Measuring and modeling the rheological data of diglycidylether of bisphenol‐A during polymerization for finite element analyses

Finite element analyses (FEA) of thermosets are often performed on the basis of rheological data for fully cured resin. For the FEA of thermosets during curing, a material model is established and a technique is demonstrated, which allows the rheological data of the linear viscoelastic material to be derived. As the moduli are sensitive to conversion, all measured properties are related to temperature and conversion. Therefore the basis for the rheological data has to be a suitable reaction kinetic. Based on the kinetics shear and compression properties are measured independently and modeled mathematically with the focus to gain a formulation, which is suitable for FEA. The considered time constants are in the range between one second and one month as these times are relevant for the investigated relaxation times. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1868–1872, 2003     

Rheological cure characterization of phosphazene–triazine polymers

Cyclomatrix phosphazene–triazine network polymers were synthesized by co‐curing a blend of tris(2‐allylphenoxy), triphenoxy cyclotriphosphazene (TAP), and tris(2‐allylphenoxy) s‐triazine (TAT) with bis(4‐maleimido phenyl) methane (BMM). The co‐curing of the three‐component resin was investigated by dynamic mechanical analysis using rheometry. The cure kinetics of the Diels–Alder step was studied by examining the evolution of the rheological parameters, such as storage modulus (G′), loss modulus (G″), and complex viscosity (η*), for resins of varying compositions at different temperatures. The curing conformed to an overall second‐order phenomenological equation, taking into account a self‐acceleration effect. The kinetic parameters were evaluated by multiple‐regression analysis. The absence of a definite trend in the cure process with blend composition ratio was attributed to the occurrence of a multitude of competitive reactions whose relative rates depend on the reactant ratio and the concentration of the products formed from the initial phase of reaction. The cure was accelerated by temperature for a given composition, whereas the self‐acceleration became less prominent at higher temperature. Gelation was accelerated by temperature. The gel conversion decreased with increase in maleimide concentration and, for a given composition, it was independent of the cure temperature. The activation energy for the initial reaction and the crosslinking process were estimated for a composition with a maleimide‐to‐allyl ratio of 2 : 1. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 908–914, 2003     

Calorimetric and rheological study of isocyanate–pyrolysis oil blends

The reaction of polymethylene diphenyl diisocyanate (pMDI) with pyrolysis oils (PO) was studied by differential scanning calorimetry (DSC) and rheology. Chemical reactions between pMDI and PO occur under 100°C, as shown in DSC scans. DSC analysis showed that the peak temperature of the reaction decreased as the PO content of the PO–pMDI blends increased. The heat of reaction is at its maximum around 30–40% PO content. A rheological study of various PO–pMDI blends was done to evaluate the evolution of viscosity with time for different PO–pMDI hybrid mixtures. The initial viscosity of the blends is directly proportional to the PO content. An exponential increase of viscosity was demonstrated for all PO–pMDI mixtures. Rheological and chemical analysis results confirmed that chemical reactions occur between pMDI and PO at room temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1362–1370, 2003     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Simple kinetic numerical model based on rheometer data for ethylene–propylene–diene monomer accelerated sulfur crosslinking

A simple numerical model for the interpretation of the reaction kinetics in ethylene–propylene–diene monomer (EPDM) vulcanized with accelerated sulfur is presented. The model is based on the assumption that during vulcanization, a number of partial reactions occurs, both in series and in parallel, which determine the formation of intermediate compounds, including activated and matured polymers. Once written a standard first‐order differential equation (DIFF‐EQ) for each partial reaction, an ordinary DIFF‐EQ system (ODEs), was obtained and solved through Runge–Kutta algorithms. Alternatively and more efficiently, a single second‐order nonhomogenous DIFF‐EQ with constant coefficients was deduced, for which a closed‐form solution was derived, provided that the nonhomogenous term was approximated with an exponential function. Kinetic constants were evaluated through experimental data fitting on standard rheometer tests. To assess model predictions, an experimental campaign at different temperatures on two EPDM compounds was performed. They exhibited moderate reversion at intermediate and high curing temperatures. A nonlinear least‐squares fitting was performed to evaluate unknown constants entering into the DIFF‐EQ model proposed. Scaled rheometer curves fit rather well, also in the presence of reversion. In addition, partial reaction kinetic constants were provided: this gave an interesting insight into the different reticulation processes occurring during vulcanization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Butadiene polymerization with vanadium–titanium–aluminum catalytic systems

