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排序方式: 共有105条查询结果,搜索用时 15 毫秒
1.
John K. Gillham 《Polymer International》1985,17(2):224-226
A generalised time-temperature-transformation cure diagram for thermosetting systems is discussed from the point of view of understanding relationships between cure, properties, and thermal degradation. 相似文献
2.
The cure of a liquid dicyanate ester monomer, which reacts to form a high-Tg (≈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. Tg 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 Tg 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 Tg and conversion from DSC studies, Tg, 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-Tg (iso-conversion) curves calculated from the kinetic model. © 1993 John Wiley & Sons, Inc. 相似文献
3.
The influence of the cis/trans ratio of 1,4-polybutadienes on the volatile products formed during temperature-programmed thermal degradation to 15% weight loss has been investigated using a mass chromatograph (a gas chromatograph which directly provides mass numbers of resolved components of a mixture) and an “on the fly” vapor-phase infrared spectrophotometer. In order of amounts, major volatile products were 4-vinyl-1-cyclohexene (dimer), 1,3-butadiene (monomer), cyclopentene, and 1,3-cyclohexadiene. With increasing trans content, the relative quantities of 4-vinyl-1-cyclohexene decreased strongly, cyclopentene increased strongly, 1,3-butadiene decreased moderately, and 1,3-cyclohexadiene increased moderately. For a high-trans polybutadiene, increasing the heating rate produced relatively more monomer and dimer but less cyclopentene. Mass chromatograms from 1,4-polybutadienes which had been heated to 15% weight loss in their prehistory were similar to those obtained from 1,2-polybutadiene, indicating that 1,4-polybutadiene undergoes isomerization prior to degradation. Mechanisms for the formation of the main volatile products of decomposition are discussed. 相似文献
4.
The presence of a high temperature (>Tg) relaxation in amorphous polystyrene has been investigated further. In the previous work,1 the techniques of differential thermal analysis (DTA) and torsional braid analysis (TBA) were employed to study polystyrene as a function of “monodisperse” molecular weight. The occurrence of the Tll transition appeared to be associated with the attainment of a critical viscosity level with also corresponded with a free volume level. An entanglement network developed at a critical value of molecular weight, Mc, giving a break in the Tll-versus-M plots. The present work deals with the influence of dispersity on the Tll transition, below and above Mc. A series of binary blends of “monodisperse” anionically polymerized polystyrenes with systematic changes in M?n and heterogeneity index (M?w/M?n) was tested by TBA. The results show that when both components have molecular weights below Mc, single and average values of Tg and Tll are observed which are linearly related to M?n?1, as predicted by free volume arguments. Although a single Tg is observed when one component has a molecular weight above and the other has a molecular weight below Mc, the components appear to undergo the Tll relaxation independently. The results indicate that both the glass transition and the Tll transition are basically governed by the same type of molecular motion but at different length ranges. 相似文献
5.
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 (Tg = 174°C, 0.3 Hz) and of a glass braid composite specimen (Tg = 182°C, 0.9 Hz) was monitored during isothermal annealing at sub-Tg temperatures (ranging to 230°C below Tg); 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 Tg and the secondary transition temperature, Tsec (≈ ?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 Tg. 相似文献
6.
Isothermal properties of thermosetting materials after cure, such as density and modulus, pass through maximum and minimum values with increasing chemical conversion. In this report observed decreases in modulus and density at isothermal temperatures below the glass-transition temperature, Tg, are termed “anomalous.” Four diepoxide (diglycidyl ether of bisphenol A) and tetrafunctional diamine (trimethylene glycol di-p-aminobenzoate) high Tg thermosetting systems with different ratios of amine to epoxy were investigated for the purpose of analyzing the evolution of the isothermal properties with increasing conversion. The density, Tg, and modulus of the materials with increasing conversion were measured by a combination of dilatometric, differential scanning calorimetry, and torsional braid analysis techniques. The results are presented in the form of conversion–temperature–property (TgTP) diagrams with modulus and density as the properties. Tg is used as a direct measure of conversion based on the one-to-one relationship between Tg and conversion. The property-conversion behavior of the systems with different ratios of amine to epoxy show similar behavior if Tg is used as the measure of conversion and the data are normalized with respect to Tg at a conversion corresponding to the lower limit of the conversion range at which a maximum in the isothermal modulus occurs. The conversion corresponding to molecular gelation, gelTg, correlates with the lower limit of the conversion range at which the maximum in isothermal modulus occurs; gelTg also marks a change in the behavior of the sub-Tg mechanical relaxations vs. conversion. The conversion corresponding to the maximum in isothermal modulus vs. conversion correlates with the conversion corresponding to the maximum in isothermal density vs. conversion. © 1995 John Wiley & Sons, Inc. 相似文献
7.
The times to gelation and to vitrification for the isothermal cure of an amine-cured epoxy (Epon 828/PACM-20) have been measured on macroscopic and molecular levels by dynamic mechanical spectrometry (torsional braid analysis and Rheometrics dynamic spectrometer), infrared spectroscopy, and gel fraction experiments. The relationships between the extents of conversion at gelation and at vitrification and the isothermal cure temperature form the basis of a theoretical model of the time–temperature–transformation (TTT) cure diagram, in which the times to gelation and to vitrification during isothermal cure versus temperature are predicted. The model demonstrates that the “S” shape of the vitrification curve depends on the reaction kinetics, as well as on the physical parameters of the system, i.e., the glass transition temperatures of the uncured resin (Tg0), the fully cured resin (Tg∞), and the gel (gelTg). The bulk viscosity of a reactive system prior to gelation and/or vitrification is also described. 相似文献
8.
Bobby Gillham Andrey Yankin Fionnan McNamara Charles Tomonto David Taylor Rocco Lupoi 《CIRP Annals》2021,70(1):171-174
Additive manufacturing techniques such as selective laser melting enable the production of customised components with high geometrical freedom. However, SLM results in a material condition with different properties to their conventionally manufactured counterparts. The presence of process-inherent defects can significantly impact the degradation of part performance. Hereby, a novel approach to assessing notched SLM Ti-6Al-4V material via a critical distance theory is presented. Geometrical notches of varying size are evaluated. Results show that the Theory of Critical Distances is appropriately applicable to fatigue prediction of SLM Ti-6Al-4V in its as-built state. 相似文献
9.
The torsional braid analysis (TBA) equipment has been used as an automated torsion pendulum to characterize prepreg materials in the form of single ply strips (2-1/2 × 1/8 in.). Compared to the use of coated glass braids, the main difference was a marked weakening of the gelation mechanical damping peak in isothermal runs. However, prepreg materials consisting of epoxy resins on glass, carbon or aramid fibers were successfully run isothermally to provide gelation and vitrification times as a function of temperature, or in constant heating rate scans to reveal the T < Tg and the Tg relaxations of the uncured resins, and at higher temperatures phenomena associated with gelation, vitrification and devitrification. 相似文献
10.