Heat-Treated Wood Reinforced High Density Polyethylene Composites Kompoziti visoke gusto ć e na bazi polietilena oja č ani toplinski modificiranim drvom

• This study investigated the effect of untreated and heat-treated ash and black pine wood ﬂ our concentrations on the selected properties of high density polyethylene (HDPE) composites. HDPE and wood ﬂ our were used as thermoplastic matrix and ﬁ ller, respectively. The blends of HDPE and wood ﬂ our were compounded using single screw extruder and test samples were prepared through injection molding. Mechanical properties like tensile strength (TS), tensile modulus (TM), elongation at break (EatB), ﬂ exural strength (FS), ﬂ exural modulus (FM) and impact strength (IS) of manufactured composites were determined. Wood ﬂ our concentrations have signi ﬁ cantly increased density, FS, TM and FM and hardness of composites while reducing TS, EatB and IS. Heat-treated ash and black pine ﬂ our reinforced HDPE composites had higher mechanical properties than untreated ones. Composites showed two main decomposition peaks; one coming from ash wood ﬂ our (353-370 °C) and black pine wood ﬂ our (373-376 °C), the second one from HDPE degradation (469-490 °C). SEM images showed improved dispersion of heat-treated ash and black pine wood ﬂ our. The obtained results showed that both the untreated and heat-treated ash/black pine wood ﬂ our have an important potential in the manufacture of HDPE composites.

The experimental design of the study is presented in Table 1. During the manufacturing process, depending on the formulation, the high density polyethylene (HDPE), untreated and heat-treated ash and black pine wood fl ours, as fi llers, were mixed in a high intensity mixer to produce homogeneous blend. The blends were compounded in a laboratory scale single screw extruder (TTB, Tecnomatic Inc, Turkey) at 40 rpm screw speed. Temperatures were set at 180 °C from feed zone to die zone. Extruded samples were collected, cooled and granulated into pellets. The pellets were then oven-dried at 80 °C for 24 h and stored in sealed plastic bags for injection molding. The pellets were injection molded into tensile, fl exural test samples using an HDX-88 injection molding machine at a barrel temperature of between 180 °C and 200 °C (injection pressure: 100 bar, injection speed: 80 mm/ sec., screw speed: 40 rpm, cooling time about 30 s).

Gustoća
Density of the manufactured polymer composites were determined using water displacement technique and analyzed utilizing central composite design (CCD) (ASTM D792-13).

Mechanical properties 2.4. Mehanička svojstva
To evaluate the effect of heat-treated and untreated wood fl ours on the mechanical, thermal and morphological properties of HDPE composites, testing of

