Utilization of flours from hemp stalks as reinforcement in polypropylene matrix

Kılıç, I., Avcı, B., Atar, I., Korkmaz, N, Yılmaz, G., and Mengeloğlu, F. (2024). “Utilization of flours from hemp stalks as reinforcement in polypropylene matrix,” BioResources 19(1), 1494-1516.

Abstract

Utilization of hemp stalks (HS) was investigated as filler and maleic anhydride-grafted polypropylene (MAPP) as a coupling agent relative to some mechanical, physical, morphological, and thermal properties of polypropylene (PP) composites. The test sample manufacturing used 10, 15, 20, 25, 30, and 35 wt% of HS and 0 wt% or 3 wt% of MAPP. Test specimens were manufactured by combining extrusion and injection molding processes. Results showed that incorporating filler and MAPP increased the mechanical properties significantly. The utilization of HS filler or MAPP provided an improvement of 11.7% (samples containing 35 wt% HS and 3 wt% MAPP) improvement in tensile strength, 44.2% (samples containing 35 wt% HS and 3 wt% MAPP) in flexural strength, and 60.2% (samples containing 30 wt% HS and 0 wt% MAPP) in impact strength compared to neat PP. The scanning electron microscopy analysis provided visual evidence that MAPP improved adhesion between the filler and the polymer matrix. This study showed that using hemp stalks in composite manufacturing provided good overall properties.

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Utilization of Flours from Hemp Stalks as Reinforcement in Polypropylene Matrix

İbrahim Kılıç,^a,* Büşra Avcı,^b İlkay Atar,^b Nesrin Korkmaz,^c Güngör Yılmaz,^dand Fatih Mengeloğlu ^b

DOI: 10.15376/biores.19.1.1494-1516

Keywords: Hemp stalk; Polypropylene; Natural fiber-based composites; Wood-plastic composites

Contact information: a: Department of Material and Energy, Hemp Research Institute, Yozgat Bozok University, Yozgat, Turkey; b: Department of Forest Industry Engineering, Kahramanmaras Sutcu Imam University, Kahramanmaras, Turkey; c: Department of Basic Sciences and Health, Hemp Research Institute, Yozgat Bozok University, Yozgat, Turkey; d: Department of Field Crops, Yozgat Bozok University, Yozgat, Turkey; *Corresponding author: ibrahim.kilic@bozok.edu.tr

INTRODUCTION

In recent years, increasing environmental awareness, concern about the depletion of oil resources, and concepts, such as sustainability and recycling, have encouraged the production of environmentally friendly materials. As a result of this new paradigm, interest in wood-plastic composites (WPCs) is also increasing. Wood-plastic composites are innovative materials in which wood or cellulosic fibers are mixed with various thermoplastics and additives and produced by extrusion, injection molding, and compression molding or thermoforming (pressing) methods (Klyosov 2007; Gardner et al. 2015; Kilic et al. 2023). Cellulosic fiber or wood refers to lignocellulosic fibers, such as flour, from various hardwood and softwood species (pine, poplar, beech, etc.), flour from timber particles, agricultural waste (wheat stalk, hemp stalk, etc.), bleached cellulose, and natural fibers (hemp, flax, kenaf, etc.) (Mohanty et al. 2002; Kim and Pal 2010; Mantia and Morreale 2011; Mengeloğlu and Çavuş 2020; Kilic et al. 2023). These fibers consist mainly of cellulose and lignin, and some of their properties, such as ease of processing, low density, low cost, and renewability, make them preferable (Mohanty et al. 2001; Mengeloğlu and Karakuş 2012; Beaugrand et al. 2014). For this reason, WPCs are widely used in many industries (automotive, aerospace, electronics, construction, outdoor deck floors, railings, fences, landscaping timbers, park benches, etc.) because of their renewability, lightweight nature, and low-cost advantages (Kim and Pal 2010; Balla et al. 2019; Wu et al. 2020).

The matrices are an essential part of WPCs. The most common matrices used in WPCs are thermoplastic polymers, which are low in density and can be processed at low temperatures. The temperature at which lignocellulosic fibers degrade is an essential parameter in matrix selection. The thermal decomposition temperature of lignocellulosic fibers is generally regarded as being around 200 °C, and therefore, polymers that can be processed at temperatures lower than 200 °C are generally used in WPC production (Kim and Pal 2010; Summerscales et al. 2010; Pickering et al. 2016). Due to this limitation, thermoplastics that soften below this temperature, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), and polystyrene (PS), are used as the matrix (Klyosov 2007; Pickering et al. 2016).

