Introduction
An adhesive is a material that can bond two surfaces together, usually in a permanent or semi-permanent manner. It works by forming a bond that holds the materials together, preventing separation and providing structural integrity. These materials include polyurethane adhesives used in everyday crafts, as well as advanced polymers necessary for building buildings, vehicles, and more. This article aims to take a deep dive into the basic properties of adhesion and cohesion. Adhesion refers to the ability of an adhesive to bond dissimilar surfaces, while cohesion describes the internal strength of an adhesive. Understanding these properties can help in selecting the right adhesive for a specific task, ensuring that the bond formed is strong and long-lasting. This article provides you with some insights into adhesion and cohesion in adhesives by exploring the basic properties of adhesives in these contexts.
Basic Concepts of Adhesion and Cohesion of adhesives
——Definition of Adhesion
Adhesion is the process of bonding two dissimilar substances together, usually through the use of adhesives. The effectiveness of adhesion depends on the ability of the adhesive to form strong molecular bonds with the materials it contacts. These bonds can be mechanical, where the adhesive physically interlocks with surface irregularities, or chemical, where molecular forces between the adhesive and the material form a strong bond.
Basic types of adhesion
| Type of Adhesion | Description | Characteristics |
|---|---|---|
| Mechanical Adhesion | Adhesive enters the microscopic pores of the surfaces being joined. | ——Effective on rough or porous surfaces – Bond strength depends on the physical interlocking |
| Chemical Adhesion | Bond formation through chemical interactions between the adhesive and the substrate. | ——Requires compatible surfaces for molecular bonding – Often results in very strong bonds |
| Dispersive Adhesion | Involves adhesion through van der Waals forces or other dispersive interactions. | ——Common in glues like PVA (polyvinyl acetate) – Effective for general-purpose applications |
| Electrostatic Adhesion | Attraction between charged particles of the adhesive and the substrate. | ——Less common, used in specialized applications – Can be influenced by humidity and temperature |
| Diffusive Adhesion | Occurs when molecules from different materials intermingle at the joint. | ——Common in welding or when adhesives dissolve into substrates – Strong and permanent, used in both solids and liquids |
——Definition of Cohesion
Cohesion refers to the internal bonding force in an adhesive that holds molecules of the same substance together. This inherent property determines the structural integrity and mechanical strength of the adhesive itself. High cohesion is essential to ensure that the adhesive does not fail under stress and maintains its shape and function over time. The cohesive strength of different adhesives varies; for example, thermosetting polymers generally have higher cohesion than simple drying adhesives.
How Adhesion And Cohesion Affect The Performance Of Adhesives.
The interplay between adhesion and cohesion is critical in determining the performance of an adhesive. Adhesion allows adhesives to adhere securely to different surfaces, ensuring that the materials remain bonded under a variety of conditions. Cohesion, on the other hand, keeps the adhesive intact within itself, preventing it from failing internally. When adhesion is strong, the adhesive forms a durable bond with the materials it contacts, but if cohesion is weak, the adhesive may tear or break under stress, resulting in bond failure. Conversely, if cohesion is too strong compared to adhesion, the adhesive may pull away from the surface while remaining intact, which can also result in bond failure. Therefore, achieving a balanced relationship between these two properties is critical to developing adhesives that not only adhere well but can also withstand forces and environmental stresses without breaking. This balance ensures that adhesives perform reliably in everything from everyday household tasks to critical industrial applications.
Types Of Adhesives Used In Industry
In industry, some common adhesives are:
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Polyvinyl Acetate (PVA): Widely used in woodworking, bookbinding, and paper products; known for its ease of use and water solubility.
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Acrylic Adhesives: Offer excellent UV resistance and are used in automotive and construction industries for bonding plastics and metals.
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Hot Melt Adhesives: Based on a variety of thermoplastics like polyethylene; used in packaging, textiles, and electronics for their quick setting times.
