Monitoring the cure of unsaturated polyester resins by pressure DSC and FTIR-PLC

A high pressure differential scanning calorimeter (DSC) and a Fourier transform infrared (FTIR) spectrometer with a prism liquid cell (PLC) were used to monitor the reaction kinetics of styrene-unsaturated polyester resins at elevated curing temperatures and pressures. The thermal method is easy to perform but provides only an overall reaction exotherm. The spectroscopic method can detect the detailed reaction mechanism of copolymerizations. It is, however, less quantitative and the calculation is much more time-consuming compared to the thermal analysis. Reactions of two unsaturated polyester resins with different molecular structure were measured by these two methods. Results showed that applying cure pressure on unsaturated polyester resins reduced the reaction rate but increased the final conversion. The styrene reaction was enhanced more than the polyester reaction at high curing temperatures.     

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.     

A kinetic model for free‐radical crosslinking co‐polymerization of styrene/vinylester resin

Vinylester resin is a major thermoset polymer used In low temperature composite manufacturing processes, such as the Seemann Composite Resin Infusion Molding Process (SCRIMP). In this study, the effect of temperature on the cure kinetics of vinylester resin in the range of 35 to 90°C was investigated using a differential scanning calorimeter (DSC) and a Fourier transform infrared spectrometer (FTIR). A mechanistic kinetic model was developed to simulate the reaction rate and conversion profiles of vinylester vinylene and styrene vinyl groups, as well as the total reaction action rate and conversion. Experimental results from DSC and FUR at different temperatures were compared with model predictions. The glass transition temperature of the vinylester resin cured at different temperatures was identified and used to monitor the final conversion change. A series of SCRIMP molding experiments were conducted. The developed kinetic model in conjunction with a heat transfer model was used to simulate the temperature and conversion changes inside the SCRIMP‐molded composites.     

Curing of unsaturated polyester resins: Effects of comonomer composition. II. High-temperature reactions

The effects of comonomer composition of the curing kinetics of unsaturated polyester (UP) resins at 100–120°C were investigated by differential scanning calorimetry (DSC) and infrared spectroscopy (IR) over the entire conversion range. One commercial UP resin, UP2821, with 6.82 unsaturated C?C bonds per polyester molecule, was used. For styrene/UP2821 reactions, experimental results of the initial and maximum reaction rates by DSC at 100–120°C revealed that the styrene content, as well as the reaction temperature, would affect the formation of microgel structures. As the initial molar ratio of styrene to polyester C?C bonds increased, the styrene swelling effect could enhance the intramicrogel crosslinking reactions, while the styrene dilution effect could diminish the intermicrogel crosslinking reactions. The competition between the two reactions would depend on the reaction temperatures. Finally, a microgel-based reaction mechanism was proposed for the high temperature reactions. © 1993 John Wiley & Sons, Inc.     

Curable resins based on recycled poly(ethylene terephthalate) for coating applications

Poly(ethylene terephthalate) waste was depolymerised in the presence of diethylene- or tetraethylene glycol and manganese acetate as a catalyst. An epoxy resin was then prepared by the reaction of these oligomers with epichlorohydrin in presence of NaOH as a catalyst. The produced oligomers were condensed with maleic anhydride and ethylene glycol to produce unsaturated polyester. The chemical structures of the resulting epoxy and unsaturated polyester resins were confirmed by ¹HNMR. The vinyl ester resins were used as cross-linking agents for unsaturated polyester resin diluted with styrene, using free radical initiator and accelerator. The 2-amino ethyl piprazine was used as hardener for epoxy resins. The curing behaviour of the unsaturated polyester resin, vinyl ester resins and styrene was evaluated at different temperatures ranged from 25 to 55 °C to calculate the curing activation energy of the system. The cured epoxy and unsaturated polyester resins were evaluated in coating application of steel.     

Effect of dual-initiator on low temperature curing of unsaturated polyester resins

In low temperature composite manufacturing processes, a major concern is how to control the resin gel time and cure time and how to achieve a high resin conversion with low residual volatile organic chemicals. In this study, a cobalt promoter catalyzed dual-initiator system was used to control the reaction rate and resin conversion of unsaturated polyester resins. A mechanistic kinetic model was developed to predict the reaction kinetics with dual initiators. This model can be used to simulate the isothermal and dynamic reaction rate and conversion profiles. It can also be utilized to predict the effect of promoter concentration on UP resin cured at low temperatures. The dual-initiator system was applied in the vacuum-assisted resin transfer molding process at room temperature. The kinetic model, in conjunction with the heat transfer analysis, was able to successfully predict the temperature profiles during the molding processes.     

