Simultaneous cationic polymerization and esterification of epoxy/anhydride system in the presence of polyoxometalate catalyst

Polyoxometalate exhibits high catalytic performance for the simultaneous cationic polymerization and esterification of epoxy resin when anhydride is introduced as a co-hardener. The selective catalysis effect of polyoxometalate and the reaction mechanism was studied by differential scanning calorimetry (DSC), mid-infrared spectroscopy (MIR), near-infrared spectroscopy (NIR) and generalized two-dimensional correlation analysis. The cationic polymerization is the dominating reaction in neat epoxy systems. Increasing the amount of polyoxometalate and the polarity of the diluents fastens the curing rate of epoxy resin. Esterification was found to be the preferred reaction once anhydride was employed. When polyoxometalate was blocked by amine to form salt, it performs as an excellent catalyst for esterification in epoxy-anhydride systems. The epoxy materials catalyzed by polyoxometalate show quite good performance compared with ordinary epoxy resins. Moreover, thermal degradation analysis (TGA) shows that polyoxometalate could significantly decrease the thermal degradation temperatures of cured epoxy resins.     

Photopolymerization kinetics of an epoxy‐based resin for stereolithography

Curing reactions of photoactivated epoxy resins are assuming an increasing relevance in many industrial applications, such as coatings, printing, and adhesives. Besides these processes, stereolithography (SL) makes use of photoactivated resins in a laser‐induced polymerization for 3D building. The kinetic behavior of photocuring is a key point for full comprehension of the cure conditions occurring in the small zone irradiated by the laser beam during the building process. Furthermore, the kinetic analysis is very important to determine the cure time needed for part building in a stereolithographic equipment. The mechanisms involved in a cationic photopolymerization are complex compared with radical photopolymerization. In this paper the photoinitiated polymerization of a commercially available epoxy‐based resin for stereolithography (SL5170) was studied by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Substantial information about the SL5170 chemical composition and curing mechanism was determined through FTIR analysis. The polymerization rate and the maximum degree of reaction were determined directly from experimental DSC curves. Kinetic characterization of epoxy photopolymerization was carried out as a function of the temperature and irradiation intensity and experimental results were compared with an original mathematical model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3484–3491, 2004     

Benchmark tests on adhesive strengths in butt, single and double lap joints and double-cantilever beams

The RC99 committee of the Japan Society for Mechanical Engineers conducted the benchmark tests on strengths of adhesive joints using different testing methods. The effects of joint configuration, loading mode, adherend yield strength and so on, on the strength and data scatter were investigated using two typical epoxy adhesives. The strengths obtained by various tests were compared with each other. The relationships among strengths of butt, single lap and double lap joints and fracture toughness were given. Thirteen member institutes of the committee participated in this project. The benchmark results allow us to recognize that the joint strengths are strongly affected by the curing process. The key to obtaining the appropriate joint strength, is precise temperature control inside the adhesive layer for curing. Toughened adhesives do not always give higher joint strengths than untoughened adhesives. The yield strength of adherends much affects the observed lap joint strength of adhesives.     

二茂铁四氟硼酸盐用作潜热阳离子引发剂对环氧树脂的热固化引发环氧化合物阳离子热聚合

A novel latent thermal initiator, ferricenium tetrafluoroborate salt (FcBF4), for cationic polymerization of epoxides is reported. The activities of FcBF4 for different epoxides, including bisphenol-A-type epoxy oligomer E44, cycloaliphatic epoxy ERL4221, and glycidylether epoxy GGE, were evaluated by differential scanning calorimetry (DSC). The results showed a moderate thermal initiation activity for E44 and GGE at 78℃ and 108℃ respectively. When adding GGE to E44, the reaction was enhanced. The temperature of thermal degradation of the cured specimens for the system of E44/FcBF4, E44 10%GGE/FcBF4 and E44 10%ERL4221/FcBF4 is higher than 350℃.     

ASPECTS OF JOINT DESIGN AND EVALUATION IN THICK-ADHEREND APPLICATIONS

Structural adhesives are gaining wide recognition by industry as they offer engineering designers greater flexibility to achieve economic and technical advantages. In the marine industry there are potential applications for adhesives in various types of construction, for example, thick steel and composite adherends, (typically 5-15 mm thick). The applications include panels and large pipes. This article is largely concerned with the use of two-part epoxy adhesives. The purpose of this article is to understand and evaluate the weaknesses of adhesives and adherends, in relation to specific applications and to use design and material selection to alleviate them. This understanding can be extended to other thick adherend applications. This article will also highlight the impact of structural epoxy adhesive technology on the design and fabrication of steel, composite, and hybrid constructions. The benefits and inherent limitations that can accrue are quantified through three case studies related to thick adherend connections.     

