Syntheses and characterizations of thermally reworkable epoxy resins II

Flip‐chip technology is a face‐down attachment of the active side of the silicon device onto the substrate. It is the ultimate packaging solution to integrated circuit devices used in 21st century electronic systems to meet the requirements of small size, high performance, and low cost. Underfill technology enhances the flip chip on board cycle fatigue life and thus dramatically extends the application of flip‐chip technology in electronics from high‐end to cost‐sensitive commodity products. Reworkable underfill is the key to addressing the nonreworkability of the underfill, so it is very important to electronic packaging. To meet the need for reworkable epoxy resins, four cycloaliphatic epoxides containing thermally cleavable carbonate linkages have been synthesized and characterized. These materials are shown to undergo curing reactions with cyclic anhydride similarly to a commercial cycloaliphatic diepoxide. Furthermore, these cured epoxides start to decompose at temperatures lower than 350 °C, the decomposition temperature for the cured sample of the commercial cycloaliphatic diepoxide. Two formulations based on two carbonate‐containing diepoxides start network breakdown around 220 °C, which is the targeted rework temperature. Moreover, these two formulations have similar properties, including the glass‐transition temperature, coefficient of thermal expansion, storage modulus, viscosity, and adhesion, compared to the standard commercial diepoxide formulation. As such, these two formulations are potential candidates for a successful reworkable underfill. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3771–3782, 2000     

Syntheses and characterizations of thermally degradable epoxy resins. III

In flip‐chip technology, the development of reworkable underfill materials has been one of the keys to the recovery of highly integrated and expensive board assembly designs through the replacement of defective chips. This article reports the syntheses, formulations, and characterizations of two new diepoxides, one containing secondary ester linkages and the other containing tertiary ester linkages, that are thermally degradable below 300 °C. The secondary and tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and Fourier transform infrared spectroscopy and formulated into underfill materials with an anhydride as the hardener and an imidazole as the catalyst. A dual‐epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, 3,4‐epoxy cyclohexyl methyl‐3,4‐epoxycyclohexyl carboxylate (ERL‐4221E), with the same hardener and catalyst. The curing kinetics of the formulas were studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis, and thermomechanical analysis. The dual‐epoxy system showed a viscosity of 18.7 and 0.87 P at 25 and 100 °C, respectively. The cured secondary, tertiary, and dual‐epoxy formulas showed decomposition temperatures around 265, 190, and 220 °C, glass‐transition temperatures around 120–140, 110–157, and 140–157 °C, and coefficients of thermal expansion of 70, 72, and 64 ppm/°C below their glass‐transition temperatures, respectively. The shear strength of the cured dual‐epoxy system decreased quickly with aging at 230 °C. The reworkability test showed that the removal of a chip underfilled with this material from the board was quite easy, and the residue on the board could be thoroughly removed with a mechanical brush without obvious damage to the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip‐chip applications. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1796–1807, 2002     

Adhesives and coatings based on phenolic/epoxy resins

Low molecular weight epoxy resin based on bis (4‐hydroxy phenyl) 1,1 cyclohexane was prepared and modified with various types of the prepared phenolic resins. Phenol–, cresol–, resorcinol–and salicylic acid–formaldehyde resins were used. The optimum conditions of formulation and curing process were studied to obtain modified wood adhesives characterized by high tensile shear strength values. This study indicated that the more suitable conditions are 1:2 weight ratio of phenol–or cresol–formaldehyde to epoxy resin in the presence of phthalic anhydride (20 wt%) of the resin content as a curing agent at 150°C for 80 min. Resorcinol–or salicylic acid–formaldehyde/epoxy resins formulated at 1:2 weight ratio were cured in the presence of paraformaldehyde (20 wt%) at 150°C for 60 min. The effect of the structure of phenolic resins on the tensile shear strength values of formulated resin samples, when mixed with the epoxy resins and cured under the previously mentioned optimum conditions for different times, was investigated. Metallic and glass coatings from the previous resins were also prepared and evaluated as varnishes or paints. Copyright © 1999 John Wiley & Sons, Ltd.     

