The Effect of Wastes of Nettle Fiber on Mechanical and Thermal Properties of Polypropylene Composite

ABSTRACT In this study, ground wastes of Nettle (Urtica dioica L.) fiber (GWN), which contains cellulose of 52.2 wt%, hemicellulose of 28.9 wt%, and lignin of 18.9 wt%, filled polypropylene (PP) composites were fabricated by using a high-speed kinetic mixer. The effect of filling GWN into PP at different weight fractions on density, crystallization behavior, coefficient of thermal expansion, and thermal stability was investigated. In addition, tensile and flexural tests were performed to determine how the mechanical properties of PP were affected. Morphological observations were conducted using scanning electron microscopy (SEM). The flexural strength values of PP increased by 10%, 15%, and 31% with the addition of 7 wt%, 14 wt%, and 21 wt% GWN, respectively. However tensile strength value of PP decreased slightly with the addition of GWN. 21 wt% GWN addition into PP led to a considerable decrease (about 29%) in the thermal expansion coefficient of PP. The degree of crystallinity of PP was not affected by the addition of GWN.


Introduction
With the increasing concerns about the use of nonrenewable resources all over the world, studies on new environmentally friendly materials have gained momentum as an alternative to traditional materials (Balla et al. 2019). Waste recycling, renewability, and sustainability have become one of the most important needs for various industries such as automotive, electrical electronics, household goods, defense, aviation, etc (Balla et al. 2019;Tarique et al. 2021).
Polymeric materials due to their lightweight, ease of processing, and low cost play an important role in meeting the challenges of the twenty-first century in engineering and high-tech applications. The number of scientific research on the development of polymeric and polymeric-composite materials with desired properties according to the application in which they will be used is quite high (Omrani, material in this study. In order to obtain the powder form of Nettle fiber waste (GWN), the waste was ground to pass the sieve size 100 microns by using Blade Grinder.

Determination of chemical composition of GWN
Chemical analysis was performed to evaluate the chemical content of GWN. Fiber samples were ovendried at 105 °C and then kept in a desiccator prior to chemical analysis processes. The procedure of the processes is given in elsewhere .

Preparation of composite
After GWN was dried at 80°C at 2 h, it was added to the mixing bowl in certain proportions (7, 14, 21 wt. % of GWN) and mixed with PP in a Thermo-kinetic mixer (Gelimat) for about 120 seconds. Before molding, the mold was coated with polytetrafluoroethylene (PTFE) fabric to prevent the material from sticking. The prepared mixture was poured into the 20 cm x 20 cm x 3 cm mold. A hot hydraulic press was applied to the sample at 180°C at 60, 90, and 120 bar pressures and for 120, 60, and 30 seconds. Then, a cold hydraulic press was applied at 120 bar pressure for 90 seconds.

Density measurement
Density measurement of the specimens was performed according to the ASTM D792-00 using Densimeter MD-300S.

Differential scanning calorimetry (DSC) analysis
DSC measurements of PP and its composites were performed by using DSC Q20 (TA Instruments Inc.). The specimens were heated from 20 to 200°C with a ramp of 10°C/min and kept at 200°C for 1 min. After being cooled to 20°C, the samples were re-heated to 200°C with a heating rate of 10°C under a nitrogen atmosphere.

Thermogravimetric (TG) analysis
TG analyses of PP and PP-GWN composites were conducted by using a thermogravimetric analyzer (TGA-Q50-TA Instruments) at a heating rate of 10 ºC/min under a nitrogen atmosphere between the temperature range of 25 to 800 ºC.

Thermomechanical analysis (TMA)
The thermal expansion coefficients (CTE) of PP and PP-GWN composites were determined by using a thermo-mechanical analyzer (TMA 400-TA Instruments). The analyses were conducted with expansion mode at a constant compression load of 0.02 N. The specimens (10 mm × 5 mm × 3 mm) were heated from −30 to 90°C at a heating rate of 5°C min −1 .

Mechanical tests
Tensile tests were performed according to ASTM D638-10 standard, with specimen type I and a crosshead speed of 50 mm/min. Three-point bending test was conducted by using the ASTM D790 standard with a support span of 64 mm and a deformation rate of 2 mm/min.

Scanning electron microscopy (SEM)
The morphological properties of GWN-filled PP composites were examined by using a scanning electron microscope (300 VP FE-SEM, Carl Zeiss AG, Germany) operated at an acceleration voltage of 3 kV. Before analysis, the surface of GWN-filled PP composites was coated with a thin layer of gold via a plasma sputtering system.

