室温固化抗剥离耐温环氧胶粘剂

研究了一种室温固化,可在１２０℃下使用的环氧树脂胶粘剂。通过ＣＴＢＮ增韧改性,获得了较高的剥离强度和剪切强度,通过加入高官能度环氧树脂和改性芳胺及催化剂,使得该体系具有一定的耐温性和可室温固化性。     

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.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service,Part II: Composition,Properties, Microstructure Relationships

Low temperature curing epoxy formulations for elevated temperature service have been previously developed and studied (Part I¹). Balanced performance with respect to shear and peel properties have been obtained for a system composed of a tetra and trifunctional epoxy blend crosslinked by a mixture of multifunctional amine and an amino-terminated elastomer. In continuation of the previous study, the present one is aimed at investigation the effect of substitution of difunctional epoxy resin and curing agent for trifunctional ones on the developing microstructure and resulting mechanical properties. Furthermore, a new type of amino-terminated-acrylonitrile (ATBN) and an epoxy-terminated silane were included in the present investigation. Experimental results show that while reduction in the overall functionality of the reactants results in a lower lap shear strength, it gives rise to enhancement in peel strength. The same effect was observed when the new ATBN was used. Thermal analysis of the polymerization processes, taking place during curing of the various low temperature curing formulations, indicates that the curing activation energies are appreciably lower compared with high temperature curing systems. Addition of silane, ATBN and substitution of the multifunctional amine curing agent by a lower functional one, resulted in a moderate increase in the activation energy. The basic formulation, comprising a tetra- and trifunctional resin blend and a multifunctional amine and ATBN crosslinking mixture, developed a typical two-phase matrix-rubber microstructure. A third phase was observed when the trifunctional epoxy resin or the multifunctional curing agent was substituted by lower functional ones. A similar three-phase morphology was obtained when the epoxy-terminated silane was added to the basic treta- and trifunctional reactant system.     

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.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service, Part II: Composition, Properties, Microstructure Relationships

Low temperature curing epoxy formulations for elevated temperature service have been previously developed and studied (Part I¹). Balanced performance with respect to shear and peel properties have been obtained for a system composed of a tetra and trifunctional epoxy blend crosslinked by a mixture of multifunctional amine and an amino-terminated elastomer. In continuation of the previous study, the present one is aimed at investigation the effect of substitution of difunctional epoxy resin and curing agent for trifunctional ones on the developing microstructure and resulting mechanical properties. Furthermore, a new type of amino-terminated-acrylonitrile (ATBN) and an epoxy-terminated silane were included in the present investigation. Experimental results show that while reduction in the overall functionality of the reactants results in a lower lap shear strength, it gives rise to enhancement in peel strength. The same effect was observed when the new ATBN was used. Thermal analysis of the polymerization processes, taking place during curing of the various low temperature curing formulations, indicates that the curing activation energies are appreciably lower compared with high temperature curing systems. Addition of silane, ATBN and substitution of the multifunctional amine curing agent by a lower functional one, resulted in a moderate increase in the activation energy. The basic formulation, comprising a tetra- and trifunctional resin blend and a multifunctional amine and ATBN crosslinking mixture, developed a typical two-phase matrix-rubber microstructure. A third phase was observed when the trifunctional epoxy resin or the multifunctional curing agent was substituted by lower functional ones. A similar three-phase morphology was obtained when the epoxy-terminated silane was added to the basic treta- and trifunctional reactant system.     

Rubber modified epoxy resin. II: Phase separation behavior

DGEBA (diglycidyl ether of bisphenol A)–ATBN (amine terminated butadiene acrylonitrile copolymer) blends exhibited upper critical solution temperature (UCST) behavior. Triethylene tetramine (TETA) was introduced as an amine curing agent of epoxy. The real-time phase separation behavior of ATBN-added epoxy system during cure was investigated using laser light scattering. SEM (scanning electron microscopy) and optical microscopy were also employed to observe the morphology of the epoxy blends. Since the DGEBA–ATBN blends showed UCST behavior, the degree of phase separation when cured at low temperature was higher than that when cured at high temperature. The domain correlation length increased as the curing temperature was lowered. Dynamic mechanical analysis (DMA) results indicated that the phase inversion occurred above 20 wt% of ATBN composition.     

