Analysis of the crosslinking process of epoxy–phenolic mixtures by thermal scanning rheometry

The curing reaction of different mixtures of an epoxy resin (diglycidyl ether of bisphenol A type) and a phenolic resin (resole type) cured with different amine concentrations (triethylene tetramine) was studied with thermal scanning rheometry under isothermal conditions from 30 to 95°C. The gel time, defined by several criteria, was used to determine the apparent activation energy of the process. Moreover, with an empirical model used to predict the change in the complex viscosity versus time until the gel time was reached, and under the assumption of first‐order kinetics, the apparent rate constant and the apparent activation energy for the curing process were calculated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 818–824, 2005     

New type of phenolic resin—The curing reaction of bisphenol A based benzoxazine with bisoxazoline and the properties of the cured resin. III. The cure reactivity of benzoxazine with a latent curing agent

The curing reaction of a bisphenol A based benzoxazine [2,2‐bis(3,4‐dihydro‐3‐phenyl‐1,3‐benzoxazine) propane (Ba)] and bisoxazoline with a latent curing agent and the properties of the cured resins were investigated. With a latent curing agent, the ring‐opening reaction of the benzoxazine ring occurred more rapidly, and then the phenolic hydroxyl group generated by the ring‐opening reaction of the benzoxazine ring also reacted with the oxazoline ring more rapidly. The cure time of molten resins from Ba and bisoxazoline with a latent curing agent was reduced, and the cure temperature was lowered, in comparison with those of resins from Ba and bisoxazoline without a latent curing agent. The melt viscosity of molten resins from Ba and bisoxazoline with a latent curing agent was kept around 50 Pa s at 80°C even after 30 min, and molten resins from Ba and bisoxazoline with a latent curing agent showed good thermal stability below 80°C. However, above 170°C, the curing reaction of Ba with bisoxazoline with a latent curing agent proceeded rapidly. Cured resins from Ba and bisoxazoline with a latent curing agent showed good heat resistance, flame resistance, mechanical properties, and electrical insulation in comparison with cured resins from Ba and bisoxazoline without a latent curing agent. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The effect of storage temperatures below 100°C on the viscosity of a novolak resin propylene glycol solution

The aging of a novolak resin solution used in iron‐making blast furnace taphole clays is reported. The novolak resin propylene glycol solution was aged at temperatures between 2 and 80°C for up to 56 days. The viscosity was measured to evaluate the change in the resin's behavior. A cure reaction was found to occur with the addition of hexamethylenetetramine (HMTA) at temperatures lower than had previously been reported. Methods for handling and storage of taphole clay to avoid excessive increases in viscosity due to aging are discussed. An approach for estimating the long term aging at temperatures of 30 to 50°C was considered using shorter term aging data obtained at 70 and 80°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 267–276, 2004     

Synthesis of novolac‐type phenolic resins using glucose as the substitute for formaldehyde

Novel Novolac type phenolic resins were prepared using glucose as the substitute for toxic formaldehyde (a carcinogenic chemical). The resins were synthesized with varying molar ratios of phenol to glucose, catalyzed by strong acid (such as sulfuric acid) at 120–150°C. Analysis of the resins using gel permeation chromatography (GPC) and proton nuclear magnetic resonance (¹H‐NMR) showed that they were broadly distributed oligomers derived from the Fridel‐Crafts condensation of phenol and glucose. Using hexamethylenetetramine (HMTA) as the curing agent, the phenol‐glucose resins could be thermally cured and exhibited exothermic peaks at 130–180°C, typical of thermosetting phenolic resins. The cured resins showed satisfactory thermal stability, e.g., they started to decompose at >280°C with residual carbon yields of above 58% at 600°C. Based on the thermal properties, phenol‐glucose resin with a molar ratio of 1 : 0.5 is promising as it could be cured at a lower temperature (147°C) and exhibited a satisfactorily good thermal stability: it started to decompose at >300°C with a residual carbon yield of >64% at 600°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of wood on the curing behavior of commercial phenolic resin systems

