Influence of nanosilica particles on the cure and physical properties of an epoxy thermoset resin

Measurements are reported on the cure and physical properties of an epoxy resin created using a functionalised nanosilica filler. The filled bisphenol A epoxy (Nanopox A410) contained 40 wt% silica nanoparticles and was blended with two bisphenol A resins of molecular weights of 355 and 1075 g mol^?1, respectively. Cure was achieved using 3,3‐diaminodiphenylsulfone. The functionality of the mixture containing the epoxy nanoparticles was determined using NMR analysis. Cure times showed a progressive decrease with increasing silica level. Dynamic mechanical thermal analysis showed a decrease in the value of the glass transition temperature (T_g) with increasing silica level. T_g was further studied using differential scanning calorimetry. The ability of the nanosilica to create a stable network structure was demonstrated by the variation of the high‐temperature modulus with silica composition. Thermomechanical analysis carried out below and above T_g showed a progressive decrease in the expansion coefficients with increasing silica level, indicating the effectiveness of the functionalised silica nanoparticles in forming a network. The network formed during cure in the nano‐modified epoxy is unable to undergo the densification possible in the pure resin material and explains the observed lowering of T_g with increasing nanosilica content. Copyright © 2009 Society of Chemical Industry     

Cure modeling and monitoring of epoxy/amine resin systems. II. Network formation and chemoviscosity modeling

The glass transition temperature (T_g) advancement and the chemoviscosity development under isothermal conditions have been investigated for four epoxy/amine systems, including commercial RTM6 and F934 resins. Differential scanning calorimetry (DSC) was the thermoanalytical technique used to determine the T_g advancement and rheometry the technique for the determination of the chemoviscosity profiles of these resin systems. The complex cure kinetics were correlated to the T_g advancement via an one‐to‐one relationship using Di Benedetto's formula. It was revealed that the three‐dimensional network formation follows a single activated mechanism independent of whether the cure kinetics follow a single or several activation mechanisms. The viscosity profiles showed the typical characteristics of epoxy/amine cure. A modified version of the Williams‐Landel‐Ferry equation (WLF) was adequate to model the viscosity profiles of all the resin systems, in the temperature range 130 to 170°C, with a very good degree of accuracy. The parameters of the WLF equation were found to vary in a systematic manner with cure temperature. Further correlation between T_g and viscosity showed that gelation, defined as the point where viscosity reaches 10⁴ Pas, occurs at a unique T_g value for each resin system, which is independent of the cure conditions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2178–2188, 2000     

The use of acrylated fatty acid methyl esters as styrene replacements in triglyceride‐based thermosetting polymers

Resins containing plant oil‐based cross‐linkers were studied with two reactive diluents: a styrene and an acrylated fatty acid methyl ester‐based (AFAME) monomer. Acrylated epoxidized soybean oil and maleinated castor oil monoglyceride were bio‐based cross‐linkers used. The viscosity and mechanical properties of the resulting polymers were measured and analyzed. Both bio‐based cross‐linkers prepared using the modified AFAME as diluent had a fairly high viscosity, so blends of AFAME and styrene were needed to meet the viscosity requirements established by the composite industry (<1000 cP at room temperature). In addition, the glass transition temperature (T_g) and stiffness of bio‐based cross‐linker/AFAME polymers were significantly lower than the resin/styrene polymers. Ternary blends of maleinated castor oil monoglyceride with AFAME and styrene improved the mechanical properties to acceptable comparable values (storage modulus at 30°C ～ 1200 MPa and T_g ～ 100°C). The addition of 5 wt% of chemically modified lignin led to an improvement in the mechanical properties of the polymeric matrix but caused an increase in the viscosity. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

The effects of alumina and silica nanoparticles on the cure kinetics of bisphenol E cyanate ester