Lignocellulosic fibers in thermoplastic polymers are used for two purposes. Firstly, the aim is to reduce the cost while providing a feature similar to neat polymer, and secondly, the aim is to increase performance properties compared to neat polymer (Kim and Pal 2010). Additionally, using lignocellulosic fibers in polymers has made them ‘environmentally friendly or greener composites’ (Netravali and Chabba 2003). There are many examples of using lignocellulosic fiber-reinforced polymer composites for these purposes (Summerscales et al. 2010; Pickering et al. 2016; Balla et al. 2019).

Lignocellulosic fibers are widely used in different industries as reinforcing agents in polymer composites, and some advantages and disadvantages of WPC are listed below (Mohanty et al. 2000, 2002; Gardner and Murdock 2010; Kim and Pal 2010; Faruk et al. 2012; Kılıç 2012; Koronis et al. 2013; Balla et al. 2019):

Advantages:

WPCs are more environmentally friendly, renewable, and potentially recyclable.
Using lignocellulosic fibers in plastic matrices makes them more rigid and can reduce overall cost;
The lignocellulosic fibers use reduces the fossil-based raw materials amount and energy consumption;
The production processes and disposal methods of WPCs are less harmful to the environment to neat plastics;
The pressure properties of WPCs are superior to solid wood material;
The maintenance cost of WPCs is lower than solid wood material;
WPCs are resistant to biological effects compared to solid wood material;
WPCs’ dimensional stability and water absorption properties are better than solid wood.

Disadvantages:

Moisture in the lignocellulosic-based areas inside WPCs may cause the formation of mold fungi;
When lignocellulosic based fibers are added to thermoplastics, bleaching caused by sunlight may occur;
WPCs are heavier and less rigid compared to solid wood material;
Lignocellulosic fibers degrade over time at temperatures of 200 °C or higher. Therefore, polymer matrix selection in WPC production is limited.

The extrusion method is widely used in the WPC industry, and the composite products produced are generally used in areas such as deck floors, park and garden furniture, siding, etc. Another method used in the production of WPCs is the injection molding method, which is a processing technique used for the production of high-volume complex products. However, injection molding of high-filled WPCs is more troublesome than that of neat polymers due to the decreased fluidity of filled polymers. However, it is thought that producing WPCs with more complex shapes suitable for injection molding will become widespread in the coming years (Montanes et al. 2019). To reveal this potential, it is necessary to increase the studies and production in this field. This study is intended to contribute to the production of WPCs by injection molding. Considering the disadvantages of injection molding in producing WPCs, the proportion of lignocellulosic filler should be kept low in the injection method compared to the extrusion method. In previous studies, it was observed that lignocellulosic filler ratios ranged between 10% and 40% by weight (Lopez et al. 2012; Yan et al. 2013; Sullins et al. 2017; Maziero et al. 2019; Kilic et al. 2023; Vallejos et al. 2023). Considering these data, this study’s hemp stalk filler level was limited to a maximum of 35%.

One of the most critical problems in WPCs production is poor bonding in the interaction of components due to poor wetting between the hydrophobic polymer matrix and hydrophilic fibers. The interface weakness between the polymer matrix and fibers can negatively affect many properties of WPCs. Because of the interfacial weakness, the zones of contact between the components fail to optimally transfer stress between the two phases when force is loaded, especially with respect to mechanical properties (Klyosov 2007). For this reason, coupling agents are used to improve compatibility and adhesion between fibers and polymers. Coupling agents are vital in improving the compatibility and adhesion between polar lignocellulosic fibers and non-polar polymeric matrices. Therefore, coupling agents utilization in WPC manufacturing is a standard method preferred for improving the mechanical properties of composites (Lu et al. 2000). Borysiak et al. (2011) reported that one of the most commonly used methods to improve adhesion between composite components is to use maleic anhydride grafted polypropylene (MAPP). In previous studies, it has been observed that coupling agents were used in the range of 2 to 10% in the production of WPCs and that the optimum properties were often found in the range of 3 to 5% (Çetin et al. 2000; Li et al. 2001; Panthapulakkal and Sain 2007; Oksman et al. 2009; Maziero et al. 2019; Díaz et al. 2020; Başboğa et al. 2022; Kilic et al. 2023; Vallejos et al. 2023). Considering the literature studies, in this study, MAPP was used at 0% and 3% by weight in the formulations of composites.

