Polyvinyl chloride (PVC) is one of the six commonly used plastics (which accounts for as much as 10% of global plastic production). PVC is a popular plastic due to its low price, durability, and good mechanical, chemical, electrical, and thermal properties. The global production of PVC in 2009 amounted to approximately 34 million tons. At the global level, PVC production in 2015 exceeded 35 million tons, and the annual growth was forecast at approximately 2%. At that time, the European PVC consumption was approximately 7 million tons per year. In 2022, PVC capacity was 59.97 million tons. The market is expected to achieve an annual growth of more than 3% during 2022–2027.

This plastic is strong, durable, long-lasting, lightweight, and versatile, so it is widely used in many industries, such as in construction, automotive industry, pipes and cables, and household goods. The service life of polyvinyl chloride in construction is more than 10 years. PVC can present a number of challenges at various stages of its life cycle, particularly at the waste stage. It is considered as the most environmentally damaging plastic and one of the most toxic substances for inhabitants of our planet. From cradle to grave, the PVC lifecycle (production, use, and disposal) results in the release of toxic, chlorine-based chemicals, and it is one of the world's largest dioxin sources. These toxins build up in water, air, and food chains. They cause severe health problems, including cancer, immune system damage, and hormone disruption. Everyone has measurable levels of chlorinated compounds (toxins) in their bodies.

Disposal Methods

Dechlorination

It seems promising to carry out complete dechlorination processes before degradation. Owing to full dechlorination, polyvinyl chloride can be treated in the same way as common halogen-free plastics. Such methods include chemical modifications, the near-critical methanol process for PVC dechlorination and recovery of additives, and the near-critical process using an aqueous ammonia solution. Among these techniques, the well-known method is the chemical modification of PVC through the substitution of some chlorine atoms with various nucleophilic reagents. Another technique used to convert waste into energy with simple, fast reactions is hydrothermal treatment. In this technique, super- or sub-critical water is used as a solvent and reagent for the reaction of organic compounds.

Moreover, despite the frequent use of certain chemicals in the hydrothermal dechlorination of polyvinyl chloride waste, their role in this process has not been fully understood. Analyses conducted by Zhao et al. with the use of Na2CO3, KOH, NaOH, NH3·H2O, CaO, and NaHCO3 in water containing Ni2+ showed that the alkalinity of the additives has a significant impact on the effectiveness of the dechlorination process. The most effective additive in these studies was Na2CO3 (concentration 0.025 M), with a maximum efficiency of 65.12%. The processes carried out using subcritical water-NaOH (CW-NaOH) and subcritical water-C2H5OH (CW- C2H5OH) proved that the main mechanism in the case of the dechlorination in CW-NaOH is the nucleophilic substitution of hydroxyl group in PVC, while in CW-C2H5OH—the nucleophilic substitution and direct dehydrochlorination were the equally significant processes. The key parameter of the dechlorination process is temperature. As the efficiency of this process also decreases with a decrease in temperature, the above-mentioned additives were used to improve the efficiency. Unfortunately, the incorporation of the additives not only increased the costs of the dechlorination process, but also generated secondary pollution. Temperature was also shown to be important in the removal of chlorine (Cl) from PVC in gas–liquid fluidized bed reactor studies where hot N2 was used as the fluidizing gas to fluidize the polymer melt.

Photodegradation

Although poly(vinyl chloride) is a commercially important polymer, it is also one of the most sensitive to UV radiation. The UVA radiation in deionized water, sea sand, and air was used to photodegrade plastics. The results showed that polyvinyl chloride effectively absorbs the UVA radiation in air, and this is where the ageing efficiency was the greatest. The ageing process included photoinitiation, chemical bond breaking, and oxygen oxidation.

Under the influence of UV radiation (in the wavelength range of 253–310 nm) and in the presence of oxygen and moisture, PVC underwent very rapid processes of dehydro-chlorination and peroxidation to form polyenes. The irradiated material crumbled, lost its stretch, elasticity, and impact resistance, and the surface of the degraded polymer was significantly modified, i.e., loss of abrasion resistance, gloss, and interfacial free energy were observed.

The use of the photodegradation process makes it easier to dispose plastics from the environment. In order to accelerate the photodegradation of plastics, semiconductor photocatalysts such as TiO2, ZnO, Fe2O3, CdS, and ZnS were also used. For example, it was observed that the addition of ZnO to polyvinyl chloride increased the decomposition of the composite by 4.13% in the case of artificial UV radiation and by 9.7% in the case of solar radiation, respectively. A photodegradable composite film was prepared by doping poly(vinyl chloride) plastic with nano-graphite (Nano-G) and a TiO2 photocatalyst. After exposure to the UV radiation (for 30 h), the weight loss rates of Nano-G/PVC, TiO2/PVC, and Nano-G/TiO2/PVC films were 7.68%, 8.94%, and 17.24%, respectively, while pure PVC decreased its weight by only 2.12%.

Reference

[1] Kudzin, M.H.; Piwowarska, D.; Festinger, N.; Chru?ciel, J.J. Risks Associated with the Presence of Polyvinyl Chloride in the Environment and Methods for Its Disposal and Utilization. Materials 2024, 17, 173.