Sådhanå (2020) 45:57 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12046-020-1294-7Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

Evaluation of the physico-mechanical properties of activated-carbon enhanced recycled polyethylene/ filament

SIEWHUI CHONG1, THOMAS CHUNG-KUANG YANG2, KUAN-CHING LEE3, YI-FAN CHEN2, JOON CHING JUAN3, TIMM JOYCE TIONG1, CHAO-MING HUANG4 and GUAN-TING PAN2,*

1 Department of Chemical and Environmental Engineering, University of Nottingham Malaysia, Semenyih, Malaysia 2 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei City, Taiwan, ROC 3 Nanotechnology and Catalysis Research Centre, University of Malaya, Kuala Lumpur, Malaysia 4 Green Energy Technology Research Center and Department of Materials Engineering, Kun Shan University, Tainan, Taiwan, ROC e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

MS received 10 March 2018; revised 16 July 2019; accepted 9 January 2020

Abstract. In this study, recycled polymer feedstocks (high-density polyethylene, HDPE and polypropylene, PP) were added with different percentages of activated carbon (AC) made from coconut fiber waste – 0, 2, 4, 6, and 8%. The melting temperatures of the recycled HDPE and HDPE/PP filaments were 113 and 170°C, respectively. The addition of AC improved the thermal stability of the recycled filaments up to 28% while decreased the crystallinity of the filament produced, resulting in a more uniform surface with less crazing. Incompatibility of the recycled HDPE and AC was observed. However, the presence of PP greatly enhanced the compatibility of AC with the HDPE polymer. With the addition of 8% AC to the recycled HDPE/PP, the elongation at break of the recycled HDPE/PP filament reached 54.2%, about 10 times higher than that without AC, which could be due to the passive local interfacial bonding of AC with the methyl group of the PP matrix. The improved elongation at break would in turn aid in 3D printing of products with better elasticity.

Keywords. Polyethylene; recycling; 3D printing; polymers; activated carbon; polymeric composites.

1. Introduction abundant recycled PE/PP plastics into 3D printing filament can contribute to solving the environmental problem of the accu- The popularity of polymer 3D printing, or known as fused mulated plastic waste [1–3]. The major problem with this filament fabrication, has increased drastically due to the environmentally friendly option of utilizing recycled PE/PP is increased affordability by public. (PLA) and that the printed objects may not have the desired mechanical Acrylonitrile Butadiene Styrene (ABS) are the two most and physical strength thus limiting its application. commonly used feedstock for polymer 3D printing. The for- A number of studies have been conducted to improve mer uses renewable sources such as cornstarch and sugarcane the physical and mechanical properties of PLA [4, 5]. as the feedstock and is biodegradable under certain conditions, There are however, very limited studies on utilizing thus is environmentally friendly compared to any mineral- recycled polymers as the feedstock. Notably, an innova- based . The use of large amount of PLA fila- tive sustainable 3D printing known as Laywoo-D3, is well ments nevertheless adds on to the current plastic waste prob- known for its close resemblance to wood due to the use of lem, as in most countries, PLA, categorized as resin code 40% recycled wood chips and polymer binders. It has number 7, is not recyclable in most countries. Among the good physical and printing properties due to its internal recyclable plastics, polyethylene (PE) and polypropylene (PP) structure [6]. Clay [7], carbon nanotubes [8] and graphene occupy the largest amount and relatively lower recycling value [9] have also been used as fillers for polymers. These than polyethylene terephthalate (PET). Upcycling these materials yielded excellent tensile strengths. But there are very limited studies reporting on the elongation at break of 3D printing polymers. In 3D printing, in addition to the *For correspondence tensile strength, elongation at break is another important 57 Page 2 of 6 Sådhanå (2020) 45:57

Figure 1. Size distribution by intensity of the activated carbon produced. factor which allows the printed products to have more Malvern Zetasizer Nano was used to determine the size flexibility. Activated carbon, due to its high specific sur- of the activated carbon produced using N-methyl-2- face area, might be able to improve the elongation at pyrrolidone as the dispersant at 25°C. Diameter of the fil- break and thus is selected as the filler in the paper. Many aments extruded was measured using electronic micrometer other recycled 3D printing filament producers are mostly with 0.01 mm accuracy. X-ray diffraction (XRD), Rigaku startup companies from the maker communities [10, 11]. RINT 2000, with CuKa radiation was used to investigate Nevertheless, not much details can be found on the the crystal structure. Differential scanning calorimeter/ company websites regarding the mechanical, albeit the thermogravimetric analyzer (DSC/TGA), Perkin Elmer, physical properties of the recycled filaments. Hoping to was used to study the thermal properties. Raman spec- provide more information on the feasibility of utilizing trometer, Dongwoo Optron, was used to identify the recycled materials as the feedstock for 3D printing, this structure of all samples under the excitation light of paper thus evaluates and compares the physical and 512 nm. Field-emission scanning electron microscopy mechanical properties of recycled HDPE and recycled (FESEM), JEOL, JSAW 6700F, at an accelerating voltage HDPE/PP filaments, using activated carbon derived from of 15 kV was used to reveal their morphologies. Finally, coconut fiber waste as the filling agent. the samples were prepared using a hot press with dumbbell shaped die at 180°C for tensile test in accordance with ASTM D638, by means of Chun Yen-Max Universal test- 2. Materials and methods ing machine.

