Effect of particle size and loading of cherry tree branch fillers on the mechanical and viscoelastic properties of polypropylene composites

Abstract

Lignocellulosic fillers derived from pruned cherry tree branches were studied relative to the mechanical and viscoelastic properties of polypropylene (PP) composites. Tree branches were collected from the orchard after pruning and the wood and bark parts were separated from each other. Both materials were processed into particles of different sizes (below 100 µm and between 100 and 250 µm) and filled into PP at different weight percentages (5%, 10%, 15%, and 20%). The mechanical performances of the biocomposites were evaluated through tensile tests, while their viscoelastic behavior was analyzed using dynamic mechanical analysis (DMA). Results revealed a decline in tensile strength with increasing filler content, which was attributed to poor interfacial adhesion between the PP matrix and fillers. However, tensile modulus increased with increasing filler content, with the highest values were observed at 20% filler loadings. The DMA showed enhanced storage and loss moduli, indicating improved stiffness and energy dissipation. Scanning electron microscopy (SEM) confirmed the presence of voids and filler agglomeration, further explaining the mechanical property reductions. These results demonstrate the potential of cherry tree pruning waste as a bio-filler for sustainable biocomposites with improved stiffness.

Full Article

Effect of Particle Size and Loading of Cherry Tree Branch Fillers on the Mechanical and Viscoelastic Properties of Polypropylene Composites

Mustafa Öncül ,* and Kutlay Sever

Lignocellulosic fillers derived from pruned cherry tree branches were studied relative to the mechanical and viscoelastic properties of polypropylene (PP) composites. Tree branches were collected from the orchard after pruning and the wood and bark parts were separated from each other. Both materials were processed into particles of different sizes (below 100 µm and between 100 and 250 µm) and filled into PP at different weight percentages (5%, 10%, 15%, and 20%). The mechanical performances of the biocomposites were evaluated through tensile tests, while their viscoelastic behavior was analyzed using dynamic mechanical analysis (DMA). Results revealed a decline in tensile strength with increasing filler content, which was attributed to poor interfacial adhesion between the PP matrix and fillers. However, tensile modulus increased with increasing filler content, with the highest values were observed at 20% filler loadings. The DMA showed enhanced storage and loss moduli, indicating improved stiffness and energy dissipation. Scanning electron microscopy (SEM) confirmed the presence of voids and filler agglomeration, further explaining the mechanical property reductions. These results demonstrate the potential of cherry tree pruning waste as a bio-filler for sustainable biocomposites with improved stiffness.

DOI: 10.15376/biores.20.1.1703-1712

Keywords: Lignocellulosic fillers; Cherry tree pruning waste; Mechanical properties; Polypropylene; Viscoelasticity; Wood; Bark

Contact information: Faculty of Engineering and Architecture, Department of Mechanical Engineering, İzmir Kâtip Çelebi University, İzmir, Turkey; *Corresponding author: mustafa.oncul@ikcu.edu.tr

GRAPHICAL ABSTRACT

INTRODUCTION

In recent years, natural fiber-reinforced polymer composites have gained significant attention due to their sustainability, eco-friendliness, and cost-effectiveness (Elfaleh et al. 2023; Palanisamy et al. 2024). Among various natural fibers, wood-based materials, such as wood fibers, offer a promising alternative to synthetic fibers in polymer composites (Skosana et al. 2024). These bio-fillers not only enhance the mechanical properties of polymers but also contribute to the reduction of plastic waste by partially replacing petroleum-based polymers. Studies have demonstrated that incorporating wood fibers into polymer matrices such as polypropylene (PP) can enhance specific mechanical properties, such as stiffness and impact resistance, while also modifying thermal, dynamic, and viscoelastic behaviors, making these biocomposites suitable for diverse industrial applications (Sanjay et al. 2016).

