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Influence synergetic effect of halloysite nanotubes and halogen-free flame-retardants on properties nitrile rubber composites
Thermochimica Acta 557 (2013) 24–30
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Influence synergetic effect of halloysite nanotubes and halogen-free flame-retardants on properties nitrile rubber composites a,∗ ´ ˙ Przemysław Rybinski , Grazyna Janowska b a b
The Jan Kochanowski University, Management of Environment Protection and Modelling, Poland Technical University of Łód´z, Institute of Polymer and Dye Technology, Poland
a r t i c l e
i n f o
Article history: Received 2 October 2012 Received in revised form 28 January 2013 Accepted 29 January 2013 Available online 15 February 2013 Keywords: Halogen-free flame retardants Halloysite nanotubes Nitrile rubbers Thermal stability Flammability
a b s t r a c t In this study, the effect of simultaneous actions of halloysite and commonly used halogen-free flameretardant compounds, such as antimony trioxide, magnesium hydroxide and melamine cyanurate on the thermal and mechanical properties as well as flammability and fire hazard of butadiene-acrylonitrile rubber (NBR) composites was examined. Based on the analysis of thermal curves (DTA, TG, DTG), it has been found that the effect of the flame-retardant compounds used on the thermal stability and processes of halloysite-filled NBR depends on the method of the elastomer cross-linking. The flameretardant compounds used significantly reduce the flammability and fire hazard of the halloysite-filled NBR. As a result of simultaneous synergic action of the flame-retardant compound and nanofiller, selfextinguishing elastomeric materials with good mechanical properties have been obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction One of the most important issues of material engineering in recent years has been the production of flame-retardant polymeric materials characterized by low emission of toxic gases and fumes. Composites with an increased resistance to the action of fire have been more and more frequently made with the use of various nanofillers, such as aluminosilitate nanoclays, nanofibers and halloysite nanotube [1,2]. A decrease in the size of filler particles incorporated into the polymeric matrix from micrometers to nanometers makes it possible to obtain composites with better, frequently quite new and unique properties. From the studies carried out so far it follows, that in the stage of ignition and fire development, halloysite nanotubes act as effective flame-retardant compounds through the formation of a thermally stable barrier boundary layer on the surface of the material being burned. Inside the composite/nanocomposite under combustion, they form a spatial network impeding the diffusion of thermal decomposition products to flame and oxygen into the polymer’s interior [3,4]. An unquestionable drawback of nanofillers, including halloysite nanotubes, is the problem in making their perfect dispersion in the polymeric matrix. The aggregates or agglomerates being formed adversely affect not only the composite physical properties but
∗ Corresponding author. Tel.: +48 413496437. ´ E-mail address: [email protected] (P. Rybinski). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.01.030
also considerably decrease the nanoparticle effectiveness of flame retardation, especially in the stage of developed fire [5]. To increase the effectiveness of nanofillers as nontoxic flame-retardant compounds, they are used with a small quantity of conventional, especially halogen-free, flame-retardant substances that due to their synergic action with nanofiller reduce material flammability as well as improve the physical and mechanical properties of the composites containing these compounds [6]. This paper presents the results of investigating the thermal and mechanical properties as well as flammability and fire hazard of butadiene-acrylonitrile rubber (NBR) containing halloysite and common halogen-free flame-retardant compounds.
2. Experimental 2.1. Materials The object of our study was butadiene-acrylonitrile rubber (NBR), NBR 2255V, containing 22% of combined acrylonitrile, from Lanxess Company. The rubber was cross-linked by means of dicumyl peroxide (DCP) in the presence of zinc oxide (ZnO) or sulfur in the presence of zinc oxide and N-cyclohexyl-2benzoylsulfenamide (Tioheksam CBS). The resultant peroxide vulcanizate was denoted with NN, while the sulfur vulcanizate with NS. Halloysite (H) derived from the Dunino mine near Legnica (Poland), intercalated with alkali (sodium hydroxide) was used as a nanofiller of elastomeric blends [7].
P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30
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Modified halloysite was incorporated into NBR in a quantity of 5 phr (phr – parts by wt. per hundred parts by wt. of rubber). Composites NNH5 and NSH5 were used as comparative material. Melamine cyanurate (POCh Gliwice), magnesium hydroxide (Martinswerk, GmbH), Magnifin H5, antimony trioxide (POCh Gliwice), were used as flame-retardant and were incorporated into NBR in a quantity of 15 or 30 phr.
2.2. Methods The photographs of modified halloysite were taken by the SEM Quanta 250 FEG microscope. The measurements of wide-angle scattering of X-radiation (WAXS) in the modified halloysite and halloysite-containing composites were performed at room temperature using an X-ray apparatus from Phillips (X-ray source operating at: U = 30 kV and I = 50 mA; wavelength= 0.15418 nm) at the Center of Molecular and Macromolecular Studies, the Polish Academy of Science, in Lodz. Diffraction patterns were recorded within the angle range 2 = 1–35◦ ; measurement step 0.05◦ ; measurement time 25 s; radiation CuK alfa ( = 0.15418 nm); operation in transmission mode. The apparatus was calibrated at the beginning of each measurement series. The samples of the composites tested were disintegrated before analysis by the CAT X520D homogenizer (revolutions 16–30 000), cooled in a liquid nitrogen, transferred onto a polystyrene holder and placed in a disposable measurement ring. The WAXS measurements of tested halloysite was used as a reference measurement for composites. Elastomeric blends were prepared at room temperature with the use of a laboratory rolling mill (D = 150 mm, L = 300 mm). The rotational speed of the front roll was 20 rpm, friction 1.1. The blends were vulcanized in steel molds placed between electrically heated press shelves. The optimal vulcanization time ( 0.9 ) at a temperature of 160 ◦ C was determined by means of a WG-2 vulcameter according to PN-ISO 3417:1994. The thermal properties of the vulcanizates obtained were tested under air within the temperature range of 25–800 ◦ C by means of a MOM derivatograph (Budapest), using Al2 O3 as reference substance. Weighed portions were 90 mg each, heating rate 7.9 ◦ C min−1 , and the sensitivities of thermal curves were as follows: TG = 100, DTA = 1/5, DTG 1/30. The flammability of the vulcanizates under investigation was tested with the use of a cone calorimeter from Fire Testing Technology Ltd. Elastomer samples with dimensions of (100 × 100 ± 1) mm and thickness of (2 ± 0.5) mm were tested at horizontal position with the heat radiant flux density 35 kW m−2 . During tests, the following parameters were recorded: initial sample weight, time to ignition (TTI), sample weight during testing, total heat released (THR), effective combustion heat (EHC), average weight loss rate (MLR), heat release rate (HRR) and sample final weight. Flammability of vulcanizations in air was also tested. Elastomer samples with dimensions of 50 × 10 × 4 mm in a vertical position were ignited with a gaseous burner supplied with LPG for 5 s and its combustion time (ts ) was measured. The strength properties of composites were determined according to PN-ISO 37:1998, using an apparatus from ZWICK, model 1435, connected to a computer with an appropriated software and dumbbell samples with a measurement section width of 4 mm and a thickness of about 1 mm.