Differential scanning calorimetry (DSC) was used to study the effect of wood on the curing behavior of two types of commercial oriented‐strand‐board phenolic resins. DSC analysis showed that the curing behavior of the core resin differed significantly from that of the face resin in terms of the peak shape, peak temperature, and activation energy. The addition of wood to the resins moved the two separated peaks in the DSC curves of the core resin adjacent to each other. It also accelerated the addition reactions in the curing processes of both the core and face resins. The two peaks in the DSC curves were the result of the high pH values of the resins. These two peaks became either jointed together or overlapped when the pH value of the resin was reduced. Wood also reduced the activation energies for both the core and face resins by decreasing the pH values of the curing systems. Moreover, the effects of wood on the curing behavior of the resins among the five species studied were similar. The lowest activation energy for a phenolic resin probably appeared at pH 10–11 under alkaline conditions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 185–192, 2005     

Synthesis and characterization of unsaturated polyesters based on the aminolysis of poly(ethylene terephthalate)

The depolymerization of poly(ethylene terephthalate) via an aminolysis process was studied. An excess of ethanol amine in the presence of sodium acetate as a catalyst was used to produce bis(2‐hydroxyl ethylene) terephthalamide (BHETA). Unsaturated polyester (UP) resins were obtained by the reaction of BHETA with different long‐chain dibasic acids such as decanedioic acid, tetradecanoic acid, and octadecanoic acid in conjunction with maleic anhydride as a source of unsaturation. The chemical structure of the UP resins was confirmed by ¹H‐NMR. The vinyl ester resins were used as crosslinking agents for UP. The curing behavior and mechanical properties of the UP resins with vinyl ester were evaluated at different temperatures ranging from 25 to 55°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dynamics of mechanical,acoustical, and electrical properties of epoxy‐amine compositions during cure

Dynamics of mechanical, acoustical, and electric properties is studied in the cure of three samples of polymeric compositions produced on the basis of epoxy oligomer with different content of the curing agent. The measurements were made in a parallel mode with an automated measuring complex. Variations in elastic modules, sound speed, attenuation coefficient, relaxation of the shear stress, electric resistance of the samples during the complete cure in the transition from a liquid into a glass state are considered. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Cure characterization of unsaturated polyester resin by fluorescence spectroscopy

A strong fluorescence emission was observed during cure of an unsaturated polyester resin containing about 30% styrene by weight. As the cure proceeded, the emission intensity at 306 nm increased. Model compound studies confirmed that the unsaturated polyester component exhibited negligible fluorescence when excited at 250 nm where styrene has strong absorption. Based on the studies of styrene/polystyrene mixtures, the fluorescence emission at 306 nm was attributed to a reduced inner‐filter effect of styrene monomer. Fluorescence intensity changes following cure at 75°C were correlated to the extent of styrene conversion determined by FT‐IR spectroscopy, demonstrating that the fluorescence measurement is more sensitive to styrene conversion in the later stages of cure than conventional IR measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2446–2450, 2004     

Preparation and properties of epoxy/phenol formaldehyde novolac/hexakis(methoxymethyl)melamine hybrid resins from in situ polymerization

Based on the self‐condensation of hexakis(methoxymethyl)melamine (HMMM), the condensation between HMMM and phenol formaldehyde novolac resin (n‐PF), and the addition reaction of diglycidyl ether of biphenyl A (DGEBA) and n‐PF, a homogeneous, transparent hybrid thermoset was prepared via in situ polymerization of DGEBA, n‐PF, and HMMM. No phase separations were observed even for the DGEBA/n‐PF/HMMM hybrid thermoset containing 40 wt % HMMM. These hybrid thermosets had high glass‐transition temperatures (98–127°C from differential scanning calorimetry and 111–138°C from dynamic mechanical analysis), excellent thermal stability with high 5 wt % decomposition temperatures (>322°C), high char yields (>24 wt %), and improved flame retardancy with high limited oxygen indices (>28.5). The excellent overall properties of these hybrid resins may lead to their applications in high‐performance “green” electronic products. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Reactive microspheres as active fillers for epoxy resins