Recent studies have explored various wood fibers and fillers to enhance the properties of PP composites. Reixach et al. (2015) investigated orange wood fibers as reinforcement for PP, finding that while the fibers did not alter transition temperatures, they increased polymer crystallinity by 7% at 50% fiber content. Similarly, Atagur et al. (2020) demonstrated that incorporating up to 20% of Ceratonia siliqua powder significantly improved tensile and flexural strength by 32% and 23%, respectively, while reducing the coefficient of thermal expansion. Hernández-Jiménez et al. (2022) examined white oak wood flour, noting increases in Young’s and flexural moduli with particle incorporation, though reduction in elongation and strength were observed. Andrzejewski et al. (2024) evaluated wood-PP composites under multiple processing cycles, observing a decline in viscosity and mechanical properties due to PP matrix degradation.

The use of pruning wastes in polymer composites has emerged as an innovative approach to both waste management and sustainable material development. Agricultural and forestry by-products, such as pruned branches, have traditionally been considered as low-value waste; they are often discarded or burned, thereby contributing to environmental pollution (Reixach et al. 2015). However, recent studies highlight their potential as reinforcements/fillers in polymer composites, particularly in enhancing mechanical and thermal properties (Maraveas 2020; Öncül et al. 2024). Incorporating pruning wastes, such as branches from cherry trees, into polymer matrices not only helps in reducing the dependency on virgin wood and synthetic fibers but it also provides a sustainable alternative for producing high-performance materials (Cabrera 2021). The environmental benefits of utilizing pruning residues extend beyond waste reduction; they also decrease the overall carbon footprint of composite production by replacing energy-intensive synthetic materials (Gupta et al. 2022). According to the Turkish Statistical Institute (2023), the total number of fruit-bearing trees in Turkey is approximately 900 million, of which 2.5% are cherry trees. In the calculation of pruning waste, the pruning coefficient, which varies according to the type of fruit tree, was reported as 5.90 for cherry trees (Bilandzija et al. 2012). According to these assumptions, the total amount of cherry tree pruning waste was estimated as 131 thousand tons/year for cherry trees.

This study addresses both environmental and material challenges by investigating the potential of using wood and bark derived from pruning waste of cherry trees, at two different particle sizes and different filling ratios, as a sustainable reinforcement in PP matrix. To our knowledge, no previous research has explored the incorporation of cherry tree branch particles into PP composites. This study examined the mechanical and viscoelastic properties of these biocomposites, focusing on key performance metrics, such as tensile strength, stiffness, and dynamic-mechanical behavior. The results will contribute to the development of sustainable, high-performance materials for industries such as automotive, construction, and packaging.

EXPERIMENTAL

Materials

The matrix material used in this study is commercial polypropylene (PP, LG M1500, South Korea), a widely utilized thermoplastic in polymer composite applications due to its favorable mechanical properties and chemical resistance. It has a melt flow index of 16 g/10 min (230 ℃, 2.16 kg) and a density of 0.9 g/cm³ as physical characteristics. Cherry tree (Prunus avium L.) pruning branches, were used as a lignocellulosic filler in this study. These agricultural wastes were collected from an orchard at an altitude of about 1600 m in the Taurus Mountains in the Mediterranean region of Turkey (Konya, Turkey). The age of the trees in the garden is in the range of 5 to 10 years old.

Methods

The wood (W) and bark (B) components of the pruned cherry branches were first separated to facilitate their processing. Following, the W and B samples were processed separately using a laboratory grinder (Mertest LB160, Mertler Machine, Eskişehir, Turkey). After grinding, the particles were sieved using a sieve shaker (Retsch RS200, Retsch GmbH, Haan, Germany), with sieving performed at both 100 µm and 250 µm. This process enabled the classification of the fillers into two distinct particle size ranges: those below 100 µm (W1 and B1) and those between 100 and 250 µm (W2 and B2). Biocomposite samples were produced using a high-speed thermokinetic mixer and a laboratory type hot-cold hydraulic press (Gülnar Machine, Gülnar, Kayseri, Turkey) (Öncül 2023). The nomenclature of the matrix material, fillers, and biocomposites used in this study is presented in Table 1.

