Effect of pH value on suspension polymerization

The pH value is too low, which affects the initiating efficiency of the initiator and the thermal stability of the resin. The pH value is too high, which will promote the continued hydrolysis of the unhydrolyzed part of PVA and affect the dispersion effect.

Will a too high ph value speed up the decomposition rate of the initiator?

The pH value affects the stability of the dispersant. A lower pH value will have a significant destructive effect on gelatin. A higher or lower pH value will cause partial alcoholysis of polyvinyl alcohol, affecting the dispersion effect and particle morphology. It will also affect the decomposition rate of the initiator.

If the pH value of the polymerization system is too high, it will affect the stability of the polymerization dispersant, produce coarse materials, and PVC resin products will also have many black and yellow spots.； If the pH value of the polymerization system is controlled too low, the thermal decomposition rate of PVC will be accelerated, the whiteness of PVC will decrease, and the PVC resin product will have more black and yellow spots.

N years ago, the operator of our company wanted to try out the consequences of not adding a pH adjuster. The result was a kettle of coarse material. From then on, no one dared to despise the pH adjuster.

If the PH value of the suspension is too high or too low, it will reduce the dispersing and protective capabilities of the dispersant, eventually causing the resin particles to become coarser. Experiments have proven that maintaining 6-9 has the best particle distribution. Of course, it's not that uncertain. There was a time when the flow meter broke and we didn't add it.

I would like to ask how NH4HNO3 adjusts pH.

The PH value of the suspension should be controlled at 6-8, which is the best and easier to control. It is difficult to say if the PH value exceeds the range of 6-8. High or too low will reduce the dispersing and protective capabilities of the dispersant, eventually causing the resin particles to become thicker and causing differences in appearance, so 6-8 is safer.

If the pH is too high, the decomposition rate of the initiator is too fast, and the reaction time is shortened above 8.5, which can increase the alcoholysis degree of PVA and cause coarse materials. If the initiator is too low, the viscosity of the kettle will be serious, and coarse materials will appear.

The dispersant for VC polymerization is mainly partially hydrolyzed polyvinyl alcohol, which is obtained by partial hydrolysis of polyvinyl acetate. After hydrolysis, hydrophilic hydroxyl groups and lipophilic ester groups exist at the same time. Due to the different proportions of the two, PVA has different water solubility and uses. The way the pH value affects the dispersant is to promote the continued hydrolysis of the remaining ester groups. The presence of hydrogen ions and hydroxyl ions can accelerate the hydrolysis reaction (the mechanism is different, please refer to organic chemistry), thereby affecting the ratio of hydroxyl groups and ester groups. The hydrophilicity and lipophilicity of further hydrolyzed PVA will change. PVA with severe hydrolysis will no longer be able to exist stably at the interface between monomer droplets and water phase. The probability of unprotected droplets gathering into large liquids will increase, and PVC particles will become thicker.