In Search of Synergy Using Conventional Fire Tests - ACS Symposium

Chapter 19

In Search of Synergy Using Conventional Fire Tests L.

J.

*

Russell, D. C. O. Marney , and V. P. Dowling

Downloaded by UNIV OF SYDNEY on May 18, 2018 | https://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch019

CSIRO Division of Manufacturing and Infrastructure Technology, P.O. Box 56, Highett, Victoria 3190, Australia

Conventional fire testing techniques including U L 94 test, cone calorimetry and oxygen index determination, were used to investigate the interaction between a brominated fire retardant and a proprietry fire performance enhancing additive within a polypropylene matrix. We have focused upon the techniques and methodologies used and the conclusions that may be drawn to gain a better understanding of the system. Analysis of the fire testing results indicates an interaction between the two additives which clearly enhances the fire performance. This work has also shown that the use of multiple fire tests representing different fire models can provide valuable information on the burning process for a particular material.

© 2006 American Chemical Society

Wilkie and Nelson; Fire and Polymers IV ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

237


238 In this paper we consider the utility of conventional fire testing methodologies to study the interaction between a proposed fire performance enhancing additive (FPEA) and a brominated fire retardant (BFR) within a polypropylene (PP) matrix and whether this interaction has an effect on the fire performance of the PP material. The details of the fire retardant additives will not be discussed due to a confidentiality agreement with an industry partner. The interaction between the two additives may impact upon the fire performance in an additional, antagonistic, or a synergistic manner (1). Troitzsch (1) and Lewin (2) have provided extensive discussions on the modes of action of flame retardants and synergists when combined with plastics. Halogenated organic compounds are well known fire retardant additives for PP. They are generally used in conjunction with antimony trioxide compounds to enhance their fire retardant efficacy (halogen-metal synergistic effect) (3). Two types of fire retardant systems will be studied. The first system consists of a BFR combined with PP, and the second system combines the FPEA and a BFR with PP. The focus of this work is on the techniques and methodologies used and the conclusions that may be drawn to gain a better understanding of the system during combustion. We present the analysis of the data from the experiments carried out using the cone calorimeter, UL 94 and the oxygen index apparatus.

Combustion of Solid Polymers - Background The steps involved in the flaming combustion of solid polymers are described schematically in Figure 1. Flaming combustion requires three coupled processes: (i) heating of the polymer, (ii) thermal decomposition / pyrolysis, and (iii) ignition of the gaseous decomposition products. The cycle is completed when an ignition source or thermal feedback of radiant energy from the flame supplies heat to the polymer surface, causing decomposition by thermolytic cleavage of primary chemical bonds in the polymer molecules. Flammable and non-flammable gaseous pyrolysis products mix and react exothermically with air in the combustion zone above the surface. Carbon dioxide, water and products of incomplete combustion such as carbon monoxide and soot are produced. For continuous burning to occur, the application of heat (step 4 to step 1) must be sufficient to decompose the material (step 1 to step 3); the temperature must be high enough to ignite decomposition products (step 2 to step 3); and the amount of heat transferred back to the polymer (step 4 to step 1) must be great enough to maintain the cycle when the initially applied source of heat is withdrawn (4). Thus, solving the problem offireretardancy involves: A. Modification of the thermal degradation process, B. Quenching of the flame, or C. Reduction of the supply of heat from the flame back to the decomposing polymer as shown in Figure 1.


239

Polymer + heat (Step 1)


Heat transfer

Thermal decomposition

Volatile flammable products + tars + char (Step 2)

B

Flame/ ignition

o/co

Heat + products of combustion (Step 4)

2

Flaming Combustion (Step 3)

Figure 1. The Polymer Burning Cycle

Experimental Materials The materials used were polypropylene, a brominated fire retardant (BFR) and the proprietary fire performance enhancing additive.

Specimen Preparation The fire retardant additives were premixed with PP in a TK Fielder powder mixer (Type: TR8, 1430 rpm) prior to feeding to a JSW twin-screw extruder set at 190 °C. The extruded material was then pelletised. Specimens for UL 94, Cone Calorimeter and Oxygen Index tests were injection moulded on a Battenfeld BA 800 CDC injection moulder at approximately 200 °C to produce specimens with dimensions specific to each test. Specimens were conditioned at 23 °C and 50 % relative humidity for a minimum of 48 hours prior to fire testing.


