**5. Optimization method for compounding α, β NAs**

Through reviewing on the crystallization kinetics, crystallization morphologies, and mechanical proprieties of iPP nucleated by α/β compounded NAs, it can be noticed that some α, β NAs can induce iPP during crystallization respectively when they are compounded, hence improve the stiffness and toughness simultaneously. While some α or β NA will play a leading role when it compounds with another NA. Thus the nucleating effect of the compounded NA appears close to the leading one, which goes against original intention of compounding α and β NA. So find out the key factor of affecting the effect of α/β compounded NAs is the precondition of successfully compounding α and β NA.

From the traditional crystallization point of view, the overall crystallization rate depends on two stages: nucleation and growth. In the nucleation process, the formation of nucleus relies on the molecular movement in the molten spontaneously. Once the nucleus came into existence, the crystal grows in the form what nucleus is. So nucleation is the precondition for crystallization. The role of NA is to provide a large number of nuclei before the self-nuclei formed, which results in changing the homogeneous nucleation into a heterogeneous one. Furthermore, several studies on crystallization kinetics show that the NA has little impact on the growth stage of crystallization (Cai et al., 2010; Huang et al., 2005; Zhao & Xin, 2010). Accordingly, we believe that the crystallization form of polypropylene depends on the NA which comes into effect first in the nucleation stage. This sequence can be judged by the crystallization temperature (Tc) of polypropylene nucleated with NA individually. The NA with higher Tc means earlier the NA nuclei could be "accepted" by polypropylene and consequently comes into effect first in compounded system. So Tc is considered as the key factor of affecting the effect of α/β compounded NAs.

Zhao et al. confirmed that the effect of compounded NA depends on which NA come into effect first during the nucleation stage (Zhao & Xin, 2010). The Tc of iPP induced by different NA individually was illustrated in Fig.7. It can be seen that Tc of iPP induced by DCHT was much higher than that of 3988. According to the mentioned assumption, when DCHT compounds with 3988, DCHT would play a leading role. Refers to the results in section 3 and section 4, it is clear that the nucleating effect such as crystallization morphologies and mechanical properties of Sorbitol/Amide compounded NA, 3988/DCHT appeared close to

strength of iPP, but was not benefit to the stiffness like tensile strength, flexural modulus (Zhao & Xin, 2010). It is exciting that the goal of enhancing the stiffness and toughness of iPP simultaneously can be reached by compounding DCHT with another α NA, NA11. The tensile strength, flexural modulus and impact strength of iPP nucleated with NA11/DCHT

Mechanical properties such as tensile strength, flexural modulus and impact strength of iPP nucleated with three kinds of α/β compounded NAs were investigated. The similar results to the study on crystallization morphologies were obtained. Incorporation with Phosphate/Carboxylate compounded NA, NA40/H-Ba only enhanced the stiffness of iPP, while with Sorbitol/Amide compounded NA, 3988/DCHT increased the toughness of iPP, which was close to iPP nucleated by DCHT individually. Compounding NA40 and DCHT could reach a good balance between stiffness and toughness of iPP. Then, what factor plays

Through reviewing on the crystallization kinetics, crystallization morphologies, and mechanical proprieties of iPP nucleated by α/β compounded NAs, it can be noticed that some α, β NAs can induce iPP during crystallization respectively when they are compounded, hence improve the stiffness and toughness simultaneously. While some α or β NA will play a leading role when it compounds with another NA. Thus the nucleating effect of the compounded NA appears close to the leading one, which goes against original intention of compounding α and β NA. So find out the key factor of affecting the effect of α/β compounded NAs is the precondition of successfully compounding α and β NA.

