Rheokinetic of a gelled resol resin curing by dynamic‐temperature rheometry based on rectangular torsion strain

The rheological behavior of a phenolic resol resin during its curing process was studied through a rheological dynamic‐temperature analysis. Two heating ramps from 0 to 120°C (1°C/min) and from 0 to 150°C (5°C/min) were performed. The resin's complex viscosity data were obtained by applying a rectangular torsion strain. The overall change of complex viscosity with temperature was due to a combination of thermal softening, described by the Andrade equation, and the resin crosslinking process. The four‐ and six‐parameter Arrhenius rheokinetic model was applied to the profiles obtained for the resin's complex viscosity, and the viscous flow and activation energies of curing kinetics were established. Two calculation methods are proposed to obtain the flow and curing parameters of the material. The six‐parameter Arrhenius model was more suitable for predicting changes in the resin's complex viscosity, obtaining an activation energy of ～ 38.0 kJ/mol for the resol resin curing process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Control of microvoids in resol phenolic resin using unsaturated polyester

A major drawback of cured phenol formaldehyde resin is the presence of microvoids, resulting from the liberation of condensation byproducts. In an attempt to rectify this, phenolic resol resin was blended with unsaturated polyester (UP). UPs with various maleic anhydride (MA) to phthalic anhydride (PA) ratios were synthesized and later mixed with resol resin in various proportions. The best MA/PA ratio was found out by determining the specific gravity, acetone‐soluble matter, and volatile content of the cast blend, cured under a satisfactory time–temperature schedule. The influence of acid value of the UP and the most desirable UP content were also investigated on the basis of the quality of the modified phenolic samples. The structural changes in the modified resin were studied using FTIR spectroscopy. Scanning electron micrographs (SEM) of the fractured surfaces were obtained to ascertain the extent of microvoids in the modified resin. Both thermogravimetric analysis results and SEM micrographs confirm the effectiveness of UP in reducing the microvoids in the cast resol resin. The tensile and impact strengths of the samples also reflect the superior quality of the resol phenolic resins that have been modified by UP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Rheological study of crosslinking and gelation in chlorobutyl elastomer systems

The multiple waveform rheological technique was used to study the crosslinking behaviour of a filled elastomeric system at various temperatures. The gel point could be precisely determined from a single experiment at each temperature using this technique. At the instant of gelation, the storage (G′) and loss (G″) moduli scale with frequency in an identical manner, i.e. ωⁿ. The relaxation exponent, n was found to be approximately constant (0.14) in the temperature range studied. Reported values of n for chemically crosslinking systems are typically higher; the low value in this case can be attributed to the high molecular weight of the prepolymer and the presence of filler. The gel times at various temperatures were used to calculate an apparent activation energy of the curing reaction, which was found to be approximately 92 kJ mol⁻¹.     

Effect of diffusional limitations on the gelation of cyanate ester resins

The effect of diffusion control-induced unequal reactivities of functional groups on the gelation of cyanate ester resins was analyzed. By introducing differentiation into the recursive approach, a gelation model was developed to calculate the gel points of systems under diffusional limitations. Modeling results for a Zn-catalyzed cyanate resin show that the unequal reactivities have a significant effect on the gel point and the gelation is delayed to a higher conversion. However, other factors, which may include localized reactions and cycle formation, also contribute to the delay of gelation. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 115–125, 1997     

Fluidized state gelation of ceramic microspheres: Feasibility study of a continuous process and analysis of the chemical dehydration phenomena

A continuous gelation process for producing aluminum and zirconium hydroxide microparticles was conceived and tested in a lab‐scale apparatus. Fluidized state was devised as an appropriate hydrodynamic configuration to maintain the reacting sol microdrops suspended: stable non‐coalescing spherical gel particles in a size range quite below 100 μm were produced via gelation. The microspheres produced by a subsequent calcination step showed a diameter ranging between 2 μm and 20 μm. This powder, without further size classification, was an optimal product to manufacture test specimens by sintering. Measurements on the mechanical properties, such as toughness, flexural strength and hardness, classified the test bars as a very promising ceramic material. In order to enlighten the chemical drying phenomenon and devise the main governing mechanisms acting in the gelation reaction, an extremely simplified experimental system was set. A single drop of sol was deposited into a given amount of the dehydrating liquid medium and observed by an optical microscope; image analysis permitted us to identify the gel formation, the size reduction kinetics, as well as secondary phenomena.     