Nanoparticles can be used as fillers to reinforce polymers, forming nanocomposites with better thermomechanical properties than composites with macrosized fillers. Furthermore, the addition of nanoparticles may influence the curing behavior of the polymer matrix during processing. In this study, the effect of various loadings of alumina or silica nanoparticles on the cure kinetics of bisphenol E cyanate ester (BECy) is investigated by differential scanning calorimetry (DSC). Alumina nanoparticles are shown to have a catalytic effect on the cure of BECy. The greater catalytic effect of alumina nanoparticles, compared with silica, is attributed to the increased number of hydroxyl groups on the surface and the Lewis acidity of γ‐phase alumina. Kinetic parameters were obtained from dynamic DSC experiments. For an autocatalytic model of the cure process, the kinetic parameters obtained from the model suggest that the addition of alumina nanoparticles changed the cure reaction mechanism of BECy. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Enhanced film forming ability of benzoxazine–urethane hybrid polymer network by sequential cure method

Uniform copolymer films of benzoxazine resin (BA‐a) and urethane prepolymer (PU) were prepared at various BA‐a/PU mass ratios (100/0, 80/20, 60/40, 40/60, 20/80, and 0/100) via sequential cure method comprising of moisture cure and thermal cure steps. In the moisture cure step, Fourier Transform Infrared (FT‐IR) spectra revealed the network formation between NCO‐terminated group and moisture to firstly produce PU solid film. Then in the thermal cure step, the change of tri‐substituted benzene ring to tetra‐substituted benzene ring was observed suggesting polybenzoxazine network formation in this step. Moreover, the spectra reveal that isocyanate groups in polyurethane structure could react with phenolic hydroxyl groups of BA‐a to form biuret and allophanate groups. Dynamic mechanical analysis (DMA) confirms a synergistic behavior in glass transition temperature (T_g) of the alloys with the highest T_g value of 275°C which is uniquely observed in these alloys obtained from traditionally thermal cure method. The proposed sequential cure method above is found to be highly useful for uniform coating or film casting process which lacks in traditional, low A‐stage viscosity, benzoxazine resin. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40502.     

Development of high Tg epoxy resin and mechanical properties of its fiber‐reinforced composites

A new epoxy resin with high glass transition temperature (T_g) (～ 180°C) and a viscosity low enough for infiltration into dry reinforcements at 40°C was developed for the vacuum‐assisted resin transfer molding process. To study the curing behavior and viscosity, several blends were formulated using multifunctional resin, aromatic hardener, and reactive diluents. Effects of these components on the viscosity and T_g were investigated by thermomechanical analysis, dynamic scanning calorimetry, and rheometer. Experimental results showed that a liquid aromatic hardener and multifunctional epoxy resin should be used to decrease the viscosity to <1 Pa·s at 40°C. Moreover, the addition of a proper reactive diluent decreased the viscosity and simultaneously minimized the deterioration of T_g. Mechanical properties of the composite produced with the optimized blend were evaluated at both room‐temperature and high‐temperature conditions. According to the results, the composite showed comparable mechanical properties with that of the current commercial resin. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Time–temperature–transformation (TTT) cure diagram: Modeling the cure behavior of thermosets

The times to gelation and to vitrification for the isothermal cure of an amine-cured epoxy (Epon 828/PACM-20) have been measured on macroscopic and molecular levels by dynamic mechanical spectrometry (torsional braid analysis and Rheometrics dynamic spectrometer), infrared spectroscopy, and gel fraction experiments. The relationships between the extents of conversion at gelation and at vitrification and the isothermal cure temperature form the basis of a theoretical model of the time–temperature–transformation (TTT) cure diagram, in which the times to gelation and to vitrification during isothermal cure versus temperature are predicted. The model demonstrates that the “S” shape of the vitrification curve depends on the reaction kinetics, as well as on the physical parameters of the system, i.e., the glass transition temperatures of the uncured resin (T_g0), the fully cured resin (T_g∞), and the gel (_gelT_g). The bulk viscosity of a reactive system prior to gelation and/or vitrification is also described.     