240 The various compositions made from PP, the BFR and the FPEA are given in Table I.

Table I. The compositions of PP/FR material


Material PP (phr) BFR (phr) FPEA (phr)

PP 100

PP+BFR 100 15

PP+BFR+FPEA 100 15 1

PP+FPEA 100 1

Combustion Tests UL 94 Test The UL 94 test (5) is commonly used by industry as a quality control or screening procedure. (6, 7, 8, 9) It provides an indication of a material's ability to self-extinguish with ratings of V-0, V-l, V-2 and not rated (NR) (10, 11, 12). For this work we used the UL 94 Vertical Burning Test in accordance with UL 94 Section 8. Tests were conducted on specimens measuring approximately 125 mm x 12 mm x 3 mm.

Cone Calorimetry The cone calorimeter can be used to examine the performance of fireretarded plastics (13). It is used to determine heat release rate and mass loss, as well as a number of other fire parameters. It uses the oxygen consumption principle (14) with the assumption that there is a constant relationship between the mass of oxygen consumed from the air and the amount of heat released. It has been demonstrated that a value of 13.1 MJ kg" oxygen consumed is appropriate for most polymers (15). By observing changes in the fire parameters measured in PP during this test, the fire retardancy effectiveness of the BFR and the FPEA can be determined. The parameters considered in this study included the following: 1

• • • • •

1

Effective heat of combustion in MJ kg" Ratio of CO to C 0 (based on a mole fraction) Heat release rate in kW m" Mass loss rate in % s" Rate of smoke formation in cm s" 2

2

1

2

1


241 A Stanton Redcroft cone calorimeter was used in accordance with ISO 5660-1 (16). The specimens measured 100 mm x 100 mm x 6 mm and were contained within an aluminium tray during testing. The aluminium tray was taller on the sides than the tray specified in ISO 5660-1 (16) to ensure that mass loss was only via volatilisation and combustion. Specimens were positioned horizontally on a load cell within the cone calorimeter and tested at a heat flux of 50 kW m" . Three specimens each of PP, PP+BFR and PP+BFR+FPEA material and two of PP+ FPEA were tested, and the results reported in the plots presented in figures 2-6 are the average of these replicates. 2


Oxygen Index Test The Oxygen Index (OI) test provides a convenient and reproducible means of assigning a numerical measure to the flammability of materials (17). It measures the minimum level of oxygen necessary to sustain combustion of a material. It is frequently used to compare the effectiveness of fire retardants (18) (19). The higher the OI value, the less likelihood there is for combustion (20). The OI was determined according to ISO 4589-2 (27). Injection moulded materials were cut into specimens measuring 80 mm x 6 mm x 3 mm (to satisfy the dimension range specified for test specimens of form IV). The ignition procedure used was type A - top surface ignition and a step size of 0.2 % for successive changes in the oxygen concentration was used.

Results and Discussion The flammability of the fire-retarded formulations was evaluated by UL 94, Oxygen Index and cone calorimeter tests. It was not the purpose of this paper to draw a comparison between these tests as has been done in previous studies (10, 22, 23). Instead, we combined the unique data obtained from each test to gain an understanding of the fire properties of the systems studied and to analyse the interaction between the FPEA and the BFR in a matrix of PP.

UL 94 Test Analysis of the UL 94 results in Table II show that the addition of BFR to PP improved the UL 94 rating from NR to V-2. The addition of the FPEA to the fire-retarded PP accelerated dripping, as shown by the shorter Time to Drip results. Despite this, the overall result was improved to the more desirable V-0 rating.


242

The t! and t results suggest that the fire retardant action of the BFR was sufficient to extinguish the flame at its source. However, with the addition of the FPEA, a more efficient flame poisoning process was possible (indicated by the failure of the cotton to ignite). This suggests that some gas phase flame poisoning may be taking place. The FPEA also appears to act on the BFR/PP system via a condensed phase action by accelerating the breakdown of the PP (i.e., chain scission (24)), as shown by the Time to Drip results. The lack of flaming drips and the non-ignition of the cotton suggest that the drips retain relatively little heat and thus there is insufficient flammable gases released to maintain the burning. This is shown graphically as stage C in the polymer burning cycle in the previously presented Figure 1. During UL 94 testing, the BFR/PP material burned with a sooty flame and a black sooty material was deposited on the specimen during combustion. This is attributed to the BFR and is an indication of an inefficient burning process and some condensed phase fire retardant action (25). This, however, was not the case when the FPEA was added to the BFR/PP material.


