The influence of degree of crystallinity on the thermal conductivity of nylon 66

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Virginia Polytechnic Institute


The use of massive nylon as a material for bearing liners is limited by its low melting point ( 260 °C) and its low thermal conductivity (0.172 Btu per hour - square foot - °F per foot). This combination of defects results in the formation of hot spots at points of stress., and since the material does not conduct away the heat from such spots rapidly enough., the nylon tends to expand and stick to the shaft. If it were not for this difficulty., more use would be made of nylon bearings., since it has excellent resistance to corrosive substances such as sea water and is capable of resisting repeated mechanical shock.

The purpose of this investigation was to determine the extent to which the degree of crystallinity of nylon could be increased by cold-rolling and annealing, and the change :in thermal conductivity resulting from these treatments.

A survey was made of the literature on the thermal conductivity of nylon in particular and non-metallic solids in general., 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 duPont Company reported the thermal conductivity of its nylon molding compounds, and stated that an average value for Nylon FMl.0001 is 0.15 Btu per hour - square foot - °F per foot.

Austin and Eucken agreed in stating that on theoretical and experimental evidence the thermal conductivity of an inorganic compound is in general higher when the substance is in crystalline form than when it is in amorphous form. Jakob stated that in nonmetallic solids heat is conducted by transfer of vibrations from molecule to molecule, and that in crystals heat is also conducted by a wave-like vibration of the lattice as a 'Whole. Austin stated that the thermal conductivity of inorganic compounds is dependent on the strength of chemical bonding in the compounds. Rehner suggested that the thermal conductivities of organic polymers are all of about the same magnitude since heat is conducted mainly along the molecular chains, which contain primary valence bonds of about the same strength.

Many investigators have shown that nylon is microcrystalline in nature, containing crystalline regions about 1000 Angstroms in diameter imbedded in a matrix which is essentially a supercooled liquid. Thus nylon is a two-phase system. The density of nylon has been investigated repeatedly and has been shown to be a measure of the degree of crystallinity. The degree of crystallinity is defined as the percentage of crystalline material present. Direct comparison of the reported densities has not been possible because the samples tested have not been completely characterized as to previous physical treatment. It has been shown that the method of forming affects the density, and samples which have been cooled slowly from the melt have a smaller number of crystal nuclei and a higher degree of crystallinity. Annealing., stretching., solvent swelling., and rolling increase the density and degree of crystallinity., and more drastic physical treatment tends to cancel out the effect of previous milder treatment.

Bunn and Garner have established tre crystal form and crystal parameters of nylon 66 and 610 and demonstrated the nature of the orientation that takes place on cold-rolling and annealing. The nylon molecules tend to lie in the plane of the rolled sheet :in the direction of rolling.

The experimental part of the work consisted of testing methods of increasing the thermal conductivity of nylon., measuring the thermal conductivity., and determining the changes in internal structure which caused the changes in conductivity.

The thermal conductivities of two samples of nylon 66 were determined. One was a slab of cast nylon 1/4-inch thick. The other was prepared from the first by cold-rolling it to half the original thickness and then annealing for two hours at 240°C.

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

The material which was tested., nylon 66., was a practically pure chemical compound. It is lmown as polyhexamethylene adipamide., and is the most common nylon of commerce.

Standard laboratory procedures were used in all 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 under the temperature difference set up by the apparatus 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 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, but has never before been used for nylon.

The results of this investigation showed that when nylon slab was rolled and annealed, the degree of crystallinity was increased, confirming the conclusions of previous investigators. On the other hand, the thermal conductivity was affected mainly by an orientation of the molecules produced by rolling rather than by the change in degree of crystallinity. Because of an orientation of the molecules in the direction of rolling, the nylon thermal conductivity became anisotropic. The conductivity was apparently increased in the direction of rolling at the expense of the other directions. These results support Rehner's hypothesis that the thermal conductivity of polymers is mainly dependent on the strength of the primary bonds in the molecular chain, and that thermal conduction in polymers is mainly molecular conduction, not lattice conduction. This hypothesis was questioned at the beginning of the investigation, but the results seem to confirm it.

Rolled nylon would be superior to cast nylon for use in bearings since it could better conduct heat away from hot spots. The lower conductivity in the direction through the thickness of the liner would not be a drawback in bearings provided with fluid lubrication since the heat generated by friction is not conducted away through the walls but is carried away by the lubricant. The nylon would have to be annealed after rolling to eliminate residual strains caused by rolling which make it brittle, even though annealing caused some decrease in the degree of orientation of the molecules, with accompanying decrease in the effect of anisotropy of conductivity.

The following conclusions were reached from tests made on nylon 66 slab characterized by an intrinsic solution viscosity of 2.53 in 90 percent formic acid, indicating a degree of polymerization of 148.