Quantitative DTA of epoxy adhesives of technological interest

1983

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The current usa of non-isothermal DTA or DSC for the investigation of polymers is based upon assumptions which allow a number of conclusions supported only by data drawn from the DTA or DSC traces of prepolymer samples.A more adequate procedure involving the analysis of the DTA or DSC traces of several samples previously cured to different polymerization degrees under isothermal conditions is proposed.A comparison of the results obtained with the two procedures is reported for two composite materials.Calorimetric investigations on polymers are currently carried out under non-isothermal conditions, viz. through DTA or DSC at a given heating rate.This kind of approach, although experimentally simple and rapid, must be considered with some caution, as it can result in misleading conclusions when the chemical process investigated occurs with a complex mechanism, as in the polymerization and reticulation of resins.The present work reports some typical examples of the misuse of non-isothermal DTA and DSC data and suggests how this kind of investigation should be employed to characterize the kinetics of a polymerization process.A direct comparison of the results obtained with the misleading and the correct approaches is also reported. General considerations Typical misuseMany authors and qualified firms report the kinetic parameters of curing processes, drawn from the evaluation of point derivatives, dQ/dT, along the profile of the exothermic peak recorded in the DTA or DSC scan for the so called prepolymer.This procedure is based on the following assumptions [1 ]:the heat Q delivered at a given time in the DSC scan is proportional to the curing degree (~ reached at that time;

Section: A Correct Approachmentioning

confidence: 99%

Epoxy polymers

1983

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“…those converging to the corresponding To, were taken into account, the remaining signals being attributable to polymer components not involved in the formation process of the final copolymer [1].…”

Section: Methodsmentioning

confidence: 99%

Epoxy polymers: Glass transition/cure degree correlation

Pezzati

1985

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The correlation between the glass transition temperature, Tg, and the cure degree, ct, of epoxy polymers has been analysed on the basis of an extension of the Vogel-Tamman-Fulcher equation for the viscosity of polymers and glasses.A physical meaning has been given to the coefficients of the series expansion of the function Tg = Tr This analysis allows one to verify that the experimental (Tg, ~) data support the conclusion that, in the case of thermoset polymers, the ideal glass transition temperature, To, appearing in the VTF equation, is larger than T~.This conclusion may be supported by the thermodynamic interpretation of T O as the temperature at which the excess entropy of the systems tends to vanish: in the case of partially cured thermosets; T O would represent the minimum temperature at which the system becomes able to enhance its cure degree.One of the main parameters allowing the characterization of any polymer is its glass transition temperature, Tg.In the case of polymers such as epoxy resins, the glass transition temperature depends upon the polymerization degree, ct, of the material considered [1]. Accordin$1y, it is of some relevance to establish the dependence of Tg on r As phe/aomenological evidence, in isothermal cure processes Ta increases up to the cure temperature, T c, with increasing at, so that, when ~ approaches unity, Tg = To. Further, both the Ta vs. ~ trend and the Tq value at ~ = 1 seem dependent upon To. One can thus define a peculiar Tg vs. ~ trend for each given Tc cure process [1, 2].The aim of the present work is to interpret the phenomenological correlations between Tg and T~, and between Tg and ct.

“…They are characterized by an epoxy matrix, which is the continuous "primary phase", and by a reinforcing or filling agent, discretely dispersed throughout the matrix, which mainly serves to improve the mechanical properties.Accordingly, the inert filling agent is generally assumed not to have a significant effect on the polymerization process of the epoxy matrix.The present study, undertaken to verify this assumption, shows that the filling agent can in fact affect the kinetics of polymerization of the epoxy host.Four CIBA materials, commonly employed as composites in the aerospace industry and here designated as A, B, C and D, respectively, were investigated by means of nonisothermal DTA, which allows monitoring of the curve process, provided that the experimental results are properly evaluated [1][2][3]. Although the phenomenological reaction order remains unchanged, always being equal to one, and although no significant difference may be observed for the kinetic constant, relevant effects have been observed for the trend of the polymerization degree vs. the time of cure.…”

mentioning

confidence: 99%

“…Four CIBA materials, commonly employed as composites in the aerospace industry and here designated as A, B, C and D, respectively, were investigated by means of nonisothermal DTA, which allows monitoring of the curve process, provided that the experimental results are properly evaluated [1][2][3].…”

mentioning

confidence: 99%

See 1 more Smart Citation

DTA traces of epoxy resins and composites

Samanni

et al. 1984

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Four CIBA epoxy composites, A, B, C and D, have been investigated with the aim of verifying the effects of the respective fibers upon polymerization rate.For material A a comparison is made between prepreg and fiber-free resin; the remaining three have been directly compared with one another, the epoxy resin being the same for all of them. Although the phenomenological reaction order remains unchanged, always being equal to one, and although no significant difference may be observed for the kinetic constant, relevant effects have been observed for the trend of the polymerization degree vs. the time of cure.A tentative interpretation, based upon the physico-chemical properties of the fibers, also seems suitable to explain the observed differences in the glass transition temperature.Epoxy prepregs are of fundamental importance in modern aero space technology, as non-metallic materials for primary structure applications. They are characterized by an epoxy matrix, which is the continuous "primary phase", and by a reinforcing or filling agent, discretely dispersed throughout the matrix, which mainly serves to improve the mechanical properties.Accordingly, the inert filling agent is generally assumed not to have a significant effect on the polymerization process of the epoxy matrix.The present study, undertaken to verify this assumption, shows that the filling agent can in fact affect the kinetics of polymerization of the epoxy host.Four CIBA materials, commonly employed as composites in the aerospace industry and here designated as A, B, C and D, respectively, were investigated by means of nonisothermal DTA, which allows monitoring of the curve process, provided that the experimental results are properly evaluated [1][2][3].