Measurement and prediction of thermal conductivity for hemp fiber reinforced composites.INTRODUCTIONIn the past few years, new research materials are being generated on the potential of cellulosic based fibers as reinforcement for plastics. All researchers who have worked in the area of natural fibers and their composites have agreed that these materials are claimed to offer environmental advantages such as lower pollutant emissions and lower greenhouse gas greenhouse gas n. Any of the atmospheric gases that contribute to the greenhouse effect. greenhouse gas emissions. Moreover, natural fibers are lowcost fibers with low density and high specific properties Specific properties of a substance are derived from other intrinsic and extrinsic properties (or intensive and extensive properties) of that substance. For example, the density of steel (a specific and intrinsic property) can be derived from measurements of the mass of a steel bar [1, 2]. The properties of fiberreinforced composites vary greatly, and depend upon their constituent materials, fiber orientation, and fiber volume fraction. Having an accurate knowledge about the material properties is essential if correct and meaningful process modeling is to be performed. Thermal properties such as thermal conductivity are important parameters to simulate the temperature variation inside the composite during a specific process. Thermal conductivity describes a material's ability to transport heat. Mangal et al. studied the effect of volume fraction of pineapple leaf fiber on thermal properties of the composite using transient plane source technique [3]. It was observed that increasing the fiber content in the matrix decreases the thermal conductivity and thermal diffusivity In heat transfer analysis, thermal diffusivity (symbol: ) is the ratio of thermal conductivity to volumetric heat capacity. Sweeting et al. developed a new experimental method to determine the inplane and thoughthickness thermal conductivities of polymer matrix composites [5]. The transient temperature gradient temperature gradient n. The rate of change of temperature with displacement in a given direction from a given reference point. temperature gradient in the given direction is recorded using thermocouples and the result is processed numerically by an inverse approach to determine the directional thermal conductivity. The validation of the method was conducted using composites with known thermal properties, and excellent correlation was found between the known and determined thermal conductivities. The thermal conductivity and thermal diffusivity of oilpalmfiberreinforced untreated and differently treated composites using transient plane source technique at room temperature was studied by Agrawal et al. [6]. They concluded that all the silane silane or silicon hydride Any of a series of inorganic compounds of silicon and hydrogen with covalent bonds and the general chemical formula Si_{n}H_{(2n + 2)}. and alkali treatments of the fibers increased the thermal conductivity and thermal diffusivity of the composites in comparison with the acetylated composite. The silane treated fiber has higher polarity because of the formation of silonal group on the surface that results the higher thermal conductivity of the silane treated composites. The alkalization alkalization the act of making alkaline. treatment removes impurities and increases the fiber surface adhesion characteristic with the resin and contribute to a higher thermal conductivity. The acetylation slightly increased the polarity of the fiber and, hence, the thermal conductivity of the composite increased marginally. Kalaprasad et al. analyzed and presented the thermal conductivity and thermal diffusivity of sisalreinforced polyethylene, glassreinforced polyethylene, and sisal/glass hybrid fiber reinforced polyethylene composites at high temperatures and with different fiber orientations [7]. The difference between the thermal conductivity properties in parallel and perpendicular directions with respect to fiber length was maximum for sisal reinforced polyethylene as a result of the anisotropic Refers to properties that differ based on the direction that is measured. For example, an anisotropic antenna is a directional antenna; the power level is not the same in all directions. Contrast with isotropic. nature of sisal fiber. An empirical equation was derived to explain the variation in thermal conductivity and thermal diffusivity with temperature. In the present work, the thermal conductivity of the hemp hemp, common name for a tall annual herb (Cannabis sativa) of the family