Hybrid bio-fiber/bio-ceramic composite materials: Mechanical performance, thermal stability, and morphological analysis

Faris M. AL-Oqla; Mohammed T. Hayajneh; Mu’ayyad M. Al-Shrida

doi:10.1515/rams-2023-0101

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Hybrid bio-fiber/bio-ceramic composite materials: Mechanical performance, thermal stability, and morphological analysis

Faris M. AL-Oqla , Mohammed T. Hayajneh and Mu’ayyad M. Al-Shrida

Published/Copyright: August 4, 2023

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From the journal REVIEWS ON ADVANCED MATERIALS SCIENCE Volume 62 Issue 1

Abstract

Hybrid composite materials are becoming more desirable for various industrial applications to enhance sustainability and develop better environmentally friendly green products. This work aims to enhance the synergy of both bio-ceramic eggshell materials and date palm leaflet (DPL) fillers to integrate their advantages in an optimized hybridization manner to enhance their significance in producing novel biomaterials with improved desired mechanical, thermal, and morphological characteristics. Different weight percentages of hybrid green reinforcement (poultry eggshells and DPLs) were utilized in various hybridization ratios (3:7, 5:5, 7:3), (15:5, 10:10, 5:15), and (20:10, 15:15, 10:20) to fabricate 10, 20, and 30 wt% novel biomaterials. The regularly chopped DPLs were immersed in various concentrations of sodium hydroxide at different soaking times to optimize and improve their bonding with the polypropylene (PP) matrix. The mechanical, thermal, and morphological properties of the fabricated hybrid composites were investigated. The results have revealed that certain hybridization ratios could improve the tensile and flexural modulus by up to 26 and 11%, respectively. According to the thermogravimetric analysis and its derivatives, hybridization was also found to have an excellent influence on the thermal stability of the PP matrix. Regarding morphological micrographs utilizing scanning electron microscopy, DPLs exhibited good bonding, whereas eggshell fillers depicted different behaviors of bonding depending on their surface topologies. It was also found that hybridization with higher eggshells had better effects on flexural strength than date palms, regardless of their weight percentages. The 30 wt% hybridization case was found to be capable of improving the modulus of elasticity of composites to 838 MPa and the flexural modulus to 735 MPa, which are suitable for various structural applications and green products.

Keywords: hybrid composite; bio-ceramics; biomaterials; eggshell; mechanical performance; natural resources

1 Introduction

The waste from synthetic polymer-based industries around the globe has exceeded acceptable quantities, which makes it a real environmental issue. It was reported that more than 6,300 million tons of waste had been produced by 2015, of which only 9% had been recycled, 12% had been incinerated, and the remainder had been dumped into landfills and the environment [1]. This has led to the release of large amounts of greenhouse gases, causing air pollution [2]. Consequently, researchers all over the world have been seeking to replace plastics with a percentage of biodegradable materials from natural resources to preserve the properties of plastic and reduce its detrimental impacts on the environment. Lignocellulosic fibers, such as green olive leaves [3], grape fibers [4], parsley [5], hay, palm, lemon, reed [6], and other mixtures of date palm parts [7], tea waste fibers [8], sugar palm [9], and cypress-pine [10], were found to have potential as reinforcement with plastic matrices. The properties of natural lignocellulosic fibers, such as biodegradability, sustainability, low density, and low cost, have made them competitive with synthetic reinforcements [11]. Among natural fibers, date palm fibers have some advantages that do not exist in other counterparts. For instance, the tensile strength and modulus of date palm leaves have reached the highest values when they are particularly compared to olive, loquat, and lemon leaves [12]. The pruning of date palm trees, in comparison with sisal, hemp, and coir, achieved the highest production waste. Moreover, it had the lowest price and density as compared with sisal, hemp, and coir [13]. Date palm leaves, in contrast with almond, apple, castor, olive, orange, and reed, contain a high weight percentage of cellulose. In addition, date palm fibers have higher thermal stability [14] than other natural fibers like Catharanthus roseus [15] and Leucas aspera [16].

