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.     

Improving glass transition temperature of unsaturated polyester thermosets: Conventional unsaturated polyester resins

Highly crosslinkable unsaturated polyester resins (UPR) have attracted many interests in the application as reinforced matrix materials. Here, we present a systematical study of the influence of different curing conditions and styrene concentrations on resin viscosity and dynamic-mechanical properties of the thermoset. The pure maleic Palapreg® P18-03 was selected as model UPR because of its broad industrial use. By applying newly developed thermal curing profiles (without thermal initiators) and by raising the styrene content, the T_g of the network could be increased up to 206/215°C (1/10 Hz). For the first time, a fast curable UPR based on propylene glycol and neopentyl glycol with a T_g of up to 215°C is described. A partial substitution of problematic styrene with methylmethacrylate, tert-butylacrylate, and maleic anhydride (MA) was studied as well. MA leads to significantly improved resin reactivity. A resin containing 42 wt% styrene and 8 wt% MA yields thermosets with remarkably improved mechanical properties and with a narrower glass transition range compared to the original P18-03.     

Crosslinking of unsaturated polyester resins studied by low-resolution 1H-NMR spectroscopy

The curing behavior of five unsaturated polyester resins with different molar ratios of styrene to the double bonds in the polyester chain (MR) was investigated. The gel time was measured according to a standard method by the Society of the Plastic Industry (SPI) and by low-resolution pulse nuclear magnetic resonance (LRP-NMR). The gel and curing times decreased and the maximum temperature of the reaction increased when the MR was decreased. The proton mobilities and populations were measured by LRP-NMR. Three components could be seen in the crosslinking reaction: styrene (very mobile, T₂ ≅ 3 s), free polyester polymers (less mobile, T₂ ≅ 150 ms), and cured resin (immobile, T₂ ≅ 0.04 ms). The third component, cured resin, could be detected at the gel point. At this point, the proton mobilities of styrene and free polyester polymers were rapidly decreasing and the proton populations of the cured resin started to increase. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 77–83, 1997     

Determination of cohesive energy densities of unsaturated polyester resins from swelling measurements

The cohesive energy densities (ced) of five unsaturated polyester resins with varying degree of crosslinks were determined by swelling measurements in 17 single solvents and 11 binary solvents of the acetone–benzene system. The solubility parameters (δ_P) of these resins were calculated by six methods. It is concluded that the values of δ_p for the unsaturated polyester resins decrease with increasing degree of crosslinks. Since the unsaturated polyester resin consists of polar and nonpolar parts in molecular structure, the choice of solvents is very important in order to obtain the maximum degree of swelling when δ_p is estimated by swelling measurements.     

Size distribution of suspension-polymerized unsaturated polyester beads

Thermosetting polyester beads in the size range of 200 to 2000 μm were prepared in a PVAl stabilized suspension polymerization system from a commercial unsaturated polyester resin. Bead size distribution curves were determined and the average surface mean diameter D_m was calculated. Agitation speed, resin volume fraction in the reaction medium and PVAl concentration were varied and their effect on D_m was studied.     

Comparison of microwave and thermal cure of unsaturated polyester resin

Abstract

A commercial unsaturated polyester resin, Beetle R 8592 from BIP Chemical Limited has been cured using a microwave oven. Thermal curing was also carried out as a comparative study. The cured resins were compared using differential scanning calorimetry, Fourier transform infrared spectroscopy, dynamic mechanical analysis, scanning electron microscopy, solid state NMR spectroscopy, and flexural properties. The DSC analysis showed that microwave curing was much faster than thermal curing. However, within the limits of experimental error, it was found that the shear modulus, the number average molecular weight M _c, flexural modulus and strength were not significantly different. Solid state NMR also showed similar spectra for both microwave and thermal cured samples, which suggests that the same curing reactions took place in each case.     

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.     

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.     

