**1. Introduction**

In recent years, the overexploitation and utilization of global fossil energy and fuels has led to environmental changes and energy shortages. Therefore, there is increasing interest for sustainable and environmentally friendly materials research [1,2]. Cellulose nanofibers (CNFs) have become a promising renewable material because of their unique structure, excellent performance, and natural abundance. It is widely found in plants and considered a nontoxic, degradable, and low-cost material, so it is a good substitute for synthesizing many products [3,4]. CNFs can be used to prepare pollution-free product components, such as membrane electrode assemblies, organic light-emitting diodes, writable touch screens, energy storage materials, and eco-friendly fabric softeners [5–8]. Thermal instability is the main problem of biomass materials because their components have a low melting temperature. Therefore, these electronic components are easily destroyed when they encounter high temperatures and need to be annealed or sterilized [9]. CNFs can be

S.; Yuan, X.; Li, M.; Wang, S. Contribution of Different Pretreatments to the Thermal Stabilityand UV Resistance Performance of Cellulose Nanofiber Films. *Coatings* **2021**, *11*, 247. https://doi.org/ 10.3390/coatings11020247

Academic Editor: Alessandro Pezzella

**Citation:** Luo, L.; Wang, X.; Zhang,

Received: 5 January 2021 Accepted: 11 February 2021 Published: 19 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

combined with other wood material components to produce a hybrid superlattice structure, reduce their thermal conductivity, and become a thermoelectric energy-harvesting material.

The thermal stability of CNFs is closely related to the lignin and hemicellulose in plant fibers [10]. Lignin and hemicellulose act as binders and fillers in lignocellulosic raw materials, and many cellulose molecular chains combine to form fiber bundles [11]. Lignin is one of the main components in lignocellulosic raw materials [12]. It is a natural high molecular weight polymer with a three-dimensional structure that is formed by connecting a phenylpropane structure through ether and carbon-carbon bonds [13]. Lignin can increase the water contact angle of cellulose fiber films, reduce their water absorption capacity, and improve the thermal stability and UV barrier properties of cellulose fiberbased films [14,15]. The degradation of wood fiber is a complex process involving a series of continuous reactions, and its thermal stability depends on the chemical composition, fiber size, crystal structure, and number of intermolecular and intramolecular hydrogen bonds. Nair et al. investigated the effect of lignin on the thermal stability of CNFs, indicating that the presence of lignin significantly improved the thermal stability of CNFs [16].

The preparation methods for CNFs also impact on their thermal stability. Pretreatments can change the cell wall structure and composition of lignocellulose, reduce energy consumption during the preparation of CNFs, and increase the yield of CNFs [17–19]. At present, the main pretreatments include acid, alkali, hot water, organosolv, and ionic liquid pretreatments [1,20,21]. Compared to gamma-valerolactone/water [22], catalyzed chemical oxidation [23] and organosolv pretreatment [24] were used to pretreat lignocellulose and prepare CNFs, and hot water pretreatment as a green and environmental method can save chemical reagen<sup>t</sup> consumption [25]. Furthermore, green liquor as a kind of alkaline liquor mainly consists of sodium carbonate and sodium sulfide, which is the liquor recovered after the combustion of black liquor in the recovery boiler of the Kraft pulp mill. Green liquor was used to pretreat lignocellulosic biomass can decrease the overall cost for the pretreatment process [26], which could also alleviate the discharge of pulping wastewater. Therefore, using the green liquor pretreated lignocellulose to prepare CNFs can realize the reusability of the wastewater in pulp and papermaking. However, there is a lack of using green liquor pretreatment to pretreat lignocellulose and then prepare CNFs, and to further investigate the effect of pretreatment on the physicochemical properties of CNFs.

Therefore, in this work, hot water, green liquor and sodium chlorite were used to pretreat sugarcane bagasse and spruce and then prepare CNFs by high-pressure homogenization. Component analysis, transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG), and ultraviolet (UV) spectroscopy were used to analyze the effect of different pretreatments on the physicochemical properties of the CNFs and CNF films. Furthermore, the effect of residual lignin and hemicellulose content on the thermal stability and UV resistance of CNF films was also investigated. This research provides the basic theory for CNFs in the application of thermal managemen<sup>t</sup> and UV resistance fields.
