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Flame retardancy of thermoplastics polyurethanes
Polymer Degradation and Stability 97 (2012) 2524e2530
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Flame retardancy of thermoplastics polyurethanes A. Toldy a, b, *, Gy. Harakály a, B. Szolnoki a, E. Zimonyi a, Gy. Marosi a a
Budapest University of Technology and Economics, Faculty of Chemical and Bioengineering, Department of Organic Chemistry and Technology, H-1111 Budapest, } egyetem rkp. 3., Hungary Mu b } egyetem rkp. 3., Hungary Budapest University of Technology and Economics, Faculty of Mechanical Engineering, Department of Polymer Engineering, H-1111 Budapest, Mu
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2011 Received in revised form 27 January 2012 Accepted 27 January 2012 Available online 13 July 2012
This study compares a wide range of potentially synergistic flame retardant combinations in thermoplastic polyurethanes, applying both additive and reactive formulations. After preliminary screening based on LOI and UL-94 results, the best additive and reactive systems were subjected to detailed (thermogravimetric, cone calorimetric, mechanical and rheological) analysis and were compared to the reference TPU and a flame retarded benchmark TPU system. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermoplastic polyurethane Phosphorus-containing additive and reactive flame retardant Intumescence Flame retarded cable insulation
1. Introduction Thermoplastic polyurethanes (TPUs) exhibit many favourable properties such as high tensile and flexural fatigue strength, excellent low temperature flexibility, outstanding wear and abrasion resistance, as well as environmental resistance (humidity, ozone, UV-radiation, microbes) which make them to ideal cable sheathing and insulation materials. However, being an organic polymer, their flame retardancy is poor, which needs to be improved in order to fulfil the safety standards of cable industry. Despite the obvious demand for flame retardancy solutions, only a few articles were published in the periodic literature dealing with this issue; nevertheless, various attempts were reported in patent literature to render TPU flame retardant using a wide range of phosphorus-, nitrogen-, silicon- and boron-containing additives. Phosphorus-containing flame retardants were usually used as part of an intumescent system containing polyolic charring agent and/or nitrogen-containing blowing agents. Siddhamalli and Brown [1] used combination of 15e25 m/m% phosphinate, 0e5 m/ m% phosphate, 2.5e10 m/m% pentaerythritol or dipentaerythritol, 0e5 m/m% inorganic filler (talc, ammonium phosphate,
* Corresponding author. Budapest University of Technology and Economics, Faculty of Chemical and Bioengineering, Department of Organic Chemistry and } egyetem rkp. 3., Hungary. Tel.: þ36 1 463 2462; Technology, H-1111 Budapest, Mu fax: þ36 1 463 1527. E-mail address: [email protected] (A. Toldy). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.015
ammonium polyphosphate, ammonium pentaborate, zinc borate, calcium carbonate, antimony oxide, clay and/or montmorillonite clay) and 0.05e2 m/m% antioxidant (hindered phenols and/or amines) to reach V-0 UL-94 rate and LOI of 35% in jackets for production of a wire and cable. Peerlings and Winkler [2] used phosphine oxide(s) in combination with melamine derivate(s), while Lee et al. [3] applied 0.5e25 m/m% red phosphorus, 0.01e20 m/m% trialkyl phosphate and triaryl phosphate and 5e50 m/m% melamine cyanurate, melamine phosphate, melamine polyphosphate and/or melamine borate to avoid dripping through formation of char. Kim et al. [4] used, besides 1e20 m/m% organic phosphate-based flame retardant and 5e50 m/m% melamine or melamine derivative, 0.1e1 m/m% of active isocyanate groups to avoid dripping. Henze et al. [5] also used isocyanate, dissolved polyurethane, as crosslinking agent besides flame retardants (ammonium phosphate, ammonium polyphosphate, antimony oxide, zinc borate, clay, montmorillonite clay, metal oxides, metal hydroxides, organic phosphinate compounds, organic phosphate compounds, multivalent alcohols, melamine compounds, chlorinated polyethylene and mixtures thereof). Combination of phosphorus-containing flame retardants with metal hydroxides was also reported: Kuwasaki and Nishiguchi [6] combined red phosphorus with metal hydrates, Yoon et al. [7] applied a mixture of red phosphorus and melamine cyanurate, and magnesium hydroxide or aluminium hydroxide as flame retardant. Application of metal hydroxides alone is not preferred given that only V-2 UL94 could be achieved even with as high rates as 80 phr (which
A. Toldy et al. / Polymer Degradation and Stability 97 (2012) 2524e2530
causes difficulties in processing) [8]. Silicon-containing additives, mainly different types of clays, are also usually applied in combination with other flame retardants (see the above examples). Hydroxyl silicone oil was used by Chen et al. [9] for microencapsulation of ammonium polyphosphate in order to give better water resistance and flame retardancy in TPU (at 20 m/m% additive level the UL-94 level could be increased from V-2 to V-0 by microencapsulation). Significant reduction in heat release rate can be achieved even when silicon-containing additives are used alone, as reported by Bourbigot et al. [10] who applied polyhedral oligomeric silsesquioxane (POSS) in TPU, however, other flame retardant classifications as UL-94 and LOI cannot be considerably ameliorated. When comparing the performance of different flame retardant systems, it has to be noted, that in case of TPU used for cable insulation low level of phosphorus-content would be more preferable, as phosphoric acid evolving during an incidental fire may cause corrosion problems in copper-containing cables. As mainly patents are dealing with the flame retardancy of TPU, no systematic research work has been published until now comparing the ideas of these promising additive combinations. Therefore the aim of our investigations was to compare a wide range of potentially synergistic flame retardant combinations, applying both additive and reactive formulations. Considering the strict requirements for cable insulating materials in terms of flame retardancy a complex question has to be answered: which compositions reach V-0 UL-94 classification even on 1 mm thick samples, while keeping the rheological and mechanical properties at an acceptable level. Two routes were considered to be followed, on the one hand application of phosphorus-containing intumescent additive systems, and on the other hand development of flame retardants systems containing reactive components with as low content of phosphorus as possible. 2. Experimental 2.1. Materials A polyether type thermoplastic polyurethane, Elastollan 1185A10 from BASF was used as base polymer matrix (density: Table 1 Summary of the types of materials applied as flame retardant (FR) component. Commercial name Abbreviation Producer Reogard 2000
Reogard
Exolit AP 422 JLS APP101
APP APP101/ME
Exolit OP 1230 Exolit OP 550
OP1230 OP550
e Dellite 67G
PER OMMT
Pangel S9 Pangel B20 HDK H15 Melapur MC e Firebrake ZB Hostavin N30
SEP OSEP
Elastan 6568/103
MDI
Magnifin
n-MH
MC MelB ZnB HALS
Chemtura
Type
Pentaerythritol phosphate alcohol-and melamine phosphate-containing intumescent FR Clariant Ammonium polyphosphate JLS Ammonium polyphosphate surface treated with melamine Clariant Phosphinate type FR Clariant Phosphorus-containing polyol type FR Montedison Pentaerythritol Laviosa Montmorillonite modified with quaternery ammonium salts Tolsa Sepiolite Tolsa Organomodified sepiolite Wacker Silica Ciba Melamine cyanurate Synthesized Melamine borate Chemarco Zinc borate Clariant Polymer of hindered amine and epichlorohydrin BASF 4,40 -diphenylmethanediisocyanate Furukawa Magnesium hydroxide
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1.12 g/cm3, Shore A type hardness: 86, extrusion temperature 180e205 C). The flame retarded benchmark material was Elastollan 1185A10FHF from BASF (density: 1.23 g/cm3, Shore A type hardness: 88, LOI: 25%, UL-94 classification: V-0 in case on 3 mm thick sample, V-2 in case of 1 mm thick sample). The types and properties of the applied flame retardants are summarized in Table 1: Besides the commercially available flame retardants, melamine borate was synthesized in our laboratory: 12.6 g (0.1 mol) melamine was dissolved in 600 ml of distilled water by continuous stirring and heating up to 80 C. Solution of 12.2 g (0.2 mol) boric acid and 100 ml of distilled water was added. Melamine borate precipitated in the form of white flakes by cooling the mixture to room temperature. The precipitation was filtered, washed with 3 100 ml cold distilled water and dried at room temperature. Melamine borate was obtained with a yield of 69.3%. 2.2. Methods 2.2.1. Sample preparation Before compounding the TPU base material was dried at 85 C for 3 h in an oven. The components were homogenized in a Brabender Plasti Corder PL 2000 compounder equipped by a 50 cm3 kneader chamber, at 190 C, with a torque of 50 Nm, for 4 min. The blends were compressed in a Collin P 200 E press at 190 C, with 30 bar for 3 min and consequently with 50 bar for 2 min. The sample thickness was 1 mm for UL-94 tests and 2 mm for rheological and mass loss calorimeter tests. The ageing of the samples was carried out in an oven at 113 C for 168 h. 2.2.2. Characterization of the samples The model reaction between polyol and isocyanate was monitored by Horiba Jobin-Yvon LabRAM type Raman micro spectrometer equipped with an external 785 nm diode laser source and an Olympus BX-40 optical microscope. Objective of 10x magnification was used for optical imaging and spectrum acquisition. Exposition time was 20 s, the accumulation was 3. Thermogravimetric measurements were carried out using Setaram Labsys TG DTA type equipment, sample mass: 10e20 mg, heating rate: 10 C/min, in air atmosphere, temperature range: 25e600 C. The fire resistance was characterized by limiting oxygen index measurement (LOI, according to ASTMD 2863), UL-94 test (according to ASTM D3801, ASTM D635, respectively) on 1 mm thick samples, and mass loss type cone calorimeter (according to ISO 13927, Fire Testing Technology, heat flux of 50 kW/m2) on 2 mm thick samples. Rheological characterization was performed on TA AR2000 type rheometer, diameter of polymer sample: 25 mm, heating rate: 10 C/min, temperature range: 25e350 C. Comparative mechanical tests were carried out by ZWICK Z020 universal tester. The tensile test speed was 1 mm/min with clamping distance of 26 mm at room temperature. The specimen size was 52 mm 4.5 mm 1 mm. 3. Results and discussion The aim of the investigations was to compare a wide range of potentially synergistic flame retardant combinations. The primary choice of combinations was based on the cited literature as most of them only include suggested concentration limits, but no comparison of the different systems in the same polymer matrix was performed.
