Effect of Different Substrate Pre-Treatments on the Resistance of Aluminum Joints to Moist Environments

The goal of this work was to evaluate and compare the performance of different surface pre-treatments applied to aluminium single lap joints exposed to warm moisture. Four different processes were considered: 1) degreasing-roughening; 2) degreasing-roughening and phosphoric acid anodising; 3) degreasing-roughening and conditioning with Picklex®20 (a water-based product for metal surface preparation); 4) degreasing-roughening and phosphoric acid anodising and contaminating the adherend surface with CaCl₂ to accelerate the tests.

After each pre-treatment, lap-shear specimens were prepared with the open face technique. The half-joints were exposed to warm moisture (50°C, 100% RH) for times up to 4 weeks, then they were completed and subjected to mechanical testing.

The results show that anodising yields the best durability, as strength remained practically constant over time; all the other groups of specimens exhibited progressive reduction in strength and wider scatter of the results. A large role is played by interfacial damage of the adherends, since, under increasing exposure time, failure tends to occur as separation at the adhesive-metal interface. The strength decreased dramatically also for the anodised specimens when the surface was contaminated, which evidences the role of the interfacial damage. The interpretation is supported also by scanning electronic microscopy observation and energy-dispersive X-ray analysis.     

The Effect of Moisture Content in Epoxy Film Adhesives on their Performance. III: Bulk Properties

Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The consequences of vacuum drying are manifested in enhancement of both the peel and shear properties of bonded joints (Part I and Part II of this series of papers) and the thermal, physical and mechanical properties of the bulk adhesive, characterized in the present study.

Experimental results have shown that the bulk properties of structural epoxy based adhesives are highly correlated with the aging processes caused by water absorption in the prepolymerized adhesive. Applying the vacuum process is harmful to fresh unaged adhesive due to devolatization of low molecular species of the film adhesive.

The characterization of bulk properties for the purpose of following the aging and recovery processes is advantageous, since the bulk is independent of geometrical and interfacial effects which dominate in the case of property evaluation of the adhesive in a bonded joint.     

Modification of Free Volume in Epoxy Adhesive Formulations

Additives are described which modify the free volume available for segmental motion in epoxy adhesives. Such a mechanism can produce an increase in the tensile modulus of conventional epoxy-amine systems of>60% (e.g. to>4.1 GPa) and in tensile strength of>50% (e.g. to 125 MPa), while also producing a ductile mode of failure (stress-strain curve has negative slope before failure). At low strains, a reduction in free volume hinders polymer segmental motion and so increases the modulus. However, these materials also exhibit a very low Poisson's ratio and strains of ca. 5% cause a sufficient increase in free volume that ductile failure can occur. Improvements in low temperature cure properties (e.g. 118 MPa tensile strength at 60°C cure) together with reductions in the coefficient of thermal expansion and water uptake are also reported. These improvements in bulk adhesive properties are shown to translate into improved adhesive joint performance.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

A Water Soluble Hydrated Metal Oxide primer for Adhesively Bonded Joints

Weight saving and manufacturing cost benefits have led to the increase in use of adhesively-bonded structures in the automotive, aerospace and marine industries. In order to be a viable alternative to, for example, metal fasteners, these adhesive bonds should maintain the strength typical of conventional fastener systems. In many applications, the bonds are put under a variety of environmental and mechanical stresses. For example, frequently these bonds are exposed over long periods of time to wet environments which can result in a loss of bond strength. The loss of strength can result from the extension of cracks and other deformations that occur in the adhesive or metal oxide which are accelerated by the moist environment. As a result of this deficiency, extensive research and development efforts have been undertaken to define methods and identify materials which improve bonded joint performance in humid conditions. For example, it is known that surface preparation is important in the bonding of aluminum and titanium, and cleanliness in the bonding of ceramic articles. Thus, it is essential that, before bonding, the adherend is cleaned and chemically pretreated to produce a surface which in combination with the adhesive develops the bond strengths which meet application requirements. The normal procedure after surface treatment is to apply a corrosion-inhibiting primer by a spray technique for surface protection prior to bonding and to insure resin penetration into the oxide structure which provides improved environmental resistance. A major drawback of spray application is the large volume of organic solvent (normally MEK) emitted to the atmosphere. A successful alternative is the recently-developed electrodeposited primer by Northrup Corp., which consists of water solubilized primer particles which migrate in an electric field to a conductive work piece where they are deposited in a dense, continuous coating.¹ The primer was developed for use with 121°C (250°F) curing epoxy adhesives. An Air Force sponsored contract is currently under way, the objective of which is to develop an electrodeposited water-based primer for use with 177°C (350°F) curing epoxy systems.² A water-based epoxy primer system for application using the more conventional spray techniques has also been decribed.³     

