STUDY OF DECOMPOSITION KINETICS OF A POLYMER USING TGA

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Introduction:

For development and selection of polymers for specific uses decomposition studies of polymer are extremely essential. This can be studied by classical method of gradually changing the temperature through the period and study the change visually. The process is incorrect and time consuming. A study of the kinetics of decomposition of a polymer is an extremely useful tool for this study.

An unknown polymer strip was received by lab for its identification. After identifying the sample, its decomposition kinetics was studied by TGA.

Experimental

1 Instrument Perkin Elmer Pyris
2 Sample Mass 18 mg
3 Heating rate 10OC/min
4 Temperature range 30OC-600OC
5 Sample Pan Covered Ceramic
6 Purge Gas Nitrogen

 

 GRAPH –A

TGA-DTA Thermogram for Identification of the sample

Graph A

From the Thermogram the sample was identified as a HDPE Sample having following characteristics

Melting Point : 135OC
Moisture Content : 0.54%
Starting Temperature for decomposition : 250⁰C
Completion of decomposition : 500⁰C
Fitter content : 2.30%
Polymer content : 96.55

 Graph -B

Study of Mass Loss w.r.t Time &Temperature

Graph BTime (min) —>

 

Mass loss as a function of time

From the study of the above two graph it was revealed that upto  230OC, it upto 22 mins,mass loss/min as also the total mass loss is small. In this temperature zone, the moisture &other volatile matters are lost, actual decomposition of the polymer has not started. Actual decomposition starts at 250OC &completed around 500OC.

GRAPH C

% Weight V/S Derivative Weight (%Wt Min-1 )

Graph C

DERIVATIVE WEIGHT (%Wt Min-1 )

The graph shows that the initial rate of decomposition is more or less constant (i.e loss of moisture and volatile matter, not the decomposition of the polymer)and independent of the total mass. When the decomposition of the polymer sets in, it is independent on the Mass% of polymer.

 

Graph-D (Rate with Time)

Graph D

 

Graph-E (Rate with Temp.)

Graph ETemperature (⁰C) —>

Chart – A

Temperature(⁰C) Time(min) %Weight Derivative Weight(wt/min)
50 2 99.57 0.021
70 4 99.41 0.025
90 6 99.25 0.024
110 8 99.1 0.026
130 10 98.9 0.032
150 12 98.62 0.028
170 14 98.51 0.015
190 16 98.43 0.01
210 18 98.35 0.018
230 20 98.23 0.022
250 22 98.03 0.058
270 24 97.35 0.22
290 26 95.94 0.24
310 28 94.66 0.26
330 30 92.73 0.46
350 32 89.78 0.57
370 34 86.68 0.6
390 36 83.1 0.72
410 38 78.62 0.94
430 40 71.18 1.86
450 42 57.69 2.79
470 44 37.69 4.98
490 46 7.9 4.31
510 48 1.49 0.25
530 50 0.208 0.15
550 52 -0.49 0.087

The two graph show that rate of decomposition with time is independent of concentration (% mass) when moisture and any other volatile matter are removed but gradually increases when actual decomposition of the polymer starts at 250⁰C reaches a maximum at 480⁰C and rate decreases thereafter and again becomes minimum when decomposition is complete.

Graph F

Study of the sp. rate of decomposition with inverse of temperature (K)

Graph F

Chart-B

Time(min) Tempareture(K) 1/T(K¯1) Specific Rate)
2 323 0.0031 0.00021
4 343 0.0029 0.00025
6 363 0.0028 0.00024
8 383 0.0026 0.00026
10 403 0.0025 0.00032
12 423 0.0024 0.00028
14 443 0.0023 0.00015
16 463 0.0022 0.0001
18 483 0.0021 0.00018
20 503 0.002 0.00022
22 523 0.0019 0.00059
24 543 0.0018 0.0023
26 563 0.0018 0.0024
28 583 0.0017 0.0027
30 603 0.0017 0.0049
32 623 0.0016 0.0063
34 643 0.0015 0.0063
36 663 0.0015 0.0069
38 683 0.0014 0.0087
40 703 0.0013 0.0119
42 723 0.0013 0.0261
44 743 0.0013 0.0484
46 763 0.0012 0.132
48 783 0.0012 0.5459
50 803  – 0.68
52 823  – 0.7212

 

Arrhenius equation for rate of change of specific rate with temperature is

d ln K/dt=E/RT2

When , E–Activation energy, T—Temperature in Kelvin, R—universal gas constant

and the Integrated form is : log K =E/2.303R*1/T+ log A

Where log A is constant

So a plot of log (sp. rate) with Temp..-1 will the a straight line with a negative slope.

In this experiment the specific rate that was calculated was for a temperature range around T±5⁰C, as no holding time was given for finding the specific rate. Still the graph in the experiment showed the same trend as predicted by Arrhenius for specific rates at a fixed Temperature.

Conclusion:

After evaluation of the TGA graphs and their derivatives an idea about decomposition of the polymer was obtained. The polymer (identified as a HDPE with moisture and some fillers) started its thermal decomposition. Initial mass loss(upto230⁰C)is due to loss of moisture and other volatile matter. After 230⁰C decomposition of the polymer starts and the monomers are carried off from the reactor ceramic pan. The decomposition rate is max at 480OC after which the rate gradually decreases and becomes steady at 500⁰C when only fillers are left behind.So the range of use of the polymer is upto 135⁰C as a solid phase, because it melts at 135OC.

 

Contributed by:  Ms. Alakta Saha under the guidance of Prof. Barun Gupta