Volume changes at macro- and nano-scale in epoxy resins studied by PALS and PVT experimental techniques

A systematic study on changes in the volumes at macro- and nano-scale in epoxy systems cured with selected aminic hardeners at different pre-cure temperatures is presented. Free- and macroscopic specific-volumes were measured by PALS and pressure–volume-temperature techniques, respectively. An analysis of the relation existing between macro- and nano-scales of the thermosetting networks developed by the different chemical structures is shown. The result obtained indicates that the structure of the hardeners governs the packing of the molecular chains of the epoxy network.     

High resolution solid‐state 13C‐NMR study of as‐cured and irradiated epoxy resins

This article presents the effects of strong ionizing radiations on the physico‐chemical modifications of aliphatic or aromatic amine‐cured epoxy resins based on diglycidyl ether of bisphenol A (DGEBA). Such epoxy resins have a considerable number of applications in the nuclear industrial field and are known to be very stable under moderate irradiation conditions. Using extensively high resolution solid‐state ¹³C‐NMR spectroscopy we show that the aliphatic amine‐cured resin (DGEBA‐TETA) appears much more sensitive to gamma rays than the aromatic amine‐cured one (DGEBA‐DDM). On the one hand, qualitative analyses of the high resolution solid‐state ¹³C‐NMR spectra of both epoxy resins, irradiated under similar conditions (8.5 MGy), reveal almost no change in the aromatic amine‐cured resin whereas new resonances are observed for the aliphatic amine‐cured resin. These new peaks were interpreted as the formation of new functional groups such as amides, acids and/or esters and to alkene groups probably formed in the aliphatic amine skeleton. On the other hand, molecular dynamics of these polymers are investigated by measuring the relaxation times, T_CH, T_1ρH and T_1C , before and after irradiation. The study of relaxation data shows the formation, under irradiation, of a more rigid network, especially for the aliphatic amine‐cured system and confirms that aromatic amine‐cured resin [DGEBA‐4,4′‐diaminodiphenylmethane(DDM)] is much less affected by ionizing radiations than the aliphatic amine‐cured resin [DGEBA‐triethylenetetramine(TETA)]. Moreover, it has been shown that the molecular modifications generated by irradiation on the powder of the aliphatic‐amine‐cured resin appear to be homogeneously distributed inside the polymers as no phase separations can be deduced from the above analyses. Copyright © 2001 John Wiley & Sons, Ltd.     

Synthesis and effect of hyperbranched (3-hydroxyphenyl) phosphate as a curing agent on the thermal and combustion behaviours of novolac epoxy resin

A novel type of hyperbranched (3-hydroxyphenyl) phosphate (HHPP) with high functionality as a curing agent of epoxy resins was synthesized and characterized by FTIR, ¹H NMR and vapor phase osmometry (VPO). The cured epoxy resin with HHPP possessed improved glass transition temperature. The thermostability and flame retardancy of O-cresol novolac epoxy resin cured with different contents of HHPP were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI) and cone calorimetry. The obtained results show that the samples containing a higher percentage of HHPP exhibit relatively lower thermostability at lower temperature while higher thermostability at elevated temperature and more char was formed compared with those containing a lower percentage of HHPP. The LOI value increased from 22.0 to 30.0 when HHPP, instead of 1,3-dihydroxybenzene, was used as a curing agent. The 25 wt% addition of HHPP in the curing agent complex effectively decreased the heat release rate and improved the char yield to the content nearly similar as those of the epoxy resin cured with pure HHPP.     