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Table 2 Composition of additive flame retardant systems prepared for the preliminary screening.
TPU-1 TPU-2 TPU-3 TPU-4 TPU-5 TPU-6 TPU-7 TPU-8 TPU-9 TPU-10 TPU-11 TPU-12 TPU-13 TPU-14 TPU-15 TPU-16 TPU-17 TPU-18 TPU-19 TPU-20 TPU-21 TPU-22 TPU-23 TPU-24 TPU-25 TPU-26 TPU-27
TPU %
Reogard %
APP %
PER %
OMMT %
SEP %
OSEP %
SiO2 %
OP1230 %
OP550 %
MC %
APP101/ME %
ZnB %
MelB %
HALS %
100 80 75 65 73 73 65 65 85 80 75 70 72.5 95 90 80 84 69 74 77 74 74 79 69 69 64 64
e 20 25 35 25 20 30 30 e e e e e e e e e e e e e e e e e e e
e e e e e 5 5 e e e e e e e e e e e e e e e e e e e e
e e e e e e e 5 e e e e e e e e e e e e e e e e e e e
e e e e 1 1 e e e e e 2.5 1.25 e e e e e e e e e e e e e e
e e e e 1 1 e e e e e 2.5 1.25 e e e e e e e e e e e e e e
e e e e e e e e e e e e e e e 5 5 10 5 2 e e e e e e e
e e e e e e e e e e e e e e 5 e e e e e e e e e e e e
e e e e e e e e 15 20 25 25 25
e e e e e e e e e e e e e e e 10 10 20 10 10 10 5 e 5 5 5 5
e e e e e e e e e e e e e e e e e e e e e e e e e e e
e e e e e e e e e e e e e e e e e e 10 10 15 20 e 20 20 25 25
e e e e e e e e e e e e e e e e e e e e e e e e 5 e 5
e e e e e e e e e e e e e e e e e e e e e e 20 5 e 5 e
e e e e e e e e e e e e e 5 5 5 1 1 1 1 1 1 1 1 1 1 1
3.1. Preliminary screening of additive flame retardant systems In order to reach V-0 classification on 1 mm thick samples necessary for cable applications of TPU extended screening of additive combinations was performed. Selected compositions are given in Table 2 including the following flame retardant systems: phosphorus-containing intumescent system in combination with nanoclays (TPU-2 e TPU-8), phosphinate-containing additive system in combination with nanoclays (TPU-9 e TPU-13), phosphorus-containing intumescent system in combination with HALS (hindered amine light stabilizer) (TPU-14 e TPU-22) phosphorus-containing intumescent system in combination with borates and HALS (TPU-23 e TPU-27). In the subgroup of phosphorus-containing intumescent system in combination with nanoclays (TPU-2 e TPU-8) first the amount of Reogard 2000 flame retardant was gradually increased up to 35 m/m%, equivalent to 5 m/m% phosphorus (TPU-2 e TPU-4). This increase did not change the UL-94 classification, because dripping was still observed, however the LOI could be increased from 23 to 32 (Table 3). The introduction of montmorillonite and sepiolite did not inhibit the material from dripping (TPU-5, TPU-6), the increase of the ratio of free phosphate groups (TPU-7) and of free polyolic groups (TPU-8) has been considered. Both of these concepts lead to the desired V-0 classification of 1 mm thick samples. In the subgroup of phosphinate-containing additive system in combination with nanoclays (TPU-9 e TPU-13) the increase of phosphorus-content (TPU-9 e TPU-11) resulted in slight enhancement of flame retardancy, but without reaching the V-0 level. Furthermore, it could be concluded that the samples containing clay had significantly higher LOI values; the increased dripping, however, deteriorated the UL-94 classifications. In the subgroup of phosphorus-containing intumescent system in combination with HALS (hindered amine light
e e e e e e e e e e e e
stabilizer) (TPU-14 e TPU-22) the application of HALS was considered, because of its radical scavenger capability, which can contribute to better FR rating by extinguishing the flame of falling drips. However, the application of HALS increased the meltdripping of the sample, which could not be avoided with additives (silica, or phosphorus-containing polyol and clay) either at 5% (TPU-14 e TPU-16) or 1% HALS content (TPU-17 e TPU-18). Therefore the phosphorus-containing polyol was combined with ammonium polyphosphate surface treated with melamine, and was tested in combination of different amount of clay (TPU-19 e TPU-22). Samples containing 2 m/m% or no clay immediately extinguished after the ignition was stopped, in case of TPU-20 the dripping flame extinguished during falling. Therefore it did not ignite the cotton below the sample and can be considered as V0 rate composition (of course avoiding the dripping at all would be more desirable). Significant increase in LOI values was only detected in case of V-0 rate TPU-20 sample, where the LOI increased from 23 to 27, in all other samples the LOI decreased or remained the same. In the subgroup of phosphorus-containing intumescent system in combination with borates and HALS (TPU-23 e TPU27) all the samples containing phosphorus and boron together extinguished immediately after the ignition was stopped, and although dripping could be still observed, it did not ignite the cotton below the sample resulting in V-0 rate. The FR effect of melamine and zinc borate was also compared (TPU-24 and TPU25, TPU-26 and TPU-27), as well as the effect of increasing phosphorus concentration while the boron content was kept constant (TPU-24 and TPU-26, TPU-25 and TPU-27). As for the horizontal burning behaviour, it could be concluded, that the amount phosphorus is determining: in case of TPU-32, where all together 30 m/ m% of phosphorus-containing FR was applied with 5 m/m% of zinc borate, not even the first mark was reached in case of horizontal burning. However, no such amelioration was detected in case of LOI: increasing the amount of phosphorus applied together with zinc borate or melamine borate did not result in any increase in LOI.
