Thermoinitiated cationic polymerization of epoxy resins by sulfonium salts

Catalytic activities of benzyl sulfonium salts with nonnucleophilic anions (BF₄^?,PF₆^?,AsF₆^?,SbF₆^?) for epoxy polymerization were examined. The order of reactivities correlated well with nonnucleophilicity of anions. Epoxy resins catalyzed by these salts are characterized by longer shelf-life and shorter gel time at high temperature. Some physical properties of cured product were compared with those cured by a commonly used BF₃ amine complex.     

Photocurable bulk epoxy resins based on resorcinol derivative through cationic polymerization

Bio-sourced epoxy resins from resorcinol diglycidyl ether (RDGE) have been obtained by using cationic photopolymerization under UV-light exposure. The photoinduced bulk resin samples were characterized by three-point bending tests, dynamic mechanical analysis, as well as differential scanning calorimetry analysis, and thermogravimetric analysis. The influence of processing parameters, that is, reactant contents, UV irradiation time, and postcuring conditions on the thermomechanical behavior has been pointed out. For instance, the flexural modulus of the most performing materials reaches 4.1 GPa with the flexural strength and the glass-transition temperature of around 105 MPa and 99°C, respectively. Interestingly, our optimized protocol has led to the synthesis of new bio-based materials with more valuable thermal and mechanical properties than those of thermocured materials obtained from petroleum-based commercial epoxy resins. Focus has been given on processing parameters to optimize the final properties of the material and to open an interesting alternative for sustainable building materials.     

Self-healing epoxy based on cationic chain polymerization

A two-component healing agent consisting of epoxy- and ((C₂H₅)₂O·BF₃)-loaded microcapsules was synthesized and applied to fabricate self-healing epoxy composites. Curing of epoxy healing agent catalyzed by (C₂H₅)₂O·BF₃ belongs to cationic chain polymerization, which is characterized by fast reaction at ambient temperature and low catalyst concentration. The experimental results showed that cracks in the composites containing the above healing system can be quickly re-bonded with satisfied healing efficiency. In the case of 5 wt% epoxy- and 1 wt% (C₂H₅)₂O·BF₃-loaded capsules, for example, a ∼80% recovery of impact strength was detected within 30 min at 20 °C. Because the healing capsules took effect at low content, mechanical properties of the matrix were largely retained. It is believed that the present healing system provides a possible solution to preparation of self-healing polymeric materials with practical applicability.     

Solid state NMR investigation of cationic polymerized epoxy resins

Cationic UV-polymerized resins, based on cycloaliphatic epoxides with aryl sulfonium salts as photoinitiators and polypropylene glycols of variable length as flexibilizers, have been investigated with respect to the network-forming reactions and the morphology of the resulting polymers. The combination of UV-curing and thermal postcure makes the chemical process complex and dependent on many variables, such as photoinitiator, exposure time, reaction temperature, and polyol components. Structure and curing of the epoxide were investigated by ¹³C CPMAS NMR. Morphology and domain sizes were studied by spin diffusion experiments using the novel “dipolar filter” technique for selective magnetization of mobile components.     

Synthesis and properties of cationic resins derived from polyether/polyester-modified epoxy resins

Four polyether and one polyester-modified cationic resins were synthesized by reacting polyether/polyester-modified epoxy resins with 2-ethylhexanol-blocked-toluene diisocyanate (2-EH-blocked TDI) and diethanolamine and subsequently neutralizing the resins with acetic acid. Four different polyethers and one polycaprolactone diol (PCP) were used to react with epoxy resin to form polyether-modified epoxy resins (1a–d) and polyester-modified epoxy resin (1e). The extent of reaction of epoxy resin and polyether or polyester was evaluated by the change of epoxy equivalent weight and the gel permeation chromatography curve of the resulting product. Cationic resins were dissolved in suitable solvents and were mixed with deionized water to form emulsions. Some factors, such as pH value of emulsion, solvent content, and applied voltage affecting the emulsion and electrodeposition properties, were investigated. Cationic resins, prepared from PPG (#1000)-modified epoxy resins, yielded a wider pH range of stable emulsion and also yielded deposited films with a pleasing appearance. PEG (#1000)-modified cationic resins produced a higher deposition yield, but higher throwing power was obtained by deposition of the PCP (#530)-modified cationic resins.     

