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Thermal degradation behaviour of isotactic polypropylene blended with lignin
Polymer Degradation and Stability 91 (2006) 494e498 www.elsevier.com/locate/polydegstab
Thermal degradation behaviour of isotactic polypropylene blended with lignin Maurizio Canetti*, Fabio Bertini, Aurelio De Chirico, Guido Audisio Istituto per lo Studio delle Macromolecole, C.N.R., Via Bassini 15, 20133 Milano, Italy Received 20 October 2004; accepted 3 January 2005 Available online 4 October 2005
Abstract The influence of lignin on the thermal degradation of isotactic polypropylene, investigated by thermogravimetric analysis, is reported in this article. Polypropylene blends containing 5 and 15 wt% of lignin were prepared by mixing the components in a screw mixer. An increase in the thermal degradation temperature of the blends was observed as a function of lignin content, in both oxidative and non-oxidative conditions. The increase is noticeably marked for the experiments carried out in air atmosphere, where the interactions between the polypropylene and the lignin lead to the formation of a protective surface able to reduce the oxygen diffusion towards the polymer bulk. Morphological analyses were carried out with optical and electronic microscopy, to evaluate the degree of dispersion of the lignin in the polypropylene matrix. X-ray techniques were employed to study the influence of lignin on the structure of the blended polypropylene. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Degradation; Lignin; Polypropylene; Thermogravimetric analysis (TGA)
1. Introduction Lignin is an amorphous polyphenolic plant constituent, able to give a high amount of char when heated at high temperature in an inert atmosphere. This peculiarity is a basic aspect of flame retardant additives, since char reduces the combustion rate of polymeric materials. The thermal degradation behaviour of lignin is influenced by its origin and structure [1,2]. Some papers described the efficiency of pure lignin and lignin mixed with triglycidylisocyanurate and/or organic and inorganic phosphates, as flame retardant additives for polypropylene (PP) [3e5]. Furthermore, the antioxidant properties of lignin were utilized to stabilize PP composites against photoand thermo-oxidation [6e8]. In a previous paper we investigated on the effect of lignin on the nucleation, kinetics of crystallization and melting behaviour of isotactic PP [9]. During isothermal crystallization,
* Corresponding author. Tel.: C39 02 23699368; fax: C39 02 70636400. E-mail address: [email protected] (M. Canetti). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.01.052
the lignin acts as a nucleant and does not interfere with the spherulitic growth rate and the three-dimensional growth of crystalline units of PP. The rate of crystallization of PP is strongly enhanced by the nucleation action of lignin particles. The number of nuclei per unit volume calculated for PP/lignin in blends was higher by some orders of magnitude than the ones calculated for plain PP. It is well known that the addition of fillers can produce changes in the crystalline structure of PP, such as the formation of b-form crystal structure. The presence of b-form crystals can improve the mechanical properties of PP [10e12]. Dynamic mechanical analyses carried out on pure PP and a PP/lignin blend (95/5 wt/wt) showed no significant differences, while a small increase in storage modulus was observed for a PP/lignin blend (85/15 wt/wt) [5]. The main goal of the paper is to study the effect of lignin, and its content, on the thermal degradation behaviour of PP/ lignin blends in oxidative and inert atmospheres. The morphology and the supermolecular structure of the PP/lignin blends, prepared by melt mixing of the components, were also investigated.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
2. Experimental 2.1. Materials The materials used in preparing the binary blends were isotactic polypropylene, PP (Moplen FLP 20, Himont, Ferrara, Italy), with a melt flow index of 12 g/10 min at 230 C, and hydrolytic lignin (Aldrich, Milan, Italy, CAS n. 8072-93-3). The blends were prepared by melt mixing the components at 190 C for 10 min at 60 rpm, in a Brabender electronic plasticorder AEV 153 mixer, with PP/lignin weight ratios of 95/5 and 85/15. Pure PP processed under similar condition was investigated as reference material. On processing dry nitrogen was continuously purged into the mixing chamber to ensure minimum thermo-oxidative degradation. 2.2. Methods
495
crystallized at 135 C from the melt by means of POM. The micrographs show the lignin as a dispersion of particles having a diameter ranging from 0.5 to 6 mm, for both blends (Fig. 1). Quantitative analysis of POM images can be used to evaluate the lignin particle’s dimensions and distribution in the blends. In Fig. 2 the amount of particles is reported as a function of constant radius intervals equal to 0.3 mm. For both blends, the particles having a radius ranging from 0.2 to 1.1 mm account for more than 80%. Nevertheless, taking into account the particle volume (VP), calculated as percentage respect to total volume of the lignin present in the blend, the contribution of higher radius particles becomes more relevant (Fig. 3). Morphological analysis performed by SEM shows good interfacial adhesion between the lignin particles and the polymer matrix, in the blends. As an example, the photomicrograph of PP/lignin 85/15, reported in Fig. 4, shows the lignin particles intimately surrounded by the PP continuous phase.
