Elsevier

Chemical Engineering Journal

Volume 323, 1 September 2017, Pages 29-36
Chemical Engineering Journal

A high heat-resistance bioplastic foam with efficient electromagnetic interference shielding

https://doi.org/10.1016/j.cej.2017.04.050Get rights and content

Highlights

  • 100% sc in the formed crystals was facilely gained in PLA foam containing CNT.

  • The formation of sc endowed the CNT/PLA foam with excellent heat resistance.

  • The incorporation of CNT promoted to lower the density of CNT/PLA foam.

  • The resultant CNT/PLA foam exhibited improved EMI shielding effectiveness.

Abstract

Owing to the growing awareness of sustainability, bioplastic based composites arouse considerable attention. However, the low use temperature (usually <100 °C) limits their applications. To improve the heat resistance and simultaneously meet the lightweight requirement for microwave shielding, a high heat-resistance crystallite, stereocomplex crystallites (sc) formed by the stereocomplexation crystallization between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA), was introduced into the conductive carbon nanotube (CNT)/poly(lactic acid) (PLA) composite foam. The composite foam was fabricated by a nonsolvent induced phase separation and freeze-drying method. An intriguing phenomenon occurred in the CNT/PLLA/PDLA/dichloromethane (DCM) solution upon addition of hexane, which not only induced the phase separation of mixed solution but also facilitated the formation of 100% sc in the formed crystals in the resultant CNT/PLA/DCM gel. The freeze-dried CNT/PLA foam exhibits a low foam density of 0.10 g/cm3 and desirable specific EMI shielding effectiveness as high as 216 dB cm3/g. More importantly, the formation of sc with high crystallinity (∼45%) and the interconnected CNT conductive networks guaranteed the dimensional stability of CNT/PLA foams, only shrinking 4.3% at 220 °C. Our work provides a facile method to fabricate a PLA based bioplastic foam and suggests high heat-resistance and efficient EMI shielding performance.

Graphical abstract

High heat-resistance poly(lactic acid) based foam with low foam density and excellent EMI shielding performance was fabricated by the nonsolvent induced phase separation and freeze-drying method.

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Introduction

Conductive polymer composite (CPC) based electromagnetic interference (EMI) shielding foams are well recognized to have potential applications to dissipate and attenuate electromagnetic radiation particularly in areas such as aircraft, spacecraft and automobiles, owing to their lightweight, material saving and easy operation [1], [2], [3], [4]. The major components for CPC foams, polymer matrices, are multifarious, ranging from thermoplastics (like isotactic polypropylene (iPP) [5], polystyrene (PS) [6], and polymethylmethacrylate (PMMA) [7]) to thermosetting plastics (like phenolic [8] and polyurethane [9]). However, the present CPC foams always involve petrochemical-derived polymers, suffering from the depletion of nonrenewable petroleum. Given the sustainable development, it is necessary to develop polymer matrix for CPC foams from sustainable sources.

The current biopolymers, like poly(lactic acid) (PLA), poly (3-hydroxybutyrate-co-3-hydroxyvalerate) and polyhydroxyalkanoate, usually exhibit relatively inferior heat resistance, hardly being used in harsh thermal environments (e.g., above 100 °C). PLA, with favorable mechanical performance and processability, is a promising bioplastic to impart improved heat resistance. This is because that stereocomplex crystallites (sc) could be formed in PLA through the stereocomplexation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) [10]. The formation of sc with high crystallinity would endow the PLA products with outstanding heat resistance [11], [12], [13], e.g., maintaining high storage modulus even at 210 °C [11]. Unfortunately, the PLA containing 100% sc in the formed crystals, denoted as scPLA, was only fabricated in solid products and scPLA related foam products were rarely reported. One reason is that the formation of sole sc in PLA is relatively difficult, especially for PLLA and high molecular weight PDLA system [14]. Another reason is that though the formation of appropriate sc (as crystal nucleating agent) in PLA can increase its melt strength and thus benefit its foaming ability, the formation of sole sc with a relatively high crystallinity will result in excessive melt strength of PLA, which in turn suppress cell growth and spoil the foaming ability of PLA [15]. Hence, the fabrication of lightweight scPLA foams is still a great challenge.

In the current work, using carbon nanotube (CNT) as the electrically conductive material, we developed a high heat-resistance scPLA composite foam for efficient EMI shielding via a nonsolvent induced phase separation (NIPS) and freeze-drying method, as illustrated in Fig. 1. CNT was uniformly distributed in the PLLA/PDLA/dichloromethane (DCM) solution through mechanical stirring plus ultrasound. The employment of hexane could promote the intermolecular interaction between PLLA and PDLA to form sc [16], and the formed sc would act as physical crosslinks to construct a three-dimensional (3D) network-like structure, namely gelation [17], [18]. Finally, a highly porous and interconnected CNT/scPLA foam was obtained through freeze-drying. The NIPS and freeze-drying method facilitate the formation of lightweight CNT/scPLA foam with a density of 0.10 g/cm3 at 1.48 vol% (30 wt%) CNT loading, avoiding the problem existing in conventional melt foaming method, i.e., the limitation of foaming process due to the high melt viscosity at high CNT content. The CNT/scPLA foam shows a high specific electromagnetic interference shielding effectiveness (EMI SE) of 216 dB cm3/g and excellent heat resistance with linear shrinkage in diameter of only 4.3% even at 220 °C for 30 min.

Section snippets

Materials

PLLA pellets were obtained from the NatureWorks (4032D) with the weight-average molecular weight (Mw) of 2.23 × 105 g/mol and the polydispersity index (PDI) of 2.10. PDLA was kindly provided by The Changchun Institute of Applied Chemistry (CIAC) (China) with Mw of 9.5 × 104 g/mol and PDI of 1.80, according to GPC tests. The Tm values of PLLA and PDLA were 167.4 and 176.0 °C, measured with differential scanning calorimetry (DSC) at a heating rate of 10 °C/min. CNTs (NC7000) supplied by Nanocyl S.A.,

Results and discussion

We first examined whether sc was effectively formed in PLA by DSC. As shown in Fig. 2, a strong endothermic peak around 220 °C appears for all the DSC curves, which is exclusively for melting temperature of sc and indicates the successful formation of 100% sc in the formed crystals in neat scPLA and CNT/scPLA foams. In addition, an endothermic shoulder around 205 °C is observed on the left side of the melting peak of sc, which can be assigned to the melting of sc formed through the cold

Conclusions

Utilizing PLA as matrix and CNT as conductive filler, a bioplastic EMI shielding foam with high heat resistance is prepared by the NIPS and the freeze-drying method. The formation of 100% sc in the formed crystals makes the scPLA foams promising use in some thermal environments. The addition of CNT benefits the formation of 3D interconnected skeletons, resulting in lower foam density, higher electrical conductivity, better EMI shielding performance and more stable geometrical shape for the

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China – China (Grant Nos. 51120135002, 51421061, 51473102, 51533004, and 51673134), the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant No. 2014TD0002), and the China Postdoctoral Science Found (Grant Nos. 2015M572474, 2016T90848).

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