Cannabinaceae, native to Asia but now widespread because of its formerly largescale cultivation for the bast fiber (also called hemp) and for the drugs it yields. fiber reinforced polymer composite at different volume fractions of the fiber is determined. The effect of fiber orientation in transverse and inplane direction is investigated. To confirm the experimental results, the heating experiments are simulated by a finite element See FEA. model (FEM FEM Female FEM Finite Element Method FEM Feminine FEM Finite Element Model FEM Fédération Européenne des Métallurgistes (European Metalworkers' Federation) FEM Faculdade de Engenharia Mecânica (Brasil) ) that incorporated the measured thermal conductivity and convection coefficients. Moreover, the experimental results of thermal conductivities of composites at different directions are compared with two reported models, ES model and rule of mixture model, to describe the variation of the thermal conductivity versus the volume fraction of the fiber. The volume fraction of the fiber was obtained using Eq. 1. EXPERIMENTAL PROCEDURE Material Preparation An environmentally friendly Environmentally friendly, also referred to as nature friendly, is a term used to refer to goods and services considered to inflict minimal harm on the environment.^{[1]} thermoset A polymerbased liquid or powder that becomes solid when heated, placed under pressure, treated with a chemical or via radiation. The curing process creates a chemical bond that, unlike a thermoplastic, prevents the material from being remelted. See thermoplastic. waterbased acrylic polymer was obtained from BASF BASF Bar Association of San Francisco (since 1872; San Francisco, California) BASF Badische Anilin und Soda Fabrik (German chemical products company) BASF Builders Association of South Florida Company, Ontario, Canada. The polymer is an aqueous solution of a polycarboxylic acid and a polyhydric polyhydric /poly·hy·dric/ (hi´drik) containing more than two hydroxyl groups. pol·y·hy·dric adj. Containing more than one hydroxyl group. alcohol as a crosslinking agent in water (52% water). The cure kinetic and conditions of the polymer were discussed in details in the previous papers [8, 9]. Hemp fibers in the form of randomly oriented and aligned were supplied by Hempline Company. The average fiber length was 9 cm and the diameter of the fiber was in the range of 10 to 300 [micro]m. Depending on the fiber diameter, the average tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its of the fiber was 2501450 MPa [10]. The randomly oriented and oriented fiber composites were prepared using the following process. Composite Preparation In this work, the composite was prepared employing a novel processing technique developed for natural fiber thermoset composites [11]. Hemp fibers were formed on a perforated screen and the acrylic resin solution was circulated to impregnate im·preg·nate v. 1. To make pregnant; to cause to conceive; inseminate. 2. To fertilize an ovum. 3. To fill throughout; saturate. the fibers with the solution (see Fig. 1). Vacuum filtration was applied to remove the excess solution. After circulation of the resin solution, the wet mat was displaced on a polyester sheet and then kept in the oven at 55[degrees]C for 48 hr to remove all moisture content. The mat would be ready for sheet molding process after the maturation period. Finally, to manufacture the composite the impregnated mat was cured at 180[degrees]C for 10 min using a hydraulic press hydraulic press Machine consisting of a cylinder fitted with a piston (see piston and cylinder) that uses liquid under pressure to exert a compressive force upon a stationary anvil or baseplate. The liquid is forced into the cylinder by a pump. to obtain the highest compaction. After the heating cycle, the composite was cooled inside the press to around 70[degrees]C using a water cooling Water cooling is a method of heat removal from components. As opposed to air cooling, water is used as the heat transmitter. Water cooling is commonly used for cooling internal combustion engines in automobiles and electrical generators. system to prevent any blister blister, puffy swelling of the outer skin (epidermis) caused by burn, friction, or irritants like poison ivy. A response of the body to protect deeper tissue, blisters generally contain serum, the liquid component of blood. formation due to any moisture content [12]. The final composite was removed from the press and prepared for thermal conductivity measurements. Figure 2 shows the SEM micrographs of the oriented and inplane randomly oriented composites. [FIGURE 