Relationship between gelation and morphology of rubber-modified epoxy resins

The addition of both a solid and a liquid rubber to an epoxy resin has been shown to strongly increase the toughness of the material. Moreover, it is well known that toughening depends on the morphology of the system, which, in turn, depends on the cure conditions. In the present work the morphologies of epoxy resins modified with mixtures of solid and liquid rubbers have been studied through transmission electron microscopy. An investigation of the influence of the elastomers and of the gel time on the morphology of the hardened material was carried out. Gel times were evaluated, through rheological measurements, at different cure temperatures.     

Isothermal curing by dynamic mechanical analysis of three epoxy resin systems: Gelation and vitrification

Times to gelation (t_gel) and times to vitrification (t_vit) during isothermal curing for the epoxy systems diglycidyl ether of bisphenol A (DGEBA)/1,3‐bisaminomethylcyclohexane (1,3‐BAC), tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM)/4‐4′‐diaminodiphenylsulfone (DDS), and TGDDM/epoxy novolac (EPN)/DDS were measured at different curing temperatures. This article reports on a method to determine t_gel and t_vit by dynamic mechanical analysis (DMA). Gelation was determined at the onset of the storage modulus or by the peak of the loss factor. Vitrification was defined as the curve of the storage modulus as the curve reached a constant level (endset) in DMA tests. The experimental values obtained for t_gel and t_vit were compared with values obtained by other experimental methods and with theoretical values (t_gel's) or indirect determinations (t_vit's). From kinetic analysis by differential scanning calorimetry, conversions corresponding to gelation were obtained for the three systems; this yielded a constant value for each system that was higher than theoretical value. Values of the apparent activation energies of the DGEBA/1,3‐BAC, TGDDM/DDS, and TGDDM/EPN/DDS epoxy systems were obtained from plots of t_gel's against reciprocal temperatures. They were 53.2, 58.2, and 46.5 kJ/mol, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 78–85, 2002     

两类水滑石对环氧树脂体系凝胶时间的影响

采用水滑石降低环氧树脂/低分子质量聚酰胺固化体系的凝胶时间。考察了水滑石和丙氨酸交换后的水滑石及其加入量对体系凝胶时间的影响,发现水滑石的加入有利于凝胶时间的降低;当低分子质量聚酰胺∶环氧树脂的质量比值>0.7时,比未改性水滑石更有利于凝胶时间的降低;丙氨酸交换后水滑石的加入量越高,凝胶时间越短。     

Rheological study on the thermoinduced gelation behavior of poly(N‐isopropylacrylamide‐co‐acrylic acid) microgel suspensions

Submicron‐sized thermoresponsive poly(N ‐isopropylacrylamide‐co ‐acrylic acid) microgels were synthesized by soap‐free emulsion polymerization. The physical state of the microgel suspensions, in a wide range of polymer concentrations (1.1–7.1 wt %), transformed from fluid to gel when the temperature was elevated across their volume phase transition temperature at pH = 3.0. Such thermoinduced gelation behavior was studied in detail by small‐deformation oscillatory rheological measurements within the linear viscoelastic region. It was found that the gelation temperature was strongly affected by the polymer concentration, decreasing as the polymer concentration increased. The gelation kinetics showed that the suspension gelled more quickly at either larger polymer concentration or higher isothermal heating temperature. In an isothermal frequency sweep for the as‐formed gels, both storage and loss moduli, G′ and G″ , exhibited a power‐law behavior, that is, G′ (ω) ～ ω^0.2–0.4 and G″ (ω) ～ ω^0.46?0.50 within the frequency range where G′ dominates G″ . In addition, the elasticity of the gels, which results from the attractive particle bonds, increased markedly with increasing polymer concentration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45259.     

Rheological study of poly(ethylene glycol)/poly(N‐isopropylacrylamide‐co‐2‐acrylamido‐2‐methylpropanesulphonic acid) semiinterpenetrating network formation

The formation of a series of semiinterpenetrating network (SIPN) hydrogels made by free‐radical copolymerization of N‐isopropylacrylamide (NIPA) and 2‐acrylamido‐2‐methylpropanesulphonic acid (AMPS) with varying comonomer mole ratios, crosslinked with N,N′‐methylene‐bisacrylamide (MBAA) in the presence of poly(ethylene glycol) (PEG) with average molecular weight 6,000 g mol^?1 was studied via determination of complex viscosity, η*, using plate–plate rheometry. The isothermal time dependence of η* at various temperatures or the variation of η* with temperature of pregel solutions was utilized to detect the onset of gelation. The SIPN systems were compared with the corresponding gels made under the same conditions in the absence of PEG. The copolymer mainchain composition has a major effect on the time or temperature for onset of gelation and in particular gelation appears to be inhibited to some extent by MBAA when the AMPS/NIPA mole ratio in the pregel solution exceeds 0.5. The presence or absence of PEG in pregel solutions has a lesser effect on gelation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2083–2087, 2004     

