Effect of catalyst on the thermal degradation of a polycyanurate thermosetting system

The thermal degradation of a polycyanurate thermosetting material was examined by monitoring glass temperature and weight loss at various temperatures ranging from 180 to 220°C. The effect of the cocatalysts, nonylphenol and copper naphthenate, which are generally used to facilitate curing, were also studied. A decrease in T_g is observed with increasing time at elevated temperatures in the systems containing copper naphthenate, with the onset of degradation occurring sooner with higher concentrations of the copper compound. No change in T_g occurred at long times in systems containing only nonylphenol. Weight loss studies were used to calculate an apparent activation energy of degradation. It was found to be approximately 50 kcal/mol. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 127–131, 1997     

Low temperature cure of epoxy thermosets attaining high Tg using a uniform microwave field

It is demonstrated for the first time that an epoxy thermoset resin can be cured at temperatures well below its T_g∞. This study compared the use of a uniform variable frequency microwave (VFM) field to standard oven curing at temperatures above and below T_g∞. Using T_g, tan δ, modulus, and FTIR measurements, it is shown that the reaction of BFDGE with MDA to attain a product with T_g∞ of 133 °C is achieved by VFM at temperatures from 100 to 140 °C; in contrast, the thermal cure normally requires 170 °C to attain the same T_g∞ and the same extent of cure. By following the pregel cure reaction with ¹³C‐NMR spectroscopy, it was determined that the lower cure temperatures of VFM cure predominately lead to chain extension and smaller amounts of crosslinking compared to the thermal cure. To explain these results, it is suggested that, after gelation, with VFM cure there is higher mobility from dipole rotations that continues the cure to completion without vitrification. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44222.     

The effects of curing cycles on properties of the epoxy system 3221/RH glass fabric composites

In this work, the epoxy system 3221 and its glass fabric laminates were thermally cured under different curing temperatures. The curing degree of the resin was increased with elevated reaction temperature. Dynamic mechanical analysis was performed on the laminate coupons and glass transition temperature (T_g) and relative stiffness (E′) of composites were measured before and after soaked in distilled water at 70°C. A shift in glass transition temperature to higher values and the splitting of the tan δ curve were observed with extent of cure under dry conditions. T_g values shifted to lower temperatures after immersion. Under wet condition, the change in T_g1 was very small when the curing degree was up to 96%. The relative stiffness experienced a reduction both in initial modulus and the initial sharp drop temperature after immersion. It also suggested that the excessively high curing temperature (>130°C) had a negative effect on the retention of relative stiffness under wet condition. Both the interlaminar shear strength and dielectric properties of laminates were determined before and after immersion. The compared results demonstrated that the elevated curing temperature played a good influence on both of the properties before aged. However, for samples cured above 130°C, lower retention of interlaminar shear strength and poor dielectric properties were observed during immersion due to their higher moisture contents. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Comparison of thermal techniques for glass transition assignment. II. Commercial polymers

The differential scanning calorimetry glass transition (DSC T_g), measured by ASTM test method E-1356, and the dynamic mechanical analysis glass transition (DMA T_g), measured using a new definition of the DMA T_g, generally agree within ±4°C for a wide variety of commercially available polymers. The DMA T_g is defined as the average of E′ and tan δ peak temperatures measured at a 1 rad/s oscillation frequency. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 191–195, 1997     

Miscibility and esterification in the poly(styrene-co-maleic anhydride)/phenoxy blends

The “miscibility” and esterification in poly(styrene-co-maleic anhydride) (PSTMA)/phenoxy blends were investigated by DSC and FTIR. The blends prepared by casting exhibited a single composition-dependent but broad T_g during the first scanning. The broadness of the T_g transition range is due to the presence of microphases in the blends, which acquired some stability because of the hydrogen-bonding interactions with the continuous phase. However, the blends displayed two distinct T_gs during the second scanning, which can be attributed to phenoxy-rich and PSTMA-rich phases dispersed one in another at a scale larger than the initial one. To investigate the effect of esterification, the samples subjected previously to two scannings have been additionally heat-treated several times between 30 and 220°C and annealed each time at 220°C for increasing periods of time. During the additional scannings, the two T_gs identified during the second scanning increased with increasing annealing time but remained distinct. The fact that the fraction soluble in tetrahydrofuran decreased with increasing annealing time indicates that crosslinking due to esterification has occurred in both phases. The two phases generated after the first scanning were stabilized by the esterification reaction at the interfaces. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:913–919, 1998     

Isothermal Cure of an Epoxy System Containing Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM), a Multifunctional Novolac Glycidyl Ether and 4,4′-Diaminodiphenylsulphone (DDS): Vitrification and Gelation

