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Determination of the thermal behaviour of PET bottle by using the Differential Scanning Calorimetry (DSC)

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Lefayet Sultan Lipol, Assistant Professor & Head,

Fareast International University

E-mail: lefayetbd@gmail.com

1. Abstract:

The aim of this experimental work is to show some mainstream thermo-dynamical methods commonly used for polymers to characterize its thermal behavior, e.g- PET (Poly Ethylene Terephthalate). The thermal characteristics of PET bottle was characterized by the apparatus named Differential Scanning Calorimetry (DSC) at the polymer lab of the University of Boras, Sweden. The sample showed a certain Tg and Tm at DSC apparatus. But when the sane material was cooled & the heating process was repeated, the PET polymer showed different Tg and Tm. The reason was considered for changed molecular orientation after cooling.

Abbreviations:

Tg= Glass Transition Temperature,

Tm= Crystalline Melting Temperature,

DSC= Differential Scanning Calorimetry.

2. Background:

The typical applications of Differential Scanning Calorimetry (DSC) apparatus is mentioned below:

  • To determine important transition temperature like Tm and Tg and study the nature of polymers.
  • To determine the heat of fusion of a crystalline phase and degree of crystallization.
  • To study the crystallization kinetics.
  • To determine heat capacity, Cp.
  • To determine the rate of the cross-linking reactions, degradation reaction etc.
  • To determine miscibility in the polymer blends.
  • To determine the structural relaxation, like enthalpy relaxation during physical aging.

Thermoplastics may be amorphous & semi-crystalline state depending on the environment. Amorphous structure is the primary structure of polymer but if we heat the polymer, it will be transformed to semi-crystalline structure (Tg). In case of the continuous increasing of heating, the polymer will be melted (Tm). The tested sample is PET here and with this a reference sample is used in DSC to control the heat and power.

In DSC apparatus, the PET polymer was heated under a controlled temperature to find out the Tg and Tm of the polymer.screenshot-91

screenshot-90

If the material is cooled from the point 1 (figure-2), the exothermic curve will appear on monitor of DSC. If we do it in the semi-crystalline structure, we will get a new bond which is different from the previous bonding.

Fractional crystalinity, Ø=DHm/DH0

DHm=for a material which have 100% crystalinity (heat of fusion).

DSC is useful to find the polymers Tg and Tm.

1/Tg=Wa/Tga+Wb/Tgb

Where, W=mg

For Co-polymers the theory is useful but not for the mixibility. But we can determine it from the graph.

3. Working Procedure:

  • Here in the machine nitrogen inert gas is used as it is required to de-activate the oxygen gas to prevent the oxidation.
  • Here the sample is very small (3-20 mg) and is placed in a small Aluminium vessel (pan). Pet-Bottle (sample), the sample is heated first then it is cooled and again it is heated at different rates. The weight of tested PET sample is 6.0 and 8.9 mg consecutively.
  • An Empty pan is always used as the reference, Aluminium (Reference)
  • When the sample is heated up a constant rate, any kind of change in its calorimetric properties will cause a temperature difference between the sample and the reference.screenshot-92
  • In the DSC the apparatus the measured temperature difference is controlling the electrical power to the sample and the reference in order to keep them at the same temperature.
  • In this technique the difference in power supply to the sample and the reference is recorded. This means that a peak area from the output recording directly corresponds to the heat consumed or produced by the sample.
  • This way we get the curve is the most distinguish feature is that the sample and the reference has the individual heaters and sensors and the measured variable is the difference in power that is supplied to keep the temperature of sample and reference the same during heating, cooling or even isothermal
  • When the polymer was heated for the first time it has a specific Tg and Tm But after the cooling when it was heated again, it showed slight different Tg and Tm. The reason may be the newly oriented polymer molecules for the heating at first time (Reference: Graph-1 & 2).screenshot-89

4. Discussion:

  • If there is any oxygen in the machine the oxidation will occur, heat flow faster and the result will be incorrect. It was seen that for 6.0 mg sample, the Tg was 970C and for the 8.9 mg, it was 97.2ºC. It may be happened for their different cooling rate.
  • As we used the same PET bottle, we cannot get combine Tg by calculating but we can get it by graph. The weights are measuring manually so there may be little mistake.
  • Here is a point in the graph showing (graph-1 & 2) the difference between endothermic regions (37.09 J/g or force among molecules) means the potential difference of the marked two points. But after the melting of polymers, there is no force among the polymers (around 0 J/g) so they are moving freely.
  • The melting temperature for the both samples was around 245°c.
  • Here three points for glass transition temperature, Tg: On-set, inflections & end.

5. Conclusion:

In Thermo-gravimetric Analyser (TGA), the polymers decompose but in DSC it does not decompose normally. So we have to know the TGA decompose temperature. Then we can use the correct temperature at the DSC from this way we can combine the two processes. Because of blow-moulding the amorphous regions are increased, this is the reason why less energy is needed to melt it. Although there was the risk of the oxidation and the correct flow of the nitrogen gas, we tried to make it perfectly. We expect that this experiment will help us in our future work.

References:

  • Lund, Anja (2008), Practical work at THS, Sweden.
  • Molecular biology (in Russian). 6. Moscow. 1975. pp. 7–33.
  • Wunderlich, B. (1990). Thermal Analysis. New York: Academic Press. pp. 137–140. ISBN0-12-765605-7.
  • Dean, John A. (1995). The Analytical Chemistry Handbook. New York: McGraw Hill, Inc. pp. 15.1–15.5. ISBN0-07-016197-6.
  • Pungor, Erno (1995). A Practical Guide to Instrumental Analysis. Florida: Boca Raton. pp. 181–191.
  • Skoog, Douglas A., F. James Holler and Timothy Nieman (1998). Principles of Instrumental Analysis (5 ed.). New York. pp. 805–808. ISBN0-03-002078-6.
  • J. O’Neill (1964). “The Analysis of a Temperature-Controlled Scanning Calorimeter”. Anal. Chem. 36 (7): 1238–1245. doi:10.1021/ac60213a020.
  • Wunderlich, Macromolecular Physics, (1980), Vol. 3, Ch. 8, Table VIII.6.

Annex:

Questions:

  1. Show the difference between DSC and TGA.
  2. Why do you have nitrogen as purge gas in the DSC?
  3. How can you gain from the possibility to use different purge gases in the TGA?
  4. Why are you looking in the literature of the heat of fusion data you can find rather different values for the same polymer?
  5. Give an example where the information from the DSC and TGA could complement each other.

Answers:

  1. In TGA, oxygen is used to decompose the polymer. It is an analysis; the mass of sample is recorded continuously as its temperature is increased linearly from ambient to high temperature. But in DSC, no oxygen is used. As a result, the heat flows slower than TGA in DSC machine.
  2. Nitrogen is mostly used. It helps to eliminate oxidation.
  3. Oxygen helps for the faster reduction. It helps to decompose the polymer quickly.
  4. If there are different amorphous, crystalinity.
  5. In TGA the polymers decompose but in DSC it does not decompose normally. So we have to know the TGA decompose temperature. Then we can use the correct temperature at the DSC from this way we can combine this two processes.
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