Halogen-free Flame Retardants that Outperform ... - SAGE Journals

Halogen-free Flame Retardants that Outperform Halogenated Counterparts in Glass Fiber Reinforced Polyamides NIHAT ALI ISITMAN,1 HUSEYIN OZGUR GUNDUZ2 CEVDET KAYNAK1,2,*

AND

1

Department of Metallurgical and Materials Engineering, Middle East Technical University, TR-06531 Ankara, Turkey 2 Department of Polymer Science and Technology, Middle East Technical University, TR-06531 Ankara, Turkey

(Received July 15, 2009)

ABSTRACT: Flammability, fire performance, and thermal stability of short glass fiber reinforced polyamide-6 and polyamide-66 containing halogenated and halogen-free flame retardants (FRs) were compared. Flammabilities were assessed by limiting oxygen index tests and UL94 classifications. Fire behavior was evaluated by mass loss cone calorimetry, a bench-scale tool, to assess fire performance of materials. Halogen-free, phosphorus-based FRs were shown to perform superior to halogenated counterparts on the basis of important fire properties, peak heat release rate, time to ignition, and fire growth index. Moreover, thermal stabilities were maintained at an acceptable level as a clear advantage of halogen-free FRs. KEY WORDS: cone calorimeter, flammability, flame retardancy, polyamide-6, polyamide-6,6.

*Author to whom correspondence should be addressed. E-mail: [email protected] JOURNAL

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FIRE SCIENCES, VOL. 28 – January 2010

0734-9041/10/01 0087–14 $10.00/0 DOI: 10.1177/0734904109349594 ß SAGE Publications 2009 Los Angeles, London, New Delhi and Singapore

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INTRODUCTION POLYAMIDE-6 (PA6) AND polyamide-6,6 (PA66) are easily ignitable and they sustain flaming combustion upon ignition. Neat PA6 and PA66 have UL94 ratings of V-2 and limiting oxygen index (LOI) values between 20% and 25% [1]. Flame-retarded polyamides are suitable for electrical applications such as connectors, terminal blocks, switch components, wire ties and many other commercial parts, which require polymers that do not ignite easily or at least do not spread the flame further. For applications requiring higher strength, stiffness, and better dimensional stability, polyamides are reinforced with short glass fibers (SGF). Halogenated compounds have long been considered as by far the most effective flame retardants (FRs), especially when used in a synergistic combination with antimony trioxide. However, it appears that the current restrictions imposed by EU REACH on the use of a number of brominated aromatic FRs will increase the demand for halogen-free FRs and parts made thereof. Accordingly, replacement of halogenated FRs used in polyamides by phosphorus- and/or nitrogen-based compounds [2–7] and metal hydroxides [7–10] has recently gained much importance. Of particular interest to our work, organophosphorus (OP) flame retardant [11] and red phosphorus (RP) [2] were shown to provide convincing flame retardancy in glass fiber (GF)-reinforced PA66. It can be inferred from the studies of Balabanovich et al. [12,13] that RP is effective at a much lower level of loading in PA66 in comparison with PA6. And, there appears no publications investigating the use of OP FRs in short GF-reinforced PA6. Therefore, in this study, OP flame retardant and RP were chosen as FRs for use in short GF-reinforced PA6 and PA66, respectively in the interest of comparing the performance of halogen-free FRs against halogenated counterparts. Interfacial interactions of short GF reinforcement with flameretarded PA6 and PA66 were discussed in our previous work [14]. Flame-retarded compositions based on halogenated compounds in combination with antimony trioxide synergist were shown to result in reduced fiber/matrix interfacial strength, lower crystallinity, and shorter residual fiber lengths, a combination of which yields large losses in tensile strength. On the other hand, tensile strength is maintained at an acceptable level owing to unaltered degree of crystallinity and strong fiber/matrix interface in short fiber reinforced PA66 composite containing RP. In the present study, short GF-reinforced PA6 and PA66 composites are evaluated by comparing common halogenated and halogen-free FRs

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on the basis of flammability ratings (LOI and UL94), fire performance (cone calorimetry), and thermal stability (thermogravimetry).

