Epoxy-amine Networks Research Articles

Ambient moisture influence on the secondary relaxations of epoxy-amine networks with different crosslink densities

Ambient moisture influence on the secondary relaxations of epoxy-amine networks with different crosslink densities

Biocidal polymer coatings based on porphyrin-modified epoxy-amine networks

For the first time, a pronounced biocidal activity of modified polymers with a porphyrin content of 21.4 μM (0.002 wt%) against Staphylococcus aureus was demonstrated. In order to develop new biocidal polymer coatings, a study was carried out on diglycidyl ether of bisphenol A (DGEBA) and porphyrin-modified oligomeric diamine systems. The study investigated the solubility of porphyrins in oligomers through experimental and theoretical means, using Van Krevelen's approach of additive group contributions. Fully cured epoxy-amine polymer materials modified with varied free-base tetraarylporphyrins were obtained. The materials were studied by means of thermogravimetry, differential scanning calorimetry, UV–Vis and fluorescence spectroscopy. The proposed approach to the formation of modified epoxy-amine materials allows preservation of the photophysical properties of porphyrins, including their photostability. The addition of modifiers in 4.28–21.4 μM range of concentrations makes it possible to keep the thermophysical and thermochemical properties of the polymer matrix.

Effect of ionic liquid on soft epoxy-amine electroactuators

Energy conversion represents a challenge in various fields of applications such as soft robotics or microfluidics technologies, particularly in terms of electromechanical coupling for actuators. The dielectric elastomers are a variety of electroactive polymers (EAP) which require high electric fields to be used as actuators. It is well-known that the presence of ionic impurities can have an influence of their electro-mechanical response under high electric field (i.e. above 1 MV/m). More precisely, it can be responsible for the bending of the sample under constant electric field. Here, we investigate the impact of a small content, i.e. from 0.1 wt% to 10 wt% of an imidazolium Ionic Liquid (IL) on the electromechanical response of a soft epoxy-amine network. The interest of this study therefore lies in its position at the frontier between dielectric polymers and ionic polymers. Dielectric spectroscopy revealed a significant increase of electric conductivity (≈2 orders of magnitude) when adding only 0.1 wt% of IL and up to 4 orders of magnitude with 10 wt% of IL at T = 20 °C for f = 10 Hz. It also evidenced the presence of the electrode polarization for all the samples doped with IL. The bending test carried out at E = 0.1 MV/m revealed no bending for pure epoxy-amine and a slow kinetics of bending for all the samples doped with IL with displacement evolving throughout the experiment (i.e. over 4 h). As evidenced by dielectric spectroscopy and bending tests, it is clear that the presence of a small quantity of ionic moieties (whether doping agent or impurity) can strongly modify the electromechanical behavior of elastomer commonly described as dielectric.

A DOPO-eugenol linear siloxane as a highly effective liquid flame retardant also imparting improved mechanical properties to an epoxy amine network

A liquid bio-derived eugenol-containing phosphorus silicone hybrid molecule (EGDS-DOPO) that endows excellent flame-retardancy and enhanced mechanical performance to an epoxy amine network at low levels of addition was successfully synthesized and characterised. The EGDS-DOPO significantly improved the flame retardancy requiring only 5 wt% of EGDS-DOPO to achieve a limiting oxygen index (LOI) of 34.9 % and a UL 94 V-0 rating. Cone calorimetry measurements reveal a reduction of the peak heat release rate, the total heat release, and total smoke production by 62.2 %, 34.0 % and 47.5 %, respectively when EGDS-DOPO concentration was 20 wt%, compared to the unmodified epoxy network. Viscosity remained low throughout cure, and despite some retardation of reactivity, the epoxy amine resin remains fast curing and suitable for liquid moulding composite fabrication technologies. Moreover, the modified EGDS-DOPO epoxy networks display a glass transition temperature (Tg) between 218.5 °C and 174.1 °C, a hydrophobic surface with water contact angle of 97.5°–106.6°, a low moisture ingress of 2.35 %–2.91 %, and improved overall mechanical properties in flexure (maximum 39.8 % increase in flexural strength and 27.0 % in flexural modulus), compression (maximum 24.5 % and 35.0 % increase in compressive strength and modulus, respectively), and fracture toughness (maximum 28.0 %).

Controlling Hybrid Polyhydroxyurethane Adhesive and Rheological Properties by Partial Carbonation of Biobased Epoxy Monomer.

