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Thermal properties of cellulose acetate and its complexes with some transition metals
Polymer Degradation and Stability 63 (1999) 293±296
Thermal properties of cellulose acetate and its complexes with some transition metals A.A. Hanna*, Altaf H. Basta, Houssni El-Saied, I.F. Abadir1 National Research Centre, P.O. Box 12311, Dokki, Cairo, Egypt Received 17 February 1998; received in revised form 26 May 1998; accepted 26 June 1998
Abstract Cellulose triacetate±metal ion complexes were prepared using dierent metal salts. The products were characterized by using IR spectra, where the complex formation is emphasized. TGA and DTA of cellulose triacetate (CTA) and its complexes with Ni(II), Co(II), Cu(II) and Cr(III) chlorides were investigated from room temperature to 550 C. The TGA curves show that these samples degrade thermally in three steps which are attributed to dehydration, fragmentation of the macromolecular structure (the main thermal degradation step), and the carbonization of the product to ash. It is found that the main degradation step follows ®rstorder kinetics. The complexation of cellulose acetate with metal ions increased its thermal stability. The calculated activation energy varied in the following sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA±Cr(III) > CTA. This observation was attributed to the electronegativity of the metal ion and the strength of the bond between the cellulose acetate and the metal ion. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Thermogravimetric analysis, TGA and DTA, are widely used as to investigate the thermal decomposition of polymers and to determine some kinetic parameters, e.g., rate of decomposition, k, order of reaction, n, and activation energy, Ea [1]. The values of these parameters can be of major importance in the elucidation of the mechanisms involved in polymer degradation [2] and in the estimation of the thermal stability of polymers. Previously, Hanna et al. [3±6] studied the thermal properties of some cellulosic materials and cellulose grafted with dierent monomers. This study indicated that the grafting increased the thermal stability of the cellulosic chain. Recently, we studied the electrical conductivity of cellulose HYPHAN1 and its complexes with Cr(III), Mn(II), Mo(V) and Hg(II) to investigate the nature of the chemical structure of the resulting complexes and their behaviour [7]. Also, El-Saied et al. [8±10] studied the characterization of carboxymethyl cellulose (CMC) with some transition metals, and the semiconductor properties of CMC±Cu (II) complexes [11]. As far as we * Corresponding author. E-mail: [email protected]. 1 Cairo Univ., Faculty of Eng. Chem. Dept., Giza, Egypt.
are aware, there is no mention in the literature concerning the thermal properties of cellulosic-metal complexes. The present work aimed to study the thermogravemetric analysis of cellulose acetate and its complexes with Ni(II), Co(II), Cu(II) and Cr(III) to throw some light on the nature of bonding between the polymer and the metal ion. Also, this study was extended to investigate the eect of complexes on the thermal stability of cellulose acetate. The IR spectra of the prepared complexes were also examined. 2. Experimental Analytical grades of NiCl2 . 6H2O, NiSO4 . H2O, Ni(NO3)2 . H2O, CoCl2 . 6H2O, Co(COOCH3)2 . 4H2O, CuCl2 . 2H2O, CuSO4 . 5H2O, and CrCl3 . 6H2O were used. Cellulose triacetate (CTA) was purchased from Fluka. The degree of substitution of CTA was determined by a conventional method [12]; D.S. 2.79. All polymer complexes were prepared by dissolving equimolar amounts of CTA and the metal salts in chloroform and methanol, respectively. The mixtures stirred thoroughly were then left for 3 h at room temperature. The clear complexes were poured into cold
0141-3910/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(98)00108 -6
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A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