Morphology of thin film samples isothermally crystallized from the melt was investigated by using a Reichert polarizing optical microscope (POM), equipped with a Mettler FP82 hot stage (precision G 0.2 C). For both blends, we captured 3e5 independent POM images and, by means of image analysis software (ImageJ) [13], calculated the particle size distribution. The particle volumes were calculated assuming spherical shape. Scanning electron microscopy (SEM) analysis was carried out on gold-coated samples fractured in liquid nitrogen, using a Philips instrument model 515, operated at 9.4 kV. The wide angle X-ray diffraction (WAXD) data were obtained at 20 C using a Siemens D-500 diffractometer equipped with a Siemens FK 60-10 2000 W tube (Cu Ka radiation, l Z 0.154 nm). The operating voltage and current were 40 kV and 40 mA, respectively. The data were collected from 5 to 40 2q at 0.02 2q intervals. Small-angle X-ray scattering (SAXS) measurements were conducted at 22 C with a Kratky Compact Camera. Monochromatized Cu Ka radiation (l Z 0.154 nm) was supplied by a stabilized Siemens Krystalloflex 710 generator and a Siemens FK 60-04, 1500 W Cu target tube operated at 40 kV and 25 mA. The scattered intensity was counted at 197 angles of measurement in the range 2q Z 0.1e3.0 , by using a step scanning proportional counter with pulse height discrimination. For all the SAXS measurements the abscissa variable was s Z 2 sin(q)/l. Thermogravimetric analyses (TGA) were performed on a PerkineElmer TGA-7 instrument with platinum pans, using 5e6 mg of material as sample. The samples were heated at 10 C/min in air or nitrogen atmosphere with a flow rate of 40 ml/min. TGA and derivative thermogravimetry (DTG) curves were recorded from 50 to 600 C. 3. Results and discussion 3.1. Morphology The dispersion of lignin in the blends was visualized carrying out a morphological analysis on the samples isothermally
Fig. 1. Polarizing optical photomicrograph of (a) PP/lignin 95/5, and (b) PP/ lignin 85/15, blends. The marked line drawn on photo (a) corresponds to 10 mm.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
496 50 45
Particles ( )
40 35 30 25 20 15 10 5 2.9-3.2
2.6-2.9
2.3-2.6
2.0-2.3
1.7-2.0
1.4-1.7
1.1-1.4
0.8-1.1
0.5-0.8
0.2-0.5
0
particle radius (µm) Fig. 2. Number of lignin particles vs. radius of lignin particle for (-) PP/lignin 95/5, and (,) PP/lignin 85/15, blends. Fig. 4. Scanning electron photomicrograph of the PP/lignin 85/15 blend.
3.2. Structure In the presence of lignin, PP can simultaneously crystallize into a and b crystalline forms. The ratio between the a- and b-form is influenced by the amount of lignin in the PP/lignin blend and by the crystallization conditions [9]. The samples of pure PP and PP/lignin blends crystallized from the melt either in isothermal condition at different crystallization temperatures (Tc), or by cooling at 5 C/min from 200 to 50 C, were submitted to WAXD and SAXS analyses. Pure PP shows the typical WAXD profile for the a crystalline form (Fig. 5). The pattern of the PP/lignin 85/15 samples shows the presence of the diffraction at 16 2q , typical of the crystalline b-form of isotactic PP. The b-form fraction (bf), calculated according to the relation proposed by Turner-Jones et al. [14] and reported in Fig. 5, is strongly influenced by the thermal treatment. The amount of b-form in the PP/lignin blends, increases with an increase in the cooling rate for the samples crystallized from the melt in non-isothermal conditions. Moreover, the isothermally crystallized PP/lignin 85/15 samples, present an increase in the b-form fraction
with a decrease in the Tc value. These findings suggest that faster crystallization rates, promote the formation of the b-form during the crystallization process. From the SAXS measurements, it is possible to determine the long period (L), defined as the distance between two adjacent lamellae. The L values were calculated from the maximum of Lorentz corrected SAXS profiles and reported in Fig. 6. L values between 15 and 16 nm were obtained for the pure PP samples crystallized either in isothermal or in non-isothermal conditions and for the PP/lignin 85/15 blend, crystallized in non-isothermal conditions. The PP/lignin 85/ 15 isothermally crystallized at Tc Z 124 C, showed an L value of 23 nm. The increase in the L value is related to the presence of the b-form in the blend [15].
