Flame retardant polymeric materials: II. The basics & recent trends in studies of flame retardant mechanisms

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1 Nippon Gomu Kyokaishi, No. 9, 2013, pp. 291 xxx Flame retardant polymeric materials: II. The basics & recent trends in studies of flame retardant mechanisms H Nishizawa Nishizawa Technical Laboratory, Kami-Iidamachi, Izumi-ku, Yokohama Selected from International Polymer Science and Technology, 40, No. 12, 2013, reference NG 13/09/291; transl. serial no Translated by K. Halpin Introduction The most important task in making a polymer material flame retardant is to understand its behaviour and to study and analyse the mechanisms involved in flame retardancy. The first of these reviews looked at the background to the development and application of flame retardant polymer materials with particular reference to flammability standards and environmental safety regulations. This second review examines the behaviour of polymer materials and basic technology of flame retardancy mechanisms, along with recent trends in research. Basic technology of polymer and flame retardancy mechanisms A polymer material starts to burn when oxygen is present and heat is available at an energy level meeting the criterion for. The combustible component, oxygen and heat are known collectively as the three elements of. More recently, the free radicals evolved in and their chain reactions have been included as a fourth element. The course of events when a polymer is heated is illustrated in Figure 1: softening, decomposition and decline in molecular weight due to scission of the molecular chains is followed by evolution of flammable gases, ignition and spread of flame. Flame retardancy mechanisms are more easily appreciated when considered separately for two sets of reactions: inhibition of in the gas phase in the ambient air, and inhibition in the solid phase at the polymer surface. For reference, the inhibitory reactions in the respective phases are set out in Tables 1 and 2. (1) Flame retardancy mechanism in the gas phase 1. Combustion-promoting OH radicals formed during com bustion are trapped and stabilised. 2. The incombustible gas generated is input to the system, diluting and excluding oxygen. 3. Heat is robbed from the system by endothermic reaction due to water fed to the system from hydrated metal compounds, etc, and by endothermic decomposition reactions of the flame retardant. Figure 1. Combustion of polymer materials and flame retardancy mechanisms 2014 Smithers Information Ltd. T/23

2 Table 1. Flame retardancy mechanisms in the gas phase in initial growth of and the effects of flame retardants Phase of Initial growth Combustion behaviour of polymer materials Melting Decrease in molecular weight (gasification) Evolution of flammable gas Ignition Generation of free radicals Heat release, smoke emission Flame spread Charring Flame, glow (Crosslinking) (Anchor effect) Flame retardancy mechanism Oxygen exclusion Oxygen dilution Heat absorption Free radical trapping effect Crosslinking Effects of flame retardants Bromine compound + metal oxide radical trap and incombustible gas formation Antimony trioxide (Sb 2 O 3 ) Antimony halide vapour Antimony oxyhalide Tin oxide SnO + OH, SnO + H STOX-501 Synergistic effect of SbO 3 50%, SiO 2 30% content due to auxiliary flame retardant Free radical trapping effect Phosphorus compounds, sulphur compounds Azoalkane compounds Hindered amine Endothermic reaction Endothermic dehydration of hydrated metal compound Endothermic decomposition of nitrogen compound (MC) Table 2. Flame retardancy mechanisms in the solid phase and effects of flame retardants Phase of Combustion behaviour of polymer material Flame retardancy mechanism Effects of flame retardants Intermediate and later stage Combustion of flammable gas Flame spread Heat release Smoke emission, gas formation Charring Char + inorganic compounds Formation of composite layer Free radical trapping effect Charring Drip prevention Flame Glow Bromine compound + metal oxide (continued) Continuation of retardancy mechanism in gas phase Phosphorus compounds Radical trapping (continued) Formation of strong acid and charring Intumescent char formation Hydrated metal oxide Char + Metal oxide composite layer Nanofiller Char + microparticulate composite layer Zinc borate, silicone Char + glass, ceramic layer Drip prevention Crosslinking reaction with PC via PTFE Silicone compounds (2) Flame retardancy mechanism in the solid phase 1. Oxygen exclusion and heat insulation effects due to a graphitic char layer or inorganic compound layer on the surface of the polymer material. 2. Prevention of flame spread due to burning fragments (drips) shed during, and rapid extinction of glowing (embers) in the residue. It is extremely important to choose the correct flame retardant exhibiting the gas phase or solid phase flame retardancy mechanism, and identify how effective the flame retardant is. The key considerations in raising flame retardant efficiency are mapped out in Figure 2, which is a schematic diagram of flame retardant mechanism plotting the pyrolysis curves of polymer and flame retardant, and the charring curve for the surface of the polymer material, against burning time. Figure 2 indicates that the key to raising flame retardant efficiency is to apply the following two basic concepts. 1. Choose a flame retardant whose pyrolysis curve matches the polymer pyrolysis curve within a temperature range of ±15-20 C. The radical trapping effect that suppresses reactions in the T/24 International Polymer Science and Technology, Vol. 41, No. 4, 2014