UVOD
Polymer composites can be manufactured using polymer matrix such as polyethylene, polystyrene, polypropylene and polyvinyl chloride and organic fi ller (wood fi ber, wood fl our and agricultural residues) or inorganic fi ller (talc, mica, calcium carbonate). Recently, the use of organic fi llers has been increased due to many advantages such as low cost, low density, high specifi c properties, non-abrasive nature, renewability, biodegradability and availability. Therefore, several studies were conducted to manufacture polymer composites using organic fi llers including wood fl our, wheat straw, nutshell fl our, sunfl ower stalk, fl ax, jute, sisal, bagasse, ramie and kapok (Yang et al., 2005;Mengeloglu and Karakus, 2008;Kaymakci et al., 2013;Donmez Cavdar et al., 2014;Aydemir et al., 2015). Some of the studies focused on industrial products and they have been applied in industrial fi elds such as outdoor furniture, automobile parts, structural panels, etc. Wood, as organic fi ller, is the most feasible material to produce the polymer composites. However, because of the moisture absorption in wood, as all organic fi llers, it suffers a number of disadvantages. Poor resistance against fungal and insect attack, swelling, and shrinkage resulting from water absorption and desorption are some of these shortcomings. Many studies have been carried out to improve the unfavorable properties of wood (Kaboorani et al., 2008;Arwinfar et al., 2016). These include chemical and thermal modifi cations. Chemical modifi cation of wood itself can be done, for example, by acetylation with acetic anhydride (Cetin and Ozmen, 2011;Ozmen et al., 2013), acetyl chloride, or isopropenyl acetate, which are usually coated on the surface of wood fi bers. Others are chemical modifi cations such as surface treatments, corona or plasma discharge, and enzymatic treatment (Follrich et al., 2010;Aragal et al., 2012). The chemical modifi cations only provide an improvement on the surface of materials used and their outdoor performances are generally not good enough in application areas. Heat treatment of wood, called thermal modifi cation, has been reported to be an effective method to provide a sustainable improvement of the physical properties such as dimensional stability and/or durability of wood. Many researchers have used heat treatment process to improve wood properties (Yildiz et al., 2006;Shi et al., 2007;Gunduz and Aydemir, 2009;Kabir et al., 2012;Segerholm, 2012;Li et al., 2013;Boruvka et al., 2015). Heat treatment of wood reduces hydrophilicity of wood. In addition, heat treatment modifi es the polar nature of wood possibly resulting in better compatibility between wood and the polymer matrix, thus leading to high quality and thermally stable composites. The changes in wood chemistry can be utilized to improve compatibility between wood and the polymer matrix (Aragal et al., 2012). Some studies focused on the use of the heat-treated wood in the production of polymer composites (Aydemir et  ..... Karakuş, Aydemir, Gunduz, Mengeloğlu: Heat-Treated Wood Reinforced High Density... the fl exural, tensile and notched impact properties were conducted in a climate-controlled testing laboratory. Flexural properties such as fl exure strength (FS) and fl exure modulus (FM) were determined in accordance with ASTM D 790. The fl exural specimens (4×15×160 mm) with 80 mm span length were tested in the three points loading with a crosshead speed of 2 mm/min on a Zwick Roell Z010 Universal testing machine. The same instrument was also used for the tensile testing. Tensile properties, such as tensile strength (TS) and tensile modulus (TM), were conducted according to ASTM D 638. Samples (4 mm × 18.6 mm × 165 mm) were tested at a crosshead speed of 5 mm/min. The tensile modulus of the samples was taken as the slope of the curve at stress levels between 0.05 % and 0.2 %. Notched impact tests (sample dimension was 4 mm × 15 mm × 50 mm) were performed according to ASTM D 256. The notches were added using a Polytest notching cutter by RayRan and notched samples were tested on a HIT5.5P impact testing machine, manufactured by Zwick. Six specimens prepared according to the applicable standards were used in all mechanical testing.

Tvrdoća
Inspired by the Shore-D method, the hardness property of specimens was tested according to ASTM D 2240. Six specimens with dimension of 50 mm × 13 mm × 5 mm were tested for each composite formulation. Thermogravimetric analysis (TGA) measurements were carried out using Shimadzu TGA-50 on samples of about 10 mg. Each sample was scanned over a temperature range from room temperature to 700 °C at a heating rate of 10 °C/min under nitrogen with a fl ow rate of 20 ml/min to avoid sample oxidation. Three samples randomly picked from the ground test specimens were used.

Scanning electron microscope (SEM) analysis 2.7. Pretražna elektronska mikroskopija (SEM)
The fractured surface of the samples was also studied by using Tescan MAIA3 XMU scanning electron microscope. All SEM characterization was conducted on the fractured section of the tensile test samples. The samples were fi rst dipped into liquid nitrogen and snapped to half to prepare the fractured surfaces.

Rendgenska difrakcija (XRD)
XRD was performed with a high resolution Xray diffractometer (Model XPert PRO, Philips PANalytical, Netherlands) with Ni-fi ltered Cu Ka (1.540562 Å) radiation source operated at 45 kV voltage and 40 mA electric current. The samples were scanned from 5° to 40° 2θ range with a step of 0.02° and a step time of 2.5 s. A silicon zero-background plate was used to make sure there was no peak associated with the sample holder. The same sample holder and the same position of the holder were used for all tests. The crystallinity index (CI) of the powdered samples was calculated as the ratio of the total area under the resolved crystalline peaks to the total area under the unresolved X-ray scattering curve (Rabiej, 2003). Three specimens for each test were scanned with XRD. CI values were found using Eq. 1: Where A c is the integrated area underneath the respective crystalline peaks, and A a is the integrated area of the amorphous halo.  obtained samples were evaluated. All samples were analyzed with the one-way variance analysis (ANO-VA), and then Duncan test was applied to determine whether the samples differed signifi cantly among the groups. All statistical analysis was conducted at 99 % signifi cance level (p < 0.01).

