On hybrid toughened DGEBA epoxy resins

Considerable improvements in the fracture resistance of epoxy resins have recently been achieved by adding either rubbery or rigid second phase dispersions, or both, to an epoxy matrix. These multiphase epoxy systems are particularly useful as high performance adhesives and as matrix materials in advanced composites. This paper describes the development of new toughened epoxy hybrids, which contain both rubbery and rigid dispersions. The latter dispersions were either zirconia particles, short alumina fibres or glassy-metal ribbons. Micromechanisms of toughening and failure processes in these new materials are identified and discussed in the light of microstructures.     

Low temperature relaxation of DGEBA epoxy resins: A thermally stimulated discharge current (TSDC) study

The effect of curing conditions on the low temperature relaxation behavior of catalyst-cured epoxy systems based on digliycidyl ether of bisphenol A (DGEBA) has been characterized by the thermally stimulated discharge current (TSDC) technique. In these chemically relatively simple epoxy systems, five relaxation processes, designated as γ, β, β′, β_OH, and β″, have been observed: Their molecular origins are discussed in detail. The results are in agreement with prior suggestions of an inhomogeneous cross-link density morphology.     

Natural and artificial weathering effects on cold‐cured epoxy resins

Three cold‐cured epoxy resins, specifically designed as structural adhesives for rehabilitation or renewal applications of civil infrastructures and cultural heritage, were submitted to natural and artificial weathering. We evaluated the variations in the thermal and mechanical properties and color changes after an artificial treatment carried out at 70°C and 75% relative humidity and after natural weathering, performed in two areas of South Italy, both located adjacent to the Mediterranean Sea. The variations in properties due to both natural exposure and artificial weathering were qualitatively similar. However, the selected artificial weathering procedure appeared excessively severe compared to the weathering that occurred after outdoor exposure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Mechanical properties of epoxy networks based on DGEBA and aliphatic amines

The mechanical properties of epoxy networks based on diglycidyl ether of bisphenol A epoxy resin cured with various linear aliphatic amines, such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and cyclic amines such as 1‐(2‐aminoethyl)piperazine and isophorone diamine, were studied. General characteristics such as T_g, density, and packing density, were determined and related to the structure and funcionality of the curing agent. Dynamic mechanical spectra were used to study both the α and β relaxations. Tensile and the flexural tests were used to determine the Young's and flexural modulus, and fracture strength all in the glassy state. Furthermore, linear elastic fracture mechanics was used to determine K_IC. As a rule, isophorone diamine network presented the higher tensile and flexure modulus while 1‐(2‐aminoethyl)piperazine gave the highest toughness properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Curing of dicyandiamide epoxy resins accelerated with substituted ureas

An epoxy resin based on bisphenol-A-diglycidylether (DGEBA) was cured with dicyandiamide (DDA) and different substituted ureas (Urones) as accelerators. Differential scanning calorimetry, size exclusion chromatography, and Fourier transform infrared (IR) analysis indicated complex reaction mechanisms. The DDA granulometry seemed not to be a predominant parameter, but its content clearly influences T_g of the network. The maximum T_g value was found for an amine-to-epoxy molar ratio of 0.6. IR spectra revealed the appearance of oxazolidone rings, which are the first product of epoxy reactions. Urone accelerators also acted as homopolymerization initiators; they modified the epoxy conversion at gelation and changed of the structure of the obtained network. Influence of uron nature and content on reaction rates were also studied. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2487–2497, 1998     

Recovery of surface defects on epoxy coatings and implications for the use of accelerated weathering