3 flammable gas evolved by pyrolysis of the polymer, and the oxygen dilution and exclusion effects of incombustible gas can then proceed effectively, enhancing flame retardation efficiency. 2. Shorten the rise time on the char formation curve and increase the yield of char, ensuring char toughness and improving stability. Recent research on flame retardant technology is progressing in this area and char properties are being analysed. To match the pyrolysis temperatures in the gas phase in (1), the TGA curves of the polymer and flame retardant are compared and a suitable flame retardant is selected. The flame retardancy mechanism most effective in the gas phase is obtained with a combination of halogen compound and antimony trioxide, a system exhibiting a synergistic effect. This is the system of greatest efficacy among the flame retardant systems currently deployed (Figure 3). Because of the escalating price of antimony trioxide due to cutbacks in rare metal supplies from China, and concerns over health hazards, alternatives are being investigated. Substitutes under investigation are zinc borate, zinc stannate, STOX-501 (a mineral powder with approximately 50% of antimony trioxide and 30% silica as the chief constituents) and PTFE. Table 3 gives details of the flame retardancy mechanisms in the gas phase in flame retardant systems inclusive of these effects [1-3]. Phosphorus compounds have an effect in both the gas phase and solid phase, but the effect in the gas phase is small and the effect in the solid phase dominates. Hindered amine compounds (NOR 116) and azoalkane compounds (Figure 4) are reported to exhibit an out standing flame retardant effect in combination with brominated flame retardants (DECA, TBBPP) and aluminium hydroxide (Table 4) [3]. The effect of azoalkane compounds differs with their structure, and it has been shown that a molecular structure R-N=N-R in which R provides symmetric aromatic groups containing a benzene nucleus is highly effective. Although the reason for this is unclear, it may be inferred that because the fire retardant mechanism is due to radical trapping, the radical generating behaviour matches the pyrolysis behaviour of the polymer. The gas phase fire retardancy mechanism of melamine compounds, which are nitrogen-based retardants, may be attributed to endothermic reaction due to the heat of sublimation and heat of decomposition during pyrolysis, and to the oxygen dilution and exclusion effects of the nitrogenous gases evolved. It has been shown that the flame retardant effect is small, and they are most often used in conjunction with phosphorus compounds to increase the flame retardant effect. Figure 2. Schematic diagram of flame retardancy mechanisms that increase flame retardant efficiency Figure 3. Synergistic mode of reaction of antimony trioxide and halogen compounds and supporting evidence from TGA and DTA Figure 4. Molecular structure of azoalkane compounds with a radical trapping effect in the gas phase 2014 Smithers Information Ltd. T/25

4 Table 3. Detailed mechanisms of flame retardancy in gas phase systems [1-3] Flame retardant system Halogen (bromine, chlorine) compounds Phosphorus compounds Hindered amine compounds Flame retardancy mechanisms Synergism with antimony trioxide (see Figure 3) HX (X: halogen) gas (radical trapping effect) SbOX (radical trap, oxygen dilution, exclusion, endothermic dehydration) SbX 3 (oxygen dilution, exclusion) Synergism with zinc stannate SnO + OH SnOH Synergism with STOX-501 Synergism of antimony trioxide component (reinforcing effect of silica in char formation is shown in the solid phase) Free radical trapping effect of phosphorus compounds H 3 PO 4 HPO + PO H + PO HPO H + HPO H 3 PO 4 OH + PO HPO (composite effect shown in char formation due to dehydration-carbonisation in the solid phase) Free radical trapping effect of hindered amine compound >NOR >OR + R >NOR >N + OR Large combined effect in use with bromine compounds, hydrated metal compounds (see Table 4) Azoalkane compounds Free radical trapping effect of azoalkane compounds (see Figure 4) R-N=N-R R-N OH, H trapping effect Large combined effect in use with bromine compounds, hydrated metal compounds (see Table 4) Melamine compounds Endothermic decomposition reactions of melamine compounds, oxygen dilution effect Exhibit the heat of sublimation, heat of decomposition, and oxygen dilution effect of MC (melamine cyanurate). Sublime above 200 C, with oxygen dilution effect at the surface of the flammable matter, with additional endothermic effect from 29 k/cal/mol heat of sublimation, 470 kcal/mol heat of decomposition. Hydrated metal compounds, inorganic compounds Endothermic reaction of aluminium hydroxide and magnesium hydroxide Al(OH) 3 Al 2 O 3 + H 2 O (205 C, heat absorbed 1.17 kj/g) Mg(OH) 2 MgO + H 2 O (345 C, heat absorbed 1.37 kj/g) (Composite effect exhibited due to formation of composite barrier layer with aluminium oxide or magnesium oxide in solid phase) Endothermic reaction with zinc borate and zinc stannate Zinc borate (260 C), zinc stannate (190 C-285 C) (Exhibit flame retardant auxiliary effect of hydrated metal compounds) Table 4. Effect of using an azoalkane compound or hindered amine compound in combination with a brominated flame retardant or aluminium hydroxide in improving the flame retardancy of polypropylene (PP) Flame retardant system UL94 vertical test PP PP + 15% TBBPP brominated flame retardant PP + 0.5% azoalkane + 14% TBBPP PP + 0.5% hindered amine NOR % TBBPP PP + 5% DECA brominated flame retardant PP + 0.5% azoalkane + 5% DECA PP + 0.5% NOR % DECA PP + 60% ATH PP + 1% azoalkane + 25% ATH PP + 1% NOR % ATH Note 1) The molecular structure of the azoalkanes is R-N=N-R where R is a benzene nuclear structure 2) NOR116 is a hindered amine flame retardant 3) ATH is an abbreviation of aluminium hydroxide Burns (fails test) Burns (fails test) Burns (fails test) V-0 Burns (fails test) T/26 International Polymer Science and Technology, Vol. 41, No. 4, 2014