Microgelation of unsaturated polyester resins by static and dynamic light scattering

The microgelation phenomenon during the curing of unsaturated polyester resin was investigated by both static and dynamic light scattering before gelation. The results of static light scattering revealed that the polymer molecular weight increased with degree of curing. The second virial coefficient, A₂, decreased slowly in the initial stage of curing and decreased dramatically at a conversion around α ∼ 8.7%, indicating a drastic decrease of compatibility between the polyesters and styrene. Two modes of the size distribution of the microgel particles during curing were observed by dynamic light scattering. The small particles consist of primary unsaturated polyester molecules. The large ones consist of microgel particles formed by linking adjacent polyester molecules. The sizes of the microgel particles increased in the initial stage of curing, then decreased slightly at a conversion of α ∼ 8.7%, which was due to the intramolecular crosslink reaction of the microgel particles. The experimental results revealed that the compatibility between polyesters and the styrene monomer became worse as the intramolecular crosslinking reaction inside the microgel particles caused a tight packing of the micro-gel molecules. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 871–878, 1998     

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.     

Application of azodiisobutyronitrile and azobisisoheptonitrile in low‐density unsaturated polyester resin manufacturing

Low‐density unsaturated polyester resin (LDUPR) is an extended application of unsaturated polyester resin (UPR) material. In this study, azodiisobutyronitrile (AIBN) and azobisisoheptonitrile (ABVN) were presented as composite foaming agents and as initiators in LDUPR manufacturing. On the basis of the kinetics of AIBN and ABVN, their optimum half‐lives (t_1/2's) for LDUPR were both 1.0 h. In this study, the mass ratio of AIBN and ABVN was chosen at 7:3, and the preferred amount of the composite foaming agent was 2 wt % resin. They were treated at a molding temperature of 78.7 ± 1.0°C. The obtained LDUPR had an apparent density of 0.37 ± 0.01 g/cm³ and a specific compression strength of 35.58 ± 1.50 MPa·g^?1·cm^?3; it approached the highest specific compression strength value of rigid polyurethane foam (28–35 MPa g^?1 cm^?3). A dual‐initiation and dual‐foaming mechanism based on the dual‐exothermic decomposition properties of the composite foaming agent was proposed with the support of the differential scanning calorimetry and scanning electron microscopy results. In the first stage, ABVN decomposed, released bubble nuclei, and initiated UPR cross‐polymerization. The bubble nuclei spread in the resin glue and grew. In the second stage, the gas in resin glue was enriched by the AIBN decomposition. The gelation time of the resin glue was influenced by AIBN and delayed. With the curing of resin, more bubbles grew up, took shape, and were retained in the UPR matrix. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40238.     

Control of gel time and exotherm behaviour during cure of unsaturated polyester resins

The curing behaviour of an unsaturated polyester resin has been studied by gel time and pseudo‐adiabatic exotherm measurements. The gel time corresponded closely with the initial rise in exotherm temperature. Incorporation of tert‐butyl catechol inhibitor increased the gel time in a linear fashion and the exotherm was similarly delayed. An increase in the concentrations of initiator (either methyl ethyl ketone peroxide or acetyl acetone peroxide) or cobalt octoate accelerator decreased the gel time in a reciprocal fashion and increased the rate of polymerization. These results are fitted to a theoretical model for inhibition and initiation. © 2001 Society of Chemical Industry     

Nanocomposites based on montmorillonite and unsaturated polyester

The concept of nanoscale reinforcement provides opportunity for synthesis of new polymer materials with unique properties. Montmorilonite (MMT) was derived from bentonite, purified, activated by sodium ions and mixed with reacting unsaturated polyester (UP). X-ray and transmission electron microscopy data were in support of the formation of a partially delaminated nanocomposite material. At an MMT content of only 1.5 vol%, the fracture energy, G_Q, of the nanocomposite was doubled, 138 J/m² as compared with 70 J/m² for the pure UP.     

The effect of resin chemistry on the curing behavior and chemorheology of unsaturated polyester resins

