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Design and Utilization of Nitrogen Containing Flame Retardants Based on N-Alkoxyamines, Azoalkanes and Related Compounds
CHAPTER
Design and Utilization of Nitrogen Containing Flame Retardants Based on N-Alkoxyamines, Azoalkanes and Related Compounds
8 Carl-Eric Wile´n
A˚bo Akademi University A˚bo, Finland
R. Pfaendner Fraunhofer Institute for Structural Durability and System Reliability LBF Division Plastics, Darmstadt, Germany
CHAPTER OUTLINE 1. Introduction ....................................................................................................... 267 2. Mechanistic considerations and studies on structureeactivity relationship of N-alkoxyamines as flame retardants ................................................................ 272 3. Azoalkanes (AZO) as flame retardants for polypropylene films and plaques............ 279 3.1 The potential of symmetric versus unsymmetric azoalkanes .................... 281 3.2 Azoxy, azine, hydrazone, and triazene compounds .................................. 282 3.3 Tetrapotassium azo diphosphonate (INAZO) as flame retardants .............. 282 3.4 Multifunctional bis(1-propyloxy-2,2,6,6-tetramethylpiperidyl)-4-diazene.. 284 4. Conclusions ....................................................................................................... 285 Acknowledgments ................................................................................................... 285 References ............................................................................................................. 286
1. Introduction Flame retardants containing nitrogen are a class of various materials among which are well-known halogen-free commercial products, namely ammonium polyphosphate (APP), melamine cyanurate (MC), melamine borate, and melamine (poly)phosphate. APP, which is available in different crystal modifications and with coatings (e.g. melamine and silicone) or in an encapsulated form, decomposes by exposition to fire or heat to ammonia, polyphosphoric acid, and phosphorus oxides. In the presence of a char-forming agent, e.g. a polyol, an intumescent barrier via phosphoric ester Polymer Green Flame Retardants. http://dx.doi.org/10.1016/B978-0-444-53808-6.00008-1 Copyright © 2014 Elsevier B.V. All rights reserved.
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
intermediates is formed [1]. The main applications of APP are coatings for steel and wood and within the plastic field unsaturated polyesters, epoxides, polyurethanes, and polyolefins. MC is a 1:1 adduct of melamine and cyanuric acid. MC decomposes endothermically into its components, and melamine decomposes further to nitrogen-containing gases such as ammonia. MC is mainly used as a flame retardant component in polyurethanes, polyesters, epoxides, and polyamides, specifically in unfilled polyamide-6. Melamine borate decomposes endothermically into its components and promotes glassy char formation mainly in combination with phosphates. Melamine phosphates cover melamine orthophosphate (1:1 salt of melamine and phosphoric acid), dimelamine orthophosphate (2:1), dimelamine pyrophosphate, melamine pyrophosphate, and melamine polyphosphate (MPP). Because of thermal stability reasons, the most interesting material in this class of plastic applications is MPP. The endothermic decomposition of MPP produces ammonia, other nitrogen species, and poly(phosphoric acid); the latter dehydrates the polymer under char formation. Application fields are polyamides (PA6, PA6.6 glass fiber reinforced) and polyesters. For example, 25% MPP provides UL 94 V-0 classification at 1.6 mm in PA6.6/20% glass fibers [2]. In combination with other phosphorus flame retardants, a synergistic effect of MPP is found, which allows one to reduce the overall flame retardant loading, e.g. combinations of MPP and a phosphinate to achieve a UL-94 V-0 classification in glass fiber-reinforced PA6.6 with an overall loading of 15e20% [3e5]. Furthermore, melamine salts other than phosphates, higher condensation products of melamine (melam, melon, melem), and closely related substances (ammeline, ammelide, urea, guanamine, guanidine, and derivatives thereof) as well as all kind of salts thereof have been proposed as flame retardants for plastic applications. For example, mixed salts of melamineemelamemelem and polyphosphate or pyrosulfate have been synthesized at condensation temperatures >300 C [6]. Phosphorus derivatives of nitrogen-containing species other than phosphates have been described, e.g. in the form of nitrilotrismethylenephosphonic acid [7,8] or as guanidine phenylphosphinate [9]. Most of the above-mentioned salt structures of melamine and related substances act in the condensed phase through endothermic decomposition of the flame retardant, dilution of the burning gases through the formation of inert gaseous molecules, and through char formation. A somewhat different activity mechanism was found by using alkoxyamines (NOR) as flame retardants forming radical species during decomposition. The use of radical generators as flame retardant components was already proposed in the 1960s [10,11], where it was shown that peroxides, azo, sulfur, bibenzyl compounds, and hydrazones act synergistically with brominated compounds. Consequently, combinations of hexabromocyclododecane and peroxides have been used as standard flame retardant (FR) systems in polystyrene foam for many years [12]. This system, however, has to be replaced due to environmental reasons in the near future. Furthermore, it was as also stated that radical generators of peroxide, hydroperoxide, and polysulfide types alone do not act as flame retardants [13].
1. Introduction
The first who realized that alkoxyamines act as potential flame retardants without any other flame retardant were inventors from Ciba Specialty Chemicals [14]. Alkoxyamines (or hydroxylamine ethers, NOR) were first introduced as noninteracting ultraviolet (UV) stabilizers with a low basicity. Such stabilizers provide protection for automotive coatings and for agricultural films even under severe conditions, e.g. in the presence of pesticides. Flame retardants such as halogens or phosphorus compounds generate thermally or photochemically acidic species, which result in inferior UV stability by deactivating hindered amine light stabilizers. This problem was solved by using alkoxyamines instead of amines in combination with brominated flame retardants. Surprisingly, it was found that the alkoxyamines do not only improve the UV stability but they also contribute to flame retardancy. Even without any additional flame retardant, it was demonstrated that alkoxyamines alone can achieve flame retardancy, e.g. in polypropylene fibers and films [15,16]. Meanwhile, various structures and process patents have been published that claim alkoxyamines to be efficient flame retardants [17,18]. The synthesis of alkoxyamines is performed usually from the corresponding sterically hindered piperidine via the nitroxyl intermediate through reaction with a hydrocarbon in the presence of an organic peroxide such as peracetic acid, hydrogen peroxide under catalysis of copper, molybdenum oxide, or iodide. A more recent process uses the coppercatalyzed reaction of aldehydes with the nitroxyl intermediate [17]. In addition, several other chemical structures related to alkoxyamines are claimed to provide flame retardancy, namely hydroxylamine esters [19], hydroxylamines, and nitroxyl radicals [20]. The first commercial product on the basis of alkoxyamines was introduced by Ciba (now BASF) in 2000 under the trade name Flamestab NOR 116 (Figure 1). R
R
HN
N
R=
N C4H9
N
N O
FIGURE 1 Structure of Flamestab NOR 116.
N
NH
H
R
N N
N
C4H9
N O
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
It was demonstrated that Flamestab NOR 116 provides flame retardancy at a concentration of only 0.5% passing the NFPA 701 fiber test [21] on polypropylene knitted socks [15]. Moreover, the addition of alkoxyamines as an FR synergist can improve the efficiency of conventional flame retardants, whereas all kinds of flame retardants and synergistic mixtures, e.g. brominated compounds [22e25], phosphorus compounds [26,27], phosphacenes, melamines [28,29], and inorganics, fluorolefins [30] are claimed. For example, the combination of NOR and decabromodiphenyloxide surpasses the flame retardants alone [31]. Therefore, it is possible with alkoxyamines to design flame retardant polyolefin molding compositions with lower levels of halogenated flame retardants, and in addition to eliminate antimony trioxide, which is often used as an FR synergist together with brominated compounds. Low FR concentrations and no antimony trioxide result in polymers with improved processability, better mechanical properties, and reduced smoke density. For example, UL 94 V-2 rating is achieved in PP injection-molded plaques at concentrations as low as 3.5% of formulations containing Flamestab NOR 116 [32]. The activity of the alkoxyamine as a flame retardant is based on the thermolysis of nitroxyl ethers, which leads to the formation of either alkoxy and aminyl radicals or alkyl and nitroxyl radicals (Figure 2). Aminyl and alkoxy radicals are very reactive and cause, on the one hand, degradation of polypropylene (and crosslinking of polyethylene). On the other hand, they are involved in the free radical chemical reactions during the combustion process [31]. In synergistic combinations, alkoxyamines can interact with brominated flame retardants and facilitate the release of bromine, consequently increasing the overall FR performance. It has been experimentally shown that alkoxyamine acts in the condensed and gas phase, modifies the decomposition pathway of phosphoric acid tris-(3-bromo-2,2-bis-bromomethylpropyl)ester and polypropylene while lowering the temperature to release HBr as flame quenching species and also the temperature
N O R
N
Aminyl radical
+
R O
Alkoxy radical
FIGURE 2 Thermolysis of alkoxyamines.
N O
Nitroxyl radical
+
R
Alkyl radical
1. Introduction
of dripping of the polymer. Decomposition experiments [33,34] of tris(3-bromo2,2-bis(bromomethyl)propyl)phosphate (TBBPP) and analysis via nuclear magnetic resonance, electron spin resonance, and gas chromatography/mass spectroscopy combination showed that there is an interaction of NOR and TBBPP below the decomposition temperature of polypropylene. In the first step, the nitroxyl radical is formed from NOR, which abstracts a bromomethyl group from TBBPP. As further intermediates, a cyclic phosphate ester and 1,3-dibromo-2,3-bis(bromomethyl)propane (mainly) and 2,3-dibromo-2-butene are generated via a radical process. These newly formed bromo compounds are thermally less stable than TBBPP and immediately release HBr as a gas phase active flame retardant, as shown in Scheme 1. Br
O
RO P
Br
+
O
O
Br
O Br
Br
O
RO
N
Br
RO Br
+
Br
Br
P RO
OH
Br
Br
O
O
O
+
+
P
N OR
Br Br
O
RO P
O
HBr Br
HBr
HBr
RO
P
OR
O
O
Br OR
+
TBBPP Br Br
Br
Br
+
Br
O
RO
+
P HO
Br
OH
Br
SCHEME 1 Various nitrogen-based classes of flame retardants
Br
Br
Br
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
The decomposition of TBBPP to the brominated species takes place in any case; however, it is shifted to lower temperatures in the presence of NOR. Obviously, this effect depends on the structure of the brominated flame retardant; however, so far, there are no other published results in this respect. Applications of alkoxyamines such as Flamestab NOR 116 are according to the manufacturer in polyolefin fibers, nonwovens, and films [35]. In addition, the hindered amine light stabilizer (HALS) molecule provides light and long-term thermal stability. Overall, nitrogen-based flame retardants still represent a small but fast growing segment of flame retardants that meet the requirements of sustainable production and products. As a consequence of this, researchers have recently started to examine the mode and action of these compounds. This review focuses on some new developments within the family of nitrogen-based flame retardants and their structuree property relationship. The general structures of these nitrogen-based flame retardant families are depicted in Figure 3.
