First commercialized by Bayer (Germany) and General Electric (USA) in the 1950s, polycarbonate (PC) is a widely used family of thermoplastic polymers. Polycarbonate is a copolymer in that it is composed of several different monomer types in combination with one another.
The grades of polycarbonate are noted for strength and particularly toughness. The family has the highest impact resistance of any thermoplastic, outstanding dimensional and thermal stability, exceptional machineability, stain resistance and is non-toxic with low water absorption. The high impact strength makes it resistant to repeated blows, shattering and spalling.
Some grades are optically transparent, with the optical properties of special grades transparent up to 32mm thick, comparable to those of the other popular clear plastic, polymethyl methacrylate (PMMA, acrylic). In fact, polycarbonate is highly transparent to visible light, with better light transmission than many kinds of glass. Polycarbonate is around 250 x stronger than glass and 30 x stronger than acrylic.
Unfortunately, although it has high impact-resistance, it has low scratch-resistance, which means that a hard coating has to be applied to polycarbonate sheet, eyewear lenses and exterior automotive parts made from polycarbonate.
Other limitations include:
- Low fatigue endurance
- Mechanical properties degrade after prolonged exposure to water at over 60°C
- Attacked by hydrocarbons and alkalis
- Proper drying before processing needed
- Yellows after long exposure to UV
- Poor creep resistance (can be improved with the addition of glass or carbon fiber reinforcement)
In addition to being stronger than acrylic, polycarbonate will hold up longer to extreme temperatures. Polycarbonate is an amorphous material, meaning that it does not exhibit the ordered characteristics of crystalline solids. Typically, amorphous plastics demonstrate a tendency to gradually soften – they have a wider range between their glass transition temperature and their melting point – rather than exhibit a sharp transition from solid to liquid, as is the case in crystalline polymers.
Polycarbonate maintains its properties over a wide range of temperatures, from -20 to 140°C. Its glass transition temperature is around 147°C, and it softens gradually above this point and flows above about 155°C.
Standard polycarbonate resins are not suitable for long term exposure to UV radiation and may embrittle. The primary resin can have UV stabilizers added.
Polycarbonates are easily worked, molded, and thermoformed. Common methods used to produce polycarbonate parts include:
- Injection molding
- Blow molding
- 3D Printing
To make strain-free and stress-free products, tools must be held at high temperatures, generally above 80°C. That said, 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 they are much more difficult to process.
Polycarbonate can be deformed plastically without cracking or breaking. Unlike most plastics, it can be processed and formed at room temperature using sheet metal techniques and 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. Acrylic is very brittle, so cannot be used in this way.
The main polycarbonate material is produced by the reaction of bisphenol A (BPA) and phosgene COCl. Some products made from polycarbonate contain the precursor monomer BPA; this has made polycarbonate containers for food storage controversial. An alternative route to polycarbonates entails transesterification from BPA and diphenyl carbonate.
Electronic components, glazing, CDs/DVDs, optical reflectors, bottles, glasses, food containers, protective eyewear, display screens, mobile phone housings, bullet-proof “glass”. Tinted PC is used for the purposes of reducing glare, for example, to cover lighted signs on the road, to protect LEDs and to reduce glare. Special grades can be sterilized and may be used for medical applications.
Alloys and Blends
Polycarbonate blends provide a balance between various properties, performance and productivity.
PC/Polyester Blends: suitable where high chemical resistance is required. PC/PBT blends offer higher chemical resistance than PC/PET blends due to PBT’s higher crystalline behavior, whereas PET blended grades offer superior heat resistance.
PC/ABS Blends: the toughness and high heat resistance of polycarbonate combine with the ductility and processability of ABS.
Researchers are also working to develop new processes for recycling polycarbonates into another type of plastic—one that does not release (BPA) into the environment when it is used or dumped into a landfill.
Many companies have developed bio-based polycarbonates but there are certain limitations regarding the cost of production.
- DURABIO by Mitsubishi Chemical Corporation
- POLYSORB Isosorbide by Roquette
- LEXAN PC resin based on Certified Renewable Feedstock by SABIC
Recently, a breakthrough has been made at the Korea Research Institute of Chemical Technology (KRICT) where researchers have created a bio-polycarbonate made largely from glucose. Unlike earlier biopolymers, the team claims that this new bio-polycarbonate has the strength and durability to match its petrochemical counterpart, paving the way for commercialization.
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