Polycarbonate Polymers: History, Synthesis, Properties, Pros and Cones, and Applications

Polycarbonate polymers (PC) have carbonate groups in their chemical structures. They are basically a type if thermoplastic polymers. These polymers are strong and tough materials and are easily thermoformed and molded, so they have a wide use in engineering applications. They are also widely used in many other wide range applications i.e. electronic, data storage, construction, aircraft etc.

Figure 1: Transparent Polycarbonates

History

The discovery of Polycarbonate dates back to 1898 when Alfred Einhorn, a German chemist, observed the formation of an insoluble, infusible solid, while endeavouring to prepare cyclic carbonates by reacting hydroquinone with phosgene.

In 1953, polycarbonate was discovered independently by Dr. H. Schnell of Bayer AG, Germany and by D. W. Fox of General Electric Company, USA. In Pittsfield, Massachusetts, a GE (General Electric) scientist, Daniel W. Fox inadvertently discovered PC while working on a wire-coating material. He synthesized a branched polycarbonate. That marked the beginning of Lexan polycarbonate. In Uerdingen, Germany, Dr. Hermann Schnell discovered polycarbonate at Bayer's main lab at a time when the company was searching for ways to advance its technology. Schnell's find marked the start of the company's Makrolon polycarbonate.

Both companies filed for U.S. patents in 1955. Bayer began commercial production under the trade name Makrolon in 1958 and GE began production under the name Lexan in 1960. In 1960, it found its use in emergency vehicles, such as police, fire and ambulance vehicles as "bubble lights".

In 1970, it began being used for astronaut helmet visors and space shuttle windshields. In 1971, the cognac-colored tint of polycarbonate is eliminated. The new material is clear as glass, allowing natural daylight to flood into rooms. In 1982, the first audio-CD was introduced to the market, quickly replacing audio tapes which was made from polycarbonate. In 1987, use of polycarbonate for a variety of medical devices, including blood oxygenators, blood reservoirs, blood filters and kidney dialysis cartridges began.

In 2010, polycarbonate grades for furniture are introduced. To this day, Bayer and GE continues developing solutions that improve the quality and ease of our lives through innovation and the realization of dreams.

Synthesis

PC is most often synthesized from Bisphenol-A and phosgene by a step-growth polymerization in which Cl- ions are eliminated every time the monomers react.

This kind of step-growth polymerization is often called a condensation process. The first step of the synthesis involves treatment of Bisphenol A with sodium hydroxide, which deprotonates the hydroxyl groups of the Bisphenol A.

The deprotonated Bisphenol A reacts with Phosgene and a catalyst at temperatures between 25 and 35ºC. This way, a Polycarbonate monomer is formed, and the catalyst (often times Pyridine), is eliminated along with the chloride anion.

The overall reaction can be written as follows:

Properties

• High Durability: Polycarbonate plastic is an extremely durable material. This makes it the material of choice where long product life and reliable performance are critical.
• Shatter Resistance: Polycarbonate is virtually unbreakable.Through its high impact resistance, it provides greater safety and comfort for applications where reliability and high performance are essential.
• Transparency: Polycarbonate is an extremely clear plastic that offers excellent visibility and transmits light better than alternative materials.
• Lightweight: Polycarbonate's light weight allows for architectural and design creativity. This leads to increased resource efficiency and reduced financial and environmental costs for transportation.
• Thermo-stability: Polycarbonate plastic provides excellent heat resistance, which facilitates good hygiene conditions when cleaning at higher temperatures.

• Toughness: Polycarbonate's toughness make it ideal for a wide variety of applications including machine guards, indoor and outdoor signs, architectural glazing, face shields, skylights, and point-of-purchase displays.
• Weight: The weight of polycarbonate is one tenth of that of glass.
• Machinability: Polycarbonate rod and plate are easy to machine and have excellent dimensional stability. Polycarbonate machining stock is used for numerous applications including electrical components, manifolds, sight glasses, and semiconductor machinery parts.
• Impact Resistance: It has high impact-resistance but it has low scratch-resistance and so a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components.
• Strength: Polycarbonate is stronger and usable in a wider temperature range.

