Mechanisms of adhesion
To better understand how or why an adhesive will stick to a surface, it helps to understand the mechanisms of adhesion. There are four commonly recognized methods through which adhesives and tapes will bond to a substrate surface.
Chemical adhesion occurs via molecular contact at the surface of the substrate. It is the strongest mechanism of adhesion, with bonds occurring between functional groups in the adhesive and atoms/molecules on the substrate surface. This mechanism contributes to the structural strength of many adhesives such as epoxies bonding aluminum.
Mechanical Interlocking Adhesion
Mechanical interlocking occurs as the adhesive flows into the pores on the substrate’s surface. The interlocking provides a physical impediment to crack propagation at the interface. It also serves to increase surface area, increasing total contact between adhesive and substrate. Liquid adhesives readily flow upon application to the substrate prior to drying or curing. Bonding tapes continue to flow slowly over time, building strength as they do.
Diffusion occurs when the adhesive polymer is capable of penetrating and entangling with a polymeric substrate. An interface is created with entangled polymer chains bonding adhesive and substrate. This mechanism is common in assemblies with difficult-to-bond low surface energy materials such as polypropylene.
Electrostatic adhesion occurs as the charged surface of the adhesive is drawn toward the oppositely charged substrate, and it is most common in tape applications. It’s the phenomenon often observed when trying to tape a box or wrap a present.
Adhesive Bonding Strength
Adhesion and cohesion work together to create a strong bond
Adhesive strength is the interfacial strength between adhesive and substrate, and usually the most important consideration when designing a strong adhesive bonded assembly. However, adhesive strength is not the only factor critical to creating an effective bond. Even when using the world’s toughest adhesive, a bond will fail if the adhesive does not bond to the surface of the substrate.
Cohesive strength is the internal strength of an adhesive – the ability of the adhesive to hold itself together under stress. The higher the cohesive strength, the stronger the adhesive. Cohesive strength is determined by the chemical composition of the adhesive. The strength of adhesives covers a wide range, from pressure sensitive adhesives to structural epoxy and acrylic adhesives.
It’s important to consider the specific types of stress that will act on an adhesive joint. Common stresses include shear, cleavage, peel and tensile. Knowing the magnitude and frequency of the stresses your application will be subjected to is helpful in choosing the adhesive with the best cohesive strength for the task at hand.