Table 1. Nomenclature of Materials

A universal testing machine (Shimadzu AG-IC, Shimadzu Corporation, Kyoto, Japan) with a 5 kN load cell was used to determine the mechanical properties of the samples. Tensile tests were conducted in accordance with the ASTM D638-14 (2014), which specifies procedures for testing the tensile properties of plastics, with a crosshead speed of 50 mm/min. Tensile tests were repeated at least five times per sample to ensure statistical reliability and minimize experimental error. Viscoelastic properties of samples were determined using a dynamic mechanical analyzer (DMA Q800, TA instruments, New Castle, DE, USA). Analyses were performed with a single cantilever clamp at a temperature range of 30 to 140 °C and heating rate of 3 °C/min. The morphological properties of the fractured surfaces of the samples were detected with a scanning electron microscope (SEM) (Carl Zeiss 300VP, Zeiss, Oberkochen, Germany) at an accelerating voltage of 5 kV. Before observation, the samples were uniformly coated with gold using a sputter coating device (Quorum Q150 Res, Quorum, East Sussex, UK) to prevent charging under the electron beam.

RESULTS AND DISCUSSION

Tensile Test

The tensile strength and tensile modulus values of PP and biocomposites are illustrated in Figs. 1. As shown in Figs. 1a and 1b, the tensile strength of PP shows a notable decline with the incorporation of fillers. Specifically, the tensile strength of PP decreased from 22.2 to 16.2 MPa for sample 20B2. This reduction in tensile strength can be attributed to the interaction between the fillers and the matrix. First, it appears that the W and B particles are not sufficiently wetted by the PP matrix, which may hinder effective stress transfer (Shumigin et al. 2011; Pickering et al. 2016). Additionally, the fillers were not uniformly dispersed within the polymer matrix, and the interfacial adhesion between the non-polar PP and the polar lignocellulosic particles was weakened. This weak interfacial bonding becomes more pronounced as the particle loading increases, as a higher filler content results in a greater interfacial surface area, exacerbating the poor adhesion between the matrix and the fillers, further compromising the tensile strength of the composite (Rosa et al. 2009; Kılınç et al. 2018; Atagur et al. 2020). Furthermore, the presence of non-wetted wood clusters may contribute to failure due to insufficient bonding between the fillers and the matrix (Hubbe et al. 2023).

Fig. 1. Tensile test results of PP and biocomposites

Conversely, changes in the tensile modulus of PP and biocomposites are presented in Figs. 1(c-d). The tensile modulus of PP was measured at 704 MPa, and an increase in tensile modulus was observed with the addition of 5% filler. This trend continued consistently with higher filler contents, reaching a peak at 20% filler loading. The highest tensile modulus was observed in the 20W1 sample, with a value of 1230 MPa, representing a 75% increase compared to PP. This important enhancement in tensile modulus is due to the lignocellulosic fillers having a much higher modulus than the polymer matrix, thereby stiffening the biocomposite as the filler content increases. Similarly, previous studies revealed a decline in tensile strength and an increase in tensile modulus with the addition of wood filler (Pérez et al. 2012; Ndiaye et al. 2013; Soccalingame et al. 2015).

Dynamic Mechanical Analysis

The variations in the storage modulus of the materials as a function of temperature are presented in Figs. 2(a–b). The results indicate that the storage modulus of the biocomposites was consistently higher than that of PP across the entire temperature range. This suggests that the addition of fillers improved the stiffness of the PP. The reduction in modulus with increasing temperature is likely due to the initiation of the relaxation process and softening of the polymer matrix, which becomes more prominent as the material heats up (Sarikanat et al. 2014). The rise in storage modulus with the incorporation of fillers is attributed to the mechanical constraints introduced by the embedded particles within the polymer matrix, which restricts chain mobility and enhances rigidity. A comparable decline in the storage modulus was also observed in the study conducted by Reixach et al. (2015).

Fig. 2. Dynamic mechanical analysis results of PP and biocomposites

Similarly, the changes in loss modulus as a function of temperature are shown in Figs. 2(c–d). The biocomposites exhibit higher loss modulus values than PP, with the highest loss modulus observed in samples filled with 20 wt% of filler, irrespective of particle size. This increase in loss modulus indicates greater energy dissipation in the biocomposites, which may be associated with enhanced internal friction and reduced energy absorption efficiency as the filler content increases (Seki et al. 2013; Nagarajan et al. 2016). Reixach et al. (2015) observed a similar trend in the loss modulus throughout the entire temperature range in the study.