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.     

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     

Kinetic analysis and mechanical properties of nanoclay reinforced unsaturated polyester (UP) resins cured at low temperatures

The reaction between unsaturated polyester (UP) resin and styrene (St) is a heterogeneous free‐radical chain‐growth cross‐linking copolymerization. Curing of the UP/St system in the presence of organically‐modified nanoclay was studied by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. A mechanistic kinetic model based on the free radical copolymerization mechanism was developed to simulate the reaction rate and conversion profiles of UP/St resin mixtures with various nanoclay contents cured at low temperatures. The model parameters were determined from several DSC experiments under isothermal conditions. The model, in conjunction with heat transfer analysis, was able to successfully predict the temperature and conversion profiles during curing in two vacuum‐infusion liquid composite molding (e.g., the Seemann composite resin infusion molding process [SCRIMP]) experiments. The presence of nanoclay particles enhanced the tensile modulus, but reduced the tensile strength of the UP nanocomposites. The fracture toughness parameter K_IC was improved by 30% with the addition of 5 wt% nanoclay. The system with mixed nanoclay and calcium carbonate was found to possess the highest K_IC without sacrificing the tensile strength. POLYM. ENG. SCI., 45:496–509, 2005. © 2005 Society of Plastics Engineers     

Comparison of unsaturated polyester and vinylester resins in low temperature polymerization

Many composite products are produced at low temperatures in processes such as resin transfer molding (RTM), vacuum infusion molding (e.g., Seemann Composite Resin Infusion Molding Process—SCRIMP), and hand lay‐up. These processes are widely used for marine, civil infrastructure, transportation and defense applications. Unsaturated polyester and vinylester resins are two major resins used in these processes due to their low cost, good performance, and processibility. In this study, the reaction kinetics and rheological changes of these two resins cured at low temperatures were studied. Effects of resin type, initiator, promoter, inhibitor and retarder on the reaction kinetics and rheological behaviors were examined using a Differential Scanning Calorimeter (DSC) and a Rheometrics Dynamic Analyzer (RDA). A model was developed to quantify the effects of resin type, temperature, and different curing agents on the gel time for both polyester and vinylester resins cured at low temperatures. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1230–1242, 2001     

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.     

A chemorheological model for the cure of unsaturated polyester resin

A chemorheological model is developed, using the free volume concept, for the prediction of viscosity during the cure of unsaturated polyester resin. We have incorporated into the development of the chemorheological model a mechanistic kinetic model of curing kinetics that predicts the degree of cure as a function of cure time. The mechanistic kinetic model uses an approach of free-radical polymerization that takes into account diffusion-controlled curing reactions, In order to test the usefulness of the chemorheological model developed, we have conducted cure experiments and measured viscosities of partially cured resin samples, using a general-purpose unsaturated polyester resin. Specifically, the following measurements were taken: (1) the quantity of ethylenic double bonds in the resin system before and after the cure reaction by infrared spectroscopy, (2) the glass transition temperature by differential scanning calorimetry (DSC) and (3) the viscosity as a function of shear rate, at several temperatures, using a cone-and-plate rheometer. It is concluded that the chemorheological model developed is very useful for predicting the variation of viscosity during the cure of unsaturated polyester resin.     

Free volume-based modelling of free radical crosslinking polymerisation of unsaturated polyesters

A new mechanistic kinetic model is presented for the cure behaviour of unsaturated polyester (UP) resins. The model is based on free radical polymerisation mechanism and the free volume concept. The quasi steady-state assumption for the free radical concentration is not used, and the decrease in initiator efficiency with conversion and radical trapping are modelled separately. The glass transition temperature of partially cured samples was measured employing differential scanning calorimetry (DSC) in conjunction with dynamic mechanical analysis (DMA), and the values obtained were incorporated into the model. DSC obtained conversion-time data for a standard commercially available UP resin under isothermal conditions. The kinetic parameters of the model were estimated using parameter optimisation procedures resulting in good agreement between model predictions and experimental data. Modelling in combination with experimental cure data showed that at higher isothermal cure temperatures a greater extent of physical trapping of radicals occurs rendering them inactive.     

