Journal of Polymer Research

, Volume 16, Issue 5, pp 489–498

Effects of additives on non-isothermal crystallization kinetics and morphology of isotactic polypropylene

Authors

  • Ali Durmus
    • Department of Chemical Engineering, Faculty of EngineeringIstanbul University
    • Department of Chemical Engineering, Faculty of EngineeringIstanbul University
Article

DOI: 10.1007/s10965-008-9252-9

Cite this article as:
Durmus, A. & Yalçınyuva, T. J Polym Res (2009) 16: 489. doi:10.1007/s10965-008-9252-9
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Abstract

In this study, effects of commercial additives such as antioxidant and stabilizer on the non-isothermal crystallization kinetics of isotactic polypropylene without nucleating agents were investigated by differential scanning calorimetry (DSC) method. Kinetic parameters by Osawa, Avrami and Liu-Mo models and apparent activation energy of the crystallization by Kissinger model were calculated. A polarized optical microscope was also used to observe crystalline morphology of the polypropylene samples crystallized at different cooling rates. On the contrary rate inducing effects of the nucleating agents on the crystallization kinetics of the polypropylene, interestingly, it was found that such types of commercial additives reduced the overall crystallization rate of the polypropylene. Based on the crystallization kinetics and morphology of the samples, it was observed that commercial additives inhibit the chain diffusion toward the growing crystal faces thus slow the crystal growth rate. Furthermore, calculated nucleation activity (ϕ) for the additives showed that they do not act as effective nucleating agents. It was found that the crystallization activation energy of additive-free sample was higher than that of the sample which has commercial additives. Activation energies were found to be 233.6 and 276.7 kJ mol−1 for the PP-1 and PP-2, respectively. Kinetic results also show importance of using of nucleating agents to increase the crystallization rate of polypropylene by increasing the nucleation and thus overall crystallization rate during polypropylene processing operations (esp. for a fast processing cycle in injection molding).

Keywords

Poly(propylene) (PP)CrystallizationAdditivesDifferential scanning calorimetry (DSC)

Introduction

Polypropylene (PP) is one of the most widely used semi-crystalline thermoplastics in many industrial applications such as extruded pipes, films and fibers, injection molded and thermoformed parts due to its physical, thermal and chemical properties including low density, high melting point, stiffness, good impact properties, chemical inertness, good clarity, excellent barrier properties and low prices. Overall crystallization process of semi-crystalline polymers consists of the nucleation and crystal growth steps. Crystallization of polypropylene can start through homogenous, heterogeneous or self nucleation [1]. It has been known that small amount of heterogeneous material in polypropylene may considerably change the crystallization temperature, crystal size, density, clarity and physical performance of the polymer. Recently, more attention has been focused on the crystallization behavior of nucleated polypropylene [211]. Isotactic PP (i-PP) has three different crystalline forms, namely the monoclinic α form, hexagonal β form and triclinic γ form [12]. Many studies have been published on the crystallization kinetics, morphology and mechanical properties of polypropylene having various types of organic or inorganic chemicals or additives called nucleating agent such as organic phosphorus salts [13], sorbitol derivatives [1416], sodium benzoate [17], alkaline dehydroabiatate [18, 19], organic pigments [20], rosin type additives [21, 22] and nanofillers [23, 24]. Generally, it was found that the nucleating agents or impurities accelerated the nucleation rate of PP and caused the higher crystallization temperature for nucleated PP than that for non-nucleated one, and sorbitol derivatives and organic phosphates are the most effective nucleating agents in PP. It has been reported that these compounds accelerate the cystallization rate of PP by reducing fold surface energy of the growing crystals. Several authors studied on effects of specific nucleating agents on the crystallization of β-PP and the effects of β crystalline phase on the mechanical properties of i-PP products [2529]. They showed that the β-iPP has better mechanical performance than that of other crystal types of i-PP. Nucleating additives are routinely used in the processing of PP to shorten injection molding cycles, improve the mechanical and optical properties by reducing spherulite size. To the best of our knowledge, there is no study yet on effects of commercial additives on the crystallization rate of polypropylene although many research efforts have been focused on the effects of nucleating agents on the crystallization behavior of i-PP in recent years.

