The Effect of Cure Conditions on a Rubber-Modified Epoxy Adhesive

The effect of cure conditions on the mechanical properties of a piperidine-cured, rubber-modified epoxy is described. The results obtained reveal that variations in cure conditions (temperature and time) have a pronounced influence on the mechanical behaviour, in particular the fracture energy obtained in both bulk and adhesive joint form. Techniques such as dynamic mechanical spectroscopy and scanning electron microscopy have been employed in an attempt to explain the trends.     

Influence of cure schedule and solvent exposure on the dynamic mechanical behavior of a vinyl ester resin containing glass fibers

This study deals with the dynamic mechanical properties of a glass-reinforced vinyl ester resin. The viscoelastic parameters of the loss factor and the storage modulus as a function of the cure temperature were used as a criterion to determine the optimum cure conditions to be employed. It is shown that the cure temperature usually used to cure these resins is not enough to reach the maximum glass transition temperature, and, therefore, an additional postcure should be used. The influence of several solvents on the dynamic mechanical behavior of these resins cured following the cure pathway proposed by the supplier was also analyzed. This behavior was compared with the solvent uptake at various exposure times, and the changes observed were related to the crosslink density as well as to the chemical structure of both the resin and the solvent. Finally, the effects of varying the exposure temperature were also investigated for the resin exposed to a liquid which simulates petroleum fluid. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2595–2602, 1998     

Formulation and mechanical characterization of PVC plastisols based on low‐toxicity additives

New formulations of plastisols based on low‐toxicity plasticizers were proposed and characterized. Traditional phthalate plasticizers were replaced in the plastisols studied in this research by polymeric plasticizers (i.e., saturated polyesters), produced by the reaction of a diol and a carboxylic acid. The main drawback for the use of these plasticizers in formulations of PVC plastisols is a significant increase of the paste viscosity, which decreases their processability; thus, the use of additional additives to reduce viscosity is recommended. This study also includes the optimization of the processing conditions (cure temperature and time) of the proposed plastisols: complete cure was obtained at 140°C and 10 min. It is reported that the final properties of plastisols are very sensitive with respect to the processing conditions; in fact, insufficient plasticization or degradation can affect the material when processed out of the optimum conditions. The influence of the plasticizer concentration on mechanical and optical properties, such as tensile strength, hardness, brightness, and the like, is also reported. In summary, the proposed plastisols, with low‐toxicity plasticizers, offer a valid alternative to traditional PVC plastisols based on phthalate plasticizers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1881–1890, 2001     

Effects of postcure temperature variation on hygrothermal‐mechanical properties of an out‐of‐autoclave polymer composite

The effects of cure temperature variation on the properties of an out‐of‐autoclave polymer composite manufactured using Cycom 5320 8HS prepreg were investigated using different postcure temperatures of a two‐stage cure cycle. In addition, the effects of adverse environmental conditions on the cure temperature variation were studied by conditioning the samples in an environmental chamber until they reached moisture equilibrium. The state of cure was obtained using a differential scanning calorimeter and dynamic mechanical analyzer. The mechanical properties were obtained using short‐beam shear (SBS) and combined loading compression (CLC) test methods. The state of cure obtained showed increases in total heat of reaction, degree of cure, and glass transition temperature as the postcure temperature increased. The SBS and CLC strengths showed an increasing trend as postcure temperature increased. Good correlations were obtained between the material's cure temperatures, state of cure, and mechanical properties for room temperature dry and hot wet conditions. The study showed that the state of cure can be used to define, monitor, and verify the cure quality. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3090–3097, 2013     

Dynamic mechanical spectrometry analysis of modifications in the cure kinetics of polyepoxy composites with particulate fillers