double distilled water. The obtained precipitate of the polymer complex was ®ltered, followed by washing with double distilled water to remove excess metal salts, and then dried at room temperature. IR spectra (4000±200 cmÿ1) were recorded on an Insco FTIR spectrophotometer using KBr disc. TGA and DTA were carried out by using an MOMDerivatograph No: 3427 from 20 C to 550 C for ®ve samples, namely CTA, CTA±Ni(II), CTA±Co(II), CTA±Cu(II) and CTA±Cr(III) derived from chloride salts. The rate of heating was 5 C minÿ1. 3. Results and discussion 3.1. IR spectra Table 1 shows that the IR spectrum of CTA has sharp strong absorption bands at 3683, 1755, and 1215 cmÿ1 characteristic of hydroxyl, ester carbonyl and ether carbon±oxygen groups, respectively. The main changes in the IR spectra of the complexes of CTA with some transition metals are summarized as follows. Water molecules, as indicated by a broad bands at 3409±3465, 900 and 670 cmÿ1, these bands are assigned to stretching, rocking and wagging (out of plane bending) vibration, respectively, of OH groups in water molecules. The band maxima correspond to stretching vibration (3618 cmÿ1) and out of plane bending (670 cmÿ1) vibration of OH groups are shifted to lower wave number, this indicates the involvement of residual hydroxyl groups of CTA (D.S. 2.79) in chelation. The extent of the shift changes with the change of the cation and anion of metal salts. The shift values () are 152.5,
208.5, 197.0, 197.0, 169.9, 188.0, 188.0 and 188.0 cmÿ1 in the case of Co(COOCH3)2, CoCl2, NiCl2, CrCl3, CuCl2, CuSO4, NiSO4 and Ni(NO3)2, as a source of metal ions. In other words, for the change of cation of metal chloride salts, the greater shift in OH maximum is noticed in the case of CTA±Co(II) complex originated from CoCl2; while OH of the band maxima of chloride salts is greater than sulfate and acetate salts. The IR spectra of CTA polymer complexes, except CTA±Cr(III) complex, have the characteristic features of coordination through the oxygen of the ether carbon±oxygen rather than the acetyl carbonyl group; whereas the ether carbon±oxygen absorption bands undergo blue shifts (from 1214±1236.15 cmÿ1); while slight change in position of ester carbonyl absorption bands is noticed in its maximum band position (change from 1755 to 1760 cmÿ1). This result suggests that the acetate group in the cellulose acetate ligand, undergoes monodendate bridging with metal ions. Therefore, for glucopyranose unit the two acetate groups or one acetate and vicinal hydroxyl are coordinated to form ®ve membered rings. However, the broadening of the band corresponding to carbonyl of acetyl groups in the case of CTA±Cr(III) complex, to low wave number indicates that the oxygen of the ester carbonyl and the ether carbon±oxygen chelate in the Cr(III)-complex, i.e., a threemembered ring is probably formed between each ester group of CTA and Cr(III) ions. CH group at 2975.6 cmÿ1, becomes narrower or a shoulder compared with non-chelating CTA. This may be attributed to changing the space between chains (reduced), due to chelation together with occurrence of coordinated water molecules. Furthermore, the IR spectra of CTA-complexes show a shift of the band at
Table 1 Main IR frequencies of the CTA and its complexes with some transition metals CTA 3683.0 3618.0 3018.0 2975.6 1754.9 ÿ 1521.0 1475.3 1423.2 1370.0 1214.9 1047.1 927.6 877.5 757.9 669.2 ÿ ÿ ÿ
CTA±NiCl2 complex
CTA±NiSO4 complex
CTA±Ni(NO3)2 complex
CTA±CoCl2 complex
CTA±Co(OAC)2 complex
CTA±CuCl2 complex
CTA±CuSO4 complex
CTA±CrCl3 complex
ÿ 3421 ÿ 2920 1760 1640 ÿ 1460 1400 ÿ 1240 1050 900 ÿ ÿ 600 ÿ 410 380
ÿ 3430 ÿ 2960 1751 1640 ÿ 1460 1440 1390 1240 1090 900 840 ÿ 607 560 450 ÿ
ÿ 3430.0 ÿ 2920.0 1750.0 1640.0 ÿ 1460.0 ÿ 1384.6 1240.0 1050.0 900.0 830.0 ÿ 600.0 ÿ 479.0 ÿ
ÿ 3409.5 ÿ 2960.0 1760.0 1640.0 ÿ 1460.0 1440.0 1380.0 1240.0 1050.0 900.0 ÿ ÿ 600.0 560.0 460.0 400.0
ÿ 3465.5 ÿ 2960.0 1760.0 ÿ 1560.0 ÿ 1436.7 1370.0 1236.0 1047.2 900.0 ÿ ÿ 603.6 580.0 450.0 ÿ
