Chemical modification of exfoliated graphite nanoplatelets with CTBN rubber and highly enhanced impact strength of vinyl ester resin by them

https://doi.org/10.1016/j.jiec.2021.07.005Get rights and content

Abstract

In the present study, exfoliated graphite nanoplatelets (EGN) were chemically modified with carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) liquid rubber via carboxylation, acylation, and subsequently esterification. Multiple characterizations by means of attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction spectroscopy, thermogravimetric analysis, and scanning electron microscopy demonstrated the chemical modification. For comparison, neat vinyl ester resin (VE), EGN/VE and CTBN-EGN/VE composites were prepared by using a free-standing Teflon mold to avoid pressure during curing. The amounts of EGN and CTBN-grafted EGN containing in EGN/VE and CTBN-EGN/VE composites were 1, 3, and 5 wt%, respectively. The impact strength of EGN/VE and CTBN-EGN/VE composites was strongly dependent upon the CTBN-grafted EGN concentration incorporated into neat VE. It was remarkably enhanced up to 220–240% even with low contents (5 wt%) of CTBN-grafted EGN in comparison to neat VE and unmodified EGN/VE. The result revealed that a small amount of EGN chemically modified with liquid rubber CTBN played an important role in increasing the impact toughness of thermosetting resin.

Graphical abstract

Exfoliated graphite nanoplatelets (EGN) were chemically modified with CTBN rubber molecules via carboxylation, acylation, and subsequently esterification, and then demonstrated by means of multiple characterizations. The Izod impact strength of vinyl ester resin (VE) was tremendously enhanced by the incorporation of a small amount of CTBN-EGN in the resin, showing about 240% increase only with 5 wt% CTBN-EGN, in comparison to neat VE and unmodified EGN/VE.

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Introduction

Exfoliated graphite nanoplatelet (EGN) with layered structure is composed of multiple graphene sheets [1], having enormous specific surface area and high aspect ratio, compared to natural graphite [2]. Appropriate surface modification or functionalization of EGN by acid treatment and grafting leads to good dispersion of EGN in a polymer matrix. As a result, EGN-filled polymer composites may exhibit improved mechanical, electrical, electromagnetic, and thermal properties, compared to the unfilled counterpart [3], [4], [5]. Hence, EGN has been increasingly used to contribute to enhancing the properties of various polymer composites for the past years [6], [7], [8], [9], [10], [11].

The more flexible molecules lead to the tougher material. Toughness is the ability of a material to absorb energy without breaking. Impact toughness strongly depends on the ability of the material to internally move or deform to accommodate the impact energy. This movement is related to the material elongation. Therefore, materials that exhibit high elongation are often tough, especially if they also have good mechanical strength [12]. The material consisting of flexible molecules will be able to absorb the external impact energy by easily elongating, making the material tough. It has been known that some experimental approaches can be used to improve the impact toughness of thermosetting resins, for example, simply adding toughening agent, rubber particles, incorporating liquid rubber, decreasing the cross-link density, forming semi-interpenetrating polymer network, etc. [13], [14], [15], [16]. One of the most effective approaches is incorporating a reactive liquid nitrile rubber into a target polymer, particularly with carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) [17], [18], [19], [20]. CTBN is one of the representative liquid rubbers with functional carboxyl groups on the end group of molecular chains. It has been reported that CTBN plays a role as a useful impact modifier in toughening thermosetting polymers such as vinyl ester and epoxy resins [17], [18], [19]. The carboxyl groups in the CTBN molecules can react with the resins, contributing to enhancing the toughness of relevant polymers.

Vinyl ester resin (VE) has been widely used as matrix in carbon fiber-reinforced plastics [21], [22], [23], [24]. They have intermediate properties and performances between epoxy and unsaturated polyester resins in terms of chemical, physical, mechanical, and curing characteristics [25]. One of the key shortcomings of VE is low impact resistance due to its brittleness. One of the simplest ways to solve such a problem is incorporating the rubber phase into VE, normally by adding a large amount of rubber particles to it. Many studies have been performed to increase the impact resistance of VE [13], [26], [27]. However, no papers have been studied on effectively enhancing the impact strength by incorporating a very small amount of EGN chemically modified with the rubber molecules into VE through the chemical functionalization of EGN with liquid rubber.

There have been many papers dealing with chemical functionalization of carbon-based nanomaterials such as carbon nanotubes and graphite nanoparticles. They reported that chemical modification of carbon-based nanomaterials with polymers by acid treatment and grafting increased the mechanical and thermal properties of resulting composites [28], [29], [30], [31], [32]. Ruan et al. [31] reported chemical functionalization of aminated graphite oxide with polyimide via reduction to develop thermally conductive composite films for thermal management. Many researchers have studied graphene as preferred nanomaterial to enhance the electromagnetic interference shielding performances and thermal conductivity of polymer composites [33], [34]. Recently, Choi et al. [29] reported that carbon fiber grafted with acyl chloride functionalized multi-walled carbon nanotubes contributed to enhancing the mechanical properties of carbon fiber/polyamide 6,6 composites due to chain entanglement between the polyamide 6,6 grafted to MWCNT-carbon fiber and polyamide matrix. Studies dealing with chemical functionalization of EGN with rubber molecules have not been scarcely found. It gave us research motivation not only to graft the EGN with a liquid rubber but also to chemically modify a thermosetting polymer, making it tougher.

Consequently, the objectives of the present study are firstly to chemically modify the EGN by grafting with liquid rubber CTBN, secondly to demonstrate the chemical modification by means of multiple characterizations such as attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction spectroscopy, thermogravimetric analysis, and scanning electron microscopy, and ultimately to explore how CTBN-grafted EGN influences the impact strength of EGN/VE and CTBN-grafted EGN/VE composites with varying the content.

Section snippets

Materials

Graphite intercalation compound (GIC 3772) was supplied from Asbury Graphite Mills, Inc., NJ, USA. It was used as a starting material for preparing EGN. Fuming nitric acid (Matsunoen Chemicals Co., Ltd, Japan), sodium chlorate (Yakuri Prechemicals Co., Ltd, Japan), thionyl chloride (Daejung Chemicals & Metals Co., Ltd, Korea), pyridine (Sigma-Aldrich Co., Ltd, USA), dimethyl formamide (Junsei Chemicals Co., Ltd, Japan), and acetone (Daejung Chemicals & Metals Co., Ltd, Korea) were used in this

Chemical evidence of EGN chemically modified with CTBN

Fig. 1 shows the ATR-FTIR spectra of (A) EGN and (B) carboxylated EGN (acid-treated for 24 h). Before acid treatment, the characteristic absorption peaks, indicating specific functional groups, were not detected from the EGN sample. Meanwhile, carboxylated EGN exhibited a broad absorption peak around 3442 cm−1 due to the O–H stretching vibration of carboxyl and hydroxyl groups. The absorption peaks at 1737 cm−1 and 1616 cm−1 were due to the Cdouble bondO stretching vibration of carboxyl groups and due to

Conclusions

EGN was chemically modified with the CTBN rubber molecules via acid treatment, acylation, and subsequently esterification. The chemical, thermal, and microstructural characteristics of carboxylated EGN and CTBN-EGN were significantly changed from those of EGN, being supported by multiple analytical demonstrations. The impact strength of EGN/VE and CTBN-EGN/VE composites strongly depended on the content of EGN and CTBN-EGN incorporated into neat VE. The impact strength of CTBN-EGN/VE composite

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Composite Materials and Structures Center at Michigan State University, East Lansing, MI, USA, particularly with thanks to Distinguished Professor Lawrence T. Drzal.

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