR bands characteristic of hydroxyl and carboxylic groups. These groups are very hydrophilic and have a strong impact on the electrical properties of the surface of the resins, especially in wet medium. Under both electric and climatic stresses, decarboxylation or decarbonylation of the photo-oxidized surface occurred at the polymer-electrode interface and were attributed to electrode reactions. Tracking was observed only after electrical aging alone. This unexpected result shows that a homogeneous, slightly conducting surface may prevent the formation of tracks on the surface of aged insulators.

Modification of Acid Anhydridecured Epoxy Resins By N-Phenyl- maleimideStyrene Copolymers and N-PhenylmaleimideStyrene p-Hydroxystyrene Terpolymers

N-Phenylmaleimide-styrene copolymers (PMS) and reactive N-phenylmaleimide-styrene-p-hydroxystyrene (HSt) terpolymers (PMSH) containing p-hydroxyphenyl groups were used to improve the toughness of bisphenol A diglycidyl ether epoxy resin cured with methyl hexahydrophthalic anhydride. PMS and PMSH were effective modifiers for epoxies. The morphologies of the modified resins depended on modifier structure and content. The most effective modification for the cured resins was attained because of the co-continuous structure of the modified resins in both PMS and PMSH modification systems. When using 15 wt% of PMS (M W 125000), the fracture toughness, K IC , for the modified resin increased by 230%, with retention of flexural modulus and glass transition temperature, but with a loss of flexural strength, compared with the values for the unmodified epoxy resin. When using PMSH as the reactive modifier, the efficiency decreased with increase in HSt content, because of the increasing extent of dispersion of the PMSH-rich continuous phases. In the modification with 10wt% PMSH (1.0 mol% HSt unit, M W 294000), the modified resin had balanced physical properties.

A Study of the Interphase Region in Carbon Fibre/Epoxy Composites using Dynamic Mechanical Thermal Analysis

Abstract We have examined the effect of fibre addition on the glass transition temperature (T g ) of two epoxy resin systems (an amine cured and an anhydride cured epoxy system) using dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC). The presence of fibres changes the glass transition temperature (T g ) of an anhydride cured epoxy resin but does not affect that of an amine cured epoxy. The data suggest that two counteracting mechanisms are responsible for these changes: firstly, the presence of fibres causes a restriction of the molecular motion in the resin system, and secondly, the presence of carboxyi and keto-enol groups on the fibre surface inhibit curing of the resin close to the fibre, i.e. in the interphase region. The former increases the T g and is a long range effect whereas the latter decreases the T g and is a localised phenomenon. Changes in the dynamic properties of the interphase region are only detected when the samples are loaded in the longitudinal ...