Kinetics of thermoset cure and polymerization in the glass transition region

A theoretical approach to thermoset cure kinetics based on Arrhenius kinetics and mobility was developed by considering the activation of the reacting group and chain mobility as elementary steps for reaction. This extended kinetic equation was successfully applied to the curing of an epoxy by an amine, the trimerization of a cyanate, and to the polymerization of methyl methacrylate. Full agreement between theory and experimental data was obtained in all cases. The activation energies for chain mobility were exceptionally low (0.3–1 kJ/mol for bisphenol-A-based epoxy and cyanate) which indicates that the structural units must undergo only small-angle rotational oscillations to allow a reaction. A theoretical time–temperature–transformation (TTT) diagram is also presented. © 1993 John Wiley & Sons, Inc.     

Vulcanization kinetics of graphene/styrene butadiene rubber nanocomposites

This paper presents the influence of graphene on the vulcanization kinetics of styrene butadiene rubber （SBR） with dicumyl peroxide. A curemeter and a differential scanning calorimeter were used to investigate the cure kinetics, from which the kinetic parameters and apparent activation energy were obtained. It turns out that with increasing graphene loading, the induction period of the vulcanization process of SBR is remarkably reduced at low graphene loading and then levels off; on the other hand, the optimum cure time shows a monotonous decrease. As a result, the vulcanization rate is suppressed at first and then accelerated, and the corresponding activation energy increases slightly at first and then decreases. Upon adding graphene, the crosslinking density of the nanocomposites increases, because graphene takes part in the vulcanization process.     

Characterization of alkyd based thermosetting resins

The rate of cure of different oxidative drying oil modified alkyd resins are investigated by DSC, when the cure is made under UV light or not. We determine, from the Kissinger equation, the apparent activation energy at different stages of the curing process. This activation energy depends on the curing conditions (temperature, illumination or not). These variations lead to the determination of a time constant, characteristic of the curing kinetics. Two particular values of the apparent activation energy are also defined. The first one called ΔE _o is a characteristic of the varnish at the liquid state, the second one called ΔE _inf characterizes the stability of the structural state of the final film.     

Effect of the segmental mobility restriction on the thermoset chemical kinetics

The cure kinetics of an epoxy–amine commercial thermoset system have been investigated with the isothermal differential scanning calorimetry technique. In particular, a kinetic study has been performed in the glass–transition zone, in which diffusion phenomena compete with the chemical transformations and the overall reaction rate is partially slowed by the reduced segmental chain mobility. A generalized form of the Vogel equation has been formulated to account for the effect of the increasing glass–transition temperature on the chain mobility and, therefore, on the overall reaction rate. The kinetic model has been expressed with two factors representing the chemical reaction rate and the segmental mobility reduction. As the main result, the activation energy relative to the diffusion phenomena has been found to be very low, having a value of 42.5 K ≈ 0.356 kJ/mol, which is compatible only with the small‐angle rotation of the reactive unit. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3757–3770, 2002     

Effect of Non-Woven Carbon Nanofiber Mat Presence on Cure Kinetics of Epoxy Nanocomposites

Summary : An investigation was carried out into the cure kinetics of carbon nanofiber (CNF) mat-epoxy nanocomposites, composed of bisphenol-A based epoxy resin and diethylene triamine as a curing agent. It was observed that the rate of cure reaction for CNF mat-epoxy nanocomposites was higher than that for neat epoxy resin at low curing temperatures and the presence of the CNF mat produced the maximum influence at a certain curing temperature and time. At high curing temperature and long curing times, the effect of CNF mat on the cure rate was insignificant. The CNF mat-epoxy composite exhibited somewhat lower value of activation energy than that of the neat epoxy system at the beginning of the curing stage. The weight fraction of CNF mat also affected the cure reaction of epoxy nanocomposites at the same curing temperature. As the amount of CNF mat increased, the cure rate was higher at the same cure time. However, at high CNF mat loading, the cure reaction was retarded since the amount of epoxy and hardener decreased dramatically at high CNF contents together with the hindering effect of the CNF mat on the diffusion of epoxy resin and the curing agent, leading to lower crosslinking efficiency. Although the curing efficiency of epoxy nanocomposites dropped at high CNF mat content, the glass transition temperature (T_g) was still high due to the ultra-high strength of the CNF mat. The cure kinetics of CNF mat-epoxy nanocomposites was in good agreement with Kamal's model.     

