Curing of unsaturated polyester resins—effects of comonomer composition. III. Medium-temperature reactions

The effects of comonomer composition on the curing kinetics of unsaturated polyester (UP) resins at 70–90°C were studied by differential scanning calorimetry (DSC) and infrared (IR) spectroscopy over the whole conversion range. One commercial UP resin, UP2660PF2, with cobalt promoter added and with 8.85 unsaturated C?C bonds per polyester molecule, was used. It was found that a marked shoulder in the initial DSC rate profile, rather than that reported after the peak of rate profile for low-temperature and high-temperature reactions, appeared when the molar ratio of styrene to polyester C?C bonds was greater than 1. With the initiator system accelerated by cobalt promoter, the formation rate of microgel particles would be enhanced at the early stage of reaction, as supported by the much higher conversion of polyester C?C bonds than that of styrene by IR spectroscopy. Those relatively greater number of microgel particles tended to facilitate the intramicrogel crosslinking reactions, which would be independently identified from the initial DSC rate profile as a shoulder. Consequently, the reaction mechanism was elucidated by decomposing the reaction rate profile into two individual profiles accounting for the intramicrogel dominated and the intermicrogel dominated crosslinking reactions, respectively. © 1993 John Wiley & Sons, Inc.     

Effects of poly(vinyl acetate) and poly(methyl methacrylate) low-profile additives on the curing of unsaturated polyester resins. I. Curing kinetics by DSC and FTIR

The effects of two low-profile additives (LPA), poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) on the curing kinetics during the cure of unsaturated polyester (UP) resins at 110°C were investigated by using a differential scanning calorimeter (DSC) and a Fourier transform infrared spectrometer (FTIR). The effects of temperature, molar ratio of styrene to polyester CC bonds, and LPA content on phase characteristics of the static ternary systems of styrene–UP–PVAc and styrene–UP–PMMA prior to reaction were presented. Depending on the molar ratio of styrene to polyester CC bonds, a small shoulder or a kinetic-controlled plateau in the initial portion of the DSC rate profile was observed for the LPA-containing sample. This was due to the facilitation of intramicrogel crosslinking reactions since LPA could enhance phase separation and thus favor the formation of clearly identified microgel particles. FTIR results showed that adding LPA could enhance the relative conversion of polyester CC bonds to styrene throughout the reaction. Finally, by use of a microgel-based kinetic model and static phase characteristics of styrene–UP–LPA systems at 25°C, the effects of LPA on reaction kinetics regarding intramicrogel and intermicrogel crosslinking reactions, relative conversion of styrene to polyester CC bonds, and the final conversio have been explained. © 1995 John Wiley & Sons, Inc.     

Effects of poly(vinyl acetate) and poly(methyl methacrylate) low-profile additives on the curing of unsaturated polyester resins. II. Morphological changes during cure

The effects of two low-profile additives (LPA), poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA), on the morphological changes during the cure of unsaturated polyester (UP) resins at 110°C were investigated by an approach of integrated reaction kinetics-morphology-phase separation measurements by using a differential scanning calorimeter (DSC), scanning electron microscopy (SEM), optical microscopy (OM), and a low-angle laser light-scattering appartus (LALLS). For the UP resins cured at 110°C, adding LPA could facilitate the phase separation between LPA and crosslinked UP phases early in the reaction, and discrete microgel particles were thus allowed to be identified throughout the reaction. Microvoids and microcracks responsible for the volume shrinkage control could also be observed evidently at the later stage of reaction under SEM. Depending on the types of LPA and the initial molar ratios of styrene to polyester C?C bonds, the morphological changes during the cure varied considerably. The progress of microstructure formation during reaction has been presented. Static ternary phase characteristics for the styrene–UP–LPA system at 25°C have also been employed to elucidate the resulting morphology during the cure in both the continuous and the dispersed phases. © 1995 John Wiley & Sons, Inc.     

