A detailed characterization of sandalwood-filled high-density polyethylene composites

Abstract

In this study, the performance of sandalwood (SW), as an efficient potential filler material for high-density polyethylene (HDPE), was investigated in detail. Firstly, the characterization of SW was conducted by the determination of chemical composition with chemical and thermal analysis methods. The distribution of SW particles, which were used in composite fabrication, was obtained by using a dynamic light scattering analyzer. Then, the composites of SW, whose weight fractions varied from 5% to 20%, with HDPE were produced in a high-speed thermokinetic mixer. The detailed characterization of composites was made by using thermogravimetric analysis, scanning electron microscopy, X-ray diffraction analysis, differential scanning calorimetry, dynamic mechanical analysis (DMA), Fourier transform infrared, thermal conductivity measurements, and tensile and three-point bending tests. From DMA, storage modulus and loss modulus values of the HDPE matrix increased with increasing the weight fraction of SW. It is clearly seen that SW incorporation into HDPE at weight fractions of 5% and 20% exhibited the best improvement in terms of tensile and flexural strengths, respectively. It can be noted that the reinforcement effect of SW for HDPE is more prominent at high temperatures.

Keywords

Compounding thermal properties polyethylene mechanical properties thermoplastic

Introduction

Arbutus is a genus of 11 accepted species of flowering plants in the family Ericaceae, native to warm temperate regions of the Mediterranean, Western Europe, and North America. Arbutus andrachne, commonly called the Greek strawberry tree, is an evergreen small tree or shrub.¹ It is native to the Mediterranean region. Moreover, it is distributed in the Middle East and Southwestern Asia. In Turkey, it is a common plant species in macchia vegetation, especially found numerously in the Aegean and Mediterranean coasts. Locally it is called as sandalwood (SW).²

Arbutus andrachne can reach height of about 10–12 m. But in dry habitat generally, it can be seen only at 3–5 m height. The smooth and bright reddish bark is one of the best characteristics for the identification of this tree. The flowers which bloom in early spring in Turkey are white or yellowish green in bell shape. Its fruits ripen in autumn. Fruits are in globular shape and dark reddish color, in 8–12 mm in diameter. They are bland and hard therefore it is not edible.² On the other hand, due to the fact that the wood of A. andrachne is of hard, durable, and water-resistant properties, the wood of A. andrachne is used in wooden boat building, furniture, and decoration. Besides, it is a good source for charcoal making.² Although SW has many different applications, it has not been used for the manufacturing of value-added products in industrial applications.

Community attentiveness is now being focused on the environmental friendly composite materials produced from natural fillers and polymeric materials.³ It is known that environmental friendly composite materials referred to as biocomposites are derived from one or more phases that derived from a biological origin.^3,4 Upon the incorporation of natural filler into polymers, the advantages such as reducing the consumption of nonrenewable sources, lower cost, reducing pollutant emission, biodegradability, increase in stiffness, and increase in stiffness came into existence.^3,5

Over the last decades, natural fillers have been utilized as a filling material for polymers in numerous studies. Especially wood fibers were compounded with thermosets⁶ or thermoplastics^7–12 to improve the properties of polymers. Natural fillers can stem from various resources, such as sisal, jute, coconut, flax, almond shells, and wood.^13–29

SW, one of the natural fillers, due to having hard and durable properties can be used as a reinforcing material for polyethylene, which has a low glass transition temperature and suits for freezer applications,³⁰ to maintain its stiffness at high temperatures. In the present research, the SW fillers, as a new filling material in polymer composites, were used to make a high-density polyethylene (HDPE)-based composite by using a high-speed thermokinetic mixer. The variation of storage modulus, which is often associated with the “stiffness” of a material,³¹ of HDPE with temperature was investigated. Moreover, the thermal, mechanical, crystallographic, chemical, and morphological properties of SW-filled HDPE composites were examined.

Materials and methods

Materials

In this study, HDPE with a density of 0.950 g cm⁻³ and a melt flow index of 0.4 g/10 min was used as a matrix material supplied from Petkim-SOCAR (Turkey). SW was supplied from Muğla Province, Datca district, Turkey, at 75 m above sea level.

