Calcium Carbonate (CaCO₃) is a common filler added to PVC formulations in pipe production. The amount of CaCO₃ used varies significantly depending on the intended application of the pipe, primarily categorized as pressure or non-pressure.
For pressure pipes, the amount of fillers like CaCO₃ that can be added is limited. This is because the primary design requirement for pressure pipes is resistance to internal pressure, which depends on the material's strength (MRS) and the pipe's wall thickness. Adding high levels of fillers can compromise these mechanical properties necessary for pressure applications. ASTM standards like D1784 set minimum properties for pressure compounds.
In non-pressure applications, such as drain, waste, and venting (DWV) and sewer pipes, stiffness is a main characteristic rather than pressure resistance. Stiffness is proportional to the E-modulus of the material. A higher CaCO₃ level increases the E-modulus, resulting in a stiffer material. For structured wall pipes (like those complying with EN13476), using a stiffer formulation with higher CaCO₃ offers a double gain: a cheaper formulation and potentially a thinner wall while meeting stiffness requirements (e.g., SN4).
Foam core pipes, a type of structured wall pipe, often utilize significant amounts of CaCO₃ in the core layer. Formulations for PVC foam core pipe often contain up to 15-20 phr CaCO₃ to compensate for the lower stiffness of the foam core, helping the pipe maintain or even exceed the stiffness of a solid wall equivalent with a similar or thinner total wall thickness. ASTM F891 for Cellular DWV pipe, which refers to ASTM D4396 for non-pressure compounds, practically limits CaCO₃ to approximately 25 phr based on cell requirements. Higher filler levels in the core can also allow for more blowing agent while maintaining a fine foam structure, contributing to further cost savings. However, high filler loads can cause processing challenges like pourability problems (bridging) and feeder stalling.
|
Pipe Application |
CaCO₃ Inclusion Level |
Primary Property Enhanced |
Relevant Standard Examples |
|
Pressure Pipe |
Limited amount |
N/A (Strength critical) |
ASTM D1784 |
|
Non-Pressure / Structured Wall Pipe |
Higher levels helpful for stiffness |
Stiffness (E-modulus) |
EN13476, ASTM D4396 |
|
PVC Foam Core Pipe |
Up to 15-20 phr in core, approx. 25 phr limit (ASTM F891/D4396), higher levels helpful for stiffness |
Stiffness (compensates for foam), allows more blowing agent |
ASTM F891 |