Rubber under fire.

This article describes the different fire and flammability tests where rubber systems are required to pass tests specified by various legal bodies and all the proposed future test requirements. Rubber manufacturers, engineers, architects and contractors should be aware of the current test requirements in this technology, thus enabling them to provide a rubber system that can meet the total spectrum of requirements including the ones for fire and flammability. The standard used to test for fire resistance in building construction is ASTM El19 - fire tests of building construction and materials and ASTM E 108 - fire tests of roof coverings.

Architects and engineers constantly require and recommend rubber systems for new construction, retrofit or renovation. Many homeowners are do-it-yourselfers with just enough knowledge to install the rubber system. Surprisingly, many professional applicators do not know very much more than the homeowners. If any of these people improperly installed the rubber system or performed poor quality work, the best result would be reduced performance. thereby forfeiting some of the original objective. The worst scenario is potentially of a greater degree.

Apparently, what can happen in some cases is that unbeknown to the person applying the rubber, a fire hazard can be created. The results created could be anything as simple as placing rubber over a heated area, such as electrical connections. hot pipes or vents. These unintentional dormant catastrophes are growing in occurrence, thus putting an onus on the government and the manufacturers. It follows that responsible professionals such as engineers and architects are also liable for their actions. Bearing this liability in mind, each of the above named parties have taken action to ensure nonculpability where there is a legal responsibility. The method used to indicate the relative fire characteristics of a material is to expose them to a series of tests.

Testing is the only method currently available that can be used to evaluate materials under different fire environments. Usually a discrete value is given as a test result, which then can be used a part of an index to evaluate numerous materials. The test results can be evaluated by a minimum rating required by a building code, which would be used to determine the suitability of the materials for that application.

Flammability testing

Flammability is the behavior of a material when that material is exposed to a fire environment and it burns; flammability can measure ignitability, burning rate, heat evolution, smoke products, products of combustion and endurance of burning. There are numerous tests which can be used to describe the properties of these materials. Many of them are ASTM standards which are listed in table 1.

The ASTM E 84 is one of the tests most often specified to determine the flammability and properties of rubber systems. The test data provide flame spread, fuel contributed and smoke developed numbers. The flame spread index is classified as follows: Class A, 0-25, Class B, 26-75 and Class C, 76-200. The greater the risk of fire hazard occupancy, the lower the number specified. So a material with an A rating would be used in a high risk category. ASTM E 84 is one of five test methods being validated in ASTM/ISR's international fire standards project.

There are currently, however, serious shortcomings with this test method. That is, even though a material has a low-flame spread number, it still provides a large contribution to growth of fire in a room. There is also a question as to the accuracy provided where the subjects are low density materials. The smoke index number may not accurately describe the volume of smoke generated. Variation in construction of the test tunnel could affect the reproducibility of the smoke developed and flame spread. Since there is no other standard, the ASTM E84 remains as the vanguard of flammability testing (ref. 1).

ASTM E 162

The tunnel specified by E 84 constitutes an expensive and cumbersome piece of apparatus. There is a need for a less expensive and more convenient flame spread test such as the E 162 radiant panel. The E 162 radiant panel test has numerous advantages over that of E 84. The sample size of the E 162 is 15.2 x 45.7 cm, while E 84 requires a sample 7.6 m x 53.3 cm, which is a rather cumbrous large size to deal with. Note, however, four samples are required for the radiant panel, whereas the tunnel uses only one sample. The ASTM E 162 could provide more accurate results in foam rubber that have ratings under 80 of the tunnel, but which prove to be as flammable as conventional materials with tunnel rating of several hundred (ref. 2).

The E 162 test method was originally published in 1960. Its unique feature is that a radiant heat source is used to expose the sample to the conditions mentioned previously and that reflects more accurately what might be found in a real fire environment.

A pilot burner is constantly left exposed on the sample to ignite any combustible gases that would be generated. The heat of this would be measured in the stack. Flame spread is observed visually and noted, based on time duration with distance. The E 162 method is typical of most flammability equipment that measures flame spread.