2. Mechanistic considerations and studies on structureeactivity relationship of N-alkoxyamines as flame retardants Inspired by the flame retardant efficacy of Flamestab NOR 116, a series of NOR analogs have been prepared, and their propertyestructure relationship has been examined. As earlier mentioned, synthetic routes for a large variety of N-alkoxyamine structures have been developed in recent years. In particularly, a great number of various NOR derivatives for nitroxide-mediated “living-free” radical polymerizations have been synthesized. The mechanisms of nitroxide-mediated “living” free radical polymerizations and their structureeproperty relationship have been well documented [36e38]. The literature available on the role of various NOR structures on fire retardant efficacy is still very limited and mostly in the form of patents [14,15]. However, a clear understanding of the mechanism of action of NOR fire retardants would be essential for their further optimization. It has been well established that the active part in NOR flame retardants is centered around the nitrogeneoxygenecarbon (NeOeC) segment of the molecule, as shown in Figure 2 [39,40]. As a consequence of this, it was investigated by GC/MS analysis as to what type of radicals are prevailing after thermally induced breakdown of NOR compounds and what is the relative extent of the two homolysis pathways [41]. Thus, a synthesized model NOR molecule (1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine) was highly diluted in a hydrogen donor solvent (9,10dihydroanthracene) in a closed vessel under vacuum. The solution was kept for 1 h at 200 C. The radicals generated from the thermal decomposition of the NOR compound were hydrogenated by the solvent. The solution was then analyzed by GC and GC/MS, and the results are shown in Figure 4. In the case of model NOR compounds, the preliminary results indicate that both types of thermal homolysis reactions take place to a similar extent.
2. Mechanistic considerations and studies on structureeactivity relationship
R2
R1 O N
N- alkoxyamines (NOR )
R1 O N
N
N
N O R1
AZONOR
KO
O
O
P N N P R2
KO
OK OK
R1 N N N R3
INAZO
Triazenes
R1 N N R2 Azoalkanes
R1
H N N R3
R2 Hydrazones
R1
N N
R2
R3
O R1 N N R2
R4 Azines
Diazene oxides
FIGURE 3 Nitrogen-based classes of flame retardants.
In order to define the key parameters in the NOR structure for an optimal flame retardancy performance, a diverse library of N-alkoxyamine additives was prepared as shown in Figure 5. Most of these chemicals are 1-alkoxy-2,2,6, 6-tetramethylpiperidine-based compounds that differ from each other either in terms of backbone structure, ring size, degree of ring substitution, or 1-alkoxy group. NOR derivatives in which any of the four methyl groups had been replaced by larger alkyl groups such as ethyl or cyclohexyl moieties were excluded from this study due to the
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
Products found by GC/MS analysis
H C I A
B D E
F
G
J
200 oC
N O
B
A
D
C
E
OH N H
Piperidine fragments
F
N
+
O
Presence of cyclohexanol (20–25% of the cyclohexyl derived fragments)
G
N O
H
N OH
N O
I
J
N O
+
Presence of TEMPO (2,2,6,6-tetramethyl-piperidin1-yl oxyl) and N-hydroxytetra-methylpiperidine (25– 30% of the piperidine derived fragments)
FIGURE 4 Homolysis products detected by GCeGC/MS from thermal decomposition of 1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine.
fact that sterically highly hindered nitroxide derivatives are generally known to exhibit rather low thermal stabilities [42,43] and are thus more suitable for living free radical polymerizations than for flame retardancy per se. The flame retardant properties were evaluated according to the UL94 protocol using rectangular 108 mm 10 mm 4-mm polypropylene bars and thermal stabilities were studied by differential scanning calorimetry analysis (Table 1). From early on, it was clear that a UL rating could not be achieved for any of the synthesized NOR compounds alone using thick section polypropylene bars. Therefore, the relative flame retardant efficacy of the various NOR additives have been based on the burning time of polypropylene plaques using 1 wt% of Flamestab NOR 116 as standard formulation. Virgin polypropylene samples without any flame retardant burned down in 132 s under these experimental conditions, whereas polypropylene bars containing 1, 2, and 10 wt% of Flamestab NOR 116 burned down in 238, 237, and 205 s, respectively. (The flame retardant experiments verify the inverse
2. Mechanistic considerations and studies on structureeactivity relationship
Effect of nitroxyl radicals
O O N
O
O
N O
O
TEMPO1
Effect of 4-substituent
O N
N
4PyNO6R
O
O O N
NH
NH
NH
NH
N O
4-UREANO6R
Effect of alkoxy ring size O R1 O N
O
O
N O R1
O
R1 =
or
,
NO5R
,
NO6R
NO7R
NO8R
FIGURE 5 Various NOR additives tested as flame retardants for polypropylene plaques.
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
Effect of ring substitution O R2 O N
O
O
N O R2
O
or
R2 =
,
,
NO6R2,6DiMe
NO6R3Me
NO6R4Me
NO6R4IsoPr
Effect of alkoxy substituent O R3 O N
O
O
N O R3
O
O R3 =
or ,
,
NOBenzyl
NOMeST
NOEster
NOIsoPr
N O R4
R4 =
Me
NOMe
,
Ethyl
NOEthyl
or
n-Propyl
NOnPropyl
FIGURE 5 cont’d.
concentration effect of Flamestab NOR 116 [15].) All other burning times are given relative to 1 wt% of Flamestab NOR 116, i.e. a value of >1 and <1 means a better or worse performance than using 1 wt% of Flamestab NOR 116, respectively. The polypropylene formulations containing 1, 2, and 3 wt% of TBBPA as a brominated reference flame retardant exhibited relative burning times of 0.95,
2. Mechanistic considerations and studies on structureeactivity relationship
Table 1 Flame Retardant Efficacies for a Series of Different N-Alkoxyamines UL 94 t1 (s)/t1 (s, NOR 116)
Additive Code
Additive %wt
DSCdDecomposition Temperature ( C)
No FR TBBPA TBBPA TBBPA NOR 116 NOR 116 NOR 116
– 1 2 5 1 2 3
– – – – 284 284 284
0.55 0.95 1.07 1.32 1 1 0.92
1
–
0.85
Nitroxyl Radical TEMPO1
4-Substituted Cyclohexyloxytetramethylpiperidines 4PyNO6R 4UREANO6R
1 1
276 –
1.25 1.13
Cycloalkyloxytetramethylpiperidine Diesters Having Various Ring Sizes NO5R NO6R NO7R NOR8R
1 1 1 1
278 295 261 236
0.85 1.13 0.88 0.86
NORs with Different Alkyl Substituents on the Cyclohexyl Ring NO6R2,6DiMe NO6R3Me NO6R4Me NO6R4IsoPr
1 1 1 1
248 288 288 277
0.71 0.81 0.93 1.15
Various Alkoxy Substituted Tetramethylpiperidines NOBenzyl NOMeST NOEster NOIsoPr NOMe
1 1 1 1 1
262 234 197 234 300
NOEthyl NOnPropyl
1 1
304 295
TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxyl; TBBPA, tetrabromobisphenol A. a Based on the burning time after second ignition.
0.86 0.81 0.8 0.76 (t1 ¼ 1 s) 1.13a 1.02 0.90
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
1.07, and 1.32, respectively. Even if one needs to admit that this method is not the ideal way to compare flame retardant efficacies, it is still a feasible method for the preliminary evaluation of the potential of various NOR structures as flame retardants for polypropylene. Overall, the flame retardant properties of different alkoxyamines mainly depend on three parameters: (1) thermal stability (homolysis temperature), (2) products of decomposition and mechanisms thereof, and (3) reactivity of the radicals generated during decomposition. The results indeed reveal that nitroxyl radicals (TEMPO1) as such also exhibit some flame retardant properties although significantly more weaker than Flamestab NOR 116. One can readily see that two 4-substituted NOR compounds (4PyNO6R and 4UREANO6R) containing the 1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine moiety showed similar thermal stabilities. Surprisingly, 4PyNO6R and 4UREANO6R exhibited even higher flame retardant properties than Flamestab NOR 116 under these experimental conditions. In general, one would expect that the substituent in the 4-position would not directly affect flame retardant properties. However, the differences in recorded flame retardant properties may arise from differences in compatibility, ease of dispersion, and volatility of these NOR compounds due to their variations of backbone structures. In order to avoid the problem of dispersion in the subsequent NOR series, compounds having the same NOR backbone were prepared (either a diester or a long alkenyl chain in the 4-position). Next, the effect of ring size on flame retardancy was investigated by comparing the flame test results obtained using 5-, 6-, 7-, and 8-membered ring 1-cycloalkyloxy-2,2,6,6-tetramethyl piperidine-4-diesters. Although, the cycloalkyloxytetramethylpiperidine diesters NO5R, NO6R, NO7R, and NO8R had relatively comparable electronic and steric properties, their flame retardancy potentials differed quite significantly. The best result was recorded for the six-membered ring N-alkoxyamine (NO6R), whereas larger or smaller ring sizes showed significantly lower flame retardant efficacies. As a consequence of this, 1-cyclohexyloxy2,2,6,6-tetra-methylpiperidine appeared to be a promising template for further optimization of the NOR structure. Therefore, as a next step, NOR additives with alkyl substitutions on the cyclohexyl ring were synthesized, and their flame retardant properties were studied. Compound NO6R2,6DiMe having methyl groups in 2 and 6 positions of the cyclohexyl group as well as NO6R3Me having a methyl group in 3 position exhibited inferior flame retardant properties compared to Flamestab NOR 116. This can be attributed to the lower thermal stability of NO6R2,6DiMe and the fact that the substituted cyclohexyl radical liberated during thermal decomposition is also less reactive than the unsubstituted cyclohexyl radical. Thus, it is understandable that NO6R2,6DiMe presented worse flame retardant properties than Flamestab NOR 116 or NO6R. Compounds NO6R4Me and NO6RIsoPr are similar additives as NO6R with alkyl substitutions of the cyclohexyl ring in the 4-position. The flame retardant performances of these two molecules are close to NO6R. Thus, the presence of alkyl substituents in the 4-position of the cyclohexyl ring does not significantly affect the flame retardant properties. However, the adjustment of the
3. Azoalkanes (AZO) as flame retardants
cyclohexyl radical volatility and diffusion properties may be fine tuned by the optimization of alkyl substitution of the cyclohexyl ring. In this study, alkoxyamines with other alkoxy substituents than cyclohexyl or related alkyl substituted cyclohexyl derivatives were also investigated. Compounds bearing isopropyloxy (NOIsoPr), ester (NOEster), or a-methyl styrene (NOMeST) exhibited fairly low potential as flame retardants since they lack sufficient thermal stabilities for processing. The processing of polypropylene was conducted at 230 C and, therefore, this type of “low thermal stability” NOR compounds may have undergone partial decomposition already during the polymer-processing step itself. This fact, explains the poor efficacies recorded for these NOR series. In contrast, in the series of 1-n-alkoxy-4-(dodec-2-enyl)-2,2,6,6-tetramethylpiperidines, all the compounds showed a comparable or even higher efficiency than Flamestab NOR 116. The results indicate that the flame retardant properties within the family of NOR compounds increased as a function of the thermal stability of the N-alkoxyamine functionality. One of the most thermally stable additives prepared was 1-methoxy-4-(dodec2-enyl)-2,2,6,6,tetra-methylpiperidine (NOMe), and it was shown to be the most efficient FR additive within the synthesized library of NOR compounds. This study clearly indicated that the formed nitroxyl and aminyl radicals of NOR thermolysis are perhaps not the only radical species involved in the retardation of the combustion process. Thus, the simultaneously formed alkyl radicals might also play a vital role as flame retardant species (Section 3). In addition, NOMe showed an extraordinarily high synergistic effect in conjunction with brominated flame retardants. Thus, the UL94 tests showed that V-0 rating in combination with an already 11 wt% of brominated flame retardant was reached when employing NOMe, whereas 14 wt% of the brominated flame retardant was needed when using Flamestab NOR 116 in order to achieve the V-O rating. The result suggests that methyl-TEMPO additives also have a higher tendency to interact/react with brominated flame retardants than those based on the more sterically hindered cyclohexyl-TEMPO analogs.