EXPERIMENTAL Materials, Compounding, and Specimen Production PA6 and PA66 used in this study were DSM Akulon K125 and DuPont Zytel FE210021, respectively. Aminosilane sized SGF were supplied by Camelyaf (Turkey) with initial lengths of 3 mm and diameter of 10.5 mm. In order to carry out a comparative study, flame-retardant additives with halogenated and halogen-free compositions were utilized in short GF-reinforced PA6 and PA66 composites. Red phosphorus masterbatch (RPM), brominated epoxy polymer (BEP, MW: 15,000 g/mol, end-group: tribromophenol), antimony trioxide (Sb2O3), and an OP compound (Clariant Exolit OP1312: a multi-component, synergistic mixture of aluminum diethylphosphinate, melamine polyphosphate and zinc borate) were obtained from various commercial sources. Compositions of the investigated materials are given in Table 1. Materials were compounded by a ZSK-70 (L/D : 40) twin-screw extruder with a temperature profile over 10 heating zones between 210–2708C and 240–2908C for PA6 and PA66, respectively. Specimens for LOI, UL94, and cone calorimetry were produced by compression molding and machining to the required dimensions mentioned in ISO 4589, UL94 (1.6 mm thickness) and ISO 5660 (4 mm thickness) standards, respectively.

Table 1. Designations and compositions of the investigated materials. Material designation PA6 PA6/GF PA6/GF-Br/Sb PA6/GF-OP PA66 PA66/GF PA66/GF-Br/Sb PA66/GF-RP

Composition (weight %) Neat 15% SGF 15% SGF, 15% SGF, Neat 25% SGF 25% SGF, 25% SGF,

20% BEP (10.6%Br), 6.5% Sb2O3 15% OP

20% BEP (10.6% Br), 6.5% Sb2O3 12% RPM (7.2% P)

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Characterization and Testing LOI values were measured by an oxygen index apparatus (Fire Testing Technology, UK) having a paramagnetic oxygen analyzer so that precise adjustments of the oxygen concentration can be performed and repeatable results are obtained. UL94 vertical burning tests were conducted on a CEAST flammability meter. Heat release rates were determined following the procedure explained in ISO 13927 using a mass loss cone calorimeter with attached chimney and thermopile (Fire Testing Technology, FTT). Heat release and mass loss rates from samples were measured at an external heat flux of 35 kW/m2. Barrier morphologies of char residues, which directly influence their ability to impede heat and mass transfer, were examined by a scanning electron microscope (SEM, JEOL JSM-6400) on gold sputtered surfaces of char residues. Thermogravimetric analyses (TGA) were performed using a TG/DTA instrument (Shimadzu DTG60H). Samples of about 15 mg weight were heated from room temperature to 9008C with a heating rate of 108C/min under flowing nitrogen at 50 mL/min.

RESULTS AND DISCUSSION Flammability Figure 1 displays the flammability properties of compounds investigated in this study. Neat PA6 and PA66 have LOI values of 24.9% and 26.8%, respectively. Both polymers burn with colorless-blue flicker flame and the molten, flammable polymer drips down leading to V-2 ratings in UL94 tests. In the presence of SGF without any flame-retardant additives, LOI is substantially decreased through the candlewick effect. It is established that GFs transport the fuel from the pyrolysis zone to the flame by capillary action, increase the rate of heat flow back to the polymer, and thus make polymers decompose faster and burn more easily [15]. On the other hand, in the presence of an effective flame retardant, GFs may have a positive contribution to flame retardancy due to increased strength of the char and reduced thermal decomposition rate [2,6]. Use of brominated epoxy polymer together with antimony trioxide (Br/Sb) is found to impart dramatic improvements in LOI and


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Figure 1. LOI values and UL94 classifications for PA6 and PA66 compounds.

