TG and DTG Study of Decomposition of Commercial PUR Cellular Materials

Transcription

1 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials BARBORA LAPCIKOVA, LUBOMIR LAPCIK, JR. Centre of Polymer Systems, Faculty of Technology, Tomas Bata University in Zlin, nám. T.G. Masaryka 5555, Zlín, Czech Republic ABSTRACT This article deals with study of thermal degradation of polymeric materials based on soft Polyurethane (PUR) foam by means of thermal and other analytical methods. Samples were of commercial grade PUR open cell foams (Gumotex, JSC, BÍeclav, Czech Republic) and their decomposition (measured on Shimadzu TG/DTA instrument) was followed in the temperature range of +40 to +550 C in the dynamic atmosphere of nitrogen. Decomposition kinetics was followed at five different heating rates for one selected PUR sample. Experimental results were described by means of formal kinetic model and activation energy of decomposition process (E a ) was evaluated. E a for the decomposition of the polymer chains of studied PUR cellular material determined by the Kissinger method was found 169 kj/mol (in the case of the first step of the obtained degradation pattern), for the total decomposition of the organic compound (second step) was found 169 kj/mol. For better characterization of the studied materials FTIR spectra, optical microscopy and pore size distribution were determined for each sample under study. KEYWORDS: Thermal analysis, TG, DTG, PUR, kinetics study, polyurethanes. INTRODUCTION The study of polymer thermal degradation is one of the major methods determining the thermal applicability of the materials. Thermogravimetric analysis (TG) is a technique widely used because for its simplicity and the information content [1-5]. Polymer is usually thermostable until a decomposition process begins. Two main types of thermal decomposition processes are usually recognized for polymer main macromolecular chain: depolymerization and random decomposition. These two processes can occur separately or in combination [6]. Decomposition reflects the behavior of the single components, and interaction is considered responsible for details of weight loss characteristics and the nature of evolved products [7]. J. Polym. Mater. Vol. 28, No. 3, 2011, MD Publications Pvt. Ltd. Correspondence author lapcikova@ft.utb.cz

2 354 Barbora Lapcikova et al. Each of the polymer undergoes heating in conformity with certain temperature-time relationship and usually passes first through the melting and then through the stages of thermal degradation. Temperature changes can stimulate a variety of physical and chemical processes in polymer systems. Important examples of these processes include thermal degradation, crosslinking, crystallization, glass transition, etc [8]. Polyurethanes are used in a wide range of industrial applications. In 1993, over 6 x 10 6 t (metric tons) of polyurethanes was consumed world-wide. Major applications of flexible polyurethane foams are applied in the furniture construction, carpet underlay and bedding and in transportation [9]. The rigid foams are mainly used in buildings construction [10-13]. Segmented type of polyurethanes consist of hard and soft segments. The hard - segment is typically realized with the urethane group, the soft segment is ether - type or ester - type polyol [14]. The polyurethane is synthesized from three separate components: a diisocyanate, a polyether or polyester, and a chain extender. Common diisocyanates (OCN-R-NCO) used in commercial polymers are 4,4'-methylenebis (phenylisocyanate) (MDI) and toluene diisocyanate (TDI) polyurethane soft segments are low molecular weight polyethers or polyesters (OH-R -OH). Typical polyether used in commercial materials is poly (propylene glycol) (PPG) and poly (tetramethylene glycol) (PTMG). Typical polyesters are poly (caprolactone) (PCL) and poly (butylene adipate) (PBA). Polyurethane hard segments consist of diisocyanate linked by small-molecule chain extenders (HO-R -OH) such as 1,4, butanediol (BDO) or hexanediol (HDO) [15]. Kinetics Reaction kinetic affords information about material ageing, oxidation stability, lifetime of materials, storage time and optimization production without the need of time consuming measurements. The kinetic modeling of the decomposition plays a central role in many cases for an accurate prediction of the materials behavior under different conditions. Many of the methods of kinetic analysis are based on the hypothesis that from a simple thermogravimetric trace, meaning values may be obtained for parameters such as activation energy and pre-exponential factor. The isoconversional methods may be best known through their most popular representatives, the methods of Friedman, Ozawa, Flynn and Wall. The methods require performing a series of experiments at different temperature programs and yield the values of effective activation energy as function of conversion. The full potential of the isocoversional methods has been appreciated as Vyazovkin Model free kinetics. The methods take their origin in the single-step kinetic equation