Butadiene polymerization in the presence of mixed vanadium–titanium–aluminum catalytic systems containing various organoaluminum compounds (OACs) was investigated. The main factors influencing the activity and stereospecificity of the [VOCl₃–TiCl₄–OAC₁]–(heating)–OAC₂ catalysts [where OAC₁ and OAC₂ were Al(i‐Bu)₃, Al(i‐Bu)₂H, or Al(i‐Bu)₂Cl] were considered. The kinetic parameters of the process were determined. The high activity and stereospecificity of the multicomponent systems probably accounted for the formation of polymerization active sites with both transition‐metal derivatives in their structure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 211–217, 2004     

Rheological investigation of cure kinetics and adhesive strength of polyurethane acrylate adhesive

In this work, we used rheological techniques to study both the cure characteristics and the degree of cure of polyurethane acrylate adhesive, a type of reactive adhesive used in hard disk component assembly. These results were then correlated with the tensile shear strengths of adhesives. Here, the cure characteristics of polyurethane adhesive were investigated at isothermal conditions ranging from 25 to 120°C. From the rheological results, the gelation time, the vitrification time, as well as the time required to reach the maximum degree of cure, decreased when increasing the curing temperature. The cure rates of adhesive increased with temperature in three temperature ranges, which were retardation zone, vitrification zone, and reaction‐controlled zone. The cure rates in these zones were controlled by slow diffusion, fast diffusion, and the rate of reaction, respectively. From the temperature sweep of fully‐cured adhesives, we found that the crosslinking level of adhesives increased with curing temperatures at different rates depending on the temperature zones as well. Moreover, the adhesive strength measured by tensile shear test was found to also increase correspondingly with the adhesives' T_g, indicating that the crosslinking level directly affected the adhesive strength. The strong dependence of adhesive strength with crosslinking level indicates that the crosslinking level was essential for high adhesive strength. The correlation of cure characteristics and adhesive strengths at various curing temperatures performed in this study can further provide useful information for planning appropriate curing schemes of polyurethane acrylate adhesives used in electronic and other industries. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of imide–oxazolidinone modification on the thermal and mechanical properties of HTPB‐polyurethanes

Imide‐ and oxazolidinone‐incorporated polyurethanes, based on hydroxy‐terminated polybutadiene (HTPB), were synthesized and characterized. Reaction of the blocked isocyanate terminals of the HTPB prepolymer with diepoxy compounds, containing preformed imide groups, was the strategy followed. The diepoxy resins were derived through reaction of an aliphatic and an aromatic dicarboxylic acid with preformed imide groups with a diepoxy resin. The intermediates and the polyurethane–imide–oxazolidinone were characterized by chemical, spectral, and elemental analyses. Incorporation of these heterocyclic groups caused dramatic improvements in the thermal and mechanical properties and the thermomechanical profile of the system. The improvements in properties were proportional to the hard‐segment content of the modified polyurethanes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1731–1738, 1999     

Low‐temperature synthesis of nano‐to‐submicron size organo‐zinc oxide and its effect on properties of polybutadiene rubber

Nano‐to‐submicron sized particles of zinc oxide (ZnO) were synthesized by low temperature hydrolysis method. Organo‐ZnO was also synthesized by the aforementioned method in presence of polyethylene glycol (PEG‐2000). The synthesized ZnO particles were characterized by infra‐red spectroscopy, X‐ray diffraction, BET surface area, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR showed that PEG was present on the ZnO surface. Organo‐ZnO exhibited floral‐shape morphology consisting of concentric nanorods. The average diameter of the nanorods was ～ 250 nm as evident from SEM. TEM showed that the nanorods were made of ～ 50 nm sized small particles. UV‐absorbance property of ZnO was unaltered even after organic coating. Curing, physico‐mechanical and thermal properties of polybutadiene rubber compounded with organo‐ZnO were compared with those of standard commercial rubber grade ZnO and nano‐ZnO prepared by high and low temperature methods. The cure‐characteristics were studied with the help of moving die rheometer as well as differential scanning calorimetry (DSC). Crosslink‐density measurement along the DSC vulcanization exotherm showed better cure efficiency of organo‐ZnO. Organo‐ZnO containing compound exhibited better mechanical and thermal properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Thermomechanical characteristics of benzoxazine–urethane copolymers and their carbon fiber‐reinforced composites