Effect of Acetic Acid as Catalyst on the Properties of Epoxy Foam

In this study, the effect of acetic acid used as catalyst in promoting the decomposition rate of blowing agent (sodium bicarbonate) in epoxy foam was done. Hard epoxy foam was produced using mechanical mixing technique. Epoxy foam has been tested with adding acetic acid at 5 phr and different content of sodium bicarbonate which are 0, 5, 10, 15, and 20 phr, respectively. The effect of acetic acid on the viscosity, density, mechanical properties, and dielectric constant has been studied. The results were compared between with and without acetic acid in the system. Viscosity reading was increased with increasing the content of sodium bicarbonate due to the rapid production of bubbles that created porosity in the structure of epoxy foam. The addition of acetic acid is able to reduce the dielectric constant. In overall, the density, flexural strength and modulus dropped for the epoxy foam with acetic acid as compared to that of without acetic acid.     

水性木器涂料在松木涂装中的应用

介绍了常用松木的特点、种类,水性木器涂料在松木涂装中的涂装类型及特点。并简述了用水性木器涂料进行松木涂装时的优缺点,以及松木的除油脂及施工水性封闭底漆的重要性。     

An Improved Method to Determine Epoxy Index of Epoxy Resins

In this communication, the effects of testing conditions, i.e., reaction time and temperature, on the accuracy for determining epoxy index of chloroepoxy propane were investigated, in order to reduce errors generated by using the current Chinese Standard GB1677-81 for determining epoxy indexes of epoxy resins. Experimental results showed that, for chloroepoxy propane reacting with hydrochloric acid-acetone for 4 h at 45°C, its epoxy index was very close to the theoretical value. The relative error of this improved method was ca. 0.40%, much lower than that determined by the method defined in GB1677-81, which results in the maximum error of 6.77%.     

Dispersion,Mechanical and Thermal Properties of Epoxy Resin Composites Filled with the Nanometer Carbon Black

The nanometer carbon black (CB) was employed to prepare epoxy resin/carbon black (EP/CB) composites by blending-casting method. The different modified methods of silicone coupling agent were used to improve the dispersion of CB in epoxy resin. The mechanical and thermal properties of EP/CB composites were investigated. Experimental results showed that the mechanical properties increased at first, but decreased with excessive addition of CB. When the mass fraction of CB was 2%, the mechanical properties were maximum. The use of modified CB significantly enhanced the mechanical properties of the composites. For given CB loading, the CB modified by pretreatment method displayed better dispersion in the epoxy resin than that of the direct mixing method. SEM observation revealed that the tensile fracture surface of the composite filled with 2 wt% modified CB held more microcracks than that of 5 wt% modified CB, and the formed microcracks could consume more energy of rupture, finally to have better tensile strength. DSC analysis showed that the glass transition temperature (Tg) of the composites increased with the increasing mass fraction of CB.     

电机用环氧灌封胶粘剂的研制

通过对环氧树脂、增韧剂以及固化剂的筛选,研制了一种可室温固化亦可中温固化的电机用透明双组分环氧灌封胶粘剂。