Polycarbonate (PC) (C15H16O2) Plastic Recycling

Topics Covered

Introduction
Chemical and Physical Properties of PC
Manufacturing Process of PC
Applications of Virgin Material
Environmental Impacts of PC
Recycling Process of PC
Applications of Recycled PC
References

Introduction

Polycarbonate or PC was developed initially in 1953 by GE and Bayer in the US and Germany, respectively. LEXAN is its most popular trade name. PC is a heterochain polymer featuring high performance. PC is an excellent material choice as it is not just high-performing but also can be recycled and produced in an eco-friendly manner.

A PC molecule includes a Bisphenol A part and a carbonate group. Bisphenol A has two aromatic rings rendering PC high strength. It does not crystallize easily due to the Bisphenol group. The polymer attains its transparency due to this amorphous structure.

Molecular formula - C₁₅H₁₆O₂

Density - 1.2 – 1.22 g/cm³

Melting point – 155°C

IUPAC ID - 4-[2-(4-hydroxyphenyl)propan-2-yl]phenol

Chemical and Physical Properties of PC

The properties of PC are listed below:

• PC features high-strength making it impact- and fracture-resistant
• It is non-toxic, can be colored easily, and transparent up to 2 in. in width.
• PC features high heat and electrical resistance
• It is inert biologically
• Can be recycled
• Features good resistance to dilute acids, alcohols, mineral oils and vegetable oils
• Reacts easily with bases, concentrated acids, esters, aromatic and aliphatic hydrocarbons, ketones, halogenated hydrocarbons and oxidizing agents
• Can be solvent bonded, joined mechanically and welded

Manufacturing Process of PC

A step-growth polymerization is used to synthesize PC from phosgene and Bisphenol A in which elimination of Cl ions are done every time the monomers react. This is popularly known as condensation.

Firstly, bisphenol A groups are made to react with proton acceptors like NaOH to result in polymerization functional groups. At a temperature range between 25 and 35°C, phosgene and a catalyst react with the deprotonated Bisphenol A. This forms a PC monomer and the catalyst mostly pyridine is removed with the chloride anion.

In an alternate method, bisphenol A and diphenyl carbonate (R-O-CO-O-R) are made to react in a temperature range between 180 and 220ºC resulting in a phenol molecule and PC. However, this process generates more impurity and incurs more expense especially since high temperatures are not needed.

PC is then converted from pellets to the desired shape by melting it and forcing it into a die or mold to provide the needed shape based on the application. This process is done repeatedly.

In the extrusion process, the molten PC is passed through a die giving the material its final shape. The melt is rapidly cooled after this. Using this process, long pipes and sheets are formed.

In the molding process, the PC melt is transferred to a mold then cooled and this process is ideal for computer and automotive components.

Applications of Virgin Material

The applications of PC are:

• Since PC has an excellent strength, it is suited for shatter-resistant or bullet-resistant glass applications
• Its relatively low weight when compared to other high-strength materials and its increased ductility enables its use in windows and lenses
• It has been also used as an electrical insulator and a flame retardant
• It can be used in small filters for the extrusion of small particles
• PC finds extensive use in manufacture of DVDs, CD’s, automotive, cell phone, and laptop components.

Environmental Impacts of PC

PC is commonly used to manufacture CDs, DVDs, automotive components and also the manufacture of baby bottles and water bottles. However, due to the presence of bisphenol A, many manufacturers have stopped using polycarbonates for applications related to human intake.

PC in landfills is definitely an environmental nightmare. It never biodegrades since it is made of petroleum. The chemicals leach into the environment causing significant damage. Many of these chemicals are consumed by marine organisms and other animals resulting in a lot of damage.

It is of concern that the accidental dumping of plastics into oceans have resulted in mammals and birds starving to death and fish ingesting toxins that may make them dangerous to eat. Annually, large amounts of CDs are discarded worldwide. The non-biodegradable polycarbonate results in serious pollution issues. Although there are methods being devised to recycle CDs, these are not very effective and novel methods are being tried out.

Recycling Process of PC

Polycarbonate is coded 7 implying that it is difficult to recycle. However, a lot of research has been done in this regard and polycarbonate bottles and CDs are being extensively recycled.

One method of recycling polycarbonate is by chemical recycling. PC is made to react with phenol in the presence of a catalyst to form BPA and DPC monomers. After purification, both these monomers are used to produce the polymer.

A CD includes at least 95% polycarbonate with the dye and reflective layers on the surface. The dye layer has organic dyes such as the metal azo and cyanine while the reflective layer contains metals such as aluminum for laser reflection. A novel recycling method was recently devised and that has been discussed below:

The CD surface is burnished and the reflective and the dye layers are burnished into powders that are collected for further treatment. The burnished disc body includes mainly polycarbonate with minimal amount of impurities like metals and dyes. The burnished CDs are then subjected to washing with ethanol that acts as a solvent for dissolving the dyes. The dyes that get dissolved in ethanol are treated with Fenton’s reagent. NaOH is added for iron ion precipitation or for any other metal ions in the solution. The remaining solution will contain very few impurities. Finally, the remains are made to react with concentrated nitric acid. By electrolysis, the metals are recovered, while polycarbonate is left as it is.

Applications of Recycled PC

Recycled PC may show lesser resilience and has reduced impact resistance when compared to newly manufactured PC. Addition of fillers and pigments may reduce the resilience of the plastic. Chemicals can be used to improve impact resistance in recycled polycarbonate and they could be used for most of the applications that virgin polycarbonate is used.

Recycled PC finds applications in manufacture of plastic profiles. They have good stiffness, impact resistance, toughness and are transparent. These profiles have a high colorability with a lovely gloss finish that can imitate metal and glass.

References

www.eng.buffalo.edu/Courses/ce435/PC_CB.pdf

http://dhss.delaware.gov/dph/files/bisphenfaq.txt

alexandria.tue.nl/extra2/200411296.pdf

resources.edb.gov.hk/~science/hkcho/11s/singyin_summary.pdf