A Study towards Thermal Kinetics of Polypropylene Grafted Dextrin
Inderjeet Kaur1, Naisergik Deepika Khanna2
1Former Professor, Department of Chemistry, H.P.U. Shimla 171005
2Assistant Professor, Department of Chemistry, NSCBM Hamirpur
*Corresponding Author E-mail: naisergikchem@yahoo.co.in
ABSTRACT:
Graft copolymerization of water soluble natural polymer, dextrin has been carried out onto preirradiated Polypropylene (PP) in an aqueous medium using benzoyl peroxide (BPO) as initiator. Graft copolymerization was studied as a function of different reaction parameters. The graft copolymers were characterized by FTIR and TGA. The kinetic parameters were evaluated from TG data by using Coats Redfern equation.
KEYWORDS: Polypropylene, Dextrin, Graft Copolymerization, TGA,
INTRODUCTION:
In the ever-growing plastic era, synthetic polymers such as polyethylene (PE) and polypropylene (PP) have almost completely inhabited the modern world. Because of their wide range of properties such as durability, resistance to chemicals, weathering and photo degradation as well as biological attack and hydrophobicity, they are used in diverse applications in various fields. But the inertness of these polymers excludes their application where chemical affinities or penetration of components is necessary, e.g. dying of fibers, printing of films, paint ability, adhesion, agricultural mulches, agricultural planting containers etc. In order to improve these and other properties such as hydrophilicity, swellibility, solubility, chemical inertness etc. of PP, modification of PP has been attempted by different groups of workers. Graft copolymerization of acryl amide (AAm) onto a water insoluble polymer backbone, isotactic polypropylene (IPP), and polymer of AAm with a water insoluble initiator, benzoyl peroxide (BPO), and a phase transfer catalyst, tetrabutylammonium bromide (Bu4N+Br-), were carried out in a water/xylene binary solvent system[1]. Graft copolymerization of acrylonitrile (AN) and its binary mixture with 4-vinyl pyridine (4-VP) onto IPP powder in aqueous medium, using γ-rays as an initiator was studied by kaur et al[2]. PP fibers were grafted with methylmethacrylate and effects of direct and preirradiation method and monomer concentration on the degree of grafting were investigated by Lopergolo et al[3]. Graft copolymerization of methacrylic acid onto polypropylene fibers by simultaneous-gamma ray irradiation technique was carried out. The effect of various solvents on grafting was studied. The results have been presented in terms of swelling behavior of polypropylene fiber and the extent of homopolymerization[4]. The tert-butoxy radical-facilitated grafting of methyl methacrylate (MMA) and several other monomer onto commercial polypropylene (PP) pellets and fiber was investigated in heterogeneous conditions[5]. Radiation graft copolymerization of co monomer mixtures of acrylic acid (AAc) and styrene (Sty) onto polypropylene (PP) films by the mutual method has been investigated[6]. Graft copolymerization of styrene onto poly(vinyl chloride) (PVC) and polypropylene (PP) was carried out in a supercritical CO2 medium using AIBN as a free radical initiator. The thermal stability of grafted copolymer of PVC was better than that of PVC, while grafted copolymer of PP had poorer thermal stability than PP[7]. From the literature survey it is inferred that the modification of PP through grafting is being successfully pursued using monomers or monomer mixtures. Not much attention is given to modification of PP by grafting a polymer through intercross linking. In the present manuscript, therefore, we report on modification of PP by grafting a natural polymer, dextrin, using chemical method. Dextrin occurs as an intermediate product of starch hydrolysis and is achieved by either enzymatic action or by heating. Dextrin is used as thickeners in textile, binders in water colours, foundries, food applications and manufacture of adhesives, envelopes, gummed tapes, postage stamps, bottle labeling[8]. Its water solubility, reactivity, and biodegradability make it a potentially useful material. Grafting of dextrin to PP is likely, to introduce hydrophilicity to a hydrocarbon polymer and improve upon the properties of the later. Various kinetic parameters such as energy of activation E*, entropy of activation S*, free energy of activation G*, and specific reaction rate constant Kr for pure and grafted sample were evaluated by applying Coats and Redfern[9] integral method for non isothermal processes to different thermal degradation kinetic models.
EXPERIMENTAL:
Commercial polypropylene (PP) in the form of beads was obtained from Thukral Trading Co. Delhi, India. The beads were recrystallized from p-xylene using methanol and the powdered PP obtained upon recrystallization was used in all grafting experiments. Irradiation of the polymer was carried from Co60 source housed in Gamma Chamber-900 at a constant dose rate of 3.40 kGy/h. Dextrin (Merck) and BPO (Merck) were used as received.
