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 method occur during the chemical change. Reagents solubility changes the softening character and develops cross-linking process. A vulcanisation method take 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 are used. For hot and cold settings, a catalyst is used in pre said compounds.
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 method of fixing each adherent by means of 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
- Strength of bond setting on drying and
The above qualities bring out the best adhesive required for 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 adhesive film:
The bond strength of an adhesive is greatly influenced by its physical characteristics such as
- Shear strength
- Compressive strength
- Modulus of elasticity
- Creep-rate and
- 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 bond. A good adhesive film should have negligible creep under the stress of structure. Thus in 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. Thermal coefficient of expansion of adhesive and the adherend surface also play an important role in determining the bond strength developed. It has been observed 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,.