How Polymer Branching & Molecular Weight Impact Processability

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When I begin my conversation with any customer, my first question is usually, “what is your main process?” Quite often the polymer process is the basis to helping us identify 1) the type of resins we are dealing with and 2) the best way to tackle cleaning that resin. Much more specific information can be obtained from learning melt temperatures and melt indexes, etc., but we can get clues to a resin type, for example, from simply understanding if it is being used for Blow Molding vs. Blown Film Extrusion.

In a previous blog focused on polymer degradation, I discussed different types of polymerization giving rise to the many types of polymers we see in the vast industry we have today. In this article, I wanted to expand on that discussion and delve a little deeper in to the molecular structure of these polymers. Very small differences in the molecular structure are what give rise to the unique properties they exhibit AND hence the ability of that polymer to then be processed in a certain way for its end use.

Let’s take a closer look at Polyethylene. This polymer in particular varies greatly in its molecular structure, and we witness varying degrees of molecular weight in one family alone i.e. HMWPE, UHMWPE, HDPE, LDPE, LLDPE. We have to remember that these types of PE all come from the same polymer backbone:

Kiran 1

 

And it is small changes in the branching structure that gives rise to the very different behaviors they exhibit.

Kiran 2

Looking at the molecular structures in more detail can help us understand the differences in their behavior, and how/why they are more likely to be utilized in a particular industry.

Low-density polyethylene (LDPE) the chain image at the top of image 2, shows a lot of branching, not only on the backbone, but also branching from other chains on the backbone. A high degree of branching gives it a less tightly packed structure (and an inherent lower density). This leads to chain entanglement making the structure a lot more flexible (not rigid). LDPE therefore exhibits a lower tensile strength but a higher resilience compared to HDPE – making it suitable for lightweight applications like plastic bags and containers.

The chain in the middle shows us a Linear-Low Density Polyethylene (LLDPE) here the chain has a lot of branching, but the branches are shorter, therefore you do not get chain entanglement, instead the chains tend to slide by one another (linear not entangled). This characteristic is what allows the LLDPE to process well in film applications as it is stiff (when in shear) and soft (when extended)- making it ideal for applications like stretch wrap or bin liners.

The bottom chain High-Density Polyethylene (HDPE) shows very little branching on the polymer backbone, this gives rise to much shorter, and therefore stiffer and stronger bonds between each molecule. The ability to create a more tightly packed structure gives rise to the higher density, and the strong bonds make the PE hard and opaque, allowing the HDPE to be used in more rigid applications like pipe and furniture applications.

These are just a few example of Polyethylene variations, there is a vast range of polymers out there, each exhibiting unique behaviors. When any recommendation is being made in the polymer processing industry, knowing and understanding your polymer and how it behaves, is extremely important.

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Topics: Troubleshooting, Color Changes, Plastics Processing, color contamination, Extrusion, Plastics Production, Melt Flow Index, reduce downtime, Extrusion Purging, Blown Film Extrusion, plastics manufacturing, Reduce Scrap-rate

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