Scanning Electron Microscopic Analysis

The lignocellulosic fillers are clearly observable within the PP matrix upon examining the fracture surfaces of the biocomposites, as shown in Figs. 3(a–d). The presence of numerous small pores around the filler particles suggests poor matrix-filler interfacial adhesion. These pores indicate weak compatibility between the matrix and the fillers, which greatly impacts the mechanical performance of the biocomposites. As the filler content increases, the dispersion of the particles within the matrix becomes less homogeneous, further aggravating the poor interfacial bonding. This is consistent with findings from other studies, which highlight that the increased filler content can lead to agglomeration, reducing the effectiveness of stress transfer across the matrix-filler interface (Babaei et al. 2014).

Fig. 3. SEM images of the fracture surfaces: a) 10W1, b) 10W2, c) 10B1, and d) 10B2

In Fig. 3d, it is evident that larger filler particles began to dominate the fracture surface as the filler content increased. This accumulation of larger particles created stress concentration points, which weakened the composite’s ability to resist tensile forces. Consequently, the tensile strength of the biocomposite decreased with higher filler loadings. Similar trends have been observed in other studies, where excessive filler addition has led to poor dispersion and weakened mechanical properties due to ineffective stress transfer and the formation of voids at the interface (Xie et al. 2010; Andrzejewski et al. 2024). This highlights the importance of optimizing filler content and particle dispersion to balance mechanical strength and filler reinforcement in polymer composites.

In the previous study, Öncül et al. (2024) observed that the cellulose content of wood was approximately 11% higher than that of bark, while the hemicellulose and lignin content of wood were 22% and 23% lower than that of bark, respectively. This finding aligns with the results of other similar studies that have also reported significant differences between wood and bark in terms of physical, chemical, and mechanical properties (Chow et al. 2008; Ruiz et al. 2015).

CONCLUSIONS

1. This study examined the influence of lignocellulosic fillers derived from pruned cherry tree branches on the mechanical and viscoelastic properties of polypropylene (PP) composites.
2. The particle size comparison of wood fillers demonstrated that the composites prepared with smaller cherry branch particles (W1) exhibited higher tensile strength due to enhanced dispersion and stress transfer within the PP. The incorporation of larger particles in W2 resulted in a notable decline in strength, predominantly due to the formation of voids and a concomitant reduction in adhesion. A similar pattern was observed for the particle size comparison of bark fillers.
3. The results of the tensile tests and dynamic mechanical analysis (DMA) consistently demonstrated that wood-filled composites outperformed bark-filled composites. The superior structure and composition of wood fibers facilitated better dispersion, resulting in better properties compared to bark fillers.
4. It can be stated that lignocellulosic fillers derived from pruned cherry tree branches, particularly those of under 100 µm particle size, demonstrate potential for use in the reinforcement/filler of PP.

ACKNOWLEDGEMENTS

This research was supported by İzmir Kâtip Çelebi University, Scientific Research Commission (Project Number: 2021-TDR-FEBE-0011).

REFERENCES CITED

Andrzejewski, J., Barczewski, M., Czarnecka-Komorowska, D., Rydzkowski, T., Gawdzińska, K., and Thakur, V. K. (2024). “Manufacturing and characterization of sustainable and recyclable wood-polypropylene biocomposites: Multiprocessing-properties-structure relationships,” Ind. Crop. Prod. 207, article ID 117710. DOI: 10.1016/j.indcrop.2023.117710

ASTM D638-14 (2014). “Standard test method for tensile properties of plastics,” ASTM International, West Conshohocken, PA, USA.