3 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 355 (1) and make use of the isoconversional principle which states that at a constant extent of conversion, the reaction rate is only function of temperature so that (2) In Equation (1) and (2), A and E a are Arrhenius parameters (the pre-exponential factor and the activation energy), respectively f(α) reaction model, R the gas constant, T temperature, t time, and α the extent of conversion, which can be determined from TGA runs as a fractional mass loss or from DSC runs as a fractional heat release [19]. The most popular isoconversional method is by Flynn, Wall [20] and Ozawa [2] given as (3) where β are heating rates, T the start of temperature of degradation, E α activation energy at conversion. These equations allow determination of different reaction rates related to the same conversion. Another from kinetic methods applying multiple heating rates for characterization of decomposition process is the Kissinger method. In this method it is assumed that the temperature of maximum deflection in DTG (derivative thermogravimetry) is also the temperature at which the reaction rate is of maximum value. The maximum rate occurs when the derivative of Eq. (4) is zero. Kissinger s method considers the maximum temperatures T m of the first derivative weight loss curves. The final expression is: (4) Therefore, the activation energy can be determined from a plot In (β/ T 2 m ) against 1/T m. This expression is also given by the American Society for Testing and Materials (ASTM) (E698-79, Appendix X3, Philadelphia, PA, 1979). The kinetics methods have limitations, like an extrapolation of the time-temperature plot, it has an approach that the reaction order is 1 and use of the onset temperature in the calculations is more susceptible to changes with the experimental conditions than the temperature at the maximum of decomposition [20]. Due to the latter mentioned facts we have used in this paper two methods for evaluation of decomposition kinetics. EXPERIMENTAL MATERIALS Tested materials were commercially available polyurethane foams Molitan grade of various types (Gumotex Brec1av, Czech Republic). Samples assigned as PUR 1-5, PUR 7 and PUR 8 were of polyether type resin, sample PUR 6 polyester type of resin. All studied PUR materials were characterized by FTIR spectroscopy. This method allows comfortable distinction between ether type and ester type of PUR resin due to the presence of a sharp peak at 1720 cm -1 wavenumber typical for C=O functional groups present in ester types of PUR resins. IR peak at 1100 cm -1 is in contrary typical for ether functional groups. Studied PUR cellular materials are used mainly for application as thermal and sound damping

4 356 Barbora Lapcikova et al. insulations. They are provided in a wide range of pore sizes and densities. In Table 1 is given labeling of studied samples with their basic physico-chemical and material characteristics [16,17]. METHODS Samples were prepared by cutting of cylinders 5mm wide in a diameter and were replacement to alumina pans before measurements. TG and DTA experiments were performed on simultaneous DTA-TG apparatus (Shimadzu DTG 60, Japan). Throughout the experiment, the sample temperature and weight-heat flow changes were continuously monitored. Conditions of measurement: Heat flow 10 C/min and dynamic atmosphere of nitrogen (N 2-50ml/min), range of temperature measurement was from 40 C to 500 C. For kinetics studies following heating rates were applied: 2, 5, 10, 15, 20 C/min at the same temperature range and atmosphere. FTIR spectra were recorded on FTIR PC (Shimadzu, Japan) by means of ATR technique (ZnSe multiple reflections crystal setup). Measurement was performed in the 4200 to 350 cm -1 wavenumber range. Microscopic measurements were performed on Eclipse 50i microscope (Nikon, Japan) at magnification zoom ranging from 40 to 400, for image processing and pore size determination program Image J - Image Processing and Analysis in Java (National Institutes of Health, USA) was used. RESULTS AND DISCUSSION Thermal stability and degradation of polyurethane foams were measured by means of the TG/DTA technique at the experimental conditions given above. Two types of PUR matrix resins were used, polyether and polyester types (for sample details see Table 1). TG summary TABLE 1. Labeling of Studied Samples with Physical-chemical, Material Characteristics. Volume Volume Pressing Tensile Permanent mass mass resistance Strength Elongation deformation Type of Sample [kg/m 3 ] [kg/m 3 ] [kpa] [kpa] [%] [%] foams labelling Brutto netto (40%) (40%) average [50%, 23 C, 72 h] S 3535 F PUR CMHR 3532 PUR H 2550 PUR N 2529 PUR H 3070 PUR HR 3532 PUR H 4055 PUR N 2227 PUR