Copolymers of polybenzoxazine (BA‐a) and urethane elastomer (PU) with three different structures of isocyanates [i.e., toluene diisocyanate (TDI), diphenylmethane diisocyanate, and isophorone diisocyanate], were examined. The experimental results reveal that the enhancement in glass transition temperature (T_g) of BA‐a/PU copolymers was clearly observed [i.e., T_g of the BA‐a/PU copolymers in 60 : 40 BA‐a : PU system for all isocyanate types (T_g beyond 230°C) was higher than those of the parent resins (165°C for BA‐a and ?70°C for PU)]. It was reported that the degradation temperature increased from 321°C to about 330°C with increasing urethane content. Furthermore, the flexural strength synergism was found at the BA‐a : PU ratio of 90 : 10 for all types of isocyanates. The effect of urethane prepolymer based on TDI rendered the highest T_g, flexural modulus, and flexural strength of the copolymers among the three isocyanates used. The preferable isocyanate of the binary systems for making high processable carbon fiber composites was based on TDI. The flexural strength of the carbon fiber‐reinforced BA‐a : PU based on TDI at 80 wt % of the fiber in cross‐ply orientation provided relatively high values of about 490 MPa. The flexural modulus slightly decreased from 51 GPa for polybenzoxazine to 48 GPa in the 60 : 40 BA‐a : PU system. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Structure and oil‐resistant properties of HTPB‐based polyurea modified with polysulfide

A series of hydroxy‐terminated polybutadiene (HTPB) polyureas modified with different liquid polysulfide content was synthesized and their structure and oil‐resistant properties were studied by attenuated total reflectance–FTIR spectroscopy, dynamic mechanical analysis, isothermal aging and differential scanning calorimetry, stress–strain analysis, oil absorption, and oil‐resistance test. The results showed that polysulfide–polybutadiene polyureas retained low temperature flexibility and had lower oil absorption and better oil resistance than that of HTPB‐based polyureas. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2672–2675, 2003     

Morphology–composition–processing relationships in poly(methyl methacrylate)–polytriethylene glycol dimethacrylate shrinkage‐controlled blends

An innovative method to control shrinkage in polymer blends, by using N,N‐dimethyl‐p‐toluidine to produce phase separation in an acrylic system, was applied to synthesize polymer blends from polymethyl methacrylate (PMMA) and polytriethylene glycol dimethacrylate (PTEGDMA). The morphology of several compositions, as analyzed by scanning electron microscopy, reveals microdomains as a function of the specific composition, in contrast to conventional MMA–TEGDMA copolymers synthesized by thermal decomposition of benzoyl peroxide, used here as reference materials. Micro‐Raman and DSC analyses were also carried out to support the electron microscopy results as well. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1254–1260, 2004     

Effect of adhesive‐coated glass fiber in natural rubber (NR), acrylonitrile rubber (NBR), and ethylene–propylene–diene rubber (EPDM) formulations. I. Effect of adhesive‐coated glass fiber on the curing and tensile properties of NR,NBR, and EPDM formulations

Adhesive‐coated glass fibers (3 and 6 mm in length) were added at loadings of 10, 20, and 30 phr in natural rubber (NR), nitrile rubber (NBR), and ethylene–propylene–diene comonomer (EPDM) formulations in both plain and carbon black mixes. The compounds were mixed in a two‐roll mill and were characterized for their cure properties, tensile, tear, and Mullin's effect. In NR mixes, all of the formulations showed reversion in cure behavior, suggesting that NR remained unaffected. In NBR and EPDM mixes, almost all of the mechanical properties of the fiber improved. The result was more significant in EPDM than in NBR. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1111–1123, 2004     

Master curve and time–temperature–transformation cure diagram of a polyfunctional epoxy acrylic resin

