Epoxy-amine System Research Articles

Effects of accelerated weathering on the chemical, mechanical, thermal and morphological properties of an epoxy/multi-walled carbon nanotube composite

The influence of weathering processes on the properties of a multi-walled carbon nanotube (MWCNT)/epoxy nanocomposite was investigated. All the samples, based on the hydrogenated gylcidyl ether of bisphenol A (HDGEBA) with and without 0.5% MWCNT, were exposed to UV radiation and elevated temperature and humidity for extended periods of time: 1, 2, 3, 4 and 6 months. The investigation included the measurement of the effects of accelerated weathering on the surface of samples for long-term conditions and for different exposure times. Chemical changes that occurred on the surface were determined using FTIR. The carbonyl and hydroxyl indexes of the nanocomposite increased much less over time than the epoxy alone, the nanocomposite also showed less loss in tensile strength and modulus and elongation on accelerated weathering, and there were few morphological changes on the composite surface compared to the unmodified epoxy. Glass transition temperatures and temperature of onset of degradation in thermal analysis decreased after accelerated weathering for all samples, but to a much greater extent for the unmodified epoxy. It appears that the incorporation of functionalized MWCNT as a modified inhibitor against accelerated weathering in the epoxy-amine system can decrease the undesirable effects arising from exposure to UV light and moisture exposure.

Kinetic studies of POSS–DGEBA precursors derived from monoamine functional POSS using dynamic dielectric sensing and nuclear magnetic resonance

ABSTRACTIncorporation of pre‐reacted monofunctional polyhedral oligomeric silsesquioxane (POSS)–epoxy adducts dramatically improves dispersion of POSS in epoxy–amine networks. The relationship between reaction kinetics and mechanism for formation of POSS–epoxy adducts versus reaction temperature was investigated. Reactivities of epoxy–monoamine functional POSS molecules were determined using in situ reaction monitoring by dynamic dielectric sensing and 29Si NMR spectroscopy. The amine‐functional POSS–epoxy isothermal reaction showed reduced reactivity due to reduced molecular mobility, that is, diffusion limitations. Kinetic parameters were determined by fitting 29Si NMR data to the model of Kamal that was extended to include diffusion. Fitting of this model to experimental data showed very good agreement over the entire conversion range for pre‐reaction between amine‐functionalized POSS and epoxy. An autocatalytic mechanism, the same as that for the neat epoxy–amine systems, was indicated. Gel permeation chromatography, scanning electron microscopy and transmission electron microscopy were used to investigate molecular weight evolution and morphology of final networks cured by 4,4′ diaminodiphenyl sulfone using pre‐reacted POSS–epoxy adducts. POSS aggregate size decreased with increased pre‐reaction temperature; more homogenous POSS dispersion was observed with higher pre‐reaction temperature. Dynamic mechanical analysis demonstrated that Tg of composites decreased slightly compared to that of the neat matrix and there appeared to be little change in microstructural heterogeneity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45994.

Development of Sustainable Thermosets from Cardanol-based Epoxy Prepolymer and Ionic Liquids

In this work, the development of new biobased epoxy networks using phosphonium-based ionic liquids combined with dicyanamide and phosphinate counteranions as initiators of epoxy prepolymers was compared with a biobased epoxy–amine system. Ionic liquids (ILs) displayed high reactivity toward cardanol-based epoxy prepolymers, and the obtained biobased epoxy/IL networks highlighted a glass transition temperature of around 30 °C, an excellent thermal stability higher than 450 °C and higher hydrophobic behavior compared to biobased epoxy–amine networks. In addition, the use of a cardanol-based epoxy prepolymer as a modifier of epoxy/IL networks was also investigated. In this case, the final properties of epoxy/IL networks were improved by the presence of biobased epoxy compounds, especially their surface energy as well as their fracture toughness (+180%), suggesting a new way to develop partially biobased epoxy coatings.

