In this blog post, we’ll explore the principles behind how adhesives bond materials and examine how they are utilized across various industries and in everyday life.
We use many tools in our daily lives. We use scissors to cut things, writing instruments to write, and glue to fix a new hairpin that has come loose. Scissors and writing instruments are tools we’ve used for centuries, with relatively simple roles and operating principles. However, glue and adhesives are a relatively recent technological development, providing significant convenience in our lives. When using glue or adhesive to bond something, we don’t just consider joining two objects; we also think about how strong that bond is and how long it can last. Countless scientific principles are hidden within these everyday actions of cutting, writing, and sticking. This article aims to explain the principle behind adhesives, one of the many tools frequently used in daily life.
You’ve probably wondered at least once how adhesives make two materials stick together. Before diving into the principles of adhesives, let’s define adhesion. Adhesion is the state where two materials become integrated through chemical, physical forces, or both. Simply put, the intermolecular forces between the polymers in the adhesive achieve adhesion. To understand this process, a basic grasp of molecular structure and interactions is essential. Polymers are, literally, molecules with high molecular weight—meaning they are long chains of atoms. Intermolecular forces refer to interactions between neighboring molecules, distinct from the forces (chemical bonds) that form atoms into molecules. The essence of these interactions is electrostatic force, which is stronger when molecules are larger and when the distance between them is shorter. Therefore, large-molecule polymers are the primary components used in adhesives.
Common adhesives are used in a dissolved state in a solvent, with water serving as the solvent in most cases. Water acts as a carrier for the polymers, transporting them and allowing them to permeate into the structure of the substrate material. Once the moisture in the adhesive evaporates, the polymers within it utilize their mutual attractive forces to firmly bond the two materials together. However, applying too much adhesive causes the polymers within to jostle each other, preventing proper intermolecular attraction. Excess adhesive also hinders the evaporation of the solvent (water). Applying adhesive too thickly actually reduces adhesion effectiveness. This is because the principle of adhesive bonding relies on intermolecular interactions, not the adhesive’s inherent viscosity.
To understand the diverse applications of adhesives, it is crucial to grasp their mechanism of action more deeply. For instance, adhesive performance can be significantly influenced by environmental factors like temperature and humidity. When using adhesives, surrounding environmental conditions must be carefully considered, and specific situations may require specialized adhesives. Since suitable usage conditions vary depending on the type of adhesive, various adhesives are being developed to meet these specific requirements.
The same principle explains why adhesive inside a container does not stick to itself or to the container walls. The polymers inside the container are dissolved in solvents, including water. At this point, the polymers are far apart from each other. As explained above, intermolecular forces are stronger when the distance between molecules is shorter. Therefore, if water is trapped between the polymers, the intermolecular forces weaken. Furthermore, adhesives generally do not use polymers directly. A low-molecular-weight substance dissolved in a solvent spontaneously undergoes a condensation reaction upon contact with oxygen in the air, converting into a polymer to achieve adhesion. Here, a condensation reaction refers to the reaction where a low-molecular-weight substance forms a high-molecular-weight substance. Therefore, when storing adhesives, a substance that inhibits the condensation reaction is added to prevent self-adhesion. In old adhesives, this condensation inhibitor evaporates, causing the adhesive to harden and deteriorate.
Adhesives can be broadly categorized into three types. First, there are adhesives that use polymers as solutions, such as starch paste and rubber paste. Second, there are adhesives that start as low-molecular-weight liquids but polymerize into polymers after application. Finally, there are adhesives that involve heating solid polymers to melt them for bonding. This diversity in adhesives stems from their unique chemical properties. Each type is tailored for different applications, with some becoming indispensable elements in specific industries.
Epoxy resin, widely used in daily life and industrial production, belongs to the second type. As a type of synthetic resin, epoxy resin encompasses a wide range of products, from liquid to solid states, differing based on melting point. The most commercially used epoxy resin is the ether type produced by the condensation reaction of ECH and BPA, widely known as DPP. Simply put, the low-molecular-weight substances ECH and BPA react under sodium hydroxide conditions to form a polymer capable of exhibiting adhesive properties. This epoxy resin provides strong adhesion and durability, making it widely used in high-tech fields such as automotive and aircraft components, construction materials, and more.
Adhesives are widely used. Epoxy resin alone, for example, has an annual production value of approximately $15 billion. Manufacturing, semiconductor processes, and military applications also require extensive bonding, with specific adhesives chosen for each situation. The adhesive industry’s sustained growth is deeply linked to new material development. As new materials emerge, research and development of adhesives to effectively bond them are actively progressing. Adhesives are expanding their potential applications beyond traditional uses into diverse new fields.
Adhesives are not only used for traditional bonding but also ingeniously applied in new domains. Ship steam engines suffer severe corrosion due to prolonged immersion in high-temperature salt solutions. Additionally, parts secured with screws accelerate corrosion under strong vibration. In such cases, applying a suitable adhesive between components before fastening with screws creates a more robust bond and prevents corrosion. Furthermore, the use of adhesives like patches has increased significantly in the biofield. From the patches people commonly apply for acne, recent inventions include adhesives suitable for cardiac bonding. Such adhesives hold promise for use in cardiac surgery, obstetrics and gynecology, and abdominal surgery. These bioadhesives are composed of substances harmless to the human body and play a crucial role in shortening recovery times and preventing infection at surgical sites. In modern medicine, adhesives have transcended their role as simple industrial tools to become essential instruments in biotechnology.