含硅磷杂化物环氧树脂固化物性能研究

首先用γ-环氧丙氧基三甲氧基硅烷(KH-560)和亚磷酸二乙酯(DEP)反应的中间产物进行水解缩合反应,合成了一种含磷低聚硅氧烷杂化物.并用FTIR,NMR,GPC对其结构及分子量进行了表征.然后将含磷低聚硅氧烷引入到双酚A环氧树脂(E-54)制备硅磷杂化物环氧树脂的固化物.对这种含硅磷杂化物环氧树脂固化物的性能研究发现其极限氧指数为23～29,DSC分析结果玻璃化转变温度(Tg)可以达到204℃,失重5%的温度(Td)5%比纯E-54提高近20℃.该固化物具有阻燃性能,同时具有较好的热性能。     

Synthesis of 4,4′‐bis (4‐oxyphenoxy)benzophenone diglycidyl ether and the thermal stability for blends with 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexanecarboxylate

We synthesized novel aromatic epoxy, 4,4′‐bis(4‐oxyphenoxy)benzophenone diglycidyl ether (DGEEK), by a three step reaction sequence and then it was blended with 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexanecarboxylate (CAE). The DGEEK structure was confirmed by FT‐IR and ¹H‐NMR measurement. Also, we investigated thermal properties of DGEEK/CAE blend epoxy by differential scanning calorimeter (DSC) and thermogravimetry analyzer (TGA). The glass transition temperature increased but initial decomposition temperature of cured epoxy decreased through the increasing amounts of ether–ether–ketone group of the epoxy network structure. By increasing the DGEEK mole fraction in the DGEEK/CAE blend epoxy matrix, the curing peak temperature decreased and the curing activation energy for DGEEK/CAE blend epoxy systems showed a considerable decrease. Copyright © 2001 John Wiley & Sons, Ltd.     

Epoxy oligomers modified with epoxyphosphazenes

Phosphazene-containing epoxy oligomers are obtained through a one-stage method via the interaction of hexachlorocyclotriphosphazene with an excess of diphenylolpropane in the medium of epichlorohydrin in the presence of a solid alkali. With the use of ³¹Р NMR and MALDI mass spectrometry, it is shown that the main components of the phosphazene fraction are penta- and tetra-aryloxysubstituted cyclotriphosphazene compounds with the corresponding numbers of epoxy groups. The maximum content of the phosphazene fraction in phosphazene-containing epoxy oligomers (~40 wt %) is attained at a hexachlorocyclotriphosphazene-to-diphenylolpropane molar ratio of 1: 8. Phosphazene-containing epoxy oligomers cured with isomethyltetrahydrophthalic anhydride have oxygen indexes of 26–28 and are self-extinguishing.     

Effect of P/Si polymeric silsesquioxane and the monomer compound on thermal properties of epoxy nanocomposite

The unique polymeric silsesquioxane/4,4′-diglycidyether bisphenol A (DGEBA) epoxy nanocomposites have been prepared by sol-gel method. The structure of nanocomposites was characterized by attenuated total reflectance (ATR) and solid state ²⁹Si NMR. The characteristic intensity of trisubstituted (T) structure was higher than that of tetrasubstituted (Q) structure from solid state ²⁹Si NMR spectra of 3-isocyanatopropyltriethoxysilane (IPTS) modified epoxy. The activation energies of curing reaction of epoxy system and IPTS modified epoxy system are 28-66 kJ/mol and 57-75 kJ/mol, respectively, by Ozawa’s and Kissinger’s methods. The triethyoxysilane side chain of IPTS modified epoxy might interfere the curing reaction of epoxy/amine and increase the activation energy of curing. The thermal degradation of nanocomposites was investigated by Thermogravimetric analysis (TGA). The char yield of nanocomposites was proportional to the 2-(diphenylphosphino)ethyltriethoxysilane (DPPETES) moiety content at high temperature. A higher char content could inhibit thermal decomposition dramatically and enhance the thermal stability. Moreover, the nanocomposites possess high optical transparency.     