Fig. 1. (A and B) Photographs of activated halloysite by alkali solution [7].
3. Results and discussion Fig. 1A and B shows SEM pictures of initial halloysite nanotubes (HNTs) activated by alkali solution [7].
Fig. 2. WAXS pattern of the composites. NNH5 peroxide vulcanizate of NBR rubber containing 5 phr of HNTs. NSH5 sulfur vulcanizate of NBR rubber containing 5 phr of HNTs.
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P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30
Fig. 4. DTA curves sulfur composites of NBR. Fig. 3. DTA curves peroxide composites of NBR.
Fig. 2 compares the X-ray diffraction spectra of halloysite nanotubes (HNTs) activated by alkali solution and NNH5, NSH5 composites. The basal space reflections of HNTs indicate a sharp peak at 2 = 12.27◦ , corresponding to a (0 0 1) basal spacing of 0.7 ± 0.3 nm. However, in the NNH5, NSH5 nanocomposites, peak at 2 = 12.27◦ was not detected. It is clear that HNTs in the nanocomposites appeared not to be intercalated by the rubber chains. The addition appropriate flame retardant to the polymer matrix does not change significantly the WAXS curves of composites. From the review of relevant literature it follows that NBR, both before and after being cross-linked with dicumyl peroxide (DCP) or sulfur, undergoes a two-stage thermal cross-linking [8,9]. The thermal cross-linking processes of NBR also proceed in the presence of halloysite (Figs. 3 and 4). From the analysis of DTA curves it follows that the effect of the flame-retardants compounds used on the thermal changes in NBR filled with halloysite depends on the method of elastomer cross-linking (Figs. 3 and 4). In the case of peroxide vulcanizates (Fig. 3), the addition of magnesium hydroxide causes that NBR filled with halloysite undergoes thermal cross-linking in a single stage at lower temperatures. Also the halloysite-filled NBR cross-linked with sulfur in the presence of the flame-retardant compounds undergoes (single stage) thermal cross-linking (Fig. 4).
The flame-retardant compounds used reduce the thermal stability of composites determined with T5 , but they do not significantly change T50 . Nevertheless, the highest values of T50 are shown by the halloysite-filled NBR cross-linked with both DCP and sulfur, in the presence of Sb2 O3 . An important parameter determining both the polymeric material thermal stability and its flammability is the rate of its thermal decomposition, dm/dt. The reduction in the decomposition rate of polymeric materials exerts a positive influence on the reduction in their flammability. This results from the formation of lower quantities of volatile and flammable products of pyrolysis passing to flame, which reduces the rate of free-radical reactions in the combustion zone. The flame-retardant compounds used ambiguously influence the thermal decomposition rate of the composites filled with them. Antimony trioxide and melamine cyanurate decrease the thermal decomposition rate of the vulcanizates investigated, which is cannot be found in the case of magnesium hydroxide (Table 1). The decrease in the decomposition rate of the composites tested (NNH5 and NSH5) under thermo-oxidative conditions compared to that of unfilled vulcanizates (NN and NS) results first of all from the barrier properties of the nanofiller used. Aluminosilicate is impermeable for vapors and gases, thus the small-molecular products of the composite thermal decomposition products can diffuse outside only through closely specified spaces [10]. The great
Table 1 Thermal analysis of composites in air atmosphere. Sample
T5 [◦ C]
T50 [◦ C]
TRMAX [◦ C]
dm × dt−1 [mm]
Pw [%]
Ts [◦ C]
P800 [%]
NN NNH5 NNH5Sb2 O3 15 NNH5Sb2 O3 30 NNH5Mg(OH)2 15 NNH5Mg(OH)2 30 NNH5CM 15 NNH5CM 30 NS NSH5 NSH5Sb2 O3 15 NSH5Sb2 O3 30 NSH5Mg(OH)2 15 NSH5Mg(OH)2 30 NSH5CM 15 NSH5CM 30
350 365 360 360 340 320 320 310 315 350 350 320 320 325 290 270
420 415 420 440 420 420 420 400 405 425 430 440 420 415 420 420
410 405 410 410 410 410 410 410 395 410 410 400 400 400 400 400
70 50 40 36 50 50 42 35 52 44 37 30 45 35 39 31
23.01 33.33 41.12 48.89 33.34 38.89 30 36.67 23.33 32.78 42.23 47.78 34.45 37.78 32.23 31.11
545 510 480 490 500 490 480 470 540 540 490 480 470 490 480 480
9.01 11.67 20 24.45 17.78 23.34 10 13.34 6.67 10.56 21.12 22.23 17.78 23.34 12.22 11.12
NN – peroxide vulcanizate of NBR rubber; NS – sulfur vulcanizate of NBR rubber; NNH5 – peroxide vulcanizate of NBR rubber containing 5 phr of HNTs; NSH5 – sulfur vulcanizate of NBR rubber containing 5 phr of HNTs; NNH5Sb2 O3 , NNH5Mg(OH)2 , NNH5CM – peroxide vulcanizate of NBR rubber containing; HNTs and respectively: antimony trioxide, magnesium hydroxide and melamine cyanurate; NSH5Sb2 O3 , NSH5Mg(OH)2 , NSH5CM – sulfur vulcanizate of NBR rubber containing HNTs and respectively: antimony trioxide, magnesium hydroxide and melamine cyanurate; 15, 30 – quantity in phr appropriate of flame retardant; T5 and T50 – temperature of sample 5% and 50% mass loss, respectively; dm × dt−1 – maximum rate of thermal decomposition of vulcanizates; TRMAX – temperature of maximum rate of thermal decomposition of vulcanizates; Pw – residue after the thermal decomposition of vulcanizates; Ts – temperature of residue burning after the thermal decomposition of vulcanizates; P800 – residue after heating up to T = 800◦ C.
