Adhesion science can be considered as a branch of the broader field of materials science. It deals with understanding and controlling the properties of things whose performance depends on the interface between two materials. An appreciation of adhesion science leads to the design and manufacture of longer-lived products with improved performance.
It is the chemical interactions at the interface of the adhesive and substrate that we call adhesion. If we look around us with an eye for all the places where one material is sticking to another, whether it’s the print on a soda pop bottle, the padded dash in your car, the display on your laptop computer, we quickly come to understand that adhesion science is critical for all industries and can teach us valuable lessons to enable better product development.
An example of an important lesson learned regards the relative unimportance of surface roughness for achieving a strong bond. Many people still carry the belief that ‘mechanical interlocking’ of an adhesive or coating with the nooks and crannies of a rough surface is necessary for a strong adhesive joint. However, adhesion relies on atomic forces, specifically intermolecular attraction, and these are not affected by surface roughness. Roughness can enhance a bond that is already strong by providing a tortuous path that interferes with the ability of a crack to propagate, but no amount of roughening will create a good bond without strong adhesion in the first place.
To immediately see that roughness is not an important factor in adhesion, simply open up any electronic device and look at the structure of the microelectronics inside. These depend on establishing strong and durable adhesive bonds between the atomically smooth surfaces of the semiconductor devices and the circuit boards they reside on.
If surface roughness were critical for adhesion, these devices would fall apart.
But why is the most common instruction that adhesive manufacturers put on their technical datasheets to roughen the surface prior to bonding? The act of roughening with an abrasive is primarily removing gross levels of contaminants and loosely adhering oxides and dirt from the surface, thereby creating a clean, strong, chemically active surface: this is what is actually enhancing the adhesion.
If we take this nicely cleaned roughened surface and simply apply a monomolecular layer of a silicone to that surface, we have not changed the roughness a bit, yet we have created a surface that is impossible to bond to. The silicone interrupts the chemical interactions that we need between the adhesive and substrate in order to establish adhesion. Roughness can enhance a bond that is already good—but roughness by itself cannot transform a bad bond into a good one. In order to create successful bonds/coatings, you need a clean, chemically active surface, not a rough one.
Adhesion science has taught us that it is the chemical interactions occurring within the top 3-5 molecular layers of the interface that are all critical in establishing strong, durable, high-performance interfaces in bonded, coated, and printed structures.Developing an appreciation for the advances in adhesion science helps product development teams leverage adhesive bonding and advanced coatings to make efficiently manufacturable realities out of the most advanced and creative designs.
Brighton Science has deep expertise in materials and adhesion science, and our goal is to help product development teams learn more about how to harness the power of adhesion in their new product development processes.