In this study, effects of commercial additives such as antioxidant, UV stabilizer etc. added to the polymer in granulation step on the crystallization rate and morphology of i-PP were investigated by Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM) methods under non-isothermal conditions. Since the solidification generally occurs under non-isothermal conditions in the actual polymer processing operations, non-isothermal crystallization often provides useful complement to understand crystallization behavior of a semi-crystalline polymer in industrial processes. Therefore, non-isothermal crystallization was chosen as the experimental method in this study.

Experimental

Isotactic polypropylenes (PP-1 and PP-2, fiber grade MH-418) were kindly supplied from the Petkim Petrochemicals, Turkey. Melt Flow rates (MFI) and density of the samples used in this study were reported as 4–6 g 10 min−1 (ASTM D1238, 230°C and 2.16 kg) and 0.89 g cm−1, respectively. PP-1 sample is in granular form and has some commercial additives; stablizer and process aid such as calcium stearate; primary and secondary antioxidants such as aryl phosphite and hindered phenols. PP-2 sample is in powder form and contains no additives. PP-2 was produced in a pilot-plant reactor in the research laboratory of the company by the same Ziegler/Natta catalyst, process and equipment design with the commercial scale system.

Non-isothermal crystallization behaviors of the samples were examined with a Setaram (DSC 131) differential scanning calorimeter. Temperature and heat flow calibrations of the instrument were achieved with high purity indium (In), tin (Sn) and lead (Pb) metals. PP samples weighing about 10–12 mg in an aluminum crucible were heated from room temperature to 220°C at a heating rate of 10°C min−1. Samples were kept at this temperature for 3 min to eliminate thermal history and unmelted crystals that may cause the self nucleation. Then they were cooled to 20°C by using liquid nitrogen cooling device at a given constant cooling rate and the crystallization exotherms were recorded. Cooling rates employed in this study were 1.5, 2.5, 5, 10 and 20°C min−1 for investigating the crystallization kinetics of PP samples. All melting and crystallization experiments were carried out under nitrogen (N2) atmosphere at a flow rate of 100 ml min−1 to prevent oxidative degradation of the samples, esp. additive-free PP (PP-2).

Spherulitic crystal images of each PP sample that crystallized at different cooling rates were photographed with an Olympus polarized optical microscope (POM).

Results and discussion

Non-isothermal crystallization kinetics

Crystallization exotherms of PP-1 and PP-2 at various cooling rates are shown in Fig. 1a and b, respectively. Crystallization peak temperatures (Tc) where the crystallization rate is maximum at all cooling rates employed are given in Table 1 and Fig. 2 for the PP samples. As expected, Tc shifts to lower temperature with an increasing cooling rate for both PP-1 and PP-2. Tc of PP-1 was not higher than that of PP-2 for each cooling rate. Therefore, it is difficult to conclude that commercial additives can act as nucleating center for PP crystallization and crystallization occurs at higher temperatures for non-isothermal conditions. On the other hand, organic phosphite salts may cause a slight increase in the crystallization temperature. Alternatively, we used another parameter to compare whole crystallization rate of the samples. This is the half-width of the crystallization exotherms (w1/2) which defined as the width of the peak at its half height [30] and schematized in Fig. 2 as an example for the crystallization exotherms of the samples at the cooling rate of 5°C min−1. w1/2 is also dependent on the crystal size distribution. From the Fig. 2 and Table 2, it can be clearly seen that the PP-1 has a broader crystallization exotherm than that of the PP-1 at all heating rates employed. We have attributed to these broader peaks to the slower and thus longer crystallization process although crytallization of the PP-1 begins early at higher temperatures.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig1_HTML.gif
Fig. 1

Crystallization exotherms of the samples recorded at various cooling rates. a PP-1, b PP-2

https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig2_HTML.gif
Fig. 2

Crystallization exotherms of the samples recorded at the cooling rates of 5 °C min−1 and given to describe various crystallization parameters

Table 1

Crystallization peak temperatures (Tc) of the PP samples

Cooling rate (°C min−1)

