Milligram Scale Flammability Testing of Flame Retardant Polyurethane

Chapter 28

Milligram Scale Flammability Testing of Flame Retardant Polyurethane Foams Downloaded by UNIV OF ARIZONA on December 19, 2012 | http://pubs.acs.org Publication Date (Web): December 18, 2012 | doi: 10.1021/bk-2012-1118.ch028

Alexander B. Morgan* University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0160 *E-mail: [email protected]

To speed up the development of new flame-retardant chemistries for polyurethane foam, there is a strong need for small-scale flammability tests, especially those which work at the milligram scale. In this paper we discuss current results using the only standard milligram heat release test (Pyrolysis combustion flow calorimetry – ASTM D7309) to study how some commercial flame retardants work in flexible polyurethane. Flexible polyurethane foams formulated to pass small flame ignition regulatory tests showed only small reductions in measured heat release when compared to non-flame retardant foams. This suggests that reductions in heat release are not the primary mechanism by which these additives pass these regulatory tests (FMVSS 302, TB-117) and additional testing in combination with ASTM D7309 may be needed to understand performance and further develop this technique for future flame retardant polyurethane chemistries.

Introduction Flexible polyurethane foams present a significant risk of catastrophic fire loss in the home, should items that contain them (bedding / furniture) catch fire and not be extinguished within 5 minutes of ignition (1–4). The reason for this high chance of fire loss due to flashover type events is that polyurethane foam, when it burns, drips and flows to form a pool-type fire which leads to rapid increases © 2012 American Chemical Society In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Downloaded by UNIV OF ARIZONA on December 19, 2012 | http://pubs.acs.org Publication Date (Web): December 18, 2012 | doi: 10.1021/bk-2012-1118.ch028

in heat release (5, 6). While flame retardants do exist for polyurethane foams, most of them are optimized against small ignition sources (cigarettes, small open flames) (7, 8) and do not address the dripping and fuel pool fire should larger ignition sources be present. However, there are robust flame retardant systems that address this dripping behavior when foams are used in aircraft (9) or in the United Kingdom (10), but for most commercial goods in the US containing flexible polyurethane foam, the dripping upon burning that leads to faster flame growth remains an unsolved problem. To that end, there is a need for new reactive flame retardants which can co-polymerize with the polyurethane backbone during foam synthesis so that they present no environmental concerns later (11–13), and also have a flame retardant mechanism that causes the polyurethane to char rather than drip and flow. While there are some new potential flame retardants which show potential for reactivity and char formation in polyurethanes (14–16), there is much more work to do before they can be considered for commercial flame retardant solutions. Specifically, these additives need to be scaled up, tested for commercial viability, and have their environmental, health, and safety (EH&S) issues tested before they can be used. This research must be done in addition to regulatory scale fire testing to confirm that the new flame retardant provides satisfactory fire safety, and all of this testing results in significant costs to the researcher, whether they be academic or industrial. Before such resources can be committed, there needs to be some confidence that the lab-scale evaluation of new flame retardants has a high chance of success at larger scale. Therefore, there is need for a small-scale test which can screen through lots of different chemical structures using commercial, EH&S, and flammability criteria before resources are consumed scaling up the flame retardant additive. Some commercial and EH&S screening models exist which work regardless of scale, but a small-scale screening test for definite flammability screening criteria does not exist today. However, one particular small-scale test which measures heat release may be appropriate in this role, and that is the pyrolysis combustion flow calorimeter, or PCFC, also known as the microcone calorimeter, MCC. PCFC is a 5-50 mg scale heat release test which uses oxygen consumption calorimetry to determine the inherent heat release of a material (17). This smallscale test is an ASTM method (ASTM D7309-07) and has been used successfully as a screening tool for several polymeric materials (18–21), but has yet to be used for polyurethane foam flame retardant development. Because of the prior success with this technique for developing flame retardant materials, and because it has been used to quantify heat release as an effect of chemical structure (22), PCFC seems as if it could be a potent screening tool for future flexible polyurethane foam development with new flame retardant chemistries. With this in mind, research was undertaken to see if the PCFC could be used to understand how commercial flame retardants work in polyurethane foam to pass existing regulatory small-scale flame tests. Specifically, do these flame retardant (FR) additives yield significant reductions in heat release which leads to the passing of the test, or do the FR additives provide a passing result via another mechanism?

