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A GUIDE TO POLYCARBONATE IN GENERAL

 

Reference Sheet
Material Safety Data Sheet
UL Information

What is polycarbonate?

Polycarbonates are long-chain linear polyesters of carbonic acid and dihydric phenols, such as bisphenol A.

Bisphenol A

What are the properties of polycarbonate?

Before starting to list properties of polycarbonate, it is probably wise to explain why the properties are the way they are. First, the technical stuff. Take a look at the above diagram. In it, you will see two six-sided structures. These are called phenyl groups. You will also see two groups identified by the label CH3. These are methyl groups. The presence of the phenyl groups on the molecular chain and the two methyl side groups contribute to molecular stiffness in the polycarbonate. This stiffness has a large effect on the properties of polycarbonate. First, attraction between of the phenyl groups between different molecules contributes to a lack of mobility of the individual molecules. This results in good thermal resistance but relatively high viscosity (i.e., low melt flow) during processing. The inflexibility and the lack of mobility prevent polycarbonate from developing a significant crystalline structure. This lack of crystalline structure (the amorphous nature of the polymer) allows for light transparency.

Now for the clearer, less technical version of the properties. Polycarbonate is naturally transparent, with the ability to transmit light nearly that of glass. It has high strength, toughness, heat resistance, and excellent dimensional and color stability. Flame retardants can be added to polycarbonate without significant loss of properties.

The general properties can be summarized as follows:

  • excellent physical properties
  • excellent toughness
  • very good heat resistance
  • fair chemical resistance
  • transparent
  • moderate to high price
  • fair processing

How do these properties compare to other materials?

One of the biggest advantages of polycarbonate is its impact strength. The following diagram compares the impact strength of polycarbonate to other commonly sold plastics.


Polycarbonate does have its disadvantages. It has only fair chemical resistance and is attacked by many organic solvents. It is also fairly expensive compared to other plastics. It has been as much as double the price of ABS. In applications where lower heat and impact are needed, ABS can be quite a bargain compared to polycarbonate.

What is a glass fiber reinforced grade of polycarbonate?

The addition of glass fibers to polycarbonate significantly increases the tensile strength, flexural strength, flexural modulus, and heat deflection temperature of the polycarbonate while causing a decrease in the impact strength and tensile elongation. (See Glossary of Plastic Properties for an explanation of each term.) The greater the amount of glass fiber added to the polycarbonate, the greater the effect on each property will be.

How is polycarbonate used?

It can be injection molded, blow molded, and extruded and is an ideal engineering plastic with good electrical insulating properties, finding applications in electric meter housings and covers, casket hardware, portable tool housings, safety helmets, computer parts, and vandal-proof windows and light globes. The price of polycarbonate restricts its use to mainly engineering applications.

Other engineering applications include the following:

  • equipment housings
  • exterior automotive components
  • outdoor lighting fixtures
  • nameplates and bezels
  • non automotive vehicle windows
  • brackets and structural parts
  • medical supply components
  • plastic lenses for eyeglasses

How is polycarbonate sold?

In general, the primary distinguishing characteristic of a grade of polycarbonate is its melt flow. The only exceptions are glass fiber reinforced grades, which are sold based on the percentage of glass fiber used in the production of the plastic. Other properties (such as UV stabilization, mold release, impact modification, and flame retardancy) typically differentiate grades of the same melt flow or percentage of glass fiber. Changes in melt flow have an effect on other physical properties.

When specifying a grade of polycarbonate, it is necessary to know the conditions under which it will be used. In the following diagram, you can see the effect of temperature on izod impact strength for three different grades of polycarbonate.

Notched Izod Impact Strength vs. Temperature


Lastly, there are two major types of polycarbonate sold. Virgin polycarbonate is polycarbonate which has not been altered from the time of original manufacturing to the purchase of the product. Polycarbonate regrind comes from polycarbonate that is taken from an end-user and ground into pellets. These new pellets can be compounded again with other material (both virgin and other regrind pellets) to produce a new product. The nature of polycarbonate is such that its properties do not deteriorate significantly after being reground if dried properly prior to processing.

What is UL approval?

UL (Underwriters Laboratories) performs various tests on products sold in the United States. One such test, UL94, is used to measure the resistance of plastics to a flame source. For polycarbonate, the test will normally result in a mark of V-0, V-1, V-2. V-0 is the most flame retardant and V-2 is the least flame retardant of the marks typically given to polycarbonate. If the material does not pass the test for V-0, V-1, or V-2; then the product is not UL approved. UL approval is given for a particular product at a measured thickness which is reported with the rating. Hence, saying that a given product is V-2 is insufficient information without also specifying at what thickness the rating is valid. UL approval is required for certain applications of polycarbonate.

CONDITION 94V-0 94V-1 94V-2
Maximum time any one specimen can burn after removal of 2nd application of flame 10 sec 30 sec 30 sec
Maximum glowing time permitted for any one sample after removal of 2nd application of flame 30 sec 60 sec 60 sec
Total flaming combustion time allowed
(maximum) for the 10 flame applications
50 sec 250 sec 250 sec
Ignition of cotton from dripping, flaming particles not permitted not permitted permitted
Flaming combustion up to the holding clamp not permitted not permitted not permitted

94HB Conditions:
  1. Shall not have a burning rate exceeding 1.5 inch/minute over a 3 inch span for specimens having a thickness of 0.120 inch and above.
  2. Shall not have a burning rate exceeding 3.0 inch/minute over a 3 inch span for specimens having a thickness of under 0.120 inch.
  3. Shall cease to burn before the flame reaches the 4 inch reference mark but shall not comply with 94 V-0, V-1, or V-2 requirements.

The second part of the UL94 test determines the Relative Thermal Index (RTI) of the material. The RTI is the temperature at which a specific property will decrease to half of its original value after 60,000 hours of exposure at that temperature. If a material has not been tested, the generic RTI for the material is assigned. The generic RTI for polycarbonate is 80°C. PTS Tristar™ polycarbonate grades have elevated RTI as shown below.

UL information for PTS Polycarbonate

What is FDA certification?

Materials that are used in applications that are in contact with food require FDA certification.

What is UV stabilization?

UV stabilizers are added to polycarbonate to help protect the material from the effects of exposure to the Sun. Radiation from the Sun, especially in the UV portion of the spectrum, degrades the properties of polycarbonate. The addition of UV stabilizers allows the material to sustain exposure to the Sun for a longer period of time.

What is mold release?

Adding a mold release agent to polycarbonate makes processing easier. It acts in a way similar to a lubricant in the injection molding process.