Sample preparation

SW was oven-dried at 80°C for 1 h, and the samples were cut to 5 mm sections. The particles were ground by Retsch RS200 model vibratory disc mill and sieved through a 500 mesh pore size sieve. The average particle size was measured by a Malvern Mastersizer 2000. The particle size distribution of SW is shown in Figure 1. According to the particle size analysis, D(0.1), D(0.5), and D(0.9) values of SW were determined to be 5.06 µm, 28.78 µm, and 73.67 µm, respectively.

Figure 1.

Particle size distribution of SW.

Manufacturing of composite materials

Manufacturing of SW-filled HDPE composites was achieved by using a high-speed laboratory-type thermokinetic mixer (Gelimat Technology, USA). Then, 70 g of SW and HDPE mixture was processed for about 15–20 s at 2000 r min⁻¹ to obtain the compound in the form of dough. Composite plates were obtained from the dough compound by using Hydraulic Hot and Cold Press (Gülnar Machinery, Turkey). After heating the hot press up to processing temperature (180°C), produced compounds were placed into the mold cavity (15 × 15 cm²) between the metal plates of hot press. Samples were held under pressure of 40 bar at 180°C for 1 min in hot press. Following hot pressing, samples were held at 120 bar at room temperature and then for 2 min in a cold press.

Characterization methods

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was carried out by using Shimadzu TGA-50 series (Japan). TGA was performed by using 10 mg of the specimen, with a temperature rate of 10°C min⁻¹ until 600°C under nitrogen atmosphere.

DSC analysis of samples

Differential scanning calorimetry (DSC) analyses of the specimens were conducted with a heating rate of 10°C min⁻¹ in the temperature range of 25–180°C under nitrogen atmosphere using DSC Q20 differential scanning calorimeter (TA Instruments-USA).

XRD analysis

X-ray diffraction (XRD) analysis was performed by X’Pert Pro Panalytical XRD system, UK with cobalt K_α radiation under 30 kV and 40 mA. Composites samples were cut into 10 × 10 × 2 mm² dimension pieces and scanned 2θ values from 10° to 110°. All results were given between 2θ values of 10–35°.

SEM analysis

The fracture surfaces of the SW-filled HDPE composites were observed using a scanning electron microscope (FEI Quanta FEG 250, USA) operated at 2 kV after tensile testing. Before scanning electron microscopy (SEM) observation, the fracture surfaces of the composite samples were coated with a thin film of gold by means of a plasma sputtering apparatus.

Thermal conductivity measurements

Thermal conductivities of HDPE and its composites were measured by C-Therm. Tci. Thermal Conductivity Analyzer, Canada at 25°C.

Dynamic mechanical analysis

Dynamic mechanical properties of HDPE and its composites were analyzed using a dynamic mechanical analyzer DMA Q800 (TA Instruments) in air atmosphere. Tests were performed using a single cantilever in multi frequency strain modulus mode. The temperature was swept from 30°C to 130°C. The specimen dimension was 13 × 6 × 2 mm³. The average of three replicates was used for each specimen.

Tensile testing

The tensile properties of HDPE and its composites were tested using a Shimadzu Autograph AG-IS Series universal testing machine (Japan) with a 5-kN load cell. Tensile tests were performed at a crosshead speed of 5 mm min⁻¹ at room temperature according to the ASTM D638 standard. The average value of at least five sample measurements of each composite was taken into consideration.

Three point bending test

Flexural properties of HDPE and its composites were determined by the ASTM D-790 standard at a crosshead speed of 1 mm min⁻¹ and span length of 32 mm. The tests were performed using a Shimadzu Autograph AG-IS Series universal testing machine with a 5-kN load cell at room temperature. Five tests were made for each specimen, and the average values of flexural strength and modulus were calculated.

Results and discussions

Chemical composition analysis of SW

Major four components of wood are extractive materials, hemicellulose, lignin, and cellulose.³² The chemical composition of these four major materials could be found out in various methods. In this study, two different methods were used for identification of the chemical composition of SW.

The first one is the chemical method which is long-lasting but accurate to determine wood extractive materials, hemicellulose, lignin, and cellulose contents.³³ The second one is a simultaneous thermal analysis (STA) method for the mixture of lignin and cellulose contents in a blend.³⁴ When compared to the chemical method, STA is a fast and easy method for determining extractive material, lignin, and cellulose contents of wood products. Two different methods were used to make a cross-check between results and compare the productivity of methods.

