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.
|1||Instrument||Perkin Elmer Pyris|
|2||Sample Mass||18 mg|
|5||Sample Pan||Covered Ceramic|
TGA-DTA Thermogram for Identification of the sample
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|
Study of Mass Loss w.r.t Time &Temperature
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.
% Weight V/S Derivative Weight (%Wt Min-1 )
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-E (Rate with Temp.)
Chart – A
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.
Study of the sp. rate of decomposition with inverse of temperature (K)
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.
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