ADHESIVES Adhesive is a substance used for sticking two unlike bodies together, due to molecular forces existing in the area of contact. All adhesives either contain polymers,or polymers are formed within the adhesive bond. The development of adhesives has continued over the centuries to meet the requirements of various civilisations;but it was not until the industrial revolution that demands were made for major advances in adhesive technology.
TYPES OF ADHESIVESAdhesives are of two types:- Thermosetting adhesives Thermoplastic adhesives
THERMOSETTING RESINS A material that hardens when heated and cannot be remolded. Silicon resins They set under the effect of atmospheric moisture, resistant to temperature, fungi, insects and chemicals. Uses For bonding of silicate containing materials, plastics, rubbers, metals etc
Phenol-formaldehyde resins This is a pressure sensitive type of adhesive. The adherend surfaces after being coated with this adhesive is subjected to curing by heat and pressure Uses This is available as solid, liquid or impregnated film and used for bonding wood and metal Polyesters Unsaturated polyesters dissolved in styrene or methacrylates are used as reactive adhesives. They possess good moisture resistance but low heat resistance. Uses Used for making laminated glass and cloth
THERMOPLASTIC RESINS A material that softens when heated and hardens again when cooled. Cellulose derivatives Cellulose nitrate is used as a raw material for solvent adhesives. Cellulose nitrate mixed with alcohol is well known as collodion. The adhesive film developed by cellulose nitrate is resistant to water but affected by strong acids and alkalies and aging. It is inflammable and in recent years it is replaced by others. Uses As multipurpose adhesive, shoe adhesive.
Starch and dextrin adhesives The raw materials used for various types of starch and dextrin based adhesives are potato starch, corn starch etc. A suspension of starch is heated for sometime to obtain the adhesive which is cheap, can be applied in cold and hot states but bond strength is low and susceptible to moisture. Uses For making envelopes, stamps, books etc. Poly-vinyls Polyvinyl acetate, polyvinyl chloride, polyvinyl propionate form hard films resistant to atmospheric conditions and chemicals. Polyvinyl acetate in combination with phenolic resins forms special adhesives. Uses For bonding of glass, metals etc. smooth surfaces and also in sealing of food materials.
PHYSICAL FACTORS INFLUENCING ADHESIVE Smoothness of the adherend surfaces: For wood surfaces smoothness of the surfaces adds to the bonding strength of adhesives. In the case of other porous materials like paper, leather etc., the large number of capillaries preferentially enclose the liquid portion of the adhesive and disturb the equilibrium distribution of the adhesive on the adherend surface leading to weak joint formation. Surface tension: The wetting characteristics of an adhesive depend on its viscosity and surface tension. The bonding between the surfaces is better when the surfaces can be adhesives uniformly wetted by the adhesive sol which is guided by the interfacial tension between the two. When this tension becomes minimum, the surface is better wetted.
. Thickness of the adhesive film: Very viscous adhesives produce thick film but do not wet the surfaces properly leaving many voids and hence the bonding becomes weak. This bond strength can be improved by applying pressure. The bonding strength can also be increased by adding solvent and/or plasticizer to decrease the viscosity. Physical characters of the adhesive film: The bonding strength of the adhesive film is influenced by the following physical characteristics of the film. Tensile strength, shear strength and compressive strength of the film: These three characteristics influence the bonding strength in a positive way
Creep rate Highly plasticized adhesives have higher creep rates under stress conditions than the rigid thermosetting materials. Creep rate under stressed condition should be lower for better adhesives. Difference in thermal coefficients of expansion of adherends and adhesive When this difference is high the bonding strength weakens under temperature variations. Substance can be added to the adhesive to alter the thermal coefficient of expansion to match with that of the adherend and hence increasing the bond strength.
Application modes of adhesives: The mode of application of adhesive on the surface tobe joined also influences the bonding strength. Pressure, temperature and time are the factorsthat affect this strength after the application of the adhesive. Sufficient time is to be allowedso that the curing time be enough to establish a strong bonding. Similarly, temperature also affects the process of bonding by allowing the solvent to evaporate. Less pressure is required in case the adherend surfaces are smooth but in the case of porous surfaces, pressure should be applied to join the surfaces together.
CHEMICAL FACTORS AFFECTINGTHE ADHESIVE STRENGTH Degree of polymerization: Bond strength of the adhesive is affected by the extent of polymerization of the monomers. Degree of polymerization best suited for adhesive action is different for different adhesives. Polarity of adhesives: For organic contacting surfaces polar-group containing adhesives provide better bond strength than non-polar adhesives. Thus, addition of polar molecules to the non-polar adhesives increases the adhesive power.
Complexity of the adhesive molecule: Complexity and chain length influence the adhesive action. In the case of phenol-aldehyde resin the higher mol. wt. products have greater adhesive action. Similarly, in the case of cellulose esters the optimum chain length of fatty acid is from 6 to 14 for better bond strength. Effect of Ph: Strong acids and strong alkalies affect the bond strength of adhesives negatively. But the effects of pH are different in the case of different type of adhesives.
ADHESIVE STRENGTH DEVELOPMENT By chemical reaction: The reactive low molecular wt. mono and/or oligomeric ingredients of thermosetting adhesives are converted by chemical reaction into high mol. wt., cross linked three-dimensional polymers with high bond strength. By solvent evaporation: The adhesive is dissolved or dispersed in a solvent and applied to the surfaces to be bonded and they are brought in contact with each other. The solvent gets evaporated and thereby gelling and finally hardening takes place leading to strong bonding between the surfaces.
By pressure application: Tacky substances in solvent-free form adhere spontaneously to the surfaces of most materials, only under pressure By cooling: Thermoplastic materials are applied to the adherend surfaces in a molten and hot condition which on cooling solidifies developing bonding strength
ADHESIVE APPLICATIONS Adhesives are dissolved in proper solvents. Solid adhesives are melted, very viscous adhesives are thinned with solvent, plasticizers, catalysts, hardners etc. added in proper proportion. Uniform addition of calculated quantity of adhesive is required for proper bonding strength, so the method of application is very important. Developing on the size and nature of the surface and rheologic properties of the adhesive, different methods of application are used as follows:-1. By manual application with brushes, casting knives, spatulas, blades etc.2. By rollers
3. Thin-flowing sprayable adhesives are distributed uniformly over large areas with spray guns.4. Highly viscous adhesives applied as coatings.5. Dry adhesives are laid as film or sheet6. For chemically reactive systems, one component is the thermosetting resin, which is applied to one surface and theother being the catalyst applied to the other surface to be bonded.7. For immediately reacting and hardeningadhesives, the components are applied tothe adherend surfaces immediately before joining by double spray.8. Cyanoacrylate adhesives and anaerobic reactive adhesives are applied by means of special metering units that delivers small metered quantities at specified timeintervals