Thermal anisotropy and conductivity studies of nylon 66

Massive nylon lends itself to many applications because of its outstanding toughness, abrasion-resistance, and chemical resistance. These properties make it a satisfactory bearing material, and in some applications it is superior to metallic bearings. However, the disadvantages of low melting point and low thermal conductivity prevent nylon from being used more extensively.

A previous investigation of the effect on thermal conductivity of change in crystallinity brough about by rolling and annealing nylon 66 indicated that it exhibits anisotropy of conductivity and that its thermal conductivity depends on primary physical factors related to its molecular structure.

The purpose of this investigation was to determine the relationships of anisotropy of thermal conductivity, bond strength, degree of crystallinity, molecular orientation, and the changes resulting from the rolling-annealing treatment of nylon 66.

A survey was made of the literature on the thermal conductivity of nylon in particular and non-metallic solids in general, of anisotropy of thermal conductivity, of the internal structure of nylon, of the effect of physical treatment on the internal structure and physical properties of nylon, and on test methods.

The experimental part of the work consisted of measuring the thermal conductivity of nylon along three directions of heat flux, and the changes in internal structure and thermal conductivity in the three directions of heat flux resulting from rolling and annealing treatment.

The thermal conductivity of six samples of nylon 66 was determined. The first three samples were cut from a slab of cast nylon ¼ inch thick. The conductivity was measured in a direction perpendicular to the plane of the sample and to the greatest length of the slab, in the plane of the sample, but perpendicular to the greatest length of the slab, and in the plane and in the direction of the greatest length of the slab. The other three samples were prepared from a ¼-inch thick cast slab which had been cold-rolled to half the original thickness and then annealed for two hours at 240 °C.

To determine the effect of physical treatment on the internal structure and thermal conductivity, the density and degree of crystallinity were determined and x-ray diffraction patterns of the samples were made.

Standard laboratory procedures were used in all of the tests. The thermal conductivity was measured using the ASTM method Cl77-45, the guarded hot plate method. In this method two sheets of nylon five inches square were sandwiched between an electrical heater and two brass cooling blocks. The quantity of heat which flowed through the samples was measured by measuring the electric power input to the heater. The temperature drop across the samples was measured by means of thermocouples. The sample thickness and area were measured, and from these quantities the thermal conductivity was calculated.

The density was determined by weighing one to six-gram samples in air and in water, according to the standard method.

The degree of crystallinity was calculated from the density of dried samples by assuming a linear relationship between density and degree of crystallinity. This method of calculation was worked out by Hermans for cellulose and used by Snow for nylon.

The results of this investigation showed that when nylon slabs were rolled and annealed, the degree of crystallinity was increased, confirming the conclusions of previous investigators. The thermal conductivity was affected mainly by an orientation of the molecule produced by rolling rather than by the change in degree of crystallinity. Because of a preferential orientation of the molecules in the direction of rolling, the nylon thermal conductivity became anisotropic. These results support Rehner’s hypothesis that the thermal conductivity of polymers is mainly dependent on the strength of the bonds in the molecular chain, and that thermal conduction in polymers is mainly molecular conduction, not lattice conduction.

The following conclusions were reached from tests made on the nylon slabs.