3. Azoalkanes (AZO) as flame retardants for polypropylene films and plaques The conclusion derived from studies of the structureeactivity relationship of NOR flame retardants and their breakdown was that two distinctly different thermolysis reaction pathways clearly take place (Figure 4). Thus, the fragmentation of the NOR molecule may occur at both the NeO and OeR bonds whereby in theory either alkyl and nitroxyl radicals or alkoxy and aminyl radicals are formed. Given the fact that these four types of radicals clearly differ from each other in terms of chemical reactivity and stability, the significance of their presence in the flame retardant process was not expected to be equivalent. In the original Flamestab NOR 116 flame retardant publications, it was thought that aminyl and alkoxy radicals would be
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the most important species for interrupting the free radical reactions during the polymer combustion process [39]. Whereas the very high synergistic effect of NOR additives with conventional brominated flame retardants was ascribed to the generation of aminyl, alkoxy, and nitroxyl radicals that in turn interact with brominated compounds and thereby facilitate the release of bromine, thus improving flame retardant performance [40]. The potential role of generated alkyl radicals was not discussed. The apparently complex decomposition of N-alkoxyamines made it worthwhile to investigate the flame retardancy properties of each of the aforementioned radical families separately. The investigation was focused on the least studied radicals, i.e. the role of alkyl radicals on flame retardancy of polypropylene. Azoalkanes were selected as the cleanest and most convenient source of alkyl free radicals [44] (Figure 6). As in the case of N-alkoxy hindered amines, the thermal stability of azoalkanes is very closely related to their chemical structure. Their thermal stability is mainly governed by the ground-state energy, stability of the incipient radicals, and orbital symmetry factors. Aromatic azo compounds and azotriphenylmethane represent extreme cases: the former decomposes at 500 C, and the latter has a transient existence only at 40 C. The Polanyi plot of activation energy for thermolysis of both symmetrical azoalkanes and N-alkoxy hindered amines versus the CeH bond dissociation energy of the corresponding hydrocarbons are in both cases essentially linear [38,45]. As a consequence of this, the thermal stability of the azoalkane can be optimized for various polymers in terms of their processing temperatures and decomposition temperatures. In analogy to N-cyclohexyloxy-2,2,6,6-tetramethylpiperidinyl-functionalized flame retardants, three azocyclohexyl compounds were synthesized, i.e. 4,40 -bis(cyclohexylazo-cyclohexyl)methane (ABO AA1), cyclohexylazooctadecane (ABO AA 2), and azocyclohexane (ABO AA3) as sources of cyclohexyl radicals [46]. For the evaluation of the flame retardancy of azocyclohexyl, ignitability tests were performed according to the DIN 4102-1/B2 standards [47]. First, polypropylene was blended with 0.1% by weight of calcium stearate, 0.2% of CibaÒ IrganoxÒ B 501 (stabilizer), and with 0, 0.25, or 0.5% of the radical precursor. The blends were melt processed into fibers, spun into socks, and finally compression molded into 200 mm films. Surface exposure of the samples to a gas flame was separately carried out, and the results are shown in Table 2 [48]. Slight differences in burn times and damage length were observed, and overall, the formulation containing the ABO AA1 additive gave a marginally better performance than the others in thin PP sections. When tested, all the samples containing azoalkanes gave a clean burn residue after the flame application. In contrast, the Flamestab NOR 116 samples exhibited N2 R
N
N
R'
R
+
FIGURE 6 Azoalkanes: “The cleanest and most convenient source of alkyl radicals”.
R'
3. Azoalkanes (AZO) as flame retardants
Table 2 Flammability Results According to DIN 4102 Part-1 Classification B2 Test Method: Face Ignition Test of 230 mm 90-mm samplesa
Formulation
Average Damage Area (cm2)
Total Burning Time (s)
Pass/Fail
Blank 0.25 wt% of Flamestab NOR 116 0.50 wt% of Flamestab NOR 116 0.25 wt% of ABO AA1 0.5 wt% of ABO AA1 0.25 wt% of ABO AA2 0.5 wt% of ABO AA2 0.25 wt% of ABO AA3 0.5 wt% of ABO AA3
100 n.d.b n.d. 28 21 39 31 39 22
100 20 22 13 8 18 13 16 10
Fail Pass Pass Pass Pass Pass Pass Pass Pass
a Substrate: polypropylene is blended with 0.1 wt% of calcium stearate; 0.2 wt% of Ciba Irganox B501; and 0 wt%, 0.25 wt%, or 0.5 wt% of the test FRs. The blends are melt processed into fibers, spun into socks, and subsequently compression molded into thin films. b n.d. ¼ not determined.
yellow chars around the burn damage area. The most striking difference in flame retardant efficacy was observed in thick polypropylene plaques of 1 mm, i.e. azo compounds (e.g. ABO AA3) had a much better FR performance than did Flamestab NOR 116 in thick PP sections (Table 3.) In addition, azo compounds resulted in less discoloration and ABO AA1 exhibited an even nonburning dripping behavior.
3.1 The potential of symmetric versus unsymmetric azoalkanes Most interestingly, many critical properties of azoalkanes of the general formula R0 eN]NeR, such as physical state, toxicity, volatility, initiator efficiency, and in particular thermal stability (in contrast to peroxides (R0 eOeOeR)) can be adjusted over a very wide range by selection of groups R0 and R. According to Engel, as a rule of thumb, the thermal stability of unsymmetrical azoalkanes (R ] R0 ) is intermediate between the stabilities of the symmetrical azoalkanes ReN]NeR and R0 eN] Table 3 FR Performance in PP Moldings (1 mm)a FR Compound 0.5 wt%
Weight Loss %
Burn Length
Pass/Fail
Control no FR NOR 116 ABO AA3
100 49.4 12.9
100 80 47
Fail Fail Pass
Compounding: W&P ZSK25 WLE, 230 C, 100 rpm, 4 kg h1.Molding: Injection molding Arburg 320S, 1-mm plaque. a Substrate: Moplen HF500N, PP homo, 0.3% Ix B225 þ 0.05% CaSt.
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NeR0 [49]. In addition, a consensus exists that most symmetrical azoalkanes lose nitrogen by the simultaneous breakage of both CeN bonds, whereas unsymmetrical azoalkanes may undergo one-bond cleavage [49]. As a natural extension of the work with ABO AA azoalkane structures, systematic structural changes of the aforementioned basic azoalkane compound were screened in terms of variations in ring size, electronic (resonance and inductive) and steric hindrance with regard to flame retardant efficiency. Therefore, a number of symmetrical and unsymmetrical azoalkanes of the general formula R0 eN]NeR were prepared, as shown in Figure 7 [46]. The experimental results show that in the series of different-sized azocycloalkanes, the flame retardant efficacy decreased in the following order: R ¼ cyclohexyl > cyclopentyl > cyclobutyl > cyclooctanyl[cyclododecanyl. However, in the series of aliphatic azoalkanes compounds, the efficacy decreased in the following order: R ¼ n-alkyl > tert-butyl > tert-octyl. As a spin-off, it was observed that unsymmetrical azoalkanes, in particularly N-cyclohexyl-N0 -t-octyldiazene, can successfully be used for manufacturing of controlled-rheology polypropylene and of crosslinked high-density polyethylene (PEX) [50,51].
3.2 Azoxy, azine, hydrazone, and triazene compounds Furthermore, structures related to azo compounds were synthesized in the form of azoxy, hydrazone, triazene as well as azine derivatives in order to assess their potential as novel flame retardants for polypropylene (Figure 7). The prepared azoxy (ReN] NOeR), azine (R]NeN]R), and hydrazone (ReN]NHeR) derivatives provided flame retardancy to polypropylene films at already very low concentrations (0.25e1 wt%). More recently, various triazene (ReN1]N2eN3R0 R0 )-based flame retardants were prepared. For example, polypropylene samples containing a very low concentration of only 0.5 wt% of bis-4,40 -(3,30 -dimethyltriazene)diphenyl ether passed the test with B2 classification. It is noteworthy that no burning dripping could be detected and the average burning times were very short with exceptionally low weight losses [52]. Therefore, triazene compounds constitute a new and interesting family of radical generators for flame retarding of polymeric materials. The high flame retardant potential of triazenes can be attributed to their ability to generate various types of radicals during their thermal decomposition. According to thermogravimetric analysis/Fourier transform infrared spectroscopy/MS analysis, triazene units are homolytically cleaved into various aminyl, resonance-stabilized aryl radicals, and different CH fragments with simultaneous evolution of elemental nitrogen, as shown in Figure 8. These radicals can effectively provide flame retardant properties.
