Properties and microstructure of expandable graphite particles pulverized with an ultra-high-speed mixer

doi:10.1016/j.powtec.2006.08.023

Powder Technology

Volume 170, Issue 3, 14 December 2006, Pages 178-184

https://doi.org/10.1016/j.powtec.2006.08.023 Get rights and content

Abstract

The expandable graphite (EG) particles were pulverized to achieve different and smaller sizes using an ultra-high-speed mixer. The microstructure of particles was observed by a scanning electron microscope (SEM). The as-received EG particles showed an irregular flake shape. With the increase of the mixing time, the EG particles tended to be circular and the collapses and cracks in the EG surfaces appeared, and their average diameter and average area rapidly reduced. At the same time, their expansion volume after thermal treatment greatly decreased, resulting from the reduction of particle sizes and the direct release of the oxidant inside the EG particles instead of exfoliating the particles. The expanded EG particles revealed the typical wormlike structure.

Graphical abstract

The expandable graphite (EG) particles were pulverized to achieve different and smaller sizes using an ultra-high-speed mixer. With the increase of the mixing time, the EG particles tended to be circular and the collapses and cracks in the EG surfaces appeared, and their average diameter and average area rapidly reduced. Their expansion volume after thermal treatment greatly decreased, resulting from the reduction of particle sizes and the direct release of the oxidant inside the EG particles instead of exfoliating the particles, but all the expanded EG particles revealed the typical wormlike structure.

Introduction

Expandable graphite (EG) is a graphite intercalation compound in which some oxidants, such as sulfuric acid, potassium permanganate, etc., are inserted between the carbon layers of the graphite [1], [2], [3], [4]. When experienced a heat source, EG, occupies hundred times its initial volume and generates a voluminous structure, thus providing fire-retardant performance for the polymeric matrix [4], [5]. Some studies implied that EG can produce good fire-retardant properties for some polymers, such as polyolefins [6], [7], polyurethane foam [5], [8], [9], coating [10], etc. In addition, EG after expansion can be used as biomedical materials due to its pore structure and absorptive capacity [3]. And for its high electrical conductivity and a unique layered nano-structure, it was compounded with some polymers, such as polymethyl methacrylate (PMMA) [11], poly(styrene-co-acrylonitrile) [1], etc., to fabricate the so-called nano-composites with excellent electrical conductivity. In our previous study [5], EG can efficiently improve the fire-retardant properties of high-density rigid polyurethane foam (RPUF) as a halogen-free fire-retardant additive. However, in the open literature, little attention has been paid to the EG pulverization, and the microstructures of finer EG particles before and after expansion, and their influences on the flame behaviors of polymers, etc. Usually a material as a filler for polymers should be fine and uniformly dispersive, that is, the finer and the more uniformly dispersive and distributive filler particles bring out better properties of composites [12], [13], [14], [15]. As sheet fire-retardant additives, the fine and uniform dispersion of EG is believed to give a considerable effect on the fire-retardant properties of RPUF. In this series studies, accordingly, the EG particles were crushed into fine ones, and subsequently added to RPUF for further improvement of fire behaviors. In the present work, an ultra-high-speed mixer, which was specially designed for dispersion of nano fillers in polymers depending on its high shear and intense mixing [15], was used for pulverization of the EG particles. After pulverization, EG particles with various sizes were obtained. The structure before and after expansion, size distribution and expansion volume of these particles have been investigated.

Section snippets

Materials

Expandable graphite (Model KP9932300) was purchased from Haida Graphite Co. (QingDao, China). The manufacturer specifies the following properties: ash, 1.0%; moisture, 1.0%; weight loss after expansion, 15%; pH value, 3.0; expansion rate, 300 ml/g.

Pulverization of expandable graphite particles

EG particles were broken up into smaller ones using an ultra-high-speed mixer whose configuration is shown in Fig. 1. This mixer can provide high speed (as high as 6000 rpm) and high shear force, with a special rotor designed to cut glomerate

Morphology of EG particles

Fig. 2 shows the SEM micrographs of EG particles. All the EG particles assume irregular flake shape. And the size of processed EG particles (Fig. 2b and c) is much smaller than that of the unprocessed one (Fig. 2a). With the increase of the mixing time, the EG particles become circular, and their size smaller (see Fig. 2c). Fig. 3 shows the surface microstructure of EG particles. Some graphite flakes exist in the surface of EG0 (Fig. 3a). These graphite flakes are well absorbed together without

Conclusions

The original EG particles show an irregular flake shape. The increase of the mixing time results in more circular EG particles. More scraps of EG flakes around the EG particles produced, and their size reduced with increasing the pulverization time. The average diameter and area rapidly decreased as the shear mixing time increases. The finer particles have a higher apparent density for EG particles due to the smaller and less spaces formed among the smaller size particles. The expansion volume

Acknowledgments

The authors gratefully acknowledge the financial support of this subject by National Natural Science Foundation of China and Engineering Physical Academy of China (Contract No.10276024). We are also indebted to Mr. Zhu Li from Analytical and Testing Center, Sichuan University, for his careful measurement.

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Properties and microstructure of expandable graphite particles pulverized with an ultra-high-speed mixer

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Pulverization of expandable graphite particles

Morphology of EG particles

Conclusions

Acknowledgments

Carbon

Polym. Degrad. Stab.

Polym. Degrad. Stab.

Compos. Sci. Technol.

Polymer

Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process.

J. Power Sources

Carbon

Carbon