The two most important characteristics of any adhesive are its ability to withstand loading and the strength of its bond to the surface to which it is applied. Adhesives that can withstand high loads, around 1000 lbs. per square inch, are referred to as structural adhesives. Adhesives with less bonding power are termed non-structural adhesives. The latter adhesives exhibit low shear strength, high creep and are sensitive to adverse environmental conditions such as high temperature, solar radiation and solvents. They are also referred to as holding adhesives .
Applications for Non-Structural Adhesives
Non-structural adhesives are most useful where a temporary bond of low to medium strength is required. They add value in a number of applications:
- To augment and reduce the number mechanical fasteners required
- To reduce vibration in mechanical joints
- To seal joints against air or water intrusion
- To provide electrical or thermal insulation
Examples of temporary adhesives include wood glues, elastomers, silicone sealants and pressure sensitive tapes or films.
How Removable Adhesives Work
The components of an adhesive bond are the adhesive itself and the material to which it is applied, called the adherend. In order for an adhesive to create a bond, it must create intermolecular bonds within itself and between itself and the adherend. The exact characteristics of the intermolecular bonds are what determine the properties of a specific adhesive. For instance, elastomer adhesives typically have cross-linked molecular structures that soften with heat but never become fluid.
Glued joints fail when either the molecular bonds within the adhesive or the bonds between adhesive and adherend fail. Removable adhesives are formulated to remain tacky without a hard set. Thus, the cohesive bonds within the adhesive are weaker than the bonds to the adherend or substrate. The adhesive’s cohesive bond is broken by tensile force without damaging the substrate. The weakening of the adhesive may be assisted with chemical solvents, water or heat.
Factors Determining the Best Removable Adhesive
Determining the most appropriate removable adhesive depends mainly on the requirements for its usage, but also on other factors:
- Cost of the adhesive itself
- Cost of additional materials, such as solvents, to prepare the adhesive
- Processing requirements for its application
- Disposal costs of adhesive waste
Often, removable adhesives are used for positioning parts to be bonded by other means. High creep, low resistance to temperature or solvents, all characteristics detrimental in a structural adhesive, are desirable when the adhesive is used as a positioning aid during product assembly. Open time and initial bond must also be considered. Three types of elastomer adhesives offer different properties for removable bonds:
- Solvent-borne adhesives can be used for pressure sensitive applications and removed by heat or solvents. They have long open times and excellent initial strength, which is useful when assembling large or 3-dimensional products. Many application processes can be used in a wide temperature range. They do require vapor control and sometimes flammability controls.
- Water-borne adhesives offer several open times from short to indefinite and can be removed without chemical solvents. Their initial strength is moderate and drying times longer compared to solvent-based adhesives, but they have wide application temperature ranges. They must be protected from freezing.
- Hot-melt adhesives are 100 % solid, so no solvent is required for application, only heat. These come in a wide variety of formulations that offer short to indefinite open times. Initial bond strength is controlled by temperature of the adhesive and substrate. They have high water-resistance, so solvents must be used for removal. Their material cost lies between solvent and water-based adhesives
Other Removable Adhesives
Although the largest use of temporary adhesives is in industrial applications, removable adhesives are also commonly found in consumer or commercial labels applied to paper, cloth or non-porous surfaces. Starch-based adhesives are often used for bottle labeling. They can be removed with water so that the bottle itself may be re-used and re-labeled.
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