One of the most common obstacles that researchers have been facing in utilizing date palm fibers with plastics, like other natural fibers, is the poor bonding behavior because of their hydrophilic nature [17]. Consequently, researchers have tried several chemical treatments to improve the adhesion between the fiber and the matrix. Chemical treatment can remove the waxy lignin layer, leading to an increase in the surface roughness of the fiber [18] and activating the hydroxyl groups, as well as enhancing the fiber–matrix interlocking [19]. Different studies have been conducted on the effects of chemical treatment on natural composites. Bachtiar et al. [20] immersed sugar palm fibers in two concentrations of NaOH solution (0.25 and 0.5 M) for three different soaking times (1, 4, and 8 h). They revealed that the tensile strength of the sugar palm–epoxy composite was improved at the lowest NaOH concentration after 1 h. Also, they noted a decrease in tensile strength at high levels of NaOH (0.5 M). They attributed this trend to damage to the fiber structure. Similar conclusions were observed in the tensile modulus at 0.5 M NaOH. Furthermore, three concentrations of NaOH (3, 6, and 9 wt%) were also used for date palm fibers for 24 h at room temperature. It was reported that the best tensile properties were achieved at 6 wt% concentration. However, the 9 wt% of NaOH had decreased the tensile properties due to the fiber structure deterioration [21]. Hemp fibers were treated by Sepe et al. [22] using two types of treatments: sodium hydroxide and silane solutions. They soaked the fibers for 0.5 h at two concentrations of NaOH (1 and 5%) and three concentrations of silane (1, 5, and 20%). Neto et al. [23] studied the effect of 2 g of NaOH, which was diluted in 100 ml of water for 1 h, on a jute fiber–epoxy matrix. Their results indicated that alkaline treatment was capable of enhancing both thermal stability and storage modulus. In addition, a comparison between hydrochloric acid (HCl) and sodium hydroxide (NaOH) treatments was performed by Vijay et al. [24] on Pennisetum Orientale fibers. Their findings have revealed that NaOH improved both tensile and thermal properties, whereas HCl weakened these properties due to its more acidic effects. Furthermore, Zanini et al. [25] treated the residue of the date palm with 4% NaOH for 1 h at 70°C. The tensile strength, tensile modulus, and thermal stability were studied. Moreover, Sh. Al-Otaibi et al. [26] improved the bonding of the date palm fiber–PP matrix by immersing the fiber in 5 wt% NaOH for 24 h at 70°C. Table 1 presents a summary of the aforementioned studies.

Table 1

Summary of the type of chemical treatment and its conditions

Fiber type	Treatment type	Concentration (%)	Time (h)	Temperature (^oC)	Response properties	Ref.
Jute	NaOH	2 g per 100 ml	1	NA	Thermal stability	[23]
Jute	NaOH	2 g per 100 ml	1	NA	Storage modulus	[23]
Sugar palm	NaOH	0.25 M	1	NA	Tensile strength	[20]
Sugar palm	NaOH	0.25 M	1	NA	Tensile modulus	[20]
Hemp	NaOH	1 and 5 wt%	0.5	NA	Tensile modulus	[22]
	Silane	1, 5, and 20 wt%			Tensile strength
					Flexural modulus
					Flexural strength
Orientale	HCl	5 wt%	2	Room temperature	Thermal stability	[24]
	NaOH	5 wt%			Tensile strength
	NaOH	5 wt%			Tensile modulus
Date palm stem	NaOH	3, 6, and 9 wt%	24	Room temp.	Tensile strength	[21]
Date palm stem	NaOH	3, 6, and 9 wt%	24	Room temp.	Tensile modulus	[21]
Palm residue	NaOH	4 wt%	1	70°C	Tensile strength	[25]
					Tensile modulus
					Thermal stability
Date palm fiber	NaOH	5 wt%	24	70°C	Tensile strength	[26]
					Tensile modulus
					Thermal stability