ASPECTS OF JOINT DESIGN AND EVALUATION IN THICK-ADHEREND APPLICATIONS

Structural adhesives are gaining wide recognition by industry as they offer engineering designers greater flexibility to achieve economic and technical advantages. In the marine industry there are potential applications for adhesives in various types of construction, for example, thick steel and composite adherends, (typically 5–15?mm thick). The applications include panels and large pipes. This article is largely concerned with the use of two-part epoxy adhesives. The purpose of this article is to understand and evaluate the weaknesses of adhesives and adherends, in relation to specific applications and to use design and material selection to alleviate them. This understanding can be extended to other thick adherend applications. This article will also highlight the impact of structural epoxy adhesive technology on the design and fabrication of steel, composite, and hybrid constructions. The benefits and inherent limitations that can accrue are quantified through three case studies related to thick adherend connections.     

Polymerization of epoxy resins initiated by metal complexes

BACKGROUND: Processing parameters and material properties of epoxy resins can be vastly influenced by choice of curing agent. In this work, metal complexes were investigated as initiators for anionic and cationic epoxide polymerization. Systems for thermally induced and electron beam‐induced curing are described. RESULTS: Zinc or cobalt imidazole complexes of the type [M(imidazole)₂(anion)₂] are efficient initiators for anionic polymerization of glycidyl‐based epoxy resins. The complexes can be employed to prepare tailored resin systems ranging from fast curing systems at slightly elevated temperatures to systems with very high thermal latencies curable at temperatures far above 150 °C. Silver complexes [Ag(L)_n]SbF₆ (L = crown ether or alkene) are highly efficient initiators for cationic curing and low initiator contents of around 1% are sufficient to reach high degrees of crosslinking. The complexes are excellent initiators for both thermally induced and electron beam‐induced polymerizations. CONCLUSION: Metal complexes are powerful initiators for the homopolymerization of epoxy resins and can be designed not only for anionic and cationic polymerization but also for thermal and radiation curing. Based on this study and additional work, a library can be compiled which allows retrieval of optimized metal–ligand–anion combinations and adjustment of the initiators to the respective processing and material demands. Copyright © 2009 Society of Chemical Industry     

Water‐soluble/dispersible cationic pressure‐sensitive adhesives. II. Adhesives from emulsion polymerization

In our previous work, we reported that cationic water‐soluble pressure‐sensitive adhesives (PSAs) could be synthesized in ethanol or methanol. These cationic water‐soluble adhesives would not cause a stickies problem during paper recycling and can be easily removed from the papermaking system by adsorbing on wood fibers. In this study we report the synthesis and application of water‐based cationic PSAs using miniemulsion polymerization. A redox initiator system of cumene hydroperoxide/tetraethylenepentamine was used to force interfacial polymerization. The end‐use properties of the PSAs were evaluated, and the repulpability of the PSAs in paper recycling was studied. It was found that the cationic PSA from miniemulsion polymerization itself was insoluble and nondispersible in water during the paper recycling process. However, if this water‐insoluble cationic PSA from miniemulsion was formulated with a water‐soluble cationic PSA made from ethanol, the solubility or dispensability of the former PSA in water was improved. The molecular weight and degree of crosslinking of the PSA polymer have significant effects on the properties and dispersability of PSA. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 347–353, 2004     

Two-Dimensional Transient Thermal Stress Analysis of Adhesive Butt Joints

Transient thermal stress distribution in an adhesive butt joint is considered. It is assumed that both the upper and lower end surfaces of the joint are maintained at different temperatures at a certain instant in time and that no heat transfers between the side surfaces of the joint and ambient air. In the analysis, two adherends were replaced with finite strips and unsteady temperature distribution in the joint was obtained theoretically. Then the transient thermal stress distribution in the joint was analyzed using a two-dimensional theory of elasticity. The effects of the ratios of the coefficient of thermal expansion and Young's modulus of the adherend to those of the adhesive on the thermal stress distribution were clarified from numerical calculations. Furthermore, the transient stress distribution in the adhesive was measured by a photoelastic experiment on a joint where the adhesive was modelled by an epoxy plate. The experimental results were consistent with the analytical results.     

