ADHESIVES, GLUES AND SEALANTS
What is the adhesion?
Currently there are several theories attempting to explain the phenomenon of adhesion of the adhesive on the substrates, there is currently no unified theory to justify all cases. Now it is required the use and combination of different theories to justify cases.
Definition of adhesion - It applies to all forces or mechanisms which hold the adhesive with each substrate, the term adhesion, refers to a thin layer (boundary layer) between the substrate and the adhesive itself.
In the definition of adhesion is used to define two important concepts:
Force or mechanism that holds the adhesive with each substrate.
Forces and mechanisms related to the forces created by intermolecular interactions, chemical bonds anchoring mechanisms by roughness, adsorption and diffusion.
The adsorption phenomena occurs when part of the adhesive polymers enter in contact with the substrate but not cross it, bonding to it via the action of intermolecular interactions and / or chemical bonds that develop in the area known as boundary layer or interface.We can define the adsorption as the adhesion of the adhesive without penetration to the substrate.
By contrast, in the diffusion phenomenon that forms part of the polymers which form the adhesive go through the substrate, the result is the binding and anchorage points intertwining the two materials. We can define the diffusion as the adhesion of the adhesive wtih penetration to the substrate.
The boundary layer refers to a thin layer corresponding to the interfaces between the substrate and the adhesive where are all the forces mentioned above.
Adhesion is parameterized in two concepts:
Adhesion energy: Represents the sum of all the energies produced by interactions (chemical bonds, dipole moments ,electrostatic forces, anchoring mechanisms, adsorption and diffusion.) These are developed in the boundary layer.
Reversible work of adhesion: Represents the work that we have to apply to exceed the sum of all interactions or forces and mechanisms that develop in the boundary layer.
As cited above, there are various theories and models that attempt to explain the phenomenon of adhesion, there is currently no unified model that explains all adhesion cases, these theories are the current models that explain the phenomenon of adhesion:
Theory of diffusion
Theory of electrostatic adhesion
Thermodynamic theory of adhesion
To ensure the quality of adhesion between two materials is important to have the following 2 parameters:
Choice of proper surface preparation and cleaning treatment to the substrate that you would like to apply to the adhesive.
Correct choice of adhesive during the design phase, both the compatibility of the adhesive with the substrate, the effects of ageing that can withstand the adhesive, and thermal expansion coefficients of the substrates.
The use of destructive testing can identify the right adhesion or right bond by type of fracture (cohesive, adhesive or mixed) and the value of fracture resistance obtained from the sample.
The most common destructive tests used to assess adherence are:
The test samples are used to do that type of test and they might be subjected to weather conditions, chemical or physical (ultraviolet light, salt spray ...) in order to assess the effect of ageing on the cohesion.
The phenomenon of adhesion is currently studied in the research and development centers, because the importance of this phenomenon in the field of adhesives.The complete knowledge of the phenomenon of adhesion allows the development of adhesives that can be attach on any type of material under any condition.
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