Numerous studies have, in addition, demonstrated that various date palm tree parts, including leaves, stems, branches, rachis, core shells, and fibers around the branch, can improve the mechanical and thermal properties of polymer matrices [7,21,25,26,27]. Because of their characteristics, chicken eggshells have recently become one of the most important competitors for other synthetic inorganic fillers such as glass and carbon fibers. To illustrate, the waste of chicken eggshells (Es) is abundant, with the world disposing of 8.6 million tons in 2018 [28]. Additionally, its low density and low price motivate the researchers to use it as a filler for composites [29]. Furthermore, its thermal stability has drawn the interest of researchers attempting to improve the thermal properties of plastics [30]. The effects of eggshells (Es) on the mechanical and thermal characteristics of polymers have been investigated by several researchers. Shin et al. [31] used different concentrations(from 10 to 40 wt%) and various sizes to improve the tensile and compressive properties of epoxy–glass fiber composites. Their results indicated that Es particles could enhance the compressive strength and tensile strength at 10 wt% of Es. Wu et al. [32], furthermore, modified poly(vinyl alcohol) composites by using five concentrations (10–50 wt%) of Es. However, the tensile strength increased up to 30 wt% and then decreased again. The improvement in tensile modulus was attributed to the stiff Es particles. Additionally, the flexural and impact strength of the hybrid composite of the epoxy matrix, glass fiber, and hemp fiber was enhanced due to the addition of Es. Conversely, Es particles decreased the tensile and shear strengths [33]. Moreover, Oladele et al. [34] hybridized sisal fibers and Es with epoxy resin and then investigated the effect of Es on flexural, tensile, impact, hardness, and wear properties. The recycled low-density polyethylene was reinforced with multi-wt% of Es nanoparticles (from 2 to 12) by Bello et al. [35]. They concluded that increasing the wt% of Es particles could improve the hardness and flexural strengths. Also, the tensile strength was enhanced due to the Es incorporation. The influences of filler (eggshells and/or lemon leaves) on flexural and tensile tests of polypropylene (PP) were also investigated by Hayajneh et al. [36]. The hybridization of various bio- and/or synthetic fibers was found in the literature [37,38,39,40]. Eggshells with other materials like CaCO₃ particles were considered with epoxy [41]. It was shown that the addition of fillers decreased the tensile strength of the composites than the original epoxy matrix. Moreover, bio-polymeric composites can be utilized in a variety of industries, including the automotive industry for automobile interiors, the household products such as moisture-resistant furniture, the packaging industry, panels in airplanes, and trains in addition to construction applications [42,43,44].

It has been reported in the literature that date palm fibers can improve the mechanical performance of polymeric matrices like tensile and flexural properties. Additionally, eggshells (Es) as a bio-ceramic material can enhance the thermal stability of plastics. Moreover, eggshells and date palms were separately used in the literature. However, the novelty of the current work is to make a new synergy and hybridization of the considered fillers to add value in the field as it considered the advantages of both treated date palm fibers and bio-ceramic eggshell materials. Due to the advantages of date palm fibers in improving the mechanical performance of the polymeric matrices and due to the inherent characteristics of the bio-ceramic eggshell material that can enhance the thermal stability of plastics, this work has filled the gap for the first time in utilizing the integrated characteristics of these available fillers to develop novel bio-composites with improved desired mechanical and thermal properties for the first time via proper and optimized hybridization synergy of reinforcement.

Therefore, this study aims to enhance the synergy of both fillers and to integrate their advantages in a novel hybridization manner utilizing optimized treated date palm leaflets (DPLs) and eggshells to enhance the overall mechanical performance of bio-composites, which have not previously been utilized. Composites with numerous hybridization ratios and weight percentages of the combined reinforcements were fabricated with PP via a screw extruder and compression molding process. Optimization of the treatment concentration and time duration of date palm fibers was performed. The mechanical, thermal, and morphological properties of the produced composites were then examined and investigated using different characterization techniques.

2 Materials and methods

2.1 Materials

PP matrix in the granules form was purchased from SABIC Company (Saudi Arabia). The DPL fibers, Figure 1(a), were extracted from nearby date trees in Jordan. To get rid of the dust and moisture, they were cleaned with distilled water and then air dried. The fiber density of DPL was measured to be 0.9 g·cm⁻³. After that, DPL fibers were cut into small, long pieces, and were then chopped into small fibers with a length of 22.3 ± 2.7 mm and a width of 1.4 ± 0.7 mm, as shown in Figure 1(b). To improve adhesion between hydrophilic DPL and hydrophobic PP, DPL was chemically treated with sodium hydroxide (SH) with purity >99%. In addition, five aqueous solutions were tested, as shown in Table 2, to examine the effects of treatment conditions (time and concentration) on the tensile properties of DPL–PP composites.

