- February 01, 2004, Per-Åke Clevenhag & Claes Oveby, Tetra Pak Converting Technologies AB
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Application: A new rheological test method can predict the processing performance of LDPE for extrusion coating.
Changes in molecular structure have great impact on the rheological and processing properties of a polymer. Therefore, these parameters are extremely significant in understanding a wide range of polymer processing operations.
In extrusion coating, a thin molten polymer film is coated on a substrate. At high extrusion coating speed, even a minor disturbance on the melt web causes major quality problems that can very rapidly lead to large quantities of waste. Higher coating speeds therefore require polymers with high and consistent quality to avoid waste due to polymer edge instability and web breaks.
The rheology-related phenomena that may cause problems in extrusion coating are neck-in (NI) and draw-down (DD). Neck-in is the reduction of film width. It can cause uncoated areas on a substrate. Neck-in is less if the melt elasticity is high. Draw-down is the ability of a melt to be drawn to thin films without breaking. A melt that is more viscous than elastic favors draw-down.
In optimizing an extrusion coating process, balancing the elasticity of a polymer has utmost importance. This paper describes how oscillatory measurements can characterize LDPE. G prime is a useful parameter to predict the extrusion coating performance of LDPE.
This study used rheological measurements performed on a commercially available controlled stress melt rheometer. Polymer pellets were melted, compressed, and cut into discs with a diameter of 25 mm and a thickness of 1 mm. Parallel plates were used for measurements under a nitrogen atmosphere. Oscillatory measurements were performed at 170∞C in the linear viscoelastic region with a frequency sweep between 20 and 0.01 Hz. When a material undergoes oscillatory stress with frequency, the response can be expressed in terms of a storage modulus, G' (G prime), and a loss modulus, G'' (G double prime).
EXTRUSION COATING TRIAL #1
Eight grades of autoclave LDPE from different suppliers were tested on a pilot extrusion coating line. All grades had melt flow range (MFR) between 7 and 9 g/10 min. and densities of 918-920 kg/m3. The melt temperature was monitored to 305∞C by means of an infrared camera. The 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. Testing was in duplicate for each grade of LDPE.
EXTRUSION COATING TRIAL #2
Five LDPE grades from extrusion coating trial #1 were tested in another pilot extrusion coating line. The methodology was similar to the one used in Extrusion Coating Trial 1. Melt temperature was monitored to 295∞C.
G' MONITORING OF TWO LDPE GRADES
To determine the product consistency and possible correlation between G' and MFR, two LDPE suppliers were selected to send samples from two lots each week during a period of four months for G' measurement. The suppliers provided the MFR data (2.16 kg; 190∞C).
RESULTS AND DISCUSSION
Storage modulus — G prime — is determined at loss modulus 500 Pa by the use of a Cole-Cole plot as Fig. 1 shows. The log loss modulus is plotted vs. the log storage modulus in the range of 200-900 Pa of the loss modulus. A linear relation is obtained. The storage modulus can be determined at loss modulus equal to 500 Pa (log 500 = 2.7).
When observed draw-down in the extrusion coating trial 1 was plotted against G', the following relationship occurred:
DD = — 6.3G' + 1050 (m/min.)(1)
The value for R2 was 0.93.
A relationship between draw-down and neck-in, with an R2 of 0.93, was also found:
NI = 0.13DD + 44 (mm)(2)
MFR showed no correlation to observed draw-down. An R2 of 0.11 was achieved when observed draw-down was plotted vs. measured MFR.
The relationship in Eq. 1 between draw-down and G' found in the first test was used to predict the draw-down in the second test. An R2 of 0.96 was found if observed draw-down was plotted vs. predicted draw-down.
Table I shows the results from the two pilot extrusion coating trials.
Table II gives the results from the monitoring of the two LDPE grades.
Both grades showed MFR 190∞C within specification. A significant difference existed in G' between the two grades both in level and variation. LDPE1 shows better consistency and can be used for higher line speeds. LDPE2 exhibits lower neck-in but shows more fluctuations.
For a long time, LDPE from high-pressure autoclave reactors for extrusion coating with an MFR from 7-9 g/10 min. and density of 917-920 kg/m3 has been considered as a uniform commodity. As evolution proceeds toward faster coating lines, detecting differences in processing performance between various LDPE grades has been possible.
The parameters of MFR and density give very limited information on the processing performance of a product. Very often a resource demanding test run of the LDPE material in a pilot or production coating line is necessary to determine processability. Extrusion coating performance can be determined and predicted conveniently and with considerable less resources by the rheological method described in this paper.