We have performed an analysis of the modifications of a polyepoxy cure kinetics due to the introduction of organic and inorganic particulate fillers. In a previous work¹ we have studied by differential scanning calorimetry the rates of reaction of a polyepoxy system reinforced with particulate fillers. This work revealed very important differences between the kinetics of the polyepoxy and those of the composites (polyepoxy reinforced). As a matter of fact, the results obtained with the composites do not fit with any literature models. For this reason, we have decided to focus our work on the various possible interactions between the polyepoxy system and the fillers. In order to bring to light elements of understanding to the origins of cure kinetics modifications, several factors have been examined by turns. On one hand, the influence of residual curing stresses, water content, variations in thermal conductivity, and viscosity has been observed. This let us show that any modification of these parameters can induce important cure kinetic modifications, but none of them is at the origin of the observed modifications. On the other hand, several TTT diagrams obtained for various fillers contents by mechanical spectrometry have been constructed. The analysis of those diagrams revealed that the presence of particulate fillers is of a great influence on the main phase transitions that occur during the crosslinking reaction. The set of results shows that some physical bonds develop between the polyepoxy network and the organic particles. None but those latter can occasion the consequent modifications of cure kinetics that have been observed.     

Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives

The effects of post-curing and cure temperature on the glass transition temperature, T _g, and the mechanical properties of epoxy adhesives were studied. T _g was measured by a dynamic mechanical analysis apparatus developed in-house and the mechanical properties of the adhesives (yield strength, Young’s modulus and failure strain) were measured by a tensile machine. The relationships between T _g and mechanical performance under various post-cure conditions were investigated. The curing process was the same for all tests, consisting of an initial stage performed at different temperatures followed by cooling at room temperature. Three sets of specimens were considered, sharing the same initial cure process, but with a different post-curing procedure. In the first set, the specimens were only subjected to a curing process; in the second set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature below the T _g of the fully cured network, T _g∞; and in the third set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature above the T _g∞. When post-cured at a temperature above T _g∞, the mechanical and physical properties tend to have a constant value for any cure temperature.     

Effect of zinc oxide nanoparticles as cure activator on the properties of natural rubber and nitrile rubber

Zinc oxide (ZnO) nanoparticles were synthesized by homogeneous precipitation and calcination method and were then characterized by transmission electron microscopy and X‐ray diffraction analysis. Synthesized ZnO was found to have no impurity and had a dimension ranging from 30–70 nm with an average of 50 nm. The effect of these ZnO nanoparticles as cure activator was studied for the first time in natural rubber (NR) and nitrile rubber (NBR) and compared with conventional rubber grade ZnO with special reference to mechanical and dynamic mechanical properties. From the rheograph, the maximum torque value was found to increase for both NR and NBR compounds containing ZnO nanoparticles. ZnO nanoparticles were found to be more uniformly dispersed in the rubber matrix in comparison with the conventional rubber grade ZnO as evident from scanning electron microscopy/X‐ray dot mapping analysis. The tensile strength was observed to improve by 80% for NR when ZnO nanoparticles were used as cure activator instead of conventional rubber grade ZnO. An improvement of 70% was observed in the case of NBR. The glass transition temperature (T_g) showed a positive shift by 6°C for both NR and NBR nanocomposites, which indicated an increase in crosslinking density. The swelling ratio was found to decrease in the case of both NR and NBR, and volume fraction of rubber in swollen gel was observed to increase, which supported the improvement in mechanical and dynamic mechanical properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Cure characteristics,swelling behaviors,and mechanical properties of carbon black filler reinforced EPDM/NBR blend system

The effect of carbon black fillers viz. semireinforcing furnace (SRF), high abrasion furnace (HAF) and intermediate super abrasion furnace (ISAF) carbon blacks on the cure, swelling and mechanical properties of 70/30 EPDM/NBR blend have been investigated. The maximum torque values have been found to be increased with increase in filler loading. Filledsystems have been found to exhibit a reduced solvent uptake tendency compared to the unfilled sample. Blends loaded with ISAF exhibited the lowest toluene uptake among the carbon black filled systems due to the better filler reinforcement. A more uniform morphology has been observed for ISAF‐filled samples compared to the other filler loaded systems. The improvement in the mechanical properties has been observed to be the highest for ISAF‐filled samples followed by HAF and SRF filled systems. This has been attributed to the smaller particle size of ISAF black. The experimental results of mechanical testing have been compared with various theoretical models. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Dielectric spectroscopy of anaerobic adhesive cure

Dielectric spectroscopy (DS) is shown to be a most useful tool for the study of the surface initiated redox cure in model anaerobic acrylic adhesives. These models are deliberately designed to exhibit different levels of cure heterogeneity. The cure is initiated by the surfaces of two substrates on either side of a bondline. The heterogeneous cure situation is referred to as low cure through volume (CTV). Poor CTV is an undesirable feature that leads to mechanical weakness in an adhesive bond. Thus, experimental techniques based on DS are developed to investigate the cure characteristics and in particular the CTV phenomenon in the model anaerobic adhesives.     