446 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


In light of the discussion about the need of a technique that can screen for flame retardant performance before scale-up of the new flame retardant chemical, it is important to say what this work is meant to accomplish and what it does not do. This work is meant as a starting point to take a look at what commercial flame retardants achieve in flexible polyurethane foam from a small scale heat release test. To date, no one has published what the heat release of commercial flame retardant polyurethane materials is when measured by PCFC, and so just obtaining that measurement and sharing it with the broader community has value. By measuring how these flame retardants affect heat release in flexible polyurethane foam, we have some starting information to begin building a model of flame retardant screening for polyurethane foams. So the work in this chapter is an initial observation that hopefully this researcher and other groups can build upon. This work is not intended to validate what amount of heat release is required to achieve a passing result in regulatory tests. Nor is this work giving any indication that the measurements within are the definitive explanation for how the flame retardants work to enable polyurethane to pass the test. Again, it is an initial investigation and the hope is that this paper will inspire others to publish their own data on flexible polyurethane foams that pass regulatory tests with the PCFC so we can see and share the wider range of heat release behavior provided by different flame retardants and flame retardant approaches. In this paper the results of such a study are presented by measuring the heat release of two sets of commercial flame retardant flexible polyurethane foams. The first set of foam studied was optimized to pass a small flame ignition, horizontal orientation, flame spread test (FMVSS 302) used to rate the fire performance of foams in automobile seating. The second set of foam studied was optimized to pass a small flame ignition, vertical orientation, flame spread test (California Technical Bulletin TB-117) used for foams sold in US-made upholstered furniture. Details on these specific regulatory tests can be found in reference #3. Since no flame spread or specific flame test data for these two regulatory tests was provided for the foams tested in this report, no attempt will be made to correlate the results of PCFC to these two tests.

Experimental Section Flame retardant polyurethane foams containing commercial flame retardants were provided by Israeli Chemical Ltd. (ICL) and Clariant GmbH, and were used as received. The exact composition of the flame retardant foams was not provided by the manufacturers, other than the general flame retardant present in the foam. PCFC testing was conducted via ASTM D7309-07, in triplicate, using Method A (pyrolysis under N2, 1 °C/sec heating rate) with the use of a MCC-1 instrument (Govmark, NJ, USA). Typical results from the PCFC, including char yield, heat release rate (HRR) peak values (in W/g) and HRR peak temperatures, were recorded.


Results and Discussion


The samples for the FMVSS 302-rated foams utilized reactive (will react into the polymer backbone during foam synthesis) phosphate-based flame retardants. These flame retardants, Exolit OP550 and Exolit OP560, are phosphate polyols, and while the exact chemical structure was unknown, some general details were provided by the manufacturer (Figure 1). The samples for the TB-117-rated foams had non-reactive flame retardants, namely an organochloro phosphate (Fyrol FR2) and a polymeric organophosphate (Fyrol PNX), which are shown in Figure 1.

Figure 1. Chemical Structures of Flexible Polyurethane Foam Flame Retardants.

With each set of foams optimized for a particular regulatory test, a set of control foams (no flame retardant) was provided for each set. First we will discuss the FMVSS 302 foams and then the TB-117 foams. For the FMVSS302 foams, there is not a great deal of difference in the measured heat release rates for these materials, as can be seen in Figures 2 and 3, when comparing the flame retardant sample to the control sample. Indeed, the only major difference noted is that the first peak of heat release is reduced in the flame retardant samples when compared to the control sample. Even the temperatures at which the peak HRR occurs are mostly the same, although they do seem to occur about 10-15 °C lower when compared to the control sample. OP550 yields a slight reduction in the second peak of HRR as well, while OP560 seems to increase the second peak of HRR slightly. Total HR is reduced though with the use of flame retardant, and char yields are slightly increased as well. While there is no definitive chemical analysis data available to comment exactly on the chemical flame retardant mechanism provided by these additives, the data suggests that the flame retardants do not seem to change the known polyurethane thermal decomposition chemistry. Flexible polyurethanes typically decompose in two steps (1, 14). The first, (lower temperature,) event is the decomposition of the urethane groups which leads to pyrolysis of the chemical structures associated with the starting isocyanate monomers used to produce the foam. The second, (higher temperature,) event is where the polyols decompose and pyrolyze. This two-step phenomena is seen in the FMVSS 302-rated samples (Figures 2 and 3), and other than some minor reductions in heat release, there is very little difference between the samples (Table 1). While the % error and uncertainty have not been officially published in the ASTM D7309, these differences in heat release, while reproducibly measured, are likely not statistically 448 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


significant and therefore cannot be used to differentiate the samples. Therefore, some other sort of mechanism is likely occurring with these polyphosphate additives that allow the foams containing them to pass the FMVSS 302 test. Since we do not have actual FMVSS 302 test data for these samples, nor any physical observations on how these samples passed the test, we cannot make any further commentary on the results.

