Preparation and properties of bisphenol A epoxy resin microcapsules coated with melamine–formaldehyde resin

Microcapsules containing epoxy resins have potential applications, such as in adhesive, electronic packaging, and self-healing polymeric composites. A series of microcapsules were prepared by in situ polymerization with poly(melamine–formaldehyde) as the shell materials and a mixture of diglycidyl ether of bisphenol A and epoxy diluent as the core substances. Morphology, chemical structure, mean particle size, and thermal properties of the microcapsules were studied by means of optical microscope, Fourier transform infrared spectroscopy, laser particle size analyzer, and microcomputer differential thermal balance, respectively. Effects of kind of epoxy diluent, surfactant type, emulsifier concentration, and emulsifying rate on the physical properties of microcapsules were investigated. Results indicate that the formation of microcapsules is affected by the epoxy diluent type and surfactant type. The highest core content of the resultant microcapsules is about 88 wt% and average diameters of the capsules range from 67 to 201 μm, which can be adjusted by changing the emulsifier concentration and emulsifying rate. Thermo gravimetric analysis indicated that the prepared microcapsules experienced excellent stability up to 235 °C.     

Investigation of the curing kinetics of alkyd–melamine–epoxy resin system

Properties of coatings based on alkyd resin can be improved via blending with other suitable resins. Recent studies assessed that many properties could be improved by blending with epoxy resins as well as with melamine resins. The aim of this work was to investigate the effect of epoxy resin content on the curing process in alkyd–melamine–epoxy three component blends. The coatings with two mixing ratios of alkyd/melamine (70:30 and 80:20) were formulated. They were made into baking enamels by blending with 3 and 5 wt% of epoxy resin on total resin solid. Curring kinetics was investigated by differential scanning calorimetry (DSC) and application of Ozawa isoconversional method. Fourier transform infrared spectroscopy (FTIR) was used to follow major curing reactions. The absorbance of –OH and –N–CH₂R, showed significant reduction and confirmed that the epoxy resin reacts and inserts in enamel structure. It was found that resin system with alkyd/melamine ratio of 70:30 and 3 wt% of epoxy resin has the lowest apparent activation energy of 141.5 kJ mol⁻¹ and needs the shortest time of 34.2 min to reach final apparent degree of cure. Isothermal DSC experiments have confirmed these findings. The samples with 30 wt% of melamine resin had higher hardness of baked enamels then samples with 20 wt%. They also showed an increase of hardness with the increase of epoxy resin content.     

Synthesis and properties of high-performance and good water-soluble melamine–formaldehyde resin

In this article, high-sulfonated melamine–formaldehyde (HSMF) resins were prepared with a sulfite/melamine (S/M1.5) molar ratio. During the sulfonation process, the reaction temperature and the added velocity of sodium bisulfite affecting the properties of the resin were studied. In the condensation stage, where the pH range is 6.0 and the temperature is about 25°C, the condensation time was prolonged above 24 h. The stability and water solubility of the resin was improved greatly. It is an effective superplasticizer at small dosages of admixture. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3268–3271, 2001     

Synergistic toughening of epoxy resin by CTBN and CM-β-CD

Carboxymethyl-β-cyclodextrin (CM-β-CD) and carboxyl terminated liquid nitrile rubber (CTBN) were used as binary component fillers in toughening the epoxy resin (E-54). For a single component filler system, the addition of CTBN resulted in significantly improved fracture toughness but reduction of glass transition temperature (T_g) and modulus of epoxy resin. On the other hand, the addition of CM-β-CD resulted in a modest increase in modulus and T_g, and significant improvement in toughness. This work provides a promising route of nanocomposites with excellent toughness. Besides the mechanism of synergistic toughening in this project was explained, and the major toughening mechanisms were attributed to interfacial micro-cracks, energy dissipation of CM-β- CD. This work gives us a further understanding of the modification effect of β- CD.     