Synthesis and properties of cationic resins derived from aniline-modified epoxy resins and various amines

Aniline-modified epoxy resin which contains tertiary amine in the middle of the polymer chain was synthesized by the reaction of aniline and epoxy resin. The resulting aniline-modified epoxy resin and two commercial epoxy resins with different epoxy equivalent weights were reacted with 2-ethylhexanol-blocked toluene diisocyanate (2-EH-blocked TDI) to obtain thermally crosslinkable epoxy resins. These epoxy resins were subsequently reacted with various secondary amines and partially neutralized with acetic acid to give thermally crosslinkable cationic resins. The resulting cationic resins were dissolved in suitable solvents and mixed with deionized water to form emulsions. The crosslinking properties, emulsion, and electrodeposition properties of these resins were studied in some detail. The electro-deposition yields of the emulsions prepared from aniline-modified epoxy resins were higher than those of other emulsions. The crosslinked films prepared from aniline-modified epoxy resins were also glossier than those prepared from commercial epoxy resins. High deposition yield and high glossiness were the characteristic properties of the aniline-modified epoxy resins. Thermal properties were not affected by aniline-modified epoxy resins.     

Thermally initiated cationic polymerization and properties of epoxy siloxane

Difunctional epoxy siloxane monomers containing disiloxane, trisiloxane, and tetrasiloxane were prepared by hydrosilylation of an α,ω‐difunctional Si? H‐terminated siloxane with a vinyl‐functional epoxide. Cationic polymerization of these monomers using 3‐methyl‐2‐butenyltetramethylenesulfonium hexafluoroantimonate and their reactivities were examined. The reactivity order was disiloxane > trisiloxane > tetrasiloxane. Thermal discoloration of these polymers increased with catalyst concentration and also with the length of dimethyl siloxane. UV discoloration was also accelerated by catalyst. From the thermo gravimetric analysis, it was found that the thermal stabilities of polymers increased with increasing the length of dimethyl siloxane chain. Mechanical properties of polymers were also tested by thermal mechanical analysis and dynamic mechanical analysis, and it was found that the flexibility of polymers was increased with increasing siloxane chain length. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2010–2019, 2006     

光引发阳离子聚合及其在环氧树脂固化研究中的进展

从光引发阳离子聚合的特点、常见阳离子型光敏引发剂及其引发聚合机理、光引发环氧树脂阳离子聚合体系及发展前景等几个方面进行了简要概述。     

Synthesis and properties of cationic resins derived from aniline/benzylamine-modified epoxy resins and diethylamines

Aniline/benzylamine-modified epoxy resins with different molecular weights, which contain tertiary amines in the middle of the polymer chain, were synthesized by the reaction of aniline/benzylamine with epoxy resin at various molar ratios. The resulting aniline/benzylamine-modified epoxy resins were reacted with diethylamine and subsequently reacted with 2-ethylhexanol-blocked toluene diisocyanate to obtain thermally crosslinkable resins which contain tertiary amines at the end and in the middle of the polymer chain. These resins were partially neutralixed with acetic acid to give thermally crosslinkable cationic resins. The resulting cationic resins were dissolved in suitable solvents and mixed with deionized water to form various emulsions. The emulsion and electrodeposition properties of these resins were studied in some detail to compare the properties of these cationic resins. The results show that the deposition yields and throwing power of the emulsions prepared from benzylaminemodified epoxy resins are higher than those of the emulsions prepared from anilinemodified epoxy resins. The emulsion having proper pH values can give a high throwing power. High throwing power is the characteristic property of these modified cationic resins. Factors determining the throwing power and deposition yield of the emulsions were also investigated.     

Effect of moisture on cationic polymerization of silicone epoxy monomers

Moisture in polymerization of a cationically cured silicone epoxy monomer blend is an important parameter that affects the resulting polymer properties. We report the kinetics of the cationic polymerization of epoxy monomers as a function of water concentration, directly quantified using Karl Fischer (KF) titration that was characterized using Fourier transform infrared (FTIR) spectroscopy and also the mechanical strength of resulting polymers via diametral tensile strength measurements. Methodology and results for a silicone epoxy monomer material were compared with the same methodology applied to a “control” monomer, 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexyane carboxylate, for which moisture effects have been previously studied. Initially, an increase in moisture during cationic polymerization of epoxy caused increased rate (ROC) and degree of conversion (DOC) that for the silicone epoxy was followed by decreased DOCs for water contents approaching saturation, i.e., [H₂O]∼0.19 wt %. Further, the rate of conversion was also affected by the presence of moisture with a trend analogous to the DOC. Diametral tensile strength measurements found that small amounts of water present during polymerization caused small changes in tensile strength but found polymer strengths to be significantly decreased if initial water concentrations approached saturation or were in excess of saturation. Lower strengths corresponded with reduced rates of conversion and DOCs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41831.     