Surface roughness, arising from photodegradation, increases overall during weathering but may relax and diminish during episodes when exposure is limited. Different ambient temperatures will change the balance between photodegradation defect size and recovery, depending on the value of the glass transition temperature of the polymer. Epoxy coatings were exposed to periods of ultraviolet irradiation, after which the recovery of the surface roughness was monitored at several temperatures, above and below their glass transition temperatures. Atomic force microscopy, as well as following the increase in roughness with exposure, showed that increased exposure made phase separated domains more distinct. Recovery of nanoindentation on un-damaged coatings produced a similar value of the glass transition temperature to that deduced from the degradation roughness recovery. This was significantly lower at the surface of the epoxy coatings than was measured for the bulk. Confocal Raman spectroscopy was unable to detect any chemical difference between the surface of any films and deeper in their bulk. This evidence suggests that the low glass transition temperature is not due to different curing chemistry at the surface of the coating, but hints that the surface of these crosslinked coatings may relax differently to the bulk or have a different physical structure. These results lead to questions about how to change accelerated testing to better serve the needs of coatings’ technology and how to make progress in the overall goal of service lifetime prediction.     

The comparative study of biocomposites based on hydrochar and chitosan-modified urea-formaldehyde resins

To provide new insight into the field of urea-formaldehyde (UF) adhesives science, in this work, for the first time, UF resin was modified with hydrochar of spent mushroom substrate (HCUF) and chitosan (CHUF) to investigate the effect of these bio-fillers on the hydrolytic and thermal stability of in situ prepared UF resins. The characterization of the modified UF biocomposites was performed using X-ray diffraction analysis (XRD), Fourier transforms infrared spectroscopy (FTIR), non-isothermal thermogravimetric analysis (TG), differential thermal gravimetry (DTG), and differential thermal analysis (DTA). Scanning electron micrographs (SEM) of the CHUF and HCUF biocomposites show a spherical structure that differs from each other because the surface of the CHUF biocomposite has pronounced pores that form a network structure. With its hydroxyl and amino groups, chitosan bonding to UF resin through hydrogen bonds, which is confirmed by FTIR analysis. The content of free FA in CHUF biocomposite is 0.06%, while that of HCUF is higher and amounts to 0.48%. The content of released FA in both modified UF biocomposites was similar (2.5% and 2.8% for CHUF and HCUF, respectively). The hydrolytic stability of CHUF is slightly higher compared to the HCUF biocomposite. Thermal analysis shows that the CHUF is thermally more stable because it starts to decompose at a slightly higher temperature than the HCUF biocomposite.     

Biolignin™ based epoxy resins

Wheat straw Biolignin? was used as a substitute of bisphenol‐A in epoxy resin. Synthesis was carried out in alkaline aqueous media using polyethyleneglycol diglycidyl ether (PEGDGE) as epoxide agent. Structural study of Biolignin? and PEGDGE was performed by solid‐state ¹³C NMR and gel permeation chromatography, respectively, before epoxy resin synthesis. Biolignin? based epoxy resins were obtained with different ratios of Biolignin? : PEGDGE and their structures were analyzed by solid‐state ¹³C NMR. The crosslinking of PEGDGE with Biolignin? was highlighted in this study. Properties of Biolignin? based epoxy resins were analyzed by differential scanning calorimetry and dynamic load thermomechanical analysis as well as compared with those of a bisphenol‐A epoxy‐amine resin. Depending on the epoxy resin formulation, results confirmed the high potential of Biolignin? as a biosourced polyphenol used in epoxy resin applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A systematic study on dispersion stability of carbon nanotube-modified epoxy resins

It is well accepted that the state of dispersion of carbon nanotubes (CNTs) considerably influences the performance of epoxy nanocomposites. Previous studies suggest that CNTs might reagglomerate during the processing of epoxy nanocomposites. Shear forces, resin viscosity, thermal excitation, curing reaction, and CNTs concentration are among the main factors suggested to influence the dispersion stability. The present work focuses on understanding the important role of each parameter on the dispersion stability and explaining the mechanisms by which the dispersion evolves during processing. In this regard, an in-situ dispersion monitoring technique was utilized to acquire dispersion micrographs from the sample of interest during a given heating profile. The results were supported by the rheological and contact angle measurements. It was found out that the increased mobility of CNTs in conjunction with the ineffective interfacial interaction between the nanotubes and resin was the main reason for the degradation of dispersion during the curing process.     