Times to gelation and vitrification have been determined at different isothermal curing temperatures between 200 and 240°C for an epoxy/amine system containing both tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a multifunctional Novolac glycidyl ether with 4,4′-diaminodiphenylsulphone (DDS). The mixture was rich in epoxy, with an amine/epoxide ratio of 0·64. Gelation occurred around 44% conversion. Vitrification was determined from data curves of glass transition temperature, T_g, versus curing time obtained from differential scanning calorimetry experiments. The minimum and maximum values T_g determined for this epoxy system were T_g0=12°C and T_gmax=242°C. Values of activation energy for the cure reaction were obtained from T_g versus time shift factors, a_T, and gel time measurements. These values were, respectively, 76·2kJmol^-1 and 61·0kJmol^-1. The isothermal time–temperature–transformation (TTT) diagram for this system has been established. Vitrification and gelation curves cross at a cure temperature of 102°C, which corresponds to glass transition temperature of the gel. © of SCI.     

Preparation and properties of ester or cyano group substituted ring-opening polymers and their hydrogenated derivatives

Ester or cyano substituted tetracyclo [4.4.0.1^2,5.1^7,10]dodec-3-enes (1) were synthesized and their metathesis ring-opening polymerization was examined. The tungsten-based ternary catalyst system polymerized them very well. The polymers showed high glass transition temperatures (T_g) and no evidence of crystallization (e.g., the T_g of the polymer derived from 8-methyl-8-methoxycarbonyl substituted monomer (1a) was 207°C, and colorless transparent films could be casted from the solution of the polymer). The stability of these high T_g polymers were too unstable, so practical thermal molding methods could not be applied to them. The hydrogenation of these polymers with a palladium catalyst decreased T_g and greatly increased thermal stability. The physical and thermal properties of the hydrogenated polymers were thoroughly investigated. Monomer 1 was successfully copolymerized with other cyclic olefins. The resultant copolymers were hydrogenated, giving thermally stable polymers. In all cases examined in this study, a decrease of T_g by hydrogenation was about 35°C, regardless of the monomer structure. These results indicate that the main-chain mobility is the major contribution to the decrease of T_g. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 367–375, 1997     

Studies on the thermal properties of poly(phenylene sulfide amide)

Thermal properties of poly(phenylene sulfide amide) (PPSA) prepared using sodium sulfide, sulfur, and thiourea as sulfur sources which reacted with dichlorobenzamide (DCBA) and alkali in polar organic solvent at the atmospheric pressure, were studied. The glass transition temperature (T_g), melting point temperature (T_m), and melting enthalpy (ΔH_m) of the related polymers were obtained by use of differential scanning calorimetry analysis. The results are: T_g = 103.4–104.5°C, T_m = 291.5–304.7°C, and ΔH_m = 104.4–115.4 J/g. Thermal properties such as thermal decomposition temperature and decomposition kinetics were investigated by thermogravimetric analysis under nitrogen. The initial and maximum rate temperatures of degradation were found to be 401.5–411.7°C and 437–477°C, respectively. The parameters of thermal decomposition kinetics of PPSAs were worked out to be: activation energy of degradation was 135 to 148 kJ/mol and the 60-s half-life temperature was 360 to 371°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1227–1230, 1997     

Conversion—temperature—property relationships in thermosetting systems: Property hysteresis due to microcracking of an epoxy/amine thermoset—glass fiber composite

A single specimen of an epoxy/amine thermoset—glass fiber composite was examined, using a freely oscillating torsion pendulum operating at ∼ 1 Hz, for different conversions (as measured by T_g) from T_g0 = 0°C to T_g∞ = 184°C during cooling and heating temperature scans. T_g was increased for successive pairs of scans by heating to higher and higher temperatures. The data were used in two ways: (i) vs. temperature for a fixed conversion to obtain transitions, modulus, and mechanical loss data, and (ii) by crossplotting to obtain isothermal values of the mechanical parameters vs. conversion (T_g). Hysteresis between cooling and subsequent heating data was observed in temperature scans of essentially ungelled material (T_g < 70°C) and was attributed to spontaneous microcracking. Hysteresis was analyzed in terms of the following three parameters: T_crack, the temperature corresponding to the onset of microcracking on cooling; T_heal, the temperature at which the specimen heals on subsequent heating; and the difference between isothermal cooling and heating data vs. conversion. Results were incorporated into a more general conversion—temperature—property diagram which serves as a framework for relating transitions (relaxations) to macroscopic behavior. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 39–53, 1997     

