Chemorheology of thermosetting resins. I. The chemorheology and curing kinetics of unsaturated polyester resin

The rheological properties and curing kinetics of a general-purpose polyester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). It was found that, as cure progresses, the steady shear viscosity increases very rapidly with cure time at all shear rates investigated, and normal forces show negative values at low shear rates and positive values at high shear rates. The observed negative normal forces are believed to result from material shrinkage during cure, and positive normal forces from the deformation of large molecules, formed by crosslinking reactions during cure. Note that, in a cone-and-plate rheometer, the shrinkage force acts in the direction opposite to that of normal forces. It is, therefore, concluded that extreme caution is needed in the interpretation of normal force measurements with thermosetting resins, subjected to steady shearing flow. Dynamic measurements seem to offer some insight on the onset of gel formation. More specifically, we have found that, when the unsaturated polyester resin was cured at a fast rate, the time at which a maximum in the loss modulus G” occurs coincides reasonably well with the time t_η∞ at which the steady shear viscosity η approaches infinity. However, at a slow rate of cure, the time at which tan δ equals unity agrees fairly well with t_η∞. DSC measurement has permitted us to determine the degree of cure as a function of cure time and the kinetic parameters in an empirical expression for the curing kinetics advanced by Kamal and co-workers. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various values of isothermal curing temperature.     

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.     

The effect of pressure on the curing behavior of unsaturated polyester resins

The effect of pressure on the curing behavior of unsaturated polyester resin was investigated, both experimentally and theoretically. The resin used was a general-purpose unsaturated polyester resin and the initiator used was t-butyl perbenzoate. A series of isothermal runs with differential scanning calorimetry (DSC) were made at various levels of cure pressure. It was found that the rate of cure was retarded under pressure, and that the ultimate degree of cure went through a maximum at a certain pressure as the cure pressure was increased from atmospheric pressure to 6.21 MPa (900 psi). It was interpreted that pressure has two competing effects on the curing behavior of unsaturated polyester resin; one is a free volume effect that hinders the curing reaction and the other is a thermodynamic effect that favors it. Therefore, when the pressure is higher than a certain level, the free volume effect becomes predominant over the thermodynamic effect, the ultimate degree of cure diminishing as the cure pressure is increased beyond that level. Theoretical interpretation of the experimental results is given, using a mechanistic kinetic model developed in our previous publication.     

Chemorheology of thermosetting resins. II. Effect of particulates on the chemorheology and curing kinetics of unsaturated polyester resin

The effect of particulates on both the rheological properties during cure and the curing kinetics of unsaturated resin has been investigated. For the investigation, a general-purpose unsaturated polyester resin was used, with calcium carbonate and clay as inorganic particulates and high-density polyethylene powder as organic particulates. It has been found that, as the particulate content increases, the resin/particulate mixture gives rise to shear-thinning behavior and the rate of cure increases. It has also been found that the CaCO₃ particles helped control shrinkage during cure when the material was subjected to steady shear deformation and that the gel time tη∞ is shorter for mixtures of resin and particulates than for the neat resin alone. Differential scanning calorimetry (DSC) is found useful for determining the curing kinetics of resin/particulate mixtures. We have combined rheological and DSC measurements to obtain a correlation between viscosity and the degree of cure during isothermal curing operations.     

Chemorheology of thermosetting resins. III. Effect of low-profile additive on the chemorheology and curing kinetics of unsaturated polyester resin

An experimental study was conducted to investigate the effect of low-profile thermoplastic additives on the rheological behavior during cure and the curing kinetics of unsaturated polyester resin. For the study, a general-purpose polyester resin was used and two different types of thermoplastic additive, poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA), were used as low-profile additives. It has been found that, during cure, the resin/PMMA system exhibits shearthinning behavior even before the cure time reaches the critical value tη∞ whereas the resin/PVAc system does not. Also, both PVAc and PMMA help reduce the shrinkage of the resin during cure. However, our study shows that shrinkage control becomes effective only when the shear rate is greater than a certain critical value. The curing behavior determined with the aid of differential scanning calorimetry (DSC) shows that the rate of cure and the final degree of cure are decreased when the amount of low-profile additive is increased.     