A. Toldy et al. / Polymer Degradation and Stability 97 (2012) 2524e2530
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Table 3 Total additive-, P-, N-, B and silicate-content; LOI and UL-94 results of additive flame retardant systems prepared for the preliminary screening. Total additives %
P%
N%
3.3 4.1 5.7 4.1 4.0 5.6
B%
Total silicate %
LOI %
U-94
UL-94 remarks (V: vertical, H: horizontal, V-0 highlighted with grey colour)
23 24 25 32 27 29 32
HB V-2 V-2 V-2 V-2 V-2 V-0
32
V-0
25 25 26
HB V-2 V-2
5.0 2.5
34 32
HB HB
Dripping flame Only vertical burning, dripping flame Only vertical burning, dripping flame Dripping flame ignites the cotton Only vertical burning, dripping flame Only vertical burning, dripping flame Charring, dripping flame does not ignite the cotton Charring, dripping flame does not ignite the cotton Ignites the cotton Ignites the cotton Ignites the cotton, but best among TPU-9 - TPU-16 Burns until clamping Flames reach clamping in short time, intensive burning, ignites the cotton V: flame drips, H: immediate extinguishing after removing the flame V: flame drips, H: immediate extinguishing after removing the flame V: flame drips during ignition H: extinguishing after removing the flame V: flame drips, H: burning speed 2 mm/s V: flame drips, H: burning speed 0.75 mm/s V: flame drips, H: second mark not reached V: flame extinguishes, H: immediate extinguishing after removing the flame V: flame drips, H: immediate extinguishing after ignition V: flame drips, H: immediate extinguishing after ignition V: flame extinguishes, H: first mark in 10 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 8 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 7 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 20 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark not reached
TPU-1 TPU-2 TPU-3 TPU-4 TPU-5 TPU-6 TPU-7
0 20 25 35 27 27 35
3.0 3.8 5.3 3.8 4.6 6.1
TPU-8
35
4.5
TPU-9 TPU-10 TPU-11
15 20 25
3.5 4.7 5.9
TPU-12 TPU-13
30 27.5
5.9 5.9
TPU-14
5
0.3
5.0
19
V-2
TPU-15
10
0.3
5.0
20
V-2
TPU-16
20
1.7
0.3
5.0
22
V-2
TPU-17
16
1.7
0.1
21
HB
TPU-18
31
3.4
0.1
10.0
19
HB
TPU-19
26
4.6
2.0
5.0
22
V-2
TPU-20
23
4.6
2.0
2.0
27
V-0
TPU-21
26
6.1
2.9
22
V-2
TPU-22
26
6.7
3.9
20
V-2
TPU-23
21
TPU-24
31
TPU-25
2.0 2.0
4.9
12.2
1.6
22
V-2
6.7
8.9
0.4
28
V-0
31
6.7
5.8
0.7
26
V-0
TPU-26
36
8.1
10.3
0.4
24
V-0
TPU-27
36
8.1
7.3
0.7
26
V-0
The screening of additive systems in TPU suggests that the HB rating achieved with unmodified TPU could be changed to V-2 with in most of the additive systems. In order to achieve V-0 with 1 mm thick samples solid phase intumescent action is required. 3.2. Preliminary screening of reactive flame retardant containing systems In order to reduce the phosphorus-content of the flame retardant system and still achieve the desired V-0 rating of 1 mm thick samples, the introduction of reactive flame retardant components was also considered. Based on former experience, a reactive FR system consisting of MDI and phosphorus-containing polyol in combination with melamine cyanurate, zinc borate and nanoclays/
nano-magnesium-hydroxide/silica was formulated and evaluated (Table 4). The reactive character of this FR system was proven by Raman spectrometry: the spectra of phosphorus-containing polyol, MDI and their mixture can be seen in Fig. 1. Beside the characteristic peaks of the starting materials (polyol: 736, 1101, 1458 cm1, MDI: 638, 865, 1185, 1618 cm1) or the ones derived from them (1258e1317 cm1 derived from 1293 cm1 peak of polyol and 1316,1254 cm1 peaks of MDI), new peaks could be also detected in the end product (undefined peak at 900 cm1 in the fingerprint region and 1715 cm1 identified as urethane carbonyl group), which suggest that the reaction between the reactants took part. The application of diisocyanate in 1:1 ratio with the linear phosphorus-containing polyol, resulted in significant decrease of
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Table 4 Composition of flame retardant systems containing reactive components.
TPU-28 TPU-29 TPU-30 TPU-31 TPU-32
TPU %
MDI %
OP550 %
ZnB %
MC %
HALS %
OSEP %
OMMT %
n-MH %
SiO2 %
70 70 70 70 70
1.92 1.92 1.92 1.92 1.92
2.08 2.08 2.08 2.08 2.08
5 5 5 5 5
20 19 19 19 19
1 1 1 1 1
e 1 e e e
e e 1 e e
e e e 1 e
e e e e 1
melt-dripping (as expected), however without further additives the TPU-28 sample did not reach the desired V-0 rate (Table 5). Therefore the effect of different nano-additives such as sepiolite, montmorillonite, nano-magnesium-hydroxide and silica were compared. The introduction of silica eliminated the dripping completely and thus lead to V-0 UL-94 rate, with as low content of phosphorus as 0.4 m/m% (the LOI remained the same as that of the reference TPU). 3.3. Detailed analysis of the chosen flame retardants systems After the preliminary screening of wider range of potentially synergistic additive and reactive flame retardant systems, an additive system of V-0 rating and of highest LOI (TPU-8, referred as TPU additive FR in the followings), and the best reactive system (TPU-32, referred as TPU reactive FR) were subjected to detailed (thermogravimetric, flame retardancy, mechanical and rheological) analysis and compared to the reference TPU and flame retarded benchmark TPU system. 3.3.1. Thermogravimetric analysis In order to compare the characteristics of thermal degradation of the chosen systems, thermogravimetric analysis was performed (Table 6). Comparing the temperatures at 5% mass loss, it can be concluded that introduction of flame retardant shifts the beginning of thermal degradation to lower temperatures in all cases: smallest effect was detected in case of the FR benchmark material, while vastest decrease was detected in case of additive FR system. However, if we compare the temperatures at 50% mass loss, it can be noticed that the trend is just the opposite: the FR benchmark
material loses 50% of its mass at almost 50 C lower temperature than the reference, while the TPU additive FR system at 20 C higher temperature. This is in good agreement with the maximum values of the degradation speed: the TPU FR benchmark material has almost double maximal degradation speed than the reference, while the additive and the reactive FR systems show somewhat lower values. As for the temperature at the maximal degradation speed, it can be concluded that the introduction of chemically bound FR caused a significant shift in temperature, which means that it successfully delayed the main degradation of TPU. Comparing the amounts of residue at 600 C, it is noticeable, that the residue of the FR benchmark material is much less than that of the TPU reference. This supports the conclusion, that although it has appropriate heat stability at the beginning of degradation, at higher temperatures it is less stable than the reference TPU, the additive or reactive FR systems. The high amount of residue in case of additive system can be attributed to the protection effect of the applied intumescent FRs. 3.3.2. Flame retardancy results Comparison in cone calorimeter (of mass loss type) was performed in order to complement the preliminary screening based on UL-94 and LOI results. Comparing the heat release rate (HRR) curves (Fig. 2), it can be concluded that the additive system was the most successful in reducing the HRR (to 20% of the original value) and THR (total heat released) of TPU. However, the thermal degradation, expressed by TTI (time to ignition), begins earlier (as suggested by TG results as well) (Table 7). The HRR curve of the reactive FR system follows the pattern of the TPU reference until 90 C (no earlier degradation occurs as in case of the additive FR). The peak of HRR is reduced in this case to
Fig. 1. Raman spectra of polyol, MDI and their mixture.
