Basic Principle Analysis of Adhesive Adhesion

The mechanism of adhesive (coating, ink) adhesion is not fully understood, but some hypothetical theories have been formed to analyze the adhesion process and the factors affecting adhesion.

1.

When two objects are put together to achieve close interface molecular contact, so that a new interface layer is formed, adhesion is generated.

When the adhesive is applied to the substrate, the adhesion is generated during the drying and curing process.

Broadly speaking, adhesion can be divided into two categories: primary valence and secondary valence.

2.

1.

Observed in a sub-microscopic state, the surface of the substrate is rough, full of holes and depressions.

As long as the coating film has a little fluidity, there will rarely be unreleasable stress.

2.

A chemical bond is generated between the interface, and the chemical groups that react with each other are firmly bonded to the substrate and the adhesive.

Adhesives containing reactive groups such as hydroxyl and carboxyl groups tend to have stronger adhesion to substrates containing similar groups.

3.

Both the adhesive and the substrate surface have residual electrons to form a charged double layer.

The electrostatic force mainly comes from the dispersion force and the interaction force caused by the permanent dipole (the positively charged region of one molecule and the negatively charged region of another molecule).

When the distance between the adhesive molecules and the substrate molecules exceeds 0.

4.
Diffusion theory

When the adhesive is in contact with the substrate, some short chains of macromolecules will diffuse to the other side of the interface to varying degrees.
That is, the chain segments diffuse into each other after passing through the interface to form a staggered network structure.

Due to different long-chain properties and low diffusion coefficients, dissimilar polymers are usually incompatible.
It is impossible for complete macromolecules to diffuse into each other across the interface.
Experiments show that local segment diffusion is easy to occur, and a diffusion interface layer of 10-1000 angstroms is produced at the interface.

Third, the formation mechanism of adhesion

1.
Mechanism description

When two dissimilar materials come into close contact, the two free surfaces in the air disappear, forming a new interface.
The nature of the interface interaction determines the strength of the bond between the coating and the substrate.
The degree of this interaction is basically determined by the wettability of one phase by the other.
When using liquid coatings, the fluidity of the liquid phase is also very good.
Great help, so wetting can be seen as the close contact between the paint and the substrate.
In order to maintain the adhesion of the coating to the substrate, in addition to ensuring the initial wetting, it is very important to maintain the bonding condition after the coating is completely wetted and cured after the film is formed.

Film forming method

(a), cooling to the melting temperature (glass transition temperature Tg) below

(b), chemical cross-linking reaction

(c) Volatilization of solvents and thinners

(d) The binder also has the ability to crosslink when it is dry.
Therefore, the wetting of the coating to the substrate is the key to the formation of adhesion bonds.

2.
Interface phenomenon

The premise of the adhesion formation mechanism is that the liquid coating produces effective wetting on the solid substrate.
The surface tension of the coating in the liquid state and the surface energy of the substrate and the solid coating film are important parameters that affect the formation of interfacial connection strength and adhesion.

When the surface tension value of the liquid is lower than the surface tension (surface free energy) value of the solid, the liquid can be effectively wetted on the solid and it can be fully spread.
The greater the difference between the two values, the better the degree of wetting and spreading.

1) Wetting

The wetting process is a process in which one fluid is replaced by another fluid at the phase interface.
The parameter to measure the degree of wetting is the contact angle.

2), net suction

The force state of the surface and internal molecules of an object is different.

As shown:

The attraction forces of a molecule on the liquid surface in various directions, the forces of A1 and A2 can cancel each other, the force of C and the downward force of B, D are F, and the horizontal components of B and D also cancel each other, so the molecules are subjected to one The “combined suction force” pointing to the inside of the liquid perpendicular to the surface of the liquid is usually called net suction.
Due to the existence of net suction, the molecules on the liquid surface tend to be drawn into the liquid.
Therefore, any liquid surface has a spontaneous shrinkage.
Tendency, this is also the reason why the surface of the liquid exhibits surface tension.

The molecules on the solid surface also have a net attraction, but the solid molecules cannot move freely like liquid molecules and cannot produce surface shrinkage, but they exist on the surface in the form of free energy.

3), surface tension

Take the spherical droplet spreading to the surface as an example

The increased surface area means that certain molecules inside the liquid are "pulled" to the surface and spread on the surface.
When internal molecules are pulled to the surface, they need to overcome the attraction of internal molecules and consume work.
Therefore, surface tension can be defined as the work consumed to increase the unit area.

The unit of surface tension is N/M.
It is the force acting on a unit length.

Intermolecular forces can cause net suction.
The net suction causes surface tension, and the surface tension is always tangent to the liquid surface and perpendicular to the net suction.

4), surface wetting

When a droplet is placed on a solid surface, two phenomena will appear: one is that the droplet will spread out immediately, that is, the solid is wetted by the liquid; the other is that the droplet will agglomerate into a spherical shape without spreading, that is, the solid will not be spread.
Wetted by liquid.

The degree of wetting of the liquid to the solid is usually expressed by the size of the angle (called the contact angle) between the liquid-solid two-phase contact interface AC and the tangent AB of the liquid surface.

The degree of wetting of the liquid to the solid

The substrate can be wetted by coatings with small surface tension and contact angle, or only when the two are close together.
Theoretically speaking: if the surface free energy of a certain object is less than 33dyn/cm, it is almost impossible to adhere to any kind of adhesive currently known.