Effects of small‐amplitude oscillatory shear on polymeric reaction

The effects of small‐amplitude oscillatory shear on the polymeric reaction between poly(butylene terephthalate) (PBT) and epoxy in melts were studied. Two kinds of samples, sandwich sample and blending sample, were prepared for reaction kinetics by using a new rheological method which correlates the change of the rheological property of reactive system with the conversion of the in situ formed copolymers. The results indicated that the reaction between PBT and epoxy was hardly affected by the small‐amplitude oscillatory shear. The increase of apparent reaction rate in sandwich sample was due to the increase of molecular diffusion by oscillatory shear. POLYM. COMPOS., 29:72–76, 2008. © 2007 Society of Plastics Engineers     

Convective and radiant (IR) curing of bulk and waterborne epoxy coatings as thin layers. Part I: Methodology

A method for studying the curing of 100 μm-thick epoxy-based layers coated on steel substrates is described. A simple waterborne epoxy reactive system based on a diglycidylether of bisphenol-A prepolymer and with a polyether triamine was set up by emulsifying the epoxy resin with the use of an ethoxylated-nonylphenol emulsifier. The parliculur phenomena involved during the isothermal curing of such an epoxy emulsion were decomposed by considering the same formulation without water and without water + emulsifier. The chemical kinetics were determined by DSC and by size exclusion chromatography. As previously proposed for numerous epoxy-amine reactive systems, for the three types of coatings, a second-order autocatalytic law was able to interpret the data. The presence of water delays the gelation characterized by rheological measurements. As a consequence, the epoxy conversion at the gel point increases. This phenomenon can be related to the inhomogeneous polymerization process during which the epoxy groups can be trapped into the initial epoxy particles. In addition, the microdielectrometry technique was used to follow the curing degree of the epoxy reactive system of thin layers during the drying and curing processes under radiant heatings such as infrared curing.     

Structural Carbon/Epoxy Prepregs Properties Comparison by Thermal and Rheological Analyses

Two different carbon/epoxy prepreg materials were characterized and compared using thermal (DSC, TGA, and DMA) and rheological analyses. A prepreg system (carbon fiber preimpregnated with epoxy resin F584) that is currently used in the commercial airplane industry was compared with a prepreg system that is a prospective candidate for the same applications (carbon fiber prepreg/epoxy resin 8552). The differences in the curing kinetics mechanisms of both prepreg systems were identified through the DSC, TGA, DMA, and rheological analyses. Based on these thermal analysis techniques, it was verified that the curing of both epoxy resin systems follow a cure kinetic of n order. Even though their reaction heats were found to be slightly different, the kinetics of these systems were nevertheless very similar. The activation energies for both prepreg systems were determined by DSC analysis, using Arrhenius's method, and were found to be quite similar. DMA measurements of the cured prepregs demonstrated that they exhibited similar degrees of cure and different glass transition temperatures. Furthermore, the use of the rheological analysis revealed small differences in the gel temperatures of the two prepreg systems that were examined.     

Crystallization kinetics and morphological behavior of reactively processed PBT/epoxy blends

Polybutylene terephthalate (PBT), a versatile engineering thermoplastic, has been processed using epoxy resin as a reactive solvent. Following processing of this blend, the epoxy was cured using a bi-functional amine curing agent, resulting in phase separation and phase inversion thus producing a different morphology. Change in crystallization kinetics of PBT in the presence of the epoxy monomer and cured epoxy resin has been studied using differential scanning calorimetry. Half time of crystallization (t_1/2) of PBT decreased in the presence of epoxy monomer while it remained constant in the presence of cured epoxy resin. The value of Avrami exponent varied between 1 and 2 in pure PBT as well as for uncured and cured blends, indicating mixed type of spherulitic growth. Morphology of the uncured and cured blends was studied using small angle light scattering (SALS) and polarizing microscopy for samples crystallized at different temperatures at all levels of the epoxy resin. Scattering pattern in H_v and V_v mode of SALS provided information about the type of spherulites as well as volume filling nature of the spherulites. In general, typical unusual type of spherulitic pattern for PBT, in which scattering lobes lie along the polar axis, changed to usual type of pattern for PBT/epoxy blends, in which scattering lobes lie at 45° to the polar axis.     