Figure 1

(a) DPL and (b) chopped leaflet fibers.

Table 2

Chemical treatment conditions

Formula	Concentration (wt%)	Time (h)
0% SH–0h	0	0
4% SH–10 h	4	10
4% SH–20 h	4	20
8% SH–10 h	8	10
8% SH–20 h	8	20

SH: sodium hydroxide.

The chemical treatment conditions were changed on 10% of the DPL–PP composite and the conditions that resulted in the best tensile characteristics were applied to the entire DPL fibers. The impact of the optimized chemical treatment on the topography of DPLs was also studied by scanning electron microscopy (SEM).

The second filler employed with the PP matrix was poultry eggshells, which comprise ∼95% calcium carbonate (CaCO₃) and the remainder is organic matter [36]. Eggshell (Es) is made up of three layers that are arranged from outside to inside: the cuticle layer (which acts as a line of defense against microbes), the palisade layer (calcium carbonate layer), and the mammillary layer, which is covered by the membrane that acts as a network of proteins [45]. As the size of the filler influences the composite’s properties [46], the Es’ fragment size was determined by passing it through two sieves (850 and 2,000 μm). Figure 2 depicts flakes of Es, which were collected, washed, dried in the sun, and then manually broken into small flakes without any chemical treatment.

Figure 2

Flakes of Es filler ranging from 850 to 2,000 µm.

2.2 Samples’ preparation

PP composites at three weight percentages (10, 20, and 30 wt%) were produced. Each weight percentage was divided into three hybridization ratios as illustrated in Table 3. The composite mixtures were injected at 180°C using a domestically built screw extruder into an aluminum mold with dimensions of 10 cm × 7 cm × 0.43 cm. Following injection, the composites were cold compressed at a pressure of 5 t. To obtain more precise findings, each composite was reproduced three times.

Table 3

Formulation of PP composites

Sample formula	Overall wt% of the hybrid filler	Hybridization ratio		PP (wt%)
Sample formula	Overall wt% of the hybrid filler	DPL (wt%)	Es (wt%)	PP (wt%)
Neat PP	0	0	0	100
7DPL–3Es	10	7	3	90
5DPL–5Es		5	5	90
3DPL–7Es		3	7	90
15DPL–5Es	20	15	5	80
10DPL–10Es		10	10	80
5DPL–15Es		5	15	80
20DPL–10Es	30	20	10	70
15DPL–15Es		15	15	70
10DPL–20Es		10	20	70

2.3 Mechanical tests

Two mechanical tests, tensile and flexural, were carried out in accordance with ASTM D3039-3039M and ASTM D790 standards, respectively, to evaluate the effect of weight percentage and the hybridization ratio of DPL and Es on the PP matrix, as shown in Figure 3. The length, width, and thickness of the tensile specimens were 80, 15, and 4.3 mm, respectively. The speed of the crosshead was 2 mm·min⁻¹. In the case of the flexural test, the dimensions of the sample were 100 mm × 10 mm × 4.3 mm and the speed of the test was 5 mm·min⁻¹ over a support span of 70 mm.

Figure 3

Mechanical tests. (a) Flexural test and (b) tensile test.

2.4 Thermal stability

The thermal stability of Es filler, DPL filler, PP matrix, and hybrid composite (20DPL-10Es) was investigated using a thermogravimetric analyzer (NETZSCH TG 209 F1 Iris). The test was carried out by heating 25 mg of the samples at a rate of 10°C·min⁻¹ from 33 to 850°C in a nitrogen atmosphere.

2.5 SEM characterization

The morphology of the inner and outer surfaces of Es fragment, Es membrane, the surface of DPL fibers (treated and untreated) as well as their cross-section and the composite of 20DPL-10Es were analyzed by SEM. Before SEM characterization, a small layer of gold was applied to all samples to achieve satisfactory conductivity.