5 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 357 Fig. 1. Thermogravimetric analysis of polyurethane foams under nitrogen atmosphere, samples 1-8 (for data see Table 3) plot of all studied samples in the temperature range from +40 to +500 C is shown in Figure 1. A typical two-step decomposition curves were obtained. Weight loss pattern of decomposition process indicated individual component material composition of cellular PUR matrix of the studied material. Studied material was stable up to +180 C followed by two step decomposition process. First decomposition step was observed at the temperature range from +180 C up to +310 C, relative weight loss for each sample under study is given in Table 2. This decomposition process reflects decomposition of diisocyantes and polyols [17,19] present in the PUR matrix evolving mostly CO and HCN in the form of gases [18,22]. Second decomposition step ascribed to the TABLE 2. Thermogravimetry Measurements Data, Weight Loss at Selected Temperature Range for 1. Step of Decomposition Kinetics Pattern. 1.step Sample Start [ C] End [ C] Onset Weight loss [%] PUR PUR PUR PUR PUR PUR PUR PUR decomposition of organic matter is observed in the temperature range from +310 C to +450 C

6 358 Barbora Lapcikova et al. TABLE 3. Thermogravimetry Measurements Data. Weight Loss at Selected Temperature Range for 2. Step of Decomposition Kinetics Pattern. 2. step Sample Start [ C] End [ C] Onset Weight loss [%] PUR PUR PUR PUR PUR PUR PUR PUR (see Table 3). DTA measurements were performed simultaneously with TG/DTA for each sample. There was obtained for majority of samples an exothermic peak at approximately +295 C and +423 C corresponding to the thermal decomposition process (see Table 4). This peak was not found for samples PUR 6, 7 and 8 (for 1. step) due to the presence of fire retardants. Weight loss pattern obtained for PUR 6 sample (polyester type resin) was different from ones obtained for polyether type of resign (samples PUR 1-5, PUR 7 and PUR 8) due to different chemical composition of the base polymer matrix. This was confirmed by TABLE 4. DTA peaks ascribed to polyurethane foams. Sample Process Peak Process Peak ( C) ( C) PUR 1 Exo 232 Exo 365 PUR 2 Exo 267 Exo 346 PUR 3 Exo 269 Exo 344 PUR 4 Exo 269 Exo 398 PUR 5 Exo 259 Exo 332 PUR 6 Endo 295 Endo 423 PUR 7 Endo 271 Endo 378 PUR 8 Endo 280 Exo 422 Fig. 2. Thermal degradation of PUR 8 for different heating rates: 2, 5, 10, 15, 20 C/min.

7 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 359 IR measurements by the presence of vibrations peak in the wavenumber 1100 cm -1 typical for ether type of PUR resins. For ester type resin a typical sharp peak at wavenumber 1720cm -1 was found, reflecting vibrations of C = O group. In detail decomposition kinetic study of sample PUR 8 for different heating rates (2, 5, 10, 15, 20 C/min) was performed under nitrogen atmosphere in the temperature range of +40 C to +500 C. There was obtained two steps decomposition process pattern (Fig. 2). Formal decomposition kinetic parameters were calculated according to the Ozawa, Flynn and Wall (OFW) method [2,4] described by the equation(3) as well as by the Kissinger method eq. (4) for the comparison (from TGA data). Activation energy was calculated from Ozawa s plots of log β vs. 1/T for selected constant values of conversion as illustrated in Fig.3 for sample PUR 8 (its 1. step decomposition part), while Fig. 3. Linear plot for thermal degradation of PUR 8, 1 step by the method OFW. in Fig 4 the 2. step data are shown. Both calculated decomposition patterns where characteristic with parallel lines where the tangent was directly proportional to the activation energy of the decomposition process. Calculated activation energy E α of sample PUR 8 according to the OFW method for the 1. step part found to be kj/mol. By means of the Kissinger s method it was found to be 169 kj/mol. The latter activation energy data are in an agreement with results found by other authors [10, 18, 20, 21]. Obtained absolute value of the activation energy is typical for degradation of polyols component of the complex PUR matrix (for the 1. step ). Second

