4.5 Adhesive joints – gluing
The essential feature of adhesive joining is that two parts are joined by placing a liquid between them, which then solidifies. If you think about it, that is exactly what happens during casting; the major difference between casting and gluing being that, in casting, it is important for the cast material to separate from the mould whereas in gluing the aim is the opposite. The strength of a glued joint depends not just on the strength of bond between each part and the adhesive layer, but also on the strength of the adhesive layer itself.
In Soldering , the layer is put in as a hot liquid that solidifies on cooling to room temperature as shown in Figure 65.
Soldering is defined as the joining of metals using a separate filler metal that melts at temperatures below the melting temperature of the metals being joined. The bond strength is relatively low. The 'traditional' solder alloy was based on a tin/lead mixture, but all solders used commercially in developed countries are now free of lead, mostly being based on mixtures of tin and copper along with other metals, such as silver.
Typical features of the soldering process are:
- The solder alloy can be significantly different from the base material because the base material does not melt.
- The strength of the alloy is substantially lower than the base metal.
- Bonding requires capillary action, where the solder liquid is drawn into the joint.
And because of these differences, the soldering process has several distinct advantages over welding:
- Virtually all metals can be joined by some type of soldering metal.
- The process is ideally suited for dissimilar metals.
- The lower temperature than that needed for welding (welding is discussed shortly) means the process is quicker and more economical.
- The low working temperature reduces problems with distortion that can occur during welding, so thinner and more complex assemblies can be joined successfully.
- Soldering is highly adaptable to automation and performs well in mass production.
Although the principles of soldering are shared with all gluing processes, the word 'adhesive' is usually taken to mean a type of polymer glue. Adhesives now come in a vast array of different types; some stick in seconds (cyanoacrylate – Superglue TM ), some take a day or so to achieve their full strength (thermosetting epoxies), others stay permanently in a soft flexible state, like silicone adhesives. Thermosetting glues , like thermosetting plastics, are made by mixing together two ingredients, a 'resin' and a 'hardener', usually in liquid form, which react chemically to form a solid.
The major advantages of adhesive bonding are:
- Almost all materials or combinations of materials can be joined.
- For most adhesives the curing temperatures are low, seldom exceeding 180 °C.
- A substantial number cure at room temperature and provide adequate strength for many applications.
- Heat-sensitive materials can be joined without damage.
- No holes have to be made as with rivets or bolts.
- Large contact areas means high joint strength.
- The adhesive will fill surface imperfections.
The major disadvantages of adhesive bonding are:
- Most adhesives are not stable above 180 °C.
- Surface preparation and curing procedures are critical if good and consistent results are to be obtained.
- Life expectancy of the joint is hard to predict.
- Depending on the curing mechanism, assembly time may be longer than alternative techniques.
- Some adhesives contain toxic chemicals and solvents.
For successful soldering or adhesion, the 'glue' material must 'wet' the surfaces of the two objects to be joined. You can see the contrast between wetting and non-wetting when washing up greasy breakfast plates. When the plates are covered with oil and fat, water just runs off without sticking. This contrasts with what happens when the fat is removed with hot water and detergent: the plate then retains a thin covering of water. We say that water is 'wetting' the clean glazed surface.
Successful joining by solders or adhesives usually requires that the surfaces to be joined are completely clean. This can be achieved by using either mechanical or chemical techniques. The mechanical method uses abrasion to clean the surface, while the chemical methods use acidic solutions that etch the surface, as well as degreasing it with solvents. After cleaning the surface it is vital that recontamination does not occur from oxidation and airborne pollution. In particular, when heat is applied during soldering, oxidation can rapidly take place; in this case a flux can be applied which prevents oxygen from reaching the prepared surface. Abrading also has the advantage that the surface is roughened, thereby increasing the surface area, which enhances the contact area of the joint.