Flammability Testing

Flammability Testing

Flammability is one of the many material properties of concern when designing a plastic part. Most plastics will burn. Material manufacturers can add various kinds of flame retardants to the plastic to alter its flammability properties. There are many ASTM tests to define a plastic's flammability characteristics, and an important set of UL tests to give the material certain ratings.

ASTM TESTS

Flammability Test (ASTM D 568 for flexible plastics and D 635 for self-supporting plastics)

ASTM D 568 supports the sample vertically, whereas D 635 supports the sample horizontally. A flame from a Bunsen burner is exposed to a plastic test bar for 30 seconds. The sample is allowed to burn until it either extinguishes itself or burns past a gage mark (100 mm) on the bar. If the sample does not burn past the 100 mm gage mark, time and extent of burning are reported. If it burns past the gage mark, and average burn rate, in centimeters per minute, is reported. Materials that do not burn to the gage mark are said to be self-extinguishing.

Oxygen Index Test (ASTM D 2863)

This tests the minimum concentration of oxygen in a pure oxygen / nitrogen environment that is necessary for the sample to maintain combustion. The sample is hung vertically in a special chamber, and then ignited. The oxygen concentration is then reduced until the combustion is just maintained. The Limiting Oxygen Index (LOI) represents this percentage of oxygen.

Radiant Panel Test (ASTM E 162)

A radiant panel is maintained at 670°C (1238°F) as a heat source to ignite a plastic sheet. The plastic sheet (152mm x 457mm, 6" x 18") is maintained at a set distance from the panel, with the top tilted at a 30° angle toward the panel. The rate of burning and the heat evolved in the burning are measured and combined to form a flame-spread index.

Smoke Density Test (ASTM D 2843)

This test measures the loss of light transmission through smoke produced from a burning plastic. A sample is burned inside of a special chamber. A light is passed between two photoelectric cell plates, and the light transmission is plotted against time. The area under this curve is the total smoke produced.

UNDERWRITERS LABORATORY TESTS

UL 94 Flammability Tests

The Underwriters Laboratories have developed a serious of flammability tests, all designated under UL 94. For a plastic to receive a certain UL rating, it must pass certain criteria of a test. The different ratings are outlined below.


UL 94 Flammability Ratings Description
5VA Surface Burn	Burning stops within 60 seconds after five applications of five seconds each of a flame (larger than that used in Vertical Burn testing) to a test bar. Test specimens MAY NOT have a burn-through (no hole). This is the highest (most flame retardant) UL94 rating.
5VB Surface Burn	Burning stops within 60 seconds after five applications of five seconds each of a flame (larger than that used in Vertical Burn testing) to a test bar. Test specimens MAY HAVE a burn-through (a hole).
V-0 Vertical Burn	Burning stops within 10 seconds after two applications of ten seconds each of a flame to a test bar. NO flaming drips are allowed.
V-1 Vertical Burn	Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. NO flaming drips are allowed.
V-2 Vertical Burn	Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. Flaming drips ARE allowed.
H-B Horizontal Burn	Slow horizontal burning on a 3mm thick specimen with a burning rate is less than 3"/min or stops burning before the 5" mark. H-B rated materials are considered "self-extinguishing". This is the lowest (least flame retardant) UL94 rating.

94HB Horizontal Burn Test

This is the easiest flammability test for a material to pass. Generally, if a 94HB requirement will be recognized if the materials passed any of the V tests. The 94HB rating is generally suitable for attended, portable, intermittent-duty, household appliance enclosures, like hair dryers.

Refer to Figure 1 for the experimental setup. A 1/2" x 5" sample is clamped on a ring stand. Marks are made on the sample 1" and 5" from the free end. A flame is applied to the sample for 30 seconds or until the sample burns past the 1" mark. The sample is allowed to burn until it stops or reaches the 5" mark. If the sample burns up to the 5" mark, a burn rate is calculated. If the sample stops burning before the 5" mark, the burn time and the length of the damaged section between the marks is reported.

A material that is less than 0.118" receives a 94HB classification if it burns at a rate of less than 3" per minute or stops burning before the 5" mark. Three samples are tested. If only one of them fails, another set of three are tested, and all must pass for the part to receive 94HB certification.

94V Vertical Burning Test

This test can yield three designations: 94V-0, 94V-1, and 94V-2. These ratings would be suitable for an unattended, portable, intermittent-duty, household appliance enclosure, like a coffee maker.

See the left side of Figure 2 for the experimental setup. A 1/2" x 5" sample is held in the vertical position with cotton placed directly under the specimen. A burner flame is then applied to free end for 10 seconds two times. The second burn is done when the flaming combustion from the first 10 second burn stops. Two sets of five specimens are tested. The following values are recorded.

Duration of flaming combustion after the first burner flame application.
Duration of flaming combustion after second burner flame application.
Duration of glowing combustion after second burner flame application.
Whether or not flaming drips ignite cotton placed below specimen.
Whether or not specimen burns up to holding clamp.

Criteria Conditions for 94V Ratings	94V-0	94V-1	94V-2
Total flaming combustion for each specimen	£ 10s	£ 30s	£ 30s
Total flaming combustion for all 5 specimens of any set	£ 50s	£ 250s	£ 250s
Flaming and glowing combustion for each specimen after second burner flame application	£ 30s	£ 60s	£ 60s
Cotton ignited by flaming drips from any specimen	NO	NO	YES
Glowing or flaming combustion of any specimen to holding clamp	NO	NO	NO

94VTM Vertical Thin Material Test

A material can be too thin for the standard 94V test because it may distort, shrink, or flex during the burn test. There is another similar test for these thin materials, which are generally less than 0.010" thick.

Refer to the right side of Figure 2. An 8" x 2" sample is wrapped around a 1/2" mandrel, and then taped on one end. The mandrel is removed, leaving a cone-shaped sample that is relatively rigid. The two flame applications are 3 seconds instead of 10 seconds. All of the other criteria from the 94V test applies, except that no specimens can have flaming or glowing combustion up to a mark 5" from the bottom of the sample.