The extractive material content of SW particles (12.051 g) was extracted with 150 mL acetone under reflux system for 3 h and then the mixture was filtered. The residue was dried at 80°C for 1 h before weighting. The remaining part was allowed to cool at room temperature and weighted. Percentage of extractive material was calculated by the following formula:

Extractive materials (%) = \frac{Initial mass of SW - final mass of SW}{Initial mass of SW} \times 100

Hemicellulose content of SW was removed by alkali medium process. Then, 150 mL of 0.5 M sodium hydroxide solution was added to 6.011 g of extractive material free SW and refluxed for 3.5 h. After the extraction process, the mixture was filtered, and the residue was washed with distilled water for removal of sodium salts from residue. Then it was dried at 105°C for 3 h and weighted at room temperature. Percentage of hemicellulose was calculated by the following formula:

Hemicellulose (%) = \frac{Initial mass of extracted SW - Final mass of extracted SW}{Initial mass of extracted SW} \times 100

Lignin content of SW was determined by using 1.10 g extractive material free SW. 30 mL of 75 wt% sulfuric acid was added, and the mixture was kept for 24 h at room temperature. Then, 300 mL of distilled water was added to the solution, and it was refluxed for 1 h. The residue was picked up and washed with distilled water for the removal of sulfide salts and ions. Drained lignin was dried under 105°C for 1 h and weighted after cooling at room temperature. Percentage of lignin was calculated by the following formula:

Lignin (%) = \frac{Mass of lignin}{Initial extracted mass of SW} \times 100

Cellulose content of SW was calculated from the following formula:

100 % = Extractive material (%) + Hemicellulose (%) + Lignin (%) + Cellulose (%)

STA of SW was made by TGA/DSC3+ Mettler Toledo instrument, Germany between 40°C and 600°C under nitrogen atmosphere with a gas flow rate of 20 mL min⁻¹ at a heating rate of 10°C min⁻¹. From 600°C to 1000°C, the air atmosphere was used for the removal of carbon residue. Three different temperature intervals were investigated for the determination of extractive material, lignin, and cellulose contents.³⁴ Extractive materials were removed from the sample between 40°C and 120°C. The temperature interval was between 280°C and 500°C, which was the thermal decomposition interval of lignin. From 340°C to 370°C was the last temperature interval which was considered as a cellulose decomposition region. The results of the chemical method and thermal analysis are summarized in Table 1.

Table 1.

The chemical composition analysis results of SW.

	Extracted material (%)	Hemicellulose (%)	Lignin (%)	Cellulose (%)
Chemical analysis	6.08	26.00	26.55	40.82
STA	5.10	NC	26.30	47.30

NC: not calculated; SW: sandalwood; STA: simultaneous thermal analysis.

As can be seen from Table 1, the lignin contents of SW are very similar for chemical and thermal analysis methods. However, cellulose contents of SW are not similar for both methods. It can be said that thermal analysis method is not sufficient for the determination of hemicellulose and extractive material contents of SW, however, it could give rough values for analyses of cellulose and lignin contents.

Thermogravimetric analysis

Table 2 presents the TGA data of SW-filled HDPE composites. As can be seen from Table 2, SW filling into HDPE did not lead to considerable variation in the degradation behavior of the HDPE matrix. The mass loss for HDPE is about 96% in the temperature range of 459–502°C. SW started to degrade at 308°C and finished the degradation process at 362°C. Between these temperatures, its mass loss was found as 59%. When the mass losses of the samples were evaluated, it was seen that while SW content was increased, mass loss decreased as expected. Besides, onset, endset, and maximum degradation temperatures increased when SW content of the composites was increased. Although the onset temperature of SW is less than the HDPE matrix, SW filling into HDPE matrix increased onset and maximum degradation temperatures of HDPE. A similar result was obtained for sawdust/polypropylene composites by Fakhrul et al.³⁵ It can be probably explained by the fact that filling of SW into the HDPE matrix reduced mass loss by volatilization. As can be seen from Table 2, there is an increase in the residues of composites with increasing weight fraction of SW, which may be ascribed to that SW involves nondecomposable minerals and metallic oxides.³⁵

Table 2.