3 Results and discussion

3.1 Results of chemical treatment

The effect of chemical treatment with sodium hydroxide (SH) on tensile strength and modulus is shown in Figure 4. It shows that using a 4% concentration of SH for 10 h optimized both tensile strength (Figure 4(a)) and tensile modulus (Figure 4(b)). Such enhancement could be seen as pits formed as a result of the partial removal of the lignin layer, as shown by SEM in the morphological analysis section. Samples that were immersed in a solution of 8% SH for 10 and 20 h showed lower strength and modulus than those immersed in 4% SH. This is due to the long soaking period and concentration that caused some degradation of the fiber structure. However, they still have better values than the untreated fibers (0% SH–0 h) due to the removal of the lignin layer. Similar results were revealed for the kenaf fibers by Asumani et al. [47].

Figure 4

The influence of chemical treatment conditions on (a) tensile strength and (b) tensile modulus.

3.2 Tensile properties of DPL–Es composites

3.2.1 Tensile properties of 10 wt% of DPL–Es composites

Figure 5 demonstrates the effect of using 10 wt% of hybrid filler (Es and DPL) on the tensile properties. From Figure 5(a), it can be observed that adding 10 wt% of hybrid filler decreased the tensile strength by nearly 20%. It can also be demonstrated that the hybridization process of 10 wt% between DPL and Es did not result in a significant difference in tensile strength, whereas hybridization either by 7:3, 5:5, or 3:7 of DPL and Es shows the same value of tensile strength. Figure 5(b) shows that 10 wt% of hybrid filler improved the tensile modulus at all hybridization ratios. The 7DPL-3Es sample hits the highest modulus to reach 721 MPa, whereas the 5DPL–5Es sample hits the lowest value to reach 689 MPa, but it is still better than the neat PP. In other words, the differences in tensile modulus values due to hybridization are marginal. The 10 wt% of the hybrid reinforcement showed better tensile modulus when compared with separate Es [48] and rice husk [49].

Figure 5

Tensile properties of 10 wt% hybrid filler. (a) Tensile strength, (b) modulus of elasticity and (c) elongation at break.

3.2.2 Tensile properties of 20 wt% of DPL–Es composites

The influences of 20 wt% of hybrid filler on tensile properties are shown in Figure 6. It is noted that incorporating 20 wt% of hybrid filler has reduced the tensile strength from 23.8 to 17.3 MPa. It is worth mentioning that the hybridization of DPL and Es at this weight percentage has not affected the tensile strength value compared to the 10 wt% cases. It can also be demonstrated that the 20 wt% hybridization has raised the modulus of elasticity of the composites to reach 735 MPa in the 15DPL–5Es composite, which is suitable for structural applications.

Figure 6

Tensile properties of 20 wt% hybrid filler at different hybridization ratios.

3.2.3 Tensile properties of 30 wt% of DPL–Es composites

In addition to the effects of fiber loading (10 and 20 wt%) on tensile characteristics, the effect of 30 wt% has been explored, and the results are presented in Figure 7. It can be illustrated that the tensile strength decreases with increasing filler loading to 30 wt% (Figure 7(a)). However, the addition of 30 wt% hybrid filler increased the modulus for all hybridization ratios. It reached 838 MPa among all hybrid composites when 20 wt% DPL and 10 wt% Es were added, as shown in Figure 7(b). It can be noticed that increasing the wt% of filler loading would make the hybridization process more effective. For example, at 10 and 20 wt% hybrid filler, the range between the highest and lowest modulus values reached 4%. However, the modulus enhancement range reached 16% at 30 wt% hybrid filler. This indicates that increasing the filler wt% would result in better modulus properties.

Figure 7

Tensile properties of 30 wt% hybrid filler. (a) Tensile strength, (b) modulus of elasticity and (c) elongation at break.

