Bonding is often the preferred joining method, particularly for dissimilar substrates and/or when mechanical fastening methods are not a good option. A large number of substrates including ceramics, glass, metals, wood, plastics and composites can be bonded with adhesives. However, there are a number of substrates and substrate combinations that tend to be difficult to bond. Low surface energy substrates such as polypropylene, polyethylene, fluoropolymers, acetal, and diene rubbers are the most common types of hard-to-bond materials. Only a small number of industrial adhesives provide durable bonds on these substrates.
(Light curing) cyanoacrylates, certain hot melts and occasionally light curing acrylic adhesives exhibit reasonable high bond strength on many hard-to-bond substrates. Polyolefin hot melts are often a good choice but are not always suitable or practical due to their limited bond strength. This family of adhesives offers good adhesion to olefins such as polypropylene and high density polyethylene. Another good option are thermosetting urethane hot melts (PURs), particularly when stress induced creep in the adhesive joint has to be avoided. These adhesives perform well on a number of hard-to-bond plastics. Unlike traditional hot melts, reactive urethanes are transformed to thermoset plastics when fully cured. They often provide durable bonds to difficult-to-bond materials and have a much lower processing temperature than traditional hot melts such as EVA, polyamide, and polyolefin hot melts. However, due to their more polar backbone, they tend to have lower bond strength on low-surface energy substrates such as PTFE, PP, HDPE, LDPE. Another class of adhesives used for hard-to-bond substrates are rubber modified cyanoacrylate adhesives. These adhesives have improved peel and impact strengths over traditional cyanoacrylates but are still rather brittle when compared to high-performance acrylic adhesives.
Thermoplastic welding is another option, however, this method is not suitable for thermosets and is often only a good option for surface edge bonding, which has only localized strength, and/or when a special joint design is chosen.
Sometimes primers are employed to promote adhesion between two non-bonding surfaces. Very popular primers are silanes. They are often used with silicone adhesives but can be used with other types of adhesives, such as epoxies, as well. The reactive silane primers typically carry two reactive groups, one that adheres to the substrate and another that is compatible with the adhesive. Often one group is hydrophilic (like silanol) and the other is hydrophobic (like n-dodecyl). These silanes create an interface that is compatible with both the substrate and the adhesive that would be incompatible without the adhesion promoter.
When priming is not an option, flame, plasma, or corona surface treatment is usually the only good option to achieve a cost effective solution. It is often believed that the main purpose of this treatment is to remove contaminations and to increase the surface tension which, in turn, improves wetting of the substrate by the adhesive. However, surface tension related adhesive forces are rather weak and have a negligible contribution to adhesion. The true purpose of this type of surface treatment is to improve interpenetration of the polymer chains across the interface with subsequent entanglement by reducing crystallinity and enhancing chain mobility in the interface region and/or to create chemical sites at the substrate surface for bonding. The later plays only a minor role for PSA's and hot melt adhesives. In the case of structural adhesives, flame treatment is often the most effective method whereas corona treatment is the most popular method because it is typically the most economical and practical method for achieving improved adhesion.
Very challenging is also the bonding of substrates with very different coefficients of thermal expansion (CTE). Some real-world hard-to-bond substrate combinations include:
To compensate for the mismatch of thermal expansion, the adhesive needs to have a low modulus (high flexibility) and often must be able to withstand temperature cycles and in some cases different chemical environments. A flexible adhesive absorbs some of the stress caused by dissimilar expansion of the adjoining materials allowing the bonded parts to move more freely with fewer constraints and thus, reducing stress in the bondline. A flexible adhesive is also more forgiving when pushed and pulled or when expanding or contracting in the joint and will not crack under extreme temperature cycles. However, these adhesives produce lower strength joints than common structural adhesives. Thus, chosing the right adhesive and optimizing the joints of the adhesively-bonded parts is crucial to achieve acceptable/optimal bond performance.
Components having ceramic-plastic or metal-plastic joints are usually only used for static applications. Many of the aforementioned adhesives can be used to join these materials. However, these material combinations are typically too brittle to be used in dynamic applications where rubber-metal composites are frequently employed. For example, rubber-metal composites are used for the isolation of noise and vibration in a multitude of automotive and engineering applications. Due to the high (cyclic) stresses (bond fatigue), a uniform, flexible, and durable bond is essential. The adhesives, primers, and bonding processes are often unique to these applications.