Use of Recycled PVC in Sandwich Cores

Recycled PVC as a Core Material in Structured Wall Pipes

Recycled PVC is effectively utilized in the production of 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.

Advantages and Rationale

The core layer of a structured wall pipe carries a significantly lower load compared to 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 and economical place for integrating 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 is also a method to achieve lower cost per pipe length.

Formulation and Processing for Recycled Cores

Foam core pipe formulations for PVC often include CaCO₃ (up to 15-20 phr) to compensate for the lower stiffness of the foamed core and maintain overall pipe stiffness, sometimes even allowing for thinner walls compared to solid wall equivalents. Adding higher levels of CaCO₃ in the core can help meet stiffness requirements. Furthermore, a higher filler level often permits the addition of more blowing agent without creating blisters, enhancing 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 moulding grade (K57). Specialized extruders and dies capable of processing and foaming recycled K67 PVC are necessary. Contaminants like sand or rubber must be managed through material sourcing, homogenization, micronizing, and appropriate processing equipment.

The ASTM F891 standard applies specifically to ABS Cellular DWV pipe, and other standards like JIS 9798, SANS 1601 exist for foam core (cellular core) pipes. EN13476 allows a minimum of 60% PVC in a sandwich core.

Technical Specifications/Considerations:

• Application: Core layer of structured wall/sandwich pipes (e.g., foam core).
• Load Distribution: Core carries lower load than skins.
• Typical Additives (Core): CaCO₃ (up to 15-20 phr), blowing agent.
• Processing Difficulty: Foaming K67 PVC is challenging.
• Standards: EN13476 (min 60% PVC in core), ASTM F891, JIS 9798, SANS 1601 for foam core.

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