Ignition activation energy of materials based on polyamide 6

Transactions of the VŠB - Technical university of Ostrava Safety Engineering Series Vol. XI, No. 1, 2016 27 IGNITION ACTIVATION ENERGY OF MATERIALS BASED ON POLYAMIDE 6 Peter RANTUCH1, Igor WACHTER2, Ivan HRUŠOVSKÝ3, Karol BALOG4 1 Slovak University of Technology, Faculty of Materials Science and Technology, Trnava, Slovak Republic, peter[email protected] 2 Slovak University of Technology, Faculty of Materials Science and Technology in Trnava, Trnava, Slovak Republic, igor[email protected] 3 Slovak University of Technology, Faculty of Materials Science and Technology, Trnava, Slovak Republic, [email protected] 4 Slovak University of Technology, Faculty of Materials Science and Technology in Trnava, Trnava, Slovak Republic, [email protected] Abstract: This contribution is aimed to compare the values of the ignition activation energies of two types of polyamide – Slovamid 6 FRB and Slovamid GF 50 LTS. Samples were isothermally stressed at fi ve different temperatures between 500 °C a 550 °C, while the time to initiation of the fl ame combustion was monitored. Subsequently from the measured times were compiled Arrhenius plots under which activation energy of ignition of both polymers were calculated. The values of activation energies were 106 kJ.mol-1 and 158.0 kJ.mol-1 for Slovamid 6 FRB 4 and Slovamid 6 GF 50 LTS. Keywords: Activation energy, Ignition, Polyamide 6, Thermal decomposition, Time to ignition. Research article Introduction Aliphatic polyamides are important engineering resins manufactured in large quantities for fi bre, fi lm and plastic applications. Polyamide 6 (PA6) together with polyamide 6.6 (PA6.6) are produced in the largest volume, whereas polyamides 7, 11, 12, 4.6, 6.10 and 6.12 are speciality products with signifi cantly lower volumes of production. Depending on the method used, the sample heating rate and the air fl ow, the ignition temperatures for polyamides were found in the range 370 °C – 540 °C. (Levchik and Weil, 2000) Thermogravimetric analysis showed that polyamides based on renewable resources (named bio-polyamides) decompose in one step in inert atmosphere and in three steps in oxidative conditions. In inert atmosphere a wide range of compounds was found to be produced during the thermal degradation with a few steps of evolution. Except of the low molecular weight volatiles, like H2O, CO, CO2 and NH3, aliphatic compounds, terminal vinyl groups, cyclic compounds and long chain alkenes were detected. Under oxidative atmosphere, the major products of degradation were carbon monoxide, carbon dioxide, water and ammonia. (Pagacz et al., 2015) Similar results were also mentioned by Herrera et al. (2001) According to their results the polyamides decompose apparently in one step in nitrogen and in two steps in air, but a detailed study confi rmed that the decomposition has two steps in nitrogen and at least three steps in air. The extra step in air is due to the degradation of the char formed in the fi rst step. PA6 is extensively used in engineering applications because of its unique characteristics such as low price, low viscosity, high toughness, shelf lubricating behaviour and high chemical resistance. (Pashaei et al., 2011) It belongs to fl ammable material with a low limiting oxygen index (LOI) value (about 21 %) and severe fl ammable dripping, which inhibit the further application of PA6 in many fi elds. (Ge et al., 2015; Novozhilov et al., 2011) During thermogravimetric testing of PA6 in nitrogen atmosphere it was determined that the decomposition of PA6 in inert atmosphere (nitrogen gas fl ow of 60 mL.min-1) occurs in the range of 345 °C – 511 °C with one DTG peak at 472 °C. (Pashaei et al., 2011) The TGA a DTG curves are shown in Fig. 1. DOI 10.1515/tvsbses-2016-0004 Transactions of the VŠB - Technical university of Ostrava Safety Engineering Series Vol. XI, No. 1, 2016 28 Fig. 1 TGA and derivative thermogram of PA6 heated at rate of 20 °C.min-1 in nitrogen stream (Pashaei et al., 2011) During the testing on cone calorimeter,the ignition time of the PA6 (exposed horizontally to an external heat fl ux of 35 kW.m-2) was 105 s, and the peak value of HRR was 1064 kW.m-2, the peak value of mass loss rate was 0.252 g.m-2s-1. (Du et al., 2007) Novozhilov et al. (2011) claim