WHY DO SHAPE OF EXTRUDATE DIFFER IN UPVC AND HDPE PIPES?

Introduction:

Today, most of the pipe manufacturers in India produce UPVC as well as HDPE pipes.

They begin with UPVC pipes and then add on HDPE pipe extrusion line to cater to the market requirements.

If they apply knowledge of PVC pipe extrusion to HDPE pipe extrusion, thinking that:

1.      The process is same and HDPE pipes are much easier to extrude.

2.      There is no compounding required and the material is available in pre compounded pellet form, and

3.      There is no troublesome degradation like U PVC pipes.

However, these are two different types of polymers..

Difference in PVC and HDPE:

1.      While PVC is predominantly amorphous having only 3-8% crystallinity, HDPE is a semicrystalline polymer having 60-80% crystallinity. The remaining part is amorphous. HDPE has both Tg as well asTm.

2.      Crystalline melting point, Tm, of PVC is 105 – 250 °C, owing to imperfections and different size, while HDPE melts at 133 – 137 °C

3.      PVC does not melt during processing temperature of 180 – 190 °C, it undergoes fusion at primary particle level; whereas HDPE melts during processing.

4.      Tg of PVC is 80 deg C, while Tg of HDPE is (–) 125 deg C.

5.      Strength in PVC pipe develops from optimum fusion at 65-70% level. HDPE pipe is softer at room temperature owing to its Tg of (–) 125 °C, and stronger due to its crystallinity.

6.      UPVC is processed in dry blend compound form for pipes, whereas HDPE is processed in pellet form.

7.      PVC melt flow is particulate consisting of three dimensional networks of fused primary particles, whereas HDPE melt flow is molecular.

8.      Unit of PVC melt is primary particle, whereas unit of HDPE melt flow is molecule in chain form.

9.      PVC is processed on twin screw vented extruder, whereas HDPE is processed on single screw extruder with a combination of barrier screw and grooved barrel.

10.  For pipes, the extrudate temperature as a thumb rule is Tm + 60 °C, or around (135 + 60) 195 °C.

For UPVC pipe, the extrudate temperature is Tg + 100 or around 180 deg C.

In comparing PVC with PE, the processing difference is so large that –       

1.      The coefficient of friction reduces with temperature for most of the thermoplastics, including HDPE. However in case of rigid PVC, it increases with temperature (due to fusion).

2.      Conventional PVC has melt viscosity three to five times higher than that of polyethylene of the same molecular weight. This leads to more shear and friction.

3.      In addition, thermal conductivity of fluxed PVC is only one - third of that of P.E. melt. Consequently the shear induced heating during processing of PVC is much greater than that experienced in P.E.

Heating PVC particles through controlled shear is the key to successful processing. Thanks to effective twin screw design.

4.      The strength of PVC comes from the optimum fusion (65-70%), while for HDPE the strength of pipe comes from recrystallization during annealing while cooling.

Difference in the extrudate shape:

For UPVC pipe, the diameter of die bush is 0-10% higher than the sizer.

On the other hand for HDPE pipe, the diameter of die bush is quite large than the diameter of the sizer.

The reason lies in the crystallinity of HDPE:

During processing of HDPE at temperature above its melting point, the original crystalline structure is destroyed and entire HDPE is converted into a disordered amorphous state.

For HDPE pipes, the extrudate minimum temperature as a thumb rule is Tm + 60 °C, or say (135 + 60) 195 °C.

This is sized and cooled in the sizer.

The original crystallinity that is lost during processing has to be regained through recrystallization during cooling, in order to provide strength to HDPE pipe.

The HDPE extrudate shrinks in air gap on two counts –

1.      Shrinkage due to crystallization

2.      Shrinkage due to thermal contraction as the extrudate cools from 200 °C to min. 71 °C, in the cooling tank.

Shrinkage due to crystallization:

For a typical formulation, melt density of UPVC is 1.32 g/cc while the product density is 1.44 g/cc (can vary depending on formulation). This amounts to 9.9% shrinkage. This shrinkage is countered by the die swell to some extent. Also, negligible recrystallization takes place for PVC due to immediate quenching.

This is the reason, why PVC pipe die designer keeps bush diameter almost same as the pipe diameter.

On the other hand, the melt density of HDPE is 0.74 g/cc, whereas the product density is 0.94 g/cc

This amounts to 27% shrinkage. Also, sizer front flange is cooled through water jacket.

Shrinkage due to Coefficient of thermal contraction:

HDPE pipes are 2.5 times thicker than PVC pipes for the same pressure rating.

Thermal contraction in the air gap is about 0.022% / deg C. Melt cooled from 200 deg C to 30 deg C – by 170 deg C will have thermal contraction = 170 x 0.022 = 3.74%.

Coefficient of thermal expansion for HDPE is three times that of PVC. Hence HDPE pipe shrinks three times as much as PVC when it gets cooled.

This is the reason why for HDPE pipe the sizer is of smaller diameter than the bush.