不同官能度环氧树脂的阳离子光固化研究

对4种不同官能度的环氧树脂在紫外光辐照下用一种二苯基碘钅翁盐或两种二烷基苯甲酰锍盐光引发剂进行阳离子光固化的体系作了系统的研究。研究结果表明,二氨基二苯甲烷环氧树脂AG-80不能阳离子光固化,而E-51,711和TDE-85均可在二苯基碘钅翁六氟磷酸盐或1-甲基-1-十二烷基苯甲酰甲基六氟锑酸锍盐作用下进行紫外光引发阳离子聚合,其中二苯基碘钅翁六氟磷酸盐引发E-51环氧树脂阳离子光固化的效果最好,该树脂体系可用做紫外光固化复合材料的树脂基体。研究还发现,在停止紫外光辐照后,由于阳离子聚合链终止困难而使碘钅翁盐引发的环氧树脂光固化体系存在后固化现象,后固化速度与后固化时的温度有关。     

Experimental study of the effect of microspheres and milled glass in the adhesive on the mechanical adhesion of single lap joints

This article describes several experiments conducted on single lap joints (SLJ) subjected to tensile mechanical loads. Two epoxy adhesives, with slow and fast curing, were used, with a weight of 0%, 3%, and 10% of glass microspheres and milled glass particles, respectively. The adherends used in the construction of the specimens were fiber-reinforced polymers. The types of failures produced in the SLJ specimens were classified according to ASTM standards. The results of the experimental tests on the SLJ with fast-curing epoxy adhesive showed that the use of milled glass and glass microspheres improved the strength of the joint compared with the neat fast-curing epoxy adhesive. As for the experimental test on the joint with slow-curing epoxy adhesive, the results showed that the use of milled glass and glass microspheres decreased its strength when using different additive concentrations compared with the neat slow-curing epoxy adhesive.     

Influence of graphene nanoplatelets (GNPs) on mode I fracture toughness of an epoxy adhesive under thermal fatigue

Abstract

The contribution of graphene nanoplatelets (GNPs) for enhancing the fracture toughness of a commonly used room-cured epoxy, used to bond E-glass/epoxy composite adherends, is evaluated. A comprehensive experimental investigation is conducted to examine the performance and degradation of adhesively bonded joints subject to cyclic thermal loading using the standard double cantilever beam (DCB) specimens. Several groups of DCB specimens were fabricated using the adhesive reinforced with four different GNPs weight-percentages (i.e. 0.0, 0.25, 0.5 and 1%). The specimens are subsequently subjected to various numbers of thermal cycles (to a maximum of 1000 heating/cooling cycles), and then tested, and the resulting mode I fracture toughness values are evaluated and compared. The extent and modes of damage captured through microscopy and scanning electron microscopy images are presented and discussed. In addition, a computational framework, using the cohesive zone modeling technique, is developed for predicting the response of the adhesives and their damage evolution.     

Investigation of the photoactivated frontal polymerization of oxetanes using optical pyrometry

An investigation of the photoactivated cationic ring-opening frontal polymerization of a series of 3,3-disubstituted oxetanes was carried out with the aid of a novel technique, optical pyrometry. Using this technique, the effects of various experimental parameters such as photoinitiator type and concentration, light intensity and sample configuration as well as the effects of monomer structure on the frontal behavior of these monomers were examined. Upon photoactivation with UV light, 3,3-disubstituted oxetanes display a long dormant period at room temperature as the result of the formation of metastable tertiary oxonium ions. Only small amount of thermal activation energy is required to induce the further reaction of these species with resulting exothermic autoaccelerated ring-opening polymerization. Frontal polymerization was observed for both mono- and difunctional oxetane monomers and the velocity of propagation of a given monomer was found to be related to its oxetane equivalent weight.     

Liquid crystalline adhesives for polypropylene joints

This study was devoted to the investigation of the adhesion of epoxy resins to polypropylene adherends. In particular, the study was carried out as follows: synthesis of both a liquid crystalline (LC) and a bisphenol-A-based resin, their characterization and their further evaluation as adhesives for polypropylene adherends. This was done in order to evaluate the difference between an LC epoxy resin and an isotropic one in adhesive applications. The adherends chosen were neat polypropylene (PP) and polypropylene reinforced with 20 and 40 wt% talc (PP20 and PP40, respectively). The effect of two different pre-treatments (simple degreasing and acid etching) on the polypropylene adherends was also evaluated. It was shown that the adhesion strength of the liquid crystalline adhesive was higher compared to that of the isotropic one. The higher adhesion was related to the extra ductility demonstrated by the LC epoxy resin, which was due to its particular microstructure. Moreover, the acid etching pre-treatment performed on polypropylene adherends prior to bonding improved the adhesion at the interface with the resin. The increased stiffness of the adherends due to the presence of talc turned out to be beneficial to obtain more resistant joints.     