2

Table II. UL 94 Test Results Material

PP PP+ FPEA PP+BFR PP+BFR+ FPEA

Rating

NR NR V-2 V-0

(')

Ave t (s)

Burning of Cotton

Time to Drip (s)

188 113 0.4 0

N/A N/A 0 0

Yes Yes Yes No

13.8 6.3 8.8 8.1

Ave ti

2

NOTE: t| = time that flaming persisted after burner had been removed for the first time; t = time that flaming persisted after burner had been removed for the second time; N/A = flame did not extinguish after first ignition; V-0 = specimen extinguished quickly enough to pass and cotton indicator below the specimen was not ignited (best rating); V-2 = specimen extinguished quickly enough to pass but cotton indicator below the specimen was ignited; NR = no rating (fail); Time to Drip = the time at which the material started to drip measured from the start of the test. 2

Cone Calorimetry

Effective Heat of Combustion (EHC) is a measure of the efficiency to which the emitted gaseous products are combusted. A low EHC indicates less efficient combustion in the gas phase suggesting better flame retardancy (26). From the


243 EHC data in Figure 2 it is clear that the effective heat of combustion plateau for PP is reduced by the addition of the BFR and FPEA. This reduction is mainly a function of the BFR, as shown by the EHC curve for PP/BFR system. The addition of the FPEA has relatively little impact upon the EHC plateau. Closer examination of these curves can also give a clue as to whether the fire inhibition mechanism is occurring in the gas or condensed phase. The addition of the FPEA to the BFR/PP system slows the initial rate at which gaseous products are combusted in comparison with the other systems, (i.e., lower EHC over the first 75 seconds). This suggests that gas-phase flame poisoning mechanisms may occur during this period. The slightly greater emission of CO with respect to C 0 from the FPEA /BFR/PP system compared to the BFR/PP system, as shown in Figure 3, supports this notion, as CO is the product of incomplete combustion.


2

50 -i

g w

-I

0

•

r

.

50

100

•

1

.

150 Time (s)

200

'

i

250

•

1

300

Figure 2. Effective heat of combustion as a function of time for polypropylene andfire-retarded polypropylene at a heat flux of 50 kWm . (Average of replicate measurements) 2

Heat release rate (HRR) is one of the most important parameters for characterising material fire behaviour. It is an indicator of the rate of fire growth and intensity of the fire (27). A more effective fire retardant has a lower HRR



244 (28). For thermoplastics, HRR in the cone calorimeter increases steadily until there is no significant quantity of material left and the peak is typically followed by an abrupt linear decline to zero, with 100% mass loss. A comparison of the peak HRR for thermoplastics is valid only if specimens are similar in thickness and mass. Peak HRR cannot therefore be used to compare thermoplastic specimens of different thickness. Figure 4 shows the HRR as a function of time. It can be seen that the fireretarded systems act to reduce the peak HRR of PP by approximately 15%. The impact of the FPEA upon the BFR system can also be seen in Figure 4. While initially the HRR curves track along similar paths, over the period ca 60 to 100 seconds, the heat release rate per unit time of the FPEA /BFR/PP system is about 50% greater. This trend continues until volatile moities have been consumed and by 125 seconds the curve returns to a similar trend as that for the system containing BFR and PP.

0

50

100

150

200

250

300

Time (s) Figure 3. CO/C0 ratio (by mole fractions) as a function of time for polypropylene andfire-retarded polypropylene at a heat flux of 50 kWm' . (Average of replicate measurements) 2

2

The HRR behaviour of the FPEA /BFR/PP material is different from the other systems in that it displays a peak plateau at 100-125 seconds. The reasons for this are not clear but appear to relate to degradation processes. This is



245 illustrated by the mass loss rate curves in Figure 5. During the early stages of combustion the presence of the BFR enhances the rate of mass loss of the PP suggesting an increase in the thermal degradation of the polymer. With the addition of the FPEA to the BFR/PP system, the rate of mass loss and hence the thermal degradation of the PP, is further enhanced. The effect of the fire retarded systems on the rate of smoke formation as a function of time is shown in Figure 6. There is an increase in the amount of smoke produced from the BFR/PP system relative to the PP system. This increase may be attributed to inefficient combustion caused by flame poisoning as a consequence of chemical interactions between the BFR and flame propagating species. Interestingly, the FPEA /BFR/PP system has a low smoke production rate which suggests a greater degree of oxidation of polymer degradation products.

