Tprinciple of wet spinning process-Part 10 Kinetics of polymer dissolution process

Kinetics of polymer dissolution process

1. Polymer dissolution process

Dissolution rate is also very important for production

This sweling rate has an important effect on the dissolution rate of the

polyment

The sweling degree polymer increases with time ,and also depend on

the properties and composition of solvent,temperature ,pressure and other

conditions  

The process by which polymers swell and then dissolve depends on the

rate of diffusion,Fickr’s best describe this process

J_v=-DV_s~

Jv is the volume flux of the diffused substance;  D is the average diffusion coefficient;  Vs is the specific volume of diffused material;  & is the thickness of polymer swelling layer;  AC is the concentration difference between the inner and outer layers of the polymer.  

The dissolution rate of polymer is directly proportional to D and AC and inversely proportional to the swelling thickness.  Since the diffusion rate of solvent is many times higher than that of polymer macromolecules, the dissolution rate of polymer is mainly determined by the diffusion rate of polymer.  The outward diffusion of the dissolved part of the polymer surface is generally achieved by either spontaneous convection (due to the density difference between the solution and the solvent) or forced convection (due to the flow of the medium).    

2 Factors affecting dissolution rate  

1) Temperature

Increasing the temperature is usually beneficial to accelerate the solvent diffusion into the polymer and the swelling process of the polymer;  At the same time, the case of wzum with polymer added by Renbo also accelerated the diffusion rate of macromolecules.  And because the viscosity of the diffusion layer is reduced, It is easier to separate from the surface layer.  So increasing the temperature can improve the dissolution rate.  Generally, there is the following exponential relationship:  

W: nephew, inside the gas constant;  Coming off  

Temperature not only affects the dissolution rate of polymer, but also affects its solubility.  

For the system with upper critical miscibility temperature, increasing the temperature can not only accelerate the dissolution, but also promote the complete dissolution.  

For the system with lower critical miscibility temperature, increasing the temperature can accelerate the dissolution, but decrease the solubility of polymer.  

2) solvent strength

Low concentration of solvent is conducive to swelling, high concentration of solvent is conducive to dissolution.  Therefore, first with low concentration of solvent swelling, and then with high concentration of solvent further dissolution, can accelerate the dissolution rate.  

3) For example, when the granular acrylic copolymer obtained by aqueous polymerization is directly dissolved in 51% NaSCN aqueous solution, the further dissolution process is slowed down due to the formation of a highly viscous solution layer on the surface of the copolymer particles.  In this way, some small micelles and swelling colloidal particles are formed in the original solution, which greatly reduces the filtration performance and spinnability of the original solution.  The formation of thick solution layer and colloidal particles on the surface of particles can be prevented if the NaSCN solution with a concentration of less than 35% is pre-dissolved and then further dissolved with 53%NaSCN solution.  In this way, not only the dissolution rate is increased, but also the quality of the original solution is better.  

4) stirring action

In addition, stirring can reduce the thickness of swelling layer and increase the concentration difference AC between inner layer and outer layer of polymer, which greatly improves the dissolution rate.  The dissolution rate increases with the increase of stirring speed.  

5) Polymer particle size

The particle size of the polymer is 1, which can reduce the thickness of the swelling layer and the dissolution speed