18
e
βf = 0.72
d
βf = 0.16
c
βf = 0.39
b
βf = 0.09
a
βf = 0.00
I (a.u.)
16 14
)
12
VP (
10 8 6 4 2 2.9-3.2
2.6-2.9
2.3-2.6
2.0-2.3
1.7-2.0
1.4-1.7
1.1-1.4
0.8-1.1
0.5-0.8
0.2-0.5
0
particle radius (µm) Fig. 3. Volume of lignin particles (VP) vs. radius of lignin particle for (-) PP/lignin 95/5, and (,) PP/lignin 85/15, blends.
10
15
20
25
30
2θ° Fig. 5. Wide angle X-ray diffractograms of (a) pure PP cooled at 5 C/min; (b) PP/lignin 85/15 cooled at 5 C/min; (c) PP/lignin 85/15 cooled at 20 C/min; (d) PP/lignin 85/15, Tc Z 137 C; (e) PP/lignin 85/15, Tc Z 124 C.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
a
497
100
d
90 80 70
Weight ( )
I (s) s2 (a.u.)
L = 22.8 nm
c L = 15.8 nm
60 50 40 30 20 10 0
L = 16.2 nm
200
b
400
500
600
500
600
Temperature (°C) L = 15.1 nm
0.08
0.12
0.16
29
0.20
s (nm-1) Fig. 6. Small-angle X-ray scattering plots of (a) pure PP cooled at 5 C/min; (b) pure PP Tc Z 124 C; (c) PP/lignin 85/15 cooled at 5 C/min; (d) PP/lignin 85/15 Tc Z 124 C.
3.3. Thermal degradation The effect of lignin’s presence and content on the stability of the blends was studied by means of thermogravimetry experiments carried out in both inert and oxidative conditions. Table 1 summarizes the TGA and DTG experimental data including the temperature corresponding to initial 2% of weight loss (Ti), the temperature of maximum rate of weight loss (Tmr) and the char yield obtained at different temperatures. For the experiments conducted in nitrogen, the residue (R) was calculated at 550 C, corresponding to the final temperature of degradation; while for the ones carried out in air, the R value was calculated at the end of the first step of the oxidative degradation (450 C), i.e. before the char oxidation step. The TGA and DTG curves, obtained under nitrogen atmosphere, of pure PP, lignin and PP/lignin blends are shown in Fig. 7. The pure PP volatilises in a single step, from 280 to 500 C, without char formation. The thermal behaviour of lignin is completely different. On heating at 10 C/min, the lignin begins to degrade at lower temperatures (about 200 C) and produces a remarkable char residue. As depicted in Fig. 7, under nitrogen, the blends PP/lignin 95/5 and PP/lignin 85/15 show slight differences in stability. In general, the thermal degradation temperature and the char yield increase with an increase in the amount of lignin in the blend. Fig. 8 reports the TGA and DTG curves, obtained under air atmosphere, of pure PP, lignin and PP/lignin blends. The thermo-oxidative degradation of pure PP takes place in one stage from 200 to 400 C. The TGA and DTG thermograms are simply shifted towards lower temperatures comparing with the ones obtained under inert atmosphere. The DTG curve of lignin in air shows two broad peaks indicating two degradation stages. Above 200 C the lignin is subjected to a slow weight loss to form a 50 wt% char residue at 415 C, which is fully oxidized between 430 and 540 C. Analogously,
24
min-1)
0.04
b
-dW/dt (
a 0.00
300
19 14 9 4 -1 200
300
400
Temperature (°C) Fig. 7. TGA (a) and DTG (b) thermograms under nitrogen atmosphere for (C) pure PP; (-) PP/lignin 95/5; (:) PP/lignin 85/15; (A) pure lignin.
