Polycarbonate – Flammability

At HighLine Polycarbonate LLC, we receive a lot of questions relating to material flammability.  These questions often occur because of the wide range of tests relating to flammability and the limited information available from manufacturers.  In this blog post we will discuss some of the most common test methods and what they mean.  It should be recognized that there are many local and national regulations that refer to different test methods and that we cannot cover all of these in a single post.  In particular, we have decided not to attempt to cover building regulations due to the multitude of local codes.  Instead we have concentrated on the transportation industry.

UL.94

The most common method of defining polycarbonate sheet flammability properties is UL.94; this test method was developed by Underwriters Laboratories in the USA.

There are multiple levels of flammability:

HB – A piece of the material to be tested is held horizontally, a flame is applied to one end of the material for 30 seconds.  When the flame is removed the material must extinguish before the flame travels 75mm along the material.

V2 – A piece of the material to be tested is held vertically, a flame is applied to the material for 10 seconds.  When the flame is removed, the material must not burn for more than 30 seconds.

V1 – This test is the same as for V2, with the additional requirement that the specimen must not drip flaming particles that ignite cotton placed under the test specimen.

V0 – This test is the same as for V1, with the additional requirement that the material must not burn for more than 10 seconds.

 The easiest of these tests to pass is the HB test and the hardest test to pass is V0.  As an indication, polycarbonate, without any flame retardant additives, would pass the tests as shown in the table below.  Please note that these figures are only be used for information and test certificates should be obtained from your polycarbonate sheet supplier.

HB      0.060” or thicker

V2       0.125” or thicker

V1       0.1875” or thicker

V0       0.25” or thicker

As can be seen, the thicker the polycarbonate, the more resistance it is to the flammability tests. 

If the design specification calls for a V0 rating at 0.125” thickness, standard polycarbonate will not be able to meet the specification.  In this case, flame retardant polycarbonate sheets would need to be considered.  Alternatively, a thicker piece of polycarbonate could be specified.  Using a thicker piece of polycarbonate would probably be cheaper if the design allows; the material would also be much more available.

Within the UL.94 standard there are two additional higher levels of flammability, 5VB and 5VA.  As these ratings are not as common we will not go into details here.  However, a quick internet-search will reveal details on these tests.

Aircraft specifications – FAR 25.853

The FAA (Federal Aviation Administration) developed a standard for materials to be used in aircraft cabin and cargo compartments.  This standard is known as FAR.25.853.  For polycarbonate sheet there are two relevant parts FAR 25.853a and FAR 25.853d relating to flammability.

FAR 25.853a measures the resistance of material to flames.  The material is held vertically and a Bunsen burner is applied for (i) 60 Seconds and (ii)12 seconds.  Three items are measured:

The flame time – the time that the specimen continues to burn after the flame is removed.

The drip flame time – the time that any flaming material continues to flame after falling from the material.

The burn length – the distance from the original specimen’s edge to the farthest evidence of damage to the specimen.

To pass the tests the material must achieve the following:

Test                                    Flame time (sec)        Drip flame time (sec)        Burn length

(i) 60 second                          < 15                            < 3                                    6"
(ii) 12 second                         < 15                            < 5                                    8"

We are aware of a number of transparent polycarbonate sheets with flame retardant additives that can pass this test.

FAR 25.853d consists of two tests.  The first, the OSU (Ohio State University) Rate of Heat release tries to limit the possibility that materials will become rapidly involved in a fire or contribute to an existing fire.  The rate of heat release is measured using the principle of oxygen consumption using the OSU calorimeter and the test method is published under ASTM E906.  The standard requires that the total heat release within the first 2 minutes is <= 65kW per square meter and that the Peal Heat Release Rate is <= 65KW per square meter.  In data sheets for flame retardant polycarbonate sheets, if a material passes this test it is often written a FAR 25.853a OSU 65/65.

At this stage we are not aware of any transparent polycarbonate sheets that can pass OSU 65/65 even with flame retardant additives.  We are aware of opaque polycarbonate sheet that can pass this test such as the Lexan XHR.6000 and the halogen free Ultem 1668A.

The second test in FAR 25.853d is the smoke density test, which is similar to ASTM E.662 (which we shall discuss later).  This test measures the amount of smoke that is generated, which could prevent passengers escaping in a fire situation.  To pass this test the 4.0 minute smoke density Ds (4 min) <= 200.  A number of transparent polycarbonate sheets, with flame-retardants, are able to pass this test.

To pass all elements of FAR 25.853 a material must list the following:

FAR 25.853a (i) 60 seconds                           Pass

FAR 25.853a (ii) 12 seconds                          Pass

FAR 25.853d Rate of Heat Release                OSU 65/65

PAR 25.853d Smoke Density Ds(4min)        <= 200

Rail car specifications – 49 CFR Part 238

The FRA (Federal Railroad Administration) developed a standard for materials to be used in rail car applications in the US.  The part of this standard that deals with flammability is known as 49 CFR Part 238.  For polycarbonate sheet Appendix B of this standard requires two tests, ASTM E.162 and ASTM E.662.

ASTM E162 provides a measure of flame spread and heat evolution.  The maximum value to pass this test Is <= 100

ASTM E.662 is similar to the test used for the aircraft industry to measure smoke density. 

To pass this test, the smoke density after 1.5 minute Ds(1.5 min) <= 100 and the smoke density after 4 minutes Ds (4 min) <= 400.  It can be seen that the 4 minute test for the FRA is not a severe as for the FAA.

In general, some of the thicker polycarbonate transparent sheet can pass these tests without the need for flame-retardants.  However, test certificates must always be provided by the manufacturer.  

Overview

Transparent polycarbonate sheet without flame retardant additives has a reasonable level of flame resistance, particularly as the thickness increases.  It is able to pass some of the UL.94 tests and can pass some of the Smoke Density, Fame spread and heat release tests required by various transport administrations. 

Where these properties are not sufficient, flame retardant additives can be used to improve the properties, but these add to the price.  Some significant improvements can be made while still allowing the material to remain transparent.

With some of the more demanding specifications, higher doses of more complex flame retardant additives are required.  Achieving some of the higher specifications are not possible while also retaining the transparency of the material.  Also, adding these flame retardant additives can impact other properties of the polycarbonate sheet.