5 Hydrated metal compounds have a flame retardancy mechanism based on endothermic dehydration in the gas phase and the formation of complex char layers comprising char plus metal oxide in the solid phase, though variation in the rate of dehydration and the nature of the dehydration reactions are likely to impact on the flame retardant effect; this is influenced by the dispersion and particle size of the hydrated metal compound in the polymer. Provided disper sibility is good, flame retardancy can be expected to occur at an average velocity, but if dispersion is poor the effect could vary considerably. For the maximum effect to be displayed, it is important to increase the probability of reaction by reducing particle size, and to minimise variability in reaction by uniform dispersion. The problem is common to all powder flame retardants, not just hydrated metal compounds. The yield of char in the solid phase in (2) readily increases at the usual polymer surface during owing to the great decrease in oxygen concentration, as shown in Figure 5 [3]. However, research is being carried out aimed at further increasing the yield of char and improving toughness and stability by tailoring the molecular structure of the polymer and the molecular structure of the flame retardant and flame retardant auxiliary. The relation between flame retard ancy and char formation has been studied for some time and two of the best known examples will be examined here. One is the study by van Krevelen [4], who measured the limiting oxygen index (the minimum oxygen concentration for to start) and the amount of residue for many different polymers, representing the relation between the two as in Figure 6, and discovered that the relation was given by Equation (1) below. As to the polymer residue CR, it was reported that the CFT (char-forming tendency), the contribu tion of molecular structure shown in Table 5, can be used to estimate the amount of residue as in Equation (2). Thus, the aromatic structures are shown to have a greater yield of residue than the aliphatic structure, and have a greater flame retardant effect in the solid phase. Figure 6. Relation between char yield (CR) and limiting oxygen index (LOI). (1) Formaldehyde resin, (2) PE, PP, (3) PS, PI, (4) PA, (5) cellulose, (6) PVA, (7) PET, (8) PAN, (9) PPO, (10) PC, (11) Nomex, (12) Polysulphone, (13) Kynol, (14) PI, (15) carbon Table 5. Contribution of polymer molecular structure to charring (CFT) Structure CFT Structure CFT Aliphatic Heterocyclic -CHOH- Others Aromatic 1 / ½ 3½ 3½ ¼ 10 Aromatic side-chain 12 >CH 2, >CH-CH CH 3-1½ 10 Figure 5. Oxygen concentration of polymer surface during >C(CH 3 ) 2 -CH(CH 3 ) Smithers Information Ltd. T/27

