excerpt from the Handbook of Plastics Testing Technology (Society of Plastics Engineers Monographs) by Vishu Shah
Plastic materials have been under considerable pressure to perform satisfactorily in situations involving FIRE, because of their increased use in homes, buildings, appliances, automobiles, aircraft and many other sectors of our lives. Before getting into discussion on FR tests and testing procedures, it is necessary to understand polymers as they relate to flammability. Polymer’s inherent flammability can be divided into three basic classes: Inherently Flame Retardant, Less Flame Retardant, and Quite Flammable.
When a polymeric material is subjected to combustion, it undergoes decomposition which produces fragments at the polymer surface. The fuel produced in this process diffuses to the flame front, where it is oxidized, producing more heat. This, in turn, causes more material decomposition. A cyclic process is established, solid material is decomposed, producing fuel which burns, giving off more heat, which results in more material decomposition. To reduce the flammability of a material, this cycle must be attacked in either the vapor phase or at the solid material surface. In the vapor phase, the cycle can be inhibited by adding certain additives to the polymer that disrupt the flame chemistry when vaporized. Bromocompounds and chlorocompounds with antimony oxide operate in this manner. Solid phase inhibition may be achieved by including additives in the polymer that promote the retention of fuel as carbonaceous char as well as providing a protective insulating layer. This layer prevents further fuel evolution. Few other solid phase approaches involve the use of heat sinks, such as hydrated alumina, which absorb heat and release water of hydration when heated, or alter the decomposition chemistry to consume additional heat in the decomposition process.
No discussion on polymer’s flammability can be considered complete without discussing the formation of smoke and the generation of toxic gases. Smoke impairs the ability of occupants to escape from a burning structure as well as the ability of fire fighters to carry out rescue operations. Many tests have been developed to measure smoke density and toxicity. The material’s ability to burn depends upon fire conditions as well as polymer composition. Actual fire conditions are difficult to simulate and therefore we are forced to rely upon small and large-scale laboratory tests to predict combustibility, smoke density and toxicity. The flammability of materials is influenced by several factors :
• Ease of ignition – How rapidly a material ignites.
• Flame spread – How rapidly fire spreads across a polymer surface.
• Fire endurance – How rapidly fire penetrates a wall or barrier.
• Rate of heat release – How much heat is released and how quickly.
• Ease of extinction – How rapidly the flame chemistry leads to extinction.
Smoke of evolution
Toxic gas generation.
Flammability Tests
|
Organization |
Name of test |
Procedure Id number |
Specimen size (in.) |
Number of specimens |
Angle of specimen |
Ignition source |
Properties measured |
|
ASTM |
Rate of burning : flexible plastics 0.050 in. and under |
D-568 |
1 X 18 |
3 |
Vertical |
Bunsen burner |
Burning rate, average time and extent of burning |
|
ASTM |
Rate of burning : Rigid plastics over 0.050 in. |
D-635 |
1/2 X 5 X thickness |
10 |
Horizontal |
Bunsen burner |
Burning rate, average time and extent of burning |
|
ASTM |
Incandescence resistance of rigid plastics |
D- 757 |
1/2(W) X 4.57(L) X 1/4(T) |
3 |
90d to source |
Silicon carbide incandescence rod |
Burning rate, average time and extent of burning |
|
UL94 Flammability Tests
UL 94, developed by Underwriters Laboratories, is one of the most widely used and most frequently cited sets of flammability tests for plastic materials. The UL flammability tests include a standard burning test applied to vertical and horizontal test bars, from which a general flammability rating is derived. There are four basic tests for classifying materials in different categories as mentioned below:
1. Horizontal burning test for classifying materials (94 HB).
2. Vertical burning test for classifying materials (94V-0, 94V-1).
3. Vertical burning test for classifying materials (94-5V).
4. Vertical burning test for classifying materials (94 VTM-0, 94 VTM-1, or 94 VTM-2).
Steps in meeting Flammability Requirements
1. The first step in meeting flammability requirements is to carefully define the application in details. This will help narrow the list agencies you may have to deal with.
2. Determine the appropriate agency that deals with your application. For example, if the application has something to do with building industry, you may want to contact one of the consulting agencies for building and construction organizations. If the application is a plastic cabinet that houses electrical components, UL is the organization to contact.
3. Once the application is defined and the governing agency is narrowed down, you may proceed with the designing and material selection. An important thing to remember at this stage is to specify the material to your design and not vice versa. The material selection process can be expedited by consulting published sources, such as UL-recognized component directory that lists plastics according to performance as tested by UL 94. Modern Plastics Encyclopedia’s flammability chart is another source of information for preliminary screening of the materials.
4. Once the preliminary decisions have been made on the type of material that will meet the requirement of the application, the material supplier or a custom compounder can be consulted for specific grade of material.
5. If the code or standard required, an independent laboratory should be consulted.
About the Author
VISHU SHAH is President and cofounder of Performance Engineered Products, Inc., a custom injection molder located at Pomona, California.
Book Details
Handbook of Plastics Testing Technology, Vishu Shah, Wiley, New York 1984,
pol.1985.130230111. ISBN, 0471078719
