Elsevier

Composites Part B: Engineering

Volume 133, 15 January 2018, Pages 203-209
Composites Part B: Engineering

Effects of bamboo cellulose nanowhisker content on the morphology, crystallization, mechanical, and thermal properties of PLA matrix biocomposites

https://doi.org/10.1016/j.compositesb.2017.09.040Get rights and content

Abstract

To improve the mechanical and thermal properties of poly (lactic acid) (PLA) composites, bamboo cellulose nanowhiskers (BCNW) were extracted and introduced into PLA composites as fillers. PLA/BCNW biofilms were fabricated by solution casting with different BCNW contents (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 wt%). The characteristics of the biofilms were investigated by scanning electron microscopy (SEM), mechanical measurements, synchrotron radiation wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The results showed that the maximum tensile modulus and elongation at break of 427.72 ± 19.32 MPa and 22.27 ± 3.50% were reached at 2.5 wt% and 1.0 wt% loadings, respectively. Both homogeneity and stereocomplexed crystallites were observed, and heterogeneous nucleation effect was confirmed. With the addition of BCNW, the crystallite size of PLA/BCNW composites increased remarkably, and the largest crystallinity was 30.7 ± 0.9% with 2.5 wt% BCNW. These results provided data support for enlarging the application of PLA.

Introduction

Poly(lactic acid) (PLA) is a green bioplastic that can be produced from natural sources such as corn and cassava. In addition, PLA degrades naturally into H2O and CO2 through composting [1], [2], [3]. With an ever-increasing public consciousness promoting environmental protection, the serious problem of “white pollution” is a cause for much concern; thus, PLA exhibits a wide degree of potential applications [4], [5], [6]. So far, the applications of PLA have involved biomedical devices, textiles, films, decorative panels, electrical components, food packages etc. [7], [8], [9], [10]. However, the poor thermal stability and high brittleness of PLA leads to difficulties in processing and decreases the product's performance [3], [11], limiting the wide application of PLA [12]. Thus, new effective methods to improve the thermal and mechanical properties of PLA are of great interest.

The employment of bio-nanoparticles as reinforcements in PLA composites has been proved to be a valid method [2]. Recent studies reported that cellulose nanowhiskers (CNW), with a large aspect ratio, high strength, and elastic modulus reinforced PLA composites and presented excellent thermal and mechanical properties [13], [14], [15], [16], [17], [18]. Moso bamboo (Phyllostachys heterocycla) grows abundantly in many tropical and subtropical regions of the world, especially in Zhejiang Province, China. It has been widely used in furniture manufacturing, construction materials and household items due to the advantages of fast growth, high strength, surface hardness and easy machinability [19], [20], [21], [22]. However, a large amount of moso bamboo processing residue is underused. The cellulose content of bamboo is 40%–65%, which is comparable to wood [23]. Thus, bamboo residue is a good resource for renewable nanobiobased filler. Brito et al. [24] and Lu et al. [25] prepared cellulose nanocrystals with a length of approximately 100 nm by the hydrolysis of bamboo fibers in the presence of sulfuric acid and phosphoric acid. Visakh et al. [26] employed bamboo cellulose nanowhiskers (BCNW) as reinforcements in rubber composites and found that the BCNW with high length and aspect ratios improved the performance of composites. Our group prepared bamboo cellulose nanowhisker from bamboo residues, and investigated the mechanism of acid hydrolysis [27]. BCNW may act as a nucleating agent or plasticizer, and has been used as an applicable nanofiller in PLA composites [28], [29]. In this regard, the crystallization performance, thermal property, and mechanical properties of the composites would be improved. Lee et al. [30] found that all properties of thermology, rheology and mechanics increased with the addition of CNW (0.1 wt% and 0.5 wt%) into PLA composites. Additionally, acetylated-CNW with different contents (1 wt%, 2 wt%, 3 wt%) reinforced PLA composites were also reported; the crystallization behavior and mechanical properties of the composites were improved. These composites can be used in artificial bone and bio-membranes, etc. [31]. Arias et al. [32] prepared PEO-coated nanocrystals, with the cellulose nanocrystals (CNC) evenly dispersed in the PLA matrix, and studied the performance of composites with different CNC contents. Arjmandi et al. [28] investigated the reinforcement mechanism of a montmorillonite/CNW/PLA ternary system and found that the three compositions with proportions of 5:1:10 presented the highest tensile properties. Sanchez-Garcia et al. [33] concluded that PLA/CNW composites had better mechanical properties with a CNW loading below 3 wt%.