The effects of filler content on cure and mechanical properties of dichlorocarbene modified styrene butadiene rubber/carbon black composites

A study has been carried out on the curing characteristics and mechanical properties of carbon black filled dichlorocarbene modified styrene butadiene rubber (DCSBR). Processing characteristics such as optimum cure time and maximum torque increases with increasing of the concentration of carbon black in DCSBR whereas scorch time decreases. The mechanical properties and resistance of the vulcanizate towards thermal, flame and oil resistance have been carried out. Variation of bound rubber content of carbon black filled DCSBR and the influence of the extracting temperature on the bound rubber content was investigated and its activation energy was calculated from the Arrhenius plot. The reinforcing nature of the filler was assessed from stress strain and swelling data. The enhancement in mechanical properties was supported by data on the increased content of crosslink density in these samples obtained from swelling and stress strain analysis. The results of the studies indicate that carbon black can be used as a good reinforcing filler for DCSBR.     

The Effects of Cure Conditions on the Relaxation Behavior of Thermosetting Adhesives

The ability to dissipate mechanical energy by plastic (viscous) deformation is an important strength consideration for structural adhesives. The bulk solid viscosity parameter, μ, used in spring-dashpot mechanical-model characterization of polymeric materials, provides a convenient measure of this capability. The viscosity parameter, in turn, can be related to the relaxation time for the material with the use of the elastic modulus.

This paper presents a study of the effects of cure temperature and time on the stress relaxation behavior of Metlbond 1113 modified-epoxy adhesive with carrier cloth. Short term (t=1/2 minute) relaxation data are successfully fitted with the modified Bingham mechanical model to obtain relaxation time and bulk solid viscosity parameter values corresponding to different cure conditions. Experimental results indicate that both the relaxation time and the bulk viscosity values behave in a bell-shaped increasing-decreasing manner with respect to the cure temperature for all cure time values. This behavior is similar to that of adhesive ultimate strength and elastic modulus as observed by us earlier.     

The Effects of Cure Conditions on the Relaxation Behavior of Thermosetting Adhesives

The ability to dissipate mechanical energy by plastic (viscous) deformation is an important strength consideration for structural adhesives. The bulk solid viscosity parameter, μ, used in spring-dashpot mechanical-model characterization of polymeric materials, provides a convenient measure of this capability. The viscosity parameter, in turn, can be related to the relaxation time for the material with the use of the elastic modulus.

This paper presents a study of the effects of cure temperature and time on the stress relaxation behavior of Metlbond 1113 modified-epoxy adhesive with carrier cloth. Short term (t=1/2 minute) relaxation data are successfully fitted with the modified Bingham mechanical model to obtain relaxation time and bulk solid viscosity parameter values corresponding to different cure conditions. Experimental results indicate that both the relaxation time and the bulk viscosity values behave in a bell-shaped increasing-decreasing manner with respect to the cure temperature for all cure time values. This behavior is similar to that of adhesive ultimate strength and elastic modulus as observed by us earlier.     

Solvent Effects on Cure 1-Benzyl Alcohol on Epoxy Cure

The effect of benzyl alcohol and phenol on the cure of epoxy resins is reported. The epoxy resin, a blend of the diglycidyl ethers of bisphenol A and bisphenol F, was cured with 4,4′-methylenebis(cyclohexylamine) diamine. Measurements of the cure and vitrification times were obtained using a vibrating plate curometer. Mechanical properties were assessed using dynamic mechanical thermal analysis (DMTA). Time temperature transformation (TTT) diagrams were constructed. Benzyl alcohol lowers the viscosity, aids cure and plasticizes the final product. Phenol and triethylenetetramine [TETA] significantly enhance the rate of cure and have a catalytic effect.     

