Thermal Conductivity of Flat-Pressed Wood Plastic Composites at Different Temperatures and Filler Content

DOI 10.1515/secm-2013-0013 Sci Eng Compos Mater 2014; 21(2): 197–204

Umberto Prisco* Thermal conductivity of flat-pressed wood plastic composites at different temperatures and filler content

Abstract: The thermal conductivity of wood flour (WF) wood plastic composite without porosity (HDPE, WM); filled high-density polyethylene composites (wood plas- PE panel(T), theoretical HDPE panel without porosity 3 tic composite, WPC) is investigated experimentally as a (HDPE); ρWF, ρWPCxx, ρPE–panel, apparent densities (g/cm ) function of filler content and temperature. Samples are of WF, WPCxx, and PE panel, respectively; ρWPCxx(T), the prepared by compression molding process of previously expected theoretical density of the WPCxx without poros- z blended and extruded WPC pellets, up to 50% weight con- ity; ϕy , volume fraction of the material y in the composite z tent of WF. The thermal conductivity is measured by the z; ωy , mass fraction of the material y in the composite z; ky, heat flow meter technique in a temperature range from thermal conductivity [W/(m K)] of the material y. -15°C to 80°C. Experimental results show that the WPC thermal conductivity decreases with temperature and WF content, with the last effect due to the increase in porosity with the filler content, as confirmed by density meas- 1 Introduction urements. Using the thermal conductivity of bare WF, the thermal conductivity of the wood material in WPC is Even though inorganic fillers presently dominate the estimated. This value successfully predicts the upper and thermoplastic industry, wood-derived fillers have been lower bounds of the WPC thermal conductivity by means obtaining much interest lately [1, 2]. Their attractiveness of the parallel and series conduction model of a mul- originates from the fact that natural fillers represent tiphase composite material. renewable and low-cost reinforcements that can improve mechanical properties such as stiffness, strength, and Keywords: particle-reinforced composites; thermal con- heat deflection temperature under load [3–5]. Nowadays, ductivity; thermal properties. wood-polymer composites are widely used for decking and automotive applications and as building material. In particular, among the wood plastic composites *Corresponding author: Umberto Prisco, Department of Materials and Production Engineering (TECN IV piano), University of Napoli (WPCs), the wood flour (WF) filled polymers have been Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy, widely studied [6–9]. Nevertheless, the thermal proper- e-mail: [email protected] ties of this type of WPC have never been truly investigated and there is a lack of scientific literature on this subject. Studies on the thermal conductivity of WF filled polymer composites are part of the research about the conductivity of particle-reinforced polymers. In this field, many experi- List of abbreviations mental as well as numerical and analytical model studies have been published [10]. So far, the fillers most fre- Materials involved in the study: A, air; WM, wood mate- quently used have been aluminum particles, copper parti- rial in the WPC; HDPE, high-density polyethylene; ρ , ρ , A WM cles, brass particles, short carbon fibers, carbon particles, ρ , densities (g/cm3) of A, WM, and HDPE, respectively. HDPE graphite, aluminum nitrides, and magnetite particles. Investigated composite materials (the constituent With the aim of covering the above-mentioned deficit, materials are indicated in parentheses): WF, oven-dry this paper studies the influence of filler content and tem- wood flour (WM, A); WPCxx, flat-pressed wood plastic perature on the thermal conductivity of WF filled polymer composite containing xx mass percentage of wood flour composites. Analyzing the experimental data through and porosity (HDPE, WM, A); PE panel, flat-pressed HDPE the parallel and series conduction model of a multiphase panel containing porosity (HDPE, A); WPCxx(T), theoretical composite material, the thermal conductivity of the wood 198 U. Prisco: Thermal conductivity of flat-pressed wood plastic composites

Table 1 Raw materials.

Material Supplier Trade name Description

HDPE Polimeri Europa Eraclene® BC 82 MFI = 0.25 g/10 min (190°C/2.16 kg, ISO 1133) Density = 0.953 g/cm3 Melting point = 135°C (ISO 3146) Beech wood flour La.So.Le. Est S.r.l. Fiber-Plast® 75 Fagus sylvatica Fraction between 45 and 180 μm = 98% by weight Maleated PE DuPont Fusabond® E100 MFI = 2 g/10 min (190°C/2.16 kg, ISO 1133) Melting point = 134°C (ISO 3146)

