Moisture issues in monomers
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There are strict requirements regarding the water content in monomers – the less, the better. However, during polymerization, under the action of strong stirring, the monomers come into full contact with water, which causes a large amount of moisture to re-enter the monomer droplets. Doesn’t this mean that the water content within the monomers should not have a significant impact on the polymerization process or on the shape of the resin particles? Then why emphasize the water content within the single body? Can it be understood as combined water and free water? What is the difference between the moisture in the single body and the water used for feeding?
Answer: 1. The main hazard of moisture is that it makes the monomer acidic, causing corrosion to equipment; moreover, in acidic conditions the monomer tends to polymerize on its own, leading to blockages in the equipment/pipelines.
Where there is moisture, acid is formed, which can corrode the equipment, especially the converters; one must be very careful.
The water in the monomer is different from the water in the dispersant. The water in the monomer is mainly bound water, and during the transport of the monomer it can corrode the pipes, resulting in the formation of iron ions which in turn affect the polymerization process. Moreover, the large amount of water in the dispersant is free water, and under stirring it does not produce acidic substances with the monomers that could corrode the equipment. Therefore, it is very important to remove water from the monomer.
The requirement for moisture control in monomers is primarily in the distillation system, as the presence of water causes the oxidation products of the monomers to hydrolyze and corrode the equipment; under acidic conditions, iron ions are generated, which in turn accelerates the oxidation of the monomers, leading to their self-polymerization and thus blocking the pipes and distillation towers.
The water in the monomer is mainly bound water; due to the presence of water, the oxide components in the monomer become acidic, which can cause corrosion of equipment and pipes during the transport of the monomer. Moreover, under acidic conditions, iron ions are likely to be generated in these equipment and pipes, thereby affecting the polymerization process. Moreover, the materials inside the equipment and pipelines may also become contaminated due to the addition of iron ions, thereby affecting the quality of the products.
The emphasis on moisture control in monomers does not actually refer to the impact of moisture itself on the polymerization process and product quality; rather, the hazard comes from the impurities contained within that moisture. The impact of certain impurities dissolved in water on the polymerization process and the quality of PVC products cannot be underestimated. As the saying goes, \"one rat’s droppings can ruin an entire pot of soup.\" Not only does hydrolysis produce acids that corrode equipment, but the resulting harmful impurities also have a negative effect on the polymerization process.
The moisture content in the monomers has a significant impact on the quality of PVC resin. The iron ions generated by water-induced corrosion of equipment enter the product, where they not only form impurity ions but, more importantly, an increase in Fe content causes PVC to release HCl, thereby lowering the pH value of the polymerization reaction system. This affects the thermal stability of the resin and can even lead to a decrease in its apparent density.