Josep Maria Salla

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.

Time-Temperature Transformation (TTT) Cure Diagram of an Unsaturated Polyester Resin

SYNOPSIS The curing of an unsaturated polyester resin was studied by differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and Fourier-transform infrared spectroscopy (FTIR). The results are presented in the form of a time-temperaturetransformation (TTT) diagram. The kinetic analysis was performed by means of the dynamic Ozawa method. This analysis was used to determine the curing times (t )a t various conversions (a) and temperatures (T ) (isoconversional lines ln tA A/ E/RT ). The equivalence of the Ozawa method and the isothermal isoconversional adjustment ln t A A / E/RT were demonstrated. The relationship between the glassy transition temperature (Tg) and the conversion a was determined by DSC. It was established that this relationship is one-to-one and independent of mass, initiation system, and curing temperature (Tc). The Tg‐a relationship was adjusted using the DiBenedetto equations and heat capacity data. Using the Tg‐a relationship and the isoconversional lines, the vitrification curve was determined and it was observed that the vitrification times obtained are consistent with those obtained experimentally when TcA Tg. Gelation was determined by TMA, the material being considered gelled when it reached sufficient mechanical stability for the TMA measuring probe to become embedded in it. At that moment the conversion reached was determined by DSC. It was seen that the material always gels at constant conversion, regardless of the curing temperature. The gelation line (gel times) were traced from the corresponding isoconversional line.

Study of lanthanide triflates as new curing initiators for DGEBA

A new class of cationic initiators, lanthanide triflates, has been studied in the curing of diglycidyl ether of bisphenol-A (DGEBA). The kinetics of this process has been evaluated by the isoconversional method that has been proved as an excellent tool to predict the technical parameters of this process. The thermal stability of the crosslinked materials has been studied by thermogravimetric analysis (TGA) and their mechanical characteristics by dynamomechanic thermal analysis (DMTA).

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.

Oxidation-induced strength degradation of WC-Co hardmetals

Abstract The oxidation behavior at 700°C and its influence on room-temperature mechanical strength of three grades of WC–Co cemented carbides containing different binder content were studied. Oxidation kinetics, determined by thermogravimetric measurement, were found to follow a linear time dependence of weight gain per unit area in all the cases. It was also observed to rise with an increase in the oxygen concentration of the atmosphere and a reduction of the cobalt content. Three oxidation times of 10 min, 1 and 6 h were chosen to evaluate changes in the mechanical strength. Fracture resistance of oxidized samples were measured at room temperature under four-point bending and compared to that of non-oxidized specimens. In all cases, a pronounced degradation in strength was observed after oxidation. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics (LEFM). It is conjectured that strength reduction results from a combination of load-bearing section reduction (extrinsic effect) and a concomitant interplay of several factors including increasing size and severity of critical flaws, wedge-effects in the case of heterogeneous oxide penetration and residual tensile stresses associated with volume and thermal differences between the substrate and the oxide scale (intrinsic effects).

Effect of the inhibitor on the curing of an unsaturated polyester resin

Abstract Differential scanning calorimetry has been used to study the influence of hydroquinone as an inhibitor on the curing of an unsaturated polyester resin using benzoyl peroxide as an initiator. The isothermal kinetic analysis was performed by isoconversional adjustment ln t = A + E/RT. This adjustment makes it possible to predict the variations in the activation energy with the degree of conversion without the need to know the rate equations. The results obtained through this adjustment were compared with those obtained by applying an autocatalytic model. The kinetic parameters obtained by both methods permitted the simulation of the curing process inside and outside the experimental range of temperatures. From the experimental results and the kinetic analysis, it was possible to elucidate the dual mechanism of inhibition and retardation exerted by hydroquinone. With the knowledge of the inhibition mechanism, we proposed a method for using the induction times to determine the kinetic parameters associated with the decomposition of the initiator and/or the instantaneous inhibitor content in unsaturated polyester resins.

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.