- 03 September 2020
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Introduction to Adhesives
Adhesives is a general term that refers to any substance that can be used to hold, fix or bond together when applied between two or more materials or surfaces. The strength of the bond created by the adhesive between surfaces is largely dependent on the properties of the adhesive, in particular adhesion and cohesion. These properties are the primary mechanisms by which adhesives succeed; Therefore, the calculation of the failure point of both properties for each available adhesive helps determine the most suitable adhesive for use in an application, given technical requirements and specifications.
Adhesives are widely used throughout the industry for permanent, semi-permanent and temporary attachment purposes in a variety of residential, commercial and industrial applications. Some of the properties by which the wide variety of available adhesives can be classified and categorized include load carrying capacity, chemical composition, reactivity or inertness and the structure it will form. Each of these adhesives exhibits different properties and offers different advantages, but as with adhesive and cohesive strengths, the suitability of a particular property or property (and the associated adhesive) depends on the application.
This article discusses adhesives in terms of the various classifications and categories available, their associated properties, advantages and disadvantages. In addition, it outlines some of the common types used and preference parameters when choosing an adhesive for an application.
Common Types of Adhesive
Based on the properties outlined above, several different types of adhesives are available. Some of the most common types of adhesives used in industry are:
• Anaerobic adhesives: Anaerobic adhesives are acrylic based adhesives that harden in the absence of air and with the presence of metal surfaces. They increase friction by making mechanical locking between the surfaces they are applied to. This type of adhesive typically has low viscosity, is available in liquid and paste forms, and is suitable for securing tight fitting structural parts and sealing threads and sealing surfaces. They prevent unintentional dismantling under vibration and dynamic loads and provide cost advantage in enterprises by preventing unplanned stops.
• Cyanoacrylate adhesives: Cyanoacrylate adhesives, also called "instant adhesives", are adhesives that usually cure in the presence of UV light for moisture and some types of surfaces. They can be single or two-component. This type of adhesive is available in a variety of formulations ranging from low viscosity to high viscosity and is suitable for porous and non-porous surfaces. They do chemical bonding, but their gap filling ability is poor. It is ideal for bonding small parts due to its fast drying properties.
• Epoxy adhesives: Epoxies are typically available in one-component (medical) and two-component (industrial) systems. These types of adhesives exhibit high shear and peel strength (even in extreme temperatures and environments) and are suitable for filling and bonding on different substrates. Filled and unfilled types are available.
Hot melt glue: Hot melt glue is a type of hot melt adhesive typically derived from thermoplastic compounds. This type of adhesive shows a high level of tackiness and is suitable for use on both porous and non-porous surfaces. Generally, hot glue solidifies by cooling, but some variants can be cured by the presence of moisture or UV light.
• White glue: White glue is a common polyvinyl acetate (PVA) adhesive traditionally used for arts and crafts. It requires contact and pressure during application and is suitable for fabric, cardboard, paper, wood and other porous substrates.
Adhesives and Sealants
Adhesives consist of similar materials, use similar mechanisms, exhibit similar properties, and are used in some of the same applications as sealants. Therefore, in a discussion about adhesives, the issue of sealants always comes up.
As mentioned earlier, adhesives are materials that can be used to hold two or more surfaces together, through adhesion and cohesion bonding. On the other hand, sealants fill the space between two or more surfaces via the surface connection. They are materials that can be used for sealing and barrier purposes against liquids, dust, noise. Sealant groups also adhere to surfaces with their adhesion ability. Usually the application and processing requirements and the type of expected performance are parameters to consider when choosing between both groups.
Industrial standards typically classify adhesives and sealants into two different and separate substances. This definition is largely due to its different primary functions. While the primary function of adhesives is to connect two or more surfaces together, sealants are primarily used to fill a gap and create a barrier between two or more surfaces.
While typically adhesives can perform both the adhesive and sealing functions, in general their primary function is not necessarily to fill gaps or create barriers between surfaces. Unlike adhesives, most sealants exhibit poorer bonding and are not suitable for bonding or bonding purposes.