TGA data of HDPE and SW-filled HDPE composites.

Samples	Maximum degradation temperature (°C)	Onset (°C)	Endset (°C)	Mass loss (up to 600°C) (%)
HDPE	480.7	459.3	502.2	95.9
SW	330.7	308.2	362.0	58.9
HDPE + 5% SW	485.9	468.5	503.1	92.1
HDPE + 10% SW	487.1	470.6	504.1	87.8
HDPE + 15% SW	488.8	473.2	505.3	84.3
HDPE + 20% SW	488.9	474.1	506.6	79.1

TGA: thermogravimetric analysis; HDPE: high-density polyethylene; SW: sandalwood.

DSC analysis

DSC curves of HDPE and its composites are presented in Figure 2, and the respective data are summarized in Table 3. Melt peak temperature (T_m) and crystallization temperature (T_c) of HDPE were obtained to be 136.5 and 199°C, respectively. With the incorporation of 20 wt% SW into HDPE matrix, while T_m value decreases from 136.5°C to 133.8°C, T_c increases from 115.3°C to 119.7°C, respectively. However, both melting enthalpy and crystallization enthalpy values decreased with increasing SW weight fraction. Percent crystallinities (X_c) of HDPE and its composites were calculated by taking 293 J g⁻¹ for the 100% crystalline material as melting enthalpy into consideration with the following equation.³⁶

Figure 2.

DSC curves of HDPE and SW-filled composites.

Table 3.

DSC data of HDPE and SW-filled HDPE composites.

Sample	T_m (°C)	Melting enthalpy (J g⁻¹)	T_c (°C)	Crystallization enthalpy (J g⁻¹)	Crystallinity (%)
HDPE	136.5	205	115.3	199	70
HDPE + 5% SW	136.0	197	117.8	189	71
HDPE + 10% SW	135.0	179	118.9	179	68
HDPE + 15% SW	134.1	168	119.6	168	67
HDPE + 20% SW	133.8	164	119.7	161	70

DSC: differential scanning calorimetry; HDPE: high-density polyethylene; SW: sandalwood; T_m: melt peak temperature; T_c: crystallization temperature.

X_{c} (%) = (\frac{Δ H_{m}}{w Δ H_{100}}) \times 100

where $Δ H_{m}$ is the melting enthalpy of the test sample, $Δ H_{100}$ is the melting enthalpy for 100% crystalline PE, and w is the weight fraction of PE.³⁷

As can be seen from Table 3, SW loading into HDPE matrix has not led to a significant variation in the crystallinity (%) of HDPE. This may be due to the fact that SW, which is able to absorb water molecules and is in direct contact with HDPE, leads to microstructural changes decreasing the crystallinity of HDPE.³⁸

XRD analysis

Figure 3 shows the XRD patterns of HDPE and its composites. As can be seen from Figure 3, the crystallinity of HDPE was reduced with increasing the weight fraction of SW into the HDPE matrix. Besides, the intensity of peak at 21.5° belonging to cellulose of SW decreased by increasing the weight fraction of SW into the HDPE matrix. This can be explained by the fact that the mixing process of SW and HDPE is very effective, which leads to the isolation of crystalline cellulose in SW. Besides, with the incorporation of SW into the HDPE matrix, XRD patterns of SW and HDPE have been superposed. In the literature, the crystallinity of different matrix polymers decreased with increasing the concentration of cellulosic content.^39,40 The amorphous phase of the polymer matrix increased when the weight fraction of SW was increased since the intensities of HDPE crystalline peaks, which were seen at 25.1° and 27.9°, reduced and disappeared, respectively.⁴¹ Cellulose has very known peaks (110) and (200) which were seen at 2θ degrees of 16.1° and 19.6°.⁴² Similarly, the peaks of cellulose were noticeable at about 16.3° and 19.6° in our study, which confirms the presence of SW into HDPE composites.

Figure 3.

XRD patterns of HDPE and SW-filled HDPE composites.