Physical aging studies in epoxy resins. I. Kinetics of the enthalpy relaxation process in a fully cured epoxy resin

The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of T_g-10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τ_eff. The rate of relaxation of the system given by 1/τ_eff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = ?333, E_H = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub-T_g peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.     

Flame retardant epoxy resins based on diglycidyloxymethylphenylsilane

Silicon‐containing epoxy resins were prepared from diglycidyloxymethylphenyl silane (DGMPS) and diglycidylether of bisphenol A (DGEBA) by crosslinking with 4,4′‐diaminodiphenylmethane (DDM). Several DGMPS/DGEBA molar ratios were used to obtain materials with different silicon contents. Their thermal, dynamomechanical, and flame‐retardant properties were evaluated and related to the silicon content. The weight loss rate of the silicon‐containing resins is lower than that of the silicon free resin. Char yields under nitrogen and air atmospheres increase with the silicon content. The LOI (limited oxygen index) values increased from 24 for a standard commercial resin to 36 for silicon‐containing resins, demonstrating improved flame retardancy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5580–5587, 2006     

Frontal cationic curing of epoxy resins

We studied the frontal curing of trimethylolpropane triglycidyl ether (TMPTGE) using two BF₃‐amine initiators and two fillers, kaolin and fumed silica. In the case of kaolin, the range of concentrations allowing for frontal polymerization to propagate was dependent on its heat absorption effect whereas in the case of silica it was a consequence of the rheological features of this additive. However, for both systems the velocity and front temperature show the same trends; in all cases front velocities were on the order of 1 cm/min with front temperatures about 200 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2000–2005, 2010     

Time evolution of dielectric parameters during the crosslink of epoxy resins

The d.c. conductivity and the dielectric constant of an epoxy resin cured with a diamine were examined in a frequency interval extended up to 10¹⁰ Hz. The analysis of the dielectric behavior has permitted gaining a better knowledge of the relationships between dielectric parameters and the physical and chemical modifications of the systems. The results indicate that the chemical kinetics of the crosslink process is closely paralleled by the change in time of the dielectric parameters so that dielectrometry provides valuable information on conversion, relaxation times, glass transition temperature and gelation.     

Direct diazotization of graphite nanoplatelets with melamine and their favorable application in epoxy resins

The ease of undesirable agglomeration and a low efficiency are two problems that restrict the application of graphite nanoplatelets (GNPs) in epoxy resins (EP). Herein, a new strategy with melamine (MEL) as the precursor to functionalize GNPs chemically, which form a bonding layer that is compatible with epoxy matrix, is reported. The MEL fragments with secondary amine groups were grafted uniformly on the GNPs surface by covalent junctions to exploit the diazonium chemistry. This behavior led to a better dispersion and a stronger interaction with the epoxy matrix and resulted in an enhanced glass transition temperature and bending strength, compared with the pure EP. When only 1 wt% functionalized GNPs (f‐GNPs) was used, the Tg of the modified EP raised of about 15°C compared with pure EP, and the bending strength increased by approximately 39%. The dielectric constant of the EP with f‐GNPs was impacted slightly, and the dielectric loss decreased. At 10⁵ Hz, the dielectric loss of the EP with 1 wt% f‐GNPs decreased by approximately 11% compared with pure EP. Therefore, diazotization modification of the GNPs is a useful approach to improve the compatibility in nanoparticle networks.     

