Adhesive Technologies

Article Series

Chapter 1

Adhesive bonding processes, one of joining technologies, have been actively used for many years in sectors such as aerospace, automotive, space, maritime, railway and medical. Joining methods can be complex by design. Because non-destructive methods from defect detection techniques do not provide definitive results and are subject to 100% inspection during and after application, they are protected by special standards. Adhesive processes are included in the "Special Processes" definition of the ISO 9001 standard. Fundamentally, the concept of adhesion is based on adhesion and cohesion forces holding two surfaces together with a non-metallic principle and establishing balance between surfaces. Their functions can be grouped as follows:

• Load transfer,
• Heat, sound and vibration insulation,
• Protection against corrosion,
• Light transmittance,
• Sealing at joint points,
• Electrical requirements,
• Compatibility with other joining techniques (hybrid technologies),
• Aesthetics

Adhesive bonding as a joining method;

- Provides a regularly distributed method of transferring load from one part to another. - Eliminates stress concentrations caused by perforated structures with mechanical fastening elements. - Distributes load transfer more evenly and uniformly to the bonding area. However, unlike welding technology which relies on melting and fusion of parts, the adhesive process requires an additional material to bond two parts together. While it is possible to eliminate some material limitations seen in welding technology with adhesives, some difficulties also arise with adhesive bonding. When the adhesive incorporated into the design is added to the parts it bonds, if we look closely at the load transfer function, we can see that different mechanisms come into play. First, the adhesive material acts as a binder through chemical bonds between surfaces. The strength of the bond depends on the adhesion process and the compatibility of the adhesive with the surfaces. Secondarily, the adhesive material demonstrates flow dependent on viscosity into the indentations and cracks of microscopically rough surfaces. The adhesive gains support from this surface roughness and mechanically interlocks in this manner, allowing a small amount of load transfer.

Surface Preparation

To understand potential failures and risks in adhesive joints, it is important to understand the fundamental steps in the adhesion process. From the perspective of surfaces to be joined, the first critical step is proper surface preparation of the parts. This system includes the following steps: - Increasing surface roughness using abrasive materials such as blasting, - Cleaning the surface using industrial solvents to remove any lubricants or foreign matter, and - Using a chemical treatment to activate the part surface, making it more susceptible to forming chemical bonds with the adhesive material.

Adhesive Application

The next step is application of the adhesive to the parts to be joined. Adhesives can be applied to surfaces in film or paste forms. After the adhesive is applied, the parts are brought together and temporarily held in place with a clamp or fixture. This is an important step because the adhesive cannot carry loads until it is completely cured and the required chemical bonds with the surfaces are established 1. Curing Monomers or prepolymer systems undergo chemical reaction with various performance-enhancing additives to reach the final strength of the polymer. This process is called curing. To accelerate this process, especially in aircraft manufacturing facilities, it is common to apply heat and pressure to effect the chemical reaction of the adhesive. Autoclaves, which are high-pressure furnaces, are frequently used to cure adhesively bonded parts and composite materials actually produced as structural parts with film adhesives. Surface preparation, the most critical step in adhesive processes, is of critical importance for ensuring chemical bond quality. Contaminants and insufficient activation of the surface to be bonded result in very low adhesion strength, leading to unacceptable failures especially for structural parts.

Occupational Health and Safety

In the surface preparation processes employed, toxic chemicals may be used for the health of workers or the environment. Restriction of hazardous processes, management with closed systems, search for chemicals with equivalent effect and performance, and continuous improvement with environmentally friendly options should be pursued.

Repeatability

Another major challenge is the repeatability of quality in adhesive processes. The clearest example seen in everyday life is adhesive notes. When several sticky notes are applied to a surface, it is expected that over time one or more of these notes may fall off. It is easy to predict that variables such as contaminants of different scales on the surface or pressure differences applied by hand force lead to the failure of notes falling, but predicting which note will fall first and when is extremely difficult. [caption id="attachment_126329" align="aligncenter"] Figure 4: Adhesive Notes (Reference: https://tr.wikipedia.org/wiki/Post-it#/media/Dosya:Wall_with_post-its.jpg)[/caption]   Failure Types: It is possible to define three main failure categories for a joint bonded with adhesive technology. 1) If the applied load exceeds the strength of one of the materials to be bonded, fracture is observed, defined as material failure. As in mechanically fastened joints, this is a desired failure type because it means a joint is designed well enough to reach the strength potential of the structures it joins. 2) Molecules within the material are held together by a force defined as cohesion force. If the load applied to the system exceeds the strength of the adhesive, fracture is observed in the adhesive material. This failure type is called cohesive failure. 3) When the chemical bonds attaching the adhesive material to the part to be bonded are overcome, adhesive failure can occur along the interface between the adhesive material and the surfaces. Adhesive failure is limited by the quality of the chemical bonds formed during the adhesion process. This failure type is unacceptable regardless of the process design and its occurrence indicates serious problems in the adhesion process itself. [caption id="attachment_126330" align="aligncenter"] Figure 5: Failure Types (Reference: https://tombrowninc.com/blog/understanding-adhesive-failures/)[/caption]   The inability to fully predict when and under what effects failures may occur complicates documentation and certification processes for adhesives in safety-critical structures. Therefore, although adhesive bonding is a chemical process that can result in a highly efficient structural bond, it is sensitive to pre-processing and process variables. Although scientists would like to take much greater practical advantage of adhesive joint efficiency, difficulties related to process variations complicate the widespread use of this technology for primary structural bonds. Adhesives are classified as chemical curing systems, physical state change systems, and pressure-applied systems that do not require solidification processes. This definition applies to the group that forms chemical bonds through reaction.

References 1) Composite Materials Handbook, CMH-17 2) Handbook of Adhesives and Surface Preparations, Ebnesajjad, S.