Preparation of spinning solution and preparation before spinning
（1）Preparation of spinning solution
Spinning solutions are prepared in two ways for solution spinning.
One-step method:polymer is used to spin directly.It is why this is called one stop method.But it must use homogeneous polymerization.Only wax nylon can be used in one step.
Two-step method:First, the solution obtained by polymerization is separated into granular or powdery fiber forming polymer.After,The solution is dissolved into spinning solution.This method is called the two-step method.
At present, among the main chemical fiber varieties produced by solution spinning method, only polyamide fiber can be spun by either one-step or two-step spinning method,Other types of fiber-forming polymers cannot be produced by one-step process.Although the separation, drying, dissolution and other processes of the polymer are omitted by one-step method, the process can be simplified and the labor production rate can be improved, but the fiber quality is not stable.
In the production of cellulose fiber, because cellulose is insoluble in common solvents, it is usually converted into derivatives (cellulose xanthate, cellulose acetate, cellulose carbamate CC, etc.) and then dissolved into spinning solution for spinning forming and post-processing. When using the new solution (N■ methylene · N■ oxide) spinning process, the cellulose can be directly dissolved in the solvent to make spinning solution.
The concentration of the spinning solution varies according to the type of fiber and the spinning method. Generally, the concentration of spinning solution used for wet spinning is 12%~25%; The concentration of the solution used for dry spinning is higher, generally between 25% and 35%
（2）preparation before spinning
In the two-step process, it is necessary to choose a suitable solvent to dissolve the fiber-forming polymer, and the resulting solution must go through mixing, filtration and defoaming processes before spinning, which are called preparation before spinning.
Mixing: the purpose of mixing is to make the properties (mainly concentration and viscosity) of the spinning solution of each batch uniform.
Filtration: The purpose of filtration is to remove impurities and undissolved polymers
Spinning solution filtration, generally using plate frame filter press, filter material selection can withstand a certain pressure, and has a certain degree of tightness of all kinds of fabrics, generally 2~4 continuous filtration. After the filter material used should be more dense than the previous one, so as to play the due effect
Defoaming: Defoaming is to remove air bubbles retained in the spinning solution.
These bubbles will cause broken end, broken filament and cavity filament in the spinning process and reduce the fiber quality, or even make spinning impossible. The defoaming process can be carried out under atmospheric pressure or vacuum. In normal pressure static defoam, because the bubble is small, the bubble rise speed is very slow, the defoam time is very long; And in the true empty state of defoaming, the higher the vacuum degree, the lower the pressure on the liquid surface, the bubble will expand rapidly, the defoaming speed can be greatly accelerated.