The components of industrial hemp that are mainly used in composites are its bast fibers (the outer part of the stalks) and hurd (the inside woody part of the stalks). Depending on the seed variety and planting conditions, hemp stalks comprise approximately 20 to 30% of bast fibers and 70 to 80% of hurds. Bast fibers are known as the most valuable part of the stalk and are high in cellulose and low in lignin. Hurds comprise the rest of the bast fibers and are higher in lignin (Borysiak et al. 2011; Shahzad 2011; Kurek 2013). Because the hurds and bast fibers have different properties, they have the potential for use in different areas. However, producing primary bast fibers is labor-intensive (Borysiak et al. 2011). For this reason, considering the difficulties and costs of obtaining these fibers separately, it becomes crucial to develop the usage areas of hemp stalks as a whole (hurd + bast) (Mengeloğlu 2020). In previous studies, generally, bast and hurd parts were evaluated separately as fillers in polymer matrices instead of utilizing hemp stalks as a whole (Beckermann and Pickering 2008; Khoathane et al. 2008; Kakroodi et al. 2012; Lopez et al. 2012; Yan et al. 2013; Beaugrand et al. 2014; Rey et al. 2017; Sullins et al. 2017; Vilasecaa et al. 2018; Panaitescu et al. 2019; Berzin et al. 2020; Díaz et al. 2020; Rouway et al. 2020; Wu et al. 2020; Vilaseca et al. 2020;Burgada et al. 2021; Han et al. 2021; Manaia and Manaia 2021; Wu et al. 2021). However, as far as the authors know, studies using whole hemp stalks together are limited. This study focused on evaluating hemp stalks as a whole (bast + hurd) as filler in wood-plastic composites (WPCs). For this purpose, flours obtained from hemp stalks were used as filler in WPCs production. The produced composites’ mechanical, physical, thermal, and morphological properties were examined.

EXPERIMENTAL

Materials

Polypropylene (PP) (MH 418; density 0.905 g/cm³, MFI 4.7 g/10 min, melting point 163 °C, Petkim Petrochemical Co., Türkiye) and hemp stalk (HS) were used as thermoplastic matrix and lignocellulosic filler, respectively. Insitu Green Technologies Industry and Trade Inc. Burdur, Türkiye, donated the hemp stalks (Cannabis sativa L.). Maleic anhydride-grafted polypropylene (MAPP) (Licocene® PP MA 7452 by Clariant, Berlin, Germany, melting point 155 to 161 °C, density 0.92 to 0.94 g/cm³) was used as coupling agent.

Methods

The HS were granulated into different mesh-sized flour by Wiley Mill (Altundal, Kahramanmaraş, Türkiye) without being separated into bast fiber and hurd. The HS flours that passed through 10-mesh screen and stayed on an 80-mesh screen were dried and used before production. Since hemp stalks were used as a whole (bast + hurd) without being separated from the bast fibers, it was observed that there were lumps formed in the sieves due to the bast fibers during the sizing process. For this reason, hemp flours in the 10 to 80 mesh screen range were used. The drying process was continued at 103 ± 2 °C for 24 h until the moisture was reduced below 1%.

Composite Compounding

The WPCs were manufactured by combining extrusion and injection molding processes. The WPCs pellets were produced by the extrusion method, and then test specimens were produced from these pellets by the injection molding method (Fig. 1). The experimental design for composite manufacturing is given in Table 1. Depending on the formulation, the blends were compounded under heat. The blends were compounded in a laboratory-type single screw extruder at 50 rpm speed, at temperatures from feed zone to die of 180, 185, 190, 195, and 200 °C. The extruded compounds were cooled in a water pool (23 °C ± 2) and then granulated into pellets. The pellets were dried below 1% in an oven at 103 (± 2 °C) for 24 h before manufacturing WPC specimens by injection molding. The test specimens (Fig. 2) were produced with injection molding (HAIDAHDX–88, Ningbo, China). Injection molding conditions were as follows: injection temperature 170, 180, 185, 190, and 190 °C (from feed zone to die), injection speed 85 mm/s, injection pressure 9 to 10 MPa, and cooling time 40 s. Before testing, the specimens were conditioned in a climate cabinet at 23 (± 2) °C temperature and 65 (± 2%) relative humidity. Five specimens for each group were tested for the determination of properties of WPCs.

Fig. 1. Flowchart of WPC production

Fig. 2. Test specimens: (a): tensile specimens; (b): flexural specimens; (c): notched impact specimens

Table 1. Experimental Design for Composites Manufacturing

Determination the Properties of WPCs

Physical properties of WPC specimens, such as density and water absorption, and mechanical properties of WPC specimens, such as tensile strength (TS), tensile modulus (TM), elongation at break (EatB), flexural strength (FS), flexural modulus (FM), and impact strength (IS-notched), values were determined according to the relevant standards. In addition, morphological and thermal analyses of WPCs specimens were determined.

Density

Density was measured according to the ASTM D792-20 (2020) water displacement technique using test specimens in the size of 20 mm × 20 mm × 4 mm. Five experimental specimens were used for each group. The weights of the specimens were determined with 0.001 g accuracy. The density was calculated through Eq. 1,

(1)

where D is density (g/cm³), a is apparent mass of specimen, without wire or sinker, in air (g), b is apparent mass of specimen completely immersed and of the wire partially immersed in liquid (g), w is apparent mass of totally immersed sinker and of partially immersed wire (g), and d_s is the density of water (0.9975 g/ cm³ at 23 °C ± 0.10).

