Problems Related to Heat Transfer

Heat transfer is often not thought of as being one of the multiple disciplines contributing to science of adhesion. One reason may be that heat transfer units are like foreign languages to many of us. It seems most engineers are comfortable working with mechanical units and terms. But heat transfer involves using terms that are more difficult to grasp such as BTU and specific heat.

In practice, heat transfer often comes along to bite us. We will look at two common problems that often plague users of adhesives because they forget to consider heat transfer.

Take for example, the case of laminating many metal sheets together. The application may be the magnetic core of a transformer, the windings of a motor, or the construction of a panel assembly. We use an adhesive to bond the laminates together, and if it is a room temperature curing adhesive, generally there is no problem - at least immediately.

Now let's consider using an adhesive that cures at elevated temperature because many of these applications require good temperature resistance. The outer plies of the laminate will see heat first. Heat will be conducted through each metal layer and through each adhesive layer until the entire mass comes up to the temperature of the heating medium (oven, press, etc.). If the adhesive is not carefully chosen, it is very possible that the outer plies will set first and the inner plies last due to heat transfer through the laminate. This means that the outer layers will be exposed to temperature and thermal stresses (due to differences in thermal expansion) before the inner regions of the laminate. As the laminate cools back down from the curing temperature to room temperature, the reverse is true. The inner laminates have less heat history. As a result of the uneven distribution of stresses, significant internal stress can be generated within the laminate that could lead to bond rupture and delmaination.

Another example of the importance of heat transfer is in the curing or drying of adhesives. Elevated temperature curing can be accomplished in several ways. The most common is by placing the assembly in a heated oven to affect the adhesive's cure. However, although this process seems to be simple and easily controlled, there are many opportunities for trouble.

If a cure of 60 min at 150°C is recommended, this does not mean that the assembly should be simply placed in a preheated 150°C oven for 60 min. The temperature is to be measured at the adhesive bond line. A large part will act as a heat sink and may require substantial time for an adhesive in the bond line to reach the necessary temperature. The number of items placed in the oven will also affect the time for the bond to get to temperature. In this example, total oven time would be 60 min in addition to whatever time is required to bring the adhesive up to 150°C. Bond line temperatures are best measured by thermocouples placed very close to the adhesive. In some cases, it may be desirable to place the thermocouple directly in the adhesive joint for the first few assemblies being cured.

Oven heating is the most common source of heat for bonded parts, even though it involves long curing cycles because of the heat-sink action of large assemblies. Ovens may be heated with gas, oil, electricity, or infrared units. Good air circulation within the oven is mandatory for uniform heating. Temperature distribution within an oven should always be checked before items are placed in the oven. Many ovens will have significant temperature distributions and dead-spaces in corners where air circulation is not uniform. The geometry and size of the part may also affect air circulation and cause variations in temperature distribution.

The combination of hot air and infrared radiation are well suited for the accelerated drying of waterborne adhesives and coatings. This is probably the most effective variation in formulation or production to gain a large improvement in speed. The hot air primarily affects the coating surface while the IR radiation, depending on the wavelength can penetrate into the deeper layers of the coating.