Physical properties of epoxy resins filled with microparticles are presented and discussed. Microparticles were synthesized in the form of micron‐sized, crosslinked spherical particles, with an excess of reactive amino groups on their outer surface, and subsequently blended with EPON828‐3,3′DDS in different weight percents (10 and 20 wt %). Differential scanning calorimetry and scanning electronic microscopy were applied to investigate microsphere properties such as morphology, shape, size, and size distribution. Electron spectroscopy for chemical analysis was applied on particles to relate surface composition and reactivity of microspheres. Rheological, dynamic–mechanical, and mechanical properties of the cured blends were analyzed and related to the pure resin and to the same resin modified with PES180. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2031–2044, 2004     

Control of microvoids in resol phenolic resin using unsaturated polyester

A major drawback of cured phenol formaldehyde resin is the presence of microvoids, resulting from the liberation of condensation byproducts. In an attempt to rectify this, phenolic resol resin was blended with unsaturated polyester (UP). UPs with various maleic anhydride (MA) to phthalic anhydride (PA) ratios were synthesized and later mixed with resol resin in various proportions. The best MA/PA ratio was found out by determining the specific gravity, acetone‐soluble matter, and volatile content of the cast blend, cured under a satisfactory time–temperature schedule. The influence of acid value of the UP and the most desirable UP content were also investigated on the basis of the quality of the modified phenolic samples. The structural changes in the modified resin were studied using FTIR spectroscopy. Scanning electron micrographs (SEM) of the fractured surfaces were obtained to ascertain the extent of microvoids in the modified resin. Both thermogravimetric analysis results and SEM micrographs confirm the effectiveness of UP in reducing the microvoids in the cast resol resin. The tensile and impact strengths of the samples also reflect the superior quality of the resol phenolic resins that have been modified by UP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanism of crosslinking Tencel woven fabric for superior easy‐care properties and analysis using fluorescence microscopy

A crosslinking treatment to impart easy‐care properties to Tencel fabric has been investigated, using dimethyloldihydroxyethylene urea (Reaktant DH) as crosslinking agent and magnesium chloride hexahydrate as catalyst. Nonconventional treatment techniques such as “flash curing,” “moist curing,” “pad‐batch‐cure,” and “pad‐dry‐dry steam cure” are used to facilitate better penetration of crosslinking monomer into the fiber interior. Easy‐care properties of Tencel fabrics using these techniques are evaluated and compared with those treated with resin monomer using conventional pad‐dry‐cure process. Analysis of treated fabric using a fluorescent labeling technique and image analysis shows that nonconventional techniques significantly improve the penetration of crosslinking agent. The results further suggest that these techniques can be used to improve abrasion resistance and fabric handle of Tencel woven fabric. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2154–2161, 2006     

Curing reaction of molybdenum–phenolic resins

The curing behavior and curing reaction kinetics of novel molybdenum–phenolic resins were studied with differential scanning calorimetry and thermogravimetry methods, the thermal degradation properties of the cured products were studied with thermogravimetry, and the mechanism of the curing reaction was investigated with Fourier transform infrared. When the mixing ratio of the molybdenum–phenolic resin (with 12% molybdenum) to the curing agent was 100/10 (w/w), the curing temperature and activation energy were at a minimum, the thermal degradation stability of the cured product was optimal, and the temperature corresponding to the maximum extent of curing was 200°C. The curing mechanism was similar to that of conventional phenolic systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1410–1415, 2003     