Water Absorption

Five specimens (20 mm × 20 mm × 4 mm) were submerged in water at room temperature (23 °C ± 1) for 2, 24, 48, 168, 336, 504, and 672 h. For each given measurement hour increment, specimens’ weight was measured after wiping the water on the specimen’s surface. The water absorption was calculated through Eq. 2,

(2)

where WA is water absorption rate (%), W_i is initial weight of the specimen (g), and W_e is weight of the specimen after absorption of water (g).

Tensile Strength, Tensile Modulus and Elongation at Break

Tensile property tests were implemented using the Zwick 10 KN (Zwick/Roell, Ulm, Germany) instrument. Tensile strength, tensile modulus, and elongation at break of the produced composite specimens were determined according to the ASTM D638 (2010) standard. The dimensions (length x width x thickness) of the test specimens for tensile properties were 165 mm x 13 mm x 4mm (Dog Bone shape). The experiments were performed in 5 replicates, while the test speed was kept at 5 mm/min. Through averaging the data, the results of the tensile properties were determined. The tensile properties were calculated through tensile strengths (Eq. 3), tensile modulus (Eq. 4), and elongation at break (Eq. 5).

(3)

In Eq. 3, TS is tensile strength (N/mm²), P_max is maximum load (N), and a*b is cross-sectional dimensions of the specimens (mm²).

(4)

In Eq. 4, TM is tensile modulus (N/mm²), ∂TS is tensile strength (N/ mm²), and ℇ is unit elongation at break during tensile test.

(5)

In Eq. 5, ℇ is elongation at break (%), ΔL is unit elongation (mm), and L₀ is specimen size (mm).

Flexural Strength and Flexural Modulus

Flexural property tests were implemented using the Zwick 10 KN (Zwick/Roell, Ulm, Germany) instrument. Flexural strength and flexural modulus of the produced composite specimens were determined according to the ASTM D790 (2010) standard. The dimensions (length x width x thickness) of the test specimens for flexural properties were 165 mm x 13 mm x 4 mm. The experiments were conducted in 5 replicates while test speed was kept at 2 mm/min. The distance between the centers of the support span on which the specimens were placed was set to 80 mm. Through averaging the data, the results of the flexural properties were determined. The flexural properties were calculated through flexural strengths (Eq. 6), and flexural modulus (Eq. 7).

(6)

In Eq. 6, FS is flexural strength (N/mm²), P_max is maximum load applied at fracture (N), L is distance between the centers of support span (mm), b is width of the specimen (mm), and h is thickness of the specimen (mm). Flexural modulus was calculated as follows,

(7)

where FM is flexural modulus (N/mm²), ΔF is force equal to the difference between the arithmetic averages of the lower and upper limits of the loading in the elastic deformation region (N), L is distance between support span, Δf is deflection in the net flexural area is the difference between the arithmetic averages of the results of the deflections measured at the lower and upper limits of the loading (mm), b is width of the specimen (mm), and h is thickness of the specimen (mm).

Impact Strength (Notched)

An HIT5, 5P (Zwick) device was used for IS testing on notched specimens. The notches were added using a Polytest notching cutter by RayRan™. Impact strength of the produced composite specimens was determined according to the ASTM D256 (2010) standard. The dimensions (length x width x thickness) of the impact test specimens were 65 mm x 13 mm x 4 mm. Five specimens were used for each group and impact strength results were determined by averaging the data. The impact strength was calculated by Eq. 8,

(8)

where IS is the impact resistance (kJ/m²), Q is energy required to break the test specimen (kJ), and a*b are the dimensions of the test specimen in radial and tangential directions, respectively (m²).

Scanning Electron Microscope

A scanning electron microscope (SEM), with the help of the Everhart–Thornley detector (ETD), model Zeiss Gemini 300 (Carl Zeiss, Oberkochen, Germany) was used to examine the morphology of the of the composites. First, the composite specimens were dipped into liquid nitrogen and then fractured surface was obtained for SEM characterization. Finally, the samples were coated with gold powders for 120 s at 10 mA using a Leica ACE600 device (Wetzlar, Germany) to provide electrical conductivity. The SEM images were taken at a magnification of 250X.

Thermogravimetric Analysis (TGA)

Thermogravimetric (TGA/DTGA) analysis of composite specimens was performed using an EXSTAR SII TG/DTA 6300 device (Labx, Midland, Canada). For analyses, the specimens were heated from 25 to 800 °C, using a nitrogen flow rate of 100 mL/min at a heating rate of 10 °C/min.

