Effects of crystal structure on the foaming of isotactic polypropylene using supercritical carbon dioxide as a foaming agent

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Abstract

This paper aims to study, for the first time, the effect of crystal structure on the cell formation in an isotactic polypropylene (iPP) during a solid-state foaming process using supercritical carbon dioxide (scCO2) as a foaming agent. Results show that the spherulite structure exerted a significant impact on the cell morphology of foamed iPP. Very interestingly, under a relatively low pressure, microcells could appear at the centers of spherulites of iPP where the melting started proceeding first. They also appeared in the amorphous domains located in between spherulites and the interlamellar regions of spherulites of iPP. The larger the size of an amorphous area, the lower the CO2 saturation pressure needed to induce cell formation. When microcells were generated in the interlamellar regions, tie fibrils bridging lamellae could be stretched. γ-Crystals were formed at very high CO2 saturation pressure.

Graphical abstract

The α-monoclinic is the most common crystal structure for isotactic polypropylene and is characterized by dominant radiating lamellae and a set of tangential lamellae. This feature is revealed by the cell structure obtained using scCO2 as a foaming agent. A cell starts nucleating and growing from the center of a spherulite. Microcells are also formed in the interlamellar regions.

Introduction

Semicrystalline polymers possess wide applications due to desirable mechanical properties they offer, such as high stiffness and high strength. Microcellular semicrystalline polymers have attracted increasing attention in recent years since they can reduce the cost and the density of products while retaining desired mechanical properties. However, the microcellular foaming behavior of semicrystalline polymers can be different from that of amorphous polymers in that crystal structures may have a large impact on the foaming behavior and foam structure.

The effect of crystal structure on the foaming behavior of semicrystalline polymers may depend on the type of the foaming process. Foaming conditions may not all be the same for batch foaming, injection molding foaming and extrusion foaming processes. In a solid-state batch foaming process, due to the structurally heterogeneous nature of semicrystalline polymers and the concomitantly non-uniform dispersion of the foaming agent in the polymer matrix, it is conceivable that the crystal structure affects both the cell nucleation and growth. Doroudiani et al. [1] observed uniform cells with fine cell size in low-crystallinity polymers while non-uniform structures were developed in high-crystallinity polymers. They attributed the above phenomena to differences in gas solubility and stiffness of the material in different regions of polymers. Baldwin et al. [2], [3] observed a higher cell density in semicrystalline polymers than in amorphous polymers and believed that interfaces between the crystalline and amorphous regions could be the preferential cell nucleation sites during the microcellular foaming process. Itoh and Kabumoto [4] indicated that when the foaming temperature of polyphenylene sulfide was above its crystallization temperature, high cell nucleation would be achieved in the surroundings of crystallized areas because the gas in the vicinity of crystallized areas was pushed out into the surroundings as crystallization proceeded. When studying the microcellular foaming behavior of polyamide-6 (PA6)/clay nanocomposites in an injection molding process, Yuan et al. [5], [6] found that the cell wall structure and smoothness were determined by the size of the crystal structure. Meanwhile, nanoclays in the microcellular injection molding process promoted the γ-form and suppressed the α-form crystal structure of the PA6. To obtain polypropylene foams with a large volume expansion ratio in a continuous extrusion foaming process, a strategy that Naguib et al. [7] proposed was to optimize processing conditions in the die to avoid too-rapid crystallization.

This work concentrated primarily on the effect of crystal structure on the foaming of isotactic polypropylene (iPP) in a solid-state foaming process using supercritical carbon dioxide (scCO2) as a foaming agent. The so-called “solid-state” means that the foaming temperature is chosen to be below the melting temperature of the iPP. Since the iPP is not completely molten during the foaming process, its crystal structure would possibly exert an impact on the cell nucleation, cell growth and cell morphology. The microstructure of iPP is complex since it may crystallize in several distinct forms (α-monoclinic, β-pseudohexagonal and γ-orthorhombic), depending on crystallization conditions and additives. The most common crystal obtained is the α-monoclinic, with β-pseudohexagonal form and γ-orthorhombic formed under special conditions [8]. The α-form of iPP tends to build spherulites upon crystallization in the melt. Spherulites themselves are semicrystal structures that consist of single-crystal lamellae radiating from a common central nucleus and interlamellar amorphous matter [9]. The spherical symmetry of α-spherulite of iPP is developed with a so-called central multi-directional growth mechanism, instead of the sheaf-like unidirectional growth mechanism [10]. As a unique feature of iPP, in addition to dominant radially growing lamellae, spherulites of iPP include a second set of tangential lamellae which are oriented in a direction perpendicular to the former [11]. Accordingly, it is necessary to study the effects of such local structures of spherulites of iPP on its foaming behavior in a solid-state foaming process. On the other hand, the aforementioned features of iPP might, in return, be revealed by the cell morphology in different parts of spherulites.

