The adhesion of a liquid on a solid utilizes two independent concepts :
- The solid liquid interaction which characterizes adhesion.
- The damping which characterizes the spreading out of the liquid on the solid.
Some considerations on absorptivity
It represents the aptitude which greatest possible surface has a liquid to occupy when one lays out it on a solid surface. We will quantify it by the introduction of the concept of energy of surface of a liquid or a solid.
Énergie superficielle d'un liquide
Surface energy of a liquid
As called surface tension, it characterizes the aptitude as for the surface of a liquid to take the smallest possible value in a given medium. It also characterizes the cohesion of a liquid since it is necessary to overcome the forces of internal cohesion of this one to increase this surface.
Mechanically, it is expressed like a force being opposed to an increase in surface and brought back to the unit of length. The unit used is N/m
| Tension superficielle forte | Tension superficielle faible ( bon pour le collage). |
It represents the quantity of work which it was necessary to spend to create the aforementioned surface, the temperature and the pressure being constant. It is expressed in J/m ².
Concepts of absorptivity
Let us pose a drop of liquid on a solid surface.
The balance of the forces gives the equation of Young :
g lv cos q + g sl = g sv
Where G S, G sl, G L represent the energy of surface of the solid, interfacial energy liquidates solid and the surface tension of the liquid.
Critical surface energy
The spreading out of the liquid is perfect only if the contact angle A is null. This value corresponds to a breaking value of the surface energy of the solid which makes it possible to predict that if :
g l < a="0,">
g l > g c alors A is positive, damping is bad.
Which are the connections which intervene ?
One thus qualifies the whole of the forces which are established between the adhesive and the surface of the solid. Several explanations exist, without one being able to generalize one or the other of those.
- Mechanical Theory. Adhesion was regarded a long time as being a simple mechanical problem, the solidity of the joint resulting from the penetration of the adhesive in the asperities of solid surface. Explain part of adhesion.
- Electric Theory. Adhesion would be due to the establishment of an electric layer to the interfaces, the forces being of electrostatic nature. Very discussed.
- Chemical Theory. It interprets the connection by the formation of covalent bonds between two involved bodies. Has place only in certain cases.
- Theory of the diffusion. There is inter-diffusion between two involved surfaces. It supposes the mutual solubility of materials. Joining of PVC for example.
- Thermodynamic Theory. It indicates the establishment of weak connections (forces of Van Der Waals) between surfaces. These forces are exerted on short distances and exist in all the cases of figure. Explain a good part of adhesion.
The forces of Van Der Waals result from the dissymmetry of burden-sharing positive and negative involving the formation of dipoles as well in polymer as in the substrate and linking themselves head-digs. These various assumptions show that the phenomena of adhesion are not yet well elucidated. It seems however that the establishment of weak but very many connections is one of the principal reasons of adhesion in much of case. |
Importance of roughness
An increase in roughness has two direct incidences:
- A better mechanical anchoring.
- A rise in the real surface of contact (10 to 100 times apparent surface). One could then expect that the resistance of the joint increases in similar proportions.
However it is not the case. Two essential reasons explain these observations.
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 Le mouillage n'est pas réalisé dans les creux. Il y a concentration de contraintes au sommet des aspérités. | Sans creux, le mouillage est mieux réalisé. Le sablage à créé des angles très ouverts, l'adhésion sera donc meilleure. |
TO RETAIN ABSOLUTELY
- Absorptivity must be the best possible one so that the maximum of connections are established between the adhesive and the substrate. The equation of Young indicates to us that a surface treatment is practically essential so that the energy of surface of the substrate is maximum. These treatments must have like principal goal to clean the solid surface of all the elements likely to establish a barrier with the establishment of the connections.
- The surface tension of the adhesives used must be lower than the energy of surface of the solids considered. This is why it is extremely difficult to stick plastics at the rough state.
- A good absorptivity is not a sufficient condition, still is necessary it that it is formed connections many and solid, which supposes that the geometry of surface is well known and that the chemistry of the adhesive and solid surface are adapted perfectly.
JOINTS
There, if I am a a little mechanic, I see well that it is necessary to avoid cleavage and peeling….But shearing ?
If I believe Volkersen of it, attention should be paid, because the constraints are not very homogeneous.
The reader will be able to refer to the two books indicated to have more information in this field, because it is true that that starts to become complicated ...... .
I retain nevertheless that the constraints must be distributed on the whole of the joint for better resisting the efforts applied
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