MATERIAL CHARACTERIZARATION
5.2 DSC Analysis 1 Method
Differential scanning calorimetry (DSC) is a technique which measure the difference in the quantity of heat required to increase the temperature of a polymer specimen and reference (an empty aluminium pan) at a constant heating rate (10ºC/min) as a function of temperature.
Figure5.2.1.1: Simple sketch of DSC instrument [90].
When the material undergoes a thermal transition, more or less heat will need to flow to it than the reference to maintain both at the same temperature (depend on the process if it is exothermic or endothermic).
If a solid sample melts to a liquid, it will require more heat flowing to the sample to increase its temperature at the same rate as the reference due to the absorption of heat by the sample as it undergoes the endothermic transition from solid to liquid (melting process). And if the sample undergoes exothermic processes (crystallization process), less heat is required to raise the sample temperature. Thus, by monitoring the difference in heat flow between the sample and reference, DSC can measure the amount of heat absorbed or released during such phase transitions.
TA instrument DSC 2010 apparatus was used to record DSC curves in the temperature range 30 to 300 ºC. A weight of 2-18 mg of each sample was used in an aluminium pan, encapsulated and transferred to the front cavity of the DSC heating head and the reference (an empty aluminium pan with a lid) is transferred to the back cavity of the DSC heating head and the area of the
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cavity was covered. The heating rate was 10 ºC/min and the test mode was ramp. Finally the data were analysed for Tg, Tc, Tm, ΔHm and ΔHc.
Glass transition may occur as the temperature of polymer sample increases, it appears as a step in the recorded DSC signal. This is due to the sample undergoing a change in heat capacity.
Figure5.2.1.2: Glass transition temperature [90].
As the temperature increases, the chains of amorphous region have enough energy to adopt the more stable conformation and arrange themselves into a crystalline form. This transition from amorphous solid to crystalline solid is known as crystallization and results in a peak (an exothermic peak) in the DSC signal.
Figure5.2.1.3: crystallisation curve [90].
With increasing temperature, the sample finally reaches its melting temperature (Tm), the chains move randomly and freely (highly mobile). When the polymer
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peak (an endothermic peak) in the DSC signal [90, 91, 92, 93].
Figure5.2.1.4: melting curve [90].
DSC is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing. One of the most significant properties of semi-crystalline polymers which contain two components: a crystalline and an amorphous, is the percent crystallinity which refers to the overall amount of crystalline component in relationship to the amorphous component.
% Crystallinity = [ΔHm – ΔHc] / ΔHmo * 100% …..5.8
Where ΔHm : Heat of melting (J/g)
ΔHc: Heat of cold crystallization (J/g)
ΔHmo: A reference value represents the heat of melting if the polymer
where 100% crystalline (ΔHmofor r-PET = 119.8 J/g) [94].
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5.2.2 Results
The phase behaviour of the r-PET flakes and pellets was analysed by DSC821 differential scanning calorimeter. The percentage of crystallinity (%Xc) for CLR- r-PET-Flakes, its pellets at 60 and 85 rpm and r-PET specimens before and after UV exposure in natural and accelerated weathering are reported in tables below.
Table 5.2.2.1: Results of DSC measurements: The percentage of crystallinity (%Xc) for CLR-r-PET-Flakes and its pellets.
Material Crystallinity, Xc (%)
CLR-r-PET-Flakes 45.77 ± 1.93 Pellets-60 rpm 17.16 ± 1.24 Pellets-85 rpm 10.50 ± 0.73
Table 5.2.2.2: The percentage of crystallinity before and after UV exposure to natural and accelerated weathering.
UV exposure time, hr Xc% for accelerated weathering Xc% for natural weathering 0 18.94 ± 2.39 18.94 ± 2.39 250 20.01 ± 2.94 19.73 ± 3.02 500 14.70 ± 2.58 16.93 ± 3.80 750 22.11 ± 6.21 15.68 ± 3.19 1000 18.74 ± 3.56 19.38 ± 2.37 2000 22.06 ± 3.84 22.42 ± 4.54 5000 21.96 ± 3.81 16.75 ± 4.04 9000 17.92 ± 4.89 21.66 ± 3.25 13000 18.66 ± 0.74 18.27 ± 2.52
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5.2.3 Discussion
DSC thermograms for CLR-r-PET-flakes and its pellets at screw speed 60 rpm and 85 rpm are shown in figures below.
Figure5.2.3.1: DSC thermogram for CLR-r-PET flakes.
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Figure5.2.3.3: DSC thermogram for r-PET pellets-85rpm
As mentioned before, during pelletizing process, there was a reduction in percentage of crystallinity 45.37%-flakes (figure5.2.3.1) to 17.04%-pellets- 60rpm (figure5.2.3.2) to 10.48-pellets-85rpm (figure5.2.3.3). This drop explains the reduction in amount of crystalline region due to the chain scission and further chain scission as the screw speed increased from 60 rpm to 85 rpm due to increasing in shear stress.
The process of chain scission produces shorter chains which are not enough to form enough folds to form crystalline segments, so the crystallinity decreased though chain breaking.
Shorter chains have more ends per unit volume than long chains. The ends of a chain can move more freely than the segments in the centre of chain and thus creating more free volume (a higher free volume with shorter chains). Also the shorter the chains, the less number of entanglement which preventing or delaying the relaxation of molecular chains. Moreover, the shorter chains have less mass, so amount of absorbed energy to relax would be less resulting in earlier relaxation of these chains at lower temperature (lower temperature for transition from glassy to rubbery state) i.e. a lower Tg as shown in figures5.2.3.1, 5.2.3.2 and 5.2.3.3; reduction in Tg: 91.11ºC (flakes) to 68.17ºC (pellets-60rpm) to 67.31 ºC (pellets-85rpm).
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After the injection moulding process of r-PET (CLR/KUDOS r-PET Pellets) the percentage of crystallinity decreased from 46.31% to 18.81% as shown in figure5.2.3.4 and figure5.2.3.5 respectively.
Figure5.2.3.4: DSC thermogram for CLR/KUDOS r-PET Pellets
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r-PET pellets suffered hydrolysis, thermal and oxidation degradation during processing which result in breaking the molecular chains, i. e. the macromolecular chains are converted into shorter chains. Thus the crystallinity of the material decreased.
Figure5.2.3.6: Crystallinity % of CLR/KUDOS r-PET Pellets and after injection moulding processing.
Figure5.2.3.7: Crystallinity percentage for r-PET samples in accelerated weathering.
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After 13000 hours of exposure to UV outdoors and inside The QUV accelerated weathering tester, the crystallinity remained unaffected as shown in figures5.2.3.7 and 5.2.3.8, explaining that the degradation is a surface effect which does not affect the whole specimens.
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