Ramie Fiber-Reinforced Polylactic-Acid Prepreg: Fabrication and Characterization of Unidirectional and Bidirectional Laminates

The fabrication of a natural prepreg with poly-lactic acid (PLA) matrix and ramie fiber reinforcement was engineered on a laboratory scale by impregnating the unidirectional and bidirectional ramie fiber with PLA matrix solvent on a glass die. The obtained composite prepreg has been stored at a very low temperature to maximize its shelf life. Tensile and biodegradability tests of the composite laminates prepared by the hot-pressing method have also been conducted. Tensile test results show that the freezer-stored bidirectional 0/90° prepreg laminate specimen has the highest tensile strength of 71.44 MPa with a modulus of 2.70 GPa on average. Meanwhile, the unstored bidirectional 0/90° prepreg laminate specimen has the highest level of elasticity, with a modulus of 1.29 GPa on average. The biodegradability test shows the decomposition process of the composite laminate under actual composting conditions. Microscopic observation of the damaged specimen results shows good adhesion between the PLA matrix and ramie fiber and the decomposition of the biodegradability test samples.

Composite; Natural prepreg; Polylactic Acid (PLA); Ramie fiber

Unfortunately, its composites still have to be fabricated in conventional procedures. It can be improved by using a prepreg or pre-impregnation during its preparation. Prepregs or pre-impregnated composites are semi-finished composite products made by impregnating a textile/fabric architecture of a fiber reinforcement with a thermoplastic or thermoset matrix (resins). Therefore, a prepreg can be defined as a preform braided structure of the reinforcement used as a composite (Potluri and Nawaz, 2011). Composite prepregs reduce the risk of poor impregnation quality by ensuring that the amount of each constituent is correct and interacts well (Duhovic and Bhattacharyya, 2011). It will also reduce the risk of possible composite processing defects, such as applying complex geometries like curvature indentations (Wang et al., 2020). Prepreg is generally used as a material for manufacturing components in the aircraft industry because of its advantages: having a high track and drape, which is useful for components with complex shapes (Seferis et al., 2011).

The reinforcing fiber in the prepreg will still be aligned as it was before during the manufacturing process. Consequently, it is considered suitable and capable of making parts with lower fiber defects with excellent characteristics (Cairns et al., 2001). Prepreg has a very good performance compared to other forms of composite materials. This material is suitable for manufacturing composite parts that are very light but can bear significant loads (Wolff-Fabris et al., 2016). Prepregs require good storage, i.e., away from direct sunlight, heat, and strong chemicals. To extend its shelf life, prepregs need to be stored at temperatures below 0°C (Bhatnagar et al., 2006). The method of prepreg preparation on composites (especially thermoplastics) with natural fiber reinforcement can be carried out by spinning the reinforcing yarn with matrix filaments (Baghaei and Skrifvars, 2016; Baghaei et al., 2015; Baghaei et al., 2013). Another study also conducted the preparation by hot-rolling a matrix sheet with reinforcing fabrics (McGregor et al., 2017). Preparation of prepregs can also be carried out by dissolving the matrix granules into a solvent compound, which is then used to impregnate the reinforcing fabrics (He et al., 2019).
      Due to the gap from the predecessor studies, this research had brought in the engineered fabrication of fully biodegradable composite materials of ramie fiber as reinforcement and PLA as the matrix on a laboratory scale by using a manual solvent casting impregnation method. Also, this research aimed to determine the characteristic of its biodegradability, interface bonding, and tensile properties.

2.1. Materials
     Ramie plain-woven fabric was supplied by Guangzhou Xinzhi Textile Co., Ltd. (China), and Bio-poly 103 PLA granules were chosen as the matrix from Shanghai Huiang Industrial Co., Ltd. (China). Meanwhile, NaOH and dichloromethane were supplied by a local distributor PT. Indogen Intertama (Indonesia).

2.2. Prepreg preparation

Two types of reinforcement were fabricated: unidirectional and bidirectional. The bidirectional will be prepared in 0/90° and ±45° fabrics. The ramie fabric used in this study is a plain weave type. The ramie woven fabric was cut into 250 × 190 mm². Then, some of those cut woven yarn is yanked in the perpendicular direction to make a unidirectional fabric reinforcement. Since the ramie fiber has hydrophilic properties and PLA has hydrophobic properties, it is necessary to apply a surface treatment to the ramie fiber to increase the interfacial adhesion between both constituents (He et al., 2019). Ramie fiber was soaked in NaOH solution (5% wt) with a ratio of fiber and solution of 1:10 for 2 hours, then rinsed until the pH reached 7. The fiber then being dried at room temperature for 12 hours.
       Figure 1 illustrates the flow of the impregnation process. The PLA granules were dissolved in dichloromethane solvent with a ratio of 1:10 using a magnetic stirrer for 2 hours at room temperature. The ramie fabric then being manually impregnated in the PLA/dichloromethane solution. The reinforcement was impregnated on a glass mold (impregnation bath) of 25.4 × 19.4 cm² until the matrix was thoroughly pervaded and the excess evaporated. After that, the resulting prepregs are taken out and covered with parchment paper, as seen in Figure 2. Then the prepregs were rolled up, and half of the 0/90° one was stored in a refrigerator freezer at -18°C for a week as a preserving act. 

Figure 1 Impregnation process flow

Figure 2 Prepared prepregs

2.3. Specimen preparation

       Figure 3 shows the flow of RFRPLA prepreg specimen fabrication. Before undergoing a tensile test, the fabricated prepregs were prepared into a plate specimen through the hot-press polymerization method. The prepreg sheets made previously were removed from the parchment paper covering, then stacked in a 25.4 × 19.4 cm² AA 6061-T6 mold, and then hot pressed at 120°C with 132 bar pressure for approximately 90 minutes. The composite laminate plate is then cut using laser cutting according to the geometry of the American Society for Testing and Materials (ASTM) D3039 standard (ASTM, 2017). Also, some residual-cut specimens will be decomposed as biodegradability test samples.

Figure 3 Specimen preparation flow

2.4. Characterization

2.4.1. Tensile test

       Tensile tests are carried out according to the ASTM D3039/D3039M standard. The RFRPLA specimens were tested on the Tinius Olsen universal uniaxial testing machine at the Metallurgy and Materials Research Center (P2MM) LIPI. The machines were equipped with a 30 kN load cell with a 2 mm/min displacement rate. The test was performed on four samples, consisting of a unidirectional (UD) sample, unstored bidirectional (BD) 0/90° sample, freezer-stored BD 0/90° sample, and BD ±45° sample. The test was also performed on 6 (six) duplicated specimens of each sample.

2.4.2. Biodegradability test

        The biodegradability test was carried out to see the decomposition process in the RFRPLA composite. This test is carried out by placing a small sample on the soil with actual composting conditions. The sample used was the unused cut of unstored 0/90° RFRPLA prepregs laminate plate. Those samples were used as the control sample depicts any other unstored samples. The test sample consists of two sizes, Sample A (5cm x 5cm) and Sample B (2.5 cm x 5 cm), with four duplications, respectively. The composting condition consisted of cow dung, wood shavings, and animal feed waste placed in a wooden box with a width of 0.5 m, length of 0.6 m, and height of 0.3 m. The decomposition process of the composite was measured by mass change and observed for 120 days to see changes in the shape and color of the sample.

3.1.  Prepreg fabrication