A. Cadenato

Curing of a thermosetting powder coating by means of DMTA, TMA and DSC

Abstract The curing of a thermosetting powder coating made up of carboxyl-terminated polyester and triglycidylisocyanurate (TGIC) was studied by means of dynamic mechanical thermal analysis (DMTA), thermal mechanical analysis (TMA) and differential scanning calorimetry (DSC). On the basis of isothermal curing of the coating on different supports with DMTA and TMA, we determine the degree of mechanical curing. The degree of chemical conversion is determined by curing the material isothermally and dynamically by DSC. In both cases, curing kinetics were established by means of isoconversional analysis and it was observed that both the technique and the support used appreciably modify the kinetics of the reaction process. Gelation was determined on the basis of the maximum for the loss tangent ( tan δ) and in TMA as the point at which the shrinkage rate drops to zero and the dimensions of the material show no appreciable change. The relationship between the glass transition and the conversion (Tg–α) was established by means of DSC, DMTA and TMA. It is demonstrated that the degree of mechanical curing, determined on the basis of dynamic mechanical measurements, is an indicator of the progress of the reaction and up to gelation is always higher than the chemical curing level. The relationship between mechanical and chemical conversion serves as a point of connection between results obtained by means of TMA and DMTA and those obtained by means of DSC. Lastly, using the calorimetric kinetic data, the Tg–α relationship and conversion at gelation, we construct the TTT diagram for the curing of the coating.

Simulation of isothermal cure of A powder coating

The curing of a thermosetting powder coating was studied by means of differential scanning calorimetry (DSC). The isothermal cure was simulated by non-isothermal experiments. The results of the simulation were compared with experimental isothermal data. From non-isothermal isoconversional procedures (free model), it was concluded that these permit simulation of the isothermal cure but do not enable us to determine the complete kinetic triplet (A preexponential factor, E activation energy, f(a) and/or g(a) function of conversion). Non-isothermal procedures based on a single heating rate or on master curves present difficulties for determination of all the kinetic parameters, due to the compensation effect between preexponential factor and activation energy. The kinetic triplet can be determined by a combination of various non-isothermal methods or by using experimental isothermal data in addition to non-isothermal data.

Polyurethane–unsaturated polyester interpenetrating polymer networks: thermal and dynamic mechanical thermal behaviour

Abstract A series of simultaneous interpenetrating polymer networks, IPNs, based on a polyurethane and an unsaturated polyester resin is studied. The curing process was followed using differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR). The IPNs were seen to crosslink completely and the kinetics of the curing process were modified greatly, accelerating with respect to the pure homopolymers. The process of styrene–polyester copolymerization varies from an azeotropic copolymerization in the pure polyester state to alternating copolymerization in the IPNs. Miscibility, phase continuity and phase separation are studied by dynamic mechanical thermal analysis (DMTA). In general, the IPNs obtained have a high degree of interpenetration and are semi-miscibles. Intermediate compositions are slightly less miscible than the outer ranges with a transition for the second component being apparent. The empirical loss modulus–composition curves are compared with those predicted by various theoretical models. In general, the IPNs follow the Budiansky model, which predicts a phase inversion at intermediate compositions. Compositions which are rich in a specific component show a continuous phase with the disperse minority component and the intermediate compositions show two co-continuous phases. It is also seen that these curves depend strongly on the temperature at which they are formed. The loss factor reveals strong synergism and the maximum of properties are found in a composition close to 40% in polyester. Comparison of simultaneous IPNs with sequential IPNs showed that the latter show a lesser tendency to phase separation in the systems studied.

Comparative studies on the non-isothermal DSC curing kinetics of an unsaturated polyester resin using free radicals and empirical models

Abstract In this paper, we study the kinetics of the curing of an unsaturated polyester resin initiated with MEKP by means of DSC. DSC runs were performed at different heating rates. The experimental curves were fitted using two kinetic models: one involving the concept of free-radical polymerization, and the other an empirical model that does not take into account the different steps involved in the curing. A computer program was developed to find the parameters involved in each kinetic model. The calculation algorithm uses the Runge-Kutta numerical integration and the “downhill simplex method”. The experimental data are very well fitted by using both models. The activation energy values are in concordance with the values tabulated by Odian. In the model theoretical calculation, it is useful to know the decomposition rate constant of the initiator. We determined the decomposition constant by using two different methods. Both methods give similar values of the activation energy Ed and they are in agreement with those tabulated in the literature.

Determination of gel and vitrification times of thermoset curing process by means of TMA, DMTA and DSC techniques

In the present work, gelation and vitrification experimental data are obtained by TMA and DMTA techniques using the same thermoset based on an epoxy-amine system. The results show that the times obtained are not equivalent and depend on the technique used. An attempt has been made to compare both determinations using the degree of cure obtained by means of DSC technique. The principal conclusion that we want to emphasize is that it is the conversion degree and not the time of the phenomenological changes that take place during cure, that is the link to connect and interrelate the results obtained with different techniques. A method is also described for constructing the TTT diagram with only DSC and TMA or DMTA data.

Thermoset Cure Kinetics by Isoconversional Methods

The curing kinetics of thermosets based on unsaturated polyester resin crosslinked with styrene was studied by differential scanning calorimetry (DSC). The isoconversional kinetic analysis was applied to non-isothermal data. The results obtained show a dependence of the activation energy (Eα) on conversion (α) that proves the existence of a multistep process and a complex kinetic scheme that must be interpreted in terms of chemical and physical mechanisms. The interrelationship of the Arrhenius parameters obtained from the isoconversional kinetic data has been used as a tool to investigate the production of free radicals by the action of a promoter (cobalt octoate) and the temperature on the initiator (methyl ethyl ketone peroxide). An optimum promoter/initiator ratio has been found.