Samples

PP-1

PP-2

1.5

123.0

121.7

2.5

118.5

120.0

5.0

117.5

116.3

10

113.3

112.7

20

108.6

110.0

Table 2

Half-width (w1/2) of the crystallization exotherms of the samples

Cooling rate (°C min−1)

Samples

PP-1

PP-2

1.5

5.55

3.82

2.5

5.42

3.95

5.0

6.34

4.70

10

6.23

5.30

20

7.51

6.78

Relative crystallinity (Xt) as a function of crystallization temperature and time can be obtained from the crystallization exotherms of samples by partially integrating of the crystallization exotherms. The relative crystallinity, Xt, as a function of crystallization temperature is defined as;
$$\chi _t = \frac{{\int\limits_{T_o }^T {\left( {\frac{{dH_c }}{{dT}}} \right)} dT}}{{\int\limits_{T_o }^{T\infty } {\left( {\frac{{dH_c }}{{dT}}} \right)} dT}}$$
(1)
where To and T represent the onset and end temperatures of the crystallization exotherm, respectively. Figure 3a and b show relative crystallinity of the PP-1 and PP-2 vs. temperature. Figure 4a and b show relative crystallinity of the samples versus time. All relative crystallinity vs. time curves at various cooling rates have the same characteristic sigmoidal shape implied that the lag effect of the cooling on crystallization. First non-linear initial part of the S shaped curves is generally considered as nucleation step of the crystallizaton process. Each curve showed a linear part considered as primary crystallization, subsequently a second non-linear part deviated off slightly and considered to be due to secondary crystallization, which was caused by the spherulite impingement in further step of the crystal growth. For the relative crystallinity vs. time curves, the higher cooling rate, the shorter time to complete crystallization. The most important rate parameter, crystallization half-time, (t1/2) the time taken the crystallinity of the sample reaches the value of 50% of the relative crystallinity, can be obtained from the relative crystallinity-time curves. t1/2 directly indicates the rate of crystallization process and usually reciprocal of crystallization half-time (1/t1/2, time−1) is used to compare crystallization rates of different systems. If t1/2 is short or reciprocal half-time is high it means crystallization is fast.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig3_HTML.gif
Fig. 3

Plots of relative crystallinity (Xt) vs. temperature (T) for non-isothermal crystallization of the samples. a PP-1, b PP-2

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Fig. 4

Plots of relative crystallinity (Xt) vs. time (t) for non-isothermal crystallization of the samples. a PP-1, b PP-2

t1/2 and 1/t1/2 values of PP-1 and PP-2 are given in Table 3. As expected, t1/2 values decreased with increasing cooling rates for the both samples. Interestingly, t1/2 values for the PP-2 are lower than that for the PP-1 at all cooling rates. It obviously indicates that the overall crystallization rate of additive-free PP sample was higher than that of the PP included commercial additives (PP-1). It can be concluded that the commercial additives do not act as a nucleation center like well-known nucleating agents and cannot accelerate the crystallization of the PP. On the contrary, conventional additives such like primary and secondary antioxidants, radical capturing additives and lubricants are transported out of the growing spherulite volume and they slow the chain transfer toward the growing surfaces of the polymer crystals.
Table 3

Crystallization rate parameters of the PP-1 and PP-2

Cooling rates (°C min−1)

PP-1

PP-2

t1/2 (min)

1/t1/2 (min−1)

t1/2 (min)

1/t1/2 (min−1)

