What’s the difference between low density and linear low density polyethylene?
On our “Big Three” page, we learned about LDPE, HDPE and Polypropylene (PP). Each have their own distinctive physical properties in terms of clarity, dart strength and tensile strength. With these properties in mind, you can make the best recommendation to your end users as to which raw material their packaging should be made from.
When it comes to the family of plastics considered “low density” polyethylene, there is more for you to know, specifically, how Linear Low Density Polyethylene (LLDPE) differs from its fraternal twin called Low Density Polyethylene (LDPE).
How about a quick chemistry lesson? Here goes. LDPE and LLDPE differ in their arrangement of polymer chains. A polymer is essentially a large molecule and when those large molecules connect, they form polymer chains. Before class is dismissed for the day, remember one last thing: for plastic bags made from either raw material, the density of polymer chains dictate the look, feel and strength of that bag.
How can you tell the difference?
Simply put, LDPE is clearer and softer than LLDPE because its polymer chains are less tightly-packed. LLDPE feels firmer, is stronger and has a slight haze to it because the polymer chains in the plastic are more densely-packed. Next time you meet with a customer and they have questions about low density and linear low density, try interlocking your hands to describe the strength and clarity. Like this:
It’s an elementary yet intuitive model. My fingers represent polymer chains. There is more space between polymer chains in LDPE, so you can see through easily, though the arrangement is clearly not as sturdy. My fingers come together much closer in the image on the right and that interlocking creates a stronger grip, just like polymer chains in an LLDPE bag.
Why not just use one material?
Each has their pros and cons. For low density, it’s difficult to extrude a bag at widths wider than 18 inches because “the bubble” that is blown loses stability. For production purposes, using Linear Low for larger bags becomes a manufacturing necessity. It’s also an assumption that larger bags, tubing or sheeting will be used in more demanding applications and therefore the added strength of linear low density is a convenient bonus.
Low density is more clear, which is a desired and necessary physical property in some situations. In fact, low density is referred to as “clarity” due to this characteristic. For this reason, it’s worth noting that the price per pound of clarity can be anywhere from 10 to 15% higher than LLDPE.
It doesn’t take a microscope to tell the difference between LDPE and LLDPE. You may get questions from your customers as to why one bag they buy from you looks different than another. Now you know! Polymer chains at the molecular level determine tensile strength, dart impact resistance and clarity. LLDPE has superior strength, but sacrifices some clarity along the way. LDPE’s clarity make it a valuable commodity and thus a more expensive option for packaging.
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