Modern Adhesives mostly classified as structural adhesives and industrial adhesives. In general, classification of adhesives done based on the mechanism of adhesion process and on the adhesive components composition.
Four classifications of adhesives:
- Chemically reactive adhesives
- Heat-sealing adhesives
- Solvent responsive adhesives
- Pressure sensitive adhesives
Chemically reactive type work based on the chemical change during the attachment of adherents. Polymerisation, condensation and cross-linking any one of the methods occur during the chemical change. Reagents solubility changes the softening character and develops cross-linking process. A vulcanisation method takes place when elements like zinc isopropyl xanthate, zinc oxide and sulphur added to rubber type. This makes the adhesive durable and attains shear strength with less creep during stress. To withstand heat and chemical effect polymerisation methods or condensation type of adhesive will work well enough. Examples of polymerisation type are monomers of methacrylic esters, actylic esters and allyl esters. Similarly, for condensation type urea-formaldehyde resin and phenol formaldehyde were used. By application of a catalyst in pre said compounds the two hot and cold settings adhesives are produced.
Heat-sealing adhesives: A sealant works as adhesive at elevated temperature and pressure. It attaches the assembled parts by fusing when heat is applied on the surfaces. Tackifying resins, waxes, cellulose ester, ether’s, polyvinyl esters, and acetals are used as heat-sealing adhesives.
Different Types of Adhesives and Characteristics of Adhesive Film:
Any substances capable of holding materials together by surface attachment are called as adhesives, for example, mastisol liquid adhesive, acrylic adhesive and headliner adhesive. Those materials that are attached by an adhesive are called adherends, while the method of fixing each adherent by means of an adhesive is known as bonding and the whole set up of the fixed adherents and adhesive is called a bond.
Let’s know how to judge the quality of an adhesive
- Degree of tackiness
- Rapidity of bonding
- A strength of bond setting on drying and
The above qualities bring out the best adhesive required for the particular application.
Different types of adhesives
The most useful method of classifying adhesives is based on the chemical nature of adhesives. The broad differentiation is
- Thermosetting synthetic resins
- Thermoplastic synthetic resins
- Natural resin adhesives
- Starch adhesives
- Inorganic adhesives
- Vegetable glues or protein glues
- Animal glues
Physical characteristics of the adhesive film:
The physical characteristics strongly influence the adhesives bond strength. Important characteristics listed below,
- Shear strength
- Compressive strength
- Modulus of elasticity
- Creep-rate and
- The thermal coefficient of expansion of the adhesive film
In general greater the tensile- strength, shear strength and compressive strength of the adhesive film, the greater is the strength of the bond. A good adhesive film should have negligible creep under the stress of structure. Thus in a structural application where an adhesive film is under great load creep under stress impairs the bond strength of the adhesive. It has been observed that high-plasticized adhesive material is bound to creep under stress more readily than stiffer and more rigid thermosetting material. The thermal coefficient of expansion of adhesive and the adherend surface also play an important role in determining the bond strength developed. Its found that the addition of finely powdered inorganic substance like aluminium oxide to the adhesive lowers its thermal coefficient of expansion to the range nearing to that of glass, metal etc.