Rheological Indicators to Predict the Extrusion Coating Performance of LDPE, 2004 PLACE Conference

Application: A new rheological test method allows prediction of processing performance of LDPE for extrusion coating.

Executive Summary: LDPE from high-pressure autoclave reactors for extrusion coating with a melt flow rate (MFR) range from 7 to 9 g/10 min with a density of 917 to 920 kg/m3 has for many years been a uniform commodity. As evolution goes toward faster coating lines, differences in processing performance between various LDPE grades have occurred. Various LDPE grades behave differently in the coating process due to different rheological properties.

Thirteen grades of autoclave LDPE from different suppliers were tested in two pilot extrusion coating lines on three different occasions. All grades had MFR between 7 and 9 g/10 min and densities of 918 to 920 kg/m3. The melt temperature was monitored to 295–305°C by means of an infrared camera. In principle, the methodology was as follows. Extruder screw rpm was set to give 10 g/m2 at 150 m/min to obtain stable processing conditions. The line speed was then increased in steps of 50 m/min until the web broke. The line speed at which the web broke (draw-down speed) and the neck-in were reported. This was made in duplicate for each grade of LDPE.

The parameters MFR and density give very limited information on the processing performance of a product. A resource demanding test run of LDPE material in a pilot or production coating line is often necessary to determine processability. Extrusion coating performance can be conveniently and considerably less resource demanding when determined and predicted by the rheological method described in this paper.