that at an external heat fl ux of 60 kW.m-2 the peak value of HRR was 863 kW.m-2. The combustion products of PA6 contained compounds with diverse structures, whereas non-fl aming combustion yielded a relatively small overall amount of volatile product. The combustion products comprise primarily nitrogen-containing compounds and no oxygen-containing oxidation products other than CO and CO2. (Levchik and Weil, 2000) Bockhorn et al. (1999) claim that the main decomposition product of PA6 in helium fl ow at heating rate of 2 K.min-1 is ε-caprolactam (92 %). Identifi ed by-products analysed by GC/MS are the cyclic dimer, i.e. 1,8-diazacyclotetradecane-2,9-dion (4 %), 5-hexene acid amide (2 %), 5-hexen nitrile (1 %), hexane nitrile (0.5 %) and 2-cyclohexen-1one (0.5 %). The toxicity of thermal decomposition products of PA6 is mentioned in the work of Esposito and Alarie (1988). The weight loss of pure PA6 takes place in a single step with a maximum rate of weight loss at 459 °C, and the char residue is 2.7 % at 700 °C (Ge et al., 2015), but the thermal degradation of PA6 involves the following processes: 1. an intramolecular backbiting process and 2. hydrogen transfer reaction leading to scission of the C-N bond β to the amide group. (Kutz, 2005) The energy barrier opposing the reation is named activation energy. (Kissinger, 1956) Activation energy of thermal decomposition of polyamides was studied in several works (Tab. 1). Tab. 1 Activation energies of various polyamides at diferent conditions Materials and methods Granulated samples of Slovamid 6 FRB 4 a Slovamid 6 GF 50 LTS were tested in this experiment. In both cases it was a material based on PA6, while the fi rst mentioned contained fi re retardants on non-halogenated bases for injection molding. Designed for the electrotechnical industry (good fl ow properties and good resistance to injection faults. Suitable for thin-walled products and wide area cable boxes. (Plastcom, 2013a) Slovamid 6 GF 50 LTS is PA6 reinforced with 50 % glass fi ber with UV and heat stabilizers for injection molding. It is used for the production of tough and high strength products in the automotive, electrical, engineering and consumer industry (holders for electric tools, hobby tools, gear units, fan parts etc.). (Plastcom, 2013b) Comparison of the basic properties of the two materials is shown in the Tab. 2. Material Atmosphere Activation energy Source PA6 N2111–276 (Du et al., 2007) PA6 N2264 (Du et al., 2007) PA6 N2276 (Du et al., 2007) Nylon vacuum 232 (Nelson, 1967) Nylon vacuum 200 (Nelson, 1967) Nylon vacuum 154 (Nelson, 1967) Nylon vacuum 155 (Nelson, 1967) Nylon vacuum 216 (Nelson, 1967) PA6 Nitrogen 162 (Herrera et al., 2001) PA6 Nitrogen 476 (Herrera et al., 2001) PA66 Nitrogen 91 (Herrera et al., 2001) PA66 Nitrogen 310 (Herrera et al., 2001) PA12 Nitrogen 2208 (Herrera et al., 2001) PA12 Nitrogen 260 (Herrera et al., 2001) PA612 Nitrogen 164 (Herrera et al., 2001) PA612 Nitrogen 400 (Herrera et al., 2001) PA6 Air 153 (Li et al., 2006) DOI 10.1515/tvsbses-2016-0004 Transactions of the VŠB - Technical university of Ostrava Safety Engineering Series Vol. XI, No. 1, 2016 29 Tab. 2 Basic properties of the tested polymers (Plastcom, 2013a; Plastcom, 2013b) For each polyamide fi ve samples were tested polyamides. Statistical evaluation of the weights is shown in Fig. 2. As apparent, the deviations from the average weights of the samples were less than 1 %, while for sample of SLOVAMID 6 GF 50 LTS the variance was minor. Fig. 2 Weight box plot of tested samples To maintain the isothermal conditions hot-air furnace with a control unit CLARE 4 was used, complying with standards ISO 871 (2010). The measurements were conducted in a stream of air. Both types of material were successively exposed to a temperature of 500 °C, resp. 510 °C to 540 °C, resp. 550 °C. At every temperature a time to initiation of fl ame combustion was monitored. Kinetic theory The dependence of the temperature and reaction rate describes the Arrhenius law, according to which the kinetics of a chemical reaction can be described as follows: (1) where k is the rate constant, A is the frequency factor, EA is the activation energy, R is the universal gas constant and T is the absolute temperature. Logarithm of equation (1) gives a linear relationship: (2) Semenov (1959) in his work deduced that in the case of characterizing reaction rate of ignition by the time to ignition of the sample under isothermal heating the relation (2) can be written as: (3) where ti is the time to ignition of the sample. If the ignition activation energy is considered as a constant characteristic of the material, it is possible to determine the dependence of the natural logarithm of time to ignition from the inverse value of the thermodynamic temperature of the heating during several temperature measurements. If the slope of mentioned dependence is labeled b then the following applies: (4) This method of calculation was mentioned by for example Martinka et al. (2015) and Balog, Tureková and Slabá (2005). Results and discussion The measured initiation times of the samples at various temperatures are shown in Tab. 3. These values indicate that it is a logarithmic dependence, corresponding to the general equation (3). Tab. 3 Time to ignition of the samples as a function of the ambient temperature From the measured values of the time to ignition of the samples and their corresponding temperatures (Tab. 3), dependence of the natural logarithm of the induction period from the inverse value of the thermodynamic temperature was crated. Trend line was covered by the points of this dependence and the slope was inserted into the equation (4). Subsequently, for each of the polymer an activation energy of ignition was calculated. Mentioned dependencies are shown in Fig. 3 and 4. Slovamid 6 FRB 4 Slovamid 6 GF 50 LTS Density [kg.m-3] 1190 1550 Melting temperature [°C] 220 220 Glow-wire fl ammability index [°C] 960 650 Moisture content [%] 0.15 0.15 E RT A kAe   ln ln A E kA RT  ln ln A i E tA RT  A EbR  Slovamid 6 FRB 4 Slovamid 6 GF 50 LTS Temperature [°C] Time to ignition [s] Temperature [°C] Time to ignition [s] 500 103 510 120 510 86 520 89 520 66 530 69 530 52 540 49 540 48 550 37 DOI 10.1515/tvsbses-2016-0004 Transactions of the VŠB - Technical university of Ostrava Safety Engineering Series Vol. XI, No. 1, 2016 30 References BALOG, K.; TUREKOVÁ, I.; SLABÁ, I. (2005): Ignition imfl ammability parameters determination of polymeric materials In: Sborník přednášek mezinárodní konference Požární ochrana 2005. Ostrava: Sdružení požárního a bezpečnostního inženýrství, 2005, s. 17-24. ISBN 80-86634-66-3. (in Slovak) BOCKHORN, H.; HORNUNG, A.; HORNUNG ,U.; WEICHMANN, J. (1999): Kinetic study on the non-catalysed and catalysed degradation of polyamide 6 with isothermal and dynamic methods. Thermochimica Acta. 1999, Vol. 337, Iss. 1–2, pp. 97-110. ISSN 0040-6031. DU, L.; QU, B.; ZHANG, M. (2007): Thermal properties and combustion characterization of nylon 6/MgAl-LDH nanocomposites via organic modifi cation and melt intercalation. Polymer Degradation and Stability. 2007, Vol. 92, Iss. 3, pp. 497-502. ISSN 0141-3910. Fig. 3 Arrhenius plot of Slovamid 6 FRB 4 Fig. 4 Arrhenius plot of Slovamid 6 GF 50 LTS Regression strips fl anking the double-sided 95 % confi dence interval are shown in both fi gures. The square of the correlation coeffi cient for Slovamid 6 FRB 4 is slightly lower, but still achieves signifi cantly high levels of 0.9798. Slovamid 6 GF 50 LTS achieves even stronger correlation with R2 equal to 0.9978. As mentioned above, the activation energies were calculated from the slope of the trend lines of the particular material. The value of the ignition activation energy of Slovamid 6 FRB 4 was 106 kJ mol-1. The ignition activation energy of Slovamid 6 GF 50 LTS was higher (158.0 kJ.mol-1). If these values are compared with those given in the literature (Tab. 1), SLOVAMID 6 GF 50 LTS achieves a typical activation energy for the decomposition of pure polyamide 6 in the air mentioned by Li et al. (2006). Observation of Yin, Krifa and Koo (2015) indicates that the presence of FR additives decreases PA6 thermal stability, particularly at the onset of decomposition, what is consistent with the ignition times indicated in the Tab. 3. Conclusion Polymeric materials currently fi nd a wide range of applications. This contribution aims to calculate the activation energy of ignition of two types of polyamide. Samples used in this experiment were Slovamid 6 FRB 4 and Slovamid 6 GF 50 LTS. 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