Isothermal physical aging of a fully cured epoxy—amine thermosetting system

The rate and effects of isothermal physical aging of a fully cured epoxy—amine/glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (−180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, T_g, was 182°C (0.9 Hz), and the principal glassy-state secondary transition temperature, T_β, was ≈ −30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (T_a) and that there are two maxima in the aging rate vs. T_a. A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about T_a. The majority of the isothermal aging effect can be eliminated by heating to temperatures above T_a, but below T_g. Theoretical and practical implications of this observation are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 55–67, 1997     

Annealing studies on a fully cured epoxy resin: Effect of thermal prehistory,and time and temperature of physical annealing

The dynamic mechanical behavior at about 1 Hz of a fully cured epoxy resin (maximum glass transition temperature, T_g∞, ca. 170°C) ahs been studied during and after isothermal annealing in terms of the influence of thermal prehistory, time of annealing, and temperature of annealing (T_a). Annealing temperatures ranged from T_g ? 15 to T_g ? 130°C. The rate of isothermal annealing was observed to decrease by a decade for each decade increase of annelaing time when the material was far from equilibrium. Annealing at high temperatures did not measurably affect the mateiral properties during cooling (for T ? T_a); similarly the effect of annealing at low temperatures was not measurale during heating (for T ? T_a).     

Cure kinetics of a thermosetting liquid dicyanate ester monomer/high-Tg polycyanurate material

The cure of a liquid dicyanate ester monomer, which reacts to form a high-T_g (≈200°C) polycyanurate network, has been investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and a dynamic mechanical technique, torsional braid analysis (TBA). The monomer is cured with and without catalyst. The same one-to-one relationship between fractional conversion and the dimensionless glass transition temperature is found from DSC data for both the uncatalyzed and catalyzed systems, independent of cure temperature, signifying that the same polymeric structure is produced. T_g is the parameter used to monitor the curing reactions since it is uniquely related to conversion, is sensitive, is accurately determined, and is also directly related to the solidification process. The rate of uncatalyzed reaction is found to be much slower than that of the catalyzed reaction. The apparent overall activation energy for the uncatalyzed reaction is found to be greater than that of the catalyzed reaction (22 and 13 kcal/mol, respectively) from time–temperature superposition of experimental isothermal T_g vs. In time data to form kinetically-controlled master curves for the two systems. Although the time–temperature superposition analysis does not necessitate knowledge of the rate expression, it has limitations, because if the curing process consists of parallel reactions with different activation energies, as is considered to be the case from analysis of the FTIR data, there should not be a kinetically-controlled master curve. Consequently, a kinetic model, which can be satisfactorily extrapolated, is developed from FTIR isothermal cure studies of the uncatalyzed reaction. The FTIR data for the uncatalyzed system at high cure temperatures, where the material is in the liquid or rubbery states throughout cure, 190 to 220°C, are fitted by a model of two parallel reactions, which are second-order and second-order autocatalytic (with activation energies of 11 and 29 kcal/mol), respectively. Using the model parameters determined from the FTIR studies and the relationship between T_g and conversion from DSC studies, T_g, vs. time curves are calculated for the uncatalyzed system and found to agree with DSC experimental results for isothermal cure temperatures from 120 to 200°C to even beyond vitrification. The DSC data for the catalyzed system are also described by the same kinetic model after incorporating changes in the pre-exponential frequency factors (due to the higher concentration of catalyst) and after incorporating diffusion-control, which occurs prior to vitrification in the catalyzed system (but well after vitrification in the uncatalyzed system). Time–temperature-transformation (TTT) isothermal cure diagrams for both systems are calculated from the kinetic model and compared to experimental TBA data. Experimental gelation is found to occur at a conversion of approximately 64% in the catalyzed system by comparison of experimental macroscopic gelation at the various curing temperatures and iso-T_g (iso-conversion) curves calculated from the kinetic model. © 1993 John Wiley & Sons, Inc.     

Time-temperature-transformation (TTT) cure diagrams: Relationships between Tg,cure temperature,and time for DGEBA/DETA systems

Isothermal curing of a bisphenol A diglycidyl ether-based epoxy-resin-based, using an aliphatic polyamine, has been performed at temperatures between 20 and 60°C. Samples were cured isothermally at various intervals of time, and analyzed by differential scanning calorimetry (DSC). The glass transition temperature (T_g) and the conversion ratio cure determined by residual enthalpy analysis is used as an isothermal cure-controlled reaction. A time-temperature-transformation (TTT) isothermal cure diagram was carried out to include the time to vitrification and iso-T_g curves. © 1996 John Wiley & Sons, Inc.     