The curing reaction of a well‐defined glycidyl methacrylate‐co‐butyl acrylate statistical copolymer, prepared by atom transfer radical polymerization, and a commercial linear diamine (Jeffamine D‐230) was studied with the objectives of constructing and discussing a time–temperature–transformation isothermal curing for this system. Thermal and rheological analyses were used to obtain the gelation and vitrification times. Differential scanning calorimetry data showed a one‐to‐one relationship between the glass‐transition temperature (T_g) and fractional conversion independent of the cure temperature. As a result, T_g was used as a measurement of conversion. We obtained a kinetically controlled master curve for isothermal curing temperatures from 50 to 100°C by shifting T_g versus the natural logarithm time data to a reference temperature of 80°C. We calculated the apparent activation energy by applying two different methods, gel time measurements versus shift factors, suggesting a good agreement between them. Isoconversion contours were calculated by the numerical integration of the kinetic model. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation and properties of unsaturated polyester–montmorillonite intercalated hybrid

An unsaturated polyester–organophilic montmorillonite hybrid was prepared by dispersing polymerizable quaternary ammonium‐modified montmorillonite in an unsaturated polyester resin, followed by cross‐linking reaction. The purpose of this investigation was to discover the role of the polymerizable group of quaternary ammonium in improving interfacial interaction between the silicate layers and polymer chains and the mechanical properties of unsaturated polyester–montmorillonite hybrids. It is found that when the content of organophilic montmorillonite is between 2 and 5%, the tensile strength, impact strength, heat resistance, and swelling resistance of the hybrid are obviously enhanced and are better than that of the composites prepared with pristine or nonpolymerizable quaternary ammonium‐modified montmorillonite. Results of X‐ray diffraction and transmission electron microscopy show that unsaturated polyester and styrene in the resin can be intercalated into the interlayer space of organophilic montmorillonites. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2038–2044, 2004     

Characterization of diene monomers as healing agents for autonomic damage repair

For effective autonomic healing of damaged polymers and composites, it is essential to understand how the encapsulated healing agent behaves during and after cure. In this study, two different diene monomers [dicyclopentadiene (DCPD), 5‐ethylidene‐2‐norbornene (ENB)] and their blends were investigated as candidate self‐healing agents, using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DSC experiments for samples showed that DCPD has a melting transition while the blends and ENB have no melting in the temperature range measured. Samples for DMA were prepared and tested by two different methods in the presence of Grubbs catalyst. In the first case (method I), monomers were mixed with the catalyst directly. In the second case (method II), the catalyst was mixed with an epoxy/amine system and cured into a film that was polished to expose the catalyst. The cure behavior of monomer samples was examined on the epoxy/catalyst film. Method II is considered to be a simulative experiment, which can occur in a real situation for damaged epoxy matrix composite. It was found that acceleration of cure reaction and reduction of catalyst concentration is possible by blending DCPD with ENB from method I. Storage modulus (G′) value after cure in method II showed that a DCPD : ENB blend ratio of 1 : 3 reached the highest G′ value at shorter cure time and lower catalyst levels than other monomer combinations. DCPD and ENB are presumably responsible for increases in rigidity and reactivity, respectively. This may improve the healing efficiency in autonomic damage repairing applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1266–1272, 2006     

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     

High‐speed photocrosslinking of thermoplastic styrene–butadiene elastomers

Photoinitiated thiol/ene polymerization was used to crosslink a triblock styrene/butadiene/styrene (SBS) polymer of low vinyl content (8%). The crosslinking process was followed by infrared spectroscopy (loss of unsaturation), insolubilization, swelling, and hardness measurements. The photogenerated thiyl radicals react with both the vinyl and the 2‐butene double bonds of the copolymer. Concentrations of less than 1 wt % in the trifunctional thiol crosslinker and in the acylphosphine oxide photoinitiator proved to be sufficient to create, within 0.5 s, a permanent chemical network in the elastomeric phase. This UV‐curing technology was successfully applied to crosslink rapidly commercial SBS–Kraton® thermoplastic elastomers. It proved also effective in the case of the much less reactive triblock styrene/isoprene/styrene (SIS) polymer which contains no vinyl double bonds. The thiol/ene polymerization was shown to be a much more efficient process to crosslink SBS and SIS thermoplastic elastomers than was the copolymerization of the rubber double bonds with a diacrylate monomer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1902–1912, 2000