Bond-joint resistance

IPC aging tests on all surface materials that come in contact with adhesives. Bond-joint resistance is measured by depositing a strip of adhesive 1 in x 0.5 in (minimum) onto gold-plated stainless steel and then applying a second gold-plated stainless-steel strip to give a one-half inch overlap. ... 

Bond Joint Resistance Test, Method TM-PT-15, Ablestik Electronic Materials (now Henkel). Sep. 15, 1993. 

Large parts can require considerable power requirements. Resistance welding has been applied to complex joints in automotive applications, including vehicle bumpers and panels, and joints in plastic pipe, and in medical devices. Resistance wire welding is not restricted to flat surfaces. If access to the heating element is possible, repair of badly bonded joints is possible, and joints can be disassembled in a reverse process to which they were made. 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

When using cyanoacrylate adhesives it should be considered that the resistance of the bonded joints against water, especially at higher temperatures, is not comparable to that of epoxy joints. In the case of common household joints, unlimited dishwasher safety cannot be taken for granted. Heat resistance is usually limited to approximately 80 °C. 

Depending on their chemical basis and the resulting thermosetting crosslinkage ratio, the adhesive layers are thermally resistant and thus perfectly suitable for transmission and engine manufacturing. If repair is required, the bonded joints can be removed by heating them to approximately 120-150 °C. 

Note In the case of metal bonded joints, the resistance to moisture and climate can decisively be influenced by the kind of surface pretreatment apart from the adhesive selection, especially in the areas adjacent to the adhesive surface (e.g., primer, sealing of gluelines). 

Thermal stress Regarding the thermal resistance of the bonded joints, thermosetting adhesives are to be preferred to thermoplastically curing adhesives. The following information is useful for the determination of thermal stress ... 

The testing of the peel resistance occurs according to the standard DIN EN 1464 Adhesives - determination of peel resistance of high-strength bonded joints - climbing drum peel test (respectively, T-peel test) and serves the determination of the resistance of bonded metal joints to peeling forces (Figure 10.5). 

Adhesion promoter Chemical compounds enhancing adhesive strength and/or ageing resistance of bonded joints in the form of adhesive additions or surface coatings (q.v. primers). 

Assembly time (closed) Period of time during which a bonded joint has to be fixed until it is strong enough to resist a displacement of the adherends by external force effects. 

Peel resistance/strength Resistance of a bonded joint against linear acting peel forces generating high stress peaks in the adhesive layer, dimension N/mm or N/cm. 

ISO 11343, 1993, Adhesives—Determination of dynamic resistance to cleavage of high strength adhesive bonds under impact conditions- Wedge impact method The method is mainly aimed for the characterisation of metal substrates suitable for automotive applications. An instrumented impact testing machine (pendulum type) of 50-300 J and. 3-5.5ms maximum capacity is required for this test. A blunt wedge of approximately 1 mm tip radius and included angle of 8 is impact driven into a bonded joint at 3 ms" for aluminum alloy adherends and 2 ms" for steel adherends. The impact event should be... 

A type of resin also used today is the so-called PMUF (or MUPF according to which author is writing) adhesives. These are fundamentally MUF resins in which a minor proportion of phenol (between 3 and 10% phenol melamine urea by weight of 10 30 60 for example) has been assumed to have coreacted with to further upgrade weather resistance of the bonded joint. Unfortunately the alleged superior performance of such resins is... 

The physical and chemical properties of both the solidified adhesive and the polymer affect the quality of the bonded joint. Major elements of concern are the corrosion resistance of the adhesive, its thermal expansion coefficient, and the glass transition temperature of the polymer relative to the adhesive. 

For the monomer polymerization at room temperature, the adhesive was augmented with a redox system of 3% BP and 0.75% DMA. To study, explain, and predict the development of the elastic failure of the polymer in the adhesive interlayer, an improved method of investigating adhesive layer crack resistance with modeling of the formation and growth of a crack at the adhesive-honded joint loading was used [119]. Five adhesive-bonded joints with the adhesive mixture compositions shown in Table 3.1 were subjected to static tests for crack resistance at room temperature. The characteristics of the static crack resistance of the adhesive-bonded joint Kic is the coefficient of the stresses intensity Gic is the intensity of the elastic energy release ic is the opening in the crack tip) were determined at the moment of onset of the crack in double-cantilever specimens DCB (Fig. 3.5). The specimen cantilevers were made of PMMA of TOCH type. 

The value of 6ic can be used not only as a comparative but also tis an estimated characteristic of the cracking resistance of adhesive-bonded joints. For this purpose it is necessary to find a relationship between the displacements 6i, load, body geometry, and crack length. The relationship for a crack with yield zone localized within its plane can be written... 

The additional introduction of the fluorinated alcohol into the adhesive enhances its water resistance. Thus, the adhesion strength of adhesive-bonded joints of steel using Sprut-4 adhesive does not change significantly over time even with many years in water. 

Let US consider a method of determining internal stresses that lacks the above shortcomings hy applying it to an adhesive-bonded joint represented by a three-layer plate (substrate-adhesive-substrate). The adhesive layer shrinkage causes deformation of the whole plate. The resistance forces appearing in this case can be represented as a system of normal forces and bending moments, which act layer by layer throughout the plate, and stresses can be represented as... 

The water resistance of adhesive-bonded joints also depends on the nature of the materials cemented. The maximal decrease of strength is characteristic for aluminum and seems to occur due to the high hydro-philicity of the porous oxide film on the aluminum surface. The water resistance of steel adhesive-bonded joints increases if the steel is etched. 

One of the principal reasons for failure of the adhesion bonds is a specific adsorption reaction of the medium with the material to be cemented at the boundary with the adhesive. There is an adsorption substitution of adhesive-substrate bonds by medium-substrate bonds. Surface structural defects that are present in each solid are the first to be subjected to adsorption. It is to be expected that the probability of appearance of such defects is higher at an interface of two materials with different properties. The rate of penetration of the medium along the polymer-substrate interface frequently substantially exceeds the rate of diffusion of the medium in pure polymer [212]. Adsorption substitution of the polymer macromolecules by water molecules on the metal surface explains the low water resistance of such adhesive-bonded joints as fluoroplastic-steel or polyethylene-steel [34]. The adhesion strength, which decreases during hold-up of adhesive-bonded joints in water, is frequently reestablished after the joints are dried [213]. 

As this brief literature review displays, at present there are no scientifically substantiated and practically applicable methods for enhancing the water resistance of adhesive-bonded joints. 

---