ÿ 3448.1 ÿ 2920.0 1751.0 1640.0 ÿ 1460.0 ÿ 1380.0 1220.0 1050.0 900.0 ÿ 770.0 606.0 550.0 ÿ 390.0
ÿ 3430.0 ÿ 2960.0 1751.0 1615.0 ÿ 1460.0 1440.0 1380.0 1240.0 1050.0 900.0 ÿ ÿ 600.0 550.0 448.0 ÿ
ÿ 3421.0 ÿ 2924.0 1760.0 1640.0 1542.0 1460.0 1423.0 1378.0 1260.0 1050.0 900.0 ÿ ÿ 600.0 ÿ 400.0 ÿ
A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
295
1215 cmÿ1 associated with the appearance of new bands in the range from 400 to 560 cmÿ1, assignable to M±O stretching vibration. 3.2. Thermal analysis The TGA curve, Fig. 1, shows that cellulose triacetate (CTA) degraded in three steps. The ®rst step from the room temperature (25 C) to 330 C, represents the volatilization of the volatile matter, and/or the evaporation of residual absorbed water. The second step starts at 330 C and ends at 500 C, and represents the main thermal degradation of the cellulose acetate chains. The third step starts at 500 C, and represents the carbonization of the products to ash. These three steps may correspond to the three steps which were suggested by Chatterjee [13] as representing the thermal degradation of the cellulosic materials. First step: Second step:
Third step:
A ! B1 volatile product or dehydration, B1 ! B2 L, B2 ! B3 L thermal degradation, Bn ! Bn1 L, Bn ! Bn1 L, Bn ! carbonization (ash),
where A denotes the initial molecules of cellulose acetate, B1 ; B2 ; . . . Bn are fragmented molecules and L denotes volatile products. The TGA curves of CTA±Cr(III), CTA±Co(II), CTA±Ni(II) and CTA±Cu(II) complexes derived from CoCl2 . 6H2O, NiCl2 . 6H2O and CrCl3 . 6H2O, . CuCl2 2H2O, respectively. Fig. 2, shows similar behaviour to that observed in the CTA; Fig. 1. The boundaries of the three steps depend on the nature of the
Fig. 2. TGA curves of CTA and its complexes.
complexes. Fig. 2 indicates that the start temperature (Tonset) of the main degradation varied in the sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA± Cr(III) > CTA. In general, the thermal degradation of polymeric materials can be represented by the following equation [14]: A Solid ! B Solid C Gas : Thus, the general rate expression may be written, Rt ÿ
dw =RT A=RH eÿEa Wn ; dt
where W is the weight of the active material remaining, RH the rate of heating, dw/dt the rate of degradation, A the frequency factor, E the activation energy, R the gas constant, T the absolute temperature, and n the order of degradation. The previous equation may be arranged in the form [15] ln Rt ln A ÿ Ea =RT n ln W;
Fig. 1. TGA and DTA curves of CTA.
where W is constant, a plot of ln Rt against 1/T gives a linear relation and the value of the activation energy, Ea, can be calculated from the slope. From the weight loss±time relation the activation energy was calculated, Table 2, as in the previous
296
A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
Table 2 The values of start temperature (Tonset), end temperature (Toutset) of the main decomposition, the activation energy (E), and the electronegativity of cellulose triacetate and its complexes Sample CTA CTA±Cr(III) complex CTA±Co(II) complex CTA±Ni(III) complex CTA±Cu(II) complex
Tonset (K)
Toutset (K)
E (kJ molÿ1)
Electronegativity cation
603 613 619 628 633
686 695 703 703 706
23.75 28.15 29.66 31.66 33.00
ÿ 1.6 1.7 1.8 1.8
The DTA curves of the CTA and its complexes with the metal ions, Fig. 3, show an exothermic reaction beginning at 330 C for CTA and ending at 410 C. This exotherm is followed by an endotherm at the higher temperature. The main dierence in the DTA curves between the CTA and CTA±metal complexes is the shifting of both the exothermic and the endothermic temperature to higher values. This shift indicates that the complexed sample is thermally more stable compared to the non-chelated CTA. For all samples, the exothermic peak may be attributed to the combustion of CTA, while the endothermic one may be attributed to the carbonization of the formed ash [17]. References
Fig. 3. DTA curves of CTA and its complexes.
