Choosing Adhesive Glue
Adhesive glue is a versatile material with multiple properties that help to create strong bonds. Some of these properties include preventing corrosion, conducting electricity, providing acoustic insulation and optical transparency.
Adhesive bonding requires proper surface preparation to produce an effective bind. This article will explore the key factors in achieving optimum results.
Adhesion
Adhesives are characterized by an array of properties and characteristics which determine their suitability for specific applications. These qualities range from load bearing capacity, chemical composition and form to reactivity or inertness and more. With so many options available, choosing the right adhesive for a project can be confusing. Fortunately, Thomas provides a comprehensive selection of quality adhesives for industrial and commercial use.
Adhesion is a property of adhesives that refers to the attractive forces between the molecules within an adhesive substance and those of the surface to which it has been applied. Two types of adhesion exist – mechanical interlocking and chemical bonding.
Mechanical interlocking occurs when liquid adhesive flows into and around the cavities and protrusions of the substrate’s surface, locking the surfaces together. A more in-depth chemical bond, known as chemisorption, takes place when the adhesive molecules adsorb onto the adherend surface and then react with it, creating a deep molecular bond.
A variety of chemical adhesives are used in industry, ranging from cyanoacrylate, commonly referred to as super glue or Krazy Glue, to epoxy for structural applications. These adhesives are typically available in spray, gel, liquid and foam formulations. They have a wide range of applications, including gap filling and bonding dissimilar substrates. However, they must be carefully dispensed and carefully applied to ensure a strong, durable bond.
Cohesiveness
Adhesives are designed to bind a variety of surfaces together by mechanical or chemical bonding. They are often used in a wide range of applications from repairing toys to building buildings. Glues are made from a combination of an adhesive agent, additives and solvents. There are natural and synthetic adhesives, most are based on carbon-based petrochemical derivatives.
Most glues are applied using brushes, rollers, films or adhesive glue pellets and spray guns. Applicators are sized according to the size of the area being bonded and the type of adhesive.
When applied, the adhesive dampens the surface of the substrate, forming weak electrostatic forces (known as Van der Waals) between the molecules of the glue and those of the surface to which it is being bonded. These forces allow the glue to stick to the substrate very thinly and evenly. They also provide an excellent buffer between the two substrates during any stress induced fracture.
Fracture of the bond may occur by the propagation of cracks within the adhesive – cohesive failure or, more commonly, by loss of integrity with the substrate – interfacial failure. The former tends to be a much stronger mode of failure.
The strength of an adhesive bond is determined by the type of adhesive and the load that it is being required to support. For example, structural adhesives such as epoxy can be used to join metals and dissimilar materials or for composite applications such as repairing damaged vehicle bodywork.
Strength
When it comes to selecting an adhesive for a particular application, the strength of the bond is a major consideration. However, there are a number of other characteristics that will affect the final performance of an adhesive. In addition to strength, flexibility and weathering resistance will also play a crucial role.
Glues come in many different varieties, including cyanoacrylates and polyurethanes. Cyanoacrylate glues are usually formulated from synthetic components, while polyurethane adhesives are manufactured from natural rubbers. Most types of glue are sold with additives, which help to enhance or modify their properties.
Adhesives are typically categorized by the method of adhesion and whether they are reactive or non-reactive. Reactive glues react chemically with surface chemicals to harden, whereas non-reactive glues do not.
The strength of an adhesive can be measured by testing its shear strength with a tensometer. Shear strength is defined as the force per unit bonding area that the bonded materials can withstand, and is expressed in megapascals (MPa). It can be further divided into tensile shear strength, compression shear strength and torsion shear strength, depending on the stress mode during testing.
A flexible adhesive can be tested for its elongation at break score, which measures how much the adhesive can stretch before breaking. The higher the score, the more flexible the adhesive is.
Optimum Surface Preparation
When adhesive bonding fails, it is almost always due to a lack of adequate or consistent surface preparation. This is especially important for dissimilar materials like metals and plastics and unique substrates like glass, copper or rubber that contain components that migrate when exposed to moisture.
Adhesives bond surfaces at a molecular level and are therefore dependent on their surface characteristics. The most critical of adhesive glue these are wettability and surface roughness. A clean, uncontaminated surface has high surface energy which attracts and spreads adhesives across the substrate. Contaminants like oil film, dust, and fingerprints have low surface energy and repel adhesives. The result is a weak, uneven, or discontinuous bond.
To avoid this, an abrasive surface treatment process is used to remove contaminants and provide a clean, dry surface for adhesion. Typically, this involves wiping the substrate with a lint-free cloth and degreaser such as 3M Prep Solvent 70 or IPA/water mixture. For heavier oils, mineral spirits, naphtha or other solvents can be used to dissolve the grease.
Rough surfaces are also more receptive to adhesive bonding, as they have more surface area and offer a mechanical grip for the adhesives to hold onto. This is particularly the case with rough materials such as wood or metals and can be achieved using corona or plasma treatments, sand blasting or grinding. This allows the adhesive to bond with more areas of the substrate for stronger, longer-lasting bonds.