Mechanism of adhesion


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Mechanism of adhesion

  1. 1. 1 TABLE OF CONTENT No. Title Pages 1 Abstract (Mechanism Of Adhesion) 2 2 Introduction 2 3 Chemical Bonding Primary Bond/ Secondary Bond 3 4 Physical Bonding Mechanical Interlocking Diffusion Theory Adsorption Electrostatic 4 5 Conclusion 6 6 References 6
  2. 2. 2 ABSTRACT MECHANISM OF ADHESION The analysis of adhesion uses various technique of thought, depending on one’s field of expertise, and therefore treatment of the phenomena observed can be considerably different. This variety of approaches is emphasized by the fact that many theoretical models of adhesion have been proposed, which together are both complementary and contradictory. Adhesion is the action of adhering to a surface or object. In the other word, it is the main application of adhesion in bonding by adhesives, this technique replacing at least partially, more classical mechanical attachment techniques such as bolting or riveting. Adhesion can be divided by two bond component; chemical bond and physical bond. In chemical bond it is consist primary and secondary bond. For the physical bond, it is consist four type bonding theory; mechanical interlocking, diffusion theory, adsorption and electrostatic theory. In fact, the term ‘adhesion’ covers a wide variety concept of bonding ideas, depending on whether the subject is broached from a molecular, microscopic or macroscopic point of view about formation of the interface of failure of the formed system. INTRODUCTION Adhesion is a process of a substance bind or glued together to the surface of another substance or substrate. The mechanism of adhesion has been explores for dotage. Many theories have been introduced for adhesion phenomena. Nonetheless, there is no complete explanation of what exactly adhesion mean. The briefly explanation of adhesion can be define as it is the bonding of an adhesive to an object or a surface is the sum of a number of mechanical, physical, and chemical forces that overlap and influence one another. It is requires two type of bonding; chemical bonding and physical bonding. It is also can be defined as the tendency of same or different substrate which is bind together by using adhesion. On the other hand, according Benjamin E. Russ, adhesion is the binding force between two different materials, whereas cohesion is the binding force between two similar materials. When two materials are brought into contact with each other, the surface molecules interact, giving rise to attractive forces that may be physical, chemical or electrostatic (corresponding to adsorption, covalent bonding or van der Waals forces, respectively). When the molecules are similar, as in the case of two 'glue molecules,' the cohesive force causes the glue to stick to itself. When the molecules are dissimilar, as in the case of a glue molecule and a molecule of the substrate (the surface the glue is sticking to), the adhesive force holds the glue to the substrate. Hence, the 'stickiness' of tape is caused by a combination of the molecular forces of the glue material sticking to itself as well as holding onto the substrate.
  3. 3. 3 CHEMICAL BONDING Chemical bonding is a form of adhesive bonding involving a reaction that results in covalent bonds between the molecules of the adhesive and the surface material. The chemical bonding mechanism suggests that primary chemical bonds may form across the interface. In chemical bond consist two category of bond which is primary and secondary bond. Chemical bonds are strong and make a significant contribution to the intrinsic adhesion in some cases. PRIMARY/SECONDARY BONDING Primary bond can be defined as a bond that forms between atoms and that involves the exchanging or sharing of electrons. Whereas, secondary bond can be explain with it relies on the mechanical linking of an adhesive to a material.
  4. 4. 4 PHYSICAL BONDING Moreover, physical bond consist four types of theory; mechanical interlocking, diffusion theory, adsorption and electrostatic. Adsorption theory is a form of adhesive bonding involving the attraction between the molecules of an adhesive and a surface material. The bonding of an adhesive to an object or a surface is the sum of a number of mechanical, physical, and chemical forces that overlap and influence one another. As it is not possible to separate these forces from one another, we distinguish between mechanical interlocking , caused by the mechanical anchoring of the adhesive in the pores and the uneven parts of the surface, electrostatic forces, as regard to the difference in electro negativities of adhering materials, and the other adhesion mechanisms dealing with intermolecular and Chemical bonding forces that occur at the interfaces of heterogeneous systems. This chemical adhesion mechanism is explained in the case of the intermolecular forces by the adsorption theory, and in the case of chemical interactions by the chemisorption theory. The processes that play a role in the bonding of similar types of thermoplastic high-polymer materials, e.g. homogeneous systems, can be determined with the diffusion theory. MECHANICAL INTERLOCKING The mechanical interlocking theory of adhesion can briefly explain that proper adhesion occurs only when an adhesive penetrates into the pores, holes and crevices and other distortion of the substrate and locks mechanically to the substrate. The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate pores and openings in a reasonable time. DIFFUSION THEORY The diffusion model explains the concept of adhesion by the compatibility between polymers and the movements that occur in the polymer chains. When two polymers are compatible, its polymer chains are able to mix up between them, resulting in partial penetration between the 2 materials, as a result of these penetrations anchorage areas and adhesion points take place. The mobility and degree of penetration of the polymers is determined directly by their molecular weight, so that short polymer chains have high mobility and penetrate into the other material before the long polymer chain. Rouse model and reptation model give a detailed explanation of the movements that occur between the polymer chains which produce the diffusion. Using this theory can explain the phenomenon of adhesion that occurs between polymeric materials, plastic welding, plastic binding with adhesives, etc...
  5. 5. 5 ADSORPTION THEORY The adsorption mechanism theory suggests that bonding is the process of intermolecular attraction (van der Waals bonding or permanent dipole, for example) between the adhesive and the adherend at the interface. An important factor in the strength of the bond according to this theory is the wetting of the adherend by the adhesive. Wetting is the process in which a liquid spreads onto a solid surface and is controlled by the surface energy of the liquid-solid interface versus the liquid- vapor and the solid-vapor interfaces. In a practical sense, to wet a solid surface, the adhesive should have a lower surface tension than the adherend. The adsorption theory or model explains the phenomenon of adhesion based on concepts such as contact angle, wet ability and surface tension. When the adhesive has a lower surface tension compared to the substrate surface energy, it is capable of wetting the surface, generating a contact angle less than 90 °, thus generating the adhesion between the adhesive and substrate. Against the mechanical model and the model of diffusion, adsorption model explains the phenomenon of adhesion without penetration by the adhesive to the substrate; the adhesion is generated by the contact between the adhesive and substrate. ELECTROSTATIC THEORY Electrostatic forces may also be a factor in the bonding of an adhesive to an adherent. These forces arise from the creation of an electrical double layer of separated charges at the interface and are believed to be a factor in the resistance to separation of the adhesive and the adherend. Adhesives and adherends that contain polar molecules or permanent dipoles are most likely to form electrostatic bonding according to this theory.
  6. 6. 6 CONCLUSION The stronger adhesion of bonds between mechanically or chemically roughened surfaces is based on the enlargement of the effective surface (contact surface between the adhesive and the substrate), and an increase in the number of active centres, e. g. edges, corners, and faulty parts which, as in the heterogeneous catalysis, increase the interactive forces in the interface adhesive/surface. REFERENCES Scientific American tm online journal; http: // /resources /adhesionguide/index.aspx?id=factors Articles/?PC_7_U00M8B1A00NI60IDFIPS8T3HR2000000_assetId=1319232951956