P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30 Table 2 Thermal analysis of flame-retardants. Flame retardant
T5 [◦ C]
T50 [◦ C]
P800 [%]
Antimony trioxide [11,12] Melamine cyanurate [11,12] Magnesium hydroxide
– 320 450
– 365 –
101.7 18.3 64.4
thermal stability of antimony trioxide and melamine cyanurate intensifies the channeling effect (Table 2) [11,12]. The lower thermal decomposition rate of the composites investigated (NNH5 and NSH5) compared to that of unfilled vulcanizates (NN and NS) is also due to the decreased segmental mobility of the elastomer chains around halloysite particles. The decrease in segmental mobility limits the amplitude of thermal vibration, and consequently also inhibits the degradation and thermal destruction processes of the composite under investigation. From the view point of flammability reduction, in addition to the thermal decomposition rate, the residue after decomposition is of paramount importance (Pw ). This parameter plays bigger and bigger role. The most effective flame-retardant compounds are those that during burning polymeric materials cause the formation of a carbonized residue with no or a minimal share of pyrolysis products. The comparative analysis of the test results obtained by the derivatography method (Table 1) leads to a conclusion that only melamine cyanurate does not increase the residue after the thermo-oxidative decomposition (Pw ) of the elastomeric materials obtained, especially in the case of the vulcanizates cross-linked with sulfur. However, the flame-retardant compounds used considerably reduce the flammability determined by the value (ts ) of
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the halloysite-filled NBR regardless of the structure of its spatial network structure. Therefore they show a synergic action with halloysite intercalated by means of sodium hydroxide (Tables 3 and 4). Under their influence the ignition of the vulcanizates obtained occurs as a result of flame action by 20 s, while the materials containing them undergo self-extinguishing (Table 3). The flame-retardant compounds used together with HNTs reduce the fire hazard of the NBR vulcanizates, also containing only appropriate flame retardant, regardless of the method of elastomer cross-linking (Tables 3 and 4). They clearly extend the time to ignition (TTI), and influence positive on the average mass loss rate (MLR) parameter. Under influence of synergistic action of flame retardants and halloysite nanotubes the values connected with the amount of heat released during the combustion of NBR composites are reduced (Tables 3 and 4). From the literature review it follows, that the activity of antimony trioxide as inhibitor of combustion processes depends on the polymer chemical structure and the composition of a polymeric blend [12,13]. It should be however clearly mentioned that the synergic action of halloysite and Sb2 O3 makes it possible to obtain self-extinguishing composites regardless of the structure of spatial network. Self-extinguishing is connected with the structure of the boundary layer formed by the strongly cross-linked rubber, aluminosilicate impermeable to vapors and gases and Sb2 O3 showing a high thermal stability (Table 2). It is worth notice that optimal content of Sb2 O3 in polymer matrix is on the level 15–20 phr. The increase in rubber the quantity of these flame retardant to 30 phr cause increasing flammability of investigated composites. Probably high level of Sb2 O3 in composite decrease barrier properties of HNTs.
Table 3 Flammability test results composites of NBR rubber. Sample NN NNH5 NNHSb2 O3 15 NNHSb2 O3 30 NNHMg(OH)2 15 NNHMg(OH)2 30 NNHCM 15 NNHCM 30 NS NSH5 NSHSb2 O3 15 NSHSb2 O3 30 NSHMg(OH) 15 NSHMg(OH)2 30 NSHCM15 NSHCM30
ts [s] 276a 336a 80b 122b,c 63b,c 30b,c 64b,c 61b,c 289a 300a 91b,c 66b,c 88b,c 50b,c 231b,c 76b,c
TTI [s]
THR [MJ m−2 ]
HRRmax [kW m−2 ]
HRR [kW m−2 ]
EHC [MJ kg−1 ]
EHCmax [MJ kg−1 ]
[%]
MLR [g m−2 s−1 ]
56 60 68 68 74 102 67 63 66 58 68 59 70 87 62 59
36.1 33.1 22.7 27.1 21.9 14.2 20.4 22.4 32.4 27.6 22.1 21.1 21.8 20.0 21.8 21.7
492.6 342.5 253 376.8 197.9 134.2 199.7 182.0 491.8 404.3 163.2 276.2 100.8 81.27 221.5 173.0
240.0 192.2 164.7 212.2 124.3 65.7 126.6 101.4 228.0 195.8 240.7 173.07 156.98 152.17 276.2 143.3
24.1 20.25 14.29 16.76 13.32 8.75 12.26 13.07 22.65 17.50 13.20 13.69 14.76 13.05 13.13 13.69
80.31 75.01 58.45 59.60 60.85 51.97 50.36 61.71 59.62 72.57 68.57 56.23 68.54 70.92 73.25 56.23
98.2 94.7 76.4 72.1 83.1 77.4 91.0 84.5 96.3 87.1 79.7 77.1 76.2 73.0 87.7 77.1
29.62 28.92 18.44 23.98 15.31 12.17 14.80 13.03 29.24 28.87 19.36 22.99 14.89 10.72 22.99 11.84
ts – combustion time of investigated vulcanizates in air; TTI – time to ignition; THR – total heat release; HRRmax – maximal heat release rate; HRR – heat release rate; EHC – effective heat of combustion; EHCmax – maximal effective heat of combustion; % – percentage mass loss; MLR – average specific mass loss. a Time of ignition equal 5 s. b Time of ignition equal 20 s. c Self extinguishing sample. Table 4 Flammability test vulcanizates of NBR rubber containing appropriate flame retardant. Sample
ts [s]
TTI [s]
THR [MJ m−2 ]
HRRmax [kW m−2 ]
HRR [kW m−2 ]
EHC [MJ kg−1 ]
MLR [g m−2 s−1 ]
NNSb2 O3 30 NNMg(OH)2 30 NNCM 30 NSSb2 O3 30 NSMg(OH)2 30 NSCM 30
67a 400 78a 98a 350 240a
60 65 63 50 70 60
28.9 20.1 21.2 23.0 21.2 21.0
410.5 263.2 280.1 342.1 210.2 261.1
210.9 170.1 169.3 197.2 153.4 164.7
23.1 16.3 14.1 19.2 13.1 13.9
27.2 30.2 15.6 28.9 31.1 14.7
Time of ignition all samples equal 20 s. a Self extinguishing sample.