Relationship between fracture toughness and domain size of liquid‐crystalline epoxy resins having polydomain structure

A liquid‐crystalline (LC) epoxy resin was cured at different temperatures and some types of curing systems having different phase structures (isotropic or polydomain, which have a microscopically ordered LC network structure) were obtained. The diameters of each domain in the polydomain system changed from the small to the larger size. The diameters of the LC domains were evaluated using a polarized optical microscope and the polarized microscopy FTIR mapping method. These systems were used to investigate the relationship between the network arrangement and mechanical properties. The fracture toughness of the cured systems was related to the enlargement of the ordered area in the network structures. With the toughness improvement, the meandering cracks were observed at the fracture surfaces. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 156–165, 2009     

Development of toughened bio‐based epoxy with epoxidized linseed oil as reactive diluent and cured with bio‐renewable crosslinker

In the current work, renewable resourced toughened epoxy blend has been developed using epoxidized linseed oil (ELO) and bio‐based crosslinker. Epoxidation of linseed oil was confirmed through FTIR and ¹H NMR spectra. The ELO bio‐resin was blended at different compositions (10, 20, and 30 phr) with a petroleum‐based epoxy (DGEBA) as reactive diluent to reduce the viscosity for better processibility and cured with cardanol‐derived phenalkamine to overcome the brittleness. The flow behavior of the neat epoxy and modified bio‐epoxy resin blend systems was analyzed by Cross model at low and high shear rates. The tensile and impact behavior studies revealed that the toughened bio‐epoxy blend with 20 to 30 phr of ELO showed moderate stiffness with much higher elongation at break 7% to 13%. Incorporation of higher amount of ELO (20 to 30 phr) increases enthalpy of curing without affecting peak temperature of curing. The thermal degradation behavior of the ELO based blends exhibits similar trend as neat epoxy. The higher intensity or broadened loss tangent curve of bio‐epoxy blends revealed higher damping ability. FE‐SEM analysis showed a rough and rippled surface of bio‐based epoxy blends ensuring effective toughening. Reduced viscosity of resin due to maximum possible incorporation of bio‐resin and use of phenalkamine as curing agent leads to an eco‐friendly toughened epoxy and can be useful for specific coating and structural application.     

Characterization of silanized poly(ether-urethane) hybrid systems using thermogravimetric analysis (TG)

The thermal behaviour of a new kind of hybrid system based on silanized poly(ether-urethanes) (SPURs) has been analyzed by thermogravimetric analysis (TG). The influence of the chemical nature of employed alkoxysilanes, polyether diol molecular weight and the physical state of the obtained hybrids (cured and non-cured) has been studied. The results show that in the non-cured state, aminosilane-based systems present a higher stability compared with those based on isocyanatesilane. However, in the cured state, both types of hybrids present a similar thermal stability, but much higher than their corresponding partners before the curing process. The presence of the inorganic silica network improves the thermal stability of all the systems studied.     

Fracture mechanism of liquid‐crystalline epoxy resin systems with different phase structures

A liquid‐crystalline epoxy resin was cured at two different temperatures. The phases of the cured systems clearly showed isotropic and nematic polydomain structures, which depended on the curing temperature. The fracture toughness of the systems was measured, and the fracture mechanism was investigated with polarized IR measurements. The nematic polydomain structure system showed considerably higher fracture toughness than the isotropic structure. Moreover, both systems exhibited a reorientation of the network chains near the fracture surface during the fracture process, and the region of the network reorientation in the nematic polydomain structure system was larger than that in the isotropic structure system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4044–4052, 2004     

PHASE STRUCTURE AND PROPERTIES OF EPOXY RESIN MODIFIED BY POLYSILOXANE BEARING PENDANT AMINO GROUPS

Polysiloxane-modified epoxy resins were prepared through the reaction of epoxy resin with polydimethylsiloxanesbearing pendant N-(β-aminoethyl)-γ-aminopropyl groups. The morphology and properties of the cured epoxy resins modifiedby the polysiloxanes were investigated. It was found that the phase structure and properties of the cured epoxy resins dependmainly on the amino group content in the polydimethylsiloxane and the level of the modifier. The change of phase structurein the cured epoxy resin systems was responsible for the dramatic change in their mechanical and surface properties.     