ASTM E 136

The E 136 standard is the behavior of material in a vertical tube furnace at 750[degree]C. This test method is used to determine combustion characteristics of building material. A 7.6 cm inside diameter ceramic tube furnace is brought to 750[degrees]C. A sample measuring 3.0 wide x 3.0 cm thick and 5.1 cm long is inserted into the furnace. The temperature of the samples is recorded and a material passes the test if at least three out of the four samples demonstrate the following:

* The temperature of the thermocouples does not exceed 54[degree]C above the furnace temperature at the beginning.

* There is no flaming of the specimen after the first 30s.

* The weight loss of the sample does not exceed 50% of the initial weight.

ASTM E 662

A test very frequently requested for rubber is ASTM E 662. This test procedure is used to determine the specific optical density of smoke generated by solid materials mounted in a vertical position in thickness not exceeding 2.5 cm. The photometric scale used closely relates to human vision. This method is fairly unique, and most called upon when a determined smoke density is needed.

ASTM E 108

This test standard is used to determine the relative fire characteristics of roof coverings under controlled fire conditions that might be found outside of a building. The roof coverings in question would be applicable for either combustible or noncombustible decks, when applied as intended. This test standard is composed of five different actual tests - intermittent flame exposure; spread of flame test; bunting brand test; flying brand test and rain test.

The results of these tests provide three classes of roof coverings - Class A tests are applicable to roof coverings that are effective against severe test exposure; Class B tests are applicable to roof coverings that are effective against moderate test exposure and Class C tests are applicable to roof coverings that are effective against light test exposure.

The test exposure for each described category becomes more severe by increasing the exposure time or the intensity of the environment.

Other flammability tests

Numerous organizations and government agencies have other flammability test requirements for rubber. The U.S. Coat Guard has fire protection requirements for commercial vessels as well as mobile offshore drilling units. The relevant approval specifications are 46 CFR 164.006 - deck coverings; 164.007; 164.008 - bulkhead panels; 164.009 - non-combustible materials; and 164.012 interior finish.

The categories 164.007 and 164.009 are the most relevant to rubber systems. The 164.007 is a fire test which uses the ASTM E 119 time-temperature curve with samples that are conditioned and have specific sizes. The 164.009 flammability test uses an apparatus similar to that of E 136 but unfortunately the test equipment and procedure are sufficiently different so that the E 136 equipment is not applicable.

Fire testing

The concept of ASTM E 119 fire testing is that the material or assembly being tested withstand fire or give protection from it. The most widely used fire test standard is the ASTM E 119 - standard method for fire tests of building construction and materials.

The ASTM E 119 was the first established fire test standard and still remains the most prominent for fire testing.

The basic test procedure is that a rubber system is exposed to a time-temperature curve as specified by the ASTM E 119 standard. The passing criteria, however, are different depending upon application. In the case of structural steel there are numerous thermocouples attached to the steel measuring the temperature of the steel. When the average of the thermocouples is 538[degrees]C, or any individual thermocouple exceeds 649[degree]C, then the test failure criteria has been exceeded. For walls, partitions and panels the failure criteria is when the average unexposed side temperature is 121 [degrees]C plus ambient.

The basis for failure for structural steel is that up to 50% of the strength of the steel is lost at 538[degree]C. The wail and floor failure criteria, as described above have unknown origins. It is known, however, that to prevent the spread of fire it is necessary to contain the heat as well as the fire and hot gases and that excessive transfer of heat can cause material on the unexposed side to combust.

There is a series of similar fire tests called room tests. A full size room test has approximate dimensions of 4.7 by 4.7 m, 3.9 by 3.9 m, or 3.9 by 4.7 m with a ceiling height of 3.1 m. A standard size doorway is located in one wall. An ignition source is located in a corner opposite the door where its effect will be greatest. The walls or ceiling or both are lined with the rubber product being tested. The room is constructed of an incombustible material such as millboard.

The room is set on fire by the ignition source, and the effects of the rubber on the fire are monitored by instrumentation of the rubber being noted by temperature rises. Further information on this test can be obtained from the ASTM E 603 - standard guide for fire experiments. Other versions of this test include half, third or quarter room, where the room size is smaller and appropriate instrumentation is set up to make all necessary measurements.