Figure 2. HRR for FMVSS 302 Control foam (left) and Flame Retardant Foam with OP550 (right).

Figure 3. HRR for FMVSS 302 Control foam (left) and Flame Retardant Foam with OP560 (right). 449 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

450


Table 1. Heat Release Data for FMVSS 302-Rated Foams

Sample

Char Yield

HRR Peak(s)

1st peak HRR

2nd peak HRR

Peak Temps

Total HR

(wt%)

Value (W/g)

% Reduction

% Reduction

(°C)

(kJ/g)

Total HR % Reduction

Non FR

0.21

157, 514

n/a

n/a

316, 408

27.0

n/a

Flex PU Foam

0.37

156, 486

n/a

n/a

312, 405

27.0

n/a

0.51

173, 496

n/a

n/a

319, 409

27.1

n/a

OP 550

1.93

107, 479

34

4

298, 415

25.7

4.9

Flex PU Foam

1.99

129, 488

20

2

300, 416

25.8

4.6

1.91

118, 490

27

2

299, 415

25.7

4.9

OP 560

0.49

125, 552

23

-11

300, 415

26.3

2.7

Flex PU Foam

0.87

139, 554

14

-11

306, 416

26.3

2.7

0.74

138, 535

15

-7

304, 416

26.3

2.7

In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


In Table 1 the measured heat release and % reductions in peak HRR and total HRR for the FMVSS 302 samples are shown. OP550 shows a 20-34% reduction in first peak HRR, only a 2-4% reduction in second peak HRR, and 4.6-4.9% reduction in total HR. OP560, on the other hand, shows a 14-23% reduction in first peak HRR, a 7-11% increase in second peak of HRR, and a 2.7% reduction in total HR. So assuming the loading of flame retardant was the same in both foams, OP550 is the superior flame retardant in regards to passing the test and reducing heat release. However, since the official % uncertainty and error for the ASTM D7309 technique has not yet been published by ASTM, no definite error bars can be given for the data. Discussions with other experts who have used the PCFC as well as the reviewer of this paper suggest that the differences are not significant. So what can be said about the results? While the differences may be within assumed % error of the technique, it is worth noting that the data is quite reproducibly different. Clearly the flame retardants have some effect on heat release, even if it is minor. So if the heat release reduction is small, and more of an effect is noted on the first peak of HRR than the second, one can conclude that these flame retardants inhibit thermal decomposition of the urethane groups in the polymer structure and subsequently delay pyrolysis of the isocyanate monomer groups. This suggests that the flame retardants slow initial thermal decomposition and mass loss of the flammable isocyanate-based monomers, which may in turn slow flame spread in the FMVSS 302 test. Admittedly this is speculation, and two things are needed to verify this point further: 1) a larger sample set of more flame retardant foams which pass FMVSS 302 with different flame retardant chemistries, and 2) chemical analysis confirming vapor-phase or condensed-phase activity, plus physical property analysis looking at polymer viscosity during burning (dripping away from flame to slow flame spread). Existing literature suggests that these polyphosphates likely have both vapor- and condensed-phase chemistry (23), so with that information, more work is needed before the PCFC data can be interpreted further.

Figure 4. Final chars for FMVSS control foam (left), Flame Retardant Foam with OP550 (center), and Flame Retardant Foam with OP560 (right).

Also of note when discussing the FMVSS 302 samples, the chars for these samples collected at the end of the PCFC test are shown in Figure 4. As can be seen, all of the samples showed very little residual char remaining, indicating that none of these materials generate a lot of char when they thermally decompose, 451 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


and that they likely flow during burning (2), which is indicated by the presence of black residue all over the PCFC test crucible by the end of the test (Figure 4). Some small “chunks” of char do remain with the OP550 and OP560 containing samples though.

Figure 5. HRR for TB-117 Control foam (left) and Flame Retardant Foam with Fyrol FR2 (right).