material in the WPC (WM) is estimated. The soundness of a counter-rotating, cylindrical extruder (ZK 35 by Dr. Collin this estimated value is evaluated on the basis of its capac- GmbH) with a screw diameter of 35 mm and a screw length- ity to forecast the thermal conductivity of the produced to-diameter ratio of 40:1 through a die with 4 × 4 mm2 WPC; from this point of view, satisfactory results seem to circular cross-section channels. In the different zones of be achieved. the extruder barrel (10 zones plus the nozzle, all equipped with a PID temperature-control system), the following temperature profile was set: 140°C, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, and 185°C in the 2 Materials and methods die. The screw rotating speed was set at 120 rpm. HDPE, WF, and MAPE were fed at the throat of the extruder by the The raw materials used in this research are listed in use of twin-screw gravimetric feeders (K-CL-SFS-KT20 by Table 1. High-density polyethylene (HDPE), Eraclene® BC K-Tron). The melt pressure at the die varied between 6 and 82 (Polimeri Europa, San Donato Milanese (MI), Italy), was 25 bar depending on material blend and extrusion condi- employed as matrix. Eraclene® BC 82 is a typical HDPE tion, and the material output was 4 kg/h. The produced resin in granule form suitable for extrusion. Hereinafter, WPC strands were quenched into a water bath and, after it will be referred to simply as HDPE. WF from European removal of the water droplets, deposited onto the extru- beech (Fagus sylvatica), Fiber-Plast® 75, supplied by La.So. dates and knife milled with a strand pelletizer (CSG 171/1 Le. Est S.r.l. (Udine, Italy; www.lasole.it), was used as filler. by Dr. Collin GmbH) into particles of approximately 3 mm An anhydride-modified HDPE, Fusabond1 E100 (DuPont, in the longest dimension. All composite pellets were dried Wilmington, DE, US) in pellet form for use in conventional at 90°C for at least 24 h to < 1% moisture content prior to extrusion was used as coupling agent. This resin is a modi- subsequent use. The WPC formulations and codes are fied polymer that has been functionalized by maleic anhy- given in Table 3. dride grafting [maleated polyethylene (MAPE)] to help Flat-pressed HDPE and WPC samples for thermal con- bond together polymers, mainly polyethylene and polypro- ductivity and density measurements were manufactured pylene, and fillers with molecular structures and nature using standardized procedures that simulated industrial different from that of polyolefins, as in the case of WF. The production at the laboratory [11]. A computer-controlled amount of coupling agent added was 5% in weight based on press (P 200 E, Dr. Collin GmbH) equipped with cooling the entire compound for all the produced WPCs, according cassettes was used. The WPC pellets were compression to the producer’s recommendation. However, the amount of MAPE included in the formulation always ensures that Table 2 Sieve analyses results for the beech wood its weight content based on WF is never below the ratio 1:10. flour (R, retained, P, pass; mesh size is in accord Both the HDPE and the MAPE were used as received, with ISO 3310-1:2000). whereas the WF was conditioned to constant weight at 100°C for 24 h in a vacuum drying oven (to obtain 1–2% Mesh size (μm) Fraction, mass percentage moisture content) and then kept in a sealed container R180 0.81 prior to any use and measurement. The particle sizes of P180–R125 36.28 the WF were analyzed with sieves conforming to ISO 3310- P125–R63 51.40 1:2000. The results are shown in Table 2. P63–R45 10.18 HDPE/WF composites containing 20%, 30%, 40%, P45–R38 0.58 P38 0.75 and 50% WF by mass were dry blended and extruded with U. Prisco: Thermal conductivity of flat-pressed wood plastic composites 199

