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) filled high-density polyethylene composites (wood plastic composite, WPC) is investigated experimentally as a function of filler content and temperature. Samples are prepared by compression molding process of previously blended and extruded WPC pellets, up to 50% weight content of WF. The thermal conductivity is measured by the heat flow meter technique in a temperature range from -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 measurements. Using the thermal conductivity of bare WF, the thermal conductivity of the wood material in WPC is estimated. This value successfully predicts the upper and lower bounds of the WPC thermal conductivity by means of the parallel and series conduction model of a multiphase composite material. Keywords: particle-reinforced composites; thermal conductivity; thermal properties. *Corresponding author: Umberto Prisco, Department of Materials and Production Engineering (TECN IV piano), University of Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy, e-mail:
[email protected]
List of abbreviations Materials involved in the study: A, air; WM, wood material in the WPC; HDPE, high-density polyethylene; ρA, ρWM, ρHDPE, densities (g/cm3) of A, WM, and HDPE, respectively. Investigated composite materials (the constituent materials are indicated in parentheses): WF, oven-dry wood flour (WM, A); WPCxx, flat-pressed wood plastic composite containing xx mass percentage of wood flour and porosity (HDPE, WM, A); PE panel, flat-pressed HDPE panel containing porosity (HDPE, A); WPCxx(T), theoretical
wood plastic composite without porosity (HDPE, WM); PE panel(T), theoretical HDPE panel without porosity (HDPE); ρWF, ρWPCxx, ρPE–panel, apparent densities (g/cm3) of WF, WPCxx, and PE panel, respectively; ρWPCxx(T), the expected theoretical density of the WPCxx without porosity; ϕyz , volume fraction of the material y in the composite z; ωyz , mass fraction of the material y in the composite z; ky, thermal conductivity [W/(m K)] of the material y.
1 Introduction Even though inorganic fillers presently dominate the thermoplastic industry, wood-derived fillers have been obtaining much interest lately [1, 2]. Their attractiveness originates from the fact that natural fillers represent renewable and low-cost reinforcements that can improve mechanical properties such as stiffness, strength, and heat deflection temperature under load [3–5]. Nowadays, wood-polymer composites are widely used for decking and automotive applications and as building material. In particular, among the wood plastic composites (WPCs), the wood flour (WF) filled polymers have been widely studied [6–9]. Nevertheless, the thermal properties 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 experimental as well as numerical and analytical model studies have been published [10]. So far, the fillers most frequently used have been aluminum particles, copper particles, brass particles, short carbon fibers, carbon particles, graphite, aluminum nitrides, and magnetite particles. With the aim of covering the above-mentioned deficit, this paper studies the influence of filler content and temperature on the thermal conductivity of WF filled polymer composites. Analyzing the experimental data through the parallel and series conduction model of a multiphase composite material, the thermal conductivity of the wood
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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
Beech wood flour
La.So.Le. Est S.r.l.
Fiber-Plast® 75
Maleated PE
DuPont
Fusabond® E100
MFI = 0.25 g/10 min (190°C/2.16 kg, ISO 1133) Density = 0.953 g/cm3 Melting point = 135°C (ISO 3146) Fagus sylvatica Fraction between 45 and 180 μm = 98% by weight 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 this estimated value is evaluated on the basis of its capacity to forecast the thermal conductivity of the produced WPC; from this point of view, satisfactory results seem to be achieved.
2 Materials and methods The raw materials used in this research are listed in Table 1. High-density polyethylene (HDPE), Eraclene® BC 82 (Polimeri Europa, San Donato Milanese (MI), Italy), was employed as matrix. Eraclene® BC 82 is a typical HDPE resin in granule form suitable for extrusion. Hereinafter, it will be referred to simply as HDPE. WF from European beech (Fagus sylvatica), Fiber-Plast® 75, supplied by La.So. Le. Est S.r.l. (Udine, Italy; www.lasole.it), was used as filler. An anhydride-modified HDPE, Fusabond1 E100 (DuPont, Wilmington, DE, US) in pellet form for use in conventional extrusion was used as coupling agent. This resin is a modified polymer that has been functionalized by maleic anhydride grafting [maleated polyethylene (MAPE)] to help bond together polymers, mainly polyethylene and polypropylene, and fillers with molecular structures and nature different from that of polyolefins, as in the case of WF. The amount of coupling agent added was 5% in weight based on the entire compound for all the produced WPCs, according to the producer’s recommendation. However, the amount of MAPE included in the formulation always ensures that its weight content based on WF is never below the ratio 1:10. Both the HDPE and the MAPE were used as received, whereas the WF was conditioned to constant weight at 100°C for 24 h in a vacuum drying oven (to obtain 1–2% moisture content) and then kept in a sealed container prior to any use and measurement. The particle sizes of the WF were analyzed with sieves conforming to ISO 33101:2000. The results are shown in Table 2. HDPE/WF composites containing 20%, 30%, 40%, and 50% WF by mass were dry blended and extruded with
a counter-rotating, cylindrical extruder (ZK 35 by Dr. Collin GmbH) with a screw diameter of 35 mm and a screw lengthto-diameter ratio of 40:1 through a die with 4 × 4 mm2 circular cross-section channels. In the different zones of 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 die. The screw rotating speed was set at 120 rpm. HDPE, WF, and MAPE were fed at the throat of the extruder by the use of twin-screw gravimetric feeders (K-CL-SFS-KT20 by K-Tron). The melt pressure at the die varied between 6 and 25 bar depending on material blend and extrusion condition, and the material output was 4 kg/h. The produced WPC strands were quenched into a water bath and, after removal of the water droplets, deposited onto the extrudates and knife milled with a strand pelletizer (CSG 171/1 by Dr. Collin GmbH) into particles of approximately 3 mm in the longest dimension. All composite pellets were dried at 90°C for at least 24 h to