3.3 Tetrapotassium azo diphosphonate (INAZO) as flame retardants With an effort to further extend the scope of azoalkane-based flame retardants, inorganic azo phosphonate compounds were synthesized, i.e. (KO)2(O)PeN] NeP(O)(OK)2$4H2O, (INAZO), and test its potential as a novel type of flame retardant additive for two-component polyurethane adhesives. The action mechanism of
3. Azoalkanes (AZO) as flame retardants
Symmetrical azoalkanes
Unsymmetrical azoalkanes
N N
N N
ABO AA3
N N
N N
N N
N N
N N
ABO AA1
ABO AA2 N N
N N
N N
N N
(CH2)15CH3
N N
N N
N
N N
N
N N
N
N N
N
N N
Related hydrazones and azine compounds
H N N
H N N
N N
FIGURE 7 Chemical structures of symmetrical and unsymmetrical azoalkanes and related compounds.
the developed INAZO flame retardant was suggested to be mainly in the condensed phase. UL 94 V-0 rating was achieved in the vertical burning test when 10 wt% loading of INAZO was used, whereas the reference flame retardant APP required a loading of 20 wt% to reach the V-0 classification [53].
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
FIGURE 8 Thermolysis of triazene compounds.
3.4 Multifunctional bis(1-propyloxy-2,2,6,6-tetramethylpiperidyl)4-diazene A disadvantage of the azoalkane-based compounds in comparison to NOR 116 is that they do not contribute to light stability. Although selective azo compounds have both the advantage of nonburning dripping properties during fire tests and excellent optical properties, they also importantly function well alone in thick polypropylene sections such as 1-mm plaques. As a consequence of this, the hypothesis of creating an optimized multifunctional flame retardant family without any of the aforementioned (as 1) lack of light stability, (2) low synergistic effect with brominated halogens, and (3) lack of flame retardant performance in thick Table 4 Flame Test on 1-mm Injection Molded PP Plaques According to the DIN 4102/B2 Test FR Compound 0.5 wt%
Weight Loss %
Burn Length
Pass/ Fail
Control NOR 116
100 49.4
100 80
Fail Fail
12.9
47
Pass
14
43
Pass
8.9
37
Pass
5.2
27
Pass
N
O N
O N
O N
N
N N
N N
N N
N O
N O
N O
3. Azoalkanes (AZO) as flame retardants
polymer plaques was explored. This was done by combining the diazene and N-alkoxyamine chemistries into a new molecule, i.e. bis(1-propyloxy2,2,6,6-tetramethylpiperidyl)-4-diazene (AZONOR) containing both of the favorable constituents [54]. Polypropylene samples containing very low concentrations of 0.25e1 wt% of this additive successfully passed not only the fire standard tests of DIN 4102 B2 and NF P92-505 but also the more challenging UL94 VTM-2 standard. By further refining the AZONOR structure, excellent flame retardant properties could be recorded in polypropylene moldings of 1 mm that clearly outperform NOR 116, as shown in Table 4. Besides relatively low levels of addition and having no detrimental effect on polypropylene appearance or its mechanical and processing properties, another great advantage offered by this flame retardant is its multifunctionality, i.e. high flame retardant durability in combination with light stability. Thus, even after 2000 h of artificial weathering, no significant decrease in flame retardant efficacy could be observed [55]. In addition, we could observe a strong synergistic effect between AZONOR and e.g. brominated flame retardants and aluminum trihydrate (ATH) for polypropylene plaques.
4. CONCLUSIONS Different nitrogen-based compounds alone or in combination with conventional flame retardant systems have demonstrated their versatility and usefulness as flame retardants for both thermoplastics and thermosets. Their main common advantages are they are effective at low concentrations, they cannot form dioxans and furans during combustion, they are noncorrosive, they exhibit good UV stability, they are recyclable due to their high thermal stabilities, some of them exhibit excellent synergistic effects with conventional flame retardants, and many of the nitrogen compounds exhibit multiple modes of action. One of the major drawbacks of today’s nitrogen flame retardants is that they require a good fit between the polymer and type of nitrogen-based flame retardant, i.e. nitrogen-based flame retardants are polymer specific. The key drivers for future research of flame retardants will be increased fire safety in combination with flame retardant formulations that fulfill the criteria of sustainable production and products. In recent years, important questions about the environmental safety of antimony, phosphate, and in particular, brominated flame retardants have been raised. All this information suggests a reappraisal of nitrogen-based flame retardants, and many improvements are expected when chemistries have been further refined and fire characterization of various polymers have been elucidated in detail. It would now appear that there are still plenty of nitrogen-based flame retardants to be discovered. Thus, continued research efforts are definitely warranted.
Acknowledgments ˚ bo Akademi University The authors are indebted to coworkers at BASF (former Ciba) and A for flame retardant synthesis and for all the fruitful discussions related to flame retardancy of
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polymers. They would especially like to thank R. Nicholas, M.Sc., Dr M. Roth, Dr H. Hoppe, Dr S. Kniesel, Dr R. Xalter, Dr R. King III, Dr R. Drewes, M. Aubert, M.Sc., T. Tirri, M.Sc., and W. Pawelec, M.Sc.
References [1] Camino G, Costa L, Martinasso G. Intumescent fire-retardant. Polym Degrad Stab 1989;23:359e76. [2] Kersjes JG, Kierkels RHM. Polyphosphate derivative of a 1,3,5-triazine compound, method for producing the same and its use. PCT Int Appl; 2000. WO 00/02869, (DSM N.V.). [3] Steenbakkers-Menting HNA, Tijssen J, Tummers DJ. Halogen-free flame retarder composition and flame retardant polyamide composition. PCT Int Appl; 2002. WO 02/28953 (DSM N.V). [4] Schlosser E, Nass B, Wanzke W. Combined flame retardant. Clariant GmbH; 2004. EP 1386942. [5] Schlosser E, Nass B, Wanzke W. Flame retardant combination. Clariant Produkte GmbH; 2001. EP 1070754. [6] Suzuki K, Shindo M, Iljima M. 1,3,5-Triazine derivative salts of polyacids comprising phosphorus sulfur, and oxygen and process for production the same. EP 0974588. Nissan Chemical Industries; 2000. [7] Wehner W, Dave T. Triazene compounds containing phosphorous as flame retardant. PCT Int Appl; 2009. WO 2009/015772 (Catena Additives GmbH). [8] Hara Y, Tamura K, Nishimura T, Matsumoto N. Flame retardant composition and flameretardant resin composition. EP 1116773. Nippon Chemical Industrial Company Ltd; 2001. [9] Xalter R, Weiss T. Guanidine phenylphosphinate flame retardant composition. PCT Int Appl; 2010. WO 2010/133570 (BASF Schweiz AG). [10] Eichhorn J. Synergism of free radical initiators with self-extinguishing additives in vinyl aromatic polymers. J Appl Pol Sci 1964;8:2497e524. [11] Eichhorn J, Bates SI. Foamed self-extinguishing alkenyl aromatic resin composition containing and organic bromide and peroxide, and method of preparation. US Pat 3058928 (Dow Chemical Corp.); 1962. [12] Modesti M, Lorenzetti A. FR design for foam materials. In: Wilkie CA, Morgan AB, editors. Fire retardancy of polymeric materials. 2nd ed. 2010. pp. 763e81. Boca Raton. [13] Chiotis A, Clouet G, Brossas J. Effect of free radicals generating compounds towards the flammability of phosphonated polystyrenes-V. Polym Bull 1984;11:275e80. [14] Horsey DW, Andrews SM, Davies LH, Dyas DD, Gray RL, Gupta A, et al. Flame retardant compositions. PCT Int Appl; 1999. WO 99/00450 (Ciba Specialty Chemicals Holding Inc.). [15] Srinivasan R, Rotzinger B. Presentation, polyolefins 2000. In: International conference on polyolefins, Houston, USA; 2000. pp. 571e81. [16] Pfaendner R. Nitroxyl radicals and nitroxylethers beyond stabilization: radical generators for efficient polymer modification. Chimie 2006;9:1338e44. [17] Basbas AI, Alvisi D, Cordova R, DiFazio MP, Fischer W, Kotrola JA, et al. Process for the preparation of sterically hindered nitroxyl amines. PCT Int Appl; 2008. WO 2008003605 (Ciba Specialty Chemicals Holding Inc.).
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[18] Pfaendner R, Roth M, Schoening KU, Weiss T, Hindalekar CB. Phospho-substituted alkoxyamine compounds. PCT Int Appl; 2011. WO 2011/086114 (BASF SE). [19] Roth M, Simon D, Leslie G, Nesvadba P, King RE. Flame retardant polymer composition containing hydroxylamine. PCT Int Appl; 2003. WO 2003087211 (Ciba Specialty Chemical Holdings Inc.). [20] Troutman MV, Ravichandran R, Srinivasan R, King RE, Horsey DV. Flame retardant composition. PCT Int Appl; 2002. WO 02/074847 (Ciba Specialty Chemicals Holdings Inc.). [21] Standard methods of fire tests for flame-resistant textiles and films. Quincy, MA, USA: National Fire Protection Association; 1996. [22] Wilen C-E, Pfaendner R. Improving weathering resistance of flame-retarded polymers. J Appl Polym Sci 2013;129:925e44. [23] Roth M. Flame retardant composition comprising sterically hindered amines. PCT Int Appl; 2009. WO 2009080554 (Ciba Specialty Chemicals Holding Inc.). [24] Kaprinidis N, Lelli N. Flame retardant polymer electrical parts. PCT Int Appl; 2004. WO 2004035673 (Ciba Specialty Chemicals Holding Inc.). [25] Kaprinidis N, Lelli N. Flame retardant composition. PCT Int Appl; 2004. WO 2004035671 (Ciba Specialty Chemicals Holding Inc.). [26] Butz V. Flame retardant composition comprising a phosphonic acid derivative. PCT Int Appl; 2010. WO 2010/026230 (Thor GmbH). [27] Xalter R, Weiss T, Roth M, Hoppe H. Phenylphosphonate flame retardant composition. PCT Int Appl; 2011. WO 2011003773 (BASF SE). [28] Kaprinidis N, Lelli N, Kierkels RHM. Flame retardant composition. PCT Int Appl; 2004. WO 2004041919 (Ciba Specialty Chemicals Holding Inc.). [29] Kaprinidis N, Chandrika G, Zingg J. Flame retardant composition. PCT Int Appl; 2004. WO 2004031286 (Ciba Specialty Chemicals Holding Inc.). [30] Kaprinidis N, Horsey DW. Flame retardant composition. PCT Int Appl; 2003. WO 03/ 016388 (Ciba Specialty Chemicals Holding Inc.). [31] Kaprinidis N, Zingg J. Overview of flame retardant compositions UV stable flame retardant products for polyolefins; halogen free melamine based flame retardants for polyamides. Presentation, Additives 2003, Executive Conference Management, San Francisco, USA; 2003. [32] Troutman MV, Ravichandran R, Srinivasan R, King RE, Horsey DV. US Pat 2006084731 (Ciba Specialty Chemicals Holdings Inc.), 2002. [33] Marney DCO, Russell LJ, Stark TM. The influence of N-alkoxy HALS on the decomposition of a brominated fire retardant. Polym Degr Stab 2008;93:714e22. [34] Marney DCO, Russell LJ, Soegang TM, Dowling VP. Mechanistic analysis of the fire performance of a fire retardant system. J Fire Sci 2007;25:471e97. [35] http://www.specialchem4polymers.com/tds/flamestab-nor-116/ciba-now-part-of-basf/ 2672/index.aspx. [36] Salomon DH, Rizzardo E, Cacioli P. Polymerization process and polymers produced thereby. US Pat 4581 429 (Commonwealth Scientific and Industrial Research Organization); 1985. [37] Hawker CJ, Bosman AW, Harth E. New polymer synthesis by nitroxide mediated living radical polymerization. Chem Rev 2001;101:3661e88 [and references therein]. [38] Ciriano MV, Korth HG, van Scheppingen WP, Mulder P. Thermal stability of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and related N-alkoxyamines. J Am Chem Soc 1999;121:6375e81.