Monitoring of photopolymerization kinetics and network formation by combined real-time near-infrared spectroscopy and ultrasonic reflectometry

For in situ monitoring of fast changes of shear modulus and chemical conversion during UV radiation curing an ultrasonic (US) reflection method was combined with real-time near infrared (NIR) spectroscopy. The combined setup has been applied to study photopolymerization of different resins as acrylates, epoxy acrylates, acrylated polyurethanes and cationic epoxy resins in order to achieve a deeper understanding of the interdependence of reaction kinetics and changes of mechanical/rheological properties. The simultaneously recorded conversion–time and modulus–time curves allow differentiating between mass and diffusion controlled polymerization regime. Light curing and dark curing phases are indicated by two distinct regions in the conversion–time curves. By rescaling the curing time by chemical conversion modulus–conversion curves were constructed, which are described by combining a viscoelastic relaxation model with the conversion dependence of relaxation times. The NIR-US setup was used to study the influence of chemical composition and curing conditions on the polymer network formation.     

Microscopic morphology,rheological behavior,and mechanical properties of polymers: Recycled acrylonitrile-butadiene-styrene/polybutylene terephthalate blends

In this study, styrene-acrylonitrile-glycidyl methacrylate (SAG) series copolymers were specially designed for producing the recycled acrylonitrile-butadiene-styrene (rABS)/poly(butylene terephthalate) (PBT)/SAG blends, which were prepared through the process of continuous melt blending and batch feeding. The effects of viscosity composition, SAG chemical composition, and SAG content on the morphology, and rheological and mechanical properties of the blends have been investigated. As demonstrated by morphological observation, the variety of viscosity composition of the blends affects the size of dispersed PBT droplets. Moreover, high viscosity of rABS matrix seems to facilitate the formation of smaller dispersed phase size of blends. Various SAG chemical compositions have different stabilities on the morphology of the blends, which affects the deformation, fragmentation, and coalescence of dispersed phase droplets. In addition, a finer phase morphology can be achieved when the density distribution of epoxy group is optimal in SAG copolymer. Rheological characterization manifested that the rheological properties of the blends depends strongly on its composition and structure, while the crosslinking degree is associated with the concentration of reactive groups and extent of reaction. Thereby, the rheological behavior of the blends during processing can be controlled by changing the reactive sequence and adding the quantity of epoxy group. The test on mechanical properties verified that a recycled product with excellent performance can be obtained by altering processing methods and the blends formula, which may be further applied to the 3D printing materials required by fused deposition modeling technology. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48310.     

Convective and radiant (IR) curing of bulk and waterborne epoxy coatings as thin layers. Part II: Infrared curing

In the first part of this paper, a method for studying the curing of 100-μm-thick epoxy-based layers on steel substrates was described. In this second part, we extend our study to radiative (infrared) heating conditions. A simple waterborne epoxy reactive system based on a diglycidylether of bisphenol-A prepolymer and a polyether triamine was used. The chemical kinetics during infrared heating were studied with the reaction kinetics recorded during convective heating. A second order autocatalytic model described the data. The curing mode had no significant influence on the heating kinetics but the heating rate had a significant influence on the curing kinetics. Higher heating rates can be achieved by infrared heating conditions compared with convective heating. The gelation phenomenon was changed for infrared conditions. Microdielectrometry can be used to record in situ the extent of reaction of the epoxy reactive system for thin layers during radiant heating.     

