Amongst the latest arrival on the scene is the ‘oily metal’ epoxy that bonds directly to oily metals as received with normal protective oil layer on them.Īlthough epoxy-based adhesives develop high shear and tensile strengths, creep and peel strengths are low. They can be cured at room temperature without any volatile by-products and can develop strengths between 15 to 30 MPa. Adhesives and their Classification:Įpoxy resins are amongst the newest thermosetting resins and are widely acclaimed as they combine the properties of excellent action, low shrinkage, high tensile strength, toughness and chemical inertness. The factors which may influence the strength of the joint include the contact angle between adhesive and metal, residual stress and the stress concentration in the adhesive. Generally strength of a lap joint increases with the amount of overlap (though the strength per unit area decreases), and decreases with the increasing thickness of the adhesive. The mechanical strength of an adhesive bonded joint depends upon the joint configuration, its dimensions, the nature of adhesive and its thickness between the adhering surfaces. A schematic representation of an adhesive bond is shown in Fig. Present theory holds that adhesion is primarily due to a chemical affinity of the adhesive for the adherend and that the mechanical action, if any, is only incidental.
Oil and grease on the surface seriously lower the surface energy of the metal surfaces and thus impair the bonding strength. To accomplish wetting of the metal surface by the adhesive the surface energy of the metal must be greater than that of the adhesive and to achieve that it is essential to provide thorough cleanliness of the metal surfaces. If the surface energy of the adhesive is greater than that of the adherend surface, the adhesive will not wet it.
When an adhesive is placed between two metal surfaces, the adhesive molecules are attracted by its neighbouring molecules as well as the metal atoms or foreign matters on the metal surfaces. The Van der Waal bond is defined as a secondary bond caused by the fluctuating diepole nature of an atom with all occupied electron shells filled. The dipole bond is a pair of equal and opposite forces that hold two atoms together and results from a decrease in energy as two atoms are brought close together. The adhesive bond is caused either by the polar forces between the adhesive and a relatively brittle oxide film (diepole bond) or by Van der Waals forces between the adhesive and the unfilmed or clean metal. Nature of Adhesive Joints:Īn adhesive bond is affected by the attractive force, generally of physical nature, between an adhesive and the base material. Also adhesives deteriorate fast under the conditions of high humidity and temperature. Need for elaborate jigs and fixtures for assembly and curing result in high cost for equipment and tooling. However, adhesive bonds do not support high peel loads above 120☌. Some adhesives can operate at temperature somewhat higher than their curing temperatures which is not possible in the case of soldered joints.
0 kommentar(er)