Statistical Analysis

Design-Expert®, version 13 (State-Ease, Inc., Minneapolis, MN, USA) software, was used for statistical analysis. Two-way analysis of variance (ANOVA) tests were performed to observe the effects of the filler amounts and coupling agent on the physical and mechanical properties of the composite specimens.

RESULTS AND DISCUSSION

In this study, some mechanical, physical (density and water absorption), morphological (SEM), and thermal (TGA, DSC) properties of PP-based composite specimens-filled hemp stalks and MAPP were determined.

Table 2. Mechanical Properties of Test Specimens

Five specimens for each group were tested, *: Average values, **: standard deviation

Mechanical Properties of WPCs

Tensile strength (TS), tensile modulus (TM), elongation at break (EatB), flexural strength (FS), flexural modulus (FM), and impact strength (IS) properties were the mechanical properties studied. The results obtained from the mechanical properties of the test specimens are shown in Table 2.

The TS, TM, and EatB interaction graphs are presented in Figs. 3a, 3b, and 3c, respectively. In all three figures, the groups without MAPP are shown with red lines, and groups with MAPP are shown with green lines.

Fig. 3. Interaction graphs of HS and MAPP loading on composites: (a): tensile strength; (b): tensile modulus; (c): elongations at break

The TS values of the test specimens were in the range of 23.8 to 32.2 MPa. The lowest and the highest TS values were observed in H₃₅M₀ and H₃₅M₃ coded specimens, respectively. Statistical analysis showed that HS loading level and MAPP significantly affected the TS values (P < 0.0001). The differential of the TS of composites compared with those of neat PP was closely related to the composition of filler, matrix, and coupling agent. The TS values were reduced with increasing concentration of HS in the polypropylene matrix. It is thought that as the increased concentration of HS in the polypropylene matrix increases the interfacial area, the weakening of interfacial bonding between hydrophilic filler and hydrophobic polymer matrix decreases the TS (Yang et al. 2004; Maziero et al. 2019; Eroğlu et al. 2023; Kilic et al. 2023). Similar strength reduction by lignocellulosic filler addition due to the worsening interfacial bond between the polymer matrix and the filler was also reported by others (Bledzki and Faruk 2003; Yang et al. 2007; Narlıoğlu et al. 2018; Çavuş 2020; Çavuş and Mengeloğlu 2020; Başboğa et al. 2022; Kilic et al. 2023). The MAPP, which was utilized as a coupling agent in the formulation to improve the adhesion between filler and polymer matrix, dramatically increasing the composites’ TS values. Compared to the TS of composites without MAPP, an average improvement of approximately 18.5% was observed with the addition of MAPP. This was attributed to the fact that MAPP creates an efficient stress transfer by increasing the adhesion strength between the filler and polymer matrix surfaces (Langhorst et al. 2018; Çavuş and Mengeloğlu 2020; Kilic et al. 2023). However, in the specimens produced with MAPP, the specimens with 15% filler ratio had tensile strength values similar to pure PP, while the specimens with 10% filler ratio provided lower tensile strength values. It is thought that this is due to the inability to provide efficient transfer from the polymer to the fibers due to the low filling ratio.

Statistical analysis showed that the HS loading level and presence of MAPP significantly affected TM values (P < 0.0001). The TM values of the test specimens ranged from 650 to 1317 MPa. The lowest and highest TM values were observed in the unfilled PP and H₃₅M₃ (35% HS filler and 3% MAPP), respectively. It is clear from Fig. 1b that the tensile modulus values of the composites increased with increasing HS filler content. The TM values of test specimens increased independently of MAPP. Lignocellulosic fibers have higher modulus compared to polymer matrix (Çavdar et al. 2011). As a result, the addition of a high-modulus lignocellulosic fiber into the polymer matrix increased the resulting composite modulus values due to the rule of mixture (Mengeloğlu and Karakuş 2012; Çavuş and Mengeloğlu 2020; Kilic et al. 2023). The presence of MAPP in the formulation increased the modulus values of composites nearly 9% due to the increased adhesion between the polymer matrix and the filler.

Manufactured test specimens had EatB values of 3.38 to 7.53%. The testing machine measured elongation up to 400%. Statistical analysis was not conducted because HS unfilled samples (H₀M₀, H₀M₃) were not broken within this elongation limit. These groups are not shown in Fig. 1c. Statistical analysis showed that both the HS rates (P < 0.0001) and MAPP (P = 0.0002) had a significant effect on EatB values. The EatB values decreased significantly with increasing HS loading levels in composite samples, a common observation in WPCs. As the percentage of lignocellulosic filler loading in WPCs increased, the composite’s brittleness increased and turned into a more rigid structure. In addition, EatB values in samples containing MAPP decreased slightly compared to samples without MAPP. This is because the samples with MAPP became more brittle due to the improved interfacial adhesion between the filler and the polymer matrix. Consequently, the elongation at break (EatB) values of WPCs decreased. Similar results were reported in previous studies (Zaini et al. 1996; Balla et al. 2019; Çavuş and Mengeloğlu 2020; Başboğa et al. 2023; Kilic et al. 2023).

