Views: 5 Author: Site Editor Publish Time: 2020-06-02 Origin: Site
How to use polyvinyl alcohol
When using PVA, the solution is generally made uniformly. According to the characteristics of different brands and different shapes of PVA, the appropriate dissolution method can be used to improve the dissolution efficiency.
2 Dissolution equipment:
Use a reaction kettle with a stirrer. The reaction kettle should be made of non-rusting and corrosion-resistant materials such as stainless steel, enamel, or synthetic resin-lined iron products to avoid contamination of the PVA solution.
Different stirring speeds can be adopted for different models of PVA and different shapes of stirrers. When using double-wing spiral stirring, the speed of complete alcoholysis PVA should be 500-1000 rpm, and the speed of partial alcoholysis PVA should be 100-300 rpm; if frame stirrer is used, 80 -Speed of 150 rpm. If the stirring speed is too low, PVA tends to settle and agglomerate, delaying the dissolution time; if the stirring speed is too high, it is easy to be involved in the air and cause foaming (especially partial alcoholy type PVA).
3 Heating method:
It is better to directly blow the steam into the heating, the steam pressure is 1-1.5KG/cm2, if it is supplemented by jacket steam heating, the dissolution time can be greatly shortened. Avoid direct heating on an open flame to avoid poor mixing and scorching at the bottom of the container.
4 Polyvinyl alcohol dissolution conditions:
The solubility of PVA is mainly determined by the degree of alcoholysis and degree of polymerization. As the degree of alcoholysis decreases, the solubility of PVA decreases and the water solubility increases. Complete alcoholysis type PVA has a great dependence on temperature, and it can not be dissolved or only swells below a certain temperature; but for partial alcoholysis type PVA, the temperature is not easy to be too high, otherwise it is easy to foam and clump.
A. Add the water needed for measurement at room temperature (about 30℃) to the dissolution tank.
B. Put the measured PVA slowly under stirring (the stirring speed is easy to be slightly larger), because part of the alcoholysis PVA is easy to agglomerate, the slower the feeding, the better.
C. After fully stirring, PVA has dispersed Pengrun as much as possible (about 30 minutes), then gradually warm up to the appropriate temperature for dissolution, continue to stir for 1-2 hours, it can be completely dissolved into a transparent solution. After filtering out impurities, it is ready for use. For some alcoholysis type PVA, the temperature rise should be slow, and the temperature rise too fast, easy to foam and overflow. [After crushing the polyvinyl alcohol, the particle size is between 20-200 mesh, the dissolution time is much shorter than the dissolution time of the particle PVA]
D. The moderate alcoholysis and partial alcoholysis type PVA are relatively easy to foam when dissolved, so it is not easy to heat up too fast, otherwise it is easy to foam and overflow. During the dissolution process, if there is more foam, turn off the steam, reduce the stirring speed, or suspend stirring. After the foaming disappears by itself, gradually increase the temperature and stir. After two or three repetitions, the foam can be greatly reduced.
5 Foaming mechanism:
PVA series porous material, air, methanol and lipid volatile matter are adsorbed in the gap. However, the residual acetic acid group in the partial alcoholysis type PVA weakens the hydrogen bonds between adjacent molecules, making the chain difficult to crystallize, the amorphous region increases, and the volume void is larger than that of the complete alcoholysis type PVA. The void contains a lot of air and volatile substances. After contact with water, hydrophilic low-molecular substances (alcohols, fats) suck water into the void and remove the air. This is the main source of gas that forms bubbles. In addition, because the surface activity of the partial alcoholysis type PVA aqueous solution is greater than that of the complete alcoholysis type, the tension of the gas-liquid two-phase interface is reduced, and the contact surface of the gas-liquid two-phase is increased. In the case of stirring, bubbles are generated condition. In addition, the solution of PVA has viscosity, and the liquid film has a certain mechanical strength, so that the bubbles are not easy to disappear. If the use conditions are not properly controlled (such as high temperature and high speed), the bubbles are serious and affect the operation.
6 Defoaming methods
When dissolving the PVA product, the appropriate temperature should be controlled first. If the bubbles are still produced by the adjustment according to the foregoing procedure, the following measures can be taken:
A. Before dissolving, the PVA is now swollen in cold water, so as to eliminate the air and volatiles in the PVA as much as possible, and gradually increase the temperature, which can effectively eliminate the generation of bubbles.
B. When the foam occurs, turn off the steam and suspend the stirring. After the bubbles disappear, the temperature will gradually increase and stir. Generally, after repeated two or three times, the bubbles can be greatly reduced.
C. No need to take chemical defoaming if necessary. Commonly used defoaming agents are (n-octanol, tributyl phosphate, polyether defoaming agents, etc.), and their dosage is generally 0.01-0.05% (based on the weight of the solution).