Glue is a polymer for the vast majority of modern adhesives. A polymer is a large molecule composed of many repeating smaller units. This molecular architecture gives glues their unique ability to bind materials together. Glue is any substance applied to surfaces that resists their separation, relying on the chemical and physical properties of these long-chain molecules.
Understanding the Polymer Foundation
The structure of a polymer is a long chain, much like a string of beads. Each bead represents a small, single molecule known as a monomer. These monomers chemically link together, end-to-end, to form the massive polymer chains.
This process results in a material with a very high molecular weight. The final properties of the adhesive, such as flexibility or stiffness, are determined by the specific type of monomer used and the arrangement of the resulting polymer chains. The long, tangled nature of these chains provides the internal structural integrity of the glue layer.
The Mechanism of Sticking
Polymer chains enable adhesion by employing two main forces: adhesion and cohesion. Adhesion is the attractive force where the glue meets the surface of the material being bonded. This attraction is achieved through intermolecular forces, such as Van der Waals forces, which collectively create a strong bond between the adhesive molecules and the substrate.
Cohesion is the internal strength of the adhesive, referring to the forces that hold the polymer chains within the glue layer together. A successful adhesive requires a balance of both forces. Weak adhesion prevents the glue from sticking to the surface, while weak cohesion causes the glue layer to break internally.
Many liquid adhesives also rely on mechanical interlocking, a physical mechanism. When applied, the liquid glue flows into the microscopic pores and irregularities of the material. As the adhesive hardens, it forms a physical anchor within these crevices, providing a mechanical lock that contributes to the overall bond strength. The polymer must first “wet” the surface, meaning it must spread out effectively to maximize contact.
Major Categories of Polymer Adhesives
Polymer adhesives are broadly categorized based on how they transition from a liquid state to a solid, bonded state, a process known as curing or setting.
Drying Adhesives
This major type cures through a physical change, specifically the evaporation of a solvent. Polyvinyl Acetate (PVA) glues, commonly used for wood and paper, are examples of drying adhesives. In PVA glue, the polymer chains are already formed and suspended in water. As the water evaporates, the chains consolidate and tangle together, leaving behind a solid film that forms the bond.
This process does not involve a chemical reaction to create new molecules. Hot-melt adhesives, which solidify upon cooling, are another example of a physical curing mechanism.
Reactive Adhesives
The second major type is the Reactive Adhesive, which undergoes a chemical change to set. These glues start as small molecules or short polymer chains and form their final, massive, cross-linked polymer network when a catalyst is introduced. Cyanoacrylate, or “super glue,” is a reactive adhesive that uses trace amounts of moisture as its catalyst to initiate rapid polymerization.
Epoxy adhesives are another common reactive type, typically consisting of two separate parts—a resin and a hardener—that are mixed just before use. This mixing triggers a chemical reaction called cross-linking. Cross-linking forms permanent chemical bonds between the chains, resulting in a rigid, heat-resistant thermoset material.