the PP/lignin blends are observed to degrade in two distinct stages. The TGA and DTG curves progressively shift towards the higher temperature with an increase in the amount of lignin in the sample. The second degradation step, occurring above 450 C, corresponds to the oxidation of the char residue formed during the first degradation stage. The Tmr values, reported in Table 1 for lignin and PP/lignin blends, refer to the first degradation step. As regard to the thermo-oxidative experiments, it is interesting to note that the presence of relatively small amounts of lignin is able to increase the Tmr value from 357 C for pure PP to 393 and 422 C for the blends PP/lignin 95/5 and PP/lignin 85/15, respectively. Theoretical TGA and Table 1 Thermogravimetric data for PP, lignin and PP/lignin blends Atmosphere
Ti ( C)
Tmr ( C)
R (wt%)
PP PP/lignin 95/5 PP/lignin 85/15 Lignin
N2 N2 N2 N2
366 359 310 207
475 479 490 374
0a 2.0a 6.3a 43.1a
PP PP/lignin 95/5 PP/lignin (85/15) Lignin
Air Air Air Air
257 280 292 205
357 393 422 356
0.8b 2.9b 7.9b 37.9b
a b
Value calculated at 550 C. Value calculated at 450 C.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
498
a
100
100
90
90
80
80
70
70
Weight ( )
Weight ( )
a
60 50 40 30
60 50 40 30
20
20
10
10
0 200
300
400
500
0
600
200
Temperature (°C)
b
300
400
500
600
500
600
Temperature (°C)
b
19
19 15
min-1)
14
9
-dW/dt (
-dW/dt (
min-1)
17
4
13 11 9 7 5 3
-1 200
1 300
400
500
600
Temperature (°C) Fig. 8. TGA (a) and DTG (b) thermograms under air atmosphere for (C) pure PP; (-) PP/lignin 95/5; (:) PP/lignin 85/15; (A) pure lignin.
relative DTG curves were calculated for PP/lignin 85/15 blend from individual weight loss data of pure components. To emphasize the synergistic effect obtained by blending the components, the theoretical TGA and DTG curves were compared with the experimental ones and shown in Fig. 9. The marked differences observed between theoretical and experimental curves, are due to the complex interactions occurring by blending PP and lignin. 4. Conclusions In this work PP/lignin blends were prepared by melt mixing and their morphology and thermal degradation behaviour were investigated. Morphological analyses show good dispersion and good adhesion between the lignin particles and the PP matrix, in the blends. In the presence of lignin, PP can crystallize into two crystalline forms, a and b. The b-form fraction, is strongly influenced by the thermal treatment. The formation of the b-form during the crystallization process was promoted by an increase in the crystallization rate. The lignin present in the blend is able to produce a high char yield that is responsible to the increase in the blend temperature degradation. The char is a carbon-based residue that undergoes slow oxidative degradation. The increase is more pronounced for the experiments carried out in air atmosphere, where, the interactions between the PP and the charring lignin
-1 200
300
400
Temperature (°C) Fig. 9. TGA (a) and DTG (b) for PP/lignin 85/15 under air atmosphere (-) experimental thermograms; (C) theoretical thermograms calculated from individual weight loss data of pure components.
