Adhesives bind materials together using physics and chemistry. The strength of the bond relies on two distinct forces: adhesion and cohesion. Adhesion refers to the attractive forces drawing the adhesive material to the surface of the object, known as the substrate. Cohesion describes the internal strength of the adhesive, specifically the molecular forces holding the glue together. An effective adhesive must possess strong adhesion to the surfaces it connects, while also maintaining high internal cohesion so the solidified material does not easily break apart.
The Fundamental Forces of Sticking
Adsorption and Wetting
The ability of an adhesive to physically stick to a surface begins at the molecular level, primarily through a mechanism called adsorption theory. This theory requires the liquid adhesive to completely spread over the substrate surface, a process known as wetting. Good wetting ensures the adhesive molecules come into intimate contact with the surface molecules, allowing short-range molecular forces to establish a strong connection.
Van der Waals Forces
The most common intermolecular forces responsible for this attraction are the Van der Waals forces. These weak, distance-dependent electrical forces are present between nearly all molecules and include London dispersion forces, which arise from temporary fluctuations in electron distribution. While individually weak, the cumulative effect of billions of these forces across the entire bonded area creates substantial adhesive strength. Since these forces are only effective over very short distances, good wetting is necessary to bring the molecules within range of each other.
Mechanical Interlocking
Mechanical interlocking is particularly effective on rough or porous substrates like wood or concrete. The low-viscosity liquid adhesive flows into microscopic pores, crevices, and irregularities on the surface. When the adhesive hardens, it forms a physical anchor, much like a microscopic rivet, physically locking the two surfaces together. Surface roughness can enhance the final bond strength by increasing the surface area and providing opportunities for this physical keying.
Chemical Bonding
In specialized applications, a stronger bond is achieved through chemical bonding. This involves the formation of primary chemical links between the adhesive and the substrate. This occurs when the adhesive and the substrate share functional groups that react to form strong covalent or ionic bonds. While less common in general consumer glues, this direct molecular connection is responsible for the durability and strength of high-performance structural adhesives used in aerospace and automotive construction.
How Different Adhesives Activate
Solvent-Based Adhesives
Adhesives transition from a liquid state into a solid, load-bearing material through a process called curing or setting. Solvent-based adhesives contain polymers dissolved in a volatile carrier, often water or an organic solvent. The adhesive sets when the solvent evaporates, leaving the solid polymer film behind to form the bond. The speed of this setting process is directly related to the evaporation rate of the specific solvent used.
Reactive Adhesives
Reactive adhesives rely on a chemical reaction to achieve their solid state, rather than simple evaporation. Two-part epoxies, for example, contain a resin and a hardener that are mixed prior to use, initiating a polymerization reaction that forms a rigid, cross-linked polymer network. Other reactive systems, such as polyurethane glues, are moisture-curing, meaning they react with humidity in the air or moisture on the substrate surface to form a durable linkage.
Specialized Reactive Systems
UV-curing systems contain photoinitiators that rapidly trigger polymerization when exposed to ultraviolet light. This allows for extremely fast setting times, often in seconds, but requires at least one substrate to be transparent so the UV light can reach the adhesive joint. Heat-activated reactive adhesives are also used, where the chemical reaction is accelerated or triggered by elevated temperatures, common in industrial applications to ensure a rapid, uniform cure.
Hot-Melt Adhesives
Hot-melt adhesives operate on a purely physical principle, requiring no chemical reaction or solvent evaporation. These materials are thermoplastic polymers that are solid at room temperature. They are heated until they melt into a low-viscosity liquid, applied to the substrates, and then rapidly solidify as they cool back down. The bond strength is achieved upon this physical phase change, making the process almost instantaneous, though the final strength is often lower than that of chemically cured reactive systems.
Factors Influencing Bond Strength
Surface Preparation
Surface preparation is one of the most important steps, as contaminants such as dust, oil, grease, or mold release agents can prevent the adhesive from achieving the necessary intimate contact for effective wetting. Cleaning the substrates with a solvent like isopropyl alcohol is often necessary to remove these weak boundary layers that would otherwise lead to failure.
Substrate Surface Energy
The surface energy of the substrate dictates how well the liquid adhesive will spread. High surface energy materials, such as metals and glass, allow for excellent wetting and tend to form stronger bonds. Conversely, low surface energy materials, like many plastics, cause the adhesive to bead up, making it difficult to achieve the molecular proximity required for strong adhesion. Specialized primers or surface treatments are sometimes needed to raise the surface energy of these plastics.
Material Nature
The nature of the substrate material itself also plays a role in the bond’s performance. Porous materials, such as wood or cardboard, can absorb too much of the liquid adhesive, starving the bond line and weakening the connection. Non-porous materials like metal or dense ceramics require an adhesive that can effectively manage the lack of mechanical interlocking opportunities. Proper material selection ensures the adhesive and substrate are chemically compatible and physically suited to each other.
Environmental Conditions
Environmental conditions during application and curing can significantly impact the bond formation. Temperature control is important, as temperatures that are too low can slow or halt the chemical reaction of reactive adhesives, while excessive heat can cause certain adhesives to degrade or soften prematurely. Humidity is also a factor, particularly for moisture-curing adhesives where insufficient ambient water vapor can prevent a full cure. A stable environment during the curing period is necessary to allow the molecular forces to fully develop their intended strength.