Figure 6. HRR for TB-117 Control foam (left) and Flame Retardant Foam with Fyrol PNX (right). For the TB-117-rated foams, as with the FMVSS 302 samples, the differences in heat release are minor. The use of flame retardant results in a reduction of the first peak of HRR, but a slight increase in the second peak of HRR is noted as well. The temperatures of the first peak of heat release are either slightly decreased by 452 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


10-15 °C (Fyrol FR2) or unchanged (Fyrol PNX). Total HR is always reduced with the TB-117-rated flame retardant samples, although char yields are not always increased with Fyrol FR2 generating considerably more char than Fyrol PNX. This is worth noting on since normally decreases in total HR are accompanied by increases in measured char yield (18, 20). Otherwise, the shapes of HRR for the flame retardant and control samples are very similar, as can be seen in Figures 5 and 6. Again, a two-peak HRR is observed, with the first peak corresponding to the thermal decomposition of the isocyanate-based monomer and the second peak to the decomposition and pyrolysis of the polyol. The measured heat release reductions are shown in Table 2, and as with the FMVSS 302 samples, reductions in peak HRR and total HR are observed for the samples that pass this test. For Fyrol PNX, there is a 19-25% reduction in first peak HRR and a 4.9-5.7 % reduction in total HR. The second peak of HRR increases, however, by about 6-10%. Fyrol FR2 shows a 14-20% reduction in first peak of HRR and a 6.8-7.1% decrease in Total HR, but second peak of HRR increases by 15-21%. Since the two flame retardants are not at equal loading, one cannot make an exact comparison of performance, but when looking at this data, Fyrol PNX is more effective at reducing the first peak of HRR, and almost as effective at reducing Total HR, at half the loading of flame retardant. The increase in second peak of HRR for these materials likely is related to the chemical mechanism of flame retardancy that these two flame retardants bring to polyurethane, but it is counterintuitive that the flame retardant reduces one aspect of heat release while increasing another. Specifically, the flame retardants seem capable of reducing the mass loss and heat release caused by the thermal decomposition of the urethane groups, but appear to accelerate the pyrolysis or decomposition of the polyol. As with the FMVSS 302 discussion, it can be argued that these results are not statistically significant, although they are reproducible measurements. Therefore, the following comments can be made about the TB-117-rated foam data:

•

•

•

The flame retardants seem to inhibit initial decomposition and pyrolysis of the urethane groups, and slowing this initial heat release may slow flame spread. Definite chemical and physical analysis of the thermally decomposed foams is needed to verify the mechanism of “passing” the TB-117 test. The data only suggests, and does not prove, that a reduction in urethane decomposition and pyrolysis occurs, but it also suggests an increase in polyol decomposition which may yield a dripping effect in the TB-117 test where the foam drips away from the flame source, thus limiting flame spread. More flame retardant foams rated to pass TB-117 with different flame retardant chemistries are needed to determine whether the measured results here are flame retardant specific, or something always required for any TB-117 foam to pass the test. 453


454


Table 2. Heat Release Data for TB-117-Rated Foams

Sample

Char Yield

HRR Peak(s)

1st peak HRR

2nd peak HRR

HRR Peak

Total HR

Total HR

(wt%)

Value (W/g)

% Reduction

% Reduction

Temp (°C)

(kJ/g)

% Reduction

1081-40-3

0.52

165, 438

n/a

n/a

312, 411

27.2

n/a

Flex - no FR

0.51

137, 456

n/a

n/a

308, 408

27.6

n/a

0.55

159, 469

n/a

n/a

309, 405

26.6

n/a

1081-40-1

1.72

124, 483

19

-6

295, 412

25.8

4.9

Flex - 6 parts

1.78

114, 497

26

-9

297, 413

25.6

5.7

Fyrol PNX

1.54

115, 498

25

-10

293, 412

25.6

5.7

1081-40-2

0.24

131, 523

15

-15

311, 412

25.3

6.8

Flex - 12 parts

0.28

132, 549

14

-21

310, 416

25.3

6.8

Fyrol FR2

0.24

123, 540

20

-19

313, 415

25.2

7.1



Final chars for the TB-117-rated foams are shown in Figure 7. These foams flowed during thermal decomposition, but did form some fragments of char, as can be seen in the crucibles. However, these fragments are quite small, and as can be seen from the data in Table 2, these foams are not robust char formers.

Figure 7. Final chars for TB-117 control foam (left), Flame Retardant Foam with Fyrol FR2 (center), and Flame Retardant Foam with Fyrol PNX (right).