Rheological modelling of thermoset composite processing

Abstract The rheological properties of epoxy matrices and adhesives during cure are modelled in this work. Three epoxy-amine systems, designed for different applications with distinct curing behaviours, were investigated. Curing times ranged from a few seconds to upwards of 220 min depending on the cure temperature and formulation of the epoxy. The presented time-dependent model adds an exponential fit to Kiuna-Fontana's master curve of dimensionless viscosity, as well as a weighting function for the overall fit, aiming at creating a more accurate model. A major benefit of the approach is that time consuming determination of the cure kinetics using differential scanning calorimetry (DSC) is not required. The developed model was also used to predict the viscosity of a typical, non-isothermal curing cycle for composite manufacturing. It proved to correlate well with both isothermal and non-isothermal processes, demonstrating that it is particularly useful model to describe viscosity for flow modelling or process optimisation.

The Effects of UV Light on the Chemical and Mechanical Properties of a Transparent Epoxy-Diamine System in the Presence of an Organic UV Absorber.

Despite several excellent properties including low shrinkage, good chemical resistance, curable at low temperatures and the absence of byproducts or volatiles, epoxy resins are susceptible to ultra violet (UV) damage and their durability is reduced substantially when exposed to outdoor environments. To overcome this drawback, UV absorbers have been usually used to decrease the rate of UV degradation. In this present study, the effects of UV light on the chemical, mechanical and physical properties of cured epoxy structure, as well as the effect of an organic UV absorber, Tinuvin 1130, on the epoxy properties were investigated. Chemical changes in a cured epoxy system as a result of the presence and absence of Tinuvin 1130 were determined using Fourier transform infrared spectroscopy (FT-IR) analyses. The effect of Tinuvin 1130 on the surface morphology of the epoxy systems was also investigated by scanning electron microscopy (SEM) imaging. Additionally, the glass transition temperatures (Tg) before and during UV radiation were measured. After an 800 h UV radiation, mechanical test results revealed that the lack of the UV absorber can lead to a ~30% reduction in tensile strength. However, in the presence of Tinuvin 1130, the tensile strength was reduced only by ~11%. It was hypothesized that the use of Tinuvin 1130, as an organic UV absorber in the epoxy-amine system, could decrease the undesirable effects, arising from exposure to UV light.

Water uptake in free films and coatings using the Brasher and Kingsbury equation: a possible explanation of the different values obtained by electrochemical Impedance spectroscopy and gravimetry

For many years, the water uptake in organic coatings was measured by EIS and/or gravimetry but differences in water content values were found in almost all studies. The Brasher-Kingsbury equation used in the electrochemical analysis (EIS) is often criticized because elementary assumptions may be unvalid. The origin of the discrepancy between both methods is still of interest because many questions remain open and this study aims to provide new insights to these questions.In this work, free films and coatings of a model epoxy-amine system were immersed in a 3wt.% NaCl solution. The water uptake in free films was evaluated using gravimetric measurements and EIS, using the Basher-Kingsbury equation. The mass of free-films used in the EIS tests was measured and compare to gravimetric measurements while the water uptake (EIS) in free films was compared to that obtained with coatings. It was found that the mass increase of free films tested with EIS was in agreement with gravimetric measurements but was always lower than the water uptake obtained by EIS. Moreover, the water uptake in free films (EIS) was different from that obtained with coatings. In all cases, it was found that the Basher-Kingsbury equation overestimated the water uptake.It appears that the differences between EIS and gravimetric measurements can be analyzed in terms of geometrical effects. Indeed, the swelling in free films and coatings can be monitored by DMA and SECM during ageing. Finally, by mixing the experimental swelling data and the Brasher-Kingsbury equation, the same value of water uptake was obtained by EIS and gravimetry for coatings.

MODIFICATION OF EPOXY-AMINE POLYMERS BY OLIGOHEXAMETHYLENEGUANIDINES

Methods of introducing a water-soluble biocide modifier, oligohexamethyleneguanidine hydrochloride, into epoxy-amine systems were studied. The obvious way is the introduction of the modifier in the waterborne epoxy-amine systems. However, this leads to cured systems with reduced mechanical properties. Introduction of oligohexamethyleneguanidine hydrochloride solutions in organic solvents into epoxy-amine systems allows obtaining a homogeneous system, but their application is limited by the presence of a volatile solvent. Finally, introduction of oligohexamethyleneguanidine hydrochloride solutions in a water-soluble amine curing agent into epoxy-amine systems has good prospects. It is shown that the analyzed modifier has good solubility in ethylenediamine, but poor solubility in oligooxypropylenediamine. It was found that the modifier is capable of interacting chemically with epoxy resins at 22°C to form an elastic insoluble material. In an epoxy-amine system with a more active hardener (ethylenediamine) oligohexamethyleneguanidine hydrochloride acts as a plastifier (flexibilizer). This significantly reduces the glass transition temperature of the matrix (from 53 to 37ºC).