不饱和聚酯树脂的常温固化

综述了不饱和聚酯树脂的固化特征、固化反应,固化机理和交联固化反应活性。介绍了4种固化反应过程、引发剂和固化反应链增长过程,同时介绍了其交联固化反应活性及影响因素。     

不饱和聚酯树脂的常温固化

(续接上期)5不饱和聚酯树脂固化网络结构分析5·1不饱和聚酯树脂交联网络结构不饱和聚酯中的双键与交联剂中的双键聚合形成不溶不熔的交联网络,网络中含有2种聚合物分子链结构。网络主体由不饱和聚酯分子链的无规线团组成,苯乙烯共聚分子链穿插其中,将不饱和聚酯分子链连接和固     

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     

Effect of resin composition on curing kinetic of nanoclay-reinforced unsaturated polyester resins

Curing behavior of two resins of different unsaturated polyesters [FARAPOL 101 (UF) and Bushpol 81715 (UB)] containing 3 wt % organically modified clay (OMC), catalyzed with methyl ethyl ketone peroxide as initiator and promoted by cobalt naphtenate accelerator was investigated by dynamic differential scanning calorimetry (DSC) and gel time test methods. Chemical structures of UF and UB resins were characterized by ¹H NMR, XRD and TEM techniques were used for morphology characterization of nanocomposites. DSC results showed that after adding OMC, the redox reaction rate of UF increased less than that of UB resin. Measurements of cloud-point temperature (T _c) indicated that the miscibility of styrene/UB alkyd chains was more than that of styrene/UF alkyd chains. Therefore, the alkyd/styrene ratio inside the platelets in UB would be more than that in the platelets in UF nanocomposite. Among the three factors in redox reaction rate of UB/OMC and UF/OMC systems namely: (1) decreasing alkyd-styrene copolymerization share among platelets of OMC, (2) decreasing the activation energy, (3) decreasing the number of collisions, the first one was more effective in UF/OMC system than in UB/OMC system. Consequently, the difference between redox reaction rates of UF/OMC and neat UF was negligible compared with the corresponding difference for UB/OMC and neat UB system.     

The effect of curing of unsaturated polyester resin on the dynamic relaxation time

The effect of hot curing of unsaturated polyester resin on the dynamic relaxation time was studied using dielectric measurements along with two dynamic mechanical measurement methods. It was found that the dynamic response during cure was a material frequency dependent property and did not depend on the measurement method. All relaxation times, measured during cure, by all three measurement methods used, converged to a single equation: τ(t)_av=at^b where t= curing time, a, b=constants. The increase of the relaxation time during cure followed the same trend as a friction factor, which was found to increase with conversion. The crosslinking density was found to increase slowly with conversion, while the relaxation time increased exponentially. These two different modes of behavior during cure explain the high resolution of dynamic measurements as a cure monitoring tool, which can easily detect small curing changes. This behavior of the relaxation time was explained by the sharp rise of activation energy due to a parallel decrease of free volume at high conversion.     

Effect of particulates and fiber reinforcements on the curing behavior of unsaturated polyester resin

The effect of particulates and reinforcement on the curing behavior of unsaturated polyester resin was investigated. Also investigated was the effect of surface treatment of particulates on the curing behavior of unsaturated polyester resin. We have found that (1) an increase in the surface area, by either increasing the loading of particulates for a fixed particle size or decreasing the size of particulates for a fixed loading of particulates, enhanced the rate of cure, and (2) the treatment of glass beads with γ-methacryloxy propyltrimethoxy silane enhanced the rate of cure of unsaturated polyester resin.     

