**4. Mechanical properties of iPP nucleated with α/β compounded NAs**

The effects of different NAs on the crystallization process of polymer reflect on its mechanical properties finally, which determines on the use value of the polymer. Mentioned in the introduction, the α NA can improve the stiffness and optical properties of iPP while decrease its toughness. The β NA will induce β-iPP during crystallization, which can improve toughness and heat distortion temperature of iPP while decrease its stiffness. Will it come true that we can balance the iPP's stiffness and toughness by compounding two kinds of nucleating agents?

Xu et al. investigate the effect of Phosphate/Carboxylate α/β compounded NA, NA40/HB on the mechanical properties of iPP (Xu et al., 2011). As shown in Tab.3, tensile strength (ASTM D-638) and flexural modulus (ASTM D-790) of iPP were improved with the presence of NA40 while the impact strength (ASTM D-256) decreased. On the contrary, the impact strength of iPP could increase to 3.4 times to that of pure iPP but tensile strength and flexural modulus was reduced as always by adding HB. Numerically the mechanical properties of iPP nucleated with NA40/HB were close to that iPP/NA40, which showed no sign of enhancing the toughness of iPP.


Table 3. Mechanical Properties of Pure iPP and iPP Nucleated with Individual α, β and α/β Compounded NAs (Xu et al., 2011; Zhao & Xin, 2010)

Similar to the effect on crystallization morphologies, different mechanical properties of iPP would be reached when the DCHT compounded with different α NAs. Incorporation with Sorbitol/Amide compounded NA, 3988/DCHT can significantly improve the impact

Controlled Crystallization of Isotactic Polypropylene

α/β compounded NAs.

two NAs is not significant.

Based on α/βCompounded Nucleating Agents: From Theory to Practice 135

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

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

Sample TC /℃ iPP 121.6 NA40/iPP 130.2 HB-a/iPP 125.1

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

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

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 was higher than those of pure iPP.

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