Statistical Models to Describe the Correlations Between the Molecular Mass Distribution and the Extrusion Coating Processability of Autoclave LDPE, 2003 European PLACE Conference Proceedings

In the past it is frequently tried to deduce correlations between molecular related parameters and the Extrusion Coating behaviour of autoclave ldPE. This investigation describes the approach to deduce, and the final obtained correlations between the Molecular Mass Distribution (MMD; incl. long chain branching) of autoclave ldPE and the extrusion coating processability.

The added value of these correlations would be a predicting test for converters and, because most suppliers have the knowledge to obtain a certain MMD of the material by modifying the process conditions, a powerful product development tool.

The processability of the various ldPE grades (Melt Index 8 dg/min.) is expressed in the properties draw down (DD) and neck in (NI). The draw down is the maximum line speed at which the PE web breaks (ability to extent a polymer melt to a thin film). The neck in is the shrinkage in width of the ldPE web. As is well known, these parameters are determined by the molecular mass distribution of the resin and are highly negatively correlated.

To build the models, a pragmatic approach is used. The relevant parameters of each technique are selected and statistical correlations (mostly linear regression models) are determined. It is well known that this “black box” approach can result in relations which are not expected / not understood from a rheological / molecular point of view. Knowledge input is required in e.g. designing the experimental set-up or selecting relevant responses as well as evaluating the results..

Finally, for the first time, quantitative correlations are derived for specific Molecular Mass data and the Extrusion Coating processability of autoclave ldPE (Fig. 1). By these models a quick ranking of the processability of different ldPE materials is possible and thus a useful tool for product development is obtained. But, of course the final test will always be at the customer, because besides draw down and neck also other parameters represent the processability .