PVC Fish-Eyes What is the PVC fish eyes phenomena. Fish-eye”’ in suspension-polymerized poly(viny1 chloride) (PVC) are particles which gel much slower than the bulk of the resin. They show up in transparent films as ungelled spots which in pigmented compositions remain co1orless.l They are a serious defect in certain applications of PVC resin such as wrapping material and bottles. The prevailing contention about the origin of “fish-eyes” is still that of Rector,* namely, that they are due to either foreign material such as dust or fibers or to admixtures of PVC of different molecular weight or different polymerization history such as crust from the reactor wall. In the present communication, an observation is reported which shows that “fish-eyes” are also caused by factors besides the above- mentioned ones. A series of suspension polymerization of vinyl chloride WBS carried out in this laboratory in a 2-liter S.F.S. glass reactor. Practically the same formulation (amount of water, monomer, initiator, and suspending agent) was used throughout, and the reactions were run at the same temperature to about the same degree of conversion. The only variable was the rate of rotation (rprn) of the agitator. The reactors were meticulously cleaned with tetrahydrofuran after each run so that both foreign PVC and dust were definitely excluded. From the resulting resins, sheets were milled together with stearic acid, calcium stearate, and carbon black for 6 and 8 min. The “fish-eyes” in these sheets show up as translucent points. The results are summarized in Table I. For most rpm, two to three runs were carried out; all counts are tabulated to indicate the degree of reproducibility Individual particles of SPVC have a diameter of about 150μm. Internally they are composed of aggregated solid PVC primary particles with a diameter of 1 to 2μm and have a porosity of 10 to 30%, depending on the grade 1 . PVC starts to thermally degrade above about 150 0 C and melts at about 200 0 C. So SPVC cannot be processed by simply heating. Plasticisers are added to the SPVC grains to produce flexible items of PVC (eg cable insulation or plastic tubing). This also reduces the melting point of PVC and eases processing. In rigid articles, with a low plasticiser content (eg pipes), lubricants must be blended into the SPVC grains, to aid extrusion. In both instances it is important that the SPVC grains have a uniform internal porosity to facilitate good mixing with added plasticizers and lubricants. Fish-eyes occur in the processed article when individual grains of SPVC fail to disperse or gel with their neighbours. Undispersed PVC grains can be felt as lumps in the processed article. If PVC film is pigmented, fisheyes will be visible as white grains in the coloured background. The problem can occur with all grades of PVC. Causes of Fish-Eyes: Fish-eyes can be caused by a number of factors: Cross Contamination: SPVC processing conditions are designed for the grade of polymer and type of article being made. The higher the molecular weight (or Kvalue) of the PVC, the more extreme are the processing conditions. High Kvalue grains of PVC, contaminating lower Kvalue polymer, will give rise to fish-eyes because the higher molecular weight PVC will not gel at the lower processing temperatures. Low molecular weight contamination will not give rise to fish-eyes, as the grains of SPVC will gel at lower temperatures than that used to process the bulk of the PVC. Similarly, contamination by higher porosity SPVC grains is unlikely to cause fish eyes. Grains of SPVC with Low Porosity: PVC grains with low porosity will be unable to absorb the plasticiser or lubricants which improve processing and will produce fish-eyes. Low porosity PVC grains can be formed in a number of ways: