Thermal conductivity of plastics

Figure 2.6 Thermal conductivity of plastics filled with glass or metal.

Thermal conductivity of plastics is very low, which makes them excellent insulators against heat and cold. On the other hand, there are times when high thermal conductivity is preferred, for example, in processing, cooking, and heating equipment. Inorganic fillers can help, in proportion to their volume concentration. Perhaps this approach can be carried further or perhaps there are totally different mechanisms waiting to be discovered. 

Heat transfer limitations are important restrictions for pyrolytic processes because of the low thermal conductivity of plastics. In general the heat flow from the surface of a plastic particle to its core (internal heat flux) is rather slow compared with the heat flow... 

Thermal conductivities of plastics are relatively low and approximate 0.0004 (cal-cm)/(°C-cm -sec). The corresponding values are 0.95 for copper, 0.12 for cast iron, 0.002 for asbestos, 0.0008 for wood, and 0.0001 for cork (SI values in J/s-m-k are obtained by multiplying by 418.7). Because of their low thermal conductivities, plastics are used for handles to cooking utensils and for automobile steering wheels. The low thermal conductivity is also responsible for the pleasant feel of plastic parts. Quite hot or quite cold objects can be handled with less difficulty if they are made of plastic, since the thermal insulation afforded by the plastic prevents a continuous rush of heat energy to (or from) the hand. 

Figure 2.21. Thermal conductivity of plasticizer -free PVC vs. temperature. [Data from Dashora, R Gupta, G., Polymer, 37, 2,231-4, 1996.]...

Cooling takes longer in the middle of the molded part. The surplus heat of melt remaining there must be dissipated through the outer layers to the mold wall. Now it becomes apparent that plastics are poor conductors of heat. The thermal conductivity of plastics is approximately 100 times poorer than that of steel. 

THERMAL CONDUCTIVITY OF PLASTICS IN THE SOLID AND MOLTEN STATES. 

THERMAL CONDUCTIVITY OF PLASTICS MIXED WITH CRYSTALLINE POWDERS. 

THERMAL CONDUCTIVITY OF PLASTICS. FROM PROCEEDINGS OF THE 5TH CONFERENCE ON THERMAL CONDUCTIVITY. 

DEPENDENCE OF THERMAL CONDUCTIVITY OF PLASTIC FOAMS ON MOISTURE CONTENT. 

MEASURING THE THERMAL CONDUCTIVITY OF PLASTICS AND RUBBER MIXTURES BY A MODIFIED BOCK TECHNIQUE. 

Stainless steel is impermeable to oxygen unftke plastics it has 10 times the thermal conductivity of plastics and does not creep at temperatures below 425°C (at room temperature for plastics). SS pipes can withstand high pressures (up to 110barfbra25 mm O.D. 300-series SS pipe vs. 5 bar for plastic pipe). Its yield and tensile stresses are an order of magnitude greater than those of thermoplastics. 

D5930-97 Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Method... 

Improved thermal conductivity in plastics is usually achieved by the addition of powder fillers into the plastic formulation. Although silica filler is often used and is effective, some other fillers have higher fhermal conductivity. Fillers that are noted to improve the thermal conductivity of plastics are metals, such as aluminum and copper metal fibers and boron and aluminum nitride. 

Plastics have thermal conductivity much lower than metals and therefore are poorer heat conductors. Thermal conductivity for plastics ranges from 3 X lO cal/-(cm s °C) for polypropylene to 12 X lO cal/(cm s °C) for HDPE. For aluminum the thermal conductivity is 0.3 cal/(cm s °C), and for steel it is 0.08 cal/(cm s °C). For closed-cell plastic foams, values of thermal conductivity are much lower than those of the unfoamed plastic because gases, and air in particular, have a lower thermal conductivity than polymers. Plastic fillers may increase the thermal conductivity of plastics. Semicrystalline plastics generally have a higher thermal conductivity than do amorphous plastics. 

J.F. Haskins, Thermal Conductivity of Plastic Foams from -423 Degrees F to 75 Degrees F., Report No. MRG-242, General Dynamics Corp., San Diego, California (1961). 

The measurement of thermal conductivity of plastics is a difficult process, and until recently, the availability of such data was quite limited. Conventional guarded hot-plate techniques have traditionally been used to make measurements of thermal conductivity. These remain the reference technique for thermal conductivity measurements, since they do not require calibration against a material of known thermal conductivity. 

From the micrographs it is possible to judge the extent of what might be loosely described as the amorphous skin, the transcrystalline zone, and the spherulitic core. From their relative proportions it is not only possible to estimate the processing conditions that were employed to produce the parts, but also to predict part performance. Particles may remain unmelted within the molten mass of plastic. These particles inhibit the formation of crystallization nuclei. Since the thermal conductivity of plastics is poor, the length of... 

A primitive variant of the hot runner is known as an insulated runner. This employs an unheated manifold with very large runners of 20 mm to 35 mm diameter, and relies on the poor thermal conductivity of plastics to ensure that a flow channel in the center of the large runner always remains molten. The insulated runner mold is not capable of precise and consistent control and is now rarely seen. 

Specimen Thickness. Thicker specimens tend to exhibit a higher heat deflection temperature. This is because of the inherently low thermal conductivity of plastic materials. The thicker specimen requires a longer time to heat through, yielding a higher heat deflection temperature. 

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