investigation [5]. The results indicate that CTA degrades in a ®rst-order reaction with a small deviation in the initial stage of decomposition and with activation energy of 23.75 kJ molÿ1, which is in agreement with the literature [16]. For the CTA±metal complexes samples, it is observed that the complex samples behave in the same way as that observed for CTA, i.e., ®rst order reaction, but the activation energy, Table 2, follows the sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA±Cr(III) > CA. In all cases, it is clear that the activation energy for the complex is greater than that for CTA. This may be due to cross-link formation by complexing or to high molecular weight molecules of complexes. The decrease of the activation energy from 33.00 kJ molÿ1 for CTA±Cu(II) complex to 28.15 kJ molÿ1 for CTA±Cr(III) complex may be attributed to the electronegativity of the metal cation, which in¯uences the chemical bond between the cellulose chain and metal ion, and is 1.8 for Cu ion and 1.6 for Cr ion.
[1] Nishizaki H, Yoshida K, Wang JH. J Appl Polym Sci 1980;25:2869. [2] Levi DW, Reich L, Tee T. Polym Eng Sci 1965;5:135. [3] Hanna AA, Abd-Elwahid A, Abass MH. Acta Polymerica 1984;35:104. [4] Hanna AA, Abd-Elwahid A, Abass MH. Acta Polymerica 1984;35:325. [5] Hanna AA, Abd-Elwahid A, Abass MH. Thermochim Acta 1983;71:119. [6] Hanna AA. Thermochim Acta 1984;73:199. [7] Hanna AA, Kenawy IMM, El-Saied KhS. Die Angew Makromol Chemie 1993;212:13. [8] El-Saied H, Basta AH, Abdel-Hadi AK, Hosny WM. Polym Int 1994;35:27. [9] Abdel-Hadi AK, Hosny WM, Basta AH, El-Saied H. Polym Plast Technol Eng 1994;33:781. [10] Hosny WM, Abdel-Hadi AK, El-Saied H, Basta AH. Polym Int 1995;37:93. [11] El-Saied H, Basta AH, Hanna AA, El-Saied AM. In press. [12] Whistler RL. ``Methods in Carbohydrate Chemistry'', Vol. III. London: Academic Press, 1963:193. [13] Chatterjee PK. J Polym Sci 1968;6:3217. [14] Reich L, Levi DW. ``Macromolecular Reviews'', vol. 1, ``Dynamic Thermogravimetric Analysis in Polymer Degradation'', 1967. [15] Hanna AA, Abdwahid A, Abass MH. Cellulose Chem Technol 1984;18:11. [16] Reardon TJ, Barker RH. J Appl Polym Sci 1974;18:1903. [17] Sha®zdeh PK, Bradburg GW. J Appl Polym Sci 1979;23:1431.
Thermal properties of cellulose acetate and its complexes with some transition metals A.A. Hanna*, Altaf H. Basta, Houssni El-Saied, I.F. Abadir1 National Research Centre, P.O. Box 12311, Dokki, Cairo, Egypt Received 17 February 1998; received in revised form 26 May 1998; accepted 26 June 1998
Abstract Cellulose triacetate±metal ion complexes were prepared using dierent metal salts. The products were characterized by using IR spectra, where the complex formation is emphasized. TGA and DTA of cellulose triacetate (CTA) and its complexes with Ni(II), Co(II), Cu(II) and Cr(III) chlorides were investigated from room temperature to 550 C. The TGA curves show that these samples degrade thermally in three steps which are attributed to dehydration, fragmentation of the macromolecular structure (the main thermal degradation step), and the carbonization of the product to ash. It is found that the main degradation step follows ®rstorder kinetics. The complexation of cellulose acetate with metal ions increased its thermal stability. The calculated activation energy varied in the following sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA±Cr(III) > CTA. This observation was attributed to the electronegativity of the metal ion and the strength of the bond between the cellulose acetate and the metal ion. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Thermogravimetric analysis, TGA and DTA, are widely used as to investigate the thermal decomposition of polymers and to determine some kinetic parameters, e.g., rate of decomposition, k, order of reaction, n, and activation energy, Ea [1]. The values of these parameters can be of major importance in the elucidation of the mechanisms involved in polymer degradation [2] and in the estimation of the thermal stability of polymers. Previously, Hanna et al. [3±6] studied the thermal properties of some cellulosic materials and cellulose grafted with dierent monomers. This study indicated that the grafting increased the thermal stability of the cellulosic chain. Recently, we studied the electrical conductivity of cellulose HYPHAN1 and its complexes with Cr(III), Mn(II), Mo(V) and Hg(II) to investigate the nature of the chemical structure of the resulting complexes and their behaviour [7]. Also, El-Saied et al. [8±10] studied the characterization of carboxymethyl cellulose (CMC) with some transition metals, and the semiconductor properties of CMC±Cu (II) complexes [11]. As far as we * Corresponding author. E-mail: [email protected]. 1 Cairo Univ., Faculty of Eng. Chem. Dept., Giza, Egypt.