Investigation of cure kinetics of epoxy resin/organic montmorillonite system by differential scanning calorimetry

The cure kinetics of the epoxy resin (EP)/organic montmorillonite, with 4-diamino diphenyl methane (DDM) as curing agent, was studied by non-isothermal differential scanning calorimetry (DSC) at four linearly programmed heating rates of 5, 10, 15, and 20 deg/min, and the effects of original montmorillonite (OMMT) on cure kinetics of epoxy resin were investigated. A two parameter (m, n) autocatalytic model was used to describe the cure kinetics of the epoxy resins. The kinetic parameters were calculated with the Malek method and the curves obtained by the Málek method showed a good agreement with experimental data for EP/DDM and EP/DDM/OMMT systems. The results, based on the isoconversional method showed that the activation energy was obvious difference with the addition of OMMT in the early stages of the cure, which indicated that the OMMT have catalytic effect on the epoxy ring-opening.     

Effect of the substituted benzene group on thermal and mechanical properties of epoxy resins initiated by cationic latent catalysts

Epoxy resins (DGEBA) were cured by cationic latent thermal catalysts, that is, N‐benzylpyrazinium hexafluoroantimonate (BPH) and N‐benzylquinoxalinium hexafluoroantimonate (BQH) to investigate the effect of substituted benzene group on cure kinetics and mechanical properties of epoxy system. Differential scanning calorimetry (DSC) was undertaken for activation energy of the system. It was also characterized in terms of flexural, fracture toughness, and Izod impact strengths for the mechanical tests. As a result, the cure reaction of both epoxy systems resulted in an autocatalytic kinetic mechanism accelerated by hydroxyl groups. Also, the conversion and cure activation energy of the DGEBA/BQH system were higher than those of DGEBA/BPH system. The mechanical properties of the DGEBA/BQH system were also superior to those of the DGEBA/BPH system, as well as the morphology. This was probably due to the consequence of the effect of the substituted benzene group of the BQH catalyst, resulting in increasing the crosslinking density and structural stability in the epoxy system studied. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2419–2429, 2004     

Cure kinetics of the ring‐opening metathesis polymerization of dicyclopentadiene

The cure kinetics of polydicyclopentadiene prepared by ring‐opening metathesis polymerization with three different concentrations of Grubbs' catalyst were examined with differential scanning calorimetry. The experimental data were used to test several different phenomenological kinetic models. The data were best modeled with a model‐free isoconversional method. This analysis revealed that the activation energy increased significantly for degrees of cure greater than 60%. The catalyst concentration had a large effect on the cure kinetics. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2373–2383, 2002     

气相生长碳纤维对环氧树脂固化反应的影响及其复合物固化动力学研究

用示差扫描量热方法研究了气相生长碳纤维作为填料对4,4′-二氨基二苯甲烷四缩水甘油环氧树脂(TGDDM)/4,4′-二氨基二苯基砜(DDS)等温固化反应的影响.与纯环氧树脂一样,气相生长碳纤维复合物的固化反应也属于自催化反应类型.气相生长碳纤维对环氧树脂的固化反应动力学影响很小.固化反应的过程可以用一种修正过的自催化动力学模型来描述,在整个固化反应过程中纯TGDDM/DDS环氧树脂及其气相生长碳纤维复合物模型拟合得到的结果和实验数据相当一致.     

Curing kinetics of epoxy resins with hyperbranched polyesters as toughening agents

The effects of hyperbranched polyesters on the cure kinetics of diglycidyl ether of bisphenol A (DGEBA) in the presence of m‐phenylene diamine were investigated with nonisothermal differential scanning calorimetry. The results showed that the addition of hyperbranched polyesters enhanced the cure reaction of DGEBA with m‐phenylene diamine, and this resulted in a reduction of the peak temperature of the curing curve and the activation energy because of the low viscosity and large number of terminal hydroxyl groups. However, when linear poly(ethylene glycol) was added, the activation energy of the blends also slightly decreased, whereas the peak temperature of the curing curve increased. The curing kinetics of the blends were calculated by the isoconversional method of Málek. The two‐parameter autocatalytic model (i.e., the ?esták–Berggren equation) was found to be the most adequate for describing the cure kinetics of the studied systems. The obtained nonisothermal differential scanning calorimetry curves showed results in agreement with those theoretically calculated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2649–2656, 2004     

Cure kinetics of liquid crystalline epoxy resins based on biphenyl mesogen

The cure kinetics of a biphenyl-based liquid crystalline (LC) epoxy resin (LCER) was studied using differential scanning calorimetry (DSC) and polarized optical microscopy. The effects of LC phase formation on the cure kinetics were investigated. Both a model-free isoconversional method and a model-fitting method were used to analyze the DSC data. Results from the isoconversional analysis were applied to develop tentative multi-step kinetic models describing the curing reaction. Kinetic analysis showed that compared to the resins cured in amorphous phase, LCERs exhibited higher values of reaction enthalpy and a complex dependence of activation energy on the degree of cure. The formation of the LC phase resulted in a decrease in activation energy, leading to higher degree of reaction.     

