Types of Adhesives

Turkchem 06 Jan 2020 50 3 dk okuma

TURKCHEM

There are many ways to classify adhesives. First, they are classified as structural or non-structural. This classification is admittedly subjective and broad, but it represents the typical application types in which the adhesive is commonly used. Adhesives can also be classified based on whether they form cross-linked molecules (for example, thermoset or thermoplastic), their flexibility (i.e., rigid or elastomeric), and their form and curing conditions. Table 1 presents some resins used in adhesive formulation [1]. Synthetic polymeric resins consist of naturally occurring resins and inorganic products. These materials form the base resin portion of the adhesive formulation. Together with additives and modifiers, the chemical properties of the adhesive system emerge [2]. Structural applications are those requiring high strength or durability. Structural adhesive bonds are expected to be functional and long-lasting. Non-structural applications are those that generally require low-strength or temporary bonding. Examples of non-structural adhesives are pressure-sensitive tapes, hot-melt or packaging adhesives. Some structural adhesives are defined as systems having tensile shear strength higher than 1,000 psi and whose properties do not change significantly in the medium term. Non-structural adhesives are those with 1,000 psi shear strength and durability that require a protected or relatively soft environment [1].

1. Structural Adhesives

Adhesives are used in the bonding of medical parts such as orthodontic wires, hearing aids, transducers, infusion devices and pumps, medical electronic devices, and the attachment of stainless steel hypodermic needles to plastic housings. Structural adhesives are used in the joining of structural components in the aerospace, automotive, sports equipment, and domestic appliance industries [3]. Epoxy: Epoxy adhesives were commercially introduced in 1946 and have found wide application in the automotive, industrial, and aerospace markets. Epoxies are a versatile adhesive family because they adhere well to many substrates and can be easily modified to achieve a wide range of properties. Some of these modifications are as follows: 1. Simple addition of organic or inorganic fillers and components. 2. Combination of resins. 3. Modification of the resin obtained by selection of curing agent and reaction mechanism [1].

Resorcinol formaldehyde and phenol resorcinol formaldehyde:

Resorcinol resins derive from the phenolic resin family. They are more expensive than their phenolic counterparts but can be utilized when hardening at room temperature is required. They are applied in the adhesion of wood, cellulose acetate, molded urea plastics, nylon, and various plastic laminates to wood.

Melamine formaldehyde and urea formaldehyde:

Melamine formaldehyde resins are used as colorless adhesives for wood. These thermoset resins are condensation products of unsubstituted melamine and formaldehyde. Due to their high cost, they are sometimes blended with urea formaldehyde. Phenolic: Phenolic resins are condensation products of phenol and formaldehyde. Phenolic or phenol formaldehyde is primarily used as an adhesive for bonding wood. Due to its good heat resistance and dimensional stability, it has also been used in brake linings, abrasive wheels, sandpaper, and casting molds. Solutions of phenolic resins filled with clay have been used as glass-to-metal adhesives for the attachment of metal bases to glass ampules. Phenolics are relatively inexpensive.

2. Non-Structural Adhesives

Non-structural adhesives are characterized by relatively low strength at somewhat elevated temperatures and poor creep resistance. The most common non-structural adhesives are elastomers and thermoplastics. Although these systems have low strength, they are generally easy to use and provide rapid bonding. Due to these properties, most non-structural adhesives are used in assembly line bonding operations where ultimate bond strength is not required and environmental conditions are not harsh and severe. Common types of non-structural adhesives are pressure-sensitive adhesives, contact adhesives, mastics, hot-melts, and thermoplastic emulsions.

Elastomeric Resins:

Natural or synthetic rubber-based adhesives generally have excellent peel strength but low shear strength. The flexibility properties of these adhesives provide good fatigue and impact characteristics. Temperature resistance is generally limited to 150-200ºF, and creep under load occurs at room temperature.

Thermoplastic Resins:

Represents an adhesive that can be set without chemical reaction. They can be melted and solidified repeatedly while largely retaining their properties. Additionally, they can be dissolved in a solvent or suspended in a water-based emulsion. For this reason, these adhesives are generally formulated to set through solvent evaporation or cooling from a hot-melt. Emine Akbulut Business Development Specialist EF Kimya Tic. and San. Ltd. Co.