It is generally accepted that crystallites are impenetrable for most non-reactive molecules including CO2. Small molecules can diffuse into the amorphous regions but not the crystalline regions [12]. With respect to spherulites, small molecules can only enter the amorphous interlayers between lamellae but not the radiating stacks of crystal lamellae. However, those considerations are largely based on experimental sorption and diffusion data with the crystallinity of semicrystalline polymers, in which crystalline regions are often represented by the dispersed impermeable spheres in a permeable matrix [13], [14], [15], [16], [17], [18]. Since cell nucleation and growth only occur in the amorphous regions where the CO2 are dissolved, the state of dispersion of scCO2 in iPP might be revealed by the foam structure: existence of cells in the amorphous regions and the absence of cells in crystalline regions.

In summary, to the best of the authors’ knowledge, this paper is the first report on the effects of the crystal structure of a semi-crystalline polymer on its foaming behavior and foam structure and provides better insight into the structure of spherulites of the iPP and the state of dispersion of scCO2 in the iPP.

Section snippets

Materials

An iPP with a melt flow index of 16.0 g/10 min was purchased from Shanghai Petrochemical Co., China. The mass-average molar mass of the iPP was 188,000 g/mol. The crystallinity and melting temperature of the iPP was 47% and 169 °C, respectively. The iPP was used as received. The CO2 (purity 99%) was purchased from Airproduct Co, Shanghai, China.

Foaming process

A depressurization batch foaming process was used to foam the iPP (Fig. 1). The central piece of the apparatus was a high-pressure vessel with an internal

DSC measurement

It is well-known that a small molecule like scCO2 induces the melt temperature depression of semi-crystalline polymers. A high-pressure differential scanning calorimeter of type Netzsch 204 HP, Germany, was used to determine the scCO2 induced melting temperature depression of the iPP used in this work. Prior to the DSC measurement, the iPP pellets were saturated with scCO2 under 10 MPa and at 155 °C for an hour using the same high-pressure vessel in which the foaming experiments were conducted.

Results and discussion

A previous work showed that at a given foaming temperature, there was a lower limit for the scCO2 saturation pressure below which the iPP could not foam because it was too stiff [19]. In this work, the foaming temperature was fixed at 156 °C, unless stated otherwise. Fig. 4 shows the cell morphology of a foamed iPP obtained at 156 °C and 10.4 MPa (according to Fig. 3, under those conditions the iPP barely started melting). Cells started to form under the above foaming conditions. From Fig. 4a,

Thermophysical properties of foamed iPP

Fig. 13 compares the DSC thermograms among the pure iPP, the iPP annealed at 156 °C and ambient pressure for 40 min (the same time as that for the scCO2 saturation of the iPP before foaming) and foamed iPP in Figs. 4, 6 and 9. The annealing effect at 156 °C and ambient pressure brought about a small increase in the melting temperature of the iPP. The melting temperature of the pure iPP was 169 °C and that of the annealed one 172 °C. On the other hand, the iPP that had been subjected to the scCO2

Conclusion

This work studied, for the first time, the effect of crystal structure on the cell formation in an iPP during a solid-state foaming process using scCO2 as a foaming agent. The results showed that the spherulite structure exerted a significant impact on the cell morphology of the resulting foamed iPP. Under a relatively low pressure, microcells could be formed from the centers of spherulites of iPP because the melting started from there. Microcells could also be formed in the amorphous areas

Acknowledgement

The authors are grateful to the National Science Foundation of China and PetroChina for the support of a joint project on multiscale methodologies (20490204), the National Science Foundation of China (50703011), Program for Changjiang Scholars and Innovative Research Team in University (IRT0721) and the 111 Project (B08021).

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