8 360 Barbora Lapcikova et al. Fig. 4. Linear plot for thermal degradation of PUR 8, 2. step by the method Kissinger. step of PUR decomposition kinetics was ascribed to the degradation of total organic matter (for activation energy data see Table 5). Fig. 5 and Fig.6 show plots of decomposition conversion versus temperature for selected of heating rates. Simultaneously with increasing Temperature Fig. 5. Conversional curve for PUR 8, 1. step.

9 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 361 Fig. 6. Linear plot for thermal degradation of PUR 8, 2. step by the method OFW. TABLE 5. Values of Activation Energy Calculation from Variety of Kinetics Model. Decomposition Activation energy Ea (kj/mol) kinetic model 1. step 2. step Ozawa-Flynn- Wall Kissinger heating rate increases decomposition rate (for PUR 8) as reflected by change of tangent in inflection point conversion curves. It is a typical non-linear autocatalytic S shape pattern with initiation period. With increasing heating temperature rate it is more steep dependency, reflecting higher decomposition rate. This behavior was found both for the 1. step as well as 2. step decomposition kinetic data (see Figs. 5 and 6). According to the Kissinger method, the data for calculation of the activation energy E a are processed as the tangent of the linear plot of In 2 β/t m versus 1/T m (K -1 ), Figs 7 and 8. The latter are obtained from DTG curves, where T m is the position of the peak temperature for different heating rate [5]. Calculated value E a very well corresponds with data of the the OFW method (Table 5). Determined kinetics equations describing the material weight loss were used by some authors for determination of lifetime prediction of polymers and their thermal stability [20,22,23]. Plot E a =f(α) was not constant in the studied range of conversions, the process of decomposition was complex and consisted of several reaction steps. Observed increasing activation energy with conversion indicated appearance of parallel reactions and random decomposition processes [21, 25] in the complex PUR matrix decomposition.

10 362 Barbora Lapcikova et al. Fig. 7. Linear plot for thermal degradation of PUR 8, 1. step by the method Kissinger. E a (kj/mol) In(beta)T m 2 1/T m (K 1 ) conversion (%) Fig. 8. E a versus conversion for degradation PUR 8, 1. and 2. step in N 2.

11 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 363 Fig. 9. Coversional curve for PUR 8, 2. step. Due to the fact, that the obtained weight loss by means of TG analysis is sensitive on density of studied materials (in our case on pore size distribution of the studied cellular PUR matrices) an optical microscopic characterization was performed. With increasing decomposition temperature the PUR matrix volume is decreased with simultaneous creation of overheated low viscosity flammable liquid of polyols [26]. Determined mean pore sizes of studied materials are summarized in Table 6 and visualized in Figure 10. It was ranging between 0.2 to 0.3 mm 2. This is an optimal pore size distribution for using as building insulation or furniture. CONCLUSIONS There was studied thermal stability and thermal decomposition of commercial PUR foams used TABLE 6. Frequency Pore Size (mm) 2 for Set of PUR Foams under Study. Range of measurement Sample Pore size [mm 2 ] mm 2 mm 2 PUR PUR PUR PUR PUR PUR PUR PUR both in building construction industry as well as in special applications (Gumotex Breclav,

12 364 Barbora Lapcikova et al. Sample PUR 1 Sample PUR 2 Sample PUR 3 Sample PUR 4 Sample PUR 5 Sample PUR 6 Sample PUR 7 Sample PUR 8 Sample PUR 9 Sample PUR 10 Fig. 10. Microscopic photographs of studied PUR foams.