Curing of unsaturated polyester resins—effects of pressure

The effects of pressure ranging from 0.1 to 6.21 MPa (0–900 psig) on the curing of unsaturated polyester resins at 110°c were investigated by an approach of integrated reaction kinetics-rheology-morphology measurements using a pressure differential scanning calorimeter (DSC), an infrared spectrophotometer (IR), a Haake rheometer, and by using scanning electron microscopy (SEM). Increasing pressure was found to delay the gel effect, and a previously unknown plateau of kinetic-controlled region in the initial portion of the DSC rate profile was observed. The plateau region was mainly attributed to the crosslinking of C ? C double bonds inside the microgel particles, as revealed by the conversions of styrene and polyester C ? C bonds measured by IR, gel conversion data and SEM micrographs. The mechanisms of reaction kinetics both at atmospheric pressure and under pressures have also been elucidated by the progress of buildup of microgel structures.     

Influence of initiator on the curing of unsaturated polyester resin at 100 °C

Sheet molding compound (SMC) parts are fiber‐reinforced unsaturated polyester (UP) composites molded at 140–170 °C under a pressure of 60–100 bar. For economic and ecological reasons, the aim of this research project is to develop new SMC formulations to modify the molding conditions. For this, SMC formulations were modified and optimized to decrease the molding temperature to 100 °C. The strategy was to change the catalytic system (peroxides) in order to obtain highly reactive formulations at 100 °C. First, the temperatures of initiation of the reaction were determined by rheological and DSC measurements for each peroxide. Second, the UP resin crosslinking kinetics were measured for the various peroxides during an isothermal curing at 100 °C. The results obtained with the three experimental methods are compared and discussed. Finally, the laboratory analyses were validated by SMC molding trials. © 2019 Society of Chemical Industry     

Effects of chemical structure of polyurethane‐based low‐profile additives on the miscibility,curing behavior,volume shrinkage,glass transition temperatures,and mechanical properties for styrene/unsaturated polyester/low‐profile additive ternary systems. I: Miscibility,curing behavior,and volume shrinkage

The effects of chemical structure and molecular weight of three series of thermoplastic polyurethane‐based (PU) low‐profile additives (LPA) on the miscibility of styrene (ST)/unsaturated polyester (UP) resin/LPA ternary systems prior to reaction were investigated by using the Flory‐Huggins theory and group contribution methods. The reaction kinetics during the cure at 110°C and the cured sample morphology were also studied by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. The phase‐separation characteristics of ST/UP/LPA systems during the cure, as revealed by the cured‐sample morphology, and the DSC reaction‐rate profile, could be generally predicted by the calculated upper critical solution temperature for the uncured ST/UP/LPA systems. Finally, based on the measurements for volume change and microvoid formation, volume shrinkage characteristics for the cured ST/UP/LPA systems have been explored. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 543–557, 2000     

Thermal cure behavior of unsaturated polyester/phenol blends

Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to detect and simulate the cure behavior of unsaturated polyester (UP), phenol, and UP/phenol blends and to calculate and predict the cure rate, cure temperature, conversion, and changes in the glass‐transition temperature along with various cure orders in order to obtain the optimum parameters for processing. With dynamic scanning and isothermal DSC procedures and Borchardt–Daniels dynamic software, cure data for the UP resin were obtained, 90% of the conversion rate at 100°C being achieved after 15 min. However, for the phenol and UP/phenol blends, gradually increasing the temperature was found to be best for curing according to the DSC and DMA test results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1041–1058, 2004     

Effects of resin chemistry on redox polymerization of unsaturated polyester resins

Unsaturated polyester resins are the most widely used thermoset resins in the composite industry. In this study, three well‐defined unsaturated polyester resins were used. These resins have similar number‐average molecular weights, and they have different numbers of C?C bonds per molecule. The reaction kinetics of unsaturated polyester resins was studied using a differential scanning calorimeter (DSC) and a Fourier transform infrared (FTIR) spectrometer. The glass transition temperature of the isothermally cured resin was also measured. Trapped radicals were observed in the cured polyester resin from electron spin resonance (ESR) spectroscopy. Considering the diffusion‐limitation effect, a simple kinetic model was developed to simulate the reaction rate and conversion profiles of polyester vinylene and styrene vinyl groups, as well as the total reaction rate and conversion. Experimental results from DSC and FTIR measurements compare favorably with the model prediction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 211–227, 2002; DOI 10.1002/app.10317     

Cure kinetic study of organoclay‐unsaturated polyester resin nanocomposites by using advanced isoconversional approach