Chemical Recycling and Kinetics of Aqueous Alkaline Depolymerization of Poly(Butylene Terephthalate) Waste

Depolymerization reactions of poly(butylene terephthalate) (PBT) waste in aqueous sodium hydroxide solution were carried out in a batch reactor at 80–140 °C at atmospheric pressure by varying PBT particle size in the range of 50–512.5 μm. Reaction time was also varied from 10–110 min to understand the influence of PBT particle size and reaction time on the batch reactor performance. Agitator speed, particle size of PBT and reaction time required were optimized. Disodium terephthalate (salt) and 1,4‐butanediol (BD) remain in the liquid phase. BD was recovered by the salting‐out method. Disodium terephthalate was separated by acidification to obtain solid terephthalic acid (TPA). The produced monomeric products (TPA and BD) and PBT were analyzed. The yields of TPA and BD were in agreement with PBT conversion. The depolymerization reaction rate was first order to PBT concentration as well as first order to sodium hydroxide concentration. The acid value of TPA changes with the reaction time as well as particle size of PBT. This indicates that PBT molecules get fragmented and hydrolyze simultaneously with aqueous sodium hydroxide to produce BD and disodium terephthalate. Activation energy, Arrhenius constant, equilibrium constant, Gibbs free energy, enthalpy and entropy were determined. The dependence of the hydrolysis rate constant on reaction temperature was correlated by the Arrhenius plot, which shows an activation energy of 25 kJ/mol and an Arrhenius constant of 438 L/min/cm².     

Chemical shrinkage and diffusion-controlled reaction in a cresol novolac epoxy

The reaction kinetics with a diffusion control mechanism, as well as the volumetric change upon curing, of a cresol novolac epoxy/o-cresol-formaldehyde novolac hardener system were studied. Simple equations to model the change in linear coefficients of thermal expansion with reacting thermosetting system conversion were also derived. Based on the heat of the reaction of monomeric monofunctional model compounds, the true degree of conversion of this crosslinking epoxy system can be obtained. The reaction is then modeled as a reaction of shifting order: it first reacts autocatalytically and later switches into diffusion control. The reaction in the diffusion-controlled region can be modeled by an n-th order kinetic equation with its rate constant described by a WLF-type equation. Both experimental linear coefficients of thermal expansion above and below the glass transition temperature decrease linearly with the degree of conversion, which agrees with the derived equations. The importance of chemical shrinkage upon curing is also discussed.     

Influence of epoxy resin on the morphological and rheological properties of PBT/ABS blends compatibilized by ASMA

Polybutylene terephthalate (PBT)/acrylonitrile–butadiene–styrene (ABS) copolymer blends compatibilized by a mixture of styrene–acrylonitrile–maleic anhydride (ASMA) copolymers and epoxy resin (EP) were prepared through melt reactive extrusion. The morphological, rheological, and mechanical properties of these blends were studied. The epoxy functional groups of EP can react with anhydride groups of ASMA and the PBT terminal groups (? OH and ? COOH) simultaneously, leading to the formation of ASMA–EP–PBT graft copolymers. Because of the effective compatibilization of these copolymers at the interface, finer dispersed phase morphologies were obtained. Compared with PBT/ABS/ASMA blends, the addition of EP induced a more stable molten phase structure, with increases of storage moduli, loss moduli, and dynamic viscosities. Results indicated that 1.5 wt% of the EP in the blends was most suited for the compatibilization. Impact properties of these blends were also investigated. POLYM. ENG. SCI., 47:1943–1950, 2007. © 2007 Society of Plastics Engineers     

Rheokinetics of the cross‐linking of melt polyethylene initiated by peroxide

The cross‐linking reaction of polyethylene/peroxide system was studied at elevated temperature. The dynamic modulus evolution was monitored within linear viscoelastic regime by parallel plate rheometer. Different strains were chosen to represent different flow fields. Based on the mechanism of cross‐linking reaction, a new expression relating recombination rate constant with rheological conversion was derived. The experimental and calculated results showed that the strain had substantial effects on the cross‐linking reaction kinetics. Consequently a formula reflecting the effect of the complex viscosity on recombination rate constant was found, which was further integrated into the classical Arrhenius kinetic equation. POLYM. ENG. SCI., 45:560–567, 2005. © 2005 Society of Plastics Engineers     

Effect of Tertiary Amines on Reactivity of Cycloaliphatic Epoxy Resins towards Methacrylic Acid

