Effects of reinforcement filler and temperature on the stability of β-crystal in glass bead filled polypropylene

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Abstract

The effects of crystallization temperature (Tc), glass bead content and its size on the formation of β-crystal and structural stability of originally formed β-crystal in glass bead filled polypropylene (PP) were examined. The differential scanning calorimetry (DSC) measurements indicated that the amount of β-phase in PP crystals was a function of the crystallization temperature and glass bead content. For a constant crystallization temperature, it was observed that the amount of β-crystal initially increased with increase in glass bead content up to 30 wt.%, and then decreased slightly with further increase in the filler content. From the DSC data, a disorder parameter (S) was derived to define the structural stability of originally formed β-crystals. The structural stability of originally formed β-crystals was enhanced with increase in either the crystallization temperature or the glass bead content. Also, the influence of glass bead size (4–66 μm) on the formation and stability of β-crystals in PP/glass bead blends was studied. Large glass bead particles suppressed the formation and decreased the stability of β-crystals.

Introduction

Semicrystalline isotactic polypropylene (iPP) can crystallize into several crystal modifications differing in their helical conformation and chain packing including monoclinic (α), hexagonal (β), and triclinic (γ) forms [1]. Since the demonstration by polarized light microscopy (PLM) [2] and X-ray diffraction studies [3] that α- and β-crystals can together form in iPP, polymorphism in iPP has been the subject of a number of investigations [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. It is known that the melting plots of PP with mixed polymorphic content exhibit a complicated melting profile because of the melting of α- and β-crystals, the βα-recrystallization phenomenon and the recrystallization within α- or β-crystal [23]. In spite of a number of studies, the mechanism of formation of α- and β-crystals and the transformation of one phase to another is still not understood. Moreover, Varga et al. [17] reported that the thermal stability of β-crystals is influenced by the nature and content of selected β-nucleating agents. Zhou et al. [24] realized that a simple K parameter (defined as the relative amount of the β-crystal) is inadequate to comprehensively characterize the crystallinity and structural disorder parameter of the β-crystal in X-ray diffraction study. Thus, a structural disorder parameter S was proposed to characterize the β-crystal, effects of crystallization temperature and β-nucleating content on K and S.

The properties and performance of PP are determined by the crystalline morphology, nature and the amount of crystalline phase, the thickness of the lamellae, and the size of the spherulites [8]. Fillers and reinforcements nucleate PP to different extent [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Talc has a strong nucleating effect, while nucleating effect of CaCO3 in PP is weak [10], [25], [26]. Recent studies indicated that layered silicates may also nucleate PP [31], [32], [33], [34]. Glass bead is a commonly used filler that significantly improves the mechanical properties of polymer blends [35], [36]. Recently, we observed that the amount of β-crystals in PP depends on the glass bead content, size, and annealing temperature, influencing the tensile properties of polypropylene/glass bead blends [37], [38].

The β-crystals of PP governs the mechanical properties, thermal properties, and processability. However, studies concerning the effects of crystallization temperature, filler content and size on the formation and structural stability of β-crystals in PP blends filled with glass bead are limited. In this study, we describe the behavior of relative amount of β-crystal in PP/glass bead blends as a function of crystallization temperature, filler content, and size using differential scanning calorimetry (DSC). Also, effects of glass bead size and content, and crystallization temperature on the structural stability of the originally formed β-crystals within PP crystals are discussed.

Section snippets

Materials and synthesis

The polypropylene with a melt flow index of 3.3 g/10 min (ASTM D1238) was produced by Liaoyang Petrochemical Fiber Co. (PR China). Its weight average molecular weight, Mw = 3.72 × 105; number average molecular weight, Mn = 6.90 × 104; molecular weight distribution, Mw/Mn = 5.4; and density was 0.9 g/cm3, respectively. The filler glass beads were obtained from Potters Industries, USA. Their properties are listed in Table 1. The surface of the glass beads was treated with a silane-coupling agent.

The PP

Results and discussion

Fig. 1(a–e) present a set of DSC heating thermograms for PP/GB1 samples with different glass bead content isothermally crystallized at specified temperatures. From Fig. 1, it is apparent that the melting profile depends considerably on the glass bead content and thermal condition of crystallization (Tc). Varga [40] found that the β-crystal would recrystallize into the α-crystal during the partial melting of the β-phase, if β-crystal samples cooled to below 100 °C. In order to determine the

Conclusion

The melting behavior of glass bead filled polypropylene was studied in terms of glass bead size and content and crystallization temperature. The results indicate that the amount of β-crystals in PP crystals increased with increase in the glass bead content up to 30 wt.%, and then decreased slightly with increase in the glass bead content for PP/glass bead (4 μm) system. Also, the structural stability of originally formed β-crystals increased with increase in crystallization temperature at a

Acknowledgements

R.D.K. Misra gratefully acknowledges University of Louisiana at Lafayette for the post-doctoral support of Qiang Yuan. The research was supported by the General Program (20274047) and the Major Program (50390090) of National Natural Science Foundation of China (NSFC), Chinese Academy of Sciences KJCX2-SW-H07, and the 973 program of MOST (No. 2003CB615600). The glass bead used in this study is kindly supplied by Prof. R.K.Y. Li.

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