are aware, there is no mention in the literature concerning the thermal properties of cellulosic-metal complexes. The present work aimed to study the thermogravemetric analysis of cellulose acetate and its complexes with Ni(II), Co(II), Cu(II) and Cr(III) to throw some light on the nature of bonding between the polymer and the metal ion. Also, this study was extended to investigate the eect of complexes on the thermal stability of cellulose acetate. The IR spectra of the prepared complexes were also examined. 2. Experimental Analytical grades of NiCl2 . 6H2O, NiSO4 . H2O, Ni(NO3)2 . H2O, CoCl2 . 6H2O, Co(COOCH3)2 . 4H2O, CuCl2 . 2H2O, CuSO4 . 5H2O, and CrCl3 . 6H2O were used. Cellulose triacetate (CTA) was purchased from Fluka. The degree of substitution of CTA was determined by a conventional method [12]; D.S. 2.79. All polymer complexes were prepared by dissolving equimolar amounts of CTA and the metal salts in chloroform and methanol, respectively. The mixtures stirred thoroughly were then left for 3 h at room temperature. The clear complexes were poured into cold
0141-3910/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(98)00108 -6
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A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
double distilled water. The obtained precipitate of the polymer complex was ®ltered, followed by washing with double distilled water to remove excess metal salts, and then dried at room temperature. IR spectra (4000±200 cmÿ1) were recorded on an Insco FTIR spectrophotometer using KBr disc. TGA and DTA were carried out by using an MOMDerivatograph No: 3427 from 20 C to 550 C for ®ve samples, namely CTA, CTA±Ni(II), CTA±Co(II), CTA±Cu(II) and CTA±Cr(III) derived from chloride salts. The rate of heating was 5 C minÿ1. 3. Results and discussion 3.1. IR spectra Table 1 shows that the IR spectrum of CTA has sharp strong absorption bands at 3683, 1755, and 1215 cmÿ1 characteristic of hydroxyl, ester carbonyl and ether carbon±oxygen groups, respectively. The main changes in the IR spectra of the complexes of CTA with some transition metals are summarized as follows. Water molecules, as indicated by a broad bands at 3409±3465, 900 and 670 cmÿ1, these bands are assigned to stretching, rocking and wagging (out of plane bending) vibration, respectively, of OH groups in water molecules. The band maxima correspond to stretching vibration (3618 cmÿ1) and out of plane bending (670 cmÿ1) vibration of OH groups are shifted to lower wave number, this indicates the involvement of residual hydroxyl groups of CTA (D.S. 2.79) in chelation. The extent of the shift changes with the change of the cation and anion of metal salts. The shift values () are 152.5,
208.5, 197.0, 197.0, 169.9, 188.0, 188.0 and 188.0 cmÿ1 in the case of Co(COOCH3)2, CoCl2, NiCl2, CrCl3, CuCl2, CuSO4, NiSO4 and Ni(NO3)2, as a source of metal ions. In other words, for the change of cation of metal chloride salts, the greater shift in OH maximum is noticed in the case of CTA±Co(II) complex originated from CoCl2; while OH of the band maxima of chloride salts is greater than sulfate and acetate salts. The IR spectra of CTA polymer complexes, except CTA±Cr(III) complex, have the characteristic features of coordination through the oxygen of the ether carbon±oxygen rather than the acetyl carbonyl group; whereas the ether carbon±oxygen absorption bands undergo blue shifts (from 1214±1236.15 cmÿ1); while slight change in position of ester carbonyl absorption bands is noticed in