聚硫醚改性环氧树脂室温固化耐高温结构胶粘剂

目前室温固化耐高温环氧树脂结构胶粘剂主要采用液体端羧基丁腈橡胶增韧环氧树脂为主体,以改性液体端胺基丁腈橡胶或聚醚胺为韧性固化剂,其最高使用温度仅120℃。聚硫橡胶作为环氧树脂增韧剂和固化剂则由于耐热性能和增韧效果差,很少用于室温固化耐热环氧树脂结构胶粘剂。通过改进聚硫橡胶的内聚强度和耐热性能,作为增韧剂,克服了聚硫橡胶耐热性能和增韧效果差的缺点,大大地提高了室温固化环氧树脂结构胶粘剂的剥离强度,通过BMI与脂肪胺加成反应,并加入叔胺固化剂,合成具有BMI结构和叔胺的固化剂,以及加入有机硅改性石棉,使室温固化环氧树脂结构胶粘剂的耐热性能达到177℃,瞬间使用温度达300℃,达到室温固化高温使用的目的。     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.     

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.     

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.

For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack growth rate over a frequency range of 1 to 5 Hz, but a significant change was found as a function of the bondline thickness. The room temperature curing adhesive evaluated herein exhibited a much lower fatigue resistance than a heat-cured commercial structural adhesive FM-73.     

NMR Study of the Epoxy Crosslinking Reactions of N,N-Dimethyl-4-Chlorophenyl Urea

The crosslinking of one-part epoxy adhesives is a complex process involving reactions of dicyandiamide and any of a variety of accelerators with epoxide functional polymers. Variations in adhesive formulation and process affect the final properties of the adhesive bond. The reactions of N,N-dimethyl-4-chlorophenyl urea, an accelerator for one-part, dicyandiamide crosslinked epoxy adhesives has been studied with Carbon-13 NMR in model systems employing phenyl glycidyl ether as the epoxy. A complex reaction mixture was observed whose composition varied with epoxy-accelerator stoichiometry and reaction temperature.

NMR peaks having chemical shifts consistent with 2-N-(4-chlorophenyl)-4-phenoxymethyl oxazolidone, a quaternary amine terminated polyether and epoxy elimination products have been observed in reaction mixtures modeling adhesive formulations. The quaternary amine terminated polyether likely results from condensation of epoxy with the dimethyl amine that is formed from N,N-dimethyl-4-chlorophenyl urea under cure conditions. Of the three products observed, only the quaternary amine terminated polyether would afford crosslinks in the actual adhesive. The other two products would consume epoxide functionality without the concurrent formation of crosslinks. The relative amounts of these three products varied as a function of reaction temperature, suggesting that variations in process conditions may affect final properties of dicyandiamide-crosslinked epoxy adhesives that are accelerated with N,N-dimethyl-4-chlorophenyl urea.     

Accelerated-curing epoxy structural film adhesive for bonding lightweight honeycomb sandwich structures

Accelerating the curing of epoxy/aromatic amine adhesives and improving their toughness are challenges in heat-resistant epoxy structural adhesives. Herein, we report an epoxy/aromatic amine adhesive accelerated curing system with an oxo-centered trinuclear (chromium III) complex, which is toughened using a thermoplastic block copolymer (TPBC). The reaction characteristics, heat resistance, microstructure, and bonding properties of the accelerated epoxy adhesives were analyzed. The reaction peak temperature of the epoxy with 3% catalyst was 113.1°C, which was 113.6°C lower than that of epoxy without catalyst, and the modified epoxy resin demonstrated a potential for rapid curing at medium temperature. The glass transition temperature of the TPBC-toughened epoxy adhesive was 125°C after curing, indicating excellent thermal stability after medium temperature curing. The introduction of the TPBC increased the single-lap shear strength of the epoxy adhesive without reducing its heat resistance. The shear strength at room temperature and 120°C of the modified epoxy adhesive with 50 phr of TPBC was 25.2 and 10.9 MPa, respectively. Moreover, the epoxy film adhesive exhibited outstanding bonding properties when used in the bonding of lightweight honeycomb sandwich structures.     

