Material of the month: PVC

Materials World magazine
,
7 May 2014

Rhiannon Garth Jones studies the history of this long lasting and durable material, found in construction, healthcare applications and packaging, as well as in our homes.

Many are the music fans who will tell you that ‘you just can’t beat vinyl’ and, while they mean the sound quality associated with old LPs, for the past 100 years it has been hard to argue against the idea that polyvinyl chloride (PVC) is a super-material. Since German inventor Friedrich Klatte patented a process to manufacture PVC in 1913, the material has become the third most produced and sold polymer globally.

It was accidentally discovered twice in the 19th Century, by French physicist and chemist Henri Victor Regnault in 1838 and by German chemist Eugen Baumann in 1872, both of whom left the newly discovered vinyl chloride gas exposed to sunlight, but on neither occasion did the breakthrough cross the so-called valley of death into commercial production. In fact, it wasn’t until the rising cost of natural rubber prompted American firm BFGoodrich to hire Waldo Semon, an industrial scientist, to develop a synthetic replacement that PVC really started to impress people. Production was threatened by the Great Depression, but sales increased again when Semon realised that PVC could be used as a water-resistant coating for fabrics. Demand rose hugely during WWII, when PVC was needed to replace traditional materials for insulating wiring on military ships.

Since then, its low cost, versatility and high durability have seen it used in various crucial applications – PVC bags were introduced to enable safe transportation of blood in 1950, PVC double-glazed windows were first installed in 1979, and more than 142,000m2 of PVC fabric was used in the construction of venues for the London 2012 Olympics. Closer to home, it has found applications in domestic flooring and the production of artificial leather. The material’s resistance to light, chemicals and corrosion has made it essential to the construction, healthcare and IT industries.

A petrochemical miracle
PVC can be produced from various hydrocarbons as well as derivatives of plants such as sugarcane, but globally most production of the thermoplastic uses ethylene, which has the chemical formula C2H4 and is made from the thermal cracking of naphtha or natural gas. PVC is therefore considered a petrochemical product. Ethylene is combined with chlorine to produce an intermediate chemical known as EDC (ethylene dichloride or 1,2-dichloroethane), which is then transformed into vinyl chloride, the molecules of which are polymerised to form chains. PVC produced in this way results in a white powder that is blended with other ingredients to give formulations for a wide range of products. Most commodity plastics have carbon and hydrogen as their main component elements. PVC differs by also containing chlorine (around 57%), which makes it flame retardant and more compatible with a wide range of other materials. It can also be used to distinguish PVC in automatic sorting systems for plastics recycling.

Petrochemical industrial facilities in western Europe are predominantly located in coastal areas or where rivers or pipelines have traditionally provided easy access to imported natural resources such as crude oil. The chlor-alkali industry is often located together with petrochemical complexes, meaning vinyl chloride monomer (VCM) and PVC plants, which use ethylene and chlorine as major raw materials, are generally also located in those areas.

Polyproperties
PVC has an amorphous structure with polar chlorine atoms in the molecular structure. Although plastics seem very similar in the context of daily use, the chemical structure of PVC means it differs significantly in performance and function from olefin plastics, which only contain carbon and hydrogen. Its amorphous structure and chlorine content enable it to mix well with various other substances, meaning its physical properties can be adjusted as necessary, increasing aspects such as impact resistance, elasticity and flexibility, by adding plasticisers and various additives, modifiers, and colouring agents. It is the only general purpose plastic that allows for such straightforward, wide adjustments.

Due to its molecular structure, where the chlorine atom is bound to every other carbon chain, PVC is highly resistant to oxidative reactions, maintaining its performance for a long time. In 1995, researchers in Germany dug up and analysed soil-buried pipes after 60 years of active use and noted they were likely to remain fit for purpose for another 50 years. PVC is resistant to acid, alkali and almost all inorganic chemicals. It is affected by aromatic hydrocarbons, ketones, cyclic ethers and swelling or dissolving, but is hard to dissolve in other organic solvents. This characteristic has contributed to its popularity in the construction industry, as it can be used in exhaust gas ducts, bottles, tubes and hoses.
Despite being a viscoelastic material, PVC suffers very little from creep deformation or changes in mechanical strength, because of its limited molecular motion in its amorphous structure at ordinary temperatures.

The future is here?
In recent years, concerns have been raised over the inherent problems in the production process. PVC is insoluble in its monomer, which means it precipitates as it forms, and the reaction is highly exothermic. Traditionally, suspension polymerisation has been used to address these issues. Monomer molecules are transformed into free radicals by an evenly distributed initiator compound, with each one adding further monomer units to become a chain. A water phase is used for distribution, which helps to control the temperature as well as ejecting the insoluble polymer grains. However, thermal runaway remains a significant safety concern. The only way to control the reaction once it has begun is by cooling, but this is an inefficient process.

A method called continuous initiator dosing (CID) has been developed to increase the efficiency of PVC production. CID controls the reaction by the rate of addition of the initiator rather than by cooling, at a rate that is determined by what is going on in the reactor. A feedback loop with the temperature and pressure of the reactor controls the rate at which the initiator is fed. Not only does this increase the rate of polymerisation by up to 30%, but the process can be stopped by turning off the flow of the initiator, making CID much safer. However, since its launch in 2007, CID hasn’t seen a great take-up.

How and when the problems with the production process will be solved remains to be seen, but it is clear that the demand for PVC isn’t going to slow down anytime soon. It is predicted that more than 800,000 tonnes of PVC will be recycled each year in Europe alone by 2020, and the Qatar Showcase Stadium for the 2022 FIFA World Cup has been designed using super-reflective, triangulated PVC fabric. Science is full of tales of the accidental wonder-discovery, and there can be no doubt that PVC deserves its place in that illustrious category.