RESULTS AND DISCUSSION
3. REZULTATI I RASPRAVA 3.1 Densities, hardness (Shore D) and mechanical properties 3.1. Gustoća, tvrdoća (Shore D) i mehanička svojstva Densities in the range of 0.87-0.93 g/cm 3 were measured based on heat-treated/untreated ash wood fl our concentrations in polymer composites. Density variations in HDPE composites are presented in Table  2. Density of polymer composites manufactured with the addition of 10 % ash wood fl our was decreased. Density of the ash wood fl our reinforced HDPE composites was increased with the other wood concentrations. However, the increases in the density of the composites were found to be not statistically signifi cant and their densities exhibited similar results.
Densities in the range of 0.87-0.93 g/cm 3 were measured based on heat-treated/untreated black pine wood concentrations in polymer composites. The densities of black pine wood reinforced HDPE composites, similar to density of ash wood fl our reinforced HDPE composites, were fi rstly decreased with the addition of 10 % black pine wood but then were increased with the addition of other wood concentrations. Compared to heat-treated and untreated black pine wood, densities of the composites were found to have similar results. Wood fl our is a compressible material and the density of the wood cell wall is about 1.44-1.50 g/cm 3 (Kellog, 1981). The porous anatomy of the solid wood results in overall densities of about 0.32-0.72 g/cm 3 , when dry (Simpson and TenWolde, 1999). However, the high pressures found during plastics processing can collapse the hollow fi bers that comprise the wood fl our or fi ll them with the molecular weight additives and polymers. Consequently, adding wood fi bers to commodity plastics increases their density (Simpson and TenWolde, 1999). Increased density of the thermoplastic composites with lignocellulosic fi ller was also reported by Rosa et al. (2009).
The effect of both heat-treated/untreated wood and wood concentration on the hardness (Shore D) of wood reinforced HDPE composites is shown in Table  2. The results showed that hardness of polymer composites manufactured with the addition of 10 % heattreated/untreated ash wood fl our was decreased but the addition of other wood concentration did not provide a signifi cant change in the hardness of HDPE composites. It is believed that hardness properties of wood reinforced HDPE composites decreased due to density of 0 ..... Karakuş, Aydemir, Gunduz, Mengeloğlu: Heat-Treated Wood Reinforced High Density... polymer composites manufactured with the addition of 10 % wood fl our. The lowest and highest hardness was determined as about 63 for the UAC1, TAC1, UBPC1 and TBPC1 and as 67 for the TBPC3, respectively. The hardness of HDPE composites showed that the fi ller concentration has a more important impact than heat treatment. Heat-treated ash and black pine wood fl our reinforced HDPE composites had a positive effect on hardness. Consequently, the heat treatment did not provide any improvement to the hardness with the fi ller concentration of 10 % and 30 %. Similar results for wood reinforced polymer composites were also observed by Hua  The obtained data for the TS of neat HDPE and the composites are presented in Table 2. Tensile strength (TS) of HDPE composites manufactured using untreated ash wood fl our was 20 MPa, 20 MPa, 19 MPa and 18 MPa for 0 %, 10 %, 20 % and 30 % wood concentration, respectively. Adding untreated ash did not provide a signifi cant increase in the tensile strength of HDPE composites and the tensile strength of HDPE composites was reduced by untreated ash wood fl our concentration (p<0.01). This reduction might be caused by the poor adhesion between hydrophilic fi ller wood and hydrophobic PE matrix. Reduced TS values due to the lack of compatibility between fi ller and polymer were also reported by others (Alsewailem and Binkhder, 2014; Obasi, 2015). The addition of heat-treated ash wood fl our into neat HDPE generally improved the tensile strength (p<0.01). In the studied heat-treated wood concentration range, the addition of more heattreated wood into HDPE composites provided higher TS values as compared to composites manufactured with untreated ash wood. Arwinfar et al. (2016) also reported that the addition of heat-treated wood into polymer matrix increased tensile strength. Tensile strength (TS) of HDPE composites manufactured using untreated black pine wood was 20 MPa, 21 MPa, 20 MPa and 19 MPa for 0 %, 10 %, 20 % and 30 % wood concentration, respectively ( Table 2). The tensile strength of HDPE composites manufactured with the addition of 10 % heat-treated/untreated black pine wood was increased but the addition of more wood concentration into HDPE matrix did not provide any improvement in TS. It is believed that tensile properties of wood reinforced HDPE composites was decreased due to density of polymer composites manufactured with the addition of 10 % wood fl our. Similar results were also reported by Robin and Breton (2001). It can be said that wood species exhibited a different effect on the tensile strength of the polymer composites. However, adding untreated ash and black pine wood fl our did not provide a signifi cant improvement in the similar tensile strength. Tensile strength of untreated and heat-treated black pine wood reinforced composites generally exhibited similar results and it can be said that both heat-treated and untreated black pine wood did not have a signifi cant effect on the tensile strength.
In the case of tensile modulus (TM), adding untreated and heat-treated wood fl our to neat HDPE gen-erally has not a signifi cant effect on TM, and TM of all HDPE composites was lower than neat HDPE expected for TAC3; TM of HDPE composites was signifi cantly increased while heat-treated and untreated ash wood fl our concentration was increased from 10 % to 30 % (p<0.01). The obtained data for TM of ash wood fl our reinforced HDPE composites is presented in Table 2. HDPE composites manufactured using heat-treated ash wood fl our showed higher TM than untreated ash wood fl our. Heat-treated ash wood fl our had a positive effect on TM. Similar to TM of ash wood fl our reinforced HDPE composites, TM was signifi cantly increased by untreated and heat-treated black pine wood concentration (p<0.01). Heat-treated black pine wood reinforced composite showed higher TM than untreated ones. Wood species did not have a signifi cant effect on the tensile modulus of the composites and TM of all HDPE composites was found to be lower than its neat HDPE. In conclusion, the TM of HDPE composite reinforced with both heat-treated/untreated ash and black pine wood fl ours was increased with wood concentrations. Similar results were also reported by Robin and Breton (2014) and Kaboorani et al. (2008).
The results for elongation at break (EatB) values can be seen in Table 2. Both ash and black pine wood fl our concentrations had a negative effect on EatB of HDPE polymer composites (p<0.01), respectively and EatB values were generally reduced with the increased concentration of ash and black pine wood fl our due to increasing stiffness of the composites. As a result, the elongation at break values reduced with adding both untreated and heat-treated wood fl our. The elongation at break decreased from 460 % to 10 % and 9 % for untreated and heat-treated ash wood fl our concentration, and 8 % for untreated and heat-treated black pine wood fl our, increasing from 0 to 30 wt% in neat HDPE, respectively. Usually in the composites, lower elongation at break values was observed with increased modulus Karakus, 2008 and2008a).
The results of the fl exural strength (FS) are presented in Table 2. The FS values of HDPE polymer composites were signifi cantly increased by untreated and heat-treated ash wood fl our concentration (p<0.01), but the TM of all composites was found to be lower than its neat HDPE. HDPE composites manufactured using heat-treated ash wood fl our generally showed higher FS than when using untreated ash wood fl our. Adding heat-treated ash wood fl our generally had a positive effect on FS. Similar to FS of ash wood fl our reinforced HDPE composites, FS was signifi cantly increased by heat-treated and untreated black pine wood concentration (p<0.01), but the FS of all composites was found to be lower than its neat HDPE. Heat-treated black pine wood reinforced composites generally provided higher FS than the composites with untreated ones, and FS values of all HDPE composites were found to be similar to each other. The addition of heattreated wood fl our in composites had a positive effect on FS, but the addition of untreated and heat-treated wood fl our did not improve FS of neat HDPE, and all FS values of the composites were found to be lower Karakuş, Aydemir, Gunduz, Mengeloğlu: Heat-Treated Wood Reinforced High Density... ..... than FS of neat HDPE. Wood species generally did not have a signifi cant effect on the FS of HDPE composites. As a result, the FS of HDPE composite reinforced with both heat-treated/untreated ash and black pine wood fl our was increased with wood concentrations. Composites produced with 30 % heat-treated ash and black pine wood fl our provided signifi cantly higher FS values compared to other composites. In previous studies, the effects of different lignocellulosic fi llers on selected properties of polymer composites were investigated and it is reported that fl exural strength of the polymer composites was increased with increasing lignocellulosic fi ller content (Kiziltas et  The results for impact strength (IS) are presented in Table 2. The results showed that IS of the composites manufactured with the addition of 10 % heat-treated/untreated ash and black pine wood fl our was increased, but it was decreased with the addition of more wood concentration into neat HDPE. Both untreated/heat-treated ash and black pine wood fl our concentration had a signifi cant effect on IS of HDPE composites (p<0.01). Generally, with the rise of both untreated/heat-treated ash and black pine wood fl our concentration, IS values were decreased. HDPE polymer composites manufactured using untreated ash and black fl our pine wood showed higher IS than heat-treated ones. Similar results were also reported by Aydemir et al. (2015) and Huang et al. (2012). They found that the increase in IS occurred because adding wood fl our played an important role in strengthening and enhanced two-phase interface area interaction of the composite. However, while wood fl our concentrations increased, interface compatibility was a major problem. Thus, impact strength of the wood polymer composites was found to decrease by Huang et al. (2012) and Tisserat et al. (2013). Boonstra et al. (2007) found that heat treatment decreases the impact strength of the wood due to the signifi cantly lower density of the treated specimens, and therefore it can be said that neither heat treatment nor wood fl our concentration have a signifi cant effect on the impact strength of HDPE composites.

