Shredding, densimetric separation, plasticizer management and washing of recycled PVC

Recycled PVC – Technical Manual - Chapter 7: Sorting and Pretreatment Technologies in PVC Recycling. Physical Control, Separation, and Material Stability

Grinding and particle size control

In mechanical PVC recycling, shredding is not a simple preliminary size reduction operation, but a structural step that profoundly impacts the entire downstream supply chain. It is during this phase that the material is physically "prepared" for all subsequent operations: separation, washing, drying, and extrusion. In the case of PVC, more than other polymers, the quality of shredding directly impacts the final quality of the granule, often irreversibly.

PVC exhibits a combination of characteristics that make shredding a delicate operation: high rigidity in the case of rigid PVC, viscoelastic behavior in plasticized PVC, sensitivity to heat, and the frequent presence of hard impurities. Unlike polymers such as PE or PP, PVC does not tolerate excessive mechanical stress accompanied by localized overheating. Aggressive shredding, poorly controlled, or unsuitable for the type of material, can trigger premature degradation, which is difficult to reverse in subsequent stages.

From an industrial perspective, PVC shredding must be designed with the end goal of recycling the material in mind, not simply to "reduce its size." The resulting particle size affects the quality of the densimetric separation, the effectiveness of the washing process, the drying speed, and the stability of the extrusion feed. An error at this stage propagates throughout the line, amplifying costs and problems.

In rigid PVC, shredding aims to obtain a sufficiently regular particle size, free of excessive fine particles. Profiles, pipes, and sheets have very different initial geometries, and non-uniform size reduction leads to an unbalanced particle size distribution. The presence of a high percentage of dust or excessively fine particles is one of the main problems in shredding rigid PVC: these particles tend to concentrate during the separation phases, absorb moisture abnormally, and create instability during extrusion.

With plasticized PVC, the problem is different but equally critical.

The softer and more flexible material tends to deform during shredding rather than fracture cleanly. This behavior often leads to the formation of irregular particles, filaments, or agglomerates that make particle size classification difficult. Furthermore, with soft PVC, the risk of local overheating is high, especially with high rotation speeds or unsuitable blades. The heat generated can promote the surface migration of plasticizers, altering the quality of the material already during the pre-treatment phase.

Controlling particle size is therefore not a secondary detail, but one of the most important indicators of pre-treatment quality. A particle size distribution that is too broad indicates poorly controlled grinding and hinders subsequent operations. Conversely, an excessively fine particle size distribution, obtained by "forcing" the grinding, increases the material's specific surface area, making it more susceptible to oxidation, contamination, and moisture absorption.

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