Lignin–Cellulose Nanocrystals from Hemp Hurd as Light-Coloured Ultraviolet (UV) Functional Filler for Enhanced Performance of Polyvinyl Alcohol Nanocomposite Films

Abstract

Lignin is a natural light-coloured ultraviolet (UV) absorber; however, conventional extraction processes usually darken its colour and could be detrimental to its UV-shielding ability. In this study, a sustainable way of fabricating lignin–cellulose nanocrystals (L-CNCs) from hemp hurd is proposed. A homogeneous morphology of the hemp particles was achieved by ball milling, and L-CNCs with high aspect ratio were obtained through mild acid hydrolysis on the ball-milled particles. The L-CNCs were used as filler in polyvinyl alcohol (PVA) film, which produced a light-coloured nanocomposite film with high UV-shielding ability and enhanced tensile properties: the absorption of UV at wavelength of 400 nm and transparency in the visible-light region at wavelength of 550 nm was 116 times and 70% higher than that of pure PVA, respectively. In addition to these advantages, the nanocomposite film showed a water vapour transmission property comparable with commercial food package film, indicating potential applications.

Comparative Preparation Method and Associated Cost of Lignin–Cellulose Nanocrystals

“…These chromophore groups lead to the increase of the UV-shielding ability in the native L-CNCs. A similar observation was also found in our previous study [14]. Though the yield of native L-CNCs was low, the generation of more chromophore groups can be confirmed by the red-brown colour perceived from native L-CNCs (Figure 3b) [7].…”

“…Though preparation of cellulose nanofibers accompanied with native lignin has been previously reported from hemp [13], our group is the first to report the fabrication of nanocrystals, i.e., native L-CNCs from hemp hurd [14]. We found that the UV-shielding ability of the fabricated native L-CNCs from hemp hurd is comparable to the traditional UV-shielding film made of ZnO/lignin/polyurethane when we used the native L-CNCs with a ratio of 20 wt.% in polyvinyl alcohol (PVA).…”

“…The hemp hurd was briefly cut into coarse particles with a cutter milling (19 Universal, Fritsch Pulverisette, Idar-Oberstein, Germany) and then milled into fine particles by using an Attritor mill (2S, Union Process, Akron, OH, USA). The preparation method of native lignin-cellulose nanocrystals (L-CNCs) was similar to our previous study [14]. Briefly, the 4 g hemp solution, which contained 0.08 g hemp hurd, was added, along with 64 wt.% sulphuric acid, to a round-bottom flask and hydrolysed at 45 • C for 7 h. To the solution was added ten-fold DI water to quench the reaction, and the solution was later centrifuged at 6000 rpm for 15 min, at 4 • C, to remove extra acid.…”

Applications of Hemp Polymers and Extracts in Food, Textile and Packaging: A Review

“…Solution casting uses low temperatures and shear stresses, thereby preventing fiber damage that occurs during blending fibers and thermoplastics via melt mixing and extrusion compounding. Luzi et al (2016) [ 41 ] prepared PLA and PLA PBS-based cellulose nanocrystal films by solvent casting, while Zhang et al (2020) and Zhang et al (2021) [ 42 , 43 ] prepared PVA and hemp fiber films by solvent casting following the method shown in Figure 4 .…”

“…Film with 10% hemp particles exhibited transmittance of 94.3% in visible light at 500 nm compared to 99.5% for pure PVA. Zhang et al (2021) [ 43 ] prepared PVA and hemp fiber films by solvent casting. Hemp/PVA composite films showed increased Young’s modulus and tensile strength.…”