Self-healing semi-IPN materials from epoxy resin by solvent-free furan–maleimide Diels–Alder polymerization

Novel self-healing Diels–Alder (DA) polymer and the corresponding semi-interpenetrated polymer networks (semi-IPNs) were synthesized and characterized. Initially, a furan-functionalized resin (FFR) was synthesized through the ring-opening reaction of a conventional epoxy resin [diglycidyl ether bisphenol A (DGEBA)] with furfuryl alcohol as a bio-based compound. Subsequently, semi-IPNs with different compositions were obtained through the blending of DGEBA, FFR, 4,4′-diaminodiphenylmethane, and 1,1′-(methylenedi-1,4-phenylene) bismaleimide in the molten state by following a predetermined time–temperature program. Fourier transform infrared and nuclear magnetic resonance analyses confirmed the successful synthesis of the materials. Thermoreversibility via retro-DA (rDA) reaction was evidenced by differential scanning calorimetry (DSC) and sol–gel transition tests. Repeated DSC cycle was successfully performed thrice on the DA polyadduct which corroborated repeatability of the DA/rDA association/dissociation. Self-healing and mechanical properties were preliminarily evaluated by scanning electronic microscopy and flexural testing analyses, respectively. The self-healing efficiencies were around 80 and 95% for semi-IPN and DA polyadduct, respectively, based on flexural strength. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48015.     

Flame-retardant epoxy resin with good smoke-suppression endowed by chitosan–cobalt/phosphorus complex

Chitosan-based flame-retardant CS–Co–DOPA (CCD) was synthesized by the neutralization reaction of 10-hydroxy-9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPA) with chitosan-cobalt complex and fully characterized by scanning electron microscopy (SEM), energy-dispersive spectrometer, x-ray diffraction, X-ray photoelectron spectroscopy (XPS), optical emission spectrometer, and Fourier transform infrared (FTIR) characterizations. The epoxy resin (EP) modified with CCD exhibited good flame retardancy. With the addition of 5 wt% CCD, the EP/CCD achieved UL-94 V-1 rating and possessed limiting oxygen index (LOI) value of 30%. Cone calorimetry (CC) test demonstrated that EP/CCD resulted in a remarkable reduction of peak smoke production rate (pSPR) and total smoke production (TSP) by 63% and 40%, respectively, showing an outstanding smoke suppression. The char residue obtained from the CC test was further characterized using SEM, FTIR, Raman, and XPS techniques. The results revealed that CCD facilitated the formation of a dense and compact char layer on EP during combustion, thereby impeding gas and heat transfer. In addition, TG-IR was employed to investigate the gas-phase flame-retardant effect of EP/CCD composites, which revealed that CCD promotes the release of water, CO_2, and other incombustible gases, altering the decomposition path of EP.     

Microstructure and thermal properties of silyl-terminated polycaprolactone–polysiloxane modified epoxy resin composites

With a direct nucleophilic addition between  OH groups of polydiol and  NCO of a silane, a blend of silyl-terminated polycaprolactone PCL-Si and silyl-terminated polydimethylsiloxane PDMS-Si oligomer, PCS-2Si, were firstly prepared, and then blended with a commercial epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) to form a ternary composite. The formed ternary composites of different content of DGEBA were cured using a polyamidoamine as a curing agent and a sol–gel process at ambient temperature. The microstructures and properties of the cured composites were investigated by SEM, TGA, and energy dispersive spectroscopy. The results showed the compatibility between DGEBA and PDMS increased with increasing content of PCS-2Si, but higher content of PCS-2Si resulted in a slight enrichment of silicon in the surface of the cured film. TGA showed that incorporating PCS-2Si into epoxy resin altered the composites' thermal stability and degradation characteristics. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation of melamine–formaldehyde resin grafted by (3-aminopropyl) triethoxysilane for high-performance hydrophobic materials

Aiming at meeting the specific market demands and expanding the downstream application of melamine–formaldehyde (MF) resins, a series of (3-aminopropyl) triethoxysilane (APTES) grafted MF (MF-Si) resins were synthesized via an effective method that minimized the hydrolysis of APTES and overcame the polarity discrepancy of APTES with MF resin matrix. The structure of MF-Si resins was characterized by FTIR spectroscopy, Raman spectroscopy, ¹H nuclear magnetic resonance (NMR), and solid state ¹³C NMR. It was found that APTES moieties in MF-Si materials afforded increased hydrophobicity, water resistance, and the thermal stability was not affected. With the increasing amount of APTES, the water contact angle of MF-Si films increased from 70.56 to 105.92°and the surface free energy decreased from 46.8 to 23.5 mN/m. The temperature of maximum weight loss rate (T_dmax) of MF-Si materials decreased slightly from 371.15 to 353.70 °C and the ultimate residual weight of MF-Si materials increased from 12.51 to 30.04% at 800 °C under N₂. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48664.     

