Material of the month – Polycarbonate
From buildings and cars to music and kitchenware, this month, Anna Ploszajski looks at the recyclable plastic polycarbonate.
Polycarbonates are a group of thermoplastic polymers that contain repeating carbonate groups in the polymer chain backbone. They were first reported in 1898 by Alfred Einhorn, a German chemist at the University of Munich, who combined hydroquinone with phosgene in an effort to produce cyclic carbonates. Unfortunately, the material failed to be commercialised. It wasn’t until 1953 that the trail was picked up again, when Hermann Schnell at Bayer AG in Germany developed a synthesis for a linear polycarbonate by combining two chemicals - bispenol A and phosgene.
Meanwhile, Daniel Fox at General Electric in New York, USA, was hot on Schnell’s heels, independently synthesising a branched polycarbonate material. Both companies filed for patents in 1955, but soon realised that only one of them could be granted it.
Before the decision came from the patent office, the two companies set up an agreement, stipulating that whoever received the US patent would grant a licence to the other for prioritised access to the technology. Bayer was granted the patent, having beaten General Electric to filing by just one week, and began commercial production in 1958 under the trade name Makrolon. As promised, General Electric used its licence to follow with Lexan in 1960.
The strength, toughness, durability, ease of forming, transparency and electrically insulating properties of early polycarbonate initally saw it applied in casings for electronics, such as telephones, plugs and the lights on emergency vehicles. By 1971, Bayer had found a way to eliminate the brown tint of transparent Makrolon and produce a material as clear and colourless as glass, but lighter in weight and with greater impact resistance. They produced this new polycarbonate in sheets for a diverse range of uses – from police riot shields to conservatories and greenhouses.
In order to widen the market, Bayer started to experiment with polycarbonate blends. In 1976, they began to market blends of polycarbonate with acrylonitrile butadiene styrene (ABS), the same material which Lego is made from, called Bayblend. This material benefited from the synergetic effect of being easier to process.
Other blends followed, allowing Bayer to pioneer components for medical applications, making use of the fact that polycarbonate can be sterilised by steam at 120°C, by gamma radiation or by the ethylene oxide method. Today, polycarbonate blends are used to make durable lightweight components in automobile parts such as bumpers, headlamps and sunroofs, as well as for reusable water bottles, furniture and housings for laptops and mobile phones.
In 1981, the first CDs, made from polycarbonate, were produced. To make a CD, polycarbonate is injection moulded using a circular die, which has a metal master containing a negative image of the digital disc data on one side. The other side is coated with a thin layer of aluminium using vacuum deposition and, finally, an acrylic coating is applied to protect the underlying data from scratch damage. Polycarbonate is still used in the production of CDs, DVDs and BluRay discs.
Where will all these discs end up, once they become a relic of a bygone era? The good news is that, as a thermoplastic, polycarbonate can be easily melted down and reshaped. The BPA component can also be recovered from used polycarbonate to make new material. There have even been instances of polycarbonate being biodegradable. In 2001, it was reported that a species of fungus – Geotrichum Candidum, consumes the polycarbonate in CDs under certain conditions.
As well as automotive applications, polycarbonate is used as a lightweight safety glass, due to its high transparency. For example, in lab safety goggles and in the lenses of swimming goggles. Polycarbonate can be laminated with glass to make bullet-proof windows for cars, screens for bank cashiers and windscreens in small vehicles such as motorcycles, golf carts and helicopters. The Lockheed Martin F-22 Raptor jet fighter is equipped with a cockpit canopy made from the largest piece of high optical quality polycarbonate ever formed.
Schnell’s original synthesis involved the reaction of bisphenol A (BPA) with phosgene (COCl2), and this route is still used today to make the three million tonnes produced annually.
Various additives can be incorporated into polycarbonate to produce different effects. For example, fatty acid esters of higher alcohols act as mould-release agents that aid the production of complicated components. Tetrabromobisphenol is added as a flame retardant, and light stabilisers, such as benzotriazole derivatives, absorb visible and UV light to prevent decomposition. Glass fibre reinforced composites improve the rigidity, stiffness and coefficient of thermal expansion of polycarbonate. Other additives include heat stabilisers, blowing agents that make foamed structures, dyes and pigments.
A second processing route exists called the melt transesterification process – the reaction between BPA and diphenyl carbonate. Although this route has not yet been scaled up for commercial production, it has gathered renewed interest recently, since it is more environmentally friendly than the phosgene route.
Polycarbonate can be fabricated into many different shapes with various techniques, including extrusion and injection moulding, as well as secondary processes, such as bending, drilling and laser cutting. It has the advantage of being able to undergo large plastic deformation without cracking or breaking, unlike most other thermoplastics. It can also be used in 3D printing for rapid prototyping. However, care needs to be taken to avoid hydrolysis during melt processing, where the product can seriously degrade if the precursor material is not dried properly.
The ease of forming polycarbonate makes it suitable for making cheap, lightweight food and drinks containers, such as injection moulded bottles, glasses or tupperware. However, the use of polycarbonate in this setting is controversial, as it degrades at high temperatures in the presence of water, resulting in the release of BPA. Many studies have sought to assess the effect of BPA on the body, with some results suggesting that it is oestrogen-mimicking and may cause hormone-related harm to consumers. Governments have reacted differently to the evidence although, as it stands, the UK Food Standards Agency states that the minute amounts released by containers in contact with food and drink is well below the safe level set by the European Food Safety Authority.
A suitable fitting
A huge consumer of polycarbonate today is the construction industry – its strength and optical properties see it used in many architectural features, including domed skylights, canopies and windows. Marion Ingle, a polymer consultant from Sandberg construction consultants, said of polycarbonate in the industry today, ‘Most of the failures of polycarbonate I’ve seen over the years relate to these issues in processing.’ Ingle went on to explain that the high viscosity of polycarbonate during injection moulding can lead to a large amount of molecular orientation during mould filling, which then gets frozen in during cooling. This results in high levels of residual stress in the product, which can lead to environmental stress cracking down the line. Ways to combat this include higher mould temperatures and sufficient cooling times, to effectively allow the material to anneal in the mold, but often commercial pressures won’t allow this, which results in inferior mouldings.
What would be her take-home message for anyone working with polycarbonate? ‘It is essential to consider every substance that the polycarbonate could come into contact with at the design stage.’ The integrity of polycarbonate can be compromised by anything from sealants and adhesives to medical liquids and cleaning fluids. If these aren’t taken into consideration from the outset, the results could be catastrophic.
Beginning with an unusual corporate negotiation, polycarbonate has become a crucial component of our environment, from buildings to cars via music and kitchenware. As a recyclable plastic, it seems likely that its position in the marketplace will only get stronger.