Conclusions Using PCFC with flame retardant flexible foams rated for regulatory tests gives some information about how the flame retardants may be working to provide a passing result, but the results are not definitive. Flexible polyurethane foams have a two-step decomposition, the first step being caused by the decomposition of the urethane groups in the polymer, and the second step being caused by polyol decomposition. This was measured with the PCFC as a two peak heat release, with the first peak coming from urethane decomposition/pyrolysis and the second peak coming from polyol decomposition and pyrolysis. In the case of the FMVSS 302 foams, which used reactive polyphosphate flame retardants (phosphates that react into the polymer backbone), there is a reduction in the first peak of heat release, but little to no reduction in the second peak of heat release. This suggests, but does not prove, that these flame retardants help the foam pass the horizontal flame spread test by slowing urethane decomposition chemistry and pyrolysis. However, since the residue in the PCFC crucibles at the end of the test suggests that the polymer flowed during decomposition, one cannot rule out that the polymer dripping away from the flame may also play a role in achieving a passing result in the FMVSS 302 test. Similar behavior of first and second peak of heat release is also seen for the TB-117 foams, which are rated to pass a vertical flame spread test. Specifically, the first peak of heat release is reduced while the second peak of heat release either is unchanged or increases. So again, the flame retardants (a chloroalkyl phosphate and a polymeric non-reactive phosphate) used to provide passing results in TB-117, suggest that they help slow down urethane group decomposition and pyrolysis, but have little effect on polyol decomposition and pyrolysis. Also similar to the FMVSS 302 samples, final residues from the PCFC crucibles suggests that the foams may flow during decomposition, and so a combination of slowing of urethane decomposition plus dripping away from the flame cannot be ruled out as the mechanism of passing TB-117 for these foams. 455 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Admittedly these conclusions are speculative and not definitive, especially in light of the fact that the % error and uncertainty for the PCFC measurements are still not officially known, and therefore the differences in the samples are believed not to be greatly significant. Therefore one can ask, “Why bother using PCFC at all for these materials?” or “Why have you wasted our time publishing this if there are no definitive results?” The answer to these questions is simply this: There is no other small-scale milligram test available to measure heat release of materials, which is a fundamental aspect of material flammability and flame retardancy; and therefore, this technique is a good starting point because there is no other milligram scale test available. Based upon the data in this paper, to advance PCFC as a tool for polyurethane development further, the following is needed: •

•

•

A larger sample set of flexible polyurethane foams in which the chemical composition is known, so chemical structure of the flame retardant and its effect on heat release (if important) can be used to predict % chance of passing a specific regulatory test. Known flame spread / fire phenomena for the samples tested in FMVSS 302 and TB-117 (or other relevant polyurethane tests) so that physical effects of flame retardant mechanism can be captured. Additional measurements to confirm which flame retardant chemistries are appropriate to evaluate with the PCFC, and which ones are not.

Returning to the introduction, if new flame retardant chemistries are to be developed, one needs to develop small-scale tests to minimize the risks and costs of synthesizing, registering, and developing new chemicals. So far, only the PCFC continues to show this potential, and indeed, the PCFC has been used with some success to predict % likelihood of passing the 12-second flame exposure, vertical orientation, Federal Aviation Administration test based upon the measured heat release of flame retardant flexible polyurethane foams used in aircraft seating (24). So with that potential being reported, it still makes sense to use the PCFC for future flame retardant polyurethane foam development. Before that potential can be fully achieved, though, more samples must be tested and a wider range of chemistries must be studied to develop a database that can predict % chance of passing larger scale regulatory tests. It is the author’s belief that the data in this paper is a starting point, and he hopes that others will build upon this data to enable the potential of the PCFC to be fully reached.

Acknowledgments The author wishes to thank Kathleen Beljan, Mary Galaska, and Kathy Schenck for their assistance in collecting the PCFC data shown in this report. Thanks also goes to Israeli Chemical Ltd. (Sergei Levchik for technical discussions, Barbara Williams and Emanuel Pinzone for foam sample fabrication) and Clariant GmbH (Timothy Reilly for project coordination, Elke Huthmacher and Frank Osterod for PU foam sample fabrication) for the kind donation of commercial flame retardants and flame retardant polyurethane foams. The author 456 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

also wishes to thank numerous reviewers of this paper (and previous versions) who have greatly improved the concept of using PCFC for flame retardant material development. Funding for this work was provided by the US taxpayer through the National Institute of Standards and Technology Fire Research Grant # 70NANB9H9183.

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Milligram Scale Flammability Testing of Flame Retardant Polyurethane

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