Properties and microstructure of expandable graphite particles pulverized with an ultra-high-speed mixer
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.
References (19)
- et al.
Characterizations of expanded graphite/polymer composites prepared by in situ polymerization
Carbon
(2004) - et al.
A comparative study of the mechanism of action of ammonium polyphosphate and expandable graphite in polyurethane
Polym. Degrad. Stab.
(2002) - et al.
Expandable graphite as an intumescent flame retardant inpolyisocyanurate-polyurethane foams
Polym. Degrad. Stab.
(2002) - et al.
Electrical conductivity and dielectric properties of PMMA/expanded graphite composites
Compos. Sci. Technol.
(2003) - et al.
A new route to polyaniline composites
Polymer
(1997) - et al.
Effect of the crystallization of bioactive glass reinforcing agents on the mechanical properties of polymer composites
Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process.
(2004) - et al.
Electrochemical properties of amorphous comb-shaped composite PEO polymer electrolyte
J. Power Sources
(2002) - et al.
Porous graphite matrix for chemical heat pumps
Carbon
(1998) - et al.
Densification of expanded graphite
Carbon
(2002)
Cited by (29)
Progress in-situ synthesis of graphitic carbon nanoparticles with physical vapour deposition
2021, Progress in Crystal Growth and Characterization of MaterialsCitation Excerpt :Similarly, the graphitic particles of 70 µm, 430 µm, and 960 µm size, present expandable ratios of 40, 380 and 400, and potential of hydrogen (pH) values of 3.25, 3.85 and 5.62, respectively [51]. The higher expandability of graphitic particles is associated with their larger particle sizes as thermographic analysis also reflects large expandability of 300 ml/g for 196 µm large size particles, whereas small size particles of 39.8 µm correspond to small expandability of only 40 ml/g [52]. Hence, the crystal structure i.e. interplanar distance, atomic arrangement, defects and physical aspects such as size, shape, surface area, and density, significantly influence the electrical, thermal, and structural properties of graphitic particles.
Preparation of the spheroidized graphite-derived multi-layered graphene via GIC (graphite intercalation compound) method
2017, Journal of Industrial and Engineering ChemistryCitation Excerpt :Smaller graphite flakes produce the expanded graphite with smaller expanded volume because trimmed GIC plane causes the splitting of the airproof GIC flakes. When GIC is rapidly heated by microwave irradiation, gaseous compounds (e.g., CO2, H2O, SO2) are generated from the reaction between intercalated sulfuric acid and graphite [12]. If graphite flake is large enough, it can entrap the pressurized gases effectively promoting highly expanded graphite production.
Characterization of graphene nanoplatelets prepared from polyimide-derived graphite
2015, Materials LettersCitation Excerpt :After intercalation of sulfuric acid into the graphitic materials, apparent color of acid intercalated specimens changed from black to blue-black representing the first-stage (i.e., status which graphite layers and intercalated layers alternate each other) characteristics explicitly. During the microwave irradiation on the acid intercalated graphite, gaseous compounds (e.g., CO2, H2O, SO2) originated from the reaction between intercalated sulfuric acid and graphite escape from the edge of flakes [12]. To compare the surface morphologies upon preparative steps, electron microscopy images on the pristine graphitized polyimide, its expanded form, and pulverized graphene platelets were presented in Fig. 3.
Chemistry of one dimensional silicon carbide materials: Principle, production, application and future prospects
2015, Progress in Solid State ChemistryPreparation of Kish graphite-based graphene nanoplatelets by GIC (graphite intercalation compound) via process
2015, Journal of Industrial and Engineering ChemistryCitation Excerpt :The smaller flake size relates to the reduced expanded volume because trimmed GIC plane results in the splitting of the airproof GIC flakes. When GIC is heated by microwave irradiation, gaseous compounds (e.g., CO2, H2O, SO2) that originated from the reaction between intercalated sulfuric acid and graphite start to escape from the edge of flakes [14]. Larger graphite flakes are expected to be more efficient to entrap the generated highly-pressurized gases longer resulting in the increased expansion volume.