The thermal conductivity of molten plastics is an important material property from the point of view of processing, since it affects temperature distribution and cooling behaviour of the melt. This piece of information is needed for a reliable process simulation of extrusion and injection moulding; its prediction and understanding can lead to increasing productivity and better quality. Using a dedicated accessory of CEAST SR50 capillary rheometer and following the ASTM D5930 transient-line source technique, we measured the thermal conductivity of selected samples of LDPE, PS and PC under different conditions of temperature and pressure. The measurements were carried out under typical conditions of temperature and pressure for plastics processing, up to 300 °C and 200 MPa (572 °F and 29,000 psi).
ASTM D5930 standard method requires a high-sensitivity probe to be plugged into the fluid sample. According to ASTM, the probe shall be in the shape of a needle with an elevated length-to-diameter ratio, hosting a thermocouple and a heating wire. The diameter of the surrounding sample shall be large enough to allow for a significant heat transmission without striking the boundaries. The CEAST system complies with ASTM D5930 and even with the more restrictive ISO 22007-1, and is managed by the same Visual Rheo software used to control the rheometer and run other types of tests. The chain of measurement is calibrated against a sample of known and certified thermal conductivity.
LDPE (low-density polyethylene) and PS (polystyrene) were tested at temperatures ranging from 180 to 200 °C, at pressures ranging from 5 to 200 MPa. For LDPE we observed an increase of thermal conductivity by approximately 45% when going from 5 to 200 MPa, while for PS the increase was around 35%. Comparing measures taken at the same pressure, the effect of different temperatures was not very significant.PC (polycarbonate) was tested at 300 °C, at pressures ranging from 60 to 200 MPa. For PC we observed an increase of thermal conductivity by approximately 15% when going from 60 to 200 MPa. In the same pressure range, the increase of thermal conductivity was 30% for LDPE and 20% for PS.
Measured values of thermal conductivity ranged from 0.22 to 0.32 W/(m*K) for LDPE, from 0.16 to 0.22 W/(m*K) for PS, from 0.24 to 0.28 W/(m*K) for PC.
For all the samples tested, an increase of pressure produces a significant increase of thermal conductivity, whereas an increase of temperature alone doesn’t have significant effects. This information can be used to optimize the choice of materials and the processing parameters.