Controlling hybrid material properties by simple monomer design offers an elegant pathway to prepare thermoset adhesives with tunable properties. Herein, biobased hybrid polyhydroxyurethane/polyepoxy is prepared starting from partially carbonated cashew nut shell epoxy derivatives (NC514) and m-xylene diamine (MXDA). The curing reactions, that is, epoxy-amine and cyclic carbonate aminolysis, monitored by ATR-IR spectroscopy at 50°C are found to be concomitant yielding highly homogeneous materials. Hybrid networks are extensively characterized by swelling tests, TGA, DMA, DSC, tensile tests, rheology, and lap-shear-test on aluminum substrates. The introduction of hydroxyurethane moieties within the epoxy-amine networks enhanced the adhesion properties (up to 20% compare to neat epoxy resins) by combining hydrogen bonding capability and vitrimeric properties (thermoset able to flow). Rheological characterizations and reprocessing tests demonstrated that hybrid adhesives with up to 47 mol% of cyclic carbonate groups are capable of covalent exchange (internally catalyzed by tertiary amine) while keeping similar thermomechanical properties and enhanced adhesion strength compare to the permanent epoxy network.

A DOPO modified cyclosiloxane reactive diluent as an effective flame retardant for a high-performance epoxy network

A novel phosphorus-siloxane containing liquid flame retardant epoxy resin was synthesized and incorporated into an epoxy amine network, maintaining low viscosity and rapid cure characteristics, good mechanical and thermal properties, and affording excellent flame retardancy at low concentration. The synthesized flame retardant incorporated a tetra-functional glycidyl ether cyclosiloxane substructure which was substituted with two DOPO segments to obtain a liquid bi-functional flame-retardant epoxy resin (BGTS-DOPO). At only 10 wt% BGTS-DOPO, the modified epoxy amine resin had a viscosity of less than 210 mPa s at 90 °C, gelled rapidly at 150 °C, (9.3 min at 150 °C), exhibited a Tg of 198.7 (extrapolated onset of E’) and 233.0 °C (tan δ max), and possessed improved flexural strength (4.5 %), modulus (4.1 %), strain (7.7 %) and fracture toughness (26.1 %). The self-extinguishing behaviour easily achieved a UL-94 V0 rating, exhibited a maximum limiting oxygen index of 36.7 %, while cone calorimetry produced a 24 % improvement in combustion behaviour of the carbon fibre composite. Presented here therefore is a new highly fire-retardant epoxy resin system that exhibited fast cure, low viscosity and excellent mechanical and thermal properties.

Design of self-healable epoxy-amine-ionic liquid thermosets using poly(urea- formaldehyde) microcapsules containing epoxy prepolymer

Extrinsic self-healing mechanism is an original route for achieving self-healing polymer materials. The use of microencapsulated diepoxized monomer as healing agent for the design of self-healable epoxy-amine networks is thus a promising strategy limiting only by the decomposition temperature of the curing agent. Therefore, this work put forward the idea of a single microcapsule system by using free ionic liquid (IL) as an initiator of the healing agent, which can be applied for epoxy-amine thermosets requiring higher curing temperatures. Thus, the synthesis of epoxy-amine-ionic liquid networks containing microcapsules was firstly investigated. Then, the morphology as well as the physical properties of these networks, i.e. the thermal stability and mechanical performances were also determined. Finally, the self-healing ability was highlighted by using over than 10 wt% of EP@PUF microcapsules.

Modification of the Dielectric and Thermal Properties of Organic Frameworks Based on Nonterminal Epoxy Liquid Crystal with Silicon Dioxide and Titanium Dioxide.

A nonterminal liquid crystal epoxy monomer is used to create an epoxy-amine network with a typical diamine 4,4'diaminodiphenylmethane. The plain matrix is compared to matrices modified with inorganic fillers: TiO2 or SiO2. Conditions of the curing reaction and glass transition temperatures in the cured products are determined through differential scanning calorimetry and broadband dielectric spectroscopy. The curing process is also followed through optical and electrical observations. The dielectric response of all investigated networks reveals a segmental α-process related to structural reorientation (connected to the glass transition). In all products, a similar process associated with molecular motions of polar groups also appears. The matrix modified with TiO2 exhibits two secondary relaxation processes (β and γ). Similar processes were observed in the pure monomer. An advantage of the network with the TiO2 filler is a shorter time or lower temperature required for optimal curing conditions. The physical properties of cured matrices depend on the presence of a nematic phase in the monomer and nonterminal functional groups in the aliphatic chains. In effect, such cured matrices can have more flexibility and internal order than classical resins. Additional modifiers used in this work shift the glass transition above room temperature and influence the fragility index in both cases.