The effects of cobalt promoter and glass fibers on the curing behavior of unsaturated polyester resin

In this study, a phenomenological autocatalytic kinetic model is used to describe the cure of a polyester system containing a cobalt-based promoter. The effect of including the promoter in the cure system is discussed regarding the changes in the kinetic parameters. It was found that the cure characteristics of the unsaturated polyester system were affected by the presence of the promoter. This effect resulted in a temperature dependency of the reaction exponents as well as in a significant increase of the Arrhenius frequency factor at low temperatures. The changes in cure characteristics of promoted polyester system caused by incorporation of chopped glass fibers were also investigated. The results indicate that unlike the promoter, the presence of glass fibers does not appreciably affect the reaction exponents. However, the presence of glass fibers results in some changes in the overall reaction rate of the cure system. To take into account the unrecorded extent of reaction at high cure temperatures, a new procedure was implemented. This procedure estimates a correction factor related to the total isothermal heat of cure obtained from DSC experiments at different isothermal cure temperatures.     

Microgelation in the curing reaction of unsaturated polyester resins

A series of unsaturated polyesters were synthesized with various chemical structures and molecular weights. These unsaturated polyesters were used to study the curing reaction with styrene by using gel permeation chromatography and differential scanning calorimetry. The variation of the size of microgel particles during the curing reaction in unsaturated polyester–styrene resins was studied by using gel permeation chromatography. The size and structure of the microgels depend strongly on the polymer chain length and the number of vinyl groups on each unsaturated polyester chain. Using the differential scanning calorimetric method, the conversion of styrene and polyester vinyl groups during the reaction was measured. The experimental results of this study revealed that microgel formation has a great effect on the curing reaction of unsaturated polyester resins. © 1994 John Wiley & Sons, Inc.     

Rheology of unsaturated polyester resins. III. Effects of filler and low-profile additive on the thickening behavior of unsaturated polyester resin

An investigation was made of the rheological behavior of unsaturated polyester resin during thickening in the presence of filler or low-profile additive alone and, also, in the presence of both filler and low-profile additive. For the study, two different types of filler (CaCO₃ and clay) and two different types of low-profile additive (PMMA and PVAc) were evaluated. Compared to the resin/thickener system, the resin/filler/thickener system exhibits shear-thinning behavior as thickening progresses, and gives rise to smaller normal stress effects. On the other hand, the resin/low-profile additive/thickener system exhibits two distinct Newtonian regimes in the viscosity-shear stress curves and gives rise to larger normal stress effects. The viscosity behavior of the resin/filler/low-profile additive/thickener system was found to be very similar to that of the resin/low-profile additive/thickener system. In all cases, when the first normal stress difference was plotted against the shear stress, a correlation was obtained which was independent of thickening period. This behavior was exactly the same as for thickening polyester resin alone, as discussed in Part II of this series.     

Rheology of unsaturated polyester resins. I. Effects of filler and low-profile additive on the rheological behavior of unsaturated polyester resin

Measurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins, using a cone-and-plate rheometer. The particulates used were clay, calcium carbonate, and milled glass fiber. With clay and milled glass fibers, shear-thinning behavior of suspensions was observed at low shear rates or low shear stresses as the concentration of particulates was increased, whereas concentrated suspensions of calcium carbonate exhibited Newtonian behavior over the range of shear stresses or shear rates investigated. The cone-and-plate rheometer was also used for measurements of the bulk rheological properties of various mixtures of polyester resin and low-profile additives. For low-profile additives, solutions, in styrene, of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used. It was found that the bulk viscosities of all mixtures of polyester resin and PVAc solution lie between those of the individual components, whereas the bulk viscosities of some mixtures of polyester resin and PMMA solution go through a minimum and a maximum, depending on the composition of the mixture. While all mixtures of polyester resin and PVAc solution exhibited negligible normal stress, some mixtures of polyester resin and PMMA solution exhibited noticeable normal stresses. It should be mentioned that polyester resin follows Newtonian behavior. It turned out that all mixtures of polyester resin and PVAc solution exhibited clear, homogeneous solutions, whereas mixtures of polyester resin and PMMA solution exhibited optical heterogeneity, i.e., turbidity. When polyethylene powders were used as low-profile additives, suspensions of polyester resin and polyethylene powders exhibited negative values of normal stress as the concentrations of suspension reached a critical value. When both filler and low-profile additive were put together in polyester resin, the rheological behavior became quite complex, indicating that some interactions exist between the filler and the low-profile additive.     