Characterisation and modelling of CNT–epoxy and CNT–fibre–epoxy composites

Abstract

This research presents an experimental and theoretical investigation on the effects of carbon nanotube (CNT) integration within neat epoxy resin (nanocomposites) and a carbon fabric–epoxy composite (multiscale composites). An approach is presented for the prediction of mechanical properties of multiscale composites. This approach combines woven fibre micromechanics (MESOTEX) with the Mori-Tanaka model which was used for the prediction of mechanical properties of nanocomposites in this research. Nanocomposite and multiscale composite samples were manufactured using cast moulding, resin infusion, and hand lay-up process. The CNT concentrations in the composite samples were from 0 to 5 wt-%. The samples were characterised using tensile, shear and flexural tests. The discrepancy between the theoretical predictions and the experimental observations was hypothesised to be due to dispersion and bonding issues and SEM images are presented in support of the hypothesis.     

Cure kinetics of melamine–formaldehyde resin/clay/cellulose nanocomposites

As a part of improving the properties of surface laminates for wood-based panel products, this study attempted to investigate cure kinetics of the melamine–formaldehyde (MF) resin/clay/cellulose nanocomposites. Three different methods (Ozawa, Kissinger, and isoconversion) of differential scanning calorimetry (DSC) were employed to study cure kinetics of the nanocomposites, using three different heating rates (5, 10 and 20 °C/min). Both Ozawa and Kissinger methods showed that the overall activation energy (E_a) of the nanocomposite at the 0.5 wt% nanoclay level reached a maximum and then decreased thereafter. But, the Ozawa method provided greater E_a values than those of the Kissinger method. The isoconversional method provided the change of activation energy (E_α) values as a function of the degree of conversion (α). The E_α values increased as the degree of conversion increased, while the influence of nanoclay levels followed a similar trend to the overall E_a values from the both Ozawa and Kissinger methods. These results indicated that the exfoliation of layered nanoclay particles into MF resin delayed the cure of MF resin/nanoclay/cellulose nanocomposites.     

Glass/epoxy composites and aluminium alloy joint using Kevlar® intermediate layer and nanoclay-reinforced epoxy adhesive

Adhesive lap joint between glass fibre/epoxy composites and aluminium alloy (2014 T4) was prepared by an in situ moulding process using a matched die mould. The surface of aluminium alloy was treated with chromic acid before adhesive bonding. Lap shear strength and fatigue life were evaluated in tensile mode and tension–compression mode (at 40% of lap shear load of adhesive joint), respectively. Knurling on the surface of aluminium alloy improved the lap shear strength of the adhesive joint but did not influence the fatigue life of the same. Lap shear strength and fatigue life of adhesive joint made with neat epoxy adhesive and reinforcement of an intermediate layer of Kevlar^® between glass/epoxy composite and aluminium alloy were observed to be 0.44?kg/mm² and 3.6?×?10⁵ cycles, respectively. In another case, lap shear strength and fatigue life of similar type of adhesive joint made from nanoclay (Cloisite 30B)-reinforced epoxy adhesive and without reinforcement of an intermediate layer of Kevlar^® were observed to be 0.38?kg/mm² and 2.3?×?10⁵ cycles, respectively. Whereas, lap shear strength and fatigue life of adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® were 0.48?kg/mm² and 3.9?×?10⁵ cycles, respectively. Therefore, adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® was the best.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

Fabrication,performances, and reaction mechanism of urea–formaldehyde resin adhesive with isocyanate

In order to improve the performances of urea–formaldehyde (UF) resin adhesive and reduce formaldehyde emission, the isocyanate was applied to modify the UF resin adhesive. The effects of the composition ratio of the isocyanate/UF resin on the thermal stability and molecular structure of the composite adhesive were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analyzer. The results showed that the appropriate addition of the isocyanate into the UF resin is favorable to improve the performances of the composite adhesive due to the chemical cross-linking reaction of the isocyanate with UF. Furthermore, the reaction mechanism of the UF resin adhesive with isocyanate was analyzed in detail. These research results can be applied to aid materials engineering design for the development of new adhesive, quality assurance, and characterization assessment of durability.     