Effects of Loading Rate and Temperature on the Mechanical Properties of Structural Adhesives Containing a Carrier

The viscoelastic related properties of four structural adhesives were studied in their bulk form. All four adhesives were based on rubber-toughened epoxy resin with a thermoplastic carrier. Two of the adhesives were commercial film adhesives (120°C curing systems) and the other two were formulated by us from commercially available constituents. The first formulation is a high-temperature-curing system based on cyclo-aliphatic resin and anhydride hardener, toughened with carboxy-terminated butadiene elastomer. The second self-prepared formulation is a special room-temperature-curing adhesive for elevated temperature service, based on a blend of trifunctional and tetrafunctional expoxies cured with triethylene tetramine toughened with amine-terminated butadiene elastomer. The latter formulation was also prepared, in addition to the carrier-containing composition, without the thermoplastic carrier.

As expected for viscoelastic materials, it was found that the yield stress and modulus decreased with temperature. The rate of loading had a pronounced effect on the yield stress which increased with increasing loading rates, and a negligible influence on the modulus. The rate-temperature effects on the yield stress were shown to obey the superposition as described by Eyring's theorem of viscosity. Consequently, the activation energy and activation volume were determined. The high-temperature-curing adhesives comprising a carrier exhibited higher activation energies compared with the room-temperature-curing formulation and other epoxy adhesives cured with aliphatic amines or polyamides reported in the literature.     

Structural Adhesive Bonds to Primers Electrodeposited on Steel

Adhesive bond strength and durability were investigated for steel substrates which had been cathodically electroprimed before bonding. Lap shear and torsional impact strengths of two model epoxy adhesives were evaluated. Very poor strengths and durability were found for one adhesive, which was cured with a mixture of three amine curing agents. Scanning electron microscopy and analysis of primer susceptibility to interaction with the curing agents suggested that, for the high concentrations of curing agent in the amine-cured adhesive, chemical and physical degradation of the primer occurred during cure at elevated temperature.

For the second adhesive, which was cured with a single imidazole catalyst, excellent strength and durability were obtained, with no evidence of primer degradation. Surprisingly, for this adhesive, strengths to primed steel were up to 88% higher than to cleaned (i.e., degreased) bare steel. The concurrent improvements in environmental durability over bare steel, as assessed by water immersion and salt spray accelerated exposures, were attributed to the more favourable surface energetics of the adhesive/primer interface.     

Simulation Analyses on the Diffusion of a Rust-Preventing Oil into an Oil-Accommodating Adhesive

In recent years bonding between two steel plates was accomplished with an oil-accommodating adhesive without requiring degreasing of the steel. In this paper, the exclusion process of the oil was investigated in this adhesive on the assumption that the oil was absorbed into the adhesive layer.

It was found that the oil layer essentially disappeared in the initial step of curing in which the temperature was raised to 180°C, because the diffusion rate of the oil into the adhesive increased abruptly with temperature. Therefore, the bonding process in this case is not influenced by the presence of oil on the steel plates.     

Applied Research and Development of Adhesives for Bonding Filled Carboxyl Terminated Polybutadienes to Various Substrates

A one-year applied research and development program was conducted on the bonding of carboxy-terminated polybutadiene (CTPB) propellant to various substrate materials encountered in solid propellant rocket motors. Under this program, in addition to CTPB liners, liners were also prepared from polyesters, polyethers, polyurethanes, polyacetal polymers, and epoxy resins. The use of various crosslinkers, emulsifiers, wetting agents, fillers, and stabilizers was also evaluated.

Four optimized liner formuations with the best all-round properties were fully characterized. The optimized formulations represented an HC liner formulation with two Ievels of glycerol additive, an HC formulation with a sorbitol additive, and a Butvar polyacetal-type liner. A standard HC-polymer liner formulation, designated as TL-H-304, was used as a control.

Unaged liner peel and shear properties were measured at -65°F, 77°F, and 160°F. Samples, aged for 30 days at 160°F, were tested at 77°F only.

The liners were tested against propellant, steel, aluminum, magnesium, titanium, epoxy-fiberglass, phenolics, polyisoprene, and butadiene-acrylonitrile as substrate materials. The steel, titanium, and polyisoprene rubber substrates gave the best adhesive results.