6 Table 6. Heat dissipation capacity, total heat dissipated, and char yield for different polymers Polymer Heat release capacity J/g.K Total heat released (HHR) kj/g Char yield % PE PS P PVC PPS PPO PEEK Molecular weight g/mol Note 1) The heat release capacity is given by the following formula and shows a relatively high correlation with UL94: h c = h 0 c (1-µ)E 2 a/ert p h c : total heat of of the pyrolysis gas, J/g µ: amount of residue after pyrolysis and, g/g E a : activation energy in pyrolysis weight loss process, J/mol T p : temperature giving maximum rate of weight loss, K b: heating rate, K/s e: constant, R: gas constant LOI = CR (1) CR = {Σ(CFTi 12)/M unit } 100 (2) Figure 7. Correlation between limiting oxygen index, UL94 vertical burn and heat release capacity (HRC) Figure 8. Models of char layers formed with a flame retardant system exhibiting solid phase flame retardancy Figure 9. Relation between dispersibility of PMMA-CNT nanocomposite and stability of barrier layer during where LOI is the limiting oxygen index, CR is the residue (barrier layer), CFTi is the contribution of molecular structure, M unit is the molecular weight of the repeating unit. Another study has been reported by Walters et al. [5] who measured the residue and total heat release for many different polymers and demonstrated an intimate relation between the two (Table 6), reporting that if the heat release capacity shown in Note 1 to Table 6 is taken as an evaluation index, a correlation is obtained with limiting oxygen index, the best represented of the flame retardancy evaluation indices, and the result of the UL94 vertical burn test (Figure 7). Compared with research on flame retardancy mechanism in the gas phase, the work on flame retardancy due to charring is more extensive and has emerged as a key topic in the development of new flame retardant technology both nationally and internationally. Table 7 and Figure 8 summarise a number of recent studies of note [6, 7]. Nanocomposite flame retardant materials are 100% based on solid phase retardancy mechanisms, where the dispers ibility of the nanofiller and high affinity at the filler surface and polymer interface are essential conditions for obtaining high flame retardancy. Figure 9 shows the relation between the dispersibility of a nanofiller (CNT) and PMMA nanocomposite and the stability of the char layer during [8]. It has also been reported that flame retardancy is greater the higher the affinity between the nanofiller and polymer interface [9]. T/28 International Polymer Science and Technology, Vol. 41, No. 4, 2014

7 Table 7. Studies of flame retardancy mechanism in the solid state Flame retardant system Flame retardancy mechanism Composition IFR (Intumescent) Chief components APP (antimony polyphosphate) + foaming agent (nitrogen compound) + char source (PER, etc) Hydrated metal compound + flame retardant auxiliary Heat-resistant phosphorus compounds (condensed phosphate esters) Insulating effect and oxygen exclusion effect of foam char (see Figure 8) Insulation and oxygen exclusion effects of graphitic char, composite char of inorganic oxide, etc (see Figure 8) Carbonisation-promoting effect of phosphorus compounds Highly effective flame retardant auxiliaries Silicone compound, nano-metal oxide (aluminium oxide) Decrease in foam cell size, uniformity of cell size and strengthening of foam film due to nanofiller (MMT, CNT, silica, etc) Highly effective flame retardant auxiliaries Densification and toughening of composite char layer with nanofiller (MMT, CNT, active silica, etc), red phosphorus, silicone compounds (active OH group modified silicone, etc), aromatic resin, aromatic engineering plastics, etc. Modification of hydrated metal compounds Enhancement of affinity of hydrated metal compound by reduction in particle size, surface treatment and polymer alloying Condensed phosphate esters (heat-resistant) Selection of phosphorus compounds of high pyrolysis temperature, highly extraction- resistant, with a strong charring effect The characteristic feature of the char layer in flame retardant nanocomposites is that flame retardancy tends to diminish if it is possible for the char formed to drip during as occurs in UL94 vertical burn and limiting oxygen index tests. Discrepancies readily arise between horizontal burn and vertical burn tests. Most recent studies are of systems combining nanofiller with conventional flame retardants, and these systems are likely to be the first to find practical application. Conclusions This second article has reviewed the basics of flame retardancy mechanism in flame retardant polymer materials and recent directions in research. Advances in flame retardant technology hinge on studies of flame retardancy mechanism. Flame retardant systems that exhibit new flame retardancy mechanisms in the gas phase in nascent need to be developed. References 1. Nishizawa H., 'Atarashii Nannenzai' Nannenka Gijutsu', Gijutsu Joho Kyokai, Tokyo (2008). 2. Cullis C. F., Hirschler M. M., 'The Combustion of Organic Polymers', Clarendon Press, Oxford (1981). 3. Aubert M., Nicolas R. C., Pawelec W., Wilen C.-E., Roth M., Pfaendner R., Polym. Adv. Technol., 22, 1529 (2011). 4. van Krevelen D. W., Polymer, 16, 615 (1975). 5. Walters R. N., Lyon R. E., J. Appl. Polym. Sci., 87, 548 (2003). 6. Nishizawa H., The 76th JECTEC Technical Seminar, Hamamatsu (2013). 7. Nishizawa H., Materials Life Gakkaishi, 16, 70 (2004). 8. Kashiwagi T., Du F., Winey K. I., Groth K. M., Shields J. R., Bellayer S. P., Kim H., Douglas J. F., Polymer, 46, 471 (2005). 9. Nishizawa H., Okoshi M., Okubo I, Proceedings of International Rubber Conference, Yokohama (IRC2005), 28-S6-03 (2005) Smithers Information Ltd. T/29

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6. Intumescent Flame Retardants

6. Intumescent Flame Retardants 6. ntumescent Flame Retardants ntumescence is an interesting phenomenon. The French verb tumere means "to swell". The Latin equivalent tumescere can be translated as "to swell up". Therefore tumid or tumescent