Thermal gelation of the 12S canola globulin

The nature of intermolecular forces responsible for thermal gelation of the 12S canola globulin was determined by preparing gels under a variety of environmental influences. The effects of pH, sodium salts and denaturing agents were evaluated by differential scanning calorimetry, small amplitude oscillatory rheology and transmission light microscopy. Gels prepared with 6% protein at alkaline pH values were superior to gels prepared under acidic conditions. Sodium salts, which promoted protein stability, had an adverse effect on gelation. The addition of guanidine hydrochloride and dithiothreitol to protein dispersions prior to heating produced inferior gels. Hydrophobic forces and electrostatic interactions were responsible for the establishment of canola gel networks. Gel stabilization and strengthening were attributed to disulfide bonding, electrostatic interactions and hydrogen bonding.     

Influence of stone particles on the rheological behavior of a novel photopolymerizable siloxane‐modified acrylic resin

A study was made of the effects of calcarenitic stone particles, known as pietra gentile, inclusions on the rheological behavior of photopolymerizable siloxane‐modified acrylic formulations, intended as protective for the calcarenitic stone structures. Different amounts of stone particles (ranging from 15% wt to 35% wt) were added to the modified acrylic mixture to achieve a natural color matching for calcarenitic stone substrates. The presence of stone particles was expected to modify the rheological behavior of the protective formulation. Therefore, the viscosity of the mixtures was studied at ambient temperature as function of the shear rate and the solid volume fraction. A relationship was obtained to predict the modification of viscosity of each formulation as a consequence of inclusions of different amounts of stone. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

MMA bulk polymerization and its influence on in situ resin viscosity comparing several chemorheological models

Earlier published rheological data of methyl methacrylate (MMA) heated and isothermally polymerized at temperatures between 50 and 80°C have been reanalyzed using three semiempirical models of viscosity advancement including a modified Boltzmann sigmoidal model, a microgel model for cure, and a first‐order isothermal kinetic model. These alternative models possessed few fitting parameters and could be used without requiring more experiments to be run. For each dataset as a function of temperature, the analysis resolved time constants associated with both the induction time for polymerization and the rate of viscosity rise, which were inversely related to the polymerization temperature. We found the sigmoidal model the most robust to accommodate nonlinearities in viscosity advancement with radical polymerization of MMA. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Urea–formaldehyde‐resin gel time as affected by the pH value,solid content,and catalyst

An experiment was conducted to investigate the effects of the resin solid content, catalyst content, and pH value obtained by the addition of two kinds of catalysts on the gel time of a urea–formaldehyde (UF) resin. Upon the addition of ammonium chloride, the pH value of the resin mixture decreased to 7 but not significantly further because of the limited free formaldehyde in the system. The pH values of the critical points, at which the resin‐curing rate dramatically increased and the gel time was reduced, were above 7 for both catalysts. To achieve the same gel time, the required pH value of the UF resin adjusted with ammonium chloride was higher than that of the resin modified by hydrochloric acid. This indicated that the main effects of ammonium chloride on the UF‐resin cure included both the release of hydrochloric acid and the catalysis of the reactants in the UF‐resin system. The gel time of the UF resin obviously decreased with increasing catalyst and resin solid contents and with decreasing pH. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1566–1569, 2007     

Modelling of resol resin polymerization with various formaldehyde/phenol molar ratios

The kinetics of polymerization of phenol and formaldehyde are modelled to account for the formation of resol resin with various initial formaldehyde/phenol molar ratios. The conversion of phenol and formaldehyde is used to determine the equilibrium constants for phenolate ion and methylene glycol. The relative concentration of each compound is obtained as a function of time. The molecular weight is also determined for each resol resin at the end of the reaction. The evaluation of substituted phenols, hydroxymethylol phenols and methylene bridges was carried out by infrared spectroscopy. The experimental tendencies are in agreement with the results of the model. © 2001 Society of Chemical Industry     

Effects of silane and the lowering of pH on the properties of phenol–formaldehyde resol resins and resin coatings

Phenol–formaldehyde resol resins were modified by the addition of silane (3‐aminopropyltriethoxysilane) and the lowering of pH (formic acid). The effects of the modifications on the properties of the resins during storage were studied through comparison with the parent resins and by viscosity measurements, NMR spectroscopy, ultraviolet–visible spectroscopy, and differential scanning calorimetry. Resin coatings on paper were prepared to determine the influence of discoloration of the resin solution on the color of the cured resin. A decrease in the pH of the NaOH‐catalyzed resin solutions lightened the color of the solutions and corresponding coatings, whereas silane additions made the coatings slightly more yellow. The lowering of pH increased the viscosities and decreased the reactivities of the resin solutions compared with the unmodified reference resins during storage. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1933–1941, 2007     