its maximum band position (change from 1755 to 1760 cmÿ1). This result suggests that the acetate group in the cellulose acetate ligand, undergoes monodendate bridging with metal ions. Therefore, for glucopyranose unit the two acetate groups or one acetate and vicinal hydroxyl are coordinated to form ®ve membered rings. However, the broadening of the band corresponding to carbonyl of acetyl groups in the case of CTA±Cr(III) complex, to low wave number indicates that the oxygen of the ester carbonyl and the ether carbon±oxygen chelate in the Cr(III)-complex, i.e., a threemembered ring is probably formed between each ester group of CTA and Cr(III) ions. CH group at 2975.6 cmÿ1, becomes narrower or a shoulder compared with non-chelating CTA. This may be attributed to changing the space between chains (reduced), due to chelation together with occurrence of coordinated water molecules. Furthermore, the IR spectra of CTA-complexes show a shift of the band at
Table 1 Main IR frequencies of the CTA and its complexes with some transition metals CTA 3683.0 3618.0 3018.0 2975.6 1754.9 ÿ 1521.0 1475.3 1423.2 1370.0 1214.9 1047.1 927.6 877.5 757.9 669.2 ÿ ÿ ÿ
CTA±NiCl2 complex
CTA±NiSO4 complex
CTA±Ni(NO3)2 complex
CTA±CoCl2 complex
CTA±Co(OAC)2 complex
CTA±CuCl2 complex
CTA±CuSO4 complex
CTA±CrCl3 complex
ÿ 3421 ÿ 2920 1760 1640 ÿ 1460 1400 ÿ 1240 1050 900 ÿ ÿ 600 ÿ 410 380
ÿ 3430 ÿ 2960 1751 1640 ÿ 1460 1440 1390 1240 1090 900 840 ÿ 607 560 450 ÿ
ÿ 3430.0 ÿ 2920.0 1750.0 1640.0 ÿ 1460.0 ÿ 1384.6 1240.0 1050.0 900.0 830.0 ÿ 600.0 ÿ 479.0 ÿ
ÿ 3409.5 ÿ 2960.0 1760.0 1640.0 ÿ 1460.0 1440.0 1380.0 1240.0 1050.0 900.0 ÿ ÿ 600.0 560.0 460.0 400.0
ÿ 3465.5 ÿ 2960.0 1760.0 ÿ 1560.0 ÿ 1436.7 1370.0 1236.0 1047.2 900.0 ÿ ÿ 603.6 580.0 450.0 ÿ
ÿ 3448.1 ÿ 2920.0 1751.0 1640.0 ÿ 1460.0 ÿ 1380.0 1220.0 1050.0 900.0 ÿ 770.0 606.0 550.0 ÿ 390.0
ÿ 3430.0 ÿ 2960.0 1751.0 1615.0 ÿ 1460.0 1440.0 1380.0 1240.0 1050.0 900.0 ÿ ÿ 600.0 550.0 448.0 ÿ
ÿ 3421.0 ÿ 2924.0 1760.0 1640.0 1542.0 1460.0 1423.0 1378.0 1260.0 1050.0 900.0 ÿ ÿ 600.0 ÿ 400.0 ÿ
A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
295
1215 cmÿ1 associated with the appearance of new bands in the range from 400 to 560 cmÿ1, assignable to M±O stretching vibration. 3.2. Thermal analysis The TGA curve, Fig. 1, shows that cellulose triacetate (CTA) degraded in three steps. The ®rst step from the room temperature (25 C) to 330 C, represents the volatilization of the volatile matter, and/or the evaporation of residual absorbed water. The second step starts at 330 C and ends at 500 C, and represents the main thermal degradation of the cellulose acetate chains. The third step starts at 500 C, and represents the carbonization of the products to ash. These three steps may correspond to the three steps which were suggested by Chatterjee [13] as representing the thermal degradation of the cellulosic materials. First step: Second step:
Third step:
A ! B1 volatile product or dehydration, B1 ! B2 L, B2 ! B3 L thermal degradation, Bn ! Bn1 L, Bn ! Bn1 L, Bn ! carbonization (ash),
where A denotes the initial molecules of cellulose acetate, B1 ; B2 ; . . . Bn are fragmented molecules and L denotes volatile products. The TGA curves of CTA±Cr(III), CTA±Co(II), CTA±Ni(II) and CTA±Cu(II) complexes derived from CoCl2 . 6H2O, NiCl2 . 6H2O and CrCl3 . 6H2O, . CuCl2 2H2O, respectively. Fig. 2, shows similar behaviour to that observed in the CTA; Fig. 1. The boundaries of the three steps depend on the nature of the
Fig. 2. TGA curves of CTA and its complexes.