Critique of dielectric cure monitoring in epoxy resins – Does the method work for commercial formulations?

Dielectric techniques can be used as an in situ monitor of resin cure. This study reports measurements on two commercial resin formulations and aims to establish the extent to which identifiable turning points in the dielectric behaviour are dependent of the formulation used. Complementary curometer and differential scanning calorimetry data are used to monitor the progress of cure and are compared with the dielectric data. System [A] is a simple epoxy amine system, whilst system [B] is a blend of both epoxy and amine resins. It is clear that without calibration of the dielectric data against other methods it is difficult to unambiguously derive absolute cure information on a system not previously studied. However, dielectric data does allow identification of the points at which gelation and vitrification occur and demonstrates its utility as an in situ monitoring method for the cure process.

Effects of the amine/epoxy stoichiometry on the curing behavior and glass transition temperature of MWCNTs-NH2/epoxy nanocomposites

In this study, amine functionalized multi-walled carbon nanotubes (MWCNTs-NH2) with surface covalently linked ethylenediamine were prepared. The effect of amine/epoxy stoichiometry on the curing behavior, cross-linking network structure and glass transition temperature (Tg) of MWCNTs-NH2 reinforced epoxy-amine systems were investigated by using non-isothermal differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) and dynamic mechanical analysis (DMA), respectively. The results indicated the catalytic effect MWCNTs-NH2 in the curing process of epoxy was weakened with the increase of amine/epoxy stoichiometry, which can affect the formation of cross-linking network and Tg of the cured MWCNTs-NH2/epoxy nanocomposites contrasted with the corresponding neat epoxy. The MWCNTs-NH2/epoxy composites with moderate excess epoxy equivalent exhibited higher Tg than that of the neat epoxy, which was attributed to the excess epoxy groups can promote the etherification reaction as well as the interfacial interaction between MWCNTs-NH2 and epoxy resin.

Novel Diels-Alder based self-healing epoxies for aerospace composites

Epoxy resins containing Diels-Alder (DA) furan and maleimide moieties are presented with the capability to self-heal after exposure to an external heat source. A conventional epoxy amine system has been combined with furfuryl and maleimide functional groups in a two-step process, to avoid major side-reactions, and the concentration of a thermo-reversibly binding cross-linker was considered to balance thermoset and thermoplastic behaviours, and the subsequent self-healing performance. In the context of self-repair technologies an inbuilt ‘intrinsic’ self-healing system is deemed favourable as the healing agent can be placed in known ‘hot spot’ regions (i.e. skin-stringer run outs, ply drops and around drilled holes) where operational damage predominately occurs in load bearing aerospace structures. In this study, the mechanical and self-healing performance of furan functionalised epoxy resins containing varying amounts (10, 20, 30 or 40 pph) of bismaleimide were investigated using a bulk epoxy polymer tapered double cantilever beam test specimen geometry. Two forms, a thin film and a bulk material, were evaluated to account for future integration methods into fibre reinforced polymer (FRP) composites. The highest healing efficiency, with respect to the obtained initial load value, was observed from the 20 pph bulk material derivative. The polymers were successful in achieving consistent multiple (three) healing cycles when heated at 150 °C for 5 min. This novel investigated DA material exhibits favourable processing characteristics for FRP composites as preliminary studies have shown successful coextrution with reinforcing fibres to form free standing films and dry fibre impregnation.

Synthesis and research of epoxy resin toughening agent.

In this paper, a synthesis method of epoxy resin toughening agent was presented, then the chemical composition and molecular number were studied, which include the DSC curves analysis, the fracture surface morphology and bonding strength. In addition, the mechanism of epoxy resin toughening agent and the effect of toughening agent’s content to bonding strength were studied. The testing results reveal that this toughening agent can form a micro two-phase structure in epoxy-amine system, which results in the stable chemical properties and excellent physical properties.

