*Article* **Barrier Properties and Hydrophobicity of Biodegradable Poly(lactic acid) Composites Reinforced with Recycled Chinese Spirits Distiller's Grains**

**Zhi-Jun Chen 1,†, Chi-Hui Tsou 1,2,3,4,\* ,†, Meng-Lin Tsai 5,†, Jipeng Guo 1,†, Manuel Reyes De Guzman <sup>1</sup> , Tao Yang <sup>1</sup> , Chen Gao <sup>1</sup> , Yan Lei <sup>1</sup> , Pei-Wen Gan <sup>1</sup> , Shuang Chen <sup>1</sup> , Lian-Jie Tu <sup>1</sup> , Chang-Lei Qu <sup>1</sup> , Ruo-Yao Wang <sup>5</sup> and Chin-San Wu 6,\***


**Abstract:** Adding natural biomass to poly(lactic acid) (PLA) as a reinforcing filler is a way to change the properties of PLA. This paper is about preparing PLA/biomass composites by physically melting and blending Chinese Spirits distiller's grains (CSDG) biomass and PLA to optimize the composite performance. Composites of modified PLA (MPLA) with varying amounts of CSDG were also prepared by the melt-mixing method, and unmodified PLA/CSDG composites were used as a control group for comparative analysis. The functional groups of MPLA enhanced the compatibility between the polymer substrate and CSDG. The composite water vapor/oxygen barrier and mechanical properties were studied. It was found that the barrier and mechanical properties of MPLA/CSDG composites were significantly improved. SEM was adopted to examine the tensile section structure of the composites, and the compatibility between the filler and the matrix was analyzed. An appropriate amount of CSDG had a better dispersibility in the matrix, and it further improved the interfacial bonding force, which in turn improved the composite mechanical properties. X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry were conducted to determine the crystalline properties and to analyze the stability of the composites. It was found that the CSDG content had a significant effect on the crystallinity. Barrier and biodegradation mechanisms were also discussed.

**Keywords:** poly(lactic acid); Chinese spirits distiller's grains; barrier; mechanical behavior; hydrophobicity; biodegradable properties

#### **1. Introduction**

Research and development on environment-friendly polymers and polymer composites based on renewable resources has attracted increasing attention [1,2]. Natural [3,4] and various functional materials used in manufacturing have gradually replaced traditional ones [5,6] because of their clean, easy-to-use, harmless, and environment-friendly features, and their usage can reduce energy consumption [7] and environmental pollution [8,9]. Poly(lactic acid) (PLA) can be naturally metabolized in the human body into

**Citation:** Chen, Z.-J.; Tsou, C.-H.; Tsai, M.-L.; Guo, J.; De Guzman, M.R.; Yang, T.; Gao, C.; Lei, Y.; Gan, P.-W.; Chen, S.; et al. Barrier Properties and Hydrophobicity of Biodegradable Poly(lactic acid) Composites Reinforced with Recycled Chinese Spirits Distiller's Grains. *Polymers* **2021**, *13*, 2861. https://doi.org/ 10.3390/polym13172861

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 24 July 2021 Accepted: 20 August 2021 Published: 26 August 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/).

carbon dioxide and water. It is called "an environmental recycling material of the 21st century" [10–12]. PLA/natural plant fiber composites have considerable performance and excellent biodegradability; they are known as "green" composites and are currently a hot spot in the field of biodegradable materials.

Cellulose sources are rich and diverse, including crop straw, bagasse, wood, bamboo, and corn cobs, and other agricultural waste [13,14]. Filling PLA with natural biomass fibers to prepare composites reduces the density of the polymer matrix, improves the strength and biodegradability of the composites, and greatly reduces the cost. Application fields include construction and civil engineering, sports and entertainment products, medical equipment, bionic products, and home office supplies, and they have expanded to various fields such as aerospace, national defense, and military industry [15,16]. This expansion not only improves the added value of plant fibers but also makes full use of degradable PLA to replace original nonrenewable petroleum-based plastics. The use of degradable plastics is of great significance to the environment and human development [17].