Although it was previously reported that the thermal and mechanical properties of a PLA composite would be enhanced to some extent by adding CNW, the agglomeration of CNW would decrease the thermal and mechanical properties of PLA in some cases due to the abundant hydroxyl groups [34], [35], [36]. Thus, the optimal content of CNW incorporated into a PLA matrix remains to be explored. In addition, the unique morphology of BCNW and the effects of different BCNW contents on the crystallization behavior and macroscopic property of PLA biocomposites still needs to be further investigated.

In this paper, BCNW were extracted from bamboo residue through hydrolysis in the presence of sulfuric acid. The morphology of the BCNW was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nanocomposite films were prepared through incorporating different contents of BCNW into a PLA matrix using a solution casting method. The reinforcement mechanism of BCNW in the PLA composites was studied. The interface characteristics of the composites were observed by SEM. The crystallite structures of the composites were investigated by synchrotron radiation wide angle X-ray scattering (WAXS). The crystallization behavior and thermal performance were characterized by differential scanning calorimetry (DSC). The tensile strength, tensile modulus, and elongation at break of the composites were investigated. On the basis of these results, the optimal BCNW content in a PLA matrix was obtained. These results provided basic reference data for further improving the properties of PLA biocomposites and achieving extensive applications in engineering fields.

Section snippets

Materials

Moso bamboo residues were kindly supplied by a local moso bamboo processing factory, Lin'an, Zhejiang Province, China. The particles were screened through a mesh size of 100 and dried at 105 °C to constant quality for further use. PLA (4032D) was produced by the NatureWorks Corporation (USA). The density was 1.25 g/cm3, and the molecular weight was 52,000 g/mol. All other reagents and solvents were used as received from the commercial source.

Preparation and characterization of bamboo cellulose nanowhiskers (BCNW)

Bamboo particles (BP) (<150 μm) were mercerized with

Morphology of bamboo cellulose nanowhisker

Fig. 1 illustrates the micro-morphology of raw bamboo particles and BCNW. We previously reported that the BCNW exhibited a large length-to-diameter ratio (L/D) and had rod-like shapes and network-like structures [27], which were different with whiskers from jute, coconut husk, rice husk and cotton [39]. The fibrillated BCNW had a higher surface area and better cross-linking characteristics when used as nano-fillers. Two types of aggregation of BCNW, namely, closely spaced crystal and isolated

Conclusions

In this paper, bamboo cellulose nanowhiskers were employed as nano-filler for PLA biocomposites. The rod-like BCNW had an average length of 455 nm, diameter of 12 nm and length-to-diameter ratio (L/D) of approximately 37. PLA/BCNW composites with 2.5 wt% BCNW content had a relatively high level of transparency and tensile modulus; excessive BCNW contents lowered the tensile property. The addition of BCNW mainly affected the homocrystallites of PLA and enlarged the crystallite size. Pure PLA

Conflict of interest

The authors declare no competing financial interest.

Acknowledgments

This work was financially supported by the Natural Science Foundation of Zhejiang Province (No. LY16E030003), the Applied Research Project on Public Welfare Technology of Zhejiang Province (No. 2016C33103), the Research Funds of NBU (No. ZX2016000752), the Foundation of Ningbo University (No. XYL17025), and sponsored by K. C.Wong Magna Fund in Ningbo University. WAXS experiments were performed at the BL16B beamline of SSRF, China.

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