lead to the formation of a protective surface shield able to reduce the oxygen diffusion towards the polymer bulk. References [1] Li J, Li B, Zhang X, Su R. Polym Degrad Stab 2001;72:493e8. [2] Li J, Li B, Zhang X. Polym Degrad Stab 2002;78:279e85. [3] De Chirico A, Audisio G, Provasoli F, Giacometti Schieroni A, Focher B, Grossi B. Angew Makromol Chem 1995;228:51e8. [4] Gallina G, Bravi E, Badalucco C, Audisio G, Armanini M, De Chirico A, et al. Fire Mater 1998;22:15e8. [5] De Chirico A, Armanini M, Chini P, Cioccolo G, Provasoli F, Audisio G. Polym Degrad Stab 2003;79:139e45. [6] Kosikova B, Demianova V, Kakurakova M. J Appl Polym Sci 1993;47:1065e73. [7] Kosikova B, Miklesova K, Demianova V. Eur Polym J 1993;29:1495e7. [8] Pouteau C, Dole P, Cathala B, Averous L, Boquillon N. Polym Degrad Stab 2003;81:9e18. [9] Canetti M, De Chirico A, Audisio G. J Appl Polym Sci 2004;91:1435e42. [10] Tjong SC, Shen JS, Li KKY. Scripta Metall Mater 1995;33:503e8. [11] Labour T, Gauthier C, Seguela R, Vigier G, Bomal Y, Orange G. Polymer 2001;42:7127e35. [12] Labour T, Vigier G, Seguela R, Gauthier C, Orange G, Bomal Y. J Polym Sci Polym Phys 2002;40:31e42. [13] http://rsb.info.nih.gov/ij/. [14] Turner-Jones A, Aizlewood JM, Beckett DR. Makromol Chem 1964;75:134e58. [15] Torre FJ, Cortazar MM, Gomez MA, Ellis G, Marco C. Polymer 2003;44:5209e17.
Thermal degradation behaviour of isotactic polypropylene blended with lignin Maurizio Canetti*, Fabio Bertini, Aurelio De Chirico, Guido Audisio Istituto per lo Studio delle Macromolecole, C.N.R., Via Bassini 15, 20133 Milano, Italy Received 20 October 2004; accepted 3 January 2005 Available online 4 October 2005
Abstract The influence of lignin on the thermal degradation of isotactic polypropylene, investigated by thermogravimetric analysis, is reported in this article. Polypropylene blends containing 5 and 15 wt% of lignin were prepared by mixing the components in a screw mixer. An increase in the thermal degradation temperature of the blends was observed as a function of lignin content, in both oxidative and non-oxidative conditions. The increase is noticeably marked for the experiments carried out in air atmosphere, where the interactions between the polypropylene and the lignin lead to the formation of a protective surface able to reduce the oxygen diffusion towards the polymer bulk. Morphological analyses were carried out with optical and electronic microscopy, to evaluate the degree of dispersion of the lignin in the polypropylene matrix. X-ray techniques were employed to study the influence of lignin on the structure of the blended polypropylene. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Degradation; Lignin; Polypropylene; Thermogravimetric analysis (TGA)
1. Introduction Lignin is an amorphous polyphenolic plant constituent, able to give a high amount of char when heated at high temperature in an inert atmosphere. This peculiarity is a basic aspect of flame retardant additives, since char reduces the combustion rate of polymeric materials. The thermal degradation behaviour of lignin is influenced by its origin and structure [1,2]. Some papers described the efficiency of pure lignin and lignin mixed with triglycidylisocyanurate and/or organic and inorganic phosphates, as flame retardant additives for polypropylene (PP) [3e5]. Furthermore, the antioxidant properties of lignin were utilized to stabilize PP composites against photoand thermo-oxidation [6e8]. In a previous paper we investigated on the effect of lignin on the nucleation, kinetics of crystallization and melting behaviour of isotactic PP [9]. During isothermal crystallization,
* Corresponding author. Tel.: C39 02 23699368; fax: C39 02 70636400. E-mail address: [email protected] (M. Canetti). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.01.052
the lignin acts as a nucleant and does not interfere with the spherulitic growth rate and the three-dimensional growth of crystalline units of PP. The rate of crystallization of PP is strongly enhanced by the nucleation action of lignin particles. The number of nuclei per unit volume calculated for PP/lignin in blends was higher by some orders of magnitude than the ones calculated for plain PP. It is well known that the addition of fillers can produce changes in the crystalline structure of PP, such as the formation of b-form crystal structure. The presence of b-form crystals can improve the mechanical properties of PP [10e12]. Dynamic mechanical analyses carried out on pure PP and a PP/lignin blend (95/5 wt/wt) showed no significant differences, while a small increase in storage modulus was observed for a PP/lignin blend (85/15 wt/wt) [5]. The main goal of the paper is to study the effect of lignin, and its content, on the thermal degradation behaviour of PP/ lignin blends in oxidative and inert atmospheres. The morphology and the supermolecular structure of the PP/lignin blends, prepared by melt mixing of the components, were also investigated.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