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[39] Kaprinidis N, Zingg J. International fire safety. Conference; papers presented at Hotel Monetleon; 2003. [40] Kaprinidis N, Shields P, Leslie G. Flame retardants; 2002. Queen Elizabeth II Conference, London, England. [41] Nicolas R, Wilen C-E. NOR structureeflame retardant property relationship. Internal Abo Akademi University Report; 2003. [42] Chauvin F, Dufils PE, Gigmes D, Guillaneuf Y, Marque SRA, Tordo P, et al. Molecular disorder and mesoscopic order in polydisperse acrylic block copolymers prepared by controlled radical polymerization. Macromolecules 2006;39:5238e50. [43] Miura Y, Nakamura N, Taniguchi I. Low-temperature living radical polymerization of styrene in the presence of nitroxides with spiro structures. Macromolecules 2001;34: 447e55. [44] Nicolas R, Wilen C-E, Roth M, Pfaendner R, King III RE. Azoalkanes: a novel class of flame retardants. Macromol Rapid Commun 2006;27:976e81. [45] Danneberg JJ, Rocklin DJ. A theoretical study of the mechanism of the thermal decomposition of azoalkanes and 1,1-diazenes. J Org Chem 1982;47:4529e34. [46] Aubert M, Nicolas R, Pawelec W, Wilen C-E, Roth M, Pfaendner R. Azoalkanesdnovel flame retardants and their structure property relationship. Polym Adv Technol 2011;22:1529e38. [47] German standard DIN 4102-1: fire behaviour of building materials and building componentsdpart 1: building materials, concepts, requirements and tests; 1998. [48] Nicolas R, Wilen C-E. PCT Int Appl; 2005. WO 2005/030852A2 (Ciba Specialty Chemicals Holding Inc.). [49] Engel PS. Mechanism of the thermal and photochemical decomposition of azoalkanes. Chem Rev 1980;2:99e150. [50] Roth M, Pfaendner R, Nicolas R, Wilen C-E. Azo compounds for polymer degradation. PCT Int Appl; 2006. WO 2006/106059 (Ciba Specialty Chemicals Holding Inc.). [51] Aubert M, Roth M, Pfaendner R, Wilen C-E. Azoalkanes: a novel class of additives for cross-linking and controlled degradation of polyolefins. Macromol Mater Eng 2007; 292:707e14. [52] Pawelec W, Aubert M, Pfaendner R, Hoppe H, Wile´n C-E. Triazene compounds as and effective class of flame retardants for polypropylene. Polym Deg Stab 2012;97:948e54. [53] Tirri T, Aubert M, Wilen C-E, Pfaendner R, Hoppe H. Novel tetrapotassium azo diphosphonate (INAZO) as flame retardant for polyurethane adhesives. Polym Deg Stab 2012;97:375e82. [54] Aubert M, Wilen C-E, Pfaendner R, Kniesel S, Hoppe H, Roth M. Bis(1-propyloxy2,2,6,6-tetramethylpiperidine-4-yl)-diazene e an innovative multifunctional radical generator providing flame retardancy to polypropylene even after extended artificial weathering. Polym Deg Stab 2011;96:328e33. [55] Roth M, Pfaendner R, Aubert M, Wilen C-E. Symmetric azo compounds in flame retardant compositions. PCT Int Appl; 2008. PCT Int Appl WO 2008/101845 (Ciba Specialty Chemicals Holding Inc.).
Design and Utilization of Nitrogen Containing Flame Retardants Based on N-Alkoxyamines, Azoalkanes and Related Compounds
8 Carl-Eric Wile´n
A˚bo Akademi University A˚bo, Finland
R. Pfaendner Fraunhofer Institute for Structural Durability and System Reliability LBF Division Plastics, Darmstadt, Germany
CHAPTER OUTLINE 1. Introduction ....................................................................................................... 267 2. Mechanistic considerations and studies on structureeactivity relationship of N-alkoxyamines as flame retardants ................................................................ 272 3. Azoalkanes (AZO) as flame retardants for polypropylene films and plaques............ 279 3.1 The potential of symmetric versus unsymmetric azoalkanes .................... 281 3.2 Azoxy, azine, hydrazone, and triazene compounds .................................. 282 3.3 Tetrapotassium azo diphosphonate (INAZO) as flame retardants .............. 282 3.4 Multifunctional bis(1-propyloxy-2,2,6,6-tetramethylpiperidyl)-4-diazene.. 284 4. Conclusions ....................................................................................................... 285 Acknowledgments ................................................................................................... 285 References ............................................................................................................. 286
1. Introduction Flame retardants containing nitrogen are a class of various materials among which are well-known halogen-free commercial products, namely ammonium polyphosphate (APP), melamine cyanurate (MC), melamine borate, and melamine (poly)phosphate. APP, which is available in different crystal modifications and with coatings (e.g. melamine and silicone) or in an encapsulated form, decomposes by exposition to fire or heat to ammonia, polyphosphoric acid, and phosphorus oxides. In the presence of a char-forming agent, e.g. a polyol, an intumescent barrier via phosphoric ester Polymer Green Flame Retardants. http://dx.doi.org/10.1016/B978-0-444-53808-6.00008-1 Copyright © 2014 Elsevier B.V. All rights reserved.
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intermediates is formed [1]. The main applications of APP are coatings for steel and wood and within the plastic field unsaturated polyesters, epoxides, polyurethanes, and polyolefins. MC is a 1:1 adduct of melamine and cyanuric acid. MC decomposes endothermically into its components, and melamine decomposes further to nitrogen-containing gases such as ammonia. MC is mainly used as a flame retardant component in polyurethanes, polyesters, epoxides, and polyamides, specifically in unfilled polyamide-6. Melamine borate decomposes endothermically into its components and promotes glassy char formation mainly in combination with phosphates. Melamine phosphates cover melamine orthophosphate (1:1 salt of melamine and phosphoric acid), dimelamine orthophosphate (2:1), dimelamine pyrophosphate, melamine pyrophosphate, and melamine polyphosphate (MPP). Because of thermal stability reasons, the most interesting material in this class of plastic applications is MPP. The endothermic decomposition of MPP produces ammonia, other nitrogen species, and poly(phosphoric acid); the latter dehydrates the polymer under char formation. Application fields are polyamides (PA6, PA6.6 glass fiber reinforced) and polyesters. For example, 25% MPP provides UL 94 V-0 classification at 1.6 mm in PA6.6/20% glass fibers [2]. In combination with other phosphorus flame retardants, a synergistic effect of MPP is found, which allows one to reduce the overall flame retardant loading, e.g. combinations of MPP and a phosphinate to achieve a UL-94 V-0 classification in glass fiber-reinforced PA6.6 with an overall loading of 15e20% [3e5]. Furthermore, melamine salts other than phosphates, higher condensation products of melamine (melam, melon, melem), and closely related substances (ammeline, ammelide, urea, guanamine, guanidine, and derivatives thereof) as well as all kind of salts thereof have been proposed as flame retardants for plastic applications. For example, mixed salts of melamineemelamemelem and polyphosphate or pyrosulfate have been synthesized at condensation temperatures >300 C [6]. Phosphorus derivatives of nitrogen-containing species other than phosphates have been described, e.g. in the form of nitrilotrismethylenephosphonic acid [7,8] or as guanidine phenylphosphinate [9]. Most of the above-mentioned salt structures of melamine and related substances act in the condensed phase through endothermic decomposition of the flame retardant, dilution of the burning gases through the formation of inert gaseous molecules, and through char formation. A somewhat different activity mechanism was found by using alkoxyamines (NOR) as flame retardants forming radical species during decomposition. The use of radical generators as flame retardant components was already proposed in the 1960s [10,11], where it was shown that peroxides, azo, sulfur, bibenzyl compounds, and hydrazones act synergistically with brominated compounds. Consequently, combinations of hexabromocyclododecane and peroxides have been used as standard flame retardant (FR) systems in polystyrene foam for many years [12]. This system, however, has to be replaced due to environmental reasons in the near future. Furthermore, it was as also stated that radical generators of peroxide, hydroperoxide, and polysulfide types alone do not act as flame retardants [13].