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Fig. 5. HRR curves of peroxide composites NBR rubber.
The flame-retardant activity of magnesium hydroxide is revealed when its content is over 50 phr. The data listed in Table 3 show that Mg(OH)2 by it synergetic action with halloysite intensively inhibits the combustion of composites already with its content amounting to 15 phr (Tables 3 and 4; Figs. 5–8). The synergic action of magnesium hydroxide and halloysite is due to the fact that [5,14–16]: - Magnesium oxide with halloysite and carbonization products forms a protective isolating layer on the surface of the polymeric material under combustion. - Water vapor released during the decomposition of Mg(OH)2 decreases the energetic balance of the material under combustion, and dilutes the volatile rubber destruction products, decreasing the concentration of flammable gases as confirmed by parameter TTI (Table 3). Taking into account the amount of released heat (THR) and mass loss rate (MLR) (Figs. 6–8), it should be noticed that melamine cyanurate acting with halloysite shows a similar effectiveness in fire extinguishing to that of magnesium hydroxide. Flammability reduction by the flame-retardant compounds containing nitrogen can be due to their action in both the solid and gaseous phases [17].
Fig. 6. MLR curves of peroxide composites NBR rubber.
Fig. 7. HRR curves of sulfur composites NBR rubber.
The flame-retarding action of melamine cyanurate is connected with its endothermic decomposition (sample cooling effect), release of non-flammable ammonia and the formation of condensation products such as: melam, melem and melon, whose presence in the boundary layer being formed during the combustion of elastomeric materials impedes the transport of mass and energy between the solid and gaseous phases (Scheme 1) [5,18–20]. As a result of the synergic action of halloysite and melamine cyanurate, the elastomer samples are both self-extinguishing and show increased ignition time from 5 to 20 s (Table 3). An important role in polymer combustion is played by reactions proceeding on the surface of the material under combustion, which affect both the processes occurring in flame and those of polymer thermal decomposition in the solid phase. The formation of gaseous products supporting combustion processes mainly results from the polymer endothermic decomposition (Figs. 3 and 4). One cannot exclude that besides conduction, convection and radiation, a significant source of heat energy, indispensable for maintaining polymer decomposition processes, are strongly exothermic oxidizing reactions in the boundary layer between the solid and gaseous phases [8,21]. A reduction in the oxidation process in the surface layer of vulcanizate containing melamine cyanurate can also contribute to flame extinguishing (Table 3).
Fig. 8. MLR curves of sulfur composites NBR rubber.
P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30
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Scheme 1. Endothermic decomposition of melamine [5,18].
Table 5 Mechanical properties of investigated composites. Sample
NN NN5H NNHSb2 O3 15 NNHSb2 O3 30 NNHMg(OH)2 15 NNHMg(OH)2 30 NNCM 15 NNCM 30 NS NS5H NSHSb2 O3 15 NSHSb2 O3 30 NSHMg(OH) 15 NSHMg(OH)2 30 NSCM 15 NSCM 30
Stress [MPa] 100 [%]
200 [%]
300 [%]
0.81 0.71 0.89 0.89 1.20 1.31 1.28 1.41 1.19 1.50 1.45 1.51 1.50 1.59 1.68 1.82
1.24 0.99 1.24 1.27 1.66 1.81 1.77 1.83 1.68 2.36 1.93 2.03 1.83 1.91 2.23 2.30
1.71 1.29 1.62 1.69 2.18 2.42 2.29 2.32 2.14 – 2.37 2.55 2.27 2.37 2.64 2.81
Ts [MPa]
Eb [%]
2.68 1.37 3.16 4.77 3.97 5.43 2.92 2.97 2.69 2.77 2.54 3.13 2.23 2.90 2.67 2.99
479.3 363.3 557.3 678.8 509.3 536.1 398.9 395.8 315.5 241.4 324.2 376.8 288.3 371.6 266.9 305.7
Ts – tension strength; Eb – elongation at break.
From a practical viewpoint, in selecting flame-retardant compounds, one should take into account a decided reduction in the flammability of polymers and maintenance of or improvement in their physical and mechanical properties at the same time. From the data listed in Table 5 it follows that simultaneous incorporation of modified halloysite and the flame-retardant compound selected into the NBR matrix, in most cases, improves the mechanical parameters of the composites obtained. It is not excluded that the improvement in mechanical properties results from the increase in the heterogeneousness degree of the filler in the elastomer matrix under the influence of the flame-retardant compound contained. 4. Summary The effect of the flame-retardant compounds used on the thermal processes of halloysite-filled NBR depends on the method of its cross-linking, and hence on its network structure.
The flame-retardant compounds used ambiguously affect the thermal decomposition rate, dm/dt, of the composites filled with them. Under their influence, both the flammability of halloysitefilled NBR and its fire hazard were considerably reduced. The simultaneous incorporation of modified halloysite and a halogen-free flame-retardant compound into the NBR matrix made it possible to obtain self-extinguishing elastomeric nanomaterials.
