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Brentwood Plastics Blog

Polyethylene Density

Posted by Joel Longstreth on Tue, Oct 25, 2011 @ 10:59 AM

The properties of polyethylene are most affected by the density of polyethylene.

Without going into the vagaries of molecular weights and molecular weight distribution, there are 2 key polyethylene properties which are a function of density.  They are secant modulus ( a fancy word for stiffness or softness ) and heat resistance.  The lower the density, polyethylene is softer.  The higher the density, polyethylene is stiffer.

Density is expressed as grams per cubic centimeter.  Polyethylene has inexact demarcations between high density ( HDPE ) over .940, medium density ( MDPE ) .939 to .927, low density     ( LDPE ) .926 to .915, very low density ( VLDPE ) .914 to .905 and ultra low density ( ULDPE ) .890 to .904.  New metallocenes are emerging all the way down to .860.  They are so new they don't have a name yet.

Polyethylene has an advantage over PVC from what is known as "yield" or area of plastic sheet derived from a pound of resin.  PVC has a density of around 1.25 gm/cc.

To relate to these properties, here are a few quick examples from everyday life:

milk crates
made from an extreme HDPE of at least .962, a milk crate is stiff and has a high melting point.

milk jugs
.957 HDPE.  Here's a fun fact:  milk bottle resin was nicknamed "linear polyethylene" in the olden days because the molecules are lined up in a linear fashion making this the original linear PE.  The molecular structure of Linear Low Density or LLDPE, is similar, hence the verbal shorthand "linear low" vis a vis the old linear which is high density ( HDPE ).

motor oil bottles
.952 a little softer and squeezable than milk jugs

dry cleaner bags
made from a .915 to .918 LDPE, dry cleaner bags are soft and pliable with a low melting point

bread bags
.923 is the most popular choice for a middle of the road PE film

soft goods overwrap
.927 to .933 density MDPE is the perennial favorite for packaging rolls of paper towels.  This film must be stiffer to process on high speed wrapping equipment.

Metallocene mPE in different density ranges exhibits these same traits.  Metallocene is taking polyethylene where no polyethylene has gone before, but that's another story.

For geeky arcane, in-depth background, visit

http://www.brentwoodplastics.com/polyethylene.html

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