Polyester resins with carbazole ring

3‐(9‐Carbazolyl)propane‐1,2‐diol was obtained in the reaction of 9‐(2,3‐epoxypropyl)carbazole with water. The obtained diol was further used for modification of unsaturated polyester resin to produce enhanced thermal and heat resistance in comparison with classic resins. The properties of polyesters and polyester resins modified with 3‐(9‐carbazolyl)propane‐1,2‐diol were studied in detail. When 3‐(9‐carbazolyl)propane‐1,2‐diol is used instead of propylene glycol the unsaturated polyester resins possess enhanced thermal stability.© 2013 Society of Chemical Industry     

Synthesis and characterization of epoxy film cured with phosphorous‐containing phenolic resin

Through the electrophilic addition reaction of ? P(O)? H and C?C, a series of novel phosphorus‐containing phenolic resins bearing maleimide (P‐PMFs) were synthesized and used as curing agent for preparing high performance and flame retardancy epoxy resins. The structure of the resin was confirmed with FTIR and elemental analysis. Thermal properties and thermal degradation behaviors of the thermosetted resin was investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The epoxy resins exhibited high glass transition temperature (143–156°C), goof thermal stability (>330°C) and retardation on thermal degradation rates. High char yields (700°C, 52.9%) and high limited oxygen indices (30.6–34.8) were observed, indicating the resins' good flame retardance for the P‐PMFs/CNE cured resins. The developed resin may be used potentially as environmentally preferable products in electronic fields. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3813–3817, 2007     

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     

The effect of the chemical structure of low-profile additives on the curing behavior and chemorheology of unsaturated polyester resin

An experimental study was conducted to investigate the effect of the chemical structure of low-profile additives on the curing behavior and chemorheology of unsaturated polyester resin during isothermal cure. For the study a general-purpose unsaturated polyester resin was cured in the presence of t-butyl perbenzoate as Initiator. The curing behavior of the resin was investigated using differential scanning calorimetry (DSC). Three different thermoplastic low-profile additives were used, namely poly(vinyl acetate) (PVAc), poly(styrene-co-butadiene), which is also known as KRATON DX-1300, and dehydrochlorinated Isobutylene/isoprene copolymer, often referred to as conjugated diene butyl (CDB) rubber. Each of the these additives, about 30 weight percent, was first dissolved in styrene. The solution was then mixed with unsaturated polyester resin and CaCO₃. The CaCO₃ particles helped stabilize the emulsions consisting of resin and KRATCN, and of resin and CDB. For each resin formulation, a series of isothermal DSC runs were made at various levels of cure pressure. It was found that for all three low-profile resins investigated, the final degree of cure went through a maximum as cure pressure was increased from atmospheric to 6.21 MPa (900 psi). We have observed evidence that in the presence of an initiator generating free radicals, the unsaturated double bonds in the KRATON and CDB undergo grafting reactions with the styrene monomers and unsaturated polyester resin, increasing the glass transition temperature of KRATON and CDB, to an extent which varies with the cure conditions employed. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer. The rheological measurements indicate that the resin/CaCO₃/KRATON and resin/CaCO₃/CDB systems give rise to gel times shorter than the resin/CaCO₃/PVAc system. It is concluded that both KRATON and CDB are more effective, both for enhancing the rate of cure of unsaturated polyester resin and imparting impact properties to the cured composites, than those thermoplastic low-profile additives that contain neither unsaturated double bonds nor a chemical structure that has rubber-like properties in the solid state.     

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     

Some filler effects on cross-linking of unsaturated polyesters

The effect of five fillers on the cross-linking macro-and microcharacteristics of simple unsaturated polyester resins was investigated by differential scanning calorimetry (DSC), reactivity tests, and gel time tests. Glass beads and silica flour appeared to have little influence on the cross-linking reaction of the resin itself, their effect being comparable to mere dilution of the resin. Kaolin presented some interaction with the resin due to its absorption characteristics and acid groups. Reground polyester/glass fiber powder and especially wood flour appeared to present clear chemical interactions with the curing behavior of the resin. Wood flour, in particular, was shown by DSC analysis to strongly co-react with the resin during cross-linking and altered markedly the resin enthalpy change and energy of activation during curing. The wood flour component causing the altered behavior of the resin appears to be lignin. DSC analysis of resins filled with three different types of isolated lignins indicated that this wood flour component reacts in a heterogeneous phase reaction with the resin during cross-linking. It appears that it is the lignin unsaturated carbon–carbon double bonds at the polyester/wood flour and at the polyester/lignin interphases that are likely to co-react by heterogeneous phase radical cross-linking with the polyester resin and styrene unsaturation, markedly changing the resin curing behavior. © 1993 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.