2. Experimental 2.1. Materials The materials used in preparing the binary blends were isotactic polypropylene, PP (Moplen FLP 20, Himont, Ferrara, Italy), with a melt flow index of 12 g/10 min at 230 C, and hydrolytic lignin (Aldrich, Milan, Italy, CAS n. 8072-93-3). The blends were prepared by melt mixing the components at 190 C for 10 min at 60 rpm, in a Brabender electronic plasticorder AEV 153 mixer, with PP/lignin weight ratios of 95/5 and 85/15. Pure PP processed under similar condition was investigated as reference material. On processing dry nitrogen was continuously purged into the mixing chamber to ensure minimum thermo-oxidative degradation. 2.2. Methods
495
crystallized at 135 C from the melt by means of POM. The micrographs show the lignin as a dispersion of particles having a diameter ranging from 0.5 to 6 mm, for both blends (Fig. 1). Quantitative analysis of POM images can be used to evaluate the lignin particle’s dimensions and distribution in the blends. In Fig. 2 the amount of particles is reported as a function of constant radius intervals equal to 0.3 mm. For both blends, the particles having a radius ranging from 0.2 to 1.1 mm account for more than 80%. Nevertheless, taking into account the particle volume (VP), calculated as percentage respect to total volume of the lignin present in the blend, the contribution of higher radius particles becomes more relevant (Fig. 3). Morphological analysis performed by SEM shows good interfacial adhesion between the lignin particles and the polymer matrix, in the blends. As an example, the photomicrograph of PP/lignin 85/15, reported in Fig. 4, shows the lignin particles intimately surrounded by the PP continuous phase.
Morphology of thin film samples isothermally crystallized from the melt was investigated by using a Reichert polarizing optical microscope (POM), equipped with a Mettler FP82 hot stage (precision G 0.2 C). For both blends, we captured 3e5 independent POM images and, by means of image analysis software (ImageJ) [13], calculated the particle size distribution. The particle volumes were calculated assuming spherical shape. Scanning electron microscopy (SEM) analysis was carried out on gold-coated samples fractured in liquid nitrogen, using a Philips instrument model 515, operated at 9.4 kV. The wide angle X-ray diffraction (WAXD) data were obtained at 20 C using a Siemens D-500 diffractometer equipped with a Siemens FK 60-10 2000 W tube (Cu Ka radiation, l Z 0.154 nm). The operating voltage and current were 40 kV and 40 mA, respectively. The data were collected from 5 to 40 2q at 0.02 2q intervals. Small-angle X-ray scattering (SAXS) measurements were conducted at 22 C with a Kratky Compact Camera. Monochromatized Cu Ka radiation (l Z 0.154 nm) was supplied by a stabilized Siemens Krystalloflex 710 generator and a Siemens FK 60-04, 1500 W Cu target tube operated at 40 kV and 25 mA. The scattered intensity was counted at 197 angles of measurement in the range 2q Z 0.1e3.0 , by using a step scanning proportional counter with pulse height discrimination. For all the SAXS measurements the abscissa variable was s Z 2 sin(q)/l. Thermogravimetric analyses (TGA) were performed on a PerkineElmer TGA-7 instrument with platinum pans, using 5e6 mg of material as sample. The samples were heated at 10 C/min in air or nitrogen atmosphere with a flow rate of 40 ml/min. TGA and derivative thermogravimetry (DTG) curves were recorded from 50 to 600 C. 3. Results and discussion 3.1. Morphology The dispersion of lignin in the blends was visualized carrying out a morphological analysis on the samples isothermally
Fig. 1. Polarizing optical photomicrograph of (a) PP/lignin 95/5, and (b) PP/ lignin 85/15, blends. The marked line drawn on photo (a) corresponds to 10 mm.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
496 50 45
Particles ( )
40 35 30 25 20 15 10 5 2.9-3.2
2.6-2.9
2.3-2.6
2.0-2.3
1.7-2.0
1.4-1.7
1.1-1.4
0.8-1.1
0.5-0.8
0.2-0.5
0
particle radius (µm) Fig. 2. Number of lignin particles vs. radius of lignin particle for (-) PP/lignin 95/5, and (,) PP/lignin 85/15, blends. Fig. 4. Scanning electron photomicrograph of the PP/lignin 85/15 blend.