SEM analysis

SEM micrographs of fractured specimens of HDPE + 5% SW, HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites are shown in Figure 4(a) to (h). The difference in the mechanical performances of HDPE + SW composites could be corroborated by morphological evidences. The micrographs clearly indicate significant differences in the particle size and distribution of the HDPE-based composites. As shown in the SEM images of HDPE + 5% SW, in Figure 4(a) and (e), it is seen that SW particles pulled out from the matrix, which indicates that the interface could not effectively transfer the stress. Besides, elongated matrix segments in Figure 4(a) and (e) can be visible. In the case of higher SW loading (Figure 4(b) and (h)), particles appear in microsize, less than about 360 µm. Moreover, elongated matrix segments cannot be seen clearly. There is no homogeneity in particle distribution. It is probable that in the HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites there are large numbers of particle pullouts. These indicate poor interfacial adhesion and inadequate wetting of the SW particles within the HDPE matrix. Micron-sized SW particles probably came into larger-sized particles due to the agglomeration of SW particles. These agglomerates and pullouts act as a stress concentrator and led to a decrease in tensile properties of HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites compared with that of HDPE + 5% SW composite.

Figure 4.

SEM micrographs of (a and e) HDPE + 5% SW, (b and f) HDPE + 10% SW, (c and g) HDPE + 15% SW, and (d and h) HDPE + 20% SW composites.

Thermal conductivity measurements

Table 4 presents the thermal conductivity and effusivity values of HDPE and its composites. The thermal conductivity of HDPE was measured as 0.561 W mK⁻¹. The thermal conductivities of HDPE + 5% SW, HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites were determined to be 0.522, 0.509, 0.503, and 0.501 W mK⁻¹, respectively. It was concluded that if the amount of SW filler increases, the thermal conductivity of composites decreases. The huge difference was firstly observed between 20% SW-filled HDPE composite and HDPE. The reason for this remarkable difference is explained by the fact that at high filler loadings, the fillers separated by polymer matrix act as a thermal barrier and become rate limiting in the thermal conduction pathway.⁴³ One can infer that SW is not a heat conductive filler as was expected. In addition to these, the reduction in thermal conductivity values may also be attributed to poor interaction between the filler and the polymer matrix. It is known that when the filler–matrix interaction becomes poor, the degree of agglomeration increases.⁴⁴ Besides, there is a study in the literature which concludes that agglomeration increases with increasing the amount of filler.⁴⁵ In this study, it was seen from SEM images that beyond the filler content of 10 wt% SW, there is an agglomeration, especially in the structure of 15 wt% and 20 wt% SW-filled composites, for which reason the filler and polymer interaction decreases. It was concluded that it creates resistance in the interface and leads to a reduction in thermal conductivity values. In addition to these results, it was observed that after SW incorporation into the HDPE matrix, the thermal effusivity value of HDPE decreased as well. It can be said that the composites of HDPE absorb less thermal energy than HDPE.

Table 4.

Thermal conductivity and effusivity values of HDPE and SW-filled HDPE composites.

Samples	Thermal conductivity (W mK⁻¹)	Thermal effusivity (Ws^1/2 m⁻² K⁻¹)
HDPE	0.561	957.5
HDPE + 5% SW	0.522	918.4
HDPE + 10% SW	0.509	903.8
HDPE + 15% SW	0.503	897.2
HDPE + 20% SW	0.501	895.9

HDPE: high-density polyethylene; SW: sandalwood.

Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) was performed between the temperatures of 30°C and 130°C to evaluate the effect of SW content on the viscoelastic behavior of the HDPE matrix. The incorporation of SW particles into the HDPE matrix significantly increased the storage modulus of the HDPE matrix. This results from the stiffness of SW particles. The storage modulus of HDPE matrix increased when SW content was increased from 5 wt% to 20 wt%, as was observed from Table 5 and Figure 5. The storage modulus values of the HDPE matrix and SW-filled composites were examined and given in Table 5 at selected temperatures. The storage modulus values at 30°C for HDPE, HDPE + 5% SW, HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW were obtained to be 1661, 1677, 1696, 1771, and 2025 MPa, respectively. Then, 20 wt% of SW incorporation into HDPE has led to a 22% increase in storage modulus at 30°C, which could be due to the ability of SW to exhibit a reinforcing effect. Besides, the storage modulus values of HDPE, HDPE + 5% SW, HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW at 120°C are determined to be 63, 158, 176, 208, and 263 MPa, respectively. Next, 20 wt% of SW incorporation into HDPE resulted in an increase in storage modulus at 120°C more than 300%, which is far higher than the increase at 30°C. From these results, it can be noted that the reinforcement effect of SW for HDPE is more prominent at high temperatures. Considering the fact that upon the incorporation of SW, the crystallinity of HDPE has not changed considerably, the increases in storage modulus can be explained by acting SW as a barrier in the polymer chain movement.⁴⁶ It was also observed that the storage modulus values of HDPE and its composites decreased as the temperature was increased. Similarly, this decreasing trend in the storage modulus over the entire temperature range was also found in the study made by Ou et al.⁴⁷

Figure 5.

The variation of storage modulus of HDPE and SW filled HDPE composites with temperature.

Table 5.

Storage modulus values of HDPE and SW-filled HDPE composites at different temperatures.

Samples	Storage modulus (MPa)
Samples	30°C	60°C	90°C	120°C
HDPE	1661	957	390	63
HDPE + 5% SW	1677	1027	471	158
HDPE + 10% SW	1696	1063	502	176
HDPE + 15% SW	1771	1145	562	208
HDPE + 20% SW	2025	1342	679	263

HDPE: high-density polyethylene; SW: sandalwood.

Figure 6 shows the variation of the loss modulus values of HDPE and SW filled HDPE composites with temperature. As can be seen from Figure 6, loss modulus of HDPE is less than those of composites. Incorporation of 5 wt% SW into the HDPE matrix made an increase in loss modulus value which is approximately the same with the value of 10 wt% SW-filled HDPE composite up to a temperature of 55°C. Beyond this temperature, 10 wt% SW-filled HDPE composite has a higher loss modulus than 5 wt% SW-filled HDPE composite. At around 55°C, the highest loss modulus belongs to 20 wt% SW-filled HDPE composite, followed by 15 wt% filled HDPE composite. At higher temperatures, it was observed that the loss modulus values of composites increased with an increase in the weight fraction of SW. The peak height of loss modulus increases with the weight fraction of SW. This may be owing to the inhibition of the relaxation process with the addition of SW into the HDPE matrix.⁴⁸

Figure 6.

The variation of loss modulus of HDPE and SW-filled HDPE composites.

As was presented in Figure 6, the loss modulus reaches a maximum value and started to decrease beyond this maximum value, which originates from the free movement of polymer chains at higher temperatures.^49,50 As shown in Figure 6, α relaxation peaks of HDPE, HDPE + 5% SW, HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites were observed at about 52.5, 52.7, 52.6, 53.0, and 54.9°C, respectively. It can be noted that α relaxation peak of HDPE was enhanced slightly by increasing the content of SW in the HDPE matrix. This may be due to large viscous dissipation of SW-filled HDPE composites than that of the HDPE matrix.

Mechanical tests

Tensile test

The effects of the weight fraction of SW on the tensile properties of the HDPE matrix are given in Table 6. As provided in Table 6, tensile strength and Young’s modulus values of HDPE were obtained to be 21.6 and 744.4 MPa, respectively. The tensile strength of the HDPE matrix is increased with the addition of 5 wt% SW. The tensile strength of HDPE + 5% SW composite is 23.8 MPa, which is 10% higher than that of the HDPE matrix. The good dispersion of SW filler into the HDPE matrix may be the main reason for better tensile strength. However, the tensile strength of the HDPE matrix decreased with the increase in SW content in the range of 5–20 wt%. When 20 wt% SW was filled into the HDPE matrix, the tensile strength of HDPE matrix decreased from 23.8 MPa to19.4 MPa. The decrement is about 10% for tensile strength of HDPE + 20% SW composite compared to that of HDPE. The decrements in tensile strength may result from the poor interfacial adhesion between hydrophobic HDPE matrix and hydrophilic SW filler. Thus it prevents efficient stress transfer from the HDPE matrix to SW particles.³⁵ Moreover, from SEM images, an agglomeration can be noticed in the fracture surfaces of HDPE + 10% SW, HDPE + 15% SW, and HDPE + 20% SW composites. This also reveals the decrements in the tensile strength of the composites.