Table 3 WPC formulation and codes. to ASTM C518-10 and related standards. Five measurements were carried out for each material to ensure repro- WPC code WF (wt%) HDPE (wt%) MAPE (wt%) ducibility and to assess experimental variance. Calibration WPC20 20 75 5 of the apparatus was carried out using a certified low- WPC30 30 65 5 = 3 WPC40 40 55 5 density polyethylene (LDPE) panel [density 0.91 g/cm , WPC50 50 45 5 thermal conductivity = 0.335 W/(m K) at 20°C] for the HDPE and WPC measurements and an expanded polystyrene panel [density = 0.035 g/cm3, thermal conductivity = 0.0275 W/(m K) at 20°C] for WF measurements. The accuracy of molded into a frame measuring 200 mm × 200 mm × 15 mm the thermal conductivity measurements is 2.5%. at a temperature of 170°C ± 5°C. The molding conditions were as follows: preheating time, 30 min, under a pressure of 0.5 MPa; heating time, 10 min, under a final pressure of 3 MPa; cooling under a slight pressure to ambient 3 Results and discussion temperature. The resulting panels were used for thermal conductivity measurements. Furthermore, several little The apparent density of the WF is determined to be parallelepipeds were cut from some of the produced HDPE 0.215 ± 0.025 g/cm3. Due to the oven treatment, this mate- and WPC panels for density measurements. Examples of rial is composed of beech wood, without water, and some the produced panels are in Figure 1. of the volatile extractives normally contained inside cavi- The apparent density of the HDPE and WPC ties and intercellular spaces, and interconnected porosity, panels was determined using a pycnometer; ethanol forming channels open at the surface and filled with air. (density = 0.789 g/cm3) was chosen as fluid. The measure- Kellogg and Wangaard [12], by means of three different ment conditions as proposed in standard ISO 1183-1:2004, techniques, found the wood material density of beech to in particular method B (liquid pycnometer method for range between 1.347 and 1.468 g/cm3, with a mean value of small pieces of finished parts), were applied. WF appar- about 1.407 g/cm3. Using this value and the formula con- ent density was determined according to the standard ISO necting WF density, ρWF, WM density, ρWM, and the volume WF 60:1977, i.e., pouring the flour into a measuring cylinder of fraction of air inside the WF, ϕA , i.e., 3 100-cm capacity and weighing the mass of the contents. WF ρ = ρϕ(1-), (1) Ten measurements were carried out for each material to WF WM A ensure reproducibility and to assess experimental variance. The accuracy of the density measurements is 0.5%. it is possible to find a mean value for the WF porosity, WF The thermal conductivity of WF, HDPE, and WPC panels i.e., ϕA =0.847. This value will be useful in the following was measured with an HFM 436/3/1E Lambda (NETZSCH discussion. Group, Selb, Germany) heat flow meter according to ISO Figure 2 reports the densities of the different panels; 8301:1991. Measurements were carried out in the tempera- for comparison, the values of the theoretical density are ture range of -15–80°C. Because the WF is a loose-fill material, an open test wooden frame was used for it, according 1.2

Experimental Theoretical 1.1 ) 3

1.0 Density (g/cm 0.9

0.8 PE-panel WPC20 WPC30 WPC40WPC50

Figure 2 Density of the flat-pressed panels (mean values with 95% Figure 1 Examples of the obtained panels. confidence intervals are reported). 200 U. Prisco: Thermal conductivity of flat-pressed wood plastic composites

also added to the plot. The HDPE theoretical density was ρρWPCxx(T) - WPCxx ϕWPCxx = . (4) provided by the producer, whereas that of the WPCs was A ρρ- WPCxx(T) A calculated by means of the formula expressing the density of a two-phase composite as a function of the proportions The value of air density, ρ , at room temperature is and density of its constituents, i.e., A 1.205 × 10-3 g/cm3 as found in [13]. A similar formula holds 1 ρ = for the PE panels. WPCxx(T) WPCxxWPCxx . ωω (2) Knowing the volume fraction of the air inside the WM + PE ρρWM HDPE flat-pressed WPC containing xx mass percentage of WPCxx WF and porosity (WPCxx), ϕA , volume fractions of other components, WM and HDPE, can be found as Measured densities are below the theoretical expected ϕWPCxxW=ϕϕPC(T)W(1-)PCxx and ϕWPCxxW=ϕϕPC(T)W(1-)PCxx . values due to the unavoidable air-filled porosity, which HDPE HDPE A WM WM A Again, a similar formula applies to PE panels, the adopted manufacturing process produces inside the ϕPE−−panels =ϕϕPE panels(T)P(1-)Ep− anels . Results of the calcula- panels. However, the porosity was isolated, i.e., not inter- HDPE HDPE A tions are in Table 4. communicating and not open at the surface; see Figure 3 It is clear that WPC porosity increases with WF content reporting the typical void distribution of the four WPC (see Figure 3). Indeed, during processing at high tem- panels. Indeed, the samples used for density measure- peratures, both plastic and wood undergo rather notice- ments did not show any weight variation after immersion able degradation and depolymerization, which leads to into the pycnometer, as it was carefully checked; this is formation of volatile organic compounds (VOCs). VOCs a necessary condition for the validity of apparent density make the material foamed, with noncontrolled porosity; measurement with a pycnometer. this noticeably decreases the density of the final WPC The volume fraction of the air inside the panels can product [3]. VOC formation is obviously expected to sig- be found from the following formula, which is valid for a nificantly increase the WF content. In particular, when solid with isolated porosity and not open at the surface, the WF content is increased from 20% to 50% by weight, expressing the density of a composite as a function of its the porosity content goes from about 4% to beyond 9%, porosity: whereas for the PE panels alone a value of around 3% is WPCxxWPCxx ρWPCxxA=⋅(1-)ϕρWPCxx(T) +ϕρAA, (3) found. The total porosity of the WPC is developed during both the extrusion and the flat pressing, so that part of the which gives porosity is already contained in the WPC pellets.