Epoxy/alumina nanoparticle composites. I. Dynamic mechanical behavior

We studied the influence that alumina nanoparticle addition has on the dynamic mechanical spectra of an amino‐cured epoxy resin. A suppression of the short‐scale cooperative motions related to the β relaxation and an increase in the activation energy for the β relaxation of the epoxy matrix was observed as the alumina content increased. This is explained in terms of an antiplasticization effect of the alumina nanoparticles on the epoxy resin. An estimation of the effective thickness of the nanoparticle–matrix interfacial region was done based on the reduced damping. The dependence of the composites' reduced modulus on the alumina nanoparticles content is very well fitted by the generalized Kerner equation. The best‐fit parameter values suggest the presence of small and strong agglomerates in the composites at room temperature. At temperatures above the T_g, these agglomerates start to behave as weak ones because of the polymer matrix softening and particle–particle and particle–matrix slippage and friction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3774–3785, 2003     

A comparative study on the properties of the different amino‐functionalized multiwall carbon nanotubes reinforced epoxy resin composites

Multiwall carbon nanotubes (MWCNTs) were amino‐functionalized by 1,2‐ethylenediamine (EDA)' triethylenetetramine (TETA), and dodecylamine (DDA), and investigated by fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA). The dispersion of the DDA functionalized MWCNT in DMF is better than that of the MWCNT functionalized by the EDA and the TETA. Carbon nanotubes reinforced epoxy resin composites were prepared, and the effect of the amino‐functionalization on the properties of the composites was investigated by differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), and TGA. The composites reinforced by the MWCNTs demonstrate improvement in various mechanical properties. The increase of T_g of the composites with the addition of amino‐functionalized MWCNT compared to the T_g of the composites with the addition of unfunctionalized MWCNT was due to the chemical combination and the physical entanglements between amino group from modified MWNTs and epoxy group from the epoxy resin. The interfacial bonding between the epoxy and the amino group of the EDA and the TETA‐modified MWCNT is more important than the well dispersion of DDA‐modified MWCNT in the composites for the improvement of the mechanical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Monitoring the reaction progress of a high‐performance phenylethynyl‐terminated poly(etherlmide). Part II: Advancement of glass transition temperature

The cure schedule for carbon fiber‐reinforced, phenylethynyl‐terminated Ultem™ (GE Plastics) composites was studied in an attempt to optimize the resultant glass transition temperature, T_g. Reaction progress and possible matrix degradation were monitored via the T_g. On the basis of previous research, matrix degradation induced T_g reduction was expected for increases in cure time or temperature beyond approximately 70 minutes at 350°C. Using the central composite design (CCD) of experiment technique, composite panels, neat resin, and polymer powder‐coated tow (towpreg) were cured following various cure schedules to allow for the measurement of the glass transition temperatures resulting fronm cure time and temperature variations. The towpreg and neat resin specimens were cured in a differential scanning calorimeter. The glass transition temperatures of all specimens were measured via differential scanning calorimetry; the composite glass transition temperatures were also measured with dynamic mechanical thermal analysis. The composite panels and towpreg specimens showed similar trends in T_g response to cure schedule variations. Composite and towpreg glass transition temperatures increased to a plateau with increasing cure time and temperature, whereas, the neat resin showed an optimal T_g followed by T_g reduction with increasing cure time and temperature. The optimal neat resin T_g occurred within a cure time and temperature significantly below that required to maximize the composite and towpreg glass transition temperatures.     

Thermal and mechanical properties of carbon nanotube/epoxy nanocomposites reinforced with pristine and functionalized multiwalled carbon nanotubes

In this study, the effects of functionalization and weight fraction of mutliwalled carbon nanotubes (CNTs) were investigated on mechanical and thermomechanical properties of CNT/Epoxy composite. Epoxy resin was used as matrix material with pristine‐, COOH‐, and NH₂‐functionalized CNTs as reinforcements in weight fractions of 0.1, 0.5, and 1.0%. Varying (increasing) the weight fraction and changing type (pristine or functionalized) of CNTs caused increment in Young's modulus and tensile strength as observed during mechanical tests. CNT reinforcement improved thermal stability of the nanocomposites as observed by thermogravimetric analysis. Thermomechanical analysis showed a slight reduction in free volume of the polymer, that is a drop in coefficient of thermal expansion, prior to glass transition temperature (T_g) beside a slight increase in T_g value. Dynamic mechanical analysis indicated an increase in storage modulus and T_g owing to the strength addition of CNT to the matrix alongside the hardener. Scanning electron microscopy analysis of the fractured surface(s) revealed that CNTs were well dispersed with no agglomeration and resulted in reinforcing the matrix. POLYM. COMPOS., 36:1891–1898, 2015. © 2014 Society of Plastics Engineers     