Chemical composition GWN fibers
The chemical composition of the plant fibers is affected by the growing region, part and age of the plant, and extraction technique (Maheshwaran et al. 2018). Plant fibers are mainly composed of cellulose, hemicellulose, and lignin. The cellulose and hemicellulose contents of GWN are 52.2 and 28.9%, respectively. The fraction of the rest of the other chemical components including lignin is 18.9%. Hemicellulose can be considered as a compatibilizer between cellulose and lignin. Cellulose consists of helically wound cellulose micro-fibrils bound together with and further embedded in lignin (Arthanarieswaran, Kumaravel, and Saravanakumar 2015).
The cellulose contents of some other fibers were listed in (Table 1). The cellulose content of GWN is lower than Thespesia populnea, Red banana peduncle, and Dichrostachys Cinerea (Baskaran et al. 2018;Ganapathy et al. 2019;Manimaran et al. 2018). It was comparable with Conium Maculatum, Curcuma longa L., Morus alba L., coir, kapok, and kenaf fibers (Chaiarrekij et al. 2012;Esmeraldo et al. 2010;Ilangovan et al. 2018). Accordingly, it is possible to reveal that GWN can be a candidate to be used as a reinforcement with its cellulose capacity.

Density
The density values of GWN-filled PP composites are given in (Table 2).
As noticed from (Table 2), the density values of the composites were obtained to be higher than the density of the neat PP. As expected, the density of the composites increased with the increase in the weight fraction of the GWN. This is due to the higher density of GWN compared to neat PP. The highest density was found to be 0.98 g/cm 3 for PP containing 21 wt.% GWN.

DSC analysis
DSC curves of PP and PP-GWN composites are shown in (Figure 1). Melting temperature (T m ) and enthalpy of melting (ΔH m ) can be obtained from the DSC curves. DSC data obtained from (Figure 1) are summarized in (Table 3). The degree of crystallinity (X c ) of the specimens can be calculated using the following equation Eq. (1) (27).
where ϕ PP is the weight fraction of PP in the composite, ΔH � m is the enthalpy of melting of fully crystalline PP, taken as 209 J/g (Kaya et al. 2018).
As can be seen from (Table 3), melting temperature does not change considerably upon GWN filling. However, an increase in melting enthalpy was observed. The degree of crystallinity of PP, PP-7GWN, PP-14GWN, and PP-21GWN values were calculated to be 40, 43, 40, and 40% respectively. It is noted that the degree of crystallinity value of the PP-7GWN composite is higher than that of neat PP. The increase in crystallinity in PP-based composites could be due to the nucleating effect of the fillers which could promote crystallization. A similar phenomenon was reported for various fillers resulting in increased crystallinity in PP composites (Altay et al. 2018). The degree of supercooling decreased with a filling of 7 and 14 wt. % of GWN, which shows the nucleating agent's ability (Beck and Ledbetter 1965).

TG analysis
TGA curves for PP and PP-GWN composites are given in (Figure 2).
As can be understood from (Figure 2), PP-GWN composites display two main weight losses around 250-350°C and 400-500°C due to the degradation of GWN and PP, respectively. Degradation temperatures at different levels of TG weight loss are listed in (Table 4).  It is presented that the maximum degradation temperatures of PP slightly increased with the addition of GWN. Salemane and Luyt (2006) and Mofokeng et al. (2012) reported similar degradation behavior when PP was filled with wood flour and sisal fiber, respectively. Besides, (Table 4) indicates that the addition of GWN into PP reduced the thermal stability of PP. It can also be observed that there is an apparent decreasing trend in the thermal stability of the composites with increasing the weight fraction of GWN. T ( 0 C) at 2 wt.% and T ( 0 C) at 5 wt.% values decreased with the weight fraction of GWN within the composites. The greatest weight fraction of GWN within PP led to the lowest T ( 0 C) at 2 wt.%, T ( 0 C) at 5 wt.%, and T ( 0 C) at 10 wt.% values.

TMA
Dimensional change versus temperature change for PP and PP-GWN composites are shown in (Figure 3).
An increasing trend in dimensional change can be realized in the studied temperature range. The coefficient of thermal expansion (CTE) values of PP and PP-GWN composites were obtained in the temperature range (−10) to 50°C and obtained results are given in (Table 5).
CTE values of PP, PP-7GWN, PP-14GWN, and PP-21GWN were obtained to be 109, 117, 120, and 77 µm/m 0 C, respectively. It is known that adding fiber to a thermoplastic polymer reduces the thermal expansion of the composite (Bogard et al. 2022). The coefficient of thermal expansion of PP-21GWN decreased considerably. The addition of 7% GWN and 14% GWN to PP may not be effective on the coefficient of thermal expansion because of the close CTE values. With the addition of 21 wt.% GWN into PP decreased the CTE value of PP by 29%. It is known that thermoplastic materials are  known with high CTE values. Therefore, lower CTE value of PP-21GWN is an advantage for many applications ).