Thermal and flame retardant properties of epoxy resin cured by a novel phosphorus-containing 4,4′-bisphenol novolac curing agent

A novel flame retardant curing agent for epoxy resin (EP), i.e., a DOPO (9,10-dihydro-9-oxa-10-phosphaphenan-threne-10-oxide)-containing 4,4'-bisphenol novolac (BIP-DOPO) was synthesized and characterized by Fourier transform infrared (FTIR), ¹H NMR, ³¹P NMR spectroscopy, and gel permeation chromatography. The epoxy resin cured by BIP-DOPO itself or its mixture with a commonly used bisphenol A-formaldehyde novolac resin (NPEH720) was prepared. The flame retardancy of the cured EP thermosets were studied by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT), and the thermal properties by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that the cured epoxy resin EPNP/BI/3/1, which contains 2.2% phosphorus, possesses a value of 26.2% and achieves the UL 94 V-0 rating. The data from cone calorimeter test demonstrated that the peak release rate, average heat release rate, total heat release decline sharply for the flame retarded epoxy resins, compared with those of pure ones. DSC results show that the glass-transition temperatures of cured epoxy resins decrease with increasing phosphorus content. TGA indicates that the incorporation of BIP-DOPO promotes the decomposition of epoxy resin matrix ahead of time and leads to higher char yield. The surface morphological structures of the char residues reveal that the introduction of BIP-DOPO benefits to the formation of a continuous and solid char layer on the epoxy resin material surface during combustion.     

Cure behavior and thermal degradation mechanisms of epoxy and epoxy–cyanate resins

The cure reactions of tetraglycidyl methylene diamine (TGMDA) epoxy cured with tetrasubstituted aromatic diamine on one hand and diglycidyl ether of bisphenol A and diglycicyl ether tetrabromobisphenol A epoxies cured with methylene bis (phenyl‐4‐cyanate) on the other hand are reported. Systematic Fourier transform infrared (FTIR) spectroscopy studies of the cure reaction of epoxy and epoxy–cyanate during thermal cycles are presented. FTIR studies indicate that the reaction of TGMDA monomer is total but the network contains a large amount of primary amine. The cyanate monomer reacts rapidly to form triazine structures. Then the epoxy monomers homopolymerize and crosslink with free cyanate groups. The gas chromatography/mass spectrometry study of volatile products evolved during the polymer thermal degradation shows the dehydration of the epoxy network and the decomposition of the amine structure. The FTIR and solid‐phase ¹³C nuclear magnetic resonance analysis revealed that the ether functions and the amine groups are temperature sensitive but the triazine structure is not. Copyright © 1999 John Wiley & Sons, Ltd.     

NMR investigation of non-covalent aggregation of coordination compounds ranging from dimers and ion pairs up to nano-aggregates

This review summarizes the results recently obtained by our research group investigating the non-covalent aggregation of coordination compounds in solution through NMR spectroscopy. First, systems that can undergo only weak non-covalent interactions, such as dispersive and dipole–dipole ones, are considered; successively, coordination compounds that are capable to establish more energetic non-covalent interactions, such as hydrogen bonding and/or extended π–π stacking interactions, are taken into account. The parallelism between the energy of non-covalent interactions and the level of aggregation is highlighted. The results concerning the latter are mainly obtained through diffusion NMR experiments. In some cases, information about the structure of non-covalent aggregation in solution, obtained through intermolecular NOE studies, is discussed and contrasted with that observed in the solid state (by means of X-ray single crystal investigations, mainly carried out by our group) and/or derived from theoretical calculations.     

A phosphate-based epoxy resin for flame retardance: synthesis, characterization, and cure properties