Concomitant cationic polymerization of a hybrid monomer and an epoxy resin

The concomitant cationic polymerization of an epoxy resin and a hybrid monomer is a new concept for toughening epoxy resins. Thermolatent super acids are superior initiators of twin monomer polymerization and copolymerization. Determination of gel yield and instrumental characterization of the formed complex polymer network indicate that copolymerization occurs, integrating the linear silicone and novolak formed during twin polymerization into the epoxy network.     

Electro-initiated cationic polymerization of α-methylstyrene by direct electron transfer

Electro-initiated cationic polymerization of α-methylstyrene in a tetrabutylammonium fluoroborate-dichloromethane system has been investigated at various temperatures by constant potential electrolysis. It is established that polymerization occurs at low temperatures with higher yields. Apparent activation energy of the electro-initiated polymerization is ?64.3 kJ mol^?1.     

Studies on epoxy resins cured by cationic latent thermal catalyst at elevated temperature

In this work, the effect of the catalyst content was investigated in terms of the thermal and mechanical properties of epoxy resins at elevated temperature. A synthesized cationic latent thermal catalyst, N‐benzylpyrazinium hexafluoroantimonate (BPH), was used to cure epoxy resins. The content of catalyst added was 0.5, 1, 2, 3 and 5 wt% and the heating time was varied from 0 to 1024 h. As a result, the mechanical properties, including flexural strength, elastic modulus in flexure and impact strength, as well as thermal‐oxidative resistance, showed a maximum value in the presence of 1 wt% BPH. With increasing elapsed time, up to 4 h, the thermal and mechanical properties of the specimens were improved. These results show that the internal structure of the epoxy system was stabilized, and post‐curing for a longer period of time, resulted in improved thermal and mechanical behaviour of the cast specimens. Copyright © 2004 Society of Chemical Industry     

Frontal cationic curing of epoxy resins in the presence of defoaming or expanding compounds

Thermal frontal polymerization was carried out with trimethylol propane triglycidyl ether using two different BF₃‐amine complexes, B‐950 and B‐110 from Leepoxy, as initiators for cationic polymerization. The amounts of filler (kaolin or fumed silica), defoaming, or expansion agents were varied to study how the compositions affected the front velocity, expansion, and flexural modulus of the resulting epoxy resins. The polymer produced with B‐950 initiator showed higher modulus than the polymers produced with B‐110. Moreover, fumed silica created stronger materials than kaolin. The presence of BYK as a defoamer or an expansion agent such as the Expancel #80 was also able to affect significantly the mechanical properties. differential scanning calorimetry studies indicated that the conversion was complete and that kaolin and silica increased the rate of reaction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40339.     

Multifunctional epoxy resins

The curing behavior of epoxy resins prepared by reacting epichlorohydrin with 4,4′-diaminodiphenyl methane (DADPM)/4,4′-diaminodiphenyl ether (DADPE) or 4,4′-diaminodiphenyl sulfone (DDS) was investigated using DDS and tris-(m-aminophenyl)phosphine oxide (TAP) as curing agents. A broad exothermic transition with two maxima were observed in the temperature range of 100–315°C when TAP was used as the curing agent. The effect of varying DDS concentration on curing behavior of epoxy resin was also investigated. Peak exotherm temperature (T_exo) decreased with increasing concentration of DDS, whereas heat of curing (ΔH) increased with an increase in amine concentration up to an optimum value and then decreased. Thermal stability of the resins, cured isothermally at 200°C for 3 h, was investigated using thermogravimetric analysis in a nitrogen atmosphere. Glass fiber-reinforced multifunctional epoxy resin laminates were fabricated and the mechanical properties were evaluated. © 1993 John Wiley & Sons, Inc.     

Radiopaque epoxy resins

Transparent, X-ray contrast (radiopaque) epoxy resins were obtained by dissolving up to 25 wt % triphenylbismuth in the commercial epoxy resin prepolymers EPON-815, DER-330, DER-383, and DEN-431 which were then hardened with diethylenetriamine. The radiopacities of the mixtures were found to be proportional to the molar concentration of the radiopaque additive. The systems follow the relationship, R = R_o + (R_a ? R_o) V _a M_a where R, R_o, and R_a are the radiopacities of the mixture, the pure epoxy resin, and triphenylbismuth, respectively (expressed in mm aluminum/mm resin); M_a and V _a denote the molar concentration and molar volume of the bismuth compound. R_a for triphenylbismuth was found to be 7.4 ± 0.2 mm Al/mm resin; the average value of R_o for the four epoxies equals 0.16 ± 0.1 mm Al/mm resin. The amount of amine required to harden the radiopaque resins was far less for the epoxy novolac resin DEN-431 than for the three bisphenol-A based epoxies. The concentration of triphenylbismuth required to impart a radiopacity equivalent to that of aluminum measures 14.6 wt % in EPON-815, 14.8 wt % in DER-330, 14.9 wt % in DER-383, and 15.9 wt % in DEN-431. The radiopaque resins remain transparent indefinitely, even when exposed to water. © 1995 John Wiley & Sons, Inc.     