The kinetics of the esterification of two cycloaliphatic epoxy resins containing glycidyl and epoxy cylohexane group, separately was studied using methacrylic acid in presence of triethyl-amine. The reaction was performed as a function of temperature to determine various kinetics and thermodynamic parameters such as reaction order, reaction rate constant, activation energy, frequency factor, entropy, enthalpy and free energy of the reaction. The reaction order was determined to be 2 and reaction rate constant for the resin containing epoxy cyclohexane group was found higher than that containing glycidyl epoxy group. The reaction was not found to proceed after ∼76% conversion due to the formation of adduct between nitrogen of amine and hydrogen of hydroxyl group of esterified resin. The rate of esterification reaction studied using higher homologue of triethyl-amine (tripropyl- and tributyl-) was found lower than that by triethyl-amine and conversion of acid into ester was ∼80% and ∼83% for tripropyl- and tributyl-amine, respectively due to the steric accessibility of nitrogen atom of higher amine. Based upon these kinetic data a suitable reaction mechanism is proposed for the system.     

基于变活化能模型的T800/环氧树脂预浸料固化反应动力学研究

为了深入了解某新型高温固化T800/环氧树脂预浸料的固化行为,借助差示扫描量热仪(DSC),采用非等温DSC法研究了T800/环氧树脂预浸料的固化反应过程。基于唯象模型,系统研究了该预浸料的固化反应特征温度及固化动力学参数,确定该预浸料中环氧树脂的固化反应动力学模型为自催化模型。采用等转化率法,分析了预浸料中环氧树脂的反应活化能随固化度的变化情况,结果表明在整个固化反应过程中,树脂固化反应活化能变化较大,传统模型法基于全固化过程活化能不变的假设无法准确描述该固化反应。采用变活化能自催化模型,利用粒子群全局优化算法,得到了T800/环氧树脂预浸料的固化动力学方程,结果表明该模型能较好地描述实验现象,可为进一步研究该预浸料的热力学性能及其成型过程中的质量控制提供理论基础。     

催化剂和扩链剂对RIM聚氨酯流变性的影响

用自行设计的绝热流变仪研究ＲＩＭ聚氨酯的流变性。绝热流变仪可同时测得精度－时间，温度－时间的关系。把温度与反应程度关联在，可理解为反应动力学与反应体系流变性的关系。相分离ＲＩＭ－ＰＵＲ的绝热流变测试表明有机锡类催化剂与胺类催化剂复合使用具有协同痊效应。二胺类扩链二醇类扩链剂反应速度更快，凝胶时间更短但转化率不高。     

环氧树脂/液晶固化剂固化反应动力学研究

通过差热分析 (DSC)研究了非等温过程环氧树脂 /液晶固化剂体系的固化反应动力学 ,研究了不同配比对固化反应的影响 ,固化反应转化率与固化温度的关系 ,计算了固化反应的活化能 ,确定了环氧树脂 /液晶固化剂的固化工艺条件 ,用偏光显微镜观察了环氧树脂 /液晶固化剂 / 4 ,4′ -二氨基二苯砜 (DDS)体系在不同温度下固化时的形态。结果表明 :液晶固化剂的加入量越大 ,固化反应速度越快 ;环氧树脂 /液晶固化剂体系固化反应的活化能为 71 5kJ/mol;偏光显微镜观察表明 :随着固化起始温度的增加 ,固化体系的形态由原来的具有各向异性的丝状结构变化为各向同性 ,液晶丝状条纹消失。     

Rubber modified epoxy resin. I: Cure kinetics and chemorheology

The curing and vitrification effect during the reaction of ATBN modified epoxy resin was studied through the dynamic differential scanning calorimetry method and a new reaction kinetic equation containing generalized WLF equation was developed to describe the reaction rate at both glassy and rubbery state. An autocatalytic mechanism was found to describe adequately the cure kinetics of the rubber modified epoxy resin and the overall order of reaction was assumed to be 2. The kinetic parameters were determined from the DSC data through the Marquardt's multivariable nonlinear regression method and Runge–Kutta integration technique. The presence of rubber indicated minor effect on the cure kinetics of epoxy resin. The Arrhenius type viscosity function was employed to establish a relationship between viscosity data measured by RMS and chemical conversion calculated from the reaction kinetic equation.     