1 OMITTED] Volume Fraction of Fiber The volume fraction of fiber ([[upsilon up·si·lon or yp·si·lon n. Symbol The 20th letter of the Greek alphabet. ].sub.f]) in the composite was determined using the following equation: [[upsilon].sub.f] = [[[omega].sub.f] [[rho].sub.m]]/[[[omega].sub.m] [[rho].sub.f] + [[omega].sub.f] [[rho].sub.m]] (1) where [[rho].sub.f] and [[rho].sub.m] are the densities of fiber and matrix and [[omega].sub.f] and [[omega].sub.m] are the weight fraction of fiber and matrix, respectively. Sample Preparation Because of anisotropic nature of the composite, the present work aims to measure the thermal conductivity in different principle directions to study the effect of fiber orientation on thermal conductivity of the composite. Three different experiments were applied to measure the transverse and inplane thermal conductivity of oriented and randomly oriented composites. Figure 3 shows different samples used to measure the thermal conductivity. For transverse measurements, the samples were cut almost in 5 X 5 X 0.2 [cm.sup.3], and for inplane measurements the dimensions of the samples were 5 X 1 X 1 [cm.sup.3]. [FIGURE 2 OMITTED] [FIGURE 3 OMITTED] Thermal Conductivity Measurements The method used to measure the thermal conductivity of composites has been adapted from a technique presented in Davidson and James work [13]. To determine the thermal conductivity, the same apparatus was used. It consists of an electrically heated thin film (Kapton KHLV202, low voltage Low voltage is an electrical engineering term that broadly identifies safety considerations of an electricity supply system based on the voltage used. While different definitions exist for the exact voltage range covered by "low voltage", the most commonly used ones include "mains heater, OMEGA) as the heat source sandwiched between two similar brickshaped specimen halves (see Fig. 4). Two steel plates with the same dimension of the specimens were placed on either side of each specimen to create a uniform heating profile and prevent any convective thermal loss directly from the samples to the environment. Thermal compound (OT201, High temperature conductive paste, OMEGA) was used to eliminate any thermal resistance between all components. The insulation was placed around the edges to create onedimensional heat flow through the specimens. Prior to each measurement, a constant temperature gradient must be established across the specimen. This is done by imposing the homogeneous rate of heat flow that equals the electrical power P fed into the resistance heater. Because of the symmetry of the apparatus, the heat flow through each sample was assumed to be half the power consumed by the heater. The thermal conductivity of the sample is then calculated using Fourier law as: [FIGURE 4 OMITTED] k = [[J.sub.q]l]/[DELTA]T (2) where k is the thermal conductivity of the sample [W [m.sup.1] [K.sup.1]], [J.sub.q] is the heat flux through the sample in W [m.sup.2], in this case [J.sub.q] = P/2A, A is the crosssectional area of the sample [[m.sup.2]], l is the thickness of the sample in the direction of the heat flow [m], and [DELTA]T is the temperature gradient across the sample [[degrees]C]. Two thermocouples were placed on both sides of one sample to monitor the temperature gradient at the surfaces and all were connected to a data acquisition system to record temperatures. Specific Heat Capacity Measurement To simulate the experimental method accurately, the specific heat capacity of the composites have to be determined. The evaluation of the specific heat capacity with temperature for hemp fibers was measured using a Differential Scanning Calorimeter calorimeter: see calorimetry. calorimeter Device for measuring heat produced during a mechanical, electrical, or chemical reaction and for calculating the heat capacity of materials. (DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interruptdriven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. ) TA instruments DSC Q 1000 following wellestablished procedures [14]. The specific heat capacity of the cured resin was obtained as well by a similar experiment. The variations of the specific heat capacity were measured in the range of 20100[degrees]C. FEM SIMULATION OF HEATING EXPERIMENTS The finite element analysis Finite element analysis (FEA) is a computer simulation technique used in engineering analysis. It uses a numerical technique called the finite element method (FEM). There are many finite element software packages, both free and proprietary. was applied to verify the temperature rise generated during the heating experiments. It