Non-isothermal degradation of a thermoset powder coating in inert and oxidant atmospheres

This paper studies the thermal stability of an epoxy powder coating. The study was carried out in a thermobalance at various heating rates and in different atmospheres: nitrogen, air and oxygen. Degradation in air and oxygen leads to a kinetic process which is clearly different from degradation in an inert atmosphere. To characterise each process, the DTG signal peaks were separated and kinetic parameters were associated to each by means of the isoconversional method and other standard methods. The results obtained were compared with the experimental results.

Calorimetric analysis of the curing behavior of an unsaturated polyester resin using different catalytic systems

Abstract Differential scanning calorimetry (DSC) has been used to study the thermodependency and thermodynamics of curing behavior of unsaturated polyester resins with different catalytic systems. The curing reaction of a thermoset resin like unsaturated polyesters with styrene as a crosslinking agent at room temperature, involves the presence of an organic peroxide initiator and a promoter that induces by chemical reduction of the peroxide, free radical production at low temperatures. Dynamic and isothermal runs were performed for each initiator/promoter combination in order to study its influence in gel and curing times, exothermic heat and cure reaction kinetics. In this paper we will present the results obtained from dynamic calorimetric experiments that show the great effect of the kind of catalytic system in the curing behavior of the resin. For a particular initiator/promoter ratio the presence of more than one exothermic peak reveals different kinetic processes that we attribute to decomposition of Initiator by two different processes: chemical reduction provoked by the promoter and thermal decomposition.

Thermal curing and photocuring of a DGEBA modified with multiarm star poly(glycidol)-b-poly(ε-caprolactone) polymers of different arm lengths

The influence of two multiarm star polymers, hyperbranched poly(glycidol)-b-poly(ε-caprolactone) of different arm lengths, on the thermal curing and the photocuring of a diglycidyl ether of bisphenol A epoxy resin (DGEBA) is studied. Star polymer with short arms PCL-10 decelerates more the thermal curing than the polymer with long arms PCL-30 because the latter is less solubilized in the epoxy matrix and its effect on the polymerization of the resin and the thermal–mechanical properties is less important. The kinetic triplet corresponding to the thermal curing of the different formulations has been determined. In the analysis of the photocuring process, we have also found that short-arm star PCL-10 is better solubilized in the epoxy matrix and its effect on the photocuring kinetics is more significant than that of the long-arm star. The effect of both polymers on the thermal–mechanical properties of the cured thermosets is less important due to the lower solubility at the relatively low photocuring temperatures.

EFFECTS OF EXPERIMENTAL SAMPLE MASS ON THE CALORIMETRIC STUDY OF THERMOSET RESINS

It is known that experimental parameters may affect peak characteristics in DSC studies. Kinetic parameters calculated from isothermal and dynamic runs, can also be affected by the choice of experimental conditions.The sample mass can affect the determination of reaction parameters because of the heat transport and the self-insulating properties of large masses.In the curing process of thermoset resins like unsaturated polyester crosslinked by styrene, the exothermal heat results from the reaction advance but it also produces an autocatalytic process that induces an increase in the reaction rate. The sample mass, in this kind of experiments, can influence the determination of reaction parameters not only through the effect of heat transport or thermal lag on the calorimetric results but also through the action that the sample mass exerts on the exothermal heat, and consequently, on the autocatalytic process.In this paper we intend to present the results obtained in the study of the curing kinetics of thermoset resins by DSC technique using different sample masses. We will point out that the reaction heat determined by dynamic measurement was found to be independent of the sample mass. We have obtained differences in the kinetic parameters like the reaction order, activation energy and preexponential factor when the sample mass was changed.ZusammenfassungEs ist bekannt, daß experimentelle Parameter die Peak-Charakteristik in DSC-Studien beeinflussen können. Aus isothermen und dynamischen Aufnahmen berechnete kinetische Parameter können außerdem durch die Wahl der Versuchsbedingungen beeinflußt werden.Wegen des Wärmetransportes und den selbstisolierenden Eigenschaften von großen Probenmassen kann die Probenmasse die Bestimmung der Reaktionsparameter beeinflussen. Beim Vernetzungsvorgang von Duroplasten wie z.B. bei der Vernetzung von ungesättigten Polyestern mit Styrol resultiert die exotherme Wärme aus dem Voranschreiten der Reaktion, es entsteht aber auch ein autokatalytischer Prozeß, der den Anstieg der Reaktionsgeschwindigkeit nach sich zieht. In derartigen Experimenten kann sich die Größe der Probenmasse nicht nur über die Beeinflussung des Wärmetransportes oder über thermische Verzögerung auf die kalorimetrischen Ergebnisse auswirken, sondern auch über jenen Einfluß der Probenmasse, den sie auf die exotherme Wärme und auf den autokatalytischen Vorgang nimmt.Es werden hier die Ergebnisse der Untersuchung der Vernetzungskinetik von Duroplasten mittels DSC-Techniken mit unterschiedlichen Probenmassen dargestellt. Dir mittels der dynamischen Messungen ermittelte Reaktionswärme war unabhängig von der Probenmasse. Unterschiede wurden dagegen für die kinetischen Parameter gefunden, wie z.B. für Reaktionsordnung, Aktivierungsenergie, präexponentieller und Faktor.