Elimination of Fish Eyes in PVCElimination of Fish-Eyes in Suspension PVC
Summary:
This note describes possible causes of the presence of “fish-eyes”, undispersed PVC particles in processed PVC articles, and how to eliminate them.
Introduction:
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.
PVC starts to thermally degrade above about 150°C and melts at about 200°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 give good processing and will produce fish-eyes. Low porosity PVC grains can be formed in a number of ways:
- Doubly Polymerised Particles. Failing to completely wash out the polymerisation reactor can leave grains of PVC inside the reactor. When the next batch of VCM is polymerized, these residual grains will absorb liquid monomer and end up as solid glassy particles of PVC.
- Inhomogeneous Grains of PVC. Suspension polymerisation conditions in the reactor should be adjusted to give uniform particles with the same porosity. Under certain conditions the product can be inhomogeneous and some grains or even parts of grains can have a lower porosity (see Fig 2), which will process badly.
Elimination of Fish-Eyes:
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 Tests which measure the "average" properties of PVC (eg particle size, porosity, bulk density of the SPVC) are usually unaffected. The problem can only be identified by processing the polymer. Thus “Fish-eye” tests involve processing the PVC sample on the laboratory scale using conditions which the 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:
- Ensure reactor charge ingredients (eg aqueous solutions, fresh and recovered VCM) are free of contamination and that all in-line filters are in position and are working well.
- Ensure the reactor and condenser (if fitted) is washed out well at the end of the previous polymerisation. Washings should be sent to drain.
- Minimise the formation of build up during polymerisations. Not only can it trap grains of SPVC in the reactor, particles of build up can give rise to fish-eyes. Use a proprietary Antifouling Agents or Build Up Suppressants.
- When grade changing, clean out all down-stream equipment including all pipe work, tanks, stripping vessel, centrifuges, dryers, hoppers and packing equipment.
- Contamination can get into the grade from very unlikely sources, so consider all possibilities. For example, in the author’s experience valves separating different grades can pass. Replace them with blanking plates.
- Design grade changes so that cross contamination will not cause fish-eye problems.
- Restrict production on individual Plant Streams to compatible grades.
Inhomogeneous Product/Polymerization Problems: 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 the external and internal structure of a few hundred grains to be examined. While statistically this is 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.
Optical Micrographs of ~150µm SPVC Grains Immersed in DOP
Fig 1 Homogeneous Grains
Fig 2 Inhomogeneous Grains
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.
CONCLUSION:
Even difficult plant problems can be solved by first determining the route cause and then eliminating the problem.
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