Rigid PVC Fusion in Extrusion Process

First of all we need to know what is the fusion of PVC ? As scientists believe, the fusion is joining the particles before melting. Sometimes it is expressed as gelation or plastication by technician. PVC is a kind of semicrystalline plastics, but it is more thermosensitive against a lot of plastics. In the other words, fusion depends on thermal history, shear tension and pressure. So, when we talk about fusion in PVC, all the parameters which might affect on it is related to grain temperature and history and tension which is applied on it.

In general, PVC is produced as emulation and suspension. Suspension PVC is also produced as powder type and granular. The suspension PVC powders are called grain by Technicians. The grain is consisted on microparticles which their diameters are around 1 to 2 micron. As these particles are located with some gaps, the grain is porous like. Each of these microparticles are made by submicro particles which are 100 to 300 Angestrum. Their crystallinity is 5 to 10 percent. For reaching to good mechanical properties, they must be packed, that means these tiny particles must join to each other and the voids between them must be omitted. The three dimension network will be created by polymeric chains. This is gelation or fusion. Against the other polymers, PVC would not be as a homogeneous melt under normal processing condition.

The additives which are used in formula, fusion temperature and tensions which would be applied during process could affect on fusion. For calculating of fusion factor this formula is always used:

F=B2/AC

In this formula A, B, C and F are Min and Max viscosity of material after putting them in in plasticorder, time between start to maximum viscosity and fusion factor.

The following picture illustrates impact of different stabilizers on fusion time.

The picture which is shown as follow is about relation between different stabilizers and fusion factor.

The following pictures are also impact of lubricant on fusion time and fusion factor respectively.

According to these diagrams, Di Basic Lead Stearate (DBLS) and Neutral Lead Stearate increase fusion time and they decrease fusion factor. But, Three Basic Lead Sulfate (TBLS) has opposite function.

Poly Ethylene Wax (PW) increases fusion time and decreases fusion factor. Stearic Acid increases fusion time then decreases it and it act as opposite way for fusion factor. Calcium Stearate decreases fusion time and increases fusion factor. 

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