Graft copolymerization of dextrin onto PP was carried out as a function of different reaction variables such as time of reaction, concentration of BPO, amount of water and temperature. The optimum conditions were evaluated for achieving the maximum percentage of grafting.
Characterization of PP Grafted Samples:
Physical characterization of PP grafted samples has been done by FTIR spectroscopy, Thermogravimetry (TG), Differential thermal analysis (DTA) and Differential thermogravimetry (DTG).
RESULTS AND DISCUSSION:
Irradiation of PP leads to the formation of hydroperoxide groups onto polymer backbone where grafting of dextrin occurred in the presence of BPO. The effect of the amount of dextrin on % apparent grafting, % grafting, and % true grafting onto PP was studied using the optimum conditions evaluated in the preceding variations. It is observed that with increasing amount of dextrin % apparent grafting, % grafting and % true grafting increases upto 0.150g giving maximum 65%, 55% and 35% respectively and decreases thereafter[10].
Evidence of Grafting:
FTIR Analysis:
FTIR spectra of pristine PP and PP-g-dextrin have been taken on Thermo-5700 Spectrophotometer. The FTIR spectrum of PP shows characteristic peaks at 2921.7 due to –CH2 (asymmetric stretching), 2956.6 cm-1 due to -CH3 (asymmetric stretching), 1377.3 cm-1 due to -CH3 (symmetric bending), 1459.1 cm-1 due to -CH3 (asymmetric bending), 1166.4 cm-1 due to another characteristic peak for -CHCH3 and 998.3 cm-1 due to helix chain of PP. The FTIR spectra of PP-g-dextrin apart from the above given peaks due to PP shows a new peak for –COC- appears at 1024.2 cm-1 and also a broad band for -OH (polymeric association of hydroxy groups) at 3400-3200cm-1 was observed. The presence of additional peaks in the grafted samples supports the formation of graft copolymer of PP and dextrin.
Fig. 1 FTIR of Polypropylene (PP)
Fig. 2 FTIR of PP-g-Dextrin
Differential Thermogravimetry:
The Derivative thermal gravimetric curves of pristine PP and PP-g-Dextrin are presented in Fig. 3 (a) and Fig. 5 (a) respectively. It is observed from figure that the pristine PP shows single stage decomposition having maximum rate of decomposition 1.816 mg/min. at 4480C whereas PP-g-Dextrin sample (Fig. 4) shows double stage decomposition with maximum rate 0.115 mg/min. and 1.072 mg/min. at 1320C and 4490C for first and second decomposition curves respectively .
Differential Thermal Analysis:
The Differential thermal analysis of pristine PP and PP-g-Dextrin is given in Fig. 3 (b) and Fig. 5 (b) respectively. The DTA curve for pristine PP gives two endothermic peaks with absorption of 103 mJ/mg and 129 mJ/mg energies at 1610C and 4510C (i.e. temperatures at maxima of peaks) respectively. On the other hand dextrin grafted PP gives one exothermic peak and two endothermic peaks (Fig. 4 (b)) at 1220C, 1520C and 4490C with energies -97.7 mJ/mg, 54.4 mJ/mg and 111 mJ/mg respectively. It is observed from the figures that the lower endothermal peak temperature is the melting temperature (Tm) and the higher is the decomposition temperature (Td).