UL94 (Figure 1). LOI values are increased from 22.4% to 28.1% in PA6 compounds and from 21.3% to 31.8% in PA66 compounds. The primary flame retardancy mechanism of halogen species is the elimination of hot H and OH radicals in the gas phase, which reduces the combustion efficiency and thus provides flame inhibition. When used in combination with antimony oxide, flame inhibition by halogen species are enhanced through the formation of metal halides, which are much more effective radical scavengers compared to hydrogen halides. The OP FR incorporated into PA6/GF provides a remarkable LOI, i.e., 29.3%, and a V-0 rating. The flame retardancy mechanism of this specific flame retardant was the discussion of our previous study [16]. It was shown that the dominant mechanism of action was in the condensed phase by means of phosphate barrier formation accompanied by secondary mechanisms of flame inhibition and dilution in the gas phase. The use of RP in PA66/GF imparts a UL94 V-0 rating and an increased LOI value of 26.7% due to the condensed phase flame retardancy of RP. This mechanism mainly depends on the formation of various phosphoric acid ester species upon reaction with polyamides, covering the burning surface. Moreover, acid promoted dehydration accelerates the formation of a consolidated char layer [5]. This intense char layer physically limits the oxygen access and heat transfer.

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Fire Performance Heat release and mass loss behaviors of investigated compounds are determined for a mild to intermediate fire scenario using mass loss cone calorimetry. Fire properties can well be followed through simultaneous evaluation of rates of heat release and mass loss from samples as a function of time, given in Figures 2 and 3 for PA6 and PA66 compounds, respectively. Neat polyamides combust with large peak heat release rates (PHRR) accompanied with fast mass loss when exposed to an external radiant heat. Comparing the curves of neat and GF-reinforced polyamides, heat release rates are lowered in the same manner as the mass loss rates in the presence of GFs. Therefore, it can be concluded that the change in the combustion behavior with the incorporation of GFs is associated with a condensed phase physical process. It was described in the study of Casu et al. [17] that, through the accumulation of GFs on the exposed surface of the specimen in the cone test, a protective layer is formed acting as a thermal insulator and a heat sink. Accordingly, the rates of polymer pyrolysis and evolution of combustible gases are reduced, and heat release and mass loss rates during flaming combustion are suppressed.

Figure 2. Mass loss cone calorimeter curves for PA6 compounds.


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Although peak mass loss rates in cone calorimetry for halogenated FRs are similar to GF-reinforced compounds, PHRR are well lower. It is a clear indication of decreased combustion efficiency and/or effective heat of combustion of volatiles by the act of flame inhibiting halogenated FRs. Halogen-free, phosphorus-based FRs provide an almost constant rate of heat release against time owing to strong char formation. Mass loss rate curves possess initial increases in the early stages of combustion followed by plateaus of lower rate of mass loss upon the establishment of protective barrier layers. Despite the notably different peak mass loss rates for halogenated and halogen-free FRs, PHRR present similar values. This could well be attributed to the dominant gas phase action of halogenated FRs contrary to the case with halogen-free, phosphorus-based FRs predominantly displaying condensed phase action. Important fire parameters are extracted from cone calorimetry and listed in Table 2. PHRR is usually addressed as an important parameter determining the severity of the flashover of a fire. It is seen that halogenfree FRs are much more effective compared to halogenated FRs in suppressing the PHRR, since reductions of 63% and 75% are obtained with OP FR used in PA6/GF and RP in PA66/GF, respectively. Increased efficiency of the halogenated FR system in PA66 compared to PA6 in terms of PHRR is attributed to higher GF content and correspondingly reduced thermal decomposition rate [6].

Figure 3. Mass loss cone calorimeter curves for PA66 compounds.

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Table 2. Mass loss cone calorimeter data for PA6 and PA66 compounds. Specimens PA6 PA6/GF PA6/GF-Br/Sb PA6/GF-OP PA66 PA66/GF PA66/GF-Br/Sb PA66/GF-RP

PHRR () (kW/m2)

266 ( 228 (

94 ( 74 (

921 611 56%) 63%) 683 293 68%) 75%)

THE (MJ/m2)

TTI (s)

FGI (kW/s m2)

Fire residue (%)

180 172 66 101 131 89 25 32

98 99 76 97 109 91 53 135

9.4 6.2 3.5 2.4 6.3 3.2 1.8 0.5

– 17.6 28.3 28.4 – 26.5 34.6 43.8

PHRR are given with respect to neat GF-reinforced composites.