A B

0.5 mm

Figure 3 Typical void distribution in the four WPC panels: (A) WPC20, (B) WPC30, (C) WPC40, (D) WPC50. U. Prisco: Thermal conductivity of flat-pressed wood plastic composites 201

Table 4 Density, volume and mass fraction (theoretical and experimental) of the different components of the flat-pressed panels.

Theoretical Experimental (mean value)

3 3 Density (g/cm ) ϕA ϕWM ϕPE Density (g/cm ) ϕA ϕWM ϕPE PE panel 0.953 0.000 0.000 1.000 0.912 0.034 0.000 0.966 WPC20 1.019 0.000 0.145 0.855 0.922 0.048 0.138 0.814 WPC30 1.055 0.000 0.225 0.775 0.906 0.061 0.211 0.728 WPC40 1.094 0.000 0.311 0.689 0.891 0.078 0.287 0.635 WPC50 1.137 0.000 0.404 0.596 0.865 0.095 0.365 0.539

Figures 4 and 6 report the measured thermal conduc- From data in [13], the following regression equation tivity of PE panels and WF at different temperatures. It is describing the thermal conductivity of air within the observed that kWF increases, whereas kPE–panel decreases, explored temperature range is assessed: with temperature. k = (0.024+7.246 × 10-5·T) [W/(m s)], From data in Figure 4, knowing the air volume frac- A 2 PE−panels -15°C ≤ T ≤ 80°C, R = 0.997. (7) tion inside the PE panels, ϕA , and the thermal conductivity of the air, kA, it is possible to estimate the thermal From Eqs. (5) and (6), we expect to find the lower conductivity of the HDPE at the experimental tempera- and upper bound of kHDPE, respectively. However, results tures. To this aim, the simplest alternatives are the parallel obtained from the series conduction model are rejected and series conduction models that, for a two-component because, being of order of magnitude of 1 W/(m s), they composite, set the materials arranged in parallel or series are not consistent with previous literature, which reports with respect to the heat flow; these models are known to a value of about 0.4 W/(m s) for the thermal conductivity give the upper and lower bounds of the composite thermal of polyethylene. On the contrary, the kHDPE calculated from conductivity [10]. For the PE panel, the above-mentioned the parallel model is in strong agreement with previous models can be written as: literature data, in particular with [14–17], that have already

PE−−panelPEpanel observed a decrease in various HDPEs’ thermal conductivity kk− =⋅ϕϕ+⋅k PE panelA AHDPEHDPE with increasing temperature until the melting point. Results

(parallel conduction model) (5) of the present calculation are in Figure 5, which also reports and the best-fit line describing the dependence of the HDPE thermal conductivity on temperature, whose equation is: ϕϕPE−−panelPEpanel 1 AHDPE = × -3 =+ (series conduction model). (6) kHDPE (0.429-0.990 10 ·T) [W/(m s)], kkk PE−panelA HDPE -15°C ≤ T ≤ 80°C, R2 = 0.999. (8)

0.600 0.600

0.500 0.500

0.400 [W/(m K)] 0.400 [W/(m K)] HDPE PE-panel k k 0.300 0.300

0.200 -15 020406080 0.200 T (°C) -15 020406080 T (°C) Figure 4 Dependence of the thermal conductivity of the PE panel on temperature (mean values with 95% confidence intervals are Figure 5 Estimated thermal conductivity of the adopted HDPE [the reported). best-fit line, Eq. (8), is reported]. 202 U. Prisco: Thermal conductivity of flat-pressed wood plastic composites

0.040 0.378

0.036 0.372 ] K)

0.032 m 0.366 /( [W [W/(m K)] WM k WF 0.028 0.360 k

0.024 0.354

0.020 0.348 -15 020406080 -15 020406080 T (°C) T (°C)

Figure 6 Thermal conductivity of the WF (mean values with 95% Figure 7 Estimated thermal conductivity of WM, the wood material confidence intervals are reported). in the WPC [the best-fit line, Eq. (9), is reported].