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Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Influence synergetic effect of halloysite nanotubes and halogen-free flame-retardants on properties nitrile rubber composites a,∗ ´ ˙ Przemysław Rybinski , Grazyna Janowska b a b
The Jan Kochanowski University, Management of Environment Protection and Modelling, Poland Technical University of Łód´z, Institute of Polymer and Dye Technology, Poland
a r t i c l e
i n f o
Article history: Received 2 October 2012 Received in revised form 28 January 2013 Accepted 29 January 2013 Available online 15 February 2013 Keywords: Halogen-free flame retardants Halloysite nanotubes Nitrile rubbers Thermal stability Flammability
a b s t r a c t In this study, the effect of simultaneous actions of halloysite and commonly used halogen-free flameretardant compounds, such as antimony trioxide, magnesium hydroxide and melamine cyanurate on the thermal and mechanical properties as well as flammability and fire hazard of butadiene-acrylonitrile rubber (NBR) composites was examined. Based on the analysis of thermal curves (DTA, TG, DTG), it has been found that the effect of the flame-retardant compounds used on the thermal stability and processes of halloysite-filled NBR depends on the method of the elastomer cross-linking. The flameretardant compounds used significantly reduce the flammability and fire hazard of the halloysite-filled NBR. As a result of simultaneous synergic action of the flame-retardant compound and nanofiller, selfextinguishing elastomeric materials with good mechanical properties have been obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction One of the most important issues of material engineering in recent years has been the production of flame-retardant polymeric materials characterized by low emission of toxic gases and fumes. Composites with an increased resistance to the action of fire have been more and more frequently made with the use of various nanofillers, such as aluminosilitate nanoclays, nanofibers and halloysite nanotube [1,2]. A decrease in the size of filler particles incorporated into the polymeric matrix from micrometers to nanometers makes it possible to obtain composites with better, frequently quite new and unique properties. From the studies carried out so far it follows, that in the stage of ignition and fire development, halloysite nanotubes act as effective flame-retardant compounds through the formation of a thermally stable barrier boundary layer on the surface of the material being burned. Inside the composite/nanocomposite under combustion, they form a spatial network impeding the diffusion of thermal decomposition products to flame and oxygen into the polymer’s interior [3,4]. An unquestionable drawback of nanofillers, including halloysite nanotubes, is the problem in making their perfect dispersion in the polymeric matrix. The aggregates or agglomerates being formed adversely affect not only the composite physical properties but
∗ Corresponding author. Tel.: +48 413496437. ´ E-mail address: [email protected] (P. Rybinski). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.01.030
also considerably decrease the nanoparticle effectiveness of flame retardation, especially in the stage of developed fire [5]. To increase the effectiveness of nanofillers as nontoxic flame-retardant compounds, they are used with a small quantity of conventional, especially halogen-free, flame-retardant substances that due to their synergic action with nanofiller reduce material flammability as well as improve the physical and mechanical properties of the composites containing these compounds [6]. This paper presents the results of investigating the thermal and mechanical properties as well as flammability and fire hazard of butadiene-acrylonitrile rubber (NBR) containing halloysite and common halogen-free flame-retardant compounds.
2. Experimental 2.1. Materials The object of our study was butadiene-acrylonitrile rubber (NBR), NBR 2255V, containing 22% of combined acrylonitrile, from Lanxess Company. The rubber was cross-linked by means of dicumyl peroxide (DCP) in the presence of zinc oxide (ZnO) or sulfur in the presence of zinc oxide and N-cyclohexyl-2benzoylsulfenamide (Tioheksam CBS). The resultant peroxide vulcanizate was denoted with NN, while the sulfur vulcanizate with NS. Halloysite (H) derived from the Dunino mine near Legnica (Poland), intercalated with alkali (sodium hydroxide) was used as a nanofiller of elastomeric blends [7].
P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30
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Modified halloysite was incorporated into NBR in a quantity of 5 phr (phr – parts by wt. per hundred parts by wt. of rubber). Composites NNH5 and NSH5 were used as comparative material. Melamine cyanurate (POCh Gliwice), magnesium hydroxide (Martinswerk, GmbH), Magnifin H5, antimony trioxide (POCh Gliwice), were used as flame-retardant and were incorporated into NBR in a quantity of 15 or 30 phr.
2.2. Methods The photographs of modified halloysite were taken by the SEM Quanta 250 FEG microscope. The measurements of wide-angle scattering of X-radiation (WAXS) in the modified halloysite and halloysite-containing composites were performed at room temperature using an X-ray apparatus from Phillips (X-ray source operating at: U = 30 kV and I = 50 mA; wavelength= 0.15418 nm) at the Center of Molecular and Macromolecular Studies, the Polish Academy of Science, in Lodz. Diffraction patterns were recorded within the angle range 2 = 1–35◦ ; measurement step 0.05◦ ; measurement time 25 s; radiation CuK alfa ( = 0.15418 nm); operation in transmission mode. The apparatus was calibrated at the beginning of each measurement series. The samples of the composites tested were disintegrated before analysis by the CAT X520D homogenizer (revolutions 16–30 000), cooled in a liquid nitrogen, transferred onto a polystyrene holder and placed in a disposable measurement ring. The WAXS measurements of tested halloysite was used as a reference measurement for composites. Elastomeric blends were prepared at room temperature with the use of a laboratory rolling mill (D = 150 mm, L = 300 mm). The rotational speed of the front roll was 20 rpm, friction 1.1. The blends were vulcanized in steel molds placed between electrically heated press shelves. The optimal vulcanization time ( 0.9 ) at a temperature of 160 ◦ C was determined by means of a WG-2 vulcameter according to PN-ISO 3417:1994. The thermal properties of the vulcanizates obtained were tested under air within the temperature range of 25–800 ◦ C by means of a MOM derivatograph (Budapest), using Al2 O3 as reference substance. Weighed portions were 90 mg each, heating rate 7.9 ◦ C min−1 , and the sensitivities of thermal curves were as follows: TG = 100, DTA = 1/5, DTG 1/30. The flammability of the vulcanizates under investigation was tested with the use of a cone calorimeter from Fire Testing Technology Ltd. Elastomer samples with dimensions of (100 × 100 ± 1) mm and thickness of (2 ± 0.5) mm were tested at horizontal position with the heat radiant flux density 35 kW m−2 . During tests, the following parameters were recorded: initial sample weight, time to ignition (TTI), sample weight during testing, total heat released (THR), effective combustion heat (EHC), average weight loss rate (MLR), heat release rate (HRR) and sample final weight. Flammability of vulcanizations in air was also tested. Elastomer samples with dimensions of 50 × 10 × 4 mm in a vertical position were ignited with a gaseous burner supplied with LPG for 5 s and its combustion time (ts ) was measured. The strength properties of composites were determined according to PN-ISO 37:1998, using an apparatus from ZWICK, model 1435, connected to a computer with an appropriated software and dumbbell samples with a measurement section width of 4 mm and a thickness of about 1 mm.