1. Introduction
The first who realized that alkoxyamines act as potential flame retardants without any other flame retardant were inventors from Ciba Specialty Chemicals [14]. Alkoxyamines (or hydroxylamine ethers, NOR) were first introduced as noninteracting ultraviolet (UV) stabilizers with a low basicity. Such stabilizers provide protection for automotive coatings and for agricultural films even under severe conditions, e.g. in the presence of pesticides. Flame retardants such as halogens or phosphorus compounds generate thermally or photochemically acidic species, which result in inferior UV stability by deactivating hindered amine light stabilizers. This problem was solved by using alkoxyamines instead of amines in combination with brominated flame retardants. Surprisingly, it was found that the alkoxyamines do not only improve the UV stability but they also contribute to flame retardancy. Even without any additional flame retardant, it was demonstrated that alkoxyamines alone can achieve flame retardancy, e.g. in polypropylene fibers and films [15,16]. Meanwhile, various structures and process patents have been published that claim alkoxyamines to be efficient flame retardants [17,18]. The synthesis of alkoxyamines is performed usually from the corresponding sterically hindered piperidine via the nitroxyl intermediate through reaction with a hydrocarbon in the presence of an organic peroxide such as peracetic acid, hydrogen peroxide under catalysis of copper, molybdenum oxide, or iodide. A more recent process uses the coppercatalyzed reaction of aldehydes with the nitroxyl intermediate [17]. In addition, several other chemical structures related to alkoxyamines are claimed to provide flame retardancy, namely hydroxylamine esters [19], hydroxylamines, and nitroxyl radicals [20]. The first commercial product on the basis of alkoxyamines was introduced by Ciba (now BASF) in 2000 under the trade name Flamestab NOR 116 (Figure 1). R
R
HN
N
R=
N C4H9
N
N O
FIGURE 1 Structure of Flamestab NOR 116.
N
NH
H
R
N N
N
C4H9
N O
269
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
It was demonstrated that Flamestab NOR 116 provides flame retardancy at a concentration of only 0.5% passing the NFPA 701 fiber test [21] on polypropylene knitted socks [15]. Moreover, the addition of alkoxyamines as an FR synergist can improve the efficiency of conventional flame retardants, whereas all kinds of flame retardants and synergistic mixtures, e.g. brominated compounds [22e25], phosphorus compounds [26,27], phosphacenes, melamines [28,29], and inorganics, fluorolefins [30] are claimed. For example, the combination of NOR and decabromodiphenyloxide surpasses the flame retardants alone [31]. Therefore, it is possible with alkoxyamines to design flame retardant polyolefin molding compositions with lower levels of halogenated flame retardants, and in addition to eliminate antimony trioxide, which is often used as an FR synergist together with brominated compounds. Low FR concentrations and no antimony trioxide result in polymers with improved processability, better mechanical properties, and reduced smoke density. For example, UL 94 V-2 rating is achieved in PP injection-molded plaques at concentrations as low as 3.5% of formulations containing Flamestab NOR 116 [32]. The activity of the alkoxyamine as a flame retardant is based on the thermolysis of nitroxyl ethers, which leads to the formation of either alkoxy and aminyl radicals or alkyl and nitroxyl radicals (Figure 2). Aminyl and alkoxy radicals are very reactive and cause, on the one hand, degradation of polypropylene (and crosslinking of polyethylene). On the other hand, they are involved in the free radical chemical reactions during the combustion process [31]. In synergistic combinations, alkoxyamines can interact with brominated flame retardants and facilitate the release of bromine, consequently increasing the overall FR performance. It has been experimentally shown that alkoxyamine acts in the condensed and gas phase, modifies the decomposition pathway of phosphoric acid tris-(3-bromo-2,2-bis-bromomethylpropyl)ester and polypropylene while lowering the temperature to release HBr as flame quenching species and also the temperature
N O R
N
Aminyl radical
+
R O
Alkoxy radical
FIGURE 2 Thermolysis of alkoxyamines.
N O
Nitroxyl radical
+
R
Alkyl radical
1. Introduction
of dripping of the polymer. Decomposition experiments [33,34] of tris(3-bromo2,2-bis(bromomethyl)propyl)phosphate (TBBPP) and analysis via nuclear magnetic resonance, electron spin resonance, and gas chromatography/mass spectroscopy combination showed that there is an interaction of NOR and TBBPP below the decomposition temperature of polypropylene. In the first step, the nitroxyl radical is formed from NOR, which abstracts a bromomethyl group from TBBPP. As further intermediates, a cyclic phosphate ester and 1,3-dibromo-2,3-bis(bromomethyl)propane (mainly) and 2,3-dibromo-2-butene are generated via a radical process. These newly formed bromo compounds are thermally less stable than TBBPP and immediately release HBr as a gas phase active flame retardant, as shown in Scheme 1. Br
O
RO P
Br
+
O
O
Br
O Br
Br
O
RO
N
Br
RO Br
+
Br
Br
P RO
OH
Br
Br
O
O
O
+
+
P
N OR
Br Br
O
RO P
O
HBr Br
HBr
HBr
RO
P
OR
O
O
Br OR
+
TBBPP Br Br
Br
Br
+
Br
O
RO
+
P HO
Br
OH
Br
SCHEME 1 Various nitrogen-based classes of flame retardants
Br
Br
Br
271
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
The decomposition of TBBPP to the brominated species takes place in any case; however, it is shifted to lower temperatures in the presence of NOR. Obviously, this effect depends on the structure of the brominated flame retardant; however, so far, there are no other published results in this respect. Applications of alkoxyamines such as Flamestab NOR 116 are according to the manufacturer in polyolefin fibers, nonwovens, and films [35]. In addition, the hindered amine light stabilizer (HALS) molecule provides light and long-term thermal stability. Overall, nitrogen-based flame retardants still represent a small but fast growing segment of flame retardants that meet the requirements of sustainable production and products. As a consequence of this, researchers have recently started to examine the mode and action of these compounds. This review focuses on some new developments within the family of nitrogen-based flame retardants and their structuree property relationship. The general structures of these nitrogen-based flame retardant families are depicted in Figure 3.
2. Mechanistic considerations and studies on structureeactivity relationship of N-alkoxyamines as flame retardants Inspired by the flame retardant efficacy of Flamestab NOR 116, a series of NOR analogs have been prepared, and their propertyestructure relationship has been examined. As earlier mentioned, synthetic routes for a large variety of N-alkoxyamine structures have been developed in recent years. In particularly, a great number of various NOR derivatives for nitroxide-mediated “living-free” radical polymerizations have been synthesized. The mechanisms of nitroxide-mediated “living” free radical polymerizations and their structureeproperty relationship have been well documented [36e38]. The literature available on the role of various NOR structures on fire retardant efficacy is still very limited and mostly in the form of patents [14,15]. However, a clear understanding of the mechanism of action of NOR fire retardants would be essential for their further optimization. It has been well established that the active part in NOR flame retardants is centered around the nitrogeneoxygenecarbon (NeOeC) segment of the molecule, as shown in Figure 2 [39,40]. As a consequence of this, it was investigated by GC/MS analysis as to what type of radicals are prevailing after thermally induced breakdown of NOR compounds and what is the relative extent of the two homolysis pathways [41]. Thus, a synthesized model NOR molecule (1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine) was highly diluted in a hydrogen donor solvent (9,10dihydroanthracene) in a closed vessel under vacuum. The solution was kept for 1 h at 200 C. The radicals generated from the thermal decomposition of the NOR compound were hydrogenated by the solvent. The solution was then analyzed by GC and GC/MS, and the results are shown in Figure 4. In the case of model NOR compounds, the preliminary results indicate that both types of thermal homolysis reactions take place to a similar extent.
2. Mechanistic considerations and studies on structureeactivity relationship
R2
R1 O N
N- alkoxyamines (NOR )
R1 O N
N
N
N O R1
AZONOR
KO
O
O
P N N P R2
KO
OK OK
R1 N N N R3
INAZO
Triazenes
R1 N N R2 Azoalkanes
R1
H N N R3
R2 Hydrazones
R1
N N
R2
R3
O R1 N N R2
R4 Azines
Diazene oxides
FIGURE 3 Nitrogen-based classes of flame retardants.
In order to define the key parameters in the NOR structure for an optimal flame retardancy performance, a diverse library of N-alkoxyamine additives was prepared as shown in Figure 5. Most of these chemicals are 1-alkoxy-2,2,6, 6-tetramethylpiperidine-based compounds that differ from each other either in terms of backbone structure, ring size, degree of ring substitution, or 1-alkoxy group. NOR derivatives in which any of the four methyl groups had been replaced by larger alkyl groups such as ethyl or cyclohexyl moieties were excluded from this study due to the
273
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
Products found by GC/MS analysis
H C I A
B D E
F
G
J
200 oC
N O
B
A
D
C
E
OH N H
Piperidine fragments
F
N
+
O
Presence of cyclohexanol (20–25% of the cyclohexyl derived fragments)
G
N O
H
N OH
N O
I
J
N O
+
Presence of TEMPO (2,2,6,6-tetramethyl-piperidin1-yl oxyl) and N-hydroxytetra-methylpiperidine (25– 30% of the piperidine derived fragments)
FIGURE 4 Homolysis products detected by GCeGC/MS from thermal decomposition of 1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine.
fact that sterically highly hindered nitroxide derivatives are generally known to exhibit rather low thermal stabilities [42,43] and are thus more suitable for living free radical polymerizations than for flame retardancy per se. The flame retardant properties were evaluated according to the UL94 protocol using rectangular 108 mm 10 mm 4-mm polypropylene bars and thermal stabilities were studied by differential scanning calorimetry analysis (Table 1). From early on, it was clear that a UL rating could not be achieved for any of the synthesized NOR compounds alone using thick section polypropylene bars. Therefore, the relative flame retardant efficacy of the various NOR additives have been based on the burning time of polypropylene plaques using 1 wt% of Flamestab NOR 116 as standard formulation. Virgin polypropylene samples without any flame retardant burned down in 132 s under these experimental conditions, whereas polypropylene bars containing 1, 2, and 10 wt% of Flamestab NOR 116 burned down in 238, 237, and 205 s, respectively. (The flame retardant experiments verify the inverse
2. Mechanistic considerations and studies on structureeactivity relationship
Effect of nitroxyl radicals
O O N
O
O
N O
O
TEMPO1
Effect of 4-substituent
O N
N
4PyNO6R
O
O O N
NH
NH
NH
NH
N O
4-UREANO6R
Effect of alkoxy ring size O R1 O N
O
O
N O R1
O
R1 =
or
,
NO5R
,
NO6R
NO7R
NO8R
FIGURE 5 Various NOR additives tested as flame retardants for polypropylene plaques.