complexes. Fig. 2 indicates that the start temperature (Tonset) of the main degradation varied in the sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA± Cr(III) > CTA. In general, the thermal degradation of polymeric materials can be represented by the following equation [14]: A Solid ! B Solid C Gas : Thus, the general rate expression may be written, Rt ÿ
dw =RT A=RH eÿEa Wn ; dt
where W is the weight of the active material remaining, RH the rate of heating, dw/dt the rate of degradation, A the frequency factor, E the activation energy, R the gas constant, T the absolute temperature, and n the order of degradation. The previous equation may be arranged in the form [15] ln Rt ln A ÿ Ea =RT n ln W;
Fig. 1. TGA and DTA curves of CTA.
where W is constant, a plot of ln Rt against 1/T gives a linear relation and the value of the activation energy, Ea, can be calculated from the slope. From the weight loss±time relation the activation energy was calculated, Table 2, as in the previous
296
A.A. Hanna et al./Polymer Degradation and Stability 63 (1999) 293±296
Table 2 The values of start temperature (Tonset), end temperature (Toutset) of the main decomposition, the activation energy (E), and the electronegativity of cellulose triacetate and its complexes Sample CTA CTA±Cr(III) complex CTA±Co(II) complex CTA±Ni(III) complex CTA±Cu(II) complex
Tonset (K)
Toutset (K)
E (kJ molÿ1)
Electronegativity cation
603 613 619 628 633
686 695 703 703 706
23.75 28.15 29.66 31.66 33.00
ÿ 1.6 1.7 1.8 1.8
The DTA curves of the CTA and its complexes with the metal ions, Fig. 3, show an exothermic reaction beginning at 330 C for CTA and ending at 410 C. This exotherm is followed by an endotherm at the higher temperature. The main dierence in the DTA curves between the CTA and CTA±metal complexes is the shifting of both the exothermic and the endothermic temperature to higher values. This shift indicates that the complexed sample is thermally more stable compared to the non-chelated CTA. For all samples, the exothermic peak may be attributed to the combustion of CTA, while the endothermic one may be attributed to the carbonization of the formed ash [17]. References
Fig. 3. DTA curves of CTA and its complexes.
investigation [5]. The results indicate that CTA degrades in a ®rst-order reaction with a small deviation in the initial stage of decomposition and with activation energy of 23.75 kJ molÿ1, which is in agreement with the literature [16]. For the CTA±metal complexes samples, it is observed that the complex samples behave in the same way as that observed for CTA, i.e., ®rst order reaction, but the activation energy, Table 2, follows the sequence: CTA±Cu(II) > CTA±Ni(II) > CTA±Co(II) > CTA±Cr(III) > CA. In all cases, it is clear that the activation energy for the complex is greater than that for CTA. This may be due to cross-link formation by complexing or to high molecular weight molecules of complexes. The decrease of the activation energy from 33.00 kJ molÿ1 for CTA±Cu(II) complex to 28.15 kJ molÿ1 for CTA±Cr(III) complex may be attributed to the electronegativity of the metal cation, which in¯uences the chemical bond between the cellulose chain and metal ion, and is 1.8 for Cu ion and 1.6 for Cr ion.
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