**2. Crystallization kinetics of iPP nucleated with α/β compounded NAs**

Crystallization process of semi crystalline polymers such as polypropylene can have a dramatic impact on the mechanical properties. Thus, we studied the crystallization kinetics of iPP nucleated by α/β compounded NAs first.

Isothermal crystallization kinetics of iPP nucleated with Phosphate/Amide compounded NA, NA40/NABW was studied by Zhang et al. (Zhang & Xin, 2007). The results showed that Avrami equation, as shown below, was quite successful for analyzing the experimental data of the isothermal crystallization kinetics.

$$1 - X\_t = \exp(-Z\_t t'') \tag{1}$$

where *Xt* is the relative crystallinity at time *t*, *n* is Avrami exponent, a constant whose value depends on the mechanism of nucleation and on the form of crystal growth, and *Zt* is a constant containing the nucleation and growth parameters. The addition of NA40/NABW could shorten crystallization half-time (t1/2) and increase crystallization rate of iPP greatly. Consequently the molding cycle time of iPP would be reduced obviously, which has great importance for polymer processing. The Avrami exponents of iPP and nucleated iPP were close to 3, indicating that the addition of nucleating agents did not change the crystallization growth patterns of iPP under isothermal conditions and the crystal growth was heterogeneous three-dimensional spherulitic growth. The Caze method was applied to study on the non-isothermal crystallization kinetics of nucleated iPP by Phosphate/Amide compounded NA, NA11/DCHT (Zhao & Xin, 2010). It can be seen from the results that the addition of the α/β compounded NAs can obviously shorten t1/2 of iPP, especially at lower cooling rates. When the cooling rate Φ is 2.5℃/min, t1/2 of nucleated iPP was 104.9s, while that of pure iPP was 135.4 s. The Avrami exponent n for nucleated iPP indicated that the α/β compounded NA acted as heterogeneous nuclei followed by three-dimensional spherical growth during non-isothermal crystallization. Therefore, the type of nucleation of iPP was significantly changed in the presence of the α/β compounded NAs while the geometry of crystal growth of iPP did not change.

Bai et al. investigated the isothermal crystallization kinetics of nucleated by Sorbitol/ Amide compounded NA, DMDBS/TMB-5 (Bai & Wang, 2009). The crystallization kinetics parameters suggested that compounded NA accelerated the crystallization process of iPP greatly. t1/2 of iPP/DMDBS/TMB-5 was much smaller than iPP, indicating the faster crystallization process by the addition of compounded NA. For all the samples, the Avrami exponent value n ranges from 2 to 3, which means spherulite development arose from an athermal heterogeneous nucleation. The fold surface free energy of virgin iPP and nucleated iPP was also calculated from the crystallization kinetics. Samples with addition of compounded NA resulted in smaller values. That means interfacial surface free energy of iPP was reduced with the presence of compounded NA. Similar results were obtained by

In this work, three kinds of well studied α/β compounded NAs, Phosphate/Amide, Sorbitol/Amide, and Phosphate/Carboxylate were selected to review. This short review aims to present some conclusions of α/β compounded NAs and to lay the foundation for

Crystallization process of semi crystalline polymers such as polypropylene can have a dramatic impact on the mechanical properties. Thus, we studied the crystallization kinetics

Isothermal crystallization kinetics of iPP nucleated with Phosphate/Amide compounded NA, NA40/NABW was studied by Zhang et al. (Zhang & Xin, 2007). The results showed that Avrami equation, as shown below, was quite successful for analyzing the experimental

where *Xt* is the relative crystallinity at time *t*, *n* is Avrami exponent, a constant whose value depends on the mechanism of nucleation and on the form of crystal growth, and *Zt* is a constant containing the nucleation and growth parameters. The addition of NA40/NABW could shorten crystallization half-time (t1/2) and increase crystallization rate of iPP greatly. Consequently the molding cycle time of iPP would be reduced obviously, which has great importance for polymer processing. The Avrami exponents of iPP and nucleated iPP were close to 3, indicating that the addition of nucleating agents did not change the crystallization growth patterns of iPP under isothermal conditions and the crystal growth was heterogeneous three-dimensional spherulitic growth. The Caze method was applied to study on the non-isothermal crystallization kinetics of nucleated iPP by Phosphate/Amide compounded NA, NA11/DCHT (Zhao & Xin, 2010). It can be seen from the results that the addition of the α/β compounded NAs can obviously shorten t1/2 of iPP, especially at lower cooling rates. When the cooling rate Φ is 2.5℃/min, t1/2 of nucleated iPP was 104.9s, while that of pure iPP was 135.4 s. The Avrami exponent n for nucleated iPP indicated that the α/β compounded NA acted as heterogeneous nuclei followed by three-dimensional spherical growth during non-isothermal crystallization. Therefore, the type of nucleation of iPP was significantly changed in the presence of the α/β compounded NAs while the

Bai et al. investigated the isothermal crystallization kinetics of nucleated by Sorbitol/ Amide compounded NA, DMDBS/TMB-5 (Bai & Wang, 2009). The crystallization kinetics parameters suggested that compounded NA accelerated the crystallization process of iPP greatly. t1/2 of iPP/DMDBS/TMB-5 was much smaller than iPP, indicating the faster crystallization process by the addition of compounded NA. For all the samples, the Avrami exponent value n ranges from 2 to 3, which means spherulite development arose from an athermal heterogeneous nucleation. The fold surface free energy of virgin iPP and nucleated iPP was also calculated from the crystallization kinetics. Samples with addition of compounded NA resulted in smaller values. That means interfacial surface free energy of iPP was reduced with the presence of compounded NA. Similar results were obtained by

1 exp( ) *<sup>n</sup> X Zt t t* (1)

**2. Crystallization kinetics of iPP nucleated with α/β compounded NAs** 

compounding α and β NAs afterwards.

of iPP nucleated by α/β compounded NAs first.

data of the isothermal crystallization kinetics.

geometry of crystal growth of iPP did not change.

the study on the non-isothermal crystallization kinetics of iPP nucleated by Sorbitol/Amide compounded NA, 3988/DCHT (Zhao & Xin, 2010).