was assumed that if the simulation of the experiment incorporating the measured thermal conductivity obtained by the method described in this paper produced accurate prediction of temperature rise in the sides of the composite, then the accuracy of the apparatus will be justified. The temperature distribution for the composite during the heating process was evaluated by solving the Fourier heat conduction Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences. equation [15]: [rho][C.sub.p] [[partial derivative]T/[partial derivative]t] = ([k.sub.x][[[[partial derivative].sup.2]T]/[[partial derivative][x.sup.2]]]) + ([k.sub.y][[[[partial derivative].sup.2]T]/[[partial derivative][y.sup.2]]]) + ([k.sub.Z][[[[partial derivative].sup.2]T]/[[partial derivative][z.sup.2]]]) (3) where [rho] and [C.sub.p] are the density and specific heat capacity of the composite, respectively, and [k.sub.x], [k.sub.y], and [k.sub.z] are thermal conductivity coefficients in the main axes. The half of the heat input from the heater was considered as a heat flux boundary condition boundary condition n. Mathematics The set of conditions specified for behavior of the solution to a set of differential equations at the boundary of its domain. for one side of the sample and the other side was transferring heat to the environment by convection. The heat transfer convection coefficient (h) was obtained from experimental results using the following equation: [FIGURE 5 OMITTED] q = [k.sub.z] [[DELTA]T/l] = h(T  [T.sub.air]) (4) where h is the air heat convective coefficient, [T.sub.air] is the temperature of environment, and k, is the transverse thermal conductivity. On the rest of the boundary, the heat transferred assumed to be zero. The domain was divided into block elements using a software package: COMSOL 3.2. The schematic of the domain is presented in Fig. 5. MODELING OF THERMAL CONDUCTIVITY To date, several models for determining the thermal conductivity of polymer composites have been proposed. There are many types of composite systems in which a polymer is reinforced with unidirectionally arranged fibers. In those systems, the thermal conductivity of the composite is the highest. This is due to the anisotropic nature of fibers; while the conductivity is good along the fiber, the heat flow across the fiber is poor. On the other hand, if the alignment of fibers is perpendicular to the direction of thermal flux, the thermal conductivity will be the lowest. The thermal conductivity of a fiberreinforced plastic in the direction parallel to the fibers is well understood in terms of the conductivities of the fiber and matrix. In a unidirectional composite the thermal conductivities of composites in parallel conductions can be estimated from the rule of mixture: [k.sub.c] = [[upsilon].sub.f] [k.sub.f] + (1  [[upsilon].sub.f])[k.sub.m] (5) where [[upsilon].sub.f] and [k.sub.f] denote the volume fraction and the thermal conductivity of fiber, respectively, while [k.sub.m] is the thermal conductivity of matrix [16]. A general expression for effective transverse thermal conductivities of unidirectional composites was derived by Zou et al. [17]. The model was presented based on the thermalelectrical analogy technique for elliptical el·lip·tic or el·lip·ti·cal adj. 1. Of, relating to, or having the shape of an ellipse. 2. Containing or characterized by ellipsis. 3. a. filament filament, in astronomy: see chromosphere. and square packing array unit cell model (ES model). The dimensionless effective transverse thermal conductivity, [k.sub.e], the ratio of the thermal conductivity of the composite, [k.sub.c], to the thermal conductivity of matrix, [k.sub.m], were expressed as a function of the ratio of thermal conductivities of filler to matrix, [beta], filler volume fraction, [[upsilon].sub.f], and the geometry ratio of the filler, [rho] = a/b, where a and b are the axial lengths of the ellipse ellipse, closed plane curve consisting of all points for which the sum of the distances between a point on the curve and two fixed points (foci) is the same. It is the conic section formed by a plane cutting all the elements of the cone in the same nappe. along the xaxis and yaxis, respectively [17]: [k.sub.e] = [k.sub.c]/[k.sub.m] = 1  [1/c] + [[pi]/2d]  [c/[d[square root of ([d.sup.2]  [c.sup.2])]]]ln[d + [square root of ([d.sup.2]  [c.sup.2])]]/c (6) where c = [square root of ([pi][rho]/[[upsilon].sub.f])]/2, d = [rho](1/[beta]  1), [beta] = [k.sub.f]/[k.sub.m]. When [rho] = 1 (i.e., a = b), the present model can be simplified as the cylindrical filaments in a square packing array unit cell model. According to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. Springer and Tsai in Ref. 7, the thermal conductivity for a unidirectional fiber composite with the heat flow making an angle (90  [theta Theta A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ])[degrees] to the fiber direction is [k.sub.c] = [k.sub.c.sub.