Fig. 1. (A and B) Photographs of activated halloysite by alkali solution [7].
3. Results and discussion Fig. 1A and B shows SEM pictures of initial halloysite nanotubes (HNTs) activated by alkali solution [7].
Fig. 2. WAXS pattern of the composites. NNH5 peroxide vulcanizate of NBR rubber containing 5 phr of HNTs. NSH5 sulfur vulcanizate of NBR rubber containing 5 phr of HNTs.
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Fig. 4. DTA curves sulfur composites of NBR. Fig. 3. DTA curves peroxide composites of NBR.
Fig. 2 compares the X-ray diffraction spectra of halloysite nanotubes (HNTs) activated by alkali solution and NNH5, NSH5 composites. The basal space reflections of HNTs indicate a sharp peak at 2 = 12.27◦ , corresponding to a (0 0 1) basal spacing of 0.7 ± 0.3 nm. However, in the NNH5, NSH5 nanocomposites, peak at 2 = 12.27◦ was not detected. It is clear that HNTs in the nanocomposites appeared not to be intercalated by the rubber chains. The addition appropriate flame retardant to the polymer matrix does not change significantly the WAXS curves of composites. From the review of relevant literature it follows that NBR, both before and after being cross-linked with dicumyl peroxide (DCP) or sulfur, undergoes a two-stage thermal cross-linking [8,9]. The thermal cross-linking processes of NBR also proceed in the presence of halloysite (Figs. 3 and 4). From the analysis of DTA curves it follows that the effect of the flame-retardants compounds used on the thermal changes in NBR filled with halloysite depends on the method of elastomer cross-linking (Figs. 3 and 4). In the case of peroxide vulcanizates (Fig. 3), the addition of magnesium hydroxide causes that NBR filled with halloysite undergoes thermal cross-linking in a single stage at lower temperatures. Also the halloysite-filled NBR cross-linked with sulfur in the presence of the flame-retardant compounds undergoes (single stage) thermal cross-linking (Fig. 4).
The flame-retardant compounds used reduce the thermal stability of composites determined with T5 , but they do not significantly change T50 . Nevertheless, the highest values of T50 are shown by the halloysite-filled NBR cross-linked with both DCP and sulfur, in the presence of Sb2 O3 . An important parameter determining both the polymeric material thermal stability and its flammability is the rate of its thermal decomposition, dm/dt. The reduction in the decomposition rate of polymeric materials exerts a positive influence on the reduction in their flammability. This results from the formation of lower quantities of volatile and flammable products of pyrolysis passing to flame, which reduces the rate of free-radical reactions in the combustion zone. The flame-retardant compounds used ambiguously influence the thermal decomposition rate of the composites filled with them. Antimony trioxide and melamine cyanurate decrease the thermal decomposition rate of the vulcanizates investigated, which is cannot be found in the case of magnesium hydroxide (Table 1). The decrease in the decomposition rate of the composites tested (NNH5 and NSH5) under thermo-oxidative conditions compared to that of unfilled vulcanizates (NN and NS) results first of all from the barrier properties of the nanofiller used. Aluminosilicate is impermeable for vapors and gases, thus the small-molecular products of the composite thermal decomposition products can diffuse outside only through closely specified spaces [10]. The great
Table 1 Thermal analysis of composites in air atmosphere. Sample
T5 [◦ C]
T50 [◦ C]
TRMAX [◦ C]
dm × dt−1 [mm]
Pw [%]
Ts [◦ C]
P800 [%]
NN NNH5 NNH5Sb2 O3 15 NNH5Sb2 O3 30 NNH5Mg(OH)2 15 NNH5Mg(OH)2 30 NNH5CM 15 NNH5CM 30 NS NSH5 NSH5Sb2 O3 15 NSH5Sb2 O3 30 NSH5Mg(OH)2 15 NSH5Mg(OH)2 30 NSH5CM 15 NSH5CM 30
350 365 360 360 340 320 320 310 315 350 350 320 320 325 290 270
420 415 420 440 420 420 420 400 405 425 430 440 420 415 420 420
410 405 410 410 410 410 410 410 395 410 410 400 400 400 400 400
70 50 40 36 50 50 42 35 52 44 37 30 45 35 39 31
23.01 33.33 41.12 48.89 33.34 38.89 30 36.67 23.33 32.78 42.23 47.78 34.45 37.78 32.23 31.11
545 510 480 490 500 490 480 470 540 540 490 480 470 490 480 480
9.01 11.67 20 24.45 17.78 23.34 10 13.34 6.67 10.56 21.12 22.23 17.78 23.34 12.22 11.12
NN – peroxide vulcanizate of NBR rubber; NS – sulfur vulcanizate of NBR rubber; NNH5 – peroxide vulcanizate of NBR rubber containing 5 phr of HNTs; NSH5 – sulfur vulcanizate of NBR rubber containing 5 phr of HNTs; NNH5Sb2 O3 , NNH5Mg(OH)2 , NNH5CM – peroxide vulcanizate of NBR rubber containing; HNTs and respectively: antimony trioxide, magnesium hydroxide and melamine cyanurate; NSH5Sb2 O3 , NSH5Mg(OH)2 , NSH5CM – sulfur vulcanizate of NBR rubber containing HNTs and respectively: antimony trioxide, magnesium hydroxide and melamine cyanurate; 15, 30 – quantity in phr appropriate of flame retardant; T5 and T50 – temperature of sample 5% and 50% mass loss, respectively; dm × dt−1 – maximum rate of thermal decomposition of vulcanizates; TRMAX – temperature of maximum rate of thermal decomposition of vulcanizates; Pw – residue after the thermal decomposition of vulcanizates; Ts – temperature of residue burning after the thermal decomposition of vulcanizates; P800 – residue after heating up to T = 800◦ C.