1.5

0.75

1.33

0.63

1.53

2.5

1.16

0.86

0.89

1.11

5

2.43

0.41

2.02

0.49

10

4.43

0.22

3.65

0.27

20

6.20

0.16

5.69

0.17

The most used kinetic approach for the non-isothermal crystallization process of polymers is Ozawa model [31]. It is based on the extended form of Avrami approximation assuming that the non-isothermal crystallization process could be composed of small isothermal steps. Ozawa equation is as follow;
$$\chi _t = 1 - \exp \left( {\frac{{ - K\left( T \right)}}{{\Phi ^m }}} \right)$$
(2)
where Xt is the relative crystallinity, K(T) is the cooling function, Φ is the cooling rate and, m is the Ozawa constant depends on the dimension of crystal growth and nucleation mechanism. If double logarithmic form of the equation is taken, a linear relationship is obtained to calculate kinetic constants.
$$\ln \left[ { - \ln \left( {1 - \chi _t } \right)} \right] = \ln K\left( T \right) - m{\text{ln}}\Phi $$
(3)
Plotting ln [–ln (1–χt)] against ln Φ at a given temperature, a straight line should be obtained. Slope of the line is Ozawa constant, m, and intercept of that is K(T). Ozawa analyses were illustrated in Fig. 5a and b for the crystallization temperatures in the range from 112°C to 122°C for the PP-1 and PP-2, respectively. As seen, Ozawa plots of the PP-1 were not straight lines. Changing slope with temperature indicates that m is not a constant during non-isothermal crystallization of the PP-1. Some reasons have been offered why the non-isothermal crystallization process of certain systems such as poly(hydroxy butyrate) (PHB), poly(ether ether ketone) (PEEK) and poly(ether ether ketone ketone) (PEEKK) do not follow the Ozawa equation [32]. The mostly offered ones were inaccurate assumptions in Ozawa crystallization model such as secondary crystallization, dependence of lamellar thickness on crystallization temperature, constant cooling function over the whole process and quasi-isothermal nature of the method. We assumed that the additives of PP-1 could initially cause a heterogeneous crystallization medium as some part of the additives behaved as impurities and also secondary crystallization decreasing the rate of crystallization as seen in values of 1/t1/2 for the PP-1 and PP-2. Thus, the above mentioned deviations from the Ozawa model were seen only for the PP-1. On the other hand, Ozawa analysis could satisfactorily deal with the non-isothermal crystallization of the PP-2. Kinetic constants determined from the intercept and slope of these curves were given in Table 4. Ozawa constant, m increased as the crystallization temperature increased for the PP-2. Values of m at higher crystallization temperatures are higher than 3.0 for the PP-2. This result implied that the PP crystals growth in three dimensional, spherulitic morphology and crystal perfection in the growth geometry is occurred.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig5_HTML.gif
Fig. 5

Osawa plots for the PP-1 (a) and PP-2 (b) at various crystallization temperatures

Table 4

Osawa kinetic constants of the PP-2

Temperature (°C)

m

K(T)