A. Toldy et al. / Polymer Degradation and Stability 97 (2012) 2524e2530
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Table 5 Total additive, phosphorus, nitrogen and boron content, LOI and UL-94 results of the reactive flame retardant system. Total additives %
P%
N%
B%
TPU-28 TPU-29
30 30
0.4 0.4
10.0 9.5
0.7 0.7
TPU-30
30
0.4
9.5
0.7
TPU-31
30
0.4
9.5
0.7
TPU-32
30
0.4
9.5
0.7
Total silicate %
LOI %
UL-94
UL-94 remarks (V: vertical, H: horizontal, V-0)
1.0
22 18
V-2 V-2
1.0
18
V-2
21
V-2
23
V-0
V: dripping, H: burning speed 0.76 mm/s, burns 2.5 cm V: dripping, H: reaches first sign in 30 s, immediate extinguishing after removing the flame V: dripping, H: reaches first sign in 23 s, immediate extinguishing after removing the flame V: dripping, H: reaches first sign in 25 s, burning speed 0.74 mm/s, burns 1.7 cm V: no dripping, H: reaches first sign in 30 s, immediate extinguishing after removing the flame
1.0
Table 6 Thermogravimetric results of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
TPU TPU TPU TPU
reference FR benchmark additive FR reactive FR
T5% C
T50% C
vmax %/min
Tmax C
Residue %
331 321 290 306
429 382 449 421
6.9 12.8 5.2 6.7
344 348 320 415
26.4 10.8 37.8 26.7
T5%: temperature at 5% mass loss; T50%: 50% temperature at 50% mass loss; vmax: maximum of the degradation speed at 10 C/min heating rate; Tmax: temperature at the maximal degradation speed, residue: amount of residue at 600 C.
Fig. 2. Heat release rate results of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
60% of the original value and was shifted in time by 60s. This result is comparable to the reduction reached by the TPU FR benchmark system. Concerning the mass loss rates (Table 7), it can be concluded that both the TPU reference and FR benchmark material left practically no residue after the calorimeter test, while the additive FR system resulted in the highest amount of residue due to the intumescent nature of the applied FRs. In case of the reactive FR system Table 7 FR characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system measured by cone calorimeter.
TPU TPU TPU TPU
reference FR benchmark additive FR reactive FR
TTI s
PHRR KW/m2
Time to PHRR s
THR MJ/m2
Residue %
39 31 18 33
876 496 167 521
142 166 203 200
103.4 94.9 51.4 109.2
0.24 0.92 14.56 3.76
TTI: time to ignition; PHRR: peak of heat release rate; time to PHRR: time to peak of heat release time; THR: total heat release; residue: amount of solid residue after cone calorimeter measurements.
Table 8 Mechanical characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system. Reference Tensile strength (MPa) Before ageing 37.0 After ageing 33.4 Elongation at break (%) Before ageing 857.7 After ageing 947.5
FR benchmark
Additive FR
Reactive FR
3.5 2.4
23.2 2.5 22.6 0.2
11.4 0.8 9.8 0.6
12.7 0.3 11.4 0.7
71.9 49.0
781.5 81.5 855.7 50.8
546.7 15.5 475.1 16.1
534.7 20.5 466.8 20.2
Fig. 3. Rheological characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
solid residue was detected despite the very low phosphoruscontent. 3.3.3. Mechanical characteristics The mechanical characteristics were compared before and after ageing in order the estimate the applicability of the developed flame retarded systems as cable material (Table 8). As expected the mechanical characteristics were deteriorated by the inclusion of flame retardants but these decreased values are still within the range acceptable for the cable-coating application. Furthermore it can be concluded, that the FR system containing reactive components resulted in higher tensile strength than the additive system. All values measured after ageing remained at least 70% of the original value, which is a requirement for cable-coating materials. 3.3.4. Rheological characteristics Rheological characterization was carried out in order to compare the flow behaviour of the chosen flame retarded systems to reference TPU and flame retarded benchmark TPU system both during the temperature range of processing and degradation (Fig. 3). According to the results at room temperature, the TPU
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reference has the highest viscosity, while the lowest one was measured in case of the reactive FR system. The viscosity of the additive and reactive FR systems starts to decrease at lower temperatures than in case of the TPU reference and FR benchmark material. All the samples have their minimum viscosity in the temperature region between 250 and 280 C. It is worth to notice, that in this region the viscosity of the reactive FR system is significantly higher than in all other cases, which may be explained by higher thermal stability and by the presence of reactive flame retardant component. Above 280 C the viscosity increases in all cases due to degradation products formed in the charring process. 4. Conclusions A wide range of additive and reactive flame retardant formulations were compared in a preliminary screening process aiming at finding compositions characterized by V-0 UL-94 classification on 1 mm thick samples, fulfilling this way one of the strictest requirements of cable insulating materials. As general conclusions, the followings can be stated in case of the additive systems: the increased phosphorus-content increased the LOI value as well, but did not change the UL-94 classifications without further measures taken to suspend the dripping of the samples, application of clays in combination with phosphoruscontaining flame retardants further increased the LOI, however, led to increased dripping by catalysing the degradation of the matrix, application of 5 m/m% hindered amine light stabilizer radical scavenger, lead also to melt-dripping of TPU, the common application of phosphorus- and boron-containing FRs successfully reduced the melt-dripping in all cases, in additive systems V-0 rate could be achieved by: B increasing of the ratio of free phosphate groups (TPU-7) and of free polyolic groups (TPU-8) in phosphorus- and nitrogencontaining additive intumescent system, B combining polyphosphate with phosphorus-containing polyol, 2 m/m% clay and 1 m/m% HALS (TPU-20), B applying polyphosphate, phosphorus-containing polyol, 5 m/m% borate and 1 m/m% HALS (TPU-24 e TPU-27). Reactive FR systems consisting of MDI and phosphoruscontaining polyol in combination with melamine cyanurate, zinc borate and nanofiller (clay, magnesium-hydroxide or silica) were also tested. Contrary to the additive systems, application of silica completely eliminated the dripping and lead to V-0 UL-94 rate of 1 mm thick samples, with as low content of phosphorus as 0.4 mass%. After the preliminary screening, the additive system with highest FR values and the best reactive system were subjected to detailed (thermogravimetric, flame retardancy, mechanical and rheological) analysis and compared to the reference TPU and flame retarded benchmark TPU system. Application of flame retardants shifted the beginning of thermal degradation to lower temperatures in all cases. However, the introduction of chemically bound FR delayed the main degradation
of TPU successfully. Comparing the heat release rate (HRR) results it could be concluded that the additive system reduced the peak of HRR to 20% of the original value, however the time to ignition became shorter. The reactive FR system did not ignite earlier than the reference and the peak of HRR was reduced to 60% of the original value (shifted in time by 60 s) with as low content of phosphorus as 0.4 mass%. The inclusion of flame retardants weakened the mechanical characteristics but only within the acceptable range. Slight advantage of the reactive system was found in tensile strength in comparison with the additive system. According to the rheological results the reactive FR system had the lowest viscosity at the temperature of processing. The viscosity of both the additive and reactive FR systems started to decrease at lower temperatures than those of the TPU reference and FR benchmark material. Between 250 and 280 C the viscosity of the reactive FR system was maintained (in contrast to other systems), probably due to higher thermal stability than other TPU’s and to the stability of the reactively bound flame retardant. Acknowledgement The activities described in this paper were supported by EU 6th Framework Programme ‘Multihybrids’; Grant Number: IP 026685-2 and Bolyai Scholarship of the Hungarian Academy of Science. This work is connected to the scientific program of the “Development of quality-oriented and harmonized RþDþI strategy and functional model at BME” project. This project is supported by the New Széchenyi Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002) and OTKA K76346.
