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Factors that influence the adhesion - Surface treatment
All surfaces exposed to the normal atmosphere
undergo gas and water adsorption in the molecular range; the surface condition
can be also changed by oxidation processes.
To ensure a good adhesion it is sometimes necessary to carry out, particularly
on metals, expensive mechanical and/or chemical pre-treatment (e.g.
sandblasting and pickling). On the other hand, inert (too little reactive)
plastics surfaces are activated by subjecting them to
specific surface treatment for plastics (eg flame treatment, corona
dischargeļ£). In principle, these processes serve to form active centres and
polar, reactive groups, which favour the wettability and the chemisorption of
suitably pretreated surfaces.
The quality of the parts being joined is paramount for the quality of the
bonded joint and, in particular, its resistance to ageing. The surface must
therefore be suitably treated before the adhesive is applied. Wide-ranging
methods of surface pretreatment exist. In every case, contaminants such as oil,
grease, drawing and releasing agents, plasticizers, etc. must be removed with
suitable cleaning agents.
| Cleaning & degreasing |
Mechanical treatment |
Chemical & physical treatment |
| Removal of dust, oxides, remnants of paints and dirt |
Processing with hard and powered brushes of varying types (after degreasing) |
Picking of aluminium, hardened and stainless steel and hard metals |
| Surface priming |
Use of abrasive belts,disks, emery paper (120 to 180grain) etc. after
degreasing |
Pickling of plastics which are difficult to bond, e.g. PTFE, POM and PP |
| Removal of unwanted contaminating films by degreasing/cleaning agents. |
Blasting treatments of all types (dry or wet) using a fine-grain sharp sand or
shot |
Flame treatment, corona pretreatment, plasma treatment of plastics which are
difficult to bond, e.g. PE, PA, PP... |
In this connection, we should also mention coupling
agents or adhesion promoters. These are in most cases bifunctional,
low-molecular substances, e. g. titanates, chlorosilanes, and chromium
complexes of unsaturated carboxylic acids, which fix the adhesive on the
surface by chemical reactions. The mode of action of these adhesion promoters
is based on their bifunctionality. One group reacts with reactive groups of the
adherends, while the second group reacts with the adhesive.
It is advisable, therefore, to use adhesion promoters whose groups react
differently or according to different types of reaction, e. g. by substitution
or radical reaction.
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