This presents a conundrum in that generally less smoke indicates better combustion. However it is clear from the UL 94 results previously shown in Table II, that the presence of the FPEA with the BFR improves the fire retardancy i.e. contributes to inefficient combustion. This is also shown by the


246 CO/C0 mole ratios previously presented in Figure 3. A possible explanation for this is that the presence of the FPEA acts to increase the thermal degradation of the PP during the early stages of combustion (as shown by the mass loss rate data). By inference, lower molecular weight pyrolysis fragments are generated and these smaller fragments are more flammable and have an increased probability of interaction with the BFR and targeted free radical species. This provides the opportunity for more complete oxidation of volatile products during the thermal degradation of the material, thereby allowing the BFR to be more effective in its role of flame poisoning. In summary, the cone calorimeter results presented previously in Figures 2, 3 and 4 as well as in Table III indicate that during the first 75 seconds the addition of the FPEA to the BFR/PP system slows the rate at which the gaseous products are combusted suggesting gas phase flame poisoning. During this early stage of the combustion process, the greater formation of CO with respect to C 0 for systems containing BFR indicates burning inefficiency and confirms gas phase action. The slightly higher CO:C0 ratio for the FPEA /BFR/PP material indicates that the FPEA acts on the BFR/PP to further increase the inefficiency of combustion.


2

2

2

Figure 5. Mass loss rate as a function of time for polypropylene andfire retarded polypropylene at a heat flux of 50 kW m . (Average of replicate measurements) 2



247

0

50

100

150

200

250

300

Time (s) Figure 6. Rate of smoke formation as a function of time for polypropylene and fire-retarded polypropylene at a heat flux ofSOkW m . (Average of replicate measurements) 2

After the first 75 seconds the CO:C0 ratio tends towards zero and there is no further increase in the EHC as it tends to plateau. This indicates the completion of the gas phase fire retardant action and a transition to some other mechanism (most probably condensed phase). It is at this stage in the combustion process that the HRR curves for the FPEA/BFR/PP system increases rapidly compared to the other systems, indicating the commencement of a different fire inhibition mechanism (presumed to be condensed phase). 2

Table III. Effect of FPEA on Fire Retarded PP Parameter PeakCO/C0 (mf) EHC (at 50 sees) (MJkg ) HRR (at lOOsecs) (kW m- ) 2

PP+BFR 0.34

PP+BFR+ FPEA 0.54

% change Up by -60%

20

14

Down by 30%

860

1145

Up by 33%

1

2


248 Oxygen Index Test


The OI value obtained for PP (17.9) shown in Table IV correlates well with a typical literature value of 17.4 (18, 19). This low OI value indicates that PP is relatively flammable. With the addition of the BFR to PP, the OI value is increased by approximately 50 % to 27.2, indicating improved fire retardancy of the PP. The addition of the FPEA to the BFR/PP system further increases the OI value to 30.7, thus enhancing the fire retardancy of the PP even more. However, unlike the UL 94 test, there was no visual evidence that the presence of the FPEA increased the flow of the polymer away from the flame.

Table IV. OI Test Results Material

OI (%)

PP PP+ FPEA PP+BFR PP+BFR+ FPEA

17.87 21.86 27.22 30.67

Conclusions The analyses carried out during the investigation of the fire retardant systems using conventional fire testing techniques, have shown that there was a interaction between the two additives which improved the fire performance of the PP. However it was unclear as to whether this interaction was synergistic or purely additive especially when considering the OI results. Further testing will be carried out in the future using techniques such as nitrous oxide index (NOI) combined with the current OI data, pyrolysis GC-MS, pyrolysis FTIR, TGAFTIR and pyrolysis NMR to determine whether there was synergy or simply an additive effect of two fire retardants. Such work will need to consider the organic chemistry mechanisms which occur between the two additives in the PP matrix in the time leading up to decomposition and during the fire. The investigation has shown that the use of multiple fire tests representing different fire models can provide valuable information on the burning process of a fire retarded polymeric system. Further, analysis of the data in a non-standard way provided useful insights into how the range of fire retardant systems functioned. The cone calorimeter data (EHC, HRR and CO:C0 ratios) suggested that the presence of the FPEA in the BFR/PP system altered the behaviour of the polymer via the gas phase during the early stages of combustion, and 2


249 subsequently via the condensed phase. The mass loss rate data demonstrated that the proposed synergist acted to accelerate the degradation of the polymer, i.e., condensed phase. The CO and C 0 gas analyses confirmed that the burning process was more inefficient in the presence of this material. Further, we have shown that cone calorimeter data can be utilised as an analytical tool to contribute to the understanding of complex interactions between fire retardant additives. As a result of considering the EHC, CO:C0 gas ratios and the HRR, we were able to comment upon, and suggest where gas and condensed phase fire retardancy occurred. The UL 94 and Oxygen Index data were strong indicators of interaction between the two fire retardant additives. Close examination of the UL 94 data indicated that this method of analysing fire properties was not just limited to a pass or fail. Instead it could be used to provide more information about the fire inhibition process by indicating the action or mechanism of the fire retardant; i.e., gas phase, condensed phase or a combination of the two and when they occur in the process. 2


2

References 1. 2. 3. 4.

