by James A. Browne AL


Comparison of Crystallization Behavior of Different-Colored Polypropylene Parts Using a Single DSC Experiment


Differential scanning calorimetry (DSC) is a powerful tool used to obtain information from materials based on the response to change in temperature, including phase changes and various kinetic events. The technique is often used to characterize and compare polymers both as a means of determining initial processing parameters and as a diagnostic tool for identifying certain processing problems. One common problem in polymer processing is variations in cycle time between plastics that are known to have the same composition and formulation. The addition of color concentrates, fillers, additives and other plastics can affect cycle times and end-use properties. When variations in processing behavior are observed, often an initial investigation is a compositional analysis that may or may not yield an explanation and is costly and time consuming. A simple DSC experiment can demonstrate significant potential processing differences between what are believed to be similar materials.


In addition to a comparison of thermal events, another way to obtain more information from the DSC experiment is to analyze the crystalliza- tion (cooling) data using the Avrami macro kinetic model. This experiment is done at a constant cooling rate, so it is nonisothermal, but the data generated can be easily converted to a time scale. This provides a use- ful comparison of crystallization half-time, rate constant and geometric exponent in addition to the crystallization temperature (TC), and is an effective way to compare two or more samples to determine if there is a need for more extensive kinetic study. From a practical standpoint, it can be used to verify analytically that there are differences between “good” and “bad” samples. Essentially, this is a single-point dynamic or noniso- thermal crystallization experiment; basic definitions are given below.


Isothermal crystallization experiment Isothermal crystallization studies of plastics and other materials by DSC


have been extensively utilized and documented in the literature, and many use the Avrami equation to fit the crystallization data. In the isothermal crystallization DSC experiment, the fraction crystallized as a function of time can be expressed by Eq. (1):


(1)


ΔHC ΔHt dHC t0


is overall enthalpy of crystallization is enthalpy crystallized at extent of conversion


is enthalpy of crystallization during infinitesimal time range (dt is time at initial crystallization


t is times during crystallization t∞


is time when crystallization process is complete.


The function of crystallized fraction X(t) can be fitted using Eq. (2), the Avrami equation:


(2)


where: X(t) is fraction crystallized at time (t) ka na


is Avrami rate constant is Avrami exponent


t is time. The linear form of the Avrami equation is shown in Eq. (3):


(3)


A plot of the log (–ln(1 – X(t)) versus log t is linear and yields the Avrami parameters ka


(antilog of intercept) and na


correlates with the nucleation growth geometry and is summarized in Table 1.


Single-point nonisothermal crystallization experiment The Avrami equation is also used in nonisothermal crystallization stud-


ies. Contrasted with the isothermal method, nonisothermal crystallization data is obtained by heating or cooling the sample at a cooling rate instead of isothermally. In this simple form of the experiment, a single DSC heating rate is utilized, and the Avrami parameters ka


and na , Avrami rate constant


and nucleation exponent are compared. An analogous expression for Eq. (1) is shown in Eq. (4) for fraction crystallized as a function of temperature:


where: X(t) is fraction crystallized at time (t)


AMERICAN LABORATORY 10 JANUARY/FEBRUARY 2017 (4) (slope). The Avrami exponent )


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