Enhanced performance of urea–glyoxal polymer with oxidized cassava starch as wood adhesive

To eliminate the hazard of formaldehyde from wood-based products to human and environment, formaldehyde was replaced by glyoxal to produce wood adhesive. Urea–glyoxal (UG) resin was environmental friendly, while its bonding strength was very poor, especially its water resistance. The object of this work was to improve the mechanical properties of UG resin by oxidized cassava starch addition. Hence, the urea–glyoxal (UG) resin was synthesized and the oxidized cassava starch was added through mechanical mixing. The bonding strength, structure distributions, and the morphology features of the cured UG resin system were investigated by producing a three-layer plywood, FTIR, and SEM analysis. The results of dry and wet shear strength of plywood indicated that there was a positive effect of oxidized cassava starch on bonding strength of a three-layer plywood, and when the oxidized cassava starch content was increased to 45%, the dry strength could reach 1.21 MPa, and the wet strength was 0.72 MPa. The FTIR results showed that chemical reaction between UG resin and oxidized cassava starch was beneficial to the branched structure formation and higher cohesion strength of UG resin. Meanwhile, the tightness structure of enhanced UG resin system was observed by SEM analysis as well. These improved properties were contributed to water resistance improvement of UG resin.

     

Strain rate and curing condition effects on the stress–strain behaviour of epoxy adhesive materials

Adhesive materials evolve properties that change significantly with the preparation procedures and curing conditions. In this study the effects of curing conditions (curing time and temperature), and strain rate on the stress–strain behaviour of the commercially available Lapox epoxy adhesive materials have been evaluated experimentally. The rectangular test specimens have been prepared with different curing temperatures and times. After preparation, the specimens have been tested in small scale tensile testing machine to investigate the stress–strain behaviour at room temperature. It has been observed that as the curing time or curing temperature is increased, the ultimate tensile strength and the elastic modulus of the material also increase. A four parameter hyperbolic tangent model has been fitted to the experimental data and the model constants have been evaluated for different curing conditions and strain rates. Furthermore, for a fixed curing time and strain rate, empirical equations have been developed for modelling the dependence of curing temperature on the stress–strain curves. Finally, the developed equations have been implemented into the finite element analysis of a lap joint to investigate the stress and strain distributions of the adhesive layer for different curing conditions (curing time and temperature).     

Flame retardancy of water-based intumescent coatings with etherified melamine–formaldehyde and polyvinyl acetate copolymer hybrid resin

This study investigated etherified melamine–formaldehyde (MF) and polyvinyl acetate copolymer hybrid resins with different intumescent formulations to improve the flame retardancy of plywood. The CO and CO₂ emissions of intumescent coatings were also investigated. The miscibility of two resins is indicated by a single cure peak. Intumescence with a hybrid resin demonstrated better flame retardancy relative to that with a pure etherified MF resin. The results of scanning electron microscopy and a cone calorimeter test indicated that a lower binder resin (BR) content enhances fire retardancy and forms an ideal char. Furthermore, an evaluation of total heat release in addition to CO and CO₂ emissions (for 300 s) revealed that the intumescent coating had the same flame retardancy when the BR content was increased by 40%. The survival duration of the chemical structures of the phosphocarbonaceous chars was verified using Fourier-transform infrared spectroscopy and solid-state phosphorus-31 nuclear magnetic resonance analyses.     

Effect of surface pre-treatment on the adhesive strength of epoxy–aluminium joints

A detailed study of the effect of pre-treatment applied on the surface characteristics of aluminium substrates and on the adhesive strength of epoxy–aluminium joints is reported. The variation of the density, composition and aspect of the adherends were analysed as a function of the applied pre-treatment. In order to determine the influence of alloying elements, two different aluminium alloys were used, A1050 and A2024. The adhesive strength was measured by the lap shear test, using several epoxy resins to analyse the influence of the adhesive nature.A chromate-free treatment based on the sulphuric acid-ferric sulphate etch provided an improved joint strength compared to dichromate-sulphuric acid etching, alkaline etching or mechanical abrasion. This increase is associated to the porous oxide layer formed, but it depends on the adhesive nature used. The joints with Al–Cu–Mg alloy substrates generally presented higher adhesive strength values than those with pure aluminium adherends, due to the selective etching of some allowing elements and intermetallic compounds, which have different electrochemical potential.     