Troitzsch, J. H. Prog. Org. Coat. 1983, 11, 41-69. Lewin, M. J. Fire Sci. 1999, 17, 3-19. Chiu, S.-H.; Wang, W.-K. Polymer. 1998, 39, 1951-1955. Grassie, N.; Scott, G. Polymer Degradation and Stabilisation; Cambridge University Press: New York, 1985. 5. Underwriters Laboratories Inc. UL 94 Standard; Northbrook, IL, 2000. 6. Use of the Cone Calorimeter for Evaluation of Plastics Used in Containers for Flammable Materials. http://www.mcclureindustries.com/pdf/Cone.pdf (accessed Jul 2004). 7. Marplex Home Page. http://www.marplex.com.au/application_content/telecornmunications.htm (accessed Aug 2004). 8. Erntec Home Page. http://erntec.net/content/products/?0,3,a001,37 (accessed Oct 2004). 9. M+H Power Systems Home Page. http://www.mhpower.com.au/Batteries/TechBrochure/techinfo 1 .htm (accessed Sep 2004). 10. Hong, S.; Yang, J.; Ahn, S.; Mun, Y.; Lee, G. Fire Mater. 2004, 28, 25-31. 11. Fernandes Jr., V. J.; Araujo, A. S.; Fonseca, V. M.; Femandes, N. S.; Silva, D. R. Thermochim. Acta. 2002, 392-393, 71-77. 12. Ebdon, J. R.; Hunt, B. J.; Joseph, P.; Konkel, C. S.; Price, D.; Pyrah, K.; Hull, T. R.; Milnes, G. J.; Hill, S. B.; Lindsay, C. I.; McCluskey, J.; Robinson, I. Polym. Degrad. Stab. 2000, 70, 425-436.



250 13. Heat Release in Fires; Babrauskas, V.; Grayson, S. J. Eds.; Elsevier Applied Science: London, 1992; pp 423-446. 14. Babrauskas, V. Fire Mater. 1984, 8, 81. 15. Huggett, C. Fire Mater. 1980, 4, 61-65. 16. International Organisation for Standardisation; International Standard ISO 5660-1; Geneva, 2002. 17. Nelson, M. Combustion of Polymers: Oxygen Index Methods; [Website] http://www.uow.edu.au/~mnelson/review.dir/oxygen.html, 2002. 18. Camino, G.; Costa, L. Polym. Degrad. Stab. 1988, 20, 271-294. 19. Fire and Polymers II: Materials and Tests for Hazard Prevention; Nelson, G. L. Eds.; ACS Symposium Series 599; American Chemical Society: Washington, D.C., 1995; pp 1-28. 20. GE Plastics Home Page. http://www.geplastics.com/resins/devprod/flammabilityt.html (accessed Sep 2004). 21. International Organisation for Standardisation; International Standard ISO 4589-2; Geneva, 1996. 22. Weil, E. D.; Hirschler, M. M.; Patel, N. G.; Said, M. M.; Shakir, S. Fire Mater. 1992, 16, 159-167. 23. Lyon, R.; Fire and Flammability. First Annual Conference for the Consortium for Fire Safety, Health and the Environment; SP Swedish National Testing and Research Institute: Boras, Sweden, 2003. 24. Kaspersma, J.; Doumen, C.; Munro, S.; Prins, A. Polym. Degrad. Stab. 2002, 77, 323-331. 25. Kuryla, W. C.; Papa, A. J. Flame Retardance of Polymeric Materials; Marcel Decker Inc.: New York, 1978. 26. Li, B.; Wang, J. J. Fire Sci. 1997, 15, 341-357. 27. Tewarson, A. Fire Mater. 1980, 4, 185-191. 28. Plastics Flammability Handbook: Principles, Regulations, Testing and Approval; Troitzsch, J. Eds.; Carl Hanser Verlag: Munich, Germany, 2004; pp 133-157.


In Search of Synergy Using Conventional Fire Tests - ACS Symposium

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