Thermal and morphological properties of wood reinforced HDPE composites 3.2. Toplinska i morfološka svojstva drvom ojačanih HDPE kompozita
The curves for thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) of HDPE composites manufactured using untreated/heattreated ash and black pine wood fl our are presented in Figure 1. The summary of thermal stability for neat HDPE, the composites with 30 % untreated ash wood fl our (UAC3) and heat-treated ash wood fl our (TAC3), and the composites with 30 % untreated black pine wood fl our (UBPC3) and heat-treated black pine wood fl our (TBPC3) are given in Table 3. Neat HDPE showed single stage degradation at the set temperature of 469 °C and total mass loss of 99 %. The TGA curves of the wood exhibit two mass loss peaks (Yang et al., 2005;Karakus et al., 2016). The fi rst occurs at about 100 °C and is mainly caused by evaporation of moisture and other volatiles from the wood. The second peak, at approximately 200 °C to 400 °C, is due to the degradation of hemicelluloses, cellulose, and lignin. Hemicellulose degrades between 150 °C and 350 °C, cellulose decomposes between 240 °C and 350 °C, and lignin between 250 °C and 500 °C (Kaboorani and Faezipour, 2009;Byren and Nagle, 1997). HDPE composites provided two main decomposition peaks. The fi rst peak of around 353-370 °C and 373-376 °C referred to ash wood and black pine wood fl our reinforced HDPE composites, respectively, while the second peak came from HDPE and was around 469-490 °C. Residue after 500 °C was increased with the addition of wood concentration to HDPE matrix. The mass loss for the samples with both untreated ash and black pine wood fl our at 500 °C was 93.8 % and 92.6 %, respectively. ..... Karakuş, Aydemir, Gunduz, Mengeloğlu: Heat-Treated Wood Reinforced High Density... Figure 1 also shows that the samples with untreated ash wood and heat-treated black pine wood fl our exhibited higher thermal stability as compared to the samples with the heat-treated ash and untreated black pine wood fl our. Thermal stability of the composites produced from heat-treated wood increased. The increase could also be attributed to better adhesion between wood and the matrix (HDPE) because the pretreatment possibly lowered the polarity of wood and made wood more compatible with the matrix. Similar results were also reported by Li et al. (2013). Morphology of the untreated/heat-treated ash and black pine wood fl our reinforced HDPE composites was also studied. SEM micrographs of neat HDPE, the samples with 30 % untreated ash and heattreated ash wood fl our, and the samples with 30 % untreated black pine and heat-treated black pine wood fl our are given in Figure 2. Fractured surface of neat HDPE is presented in Figure 2a. Furthermore, neat HDPE exhibits ductile mode of failure. Figure 2b and 2c shows the wood fi llers and their size in the samples with the heat-treated and untreated ash wood fl our. The mode of failure becomes more brittle with HDPE composites with untreated wood (Ghasem, 2013;Atli et al., 2018). In addition, the surfaces of the composites with untreated wood have prominent holes due to particle pull out resulting from poor adhesion ( Figure  2b and 2d) (Mengeloglu and Karakus, 2012). Under tensile stress, the particles were easily pulled out from the matrix. This may mean that the interface could not effectively transfer the stress. This observation is in agreement with the lower modulus values recorded for the untreated wood composites (Table  2). Furthermore, for the heat-treated ash and black pine wood fl our reinforced composites (Figure 2d and 2e), the holes are not quite prominent and particles pull out appears relatively less compared to the untreated ones. The particle surface is slightly rough and nearly uniformly dispersed and embedded within the matrix. Similar results for untreated and heat-treated wood reinforced HDPE composites were also reported by Arwinfar