The test specimens’ FS and FM interaction graphs are presented in Fig. 4a and 4b, respectively. The groups without MAPP are shown with red lines in the two figures, and groups with MAPP are shown with green lines. The FS values of the test specimens ranged from 40.0 to 58.0 MPa. Statistical analysis showed that both the percentage of HS loading and MAPP significantly affected FS values (P < 0.0001). Despite MAPP, the FS values enhanced considerably with an increasing percentage of HS loading in the PP matrix. Compared to the FS values of unfilled and HS-filled PP, an average improvement of 17% was observed in the groups without MAPP and an average improvement of 34% in the groups with MAPP. It is thought that after a lignocellulosic fiber with higher bending strength is compounded into the polymer matrix, effective load transfer from the polymer to the fiber occurs when force is applied (Başboğa 2023). Furthermore, MAPP addition to HS-filled formulations resulted in an average improvement of approximately 14.5% in FS values. This is attributed to the improved adhesion strength between filler and polymer matrix. It has been reported that effective stress transfer from the PP matrix to lignocellulosic fibers occurs when there is significant binding of MAPP in WPCs (Langhorst et al. 2018; Kilic et al. 2023). Manaia and Manaia (2021) reported that the improved interfacial adhesion between the polymer matrix and hemp fiber largely determines the flexural strength of composites, as the load in flexural tests is transferred through the matrix-fiber interface. Other studies reported similar results using different lignocellulosic fillers (Bledzki and Faruk 2003; Kim et al. 2007; Çavuş and Mengeloğlu 2020; Kilic et al. 2023).

Fig. 4. Interaction graphs of HS and MAPP loading on composites: (a): flexural strength; (b): flexural modulus

The FM values of the manufactured specimens ranged from 1188 to 3070 MPa. Statistical analysis demonstrated that HS loading level and MAPP significantly affected FM (P < 0.0001). Independent of MAPP, FM values increased substantially with increasing HS filler compared to unfilled PP. Lignocellulosic fibers are a higher modulus material than polymers, and higher fiber concentration demands higher stress for the same deformation (Karmarkar et al. 2007; Kilic et al. 2023). Furthermore, adding MAPP to the formulations slightly increased the FM values by the improved interfacial adhesion between the hemp fiber and the polymer matrix, enabling increased stress transfer from the matrix to the filler. As a result, lignocellulosic fiber-reinforced polymer composites can achieve a higher flexural modulus than unfilled polymers. Similar results have been reported in other studies (Espinach et al. 2013; Mengeloğlu and Çavuş 2020; Başboğa 2023).

The interaction graphs of the notched IS are presented in Fig. 5. The IS values of the manufactured composite specimens ranged from 2.59 to 4.15 kJ/m². The groups without MAPP are shown with a red line, and groups with MAPP are shown with a green line. Statistical analysis showed that the HS filler loading level and MAPP significantly affected IS values (P < 0.0001). The IS values enhanced within a creased percentage of HS loading to the polymer matrix. An approximate 60% increase in IS values was observed in 30% of HS filled-PP composite samples compared to unfilled-PP. Impact strength is related to the toughness of the composite and is a measure of the ability of the composite to resist breakage under load applied (Panthapulakkal and Sain 2007). Yuan et al. (2008) reported that more energy is needed to break the fillers when breaking fiber-filled polymers than unfilled polymers. Burgada et al. (2021) reported that the ability to absorb and dissipate energy in the polymer matrix could be attributed to fiber fillers. Panthapulakkal and Sain (2007) also reported similar results and attributed the ability of fibers acting as stress-transferring to play an essential role in the impact strength of WPCs. Although the IS values increased with the HS filler, lower IS values were obtained in specimens with MAPP than in specimens without MAPP. It is thought that with the addition of MAPP in WPCs, the composite’s brittleness increases and the IS values decrease slightly. Other studies have also reported that coupling agents reduce IS values in WPCs (Mengeloğlu and Karakuş 2008; Langhorst et al. 2018; Kilic et al. 2023).

Fig. 5. Interaction graph of HS and MAPP loading on composites notched impact strength

Physical Properties of WPCs

Density values and water absorption rates were the physical properties studied. The density values and water absorption percentage are presented in Tables 3 and 4, respectively.

