Structural Adhesives in Composite Materials

There are numerous adhesive alternatives available for bonding composite materials. Each composite part has different requirements during production. Sometimes full adhesion must be achieved within seconds, sometimes within hours, and the adhesive must be resistant to certain properties it will be exposed to during use. Adhesives are generally defined as a non-dissolvable bond created with a typically synthetic-based material that joins two parts together. The adhesive, which creates a very thin layer between the parts, hardens through the formation of macromolecules as a result of chemical reactions. The adhesive provides resistance to external forces through both its internal strength (cohesive resistance) and its adhesion to part surfaces (adhesive resistance). Adhesives are available in single and two-component formulations. In single-component adhesives, all necessary components are contained within the adhesive material. According to their chemical structure, pressure is applied during hardening to prevent water discharge during spreading on the surface. Another type is two-component adhesives, which are preferred in the composite sector. The most commonly preferred two-component adhesives are epoxy, acrylic, and polyurethane-based ones. Because they are two-component, the A and B components must be thoroughly mixed before bonding. These mixtures are applied either manually or with the aid of static mixing nozzles during application. Due to the high peel and shear strength values of these types of adhesives, this group of adhesives is defined as "Structural Adhesive." Because of their structural nature, they have high strength values when exposed to loads, providing protection against separation, peeling, tearing, and other failures. Structural adhesives preferred in the composite sector enable the assembly of parts made from identical or different materials. Different adhesives are available for bonding composites to metal, wood, or polymer-based materials. Joining methods other than bonding, such as drilling and welding, can damage the materials to be joined. Especially rust and corrosion that can develop over time during use reduce material lifespan. For this reason, structural adhesives are the most preferred adhesives for joining composite materials due to their resistance to chemical and environmental factors. Because structural adhesives are manufactured with 100% solids, there is no solvent emission during their use, and application and curing times can be adjusted according to preference. For curing, they do not require exposure to air and moisture like single-component silicones and polyurethanes, so curing is completed down to the deepest points. Because structural adhesives have a very wide range of characteristics and applications, engineers sometimes have difficulty in their selection. With the development of adhesive technologies, the load-carrying values of bonded parts have also increased. Manufacturer guidance is important in selecting structural adhesives. When choosing an appropriate adhesive, it is first necessary to consider the technical properties and conditions required in the final application. The heat, humidity, and UV light exposure to which the material to be bonded will be subjected are important factors. In addition, the required chemical resistance, environmental factors, and vibration, impact, and fatigue data of the part should be evaluated. For materials in rail systems and aviation sectors, vibration values to which the materials will be exposed should be calculated and adhesives with resistance to these properties should be selected. The most commonly preferred structural adhesive types are single or two-component epoxies, acrylics, urethanes, and cyanoacrylate-based adhesives defined as fast-setting adhesives. Although acrylic-based structural adhesives have the highest bond strength values in joining plastic materials, they have lower values compared to epoxy adhesives in terms of impact and vibration resistance. Urethane-based adhesives are more flexible than others but have lower strength. They are a suitable solution for bonding rubber and plastic materials. In conclusion, in making the correct adhesive selection, the end user's demands and requirements must first be evaluated. Based on this information, a final decision should be made by comparing the evaluated adhesive properties. Before proceeding to use in the production environment, testing and approval on samples is always recommended. Samples bonded one on top of another should undergo peel, tearing, and if necessary vibration and impact tests, and the results should be evaluated. This type of technical support can be obtained through collaborative work with adhesive manufacturers. Recai Turan - Mechanical Engineer / Sales Representative - Omnis Kompozit