Hydroxyl terminated PEEK-toughened epoxy–amino novolac phthalonitrile blends – Synthesis,cure studies and adhesive properties

Self cure promoting, amine-containing novolac–phthalonitrile (APN) resins of varying compositions were synthesized and characterized. APN possessing amine functionalities reduced the cure initiation temperature from 310 °C (typical of pure phthalonitrile systems) to 180 °C. It showed excellent thermal stability up to 420 °C and high char residue of 77–79 %. Co-reaction of APN with diglicydyl ether of bisphenol A (DGEBA) led to a decrement in their thermal stability though improved their adhesive properties. Evidences were obtained for epoxy–amine, epoxy–phthalonitrile and amine–phthalonitrile reactions. The latter reactions led to formation of oxazoline, triazine and phthalocyanine groups in the network. These were rationalized by density functional theory studies on model compounds. The extents of epoxy–amine and epoxy–phthaonitrile reactions were quantified. Introduction of hydroxyl terminated poly ether ether ketone (PEEK) reduced the brittleness of the blends and improved their lap shear strength. Toughening of epoxy–amino novolac phthalonitrile networks occurred through phase separation of PEEK segments in cured matrix.     

Electrical properties of a composite comprising epoxy resin and α-hematite nanorods

Electrical properties of pure epoxy and epoxy-hematite nanorod composites have been investigated. The nanorods were synthesized by the forced hydrolysis method and further mixed with epoxy to obtain the nanocomposite. TEM analysis revealed that they have an average diameter of about 8 nm, with an average aspect ratio of 25. DC-conductivity and DC-current relaxation measurements showed a significant influence of Fe₂O₃ nanorods on the DC-electrical properties of the epoxy matrix. However, the observed effects of the filler below and above the glass transition are different. Because of their high specific surfaces, nanorods affected segmental mobility of epoxy molecules to a large extent, which resulted in an increase in the glass transition temperature (T_g) and a decrease in the real part of dielectric permittivity in high frequency/low temperature region. It is further observed that at elevated temperatures (above T_g) and low frequencies the real part of dielectric permittivity of the nanocomposite exceeds that of the pure matrix, i.e. there is a transition towards microcomposite-like dielectric behaviour.     

Effect of proanthocyanidin incorporation into dental adhesive on durability of resin–dentin bond

BackgroundProanthocyanidin has shown to have beneficial effects on dentin bonding via its collagen cross-linking and protease inhibitory effects.ObjectiveThis study evaluated the effect of incorporation of 1–3% PA into a dental adhesive on durability of resin–dentin bond.Materials and methodsThe experimental adhesive was first formulated by combining 50 wt% comonomer mixtures with 50 wt% ethanol. PA was then added to the ethanol-solvated adhesive to yield three groups of adhesives at concentrations of 1.0 wt%, 2.0 wt% and 3.0 wt%. The PA-free adhesive served as control. Flat dentin surfaces from forty extracted third molars were etched with 32% phosphoric acid and the specimens were randomly assigned to one of the four adhesive groups. Two layers of experimental adhesives were applied to etched dentin and light-cured for 20 s after solvent evaporation. Composite build-ups were performed using Filtek Z250 (3M ESPE). The bonded teeth were divided into three subgroups for different methods of storage: (1) 24 h indirect water exposure (IE), (2) 6 M IE and (3) 6 M direct water exposure (DE). After the designated period of water storage, the bonded teeth were sectioned into 0.9 mm×0.9 mm beams for bond strength testing. Bond strength data were evaluated by two-way ANOVA and Tukey׳s tests (α=0.05). Interfacial nanoleakage was examined using a field-emission scanning electron microscopy. Two-way ANOVA and Tukey׳s tests were used to examine the effects of PA concentration and water exposure on bond strength and percentage of nanoleakage (α=0.05).ResultsTwo-way ANOVA showed that the factors, water exposure and PA concentration had a significant effect on bond strength (p<0.001). Interaction between the two factors was also significant (p<0.001). Bond strength of all four adhesives decreased with PA concentrations and ageing. Type of water exposure had no effect on the bond strength of PA-incorporated adhesive; while direct water exposure significantly reduced the bond strength of PA-free adhesive. Conversely, the factors, water exposure and PA concentration showed a significant effect on nanoleakage percentage (p<0.001). Interaction between the two factors was not significant (p>0.05).ConclusionIncorporation of proanthocyanidin into a dental adhesive did not prevent resin–dentin bond degradation over time.