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- Publisher
- Hindawi Publishing Corporation
- ISSN
- 1687-8434
- eISSN
- 1687-8442
- DOI
- 10.1155/2023/7042932
- Publisher site
- See Article on Publisher Site
Abstract
Hindawi Advances in Materials Science and Engineering Volume 2023, Article ID 7042932, 14 pages https://doi.org/10.1155/2023/7042932 Research Article Investigating the Dimensional, Mechanical, and Morphological Properties of Composites Reinforced with Sisal Fibers and Polylactic Acid in Response to Water Absorption 1,2 1 2 3 Eshetie Kassegn , Belete Sirhabizu , Temesgen Berhanu, Bart Bufel, and Frederik Desplentere Department of Mechanical Engineering, Addis Ababa Science and Technology University, Addis Ababa 16417, Ethiopia School of Mechanical and Industrial Engineering, Mekelle University, Mekelle 231, Ethiopia ProPoLiS Research Group, KU Leuven Bruges Campus, Spoorwegstraat 12, B-8200 Bruges, Belgium Correspondence should be addressed to Eshetie Kassegn; keshetie61@yahoo.com Received 25 February 2023; Revised 16 May 2023; Accepted 24 May 2023; Published 2 June 2023 Academic Editor: Fabrizio Sarasini Copyright © 2023 Eshetie Kassegn et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Biocomposites are promising candidates for some engineering applications owing to the growing environmental and economic challenges to replace petrochemical-based polymers with biodegradable polymers. Herein, sisal fber reinforced polylactic acid (PLA) composite specimens were fabricated using an injection molding machine with and without plasticizer. Te weight percentage (wt%) of the sisal fber varied between 5% and 20%. Te efect of the sisal fber wt% on water absorption resistance and mechanical properties was investigated experimentally using water ageing, mechanical, and morphological studies. Te results revealed that tensile and fexural specimens after water ageing exhibited lower tensile and fexural strengths with higher water absorption behavior at 20 wt% sisal fber than 5 wt% sisal fber, while the impact strength increased after water ageing. In addition, the sisal fber/PLA composites exhibited higher water absorption behavior and lower strength and modulus at 20 wt% sisal fber after water ageing. Moreover, the water absorption decreased with the incorporation of the plasticizer. composites because water attacks the hydrophilic natural 1. Introduction fbers and the fber-matrix interface [4]. Natural fber-reinforced thermoplastic polymers have been Tere are some biodegradable thermoplastic polymers utilized in the composite industry because of their in- commercially available. Among the biodegradable polymers, teresting physical and mechanical properties, and envi- polylactic acid (PLA) is one of the most widely used poly- ronmental advantages over synthetic fbers [1]. Most mers because of its properties such as good biocompatibility, recently, the development and use of biocomposites from nontoxic byproducts, excellent transparency, high strength, and modulus [5, 6]. PLA has properties competitive with biodegradable polymers and natural fbers have received increasing attention in the composite industry due to their many other polymers such as polystyrene (PS), poly- ability to fully degrade in the soil or dedicated industrial hydroxyalkanoates (PHA), polyhydroxyhexanoate (PHH), facilities [2, 3]. Despite the attractive mechanical properties polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and environmental advantages of natural fber composites, polycaprolactone (PCL), polybutylene succinate (PBS), and their poor hygrothermal resistance compared to synthetic polyethylene terephthalate (PET) [3, 7]. But PLA’s brittle- fber composites restricts their outdoor applications. Pre- ness limits its outdoor application to some extent. Bio- vious studies indicate that water absorption deteriorates the degradable plasticizers can be added to PLA to improve its mechanical properties of natural fber-reinforced elasticity in composites [8]. Biocompatible plasticizers are 2 Advances in Materials Science and Engineering used in composites for the excellent fexibility of bio-based study. SF was used as reinforcement in PLA composites. Te polymers at low temperatures, improved stability at high fber was obtained from Abala, in Ethiopia’s Afar region. To temperatures, and good ultraviolet (UV) stability [9]. develop the composites and test water absorption and ef- Among the most common natural fbers, sisal fber (SF) fects, SFs with diferent weight percent (wt%) were used. is abundantly available and has been used as reinforcement Before processing, the fbers were cut to approx. 5 mm. due to its excellent properties such as specifc strength and Tributyl 2-acetylcitrate plasticizer (PLAST) was in- modulus, high durability, ability to stretch, and resistance to corporated into compounds to enhance PLA’s elasticity in deterioration in salt water [2]. SF is widely cultivated in East SF-reinforced composites (Figure 1). Africa and East Asian countries such as Ethiopia, Kenya, and India [10]. Ethiopian SF is cultivated to make traditional 2.2. Specimen Preparation. Production of composites goods such as ropes, bags, hats, and fshing nets. Te fbers through injection molding needs a pre-process to melt and have an approximate density of 1.45 g/cm (Table 1). Its blend the polymer with the fber reinforcement. Tis tensile strength and tensile modulus are 400 to 700 MPa and compounding step of the short SF (approx. 5 mm fber 9 to 38 GPa, respectively [12, 18]. Te approximate chemical length) with PLA was carried out through a corotating twin compositions of SF are 65% cellulose, 12% hemicellulose, screw extruder (Leistritz ZSE18maxx). In preparation for the 9.9% lignin, 2% wax, and 1.1% others [19]. compounding step, the fbers and polymer materials were To evaluate and improve the exterior use of natural fber- dried at 80 C for 8 h with a pressurized air dryer (Moretto). reinforced polymer composites, water absorption and its Tis was done to avoid moisture turning into a stream efects on mechanical properties are important studies. during compounding, as well as hydrolysis efects along the Water absorption depends on polymer and fber types, interfacial bonding between fbers and matrix. Fiber con- natural fber loadings, methods of composite processing, and tents used in compound development were 5 wt% and 20 wt environmental conditions. Water absorption by polymer %. In addition, neat