Zhao et al. [18] carried out surface modification treatment on rice straw fiber (RSF) and on combined modified and unmodified RSF with PLA, and they compared and analyzed different samples. The results showed that modified RSF had obvious improvement in the composite tensile strength. When the RSF content was 30%, the elongation of the modified sample was 60.8% higher than that of the unmodified one. Yaacab et al. [19] used rice straw powder (RSP) as filler and PLA as matrix, and they mixed them with hot pressing, followed by high-pressure cooling, to obtain PLA/RSP biocomposites. The results indicated that the tensile strength of PLA/RSP composites decreased as the RSP content increased, but the tensile modulus and stiffness increased significantly. Increase in RSP reduced the thermal stability and crystallinity.

Way et al. [20] applied some maple wood fiber (WF) as a reinforcing filler to prepare PLA/WF composites by melt blending. They studied the composite mechanical and thermodynamic properties, and they reported that the interfacial compatibility between PLA and WF was enhanced, and the mechanical properties of the composites were improved as a result of the chemical treatment with a silane coupling agent. Yusoff et al. [21] adopted an effective method to extract bamboo fiber and subsequently characterized its mechanical properties. Bamboo, coconut, or kenaf fiber was used as reinforcement to prepare an environment-friendly composite with PLA. The mechanical properties were tested, and the fiber structure was observed using SEM. The results showed that when three kinds of fibers were added, the mechanical properties or strength of the composites were improved. Wassamon et al. [22] combined natural fibers, such as bamboo, vanilla, and coconut, with modified PLA to prepare a new type of environment-friendly functional materials. Their study demonstrated that the rigidity of composite materials increased significantly.

Bourmaud et al. [23] used maleic anhydride-grafted PLA (PLA-g-MAH) as a compatibilizer in PLA/reed fiber composites, and they found that adding the compatibilizer enhanced the reed fiber adhesion to the polymer matrix. The degree of fiber dispersion significantly improved the compatibility between the fiber and PLA. Zhang et al. [24] chose PLA-g-MAH as a compatibilizer for wood flour-reinforced PLA composites, and they observed that when the mass fraction of PLA-g-MAH was 30 wt %, the mechanical properties were significantly improved compared with those of the composites without a compatibilizer.

A category of biomass is a by-product mainly obtained from wine industries, known as distiller's grains, one type of which is referred to as Chinese Spirits distiller's grains (CSDG). The majority of CSDG is burned or buried as waste, which not only wastes resources but also causes environmental pollution. However, biomass can be used as fillers to make biodegradable materials. For example, plant fibers can act as nucleating agents in the crystallization process of different thermoplastic polymers and interfere with their supramolecular structure [25]. However, plant fibers also have disadvantages such as poor water resistance and poor plasticity. CSDG can be an alternative, as it is also an important part of biomass, and it is likely to replace glass fibers, inorganic fibers, and other fillers commonly used in plastics to prepare environment-friendly composites. Therefore, CSDG may be combined with PLA to improve the water resistance of composites. At the same time, it may expand further the application of green plastics or the application fields of natural fibers.

CSDG, an unfermented grain, is a by-product from ethanol fuel industries. At present, there is little research on the application of CSDG in PLA composites. Only Lu et al. [26] prepared biodegradable composites in 2014 by mixing PLA with dried distiller's grains. Only the two materials—PLA and dried distiller's grains—were mixed. Neither PLA nor distiller's grains was modified, and the change in mechanical strength was not mentioned in the study. It was only estimated that the mechanical properties should not be ideal. Moreover, the research was mainly focused on biodegradable and thermal properties, and the content of distiller's grains was only 20%. There is still much room for discussion and further studies. Interestingly, CSDG contains a lot of grains, and it is more complicated than distiller's grains in general. In addition, CSDG has never been reported in studies dealing with PLA. In this present work, CSDG was used as a reinforcing filler, and a composite material was obtained by melt blending it with PLA. A modified form of PLA (MPLA) was also considered to improve the interfacial compatibility between CSDG and PLA. The hydrophobicity, biodegradability, as well as the mechanical, barrier, crystallization, and thermodynamic properties of the composites were characterized and analyzed. The effect of varying amounts of CSDG on the physical and mechanical properties of the composites was investigated to explore the potential application of CSDG in the field of green plastics.
