Theory of film formation

Generally, A leather has five  parts. they are-

1. 
   Butt
2. Shoulder
3. Neck
4. Belly
5. Shank

Theory of film formation

The molecules Film forming materials together with other molecules of non-volatile materials present thus approach each other and since the intramolecular forces. known as van der waals forces. With the decrease of molecular distance, the force  of attraction increases very rapidly. Many practical tanners wrongly believed that, the molecules come close to the nearest approaches they chemically react (polymerise) and form the film. 

But this is true when polyurethane, linsed mucilage etc. are used for patent leather finishing (Chemical reaction) but with other materials like casein, shellac, acrylic or methacrylic resins synthetic rubbers based on butadine, vinyl resins, microcellulose lacquers etc, such chemical reactions never take place during film formation. 

The molecules in such films remain together due to van waals forces of attraction which is also also called residual valence forces or secondary valence forces.

In the liquid or solid states the term internal pressure is also applied. The tensile strength, flexibility, wear resistance and practically all other physical properties of film naturally depend to a largo extent on the strength of this secondary valence force.

The secondary force which keeps the molecules in matters together just like the atoms in molecules remain together by the influence of primary valence force.

The forces due to ionic, covalent, co-ordinate, metallic bonds resonance in the molecules are the primary valence forces or interatomic forces whereas keesom force, London force and Debye force together which  are also known as  van der waals forces are the secondary valence forces.

Hydrogen bonding is also a kind of secondary  valency. The secondary valence force are actually due to residual fields left about the molecules as a whole after two or more atoms have combines together to form a molecule through primary valence forces. As is found in ionic bonds, when electrons are transferred from one atom to another, to form a molecule, the latter becomes a dipole with permanent dipole moment.

Similarly when a molecule with dissimilar atoms is formed trough covalent bonds , the centers of action of positive charges may not coincide of action of negative charges, the whole system thus will show a dipole moment.

The same rule can be attributed when molecules are formed from atomic stages through other type of bonds.

There may be several  dipoles in a molecule but the overall dipole moment of the molecule is the resultant of all the dipole moments in it.

For simplicity consider two different arrangements of equal and opposite charges at the comers of a square as shown in figures (a) and (b)

Principles of Tanning process and features of tanning