The Role of inclusion size in toughening of epoxy resins by spherical micelles

We report our finding of an optimal length scale for toughening of epoxies using spherical micelles formed by block copolymers. The amphiphilic diblock copolymer poly(hexylene oxide)‐poly(ethylene oxide) (PHO‐PEO) with 30 wt % PEO self‐assembled to form spherical micelles in a bisphenol A epoxy resin with a phenol novolac hardener. We systematically increased the size of the spherical micelles from 20–30 nm to 0.5–10 μm by swelling their PHO core using PHO homopolymer. Although all the blends were tougher than the unmodified epoxy, the largest enhancement of fracture resistance was measured in blends containing 0.1–1 μm spherical inclusions. This enhanced toughness was correlated with plastic deformation by shear banding in tensile test and greater roughness of the fracture surface. Smaller micelles neither induced plastic deformation nor contributed to surface roughness significantly whereas larger micelles acted as local defects resulting in early failure. These findings provide a framework in assessing the toughening effects of blended block copolymers on epoxy resins. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1125–1129, 2009     

Curing of liquid crystalline epoxy resins with a biguanide

This work extends the authors' investigations on liquid crystalline epoxy resins prepared from diglycidyl ether of 4,4′-dihydroxybiphenyl (DGE-DHBP) and aliphatic dicarboxylic compounds (ADC) or difunctional aromatic compounds. Syntheses and properties of these liquid crystalline epoxy resins are described elsewhere. In this paper a study on the curing reaction of the above mentioned liquid crystalline epoxy resins is presented. Ortho-tolylbiguanide was applied as the curing agent. The curing reactions were investigated by differential scanning calorimetry, microscopic observations and IR spectroscopy. Depending upon the temperature program of curing, it was possible to obtain polymeric networks with liquid crystalline order. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2739–2745, 1997     

Thermoplastic toughening of epoxy resins: a critical review

Thermoplastic toughening of epoxy resins has been actively studied since the early 1980s with considerable progress in property improvement and understanding having been made since then. The main advantage in using thermoplastics to toughen epoxy resins is that their incorporation need not result in significant decreases in desirable properties such as modulus and yield strengths as is generally the case when rubbers are used as toughening agents. However, the predominant criteria for achieving optimum toughness enhancement in the thermoplastic toughening of epoxy resins are still not all that clear from the literature. This review has focused upon the importance of the thermoplastic endgroups, the material's morphology, the ductility of the matrix and the chemical structure of the thermoplastic, it summarizes what the authors believe are the important requirements for good thermoplastic toughening. © 1998 John Wiley & Sons, Ltd.     

Studies on the curing kinetics of epoxy resins using silicon containing amide-amines

Curing kinetics of diglycidyl ether of bisphenol-A (DGEBA) in the presence of novel silicon containing amide-amines were investigated by the dynamic differential scanning calorimetry. Silicon containing amide-amines were prepared by reacting 2.5 moles of 4,4'-diaminodiphenyl ether (E)/4,4'-diaminodiphenyl methane (M)/3,3'-diaminodiphenyl sulfone (mS)/bis(m-aminophenyl) methyl phosphine oxide (B) with one mole of bis(4-chlorobenzoyl) dimethyl silane. The multiple heating rate method (5, 10, 15 and 20°C min^-1) was used to study the curing kinetics of epoxy resins in the presence of stoichiometric amounts of amide-amines having molecular masses in the range of 660 to 760 g mol^-1. The peak exotherm temperature depends on the heating rate as well as on the structure of amide-amines. Activation energy of curing reaction as determined in accordance to the Ozawa's method was found to be dependent on the structure of amine. The thermal stability of the isothermally cured resins was also evaluated using dynamic thermogravimetry in a nitrogen atmosphere. The char yield was the highest in case of resins cured with amide-amines having both phosphorus and silicon atoms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Curing behavior and toughening performance of epoxy resins containing hyperbranched polyester

The effects of the hyperbranched polyester with hydroxyl end groups (HBPE‐OH) on the curing behavior and toughening performance of a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) were presented. The addition of HBPE‐OH into DGEBA strongly increased its curing rate and conversion of epoxide group due to the catalytic effect of hydroxyl groups in HBPE‐OH and the low viscosity of the blend at curing temperature. The improvements on impact strength and critical stress intensity factor (or fracture toughness, K_1c) were observed with adding HBPE‐OH. The impact strength was 8.04 kJ m^?1 when HBPE‐OH reached 15 wt% and the K_1c value was approximately two times the value of pure epoxy resin when HBPE‐OH content was 20 wt%. The morphology of the blends was also investigated, which indicated that HBPE‐OH particles, as a second phase in the epoxy matrix, combined with each other as the concentration of HBPE‐OH increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Improved thermosets obtained from cycloaliphatic epoxy resins and γ‐butyrolactone with lanthanide triflates as initiators. I. Study of curing by differential scanning calorimetry and Fourier transform infrared