Selection of thinner for epoxy–amine system used as binder for polymeric mortars

ABSTRACTEpoxy resin mortars are mainly used in the implementation of precast items used in the civil engineering field. The workability of these mortars represents a major obstacle to the development of this field. In order to improve this property, different types of thinners are selected to study their effect on the rheological properties of the epoxy–amine system. Rheological tests are performed in order to estimate the resin gel point and to measure its viscosity during the crosslinking reaction. It appears that viscosity is the lowest for mixtures prepared with methyl octanoate, o‐xylene, and butyl glycidyl ether. Methyl octanoate is chosen as an ecofriendly thinner. The curing process is evaluated by varying the amount of the selected thinner using isothermal and dynamic DSC to determine the end of crosslinking. The mechanical properties using dynamic mechanical analysis (DMA) and tensile resistance testing of the cured epoxy resin with different amounts of methyl octanoate are also investigated. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43970.

Dispersion stability of functionalized MWCNT in the epoxy–amine system and its effects on mechanical and interfacial properties of carbon fiber composites

There are many complex interactions between the functionalized multi-walled carbon nanotube (MWCNT) and curing agent/resin molecules, which result in the unstable and unsatisfactory enhancing effects of nanoparticles in epoxy–amine system as a matrix of carbon fiber composites. In this paper, the surface chemical characteristics of carboxyl-functionalized MWCNT (MWCNT-COOH) and amino-functionalized MWCNT (MWCNT-NH2) were analyzed. The reactivity of MWCNT-COOH and MWCNT-NH2 in the epoxy matrix before and after adding amine curing agent was discussed through non-isothermal DSC analyses and micro-morphology observation. The results showed the surface functional groups affected the dispersion stability of MWCNT during the mixing and curing process of the epoxy–amine matrix. The potential chemical mechanisms behind the improved properties of epoxy resin containing MWCNT were specifically studied. The MWCNT-COOH tended to re-aggregate during the fabrication process of epoxy matrix, whereas MWCNT-NH2 can maintain the dispersion stability due to the presence of amine curing agents, which would ensure MWCNT-NH2 more efficiently participated in the cross-linking reaction of epoxy resin and the interface strengthening between carbon fiber and resin matrix, and then resulting in the improved mechanical performances compared to the carbon fiber composites based on the neat epoxy and MWCNT-COOH reinforced ones.

Diels-Alder based, thermo-reversible cross-linked epoxies for use in self-healing composites

Epoxy resins are functionalized with Diels-Alder based thermo-reversible crosslinks to enable the fabrication of composites that are capable of multiple self-healing-repair processes. The key challenge in realizing a structural combination of conventional epoxy amine systems with furfuryl and maleimide functional groups is to achieve a high conversion in the reversible cross-linked polymer network formation while avoiding the major side-reactions.We have developed a two-step process consisting of a bulk prepolymerization reaction followed by a network formation step. The second step is performed in an extruder to realize a good mixing at high temperatures, while keeping the residence time at that temperature short to avoid maleimide side-reactions. The obtained oligomers are characterized using NIR, NMR, DSC, GPC analyses while solvent exposure tests and rheological measurements are performed to prove the formation of a thermo-reversible network and study their behavior in relation to the cross-linker concentration.Densely cross-linked, solvent resistant polymeric networks are obtained that maintain the ability to un-crosslink and thus regaining fluid behavior at elevated temperatures for at least five subsequent heat cycles. Glass fiber reinforced polymer composite films, which are fabricated using these resins, demonstrate that the thermo-reversible effect is strong enough to achieve full self-healing of a severely cracked and delaminated test specimen.

Relaxation transitions in mixed epoxy networks based on diane and aliphatic epoxy resins

Spectra of the internal friction of cured epoxy-amine networks based on diane and an aliphatic epoxy oligomers are studied using an aliphatic oligoamine hardener. It is shown that secondary relaxation is observed in the spectra for diane oligomer in the investigated range of temperatures, but not for aliphatic oligomers or cured epoxy amine systems based on them. Using van Krevelen calculations based on group contributions, is confirmed that the secondary relaxation is caused by the motion of aliphatic (glyceryl and oxypropylene) fragments, and the degree of curing does not affect them prior to gelation. It is found that the secondary relaxation of aliphatic fragments is hampered after the gel point, raising the temperature and activation energy of the transition. An original method for calculating the number of structural elements involved in the elementary act of primary relaxation is proposed that allows for the possible movement of aliphatic and diphenylolpropane fragments.

