Polycarbonates, known by the trademarked names Lexan, Makrolon, Makroclear, arcoPlus® and
others, are a particular group of thermoplastic polymers. They are easily worked, molded,
and thermoformed. Because of these properties, polycarbonates find many applications. Polycarbonates
do not have a unique resin identification code and are identified as Other, 7. Items
made from polycarbonate can contain the precursor monomer bisphenol A.
Structure Polycarbonates received their name because
they are polymers containing carbonate groups–O–). Most polycarbonates of commercial interest
are derived from rigid monomers. A balance of useful features including temperature resistance,
impact resistance and optical properties position polycarbonates between commodity plastics
and engineering plastics. Production
The main polycarbonate material is produced by the reaction of bisphenol A and phosgene
COCl 2. The overall reaction can be written as
follows:
The first step of the synthesis involves treatment of bisphenol A with sodium hydroxide, which
deprotonates the hydroxyl groups of the bisphenol A.
(HOC6H4)2CMe2 + 2 NaOH → Na2(OC6H4)2CMe2 + 2 H2O
The diphenoxide2CMe2) reacts with phosgene to give a chloroformate, which subsequently
is attacked by another phenoxide. The net reaction from the diphenoxide is:
Na2(OC6H4)2CMe2 + COCl2 → 1/n [OC(OC6H4)2CMe2]n + 2 NaCl
In this way, approximately one billion kilograms of polycarbonate is produced annually. Many
other diols have been tested in place of bisphenol A, e.g. 1,1-bis(4-hydroxyphenyl)cyclohexane
and dihydroxybenzophenone. The cyclohexane is used as a comonomer to suppress crystallisation
tendency of the BPA-derived product. Tetrabromobisphenol A is used to enhance fire resistance. Tetramethylcyclobutanediol
has been developed as a replacement for BPA. An alternative route to polycarbonates entails
transesterification from BPA and diphenyl carbonate:
(HOC6H4)2CMe2 +2CO → 1/n [OC(OC6H4)2CMe2]n + 2 C6H5OH
The diphenyl carbonate was derived in part from
carbon monoxide, this route being greener than the phosgene method.
Properties and processing Polycarbonate is a durable material. Although
it has high impact-resistance, it has low scratch-resistance and so a hard coating is
applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components.
The characteristics of polycarbonate are quite like those of polymethyl methacrylate, but
polycarbonate is stronger and usable over a greater temperature range. Polycarbonate
is highly transparent to visible light, with better light transmission than many kinds
of glass. Polycarbonate has a glass transition temperature
of about 147 °C, so it softens gradually above this point and flows above about 155 °C.
Tools must be held at high temperatures, generally above 80 °C to make strain- and stress-free
products. Low molecular mass grades are easier to mold than higher grades, but their strength
is lower as a result. The toughest grades have the highest molecular mass, but are much
more difficult to process. Unlike most thermoplastics, polycarbonate
can undergo large plastic deformations without cracking or breaking. As a result, it can
be processed and formed at room temperature using sheet metal techniques, such as bending
on a brake. Even for sharp angle bends with a tight radius, heating may not be necessary.
This makes it valuable in prototyping applications where transparent or electrically non-conductive
parts are needed, which cannot be made from sheet metal. Note that PMMA/Plexiglas, which
is similar in appearance to polycarbonate, is brittle and cannot be bent at room temperature.
Main transformation techniques for polycarbonate resins:
extrusion into tubes, rods and other profiles including multiwall
extrusion with cylinders into sheets) and films), which can be used directly or manufactured
into other shapes using thermoforming or secondary fabrication techniques, such as bending, drilling,
routing, laser cutting etc. injection molding into ready articles
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 can also serve as a dielectric in high-stability capacitors. However, commercial
manufacture of polycarbonate capacitors mostly stopped after sole manufacturer Bayer AG stopped
making capacitor-grade polycarbonate film at the end of year 2000.
Construction materials
The second largest consumer of polycarbonates is the construction industry, e.g. for domelights,
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 molding polycarbonate into a mold cavity that
has on one side a metal stamper containing a negative image of the disc data, while the
other mold side is a mirrored surface. Typical products of sheet/film production include
applications in advertisement. Automotive, aircraft, and security components
In the automotive industry, injection-molded polycarbonate can produce very smooth surfaces
that make it well-suited for sputter deposition or evaporation deposition of aluminium without
the need for a base-coat. Decorative bezels and optical reflectors are commonly made of
polycarbonate. Due to its low weight and high impact resistance, polycarbonate is the dominant
material for making automotive headlamp lenses. However, automotive headlamps require outer
surface coatings because of its low scratch resistance and susceptibility to ultra violet
degradation. The use of polycarbonate in automotive applications is limited to low stress applications.
Stress from fasteners, plastic welding and molding render polycarbonate susceptible to
stress corrosion cracking when it comes in contact with certain accelerants such as salt
water and plastisol. 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.
So-called "theft-proof" large plastic packaging for smaller items, which cannot be opened
by hand, is uniformly made from 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.