The effect of the structure of unsaturated polyester resin on its curing and rheological behavior during isothermal cure has been investigated, using three different grades of resin. In the investigation, the structure of the resins was determined, using nuclear magnetic resonance spectrometry (NMR), together with chemical analysis. Both a differential scanning calorimeter (DSC) and an infrared (IR) spectrometer were used to determine the curing kinetics, and a cone-and-plate rheometer was used to determine the variation of rheological properties during isothermal cure. On the basis of the experimental study, we have concluded: (1) at the same styrene-to-fumarate mole ratio, the resin having isophthalates cures slower than the one having none; (2) everything else being equal, the resin having a high styrene-to-fumarate mole ratio cures faster than the resin having a low one; (3) the higher the concentration of initiator, the faster a resin cures. It has been found that a resin that cures faster does not necessarily achieve a higher final degree of cure than one that cures slowly. We have found that a mechanistic kinetic model developed in our previous investigation is very useful for investigating the reactivity of unsaturated polyester resin, by determining the rate constant and activation energy of the propagation reaction. On the basis of rheological investigation, we have concluded that both t_η∞ determined from steady shearing flow measurement and t_{tan δ = 1} determined from oscillatory shearing flow measurement may be used as a measure of gel time.     

Infusible acrylic thermoplastic resins: Tailoring of chemorheological properties

Infusible liquid resins that polymerize into a thermoplastic are desirable for many applications. Similar to unsaturated and vinylester thermosetting systems, they consist of polymers dissolved in reactive monomer. This work presents a method to decrease cycle time by tuning the molecular weight and concentration of the predissolved polymer in the resin. Variation of these properties allows precise control of the viscosity which in turn controls the time at which peak exotherm is reached, the maximum temperature for a given part thickness, and cure time. Predictive models for the viscosity dependence on molecular weight, polymer concentration, shear rate, and temperature are developed. Two fiberglass panels are fabricated and tested; one with a lower molecular weight (M_w~30 kg mol⁻¹) poly(methyl methacrylate) dissolved in methyl methacrylate and the second with higher molecular weight (M_w~500 kg mol⁻¹) predissolved polymer in the resin. Mechanical properties are indistinguishable but the lower molecular weight panel cures up to three times faster. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48006.     

Investigation of readily processable thermoplastic‐toughened thermosets: IV. BMIs toughened with hyperbranched polyester

This is the fourth article in a series describing efforts to produce tough, high‐performance thermosets from very low viscosity prepolymers which are autoclave processable. Hydroxy‐terminated hyperbranched polyester (HBP) with a systematically increased molar mass was used to toughen bismaleimide (BMI). HBP was dissolved in the allyl phenol component, B, of a two‐part BMI, to yield homogeneous solutions. The BMI monomer, A, was dissolved in the solution of HBP in B to give homogeneous prepolymers. The fracture toughness (K_Ic) of neat resin plaques was measured by compact tension, while the T_g and storage moduli (E′, at 55 and 200°C) were determined by DMA. At 9% loading, the K_Ic of the BMI increased steadily with HBP molecular weight up to 138% over the control with G5 HBP (M_n ∼ 14,000 g/mol); however, significant decreases in both the T_g and E′ resulted, indicating incomplete phase separation of the thermoplastic. A linear hydroxy‐terminated polyester (M_n ∼ 5400 g/mol) with a repeat unit structure which was similar to the HBP's was prepared and used as a control. The linear polyester (LPE) toughened the BMI nearly as effectively as did the HBP and caused a smaller decrease in the T_g and E′. The viscosity of solutions of HBP and LPE in B were essentially the same at lower loadings in B, but at higher loadings, the HBP viscosity increased faster than did that of the LPE. The viscosity increase was end group‐dependent. Preliminary morphological results are presented to show the effect of the thermoplastic architecture, loading, and end group on the cured thermoset. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1809–1817, 1999     

Condensation polymerization of multifunctional monomers and properties of related polyester resins. I. Polyester varnishes

This article discusses optimum conditions for polycondensation of multifunctional monomers. Reaction variables such as monomer concentration, temperature, and time were studied to optimize reaction conditions. The influence of the nature and concentration of catalysts was also investigated. The optimum reaction conditions consisted of polymerization in an 82% w/w m-cresol solution at 160°C for 2 h, continuous reaction at 200°C for 1.5 h at 230°C for 3.5 h, followed by 40 min under reduced pressure and at 200°C. The catalyst was 1.4 × 10^?3 mol % Zn(OAc)₂ · 2H₂O in dimethyl terephthalate. Polycondensation of ethylene glycol, 1,4-butanediol, pentaerythritol, and trifunctional monomers (glycerin or trimethanolpropane) with dimethyl terephthalate was carried out in m-cresol to produce polyester prepolymers. The synthesized prepolymers were mixed with commercial Desmodur CT-stable (a phenol-blocked polyisocyanate) to form one-component varnishes which were characterized by thermogravimetric analysis. Polyester modified with trimethanolpropane has higher thermal stability than the one modified with glycerin. After being coated onto treated copper wires, the magnetic wires were characterized according to the specification of Japanese Industrial Standards (JIS-C-2358) and were found to be acceptable.     

