Recycled PVC is used effectively in producing structured wall pipes, specifically as the core material in multi-layer "sandwich" constructions. Foam core pipe is a prime example of this application. This approach leverages the principle that in certain pipe designs, the inner layers contribute less to stiffness and strength than the outer layers.
The core layer of a structured wall pipe carries a much lower load than the outer and inner skins. The source material explicitly states that the inner 50% of the pipe wall cross-section is less effective than the outer 50% in resisting bending loads; the outer 50% contributes 87.5% of the stiffness in a solid wall pipe. This load distribution makes the core a suitable, economical place to integrate recycled or lower-grade materials.
Using recycled PVC in the core layer contributes directly to sustainability by reducing the amount of virgin polymer required. It also helps lower cost per pipe length.
Foam core pipe formulations for PVC often include CaCO₃ (up to 15-20 phr) to compensate for the foamed core's lower stiffness and maintain overall pipe stiffness, sometimes even allowing thinner walls than solid-wall equivalents. Adding higher levels of CaCO₃ in the core can help meet stiffness requirements. Furthermore, a higher filler level often allows more blowing agent without creating blisters, improving weight and cost savings.
Processing recycled PVC, particularly post-consumer material, for use in foam cores presents specific challenges. Pipe-grade PVC (K67), often the source of post-consumer recycled pipe, is harder to foam than injection-molding-grade (K57). Specialized extruders and dies are needed to process and foam recycled K67 PVC. Contaminants like sand or rubber must be managed through material sourcing, homogenization, micronizing, and appropriate processing equipment.
ASTM F891 applies specifically to ABS Cellular DWV pipe, and other standards like JIS 9798 and SANS 1601 cover foam-core (cellular-core) pipes. EN13476 allows a minimum of 60% PVC in a sandwich core.