Processing robustness for a phenylethynyl‐terminated polyimide composite

The processability of a phenylethynyl‐terminated imide resin matrix (PETI‐5) composite was investigated. Unidirectional prepregs were made through the coating of an N‐methylpyrrolidone solution of an amide acid oligomer (PETAA‐5/NMP) onto unsized IM7 fibers. Two batches of prepregs were used: one was made by the National Aeronautics and Space Administration in house, and the other was from an industrial source. The composite processing robustness was investigated with respect to the prepreg shelf life, the effect of B‐staging conditions, and the optimal processing window. The prepreg rheology and open hole compression (OHC) strengths were not to affected by prolonged ambient storage (i.e., up to 60 days). Rheological measurements indicated that the PETAA‐5/NMP processability was only slightly affected over a wide range of B‐stage temperatures (from 250 to 300°C). The OHC strength values were statistically indistinguishable among laminates consolidated under various B‐staging conditions. An optimal processing window was established with response surface methodology. The IM7/PETAA‐5/NMP prepreg was more sensitive to the consolidation temperature than to the pressure. A good consolidation was achievable at 371°C (700°F)/100 psi, which yielded a room‐temperature OHC strength of 62 ksi. However, the processability declined dramatically at temperatures below 350°C (662°F), as evidenced by the OHC strength values. The processability of the IM7/PETI‐5 prepreg was robust. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3212–3221, 2006     

Interdependence between the curing,structure, and the mechanical properties of phenolic resins

The interdependence between the curing conditions, structure, and the mechanical properties of tow neat phenolic resin systems was investigated. Changes of the distribution of the void diameters were characterized by light‐ an scanning electron microscope analyses. Tensile tests and dynamic mechanical thermo analysis were performed to determine the influence of the hardener concentration and the curing temperature on the mechanical and the thermomechanical properties. The study reveals that the hardener concentration predominately influenced the microscopic structure, and thus the mechanical properties of the phenolic resin systems. By varying the postcuring times, it can be shown that independent from the microstructure of the phenolic resin system, the degree of cure has a strong influence on the mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3173–3185, 1999     

Curing and thermal behavior of diglycidyl ether of bisphenol A in the presence of a mixture of amines

The curing behavior of diglycidyl ether of bisphenol A (DGEBA) was investigated by differential scanning calorimetry with mixtures of silicon‐containing amide–amines and diaminodiphenyl sulfone (DDS). Silicon‐containing amide–amines were prepared by the reaction of 2.5 mol of 4,4′‐diaminodiphenyl ether (E), 4,4′‐diaminodiphenyl methane (M), 3,3′‐diaminodiphenyl sulfone (mS), 4,4′‐diaminodiphenyl sulfone (pS), bis(3‐aminophenyl) methyl phosphine oxide (B), or tris(3‐aminophenyl) phosphine oxide (T) with 1 mol of bis(4‐chlorobenzoyl) dimethyl silane. Mixtures of the amide–amines and DDS at ratios of 0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25, and 1:0 were used to investigate the curing behavior of DGEBA. A single exotherm was observed on curing with a mixture of amide–amine and DDS. This clearly shows that the mixture participated in the cocuring reaction. The peak exotherm temperature depended on the structure and the molar ratio of amide–amines. With all of the amide–amines and DDS, a significant decrease in the kick‐off temperature of the curing exotherm was observed on the incorporation of a 0.25 molar fraction of amide–amines. Thus, with the mixture, the curing temperatures were reduced and were lowest for ether‐containing amide‐amines and highest for methylene‐containing amide–amines. The char yield was almost similar in the samples cured with amide–amines (E, pS, or mS) or DDS. The char yield was higher than for either of the constituents when a mixture was used. A synergistic behavior was observed when a mixture of E, M, mS, or pS and DDS was used, whereas mixture of B or T and DDS showed antigonism in the char yield. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1739–1747, 2003     

Comparison of microwave and thermal cure of epoxy–anhydride resins: Mechanical properties and dynamic characteristics