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Polyurethane Adhesives: Offers great flexibility and excellent bond strength to a variety of materials, including wood, plastics, and metals. This adhesive is resistant to moisture and temperature, making it ideal for outdoor applications. In the construction industry, for example, polyurethane is often used for panel bonding and flooring installations because it absorbs shock and vibration while maintaining a strong bond.
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Epoxy Adhesives: Known for their high strength and chemical resistance, used in automotive, aerospace, and construction applications.
Mechanisms of Adhesion
There are many different bonding mechanisms, each of which plays a key role in how well an adhesive performs in different applications. Mechanical bonding is one of the most straightforward mechanisms, where the adhesive fills in gaps and irregularities in the surface to which it is applied. This process creates a physical interlock between the adhesive and the surface, like puzzle pieces fitting together, which enhances the strength and durability of the bond. Chemical bonding, on the other hand, involves molecular interactions at the interface of the adhesive and substrate. This type of bonding relies on the formation of covalent or ionic bonds, so the resulting bond is generally stronger and more resistant to environmental factors than mechanical bonds alone.
In addition, dispersion forces, such as van der Waals forces, also play an important role in bonding. These forces are weaker than chemical bonds, but are essential for the adhesive to initially adhere to the surface, helping to form a stronger bond later. Diffusion is another mechanism, where the molecules of the adhesive and substrate actually mix together, and is particularly common in solvent-based or polymer adhesives. This mixing can create a very strong bond because the materials are effectively partially fused at the molecular level. Each of these mechanisms contributes to the overall effectiveness of the adhesive, ensuring it performs optimally in its intended environment.
Differences Between Adhesion and Cohesion of adhesives
| Aspect | Adhesion | Cohesion |
|---|---|---|
| Definition | The bonding force between two different materials through an adhesive. | The bonding force within the same material, particularly within the adhesive. |
| Purpose | To bond two surfaces of different materials together. | To keep the adhesive itself intact and maintain its structural integrity. |
| Influence on Performance | Determines how well the adhesive sticks to different substrates. | Influences the mechanical strength and durability of the adhesive. |
| Failure Mode | Adhesive failure occurs when the bond between the adhesive and the substrate breaks. | Cohesive failure occurs when the adhesive itself breaks down internally. |
| Examples | Bonding glass to metal, attaching a label to a bottle. | The internal strength that prevents an epoxy or a glue from cracking or splitting. |
Factors Influencing Adhesion And Cohesion
The effective adhesion and cohesion of an adhesive is influenced by a number of key factors, each of which plays a key role in ensuring that materials bond well and that the bond lasts.
Surface Preparation: Surface preparation is the basis for successful bonding. Surfaces must be clean and free of oil, dust and other contaminants. Often, surface preparation, such as sanding, priming, or the use of chemical etchants, may be required to increase the surface area available for bonding and enhance the adhesive’s ability to form a strong bond.
Environmental Conditions: The environment in which the adhesive is used and operated can significantly affect adhesion and cohesion. Temperature, humidity and pressure can alter the adhesive’s curing process and change its viscosity, affecting its bonding effectiveness and long-term integrity. For example, high temperatures can weaken the bond, while extreme cold can make some adhesives brittle. Likewise, high humidity affects the setting time and final strength of many adhesives.
Material Properties: Different materials (such as metals, plastics, and ceramics) have different properties, such as surface energy, thermal expansion, and porosity, all of which affect adhesion. Adhesives that work well with metal may not work well with plastics due to differences in surface properties and how the material reacts to environmental factors.
Therefore, understanding these factors can help in choosing the right adhesive and using it correctly, ensuring that the bond is both effective and durable.
Conclusion
Adhesion and cohesion are fundamental to adhesive performance, playing different but complementary roles. Adhesion ensures that the adhesive can effectively adhere to different surfaces, forming a reliable bond. On the other hand, cohesion maintains the internal strength of the adhesive, preventing it from failing under pressure. The balance between these two properties determines the durability and versatility of the adhesive in a variety of applications.
Related reference
Detailed explanation of the relationship between adhesion and cohesion
Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
Wikipedia:Adhesion
Wikipedia:Cohesion_(chemistry)