The substitution of asbestos and Cab-O-Sil for the Thermax filler in the liner gave comparable adhesive results while the substitution of clay fillers gave poor results.

This program was performed while at the Elkton Division of the Thiokol Chemical Corporation in fulfillment of the requirement of Contract N123 (60530-53329A) U.S. NAVAL ORDNANCE TEST STATION, China Lake, California, reported previously in U.S. Naval Report NOTS-TP4283.     

Dynamic mechanical properties and adhesive strengths of epoxy resins modified with liquid rubber. I. Modification with ATBN

The dynamic mechanical properties and the adhesive strengths of Epikote 828 and Epikote 828-ATBN blend systems were investigated. The ATBN blend systems were proved to be completely incompatible with the dynamic mechanical measurement and also fitted well with Takayanagi's model which was designed for completely incompatible two-phase systems. The epoxy resin had a nonreacted part when cured at room temperature. The blending of ATBN reduced the nonreacted part of the epoxy resin, and made contributions to the adhesive strengths. In the case of tensile test of crosslap specimens using aluminium as adherends, the adhesive strengths of ATBN blend systems were almost 1.5-fold of those of epoxy resin without blending of ATBN. As for wood adherends, the maximum of the adhesive strengths was found at 60°C for epoxy resin without blending of ATBN, and at 0°C for ATBN blend systems. The facts meant that there were mutual interactions between the adhesive strengths and the viscoelastic behavior of the adhesive polymers in the two-phase systems as observed in completely miscible polymer blends. There was not pronounced distinction between epoxy resins without blending of ATBN and ATBN blend system, as to the shear adhesive strengths.     

Effects of amino‐functionalized carbon nanotubes on the properties of amine‐terminated butadiene–acrylonitrile rubber‐toughened epoxy resins

Amino‐functionalized multiwalled carbon nanotubes (MWCNT‐NH₂s) as nanofillers were incorporated into diglycidyl ether of bisphenol A (DGEBA) toughened with amine‐terminated butadiene–acrylonitrile (ATBN). The curing kinetics, glass‐transition temperature (T_g), thermal stability, mechanical properties, and morphology of DGEBA/ATBN/MWCNT‐NH₂ nanocomposites were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis, a universal test machine, and scanning electron microscopy. DSC dynamic kinetic studies showed that the addition of MWCNT‐NH₂s accelerated the curing reaction of the ATBN‐toughened epoxy resin. DSC results revealed that the T_g of the rubber‐toughened epoxy nanocomposites decreased nearly 10°C with 2 wt % MWCNT‐NH₂s. The thermogravimetric results show that the addition of MWCNT‐NH₂s enhanced the thermal stability of the ATBN‐toughened epoxy resin. The tensile strength, flexural strength, and flexural modulus of the DGEBA/ATBN/MWCNT‐NH₂ nanocomposites increased increasing MWCNT‐NH₂ contents, whereas the addition of the MWCNT‐NH₂s slightly decreased the elongation at break of the rubber‐toughened epoxy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40472.     

Comparison of the Degradation Mechanisms of Zinc-Coated Steel, Cold-Rolled Steel, and Aluminium/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and zinc, zinc-coated steel, two different aluminium alloys or cold-rolled steel metal coupons have been investigated. The influence of the dicyandiamide content of the adhesive on the durability properties-has been assessed by salt spray testing or by storing the joints in water at 70°C or 90°C for periods of time up to five weeks. The degradation products formed during ageing of the epoxy adhesive in water have been investigated using high performance liquid chromatography (HPLC) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The degradation mechanisms of aluminium/epoxy bonded joints have been thoroughly studied using X-ray photoelectron spectroscopy.

The performances of the bonded joints under a pure corrosive environment have been found to be little influenced by the quantity of dicyandiamide in the adhesive. When the bonded joints were aged in hot water, the stability of the interface toward an excess of dicyandiamide directly followed the sensitivity of the oxide layer at high pH values. Optimal durability properties without peel strength losses of the adhesive were aehieved both with zinc and aluminium-coated substrates by reducing the quantity of dicyandiamide in the epoxy adhesive by 20% (the initial dicyandiamide content in the commercial adhesive being ca. 9%, with respect to the epoxy resin).