Pros and Cones of Polycarbonates

Pros (Advantages)

1. Excellent impact resistance and strength at low and high temperatures.
2. Available in visually clear grades.
3. Good heat strength, flame retardant, RoHS compliant, mould release, good lubricated, dimensional stability, good impact resistance, good process ability, good heat resistance and high viscosity.
4. Polycarbonates are virtually unbreakable and can withstand massive force. For this reason, they are used in the construction of bulletproof windows and police shields.
5. Ultraviolet blockage is another great plus with polycarbonates. This quality makes these panels the material of choice for constructing overhead covers and awnings in commercial establishments.
6. Polycarbonates are very light when compared to glass, acrylic or other plastics. This results in easy transportation, installation and lower labor costs.

1. Polycarbonates are highly resistant to heat and cold. These are ideal for use in any kind of harsh environment, where they can last longer than other standard construction materials.
2. Another advantage is the fact it can be injected into various objects which allows it to be used for discs, bottles, glasses, lenses, audio player cases, and lab equipment.
3. Polycarbonate materials are preferred material of choice for constructing greenhouses and plant nurseries due to excellent optical properties of polycarbonate which enable keeping out ultraviolet rays while letting in adequate sunlight and heat.
4. Polycarbonate factors are resistant to sunlight, rain and snow. This enables the polycarbonate factors to last in the outdoors for many years without showing signs of fading, yellowing or discoloration.

Cons (Disadvantages)

1. Polycarbonate is recommended only for those products that are used once, not repeated because of its chemical makeup.
2. Bisphenol A (primary component of polycarbonate) may have unpredicted effects on humans, repeated use of the one object may be potentially hazardous to the health.
3. High price is one of the major disadvantages of polycarbonates. It is much more expensive than glass and other plastics.
4. Polycarbonates are not very resistant to scratching, marring and abrasive surfaces. As a result, denting is possible on the surface if care is not taken.

1. Polycarbonates are highly sensitive to abrasive cleaners, alkaline cleaning products and solvents. Therefore, you must be very careful to avoid substances containing acetone, benzene or any other such organic solvents.
2. The manufacturing process for polycarbonates is not very environmentally friendly, requires very high processing temperatures and is also very expensive. Also, it requires phosgene, which is known for its ill effects on human health and chlorine, which is also environmentally unfriendly.
3. Polycarbonate is susceptible to degradation when exposed to processing equipment for an extended period.
4. Polycarbonate possesses only fair resistance to chemicals. Because of this lower resistance factor, polycarbonate deteriorates when exposed to many organic solvents.
5. Polycarbonate material exhibits aromatic sensitivity or is prone to absorb odours.
6. Although polycarbonate rates high for impact strength when compared to ABS, polyvinyl chloride (PVC) or acrylic, it is subject to stress cracking.

Applications

Electronic components

Polycarbonate is mainly used for electronic applications that capitalize on its collective safety features. Being a good electrical insulator and having heat resistant and flame retardant properties, it is used in various products associated with electrical and telecommunications hardware. It also serves as dielectric in high stability capacitors.

Construction materials

The second largest consumer of polycarbonates is the construction industry, e.g. for dome lights, flat or curved glazing, and sound walls.

Data storage

A major application of polycarbonate is the production of Compact Discs, DVDs, and Blu-ray Discs. These discs are produced by injection moulding polycarbonate into a mould cavity that has on one side a metal stamper containing a negative image of the disc data, while the other mould side is a mirrored surface.

Automotive, aircraft, and security components

In the automotive industry, injection moulded polycarbonate can produce very smooth surfaces that make it well suited for direct metalized parts such as decorative bezels and optical reflectors.

Its uniform mould shrinkage results in parts with greater accuracy than those made of polypropylene. It can be laminated to make bullet-proof glass, although bullet-resistant is more accurate for the thinner windows, such as are used in bullet-resistant windows in automobiles. The thicker barriers of transparent plastic used in teller's windows and barriers in banks, are also polycarbonate.

The cockpit canopy of the F-22 Raptor jet fighter is made from a piece of high optical quality polycarbonate, and is the largest piece of its type formed in the world.

Niche applications

Many kinds of lenses are manufactured from polycarbonate, including automotive headlamp lenses, lighting lenses, sun-glass/eyeglass, lenses, and safety glasses. Other miscellaneous items: MP3/Digital audio player cases, Ocarinas, computer cases, riot shields, visors, instrument panels, blender jars. Many toys and hobby items are made from polycarbonate parts, e.g. fins, gyro mounts, and fly bar locks for use with radio-controlled helicopters.