Glass and sub-glass relaxation changes induced by thermal ageing of epoxy-amine polymer networks – A DMA study

Four epoxy-amine networks were characterized by Dynamical Mechanical Analysis (DMA) and by Dielectric Relaxation Spectroscopy (DRS). The epoxy-amine networks were cured with DGEBA and four aliphatic amine hardeners: three ethylamine hardeners of increasing complexity: ethylene diamine (EDA), diethylene triamine (DETA), triethylene tetramine (TETA), and 4,7,10-Trioxa-1,13-tridecanediamine (TTDA). DGEBA-DETA was aged at 160 °C. Its α relaxation temperature was shown to decrease with time, implying a predominant chain scissions mechanism through the thermal oxidation. DGEBA-DETA and DGEBA-TETA were also shown to display a sub-glassy relaxation made of two components. After ageing at 160 °C, the lowest one around −50 °C, associated to the local motions of hydroxypropyl ether segments, increases in intensity. The second one around 30 °C, with unknown origin but probably from motions in more densely crosslinked areas within the network, disappears with ageing. Those results are discussed regarding the molecular aspects of oxidation.

Xylan-tannic acid adhesive combined activated wood interface to construct ultrastrong cross-linking network bonding interface

Biomass adhesives have long been a subject of widespread concern and study due to their cost-effectiveness and minimal environmental impact. In this study, an activated wood surface with epoxy groups (AWI) was created by uniformly applying 3-glycidoxypropyltrimethoxysilane (KH560) onto poplar veneer. Then the aminated xylan-oxidized tannic acid adhesive (2AX-OTA) was prepared based on the Schiff base reaction between the quinone of oxidized tannic acid (OTA) and the amine group of aminated xylan (AX). Eventually, the high-performance bonding interface was co-built by 2AX-OTA and AWI. The dry bonding strength of 2AX-OTA-AWI plywood achieved 3.86 MPa, while the wet bonding strength was measured at 2.41 MPa and 2.15 MPa after immersion in hot (63 °C)/boiling water for 3 h. FT-IR, 13C NMR, and XPS chemical analyses demonstrated that a triple cross-linked network structure of the bonding interface was co-constructed by imine covalent chain bridges and epoxy-amine network. This triple cross-linked network structure plays a crucial role in enhancing water resistance. This study presents a novel approach to designing multi-cross-linked networks for the synthesizing of eco-friendly and high-performance bio-based adhesives.

Design for disassembly of composites and thermoset by using cleavable ionic liquid monomers as molecular building blocks

Aircraft, automotive industries and the increasing numbers of wind turbine blade lead to a huge amount of carbon fiber reinforced polymers requiring to be recycled and/or reused in a closed loop supply chain or circularity of materials. Herein, we have designed and synthesized a tetra-epoxidized imidazolium ionic liquid (Tetra-IL) monomer containing ester-cleavable groups. This monomer was incorporated as a molecular brick platform into conventional epoxy-amine networks in order to tailor the physical properties as well as the end-of-life of the resulting networks. Thus, the introduction of only 10% of IL-based comonomers significantly reduced the gel time by 85% opening perspectives in the field of fast-cure epoxy resins. Overall, all the networks designed in this work presented high thermal stability (>350 °C), higher Tg included between 180 and 230 °C combined with hydrophobic behavior. Increasing the amount of IL monomer in the networks improved the homogeneity of thermosets and wettability of the epoxy resins with the carbon fibers (CF). Most importantly, the use of Tetra-IL led to the development of degradable networks under mild conditions within a brief timeframe (4.5 h) and allowing to recover the carbon fibers. In summary, this study highlighted the great potential of IL-based comonomers as molecular brick into conventional epoxy thermosets as a promising strategy to tailor the physical properties versus sustainability/degradability for the development of high-performance thermosets and composites.

Microencapsulated Diepoxy-Functionalized Ionic Liquids to the Design of Self-Healable Epoxy Networks

An extrinsic self-healing mechanism based on microencapsulated healing agents represents an original way to produce self-healable thermosetting materials without modifying the structural architecture of the co-monomers. In this work, self-healing was achieved through poly(melamine–formaldehyde) (PMF) microcapsules containing a polymerizable diepoxidized ionic liquid monomer denoted as ILEM. First, a synthetic route to design ILEM@PMF microcapsules via in situ polymerization was developed and optimized through the choice of surfactants, core/shell ratios, and stirring speeds. Then, the obtained microcapsules (10 wt %) were incorporated into three different epoxy–amine networks and their effects on the morphology, thermal behavior, i.e., glass transition temperature (Tg) and degradation temperature (Td), as well as on the mechanical properties were investigated. In addition, a pre-crack was generated with a fresh razor blade into the center groove of the epoxy networks and their self-healing performances were observed by scanning electron microscopy before and after the curing process.

