AL 4. Temperature and heat flow calibration: indium and sapphire standards
5. Method: Samples were heated to 235 °C and subsequently cooled at 10 °C/min to 0 °C.
Results and discussion Figure 2 shows an overlay of nonisothermal crystallization exotherms. It is
obvious that the different-colored caps yield a substantially different array of exotherms of various shapes, symmetries and crystallization tempera- tures. The fraction crystallized as a function of temperature, generated from Eq. (4), is shown in Figure 3. Figure 4 depicts the fraction crystallized after converting to time domain (Eq. [6]) and applying Eq. [1]. It is appar- ent that the sample with the highest crystallization temperature is not necessarily the fastest to crystallize.
The single-point Avrami analysis is summarized in Table 2 and Figure 5. Each color sample and the unpigmented reference show different crystal- lization characteristics. The Avrami exponent n is related to the nucleation geometry. It does not hold the same meaning as in the isothermal ex- periment, but can be used empirically to compare similar samples. The different values for n observed in the samples indicate that the materials crystallize differently and may have different physical properties. Each of the samples has a rate constant k that is substantially higher than the sample with no colorant.
Conclusion The samples analyzed in this experiment yield varying Avrami parameters and will likely process differently. For polypropylene parts, the effect of additives on the crystallization performance can be compared empirically with a single-point Avrami analysis.
Additional reading 1. Hongfang Zhang, H.; Mo, Z. et al. Polym. Testing 2005, 24, 873–85. 2. Supaphol, J. Appl. Polym. Sci. 2000, 78, 338. 3. Supaphol, J.; Thanomkiat, P. et al. Polym. Testing 2004, 23, 881.
James A. Browne is senior applications scientist, TA Instruments, 159 Lukens Dr., New Castle, Del. 19720, U.S.A.; tel.: 302-427-4000;
www.tainstruments.com
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Figure 4 – Fraction crystallized as a function of time.
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www.carverpress.com Figure 5 – Avrami comparison of colored pen caps. AMERICAN LABORATORY 13 JANUARY/FEBRUARY 2017
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