Total heat evolved (THE), a measure of total fire load, is reduced to a great extent with halogen-free FRs, though not as much as with halogenated FRs. It may be concluded that compounds with halogenfree FRs used in this study burn with a lower PHRR but for much longer times, and thus generate larger fire loads compared to those with flame inhibiting halogenated FRs. Amounts of char residues obtained at the end of cone calorimetry show that halogenated and halogen-free FRs used in this study promote charring (Table 2). Significantly larger amount of residue is observed for PA66/GF-RP compared to PA66/GF-Br/Sb as an indication of effective condensed phase activity of RP. The reduction in THE, associated with a noticeable increase in the fire residue confirms the dominant condensed phase action of RP in GF-reinforced PA66. RP was also shown to act in the gas phase by scavenging of hot radicals as explained by Levchik et al. [1]. The OP flame retardant operates mainly in the condensed phase inferred from the reduced values of THE, and increased fire residues compared to PA6/GF. Significant increases in the amounts of fire residues with halogenated FRs suggest a condensed phase action as an additional mechanism to dominant flame inhibition in the gas phase. Contrary to most of the other FRs, RP significantly increases time to ignition (TTI), ascribed to thermo-oxidative decomposition reactions that take place on the surface of the sample forming a black skin before ignition [18]. Fire growth index (FGI), defined by the ratio PHRR/TTI [19], makes a reasonable attempt at assessing the flame spread as a fire hazard. Owing to delayed ignition and suppressed PHRR, halogen-free FRs provide significantly slower flame spread compared to halogenated FRs. Effectiveness of the formed barriers in impeding heat and mass transfer during flaming can be observed from SEM micrographs


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Figure 4. SEM micrographs of (a) PA6/GF-Br/Sb, (b) PA6/GF-OP, (c) PA66/GF-Br/Sb, and (d) PA66/GF-RP showing the stronger chars formed from halogen-free, phosphorus-based FRs (b and d) compared to halogenated FRs (a and c).

displayed in Figure 4. It is clear that the residues of compounds with halogenated FRs possess open/loose structures, whereas those from halogen-free, phosphorus-based FRs are consolidated and act efficiently as protective barriers. These protective barriers successfully inhibit oxygen diffusion and escape of flammable volatiles, and thermally shield the underlying material against the flame. Thermal Stability TGA were carried out on representative samples to investigate the effects of employed FRs on the thermal degradation of PA6 and PA66 compounds. Figures 5 and 6 display the thermogravimetric (TG) and differential thermogravimetric (DTG) curves of PA6 and PA66 compounds, respectively. Neat polyamides decompose in a single step, practically giving no char residue. Compounds with halogenated FRs have two distinct decomposition steps. The former steps where major mass loss occurs can be attributed to polymer main-chain decomposition. It was described previously by Sallet et al. [20] that a complex salt

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Figure 5. TG and DTG analysis of PA6 compounds.

that contains bromine, antimony, and ammonium was formed upon the reactions between the aromatic brominated FR, antimony trioxide, and nylon degradation products. Accordingly, the latter step may be attributed to the decomposition of the formed residue, as observed in a similar work by Ma et al. [21]. The decomposition pathway of PA66/ GF-RP consists also of two steps; however, the decomposition steps overlap and a shoulder appears in DTG curve given in Figure 6. Overlap of the two decomposition steps with RP flame-retarded PA66 has been discussed by Schartel et al. [18] and Balabanovich et al. [13]. Table 3 summarizes TG and DTG data of the investigated compounds. With the use of halogen-free FRs, thermal stabilities are affected to a minor extent, i.e., the maximum degradation (Tmax) and onset of degradation (To) temperatures are lowered by 258C and 108C for PA6/GF, and 188C and 118C for PA66/GF. Contrarily, substantial decreases in thermal stability are encountered with halogenated FRs. Tmax and To are lowered by 928C and 598C with respect to PA6/GF, and 938C and 628C with respect to PA66/GF. Decreased thermal stabilities in the presence of halogenated FRs may be attributed to the early