XRD Analysis of neat HDPE and wood
reinforced HDPE polymer composites 3.3. XRD analiza čistog HDPE-a i drvom ojačanih HDPE kompozita X-ray diffractograms of HDPE and its composites are presented in Figure 3. Neat HDPE and its composites showed peaks around 2θ = 22.1° and 24.50°. The peaks for the (110) and (200) planes of HDPE shifted little when the wood fl our was added and an important change in the diffraction peak intensity was not observed with the presence of wood fl our.
Crystallinity index (CI) of neat HDPE and its composites was calculated with XRD peaks. With the addition of wood fl our, the CI changed in all the XRD. CI was calculated as 61.2 % for neat HDPE, 41.6 % for untreated ash wood fl our reinforced HDPE polymer composites (UAC3), 44.8 % for heat treated ash wood fl our reinforced HDPE composites (TAC3), 43.9 % for untreated black pine wood reinforced HDPE composites (UBPC3), and 46.2 % for heat treated black pine wood reinforced HDPE composites (TBPC3). It was determined that crystallinity of cellulose in wood increased with thermal modifi cation. It is believed that mechanical properties of wood fl our reinforced HDPE composites had a positive effect due to high crystallinity properties of polymer composites manufactured with heat treated wood. After thermal modifi cation of wood at high temperature (at 180 °C and above), the crystallinity ratio of the heat-treated wood increased slightly due to the degradation of the hemicelluloses and the crystallinity ratio of softwood was found to increase more in comparison with hardwood (Aydemir  et al., 2015). Consequently, it can be said that the crys-tallinity of black pine wood increased with thermal modifi cation more than that of ash wood and, therefore, the effect of pine wood on the mechanical properties of the composites was determined to be higher than that of ash wood. ..... Karakuş, Aydemir, Gunduz, Mengeloğlu: Heat-Treated Wood Reinforced High Density...