The density values obtained from the test specimens ranged from 0.879 to 0.984 g/cm³. The lowest and the highest density values were observed in H₀M₀ and H₃₅M₃ coded specimens, respectively. The interaction graph of the HS fillers and MAPP on the density of composite specimens is presented in Fig. 6. Statistical analysis showed that filler loading level (P < 0.0001) and MAPP (P = 0.0150) had a significant effect on density. Higher density values were obtained in composite specimens as the percentage of HS filler loading increased. This increase is thought to be due to the higher cell wall density of lignocellulosic fibers (approximately 1.5 g/cm³) than polymer (Stokke and Gardner 2003; Mengeloğlu and Karakuş 2008; Islam et al. 2013; Mengeloğlu et al. 2015; Kilic et al. 2023). Clemons and Caufield (2005) reported high-pressure processes, such as injection molding, cause the lignocellulose’s hollow fibers to collapse or to become filled with low molecular weight plastics.

Table 3. Average Density Values of Test Specimens

Five specimens for each group were tested, *: Average values, **: standard deviation

Fig. 6. Interaction graph of FFWS and MAPP loading on composites’ density values

Table 4. Water Absorption Percentages of Test Specimens

Note: Values are percentage change by weight (wt%)

The HS-filled PP composites’ water absorption percentages after 2 h, 24 h, 48 h, 72 h, 168 h, 336 h, 504 h, and 672 h are given in Fig. 7. Because the data relating to PP water absorption is considered zero, water absorption data of unfilled PP are not included. In the columns in Fig. 7, the shaded columns show groups with MAPP, and the empty columns show groups without MAPP.

Fig. 7. The water absorption of the HS reinforced PP composites during 28 days of water immersion

The water absorption of test specimens ranged from the lowest (0.08%) to the highest (3.71%). Increasing filler content in composite specimens resulted in higher water absorption. The highest water absorption was obtained from the specimens consisting of 35%; the time the specimens stayed in water increased the water absorption. A more significant increase was observed in the water absorption, especially of the highly filled specimens (30% and 35%) after 72 h. The water absorption of WPCs is mainly affected by the structure of lignocellulosic fiber, and they may absorb more water than neat thermoplastics. Lignocellulosic fibers have hydroxyl groups that interact with water molecules by hydrogen bonding. Thus, the results are sensitive to water (Çavdar et al. 2011). Lopez et al. (2012) attributed that to forming of an incompatible mixture of the two components and incorporating hydrophilic fibers inside a hydrophobic polymer matrix. Çavdar et al. (2023) reported that the incompatibility of the filler with the polymer matrix causes voids and increases the water absorption of the WPCs. However, if the lignocellulosic fiber is properly encapsulated during processing, the WPCs will absorb less water (Stokke and Gardner 2003). This has been observed in composites containing MAPP. Because the inclusion of MAPP in the formulations increased the compatibility between the fiber and the polymer matrix, the water absorption percentages of the composites were slightly improved. The improved adhesion between filler and polymer matrix in groups with MAPP is presented in SEM images.

Morphological Properties of WPCs

The morphology of the test specimens is presented in Fig. 8. Parts (a), (b), (c), and (d) of the figure show SEM images of the H₂₀M₀, H₂₀M₃, H₃₅M₀, and H₃₅M₃, respectively. In composites, samples without MAPP (Fig. 8a and 8c) show pullout, debonding, or gaps throughout the interphase between fibers and matrix. These gaps manifest the weak adhesion between fiber and polymer matrix. However, in composites with MAPP (Fig. 8b and 8d), a significant improvement in fiber/matrix adhesion and a compact interphase with fewer voids were observed. Fig. 8b shows a fiber entirely covered by the matrix, indicating the presence of improved adhesion between the filler and the polymer matrix. This result was also consistent with the improved mechanical properties (e.g., tensile and flexural strength) of composites with MAPP. Previous studies also reported that the use of compatibilizers in WPCs improves the performance properties of composites (Kakroodi et al. 2012; Panaitescu et al. 2019; Çavus 2020; Çavus and Mengeloğlu 2020; Díaz et al. 2020; Kilic et al. 2023).

Fig. 8. SEM images of the sections broken under impact for composites: (a): H₂₀M₀; (b): H₂₀M₃; (c): H₃₅M₀; and (e): H₃₅M₃

Thermal Properties of WPCs

The composite specimens’ TGA and DTGA (derivative thermogravimetric analysis) curves are presented in Figs. 9a and 9b, respectively. Fig. 9a shows the weight loss versus temperature curves, while Fig. 9b illustrates the corresponding first derivative curves. The TGA analyses were performed for H₀M₀, H₂₀M₀, H₂₀M₃, H₃₅M₀, and H₃₅M₃. In addition, the degradation onset temperature (T_onset), the temperature of degradation endset (T_endset), and degradation percentages at different stages of degradation of the composite specimens are presented in Table 5. A single-stage thermal degradation was observed in the specimens in TGA analysis. Considering the values in Table 5, the decomposition for the unfilled-PP started at approximately 216 °C and ended at around 479 °C. At the end of the second stage, the unfilled-PP exhibited 99.5% degradation. At the end of the test at 700 °C, neat polypropylene reached maximum degradation, and residue content reached 3%. A similar result was reported: unfilled PP was degraded entirely (Silveira et al. 2023). Additionally, at the end of the test at 700 °C, the H₂₀M₀, H₂₀M₃, H₃₅M₀, and H₃₅M₃ residue contents were 1.6, 1.6, 0.6, and 0.8%, respectively. Consequently, using MAPP in HS-filled composites showed no change in residue amounts.

