J.M. Morancho

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.

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.

Relaxation in partially cured samples of an epoxy resin and of the same resin modified with a carboxyl-terminated rubber

Abstract This study examines the relaxation process at different temperatures in partially cured samples of an epoxy resin and of the same resin modified with 11.1% carboxyl-terminated butadiene acrylonitrile copolymer (CTBN). A differential scanning calorimeter (DSC) was used, which enabled the relaxation enthalpy, the glass transition temperature ( T g ) and the relaxation peak temperature to be found. CTBN modifies the cure reaction and the relaxation process of the epoxy resin. The study of the relaxation process has a practical interest in addition to a theoretical one. It is known that, with these thermosets, a fully cured sample cannot be obtained at the first step. The introduction of an elastomeric modifier is a normal way of increasing the toughness of these materials.

Relaxation in a neat epoxy resin and in the same resin modified with a carboxyl-terminated copolymer

In this work we study the effect of a rubber-modifier (CTBN) on the relaxation process of an epoxy resin. CTBN decreases the apparent activation energy and increases the non-linearity parameter (x). It increases the non-exponentiality parameter (β), as we have seen, by means of the normalized upper peak height.

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.

Thermal curing of an epoxy-anhydride system modified with hyperbranched poly(ethylene imine)s with different terminal groups

New hyperbranched polymers (HBP) have been synthesized by reaction of a poly(ethylene imine) with phenyl and t-butyl isocyanates. These HBPs have been characterized by 1H-NMR (nuclear magnetic resonance of hydrogen) and Fourier transform infrared spectroscopy. Their influence on the curing and properties of epoxy-anhydride thermosets has been studied by different techniques: differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermogravimetry (TG). The curing kinetics has been studied with DSC. Integral isoconversional method and the Šesták–Berggren model have been used to determine the activation energy and the frequency factor. The kinetic parameters are very similar for all the studied systems at the middle stage of the process, but changes are observed at the beginning and at the end of the process when these modifiers are used. The HBPs reduce the glass transition temperature of the cured materials. In addition, from the DMA analysis it can be seen that the HBP modifier obtained from phenyl isocyanate hardly changes the storage modulus, but the obtained ones from t-butyl isocyanate decrease it. TG analysis reveals a decrease in the onset temperature of the degradation process upon addition of the HBPs.

Curing of off-stoichiometric amine–epoxy thermosets

The kinetics of the epoxy–amine polycondensation and the epoxy homopolymerization in off-stoichiometric epoxy/amine formulations with excess of epoxy groups, and in the presence of an anionic initiator have been investigated. Diglycidyl ether of bisphenol A (DGEBA) and diethylenetriamine (DETA) have been used as epoxy and amine reagents, respectively, and 2-methylimidazole (2MI) has been used as anionic initiator. This study has been carried out using a differential scanning calorimeter (DSC). The thermal–mechanical properties of the partially cured and fully cured materials with and without initiator have been determined by DSC and dynamic-mechanical analysis. First, off-stoichiometric DGEBA/DETA mixtures with excess of DGEBA, with and without 2MI, have been reacted isothermally at low temperatures, where only the epoxy–amine condensation takes place, because the epoxy homopolymerization has a very low curing rate. Afterward, samples containing 2MI have been heated at different heating rates to study the homopolymerization process of the epoxy excess. The kinetics of both processes have been analyzed with an isoconversional method to determine the activation energy, and the Šesták–Berggren equation has been applied to determine the frequency factor and the orders of reaction. In the isothermal curing, amine–epoxy condensation, the activation energy and the frequency factor decrease with increasing degree of conversion, but in the homopolymerization process, both magnitudes increase with the degree of conversion. Results show that the dual-curing character of off-stoichiometric DGEBA/DETA thermosets with 2MI as anionic initiator renders them suitable for multistage curing processes in which the degree of cure and material properties in the intermediate stage can be controlled easily and final material properties can be enhanced.

Influence of a carboxyl-terminated modifier (CTBN) on the cure of an epoxy resin☆

Abstract Differential scanning calorimetry (DSC) was used to monitor the degree of cure within epoxy-carbonyl-terminated modifier (CTBN) samples cured at 50 ± 1°C. At this temperature, all samples were found to achieve 80% of conversion within 1000 min or less. CTBN accelerates the curing process and modifies the cross-linked network. The DSC was also used to measure the extent of ‘physical aging’ which took place during the cure process.