3.2.4 Comparison between different weight percentages of hybrid fibers

Figure 8 depicts the changes in the tensile strength and modulus caused by the incorporation of different amounts (10, 20, and 30 wt%) of hybrid filler. The hybridization ratio that contains the most DPL by weight has a positive influence on the tensile modulus but has an adverse effect on tensile strength. Figure 8(a) shows that increasing the wt% of the hybrid filler from 10% in the 7DPL–3Es composite to 30% in the 20DPL–10Es composite () results in a slight decrease in the tensile strength from 19.1 to 16.8 MPa. This pattern is evident at all hybridization ratios, i.e., the tensile strength decreases as the wt% increases in general, regardless of the method of hybridization. In the case of the modulus of elasticity, it can be shown that increasing the amount of hybrid filler enhanced the modulus. For example, 10 wt% of the hybrid filler (7DPL–3Es) increased the modulus of PP by 9%. In comparison, as indicated in Figure 8(b), the increase owing to embedding PP by 30 wt% of the hybrid filler (20DPL–10Es) has reached 25%.

Figure 8

Tensile properties of single hybridization ratios of 10, 20, and 30 wt% of hybrid DPL–Es filler. (a) Tensile strength and (b) modulus of elasticity.

3.2.5 Stress–strain curves of the mechanical performance

Figure 9 shows the stress–strain behavior of the 7Es–3DPL composite. Each curve in the figure represents a replication of the tensile test. There are some variations in tensile properties between the three replications. This trend is common in polymer composites. This can be attributed to the mixing conditions in the single-screw extruder. The curve also reveals a step that occurred at the start of the test due to sample slippage between the jaws of the universal testing equipment.

Figure 9

Stress–strain curve of 10 wt% hybrid filler (7Es–3DPL).

3.3 Flexural properties of DPL–Es composites

3.3.1 Flexural properties of 10 wt% of DPL–Es composites

Figure 10(a) shows that adding 10 wt% of hybrid filler conserved the flexural strength of the PP matrix in the case of the 7DPL–3Es composite, reduced it in the 5DPL–5Es composite case, and enhanced it in the 3DPL–7Es composite. However, the differences caused by the hybridization process are negligible; the variations in the hybridization ratios have resulted in a difference in the flexural strength of 4.5%. It can be inferred that the hybrid fillers have improved the flexural modulus, as shown in Figure 10(b), at all filler weight percentages.

Figure 10

Flexural properties of 10 wt% DPL–Es. (a) Flexural strength and (b) flexural modulus.

3.3.2 Flexural properties of 20 wt% of DPL–Es composites

Figure 11(a) shows that the addition of hybrid fillers to the PP matrix resulted in a reduction in the flexural strength. It decreased from 27 MPa in the case of neat PP to 23 MPa in the 15DPL–5Es composite before gradually increasing from 23 MPa to reach its greatest value of 24.4 MPa in the 5DPL–15Es composite. Specifically, the percentage of increase within the same percentage (i.e., 20 wt%) with varied hybridization ratios does not exceed 6%, which is the same percentage of increase in flexural modulus, as shown in Figure 11(b).

Figure 11

Flexural properties of 20 wt% DPL–Es. (a) Flexural strength and (b) flexural modulus.

3.3.3 Flexural properties of 30 wt% of DPL–Es composites

As shown in Figure 12(a), it is evident that the 30 wt% hybrid filler has not added any improvement to the flexural strength. Conversely, it has worsened the results of flexural strength regardless of the method of hybridization. This means that the highest reduction in PP’s flexural strength has reached 12% due to the addition of hybrid fillers. Despite this, the hybrid filler has enhanced the flexural modulus, and the highest increase has been in the 15DPL–5Es composite at 735 MPa, as shown in Figure 12(b). As shown in Figures 10–12, it is found that the composite has a higher wt% of Es than DPL and has a higher flexural strength. In other words, 3DPL–7Es, 5DPL–15Es, and 10DPL–20Es composites have higher flexural strength than their competitors at each wt%. Also, it can be noted that the difference between the highest and the lowest value of flexural strength within the same percentage at different hybridization ratios increased steadily with increasing wt%.

Figure 12

Flexural properties of 30 wt% DPL–Es. (a) Flexural strength and (b) flexural modulus.