P. Rybi´ nski, G. Janowska / Thermochimica Acta 557 (2013) 24–30 Table 2 Thermal analysis of flame-retardants. Flame retardant
T5 [◦ C]
T50 [◦ C]
P800 [%]
Antimony trioxide [11,12] Melamine cyanurate [11,12] Magnesium hydroxide
– 320 450
– 365 –
101.7 18.3 64.4
thermal stability of antimony trioxide and melamine cyanurate intensifies the channeling effect (Table 2) [11,12]. The lower thermal decomposition rate of the composites investigated (NNH5 and NSH5) compared to that of unfilled vulcanizates (NN and NS) is also due to the decreased segmental mobility of the elastomer chains around halloysite particles. The decrease in segmental mobility limits the amplitude of thermal vibration, and consequently also inhibits the degradation and thermal destruction processes of the composite under investigation. From the view point of flammability reduction, in addition to the thermal decomposition rate, the residue after decomposition is of paramount importance (Pw ). This parameter plays bigger and bigger role. The most effective flame-retardant compounds are those that during burning polymeric materials cause the formation of a carbonized residue with no or a minimal share of pyrolysis products. The comparative analysis of the test results obtained by the derivatography method (Table 1) leads to a conclusion that only melamine cyanurate does not increase the residue after the thermo-oxidative decomposition (Pw ) of the elastomeric materials obtained, especially in the case of the vulcanizates cross-linked with sulfur. However, the flame-retardant compounds used considerably reduce the flammability determined by the value (ts ) of
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the halloysite-filled NBR regardless of the structure of its spatial network structure. Therefore they show a synergic action with halloysite intercalated by means of sodium hydroxide (Tables 3 and 4). Under their influence the ignition of the vulcanizates obtained occurs as a result of flame action by 20 s, while the materials containing them undergo self-extinguishing (Table 3). The flame-retardant compounds used together with HNTs reduce the fire hazard of the NBR vulcanizates, also containing only appropriate flame retardant, regardless of the method of elastomer cross-linking (Tables 3 and 4). They clearly extend the time to ignition (TTI), and influence positive on the average mass loss rate (MLR) parameter. Under influence of synergistic action of flame retardants and halloysite nanotubes the values connected with the amount of heat released during the combustion of NBR composites are reduced (Tables 3 and 4). From the literature review it follows, that the activity of antimony trioxide as inhibitor of combustion processes depends on the polymer chemical structure and the composition of a polymeric blend [12,13]. It should be however clearly mentioned that the synergic action of halloysite and Sb2 O3 makes it possible to obtain self-extinguishing composites regardless of the structure of spatial network. Self-extinguishing is connected with the structure of the boundary layer formed by the strongly cross-linked rubber, aluminosilicate impermeable to vapors and gases and Sb2 O3 showing a high thermal stability (Table 2). It is worth notice that optimal content of Sb2 O3 in polymer matrix is on the level 15–20 phr. The increase in rubber the quantity of these flame retardant to 30 phr cause increasing flammability of investigated composites. Probably high level of Sb2 O3 in composite decrease barrier properties of HNTs.
Table 3 Flammability test results composites of NBR rubber. Sample NN NNH5 NNHSb2 O3 15 NNHSb2 O3 30 NNHMg(OH)2 15 NNHMg(OH)2 30 NNHCM 15 NNHCM 30 NS NSH5 NSHSb2 O3 15 NSHSb2 O3 30 NSHMg(OH) 15 NSHMg(OH)2 30 NSHCM15 NSHCM30
ts [s] 276a 336a 80b 122b,c 63b,c 30b,c 64b,c 61b,c 289a 300a 91b,c 66b,c 88b,c 50b,c 231b,c 76b,c
TTI [s]
THR [MJ m−2 ]
HRRmax [kW m−2 ]
HRR [kW m−2 ]
EHC [MJ kg−1 ]
EHCmax [MJ kg−1 ]
[%]
MLR [g m−2 s−1 ]
56 60 68 68 74 102 67 63 66 58 68 59 70 87 62 59
36.1 33.1 22.7 27.1 21.9 14.2 20.4 22.4 32.4 27.6 22.1 21.1 21.8 20.0 21.8 21.7
492.6 342.5 253 376.8 197.9 134.2 199.7 182.0 491.8 404.3 163.2 276.2 100.8 81.27 221.5 173.0
240.0 192.2 164.7 212.2 124.3 65.7 126.6 101.4 228.0 195.8 240.7 173.07 156.98 152.17 276.2 143.3
24.1 20.25 14.29 16.76 13.32 8.75 12.26 13.07 22.65 17.50 13.20 13.69 14.76 13.05 13.13 13.69
80.31 75.01 58.45 59.60 60.85 51.97 50.36 61.71 59.62 72.57 68.57 56.23 68.54 70.92 73.25 56.23
98.2 94.7 76.4 72.1 83.1 77.4 91.0 84.5 96.3 87.1 79.7 77.1 76.2 73.0 87.7 77.1
29.62 28.92 18.44 23.98 15.31 12.17 14.80 13.03 29.24 28.87 19.36 22.99 14.89 10.72 22.99 11.84
ts – combustion time of investigated vulcanizates in air; TTI – time to ignition; THR – total heat release; HRRmax – maximal heat release rate; HRR – heat release rate; EHC – effective heat of combustion; EHCmax – maximal effective heat of combustion; % – percentage mass loss; MLR – average specific mass loss. a Time of ignition equal 5 s. b Time of ignition equal 20 s. c Self extinguishing sample. Table 4 Flammability test vulcanizates of NBR rubber containing appropriate flame retardant. Sample
ts [s]
TTI [s]
THR [MJ m−2 ]
HRRmax [kW m−2 ]
HRR [kW m−2 ]
EHC [MJ kg−1 ]
MLR [g m−2 s−1 ]
NNSb2 O3 30 NNMg(OH)2 30 NNCM 30 NSSb2 O3 30 NSMg(OH)2 30 NSCM 30
67a 400 78a 98a 350 240a
60 65 63 50 70 60
28.9 20.1 21.2 23.0 21.2 21.0
410.5 263.2 280.1 342.1 210.2 261.1
210.9 170.1 169.3 197.2 153.4 164.7
23.1 16.3 14.1 19.2 13.1 13.9
27.2 30.2 15.6 28.9 31.1 14.7
Time of ignition all samples equal 20 s. a Self extinguishing sample.
