This note describes possible causes of the presence of "fish-eyes", undispersed PVC particles in processed PVC articles, and how to eliminate them.
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 grade1.
PVC starts to thermally degrade above about 1500C and melts at about 2000C. 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.
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:
Testing for Fish-eyes: Fish-eyes, like black specs, are a contamination problem. A few SPVC grains in many millions are sufficient to cause the problem. Normal QC properties (eg particle size, porosity, bulk density of the SPVC) are often unaffected. The problem can only be identified by processing the polymer. Thus “Fish-eye” Tests involve carrying out Laboratory Processing on the sample which mimic customers’ processing conditions.
Cross Contamination: Because the concentration of contaminating particles is so small, sources of contamination can be extremely difficult to identify.
If fish-eyes become a problem:
One useful characterization test is to disperse the SPVC grains in plasticiser and after about 30 minutes observe them under an optical microscope. This enables you to examine the external and internal structure of a few hundred grains. While statistically this is still not significant and it is very unlikely you will see a grain which will cause a “fish-eye“, one can get a qualitative impression of particle structure. With experience it is possible to infer a lot about the particle formation processes occurring in the reactor.
SPVC grains with the appearance shown in Fig 1 have a uniform size and porosity and should not cause fish-eye problems. The micrograph in Fig 2 shows a clear grain (a non porous PVC particle) and dark areas which are probably isolated porous regions. It is quite possible the PVC in Fig 2 will be susceptible to fish-eyes. Modification of polymerisation conditions (suspension stabilisers, reactor agitation, condenser operation, etc) is needed to improve grain homogeneity.
Many Plant Problems at first seem difficult to solve. The first step is to determine the route cause and then to eliminate the problem.
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"Vinyl Chloride Polymers", Encyclopedia of Polymer Science and Engineering, John Wiley & Sons Inc., Vol. 17, (1989)