Effect of physical annealing on the dynamic mechanical properties of a high-Tg amine-cured epoxy system

Physical annealing of a fully cured amine/epoxy system has been investigated using the freely oscillating TBA torsion pendulum technique. The material densifies spontaneously during annealing in an attempt to reach equilibrium, thereby changing material behavior. The dynamic mechanical behavior of a film specimen (T_g = 174°C, 0.3 Hz) and of a glass braid composite specimen (T_g = 182°C, 0.9 Hz) was monitored during isothermal annealing at sub-T_g temperatures (ranging to 230°C below T_g); after annealing, the behavior was measured vs. temperature and compared with that of the unannealed state. Isothermally, the storage modulus (G′) of the film specimen and the relative rigidity (1/P²) of the composite specimen increased almost linearly with log time, whereas the logarithmic decrement (Δ) decreased with time. The isothermal rates of annealing were determined from the rates of changes in G′ and in 1/P² for the film and composite specimens, respectively. In a wide temperature range between T_g and the secondary transition temperature, T_sec (≈ ?30°C, 2.3 Hz by TBA), the isothermal rates of annealing at the same annealing time appeared to be the same. Thermomechanical spectra of the isothermally annealed material revealed a maximum deviation in thermomechanical behavior from the unannealed material in the vicinity of the annealing temperature. The effects of physical aging were the same for the film and composite specimens. Effects of sequential annealing at two isothermal temperatures on the thermomechanical behavior were also investigated; when the second temperature was higher than the first, the effect of only the high-temperature annealing was evident, whereas the effect of annealing at both temperatures was revealed when the second temperature was lower than the first. Results suggest that physical annealing at different temperatures involves different length scales of chain segment relaxation and that the effects of isothermal aging can be eliminated by heating to below T_g.     

Glass transition phenomena in melt‐processed polystyrene/polypropylene blends

Blends of an amorphous and a semi‐crystalline polymer—polystyrene and polypropylene, respectively—were prepared by melt processing in an extruder at 220°C. These polymers are known to be immiscible and the composite morphologies were characterized by electron microscopy and thermal analysis. Fine micron‐scale morphologies, ranging from 0.5 to 20 microns were observed. Thermal analysis and dynamic mechanical analysis showed changes in both the polystyrene and polypropylene glass transition temperatures (T_g) over the composition range. The major effect was a sharp increase in polystyrene T_g with increasing polypropylene content in the blend. A T_g elevation of 5.5°C was observed at 85% polypropylene. The polypropylene T_g also increases with increasing polypropylene content, starting at a depressed value in discrete polypropylene domain environments and approaching the bulk polypropylene value after the phase inversion is crossed. Qualitative structural models are proposed based on spatial and mechanical interactions between the components. POLYM. ENG. SCI., 45:1187–1193, 2005. © 2005 Society of Plastics Engineers     

Cure of epoxy novolacs with aromatic diamines. I. Vitrification,gelation, and reaction kinetics

The cure reaction of a commercial epoxidized novolac with 4,4' diaminodiphenylsulfone (DDS) was studied at constant cure temperatures in the range 120–270°C, as well as at constant heating rates (differential scanning calorimetry, DSC). Stoichiometric formulations did not attain complete conversion due to the presence of topological restrictions. The limiting conversion was x_max = 0.8. Samples containing an amine excess (≥ 20%) could be completely reacted, whereas this was not possible for formulations containing an epoxy excess. Samples containing a 20% amine excess showed the maximum value of the glass transition temperature (T_g230°C). Cure took place by epoxy-amine hydrogen reactions catalyzed by (OH) groups. A reactivity ratio of secondary to primary amine hydrogens equal to 0.2 was found. The activation energy was E = 61 kJ/mol, as arising from T_g versus time shift factors and time to gel measurements. A unique relationship between T_g and x could be obtained. Gelation took place at x_gel = 0.45 and the maximum T_g for the stoichiometric system was T_gmax = 215°C for x = 0.8. A conversion versus temperature transformation diagram was used to represent conditions where gelation, vitrification, degradation, and topological limitations took place. © 1993 John Wiley & Sons, Inc.     

