A technical synopsis of different methods of adhesive joining
Adhesives are considered as one of the most reliable options for joining two substrates together. Unlike other methods of bonding, adhesives keep the geometry and properties of the materials intact. The technique has considerably matured over the years and every industry relies on adhesive bonding to produce high quality products.
Most substrates like metals and polymers are not wettable by adhesives and require some pre-adhesion treatment. There are a number of surface preparation techniques for making materials suitable for adhesions. Some of the commercially used techniques are:
1. Chemical Treatment
This is one of the oldest methods of surface preparation. Usually strong acids are used as surface treatment agents. Acids form different functional groups on the surface of a substrate thus increasing the surface energy of the material. Most small scale industries rely on this adhesion technique, as it is considerably cost effective than other techniques.
2. Plasma Treatment
Unlike chemical treatment, plasma treatment does not modify the physical and chemical properties of a material. Plasma consists of high energy ionized particles. When a material to be treated is placed in a plasma medium, plasma particles transfer their energy to the surface particles of the material, thus increasing its surface tension and making it ideal for adhesion. This technique requires a plasma treatment plant for plasma generation. Automotive industry relies heavily on plasma treatment technique for adhesive bonding.
3. UV Treatment
This technique is generally used for preparing metallic surfaces for adhesion. In this technique the surface tension is increased by exposing a material to intense beam of UV radiations. The method requires expensive equipment therefore large scale industries like aircraft manufacturing industry, relies on this method of surface preparation.
For effective and reliable adhesive bonding, following considerations should be kept in mind:
1. Service Temperature
It is the temperature at which an adhesive can maintain its maximum strength. Service temperature varies with the type of adhesive. For most adhesives service temperature can range from 65oC to 230oC. Adhesives with exceptionally high temperature resistance have low peel strength.
2. Curing Temperature
Curing temperature is the point at which an adhesive starts hardening and performing the bonding action. Although every adhesive has its own curing temperature, it can range from 120oC to 180oC. Factors like type of equipment, type of material and energy economy directly influence the curing temperature of adhesives.
3. Film Weight
Adhesive film should not be ticker than 400g/m2. Although thickness of the film depends on the substrate type, thicker films have low resistance. Moreover thick adhesive films can squeeze out from the joints if the pressure is increased during heating.
Adhesives serve as an intermediate layer between two substrates. In order to join two substrates of different nature (for example, plastic and metal), the adhesive chosen should be compatible with both of the materials.
Due to advanced equipments and improved quality adhesive bonding has become one of the most reliable joining techniques.