Influence of ZnO nanoparticles on the cure characteristics and mechanical properties of carboxylated nitrile rubber

Zinc oxide (ZnO) nanoparticles assembled in one dimension to give rod‐shaped morphology were synthesized. The effect of these ZnO nanoparticles (average particle size ～ 50 nm) as the curing agent for carboxylated nitrile rubber was studied with special attention to cure characteristics, mechanical properties, dynamic mechanical properties, and swelling. These results were compared with those of the conventional rubber grade ZnO. The study confirmed that the ZnO nanoparticles gave a better state of cure and higher maximum torque with a marginal decrease in optimum cure time and scorch time. The mechanical properties also showed an improvement. There was an increase in tensile strength by ～ 120%, elongation at break by ～ 20%, and modulus at 300% elongation by ～ 30% for the vulcanizate cured with ZnO nanoparticles, as compared with the one containing rubber grade ZnO. Dynamic mechanical analysis revealed that the vulcanizates exhibited two transitions—one occurring at lower temperature due to the T_g of the polymer, while the second at higher temperature corresponding to the hard phase arising due to the ionic structures. The second transition showed a peak broadening because of an increase in the points of interaction of ZnO nanoparticles with the matrix. The tan δ peak showed a shift towards higher T_g in the case of ZnO nanoparticle‐cured vulcanizate, indicating higher crosslinking density. This was further confirmed by volume fraction of rubber in the swollen gel and infrared spectroscopic studies. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Application of rice husk ash as fillers in the natural rubber industry

Rice husk ash (RHA) obtained from agricultural waste, by using rice husk as a power source, is mainly composed of silica and carbon black. A two‐stage conventional mixing procedure was used to incorporate rice husk ash into natural rubber. For comparison purposes, two commercial reinforcing fillers, silica and carbon black, were also used. The effect of these fillers on cure characteristics and mechanical properties of natural rubber materials at various loadings, ranging from 0 to 40 phr, was investigated. The results indicated that RHA filler resulted in lower Mooney viscosity and shorter cure time of the natural rubber materials. The incorporation of RHA into natural rubber improved hardness but decreased tensile strength and tear strength. Other properties, such as Young's modulus and abrasion loss, show no significant change. However, RHA is characterized by a better resilience property than that of silica and carbon black. Scanning electron micrographs revealed that the dispersion of RHA filler in the rubber matrix is discontinuous, which in turn generates a weak structure compared with that of carbon black and silica. Overall results indicate that RHA can be used as a cheaper filler for natural rubber materials where improved mechanical properties are not critical. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 34–41, 2005     

Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation

Although ultrasonic wave propagation is a well-known technique for nondestructive analysis, it can be also applied for dynamic mechanical characterization (DMA) of polymers and composites. Most of DMA characterizations at ultrasonic frequencies are performed on thermoplastics and only a few articles are available on the characterization of the reactive properties of thermosetting resins. Therefore, in this work a complete characterization of the cure of a model epoxy system is presented, by comparing isothermal and nonisothermal data. The propagation of ultrasonic waves acting as a dynamic mechanical deformation at high frequencies can be used for the calculation of complex longitudinal bulk moduli during the cure of the epoxy resin. The evolution of attenuation and velocity during reaction is related to the strong physical changes occurring during the cure process. Furthermore, a comparison between the degree of reaction measured by Differential Scanning Calorimetry and ultrasonic data is proposed. The ultrasonic velocity (or the bulk longitudinal modulus) can be considered the most interesting parameter for cure monitoring because it follows the growth and evolution of the mechanical stiffness of the resin during cure. In particular, the obtained results suggest that the measurement of longitudinal velocity or L′ could be exploited for on-line measurements of post-gel properties. Finally, an immediate correlation is also proposed between the gel time and the end of cure and the ultrasonic data. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1969–1977, 1999     