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
Effect of ring substitution O R2 O N
O
O
N O R2
O
or
R2 =
,
,
NO6R2,6DiMe
NO6R3Me
NO6R4Me
NO6R4IsoPr
Effect of alkoxy substituent O R3 O N
O
O
N O R3
O
O R3 =
or ,
,
NOBenzyl
NOMeST
NOEster
NOIsoPr
N O R4
R4 =
Me
NOMe
,
Ethyl
NOEthyl
or
n-Propyl
NOnPropyl
FIGURE 5 cont’d.
concentration effect of Flamestab NOR 116 [15].) All other burning times are given relative to 1 wt% of Flamestab NOR 116, i.e. a value of >1 and <1 means a better or worse performance than using 1 wt% of Flamestab NOR 116, respectively. The polypropylene formulations containing 1, 2, and 3 wt% of TBBPA as a brominated reference flame retardant exhibited relative burning times of 0.95,
2. Mechanistic considerations and studies on structureeactivity relationship
Table 1 Flame Retardant Efficacies for a Series of Different N-Alkoxyamines UL 94 t1 (s)/t1 (s, NOR 116)
Additive Code
Additive %wt
DSCdDecomposition Temperature ( C)
No FR TBBPA TBBPA TBBPA NOR 116 NOR 116 NOR 116
– 1 2 5 1 2 3
– – – – 284 284 284
0.55 0.95 1.07 1.32 1 1 0.92
1
–
0.85
Nitroxyl Radical TEMPO1
4-Substituted Cyclohexyloxytetramethylpiperidines 4PyNO6R 4UREANO6R
1 1
276 –
1.25 1.13
Cycloalkyloxytetramethylpiperidine Diesters Having Various Ring Sizes NO5R NO6R NO7R NOR8R
1 1 1 1
278 295 261 236
0.85 1.13 0.88 0.86
NORs with Different Alkyl Substituents on the Cyclohexyl Ring NO6R2,6DiMe NO6R3Me NO6R4Me NO6R4IsoPr
1 1 1 1
248 288 288 277
0.71 0.81 0.93 1.15
Various Alkoxy Substituted Tetramethylpiperidines NOBenzyl NOMeST NOEster NOIsoPr NOMe
1 1 1 1 1
262 234 197 234 300
NOEthyl NOnPropyl
1 1
304 295
TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxyl; TBBPA, tetrabromobisphenol A. a Based on the burning time after second ignition.
0.86 0.81 0.8 0.76 (t1 ¼ 1 s) 1.13a 1.02 0.90
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
1.07, and 1.32, respectively. Even if one needs to admit that this method is not the ideal way to compare flame retardant efficacies, it is still a feasible method for the preliminary evaluation of the potential of various NOR structures as flame retardants for polypropylene. Overall, the flame retardant properties of different alkoxyamines mainly depend on three parameters: (1) thermal stability (homolysis temperature), (2) products of decomposition and mechanisms thereof, and (3) reactivity of the radicals generated during decomposition. The results indeed reveal that nitroxyl radicals (TEMPO1) as such also exhibit some flame retardant properties although significantly more weaker than Flamestab NOR 116. One can readily see that two 4-substituted NOR compounds (4PyNO6R and 4UREANO6R) containing the 1-cyclohexyloxy-2,2,6,6-tetramethylpiperidine moiety showed similar thermal stabilities. Surprisingly, 4PyNO6R and 4UREANO6R exhibited even higher flame retardant properties than Flamestab NOR 116 under these experimental conditions. In general, one would expect that the substituent in the 4-position would not directly affect flame retardant properties. However, the differences in recorded flame retardant properties may arise from differences in compatibility, ease of dispersion, and volatility of these NOR compounds due to their variations of backbone structures. In order to avoid the problem of dispersion in the subsequent NOR series, compounds having the same NOR backbone were prepared (either a diester or a long alkenyl chain in the 4-position). Next, the effect of ring size on flame retardancy was investigated by comparing the flame test results obtained using 5-, 6-, 7-, and 8-membered ring 1-cycloalkyloxy-2,2,6,6-tetramethyl piperidine-4-diesters. Although, the cycloalkyloxytetramethylpiperidine diesters NO5R, NO6R, NO7R, and NO8R had relatively comparable electronic and steric properties, their flame retardancy potentials differed quite significantly. The best result was recorded for the six-membered ring N-alkoxyamine (NO6R), whereas larger or smaller ring sizes showed significantly lower flame retardant efficacies. As a consequence of this, 1-cyclohexyloxy2,2,6,6-tetra-methylpiperidine appeared to be a promising template for further optimization of the NOR structure. Therefore, as a next step, NOR additives with alkyl substitutions on the cyclohexyl ring were synthesized, and their flame retardant properties were studied. Compound NO6R2,6DiMe having methyl groups in 2 and 6 positions of the cyclohexyl group as well as NO6R3Me having a methyl group in 3 position exhibited inferior flame retardant properties compared to Flamestab NOR 116. This can be attributed to the lower thermal stability of NO6R2,6DiMe and the fact that the substituted cyclohexyl radical liberated during thermal decomposition is also less reactive than the unsubstituted cyclohexyl radical. Thus, it is understandable that NO6R2,6DiMe presented worse flame retardant properties than Flamestab NOR 116 or NO6R. Compounds NO6R4Me and NO6RIsoPr are similar additives as NO6R with alkyl substitutions of the cyclohexyl ring in the 4-position. The flame retardant performances of these two molecules are close to NO6R. Thus, the presence of alkyl substituents in the 4-position of the cyclohexyl ring does not significantly affect the flame retardant properties. However, the adjustment of the
3. Azoalkanes (AZO) as flame retardants
cyclohexyl radical volatility and diffusion properties may be fine tuned by the optimization of alkyl substitution of the cyclohexyl ring. In this study, alkoxyamines with other alkoxy substituents than cyclohexyl or related alkyl substituted cyclohexyl derivatives were also investigated. Compounds bearing isopropyloxy (NOIsoPr), ester (NOEster), or a-methyl styrene (NOMeST) exhibited fairly low potential as flame retardants since they lack sufficient thermal stabilities for processing. The processing of polypropylene was conducted at 230 C and, therefore, this type of “low thermal stability” NOR compounds may have undergone partial decomposition already during the polymer-processing step itself. This fact, explains the poor efficacies recorded for these NOR series. In contrast, in the series of 1-n-alkoxy-4-(dodec-2-enyl)-2,2,6,6-tetramethylpiperidines, all the compounds showed a comparable or even higher efficiency than Flamestab NOR 116. The results indicate that the flame retardant properties within the family of NOR compounds increased as a function of the thermal stability of the N-alkoxyamine functionality. One of the most thermally stable additives prepared was 1-methoxy-4-(dodec2-enyl)-2,2,6,6,tetra-methylpiperidine (NOMe), and it was shown to be the most efficient FR additive within the synthesized library of NOR compounds. This study clearly indicated that the formed nitroxyl and aminyl radicals of NOR thermolysis are perhaps not the only radical species involved in the retardation of the combustion process. Thus, the simultaneously formed alkyl radicals might also play a vital role as flame retardant species (Section 3). In addition, NOMe showed an extraordinarily high synergistic effect in conjunction with brominated flame retardants. Thus, the UL94 tests showed that V-0 rating in combination with an already 11 wt% of brominated flame retardant was reached when employing NOMe, whereas 14 wt% of the brominated flame retardant was needed when using Flamestab NOR 116 in order to achieve the V-O rating. The result suggests that methyl-TEMPO additives also have a higher tendency to interact/react with brominated flame retardants than those based on the more sterically hindered cyclohexyl-TEMPO analogs.
3. Azoalkanes (AZO) as flame retardants for polypropylene films and plaques The conclusion derived from studies of the structureeactivity relationship of NOR flame retardants and their breakdown was that two distinctly different thermolysis reaction pathways clearly take place (Figure 4). Thus, the fragmentation of the NOR molecule may occur at both the NeO and OeR bonds whereby in theory either alkyl and nitroxyl radicals or alkoxy and aminyl radicals are formed. Given the fact that these four types of radicals clearly differ from each other in terms of chemical reactivity and stability, the significance of their presence in the flame retardant process was not expected to be equivalent. In the original Flamestab NOR 116 flame retardant publications, it was thought that aminyl and alkoxy radicals would be
279
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
the most important species for interrupting the free radical reactions during the polymer combustion process [39]. Whereas the very high synergistic effect of NOR additives with conventional brominated flame retardants was ascribed to the generation of aminyl, alkoxy, and nitroxyl radicals that in turn interact with brominated compounds and thereby facilitate the release of bromine, thus improving flame retardant performance [40]. The potential role of generated alkyl radicals was not discussed. The apparently complex decomposition of N-alkoxyamines made it worthwhile to investigate the flame retardancy properties of each of the aforementioned radical families separately. The investigation was focused on the least studied radicals, i.e. the role of alkyl radicals on flame retardancy of polypropylene. Azoalkanes were selected as the cleanest and most convenient source of alkyl free radicals [44] (Figure 6). As in the case of N-alkoxy hindered amines, the thermal stability of azoalkanes is very closely related to their chemical structure. Their thermal stability is mainly governed by the ground-state energy, stability of the incipient radicals, and orbital symmetry factors. Aromatic azo compounds and azotriphenylmethane represent extreme cases: the former decomposes at 500 C, and the latter has a transient existence only at 40 C. The Polanyi plot of activation energy for thermolysis of both symmetrical azoalkanes and N-alkoxy hindered amines versus the CeH bond dissociation energy of the corresponding hydrocarbons are in both cases essentially linear [38,45]. As a consequence of this, the thermal stability of the azoalkane can be optimized for various polymers in terms of their processing temperatures and decomposition temperatures. In analogy to N-cyclohexyloxy-2,2,6,6-tetramethylpiperidinyl-functionalized flame retardants, three azocyclohexyl compounds were synthesized, i.e. 4,40 -bis(cyclohexylazo-cyclohexyl)methane (ABO AA1), cyclohexylazooctadecane (ABO AA 2), and azocyclohexane (ABO AA3) as sources of cyclohexyl radicals [46]. For the evaluation of the flame retardancy of azocyclohexyl, ignitability tests were performed according to the DIN 4102-1/B2 standards [47]. First, polypropylene was blended with 0.1% by weight of calcium stearate, 0.2% of CibaÒ IrganoxÒ B 501 (stabilizer), and with 0, 0.25, or 0.5% of the radical precursor. The blends were melt processed into fibers, spun into socks, and finally compression molded into 200 mm films. Surface exposure of the samples to a gas flame was separately carried out, and the results are shown in Table 2 [48]. Slight differences in burn times and damage length were observed, and overall, the formulation containing the ABO AA1 additive gave a marginally better performance than the others in thin PP sections. When tested, all the samples containing azoalkanes gave a clean burn residue after the flame application. In contrast, the Flamestab NOR 116 samples exhibited N2 R
N
N
R'
R
+
FIGURE 6 Azoalkanes: “The cleanest and most convenient source of alkyl radicals”.
