Details
Structural adhesives fall into four broad polymer families: epoxies, cyanoacrylates, silicones, and acrylics. With the exception of silicones, these polymers have bond strengths on the order of 2,500 to 7,500 psi in tensile-shear mode. They also tolerate temperature swings as wide as 3500°F and endure impact loads of 10 ft-lb/in2 or greater. Industrial adhesives have several major benefits over other joining methods like brazing and mechanical fastenings. Joint stress is reduced by evenly distributing the load over a broad area. Adhesives are invisible because they are applied inside the joint, and they resist flex and vibration stresses by forming a seal that can protect the joint from corrosion.
Adhesives are also a perfect choice for joining irregularly shaped surfaces, which may prove problematic for brazing. Minimal weight is added to an assembly and there is virtually no change in part dimensions or shape.
Some adhesive limitations include a potential need to disassemble the joint, curing time, and surface prep requirements.
Adhesives and mechanical fasteners, when used together, form a stronger bond than when used separately. For example, a bolt that is tightened to the correcque setting and has a thread-locking adhesive applied to the threads will improve the strength of the assembly. The thread-locking adhesive ensures the assembly will not loosen and corrosoin will also be minimized.
Some adhesives require the addition of a hardening agent, often referred to as a catalyst or activator. Others require only heat to obtain the bond. When a catalyst is required, a variable is introduced into the completeness of the mixing. There is a limited pot life, or time during which the epoxy can be applied. In some cases, this can be extended by refrigeration. Most adhesives requiring hardeners have a specified shelf life and the manufacturer should be consulted on their recommendations.
Heat and humidity usually have the most damaging effects on bonded joints, although exposure to solvents and ultraviolet light also take a toll. Operating temperature is the most important variable that qualifies an adhesive for a particular application. While a device mounted outside is exposed to cold, wet, sunlight, and other conditions, the maximum temperature is not likely to exceed 60°C (140°F). Therefore, an outdoor environment does not eliminate any of the potential adhesive chemistries described above.
Thermal Cycling
When devices operate in environments that cycle between extremes of heat and/or humidity, they experience thermal cycling or thermal shock. All materials expand when heated and shrink when cooled. This rate of dimensional change is called the coefficient of thermal expansion. Differences in the coefficient of thermal expansion produce stress on the bond joint.
Resistance to thermal cycling is generally achieved in two basic ways:
• A very high strength, rigid adhesive may resist the applied stress. Classic rigid chemistries include acrylics and epoxies, but many urethane-modified or elastomer-modified formulations are available.
• A softer, more flexible adhesive can absorb the applied stress by flexing or moving rather than cracking. Silicones and urethanes are typical of these softer and more flexible chemistries.
Surface Preparation
Proper surface preparation is key to ensuring a good bond. It can be as simple as cleaning the surfaces with a solvent to remove oils, greases, and other potential contaminants that could hinder bond strength. Other applications may require surface abrasion or grit blasting to enable proper adhesion. Chemical cleaning with an approved solvent after grit blasting is recommended for a better bond.
Adhesive Recommendations
The most frequent causes of adhesive joint failures do not involve adhesive strength. Typically, adhesive joint failure may be attributed to poor design, inadequate surface preparation, or improper adhesive selection for the substrate and the operational environment. A competent carbide application engineer, familiar with successful assembly techniques will be able to provide the optimum bonding technique for a specific application. Testing under load my be necessary to ensure success of an adhesive assembly.
The joint should be from 0.003" to 0.006" in thickness to assure maximum strength in the bond. The facing surfaces must be clean and free from dirt, grease, and scale.