3.2. Structure In the presence of lignin, PP can simultaneously crystallize into a and b crystalline forms. The ratio between the a- and b-form is influenced by the amount of lignin in the PP/lignin blend and by the crystallization conditions [9]. The samples of pure PP and PP/lignin blends crystallized from the melt either in isothermal condition at different crystallization temperatures (Tc), or by cooling at 5 C/min from 200 to 50 C, were submitted to WAXD and SAXS analyses. Pure PP shows the typical WAXD profile for the a crystalline form (Fig. 5). The pattern of the PP/lignin 85/15 samples shows the presence of the diffraction at 16 2q , typical of the crystalline b-form of isotactic PP. The b-form fraction (bf), calculated according to the relation proposed by Turner-Jones et al. [14] and reported in Fig. 5, is strongly influenced by the thermal treatment. The amount of b-form in the PP/lignin blends, increases with an increase in the cooling rate for the samples crystallized from the melt in non-isothermal conditions. Moreover, the isothermally crystallized PP/lignin 85/15 samples, present an increase in the b-form fraction
with a decrease in the Tc value. These findings suggest that faster crystallization rates, promote the formation of the b-form during the crystallization process. From the SAXS measurements, it is possible to determine the long period (L), defined as the distance between two adjacent lamellae. The L values were calculated from the maximum of Lorentz corrected SAXS profiles and reported in Fig. 6. L values between 15 and 16 nm were obtained for the pure PP samples crystallized either in isothermal or in non-isothermal conditions and for the PP/lignin 85/15 blend, crystallized in non-isothermal conditions. The PP/lignin 85/ 15 isothermally crystallized at Tc Z 124 C, showed an L value of 23 nm. The increase in the L value is related to the presence of the b-form in the blend [15].
18
e
βf = 0.72
d
βf = 0.16
c
βf = 0.39
b
βf = 0.09
a
βf = 0.00
I (a.u.)
16 14
)
12
VP (
10 8 6 4 2 2.9-3.2
2.6-2.9
2.3-2.6
2.0-2.3
1.7-2.0
1.4-1.7
1.1-1.4
0.8-1.1
0.5-0.8
0.2-0.5
0
particle radius (µm) Fig. 3. Volume of lignin particles (VP) vs. radius of lignin particle for (-) PP/lignin 95/5, and (,) PP/lignin 85/15, blends.
10
15
20
25
30
2θ° Fig. 5. Wide angle X-ray diffractograms of (a) pure PP cooled at 5 C/min; (b) PP/lignin 85/15 cooled at 5 C/min; (c) PP/lignin 85/15 cooled at 20 C/min; (d) PP/lignin 85/15, Tc Z 137 C; (e) PP/lignin 85/15, Tc Z 124 C.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
a
497
100
d
90 80 70
Weight ( )
I (s) s2 (a.u.)
L = 22.8 nm
c L = 15.8 nm
60 50 40 30 20 10 0
L = 16.2 nm
200
b
400
500
600
500
600
Temperature (°C) L = 15.1 nm
0.08
0.12
0.16
29
0.20
s (nm-1) Fig. 6. Small-angle X-ray scattering plots of (a) pure PP cooled at 5 C/min; (b) pure PP Tc Z 124 C; (c) PP/lignin 85/15 cooled at 5 C/min; (d) PP/lignin 85/15 Tc Z 124 C.