[parallel]] [sin.sup.2][theta] + [k.sub.c[perpendicular to]] [cos.sup.2][theta] (7) where [k.sub.c[parallel]] and [k.sub.c[perpendicular to]] are the thermal conductivities of the composite parallel and perpendicular to the fibers, respectively. The above mentioned Eqs. 57 will be used to describe the variation of the thermal conductivity of the hemp fiber/acrylic composite versus volume fraction of the fiber. RESULTS AND DISCUSSIONS Specific Heat Capacity Measurements The variation of specific heat capacity of hemp fiber and cured resin obtained from DSC measurements are shown in Figs. 6 and 7, respectively. Figure 6 shows that the specific heat capacity of fiber increases from 2.2 J [g.sup.1] [K.sup.1] to almost 3.4 J [g.sup.1] [K.sup.1] with increasing temperature from 20 to 100[degrees]C. The average variation of the specific heat capacity of fiber could be fitted quite well by a polynomial polynomial, mathematical expression which is a finite sum, each term being a constant times a product of one or more variables raised to powers. With only one variable the general form of a polynomial is a_{0}x^{n}+a_{} of the second order as [FIGURE 6 OMITTED] [C.sub.pf] = 5[e.sup.6][T.sup.2] + 0.0149T + 1.78. (8) Figure 7 shows that the specific heat capacity of the cured resin also increased linearly with temperature in the range of 20100[degrees]C. The values vary from 1.98 J [g.sup.1] [K.sup.1] to 2.7 J [g.sup.1] [K.sup.1] approximately. These variations could not be neglected in the model and the following linear relationship can be fitted for the average heat capacity of the cured resin as [C.sub.pr] = 0.011T + 1.5. (9) To predict accurately the temperature variations in the composite the specific heat capacities of the composite materials are needed. Therefore, the evaluation of the specific heat capacity of the composite can be determined by combining Eqs. 8 and 9 using the rule of mixture as [FIGURE 7 OMITTED] [FIGURE 8 OMITTED] [C.sub.pc](T) = [v.sub.f][C.sub.pf](T) + (1  [[upsilon].sub.f])[C.sub.pr](T) (10) where [[upsilon].sub.f] is the volume fraction of the fiber. Thermal Conductivity Measurements Transverse Measurements. The measured transverse thermal conductivities of randomly oriented composites, [k.sub.c[perpendicular to]], (Fig. 3a) are displayed as a function of volume fractions of the fiber in Fig. 8. It is clear that as the volume fraction of hemp fiber increases the thermal conductivity of the composite decreases. Direct measurement of the thermal conductivity of fiber is extremely difficult. The uncured polymer contains high percentage of water that cannot be easily removed and for that reason it leads to the present of voids for a bulk of cured resin. Therefore, the calculation of thermal conductivity of the pure cured polymer is also doubtful. Hence, the transverse thermal conductivity of the fiber ([k.sub.[perpendicular to]]) and polymer have been evaluated by extrapolating the linear regression Linear regression A statistical technique for fitting a straight line to a set of data points. of thermal conductivity values of the composite to 100% fiber and 0% fiber. It is found that the thermal conductivities of fiber and polymer are 0.115 W [m.sup.1] [K.sup.1] and 0.42 W [m.sup.1] [K.sup.1], respectively. The behavior of the thermal conductivity of different composites can now be explained using the thermal conductivity values of fiber and polymer. As 50% of the polymer replaced by the hemp fiber, thermal conductivity value of the composite decreases from that of the matrix. Further increasing of the fiber content in the composites shows a decreasing behavior trend of the transverse thermal conductivity as compared with the thermal conductivity of polymer matrix. This behavior of polymer composites seems to be justified because the fiber filled in the polymer matrix has a lower thermal conductivity. Each test was then simulated using a FEM. Applying the same amount of heat to the sample and employing the thermal conductivity and the heat transfer convection coefficient obtained from the experimental results, the temperature variation with time at different locations were obtained. To quantify the agreement between the measured temperature data and the predicted results, the temperature distribution in the hot and cold sides of