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     

Glass transition temperature (Tg) determination of partially cured thermosetting systems

Previous experiments have shown that T_g of a partially cured thermosetting system can be measured with conventional thermal scan methods only if no appreciable additional cure occurred during the scan. For high temperature performance systems, the partially cured T_g is often at a temperature where kinetics rate is very rapid, causing either an observation of a more advanced cured T_g, or only the completely cured system's T_g [T_g (∞)]. Two methods to interpolate the T_g information in spite of additional cure are presented and illustrated with an epoxy resin as a testing material. The Isocure State Curve method is demonstrated with a two-step curing experiment with the Torsion Impregnated Cloth Analysis (TICA) technique, which is a forced torsion measurement on the Rheometrics mechanical spectrometer, with the resin impregnated on a glass cloth specimen. The calibration method is demonstrated by the post cure experiments of TICA, using the time to loss modulus maximum, and the softening parameter R as the calibration parameters.     

Fatty acid‐based comonomers as styrene replacements in soybean and castor oil‐based thermosetting polymers

In this study, a fatty acid‐based comonomer is employed as a styrene replacement for the production of triglyceride‐based thermosetting resins. Styrene is a hazardous pollutant and a volatile organic compound. Given their low volatility, fatty acid monomers, such as methacrylated lauric acid (MLA), are attractive alternatives in reducing or eliminating styrene usage. Different triglyceride‐derived cross‐linkers resins were produced for this purpose: acrylated epoxidized soybean oil (AESO), maleinated AESO (MAESO), maleinated soybean oil monoglyceride (SOMG/MA) and maleinated castor oil monoglyceride (COMG/MA). The mechanical properties of the bio‐based polymers and the viscosities of bio‐based resins were analyzed. The viscosities of the resins using MLA were higher than that of resins with styrene. Decreasing the content of MLA increased the glass transition temperature (T_g). In fact, the T_g of bio‐based resin/MLA polymers were on the order of 60°C, which was significantly lower than the bio‐based resin/styrene polymers. Ternary blends of SOMG/MA and COMG/MA with MLA and styrene improved the mechanical properties and reduced the resin viscosity to acceptable values. Lastly, butyrated kraft lignin was incorporated into the bio‐based resins, ultimately leading to improved mechanical properties of this thermoset but with unacceptable increases in viscosity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Addition‐cure‐type phenolic resin based on alder‐ene reaction: Synthesis and laminate composite properties

A maleimide‐functional phenolic resin was reactively blended with an allyl‐functional novolac in varying proportions. The two polymers were coreacted by an addition mechanism through Alder‐ene and Wagner–Jauregg reactions to form a crosslinked network system. The cure characterization was done by differential scanning calorimetry and dynamic mechanical analysis. The system underwent a multistep curing process over a temperature range of 110–270°C. Although the cure profiles were independent of the composition, the presence of maleimide led to a reduced isothermal gel time of the blend. Increasing the allylphenol content decreased the crosslinking in the cured matrix, leading to enhanced toughness and improved resin‐dominant mechanical properties of the resultant silica laminate composites. Changing the reinforcement from silica to glass resulted in further amelioration of the resin‐reinforcement interaction, but the resin‐dominant properties of the composite remained unaltered. Increasing the maleimide content resulted in enhanced thermal stability. Integrating both the reactive groups in a single polymer and its curing led to enhanced thermal stability and T_g, but to decreased mechanical properties of the laminate composites. This can be attributed to a brittle matrix resulting from enhanced crosslinking facilitated by interaction of the reactive groups located on the polymer of an identical backbone structure. The cured polymers showed a T_g in the range of 170–190°C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 737–749, 2001     

A new extra situ sol–gel route to silica/epoxy (DGEBA) nanocomposite. A DTA study of imidazole cure kinetic