PLA and plasticized PLA compounds composites reinforced with natural fbers is attributed to the were produced. To conduct the study, composite specimens presence of hydroxyl groups which attract and bond water of SF/PLA/PLAST with respective wt% of 0/100/0, 0/94/6, 5/ through the formation of hydrogen bonds [20]. Due to the 89/6, 20/75/5 and 20/80/0 were used. Diferent amounts of hygroscopic properties of natural fbers, they are sensitive to plasticizer were used in the diferent composites to keep the moisture. In addition, natural fbers are sensitive to moisture ratio of PLA to plasticizer constant. It was important to due to their hollow morphology. Te higher the weight analyze the efect of the plasticizer on the composites at percentage (wt%) of natural fbers in composites, the higher a constant ratio with PLA. the water absorption [21]. Composite manufacturing Prior to injection molding, compounded pellets were methods can also afect water absorption characteristics. dried at 60 C for 8 h using the same pressurized air dryer. Injection molding is one of the most efcient methods of Higher drying temperatures would result in sticking to- processing polymer composites. It produces products with gether of the compounds due to plasticizer presence. Te good fber-matrix intermixing and geometrically intricate barrel temperature of injection molding equipment was shapes. In addition, injection-molded composite products independently controlled in six zones in the following se- absorb less water than composite products processed by quence: 30, 155, 170, 180, 190, and 190 C from the feed zone other methods such as compression molding and extrusion to the injection nozzle. Test specimens were fabricated using [22]. Te efect of water on composites depends on the an Arburg320S-50T injection molding machine with tensile amount of water absorbed [23]. Some papers studied the bar geometry (ISO 527 specimen A, Figure 2) mold. Te efect of water absorption on SF composites with PLA production of composite specimens using injection molding matrix, and analyzed their water absorption capacities and was performed using the following process parameters: melt efects on physical properties [24–27]. However, there is no temperature and mold cooling water temperature were previous study investigating the mechanical performances of ° ° 190 C and 25 C, respectively. Te injection fow rate and SF/PLA composites after water absorption and desorption packing pressure were 30 cm /s and 700 bar, respectively. (water ageing) [28]. Cooling time was 20 to 30 s based on fber wt% in the Tis study investigates the efect of water absorption on composites. Te switch-over point was 12 cm with a total the dimensional, mechanical, and morphological properties dosing volume of 43 cm . of SF/PLA/plasticizer biocomposites at various fber wt%. It is crucial that the study be carried out to determine whether SF-reinforced composites are applicable for exterior use 2.3. Moisture Content of SF. To study the efect of water under water exposure. In order to extend the importance of absorption on SF-reinforced PLA composites, the moisture the study on water absorption by SF-reinforced PLA content of SF was investigated. Tis was important to composites, neat PLA and plasticized PLA were investigated. measure the moisture content of fbers and composites. Te moisture content of SF was determined using a moisture analyzer and a Karl–Fisher titrator. SF with approximate 2. Materials and Methods length of 5 mm was used for measuring the moisture content 2.1. Materials. PLA pellets (Natureworks 4043D) with 95% of the fbers. Measurements of the moisture content of the L-lactic acid and 5% D-lactic acid content and a molecular fbers were conducted by a Mettler Toledo HX204 moisture weight of around 110.000 g/mol [29] were used for this analyzer at a drying temperature of 110 C and drying time of Advances in Materials Science and Engineering 3 Table 1: Fundamental properties of the most common natural fbers used in composite materials. Fiber Density (g/cm ) Tensile strength (MPa) E-modulus (GPa) Reference Jute 1.46 393 14 [11] Sisal 1.45 550 24 [12] Bamboo 1.10 262 10 [13] Flax 1.47 305 31 [14] Hemp 1.35 845 45 [15] Kenaf 1.40 250 4 [16] Banana 1.25 600 18 [17] SF Compounding Specimen Performing of SF/PLA production tests for PLA with/without by injection mechanical PLAST molding properties PLAST Figure 1: Schematic representation for specimen production and testing. accordingly. Te actual wt% of the fbers in the compounds was checked against the compounding wt% set points. Te actual wt% of the fbers was determined by dissolving PLA and plasticizer in the compounds using chloroform (CHCL3-for analysis-ISO-ACS-Reag.Ph.Eur., UN-1888). A 150 ml of chloroform was used to dissolve PLA and plas- ticizer in a 500 mg of compound. Te mixture was allowed to R 20 dissolve the components for 24 h. Te fbers were then fl- tered from the dissolved PLA and plasticizer through flter 150 paper with a vacuum pump. Whatman qualitative flter paper grade 2 circle with 9 cm diameter was used. All dimensions in mm. 2.6. Water Absorption of Specimens. Water absorption of tensile specimens was studied through submersion experi- ments. Before immersion, all specimens were dried at room Figure 2: Specimen geometry used for tensile tests (ISO 527-type temperature to remove moisture. Specimens were fully 1A). immersed in distilled water for 33 consecutive days. During the water absorption test, specimens were periodically (every 24 h) taken out of the distilled water bath to measure the 25 min. Te moisture content of the fbers was also de- mass changes. To assess the mass changes, specimens were termined by Karl–Fisher titration and compared with moisture analyzer results. Te moisture contents were weighted using a digital balance (Mettler Toledo, IM1504) after all surfaces were meticulously wiped dry with clean measured after exposing fbers to 40% relative humidity and 23 C temperature between 24 h and 120 h. cloth. After weighting, these specimens were again im- mersed in water (Figure 3). Water molecules difuse through all surfaces of the 2.4. Moisture Content of Compounds. Te moisture content specimens during water immersion treatment. Tis is be- of diferent compounds was measured on samples exposed cause an injection-molded composite specimen of natural to environmental conditions with an average relative hu- fber-reinforced thermoplastic polymer is assumed to be an midity of 40% and an average temperature of 23 C for 24 h. isotropic material. It is not common to seal surfaces of Compounds of SF blended with PLA, neat PLA, and plas- isotropic material specimens in order to allow water to pass ticized PLA were used for moisture content measurement. only through unsealed surfaces [30]. After water absorption Te measurements were performed using a Mettler Toledo was performed, water desorption was performed for 24 HX204 moisture analyzer with 110 C drying temperature consecutive days. During water desorption, weight mea- and 30 min drying time. surements of the specimens were carried out every 24 h. Te percentage of water uptake (W ) after water absorption uptake was determined by equation (1). 2.5. Actual wt% of Fibers. As mentioned above in the composite preparation section, diferent PLA compounds W − W t o were produced through a compounding process. Te wt% of W [%] � ∗ 100, (1) Uptake each constituent was set on the machine and processed o 4 Advances in Materials Science and Engineering Specimens before Specimens Wiped specimens for Specimens dried at Performing immersing while immersing weight measurement room temperature tensile testing Figure 3: Procedures applied to perform water absorption and tensile testing of specimens. where W and W are weights of the specimen before and and after water ageing were used for Charpy notched impact o t after water absorption, respectively. Te changes in speci- strength tests, and the results were compared. Te tests were men dimensions due to water absorption were determined performed at an average temperature of 23 C and relative using a sliding caliper. Te dimensional changes of the humidity of approx. 40%. specimens after water absorption were measured before the desorption process took place. Te thickness and width 2.8. Morphological Study. Te morphology of SF and the measurements were made on the 80 mm gauge length of the efect of the injection molding process on the microstruc- tensile specimens. Both water absorption and desorption tures of SF/PLA composites were studied. Te efect of water (water ageing) were performed at an average temperature of absorption on composite microstructures was also in- 23 C and relative humidity of approx. 40%. vestigated. Fiber orientation and dispersion, as well as fber microstructures before and after injection molding, were 2.7. Mechanical Characterization. Tensile specimen prepa- observed. Fiber-matrix interfacial adhesion before and after ration and testing were performed according to ISO 527 water ageing was also observed. Both SF and a piece of the standard requirements [31]. Te tests were performed using composite specimen were embedded with resin (a mixture of an Instron 3367 universal testing machine with a load cell of 15 ml Epofx resin and 2 ml Epofx hardener) and cured 30 kN and a displacement rate of 5 mm/min. An exten- inside a desiccator for 24 h. Te specimens were approx. someter was used to measure the initial strain of specimens. 25 mm long. Te specimens were ground and polished with It was removed from the specimen at 1% strain. Te tests various sizes of sand paper and diamond paste, respectively. were conducted at an average temperature of 23 C and Struers DP-Paste M, 6 μm diamond was used for the fnal relative humidity of approx. 40%. Tensile specimens before polishing of the specimens. Ground and polished specimens and after water ageing were used for the tensile tests to were used for microscopic imaging. Te microscopic images identify the efect of water absorption on the tensile prop- of the specimens were captured by Keyence laser scanning erties of the composites, and the results were compared. microscopy (LSM) with 20x magnifcation. General specifcations of the tensile specimens used for the study were defned as 80 mm grip separation, 10 mm width, 3. Results and Discussion and 4 mm thickness (Figure 2). Te fexural testing (three-point bending) was per- 3.1. Actual wt% of Fibers. Te actual fber wt% in the dif- formed using an Instron 3367 universal testing machine at ferent compounds was determined. As shown in Table 2, the an average temperature of 23 C and relative humidity of actual wt% of the fbers difers from the intended wt%. Tis is approx. 40%. Te maximum displacement was set at 10 mm due to inconsistent fber feeding while compounding. Te for a span length of 64 mm. Te test was performed with an inconsistent fber feeding happened due to the nonuniform Instron testing machine of a 30 kN load cell and displace- fber feeding rate through the screws and process in- ment rate of 2 mm/min. Specimen preparation and testing terruption when composite rods are broken at the com- were conducted according to ISO 178 standard requirements pounding outlet. Te amount of SF in the hopper also afects [32]. Flexural specimens before and after water ageing were the fber feeding rate; the lower amount of SF in the hopper used to perform the tests, and the results were compared. leads to a lower fber feeding rate through the screws of the Te impact testing was performed using a Ceast resil compounding machine. impactor Charpy impact test set-ups. Te hammer was released from an angle of 150 and hit the specimen. Te test was performed with hammer impact energy of 3.2. Moisture Content and Water Absorption. Te moisture 2.750 J. Specimen preparation and testing were performed content of SF was measured by a moisture analyzer and based on ISO 179 standard requirements [33]. Te impact Karl–Fisher titrator after exposing the fbers to humidity for specimens had 2 mm notch depth. Impact specimens before 24 h and 120 h. Te results show that the moisture contents Advances in Materials Science and Engineering 5 Table 2: Te wt% of SF in diferent compounds. Table 3: Moisture content of diferent compounds. Intended Moisture content (ppm) using SF/PLA/PLAST (wt%) Actual fber wt% (%) fber wt% (%) moisture analyzer SF/PLA/PLAST (wt%) 5/89/6 5 7 Before After exposing to 5/95/0 5 8 exposing to humidity humidity for 24 h 20/75/5 20 22 0/100/0 1980 4070 20/80/0 20 20 0/94/6 1870 3830 5/95/0 2460 7190 5/89/6 2320 6940 of the fbers using the moisture analyzer after exposure to 20/75/5 8240 13370 humidity for 24 h and 120 h were 39190 ppm and 20/80/0 8450 13940 44250 ppm, respectively. Similarly, the moisture contents of the fbers using the Karl–Fisher titrator after exposure to 3.9 humidity for 24 h and 120 h were 45050 ppm and 3.6 48910 ppm, respectively. Te moisture content of the un- 3.3 compounded PLA after exposing it to open air for 24 h at 3.0 a relative humidity of 40% was 1250 ppm. In industry, 2.7 a moisture level of 100 ppm is applied prior to processing. In 2.4 addition, the compounds’ moisture content was measured 2.1 and compared. Table 3 shows the moisture content of 1.8 compounds before and after humidity exposure for 24 h. Te 1.5 results indicate that the moisture contents before and after 1.2 exposing the compounds to humidity for 24 h were in- 0.9 creased linearly with increasing fber wt%. 0.6 0.3 Water absorption from injection-molded tensile speci- 0.0 mens was investigated. Water absorption evolution was 0 2 4 6 8 10121416182022242628 infuenced by the fber wt% of composite specimens (Fig- 1/2 Time (h ) ure 4), that is, a composite with higher fber wt% absorbs more water than a composite with less fber wt% because 20/80/0 20/75/5 natural fbers are more hygroscopic than polymers. PLA is 5/95/0 5/89/6 a fairly polar molecule due to the presence of a small number 0/100/0 0/94/6 of –OH groups in the chemical structures (Figure 5(a)), and Figure 4: Water absorption behavior of composite specimens. it can absorb water [34]. Natural fbers, however, are more polar due to the presence of many –OH groups in the chemical structures and are hydrophilic in nature [35]. As the highest water desorption was measured during the frst 12 h (Figure 6), respectively. Te water absorption of the a result, natural fbers absorb water better than PLA. Water difuses into polymer composites through three mechanisms specimens was plotted versus the square root of time, [36]. First, water molecules difuse within microgaps be- according to the Fickian moisture difusion law [38]. Sim- tween polymer chains. Second, water molecule capillary ilarly, the water desorption of the specimens was plotted difuses into the gaps and faws at the interfaces between versus the square root of time. fbers and the polymer matrix. Tird, water molecules difuse Figure 7 illustrates the specimen dimensions before and through microcracks in the matrix propagated by fber after water absorption. Te specimen dimensions of diferent swelling. Figure 4 shows that neat PLA specimens absorb composites were measured before water absorption. Te water slightly more than plasticized PLA specimens. Tis is results show that the thickness and width of specimens with due to the lower water absorption of plasticizer compared to high fber wt% were signifcantly higher than specimens with PLA. It can be observed that the tributyl 2-acetylcitrate low fber wt%. Specimens with fber content had an in- creased thickness and width than specimens without fber plasticizer (PLAST) used is a nonpolar molecule and a hy- drophobic plasticizer (Figure 5(b)). Te plasticizer does not content. Tis is based on the fact that specimens with higher have an –OH group in its chemical structure. Moreover, it fber wt% in the composite exhibit less shrinkage than has ester functional groups (−COOR), and esters are non- specimens with lower fber wt% during cooling of injection- polar molecules. Furthermore, the plasticizer occupies most molded specimens. Tis can happen due to the insignifcant of the available space between the polymer chains and thus shrinkage behavior of fbers in polymer composites during causes less space for the water molecules. Kang et al. [37] and after injection molding. Fibers are dimensionally stable investigated that small molecules of plasticizers move into at elevated temperatures. Specimens of composites at higher PLA polymer chains, allowing PLA to become crystalline fber wt% were thus dimensionally more stable than spec- imens at lower fber wt%. Neat PLA specimens, however, since plasticizers act as a nucleating agent. With increased crystallinity, water molecules cannot penetrate polymer showed greater dimension loss during cooling. After im- chains easily due to their tight packing. Te highest water mersing specimens in water, the fnal dimensions were absorption was measured on the frst day (Figure 4), whereas measured and compared with the dimensions obtained Water absorption (%) 6 Advances in Materials Science and Engineering CH O CH O 3 3 CH O OH O 3 HO O H C O O CH 3 3 O CH O O O O H C (a) (b) Figure 5: Chemical structures of (a) polylactic acid (PLA) and (b) tributyl 2-acetylcitrate. were water aged. Te reduction in tensile strength and 3.9 modulus of specimens after water ageing is attributed to the 3.6 3.3 debonding efect of water molecules [39]. Natural fbers are 3.0 more hygroscopic and absorb more water. Water absorption 2.7 in composites causes fbers to swell, resulting in debonding 2.4 of the fbers and matrix. In addition, water absorption leads 2.1 1.8 to microcracks in the matrix and weakening of the fbers due 1.5 to irreversible damages [23]. Te efects of fber debonding 1.2 and weakening, as well as matrix cracking are signifcant 0.9 when the composite is exposed to water absorption for 0.6 a prolonged duration. Prolonged exposure to water will 0.3 0.0 often result in irreversible physical and chemical changes 02468 10 12 14 16 18 20 22 24 within the composite, leading to degradation in tensile 1/2 Time (h ) strength and modulus; that means that both the stress transfer and load bearing ability of the composite degrade 20/80/0 20/75/5 5/95/0 5/89/6 due to water absorption. Conversely, the strain at failure of 0/100/0 0/94/6 specimens increases after water ageing because of the plasticization efect of water absorption [40, 41]. Tis means Figure 6: Water desorption behavior of composite specimens. that water immersion treatment can be considered as a plasticizer for natural fber-reinforced polymer composites [4]. Plasticization is a physical change that occurs through before water absorption. Te results revealed that the the interaction of water molecules with polar groups in the specimen dimensions increased by approx. 1% after water polymer matrix [42]. Tough water desorption took place on absorption. Te width and thickness of specimens afect specimens at 23 C, the moisture content was not expected to water absorption evolution when immersed in water. Fur- fully drain out and the efect would not be reversed. Te thermore, the results indicate that the fber wt% in the results of the tensile properties of the diferent composite composites afects the dimensional characteristics of the specimens before and after water ageing are shown in specimens. Tis is due to natural fbers being more hy- Figure 8. drophilic and absorbing more water. Te presence of Te study of the tensile properties of composite speci- plasticizer in composite specimens resulted in lower di- mens before and after water ageing is summarized in Table 4. mension changes than specimens without plasticizer. Te Te experimental results indicate that plasticized composite plasticizer efect on the dimensional characteristics of the specimens with 5 wt% and 20 wt% of fber after water ageing specimens is shown in graphs of 0/100/0 versus 0/94/6, 5/95/ exhibited 19% and 29% reductions in tensile strengths, re- 0 versus 5/89/6, and 20/80/0 versus 20/75/5 in Figure 7. Te spectively. Te tensile strengths of specimens with 20 wt% of specimen dimensional value evolution (in percentage) after fber in 80% PLA and neat PLA were reduced by 16% and water absorption was added above each bar in the histogram. 12%, respectively. Te value evolution seems insignifcant, but a small change in the dimensions of a composite product has a signifcant impact on its mechanical performance (investigated and 3.3.2. Flexural Properties. Te results of fexural testing discussed in Section 3.3). indicate that the fexural strengths of neat PLA, plasticized PLA, and PLA reinforced with lower fber wt% increased after water ageing. In contrast, the fexural strength of 3.3. Mechanical Characterization specimens with higher fber wt% decreased after water 3.3.1. Tensile Properties. Te efect of water absorption on ageing. Te reduction in fexural strength of specimens with the mechanical properties of injection-molded specimens of higher fber wt% after water ageing is attributed to higher the PLA matrix flled with SF was studied. Te results of water absorption than specimens with lower fber wt%. Te tensile testing indicate that the tensile strength and modulus higher water absorption is attributed to the higher fber wt% of the specimens after water ageing decreased. In contrast, in the composites. Since water absorption of neat PLA is less the strain at failure of the specimens increased after they than 1%, the overall result can be interpreted as a result of Water desorption (%) Advances in Materials Science and Engineering 7 10.3 4.3 0.92 2.37 1.19 2.43 10.2 0.80 0.51 4.2 0.97 0.76 10.1 0.86 0.35 0.49 0.52 4.1 10.0 9.9 4.0 9.8 3.9 9.7 3.8 9.6 9.5 3.7 0/100/0 0/94/6 5/95/0 5/89/6 20/80/0 20/75/5 0/100/0 0/94/6 5/95/0 5/89/6 20/80/0 20/75/5 Before water ageing Before water ageing Afer water ageing Afer water ageing (a) (b) Figure 7: Dimensional comparison of the specimens before and after water absorption. 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.0 Before ageing Afer ageing Before ageing Afer ageing 0/100/0 5/89/6 0/100/0 5/89/6 0/94/6 20/80/0 0/94/6 20/80/0 5/95/0 20/75/5 5/95/0 20/75/5 (a) (b) 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Afer ageing Before ageing 0/100/0 5/89/6 0/94/6 20/80/0 5/95/0 20/75/5 (c) Figure 8: Efects of water ageing on tensile: (a) strength, (b) strain at failure, and (c) modulus. Tensile strength (MPa) Width (mm) Modulus (GPa) Tensile strain (%) Tickness (mm) 8 Advances in Materials Science and Engineering Table 4: Performance evaluation on tensile specimens after water ageing. SF/PLA/PLAST (wt%) Tensile strength (MPa) Strain at failure (%) E-modulus (GPa) 0/100/0 12% decreased 9% increased 6% decreased 0/94/6 11% decreased 2% increased 10% decreased 5/95/0 14% decreased 48% increased 15% decreased 5/89/6 19% decreased 13% increased 21% decreased 20/75/5 29% decreased 27% increased 29% decreased 20/80/0 16% decreased 20% increased 22% decreased water absorption at the SF and SF/PLA interfaces. Te re- nonplasticized composite specimens after water ageing. Tis can be due to the fact that water plasticizes plasticized duction in fexural strength of the specimens with higher fber wt% as a result of water absorption could be due to the composites more than nonplasticized composites, that is, the fact that water absorption afects the interfacial adhesion glass transition temperature of plasticized composites drops between fbers and matrix, leading to a decrease in me- when immersed in water for a prolonged time, which leads chanical properties of the composites [36]. On the other to an increase in plasticization. Furthermore, the water hand, the increase in fexural strengths of neat PLA, plas- plasticization efect is higher for crystalline materials than ticized PLA, and PLA composites with lower fber wt% after for amorphous materials [45]. It is a fact that plasticized water ageing could be due to the increase in PLA’s crys- polymers are more crystalline than neat polymers due to the tallinity [43]. As investigated by Jiang et al. [44], water crystallization efect of plasticizers [46]. Accordingly, the impact strengths of plasticized and nonplasticized composite absorption acts as an efective plasticizer, increasing the aggregation of PLA chains and promoting nucleation, specimens with 20% fber wt% were increased by 24% and 6% after water ageing, respectively. Similarly, the impact leading to the crystallization of PLA molecules. Accordingly, an increase in crystallinity of PLA in composites can im- strengths of plasticized PLA and neat PLA were increased by prove the regularity of the molecular chain, improving 19% and 15% after water ageing, respectively. Te impact compression load carrying resistance, particularly fexural strengths of plasticized and nonplasticized specimens with strength. Te increase in fexural strength at low fber 5% fber wt% were increased by 32% and 3% after water contents indicates that an increase in PLA crystallinity ageing, respectively (Table 6). However, many studies in- compensates for the negative efect of water absorption on dicate that impact strength may decrease with further in- the fber-matrix interface to a certain extent. Te fexural creases in water immersion time after reaching maximum modulus of specimens decreased after water ageing. Te impact strength [23]. Furthermore, the SF/PLA composite specimens exhibited higher impact strength than neat PLA decrease in fexural modulus after water ageing is attributed to the weak bond between the fbers and the matrix due to and plasticized PLA before and after water ageing. Tis is due to the enhancement of SF/PLA composites’ ability to water absorption. Similar to the tensile strain at failure, the fexural strain at failure of specimens increased after water absorb impact energy through increased toughness. On the ageing. Tis is due to the efect of water as a plasticizer on other hand, the damping capacity is sensitive to changes in composites. Te efect of PLAST and water on composite material stifness due to the water plasticization efect [47], plasticization is barely visible from strain results. It was that is, damping increases with water absorption [48]. found that water plasticized better than PLAST plasticizer. Composite constituents are more plasticized with the in- Plasticized and nonplasticized composites showed higher fltration of water molecules. Water absorption can also lead to frictions mainly inside fbers at diferent scales and at the plasticization efects after water ageing. Te fexural prop- erties of diferent specimens were determined before and fber-matrix interface, which increases energy dissipation and damping capacity [48]. after water ageing (Figure 9). Te study of the fexural properties of composite spec- imens before and after water ageing is summarized in Ta- 3.4. Morphological Observations. Figure 11 shows micro- ble 5. Te results show that the fexural strengths of scopic images of the SF and SF/PLA composite surfaces. Te plasticized composite specimens with 5 wt% and 20 wt% of microscopic images revealed the multicellular nature of SF. fber were 20% increased and 17% decreased, respectively. As shown in Figures 11(a)–11(d), groups of fber cells and Te fexural strengths of specimens with 20 wt% of fber in lumen structures are observed in each fber. It has a surface 80% PLA and neat PLA were decreased and increased by packed in a joined fber bundle in a parallel arrangement 13% and 11%, respectively. (Figures 11(e) and 11(f)), which is the nature of SF. After the injection molding process, some fber bundle separations 3.3.3. Impact Strength. Te results of impact tests indicate were observed (Figure 11(g)), which leads to a better dis- that the impact strength of the specimens increased after persion of elementary fbers within the composites. Bundle separation is an efect of the compounding and injection water ageing (Figure 10) due to the plasticization efect of water. Previous studies showed that water is used as molding processes. Better dispersion of elementary fbers within composites can improve the homogeneity of me- a crystallizing plasticizer [45]. In addition, plasticized composite specimens exhibited higher impact strength than chanical properties. Moreover, the contact area between the Advances in Materials Science and Engineering 9 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.0 Before ageing Afer ageing Before ageing Afer ageing 0/100/0 5/89/6 0/100/0 5/89/6 0/94/6 20/80/0 0/94/6 20/80/0 5/95/0 20/75/5 5/95/0 20/75/5 (a) (b) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Before ageing Afer ageing 0/100/0 5/89/6 0/94/6 20/80/0 5/95/0 20/75/5 (c) Figure 9: Efects of water ageing on fexural: (a) strength, (b) strain at failure, and (c) modulus. Table 5: Performance evaluation on fexural specimens after water ageing. SF/PLA/PLAST (wt%) Flexural strength (MPa) Strain at failure (%) F-modulus (GPa) 0/100/0 11% increased 10% increased 4% decreased 0/94/6 20% increased 10% increased 3% decreased 5/95/0 9% decreased 21% increased 12% decreased 5/89/6 20% increased 26% increased 2% decreased 20/75/5 17% decreased 40% increased 30% decreased 20/80/0 13% decreased 26% increased 9% decreased fbers and the matrix increases when bundled fbers are with PLA at 190 C. Tis is due to the fact that the viscosity of separated and well dispersed throughout the matrix. Tis molten PLA at higher temperatures becomes low and can can increase the mechanical strength and modulus of penetrate into fber lumens. On the other hand, large voids composites. were observed in an internal part of PLA (Figure 11(i)). Te It is clearly seen in microscopic images that molten PLA voids could be created by air trapped in an internal part of an penetrated into the lumens of the fbers (Figure 11(h)). PLA injection-molded specimen due to fast cooling and solidi- penetration into SF occurred during melt blending of fbers fying. It can also be caused by insufcient holding pressure Flexural strength (MPa) Flexural modulus (GPa) Flexural strain (%) 10 Advances in Materials Science and Engineering 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Before ageing Afer ageing 0/100/0 5/89/6 0/94/6 20/80/0 5/95/0 20/75/5 Figure 10: Efects of water ageing on impact strength of composites. Table 6: Performance evaluation on impact strength after water ageing. SF/PLA/PLAST (wt%) Impact strength (kJ/m ) 0/100/0 15% increased 0/94/6 19% increased 5/95/0 3% increased 5/89/6 32% increased 20/75/5 24% increased 20/80/0 6% increased 20 μm 100 μm (a) (b) Middle lamella Tracheid Lumen wall 100 μm 20 μm (c) (d) Figure 11: Continued. Impact strength (kJ/m ) Advances in Materials Science and Engineering 11 50 μm 50 μm (e) (f) Elementary fbers Polymer in lumens Elementary fbers PLA matrix 100 μm 20 μm (g) (h) Various fber orientations Air voids 100 μm 100 μm (i) (j) Fiber Matrix Poor interfacial bonding Good interfacial bonding 20 μm 50 μm (k) (l) Figure 11: LSM images of SF and SF/PLA composites: (a) fber cross sectional shapes, (b–f) fber microstructures, (g) fber bundles separation during compounding and injection molding, (h) PLA penetration into fber lumen, (i) air voids in injection-molded specimen, (j) fber orientation, (k) fber-matrix interfacial before water ageing, and (l) fber-matrix interfacial after water ageing. to condense the molten polymer inside the mold. As shown because the fbers were 5 mm long and rotate easily during in Figure 11(j), images of fber orientation in composites material fow into the mold cavity. were captured. Te images show that the fbers were oriented Te efect of water absorption on composite micro- randomly; some fbers were oriented unidirectional and structures was studied. Te microscopic images show that others were oriented transversely or at an angle of θ. Tis is the interfacial adhesion between the fbers and the matrix 12 Advances in Materials Science and Engineering was improved before the water absorption experiment took collaboration of the Ministry of Education (Ethiopia) and place (Figure 11(k)). After water absorption, however, KU Leuven (Belgium). microcracks between the fbers and the matrix interface were developed (Figure 11(l)). Tis is because natural fbers ab- References sorb high amount of water since they