3,4‐Epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate was cured with different proportions of γ‐butyrolactone with lanthanum, samarium, and ytterbium triflates as catalysts. The curing was studied with differential scanning calorimetry (DSC) and Fourier transform infrared in the attenuated‐total‐reflection mode (FTIR/ATR). FTIR/ATR was used to monitor the competitive reactive processes and to quantify the evolution of the epoxide, lactone, and intermediate spiroorthoester groups. The glass‐transition temperature of the crosslinked materials was high and increased when the proportion of lactone decreased. The kinetics were studied with DSC experiments and were analyzed with isoconversional procedures. The differences in the reactivities of the systems were related to the Lewis acidity of the lanthanide salt used as the initiator. An increase in the proportion of lactone produced an increase in the reaction rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2337‐2347, 2005     

Thermal degradation and flame retardancy of epoxy resins containing intumescent flame retardant

Pentaerythritol diphosphonate melamine-urea-formaldehyde resin salt, a novel cheap macromolecular intumescent flame retardants (IFR), was synthesized, and its structure was a caged bicyclic macromolecule containing phosphorus characterized by IR. Epoxy resins (EP) were modified with IFR to get the flame retardant EP, whose flammability and burning behavior were characterized by UL 94 and limiting oxygen index (LOI). 25 mass% of IFR were doped into EP to get 27.2 of LOI and UL 94 V-0. The thermal properties of epoxy resins containing IFR were investigated with thermogravimetry (TG) and differential thermogravimetry (DTG). Activation energy for the decomposition of samples was obtained using Kissinger equation. The resultant data show that for EP containing IFR, compared with EP, IFR decreased mass loss, thermal stability and R _max, increased the char yield. The activation energy for the decomposition of EP is 230.4 kJ mol⁻¹ while it becomes 193.8 kJ mol⁻¹ for EP containing IFR, decreased by 36.6 kJ mol⁻¹, which shows that IFR can catalyze decomposition and carbonization of EP.     

Liquid crystalline epoxy resins by polyaddition of aliphatic diepoxides with 4,4′-dihydroxybiphenyl

Liquid crystalline epoxy resins were synthesized from 4,4′-dihydroxybiphenyl (DHB), which was used as a mesogenic component, and diglycidyl ethers of aliphatic glycols (ethylene glycol and 1,6-hexanediol) which were used as flexible spacers. The synthesis was carried out by the catalytic polyaddition in the melt. Triphenylphosphine was used as the catalyst. The course of the polyaddition was investigated at various molecular ratios of the reactants. It was found that both linear and branched structures were formed in the course of the synthesis. The rates of the formation of the structures were calculated. The epoxy oligomers were investigated by DSC, polarizing microscope, and x-ray and IR spectroscopy. The molecular weight distribution was determined by GPC. The dependence of liquid crystalline properties of the obtained epoxy resins on the molecular weight and on the chain length of the flexible spacer was investigated. The molecular weight of the epoxy oligomers and the length of flexible spacer influence the phase transition temperatures. © 1995 John Wiley & Sons, Inc.     