Mechanical properties
The values of tensile and flexural strength of the GWN-loaded PP are given in Figures 4 and 5, respectively.   Tensile strength values of PP, PP-7GWN, PP-14GWN, and PP-21GWN composites were obtained to be 22. 05, 20.55, 19.02, and 18.09 MPa, respectively. At the whole weight fraction of GWN, the tensile strength values of GWN-loaded PP composites are lower than that of PP.
The decrements in tensile strength may result from the poor interfacial adhesion between the hydrophobic PP matrix and hydrophilic lignocellulosic GWN filler. Thus, it prevents efficient stress transfer from the PP matrix to GWN particles ( (Premalal, Ismail, and Baharin 2002) (Ab Ghani and Ahmad 2011;Fu et al. 2008)). Reductions in the tensile strength may be prevented if a compatibilizer is used to increase the compatibility between the additives and matrix.
According to (Figure 5), the flexural strength values for PP, PP-7GWN, PP-14GWN, and PP-21GWN were determined to be about 46.4, 51.1, 53.2, and 61.0 MPa, respectively. The flexural strength values of PP increased by 10%, 15%, and 31% with the addition of 7 wt%, 14 wt%, and 21 wt% GWN, respectively. This indicates that as the weight fraction of GWN was increased, the flexural strength values increased. The highest flexural strength value was achieved at 21 wt.% GWN filling (Fischer, Werwein, and Graupner 2012). Fischer, Werwein, and Graupner (2012), obtained that  reinforcement by 30 wt-% nettle resulted in a tensile strength 33% lower than PP and a flexural strength 60% below PP, which is due to poor interaction between the nonpolar polymer matrix and the polar nettle fibers (Fischer, Werwein, and Graupner 2012). Kumar and Das (2017) showed that the tensile strength, elongation-at-break, Young's modulus, flexural strength, and impact strength of the  biocomposites increased initially with the increase of nettle fiber content and decreased afterward (Kumar and Das 2017).
As can be realized from Figures 6 and 7, the tensile and flexural modulus of PP increased with the addition of GWN fillers in the studied concentration range (7-21 wt.%). PP has tensile and flexural moduli of about 745 and 1619 MPa, respectively. Filler loading affects the stiffness properties of the matrix material. The increase in the tensile and flexural modulus of the PP matrix may be due to the higher modulus of the natural fillers. It is well known that the addition of rigid filler is anticipated to increase the tensile and flexural modulus of the composites and yield a greater stiffness the composites. When 21 wt. % GWN was added, and the tensile and flexural moduli increased by about 101% and 123%, respectively (Jamil, Ahmad, and Abdullah 2006). Showed that the addition of the filler increases the tensile and flexural moduli of composites. This results from the addition of rigid fillers into the matrix (Jamil, Ahmad, and Abdullah 2006).

SEM
GWN at different particle sizes within the PP matrix can be given in Figure 8a-c Powder pull-outs can be tracked especially at a higher weight fraction of GWN (Figures 8b-c) This indicates a poor dispersion between PP and GWN particles. However, aggregated filler powder acting as a stress concentrator, which led to drops in tensile strength, is not commonly seen in the SEM images of GWN-added PP composites Kaya et al. 2018;Maheshwaran et al. 2018).

Conclusion
In this study, GWN-filled polypropylene composites at different weight fractures of GWN were manufactured. There were certain effects of GWN on the thermal, mechanical, and physical properties of polypropylene composites. Some specific conclusions could be drawn as follows: (1) The degree of crystallinity values of 40, 43, 40, and 40% were calculated for PP, PP-7GWN, PP-14GWN, and PP-21GWN composites respectively. (2) The greatest weight fraction of GWN within PP led to the lowest T( 0 C) at 2 wt.% and T( 0 C) at 5 wt.%, T( 0 C) at 10 wt.% values. (3) PP-7GWN and PP-14GWN exhibited an increasing trend in CTE values. However, with the addition of 21 wt.% GWN into PP decreased the CTE value of PP by 29%. (4) The flexural strength value of PP increased by about 10, 15, and 31% with the filling of 7, 14, and 21 wt. % of GWN, respectively.
From the conducted study it can be concluded that GWN as a bio-waste material, without any pretreatment, enhanced not only modulus values but also improved the coefficient of thermal expansion and flexural strength values of the polymer. GWN's low cost, biodegradability, and satisfactory performance make it a promising natural fiber to replace synthetic fibers in thermoplastic-based composites for numerous applications.

Highlights
• Nettle-reinforced polypropylene composites were prepared from a bio-waste material by using high-speed kinetic mixer. • The effect of filling nettle fiber waste into polypropylene at different weight fractions was investigated in detailed.
• Nettle fiber waste as a bio-waste material, without any pre-treatment, enhanced modulus values, coefficient of thermal expansion and flexural strength values of polypropylene.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This research received no external funding.

Ethical approval
We confirm that all the research meets ethical guidelines and adheres to the legal requirements of the study country. The research does not involve any human or animal welfare-related issues.