Off-Optimum Cure of Aerospace Epoxy Adhesives

Four epoxy film adhesives used in aircraft manufacture and repair have been examined to establish the effect of deviation from the cure cycle specified by the manufacturer. In addition to the variation of the cure cycle, two surface preparations of the aluminium adherends (chromic acid etch or grit blast followed by silane treatment) were evaluated. Thermal analysis was used to examine the cure envelope of the adhesive, and its extent of cure and glass transition temperature. The adhesive properties were assessed by shear strength (in both single lap joints and in Iosipescu configuration), durability (Boeing wedge test) and chemical resistance to selected aggressive fluids. The sensitivity of the performance of a particular adhesive to offoptimum cure conditions depends on its composition and needs to be determined, not predicted.     

Model Adherend Surface Effects on Epoxy Cure Reactions

Adherend surface effects on the amine cure of epoxy resins were investivated using finely divided aluminum oxide as high surface area models for aluminum. Calorimetric analysis of simplified crosslinking systems revealed significantly faster reactions which led to lower glass transition temperature materials for activated aluminum oxide filled samples. A monofunctional amine and epoxy were then utilized to obtain soluble reaction products amenable to molecular characterization. These studies similarly showed an increase in the rate of epoxy consumption in the presence of activated aluminum oxide which was attributed to both an increase in the rate of amine addition to epoxy as well as to epoxy homopolymerization. The latter was not observed in the unfilled mixtures. Such changes in reaction mechanism at the adherend surface have implications for the strength and durability of actual adhesive bonds.     

Model Adherend Surface Effects on Epoxy Cure Reactions

Adherend surface effects on the amine cure of epoxy resins were investivated using finely divided aluminum oxide as high surface area models for aluminum. Calorimetric analysis of simplified crosslinking systems revealed significantly faster reactions which led to lower glass transition temperature materials for activated aluminum oxide filled samples. A monofunctional amine and epoxy were then utilized to obtain soluble reaction products amenable to molecular characterization. These studies similarly showed an increase in the rate of epoxy consumption in the presence of activated aluminum oxide which was attributed to both an increase in the rate of amine addition to epoxy as well as to epoxy homopolymerization. The latter was not observed in the unfilled mixtures. Such changes in reaction mechanism at the adherend surface have implications for the strength and durability of actual adhesive bonds.     

Silane Coupling Agents as Adhesion Promoters for Aerospace Structural Film Adhesives

The performance of eight organofunctional silane coupling agents as adhesion promoters for the bonding of aluminium with two 121°C and two 177°C curing structural film adhesives was investigated and compared to the chromic acid (FPL) etch pre-treatment process and two non-chemical pretreatments. Aspects considered were shear strength of joints at ambient and elevated temperatures and durability, as judged by the wedge test.

The epoxy silane, γ-glycidoxypropyltrimethoxy silane, was found to be a very efficient adhesion promoter with all film adhesives evaluated. The cationic styryl silane, a neutral diamine monohydrochloride, showed promise with two adhesive systems. Four other neutral silanes were less effective.

Performance of amine functional silanes was mixed. Although the shear strength of joints with the primary amine silane at its natural pH of ∼10.3 was relatively good, durability was poor. However, good durability was obtained if the primer was first adjusted to pH 8 with hydrochloric acid, but not if acetic or phosphoric acids were used. Diamine silane was not an effective adhesion promoter at either its natural pH or when acidified with hydrochloric acid.     

Aspects of Latency in Epoxy Film Adhesives

Studies have shown that the cure of epoxy resins with urea hardeners, either alone or in conjunction with dicyandiamide, is influenced by the ability of adventitious agents, products, or by-products to volatilise from the reacting system. Although a generalised mechanism has not been identified, the phenomenological findings are important in evaluation and characterisation of adhesives and cure schedules, particularly by thermal analysis, and need to be considered in any detailed chemical study of these systems. Propenal has been identified as a significant by-product of cure under certain conditions.     

