Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
the technique can be applied to a wide range of thermoplastics, if necessary
in the form of large complex components. There is a danger of entrapped air
pockets when this technique is used, with a resulting risk of fracture in
service if the polymer is notch-sensitive.
5.5.2 Adhesive bonding
Adhesive bonding was discussed in Chapter 3 in the context of the joining of
metals, but its use is also widely encountered in the joining of polymers.
Good wetting of the polymer by the adhesive is required and this will be
achieved if there is a strong chemical bond formed between the adhesive and
the adherands.
There are three important categories of adhesives which may be used for
the joining of polymers:
1. Hot melt adhesives. These are thermoplastics such as PE or PET, which
are melted and applied to the adherands, which are then squeezed together
during the cooling cycle. Although good bond strengths may be achieved,
the joint may creep in service if the temperature is not low.
2. Solvent-based adhesives. Amorphous plastics are the most likely to dissolve
in appropriate solvents and these include materials such as PS, ABS
(alkylbenzene sulphonate), PVC and PC. Solvent alone will form an
adhesive joint, but polymer solutions have better gap-filling properties
and are available in various viscosities depending on the application.
Many of these adhesives are based on rubber and are used as ‘contact’
adhesives to form tough joints of fairly low strength.
3. Reaction cured adhesives. Very high bond strengths are achievable with
this class of adhesive: polymerization and cross-linking takes place after
mixing low-viscosity precursors. Such adhesives are usually temperature
and solvent resistant, the main types being:
(i) epoxies
(ii) phenolics
(iii)
(iv)
cyanacrylates, widely used in the bonding of rubber, and
anaerobics, which cure when air is excluded (useful in threadlocking
applications).
5.5.3 Mechanical fastening
Snap fitments
Designs of this nature are possible using semicrystalline thermoplastics
(e.g. PP, PE and nylons) which exhibit resilience and high elastic strains.