R'
3. Azoalkanes (AZO) as flame retardants
Table 2 Flammability Results According to DIN 4102 Part-1 Classification B2 Test Method: Face Ignition Test of 230 mm 90-mm samplesa
Formulation
Average Damage Area (cm2)
Total Burning Time (s)
Pass/Fail
Blank 0.25 wt% of Flamestab NOR 116 0.50 wt% of Flamestab NOR 116 0.25 wt% of ABO AA1 0.5 wt% of ABO AA1 0.25 wt% of ABO AA2 0.5 wt% of ABO AA2 0.25 wt% of ABO AA3 0.5 wt% of ABO AA3
100 n.d.b n.d. 28 21 39 31 39 22
100 20 22 13 8 18 13 16 10
Fail Pass Pass Pass Pass Pass Pass Pass Pass
a Substrate: polypropylene is blended with 0.1 wt% of calcium stearate; 0.2 wt% of Ciba Irganox B501; and 0 wt%, 0.25 wt%, or 0.5 wt% of the test FRs. The blends are melt processed into fibers, spun into socks, and subsequently compression molded into thin films. b n.d. ¼ not determined.
yellow chars around the burn damage area. The most striking difference in flame retardant efficacy was observed in thick polypropylene plaques of 1 mm, i.e. azo compounds (e.g. ABO AA3) had a much better FR performance than did Flamestab NOR 116 in thick PP sections (Table 3.) In addition, azo compounds resulted in less discoloration and ABO AA1 exhibited an even nonburning dripping behavior.
3.1 The potential of symmetric versus unsymmetric azoalkanes Most interestingly, many critical properties of azoalkanes of the general formula R0 eN]NeR, such as physical state, toxicity, volatility, initiator efficiency, and in particular thermal stability (in contrast to peroxides (R0 eOeOeR)) can be adjusted over a very wide range by selection of groups R0 and R. According to Engel, as a rule of thumb, the thermal stability of unsymmetrical azoalkanes (R ] R0 ) is intermediate between the stabilities of the symmetrical azoalkanes ReN]NeR and R0 eN] Table 3 FR Performance in PP Moldings (1 mm)a FR Compound 0.5 wt%
Weight Loss %
Burn Length
Pass/Fail
Control no FR NOR 116 ABO AA3
100 49.4 12.9
100 80 47
Fail Fail Pass
Compounding: W&P ZSK25 WLE, 230 C, 100 rpm, 4 kg h1.Molding: Injection molding Arburg 320S, 1-mm plaque. a Substrate: Moplen HF500N, PP homo, 0.3% Ix B225 þ 0.05% CaSt.
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
NeR0 [49]. In addition, a consensus exists that most symmetrical azoalkanes lose nitrogen by the simultaneous breakage of both CeN bonds, whereas unsymmetrical azoalkanes may undergo one-bond cleavage [49]. As a natural extension of the work with ABO AA azoalkane structures, systematic structural changes of the aforementioned basic azoalkane compound were screened in terms of variations in ring size, electronic (resonance and inductive) and steric hindrance with regard to flame retardant efficiency. Therefore, a number of symmetrical and unsymmetrical azoalkanes of the general formula R0 eN]NeR were prepared, as shown in Figure 7 [46]. The experimental results show that in the series of different-sized azocycloalkanes, the flame retardant efficacy decreased in the following order: R ¼ cyclohexyl > cyclopentyl > cyclobutyl > cyclooctanyl[cyclododecanyl. However, in the series of aliphatic azoalkanes compounds, the efficacy decreased in the following order: R ¼ n-alkyl > tert-butyl > tert-octyl. As a spin-off, it was observed that unsymmetrical azoalkanes, in particularly N-cyclohexyl-N0 -t-octyldiazene, can successfully be used for manufacturing of controlled-rheology polypropylene and of crosslinked high-density polyethylene (PEX) [50,51].
3.2 Azoxy, azine, hydrazone, and triazene compounds Furthermore, structures related to azo compounds were synthesized in the form of azoxy, hydrazone, triazene as well as azine derivatives in order to assess their potential as novel flame retardants for polypropylene (Figure 7). The prepared azoxy (ReN] NOeR), azine (R]NeN]R), and hydrazone (ReN]NHeR) derivatives provided flame retardancy to polypropylene films at already very low concentrations (0.25e1 wt%). More recently, various triazene (ReN1]N2eN3R0 R0 )-based flame retardants were prepared. For example, polypropylene samples containing a very low concentration of only 0.5 wt% of bis-4,40 -(3,30 -dimethyltriazene)diphenyl ether passed the test with B2 classification. It is noteworthy that no burning dripping could be detected and the average burning times were very short with exceptionally low weight losses [52]. Therefore, triazene compounds constitute a new and interesting family of radical generators for flame retarding of polymeric materials. The high flame retardant potential of triazenes can be attributed to their ability to generate various types of radicals during their thermal decomposition. According to thermogravimetric analysis/Fourier transform infrared spectroscopy/MS analysis, triazene units are homolytically cleaved into various aminyl, resonance-stabilized aryl radicals, and different CH fragments with simultaneous evolution of elemental nitrogen, as shown in Figure 8. These radicals can effectively provide flame retardant properties.
3.3 Tetrapotassium azo diphosphonate (INAZO) as flame retardants With an effort to further extend the scope of azoalkane-based flame retardants, inorganic azo phosphonate compounds were synthesized, i.e. (KO)2(O)PeN] NeP(O)(OK)2$4H2O, (INAZO), and test its potential as a novel type of flame retardant additive for two-component polyurethane adhesives. The action mechanism of
3. Azoalkanes (AZO) as flame retardants
Symmetrical azoalkanes
Unsymmetrical azoalkanes
N N
N N
ABO AA3
N N
N N
N N
N N
N N
ABO AA1
ABO AA2 N N
N N
N N
N N
(CH2)15CH3
N N
N N
N
N N
N
N N
N
N N
N
N N
Related hydrazones and azine compounds
H N N
H N N
N N
FIGURE 7 Chemical structures of symmetrical and unsymmetrical azoalkanes and related compounds.
the developed INAZO flame retardant was suggested to be mainly in the condensed phase. UL 94 V-0 rating was achieved in the vertical burning test when 10 wt% loading of INAZO was used, whereas the reference flame retardant APP required a loading of 20 wt% to reach the V-0 classification [53].
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CHAPTER 8 Design and Utilization of Nitrogen Containing Flame Retardants
FIGURE 8 Thermolysis of triazene compounds.
3.4 Multifunctional bis(1-propyloxy-2,2,6,6-tetramethylpiperidyl)4-diazene A disadvantage of the azoalkane-based compounds in comparison to NOR 116 is that they do not contribute to light stability. Although selective azo compounds have both the advantage of nonburning dripping properties during fire tests and excellent optical properties, they also importantly function well alone in thick polypropylene sections such as 1-mm plaques. As a consequence of this, the hypothesis of creating an optimized multifunctional flame retardant family without any of the aforementioned (as 1) lack of light stability, (2) low synergistic effect with brominated halogens, and (3) lack of flame retardant performance in thick Table 4 Flame Test on 1-mm Injection Molded PP Plaques According to the DIN 4102/B2 Test FR Compound 0.5 wt%
Weight Loss %
Burn Length
Pass/ Fail
Control NOR 116
100 49.4
100 80
Fail Fail
12.9
47
Pass
14
43
Pass
8.9
37
Pass
5.2
27
Pass
N
O N
O N
O N
N
N N
N N
N N
N O
N O
N O
3. Azoalkanes (AZO) as flame retardants
polymer plaques was explored. This was done by combining the diazene and N-alkoxyamine chemistries into a new molecule, i.e. bis(1-propyloxy2,2,6,6-tetramethylpiperidyl)-4-diazene (AZONOR) containing both of the favorable constituents [54]. Polypropylene samples containing very low concentrations of 0.25e1 wt% of this additive successfully passed not only the fire standard tests of DIN 4102 B2 and NF P92-505 but also the more challenging UL94 VTM-2 standard. By further refining the AZONOR structure, excellent flame retardant properties could be recorded in polypropylene moldings of 1 mm that clearly outperform NOR 116, as shown in Table 4. Besides relatively low levels of addition and having no detrimental effect on polypropylene appearance or its mechanical and processing properties, another great advantage offered by this flame retardant is its multifunctionality, i.e. high flame retardant durability in combination with light stability. Thus, even after 2000 h of artificial weathering, no significant decrease in flame retardant efficacy could be observed [55]. In addition, we could observe a strong synergistic effect between AZONOR and e.g. brominated flame retardants and aluminum trihydrate (ATH) for polypropylene plaques.
4. CONCLUSIONS Different nitrogen-based compounds alone or in combination with conventional flame retardant systems have demonstrated their versatility and usefulness as flame retardants for both thermoplastics and thermosets. Their main common advantages are they are effective at low concentrations, they cannot form dioxans and furans during combustion, they are noncorrosive, they exhibit good UV stability, they are recyclable due to their high thermal stabilities, some of them exhibit excellent synergistic effects with conventional flame retardants, and many of the nitrogen compounds exhibit multiple modes of action. One of the major drawbacks of today’s nitrogen flame retardants is that they require a good fit between the polymer and type of nitrogen-based flame retardant, i.e. nitrogen-based flame retardants are polymer specific. The key drivers for future research of flame retardants will be increased fire safety in combination with flame retardant formulations that fulfill the criteria of sustainable production and products. In recent years, important questions about the environmental safety of antimony, phosphate, and in particular, brominated flame retardants have been raised. All this information suggests a reappraisal of nitrogen-based flame retardants, and many improvements are expected when chemistries have been further refined and fire characterization of various polymers have been elucidated in detail. It would now appear that there are still plenty of nitrogen-based flame retardants to be discovered. Thus, continued research efforts are definitely warranted.
Acknowledgments ˚ bo Akademi University The authors are indebted to coworkers at BASF (former Ciba) and A for flame retardant synthesis and for all the fruitful discussions related to flame retardancy of
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polymers. They would especially like to thank R. Nicholas, M.Sc., Dr M. Roth, Dr H. Hoppe, Dr S. Kniesel, Dr R. Xalter, Dr R. King III, Dr R. Drewes, M. Aubert, M.Sc., T. Tirri, M.Sc., and W. Pawelec, M.Sc.
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