Introduction:
PVC is a unique polymer that cannot be processed without compounding. The strength in PVC products is derived from the optimum fusion that takes place during processing in three stages –
1. Breakdown of 90 – 250 µ PVC grains (secondary particle) to 1 µ primary particles.
2. Entanglement of PVC chains, partially released from the primary particles due to melting of some of the crystallites, to form a three dimensional network of primary particles.
3. Recrystallization during cooling of shaped product.
It is interesting to note that in spite of intensive working during high speed mixing, PVC grain does not breakdown. PVC particles only soften and approach spherical shape due to surface tension, thus increasing the bulk density of the compound. Breaking of PVC grains occurs during twin screw extrusion under the influence of shear and temperature. Unless PVC grain breaks down to primary particle, the next step of fusion cannot start.
The situation at the milling gap:
If the flights two intermeshing identical screws intersect in such a manner that the tip of flights of one screw nearly contacts the channel bottom or root of the other screw, the screws are called, fully intermeshing. The gap is known as milling or calendaring gap.
The size reduction (dispersion) is achieved mainly during milling action (shear) in the milling gap.
For shear to take place, two surfaces must move with different speeds so that the material caught between the two shearing surfaces reduces in particle size.
It is interesting to note that in co-rotating twin screws, the surfaces of two screws at the intermeshing point are moving in opposite direction. Material hardly enters in this region. Material is transferred from one screw to another.
However in case of counter rotating twin screws, the surfaces at the intermeshing point are moving in the same direction, providing calendaring effect.
Many people fail to understand that at the intermeshing point when two screws are rotating at the same RPM and the material is simply passing through the calendaring gap, how can there be shear?
The reason for shearing in the milling gap:
In twin screw extruders, the diameters of two surfaces are different. The flight diameter of one screw is more and the root diameter of other screw is less. Though the screws are rotating at same RPM, the linear speeds of two surfaces are different that causes the shear for PVC in milling gap.
The amount of shear put into the material, with consequent temperature rise, is controlled by –
1. The size of the milling gap (If the gap is more (0.5-5mm) mixing action is promoted and if it is less (0.05-0.5mm), shear is promoted), and
2. The screw RPM (higher the RPM, more is the shear)
Shear rate in counter rotating twin screws:
If at the point of screw intermeshing,
D = Diameter of the flight of one screw,
d = Root diameter of the other screw, and
N = Screw RPM
Then the shear rate can be expressed by: Sg (max) = (V1 – V0) / g
Where,
V1 = Linear velocity of the flight tip = (πDN)
V0 = Linear velocity of the channel bottom = (πdN)
g = Gap between screw tip and the channel bottom (milling or calendaring gap)
Let us understand the effect of various parameters on shear rate:
1. If we increase the RPM, then (V1-V0) factor will be higher and shear will be more. This will result in effective dispersion of PVC grain.
This also indicates that twin screw extruders should not be run at lower speeds, rather it should be run at standard designated speed. At such higher speed –
a. The output will be more and hence cost of production will be less.
b. The residence time for the compound in extruder will be less,
c. Hence entire thermal stabilizer will not be consumed, and in turn,
d. UV degradation will be less.
2. If the milling gap “g” is more due to wear and tear (mainly caused due to separating forces between the screws, use of more filler level, improper screw & barrel metallurgy & improper running of extruder), then the shear rate will be less and PVC grains will not be dispersed effectively.
This result in –
a. Improper fusion causing inferior mechanical properties,
b. Ungelled hard particles appearing on the extrudate surface due to improper dispersion,
c. More residence time for compound in barrel,
d. More demand for thermal stabilizer,
e. Less output, and
f. Increase in cost of production.
The operator usually misinterprets this phenomenon. Looking at the ungelled surface, he tries to increase temperatures that results in degradation and invariably the operator opens the die and fixes is back after cleaning. The root cause is inadequate fusion due to lack of breakdown of secondary particle to primary particle.
3. If we increase the diameter of the screws, then (V1-V0) factor will be higher and shear will be more.
This aspect is effectively used in conical twin screw extruders:
In conical extruders, the extruder is designated by the screw diameter at the end of the metering section.
It is claimed that 50 mm conical extruder matches the output of 65 mm parallel screw extruder.
For such a conical extruder, the diameter of screw in feed section is 100 mm.
In feed section, the plasticizing is equal to 100 mm parallel screws, while in the metering section, the friction and shear matches 50 mm parallel screws.
The lower diameter of 50 mm in the metering section results in lower axial force on the screws, which is a beneficial factor.
In the plasticizing section, the intermeshing surface in case of conical screws is larger than in parallel screws.
This results in higher but controlled energy inputs through shear.
On the other hand in the metering section-
1. The intermeshing surface of the conical screws is less than the parallel screws. This provides lower shear energy inputs.
2. Smaller diameter at the metering section further reduces the shear rate.
Thus, unwanted energy input into melt is avoided and pressure build up is achieved with less stress on the material.
Lower shear rate means lower temperature rise / lower degradation / and lower die swell.
This arrangement facilitates processing of C – PVC.
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