PVC dry blend, used for producing pressure pipes, is a mix of PVC powder and various additives. These additives are crucial for processing and achieving the required final pipe properties.
Heat stabilizers, like Tin (Sn), Calcium Zinc (CaZn), and Organic Based Stabilizers (OBS), are main components mixed with PVC powder. They are essential because PVC degrades quickly at high temperatures experienced during extrusion. Stabilizers protect the polymer from heat degradation, contributing to the long-term durability and performance of the pipe. CaZn and OBS formulations are increasingly replacing older Lead (Pb) systems.
Lubricants, including stearates and low and higher melting waxes, are vital for smooth and reliable PVC processing. They ensure sufficient wall slip to prevent burning caused by local friction, especially critical at higher output rates and with complex dies like double compression tooling. Adjusting the balance between low and higher melting waxes can significantly improve process stability and prevent issues like beard grow and burned particles, which otherwise would lower pipe performance in tests like burst pressure and impact.
Impact modifiers are added to modify PVC properties, such as in Modified PVC (MPVC). Adding impact modifier makes PVC more ductile and improves resistance to point loads, important for buried pressure pipes. MPVC pipes process well and can have lower safety factors (1.4 to 1.6) compared to standard UPVC (2-2.5), though they typically have a slightly lower Minimum Required Strength (MRS) of approximately 22 MPa compared to 25 MPa or higher for UPVC. Quick gelling materials containing impact improvers process well with early venting.
Fillers like Calcium Carbonate (CaCO₃) can be added to PVC formulations. However, for pressure pipes, the amount is generally limited to maintain the required strength and performance characteristics defined by standards like ASTM D1784. Higher filler levels are more common in non-pressure and structured wall pipes where stiffness is the primary requirement.
|
Property |
UPVC (Standard) |
MPVC (Modified PVC) |
|
Minimum Required Strength (MRS) |
25 MPa (250 bar) or higher |
Approx. 22 MPa |
|
Safety Factor (Overall Design Coefficient) |
2 - 2.5 |
1.4 - 1.6 |
|
Ductility |
Standard |
Higher |
|
Point Load Resistance |
Standard |
Improved |
|
Processing (Gelling) |
Standard |
Quicker (with impact improvers) |
|
CaCO₃ Inclusion |
Limited amount for pressure pipe |
Limited amount for pressure pipe |