References [1] Siddhamalli Sridhar K, Brown Carl A. Non halogen flame retardant thermoplastic polyurethane, WO/2006/121549; 2007. [2] Peerlings H, Winkler J. Self-extinguishing thermoplastic polyurethane employing incorporable organic phosphine oxide and melamine derivative, Korean Patent 10200701228652007, 2007. [3] Lee HY, Lee TW, Kim DS, Kim SK. Halogen-free flame retardant thermoplastic polyurethane composite resin composition prepared from thermoplastic polyurethane, red phosphorus and melamine derivative, KR2007055886-A; 2008. [4] Kim DS, Kim NJ, Kim YW. Halogen-free flame retardant thermoplastic polyurethane composition having excellent mechanical properties without containing a halogen-based flame retardant, KR1020080057636; 2008. [5] Henze OS, Hansen M, Hackmann K, Meier D, Beckmann C, Mühren O. Halogenfree flame-retardant TPU, WO/2009/103765; 2009. [6] Kuwasaki Y, Nishiguchi M. Flame retardant optical-fiber strand for wire-cable, has secondary coating layer consisting of inner layer and outer layer comprising resin composition containing preset amount of metal hydrate, triazine derivative compound and clay, JP2007219325-A; 2007. [7] Yoon SH, Eom YB, Sung IK. Thermoplastic polyurethane having good selfextinguishing property or excellent flame retardancy, and a preparation method thereof, Korean Patent 1020080095097, 2008. [8] Pinto UA, Yuan Visconte LL, Gallo J, Reis Nunes RC. Flame retardancy in thermoplastic polyurethane elastomers (TPU) with mica and aluminum trihydrate (ATH). Polym Degrad Stab 2000;69(3):257e60. [9] Chen X, Jiao C, Zhang J. Microencapsulation of ammonium polyphosphate with hydroxyl silicone oil and its flame retardance in thermoplastic polyurethane. on-line available. J Therm Anal Calorim 2011. http://dx.doi.org/ 10.1007/s10973-011-1347-6. [10] Bourbigot S, Turf T, Bellayer S, Duquesne S. Polyhedral oligomeric silsesquioxane as flame retardant for thermoplastic polyurethane. Polym Degrad Stab 2009;94(8):1230e7.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Flame retardancy of thermoplastics polyurethanes A. Toldy a, b, *, Gy. Harakály a, B. Szolnoki a, E. Zimonyi a, Gy. Marosi a a
Budapest University of Technology and Economics, Faculty of Chemical and Bioengineering, Department of Organic Chemistry and Technology, H-1111 Budapest, } egyetem rkp. 3., Hungary Mu b } egyetem rkp. 3., Hungary Budapest University of Technology and Economics, Faculty of Mechanical Engineering, Department of Polymer Engineering, H-1111 Budapest, Mu
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2011 Received in revised form 27 January 2012 Accepted 27 January 2012 Available online 13 July 2012
This study compares a wide range of potentially synergistic flame retardant combinations in thermoplastic polyurethanes, applying both additive and reactive formulations. After preliminary screening based on LOI and UL-94 results, the best additive and reactive systems were subjected to detailed (thermogravimetric, cone calorimetric, mechanical and rheological) analysis and were compared to the reference TPU and a flame retarded benchmark TPU system. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermoplastic polyurethane Phosphorus-containing additive and reactive flame retardant Intumescence Flame retarded cable insulation
1. Introduction Thermoplastic polyurethanes (TPUs) exhibit many favourable properties such as high tensile and flexural fatigue strength, excellent low temperature flexibility, outstanding wear and abrasion resistance, as well as environmental resistance (humidity, ozone, UV-radiation, microbes) which make them to ideal cable sheathing and insulation materials. However, being an organic polymer, their flame retardancy is poor, which needs to be improved in order to fulfil the safety standards of cable industry. Despite the obvious demand for flame retardancy solutions, only a few articles were published in the periodic literature dealing with this issue; nevertheless, various attempts were reported in patent literature to render TPU flame retardant using a wide range of phosphorus-, nitrogen-, silicon- and boron-containing additives. Phosphorus-containing flame retardants were usually used as part of an intumescent system containing polyolic charring agent and/or nitrogen-containing blowing agents. Siddhamalli and Brown [1] used combination of 15e25 m/m% phosphinate, 0e5 m/ m% phosphate, 2.5e10 m/m% pentaerythritol or dipentaerythritol, 0e5 m/m% inorganic filler (talc, ammonium phosphate,
* Corresponding author. Budapest University of Technology and Economics, Faculty of Chemical and Bioengineering, Department of Organic Chemistry and } egyetem rkp. 3., Hungary. Tel.: þ36 1 463 2462; Technology, H-1111 Budapest, Mu fax: þ36 1 463 1527. E-mail address: [email protected] (A. Toldy). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.015
ammonium polyphosphate, ammonium pentaborate, zinc borate, calcium carbonate, antimony oxide, clay and/or montmorillonite clay) and 0.05e2 m/m% antioxidant (hindered phenols and/or amines) to reach V-0 UL-94 rate and LOI of 35% in jackets for production of a wire and cable. Peerlings and Winkler [2] used phosphine oxide(s) in combination with melamine derivate(s), while Lee et al. [3] applied 0.5e25 m/m% red phosphorus, 0.01e20 m/m% trialkyl phosphate and triaryl phosphate and 5e50 m/m% melamine cyanurate, melamine phosphate, melamine polyphosphate and/or melamine borate to avoid dripping through formation of char. Kim et al. [4] used, besides 1e20 m/m% organic phosphate-based flame retardant and 5e50 m/m% melamine or melamine derivative, 0.1e1 m/m% of active isocyanate groups to avoid dripping. Henze et al. [5] also used isocyanate, dissolved polyurethane, as crosslinking agent besides flame retardants (ammonium phosphate, ammonium polyphosphate, antimony oxide, zinc borate, clay, montmorillonite clay, metal oxides, metal hydroxides, organic phosphinate compounds, organic phosphate compounds, multivalent alcohols, melamine compounds, chlorinated polyethylene and mixtures thereof). Combination of phosphorus-containing flame retardants with metal hydroxides was also reported: Kuwasaki and Nishiguchi [6] combined red phosphorus with metal hydrates, Yoon et al. [7] applied a mixture of red phosphorus and melamine cyanurate, and magnesium hydroxide or aluminium hydroxide as flame retardant. Application of metal hydroxides alone is not preferred given that only V-2 UL94 could be achieved even with as high rates as 80 phr (which
A. Toldy et al. / Polymer Degradation and Stability 97 (2012) 2524e2530
causes difficulties in processing) [8]. Silicon-containing additives, mainly different types of clays, are also usually applied in combination with other flame retardants (see the above examples). Hydroxyl silicone oil was used by Chen et al. [9] for microencapsulation of ammonium polyphosphate in order to give better water resistance and flame retardancy in TPU (at 20 m/m% additive level the UL-94 level could be increased from V-2 to V-0 by microencapsulation). Significant reduction in heat release rate can be achieved even when silicon-containing additives are used alone, as reported by Bourbigot et al. [10] who applied polyhedral oligomeric silsesquioxane (POSS) in TPU, however, other flame retardant classifications as UL-94 and LOI cannot be considerably ameliorated. When comparing the performance of different flame retardant systems, it has to be noted, that in case of TPU used for cable insulation low level of phosphorus-content would be more preferable, as phosphoric acid evolving during an incidental fire may cause corrosion problems in copper-containing cables. As mainly patents are dealing with the flame retardancy of TPU, no systematic research work has been published until now comparing the ideas of these promising additive combinations. Therefore the aim of our investigations was to compare a wide range of potentially synergistic flame retardant combinations, applying both additive and reactive formulations. Considering the strict requirements for cable insulating materials in terms of flame retardancy a complex question has to be answered: which compositions reach V-0 UL-94 classification even on 1 mm thick samples, while keeping the rheological and mechanical properties at an acceptable level. Two routes were considered to be followed, on the one hand application of phosphorus-containing intumescent additive systems, and on the other hand development of flame retardants systems containing reactive components with as low content of phosphorus as possible. 2. Experimental 2.1. Materials A polyether type thermoplastic polyurethane, Elastollan 1185A10 from BASF was used as base polymer matrix (density: Table 1 Summary of the types of materials applied as flame retardant (FR) component. Commercial name Abbreviation Producer Reogard 2000
Reogard
Exolit AP 422 JLS APP101
APP APP101/ME
Exolit OP 1230 Exolit OP 550
OP1230 OP550
e Dellite 67G
PER OMMT
Pangel S9 Pangel B20 HDK H15 Melapur MC e Firebrake ZB Hostavin N30
SEP OSEP
Elastan 6568/103
MDI
Magnifin
n-MH
MC MelB ZnB HALS
Chemtura
Type
