Hybrid Dual-Curing Adhesives

Most manufacturing companies prefer light-curing adhesives if possible to achieve high productivity levels. The use of these systems is however subject to limitations. So, in recent years, adhesive manufacturers like DELO have focused on pushing these limits considerably further by developing several dual-curing products.

di Stefano Farina

In many industries, from automotive and electronics to mechanical engineering, bonding has become more important due to light-weight construction, miniaturization and multi-material design. Adhesives are also increasingly used as sealants to protect components from environmental influences.
Companies producing mass goods prefer light-curing adhesives, if possible, for productivity reasons. Another benefit is high positioning accuracy, as components can be initially fixed on demand, and once applied the adhesive does not flow, which can happen in the case of heat-cured products in the oven.
Light-cured adhesives achieve full strength within a couple of seconds when irradiated, in specials cases even less than a second. This is achieved via high-energy LED lamps, which generate 100 to 1000 times the intensity of normal daylight within their specific light spectrum. These systems materials are subject to limitations given maximum implementation temperatures of more than 150 °C and regular contact with aggressive chemicals, oil and acid.

Where there is light there is also shadow
When two components are bonded, it is important that all the adhesive is fully cured. If the light only reaches some of the adhesive, it will remain liquid in the so-called 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 therefore be avoided from the outset at the design stage if light-cured adhesives are used.
The adhesives industry has developed many new dual-curing products for situations in which this is not possible or very difficult. Aside from light they use a second curing mechanism so that adhesives can bond reliably even in shadowed areas. This may either be humidity, air exclusion or heat. Each option fulfils different requirements and permits various manufacturing processes. All are one-component products which are isocyanate- and silicone-free, with the exception of UV silicones of course.

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. In addition, bonded components can be further processed immediately.
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, i.e. 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 around 300 °C. However, due to their low strength, they are only suitable as sealants and also possess the typical disadvantages of silicones like their swelling behavior and contamination of production plant.

Light-/anaerobic-curing adhesives
If requirements are higher, anaerobic curing instead of humidity is used as the second mechanism. Light-/anaerobic-curing adhesives offer high strength levels and temperature ranges up to 180 °C, so they can be used for challenging applications in electric motors for instance, with high heat dissipation levels. They are also resistant to chemicals in the automotive sector like brake fluid, oil and road salt.
These products are based on the metal adhesives widely used in industry. They therefore need metal ions and oxygen exclusion to fully cure in shadowed areas. However, they offer two benefits compared to traditional metal adhesives: on the one hand, productivity is higher due to fast light fixation, and on the other, the adhesive also 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, as it offers products based on epoxy resins, acrylates and other chemicals, with the latter mainly being used in optoelectronics thanks to its high transparency and low yellowing and outgassing.
Epoxides tend to display higher strengths; they are harder and on account of their denser network, resistant to chemicals and high temperatures. Some of these products are so resilient that they can be used in modules, which 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 groups of products share a common feature in that they fix components with UV or visible light within a few seconds and thus ensure high production precision compared to standard products. Component shifting on the way to or during heat curing is thus 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. High precision, defined production processes and short cycle times can be combined with low thermal stress.

Conclusions
Dual-curing adhesives offer the benefits of light-curing systems without compromising on reliability, bond strength and processing quality. They also ensure that the adhesive in the finished product is fully cured and permit maximum bonding precision in complex modules. They offer a high degree of flexibility in production and thus allow users more freedom in the development of their production processes.

(Stefano Farina is Regional Sales Manager at DELO)