Curing behavior of an unsaturated polyester (UP) resin containing 3 wt % Cloisite 10A (UP/10A) and 3 wt % Cloisite 30B (UP/30B) catalyzed with methyl ethyl ketone peroxide (MEKP) as initiator and promoted by cobalt naphthenate as accelerator was investigated by dynamic differential scanning calorimetry (DSC) at heating rates of 2, 2.5, 3, and 3.5°C min⁻¹. X‐ray diffraction and transmission electron microscopy were utilized to evaluate the morphology of UP/10A and UP/30B nanocomposites. Kinetic parameters of cure reactions were evaluated using the advanced isoconversional method. The addition of nanoclay resulted in a decrease in the activation energy of the redox reaction compared to that of the neat UP resin. The pre‐exponential factor of the redox reaction for UP/10A and UP/30B was less than that of the neat UP. Results showed an increase in the concentration of styrene between Cloisite 10A platelets leading to a decrease in the intralayer styrene content. The high concentration of styrene between nanoclay layers may lead to the formation of polystyrene chains grafted on the alkyds chains. This homo‐polymerization was also observed in the variation of activation energy of UP/10A specimen versus the degree of conversion for 0.42 ≤ α ≤ 0.6 which is very close to the activation energy of free radical homo‐polymerization of styrene. POLYM. COMPOS., 34:1824–1831, 2013. © 2013 Society of Plastics Engineers     

Effects of poly(methyl methacrylate)‐based low‐profile additives on the properties of cured unsaturated polyester resins. I. Miscibility,curing behavior,and glass‐transition temperatures

The effects of three series of self‐synthesized poly(methyl methacrylate) (PMMA)‐based low‐profile additives (LPAs), including PMMA, poly(methyl methacrylate‐co‐butyl acrylate), and poly(methyl methacrylate‐co‐butyl acrylate‐co‐maleic anhydride), with different chemical structures and MWs on the miscibility, cured‐sample morphology, curing kinetics, and glass‐transition temperatures for styrene (ST)/unsaturated polyester (UP) resin/LPA ternary systems were investigated by group contribution methods, scanning electron microscopy, differential scanning calorimetry (DSC), and dynamic mechanical analysis, respectively. Before curing at room temperature, the degree of phase separation for the ST/UP/LPA systems was generally explainable by the calculated polarity difference per unit volume between the UP resin and LPA. During curing at 110°C, the compatibility of the ST/UP/LPA systems, as revealed by cured‐sample morphology, was judged from the relative magnitude of the DSC peak reaction rate and the broadness of the peak. On the basis of Takayanagi's mechanical models, the effects of LPA on the final cure conversion and the glass‐transition temperature in the major continuous phase of ST‐crosslinked polyester for the ST/UP/LPA systems was also examined. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3369–3387, 2004     

On the curing kinetics of unsaturated polyesters with styrene

The curing kinetics of a general purpose unsaturated polyester (UP) with styrene was studied by differential scanning calorimetry and dielectric capacitance measurements. Benzoyl peroxide was used as initiator. Results showed that there is a complete change in the phenomenological kinetics, in different temperature ranges. At low temperatures (70–90°C), the rate went through a maximum and then showed a first-order decay. At high temperatures (100–160°C) a second-order kinetics was suitable for all the conversion range. At T > 160°C another mechanism took place when the initiator amount was less than a critical value. From the changes in the dielectric capacitance it was inferred that the conversion rate of UP unsaturations followed a first-order decay after a certain conversion, with an activation energy close to values reported for diffusion of UP radicals. Possible free radical mechanisms accounting for experimental observations are discussed.     

Radiation and postirradiation crosslinking and structure of two unsaturated polyester resins

Radiation and postirradiation crosslinking of two unsaturated polyester (UP) resins were monitored, and substantial differences in the reaction course and extents were observed. DSC thermograms of one of the resins showed double peaks and significantly lower residual reaction heats. Extraction revealed that gelation dose of the resin with double peak was twice the gelation dose of the other resin that had single peak in DSC thermograms. Although other components of the polyesters were identical, NMR spectra of the resin with a single peak revealed isophthalic units while in the polyester of the resin having double DSC peaks orthophthalic units were detected. Orthophthalate reduced the compatibility of polyester and styrene and caused the reaction‐induced phase separation, influencing gel structure that was visible in scanning electron microscope micrographs. Previously, the double peaks in crosslinking thermograms of UP resins were usually attributed to initiator effects, but here no initiator was used, and, in the literature, we found that the double peaks are almost exclusively present in the thermograms of UP resins containing orthophthalates, whereas in resins with isophthalates double peaks almost never appear. Crosslinking extents were significantly higher in the resin‐containing isophthalate and in both cases enhanced by postirradiation reaction that is often neglected. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Mechanical,thermal, and morphological properties of bismaleimide/unsaturated polyester copolymers

