Each GRACE newsletter will explore the recyclability of a plastic used in the Large Format Print & Sign sector.
Today we look at the recyclability of polyvinyl chloride (PVC), also known as vinyl.
This material is one of the most widely used plastics in the world and is used in pipes, window frames, cables, flooring, medical equipment and visual communication.
Within the LFP&S sector a wide range of substrates are made from PVC or contain a PVC coating examples are foamed sheets (Forex®), mount films, shrink films, blockout banner (PVC coated banner), frontlit (PVC coated banner), mesh banner, etc.
The reason for this common use is the combination of:
- Price: PVC is a relatively cheap material, cost/kg resembles PP or PE
- Lightness: PVC is heavy for a plastic material (density of 1,4 g/cm³) but still light compared to for example metallic substrates. Additionally PVC is commonly foamed to reduce the density and further decrease the price.
- Durability: PVC has a good resistance to light and a wide range of chemicals making it an excellent choice for durable in- and outdoor materials.
- Flame retardancy: PVC is intrinsically flame retardant
- Versatility: PVC can be tailored from rigid to flexible by varying the plasticizer (an additive that improves flexibility) content.
However, PVC also has some limitations. It has a limited temperature stability which means it requires the addition of stabilizers to process the material. In the case of flexible PVC, the flexibility of the material will decrease over time due to migration of the plasticizer.
When PVC reaches its end of life it can be recycled, due to its common use PVC has a long history of specialized mechanical recycling. However, due to the long lifespan of PVC it can contain “legacy chemicals” (additives that can no longer be used, such as lead as stabilizer).
These contaminated streams of PVC need to be cleaned during the recycling processes or those PVC’s need to go towards energy recovery instead of recycling. Important to note is that PVC contains Chloride, which causes the release of hydrochloric acid during the incineration or recycling of PVC.
Energy recovery and recycling facilities must and will be adapted to the release of this acid.
The PVC recycling process can be separated in 4 steps
1) Collection and sorting
Waste materials need to be collected and sorted to isolate the PVC fraction. Difficulties related with collecting and sorting will be addressed in the future within the GRACE project.
2) Shredding materials
The aim of this step is to reduce the size of materials down to flakes of 4-6mm so they can be fed into the subsequent processes. During this step the PVC can also be washed to remove dirt, additionally this washing step also allows to separate PP and PE contaminations since PP and PE will float in water while PVC will precipitate.
3) Recycling
Recycling can be split into mechanical recycling (recovers plastic) and chemical recycling (breaks down plastic into building blocks that can be used to make virgin plastic).
a. Mechanical recycling
Mechanical recycling by extrusion has been an established was to recycle PVC for decades. During this process the PVC is molten and reprocessed into ready to use pellets. During this process PVC will slowly degrade, but due to the presence of stabilizers and good process control PVC can be recycled 10 times before it loses its desired technical performance. Additionally this process can be combined with a chemical extraction to extract legacy chemicals or plasticizers but those processes are not conventional.
Another mechanical recycling process is by dissolution, in which the PVC is dissolved in solvent and subsequently precipitated in water, which allows the removal of other materials and impurities (such as colourants or inks). Afterwards the cleaned PVC is pelletised for further processing. An example of this process is the “project circle” by INEOS.
b. Chemical recycling
During the chemical recycling process the PVC is broken down completely at high temperature into syngas, a feedstock for a wide range of chemicals, and hydrochloric acid. An example of this process is SOLVAir, which converts PVC into syngas and sodiumchloride (table salt). In general chemical recycling is only advised when a material cannot be recycled mechanically, for example due to the amount of contaminations or degradation of the PVC. In these cases, breaking down the PVC into syngas and salt is a viable method to give these materials a new life.
4) Extruding into material
Finally the mechanically recycled PVC can be extruded into new products, if this r-PVC contains solid particles (such as ink residues) it will be important to determine their size. Depending on thickness of the PVC material to be produced these solid contaminations will block the filters and dies during the extrusion process. As a rule of thumb, the contaminations should be 3 times smaller than the extruders filter or die openings. Additionally the PVC purification could have resulted in the extraction of plasticizers or other desired additives, if needed these should be readded into the PVC before producing new products.