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Figure 6. TG and DTG analysis PA66 compounds. Table 3. Thermogravimetric data for PA6 and PA66 compounds. Specimens PA6 PA6/GF PA6/GF-Br/Sb PA6/GF-OP PA66 PA66/GF PA66/GF-Br/Sb PA66/GF-RP

Tmax (8C)

To (8C)

Residue (%)

DTG Peaka (% / s)

443 450 358 425 452 443 350 425

410 409 350 399 412 411 349 400

0.7 13.8 13.0 18.8 0.5 25.9 27.1 32.3

0.31 0.26 0.43 0.21 0.27 0.21 0.41 0.18

a The maximum degradation rate in thermogravimetry. Tmax is taken as the DTG peak temperature; To is taken as the 10% mass loss temperature.

dehydrogenation of the polymer [20], followed by the formation of hydrobromic acid and subsequent acid catalyzed dehydration of polyamides. Considering the peak values of DTG given in Table 3, it is clear that the OP and RP FRs suppress the maximum degradation rates in TG

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with respect to neat GF-reinforced composites, in contrast to the significantly increased values with halogenated FRs. This may be attributed to the formation of an effective barrier layer by the act of halogen-free FRs, even under the inert conditions of TG, which retards the evolution of volatilized species in addition to the promotion of charring inferred from char residue percentages. CONCLUSION Two distinct classes of FRs for short GF-reinforced PA6 and PA66, namely halogenated and halogen-free FRs, were compared in terms of flammability, fire performance, and thermal properties. Cone calorimetry showed that halogen-free FRs act much more effectively compared to halogenated FRs in terms of reductions brought in the rates of heat release and fire growth during a simulated mild to intermediate fire. Polyamides with halogenated FRs were found to exhibit remarkable ease of ignition upon exposure to radiant heat. Moreover, thermal stabilities were maintained with the use of halogen-free FRs, unlike the case with halogenated FRs, which led to notable catalysis of polymer degradation. Considering the ecological and human health concerns claimed for halogenated FRs, phosphorus-based halogen-free FRs are becoming a promising class of FRs for use in GF-reinforced polyamides. ACKNOWLEDGMENTS Partial financial support of this research as part of the Project 107M347 by TUBITAK, the Scientific and Technological Research Council of Turkey, is gratefully acknowledged. Authors also wish to acknowledge EUROTEC Engineering Thermoplastics Inc. (Turkey) for providing twin-screw extrusion and UL94 equipments. REFERENCES 1. Levchik, S.V. and Weil, E.D. (2000). Combustion and Fire Retardancy of Aliphatic Nylons, Polymer International, 49(10): 1033–1073. 2. Jou, W.S., Chen, K.N., Chao, D.Y., Lin, C.Y. and Yeh, J.T. (2001). Flame Retardant and Dielectric Properties of Glass Fiber Reinforced Nylon-66 Filled with Red Phosphorous, Polymer Degradation and Stability, 74(2): 239–245. 3. Jahromi, S., Gabrielse, W. and Braam, A. (2002). Effect of Melamine Polyphosphate on Thermal Degradation of Polyamides: A Combined X-ray Diffraction and Solid-state NMR Study, Polymer, 44(1): 25–37.


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BIOGRAPHIES Mr. Nihat Ali Isitman Nihat Ali Isitman is a PhD research assistant at the Polymers and Nanocomposites Laboratory of the Department of Metallurgical and Materials Engineering, Middle East Technical University since three years. His recent publications are concerned with interfaces in short fiber-polymer composites and nanocomposites, flame retardant polymers, and polymer nanocomposites. Mr. Huseyin Ozgur Gunduz Huseyin Ozgur Gunduz has taken his MS degree at the Department of Polymer Science and Technology in Middle East Technical University. His thesis was about the flame retardancy of glass fiber reinforced polyamides, and nanoclay synergy in flame retarded/glass fiber reinforced polyamides. Prof. Cevdet Kaynak Cevdet Kaynak is a full professor at the Department of Metallurgical and Materials Engineering, and Department of Polymer Science and Technology in the Middle East Technical University, Ankara, Turkey. He is mainly working on mechanical, thermal, and flammability behavior of polymers, polymer composites, and nanocomposites. He is the co-author of more than 30 journal articles and 40 conference proceedings