Deoxybenzoin-based epoxy resins

The diepoxide of bishydroxydeoxybenzoin, termed BEDB, was prepared and used as a diepoxide in adhesive formulations with various aromatic diamine cross-linkers. These novel epoxy resins were characterized and compared to the properties of bisphenol A (BPA)- and 3,3′,5,5′-tetrabromobisphenol A (TBBA)-based epoxies in terms of their thermal and mechanical properties. Cured formulations were characterized to determine glass transition temperatures by differential scanning calorimetry (DSC). The char residue, heat release capacity, dynamic mechanical properties, fracture toughness, and adhesion strength of the cured resins were investigated by thermogravimetric analysis (TGA), microscale combustion calorimetry, dynamic mechanical analysis (DMA), plain-strain fracture toughness tests, and lap shear tests. The BEDB-based resins exhibited significantly higher fracture toughness and adhesion strength compared to the BPA-epoxy resins, as well as low heat release properties (i.e., lower flammability) despite the absence of halogen.     

Fracture and mechanical properties of epoxy resins and rubber-modified epoxy resins

Fracture and mechanical property data on a wide range of epoxy resin systems are presented. The extent to which toughening can be induced by heterophase rubber inclusions depends more on the curing agent used than on the resin component. The greatest improvements in toughness were obtained by rubber modification of epoxy resins cured with an anhydride. A preformed ABS polymer can be used to toughen many epoxy resin systems. With one major exception (where a large improvement was found) only small changes in tensile properties occur when small amounts of rubber are present.     

Polyurethane-ductilized epoxy resins

Amine-cured epoxy resins were modified to improve their impact properties. Urethane prepolymers (PUs), in which terminal isocyanate groups were blocked with nonylphenol (NP) for easy handling, were used as modifiers. The synthesis of the elastomers were carried out at different NCO : OH ratios: 1 : 1, 2 : 1, and 3 : 1 (PU1, PU2, and PU3). Characterization of these materials by GPC and FTIR indicated that PU1 has a negligible amount of NCO-terminated chains and no unreacted toluenediisocyanate (TDI). PU2 and PU3 have free-blocked TDI in the mixture, even after distillation under a vacuum. The molecular weight and polydispersity of the prepolymer increases as PU3 < PU2 < PU1. Copolymerization was carried out by crosslinking with a mixture of cycloaliphatic amines, which react with the epoxy ring and with the NCO groups by deblocking and urea formation. Dynamic mechanical tests were used to measure the glass transition temperatures (T_g) of the copolymers. Two T_g were found if PU1 was the epoxy modifier, indicating that phase separation took place. This was correlated with a structure of PU1 of linear chains with a negligible amount of reactive groups. Flexural and compression properties showed negligible changes for PU2- and PU3-modified epoxy, but the critical strain energy release rate (G_1C) was improved if PU2 was the modifier. This behavior was explained by the linkage of elastomeric chains into the epoxy network. The PU1–epoxy copolymer showed a completely different behavior, with the bending modulus (E_b) reduced to almost one-half with respect to that of the epoxy matrix and with largely improved impact properties. This difference was attributed to the separation of an elastomeric phase, which favors the formation of shear bands in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1781–1789, 1998     

Effects of ultraviolet and electron radiations on graphite-reinforced polysulfone and epoxy resins

Degradation mechanisms have been investigated for graphite/polysulfone and graphite/epoxy laminates exposed to ultraviolet and high-energy electron radiations in vacuum up to 960 equivalent sun hours and 10⁹ rads, respectively. Based on GC and combined GC/MS analysis of volatile by-products evolved during irradiation, several free radical mechanisms of composite degradation have been identified. All the composite materials evaluated have shown high electron radiation stability and relatively low ultraviolet stability as indicated by low G values and high quantum yields for gas formation. Mechanical property measurements of irradiated samples did not reveal significant changes, with the possible exception of UV exposed polysulfone laminates. Hydrogen and methane have been identified as the main byproducts of irradiation, along with unexpectedly high levels of CO and CO₂. Initial G values for methane relative to hydrogen formation are higher in the presence of isopropylidene linkages, which occur in bisphenol-A based resins.