Reaction-Induced structural and compositional heterogeneity in amine-cured epoxy/epoxy thermosets: Visualization of heterogeneity using fluorescence lifetime imaging microscopy (FLIM)

The strategic integration of a rhodamine-spirolactam (RS) mechanophore into transparent epoxy-amine (E-A) networks has revealed using fluorescence lifetime imaging microscopy (FLIM), multiple domains in the cured epoxies that are different (compositional and structural heterogeneity) even though only a single glass transition temperature (Tg) is observed in the solid rheology data. For the E-A networks investigated in this study, the FLIM data suggests that heterogeneity exists at two levels (molecular and macroscopic). Heterogeneity in the network composed of diglycidyl ether of bisphenol-A (DGEBA) epoxy cured with meta-phenylenediamine (m-PDA) is associated with the network being under-cured. In the network composed of the diglycidyl ether of 1,4-butanediol (DGEBD) cured with m-PDA, heterogeneity is linked to the oligomers that likely self-associate in the 60 mass % DGEBD medium, thereby creating discrete regions of low crosslink density in the cured network. In the DGEBA (80 mass %)/DGEBD (20 mass %) epoxy blend cured with m-PDA, the heterogeneous morphology in this fully cured network is caused by reaction kinetics differences of the two miscible bis-epoxides with m-PDA at the molecular level and the self-association of the oligomers found in the 60 mass % DGEBD medium at the macroscopic level.

Recyclable thermosets based on modified epoxy-amine network polymers.

The development of high performance, recyclable thermoset materials for applications in plastics, composites, coatings and adhesives requires a synthetic approach where recyclability is designed into the molecular structure of the material. This paper describes a single stage process for the creation of materials from simple, low-cost molecular building blocks, where the polymerisation of liquid epoxy resins and aliphatic amines in the presence of an n-butyl diboronic ester, delivers epoxy-amine-dioxazaborocane materials with tunable physical properties including glass transition temperature (Tg). Mechanical (thermal) recycling and reprocessing of the epoxy-amine-dioxazaborocane thermoset is demonstrated, with retention of Young's modulus and ultimate tensile strength. Most notably, an efficient and low-cost process for the chemical recycling, disassembly and dissolution of the thermoset is demonstrated via two complementary processes using either pinacol (diol) or mono-functional phenylboronic ester.

Structure-Properties Relationships Involved in the Embrittlement of Epoxies.

This paper illustrates a study of the thermal oxidation of several epoxy amine networks. Oxidation was followed at the molecular scale using Fourier Transform InfraRed spectroscopy (FTIR) and at the macromolecular scale using tensile tests. FTIR showed the major formation of amides, while tensile tests showed the progressive increase in the elastic modulus (~0.5 GPa for room temperature Young modulus) and decrease in ultimate strain and volumic energy for failure (assessed using the integrals of stress-strain curves). Both ultimate strain and volumic energy were divided by more than two. Linear correlations between oxidation trackers (amide concentration) and changes in mechanical parameters are presented and discussed.

Experimental and theoretical insights on the thermal oxidation of epoxy-amine networks

This paper reports an investigation of the oxidation of three epoxy-amine networks obtained from DGEBA cured with three sorts of aliphatic hardeners ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetraamine (TETA). Oxidation was monitored by FTIR with an innovative discussion of CH2 consumption. Methylene groups held by hardeners groups were shown to be more sensitive to oxidation, which was confirmed by a discussion of Bond Dissociation Energies (BDE) computed by a machine-learning approach.

Functionalization of cellulose with amine group and cross-linked with branched epoxy to construct high-performance wood adhesive

Sustainable biomass resources are favored by researchers on account of their biodegradability and biocompatibility, which is a replacement for non-renewable fossil fuels. The development of low-carbon, green, and high-value bio-based adhesives are the inevitable trend of the industry development. However, the main factors limiting their application are poor water resistance and low bonding performance. Herein, the crosslinking network was constructed based on the reaction between the epoxy groups of trimethylolpropane glycidyl ether (TMPEG) and the amino groups of the synthesized aminated cellulose (AC) to form an interlocking bond. Through the synergy of covalent bond, electrostatic interaction, and hydrogen bond, the bonding strength and water resistance of the proposed adhesive can be effectively improved. Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and solid-state nuclear magnetic resonance spectroscopy (13C NMR) demonstrated the formation of epoxy-amine network. The excellent bonding strength and water resistance of the adhesive made with AC and TMPEG (AC-TMPEG) are mainly reflected by the dry lap shear strength of 2.56 MPa and the wet lap shear strength of 1.94/2.09 MPa after soaking in 63 °C/boiling water for 3.0 h. This study reveals an approach for manufacturing wood adhesive with superior bonding performance and exceptional water resistance.