Novel epoxy resins based on cyclohexanone-aldehyde condensation products

Solid ketone-formaldehyde resins are used in certain coating formulations in order to improve hardness, gloss, and light stability. They are soluble, thermoplastic by nature, and contain limited amounts of hydroxyl groups. We found that their primary hydroxyls can be etherified with epichlorohydrin (ECH) either by a two-step ECH-addition/dehydrohalogenation procedure or by a one-step phase-transfer process. An intermediate of particular usefulness is the crystalline 2,2,6,6-tetramethylol-cyclohexanol (TMCH) made from cyclohexanone and 5 mol formaldehyde, yielding low colored epoxy resins with epoxy values up to 7.5 eq/kg. Depending upon the nature of the curing agent, high T_g solids as well as tough and flexible coatings with good outdoor stability can be made. Upon decreasing the formaldehyde–cyclohexanone ratio, solid condensation polymers melting up to 150°C can be obtained. Phase-transfer glycidylation yielded solid thermoset glycidyl ether resins with M?_n up to 1600, M?_ω up to 13,000, epoxy values up to 3.6 eq/kg, and softening points between 80 and 160°C. Powder coatings formulated with carboxyterminated polyesters are hard, glossy, solvent-resistant but somewhat brittle. In order to overcome this drawback, polycycloacetals have been produced from TMCH and glutardialdehyde, which are terminated by pairs of methylol groups. Powder coatings of the corresponding glycidylethers with carboxyl-terminated polyesters exhibited excellent flexibility and impact strength.     

Self-stratifying coatings based on Schiff base epoxy resins

Schiff base epoxy resin was used as a component of self-stratifying compositions, in which different acrylic resins were used. The degree of stratification was determined using the ATR FTIR spectroscopy and microscopy technique. Results were compared with those for commercial epoxy resins with a comparable epoxy number. A determination was made of the mechanical properties of the coatings obtained. Based on the previous studies, it was concluded that Schiff base epoxy resin in a film-forming composition behaves in a similar manner to BPA-based epoxy resins. The best mechanical properties and a high degree of stratification were achieved using a Schiff base epoxy resin/poly(isobutyl methacrylate) composition. Use of a self-stratifying composition not only reduces the time and cost of preparing a multilayer coating system, thanks to the formation of two layers in one application, but also causes the Schiff based epoxy coatings to become more flexible.     

Fast-curing adhesive compositions based on modified epoxy resins

The properties of two new fast-curing adhesive materials based on modified cold-curing epoxy resins are studied. The studied materials are used for the repair and production of complex technological equipment. The properties of fast-curing adhesives developed in Germany and Russia (OAO Kompozit) are compared, and it is shown that the strength properties of new adhesive materials are not inferior to foreign analogs and are distinguished by a high curing rate at room temperature; they display good service properties at high temperatures and enhanced humidity. The fields of application of the adhesives for bonding parts made of aluminum alloys and glass-and carbon-reinforced plastics are indicated.     

Nanoclay and long-fiber-reinforced composites based on epoxy and phenolic resins

In this study, high-performance thermoset polymer composites are synthesized by using both long fibers and nanoclays. Epoxy and phenolic resins, the two most important thermoset polymers, are used as the polymer matrix. The hydrophobic epoxy resin is mixed with surface modified nanoclay, while the hydrophilic phenolic resin is mixed with unmodified raw nanoclay to form nanocomposites. Long carbon fibers are also added into the nanocomposites to produce hybrid composites. Mechanical and thermal properties of synthesized composites are compared with both long-fiber-reinforced composites and polymer- layered silicate composites. The optimal conditions of sample preparation and processing are also investigated to achieve the best properties of the hybrid composites. It is found that mechanical and thermal properties of epoxy and phenolic nanocomposites can be substantially improved. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

FRP composites based on different types of glass fibers and matrix resins: A comparative study

