**1. Introduction**

Petroleum-based packaging films, such as polyethylene and polypropylene, are a major source of pollution and have a negative impact on the environment [1,2]. Almost all plastic films are designed to be used once before being destroyed, resulting in a large volume of plastic film waste that poses a direct threat to the environment [3]. Thus, several researchers have attempted to develop a new solution to replace conventional plastic film with a fully biodegradable one.

Biodegradable plastic, also known as bioplastic, is defined by its ability to degrade naturally, such as polybutylene succinate (PBS) and poly(lactic acid) (PLA). PBS is mentioned as one of the possibilities appropriate for utilization as a bioplastic because of its outstanding mechanical characteristics, heat resistance, and processibility [4,5]. However, its properties are insufficient for daily use.

PBS has recently been enhanced by the use of inorganic materials [6]. However, it has a direct impact on humans, such as health problem explanation. Many research groups utilize cellulose in order to prevent these issues. It is a well-known natural-based additive used to improve the properties and biodegradability of several substances [7].

**Citation:** Martwong, E.; Tran, Y.; Natsrita, N.; Kaewpang, C.; Kongsuk, K.; Nakaramontri, Y.; Sukhawipat, N. A Potential of New Untreated Bio-Reinforcement from *Caesalpinia sappan* L. Wood Fiber for Polybutylene Succinate Composite Film. *Polymers* **2022**, *14*, 499. https:// doi.org/10.3390/polym14030499

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 6 January 2022 Accepted: 24 January 2022 Published: 26 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

However, in order to improve the polymer matrix properties, cellulose-based fiber must first be treated before use, either chemically [8] or physically [9]. Almost every piece of research [10,11] has reported that treating cellulose with a chemical reagent or adding a compatibilizer enhanced the interfacial interaction between the polymer and natural-based cellulose. Finding a new bio-reinforcement without treating it or adding compatibilizers is a promising and desired alternative to save time and cost.

*Caesalpinia sappan* L. wood fiber (CSWF) is cellulose derived naturally from a Caesalpiniaceae plant. The mechanical characteristics are comparable to those of cellulosic materials. Brazilin and its derivatives, a heterotetracyclic structure with a hydroxyl functional group, are found in CSWF [12–14]. Thus, we hypothesized that CSWF may be a good reinforcement, even without any surface treatment or compatibilizers. Untreated CSWF is focus on being used as a bio-reinforcement for the PBS composite film to examine this hypothesis.

Therefore, the potential of untreated CSWF as a new reinforcement in a novel biocomposite film was investigated. The influences of CSWF content on the morphology, mechanical properties and biodegradability of PBS/CSWF composite films were studied.

#### **2. Materials and Methods**

#### *2.1. Materials*

PBS grade FZ71PM with a density of 1.24 g/cm<sup>3</sup> and MFR (190 ◦C, 2.16 kg.) of 22 g/10 min was purchased from PTTMCC Biochem in Rayong, Thailand.

The CSWF originated from the OTOP center in Nan, Thailand. To obtain mesh size 100, CSWF was ground and sieved by mesh sieve size number 100. CSWF was less than 150 m long and 30 m in diameter. All of the components were dried for 12 h at 80 ◦C before use.

#### *2.2. Method*

2.2.1. Preparation of PBS/CSWF Composite Films

To control the mixing conditions, a mixing temperature of 160 ◦C and a rotor speed of 50 rpm were utilized. The formula for PBS/CSWF composites is shown in Table 1. An additional minute was spent in an internal mixer preheating a PBS pellet. The CSWF was then added to the melted PBS at 0, 5, 10, and 15 phr and continuously mixed for 3 min. After that, the PBS/CSWF composite was dumped and rapidly cooled to ambient temperature. Finally, the composite was crushed by a granulator machine before being blown up.

**Table 1.** Compositions of the composite films.


The PBS/CSWF composite film was formed by blow film extrusion (type LF-400, Labtech, Sorisole (BG), Italy). Extruder temperature profiles from zone 1/zone 2/ zone 3/zone 4 were 130/135/135/140 ◦C, respectively. The die temperature and screw rotational speed were 145 ◦C and 40 rpm, respectively. The nip-roll and windup rates were 4.2 and 4.4 mm/min, respectively. The lay flat width of composite films was fixed at 13 cm. To prevent humidity absorption, all of the composite films were kept in a desiccator.

An overview of PBS/CSWF composite preparation process is illustrated in Figure 1.

#### 2.2.2. Characterization

The density of the composite was determined using ASTM D4635-16. A Vernier calliper was used to accurately measure the length and thickness of composite films. The specimens were precisely weighed in order to calculate the density using the equation below.

> Density (g/cm<sup>3</sup> ) = Mass (g)/Volume (cm<sup>3</sup> )

The morphology of PBS/CSWF composite films was studied with an optical microscope at a magnification of 10× and 40×, using Xenon (DN-117M, Nanjing Jiangnan Novel Optics, Nanjing, China).

Following imaging, measurements of each sample were completed using ImageJ. The fiber size, aggregation area of fiber in PBS film composite and total area were analyzed. The statistical analysis of aggregation area, total area, and % area of fiber has been accomplished and reported.

The mechanical properties of the composite films were measured using Testometric (M500-25AT) (Testometric, Rochdale, UK) on a universal testing machine in accordance with ASTM D638. The size of the specimen was 130 <sup>×</sup> 10 mm<sup>2</sup> . The instrument included a 1 kN load cell and a 25 mm gauge length extensometer with a crosshead speed of 10 mm/min. Tensile strength and elongation at were reported.

Biodegradation was studied via soil burying in an ambient environment, and the weight change before and after soil burying was measured. The weight change was measured once a month for three months under ambient conditions. The Equation (1) was used to determine the film degradation.

$$\mathbf{W\_{loss}} \text{ (\%)}=(\mathbf{W\_{f}} - \mathbf{W\_{i}}) / \mathbf{W\_{i}} \times 100\tag{1}$$

where: Wloss (%) is weight loss percentage, W<sup>i</sup> is the weight of the composite film before the burial test, and W<sup>f</sup> is the weight of the composite film after the soil burial test.

#### **3. Results and Discussion**

*3.1. Properties of PBS/CSWF Composite Films*

The thicknesses of PBS/CSWF composite films were determined by setting the flat lay width to 13 cm as shown in Figure 2. It was discovered that the thicknesses of composite films for neat film, PBS/CSWF5, PBS/CSWF10, and PBS/CSWF15 were 0.031, 0.043, 0.052, and 0.069 mm, respectively. By increasing the CSWF component, the thickness of the PBS/CSWF composite film was marginally increased. It was attributed to agglomeration effects and limited dispersion in CSWF with a higher content.

**Figure 2.** Thickness of PBS/CSWF composite films.

Figure 3 illustrates the density of PBS/CSWF composite films. It was marginally reduced from 1.24, 1.20, 1.19, and 1.16 g/cm<sup>3</sup> by increasing the CSWF content from 0, 5, 10, and 15 phr, respectively. Because CSWF has a lower density than neat PBS, the density tendency of PBS/CSWF composite film was therefore reduced in the formulation with a higher content of CSWF. As a result, by increasing the CSWF content, the density of the PBS/CSWF composite in this study was reduced significantly.

**Figure 3.** Density of PBS/CSWF composite films.