are more hydrophobic. High water absorption leads to fber swelling, which leads to [1] A. Praveen Kumar and M. Nalla Mohamed, “A comparative weak fber-matrix interfacial adhesion. Te reduction of analysis on tensile strength of dry and moisture absorbed hybrid kenaf/glass polymer composites,” Te Journal of In- interfacial adhesion between the fbers and the matrix leads dustrial Textiles, vol. 47, no. 8, pp. 2050–2073, 2018. to the reduction of mechanical properties. Indeed, this [2] G. Rajesh, A. R. Prasad, and A. Gupta, “Mechanical and scenario agrees with the results of the mechanical properties degradation properties of successive alkali treated completely of the composite specimens after water ageing. biodegradable sisal fber reinforced poly lactic acid com- posites,” Journal of Reinforced Plastics and Composites, vol. 34, 4. Conclusion no. 12, pp. 951–961, 2015. [3] R. Gunti, A. V. Ratna Prasad, and A. V. S. S. K. S. Gupta, Te present study discusses the infuences of water ageing on “Mechanical and degradation properties of natural fber- the mechanical properties of short SF-reinforced PLA matrix reinforced PLA composites: jute, sisal, and elephant grass,” composites. Te composite specimens were developed with Polymer Composites, vol. 39, no. 4, pp. 1125–1136, 2018. and without plasticizer and the fbers at diferent wt% to [4] S. Panthapulakkal and M. Sain, “Studies on the water ab- investigate the efect of water ageing on tensile, fexural, and sorption properties of short hemp-glass fber hybrid poly- impact properties. Te results indicate that the tensile propylene composites,” Journal of Composite Materials, vol. 41, no. 15, pp. 1871–1883, 2007. strength and modulus were decreased, whereas the strain at [5] H. Zhang, S. Xiang, Q. Luan et al., “Development of poly failure under tensile test conditions was increased due to the (lactic acid) microspheres and their potential application in efect of water ageing. Te fexural strength of composite Pickering emulsions stabilization,” International Journal of specimens with higher fber wt% was decreased due to the Biological Macromolecules, vol. 108, pp. 105–111, 2018. efect of water on the fber-matrix interfacial adhesion. On [6] U. Saeed, M. A. Nawaz, and H. A. Al-Turaif, “Wood four the other hand, the fexural strength of composite specimens reinforced biodegradable PBS/PLA composites,” Journal of with lower fber wt% was increased due to the increase in Composite Materials, vol. 52, no. 19, pp. 2641–2650, 2018. crystallinity of PLA after water ageing. Te fexural modulus [7] K. Madhavan Nampoothiri, N. R. Nair, and R. P. John, “An of all composite specimens decreased after water ageing. overview of the recent developments in polylactide (PLA) Similar to the strain at failure in tensile testing, the strain at research,” Bioresource Technology, vol. 101, no. 22, failure of all composite specimens under the fexural testing pp. 8493–8501, 2010. [8] U. C. Paul, D. Fragouli, I. S. Bayer, A. Zych, and condition was increased as a result of the plasticizing efect of A. Athanassiou, “Efect of green plasticizer on the perfor- water ageing. Moreover, the impact strength of all the mance of microcrystalline cellulose/polylactic acid bio- composite specimens increased after water ageing, since composites,” ACS Appl. Polym. Mater.,vol. 3, no. 6, water acts as a plasticizer. Water ageing also afects the pp. 3071–3081, 2021. morphological properties of fbers and composites, leading [9] T. Datasheet, “Proviplast 2624 proviplast 2624 properties ® ® to a reduction in their mechanical properties. compatiblity with other products,” pp. 1–3, 2020, https:// proviron.com/products/proviplast-2624/. Data Availability [10] J. K. Fink, Reactive Polymers: Fundamentals and Applications, Elsevier Inc, Amsterdam, Netherlands, 3rd edition, 2017. Te data used to support the fndings of this study are [11] S. Dwivedi, S. Pattnaik, and M. Kumar Sutar, “Tensile available from the corresponding author upon request. properties and regression analysis of natural fber and intralaminar mat reinforcement,” Materials Today Pro- ceedings, vol. 44, 2021. Conflicts of Interest [12] J. Naveen, M. Jawaid, P. Amuthakkannan, and Te authors declare that they have no conficts of interest M. Chandrasekar, Mechanical and Physical Properties of Sisal with respect to the research, authorship, and publication of and Hybrid Sisal Fiber-Reinforced Polymer Composites, Elsevier Ltd, Amsterdam, Netherlands, 2018. this article. [13] K. Zhang, F. Wang, W. Liang, Z. Wang, Z. Duan, and B. Yang, “Termal and mechanical properties of bamboo fber rein- Acknowledgments forced epoxy composites,” Polymers, vol. 8, p. 6, 2018. [14] D. Depuydt, K. Hendrickx, W. Biesmans, J. Ivens, and Te authors would like to acknowledge the support of A. W. Van Vuure, “Digital image correlation as a strain IUPEPPE Project for fnancing the research. Secondly, the measurement technique for fbre tensile tests,” Composites authors would like to appreciate the members of the research Part A Appl. Sci. Manuf.,vol. 99, pp. 76–83, 2017. group Propolis at KU Leuven Bruges campus for their [15] T. Gurunathan, S. Mohanty, and S. K. Nayak, “A review of the support while specimen preparation and performing ex- recent developments in biocomposites based on natural fbres perimental tests. Lastly, the authors want to thank all the KU and their application perspectives,” Composites Part A Appl. Leuven community for their contribution in one or another Sci. Manuf.,vol. 77, pp. 1–25, 2015. way while doing the research work. Tis work was fnanced [16] M. R. Sanjay, P. Madhu, M. Jawaid, P. Senthamaraikannan, by the IUPEPPE project under the framework of S. Senthil, and S. Pradeep, “Characterization and properties of Advances in Materials Science and Engineering 13 natural fber polymer composites: a comprehensive review,” [30] B. C. Duncan and W. R. Broughton, “Measurement good Journal of Cleaner Production, vol. 172, pp. 566–581, 2018. practice guide No. 102 absorption and difusion of moisture in [17] P. Dilleswara Rao, D. Venkata Rao, A. Lakshumu Naidu, and polymeric materials,” Meas. Good Pract. Guid.vol. 102, 2007. M. Raju Bahubalendruni, “Mechanical properties of banana [31] ISO, “Plastics—Determination of Tensile Properties—Part 1: fber reinforced composites and manufacturing techniques: General Principles,” ISO, Geneva, Switzerland, ISO 527-1: a review,” Int. J. Res. Dev. Technol.,vol. 8, no. 5, pp. 2349–3585, 2019, 2019. [32] ISO, “International Standard Plastics — Determination of [18] J. L. Tomason, J. Carruthers, J. Kelly, and G. Johnson, “Fibre fexural properties,” vol. 2019, ISO, Geneva, Switzerland, cross-section determination and variability in sisal and fax 2019, Iso 178. and its efects on fbre performance characterisation,” Com- [33] ISO, “Plastics - Determination of charpy impact properties,” posites Science and Technology, vol. 71, no. 7, pp. 1008–1015, vol. 1110pp. 1–11, ISO, Geneva, Switzerland, 2010, ISO 179-1. [34] E. Castro-Aguirre, F. Iñiguez-Franco, H. Samsudin, X. Fang, [19] K. V. Sabarish, P. Paul, Bhuvaneshwari, and J. Jones, “An and R. Auras, “Poly(lactic acid)—mass production, process- experimental investigation on properties of sisal fber used in ing, industrial applications, and end of life,” Advanced Drug the concrete,” Materials Today Proceedings, vol. 22, pp. 439– Delivery Reviews, vol. 107, pp. 333–366, 2016. 