白炭黑部分替代莫来石填充丁苯橡胶的性能研究

研究了白炭黑部分替代莫来石填充丁苯橡胶（SBR）对胶料的硫化特性和硫化胶力学性能的影响。结果表明：白炭黑可以减缓莫来石对SBR的促进硫化作用;白炭黑的少量加入可以促使莫来石粉体在SBR基体中的均匀分散,表现为硫化胶除了拉伸永久变形外,拉伸性能、撕裂性能和硬度增加,具有良好的协同效果。     

Influence of cure via network structure on mechanical properties of a free-radical polymerizing thermoset

An improved understanding has been achieved regarding the relationships among cure chemistry, network structure, and final physical properties of vinyl ester (VE) resins, a thermoset polymer often used as the matrix of fiber reinforced polymers. Mechanical properties of the polymer are found to depend on both cure schedule and cure formulation. The possibilities of phase separation and micro-gel formation being the cause for these differences in mechanical properties are examined. The VE/styrene (S) system does not phase separate under the conditions studied. Though bulk properties of the resin are unaffected by the details of the cure, the microscopic morphology, in particular the type of cross-link formed (intermolecular bond or intramolecular bond), is sensitive to both cure temperature and initiation mechanism as determined by cure formulation. An analysis of cure kinetics shows that both temperature and initiation mechanism affect the apparent ‘reaction order’ of the VE/S system as determined by the autocatalytic equation. This apparent reaction order is interpreted as being an indication of the degree of heterogeneity in the resin. By controlling cure temperature and cure formulation, it is possible to minimize the apparent reaction order and thereby optimize physical properties. Finally, a theory is adapted from other non-network polymer systems to qualitatively describe how cure temperature and initiation mechanism may alter the heterogeneity in network structure via micro-gel formation and how these changes in structure affect changes in the mechanical properties.     

Molecular modeling of epoxide-amine systems: Topological cure conversion limit and its influence on material properties

The cure conversion is a key determinant of the characteristics of many thermosetting polymers. Their curing process can be monitored using experimental tools such as FTIR and DSC; however, these techniques can only provide a qualitative measure of the extent of curing reaction rather than a quantitative determination of cure conversion. In fact, the actual cure conversion in epoxy systems is not known. Many molecular simulation works assume that the cure conversion limit approaches 100% although it is unrealistic. In this work, a parametric study based on model size and a radial distribution function (RDF) of relevant functional groups reveals that a topological cure conversion limit may exist and is estimated to be about 70% for a typical epoxide-amine system based on the chosen force field and simulation conditions. Beyond this, atoms within the model may form isolated clusters and the molecular model fails to converge to an equilibrated topology with structural homogeneity. It is found that the predicted dependencies of thermal and mechanical properties on cure conversion are in concurrence with accepted norms and only show deviation when the cure conversion exceeds the limit.     

Build-up of structure and viscoelastic properties in epoxy and acrylate resins cured below their ultimate glass transition temperature

The build-up of structure and viscoelastic properties with conversion during cure below the ultimate glass transition temperature of epoxy and acrylate resins has been investigated. Using a torsional dynamic mechanical analyser, dynamic shear modulus and change in sample thickness was monitored simultaneously, thus giving information on both the physical properties (stiffness) and the progress of the reaction (shrinbdkage) in one experiment. Two step-wise curing epoxy systems and two chain-wise curing acrylate systems with different crosslink densities were studied and compared. The results showed that in the epoxies vitrification was a distinct event, occurring separately from gelation and ending with the end of the cure reaction. In the acrylates vitrification began immediately after gelation, the two events being indistinguishable, and lasted until the end of the reaction, leaving the sample in its transition zone. Scaling of modulus—cure time data obtained at different frequencies showed that the data for each system followed one single curve, independent of frequency over five decades. This made it possible to estimate the modulus development at low frequencies early in the reaction, which is difficult to measure directly. From the shrinkage and storage moduli approximate values of the relaxation modulus as a function of chemical conversion were calculated. The relaxation modulus curves at different conversions were then shifted along the time axis to provide a relaxation master curve. The data and understanding gained in this work provide the basis for analysing the time-dependent mechanical behaviour during cure, e.g. build-up and relaxation of residual stresses.