Pentaerythritol phosphate alcohol-and melamine phosphate-containing intumescent FR Clariant Ammonium polyphosphate JLS Ammonium polyphosphate surface treated with melamine Clariant Phosphinate type FR Clariant Phosphorus-containing polyol type FR Montedison Pentaerythritol Laviosa Montmorillonite modified with quaternery ammonium salts Tolsa Sepiolite Tolsa Organomodified sepiolite Wacker Silica Ciba Melamine cyanurate Synthesized Melamine borate Chemarco Zinc borate Clariant Polymer of hindered amine and epichlorohydrin BASF 4,40 -diphenylmethanediisocyanate Furukawa Magnesium hydroxide
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1.12 g/cm3, Shore A type hardness: 86, extrusion temperature 180e205 C). The flame retarded benchmark material was Elastollan 1185A10FHF from BASF (density: 1.23 g/cm3, Shore A type hardness: 88, LOI: 25%, UL-94 classification: V-0 in case on 3 mm thick sample, V-2 in case of 1 mm thick sample). The types and properties of the applied flame retardants are summarized in Table 1: Besides the commercially available flame retardants, melamine borate was synthesized in our laboratory: 12.6 g (0.1 mol) melamine was dissolved in 600 ml of distilled water by continuous stirring and heating up to 80 C. Solution of 12.2 g (0.2 mol) boric acid and 100 ml of distilled water was added. Melamine borate precipitated in the form of white flakes by cooling the mixture to room temperature. The precipitation was filtered, washed with 3 100 ml cold distilled water and dried at room temperature. Melamine borate was obtained with a yield of 69.3%. 2.2. Methods 2.2.1. Sample preparation Before compounding the TPU base material was dried at 85 C for 3 h in an oven. The components were homogenized in a Brabender Plasti Corder PL 2000 compounder equipped by a 50 cm3 kneader chamber, at 190 C, with a torque of 50 Nm, for 4 min. The blends were compressed in a Collin P 200 E press at 190 C, with 30 bar for 3 min and consequently with 50 bar for 2 min. The sample thickness was 1 mm for UL-94 tests and 2 mm for rheological and mass loss calorimeter tests. The ageing of the samples was carried out in an oven at 113 C for 168 h. 2.2.2. Characterization of the samples The model reaction between polyol and isocyanate was monitored by Horiba Jobin-Yvon LabRAM type Raman micro spectrometer equipped with an external 785 nm diode laser source and an Olympus BX-40 optical microscope. Objective of 10x magnification was used for optical imaging and spectrum acquisition. Exposition time was 20 s, the accumulation was 3. Thermogravimetric measurements were carried out using Setaram Labsys TG DTA type equipment, sample mass: 10e20 mg, heating rate: 10 C/min, in air atmosphere, temperature range: 25e600 C. The fire resistance was characterized by limiting oxygen index measurement (LOI, according to ASTMD 2863), UL-94 test (according to ASTM D3801, ASTM D635, respectively) on 1 mm thick samples, and mass loss type cone calorimeter (according to ISO 13927, Fire Testing Technology, heat flux of 50 kW/m2) on 2 mm thick samples. Rheological characterization was performed on TA AR2000 type rheometer, diameter of polymer sample: 25 mm, heating rate: 10 C/min, temperature range: 25e350 C. Comparative mechanical tests were carried out by ZWICK Z020 universal tester. The tensile test speed was 1 mm/min with clamping distance of 26 mm at room temperature. The specimen size was 52 mm 4.5 mm 1 mm. 3. Results and discussion The aim of the investigations was to compare a wide range of potentially synergistic flame retardant combinations. The primary choice of combinations was based on the cited literature as most of them only include suggested concentration limits, but no comparison of the different systems in the same polymer matrix was performed.
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Table 2 Composition of additive flame retardant systems prepared for the preliminary screening.
TPU-1 TPU-2 TPU-3 TPU-4 TPU-5 TPU-6 TPU-7 TPU-8 TPU-9 TPU-10 TPU-11 TPU-12 TPU-13 TPU-14 TPU-15 TPU-16 TPU-17 TPU-18 TPU-19 TPU-20 TPU-21 TPU-22 TPU-23 TPU-24 TPU-25 TPU-26 TPU-27
TPU %
Reogard %
APP %
PER %
OMMT %
SEP %
OSEP %
SiO2 %
OP1230 %
OP550 %
MC %
APP101/ME %
ZnB %
MelB %
HALS %
100 80 75 65 73 73 65 65 85 80 75 70 72.5 95 90 80 84 69 74 77 74 74 79 69 69 64 64
e 20 25 35 25 20 30 30 e e e e e e e e e e e e e e e e e e e
e e e e e 5 5 e e e e e e e e e e e e e e e e e e e e
e e e e e e e 5 e e e e e e e e e e e e e e e e e e e
e e e e 1 1 e e e e e 2.5 1.25 e e e e e e e e e e e e e e
e e e e 1 1 e e e e e 2.5 1.25 e e e e e e e e e e e e e e
e e e e e e e e e e e e e e e 5 5 10 5 2 e e e e e e e
e e e e e e e e e e e e e e 5 e e e e e e e e e e e e
e e e e e e e e 15 20 25 25 25
e e e e e e e e e e e e e e e 10 10 20 10 10 10 5 e 5 5 5 5
e e e e e e e e e e e e e e e e e e e e e e e e e e e
e e e e e e e e e e e e e e e e e e 10 10 15 20 e 20 20 25 25
e e e e e e e e e e e e e e e e e e e e e e e e 5 e 5
e e e e e e e e e e e e e e e e e e e e e e 20 5 e 5 e
e e e e e e e e e e e e e 5 5 5 1 1 1 1 1 1 1 1 1 1 1
3.1. Preliminary screening of additive flame retardant systems In order to reach V-0 classification on 1 mm thick samples necessary for cable applications of TPU extended screening of additive combinations was performed. Selected compositions are given in Table 2 including the following flame retardant systems: phosphorus-containing intumescent system in combination with nanoclays (TPU-2 e TPU-8), phosphinate-containing additive system in combination with nanoclays (TPU-9 e TPU-13), phosphorus-containing intumescent system in combination with HALS (hindered amine light stabilizer) (TPU-14 e TPU-22) phosphorus-containing intumescent system in combination with borates and HALS (TPU-23 e TPU-27). In the subgroup of phosphorus-containing intumescent system in combination with nanoclays (TPU-2 e TPU-8) first the amount of Reogard 2000 flame retardant was gradually increased up to 35 m/m%, equivalent to 5 m/m% phosphorus (TPU-2 e TPU-4). This increase did not change the UL-94 classification, because dripping was still observed, however the LOI could be increased from 23 to 32 (Table 3). The introduction of montmorillonite and sepiolite did not inhibit the material from dripping (TPU-5, TPU-6), the increase of the ratio of free phosphate groups (TPU-7) and of free polyolic groups (TPU-8) has been considered. Both of these concepts lead to the desired V-0 classification of 1 mm thick samples. In the subgroup of phosphinate-containing additive system in combination with nanoclays (TPU-9 e TPU-13) the increase of phosphorus-content (TPU-9 e TPU-11) resulted in slight enhancement of flame retardancy, but without reaching the V-0 level. Furthermore, it could be concluded that the samples containing clay had significantly higher LOI values; the increased dripping, however, deteriorated the UL-94 classifications. In the subgroup of phosphorus-containing intumescent system in combination with HALS (hindered amine light
e e e e e e e e e e e e
stabilizer) (TPU-14 e TPU-22) the application of HALS was considered, because of its radical scavenger capability, which can contribute to better FR rating by extinguishing the flame of falling drips. However, the application of HALS increased the meltdripping of the sample, which could not be avoided with additives (silica, or phosphorus-containing polyol and clay) either at 5% (TPU-14 e TPU-16) or 1% HALS content (TPU-17 e TPU-18). Therefore the phosphorus-containing polyol was combined with ammonium polyphosphate surface treated with melamine, and was tested in combination of different amount of clay (TPU-19 e TPU-22). Samples containing 2 m/m% or no clay immediately extinguished after the ignition was stopped, in case of TPU-20 the dripping flame extinguished during falling. Therefore it did not ignite the cotton below the sample and can be considered as V0 rate composition (of course avoiding the dripping at all would be more desirable). Significant increase in LOI values was only detected in case of V-0 rate TPU-20 sample, where the LOI increased from 23 to 27, in all other samples the LOI decreased or remained the same. In the subgroup of phosphorus-containing intumescent system in combination with borates and HALS (TPU-23 e TPU27) all the samples containing phosphorus and boron together extinguished immediately after the ignition was stopped, and although dripping could be still observed, it did not ignite the cotton below the sample resulting in V-0 rate. The FR effect of melamine and zinc borate was also compared (TPU-24 and TPU25, TPU-26 and TPU-27), as well as the effect of increasing phosphorus concentration while the boron content was kept constant (TPU-24 and TPU-26, TPU-25 and TPU-27). As for the horizontal burning behaviour, it could be concluded, that the amount phosphorus is determining: in case of TPU-32, where all together 30 m/ m% of phosphorus-containing FR was applied with 5 m/m% of zinc borate, not even the first mark was reached in case of horizontal burning. However, no such amelioration was detected in case of LOI: increasing the amount of phosphorus applied together with zinc borate or melamine borate did not result in any increase in LOI.