In the same way, for the explored temperature range, range. All kWPCxx values decrease with temperature due it is possible to assess a value of thermal conductivity of to the predominant effect of the HDPE. Furthermore, it is

WM from the measured kWF reported in Figure 6. To the evident that the WPC thermal conductivity decreases as author’s knowledge, the thermal conductivity of the WM the WF content increases because of the porosity incre- material of a wood has never been experimentally evalu- ment brought about by the WF degradation and subse- ated or theoretically assessed; thus, this is the first attempt quent VOC formation. to estimate such a value. Again, the formulae for the par- The theoretical curves added to Figure 8 are obtained allel and series conduction are used to model the thermal using the previously estimated kWM, reported in Figure conductivity of WF, which is a two-component composite 7, and the parallel and series conduction models; as material. Calculation is performed using kA, from Eq. (7), expected, the series conduction model underestimates the WF and ϕA . As before, two values for the kWM are obtained; thermal conductivity of the WPC panels, whereas the par- the one from the parallel model is discarded as it is physi- allel model curves, although lying above the experimen- cally meaningless, being of order of magnitude of 0.01 tal data, supply a fairly good forecast of the experimental W/(m s), which is inconsistent with thermal conductiv- data, with a deviation from the experimental average ity of WM constituents. Indeed, it is known [18, 19] that smaller than +10%; this means that the WPC panels are dry WM is composed of hemicellulose [khemicellulose = 0.34 well described by a parallel arrangement of the fiber, poly

W/(m s) at room temperature], lignin [klignin = 0.39 W/(m s) ethylene, and air. However, values of the WPC thermal at room temperature], and cellulose [at room temperature, conductivity predicted by the parallel conduction model \\ amorphous kcellulose = 0.34 W/(m s); kcellulose =1.04 W/(m s), close to the upper bound of the experimental range at ⊥ kcellulose =0.26 W/(m s), parallel and perpendicular to the each temperature are awaited considering that kWM was fibers, respectively]. Then, a value compatible with the calculated using the model that provides its maximum previous ones is expected for kWM; such a value is provided value, i.e., the parallel conduction model of the two-com- by the series conduction model. ponent composite WF.

Figure 7 and Table 5 report the kWM obtained from the series conduction model; a regression best-fit line is added to the plot, and its equation is Table 5 Estimated thermal conductivity of WM.

-4 kWM = (0.361+1.876 × 10 ·T) [W/(m s)], T (°C) k -15°C ≤ T ≤ 80°C, R2 = 0.884. (9) WM -15 0.355 Naturally, the soundness of the values in Figure 7 will 0 0.364 be evaluated on the basis of their prediction capacity of 20 0.366 the thermal conductivity of the WPC; see below. 40 0.371 Finally, Figure 8 shows the thermal conductivity 60 0.372 80 0.375 of the WPC panels within the investigated temperature U. Prisco: Thermal conductivity of flat-pressed wood plastic composites 203

0.4 0.4 ] ] K) K) 0.3 0.3 m m /( /( k [W k [W 0.2 0.2

WPC20 WPC30 0.1 0.1 -150 20 40 60 80 -150 20 40 60 80 T (°C) T (°C)

WPC40 WPC50 0.4 0.4 ] ] K) 0.3 0.3 K) m m /( /( k [W 0.2 0.2 k [W

0.1 0.1 -150 20 40 60 80 -150 20 40 60 80 T (°C) T (°C)

Figure 8 Thermal conductivity of the WPCs (mean values with 95% confidence intervals are reported). The continuous and dashed lines are the parallel and series conduction models, respectively.

From Figure 8, it is clear that the assessed kWM values conductivity of WM, the beech wood material in the in Eq. (9) provide a fairly good estimate of the thermal WPC, is estimated. WM conductivity linearly increases conductivity of WM in view of their predictive capacity as temperature increases, with a regression coefficient of the bounds of the thermal conductivity, kWPCxx, of the for temperature of the order of magnitude equal to different composites. 10-4 W/(m K2). HDPE thermal conductivity decreases with temperature, showing a constant temperature gradient of the order of magnitude of -10-3 W/(m K2); 2. The WPC thermal conductivity decreases with 4 Conclusion temperature and WF content; the former effect is due to the predominant decrease of the HDPE thermal The thermal conductivity of WF filled HDPE composites is conductivity with temperature, whereas the latter is a investigated experimentally as a function of filler content result of the porosity growth with the filler content, as and temperature by means of the heat flow meter technique confirmed by density measurements. in the temperature range -15°C–80°C. The following results, valid in the studied temperature range, are obtained: 1. Using thermal and density measurement on WF, Received January 10, 2013; accepted June 8, 2013; previously pub- HDPE, and WPC and literature data, the thermal lished online July 31, 2013

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