不饱和聚酯树脂固化动力学研究进展

介绍了不饱和聚酯树脂(UPR)固化反应动力学的n级反应模型和自催化模型,指前因子(A)和表观活化能(E)的求解方法:Kissinger法,Ozawa法和Friedman法以及由Crane方程或形状指数Si求解反应级数(n)的方法,综述了目前国内外DSC法研究UPR固化动力学的进展。     

The curing of unsaturated polyester resins in adiabatic reactors and heated molds

The influence of several process parameters on the curing of multipurpose unsaturated polyesters with styrene, in heated molds, is discussed. The polymerization kinetics was studied in a quasi-adiabatic reactor taking into account corrections for heat losses. The following expression resulted: dx/dt = k′[I](1 ? x)³ exp(?16.6/RT), where x is the conversion of unsaturated bonds and [I] is the initial initiator concentration (benzoyl peroxide); cobalt octoate was used as an accelerator. Specimens were cured in two kinds of heated molds (one jacketed and the other electrically heated), and temperature profiles recorded. The temperature increase at the midplane showed a maximum when plotted as a function of the reaction rate (Arrhenius preexponential factor), in agreement with previous theoretical predictions. The temperature increase was enhanced for a thermally initiated reaction.     

Rheology of unsaturated polyester resins. II. Thickening behavior of unsaturated polyester and vinyl ester resins

The rheological properties of mixtures of unsaturated polyester resin and viscosity thickener were determined as thickening progressed. Two commercially available resins were used: (1) general purpose unsaturated polyester resin (Ashland Chemical, Aropol 7030), and (2) vinyl ester resin (Dow Chemical, XD-7608.05). As thickening agent, a magnesium oxide (MgO) paste dispersed in styrene monomer was used. No fillers, pigments, or other additives were used. During thickening, the following measurements were also made: (1) acid number by titration and (2) molecular weights by gel permeation chromatography (GPC). For the Ashland Chemical polyester resin, it was found that, over a period of 300 h, the titration method indicated that the number-average molecular weight (M _n) increased by a factor of 2 and the weight-average molecular weight (M _w) increased by a factor of 3. The GPC measurements, however, showed that M _n increased very little whereas M _w increased by a factor of about 2. Over the same period, the viscosity of the Ashland Chemical polyester resin increased from 0.9 N·s/m² (9 P) to 10⁴ N·s/m² (10⁵ P), and the viscosity of the Dow Chemical vinyl ester resin increased from 0.7 N·s/m² (7 P) to 2 × 10³ N·s/m² (2 × 10⁴ P). Such a large increase in viscosity cannot be explained by the existing molecular theory, in view of the fact that the molecular weights increased relatively little. We speculate that the exceedingly large increase in viscosity during thickening is attributable primarily to ionic associations between the carboxylic anions and the magnesium ions, rather than to the formation of chain branching suggested in the literature. It was found further that mixtures of polyester resins and viscosity thickener exhibit normal stress effects, increasing with thickening time and following the behavior of a second-order fluid when the first normal stress difference was plotted against shear rate. Interestingly enough, however, plots of first normal stress difference vs. shear stress yield a correlation which becomes independent of thickening time. An explanation is offered to the correlation obtained.     

Chemorheology of thermosetting resins. IV. The chemorheology and curing kinetics of vinyl ester resin

The rheological properties and curing kinetics of a vinyl ester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). Also determined were the rheological properties and curing kinetics of the resin when it had been thickened using magnesium oxide (MgO), in the presence of calcium carbonate (CaCO₃) as filler and polyvinyl acetate (PVAc) as low-profile additive. The steady shearing flow behavior observed with the vinyl ester resin was found to be very similar to that observed with a general-purpose polyester resin, reported in Paper I of this series [C. D. Han and K. W. Lem, J. Appl. Polym. Sci., 28 , 3155 (1983)]. However, a significant difference in the oscillatory shearing flow behavior was found between the two resins. We have concluded that dynamic measurement is much more sensitive to variations in resin chemistry than steady shearing flow measurement. DSC measurement has permitted us to determine the degree of cure as a function of cure time. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various isothermal curing temperatures.