PVC Extrusion vs. PO Extrusion

Plastic pipes offer distinct advantages over traditional materials due to properties like weight, corrosion resistance, flexibility, and ease of connection. This chapter primarily focuses on the extrusion of pipes from high-volume commodity polymers: UPVC (Un-plasticised Poly Vinyl Chloride) and Polyolefins (PO), specifically HDPE and PP. These are further referred to as PVC, PE, and PP.

Processing Differences

The core difference in processing lies in the extruder type used and the material form. PVC resin pipe grades are typically processed in dry blend form using a double screw extruder. In contrast, PO resin pipe grades are processed in granular form on single screw extruders. Double screw extruders are well-suited for PVC because it exhibits more wall slip than PO. Single screw extruders are commonly used for PO because it needs substantial heat input (approximately 3 times more than PVC) and isn't as prone to rapid degradation as PVC. Intermeshing double screws transport material via cavities independent of barrel friction, offering a higher pump efficiency compared to single screws which rely on friction with the barrel. Dry blend is optimally processed on double screw extruders; using it on single screw extruders with a smooth intake is too dependent on friction for consistent output. Granulated PVC can be used on grooved single screws, but double screw extruders avoid the granulation step and are self-wiping, leading to cost advantages. PO extrusion involves processing a real melt that crystallizes upon cooling. PVC at extrusion conditions is more of a rubber-like solid, not a true melt, forming an entanglement of tie molecules and crystallites in the pipe wall. Modern high-grade PO extrusion on single screws shares similarities with PVC extrusion, using forced transport via a grooved intake and short residence time at low shear in the die head to achieve a network of tie molecules.

Material Property Differences

Material properties significantly differ between PVC and PO. Generally, PVC is 2-3 times stronger and stiffer than PO, but its impact resistance is lower. Creep is higher for PO, which is relevant for buried pipes and seals. Diffusion through the pipe wall is lower for PVC, important for drinking water pipes in contaminated soil (brown fields). PVC pipe also has less risk of bio-film in the bore and is not vulnerable to chlorinated water, unlike PE pipe. PO materials are not impermeable to oxygen diffusion, requiring barrier layers for hot and cold water applications to prevent rust. CPVC is widely used in the USA and India for hot and cold applications without needing a barrier layer.

Technical Specifications

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