13 TG and DTG Study of Decomposition of Commercial PUR Cellular Materials 365 JSC, Czech Republic). Pyrolysis of PUR foams was measured by TG/DTA technique. There were observed two steps decomposition process patterns typical for PUR systems. Materials under study were stable up to +180 C temperature, consequently were decomposed to polyols and disocyanate random degradation residues. Thermogravimetric measurements were used to calculate formal kinetic parameters of degradation of studied PUR cellular matrices. Obtained kinetic data were numerically processed by means of OFW and Kissinger kinetic models. Calculated activation energies of the degradation 1. step were found to be kj/mol and 169 kj/mol, for the 2. step 162 kj/mol and 169 kj/mol. ACKNOWLEDGEMENT This article was created with support of Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF) and national budget of Czech Republic within the framework of the Centre of Polymer Systems project (reg. number CZ. 1.05/ / ). REFERENCES 1. Y-H Hu., Ch-Z. Chen and Ch. Ch. Wang Polym Degrad Stab 84 (2004) T. Ozawa Bull Chem. Soc Japan 38 (1965) S. Vyazovkin, C.A. Wight Thermochimica Acta (1999); : J. H. Flynn, L.A. Wall, General treatment of the thermogravimetry of polymers. J Res Natl Bureau Standards, Section A: Physics and Chemistry 70A(6) (1966); H. E. Kissinger Anal Chem 29 (1957) W. Groenwoud Characterization of Polymers by Thermal Analysis. Amsterdam: Elsevier, M. Ravey, E. M. Pearce, J. Appl Polym Sci 63(1): (1997) J. Lefebvre, Y. Mamleev, M. Le Bras, S. Bourbigot, Polym Degrad Stab 88(1) (2005) R. Font, A. Fullana, J. A. Caballero, J. Candela, A. Garcia, J. Anal Applied Pyrolysis (2001); 58-59: C. Branca, C. Di Blasi, A. Casu, V. Morone, C. Costa, Termochimica Acta 399(1-2): (2003) L. Lapcik, Jr., V. Cetkovsky, B. Lapcikova, S. Vasut, Chem. Listy 94(2): (2000) V. Yocik, B. Lapcikova, L. Lapcik, Jr., R. Asuquo Plasty a Kaucuk 39(6): (2002) L. Lapcik, Jr., R. Asuquo, CZ Pat. (2005) F. S. Chuang, Polym Degrad Stab 2007; 92(7): R. P. Lattimer, M. J. Polce, C. Wesdemiotis, J Anal Applied Pyrolysis (1998) 48(1): < ]. 17. < ]. 18. H. Hatakeyema, N. Tanamachi, H. Matsumura, S. Hirose, T. Hatakeyama,Thermochimica Acta 431(1-2): (2005) S. Vyazovkin, N. Sbirrazzuoli, Macromol Rapid Commun 27(18): (2006) L. G. Lage, Y. Kawano, J Appl Polym Sci 79(5): (2001) Y. Zhang, Z. Xia, H. Huang, H. Chen, Polymer Testing 28(3): (2009) M. Herrera, G. Matuschek, A. Kettrup, Polym Degrad Stab 78(2): (2002)

14 366 Barbora Lapcikova et al. 23. R. Bilbao, J. F. Mastral, J. Ceamanos, M. N. Aldea J Anal App PyroI 37(1): (1996) R. Ramani, S. Alam, Thermochimica Acta 511 (1-2): (2010) Vyazovkin S. Mettler Toledo User Com 1999; 2/ R. H. Kramer, M. Yammarano, G. T. Linteris, U. W. Gedde, J. W. Gilman, Polymer Degrad Stab 95(6): (2010) P. E. Sánchez-Jiménez, L. A. Pérez-Maqueda, A. Perejón, J. M. Criado, Polymer Degrad Stab 95(5): (2010) RECEIVED : 02 February 2011 ACCEPTED : 02 May 2011