An unsaturated polyester (UP) resin was modified by the addition of a thermosetting bismaleimide (BMD) as a second coreactive monomer. The copolymers were characterized in terms of mechanical, thermal, and morphological properties by tensile, bend, and impact testing; thermogravimetric analysis; heat deforming temperature analysis; dynamic mechanical analysis; and scanning electron microscopy. In addition, Fourier transform infrared spectroscopy of modified resin indicated that crosslinking networks were formed between BMD and UP. The properties of the modified resins were compared with those of unmodified resins. The results indicate that the addition of BMD not only improved the thermal decomposition temperature and heat deforming temperature but also caused small changes in the mechanical properties. The effect of the construct of BMD and the reactions among BMD, UP, and styrene were analyzed. The results show that BMD has great potential to improve the properties of UP. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 593–598, 2006     

Effects of reactive low‐profile additives on the volume shrinkage and internal pigmentability for low‐temperature cure of unsaturated polyester

The effects of reactive poly(vinyl acetate)‐block‐poly(methyl methacrylate) (PVAc‐b‐PMMA) and poly(vinyl acetate)‐block‐polystyrene (PVAc‐b‐PS) as low‐profile additives (LPA) on the volume shrinkage characteristics and internal pigmentability for low‐shrink unsaturated polyester resins (UP) during the cure at 30°C were investigated. These reactive LPAs, which contained peroxide linkages in their backbones, were synthesized by suspension polymerizations, using polymeric peroxides (PPO) as initiators. Depending on the LPA composition and molecular weight, the reactive LPA could lead to a reduction of cyclization reaction for UP resin during the cure, and would be favorable for the decrease of intrinsic polymerization shrinkage after the cure. The experimental results have been explained by an integrated approach of measurements for the static phase characteristics of the styrene (ST)/UP/LPA system, reaction kinetics, cured sample morphology, and microvoid formation by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), optical microscopy (OM), and image analysis. Based on the Takayanagi mechanical model, factors leading to both a good volume shrinkage control and acceptable internal pigmentability for the molded parts have been explored. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 967–979, 2006     

Kinetic modeling of nanoclay‐reinforced unsaturated polyester resin

Curing behavior of an unsaturated polyester (UP) resin containing 1 wt% organically modified clay (OMC) catalyzed with methyl ethyl ketone peroxide (MEKP) initiator and promoted by cobalt naphthenate accelerator was investigated by dynamic differential scanning calorimetry (DSC) at five different heating rates of 5, 10, 15, 20, and 25°C/min. X‐ray diffraction and transmission electron microscopy were used to evaluate the morphology of UP/OMC composites. Results showed a mixture of intercalated and exfoliated morphology. The dynamic DSC curing curves showed a bimodal exothermic peak; therefore, two independent reactions, namely, redox and thermal copolymerizations were assumed. Kinetic parameters were calculated by using autocatalytic model and using Down hill simplex method and Runge–Kutta algorithm for each reaction. The addition of nanoclay resulted in decrease of the activation energy of the redox reaction compared to that of the neat UP resin. Also, the pre‐exponential factor of the first reaction for UP/OMC was less than that of the neat UP. Two factors including decreasing the activation energy and decreasing the number ofcollisions of reactionary components finally resulted in increasing the reaction rate of the first reaction out of the whole reaction in the system containing nanoclay compared to the neat UP resin. It is interesting that nanoclay has no effect on the thermal decomposition reaction. POLYM. COMPOS., © 2011 Society of Plastics Engineers.     