Table 5. Thermal Parameters Obtained by TGA of Composites

When Figs.9a, 9b, and Table 5 were examined, it was observed that the thermal degradation of the composite specimens occurred in two different stages.

Fig. 9. Comparative TGA graphs of composites: (a): TGA curves; (b): DTGA curves

The first stage, which started at room temperature and was completed at T_onset, is generally associated with moisture loss in the composites. A similar result was reported by Silveira et al. (2023). With the increase of the HS filler ratio, the degradation rate in T_onset increased, resulting in a 2.4% loss in 35% HS filled-PP. It is thought that this is because HS-filled composites contain more moisture. The second stage of the degradation started when the temperature reached approximately 215 °C, and above this temperature (T_onset–T_endset range), a rapid increase in the decomposition curve was observed.

While the T_onset temperature of unfilled-PP was approximately 216 °C, the T_onset temperatures of HS-filled-PP ranged between 216 and 251 °C. This difference is thought to be due to cellulose, hemicellulose, and lignin, the main components of hemp stalks. Others have also reported that these three components increase the thermal decomposition temperature in WPCs (Şeker Hirçin et al. 2021; Başboğa 2023). The second stage of the degradation started when the temperature reached approximately 215 °C, and above this temperature (T_onset–T_endset range), a rapid increase (Fig. 7a) in the decomposition curve was observed. The second degradation stage (T_endset) temperature in composite specimens ranged from 479 to 503 °C. With the addition of hemp stalk, the T_endset temperature value increased approximately 20 °C compared to the unfilled-PP.

CONCLUSIONS

This study evaluated the effect of hemp stalk flours and maleic anhydride-grafted polypropylene (MAPP) participation percentage on the mechanical, physical, morphological, and thermal properties of hemp stalk (HS) reinforced polypropylene (PP)-based composites. In the production of composites, 0, 5, 10, 15, 20, 25, 30, and 35% hemp stalk (HS) was loaded into the polypropylene matrix as filler, respectively, and 0 or 3% MAPP was used as a coupling agent. HS-filled polypropylene composites were successfully produced, and the following conclusions were reached:

Adding HS filler to the PP matrix increased the mechanical properties of the produced composites compared to neat PP.
Increasing the amount of HS filler increased the flexural strength, flexural modulus, tensile modulus, impact resistance, and density values of PP.
The tensile strength decreased with increasing HS amount, but the addition of MAPP caused a dramatic increase in tensile strength.
The flexural strength, flexural modulus, tensile strength, tensile modulus, and density values of composites with MAPP were higher than those without MAPP.
When the tensile strength (TS) and flexural strength (FS) of composites containing MAPP were compared to composites without MAPP, an average improvement of about 18.5% and 14.5%, respectively, was observed.
Increasing the amount of HS filler increased the water absorption percentages, but the inclusion of MAPP in the formulations slightly improved the water absorption rates of the composites.
Thermogravimetric analyses (TGA) were performed on groups selected among the produced composites. Weight losses in the first stage were observed as 0.5%, 1.2%, 1.5%, 2.4%, and 2.4% in the groups coded H₀M₀, H₂₀M₀, H₂₀M₃, H₃₅M₀, and H₃₅M₃, respectively. The amount of residue at 700 °C was observed as 0%, 1.6%, 1.6%, 0.6%, and 0.8% in the groups coded H₀M₀, H₂₀M₀, H₂₀M₃, H₃₅M₀, and H₃₅M₃, respectively.
The scanning electron microscopy (SEM) analysis showed that MAPP improved adhesion between the HS filler and the polypropylene matrix.

ACKNOWLEDGMENTS

This study was supported by The Scientific and Technological Research Council of Türkiye (TUBITAK—project Number 122O317).

We thank “Insitu Green Technologies Industry and Trade Inc.” for providing hemp stalks.

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Article submitted: December 5, 2023; Peer review completed: January 3, 2024; Revised version received: January 4, 2024; Accepted: January 11, 2024; Published: January 16, 2024.

DOI: 10.15376/biores.19.1.1494-1516