r2

112

1.74

43.54

0.9888

114

2.44

86.87

0.9977

116

2.87

67.12

0.9983

118

3.56

66.68

0.9986

120

3.13

10.41

0.9753

122

3.76

4.34

0.9759

An alternative approach, Avrami model [33], was used in this study to compare crystallization rates of the samples in detail. However Avrami model is generally used to analyze the isothermal crystallization kinetics, it is accepted that the model can be directly applied to non-isothermal crystallization kinetics of polymers [34, 35]. According to the model, relative crystallinity develops with time as follows:
$$\chi _t = 1 - \exp \left( { - Z_t t^n } \right)$$
(4)
where n is Avrami constant depending on crystal growth mechanism, Zt is the rate constant involving both nucleation and growth rate parameters. Taking double logarithmic form of the equation
$$\ln \left[ { - \ln \left( {1 - \chi _t } \right)} \right] = \ln Z_t + n{\text{ln}}t$$
(5)
and plotting ln[–ln(1–χt)] versus ln t for each cooling rate, straight line should be obtained to determine kinetic constants. Avrami plots generally fit the experimental data linearly at low degree of crystallinity and deviate from the linear regression at higher crystallization ratio as possibly, it does not account the secondary crystallization. In non-isothermal crystallization, temperature change at a given constant cooling rate affects the rate of both nucleation and spherulite growth which are temperature dependent parameters. Considering the temperature dependent character of the non-isothermal crystallization process, the rate parameter, Zt was modified by Jeziorny [36].
$$\ln Z_c = \frac{{\ln Z_t }}{\Phi }$$
(6)
Avrami plots of the samples were shown in Fig. 6 and the kinetic constants by Jeziorny method were given in Table 5. n and Zc values of the PP-2 are slightly higher than those of the PP-1 at the same cooling rate. Value of Zc increased with increasing cooling rate, as expected. Results obtained from the Avrami model implied that the crystallization rate of PP-2 was higher than that of the PP-1 and the additives inhibited the chain transfer process resulting lower crystallization rate. It can be inferred that the rate limiting step of the crystallization process is chain transfer through the crystal growth face of the PP-1 crystals. Another method developed by Liu and Mo [37] was also used to describe the non-isothermal crystallization process. Liu et al. offered a new method combining the Avrami and Ozawa equation at a given value of relative crystallinity as follows;
$$\ln K\left( T \right) - m{\text{ln}}\Phi {\text{ = ln}}Z_t + n\ln t$$
(7)
$$\ln \Phi = \ln F\left( T \right) - a\ln t$$
(8)
where the parameter F(T)=[K(T)/Zt]1/m refers to the cooling rate, a is the ratio of the Avrami constant n, to the Ozawa constant m. According to Liu model, plotting ln Φ versus ln t, a series of straight lines are obtained at a given value of the relative crystallinity. The kinetic parameters, F(T) and a, could be determined by intercept and slope of these lines, respectively. Liu plots of the samples were shown in Fig. 7 and the kinetic parameters were listed in Table 6. At a certain value of the relative crystallinity, Xt, higher value of F(T) means high cooling rate needed to reach this Xt in a unit time which indicates the difficulty in crystallization process. As shown in Table 6, F(T) increased with an increasing relative crystallinity for both samples but the F(T) value of PP-2 was lower than that of the PP-1 at a given relative crystallinity. Liu model was satisfactorily successful to examine non-isothermal crystallization of the PP samples and showed that crystallization of the PP-1 is more difficult than that of the PP-2 due to the effects of commercial additives. a values are slightly increased with increasing in Xt. a values can also be compared with the calculated kinetic constants of the Ozawa and Avrami models. If the values of 3.5 as average Ozawa constant and 4.5 as average Avrami constant for the non-isothermal crystallization of PP-2 are taken, (generally m = n−1 is also well known relationship) value of a can be obtained to be 1.28. It was relatively consistent with the results given above.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig6_HTML.gif
Fig. 6

Avrami plots for the PP-1 (a) and PP-2 (b) at various cooling rates

https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig7_HTML.gif
Fig. 7

Lui-Mo plots for the PP-1 (a) and PP-2 (b)

Table 5

Avrami kinetic constants of the samples

Cooling rate (°C min−1)

PP-1

PP-2

1.5

2.5

5

10

20

1.5

2.5

5

10

20

N

4.29

4.69

4.63

3.56

4.39

5.04

4.95

4.66

3.43

4.46

Zc

4.5

54.8

407.5

910.3

1032.4

2.4

68.2

472.9

993.6

1078.8

r2

0.9988

0.9961

0.9994

0.9967

0.9902

0.9968

0.9944

0.9985

0.9965

0.9941

Table 6

Kinetic parameters by the Liu model

Xt (%)

PP-1

PP-2

20

40

60

80

20

40

60

80

F (T)