Rheological study on the cure kinetics of two-component addition cured silicone rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (t _gel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (t _nc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to t _nc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than t _nc, such as to the end of reaction (t _e), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (k _r) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lnt _gel, lnt _nc, and lnk _r versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.     

Different catalytic systems on hydroxyl-terminated GAP and PET with poly-isocyanate: Curing kinetics study using dynamic in situ IR spectroscopy

Reactions of hydroxyl-terminated glycidyl azide polymer (GAP) or poly(ethylene oxide-co-tetrahydrofuran) (PET) polymers with poly-isocyanate (N100) were monitored by dynamic in situ Fourier transform infrared spectroscopy. The influence of catalytic systems on the cure kinetics of polyurethane reaction was investigated. From the comparison between GAP/N100 and PET/N100 systems, it was found that primary and secondary hydroxyl groups were differentiated due to the effects of steric hindrance. Using Arrhenius law and Eyring equation, the activation parameters of polyurethane reaction were calculated at different catalytic systems. The negative value of the activation entropy demonstrated an associative mechanism within the transition state.     

Oxidative destruction of polymers under mechanical load

An influence of tensile stress on the rate of oxidative destruction of polydienes and polyolefins under the action of ozone has been found; the influence of molecular and submolecular parameters and macrochain segmental mobility on the kinetics of oxidation of polymers is discussed. It is shown that chemical processes, with sp³ to sp² carbon atom rehybridization in the main chain, can change the reactivity of molecules under the action of structural deformations. Both acceleration and deceleration of bimolecular reactions have been found.     

Curing reaction of o-cresol novolac epoxy resin according to hardener change

The investigations of cure kinetics and glass transition temperature (T_g) versus reaction conversion (α) of o-cresol novolac epoxy resin with the change of hardener were performed. All kinetic parameters of the curing reaction such as the reaction rate order, activation energy, and frequency factor were calculated. The curing mechanisms were classified into two types. One was an autocatalytic mechanism and the other was a nth order kinetic mechanism. The constants related to the chain mobility of polymer segments were obtained by using the DiBenedetto equation. We have tried to correlate the relationships between curing mechanism and molecular structures of hardeners from these results. © 1993 John Wiley & Sons, Inc.     

Cationic cure kinetics of a polyoxometalate loaded epoxy nanocomposite

The reaction cure kinetics of a novel polyoxometalate (POM) loaded epoxy nanocomposite is described. The POM is dispersed in the epoxy resin up to volume fractions of 0.1. Differential scanning calorimetry measurements show the cure of the epoxy resin to be sensitive to the POM loading. A kinetics study of the cure exotherm confirms that POM acts as a catalyst promoting cationic homopolymerization of the epoxy resin. The cure reaction is shown to propagate through two cure regimes. A fast cure at short time is shown to be propagation by the activated chain end (ACE) mechanism. A slow cure at long time is shown to be propagation by the activated monomer (AM) mechanism. The activation energies for the fast and slow cure regimes agree well with other epoxy based systems that have been confirmed to propagate by the ACE and AM mechanisms.© 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Rheokinetic cure characterization of epoxy–anhydride polymer system with shape memory characteristics

An epoxy resin capable of exhibiting shape memory property was derived by curing diglycidyl ether of bisphenol A (DGEBA) with a blend of carboxy telechelic poly(tetramethyleneoxide) (PTAC) and pyromellitic dianhydride (PMDA). The cure kinetics of DGEBA/PTAC/PMDA blend of varying compositions was investigated using isothermal rheological analysis. The overall reaction conformed to a second-order autocatalytic model. The kinetic parameters including reaction order, kinetic constants and activation energy were determined. The results showed that increase of PTAC decreased the overall activation energy and frequency factor of the cure reaction. This effect resulted in a diminution of the overall rate of curing. The catalysis by PTAC has its origin from the activation of epoxy groups by the protons of the COOH groups. The autocatalysis was caused by the COOH groups generated by the reaction of alcohol groups with anhydride. The activation energy for the autocatalysis was more than that for the primary reaction as the COOH groups responsible for autocatalysis were generated on a sterically hindered polymer backbone. The kinetics helped generate a master equation conforming to second-order autocatalytic model that could predict the cure profile of a specified resin system at a given temperature, leading to cure optimization.     

Effect of carbon on sulfuration under a binary accelerator system

The continuous measurements of cure characteristics were monitored with a Monsanto rheometer and compared with the volume swell measurement of cross-link density in benzene. Both methods of measurement indicated the formation of a complex of MBT and DPG in equimolar proportions, confirmed by the chemical probe technique and activation energy data. Presence of HAF did not alter vulcanization; that is, HAF has almost no qualitative effect on the kinetics of sulfuration but some on the state of the cure, optimum cure time, and onset and rate of vulcanization. Activation energy remained unchanged by the addition of HAF black.