Improving mechanical properties and processability of a very high Tg epoxy amine network via anti‐plasticizer fortification

AbstractIn this work, molecular fortifiers are added to a highly aromatic and rigid epoxy monomer bis(2,7 diglycidyl ether naphthalenediol) methane (NNE) possessing a very high glass transition temperature (Tg) when cured with 4,4′‐diaminodiphenyl sulfone (DDS) to explore their impact upon mechanical and thermal properties and reactivity. The molecular fortifiers used are the nonfunctional naphthalene (NAPH), the reactive diluent o‐cresyl glycidyl ether (CGE) and an adduct of dihydroxy naphthalene and CGE (molecular fortifier naphthalene, MFN), a variant on the partially reacted substructures approach. The fortifiers are found to affect NNE/DDS reactivity and increase processability depending upon their propensity to attach to the network either through hydrogen bonding or pi‐pi electron interactions. Thermal analysis shows that the fortifiers increased cure conversion although the Tgs of the networks were generally unaffected until higher levels of addition. The fortifiers reduce moisture ingress and suppress glassy state β relaxations while increasing modulus significantly. Although there is little improvement in toughness overall, some evidence for higher fracture toughness is observed for the MFN and NAPH modified networks. This work highlights the effectiveness of different molecular level fortifiers on improving properties, in particular the rigidity of highly crosslinked networks.

The effect of DOPO concentration and epoxy amine stoichiometry on the rheological, thermal, mechanical and fire‐retardant properties of crosslinked networks

AbstractPresented here is an investigation into the effect of a phosphorus‐based fire retardant on the thermal, rheological and mechanical properties and flammability of an epoxyamine network prepared using stoichiometric and off‐stoichiometric epoxyamine compositions. The flame retardant 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) was reactively incorporated into a blend of a bisphenol‐F diglycidyl ether epoxy resin and a polyfunctional epoxidized phenol Novolac resin, then cured with diethyltoluenediamine and accelerated using imidazole. Increased DOPO concentration improved fire retardancy, but reduced reactivity and thermal and mechanical properties. An increasing amino excess stoichiometry, however, comparatively increased reactivity and improved thermal and mechanical properties. Despite evidence of an earlier onset of degradation and higher char yield, enhanced graphitisation and reduced volatile emissions for the off‐stoichiometric networks, no significant improvements in fire resistance as a function of stoichiometry were observed using UL‐94 and limiting oxygen index measurements. © 2022 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Reactive Molecular Dynamics Study of Hygrothermal Degradation of Crosslinked Epoxy Polymers

Epoxy thermosets are often exposed to high humidity environments in various applications, undergoing reversible and irreversible degradation depending on the environment. This study presents a reactive molecular dynamics (MD) simulation framework to gain deeper insights into the hygrothermal aging process, which is essential to develop a targeted approach to combat water-assisted degradation in epoxy thermosets. By applying ReaxFF potential, an epoxy–amine network is created at low temperatures to avoid unwanted high-temperature side reactions, where the water molecules are added to achieve the desired degree of moisture contamination. The simulations show that in addition to the plasticization effect from the moisture ingress, the epoxy network shows recovery in mechanical properties and density due to the multi-site interaction of the water molecule with the electronegative sites within the network. Moreover, long-term exposure to humidity or direct exposure due to cracking can induce irreversible changes in the epoxy–amine network. The protonation of the water molecule and nucleophilic attack on the C–O bond of the ether linkages in the epoxy–amine networks are successfully simulated by applying reactive MD simulations. Remarkably, the simulations show that the selectivity of water molecules for the hydrolysis reaction in the epoxy network depends on the spatial arrangement and the steric hindrance of the network. This work provides molecular level insights into hygrothermal aging by elucidating the interplay between free volume and polarity of the network in the physical aging of the moist epoxy networks, paving a way for advanced design strategy toward better durability and performance of epoxy thermosets in humid environments.