Method for the Control of “Fish Eyes”in PVC Resin

1. Characteristics of “Fish Eyes”

Essentially, “ Fish Eyes” are PVC granules having ultra-high molecular weight generated in the process of polymerization.

“Fish Eyes” cannot be swelled but be plasticized by plasticizers under normal processing conditions due to its poor absorbancy of plasticizers. However, a big part of “Fish Eyes” encountered in processing are false “Fish Eyes”. This type of false “Fish Eyes” are linear-structured PVC molecule with higher molecular weight. Because of difference in molecular weight of PVC Resin, under the same processing conditions, polymerization speed of PVC Resin with low molecular weight is fast, while polymerization speed of PVC Resin with high molecular weight is slow, thus false “Fish Eyes” coming into being.

2. Effect of “Fish Eyes” on Performances of PVC Finished Products

”Fish Eyes” are gel particles. It will exert bad effect on the strength of PVC film, insulation of PVC cable and tone quality of albums.

Too many “Fish Eyes” will make the surface of finished products become rough and reduce the physical properties of products, mainly mechanical strength of products. Meanwhile, “Fish Eyes” will be the core of degradation, thus causing the discoloring of products.

Too many “Fish Eyes” contained in PVC Resin will also make the process control much harder. Damage of blowing PVC film will occur and the uniformity degree of films will be hard to control. “Fish Eyes” exist in PVC sheets and plates in the form of “crystal point” and they will badly affect the appearance and physical performances.

3. Measures for Prevention of “Fish Eyes”

A. Reaction Pot shall be washed and cleaned in the production, and time for washing and cleaning shall be adjusted as per real situations and manual cleanup shall be carried out when necessary.

B. Temperature of polymerization reaction shall be stablized, which will make the distribution of molecular weight of PVC Resin balanced, thus reducing false “Fish Eyes”.

C. PH value of polymerization system shall be determined and adjusted as per real situations.

D. Monomer mass shall be strictly controlled, especially the content of high-boiling residues.

E. Reaction time can be shortened as per real situations to lower the polymerization conversion rate.