Reaction‐induced phase separation in poly(vinyl acetate)/polyester blend

In blends of unsaturated polyester (UP), poly (vinyl acetate) (PVAc), and styrene, a reaction‐induced phase separation occurs upon curing that is due to the crosslinking between styrene and the UP molecules. The evolution of the morphology was observed by optical microscopy on a heated stage. Light transmission was used in parallel to precisely detect the onset of phase separation and the formation of microvoids. Using Fourier transform IR spectroscopy in the same conditions, the conversions at phase separation and at microvoiding were evaluated. Phase separation occurs at a very low degree of conversion and microvoiding develops at around 60% of conversion. The final morphology of the blend was investigated by scanning electron microscopy. The relative influences of the cure temperature, the concentration in PVAc, and the molecular weight of PVAc were investigated. It was confirmed that the early stages of the reaction at high temperature determine the final morphology of the blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3877–3888, 2006     

Comparative study by DSC and FTIR techniques of an unsaturated polyester resin cured at different temperatures

The polymerization of a commercial polyester resin was investigated by differential scanning calorimetry (DSC). The conversion profiles were obtained in the temperature range 60–80°C. The autocatalytic model satisfactorily describes the experimental data. Fourier transform infrared spectroscopy (FTIR) measurements were also made in order to obtain both the styrene and polyester unsaturations conversions, which were compared to the overall conversion obtained by DSC. Overall conversion measured by DSC lies between styrene and polyester C=C bond conversion obtained by FTIR. © 1998 SCI.     

Cure behavior of unsaturated polyester/modified montmorillonite nanocomposites

Dynamic rheology, differential scanning calorimetry and in situ Fourier transform infrared spectroscopy were used to study the cure behavior of unsaturated polyester/modified montmorillonite nanocomposites. The results showed that their gel times increased markedly at the same cure temperature, and that the activation energy of the nanocomposites was higher than that of the pure unsaturated polyester. Their peak temperatures of enthalpy increased as well. The total exotherm of the cure reaction declined and the cure rate decreased. In the curing process of pure unsaturated polyesters, the conversion of styrene was higher than that of the double bonds on unsaturated polyester macromolecular chains. The cure mechanism of the unsaturated polyester/modified montmorillonite changed because of the presence of double bonds in the layers of modified montmorillonite. However, the conversion of styrene in the nanocomposites was lower than that of double bonds on unsaturated polyester chains during cure at room temperature, and the conversion of styrene was increased after post‐curing and was higher than that of the double bonds on unsaturated polyester chains at the end of the cure reaction. Moreover, the degree of reaction of double bonds on unsaturated polyester chains of the nanocomposites was higher than that of unsaturated polyesters. Copyright © 2006 Society of Chemical Industry     

Mechanical characterization and chemical resistances of cured unsaturated polyester resins modified with vinyl ester resins based on recycled poly(ethylene terephthalate)

Unsaturated polyester resin (UP) was prepared from glycolyzed oligomer of poly(ethylene terephthalate) (PET) waste based on diethylene glycol (DEG). New diacrylate and dimethacrylate vinyl ester resins prepared from glycolysis of PET with tetraethylene glycol were blended with UP to study the mechanical characteristics of the cured UP. The vinyl ester resins were used as crosslinking agents for unsaturated polyester resin diluted with styrene, using free‐radical initiator and accelerator. The mechanical properties of the cured UP resins were evaluated. The compressive properties of the cured UP/styrene resins in the presence of different vinyl ester concentrations were evaluated. Increasing the vinyl ester content led to a pronounced improvement in the compression strength. The chemical resistances of the cured resins were evaluated through hot water, solvents, acid, and alkali resistance measurements. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3175–3182, 2007     

Curing of styrene‐free unsaturated polyester alkyd: synthesis,characterization and simulation

Experimental and simulation studies of the crosslinking process of styrene‐free unsaturated polyester (UP) alkyd chains are presented. The thermal and mechanical properties of the crosslinked UP alkyd are studied as a function of the peroxide concentration. The characterized and simulated thermoset matrix properties are compared. Simulation of the crosslinking reaction is used to improve the understanding of the process and to define the species involved in it. The main experimental characterization tools used were differential scanning calorimetry and dynamic mechanical analysis. The main simulation tools used were a Monte Carlo procedure for the crosslinking process and a density functional theory‐based quantum code for the scission process. Good agreement between the experimental and simulation results was achieved. Copyright © 2010 Society of Chemical Industry