Atagur, M., Seki, Y., Oncu, O., Server, K., Seki, Y., Sarikanat, M., and Altay, L. (2020). “Evaluating of reinforcing effect of Ceratonia siliqua for polypropylene: Tensile, flexural and other properties,” Polymer Testing 89, article ID 106607. DOI: 10.1016/j.polymertesting.2020.106607

Babaei, I., Madanipour, M., Farsi, M., and Farajpoor, A. (2014). “Physical and mechanical properties of foamed HDPE/wheat straw flour/nanoclay hybrid composite,” Composites Part B: Engineering 56, 163-170. DOI: 10.1016/j.compositesb.2013.08.039

Bilandzija, N., Voca, N., Kricka, T., Matin, A., and Jurisic, V. (2012). “Energy potential of fruit tree pruned biomass in Croatia,” Span. J. Agric. Res. 10(2), 292-298. DOI: 10.5424/sjar/2012102-126-11

Cabrera, F. C. (2021). “Eco-friendly polymer composites: A review of suitable methods for waste management,” Polymer Composites 42(6), 2653-2677. DOI: 10.1002/pc.26033

Chow, P., Nakayama, F. S., Blahnik, B., Youngquist, J. A., and Coffelt, T. A. (2008). “Chemical constituents and physical properties of guayule wood and bark,” Ind. Crop. Prod. 28(3), 303-308. DOI:10.1016/j.indcrop.2008.03.006

Elfaleh, I., Abbassi, F., Habibi, M., Ahmad, F., Guedri, M., Nasri, M., and Garnier, C. (2023). “A comprehensive review of natural fibers and their composites: an eco-friendly alternative to conventional materials,” Results in Engineering 19, article ID 101271. DOI: 10.1016/j.rineng.2023.101271

Gupta, J., Kumari, M., Mishra, A., Akram, M., and Thakur, I. S. (2022). “Agro-forestry waste management – A review,” Chemosphere 287, article ID 132321. DOI: 10.1016/j.chemosphere.2021.132321

Hernández-Jiménez, J. A., Jiménez-Amezcua, R. M., Lomelí-Ramírez, M. G., Silva-Guzmán, J. A., Torres-Rendón, J. G., and García-Enriquez, S. (2022). “Utilization of wood flour from white oak branches as reinforcement in a polypropylene matrix: Physical and mechanical characterization,” Journal of Composites Science 6(7), 184. DOI: 10.3390/jcs6070184

Hubbe, M. A., Trovagunta, R., Zambrano, F., Tiller, P., and Jardim, J. (2023). “Self-assembly fundamentals in the reconstruction of lignocellulosic materials: A review,” BioResources 18(2), 4262-4331. DOI: 10.15376/biores.18.2.Hubbe

Kılınç, A. Ç., Köktaş, S., Seki, Y., Atagür, M., Dalmış, R., Erdoğan, Ü. H., Göktaş, A. A., and Seydibeyoğlu, M. Ö. (2018). “Extraction and investigation of lightweight and porous natural fiber from Conium maculatum as a potential reinforcement for composite materials in transportation,” Composites Part B: Engineering 140, 1-8. DOI: 10.1016/j.compositesb.2017.11.059

Maraveas, C. (2020). “Production of sustainable construction materials using agro-wastes,” Materials 13(2), article 262. DOI: 10.3390/ma13020262

Nagarajan, T., Babu, A. S., Palanivelu, K., and Nayak, S. (2016). “Mechanical and thermal properties of PALF reinforced epoxy composites,” Macromolecular Symposium 361, 57-63. DOI: 10.1002/masy.201400256

Ndiaye, D., Gueye, M., and Diop, B. (2013). “Characterization, physical and mechanical properties of polypropylene/wood-flour composites,” Arabian Journal for Science and Engineering 38, 59-68. DOI: 10.1007/s13369-012-0407-y

Öncül, M., Atagür, M., Atan, E., and Sever, K. (2024). “A preliminary evaluation of bing cherry tree (Prunus avium L.) pruning waste as an alternative lignocellulosic filler for light weight composite material applications,” Polymer Composites 1-13. DOI: 10.1002/pc.29197

Öncül, M. (2023). Influence of Lignocellulosic Fillers and Interfaces on Thermal and Mechanical Properties of Biocomposites, Ph.D. Dissertation, İzmir Katip Çelebi University, İzmir, Turkey.

