Spin-coating: A new approach for improving dispersion of cellulose nanocrystals and mechanical properties of poly (lactic acid) composites

Highlights

• Composites were successfully manufactured using masterbatch approach.

• Masterbatches were prepared using solvent casting and spin-coating technique.

• Composites with spin-coated masterbatch had superior mechanical properties.

• A huge advance in degree of crystallinity was seen in spin-coated composites.

Abstract

This study systematically evaluated the influence of masterbatch preparation techniques, solvent casting and spin-coating methods, on composite properties. Composites were manufactured by combining CNCs masterbatches and PLA resin using twin screw extruder followed by injection molding. Different microscopy techniques were used to investigate the dispersion of CNCs in masterbatches and composites. Thermal, thermomechanical, and mechanical properties of composites were evaluated. Scanning electron microscopy (SEM) images showed superior dispersion of CNCs in spin-coated masterbatches compared to solvent cast masterbatches. At lower CNCs concentrations, both SEM and optical microscope images confirmed more uniform CNCs dispersion in spin-coated composites than solvent cast samples. Degree of crystallinity of PLA exhibited a major enhancement by 147% and 380% in solvent cast and spin-coated composites, respectively. Spin-coated composites with lower CNCs concentration exhibited a noticeable improvement in mechanical properties. However, lower thermal characteristics in spin-coated composites were observed, which could be attributed to the residual solvents in masterbatches.

Introduction

During the past decade, the use of biocompatible and biodegradable materials has grown, primarily motivated by environmental consciousness and use of renewable resources (Cheng, Deng, Chen, & Ruan, 2009). Among the bio-based polymers, polylactic acid (PLA), which is derived from renewable resource, is widely used for commercial products (Drumright, Gruber, & Henton, 2000; Jamshidian, Tehrany, Imran, Jacquot, & Desobry, 2010; Lim, Auras, & Rubino, 2008; Masutani & Kimura, 2014; Nam, Sinha Ray, & Okamoto, 2003). Its biocompatible nature, relatively high strength, great wrinkle resistance and low toxicity are some beneficial characteristics that make PLA a promising alternative to petrochemical plastics (Domenek & Ducruet, 2016; Gupta, Simmons, Schueneman, Hylton, & Mintz, 2017; Oksman, Skrifvars, & Selin, 2003). However, its shortcomings such as low impact strength, high brittleness, low tensile elongation, and low heat degradation temperature limit the application of PLA (Li et al., 2014; Ljungberg & Wesslén, 2005; Shi, Jiang, Sun, & Gan, 2011; Yoon & Ji, 2003). Therefore, numerous methods have been tried to overcome these aforementioned weaknesses by using multiple reinforcing agents (De France, Chan, Cranston, & Hoare, 2016; Martínez-Sanz, Lopez-Rubio, & Lagaron, 2012; Shi et al., 2012; Yu et al., 2012).

Cellulose nanocrystals (CNCs) have attracted great interest due to their biocompatibility, low density, abundance, large surface area, and high strength in the composites field (Brinchi, Cotana, Fortunati, & Kenny, 2013; Lin, Huang, Chang, Feng, & Yu, 2011; Sullivan, Moon, & Kalaitzidou, 2015). Incorporation of CNCs in a polymeric matrix is expected to improve the mechanical and barrier properties of the matrix (Duran, Paula Lemes, & Seabra, 2012; Hamad, 2006; Klemm et al., 2011).

One of the main problems related to the use of CNCs as reinforcing agents is their high hydrophilicity and strong tendency to form aggregates in hydrophobic media (López de Dicastillo et al., 2017). Therefore, several techniques have been experimented to improve the dispersion of CNCs in polymer matrices. The most extended approach to promote the dispersion of CNCs through PLA matrix is the use of surface modification methods. The most widely explored surface modification techniques are acetylation (Cetin et al., 2009; Lin et al., 2011), esterification (Spinella et al., 2016; Yuan, Nishiyama, Wada, & Kuga, 2006), silylation (Andresen, Johansson, Tanem, & Stenius, 2006; Yu et al., 2015) and carboxymethylation (Aguir & M’Henni, 2006; Aulin, Johansson, Wågberg, & Lindström, 2010).

In addition to surface modification methods, several manufacturing processes have been investigated to promote the uniform dispersion of cellulose nanocrystals into hydrophobic polymers. Liquid feeding of cellulose-based nanoparticles into the PLA polymer during the extrusion process and the application of masterbatch films with high nanofiller concentration have been studied to improve the dispersion of nanoparticles in polymer matrix (Jonoobi, Harun, Mathew, & Oksman, 2010; Oksman, Mathew, Bondeson, & Kvien, 2006; Yang, Gardner, & Nader, 2013).

High concentrated masterbatch is mostly prepared through solvent casting technique. Solvent casting is a century-old methodology for nanocomposite films production. In this method, a polymer is first dissolved in a solvent and then fibers or filler are added to the solution. The solvent is then evaporated leaving behind thin nanocomposite film. Thin films with uniform thickness, maximum optical clarity, and low haze are some advantages of solvent casting technology (Anbukarasu, Sauvageau, & Elias, 2015; Hsu & Yao, 2014; Siemann, 2005).

Spin-coating method is extensively employed for fabrication of smooth polymeric coatings on flat substrates. In this method, the polymer solution is applied on a substrate which can be either static or rotating at a low speed (Hall, Underhill, & Torkelson, 1998; Mellbring, Kihlman Øiseth, Krozer, Lausmaa, & Hjertberg, 2001; Norrman, Ghanbari-Siahkali, & Larsen, 2005). Spin-coating has also been used to prepare open films of cellulose nanoparticles on different substrates to examine the influence of the substrate on the nanoparticles sub-monolayer (Herrera, Mathew, & Oksman, 2014; Kontturi, Thüne, Alexeev, & Niemantsverdriet, 2005). In the spin-coating method, thin films with the thickness of the order of micrometers are spread evenly over the substrate surface owing to centrifugal force and the surface tension of the solution. In this method, solvent evaporates simultaneously as the solution is applied on the substrate (Hall et al., 1998).

In spite of the broad use of the spin-coating method in thin film preparation, to the best of our knowledge, no one has used this technique for preparing masterbatches. Spin-coating method was introduced for the first time in this paper as masterbatch preparation technique and solvent casting method was considered as the reference method. Modified CNCs were prepared through physical attachment method introducing poly (ethylene oxide) to attach on the surface of CNCs. Composites were prepared using high shear melt compounding technique through extrusion followed by injection molding process. The agglomeration of cellulose nanocrystals in masterbatches and the corresponding composites were studied using scanning electron microscopy. In addition, crystallinity, thermal properties, and mechanical behavior of resultant composites were studied experimentally.