The objective of this work was to compare the mechanical properties of epoxy resins cured by thermal heating and microwave heating. Epoxy–anhydride (100:80) resins were cured in a domestic microwave oven and in a thermal oven. The hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Three types of accelerators were employed. Thermal curing was performed at 150°C for 20 and 14 min for resins containing 1 and 4% accelerator, respectively. Microwave curing was carried out at a low power (207 or 276 W) for 10, 14, and 20 min. All cured resins were investigated with respect to their tensile properties, notched Izod impact resistance, and flexural properties (three‐point bending) according to ASTM standards. The tan δ and activation energy values were investigated with dynamic mechanical thermal analysis, and the extent of conversion was determined with differential scanning calorimetry. The differences in the mechanical properties of the thermally cured and microwave‐cured samples depended on the resin formulation and properties. Equivalent or better mechanical properties were obtained by microwave curing, in comparison with those obtained by thermal curing. Microwave curing also provided a shorter cure time and an equivalent degree of conversion. The glass‐transition temperatures (tan δ) of the thermally and microwave‐cured resins were comparable, and their activation energies were in the range of 327–521 kJ/mol. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1442–1461, 2005     

Cure kinetics and thermal properties of tetramethylbiphenyl epoxy resin/phthalazinone‐containing diamine/hexa(phenoxy)cyclotriphophazene system

Inherently flame retardant epoxy resin is a kind of halogen‐free material for making high‐performance electronic materials. This work describes an inherently flame retardant epoxy system composed of 4,4′‐diglycidyl (3,3′,5,5′‐tetramethylbiphenyl) epoxy resin (TMBP), 1,2‐dihydro‐2‐(4‐aminophenyl)‐4‐(4‐(4‐aminophenoxy) phenyl) (2H) phthalazin‐1‐one (DAP), and hexa(phenoxy) cyclotriphophazene (HPCTP). The cure kinetics of TMBP/DAP in the presence or absence of HPCTP were investigated using isoconversional method by means of nonisothermal differential scanning calorimeter (DSC). Kinetic analysis results indicated that the effective activation energy (E_α) decreased with increasing the extent of conversion (α) for TMBP/DAP system because diffusion‐controlled reaction dominated the curing reaction gradually in the later cure stage. TMBP/DAP/HPCTP(10 wt %) system had higher E_α values than those of TMBP/DAP system in the early cure stage (α < 0.35), and an increase phenomenon of E_α ～ α dependence in the later cure stage (α ≥ 0.60) due to kinetic‐controlled reaction in the later cure stage. Such complex E_α ～ α dependence of TMBP/DAP/HPCTP(10 wt %) system might be associated with the change of the physical state (mainly viscosity) of the curing system due to the introduction of HPCTP. These cured epoxy resins had very high glass transition temperatures (202–235°C), excellent thermal stability with high 5 wt % decomposition temperatures (>340°C) and high char yields (>25.6 wt %). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Study of curing of layered silicate/trifunctional epoxy nanocomposites by means of FTIR spectroscopy

A study was conducted on the curing process of a nanocomposite consisting of a trifunctional epoxy resin, a hardener containing reactive primary amine groups, and montmorillonite (MMT) nanoparticles, previously treated with octadecyl ammonium. Three levels of MMT content were used: 2, 5, and 10%. The curing was carried out following the cycle: 4 h at 100°C, 2 h at 150°C, and 2 h at 200°C. Isothermal trials were also considered at three levels (120, 150, and 200°C) to conduct a kinetic study. The curing conversion was determined by FTIR spectroscopy by selecting the suitable bands for epoxide and primary amine functional groups. The study demonstrated that the MMT nanoparticles accelerate the curing process, especially at the initial stages of the thermal cycle, being this influence quasi negligible at the end of the cycle. Curing conversions were also evaluated by differential scanning calorimetry and compared to those obtained by FTIR spectroscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Monitoring of cure‐induced strain of an epoxy resin by fiber Bragg grating sensor

In this study, we present a method to detect cure‐induced strain in an epoxy resin (EP) with a Fiber Bragg Grating (FBG) sensor. By embedding the optical fiber into the EP resin the characteristics during isothermal cure (gel point, vitrification) could be precisely detected due to changes in the fiber strain. In a follow up dynamic temperature scan the coefficient of thermal expansion and the glass transition temperature (T_g) of the fully cured EP were determined by the FBG sensor technique. All results obtained by the fiber optical method showed a very good agreement with those deduced by independent techniques, viz. rheological measurements, differential scanning calorimetry, and thermomechanical analysis. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008