3.4 Thermogravimetric analysis (TGA)

3.4.1 TGA of DPL and Es fibers

A thermogravimetric (TG) thermogram of both DPL and Es fibers is shown in Figure 13. The initial mass loss of Es filler was 1.5%, and it occurred at a temperature of about 230°C. This mass loss can be attributable to the decomposition of the organic Es membrane. A similar result was obtained by Nnodu et al. [50]. By contrast, the principal mass loss started at a temperature of 640°C and ended at a temperature of 785°C. It is important to note that, as shown in Figure 13, the temperature cannot be measured after the mass loss has reached 50% because the decomposition process has already been completed. Table 4 displays the char residues of Es at 600°C. There are two peaks, as can be seen from the derivative thermogravimetric curve (DTG) depicted in Figure 14. The temperature at which the rate of deterioration of organic materials, such as the membrane, reaches its maximum is shown by the first peak (P1), which occurs at 330°C. However, the second peak (P2) at 762°C is attributed to the conversion of calcium carbonate to calcium oxide. A similar conclusion was recorded by Rohim et al. [51].

Figure 13

TG thermogram of DPL and Es fibers.

Table 4

Analysis of TG and DTG thermograms of DPL, Es, PP, and 20DPL–10Es composite

Sample	T _i (°C)	T _50% (°C)	T _f (°C)	T _max (°C)	Residue after 600°C (%)
Es	640	—	785	760	95
DPL	227	342	354	326	27
PP	321	403	430	410	∼0
20DPL–10Es	388	447	470	448	22.5

Figure 14

DTG thermogram of DPL and Es fibers.

In the case of DPL fibers, it can also be noted that there are three prominent phases of mass loss. According to the TG thermogram in Figure 13, the initial mass loss of 7.5% has occurred as a result of moisture content evaporation. This occurred at a temperature of <125°C. Comparatively, the second mass loss of DPL fiber corresponds to the decomposition of cellulose and hemicellulose. The second mass loss starts at an onset temperature of 227°C and completes at an end set temperature of 354°C. The DPL lost 54% of its mass at the end-set temperature. However, non-cellulosic materials started to decompose after ∼350°C. [52]. The DTG thermogram of DPL fibers, as shown in Figure 14, exhibits three peaks (P1, P2, and P3)and represents three different processes: water evaporation, cellulose degradation, and non-cellulose degradation. The analysis of peaks in the DTG curve, shown in Table 4, is used to determine the temperature at which the greatest rate of mass loss reaches the apex. According to the aforementioned results, it can be observed that DPL and Es fibers can withstand the processing temperature of 180°C of bare PP.

3.4.2 TGA of PP and the 20DPL–10Es composite

It can be observed from the TG curve in Figure 15 that the neat PP breakdown took place in a single stage of degradation. The temperatures at which it begins and ends are 321 and 430°C, respectively. The composite (20DPL–10Es) broke down into three steps: in the first step, degradation of the Es membrane and DPL fiber occurs; in the second step, PP degradation occurs; and in the third step, the conversion of Es (which contains mainly calcium carbonate) to calcium oxide takes place. These steps are clear in the DTG curve shown in Figure 16. In any case, the temperature at which the maximum rate of degradation occurs can be observed in the DTG curve. These temperatures are recorded in Table 4. Moreover, it can be noted from Figure 16 that the hybrid filler shifts the peak of the PP matrix from 410 to 480°C. In addition, it decreases the rate of degradation of PP by 21%. It can also be observed that the onset temperature (T _onset), the end-set temperature (T _endset), the temperature at which the mass loss is 50% (T ₅₀), the temperature at which the mass loss rate reaches the highest peak (T _max), and the char residue after 600°C of the composite (20DPL–10Es) are higher than those in the PP matrix. This indicates that the composite is much more thermally stable than its matrix.

Figure 15

TG thermogram of PP and 20DPL–10Es composite.

Figure 16

DTG thermogram of PP and 20DPL–10Es composite.

3.5 Morphological analysis

3.5.1 Characterization of treated and untreated DPL fibers

The effect of the 4% SH–10 h treatment on DPL fibers is shown in Figure 17. As shown in Figure 17(a), the DPL fibers that were not chemically treated exhibit a large number of impurities on their surface. Furthermore, the untreated fibers show a layer of lignin that is free of holes. Figure 17(b) demonstrates that the lignin layer has developed some holes as a result of the sodium hydroxide chemical treatment. The DPL surface becomes rougher as a result of some of the lignin layer being removed, which improves interlocking with the PP matrix. This confirms the improvement that had occurred in the tensile strength and modulus of treated DPL fibers.

