Thermal analysis of Polymers

Thermal Analysis of Polymers – Aramids and its blends

Introduction (Thermal Analysis of polymers)

The changes in material properties with a change in temperature under controlled temperature programs are studied in the thermal analysis of polymers. These techniques include thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTGA), and differential scanning calorimetric (DSC), differential thermal analysis of polymers (DTA). These techniques help us to understand the behavior of materials such as phase change, crystallization, and melting behavior with temperature.

The two most commonly used thermal analysis of polymers techniques are TGA and DSC and they can be run simultaneously in one instrument. In TGA the mass loss during heating is monitored under controlled temperature which provides information about thermal stability and changes in the composition of the material, while in DSC the heat flow due to transitions in material as a function of time and temperature (Fig. 01).  In DSC, the heat of fusion, melting temperature, and glass transition temperature, latent heat of fusion, and specific heat capacity.

Setup of TGA and DSC for thermal analysis of polymers.
Setup of TGA and DSC for thermal analysis of polymers.

Thermogravimeteric Analysis of Aramid and its Polymer Blend

Aramids are polymers having an amide group in their repeating unit. They are being used in the form of Aramid fiber, Twaron fibers, and KEVLAR fibers. Aramid blends are created by the addition of hyperbranched polymers along with plasticizers to improve the processability of Aramids. Thermal characterization process is also being used for Polyimides.

The thermal analysis of polymers (pure aramid) and some of the blend composition showed the thermal stability of the polymers and effect of blending on the thermal profile of a pure aramid (Table. 01). The effect of temperature on the polymers was studied in 50ml/min of N2 at a heating rate of 10oC per minute from room temperature to 1000oC. The results obtained from a thermal analysis of polymers showed high thermal stability of 100% aramid with T10 of 186oC at which 10% weight loss occurred, Tmax ranging from 481oC to 676oC.The char yield as 10% at 700oC which is sufficient evidence of excellent high-temperature properties this can be ascribed to the presence of aromatic ring and para-aramid in the structure.

Thermal profile of aramid and different blend compositions. (thernal analysis of polymers)
Thermal profile of aramid and different blend compositions.

aT10: Temperature for 10% weight loss.

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bTmax: Maximum decomposition temperature.

c Yc: Char yield; weight of polymer remained.

The thermal profile during the thermal analysis of polymers of the blend compositions (40%, 60%, 70%, 80% aramid) was analyzed by TGA. It can be seen from obtained results that blend having 70% aramid and 30% HBPAE has thermal profile almost close to a pure one.

The maximum decomposition temperature was observed to be 666oC and char yield of 9 at 700oC and these values are almost equal to the pure aramid. The incorporation of heterocyclic pyrimidine and amide groups from the HBPAE resulted in the formation of intermolecular hydrogen bonding between amide NH and pyrimidine Nitrogen as well as aromatic rings and para-amide groups from aramid complemented the high thermal stability.

Structural repeating units of blended aramid.
Structural repeating units of blended aramid.

The other blend compositions with 40%, 60%, and 80% aramid showed deviation from the pure aramid. The blend having 40% aramid has the lowest char yield at 700oC i.e. 4, which can be very well justified by a decrease in aramid content and increase in HBPAE content. The increase in HBPAE content in the structure lowered the thermal properties as the branching was increased and the crystalline content due to aramid was decreased.

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Thermographs of aramid and different blend compositions at heating rate of 10 C /min in N2.
Thermographs of aramid and different blend compositions at heating rate of 10 C /min in N2.

So the TGA analysis of the blends gave us the idea that a blend of 70% aramid and 30% HBPAE is the desired composition with optimum results and fewer deviations of thermal properties from pure aramid (Fig. 03). Blending aramid with 30% HBPAE does not significantly affect its high thermal stability but can lower the Tg required for enhanced processability of the aramid. This blend composition rendered maximum decomposition temperature of 666oC and char yield (700oC) of 9% and these values are almost equal to the pure aramid. The incorporation of heterocyclic pyrimidine and amide groups from the HBPAE resulted in the formation of intermolecular hydrogen bonding between amide NH and pyrimidine nitrogen thus substantiating the high thermal stability.

  • Viscometery

The viscosity of HBPAE was suggested lower than pure aramid due to high degree of branching which result in less entanglement of chains. For HBPAE blended in the pure aramid the viscosity was found to be 0.25 dL/g.

  • Differential Scanning Calorimetric (Thermal analysis of polymers)

Polymers act as glassy solids below glass transition temperature; this is because segments of polymers do not possess energy high enough for rotation. When heat is provided, energy is obtained as the volume is increased and chains of the polymers start to flow which transforms the glassy state of the polymer into the rubbery state. The temperature at which this transformation takes place is known as glass transition temperature (Tg).

DSC curves of aramid and different blend compositions at heating rate of 10oC per minute in N2.
DSC curves of aramid and different blend compositions at heating rate of 10oC per minute in N2.

The effect of temperature on the polymers was studied in 50ml/min of N2 at a heating rate of 10oC per minute from -50 to 300oC (Fig. 04). The heat flow plotted against temperature gives the Tg value which is noted to be 151 °C for aramid and 79oC for blend composition with 30 wt% HBPAE and 70 wt% aramid.

Tg and Tm of aramid and blends. (thermal analysis of polymers)
Tg and Tm of aramid and blends.

This result proves that due to the plasticization effect of HBPAE on aramid, Tg can be adjusted so as to make the aramid process-able.

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Reference

Thermal Analysis of Polymers: Fundamentals and Applications