Specifying fire and flammability tests

In building construction the first place to check for the necessary fire/flammability requirement of a rubber system is the building codes. Most building departments clearly specify their minimum requirements for rubber systems. This does not mean that the architect or engineer or both (A & E) should settle for the minimum necessary requirements. The A&E is responsible to the owner for maximum safety with cost in mind. Also, the A&E could be able to realize additional savings for the owner by obtaining lower insurance costs, which are due directly from additional fire safety. Here again, the A&E must meet or exceed the minimum fire safety standards, while still following the cost versus return criteria of the owner.

The fire/flammability specification becomes more difficult when the code does not exactly define the requirement or there is no test standard for what is required.

Many A&Es find acceptable fire test data on a small scale ASTM E 119 test, (which indicates the integrity of the rubber system during and after the test), and not a time-temperature requirement (ref. 3).

Future fire and flammability requirements for rubbers

Future fire requirements of rubber systems can only change as quickly as the fire technology, with its associated testing, does. The complete understanding of fires is still under investigation by scientists, with no new major breakthroughs seen at this time. However, the National Institute of Standards and Technology (NIST) is developing new methods of fire and flammability testing. Their programs are interwound with computer models producing results that can be directly applied to real life conditions.

The most advanced piece of flammability equipment developed by Dr. Vytenis Babrauskas (ref. 4) at NIST is the cone calorimeter. This piece of equipment is used to measure rate of heat release (RHR) in samples measuring 10 by 10 cm. The primary purpose of the calorimeter is to measure RHR of products in order to estimate their contribution to a room fire (as opposed to characterizing in detail their combustion chemistry behaviors). The rate of heat release is determined by measuring combustion product gas flow and oxygen depletion, while mass loss is simultaneously recorded (directly). The cone calorimeter was approved as ASTM E 1354, test method for heat and visible smoke release rates for material and products using an oxygen consumption calorimeter.

The ASTM has a subcommittee E5.11 ,which is reviewing the ASTM E 119 test method. One of the proposed revisions is to modify the time-temperature curve, so that it is more severe.

Other proposed changes include furnace calibration and procedural methods which do not directly alter the fire environment. They are more for the furnace operator (ref. 5).

Conclusions

It is necessary for engineers, architects, designers and manufacturers of rubber systems to understand the requirements for (a) resistance to ignition, and, once combustion has begun, (b) resistance to the spread of flame. As a start, this group and any others concerned with rubber should understand the primary differences between (a) and (b). The rubber system that is specific should be tested according to the appropriate method (tel. 1 ).

Very often, the limiting factor for fire protection is specified by building codes. However, fire protection can go beyond this minimum requirement. Manufacturers should become aware of changing fire/flammability tests so that their products are designed with minimum fire hazard and can pass tests when these tests become the required minimum. Professionals specifying rubber systems should use applicable fire/flammability test standards that provide optimum information about the combustibility of the rubber.

ASTM E 84 Test method for surface burning characteristics of building materials 

ASTM E 162 Surface flammability of materials using a radiant heat energy source

ASTM E 136 Behavior of materials in a vertical tube furnace at 750[degrees]C

ASTM E 662 Specific optical density of smoke generated by solid materials

ASTM E 108 Fire tests of roof coverings

References

1. Enright, C.F., ASTM Standardization News, October 1984, pp. 34-36.

2. McGuire, J.H. and D'Sovza, M.V. "The E 162 radiant panel flammability test and foamed plastics," Fire Technology, November 1979, pp. 23,24.

3. Schultz, N, "Acoustic materials and products - can they take the heat?," The Construction Specifier, April 1989, pp. 68,69.

4. Babrauskas, V., "Development of the cone calorimeter-a bench-scale heat release rate apparatus based on oxygen consumption," November 1984. NBSIR 82-2611, National Bureau of Standards, Gaithersburg, MD.

5. Harmathy, T.Z., "Latest draft of revision of test method E 119," National Research Council of Canada, November 1984.

Neil Schultz is one of the founders and the executive director of VTEC Laboratories, a commercial testing and research lab specializing in the field of flammability technology. He is an adjunct professor of thermodynamics and environmental engineering at New York City. Technical College.