Figure 17

SEM micrographs of the (a) untreated DPL, (b) treated DPL, and (c) cross-sectional view of untreated DPL.

Figure 18 shows the topography of the membrane-containing Es flakes. It is evident that Es has a complex structure on both its inner and outer surfaces, which is crucial to the bonding behavior. Figure 18(a) and (b) presents the outer surface of Es with and without the protective layer, where (a) shows the cuticle layer that covered the palisade layer (CaCO₃ layer) (b). However, the mammillary layer (the inner surface) covered with a network of proteins (the membrane) is displayed in Figure 18(c) and (d). The differences in the structure of Es lead to different adhesion behaviors with the PP matrix.

Figure 18

SEM micrographs of the (a) cuticle layer, (b) palisade layer, (c) mammillary layer, and (d) membrane.

3.5.2 Morphology characterization of the hybrid (20DPL–10Es) composite

The SEM micrographs of the cross-section of the composite after the tensile test are presented in Figure 19(a)–(d). Some voids in the PP matrix have arisen due to the agglomeration of DPL fibers. The agglomeration occurred due to poor mixing. Such a phenomenon prevents matrix–fiber contact, causing weak points in the composite and decreasing its mechanical properties. To get a better idea about the nature of adhesion between the fibers and the matrix, additional micrographs were taken from various locations of the composite. Figure 19(b) shows that there is good adhesion between the PP and DPL. Another point that indicates that DPL has good bonding is presented in Figure 19(a), where the DPL fiber has broken rather than pulled out. In the case of Es, Figure 19(c) shows that the bonding behavior of Es differs according to the type of surface. In other words, the adhesion between the inner surface containing the Es membrane and the PP is much better than the adhesion with the outer surface. Also, the detachment of Es from PP begins at the outer surface. Figure 19(d) depicts the bonding between the Es membrane at higher magnification.

Figure 19

SEM micrographs of the (a) 20DPL–10Es composite, (b) DPL–PP bonding, (c) Es–PP bonding, and (d) Es–PP bonding at higher magnification.

4 Conclusions

This work studied the impacts of numerous hybridization ratios of eggshell and DPL in reinforcing PP matrix. Three concentrations (10, 20, and 30 wt%) with nine hybridization ratios were utilized. Different concentrations of sodium hydroxide and different durations were found to improve the bonding between the fiber and matrix. It was found that 4 wt% of sodium hydroxide optimized both the tensile modulus and strength at 10 h treatment. Reinforcing PP with the considered hybrid reinforcements increased the tensile and flexural modulus properties for all weight percentages. The 20DPL–10Es composite was the best regarding tensile modulus. However, the considered hybridization has some negative effects on the tensile and flexural strengths. Furthermore, TGA and its derivatives have revealed that 20 wt% of DPL and 10 wt% of Es hybridization cases were capable of improving the thermal stability of PP by decreasing the rate of degradation by 21% and increasing the char residue at 600°C. By the morphological analysis, it was proven that DPL had good bonding in the composite, and the Es membrane had excellent adhesion with the PP.

Acknowledgments

The authors would like to thank the Deanship of Scientific Research at the Jordan University of Science and Technology for supporting this research.

Funding information: This work was supported by a grant from the Deanship of Scientific Research at the Jordan University of Science and Technology (JUST) with grant no. 448/2019.
Author contributions: Conceptualization, formal analysis, and investigation: Faris M. AL-Oqla and Mu’ayyad M. Al-Shrida; methodology, resources, validation, writing – original draft and writing – review, and editing: Faris M. AL-Oqla, Mohammed T. Hayajneh, and Mu’ayyad M. Al-Shrida; supervision: Faris M. AL-Oqla and Mohammed T. Hayajneh. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2023-01-23

Revised: 2023-07-13

Accepted: 2023-07-14

Published Online: 2023-08-04

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/rams-2023-0101

Keywords for this article

hybrid composite; bio-ceramics; biomaterials; eggshell; mechanical performance; natural resources

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