Viscoelastic and Nonlinear Adherend Effects in Bonded Composite Joints

An experimental and theoretical investigation is described on the effects of viscoelasticity and geometric and material nonlinearity in rubber-toughened graphite/epoxy adherends. Single-lap joints, with adherends of a matrix-dominated layup and a brittle epoxy adhesive layer, were tested under two constant loading rates to failure; axial strains were measured at several locations on the surface of the adherends. Aluminum and fiber-dominated laminate adherends were also studied for comparison. Finite element analyses of the adhesive joint were made using linear and nonlinear viscoelastic characterizations of the composite. The experimental work is discussed first. Then we describe the constitutive theory and its implementation in the finite element analysis, after which the theoretical ane experimental results are presented and compared.     

Viscoelastic and Nonlinear Adherend Effects in Bonded Composite Joints

An experimental and theoretical investigation is described on the effects of viscoelasticity and geometric and material nonlinearity in rubber-toughened graphite/epoxy adherends. Single-lap joints, with adherends of a matrix-dominated layup and a brittle epoxy adhesive layer, were tested under two constant loading rates to failure; axial strains were measured at several locations on the surface of the adherends. Aluminum and fiber-dominated laminate adherends were also studied for comparison. Finite element analyses of the adhesive joint were made using linear and nonlinear viscoelastic characterizations of the composite. The experimental work is discussed first. Then we describe the constitutive theory and its implementation in the finite element analysis, after which the theoretical ane experimental results are presented and compared.     

Epoxy‐Based Azopolymers with Enhanced Photoresponsive Properties Obtained by Cationic Homopolymerization

Azopolymers are highly versatile materials due to their unique photoresponsive properties. In this contribution, a novel azo‐modified epoxy network is synthesized by cationic homopolymerization with boron trifluoride monoethylamine (BF₃.MEA) complex as initiator. The effect of the addition of a fixed content of amino‐functionalized azo chromophore, Disperse Orange 3, into the polymer matrix is studied in detail. First of all, the thermal curing cycle is optimized by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) measurements. Then, the resulting bulk azo‐modified epoxy networks are characterized by means of thermogravimetric analysis (TGA), FTIR, DSC, UV–vis spectroscopy, and rheological measurements. Finally, the optical response of thin films of these materials is determined. The results evidence that azo‐modified epoxy networks obtained by cationic polymerization with optimized curing cycle display high T _g values, high maximum photoinduced birefringence, fast writing speed, and exceptionally high remnant anisotropy. Therefore, this material is a promising candidate to be used for optical storage applications.     

Stress-free temperature in a mixed-adhesive joint

Adhesive joints used in supersonic aircraft fuselage need to withstand low (?55°C), as well as high (200°C) temperatures. However, there are no adhesives suitable for the whole temperature range. A solution would be a joint with a combination of a low-temperature adhesive and a high-temperature adhesive, called a mixed-adhesive joint. In a bonded joint, the thermal stresses are generated essentially by the different thermal expansion properties of the adhesive and the adherends and, to a lesser extent, by the shrinkage of the adhesive produced by curing. The case of a mixed-adhesive joint is more complicated because there are two adhesives with different glass transition temperatures (T _g). To determine the stress-free temperature in a mixed adhesive joint, sandwich specimens of aluminium–adhesive–CFRP (carbon-fibre-reinforced plastic) were fabricated and the thermal strains were measured with strain gauges. In a mixed adhesive joint, two stress-free temperatures were found: the stress-free temperature of the high temperature adhesive, which is its cure temperature, and the stress-free temperature of the low temperature adhesive, which is its T _g.     

Thermal peel,warpage and interfacial shear stresses in adhesive joints

Thermal stresses are determined in a single lap joint with identical adherends, which are due solely to temperature changes. The simple bending model used here includes bending and extension of the adherends and extensional and shear strains in the adhesive. The analytical solution shows 'sinusoidal' deformation consistent with warpage (bending) of the adherends due to thermal mismatch. While a modified shear lag model (MSLM) with no adherend bending leads to peak bondline shear stresses which occur only at the ends of the overlap, the bending model shows that such stresses occur not only near the ends, but also at interior points of the overlap region. Results for aluminum adherends and an epoxy adhesive show how the peel, warpage and interfacial shear stresses are distributed over the overlap region.     

Strength prediction of bonded single lap joints by non-linear finite element methods

A non-linear finite element technique has been used to predict the mode of failure and failure load of single lap joints made from three aluminium alloys and four epoxy adhesives, and the results compared with those obtained from experiment and closed-form analyses. The finite element program used was able to account for the large displacement rotations that occur in a single lap joint under load, and allowed the effects of elasto-plasticity in both the adhesive and adherends to be modelled. A failure criterion based on the uniaxial tensile properties of the adhesive was used: for two untoughened adhesives a maximum stress criterion was found to be appropriate while for two toughened adhesives a maximum strain criterion was employed.