Why Are HDPE and PP Not Mixed During The Extrusion Process // Bausano Blog

In extrusion, it is common to evaluate the use of different materials to improve performance, reduce costs or increase recycled content. However, it is important to remember a fundamental principle. Polymers with apparently similar characteristics are not necessarily compatible during the extrusion process. Bausano, extrusion experts, explain what can cause incompatibility as well as how to approach the introduction of new materials. 

Take two widely used materials as an example: PP and HDPE. Both belong to the polyolefin family, but they have different molecular structures that lead to different behaviour during melting.

This means that simple blending does not automatically guarantee a material with intermediate characteristics. On the contrary, issues may arise that compromise both the process and the performance of the finished product.

There are three aspects to consider:

Chemical Compatibility

Two materials may melt at the same temperature without being truly compatible. When the polymer phases do not interact correctly, the resulting material often exhibits mechanical properties that are inferior to those of the individual components.

Behaviour During Processing

Viscosity, melt flow index and rheological behaviour directly influence plasticisation, pressure and process stability. Different materials respond differently even when processed on the same line.

Performance of The Final Product

Rigidity, impact resistance, long-term behaviour and surface quality all depend on the balance of the entire formulation. A blend that has not been correctly designed can compromise the very performance it was intended to improve.

A Concluding Consideration

When evaluating the introduction of a new material or a polymer blend, the question should not be:

‘Is it possible to extrude it?’

The correct question is:

‘How will it behave throughout the entire process and what effects will it have on the final product?’

It is precisely this preventive analysis that makes it possible to reduce trials, scrap and machine downtime, making the process more predictable and repeatable.

Read more from Bausano here.

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