Silica nanoparticles were obtained through the Stöber method, from mixtures of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTS). The nanoparticles were dispersed in tetrahydrofuran (THF) and coupled to bisphenol A epoxy resin (DGEBA) through surface amino groups. After removing THF non-isothermal cure was performed at different heating rates (2–20°C/min), using imidazole (2–4 wt%) as curing agent. For the sake of comparison bare DGEBA epoxy polymers were also prepared with similar schedule A nanocomposite of well-dispersed silica nanoparticles (5 wt%) in a fully cured epoxy matrix was easily obtained. Lower cure kinetics were observed with silica addition. This was attributed to reduction of the imidazole volume concentration. Cure activation energy was not influenced by silica presence, whereas it changed with the imidazole content. Therefore, experimental results suggested that silica had only an indirect effect (the reduction of the imidazole molar concentration) on the epoxy matrix cure kinetics. Glass transformation temperatures, T _g, as high as 175°C were recorded. The nanocomposite glass transformation temperature depended on the heating rate of the cure process, the imidazole and silica content. T _g changes as high as 40°C were detected as a function of the heating rate. At higher imidazole content no differences in T _g values between bare polymer and the nanocomposite were observed. This suggests that a higher imidazole content assures a better interconnection between the compatibilizing epoxy shell around the nanoparticles and the epoxy matrix. The new proposed methodology is an easy route to engineer both nanocomposites structure and interfacial interactions, thus tailoring their properties.     

Toughening of epoxy resin systems using low‐viscosity additives

This study has evaluated three low‐viscosity epoxy additives as potential tougheners for two epoxy resin systems. The systems used were a lower‐reactive resin based upon the diglycidyl ether of bisphenol A (DGEBA) and the amine hardener diethyltoluene diamine, while the second epoxy resin was based upon tetraglycidyl methylene dianiline (TGDDM) and a cycloaliphatic diamine hardener. The additives evaluated as potential tougheners were an epoxy‐terminated aliphatic polyester hyperbranched polymer, a carboxy‐terminated butadiene rubber and an aminopropyl‐terminated siloxane. This work has shown that epoxy‐terminated hyperbranched polyesters can be used effectively to toughen the lower cross‐linked epoxy resins, i.e. the DGEBA‐based systems, with the main advantage being that they have minimal effect upon processing parameters such as viscosity and the gel time, while improving the fracture properties by about 54 % at a level of 15 wt% of additive and little effect upon the T_g. This result was attributed to the phase‐separation process producing a multi‐phase particulate morphology able to initiate particle cavitation with little residual epoxy resin dissolved in the continuous epoxy matrix remaining after cure. The rubber additive was found to impart similar levels of toughness improvement but was achieved with a 10–20 °C decrease in the T_g and a 30 % increase in initial viscosity. The siloxane additive was found not to improve toughness at all for the DGEBA‐based resin system due to the poor dispersion within the epoxy matrix. The TGDDM‐based resin systems were found not to be toughened by any of the additives due to the lack of plastic deformation of the highly cross‐linked epoxy network Copyright © 2003 Society of Chemical Industry     

Shape memory epoxy nanocomposites with carbonaceous fillers and in‐situ generated silver nanoparticles

Aim of this work is to develop a novel epoxy based nanocomposite and to analyse its shape memory behavior. In particular, silver nanoparticles are in‐situ generated within an epoxy resin subsequently filled with both carbon black (CB) and carbon nanofibers (NFs) at different ratios and at a total amount of 4 wt%. Differential scanning calorimetry shows how the introduction of both CB and NF induces a slight decrease of the glass transition temperature (T_g) of the samples. The T_g drop due to nanofiller addition determines a decrease of both flexural modulus and stress at yield with respect to the neat resin, especially at elevated CB concentrations, while the presence of Ag nanoparticles plays a positive effect on the flexural properties. The best mechanical properties can be detected at a CF/NF ratio of 50%, coupled with a noticeable decrease of the electrical resistivity down to 10² Ω·cm and an interesting heating capability through Joule effect. The electro‐mechanical shape‐memory characterization under bending configuration demonstrates how it is possible to obtain an almost complete shape recovery after 60 s under an applied voltage of 220 V. POLYM. ENG. SCI., 59:694–703, 2019. © 2018 Society of Plastics Engineers     

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     

Influence of cure conditions on glass transition temperature and density of an epoxy resin

The glass transition temperature T_g and density of a TGDDM-DDS epoxy resin system were studied as a function of cure times at various cure temperatures. Both T_g and density asymptotically increased toward a maximum value with increasing cure time. The T_g and density measurements were related to the extent of cure, and the relationship in both cases was found to be independent of cure temperature.