the samples were compared with the experimental values (see Fig. 9). It can be observed that initially the temperature increases sharply and differences between the measured and predicted temperature is around 12[degrees]C up to almost 2 hr. At longer time, it reaches a plateau and the steady state temperature shows a very good agreement between the experimental and predicted results which can validate the thermal conductivity obtained from the experiment method. [FIGURE 9 OMITTED] Figure 8 shows the theoretical value of transverse thermal conductivity of composites obtained through the ES model. Although the fibers are not remained directionally symmetric and the random orientations of fibers are visible in the composite, the model predicts the value of thermal conductivity in close agreement with the experimental values ([R.sup.2] = 0.99). Inplane Measurements. The measured inplane thermal conductivities of oriented composites, [k.sub.c[parallel]], (Fig. 3b) are plotted as a function of volume fraction of fibers in Fig. 10. It can be noticed that unlike transverse measurements, as the volume fraction of hemp fibers increases the inplane thermal conductivity of the oriented composite increases. By extrapolating of the thermal conductivity values of the composite to 100% fiber and 0% fiber, the longitudinal thermal conductivity of the fiber ([k.sub.[parallel]]) and the thermal conductivity of polymer were obtained to be 1.48 W [m.sup.1] [K.sup.1] and 0.42 W [m.sup.1] [K.sup.1], respectively. As it is obvious, there is a significant difference in thermal conductivities in the longitudinal and transverse directions of the fiber. Since natural fibers consist of crystalline cellulose lattice that are radially arranged around its axis, they are highly anisotropic, which give less thermal resistance along the axis compare with across the axis. Moreover, the number of fiber walls in plane oriented direction would be much less than the number of fiber walls in transverse direction. Therefore, there are more fiber walls to cross in the transverse direction and consequently more resistance to the heat flow. [FIGURE 10 OMITTED] [FIGURE 11 OMITTED] The predicted values for the inplane oriented thermal conductivity obtained from the rule of mixture model (Eq. 5) were shown in Fig. 10. It is found that the obtained values of thermal conductivities are in good agreement with the experimentally measured values ([R.sup.2] = 0.95). Figure 11 shows the experimental inplane thermal conductivities of random oriented composites, [k.sub.c], (Fig. 3c). In this case, fibers are oriented in different angles (from 0 to 90[degrees]) with the direction of the heat flow. It can be seen that as the fiber volume fraction increases, the thermal conductivity also increases, which indicates that fibers are mostly oriented in the same direction of the heat flow. The thermal conductivity of the polymer was obtained by extrapolating a linear regression of experimental values ([R.sup.2] = 0.92) to 0% fiber and compared with other values that were determined in the above mentioned experiments (Table 1). Comparison of the results obtained in different experiments shows approximately the same value for the thermal conductivity of polymer which validates the experimental data. The small deviation could be due to the experimental errors. Inserting the inplane thermal conductivity of randomly oriented composites, [k.sub.c], the inplane thermal conductivity of inplane oriented composite, [k.sub.c[parallel]], and the transverse thermal conductivity of the composite, [k.sub.c[perpendicular to]], at different volume fractions into Eq. 7, the average of the fiber angles with heat flow direction in randomly oriented composite at different volume fractions of fiber obtained to be around 31[degrees]. An empirical equation was developed to describe the variation of the inplane thermal conductivity of randomly oriented composite, [k.sub.c], with volume fractions of fiber through linear regression of the experimental values. It can be expressed by the following linear relationship ([R.sup.2] = 0.92). [k.sub.c] = 0.72[[upsilon].sub.f] + 0.40. (11) CONCLUSION A simple method has been employed to measure the transverse and inplane thermal conductivities of hemp fiber reinforced composites. The transverse and inplane thermal conductivities for oriented and randomly oriented composites for different volume fractions of fiber were investigated. Experimental results show that the orientation of fibers has a significant effect on the thermal conductivity of composites. For transverse direction, because of the lower