Flexural properties, impact energy, heat deflection temperature, and resistance to thermal and hydrothermal degradation of composites based on E-glass and N-glass fibers as the reinforcing agents, and epoxy, unsaturated polyester, phenolic, and epoxy-phenolic resin systems as the matrix materials were studied and compared. As a reinforcing agent E-glass fiber is superior to N-glass fiber, particularly with respect to development of flexural strength and modulus, impact strength, and thermal resistance; N-glass fiber, however, imparts to the composites substantially higher resistance to hydrothermal degradation under boiling conditions in different chemical environments. For use of both E-glass and N-glass fibers as reinforcing agents, the general order of resistance to hydrothermal degradation for the composites based on different matrix resins is epoxy > phenolic > unsaturated polyester resin. Incorporation of a low dose of a rubbery polymer, such as styrene butadiene rubber (0.1–0.2%) and liquid polybutadiene (0.5–0.75%), in unsaturated polyester resin as the matrix resin measurably enhances impact energy of the composite. © 1995 John Wiley & Sons, Inc.     

脂环族环氧树脂／双酚A环氧树脂共混改性研究

采用脂环族环氧树脂（EPL-4221）与双酚A型环氧树脂E-51共混,用酸酐固化剂和促进剂使其固化,研究脂环族环氧树脂的用量对共混树脂性能的影响及其规律性,包括冲击强度、弯曲强度、拉伸强度、维卡软化点温度、漆膜的粘附力、铅笔硬度、耐磨性以及拉伸剪切强度。结果表明,随脂环族环氧树脂用量的增加,共混树脂的综合性能先增加后降低,脂环族环氧树脂的质量分数为15％-20％时,具有最大值。     

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.     

Novel thermosets based on DGEBA and hyperbranched polymers modified with vinyl and epoxy end groups

The cationic polymerization of DGEBA with two hyperbranched polymers (HBPs) with epoxy or vinyl end groups, using ytterbium triflate as initiator, has been studied. These HBPs have been obtained from commercial Boltorn H30 of which terminal hydroxyl groups have been replaced with long aliphatic ester chains having vinyl or epoxy end groups. Differences between the HBPs added as modifiers are observed with respect to the curing kinetics, network development, properties and morphology of the cured materials. While terminal epoxy groups ease the solubility of the HBP in DGEBA and allows its covalent incorporation into the network structure, the HBP with vinyl terminal groups is only miscible at high temperature and phase-separates during curing. As a consequence, morphology and thermal–mechanical properties are strongly dependent on the HBP employed.     

High temperature dielectric study of epoxy resins

Four common 177°C-curing organic resin formulations were studied dielectrically. Results and discussion are presented on the variation of resin electronic properties during cure. In particular, capacitance and dissipation curves are discussed as a function of time in the cure cycle. Variations in frequency are shown to affect the other electronic properties and a frequency “shift” in dissipation sensitivity is noted.     

研究环氧树脂聚酰亚胺树脂凝胶行为的新方法

为了优化双组分环氧树脂和PMR型聚酰亚胺树脂的固化工艺,选用DMA圆盘压缩夹具,采用等温模式分别测定了这2种树脂的凝胶行为,给出了其凝胶动力学数学模型,计算了树脂的凝胶活化能为75.68 kJ/g。在聚酰亚胺树脂凝胶实验结果的基础上,推测该聚酰亚胺体系在310℃前后可能存在不同的固化反应机理。     

超支化聚合物改性环氧树脂的研究进展

超支化聚合物因其独特的结构和性能特点,已在众多领域得到了广泛的应用,尤其是在热固性树脂的改性中的应用,可作为热固性树脂的增韧剂。该文介绍了环氧树脂的性能特点及应用,超支化聚合物的结构及特点,着重论述了近年来超支化聚合物在改性环氧树脂力学性能、固化行为及热性能方面的研究进展,并指出了超支化聚合物在环氧树脂和其它热固性树脂改性方面的发展方向。