Except for Amide NA, Carboxylate NA is proved to be another highly effective β NA for iPP. Xu gave us the picture of non-isothermal crystallization kinetics of iPP nucleated by Phosphate/Carboxylate compounded NA, NA40/H-Ba (Xu, 2010). From the point view of crystallization temperature, the addition of NA40/H-Ba enhanced the crystallization rate of iPP. Judging from the Avrami exponent, the spherulite of iPP grew in the way of threedimensional during non-isothermal crystallization with the presence of NA40/H-Ba, which was in accordance with the other two α/β compounded NAs.


Table 1. Isothermal crystallization kinetics parameters of pure iPP and nucleated iPP (Bai & Wang, 2009; Zhang & Xin, 2007)

Controlled Crystallization of Isotactic Polypropylene

dimension spherical growth.

NA40 individually.

crystallized at 135 °C (Xu et al., 2011)

Based on α/βCompounded Nucleating Agents: From Theory to Practice 129

NAs will increase the crystallization temperature of iPP, shorten the crystallization halftime. Consequently the molding cycle time of iPP will be reduced obviously, which has great importance for polymer processing. The obtained Avrami exponents indicated that the type of nucleation of iPP is changed from homogeneous to heterogeneous in the presence of the α/β compounded NAs while the geometry of crystal growth of iPP remains three-

**3. Crystallization morphologies of iPP nucleated with α/β compounded NAs**  The spherulite size of iPP can be decreased by cooperation with any kinds of NAs. But the morphology of nucleated iPP largely depends on the types of NA. The α NA will only induce α form iPP while β form iPP can be obtained by incorporating with β NA. Then what

Polarized optical microscope was used to investigate the crystallization morphologies of iPP nucleated with Phosphate/Carboxylate compounded NA, NA40/H-Ba by Xu et al. (Xu et al., 2011). As shown in Fig.1, in nucleated iPP, a large number of nuclei would be produced due to the existence of NAs. Therefore the spherulites cannot grow large enough to overlap, the size of spherulites in nucleated iPP would be much smaller than those in pure iPP. But as to the morphologies of the samples, iPP nucleated with NA40/H-Ba showed no sign of bright and colorful β crystals, appeared much close to the morphology of iPP induced by

Fig. 1. Polarized light microphotographs for pure iPP and nucleated iPP samples

The crystallization morphologies of pure iPP and iPP induced by Sorbitol/Amide compounded NA, 3988/DCHT were shown in Fig.2 (Zhao & Xin, 2010). From figure, it can be seen that with the addition of the α/β compounded NA, the spherulite size decreased

about the morphologies of iPP nucleated by α/β compounded NAs?


Table 2. Non-isothermal crystallization kinetics parameters of pure iPP and nucleated iPP (Xu, 2010; Zhao & Xin, 2010)

Isothermal and non-isothermal crystallization kinetics of iPP nucleated by three kinds of α/β compounded NAs were reviewed in this section. It can be concluded that compounded

Cooling rate

<sup>Φ</sup>/(℃/min) TC/℃ t1/2/s n

3.75±0.03

3.66±0.11

3.75±0.03

2.88±0.25

3.67±0.09

4.52±0.04

2.5 121.4 135

5 118.8 78 10 115.9 44 20 112.7 23 40 108.9 14

2.5 133.7 104

5 131.2 60 10 128.5 25 20 125.6 16 40 121.7 8

2.5 121.4 135

5 118.8 78 10 115.9 44 20 112.7 23 40 108.9 14

2.5 128.7 121

5 124.6 69 10 120.2 43 20 115.4 27 40 111.9 17

2.5 127.8 127

5 124.7 124 10 121.6 121 15 119.8 119 20 118.4 118

2.5 137.9 115

5 135.3 60 10 132.6 33 15 130.8 23 20 129.7 18

Compounded NAs

iPP

NA11/DCHT

iPP

3988/DCHT

iPP

NA40/H-Ba

Table 2. Non-isothermal crystallization kinetics parameters of pure iPP and nucleated iPP

Isothermal and non-isothermal crystallization kinetics of iPP nucleated by three kinds of α/β compounded NAs were reviewed in this section. It can be concluded that compounded

Phosphate/Amide

Sorbitol/Amide

Phosphate/Carboxylate

(Xu, 2010; Zhao & Xin, 2010)

NAs will increase the crystallization temperature of iPP, shorten the crystallization halftime. Consequently the molding cycle time of iPP will be reduced obviously, which has great importance for polymer processing. The obtained Avrami exponents indicated that the type of nucleation of iPP is changed from homogeneous to heterogeneous in the presence of the α/β compounded NAs while the geometry of crystal growth of iPP remains threedimension spherical growth.
