Polyethylene thermal diffusivity

Figure 4. Absorption Spectra of the Disperse Blue 60/polyethylene-terephthalate system. (a) Evaporated film, (b) Thermally diffused,...

Table 7.2 shows the viscosity, mutual diffusion coefficient, and thermodynamic factor for aqueous solutions of ethylene glycol and polyethylene glycol (PEG) at 25°C the diffusivity decreases considerably with increasing molecular weight, while the viscosity increases. Table 7.2 shows the thermal diffusion ratios for liquids and gases at low density and pressure the thermal diffusion ratios are relatively larger in liquids. 

Table 7.2b. Thermal diffusion ratio, KJt thermal diffusion coefficients Dj, and heats of transport Q for aqueous ethylene glycol and polyethylene glycol (PEG) solutions at 25°Ca...

Transient conduction conditions occur in polymer processing. Appendix A derives Eq. (A.14) for one-dimensional transient heat flow, which contains the thermal diffusivity a. This is the combination k/pCp of the thermal conductivity k, density p and specific heat Cp. For most polymer melts a is approximately equal to O.lmm s" (Fig. 5.3). For the melting of low-density polyethylene in an extruder, typical conditions are a barrel temperature of To = 220 °C, an initial polymer temperature Tp = 20 °C, and a melting process complete at T = 120 °C. Consequently, using Eq. (C.19), after a contact time t, the melt front is at a distance from the barrel given by... 

Variation of thermal diffusivity with temperature, for amorphous polycarbonate and semi-crystalline polyethylene. 

Table 7.5 Soret Coefficients, Thermal Diffusion Coefficients, and Heats of Transport for Aqueous Ethylene Glycol and Polyethylene Glycol (PEG) Solutions at 25 °C (Chan et al., 2003)...

G. Kalaprasad, P. Pradeep, G. Mathew, C. Pavithran, and S. Thomas, Thermal conductivity and thermal diffusivity analyses of low-density polyethylene composites reinforced with sisal, glass and intimately mixed sisal/glass fibres. Compos. Sci. Technol. 60, 2967-2977 (2000). 

Various methods have been described for the determination of thermal conductivity. Capillarity has been used to measure the thermal conductivity of low-density polyethylene, high-density polyethylene, and polypropylene at various temperatures and pressure [50]. A transient plane source technique has been applied in a study of the dependence of the effective thermal conductivity and thermal diffusivity of polymer composites [51]. 

A laser flash technique has been used to determine the diffusivity of pyroelectric polymers such as polyvinylidene fluoride [83], whereas hot-wire techniques have been used to determine the thermal diffusivity of high-density polyethylene, low-density polyethylene propylene, and polystyrene [83], Dos Santos and coworkers [84] utilized the laser flash technique to study the effect of recycling on the thermal properties of selected polymers. Thermal diffusivity expresses how fast heat propagates across a bulk material, and thermal conductivity determines the woiking temperature levels of a material. Hence, it is possible to assert that those properties are important if a polymer is used as an insulator, and also if it is used in applications in which heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications (including food wrapping) were selected for this study. 

Hay and co-workers refer to this quantity as the Nusselt number, but the Nusselt number is properly dehned using the thermal conductivity of the ambient medium, not of the polymer.) Bi is a measure of the efficiency of heat transfer to the surroundings (Uh) relative to heat transfer by conduction through the polymer (k/H). The thermal diffusivity KfpCp) of most polymers is about lO m /s, giving a value of K of about 0.27 W/(m K) for polyethylenes. The Biot number would thus be of order 10 for 17 0.1, but of order unity for U 10. The latter value seems more realistic. Note that the coefficient of Bi is insensitive to p, varying only between 0.5 and about unity and approaching the asymptotic value of 0.75 for p 1. 

Kilian, H.-G., PietraUa, M. (1978). Anisotorpy of thermal diffusivity of uniaxial stretched polyethylenes. Polymer, 19,664-672. 

This effect can be observed in the three PA spectra shown in Figure 20.5, which shows how the spectmm of polyethylene (PE) changes as the optical velocity, V (given in terms of the HeNe laser frequency in this figure) is changed from 200 Hz to 20 kHz [5]. When the laser frequency is 200 Hz, the modulation frequencies at 2900, 1460, and 720 cm are about 37, 18, and 9 Hz, respectively. The actual thermal diffusivity of polyethylene varies with its source but is assumed... 

ThFFF, SEC, and light scattering have been used to determine the MWD of anionically polymerised p-methoxystyrene, p-methylstyrene, p-chlorostyrene, and p-cyano-styrene. ThFFF and SEC/multiangle laser light scattering have been used to study the thermal diffusion coefficients of PS, poly( er -butylstyrene) (PtBS) and PS/PtBS copolymer microgels [138]. The retention behaviour of poly(styrene-co-methyl-methacrylate) and poly(styrene-f -iosprene) in ThFFF and SEC have been studied [139]. SEC fractions of blends and copolymers of PS and polyethylene oxide were cross fractionated by ThFFF [140]. 

Fig. 6. Thermal diffusivity of 98 vol % crystalline high density polyethylene (HDPE) and amorphous epoxy resins (EP) vs temperature.

H. G. Kilian, M. Pietralla (Polymer 7P, 664-672, 1978) derive from the anisotropy A of the thermal diffusivity a = X/cpp of oriented polyethylenes that the intrinsic anisotropy Ai of the orienting, partly crystalline lamellar clusters increases with the degree of crystallinity (Aj = 7 to 26, linear extrapolation yields Aj = 2 for the fully amorphous and Aj = 50 for a completely crystalline cluster) the average degree of orientation of the lamellae, , as determined from thermal measurements agrees very well with X-ray data the observed increase of orientation with draw ratio is more rapid than it would be in affine deformation. 

Diffusion of Heat. In dynamic equilibrium, a transfer of vapor from liquid through a vapor phase to a second liquid (the two liquids being thermally connected only across the thin gap) will require reverse transfer of the heat of vaporization. This will accompany a temperature difference determined by the ratio of heat flow to the thermal conductance of the two heat paths. These two are the diffusion vapor gap and the series of salt water and plastic films. For the diffusion gap the c.g.s. air value 5.7 x 1(H is chosen for the thermal conductivity (neglecting the separating powder), while for the series polyethylene (50 X 10-4 cm. thick), wet cellophane (50 X 10"4 cm. thick), and water (200 X 10-4 cm. thick) the respective thermal conductivities are 3.5 X 10"4, 4 X 10-4, and 14 X 10 4. 

Keywords polyethylene, modification, thermal properties, diffusion, structure - property relation. 

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