G／Epoxy复合材料胶接强度研究

使用G／Epoxy作为底材研究了垫板、结构胶黏剂厚度和底材表面处理对拉伸剪切强度的影响。使用光学显微镜观察了断口形貌。结果表明加垫板能减小试验过程中由于加载偏心引起剥离应力,测试结果较大;结构胶黏剂的厚度和底材表面处理对拉伸剪切强度影响十分明显,随着厚度的增大而减小,经打磨表面裸露出纤维的试样拉伸剪切强度很低。结构胶黏剂厚度较小时以内聚破坏为主,随着厚度的增加破坏模式转变为粘接破坏。     

UV-Curable Transparent Adhesives for Fabricating Precision Optical Components

UV-curable transparent epoxy adhesives have been specifically developed for the fabrication of optical communictions precision devices. The newly developed adhesives using cylohexane type fluoro-epoxy as the base resin and speherical quartz filler have extremely low volume shrinkage of 1.2% during curing and the cured adhesives have low thermal expansion coefficient of less than 2 × 10^-5/°C. Sheets of the adhesives are colorless and transparent to visible light because the refractive index of the epoxy matrix resin is matched to that of the quatz filler. These highly transparent adhesives can be cured to a depth of more than 5 mm by using 10 mW/cm² UV-irradiation for 30 min. They also have high adhesive strength and good durability. Therefore, they can be used in the fabrication of optical components that require submicron positioning accuracy.     

The Effect of Cure Conditions on a Rubber-Modified Epoxy Adhesive

The effect of cure conditions on the mechanical properties of a piperidine-cured, rubber-modified epoxy is described. The results obtained reveal that variations in cure conditions (temperature and time) have a pronounced influence on the mechanical behaviour, in particular the fracture energy obtained in both bulk and adhesive joint form. Techniques such as dynamic mechanical spectroscopy and scanning electron microscopy have been employed in an attempt to explain the trends.     

The Effect of Cure Conditions on a Rubber-Modified Epoxy Adhesive

The effect of cure conditions on the mechanical properties of a piperidine-cured, rubber-modified epoxy is described. The results obtained reveal that variations in cure conditions (temperature and time) have a pronounced influence on the mechanical behaviour, in particular the fracture energy obtained in both bulk and adhesive joint form. Techniques such as dynamic mechanical spectroscopy and scanning electron microscopy have been employed in an attempt to explain the trends.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g ), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service. Part III. The Effect of Silane Coupling Agents

The effect of silane coupling agents incorporated into the bulk of previously-developed room-temperature-curing epoxy adhesives^8,9,10 was studied. The physical and mechanical properties of corresponding aluminum bonded joints were characterized in ambient and humid-hot environments. Experimental results have demonstrated significant advantages of silane addition to the performance of these epoxy adhesives, especially under exposure to humid atmosphere. Thermal analysis of the polymerization processes, taking place during curing of the various low-temperature-curing formulations containing silane coupling agents, indicates that curing is not complete after seven days at room temperature, showing an exotherm at 80-100°C and a residual small one at 120°C. The basic formulation, comprising a tetra- and trifunctional epoxy resin blend and a multifunctional amine and ATBN cross-linking mixture, developed a three-phase matrix-rubber microstructure when the silane was added to the system.     

高性能胶黏剂密封剂在航天工业领域的应用

对航天产品用胶黏剂密封剂的特殊要求及热防护层粘接剂、防热密封剂、耐低温胶黏剂、耐空间环境胶黏剂、功能性胶黏剂和复合材料结构胶黏剂等高性能胶黏剂密封剂的性能及其在运载火箭、卫星、飞船和空间站等航天产品上的应用进行了综合评述。     

Bonding of Aluminium Alloy with some Phenolic Adhesives and a Modified Epoxide Adhesive, and Strength Changes on Exposure to Moist Air at 50°C

Single lap joints of aluminium alloy bonded with three phenol-based and one modified epoxide adhesive have been aged at 100% relative humidity (r.h) or at 50% r.h. for up to 10,000 hours at 50°C. Whilst joints are not significantly weakened on exposure at 50% r.h., at 100% r.h. strength falls over about 2000 h and then tends to remain steady. This fall in strength is controlled by the rate at which water enters the adhesive layer. Some joints were exposed firstly at 100% r.h. for 5000 h., and then at 50% r.h. for a further 5000 h whereupon some recovery of strength took place. The effect of moisture on joint strengths can be interpreted in terms of water, by virtue of its high permittivity, weakening ion-pairs at the interface.

X-ray photoelectron spectroscopy has been used to examine fracture surfaces at all stages, showing that visual inspection can lead to false conclusions about the mode of failure.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service. Part III. The Effect of Silane Coupling Agents

The effect of silane coupling agents incorporated into the bulk of previously-developed room-temperature-curing epoxy adhesives^8,9,10 was studied. The physical and mechanical properties of corresponding aluminum bonded joints were characterized in ambient and humid-hot environments. Experimental results have demonstrated significant advantages of silane addition to the performance of these epoxy adhesives, especially under exposure to humid atmosphere. Thermal analysis of the polymerization processes, taking place during curing of the various low-temperature-curing formulations containing silane coupling agents, indicates that curing is not complete after seven days at room temperature, showing an exotherm at 80-100°C and a residual small one at 120°C. The basic formulation, comprising a tetra- and trifunctional epoxy resin blend and a multifunctional amine and ATBN cross-linking mixture, developed a three-phase matrix-rubber microstructure when the silane was added to the system.     