Chemical interpretation of the relaxed permittivity during epoxy resin cure

The isothermal cure of a diglycidyl ether of Bisphenol-A (DGEBA) epoxy resin with diaminodiphenylsulfone has been characterized by microdielectrometry and differential scanning calorimetry. The cure temperatures ranged from 410 to 460K. The behavior of the relaxed (or static) dielectric permittivity vs. cure time and temperature was determined from the microdielectrometry data. The DSC data was fit to an autocatalyzed reaction kinetics model, which was then used to predict reactive group concentrations as a function of cure time and temperature. The temperature dependence of the relaxed permittivity at constant chemical conversion was examined in the context of the Onsager theory for the relaxed permittivity of a system of independent dipoles. This analysis indicated that the dipoles in the resin are not independent, as assumed by the Onsager theory, and that the behavior is similar to that observed in polyethers. An empirical modification to the Onsager theory was used in conjunction with the kinetic model to estimate dipole moments for the epoxide, primary amine, and reacted (secondary and tertiary) amine groups. The relative and absolute values of the dipole moments were in good agreement with estimates based on the structures, leading to the conclusion that the observed decrease of the relaxed permittivity during cure of this epoxy/amine system is due to the changing concentrations of polar reactive groups.     

A study on the cure characteristics of epoxy/diaminodiphenylmethane system prepared at room temperature

The curing behavior and kinetics of epoxy resin with diaminodiphenylmethane (DDM) as the curing agent was studied by many researchers, however all of them prepared the system at a high‐temperature condition (i.e., T ≥ 80°C). In this study, a mixture of epoxy/DDM was prepared at ambient temperature and its curing characteristics were studied by using differential scanning calorimetry (DSC). The autocatalytic model was used to calculate the kinetic factors in the dynamic experiments. The kinetics of the curing reaction was also evaluated by two different isoconversional models; namely Friedman method and the Advanced Isoconversional method proposed by Vyazovkin to investigate the activation energy behavior during the curing reaction. The activation energy of the curing reaction was found to be in the range of 48 ± 2 kJ/mol and might be considered to be constant during the curing. In fact, our findings were different from the result reported by other researchers for the system which was prepared at elevated temperature. Therefore, it seems that the preparation temperature of the samples influenced considerably on the curing behavior of epoxy with DDM. Finally, a time–temperature–transformation (TTT) diagram was established to determine the cure process and glass transition properties of the system. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

MONITORING THE SURFACE TENSION OF REACTIVE EPOXY-AMINE SYSTEMS UNDER DIFFERENT ENVIRONMENTAL CONDITIONS

Two commercially available amine-cured epoxy resin formulations were studied under different environmental conditions with regard to the surface tension evolution using axisymmetric drop shape analysis (ADSA). By employing a new strategy, ADSA was used to monitor simultaneously the surface tension and the density of these reactive mixtures from sessile drops. The kinetics of the bulk reactions were quantified by Fourier transform infrared (FTIR) spectroscopy, and the changes in the molecular composition of the surface region were studied by X-ray photoelectron spectroscopy (XPS).

In both formulations, the surface tension values of the amine hardeners were lower than those of the epoxy resins. For one system, the surface tension of the mixture was similar to the surface tension of the hardener. In this case, the hardener migrates to the surface and determines the surface tension of the mixture, as could be proved by XPS measurements. In the other case, the surface region contained only a very small amount of nitrogen, indicating that the nitrogen-containing groups of the hardener were not enriched in the surface region of this mixture. Its surface tension was similar to that of the pure epoxy resin.

In a controlled argon atmosphere, the surface tension of the reactive epoxy-amine systems considered here changed very little as the curing reaction proceeded. The time-dependent changes of the surface tension of the mixtures were caused by environmental factors, particularly the presence of carbon dioxide and water. Such factors can produce complicated surface tension responses due to surface reactions with the amine hardener. The extent of these changes can be controlled by the migration of the hardener to the surface region.