In a vast number of industries, including automotive, electronics, and mechanical engineering, bonding has become more important due to light-weight construction, miniaturization, and multi-material design. Adhesives are also increasingly being used as sealants to protect components from environmental influences.
For productivity reasons, most manufacturing companies—as well as those producing mass goods—prefer light-curing adhesives to achieve high productivity levels. A light-curing adhesive helps to provide high positioning accuracy, as components can be initially fixed on demand. Once applied, the adhesive does not flow, which can happen when using heat-cured products in the oven.
However, these systems are subject to limitations. Light-cured adhesives achieve full strength within a couple of seconds when irradiated (in specials cases, even less than a second). This is achieved with high-energy LED lamps that generate 100 to 1000 times the intensity of normal daylight within their specific light spectrum. The materials used in these systems are subject to limitations, given maximum implementation temperatures of more than 150°C and regular contact with aggressive chemicals, oil, and acid.
In recent years, adhesive manufacturers have focused on pushing these limits further by developing several dual-curing products. Dual-curing adhesives offer the benefits of light-curing systems without compromising on reliability, bond strength, and processing quality. They ensure that the adhesive in the finished product is fully cured and also permit maximum bonding precision in complex modules. In addition, they offer a high degree of flexibility in production while allowing users more freedom in the development of their production processes.
Light-curing adhesives are a great choice for mass goods, but shadowed areas that cannot be reached by light present a challenge for them. (Image source: DELO)
Where There Is Light, There Is Also Shadow
When two components are bonded, it is important that all of the adhesive is fully cured. If the light only reaches some of the adhesive, it will remain liquid in the shadowed areas. This exposes components to the risk of corrosion or, in the case of optical products, an undesirable effect on the light path. Shadowed areas should be avoided from the start of the design stage if light-cured adhesives are used.
The adhesives industry has developed many new dual-curing products for situations in which it is not possible or very difficult to avoid shadowed areas. Aside from light, a second curing mechanism—either humidity, air exclusion, or heat—is used so that adhesives can bond reliably, even in shadowed areas. There are three main options, each of which fulfills different requirements and permits various manufacturing processes. All are one-component products that are isocyanate- and silicone-free, with the exception of UV silicones.
Using Natural Humidity in the Air
After initial fixation, light-/humidity-curing adhesives react with the natural humidity in the air in the shadowed areas. One benefit is that no additional equipment is necessary and no other curing process step is required after light curing. Bonded components can be further processed immediately.
Light-/humidity-curing adhesives are based on acrylates. In shadowed areas, they polymerize with humidity. (Image source: DELO)
In chemical terms, light-/humidity-curing adhesives are closely related to conventional light-curing acrylates and possess similar properties. Due to the simplicity of the process, this product group is selected for medium requirements—at maximum temperatures of use of 120-150°C and moderate chemical impact. UV silicones work on the same principle and can even be used in temperatures of up to about 300°C. However, due to their low strength, they are only suitable as sealants and also possess the typical disadvantages of silicones, such as swelling and contamination of production plants.
If requirements are higher, anaerobic curing is used as the second mechanism instead of humidity. Light-/anaerobic-curing adhesives offer high strength levels and temperature ranges up to 180°C. They can be used for challenging applications in electric motors with high heat dissipation levels. They are also resistant to chemicals like brake fluid, oil, and road salt that are encountered in the automotive sector.
Light-/anaerobic-curing adhesives are often used in mechanical engineering. They cure in shadowed areas viacontact with metal ions and oxygen exclusion. (Image source: DELO)
Light-/anaerobic-curing adhesives are based on widely used metal adhesives. Therefore, they need metal ions and oxygen exclusion to fully cure in shadowed areas. They offer two benefits compared to traditional metal adhesives. Productivity is higher due to fast light fixation. In addition, the adhesive cures on the fillet, where otherwise air is frequently found. Both the purely light- and anaerobic-cured areas are well-cured and share similar properties. If sufficient metal ions are available, these products do not require a further process to cure in shadowed areas.
Full Power: Light Plus Heat
The third option is light-/heat-curing materials, in which heat is applied to achieve full bond strength. This group is the most diverse. It offers products based on epoxy resins, acrylates, and other chemicals, with the latter mainly used in optoelectronics, thanks to its high transparency and low yellowing and outgassing.
Light-/heat-curing adhesives combine the bonding precision of light-curing products with the resilience of heat-curing materials. (Image source: DELO)
Epoxides tend to display higher strengths. They are harder and, because of their denser network, resistant to chemicals and high temperatures. Some of these products are so resilient that they can be used in modules that are permanently in contact with hot transmission oil. Acrylates are softer and therefore more flexible and tension-equalizing, letting them better compensate dynamic stress. An example of this is the attachment of decorative trims and cockpit elements in cars, where component tension needs to be equalized in a temperature range of -40 to -100°C.
These product groups share a common feature in that they fix components with UV or visible light within a few seconds, thus ensuring high production precision compared to standard products. Component shifting on the way to or during heat curing is avoided.
Convection ovens are usually used to supply the necessary heat for final curing. Alternatively, tunnel ovens, induction, or thermodes can be used. Typical temperatures are around 100°C, while the highest reliability products need at least 120°C and temperature-sensitive components can be bonded with other materials at as little as 60°C. As a result, high precision, defined production processes, and short cycle times can be combined with low thermal stress.
Dr. Karl Bitzer is a product manager at DELO Industrial Adhesives.
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