A. Toldy et al. / Polymer Degradation and Stability 97 (2012) 2524e2530
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Table 3 Total additive-, P-, N-, B and silicate-content; LOI and UL-94 results of additive flame retardant systems prepared for the preliminary screening. Total additives %
P%
N%
3.3 4.1 5.7 4.1 4.0 5.6
B%
Total silicate %
LOI %
U-94
UL-94 remarks (V: vertical, H: horizontal, V-0 highlighted with grey colour)
23 24 25 32 27 29 32
HB V-2 V-2 V-2 V-2 V-2 V-0
32
V-0
25 25 26
HB V-2 V-2
5.0 2.5
34 32
HB HB
Dripping flame Only vertical burning, dripping flame Only vertical burning, dripping flame Dripping flame ignites the cotton Only vertical burning, dripping flame Only vertical burning, dripping flame Charring, dripping flame does not ignite the cotton Charring, dripping flame does not ignite the cotton Ignites the cotton Ignites the cotton Ignites the cotton, but best among TPU-9 - TPU-16 Burns until clamping Flames reach clamping in short time, intensive burning, ignites the cotton V: flame drips, H: immediate extinguishing after removing the flame V: flame drips, H: immediate extinguishing after removing the flame V: flame drips during ignition H: extinguishing after removing the flame V: flame drips, H: burning speed 2 mm/s V: flame drips, H: burning speed 0.75 mm/s V: flame drips, H: second mark not reached V: flame extinguishes, H: immediate extinguishing after removing the flame V: flame drips, H: immediate extinguishing after ignition V: flame drips, H: immediate extinguishing after ignition V: flame extinguishes, H: first mark in 10 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 8 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 7 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark in 20 s, immediate extinguishing after removing the flame V: flame extinguishes, H: first mark not reached
TPU-1 TPU-2 TPU-3 TPU-4 TPU-5 TPU-6 TPU-7
0 20 25 35 27 27 35
3.0 3.8 5.3 3.8 4.6 6.1
TPU-8
35
4.5
TPU-9 TPU-10 TPU-11
15 20 25
3.5 4.7 5.9
TPU-12 TPU-13
30 27.5
5.9 5.9
TPU-14
5
0.3
5.0
19
V-2
TPU-15
10
0.3
5.0
20
V-2
TPU-16
20
1.7
0.3
5.0
22
V-2
TPU-17
16
1.7
0.1
21
HB
TPU-18
31
3.4
0.1
10.0
19
HB
TPU-19
26
4.6
2.0
5.0
22
V-2
TPU-20
23
4.6
2.0
2.0
27
V-0
TPU-21
26
6.1
2.9
22
V-2
TPU-22
26
6.7
3.9
20
V-2
TPU-23
21
TPU-24
31
TPU-25
2.0 2.0
4.9
12.2
1.6
22
V-2
6.7
8.9
0.4
28
V-0
31
6.7
5.8
0.7
26
V-0
TPU-26
36
8.1
10.3
0.4
24
V-0
TPU-27
36
8.1
7.3
0.7
26
V-0
The screening of additive systems in TPU suggests that the HB rating achieved with unmodified TPU could be changed to V-2 with in most of the additive systems. In order to achieve V-0 with 1 mm thick samples solid phase intumescent action is required. 3.2. Preliminary screening of reactive flame retardant containing systems In order to reduce the phosphorus-content of the flame retardant system and still achieve the desired V-0 rating of 1 mm thick samples, the introduction of reactive flame retardant components was also considered. Based on former experience, a reactive FR system consisting of MDI and phosphorus-containing polyol in combination with melamine cyanurate, zinc borate and nanoclays/
nano-magnesium-hydroxide/silica was formulated and evaluated (Table 4). The reactive character of this FR system was proven by Raman spectrometry: the spectra of phosphorus-containing polyol, MDI and their mixture can be seen in Fig. 1. Beside the characteristic peaks of the starting materials (polyol: 736, 1101, 1458 cm1, MDI: 638, 865, 1185, 1618 cm1) or the ones derived from them (1258e1317 cm1 derived from 1293 cm1 peak of polyol and 1316,1254 cm1 peaks of MDI), new peaks could be also detected in the end product (undefined peak at 900 cm1 in the fingerprint region and 1715 cm1 identified as urethane carbonyl group), which suggest that the reaction between the reactants took part. The application of diisocyanate in 1:1 ratio with the linear phosphorus-containing polyol, resulted in significant decrease of
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Table 4 Composition of flame retardant systems containing reactive components.
TPU-28 TPU-29 TPU-30 TPU-31 TPU-32
TPU %
MDI %
OP550 %
ZnB %
MC %
HALS %
OSEP %
OMMT %
n-MH %
SiO2 %
70 70 70 70 70
1.92 1.92 1.92 1.92 1.92
2.08 2.08 2.08 2.08 2.08
5 5 5 5 5
20 19 19 19 19
1 1 1 1 1
e 1 e e e
e e 1 e e
e e e 1 e
e e e e 1
melt-dripping (as expected), however without further additives the TPU-28 sample did not reach the desired V-0 rate (Table 5). Therefore the effect of different nano-additives such as sepiolite, montmorillonite, nano-magnesium-hydroxide and silica were compared. The introduction of silica eliminated the dripping completely and thus lead to V-0 UL-94 rate, with as low content of phosphorus as 0.4 m/m% (the LOI remained the same as that of the reference TPU). 3.3. Detailed analysis of the chosen flame retardants systems After the preliminary screening of wider range of potentially synergistic additive and reactive flame retardant systems, an additive system of V-0 rating and of highest LOI (TPU-8, referred as TPU additive FR in the followings), and the best reactive system (TPU-32, referred as TPU reactive FR) were subjected to detailed (thermogravimetric, flame retardancy, mechanical and rheological) analysis and compared to the reference TPU and flame retarded benchmark TPU system. 3.3.1. Thermogravimetric analysis In order to compare the characteristics of thermal degradation of the chosen systems, thermogravimetric analysis was performed (Table 6). Comparing the temperatures at 5% mass loss, it can be concluded that introduction of flame retardant shifts the beginning of thermal degradation to lower temperatures in all cases: smallest effect was detected in case of the FR benchmark material, while vastest decrease was detected in case of additive FR system. However, if we compare the temperatures at 50% mass loss, it can be noticed that the trend is just the opposite: the FR benchmark
material loses 50% of its mass at almost 50 C lower temperature than the reference, while the TPU additive FR system at 20 C higher temperature. This is in good agreement with the maximum values of the degradation speed: the TPU FR benchmark material has almost double maximal degradation speed than the reference, while the additive and the reactive FR systems show somewhat lower values. As for the temperature at the maximal degradation speed, it can be concluded that the introduction of chemically bound FR caused a significant shift in temperature, which means that it successfully delayed the main degradation of TPU. Comparing the amounts of residue at 600 C, it is noticeable, that the residue of the FR benchmark material is much less than that of the TPU reference. This supports the conclusion, that although it has appropriate heat stability at the beginning of degradation, at higher temperatures it is less stable than the reference TPU, the additive or reactive FR systems. The high amount of residue in case of additive system can be attributed to the protection effect of the applied intumescent FRs. 3.3.2. Flame retardancy results Comparison in cone calorimeter (of mass loss type) was performed in order to complement the preliminary screening based on UL-94 and LOI results. Comparing the heat release rate (HRR) curves (Fig. 2), it can be concluded that the additive system was the most successful in reducing the HRR (to 20% of the original value) and THR (total heat released) of TPU. However, the thermal degradation, expressed by TTI (time to ignition), begins earlier (as suggested by TG results as well) (Table 7). The HRR curve of the reactive FR system follows the pattern of the TPU reference until 90 C (no earlier degradation occurs as in case of the additive FR). The peak of HRR is reduced in this case to
Fig. 1. Raman spectra of polyol, MDI and their mixture.