Time-temperature–transformation (TTT) diagrams of high Tg epoxy systems: Competition between cure and thermal degradation

The cure behavior and thermal degradation of high T_g epoxy systems have been investigated by comparing their isothermal time-temperature-transformation (TTT) diagrams. The formulations were prepared from di- and trifunctional epoxy resins, and their mixtures, with stoichiometric amounts of a tetrafunctional aromatic diamine. The maximum glass transition temperatures (T_g∞) were 229°C and > 324°C for the fully cured di- and trifunctional epoxy materials, respectively. Increasing functionality of the reactants decreases the times to gelation and to vitrification, and increases the difference between T_g after prolonged isothermal cure and the temperature of cure. At high temperatures, there is competition between cure and thermal degradation. The latter was characterized by two main processes which involved devitrification (decrease of modulus and T_g) and revitrification (char formation). The experimentally inaccessible T_g∞ (352°C) for the trifunctional epoxy material was obtained by extrapolation from the values of T_g∞ of the less highly crosslinked systems using a relationship between the glass transition temperature, crosslink density, and chemical structure.     

Glass‐transition behavior of poly(methacrylonitrile)

The glass‐transition temperatures (T_g) of some polymers reported in the literature have always been a source of great uncertainty. The values reported for poly(methacrylonitrile) (PMAN, 100 and 120°C) are well above the value determined in this study (67°C). It is clearly shown by FTIR and DSC work that formation of cyclic structures during the drying of PMAN, even at low temperatures, is the main reason for the high T_g values observed. The contributions of naphthydrine type cyclic structures and intermolecular crosslinks in the increase of the T_g are determined over an aging temperature interval of 90–300°C. The combined effects of intra‐ and intermolecular linking cause an increase in the T_g from 67 to 116°C. The hardness measurements also confirm the value determined by DSC. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1936–1943, 2001     

Network polyester films from glycerol and dicarboxylic acids

This paper presents information on the preparation of the network polyester films from glycerol (Y_g) and aromatic dicarboxylic acids of phthalic anhydride (P), dimethyl isophthalate (I) and dimethyl terephthalate (T), as well as aliphatic dicarboxylic acids of adipic, sebacic, 1, 10-decanedicarboxylic and 1, 12-dodecanedicarboxylic acids, and their properties. Y_g and dicarboxylic acid were polycondensed immediately before the gelation started. The prepolymers obtained were cast from DMF solution and successively post-polymerized at various temperatures and times to form networks. The resultant films were transparent, flexible and insoluble in organic solvents. Heat distortion temperature (T_h) measured by a penetration mode of thermomechanical analysis increased with increasing post-polymerization temperature and time, and then leveled out. T_h values corresponded well to the glass transition temperature (T_g) measured by differential thermal analysis (DTA). T_h was 152°C, 162°C and 197°C for Y_gP, Y_gI and Y_g T post-polymerized at 270°C for 6 h, respectively. T_h values of network films made from aliphatic dicarboxylic acids could not be observed until complete probe penetration occurs, as a result of thermal decomposition because the T_g is lower than room temperature. The degree of reaction estimated from the IR absorbance of hydroxyl and methylene groups was in the range of 60–80%. Two diffraction peaks appeared in the wide-angle X-ray scattering pattern, suggesting some ordered structure owing to the regular networks. Density decreased with increasing post-polymerization time and temperature, in the order Y_gP > Y_gI > Y_gT. Network films made from aliphatic dicarboxylic acids had much lower tensile strength and Young's modulus, and greater elongation, than those made from dicarboxylic acids, as a result of the T_g being below room temperature.     

Hybrid Sol–Gel Glasses with Glass‐Transition Temperatures Below Room Temperature

Melting gels are hybrid gels that have the ability to soften and flow at around 100°C for some combinations of mono‐ and di‐substituted alkoxysiloxanes, where substitutions are either all aromatic or all aliphatic. In this study, melting gels were prepared using phenyltriethoxysilane (PhTES) and dimethyldiethoxysilane (DMDES), meaning both an aromatic and aliphatic substitution. Differential scanning calorimetry was performed to identify glass‐transition temperatures, and thermal gravimetric analysis coupled with differential thermal analysis (TGA‐DTA) was performed to measure weight loss. The glass‐transition temperatures (T_g) ranged from ?61°C to +5.6°C, which are between the values in the methyl only system, where all T_g values are less than 0°C, and those values in the phenyl only system, where T_g values are greater than 0°C. The T_g decreased with an increase in the DMDES fraction. Below 450°C, the gels lost little weight, but around 600°C there was a drop in weight. This temperature is lower than the temperature for gels prepared with only aromatic substitutions, but higher than that for gels prepared with only aliphatic substitutions. Final heat treatment was carried out at 150°C for the gel with 80%PhTES‐20%DMDES (in mol%), and the consolidation temperature increased with increasing DMDES content to 205°C for the gel with 50%PhTES‐50%DMDES. After this heat treatment, the melting gels no longer soften.