9.9

12.4

14.5

17.2

8.8

10.7

12.4

14.9

a

1.16

1.17

1.17

1.20

1.16

1.17

1.19

1.22

Nucleation activity

A simple method was suggested by Dobreva and Gutzow [38, 39] to calculate the nucleation activity of foreign substrates, additives etc. in a polymer melt. This method has been also used for silica nanoparticle-filled PEN [40], surface modified talc-PP composites [41] and PP/SiO2 nanocomposites [42]. Nucleation activity (ϕ) can be defined as a factor by which the work of three-dimensional nucleation decreases with the addition of a foreign substrate. If the foreign substrate is extremely active for nucleation, value of ϕ approaches 0, while it is about 1 for inert particles. Mathematically, the nucleation activity is the ratio of B parameters in heterogeneous and homogenous medium;
$$\varphi = \frac{{B^* }}{B}$$
(9)
B parameter is defined as;
$$B = \frac{{\omega \sigma ^{3} V^{2}_{{^{m} }} }} {{3nk_{B} T^{0}_{m} \Delta S^{2}_{m} }}$$
(10)
where ω is a geometric factor, σ is a specific energy, Vm is the molar volume of the crystallizing substance, n is the Avrami exponent, ΔSm is the melting entropy and \(T_{\text{m}}^{\text{0}} \) the equilibrium melting temperature.
But B parameter could be determined experimentally by using the simple definitions of non-isothermal crystallization. For homogenous nucleation, B parameter can be calculated from the following equation;
$$\ln \Phi = C - \frac{B}{{\Delta T_c^{\text{2}} }}$$
(11)
where Φ is the cooling rate, C is a constant and ΔTc is the supercooling (TmTc). For heterogeneous nucleation Eq. 11 becomes:
$$\ln \Phi = C - \frac{B}{{\Delta T_c^{\text{2}} }}$$
(12)
B can be obtained by plotting ln Φ against the \({1 \mathord{\left/ {\vphantom {1 {\Delta T_{\text{c}}^{\text{2}} }}} \right. \kern-\nulldelimiterspace} {\Delta T_{\text{c}}^{\text{2}} }}\). These plots of the PP-1 and PP-2 are given in Fig. 8. From the calculation, values of B and B* were found to be 1.0 (r2 = 0.985) and 0.88 (r2 = 0.959), respectively. From the Eq. 9, nucleation activity of commercial additives in PP-2 was calculated as 0.88. This value suggests that this type of additives does not act effectively as a nucleating agent.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig8_HTML.gif
Fig. 8

Plots of ln Φ vs. \({1 \mathord{\left/ {\vphantom {1 {\Delta T_{\text{p}}^{\text{2}} }}} \right. \kern-\nulldelimiterspace} {\Delta T_{\text{p}}^{\text{2}} }}\) for the samples

Crystallization activation energy

In DSC analysis, the activation energy of the crystalization process could be determined by the Kissinger equation [43] considering the variation of the crystallization peak temperature Tc, with the cooling rate Φ. Kissinger equation is;
$$\frac{{d\left[ {ln\left( {\frac{\Phi }{{T_c^{\text{2}} }}} \right)} \right]}}{{d\left[ {\frac{{\text{1}}}{{T_c }}} \right]}} = - \frac{{\Delta E}}{R}$$
(13)
where Φ is the cooling rate (°C min−1), Tc is the crystallization peak temperature (K), ΔE is the activation energy of crystallization process (kJ mol−1) and R is the universal gas constant (8.314 kJ mol−1 K−1). When the parameter \(\ln \left( {{\Phi \mathord{\left/ {\vphantom {\Phi {T_c^{\text{2}} }}} \right. \kern-\nulldelimiterspace} {T_c^{\text{2}} }}} \right)\) is plotted versus 1/Tc, slope of the curve gives activation energy of the process. Kissinger plots of the samples were given in Fig. 9a and b for the PP-1 and PP-2, respectively. From the calculation based on the Kissinger approach, activation energies were found to be −233.6 kJ mol−1 (r2 = 0.988) for the PP-1 and −276.7 kJ mol−1 (r2 = 0.996) for the PP-2. Calculated EA values of the samples are somewhat higher than the values reported in literature under non-isothermal conditions, but very similar to values reported by Papageorgiou et al. [42], Qian et al. [44] and Zhang et al. [45]. This difference could be originated from the structural characteristics of different polypropylenes such as molecular weight, molecular weight distribution, tacticity etc. Activation energy of the PP-1 was lower than that of the PP-2. This result can be explained by the nucleation effects. The crystallization activation energy is sum of the nucleation activation energy and chain transfer activation energy. As we mentioned in kinetic part, some of the commercial additives may act as a nucleating center (this phenomena will be discussed in the next part) and for the non-isothermal crystallization of PP-1. But this effect is not dominant to increase the overall crystallization rate. If we compare the crystallization peak temperatures of the samples given in Table 1, it could be deduced that additives slightly increased the crystallization temperature of PP. The same trend can be observed in the very early stage of the crystallization. Temperatures at the relative crystallinity 0.01 and different cooling rates given in following are 129.5 (1.5°C min−1), 124.7 (5°C min−1) and 115.85 (20°C min−1) for the PP-1 and 127.4 (1.5°C min−1), 121.8 (5°C min−1) and 116.2 (20°C min−1) for the PP-2. It can be said that some parts of the additives (e.g. stearate salts) may show nucleation effects and reducing in nucleation activation energy may affect the overall activation energy. Therefore, we believe that the higher activation energy for the additive-free sample, PP-2, originates from the difficulty of its self-nucleation.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig9_HTML.gif
Fig. 9