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Fig. 5. HRR curves of peroxide composites NBR rubber.
The flame-retardant activity of magnesium hydroxide is revealed when its content is over 50 phr. The data listed in Table 3 show that Mg(OH)2 by it synergetic action with halloysite intensively inhibits the combustion of composites already with its content amounting to 15 phr (Tables 3 and 4; Figs. 5–8). The synergic action of magnesium hydroxide and halloysite is due to the fact that [5,14–16]: - Magnesium oxide with halloysite and carbonization products forms a protective isolating layer on the surface of the polymeric material under combustion. - Water vapor released during the decomposition of Mg(OH)2 decreases the energetic balance of the material under combustion, and dilutes the volatile rubber destruction products, decreasing the concentration of flammable gases as confirmed by parameter TTI (Table 3). Taking into account the amount of released heat (THR) and mass loss rate (MLR) (Figs. 6–8), it should be noticed that melamine cyanurate acting with halloysite shows a similar effectiveness in fire extinguishing to that of magnesium hydroxide. Flammability reduction by the flame-retardant compounds containing nitrogen can be due to their action in both the solid and gaseous phases [17].
Fig. 6. MLR curves of peroxide composites NBR rubber.
Fig. 7. HRR curves of sulfur composites NBR rubber.
The flame-retarding action of melamine cyanurate is connected with its endothermic decomposition (sample cooling effect), release of non-flammable ammonia and the formation of condensation products such as: melam, melem and melon, whose presence in the boundary layer being formed during the combustion of elastomeric materials impedes the transport of mass and energy between the solid and gaseous phases (Scheme 1) [5,18–20]. As a result of the synergic action of halloysite and melamine cyanurate, the elastomer samples are both self-extinguishing and show increased ignition time from 5 to 20 s (Table 3). An important role in polymer combustion is played by reactions proceeding on the surface of the material under combustion, which affect both the processes occurring in flame and those of polymer thermal decomposition in the solid phase. The formation of gaseous products supporting combustion processes mainly results from the polymer endothermic decomposition (Figs. 3 and 4). One cannot exclude that besides conduction, convection and radiation, a significant source of heat energy, indispensable for maintaining polymer decomposition processes, are strongly exothermic oxidizing reactions in the boundary layer between the solid and gaseous phases [8,21]. A reduction in the oxidation process in the surface layer of vulcanizate containing melamine cyanurate can also contribute to flame extinguishing (Table 3).
Fig. 8. MLR curves of sulfur composites NBR rubber.
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Scheme 1. Endothermic decomposition of melamine [5,18].
Table 5 Mechanical properties of investigated composites. Sample
NN NN5H NNHSb2 O3 15 NNHSb2 O3 30 NNHMg(OH)2 15 NNHMg(OH)2 30 NNCM 15 NNCM 30 NS NS5H NSHSb2 O3 15 NSHSb2 O3 30 NSHMg(OH) 15 NSHMg(OH)2 30 NSCM 15 NSCM 30
Stress [MPa] 100 [%]
200 [%]
300 [%]
0.81 0.71 0.89 0.89 1.20 1.31 1.28 1.41 1.19 1.50 1.45 1.51 1.50 1.59 1.68 1.82
1.24 0.99 1.24 1.27 1.66 1.81 1.77 1.83 1.68 2.36 1.93 2.03 1.83 1.91 2.23 2.30
1.71 1.29 1.62 1.69 2.18 2.42 2.29 2.32 2.14 – 2.37 2.55 2.27 2.37 2.64 2.81
Ts [MPa]
Eb [%]
2.68 1.37 3.16 4.77 3.97 5.43 2.92 2.97 2.69 2.77 2.54 3.13 2.23 2.90 2.67 2.99
479.3 363.3 557.3 678.8 509.3 536.1 398.9 395.8 315.5 241.4 324.2 376.8 288.3 371.6 266.9 305.7
Ts – tension strength; Eb – elongation at break.
From a practical viewpoint, in selecting flame-retardant compounds, one should take into account a decided reduction in the flammability of polymers and maintenance of or improvement in their physical and mechanical properties at the same time. From the data listed in Table 5 it follows that simultaneous incorporation of modified halloysite and the flame-retardant compound selected into the NBR matrix, in most cases, improves the mechanical parameters of the composites obtained. It is not excluded that the improvement in mechanical properties results from the increase in the heterogeneousness degree of the filler in the elastomer matrix under the influence of the flame-retardant compound contained. 4. Summary The effect of the flame-retardant compounds used on the thermal processes of halloysite-filled NBR depends on the method of its cross-linking, and hence on its network structure.
The flame-retardant compounds used ambiguously affect the thermal decomposition rate, dm/dt, of the composites filled with them. Under their influence, both the flammability of halloysitefilled NBR and its fire hazard were considerably reduced. The simultaneous incorporation of modified halloysite and a halogen-free flame-retardant compound into the NBR matrix made it possible to obtain self-extinguishing elastomeric nanomaterials.
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