Basic Mechanisms of Adhesives
As mentioned earlier, adhesion and cohesion are the primary mechanisms underlying adhesives. Regardless of type and design, all adhesives operate according to these same basic characteristics that define the interactions between the surface and the adhesive; components of the adhesive; and components of the surface.
Adhesion
Adhesion is a measure of the forces of attraction that hold two surfaces together. When calculating the adhesion level indicated by an adhesive, the strength of the bond created between the adhesive and the surface (adhesion) and the measure of the forces between the internal bonds of the adhesive (cohesion) should be considered together.
Several theories have been developed to describe and explain this property. Some of the most common adhesion theories include:
• Adsorption: Adsorption theory states that adhesion results from intermolecular contact between the surfaces of two substances - that is, attractive forces resulting from contact between the adhesive and the surface - for example, chemical bonds, van der Waals forces, etc. - Holds two surfaces together.
Mechanical clamping: The mechanical clamping theory states that adhesion is caused by the flow of adhesive into and around the voids and protrusions of the surfaces of both substrates. The adhesive mechanically holds the two substrates together after curing.
• Transfusion: Intermediate diffusion theory states that the adhesion is caused by the penetration of the adhesive and surface molecules. In some cases, contact of the adhesive molecules with the surface may cause a chemical reaction (eg melting). Diffusion of molecules followed by hardening of the adhesive leads to the formation of a bond that effectively binds the adhesive and the two surfaces together.
• Electrostatic attraction: The theory of electrostatic attraction states that adhesion is caused by the development of electrostatic forces at the contact point (ie interface) between the adhesive and the surface, given different electrostatic force structures. These attractive forces create a connection between the adhesive and the surface - the connection resists separation even if it is temporary.
Cohesion
Cohesion is a measure of the internal attraction forces that hold the adhesive molecules together. This value can express the strength of the bonds between the components of the adhesive or components of the surface. Some of the factors that can affect the cohesive properties of an adhesive are the chemical and intermolecular bonds between the atoms of the components of the adhesive and the crosslinking of molecules (short chain to long chain).
Adhesive and Adhesive Defect Modes
In most cases, the failure of an adhesive can be attributed to a problem with the adhesive itself in terms of its adhesive or cohesive properties. However, cohesion failure in the adhesive part can also cause an adhesive bond to fail.
• Adhesive failure occurs as a result of insufficient bonding between the adhesive and the bonded surface. For example, given two surfaces that have been glued with an adhesive and then pulled apart, if the adhesive has fully adhered to one surface but failed to adhere on the other surface, this would be considered a bonding error. However, it is not the adhesive itself that needs to be examined here, but the surface where adhesion does not occur. Surface polyolefin, PTFE, silicone etc. may contain one of the non-stick surfaces. Standard adhesives can be solved with the help of auxiliary chemical products on surfaces that cannot be bonded alone.
• Cohesive rupture occurs when the bond strength of the adhesive to the surface (adhesion) is stronger than the bonds between the atoms and molecules in the adhesive. For example, returning to the previous situation where the two glued surfaces were separated, if some of the adhesive remains on the surface of both parts, this will be considered as the cohesion error of the adhesive. In this case, an alternative product can be determined by reviewing the strength required from the bonding work, the working modes of the glued part and the loads it is exposed to.
• Cohesive rupture occurs on the bonded surface when the adhesive and cohesive bonds between the molecules of the adhesive exceed the bonds between the atoms and molecules of the bonded surfaces. This phenomenon, called cohesive failure of the bonded surface, typically occurs at the junction formed between the surfaces. As the surface itself succumbs to structural fatigue, the adhesive that remains intact shows itself. Some tearing or breakage occurs within the surface itself.
The three conditions mentioned above and shown in Figure 1 below represent the three main types of adhesive bond break modes. Of these three modes of degradation, the cohesive failure of the surface is typically the most ideal as it indicates the correct selection and application of the adhesive for the surface with a certain structural integrity.
Figure 1- Adhesive Fail Modes
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Raziye Gündeş – Technical Manager