Palanisamy, S., Murugesan, T. M., Palaniappan, M., Santulli, C., Ayrilmis, N., and Alavudeen, A. (2024). “Selection and processing of natural fibers and nanocellulose for biocomposite applications: A brief review,” BioResources 19(1). 1789-1813. DOI: 10.15376/biores.19.1.Palanisamy

Pérez, E., Famá, L., Pardo, S. G., Abad, M. J., and Bernal, C. (2012). “Tensile and fracture behaviour of PP/wood flour composites,” Composites Part B: Engineering, 43(7), 2795-2800. DOI: 10.1016/j.compositesb.2012.04.041

Pickering, K. L., Efendy, M. G. A., and Le, T. M. (2016). “A review of recent developments in natural fibre composites and their mechanical performance,” Composites Part A: Applied Science and Manufacturing 83, 98-112. DOI: 10.1016/j.compositesa.2015.08.038

Reixach, R., Puig, J., Méndez, J. A., Gironès, J., Espinach, F. X., Arbat, G., and Mutjé, P. (2015). “Orange wood fiber reinforced polypropylene composites: Thermal properties,” BioResources 10(2), 2156-2166. DOI: 10.15376/biores.10.2.2156-2166

Rosa, S. M. L., Santos, E. F., Ferreira, C. A., and Nachtigall, S. M. B. (2009). “Studies on the properties of rice-husk-filled-PP composites: Effect of maleated PP,” Materials Research 12, 333-338. DOI: 10.1590/S1516-14392009000300014

Ruiz-Aquino, F., González-Peña, M. M., Valdez-Hernández, J. I., Revilla, U. S., and Romero-Manzanares, A. (2015). “Chemical characterization and fuel properties of wood and bark of two oaks from Oaxaca, Mexico,” Ind. Crops Prod. 65, 90-95. DOI: 10.1016/j.indcrop.2014.11.024

Sanjay, M. R., Arpitha, G. R., Naik, L. L., Gopalakrishna, K., and Yogesha, B. (2016). “Applications of natural fibers and its composites: An overview,” Natural Resources 7(3), 108-114. DOI: 10.4236/nr.2016.73010

Sarikanat, M., Seki, Y., Sever, K., and Durmuşkahya, C. (2014). “Determination of properties of Althaea officinalis L. (Marshmallow) fibres as a potential plant fibre in polymeric composite materials,” Composites Part B: Engineering 57, 180-186. DOI: 10.1016/j.compositesb.2012.09.029

Seki, Y., Sarikanat, M., Sever, K., and Durmuşkahya, C. (2013). “Extraction and properties of Ferula communis (chakshir) fibers as novel reinforcement for composites materials,” Composites Part B: Engineering 44(1), 517-523. DOI: 10.1016/j.compositesb.2012.03.013

Shumigin, D., Tarasova, E., Krumme, A., and Meier, P. (2011). “Rheological and mechanical properties of poly (lactic) acid/cellulose and LDPE/cellulose composites,” Materials Science 17(1), 32-37. DOI: 10.5755/j01.ms.17.1.245

Skosana, S. J., Khoathane, C., and Malwela, T. (2024). “Driving towards sustainability: A review of natural fiber reinforced polymer composites for eco-friendly automotive light-weighting,” Journal of Thermoplastic Composite Materials 2024, available online. DOI: 10.1177/08927057241254324

Soccalingame, L., Bourmaud, A., Perrin, D., Bénézet, J. C., and Bergeret, A. (2015). “Reprocessing of wood flour reinforced polypropylene composites: Impact of particle size and coupling agent on composite and particle properties,” Polymer Degradation and Stability 113, 72-85. DOI: 10.1016/j.polymdegradstab.2015.01.020

Turkish Statistical Institute (2023). TURKSTAT Report, Turkish Statistical Institute, (https://biruni.tuik.gov.tr/medas/?kn=92&locale=tr), Accessed 26 Nov 2024.

Xie, Y., Hill, C. A., Xiao, Z., Militz, H., and Mai, C. (2010). “Silane coupling agents used for natural fiber/polymer composites: A review,” Composites Part A: Applied Science and Manufacturing 41(7), 806-819. DOI: 10.1016/j.compositesa.2010.03.005

Article submitted: October 17, 2024; Peer review completed: November 23, 2024; Revised version received and accepted: December 10, 2024; Published: December 20, 2024.

DOI: 10.15376/biores.20.1.1703-1712