The impact of PVC stabilisers in recycling and waste disposal
Evidence does not suggest that disposal of PVC waste products, by means such as landfill, gives rise to hazardous soil contamination. Nevertheless, recovery schemes are being developed which maintain control of the additives and prevent their release into the environment.
Separated and cleaned post-use PVC products can easily be recycled into new products. The applications which incorporated heat stabilisers based on lead and cadmium were almost all long-life applications but, in many cases, are already entering the waste stream and are being recycled to some extent. For example, the PVC pipe, flooring, roofing membranes and window industries in Europe already have schemes developed which are aimed at collecting and recycling these products when they have reached the end of their useful life. Detailed information on these schemes can be found on the site https://vinylplus.eu.
The recyclate is then be incorporated into the same or similar long-life products. In the case of waste containing mixed plastics or products of a multi-material construction thatcannot, due to economic reasons, be be separated into pure polymers, such as coated fabrics, other methods such as feedstock recycling are being developed. The Vinyloop® process, which was applied to a commercial scale, produces recycled PVC compound from PVC waste contaminated with other polymers and can also handle some composites. Detailed information can be found at www.vinyloop.com. Feedstock recycling remains an option for waste which is too complex or too contaminated for mechanical recycling.
A number of European standards are being written that cover the properties of PVC construction products made,a sector which constitutes more than two thirds of the PVC market. These now recognise the fact that recyclate, which inevitably will contain a range of stabilisers, will become available for use in these products. The European standards also include test requirements to ensure the appropriate performance of the final product.
Feedstock recycling must be seen as complementary to mechanical recycling for two reasons: first to have technologies in place which are less sensitive to unsorted or contaminated waste products; and, second, to enlarge the overall recycling capacities for the larger waste quantities of the future.
In fact, there are a number of compositions or multiple material products which, when returned for recycling, cannot be economically sorted into generic (single) polymer streams. Examples of these are laminated films, ‘leather cloth', footwear or car dashboards, where the construction includes a number of different plastic and non-plastic materials.
These materials are intimately connected to each other for performance reasons but economic separation is not yet feasible.
The feedstock recycling processes operating today treat mixed plastics waste from packaging sources. This means with a PVC-content up to 10 per cent. The ‘thermal cracking’ of this plastic waste stream can be done via hydrogenation, pyrolysis or gasification. Since the recovered hydrocarbon products are mostly used in petrochemical processes, the specifications limit the amounts of halogens in the waste below limits varying usually between 0.1 and 1%. One way to achieve this is to pre-treat the waste.
Such pre-treatment consists of a sorting or separation step of the used input - that allows adjusting the chlorine content of the main waste stream. A second possibility is thermal or chemical dehalogenation before the pre-treated product is further processed. This dehalogenation takes place either in a liquid or in a fluidised bed pyrolysis. The hydrochloric acid produced is neutralised or separated for industrial use.
The chlorine content of the resulting hydrocarbon feed can vary depending on the final processing step which is the determining factor. It is also common to dilute the chlorine-containing hydrocarbon feed with chlorine-free petroleum fractions coming from refineries. Thus relatively low PVC/chlorine content, as found in mixed plastics waste (coming mainly from short life applications such as packaging etc) is acceptable for the existing feedstock recycling processes as long as an appropriate pre-treatment of the plastics waste is guaranteed.
The feedstock recycling of a waste stream in which PVC is the dominant material (>30 per cent) - for example the multiple-material products as described earlier - has to be designed primarily for the recovery of hydrochloric acid, but also to recover the hydrocarbon content and/or energy.
Some commercial installations capable of handling large amounts of chlorinated waste are operating in Europe. A number of other processes have been proposed to specifically deal with this matter, such as, gasification in a metal or slag bath, and pyrolysis in a circulating fluidised bed. Most are at the concept or early development stage, but some have reached pilot plant stage, notably the process developed by Sumitomo Metals in Japan. A major operating step is to purify the HCI so that it can be fed as a gas to the oxychlorination unit in vinyl chloride monomer plant, the feed product for making new PVC, or to other chemical processes.
More information can be found in the PVC recycling brochure of VinylPlus.
Incineration with energy recovery
The use of incineration, together with the associated energy recovery, plays a complementary role to recycling. Heat stabilisers found in PVC can potentially cause residual ash to emit dust and leachate containing harmful quantities of toxic metals. A number of independent studies have shown that relevant control technology has to be used whether PVC waste is present or not, as waste sources other than waste PVC also contain metals producing toxic emissions. It is also relevant to note that the vast majority of PVC waste fed to incinerators originates from PVC packaging applications which do not, and have never, contained lead or cadmium.
PVC waste in landfill sites predominantly originates from household and packaging applications that contain calcium/zinc and tin stabilisers, but which have never contained lead or cadmium based heat stabilisers. In any case, the amount of PVC found in a typical waste site is less than 1 per cent of the total waste, and the content of lead or cadmium based stabilisers will generally not exceed 2 % of the total PVC quantity. Even when waste from construction products is present, such as pipes and window profiles that were stabilised with lead and/or cadmium compounds, studies have shown that minimal to no leaching into the soil occurs, even under aggressive soil conditions. The levels detected of leaching elements are well below the natural levels of these elements found in the soil. Furthermore, certain co-stabilisers used in calcium/zinc and barium/zinc systems are known to biodegrade in landfill sites.
The PVC industry commissioned a three year research programme conducted by independent institutes in Germany and Sweden on the long-term performance of PVC waste in landfill, covering a wide range of applications and stabiliser systems. The results showed that the contribution of PVC products to the quantity of heavy metals in municipal solid waste is low. Despite the fact that organotin compounds (tin) do originate from PVC, the levels found in landfill leachate (water that exits landfills) do not exceed the Predicted No-Effect Concentration (PNEC).