Infrared laser‐ignited horizontal frontal polymerization of versatile unsaturated polyester resins

Infrared laser‐ignited horizontal frontal polymerization is applied for the synthesis of unsaturated polyester resin (UPER) by copolymerizing unsaturated polyester with styrene (St), methyl methacrylate (MMA), and 2‐dimethylamino ethyl methacrylate (DMAEMA) monomers. Dependence of frontal velocity and temperature on the initiator and monomer concentration is discussed for St‐based UPER. These resins have also been characterized by Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy. The results reveal St‐based resins have superior crosslinked networks. Besides, the thermo‐pH responsiveness behaviors of DMAEMA‐based resins are demonstrated by swelling measurements under the conditions of different temperatures and pH values. Moreover, by introducing CdSe@ZnS quantum dots and CsPbBr₃ perovskites into St‐ and MMA‐based resins, respectively, we realize the in situ generation of CdSe@ZnS‐UPER and CsPbBr₃‐UPER composites with good fluorescence properties and fluorescent stability, which have potential application in optoelectronic devices such as light‐emitting diode and perovskite solar cells. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45935.     

Synthesis and characterization of a novel aliphatic polyester based on itaconic acid

A novel potentially biobased aliphatic polyester poly (butylene 2‐methylsuccinate) (PBMS) was synthesized from 1,4‐butanediol (BDO) and 2‐methylsuccinate acid (MSA) via direct esterification and polycondensation route using tetrabutyl titanate (TBT) as catalyst. The reaction conditions were investigated in detail. The proper molar ratio (1.08:1) of BDO to MSA was determined through balancing the reaction efficiency and costs of reactants. TBT was found to be an effective catalyst, and its content (0.1 mol% of MSA) was optimized based on the esterification ratio and intrinsic viscosity. The molecular weight of PBMS polymers was governed by the polycondensation time. The weight average molecular (M_w) characterized by gel permeation chromatography (GPC) ranged from 5,800 to 8,700 g/mol. The polyester was also characterized by nuclear magnetic resonance spectroscopy (¹H NMR), differential scanning calorimeter and thermogravimetric analysis. The results showed that the glass transition temperature continuously increased with molecular weight. The polyester had excellent thermal stability, and its decomposition temperature increased with the molecular weight. As new potentially biobased polyester plasticizer, desirable mechanical properties were achieved at the weight ratio of PBMS was 80/100 and 50/100. In addition, the PBMS/poly(vinyl chloride) (PVC) blends had superior migration‐resistant property to the low‐molecular weight plasticizer dioctyl phthalate for PVC. POLYM. ENG. SCI., 54:2515–2521, 2014. © 2013 Society of Plastics Engineers     

Synthesis and applications of unsaturated polyester resins based on PET waste

Three types of unsaturated polyester resins were synthesized from the glycolysis of polyethylene terephthalate (PET) plastic waste, considering environment, cost and properties for their applications. These synthesized unsaturated polyester resins could be used for various construction processes and materials such as no dig pipelining (NDR-1), pultrusion (PLR-1) and polymer concrete (PCR-1). PET was taken from common soft-drink bottles, and ethylene glycol (EG), diethylene glycol (DEG) and MPdiol glycol mixtures were used for the depolymerization at molar ratios. The glycolyzed PET 1 ^st products (oligomers) were reacted with maleic anhydride, phthalic anhydride and dicyclopentadiene (DCPD) (especially for polymer concrete) to form unsaturated polyester resins with mixed styrene. The lab scale (1–5 kg) and pilot plant scale-up tests (200 kg) were experimented to evaluate the processing characteristics, viscosity, acid number and curing behaviors. The main properties such as hardness, flexural strength, tensile strength, heat distortion temperature, elongation, and chemical resistance were determined based on the various uses of the three resins. Furthermore, the applicability and the properties of these developed resins were verified through many real application tests.