South African security companies have launched the 'transparent burglar bar' under a variety
of retail names, made from polycarbonate. Niche applications
Polycarbonate, being a versatile material with attractive processing and physical properties,
has attracted myriad smaller applications. The use of injection molded drinking bottles,
glasses and food containers is common, but the use of BPA in the manufacture of polycarbonate
has stirred serious controversy, leading to development and use of "BPA-free" plastics
in various formulations.
Polycarbonate is commonly used in eye protection, as well as in other projectile-resistant viewing
and lighting applications that would normally indicate the use of glass, but require much
higher impact-resistance. Polycarbonate lenses also protect the eye from UV light. Many kinds
of lenses are manufactured from polycarbonate, including automotive headlamp lenses, lighting
lenses, sunglass/eyeglass lenses, swimming goggles and SCUBA masks, and safety glassesvisors
including visors in sporting helmets/masks and police riot gear. Windscreens in small
motorized vehicles are commonly made of polycarbonate, such as for motorcycles, ATVs, golf carts,
and small planes and helicopters. The light weight of polycarbonate as opposed
to glass has led to development of electronic display screens that replace glass with polycarbonate,
for use in mobile and portable devices. Such displays include newer e-ink and some LCD
screens, though CRT, plasma screen and other LCD technologies generally still require glass
for its higher melting temperature and its ability to be etched in finer detail.
As more and more governments are restricting the use of glass in pubs and clubs due to
the increased incidence of glassings, polycarbonate glasses are becoming popular for serving alcohol
because of their strength, durability, and glass-like feel.
Other miscellaneous items include durable, lightweight luggage, MP3/digital audio player
cases, ocarinas, computer cases, riot shields, instrument panels, tealight candle containers
and blender jars. Many toys and hobby items are made from polycarbonate parts, e.g. fins,
gyro mounts, and flybar locks for use with radio-controlled helicopters.
Standard Polycarbonate resins are not suitable for long term exposure to UV radiation. To
overcome this the primary resin can have UV Stabilisers added. These grades are sold as
UV Stabilised Polycarbonate to Injection Moulding and Extrusion companies. Other applications
including Polycarbonate sheet may have the anti-UV layer added as a special coating or
a coextrusion for enhanced weathering resistance. Polycarbonate is also used as a printing substrate
for nameplate and other forms of industrial grade under printed products. The polycarbonate
provides a barrier to wear, the elements, and fading.
Medical applications Many polycarbonate grades are used in medical
applications and comply with both ISO 10993-1 and USP Class VI standards. Class VI is the
most stringent of the six USP ratings. These grades can be sterilized using steam at 120 °C,
gamma radiation, or by the ethylene oxide method. However, scientific research indicates
possible problems with biocompatibility. Dow Chemical strictly limits all its plastics
with regard to medical applications. Phones
Some major smartphone manufacturers use polycarbonate. Nokia has used polycarbonate in their phones
starting with the N9's unibody case in 2011. This practice continues with various phones
in the Lumia series. Samsung has started using polycarbonate with Galaxy S III's battery
cover in 2012. This practice continues with various phones in the Galaxy series. Apple
has started using polycarbonate with iPhone 5c's unibody case in 2013.
Primary resin manufacturers and brands Semi-finished products produced from polycarbonate
History Polycarbonates were first discovered in 1898
by Alfred Einhorn, a German scientist working at the University of Munich. However, after
30 years of laboratory research, this class of materials was abandoned without commercialization.
Research resumed in 1953, when Hermann Schnell at Bayer in Uerdingen, Germany patented the
first linear polycarbonate under the trade name Makrolon.
Also in 1953, and one week after the invention at Bayer, Daniel Fox at GE in Schenectady,
New York, independently synthesized a branched polycarbonate. Both companies filed for U.S.
patents in 1955, and agreed that the company lacking priority would be granted a license
to the technology. Once patent priority was resolved, in Bayer's
favor, Bayer began commercial production under the trade name Makrolon in 1958 and GE began
production under the name Lexan in 1960. Potential hazards in food contact applications
The use of polycarbonate containers for the purpose of food storage is controversial.
The basis of this controversy is their hydrolysis occurring at high temperature, releases bisphenol
A: 1/n [OC(OC6H4)2CMe2]n + H2O →2CMe2 + CO2
More than 100 studies have explored the bioactivity of bisphenol A derived from polycarbonates.
Bisphenol A appeared to be released from polycarbonate animal cages into water at room temperature
and it may have been responsible for enlargement of the reproductive organs of female mice.
However, the animal cages used in the research were fabricated from industrial grade polycarbonate,
rather than FDA food grade polycarbonate. An analysis of the literature on bisphenol
A leachate low-dose effects by vom Saal and Hughes published in August 2005 seems to have
found a suggestive correlation between the source of funding and the conclusion drawn.
Industry-funded studies tend to find no significant effects whereas government-funded studies
tend to find significant effects. Sodium hypochlorite bleach and other alkali
cleaners catalyze the release of the bisphenol A from polycarbonate containers. A chemical
compatibility chart shows that polycarbonate is incompatible with ammonia and acetone because
it dissolves in their presence. Alcohol is one recommended organic solvent for cleaning
grease and oils from polycarbonate. Bioremediation
A species of fungus found in Belize, Geotrichum candidum, has been found to consume the polycarbonate
found in CDs. See also
CR-39 Organic electronics
Vapor polishing References
External links