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Table 5 Total additive, phosphorus, nitrogen and boron content, LOI and UL-94 results of the reactive flame retardant system. Total additives %
P%
N%
B%
TPU-28 TPU-29
30 30
0.4 0.4
10.0 9.5
0.7 0.7
TPU-30
30
0.4
9.5
0.7
TPU-31
30
0.4
9.5
0.7
TPU-32
30
0.4
9.5
0.7
Total silicate %
LOI %
UL-94
UL-94 remarks (V: vertical, H: horizontal, V-0)
1.0
22 18
V-2 V-2
1.0
18
V-2
21
V-2
23
V-0
V: dripping, H: burning speed 0.76 mm/s, burns 2.5 cm V: dripping, H: reaches first sign in 30 s, immediate extinguishing after removing the flame V: dripping, H: reaches first sign in 23 s, immediate extinguishing after removing the flame V: dripping, H: reaches first sign in 25 s, burning speed 0.74 mm/s, burns 1.7 cm V: no dripping, H: reaches first sign in 30 s, immediate extinguishing after removing the flame
1.0
Table 6 Thermogravimetric results of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
TPU TPU TPU TPU
reference FR benchmark additive FR reactive FR
T5% C
T50% C
vmax %/min
Tmax C
Residue %
331 321 290 306
429 382 449 421
6.9 12.8 5.2 6.7
344 348 320 415
26.4 10.8 37.8 26.7
T5%: temperature at 5% mass loss; T50%: 50% temperature at 50% mass loss; vmax: maximum of the degradation speed at 10 C/min heating rate; Tmax: temperature at the maximal degradation speed, residue: amount of residue at 600 C.
Fig. 2. Heat release rate results of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
60% of the original value and was shifted in time by 60s. This result is comparable to the reduction reached by the TPU FR benchmark system. Concerning the mass loss rates (Table 7), it can be concluded that both the TPU reference and FR benchmark material left practically no residue after the calorimeter test, while the additive FR system resulted in the highest amount of residue due to the intumescent nature of the applied FRs. In case of the reactive FR system Table 7 FR characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system measured by cone calorimeter.
TPU TPU TPU TPU
reference FR benchmark additive FR reactive FR
TTI s
PHRR KW/m2
Time to PHRR s
THR MJ/m2
Residue %
39 31 18 33
876 496 167 521
142 166 203 200
103.4 94.9 51.4 109.2
0.24 0.92 14.56 3.76
TTI: time to ignition; PHRR: peak of heat release rate; time to PHRR: time to peak of heat release time; THR: total heat release; residue: amount of solid residue after cone calorimeter measurements.
Table 8 Mechanical characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system. Reference Tensile strength (MPa) Before ageing 37.0 After ageing 33.4 Elongation at break (%) Before ageing 857.7 After ageing 947.5
FR benchmark
Additive FR
Reactive FR
3.5 2.4
23.2 2.5 22.6 0.2
11.4 0.8 9.8 0.6
12.7 0.3 11.4 0.7
71.9 49.0
781.5 81.5 855.7 50.8
546.7 15.5 475.1 16.1
534.7 20.5 466.8 20.2
Fig. 3. Rheological characteristics of the chosen flame retarded systems compared to reference TPU and flame retarded benchmark TPU system.
solid residue was detected despite the very low phosphoruscontent. 3.3.3. Mechanical characteristics The mechanical characteristics were compared before and after ageing in order the estimate the applicability of the developed flame retarded systems as cable material (Table 8). As expected the mechanical characteristics were deteriorated by the inclusion of flame retardants but these decreased values are still within the range acceptable for the cable-coating application. Furthermore it can be concluded, that the FR system containing reactive components resulted in higher tensile strength than the additive system. All values measured after ageing remained at least 70% of the original value, which is a requirement for cable-coating materials. 3.3.4. Rheological characteristics Rheological characterization was carried out in order to compare the flow behaviour of the chosen flame retarded systems to reference TPU and flame retarded benchmark TPU system both during the temperature range of processing and degradation (Fig. 3). According to the results at room temperature, the TPU
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reference has the highest viscosity, while the lowest one was measured in case of the reactive FR system. The viscosity of the additive and reactive FR systems starts to decrease at lower temperatures than in case of the TPU reference and FR benchmark material. All the samples have their minimum viscosity in the temperature region between 250 and 280 C. It is worth to notice, that in this region the viscosity of the reactive FR system is significantly higher than in all other cases, which may be explained by higher thermal stability and by the presence of reactive flame retardant component. Above 280 C the viscosity increases in all cases due to degradation products formed in the charring process. 4. Conclusions A wide range of additive and reactive flame retardant formulations were compared in a preliminary screening process aiming at finding compositions characterized by V-0 UL-94 classification on 1 mm thick samples, fulfilling this way one of the strictest requirements of cable insulating materials. As general conclusions, the followings can be stated in case of the additive systems: the increased phosphorus-content increased the LOI value as well, but did not change the UL-94 classifications without further measures taken to suspend the dripping of the samples, application of clays in combination with phosphoruscontaining flame retardants further increased the LOI, however, led to increased dripping by catalysing the degradation of the matrix, application of 5 m/m% hindered amine light stabilizer radical scavenger, lead also to melt-dripping of TPU, the common application of phosphorus- and boron-containing FRs successfully reduced the melt-dripping in all cases, in additive systems V-0 rate could be achieved by: B increasing of the ratio of free phosphate groups (TPU-7) and of free polyolic groups (TPU-8) in phosphorus- and nitrogencontaining additive intumescent system, B combining polyphosphate with phosphorus-containing polyol, 2 m/m% clay and 1 m/m% HALS (TPU-20), B applying polyphosphate, phosphorus-containing polyol, 5 m/m% borate and 1 m/m% HALS (TPU-24 e TPU-27). Reactive FR systems consisting of MDI and phosphoruscontaining polyol in combination with melamine cyanurate, zinc borate and nanofiller (clay, magnesium-hydroxide or silica) were also tested. Contrary to the additive systems, application of silica completely eliminated the dripping and lead to V-0 UL-94 rate of 1 mm thick samples, with as low content of phosphorus as 0.4 mass%. After the preliminary screening, the additive system with highest FR values and the best reactive system were subjected to detailed (thermogravimetric, flame retardancy, mechanical and rheological) analysis and compared to the reference TPU and flame retarded benchmark TPU system. Application of flame retardants shifted the beginning of thermal degradation to lower temperatures in all cases. However, the introduction of chemically bound FR delayed the main degradation
of TPU successfully. Comparing the heat release rate (HRR) results it could be concluded that the additive system reduced the peak of HRR to 20% of the original value, however the time to ignition became shorter. The reactive FR system did not ignite earlier than the reference and the peak of HRR was reduced to 60% of the original value (shifted in time by 60 s) with as low content of phosphorus as 0.4 mass%. The inclusion of flame retardants weakened the mechanical characteristics but only within the acceptable range. Slight advantage of the reactive system was found in tensile strength in comparison with the additive system. According to the rheological results the reactive FR system had the lowest viscosity at the temperature of processing. The viscosity of both the additive and reactive FR systems started to decrease at lower temperatures than those of the TPU reference and FR benchmark material. Between 250 and 280 C the viscosity of the reactive FR system was maintained (in contrast to other systems), probably due to higher thermal stability than other TPU’s and to the stability of the reactively bound flame retardant. Acknowledgement The activities described in this paper were supported by EU 6th Framework Programme ‘Multihybrids’; Grant Number: IP 026685-2 and Bolyai Scholarship of the Hungarian Academy of Science. This work is connected to the scientific program of the “Development of quality-oriented and harmonized RþDþI strategy and functional model at BME” project. This project is supported by the New Széchenyi Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002) and OTKA K76346.
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