环氧胶粘剂增韧改性研究进展

概述了关于近些年环氧树脂以及环氧树脂胶粘剂增韧改性的研究进展情况,简单地介绍了以无机刚性填料(颗粒)增韧改性环氧胶粘剂的情况,比较详细地叙述了以橡胶弹性体和热塑性树脂增韧改性环氧胶粘剂的一些重要研究情况及相关的增韧机理,指出了国内外之间环氧胶粘剂发展的差距,同时提出了环氧胶粘剂将来的重点和发展趋势。     

Sustainable UV-Crosslinkable Acrylic Pressure-Sensitive Adhesives for Medical Application

This study aimed to investigate the potential of photoreactive acrylate patches as systems for transdermal drug delivery, in particular, using more renewable alternatives and more environmentally friendly synthesis routes of transdermal patches. Therefore, the aim of this study was to develop a transdermal patch containing ibuprofen and investigate its performance in vitro through the pigskin. Transparent patches were prepared using four acrylate copolymers with an incorporated photoinitiator. Two types of transdermal patches based on the photocrosslinking acrylic prepolymers with isobornyl methacrylate as biocomponent and monomer increasing Tg (“hard”) were manufactured. The obtained patches were characterized for their adhesive properties and tested for permeability of the active substance. It turns out that patches whose adhesive matrix is photoreactive polyacrylate copolymers have a higher cohesion than patches from commercial adhesives, while the modification of the copolymers with isobornyl methacrylate resulted in an improvement in adhesion and tack. This study demonstrates the feasibility of developing photoreactive acrylic-based transdermal patches that contain biocomponents that can deliver a therapeutically relevant dose of ibuprofen.     

Shear Strength of Pressure Sensitive Adhesives and its Correlation to Mechanical Properties

Correlations between shear resistance and the mechanical properties of pressure sensitive adhesives are studied by measuring the deformation behaviour in the static and the dynamic shear test and determining the dynamic shear modulus of the adhesive. For polymers with low or moderate viscosities, the shear strain vs. time characteristics in a static shear test and, accordingly, the static shear strength, can be evaluated from the master curves of the dynamic shear modulus or the dynamic viscosity. The dynamic shear strength also can be calculated. These exact calculations cannot be applied to highly viscous or slightly crosslinked polymers. On the basis of the model experiments, empirical correlations between shear strength and the dynamic shear modulus are established which seem to be generally valid.     

Transmission and Mechanical Properties of Optical Adhesives

The optical, mechanical and durability performance of selected epoxy, polyester, UV-curable acrylic, cyanoacrylate and silicone adhesives were evaluated and measured for bonding applications of optically transparent glasses in the visible and infra-red regions of the electromagnetic spectra.

From the initially selected adhesives only the UV-curable modified acrylic, two-component silicone and room temperature cured epoxy, were found to be of high performance characteristics, having good transmission properties and enhanced endurance in a combination of heat and humidity and following thermal cycling.

Sodium chloride substrates served as adherends for the transmission characterization of the optical adhesives, due to their high transmission properties in the 0.4-10 m μ spectral range. A modified lap shear specimen was designed for studying the mechanical properties and failure mechanisms of the adhesives and their durability in a humid and not environment. Finally, a two-piece glass doublet was used for investigating the optomechanical characteristics of the optical adhesive following environmental conditioning and thermal shock cycling.

Due to the inherent C-C bond, polymer adhesives are limited in utility, as far as transparency is concerned, close to 3.5 μm and in most of the 8-12 μm spectral range.     

Mechanical Behavior of Butyl Adhesives

Butyl adhesives are widely used to bond elastomeric membranes. The results of uniaxial tension tests and simple shear tests conducted on bulk samples of butyl adhesives are presented in this paper. Butyl adhesives were found to have a rate-sensitive mechanical response with very low tensile and shear strengths. The stress-strain curves of the adhesive are characterized by an initial elastic response followed by a region of large plastic flow. A three-element viscoelastic model was used to model the stress-strain behavior of the adhesive. The model effectively combines a viscous dashpot and a network spring to capture the plastic flow in the material after the initial yield. The parameters of the model are calibrated to simulate a wide range of mechanical response of the adhesive.