Recycled HDPE pellets were purchased from Bear Mama DIY, Taipei, Taiwan, and the recycled HDPE/PP pellets 3. Results were supplied by DA.AI Technology, Taipei, Taiwan. To obtain activated carbon (AC), coconut fiber waste was 3.1 Component identification heated at 700°C under nitrogen for 5 hours. The carbonized fiber was then activated by mixing with potassium Figure 1 shows that the activated carbon produced from hydroxide (KOH) at a KOH: fiber waste ratio of 4: 1 at coconut fiber was having a diameter of 154 nm. As shown 85°C for 1 hour followed by drying. The dried powder was in the XRD patterns in figure 2(a), all recycled HDPE fil- then heated at 800°C for 1 hour, and washed by 0.1 M aments with different percentages of AC show diffraction hydrochloric acid and DI water to neutralize pH. Activated peaks at 21.44 and 23.99°, confirming the plastic type as carbon at percentages of 0, 2, 4, 6, 8% were mixed with the HDPE [12, 13]. As shown in figure 2(b), apart from 21.44 recycled HDPE/PP pellets. The mixed pellets were then and 23.99°, other diffraction peaks occur at 14.1, 16.8, 21.2, used as the feedstock for Filastruder filament extruder, and 25.5°, confirming the recycled HDPE/PP plastic type as U.S.A. with 1.75-mm nozzle at 180–190°C. The filaments a poly-blend mixture of HDPE and a-type PP [14]. produced from the recycled PE and PE/PP pellets were In conformation with the Raman spectra in figure 3, denoted respectively as r-HDPE/X% AC, and r-HDPE/PP/ increasing AC content decreases the crystallinity of the X% AC, with X representing the percentage of AC added. filament materials, as indicated by the drop in both XRD Each filament sample was produced with at least 3 m of and Raman peak intensities. The Raman spectra of the as- length, with the first 1 m omitted. prepared HDPE filament products and recycled HDPE/PP Sådhanå (2020) 45:57 Page 3 of 6 57

Figure 2. XRD results of filaments (a) r-HDPE and (b) r-HDPE/ Figure 3. Raman spectra for filaments (a) r-HDPE and PP. (b) r-HDPE/PP.

filament products show the peaks at 1062 cm-1, similar lamellar structure of the fractured surfaces of the 1128 cm-1, 1294 cm-1, and 1462 cm-1, indicating asym- filament products. metric vibrations of d(CH2) and d(CH3). There is an addi- tional broad peak in figure 3(b) at around -1 1220*1250 cm indicating Si-CH2CH3 [15], in other 3.3 Mechanical properties words, the recycled HDPE/PP pellets used contained silica material as the binder. Comparing figure 6, the two recycled r-HDPE and r-HDPE/ PP products without AC addition have similar tensile properties, with ultimate tensile strength of 13–17 MPa, 3.2 Physical properties elongation at break of 5.7–6.4%, and Young modulus of 5.1–5.4 GPa. For r-HDPE, AC addition has not resulted in Figure 4 shows the melting points and thermal stabilities of significant change in the tensile properties for r-HDPE. 8% the filaments produced. The melting points of the recycled AC resulted in 1.4 times higher ultimate tensile strength HDPE products were found to be 111–114°C (figure 4(a)) and Young modulus compared to that without AC addition. and that of recycled HDPE/PP products was around For r-HDPE/PP, the addition of AC has drastically 167–173°C (figure 4(b)). The TGA results in fig- enhanced the elongation at break of the r-HDPE/PP prod- ure 4(c) and 4(d) indicate that the thermal stability ucts, from the initial 5.7%, to 13.6, 23.1, 39.4, and 54.2% increases with the amount of AC content. Especially for subject to 2, 4, 6 and 8% AC content respectively, i.e., recycled HDPE/PP products, due to the presence of PP, the filament r-HDPE/PP/8% AC resulted in about 10 times use of 8% AC enhances T75% by 108°C. The diameters of higher in elongation at break than that without AC addition. the filament products were found to be 1.71–1.76 cm, with As a compromise, the enhancement in elongation at break tolerance less than 0.12 cm. FESEM in figure 5 reveals the reduced the tensile strength and Young modulus. 57 Page 4 of 6 Sådhanå (2020) 45:57

Figure 4. DSC of Filaments (a) r-HDPE, (b) r-HDPE/PP; TGA Figure 5. FESEM micrographs of filament surface: (a) r-HDPE; of Filaments, (c) r-HDPE and (d) r-HDPE/PP. (b) r-HDPE/8% AC; (c) r-HDPE/PP, and (d) r-HDPE/PP/ 8% AC. Sådhanå (2020) 45:57 Page 5 of 6 57

tends to reduce the tensile strength and Young modulus of the filament product even at an insignificant content (less than 5%). Also revealed by the Raman spectra as in figure 3 which correspond to the FESEM images in figure 5, increasing AC content decreases the crystallinity of the product, resulting in more uniform surfaces. However, FESEM of filament r-HDPE/8% AC shows that the recycled HDPE and AC are incompatible as AC can be seen in the form of agglomerated granules with size of about 200 nm. Inter- estingly, in contrast, FESEM of filament r-HDPE/PP/8% AC shows that the r-HDPE/PP and AC are partially com- patible. We postulate that the compatibility could be due to the passive local interfacial bonding between AC and the methyl group of PP, resulting in synergistic interaction [18, 19], thus contributing to the enhancement of elongation at break. By comparing with the two commonly used filament types in the market – PLA and ABS filaments made by Ultimaker (one of the biggest 3D printer manufacturers [16]), as shown in table 1, the recycled r-HDPE/8% AC has the highest elongation at break, 60% higher than the Ulti- maker ABS filament, at the expense of lower tensile strength. Addition of AC has also improved the thermal stability of the filaments. These enhancements indicate that recycled HDPE/PP material with activated carbon addition Figure 6. Mechanical properties for filaments (a) r-HDPE and is a viable filament option for smoother printing of products (b) r-HDPE/PP. with improved elasticity, less crack formation, and improved tolerance to hot environment. Table 1. Comparison of filament products.

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