Curing studies of epoxy resins with phosphorus‐containing amines

The curing system of diglycidyl ether of bisphenol A (DGEBA) with two phosphorus‐containing amine compounds—bis(3‐aminophenyl)methyl phosphine oxide and bis(4‐aminophenyl)‐bis(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl)methane—was studied with differential scanning calorimetry under isothermal and nonisothermal conditions and compared with the DGEBA/diamino diphenyl methane system. The isoconversional method was used to evaluate the dependence of the effective activation energy on the extent of conversion. Modulated differential scanning calorimetry and dynamic mechanical thermal analysis were used to study the phenomena of vitrification and gelation. The thermal and flame‐retardant properties were evaluated, and the limiting oxygen index values of the phosphorylated resins, above 30, confirmed that phosphorus‐containing epoxy resins are effective flame retardants. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1676–1685, 2006     

Comparative studies on the curing kinetics and thermal stability of tetrafunctional epoxy resins using various amines as curing agents

The curing reactions of the epoxy resins tetraglycidyl diaminodiphenyl methane (TGDDM) and tetraglycidyl methylenebis (o-toluidine) (TGMBT) using diaminodiphenyl sulfone (DDS), diaminodiphenyl methane (DDM) and diethylenetriamine (DETA) as curing agents were studied kinetically by differential scanning calorimetry. The dynamic scans in the temperature range 20°–300°C were analyzed to estimate the activation energy and the order of reaction for the curing process using some empirical relations. The activation energy for the various epoxy systems is observed in the range 71.9–110.2 kJ·mol^–1. The cured epoxy resins were studied for kinetics of thermal degradation by thermogravimetry in a static air atmosphere at a heating rate of 10 deg·min^–1. The thermal degradation reactions were found to proceed in a single step having an activation energy in the range 27.6–51.4 kJ·mol^–1.

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Zusammenfassung Die Vernetzungsreaktionen der Epoxidharze Tetraglycidyl-diamino-diphenyl-methan (TGDDM) und Tetraglycidyl-methylen-bis(o-toluidin) (TGMBT) unter Verwendung von Diaminodiphenylsulfon (DDS), Diaminodiphenylmethan (DDM) und Diethylentriamin (DETA) als Vernetzungsmittel wurden kinetisch mittels DSC untersucht. Die dynamischen Scans im Temperaturbereich 20°–300°C wurden analysiert, um unter Anwendung einiger empirischer Gleichungen die Aktivierungsenergie und die Reaktionsordnung des Vernetzungsprozesses zu ermitteln. Die Aktivierungsenergie der einzelnen Epoxy-Systeme liegt im Bereich 71.9–110.2 kJ·mol^–1. An der ausgehärteten Harze wurde mittels TG in einer statischen Luftatmosphäre un deiner Aufheizgeschwindigkeit von 10 Grad/min die Kinetik des termischen Abbaues untersucht. Man fand, daß die thermiscehn Abbaureaktionen in einem Schritt ablaufen und ihre Aktivierungsenergie im Intervall 27.6–51.4 kJ·mol^–1 liegt.

     

Synthesis, characterization, and cure properties of phosphorus-containing epoxy resins for flame retardance

An organophosphorus compound, 10-(2,5-dihydroxyl phenyl)-9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DHPDOPO), was synthesized through the reaction of 9,10-dihydro-9-oxa-10-phosphaphnanthrene-10-oxide (DOPO) and p-benzoquinone, and characterized by elemental analysis, Fourier transform infrared spectrum (FTIR), and ¹H-NMR and ³¹P-NMR spectroscopes. Consequently, the phosphorus-containing epoxy resins with phosphorus content of 1 and 2 wt.% were prepared via the reaction of diglycidyl ether of bisphenol-A with DHPDOPO and bisphenol-A, and confirmed with FTIR and gel permeation chromatography (GPC). Phenolic melamine, novolak, and dicyanodiamide (DICY) were used as curing agents to prepare the thermosetted resins with the control and the phosphorus-containing epoxy resins. Thermal properties and thermal degradation behaviors of these the thermosetted resins were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Phenolic melamine-cured resins exhibited higher glass transition temperatures than the other cured resins due to the high rigidity of their molecular chain. TGA studies demonstrated that the decomposition temperatures of the novolak-cured resins were higher than those of the others. A synergistic effect from the combination of the phosphorus-containing epoxy resin and the nitrogen-containing curing agent can result in a great improvement of the flame retardance for their thermosetted resins.