Tech Briefs

Assessing the performance of polymers used in structural armor and filament winding applications.

Thermoset polymers are good electrical insulators that are used in applications ranging from electronics to composite armor. They are rather poor thermal conductors, however.

For encapsulation of electronic components and devices, there is a huge amount of literature on filled polymers, particularly epoxies. One source provides a recent review of thermal conductivity (Kt) of filled polymers with emphasis on carbon nanotube loading. As an example of a low Kt polymer, a Kt of 0.3 watts per meter per degree kelvin (W/MK) for unfilled polyurethane from an unidentified source has been cited, with higher values for filled resin. Measurements have also been reported on particle-filled polypropylene with a neat resin Kt of 0.27 W/MK. These thermal conductivities are higher than those of most epoxies, and much lower than those of metals and crystalline ceramics.

Of particular interest are Kt results for various filled diglycidyl ether of bisphenol-A or F (DGEBP-A or F) epoxies. The references for Kt of filled epoxies contain results at room temperature for the neat resin, but no other data on temperature dependence of Kt for the neat resin. One exception shows that Kt for the neat resin increases slightly with temperatures from 0–60°C. The accompanying table presents a summary of 20°C results. All values are ±5%, a fairly standard accuracy for Kt measurements of polymeric materials. All of the research to date is more concerned with the effects of fillers on Kt than with Kt for the base polymers.

Some available research also discusses the synthesis of an epoxy, diglycidyl ether of terephthalylidene-bis-(4-amino-3-methylphenol) (DGETAM) (see accompanying figure). This epoxy exhibits partial liquid crystal (LC) behavior, which has a Kt of 0.35 W/MK in an isotropic phase, and 0.38 W/MK in a polydomain LC phase. Relevant information is also available for other special epoxies with neat resin Kt values nearly as high as 1 W/MK. These are high-temperature epoxies.

Modeling the Kt of polymers would seem to be a nearly hopeless task, even with today’s modeling capabilities. However, one researcher, E. Algaer, was able to model the Kt of polystyrene and amorphous polyamide (nylon) 6-6 within a factor of 2. The results depend very strongly on the intermolecular potential assumed in the molecular dynamics calculations. In contrast, results for Kt of several liquids were much better, often within 10% of experimental values.

This research presents results of thermal conductivity measurements of some polymers used in applications such as structural armor and filament winding and in basic studies of polymer networks in epoxies at the US Army Research Laboratory (ARL). For epoxies, the effects of the glass transition temperature (TG), type of curing agent (amine, polyamide, or anhydride), toughening, and moisture were examined. Two polyurethanes, poly-dicyclopentadiene (p-DCPD) and polyethylidiene-norbornene (p-ENB) were also studied, as were composites of the epoxies with E- and S2-glass fibers in various woven fabrics. In addition, a uniaxial carbon fiber-epoxy composite and an S2-glass-phenolic composite were explored.

This work was done by William A Spurgeon for the Army Research Laboratory. ARL-0221

This Brief includes a Technical Support Package (TSP).

Thermal Conductivities of Some Polymers and Composites (reference ARL-0221) is currently available for download from the TSP library.

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