Kissinger plots of the samples a PP-1, b PP-2

Morphology

Optical micrographs of the PP-1 crystallized at the cooling rates of 1.5, 5 and 10°C min−1 and those of the PP-2 crystallized at the same cooling rates are given in Fig. 10. Pictures of the spherulites were observed in a polarized optical microscope with a filter and photographed to discuss effects of the additives on the spherulite size at a given constant cooling rate. As expected, spherulite size decreased with increasing cooling rate for the both PP samples. It is obvious that spherulites of the PP-2 were bigger than those of the PP-1 at any cooling condition. For the same cooling rate, it can be assumed that dynamic effects of temperature and cooling conditions must be equal on the crystallizing system. Thus other parameters, such as molecular structure, molecular weight, additives and impurities which may cause heterogeneous nucleation and affect the crystallization rate could be discussed. As can be seen from the micrographs of PP-1, additives resulted in decreasing the spherulite size and less perfection in crystal geometry. According to Mercier [46], a good nucleating agent reduces the spherulite diameter from 1/5 to 1/10 that of the unseeded polymer. In this study, much smaller spherulites for PP including the commercial additives were not observed. Beck [47] studied to evaluate a series of compounds as nucleating agents for PP crystallization and reported that some obvious correlations between each compound and its structural characteristics. Well-known characteristics of a good nucleating agent for PP should possess capability of decreasing the interfacial surface energy, insolubility in polymer at and below the polymer melting temperature, stability above the polymer melting point and similarity in the crystalline structure to that of polymer if possible. Beck also reported that the presence of unsaturated carbon groups in the aliphatic chain of an additive appear to inhibit nucleation somewhat compared with the saturated analogs and the molecular geometry of additives is important based on the most symmetrical compounds crystallized the fastest. Additives of the PP-1 sample used in this study are mainly large organic compounds having large amount of unsaturation due to its antioxidant capability and many aromatic groups which are not similar to the chain structure of the polypropylene. We conclude that these additives could not possess a favorable surface to increase the crystallization rate of PP based on our kinetic results. Smaller spherulite size of the PP-1 comparing with the PP-2 at a given cooling rate could be attributed to the fact that the crystal growing rate (G) of PP-1 is lower than that of the PP-2. Furthermore, one of commercial additives, aliphatic stearate salt, might also affect to nucleation rate due to similarity between molecular structures of stearate salt and PP. This behavior can be clarified by studying isothermal crystallization kinetics of the samples and determining the nucleation (I) and crystal growth rate (G) individually by models such as modified Avrami and Lauritzen-Hoffman.
https://static-content.springer.com/image/art%3A10.1007%2Fs10965-008-9252-9/MediaObjects/10965_2008_9252_Fig10_HTML.gif
Fig. 10

Polarized optical microscopy (POM) images of the PP-1 (left column) and PP-2 (right column) at various cooling rates, a 1.5°C min−1, b 5°C min−1 and c 10°C min−1. Scale bars indicate 100 μm

Conclusion

In this paper, effects of very small amount of commercial additives such as antioxidant, stabilizer etc. having chemical structure different from the nucleating agents, on the crystallization rate of i-PP were investigated by DSC method. It was found that these type of additives do not behave as an effective nucleator and do not cause an increase in the overall crystallization rate of PP. Contrarily, we observed that these additives reduced the crystallization rate of PP. Based on the kinetic parameters and morphological observation, we conclude that they limit the crystal growth rate by slowing the chain transfer toward the developing crystal face. Our results also indicate the importance of nucleating agents for PP processing in order to accelerate overall crystallization rate.

Acknowledgement

This study was supported by the Research Fund of Istanbul University (Project number: T-835/07032000). The authors thank to Petkim Petrochemicals for supplying samples used in this work and Professor Dr. Serhat Pabuccuoglu, Istanbul University, for his helps in POM study.

Copyright information

© Springer Science+Business Media B.V. 2008