443, 2020. [35] K. Zhang, W. Liang, F. Wang, and Z. Wang, “Efect of water [20] S. M. S. Abdel-Hamid, O. Al-Qabandi, E. N.a.s. et al., absorption on the mechanical properties of bamboo/glass- “Fabrication and characterization of microcellular poly- reinforced polybenzoxazine hybrid composite,” Polymers and urethane sisal biocomposites,” Molecules, vol. 24, no. 24, Polymer Composites, vol. 29, no. 1, pp. 3–14, 2021. pp. 4585–4616, 2019. [36] S. Sanjeevi, V. Shanmugam, S. Kumar et al., “Efects of water [21] S. F. K. Sherwani, S. M. Sapuan, Z. Leman, E. S. Zainudin, and absorption on the mechanical properties of hybrid natural A. Khalina, “Physical, mechanical and morphological prop- fbre/phenol formaldehyde composites,” Scientifc Reports, erties of sugar palm fber reinforced polylactic acid com- vol. 11, no. 1, Article ID 13385, 2021. posites,” Fibers and Polymers, vol. 22, no. 11, pp. 3095–3105, [37] H. Kang, Y. Li, M. Gong et al., “An environmentally sus- tainable plasticizer toughened polylactide,” RSC Advances, [22] N. M. Stark, L. M. Matuana, and C. M. Clemons, “Efect of vol. 8, no. 21, Article ID 11643, 2018. processing method on surface and weathering characteristics [38] ASTM, “Standard test method for moisture absorption of wood-four/HDPE composites,” Journal of Applied Polymer properties,” Annual Book of ASTM Standards, vol. 92, ASTM, Science, vol. 93, no. 3, pp. 1021–1030, 2004. West Conshohocken, PA, USA, 2004. [23] C. P. L. Chow, X. S. Xing, and R. K. Y. Li, “Moisture ab- [39] H. N. Dhakal, C. Jiang, M. Sit, Z. Zhang, M. Khalfallah, and sorption studies of sisal fbre reinforced polypropylene composites,” Composites Science and Technology, vol. 67, E. Grossmann, “Moisture absorption efects on the me- no. 2, pp. 306–313, 2007. chanical properties of sandwich biocomposites with cork core [24] Y. Ratna Kumari, K. Ramanaiah, A. V. Ratna Prasad, and fax/PLA face sheets,” Molecules, vol. 26, pp. 7295–7323, K. Hemachandra Reddy, S. Prasad Sanaka, and A. Kalyan Prudhvi, “Experimental investigation of water absorption [40] N. Jiang, T. Yu, Y. Li, T. J. Pirzada, and T. J. Marrow, behaviour of sisal fber reinforced polyester and sisal fber “Hygrothermal aging and structural damage of a jute/poly reinforced poly lactic acid composites,” Materials Today: (lactic acid) (PLA) composite observed by X-ray tomogra- Proceedings, vol. 44, pp. 935–940, 2021. phy,” Composites Science and Technology, vol. 173, pp. 15–23, [25] B. Wang, K. Hina, H. Zou, L. Cui, D. Zuo, and C. Yi, “Mechanical, biodegradation and morphological properties of [41] Kusmono, H. Hestiawan, and Jamasri, “Te water absorption, sisal fber reinforced poly(lactic acid) biocomposites,” Journal mechanical and thermal properties of chemically treated of Macromolecular Science, Part B, vol. 58, no. 2, pp. 275–289, woven fan palm reinforced polyester composites,” Journal of Materials Research and Technology, vol. 9, no. 3, pp. 4410– [26] A. Belaadi, A. Saaidia, M. Boumaaza, H. Alshahrani, and 4420, 2020. M. Bourchak, “Modeling moisture absorption of fax/sisal [42] S. U. Hamim and R. P. Singh, “Efect of hygrothermal aging reinforced hybrid biocomposites using fck’s and ANN on the mechanical properties of fuorinated and non- methods,” Journal of Natural Fibers, vol. 20, no. 1, 2023. fuorinated clay-epoxy nanocomposites,” International [27] C. Moliner, E. Finocchio, E. Arato, G. Ramis, and A. Lagazzo, Scholarly Research Notices, vol. 2014, Article ID 489453, “Infuence of the degradation medium on water uptake, 13 pages, 2014. morphology, and chemical structure of Poly(Lactic Acid)- [43] Y. Yuryev, A. K. Mohanty, and M. Misra, “Hydrolytic stability Sisal bio-composites,” Materials, vol. 13, pp. 3974–4018, 2020. of polycarbonate/poly(lactic acid) blends and its evaluation [28] M. K. Gupta and R. Singh, “PLA-coated sisal fbre-reinforced via poly(lactic) acid median melting point depression,” polyester composite: water absorption, static and dynamic Polymer Degradation and Stability, vol. 134, pp. 227–236, mechanical properties,” Journal of Composite Materials, vol. 53, no. 1, pp. 65–72, 2019. [44] N. Jiang, T. Yu, and Y. Li, “Efect of hydrothermal aging on [29] E. H. Backes, L. N. Pires, L. C. Costa, F. R. Passador, and injection molded short jute fber reinforced poly(lactic acid) L. A. Pessan, “Analysis of the degradation during melt pro- cessing of pla/biosilicate composites,” J. Compos. Sci.,vol. 3, (PLA) composites,” Journal of Polymers and the Environment, no. 2, p. 52, 2019. vol. 26, no. 8, pp. 3176–3186, 2018. 14 Advances in Materials Science and Engineering [45] H. Levine and L. Slade, Water as a Plasticizer: Physico- Chemical Aspects of Low-Moisture Polymeric Systems, Cam- bridge University Press, Cambridge, UK, 2017. [46] J. A. da Silva, C. Dalmolin, W. M. Pachekoski, and D. Becker, “Te combined efect of plasticizers and graphene on prop- erties of poly(lactic acid),” Journal of Applied Polymer Science, vol. 135, no. 41, Article ID 46745, 2018. [47] T. Liu, P. Butaud, V. Placet, and M. Ouisse, “Damping be- havior of plant f ber composites: a review,” Composite Structures, vol. 275, Article ID 114392, 2021. [48] K. Cheour, M. Assarar, D. Scida, R. Ayad, and X. Gong, “Long-term immersion in water of fax-glass fbre hybrid composites: efect of stacking sequence on the mechanical and damping properties,” Fibers and Polymers, vol. 21, no. 1, pp. 162–169, 2020.
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Advances in Materials Science and Engineering – Hindawi Publishing Corporation
Published: Jun 2, 2023