Fig. 3 Thermal analysis of PP
Fig. 4 Thermal Analysis of Dextrin
Fig. 5 Thermal Analysis of PP-g-Dextrin
Thermogravimetric Analysis:
Table 1 Thermogravimetric analysis of PP and PP grafted sample
|
Sample |
IDT (0C) (at % residue left) |
FDT (0C) (at % residue left) |
DT (0C) at every 10% wt. loss |
|||||||||
|
10% |
20% |
30% |
40% |
50% |
60% |
70% |
80% |
90% |
100% |
|||
|
Pristine PP |
425.6 (99.98) |
470.5(0.35) |
414.1 |
428.8 |
436.0 |
440.8 |
444.5 |
447.8 |
451.1 |
454.7 |
459.1 |
470.6 |
|
Dextrin |
291.27(13.9) |
326.49(78.5) |
223.8 |
280.9 |
300.0 |
304.7 |
319.0 |
323.8 |
328.5 |
357.1 |
447.6 |
|
|
PP-g-Dextrin |
417.40(80.5) |
464.3(1.02) |
205.5 |
383.9 |
412.0 |
424.9 |
433.3 |
439.9 |
445.7 |
451.4 |
458.0 |
526.7 |
Table 2 Algebraic expressions f(α) and g(α) for the most frequently used mechanisms of solid state processes
|
Symbol |
Solid state process |
f(α) |
g(α) |
|
Sigmoidal curves |
|||
|
A2 |
Nucleation and growth [Avrami-Erofeev Eq. (1)] |
|
|
|
A3 |
Nucleation and growth [Avrami-Erofeev Eq. (2)] |
|
|
|
A4 |
Nucleation and growth [Avrami-Erofeev Eq. (3)] |
|
|
|
Deceleration curves |
|||
|
D1 |
One-dimensional diffusion |
|
|
|
D2 |
Two-dimensional diffusion |
|
|
|
D3 |
Three-dimensional diffusion (Jander equation) |
|
|
|
D4 |
Three-dimensional diffusion (Ginstling Rounshtein equation) |
|
|
|
F1 |
Random nucleation with one nucleus on the individual particle |
|
|
|
F2 |
Random nucleation with two nuclei on the individual particle |
|
|
|
F3 |
Random nucleation with three nuclei on the individual particle |
|
|
|
R2 |
Phase boundary controlled reaction: Cylindrical Symmetry (contracting area) |
|
|
|
R3 |
Phase boundary controlled reaction: Spherical symmetry (contracting volume) |
|
|
Table 3 Kinetic Parameters for Pristine PP
|
Models |
Slope |
Intercept |
Correlation |
E*(J/mol) |
A(sec-1) |
K |
S*(J/mol) |
G*(J/mol) |
H*(J/mol) |
|
D1 |
-48083.67 |
52.3525 |
-0.9946 |
399767.66 |
4.3683x1026 |
0.00000024 |
258.89 |
237182.12 |
399767.66 |
|
D2 |
-54411.68 |
60.7940 |
-0.9975 |
452378.69 |
2.2915x1030 |
0.00000005 |
330.10 |
245072.78 |
452378.69 |
|
D3 |
-65975.74 |
75.9059 |
-0.9934 |
548522.29 |
1.0158x1037 |
0.00000000 |
457.35 |
261308.07 |
548522.29 |
|
D4 |
-57740.56 |
64.0782 |
-0.9979 |
480055.00 |
6.4895x1031 |
0.00000001 |
357.90 |
255291.71 |
480055.00 |
|
A2 |
-20567.77 |
15.5397 |
-0.9496 |
171000.44 |
1.9227x1010 |
0.00011488 |
-54.23 |
205055.37 |
171000.37 |
|
A3 |
-13237.61 |
5.3148 |
-0.9463 |
110057.47 |
4.4869x105 |
0.00031438 |
-142.90 |
199799.13 |
110057.28 |
|
A4 |
-9572.53 |
0.2024 |
-0.9427 |
79585.99 |
1.9537x103 |
0.00046878 |
-188.10 |
197713.14 |
79585.69 |
|
F1 |
-42558.26 |
46.2143 |
-0.9526 |
353829.34 |
8.3467x1023 |
0.00000309 |
206.85 |
223929.87 |
353829.34 |
|
F2 |
-85424.59 |
109.1499 |
-0.6622 |
710220.07 |
3.6035x1051 |
0.00000003 |
735.89 |
248083.51 |
710220.07 |
|
F3 |
-172271.90 |
233.4347 |
-0.6653 |
1432268.60 |
6.8796x10105 |
0.00000000 |
1775.02 |
317554.88 |
1432268.60 |
|
R2 |
-29255.27 |
26.4514 |
-0.9974 |
243228.27 |
1.4991x1015 |
0.00000880 |
39.42 |
218471.54 |
243228.27 |
|
R3 |
-91203.18 |
-143.8118 |
0.6864 |
758263.27 |
0.00000000 |
0.00000000 |
-1366.69 |
1616546.59 |
758263.27 |
Kinetic Models of Degradation:
The rate = dα/dt = k(T).g(α) of thermal decomposition reaction of a solid is expressed by Arrhenius equation as k = Ae-E*/RT where α is the fraction of material which has reacted at time t and g(α) is a function which depends on the actual reaction mechanism and corresponds to some thermal decomposition mechanisms (Table 2) and other terms has usual meanings. The Arrhenius equation can be modified and written as dα/dT = (A/β).e-E*/RT. g(α) or 0∫α1/g(α).dα = To∫T(A/β) e -E*/RT.dT where β = dT/dt is the heating rate at a higher temperature. Differentiation of logarithmic form of this new equation yields:
Where f(α) is the integral form of 1/g(α) pertaining to the broad classification of the solid state rate mechanisms. Since the term [1-2RT/E*]<<1, so it can be neglected. The above equation on taking natural logarithms and rearranging, leads to:
Which is a form of Coats-Redfern equation. The unknown in this equation are A, E* and f(α).