ZAKLJUČAK
This study evaluated the effect of untreated and heat-treated ash and black pine wood fl our concentrations on the physical, mechanical, thermal and morphological properties of HDPE composites. Physical properties (density), mechanical properties (tensile, fl exural, impact strength and hardness), thermal properties (TGA) and morphological properties (SEM) were determined. According to the obtained results, mechanical properties of all HDPE composites were found to be lower than those of neat HDPE. Statistical analysis showed that density, fl exural strength, fl exural modulus tensile modulus and hardness values of polymer composites signifi cantly increased with the rising percentages of ash and black pine wood fl our. Both heat-treated ash and black pine wood fl our reinforced HDPE composites had a positive effect on hardness. Heat-treated ash and black pine wood fl our reinforced HDPE composites had a positive effect on mechanical properties compared to untreated ones. The increase in wood concentration improved fl exural strength, tensile modulus, fl exural modulus and hardness while reducing tensile strength, elongation at break and impact strength. However, impact strength values of HDPE composites produced from heat-treated ash and black pine wood fl ours were slightly lower. Heat treatment had a negative effect on impact strength. HDPE showed single stage degradation while both ash and black pine wood fl our reinforced HDPE composites showed a two-step mass loss thermal degradation. The fi rst peak of around 353-370 °C and 373-376 °C referred to ash wood and black pine wood fl our reinforced HDPE composites, respectively, while the second peak came from HDPE and was around 469-490 °C. Residue after 500 °C was increased with the addition of wood concentration to HDPE matrix. The crystallinity of the composites with heat-treated wood was found to be higher than that of untreated wood-HDPE composites according to the data obtained with XRD. According to the study results, it can be concluded that heat-treated wood fl our can be used as an alternative raw material in the application areas of wood-HDPE composites.