Table 6.

Tensile properties of HDPE and SW-filled HDPE composites.

Sample name	Tensile strength (MPa)	Young’s modulus (MPa)
HDPE	21.6 ± 0.5	744.4 ± 83.6
HDPE + 5% SW	23.8 ± 0.5	1040.6 ± 16.0
HDPE + 10% SW	22.1 ± 0.3	1132.9 ± 35.5
HDPE + 15% SW	21.1 ± 0.3	1189.9 ± 57.0
HDPE + 20% SW	19.4 ± 0.2	1282.0 ± 67.3

HDPE: high-density polyethylene; SW: sandalwood.

As can be seen from Table 6, Young’s modulus of the HDPE matrix increases with the addition of SW fillers in the studied concentration range (5–20 wt%). Filler loading affects the stiffness properties of the matrix material. The improvement in Young’s modulus of the HDPE matrix may be due to the higher modulus of the SW fillers. When 20 wt% SW was filled, Young’s modulus was increased by about 72%. It is well-known that the addition of rigid filler is anticipated to improve Young’s modulus of the composites and gives a greater stiffness to the composites. Jamil et al. showed that the addition of the filler increases the modulus of composites. This results from the inclusion of rigid fillers into the matrix.⁵¹ The addition of SW particles, which can be considered as a rigid filler, into softer HDPE matrix increases the stiffness of the HDPE matrix and decreases the mobility of HDPE chains.

Flexural test

The flexural strength and modulus values of the HDPE matrix and SW-filled HDPE composites at different filler contents are listed in Table 7. Flexural strength and modulus values of the HDPE matrix were obtained to be 25.7 and 759.1 MPa, respectively. Flexural strength and modulus values of the composites increased with increasing SW filler content up to 20 wt%. The flexural strength and modulus values for the HDPE matrix were increased from 25.7 MPa and 759.1 MPa to 33.00 MPa and 1382.6 MPa for HDPE + 20% SW composite, respectively. It is clearly seen that among SW-filled HDPE composites, 20 wt% SW-filled HDPE composite showed the best improvement in terms of flexural strength and modulus.

Table 7.

Flexural properties of HDPE and SW-filled HDPE composites.

Sample name	Flexural strength	Flexural modulus
HDPE	25.7 ± 2.6	759.1 ± 58.4
HDPE + 5% SW	30.7 ± 0.6	888.8 ± 17.9
HDPE + 10% SW	32.1 ± 2.6	1019.3 ± 68.1
HDPE + 15% SW	32.7 ± 0.2	1185.3 ± 43.0
HDPE + 20% SW	33.0 ± 0.8	1382.6 ± 53.5

HDPE: high-density polyethylene; SW: sandalwood.

Conclusion

SW filler, which has a particle size (D0.5) of 28.78 µm and whose composition was determined by chemical and thermal analysis methods in this study, addition into HDPE matrix has changed the thermal conductivity, thermal stability, viscoelastic, crystallographic, and mechanical properties of the HDPE matrix. SW filling into the HDPE matrix increased the onset and maximum degradation temperatures of the HDPE matrix. Then, 20 wt% SW addition into HDPE matrix decreased the thermal conductivity of the HDPE matrix by 11.5%. From DMA, storage modulus and loss modulus values of the HDPE matrix increased as the weight fraction of SW filler was increased from 5 wt% to 20 wt%. Thus, it can be reported that the reinforcement effect of SW for HDPE is more prominent at high temperatures. The tensile strength of HDPE + 5% SW composite was determined to be 23.8 MPa, which indicates an increase of 10% compared to HDPE. It can be noted that flexural strength and modulus values of the composites increased by 28% and 82% with the addition of 20 wt% SW into the HDPE matrix. From the conducted study, it can be reported that SW can be used as an alternative filler material for the HDPE matrix material to maintain its stiffness at high temperatures.

Footnotes

Acknowledgements

The authors would like to acknowledge İzmir Eğitim Sağlık Sanayi Yatırım A.Ş. for their support in DSC analyses.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Yoldaş Seki

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