Considering to different mechanistic models listed in the Table 2 the experimental α values are converted into f(α) values and the plot results into a straight line. The activation energy E* and frequency factor A are calculated from slope and intercept respectively. One can select the actual mechanism corresponding to f(α) values by finding out the maximum correlation co-efficient value from all the mechanisms of decomposition employed. Seven kinetic models were applied in the present study and interestingly both the samples follow the same mechanism (Table 3 and 4) irrespective of different stages of decomposition. Based upon the kinetic and thermodynamic parameters, the following conclusions are drawn: The low value of rate constant show that rate of degradation is slow. The best model is one in which correlation coefficient has maximum positive value nearest to one. All the samples shows R3 (spherical symmetry) mechanism for degradation in nitrogen atmosphere. The order of stability of the samples is based on the E* in KJ/mol as follows the order: PP (758.263) > PP-g-Dextrin (54.093).
Table 4 Kinetic Parameters for PP-g-Dextrin
|
Models |
Slope |
Intercept |
Correlation |
E*(J/mol) |
A(sec-1) |
K |
S*(J/mol) |
G*(J/mol) |
H*(J/mol) |
|
D1 |
-1948.16 |
-10.5822 |
-0.6885 |
16197.03 |
0.00823717 |
0.00037027 |
-291.00 |
198944.79 |
16196.80 |
|
D2 |
-2150.61 |
-10.6873 |
-0.7153 |
17880.17 |
0.00818580 |
0.00026657 |
-291.05 |
200660.59 |
17880.00 |
|
D3 |
-2384.94 |
-11.5225 |
-0.7426 |
19828.39 |
0.00393781 |
0.00008830 |
-297.14 |
206429.56 |
19828.34 |
|
D4 |
-2228.34 |
-11.9693 |
-0.7249 |
18526.39 |
0.00235341 |
0.00006772 |
-301.42 |
207815.21 |
18526.35 |
|
A2 |
99.43 |
-13.0221 |
0.1648 |
-826.66 |
-0.00003665 |
-0.00004293 |
- |
- |
-826.64 |
|
A3 |
402.63 |
-13.5100 |
0.6938 |
-3347.43 |
-0.00009110 |
-0.00017297 |
- |
- |
-3347.33 |
|
A4 |
554.22 |
-13.7539 |
0.8592 |
-4607.82 |
-0.00009826 |
-0.00023749 |
- |
- |
-4607.67 |
|
F1 |
-810.16 |
-11.5585 |
-0.5827 |
6735.65 |
0.00129043 |
0.00035520 |
-306.41 |
199161.85 |
6735.43 |
|
F2 |
249.45 |
-12.3369 |
0.6612 |
-2073.93 |
-0.00018242 |
-0.00027138 |
- |
- |
-2073.76 |
|
F3 |
-510.12 |
-10.1880 |
-0.6710 |
4241.12 |
0.00319884 |
0.00141978 |
-298.86 |
191927.44 |
4240.23 |
|
R2 |
-630.15 |
-12.7639 |
-0.4994 |
5239.03 |
0.00030067 |
0.00011023 |
-318.52 |
205271.04 |
5238.96 |
|
R3 |
-1922.87 |
-17.2152 |
0.9834 |
15986.78 |
0.00001070 |
0.00000050 |
-346.25 |
233434.92 |
15986.78 |
CONCLUSION:
Maximum percentage of grafting (55%) of dextrin onto PP was obtained at optimum conditions of [BPO] = 5.165 X 10-2 mol/L in 120 min at 600C using 15 ml of water. Polypropylene has been successfully intercross linked with dextrin through chemical method. A contrast difference in the thermal behavior of pristine polypropylene and the PP-g-Dextrin samples has been observed. Pristine PP and PP-g-Dextrin decompose by a type R3 (spherical symmetry) mechanism. Pristine polypropylene shows much higher thermal stability as indicated by order than PP-g-Dextrin.
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Received on 21.11.2016 Modified on 25.11.2016 Accepted on 28.11.2016 ©A&V Publications All right reserved DOI: 10.5958/2349-2988.2017.00012.2 Research J. Science and Tech. 2017; 9(1):76-80.
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