3.3. Thermal degradation The effect of lignin’s presence and content on the stability of the blends was studied by means of thermogravimetry experiments carried out in both inert and oxidative conditions. Table 1 summarizes the TGA and DTG experimental data including the temperature corresponding to initial 2% of weight loss (Ti), the temperature of maximum rate of weight loss (Tmr) and the char yield obtained at different temperatures. For the experiments conducted in nitrogen, the residue (R) was calculated at 550 C, corresponding to the final temperature of degradation; while for the ones carried out in air, the R value was calculated at the end of the first step of the oxidative degradation (450 C), i.e. before the char oxidation step. The TGA and DTG curves, obtained under nitrogen atmosphere, of pure PP, lignin and PP/lignin blends are shown in Fig. 7. The pure PP volatilises in a single step, from 280 to 500 C, without char formation. The thermal behaviour of lignin is completely different. On heating at 10 C/min, the lignin begins to degrade at lower temperatures (about 200 C) and produces a remarkable char residue. As depicted in Fig. 7, under nitrogen, the blends PP/lignin 95/5 and PP/lignin 85/15 show slight differences in stability. In general, the thermal degradation temperature and the char yield increase with an increase in the amount of lignin in the blend. Fig. 8 reports the TGA and DTG curves, obtained under air atmosphere, of pure PP, lignin and PP/lignin blends. The thermo-oxidative degradation of pure PP takes place in one stage from 200 to 400 C. The TGA and DTG thermograms are simply shifted towards lower temperatures comparing with the ones obtained under inert atmosphere. The DTG curve of lignin in air shows two broad peaks indicating two degradation stages. Above 200 C the lignin is subjected to a slow weight loss to form a 50 wt% char residue at 415 C, which is fully oxidized between 430 and 540 C. Analogously,
24
min-1)
0.04
b
-dW/dt (
a 0.00
300
19 14 9 4 -1 200
300
400
Temperature (°C) Fig. 7. TGA (a) and DTG (b) thermograms under nitrogen atmosphere for (C) pure PP; (-) PP/lignin 95/5; (:) PP/lignin 85/15; (A) pure lignin.
the PP/lignin blends are observed to degrade in two distinct stages. The TGA and DTG curves progressively shift towards the higher temperature with an increase in the amount of lignin in the sample. The second degradation step, occurring above 450 C, corresponds to the oxidation of the char residue formed during the first degradation stage. The Tmr values, reported in Table 1 for lignin and PP/lignin blends, refer to the first degradation step. As regard to the thermo-oxidative experiments, it is interesting to note that the presence of relatively small amounts of lignin is able to increase the Tmr value from 357 C for pure PP to 393 and 422 C for the blends PP/lignin 95/5 and PP/lignin 85/15, respectively. Theoretical TGA and Table 1 Thermogravimetric data for PP, lignin and PP/lignin blends Atmosphere
Ti ( C)
Tmr ( C)
R (wt%)
PP PP/lignin 95/5 PP/lignin 85/15 Lignin
N2 N2 N2 N2
366 359 310 207
475 479 490 374
0a 2.0a 6.3a 43.1a
PP PP/lignin 95/5 PP/lignin (85/15) Lignin
Air Air Air Air
257 280 292 205
357 393 422 356
0.8b 2.9b 7.9b 37.9b
a b
Value calculated at 550 C. Value calculated at 450 C.
M. Canetti et al. / Polymer Degradation and Stability 91 (2006) 494e498
498
a
100
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90
90
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60 50 40 30
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-dW/dt (
-dW/dt (
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13 11 9 7 5 3
-1 200
1 300
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Temperature (°C) Fig. 8. TGA (a) and DTG (b) thermograms under air atmosphere for (C) pure PP; (-) PP/lignin 95/5; (:) PP/lignin 85/15; (A) pure lignin.
relative DTG curves were calculated for PP/lignin 85/15 blend from individual weight loss data of pure components. To emphasize the synergistic effect obtained by blending the components, the theoretical TGA and DTG curves were compared with the experimental ones and shown in Fig. 9. The marked differences observed between theoretical and experimental curves, are due to the complex interactions occurring by blending PP and lignin. 4. Conclusions In this work PP/lignin blends were prepared by melt mixing and their morphology and thermal degradation behaviour were investigated. Morphological analyses show good dispersion and good adhesion between the lignin particles and the PP matrix, in the blends. In the presence of lignin, PP can crystallize into two crystalline forms, a and b. The b-form fraction, is strongly influenced by the thermal treatment. The formation of the b-form during the crystallization process was promoted by an increase in the crystallization rate. The lignin present in the blend is able to produce a high char yield that is responsible to the increase in the blend temperature degradation. The char is a carbon-based residue that undergoes slow oxidative degradation. The increase is more pronounced for the experiments carried out in air atmosphere, where, the interactions between the PP and the charring lignin
-1 200
300
400
Temperature (°C) Fig. 9. TGA (a) and DTG (b) for PP/lignin 85/15 under air atmosphere (-) experimental thermograms; (C) theoretical thermograms calculated from individual weight loss data of pure components.
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