Rheology of polyflavonoid tannin–Formaldehyde reactions before and after gelling. I. Methods

The gelling and hardening reactions of different, commercial polyflavonoid tannin extracts of the prorobitenedin/profisetinidin, procyanidin, and prodelphinidin types with formaldehyde were studied by parallel‐plate rheometry. To do this, methods to determine the rheological characteristics of tannin–formaldehyde polycondensation reactions both before and after the gel point were developed. The validity of some modifications of known methods was checked on the best‐known commercial tannin extract, namely, the reaction of the mimosa tannin extract with 5% paraformaldehyde, over a range of different temperatures. The gel point was determined by three different methods, namely, by the crossover point of viscous and elastic moduli, by the extrapolation to ∞ of the zero‐frequency viscosity, and by determining the point at which the value of tan δ is constant whatever frequency is used in the measurement. The results obtained were critically compared. The comparisons obtained were good and were reproducible when the first two methods were used, but not when the third one was used. This allowed the determination of the energy of activation at different stages in the tannin–aldehyde polycondensation reaction and the calculation of the degree of conversion of the tannin–aldehyde polycondensation before, at, and after the gel point. Mathematical expressions defining the degree of conversion for tannin–aldehyde reactions before, at, and after the gel point as a function of different moduli were developed and checked with the experimental data obtained. Methods and mathematical expressions for the determination of the rate constants of the different stages of the reaction were also developed and checked. After the gel point, the system was modeled using both a diffusion‐controlled model and a second‐order kinetic law. The applied results obtained indicated that, although a certain element of diffusion control is present, a second order, diffusion‐independent kinetic is more valid under the experimental conditions used. This indicates that the rheometry approach to a polycondensation network is limited mostly by the capability of the equipment to the region after the gel point, where diffusion control does not as yet play a predominant role. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 852–863, 2002     

甲基丙烯酸酯/二甲基丙烯酸酯自由基共聚合体系中的自加速和凝胶行为

对甲基丙烯酸聚乙二醇单醚酯/二甲基丙烯酸聚乙二醇酯共聚体系分别实施常规自由基聚合（FRP）,原子转移自由基聚合（ATRP）和可逆加成-断裂链转移（RAFT）自由基聚合,通过观察聚合速率、双键转化率、凝胶点以及交联网络的发展,比较FRP、ATRP和RAFT共聚合体系的反应动力学和交联行为。3个聚合体系均出现了自加速现象,ATRP体系的自加速由扩散控制的自由基脱活造成,RAFT体系的自加速来自于扩散控制的自由基加成。在ATRP和RAFT交联体系中,初级链的缓慢增长和充分松弛减少了分子内环化,抑制了微凝胶形成,因此其凝胶点远低于FRP体系。ATRP和RAFT交联网络通过凝胶自由基与单体加成以及支化链的结合而不断发展,导致凝胶含量和交联网络密度随转化率不断增大。     

管材专用PPR的等温结晶动力学

研究了管材专用无规共聚聚丙烯(PPR)的结晶温度和等温结晶行为,利用Avrami方程对等温结晶过程和动力学进行了分析。结果表明:北欧化工公司的PPR1具有更高的结晶温度、结晶度以及更快的结晶速率。同一结晶温度条件下,国产PPR3的结晶度达到一半的时间略高于其他PPR。利用Hoffman-Lauritzen结晶动力学理论计算得到了PPR的成核常数和结晶生长时大分子在垂直于分子链方向的折叠表面自由能(σ_e),与其他试样相比,PPR1的σ_e最低,PPR3的σ_e最高,结晶速率最慢。通过偏光显微镜照片可以发现,PPR1的球晶尺寸最小,PPR3和PPR4的球晶较大,球晶尺寸比较接近。     

PAN／DMSO溶液凝胶化行为的流变学研究

通过动态流变测试,研究PAN／DMSO溶液的热致变凝胶化过程和测定PAN／DMSO溶液的凝胶点温度。主要通过测量动态流变学参数（动态模量G′i和G″ii、损耗角正切tanδ、复数粘度η^＊）来表征PAN溶液的凝胶化行为,分析了含水量对溶液凝胶化行为和凝胶结构的影响。结果发现,虽然PAN溶液的凝胶点温度随着溶液中含水量的增加而升高,但是由松弛指数n和分形维数值df表征的凝胶结构几乎不受溶液中含水量的影响。