Accelerated adhesive curing

In adhesive bonding technology, adhesive curing is often one of the most time-consuming process steps. However, the well adapted addition of heat can accelerate the curing of many classic adhesives and thus significantly reduce the process time. 

While adhesive bonding offers a number of advantages over other joining technologies, the time required for adhesive curing often poses a particular challenge for process design. Additional buffer sections for long fixing times and molds with multiple mounts are typical approaches when high volumes are to be produced. In the worst case, adhesive bonding technology is even ruled out as a possible joining technology for the manufacturing process for these reasons.

 

Fast-curing adhesives for short cycle times

If the cycle times in an adhesive bonding process are to be short, this can be achieved e.g. by selecting fast-curing adhesive systems. 

With UV-curing adhesives, curing times of a few seconds can usually be achieved. However, these relatively cost-intensive adhesives require the adhesive to be accessible to UV radiation, which is used to trigger the curing reaction (e.g. at least one UV-permeable component). In addition, these adhesives are special classes of adhesives whose end property profile covers only a small part of the performance spectrum of the adhesive classes available on the market.

Fast-curing versions of many other adhesive classes are also available. For example, there are 2K epoxies, polyurethanes and acrylates that achieve handling strength and even final strength within minutes to hours. Here, however, the likewise very short pot life and processing time must be taken into account. If an ongoing supply with new component cannot be maintained within a narrow time window, flushing times of the mixer or its replacement are necessary, which can result in high costs.

Hot-curing adhesives are also available as faster-curing versions. However, a faster curing reaction or one that starts at a lower temperature generally results in a reduced shelf life or the need for cold storage.

 

Thermal acceleration of adhesive curing

If neither buffer sections for adhesive curing nor fast adhesive systems can or should be used for the adhesive bonding process, there is often the possibility of thermally accelerating the curing reaction.

In many adhesive systems, the chemical curing reaction is accelerated by the addition of heat. For adhesive systems such as 2K epoxies, polyurethanes, acrylates and silicones with moderate pot and processing times, our experts have been able to accelerate curing significantlys in numerous projects.

In many cases, it could also be realized to accelerate the curing process of already heat-curing adhesive systems. This was then mainly based on a locally limited heat input, and its adaption to the curing reaction of the respective adhesive.

 

Heat input into the component

Different methods can be used to introduce heat into the component or the adhesive layer.

A comparatively simple method of introducing heat into the component or the adhesive to be cured is to heat the entire component in an oven. In case that the same components are always heated in the same oven at a given temperature and dwell time, it can be assumed that the adhesive is sufficiently heated and thus cured.

However, if large components are involved to which only small attachments are bonded, this process is very energy-intensive and a long heating and cooling phase must be planned. Temperature-sensitive attachments also limit the maximum applicable temperature, which leads to a longer dwell time in the oven but also exclusion of certain adhesive systems.

With other heating methods, we are able to apply the heat locally. For larger components, this means that only the area of the bond is heated without, for example, heating temperature-sensitive add-on parts. In this way, also a certain independence of the adhesive bonding process from the ambient temperature can be achieved if, for example, a 2-component adhesive system requires a specific temperature window for curing but the production environment is not sufficiently heated.

 

Process development for thermally accelerated adhesive curing

For the development of a bonding process with thermally accelerated adhesive curing, the Fraunhofer IFAM possess various methods to introduce heat into the components and the adhesive. Beside the classic oven, components can also be heated locally using the following methods:

• Induction
• Infrared radiation
• Hot air
• Thermode

When selecting the method for the particular adhesive bonding process, we take into account aspects such as the material of the component, the component geometry, the adhesive, the targeted process time and the conditions in production (space, equipment integration, dissipated heat...). In this way, an individual solution for the respective adhesive bonding process is created.

 

Adhesives qualification

If the adhesive shall be cured thermally accelerated in an adhesive bonding process, a corresponding adhesive qualification must be carried out for this particular bonding process.

Even if many adhesives are generally suitable to be cured thermally accelerated, the curing process cannot be accelerated at will.

We therefore give particularly high priority to matching the temperature regime to the adhesive system to be used. For each adhesive, a process window must be maintained with regard to the maximum heating rate and the associated curing temperature. This is individual for each adhesive and is determined by us experimentally.

Above a critical heating temperature, damage to the adhesive can occur, which reduces the strength and long-term resistance of the bond. If the curing temperatures and times are too low, the adhesive is not fully cured and does not achieve the desired strength.

When developing an adhesive bonding process with thermally accelerated adhesive curing, we therefore check the adhesives in this regard as part of the adhesive selection process and already compare at this point whether the targeted process time can be achieved with them.

 

Fraunhofer IFAM is continuously developing the topic of thermally accelerated adhesive curing within the framework of publicly funded projects and develops customized rapid curing processes for its industrial customers. This includes the selection of suitable methods, the design of the adhesive bonding process, and the selection and qualification of adhesives.