transverse thermal conductivity of the fiber compare with the polymer as the volume fraction of fibers increases, the thermal conductivity of the composite decreases. On the other hand, for inplane direction, because of the higher longitudinal thermal conductivity of fiber compare with polymer, the thermal conductivity of oriented composites enhanced with increasing the volume fraction of fiber. To validate the experimental results, the heating experiments for thermal conductivity measurement were simulated using a FEM that incorporated the measured thermal conductivity and the heat transfer convection coefficients along with specific heat capacity from DSC measurements and density. Predicted temperatures show close agreement with experimental data, which confirm the obtained thermal conductivity. Moreover, the experimental results of thermal conductivities of composites at different directions were compared with two different models (ES model and rule of mixture model). Good agreement between theoretical and experimental results has been found. An empirical relationship In science, an empirical relationship is one based solely on observation rather than theory. An empirical relationship requires only confirmatory data irrespective of theoretical basis. was obtained for the inplane thermal conductivity of randomly oriented hemp fiber composites. ACKNOWLEDGMENTS The assistance of S.R. Davidson for providing the experimental setup and performing the thermal conductivity measurements is gratefully acknowledged. Authors are thankful to NCE NCE Networks of Centres of Excellence NCE New Chemical Entity (pharmaceutical research) NCE Normal Curve Equivalent NCE New Civil Engineer (UK Journal) NCE NonCommercial Educational NCE New Century Energies Auto21, Canada, for their financial support in this work. REFERENCES 1. D.N. Saheb and J.P. Jog, Adv. Polym. Technol., 18, 351 (1999). 2. A.K. Mohanty, M. Misra, and G. Hinrichsen, Macromol. Mater. Eng., 276/277, 1 (2000). 3. R. Mangal, N.S. Saxena, M.S. Sreekala, S. Thomas, and K. Singh, Mater. Sci. Eng., A339, 281 (2003). 4. R.E. Meredith and C.W. Tobias, J. Appl. Phys., 31, 1270 (1960). 5. R.D. Sweeting and X.L. Liu, Compos. A: Appl. Sci. Manuf., 35, 933 (2004). 6. R. Agrawal, N.S. Saxena, M.S. Sreekala, and S. Thomas, J. Polym. Sci.: Part B: Polym. Phys., 38, 916 (2000). 7. G. Kalaprasad, P. Pradeep, G. Mathew, C. Pavithran, and S. Thomas, Compos. Sci. Technol., 60, 2967 (2000). 8. T. Behzad and M. Sain, J. Appl. Polym. Sci., 92, 757 (2004). 9. T. Behzad and M. Sain, Polym. Polym. Compos., 13, 235 (2005). 10. D. Gulati, Master thesis, University of Toronto Research at the University of Toronto has been responsible for the world's first electronic heart pacemaker, artificial larynx, singlelung transplant, nerve transplant, artificial pancreas, chemical laser, Gsuit, the first practical electron microscope, the first cloning of Tcells, , Canada, 42 (2005). 11. T. Behzad and M. Sain, U.S. Patent 0,245,161 (2005). 12. T. Behzad, M. Sain, "Process for Manufacturing a High Performance Natural Fiber Composite by Sheet Molding," in SMe Technical Papers, SMe, 1 (2005). 13. S.R.H. Davidson and M.D. Sherar, Int. J. Hyperthermia hyperthermia /hy·per·ther·mia/ (ther´meah) hyperpyrexia; greatly increased body temperature.hyperther´malhyperther´mic malignant hyperthermia ., 19, 551 (2002). 14. E. A. Turi, Thermal Characterization of Polymeric Materials, Academic Press, New York New York, state, United States New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of (1981). 15. E.R.G. Eckert and R.M. Drake, Analysis of Heat and Mass Transfer, McGrawHill, Dearborn, Michigan Dearborn is a city in the U.S. state of Michigan. It is located in the Detroit metropolitan area and Wayne County, and is the tenth largest city in the U.S. state of Michigan. As of the 2000 census, it had a population of 97,775. (1972) 16. R. Rolfes and U. Hammerschmidt, Compos. Sci. Technol., 54, 45 (1995). 17. M. Zou, B. Yu, D. Zhang, and Y. Ma, J. Heat. Transfer., 125, 980 (2003). T. Behzad, (1) M. Sain (2) (1) Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto M5S 3E5, Canada (2) Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto M5S 3B3, Canada Correspondence to: M. Sain; email: m.sain@utoronto.ca Contract grant sponsor: NCE Auto21, Canada. TABLE 1. Estimated thermal conductivities of fiber and polymer. Thermal conductivity (W [m.sup.1] [K.sup.1]) Direction of measurement Fiber Polymer Transverse measurement 0.115 0.423 Inplane oriented measurement 1.48 0.427 Inplane randomly oriented measurement  0.405 

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