A phosphorus-containing oligomer, bis(3-hydroxyphenyl) phenyl phosphate (BHPP), was synthesized through the reaction of phenyl dichlorophosphate and 1,3-dihydroxybenzene, and characterized by elemental analysis, Fourier transform IR spectroscopy, and ¹H NMR and ³¹P NMR spectroscopy. Consequently, the phosphate-based epoxy resins with a phosphorus content of 1 and 2 wt % were prepared via the reaction of diglycidyl ether of bisphenol-A with BHPP and bisphenol-A, and were confirmed with Fourier transform IR spectroscopy and gel permeation chromatography. Phenolic melamine, Novolak, and dicyanodiamide were used as curing agents to prepare the thermoset resins with the control and the phosphate-based epoxy resins. Thermal properties and thermal degradation behavior of these thermoset resins were investigated by using differential scanning calorimetry and thermogravimetric analysis. The thermoset resins cured with phenolic melamine exhibited higher glass-transition temperatures than the other cured resins owing to the high rigidity of their molecular chain. Thermogravimetric analysis studies demonstrated that the decomposition temperatures of the thermoset resins cured with Novolak were higher than those of the others. A synergistic effect from the combination of the phosphate-based epoxy resin and the nitrogen-containing curing agent can result in a great improvement of the flame retardance for their thermoset resins.     

Study on the sorption of water into epoxy resins by means of electrochemical impedance spectroscopy

In this work the relationship between structure and water sorption for epoxy films was studied using electrochemical impedance spectroscopy. It is shown that the electrochemical impedance spectroscopy can be applied to obtain information about the amount of water absorbed by the epoxy film and the water permeation through the film, while only the amount of water absorbed is obtained from the “sorption method”. The free volume is the decisive factor for both the amount of water in the cured epoxy film and water permeation through the film.     

基于可逆动态共价化学的新型可修复、可回收、可加工环氧树脂

利用生物来源的二聚脂肪酸为原料,合成了二聚酸酰肼和二聚酸酰腙两种衍生物,并进一步以其作为环氧E-44树脂固化剂,得到了新型的含动态共价连接的热固性环氧树脂。采用傅里叶红外光谱(FT-IR)、差式扫描量热(DSC)、扫描电子显微镜(SEM)、热重(TG)和动态力学分析(DMA)等多种测试手段对环氧树脂固化过程以及固化后材料的结构与性能关系进行了详细表征,特别研究了动态亚胺键对热固性环氧树脂性能的独特影响。结果表明:与传统环氧树脂相比,改性后的环氧树脂有更好的韧性,且其玻璃化转变温度及热稳定性没有明显下降。在升温和加压的条件下,酸可催化亚胺键的动态交换反应,赋予传统环氧树脂以全新的可修复、可回收与可多次加工性能。     

The heterogeneity of the network structure of epoxy polymers studied by dynamic and DSC tests

The final network structure of epoxy polymers cured with a crosslinking agent is strongly connected with the type and amount of the curing agent and conditions. Concerning the morphology of this network, it has been reported that cured epoxies are two-phase systems, containing roughly spherical floccules arranged in a matrix resembling the constituent phases. In this paper and by using an epoxy-polymer cured with different amounts (i.e., different than the stoichiometric one) of a curing agent, strong indications of their network-structure heterogeneity have been disclosed. For this study, two different experimental techniques, such as dynamic mechanical measurements and DSC tests have been utilised. It was found that cured epoxy-polymers consist of two extreme regions, with low and high crosslink densities, whose volume fractions have been estimated by using a mechanical model applicable for two-phase systems.     

Mechanical,thermal and morphological behavior of bismaleimide modified polyurethane‐epoxy IPN matrices

An intercrosslinked network of bismaleimide modified polyurethane‐epoxy systems were prepared from the bismaleimide having ester linkages, polyurethane modified epoxy and cured in the presence of 4,4′‐diaminodiphenylmethane. Infrared spectral analysis was used to confirm the grafting of polyurethane into the epoxy skeleton. The prepared matrices were characterized by mechanical, thermal and morphological studies. The results obtained from the mechanical and thermal studies reveal that the incorporation of polyurethane into the epoxy skeleton increases the mechanical strength and decreases the glass transition temperature, thermal stability and heat distortion temperature. Whereas, the incorporation of bismaleimide having ester linkages into polyurethane modified epoxy systems increases the thermal stability, tensile and flexural properties and decreases the impact strength, glass transition temperature and heat distortion temperature. Surface morphology of polyurethane modified epoxy and bismaleimide modified polyurethane‐epoxy systems were studied using scanning electron microscopy. Copyright © 2003 John Wiley & Sons, Ltd.