From the traditional crystallization point of view, the overall crystallization rate depends on two stages: nucleation and growth. In the nucleation process, the formation of nucleus relies on the molecular movement in the molten spontaneously. Once the nucleus came into existence, the crystal grows in the form what nucleus is. So nucleation is the precondition for crystallization. The role of NA is to provide a large number of nuclei before the self-nuclei formed, which results in changing the homogeneous nucleation into a heterogeneous one. Furthermore, several studies on crystallization kinetics show that the NA has little impact on the growth stage of crystallization (Cai et al., 2010; Huang et al., 2005; Zhao & Xin, 2010). Accordingly, we believe that the crystallization form of polypropylene depends on the NA which comes into effect first in the nucleation stage. This sequence can be judged by the crystallization temperature (Tc) of polypropylene nucleated with NA individually. The NA with higher Tc means earlier the NA nuclei could be "accepted" by polypropylene and consequently comes into effect first in compounded system. So Tc is considered as the key

Zhao et al. confirmed that the effect of compounded NA depends on which NA come into effect first during the nucleation stage (Zhao & Xin, 2010). The Tc of iPP induced by different NA individually was illustrated in Fig.7. It can be seen that Tc of iPP induced by DCHT was much higher than that of 3988. According to the mentioned assumption, when DCHT compounds with 3988, DCHT would play a leading role. Refers to the results in section 3 and section 4, it is clear that the nucleating effect such as crystallization morphologies and mechanical properties of Sorbitol/Amide compounded NA, 3988/DCHT appeared close to

a dominant role when compounding α, β two kinds of nucleating agents?

**5. Optimization method for compounding α, β NAs** 

factor of affecting the effect of α/β compounded NAs.

was higher than those of pure iPP.

that of DCHT. It can be noticed from Fig.7 that Tc of NA11 showed little difference to that of DCHT. During crystallization competitive nucleation takes place between two NAs, which results in the combined crystallization morphology and simultaneously increasing tensile strength, flexural modulus and impact strength of iPP. Xu et al. came to the same result by investigating Phosphate/Carboxylate compounded NA, NA40/H-Ba (Xu et al., 2011). NA40, the one with higher Tc plays a leading role in the crystallization while H-Ba showed no effect on inducing iPP. Therefore Tc is proved to be the key factor of affecting the effect of α/β compounded NAs.

Fig. 7. DSC melting curves of iPP nucleated with individual α or β NAs (Zhao & Xin, 2010)


Table 4. Crystallization temperature of iPP nucleated with different NAs (Xu et al., 2011)

The key factor of affecting the α/β compounded NAs was summarized in this part. That is the crystallization temperature of polypropylene nucleated with NA individually. The NA with higher Tc plays a leading role in the crystallization process. Consequently the mechanical properties, crystallization properties and crystallization morphologies of iPP appear close to it. Competitive nucleation will occur when the difference of Tc between the two NAs is not significant.

According to this, Tc becomes the one we can adjust that controls the crystallization behaviors of iPP based on α/β compounded NA. It can be easily conclude that the principle of compounding α and β NA is to make Tc of two NAs as close as possible, so as to let competitive nucleation happen. As known to all, the Tc of a NA depends on not only the species but the content of it as well. That is to say various addition amount of the same NA leads different Tc. Then, method for compounding α and β NAs can be developed according

Controlled Crystallization of Isotactic Polypropylene

(addition amount 0.2 wt %)

Based on α/βCompounded Nucleating Agents: From Theory to Practice 137

Fig. 9. TC <sup>α</sup> and TC <sup>β</sup> at different ratio of NA40/H-Ba and NA40/PA-03 compounded NAs

Fig. 10. Effect of NA40/H-BA and NA40/PA-03 compounded NAs with different ratio on

mechanical properties of iPP (addition amount 0.2wt %) (Shi & Xin, 2011)

to this: First, we shall obtain Tc of iPP nucleated with different addition amounts of α and β NAs individually by DSC. Then list TC <sup>α</sup> and TC β at each ratio of compounded α/β with a fixed concentration. The ratio which contains TC <sup>α</sup> = TC <sup>β</sup> will be the optimal compounded ratio of these two α and β NAs at this concentration. In this context, competitive nucleation will occur during crystallization.
