37. Preparation for Bonding

Adhesives are used today with increasing volumes in many applications in different industries and are supplied to the end customers in various forms. Very common are liquid adhesives, pastes, mastics, as well as adhesive films or tapes. Less common are powder systems for coating purposes. Adhesives are used where more conventional mechanical or thermal joining technologies can’t be used because the joint area is not accessible (e.g., for a spot welding or riveting gun), entire areas need to be bonded (tile or carpet bonding), or if improved mechanical joint performance is required. Improved mechanical performance could be increased joint stiffness, longtime durability, or corrosion resistance under environmental exposure.

Bonding with adhesives is not a simple operation, and a lot of preparation work is necessary before the adhesive can be applied to the designated bonding area (Shields J, Adhesives handbook. Butterworths, London, 1985). A chain of preparation processes is required. It starts with the mixing of the adhesive and its storage before the transfer to the customer. At the customer, the adhesives need to be transferred to the operation area. The substrates to which the adhesive will be applied have to be well prepared. The final process step includes the dispensing of the adhesive to the substrate and the joining of the parts which are intended to be bonded. Some adhesive formulations require an accurate metering of different components. Some, sensitive adhesive operations require a monitoring of the application, e.g., by vision systems, to guarantee the correctness of the application. Each of the steps is important and contributes equally to the quality of the final adhesive application. See below the summary of the different process preparation steps:

Depending for which application, in which industry, and by which end customers adhesives are used, different application techniques and dispensing and metering devices are chosen.

For smaller bonding operations like sealing work at construction sites, repair work at car garages, or to fix tiles to a floor, adhesives are typically manually applied out of smaller containers like cartridges. If adhesives are used in a larger scale, like at an automotive line, for example, for metal bonding reasons or to bond windows to the frame of a car, a robot application is used, which pumps the adhesive out of drums and transfers it to a dozer and a gun for the final application. The adhesive itself can be applied in different ways like as a simple bead, sprayed, extruded, or by air assistance, called swirling. Which technique is used depends on factors like the application, the rheological property of the adhesive, and the application temperature. Therefore, the rheological properties of the adhesive formulation need to be tailored to the application. Lower viscous adhesives can be applied by spraying or extrusion beside the simple bead application. Higher viscous adhesives are often limited to bead application, or the application temperature needs to be significantly increased to be applicable by other methods.

This chapter will discuss in further detail each item of the process chain: the storage of adhesives, the transfer to the application area, the different metering and dispensing technologies, the used mixing equipments, the substrate preparation, and the quality control. Proposals are made regarding people education and how to create a safe working environment.