Modification of epoxy binders using cycloaliphatic epoxy imides

The influence of epoxy imide cycloaliphatic epoxy resin EPOX-01 on the rheological properties of the epoxy amine system was investigated. The effect of the amount of amine hardener for solidification process of modified system is evaluated. It is shown that the use of epoxy imide cycloaliphatic resin allows one to obtain materials with improved technological characteristics of film technology (RFI).

Cure kinetics of several epoxy–amine systems at ambient and high temperatures

Epoxy-crosslinker curing reactions and the extent of the reactions are critical parameters that influence the performance of each epoxy system. The curing of an epoxy prepolymer with an amine functional group may be accompanied by side reactions such as etherification. Commercial epoxy prepolymers were cured with different commercial amines at ambient as well as at elevated temperatures. Singularly, only epoxy–amine reactions were observed with diglycidyl ether of bisphenol-A (DGEBA)-based epoxides in our research even upon post-curing at 200°C. Etherification side reaction was found to occur at a cure temperature of 200°C in epoxides possessing a tertiary amine moiety. A combined goal of our research was to understand the effect of tougheners on the cure of epoxy–amine blend. To discern the effect of tougheners on the cure, core–shell rubber (CSR) particles were incorporated into the epoxy–amine blend. It was observed that CSR particles did not restrict the system from proceeding to complete reaction of epoxy moieties. Besides, CSR particles were found to accelerate the epoxy-amine reaction at a lower level of epoxy conversion. The lower activation energy of epoxy–amine reaction of CSR incorporated system compared to control supported the catalytic effect of CSR particles on the epoxy-amine reaction of epoxy prepolymer and amine blends.

Self-healing mortars based on hollow glass tubes and epoxy–amine systems

In the present work key parameters of different epoxy systems (such as viscosity and gel time) were evaluated to be used as healing agents when were included in a cement matrix. Epoxy systems were encapsulated in hollow glass tubes and were introduced in a mortar matrix. Samples were preloaded under three point bending in order to create a crack and release the healing system. After that, they were loaded to measure the residual strength and estimate the healing efficiency. The influence of temperature and the volume of the glass tubes were examined. Regarding the healing efficiency, a higher temperature led to an improvement of autogenous healing of the mortar matrix and a higher degree of crosslinking of the healing agent. For the studied systems, the use of glass tubes with smaller diameter containing the healing system seemed to be better in order to maintain the mechanical properties of the mortar-based composite.

Synthesis, characterization and some properties of epoxy resins containing azomethine bonding

New epoxy resins were prepared from hydroxyl substituted Schiff base monomers in two steps. The first step is based on the synthesis of hydroxyl substituted Schiff base monomers via condensation reaction. The second step includes the reaction between Schiff base monomers with epichlorohydrine (EPC) to obtain epoxy resins. The structures of resulting compounds were confirmed by FTIR and 1H-NMR. TG-DTA and DSC measurements were made for thermal characterizations of the compounds. Chemical resistances of the cured epoxy-amine systems in acidic, alkaline and organic solvents were determined for coating applications. HCl (aqueous solution, 10%), NaOH (aqueous solution, 10%), DMSO, DMF, N-methylpyrrolidone, ethanol, THF and acetone were used for corrosion tests. Chemical resistance data show that the synthesized resins have good chemical resistance against various acid, alkaline and common organic solvents.

Rheological study of the SAN modified epoxy–DDM system: relationship between viscosity and viscoelastic phase separation

The rheological behavior and structural transitions in an epoxy–amine system and poly(styrene-acrylonitrile) SAN modified epoxy–amine systems were studied by, optical microscopy and rheometry. The SAN modified epoxy blends undergo viscoelastic phase separation. The concentration of SAN has a profound effect on viscoelastic phase separation and hence the rheological behavior of modified epoxy systems. The evolution of complex viscosity is closely related to viscoelastic phase separation. The increase in complex viscosity shifted to shorter times upon increasing the concentration of SAN due to viscoelastic phase separation. The complex viscosity profiles follow an exponential growth with phase separation at various cure temperatures. The relaxation time of viscosity growth depends on both the concentration of SAN and cure temperature. The relaxation time of viscosity growth can be described by the WLF equation.