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Brentwood Plastics Blog

LLDPE Linear Low Density Polyethylene varieties

Posted by Joel Longstreth on Tue, Nov 27, 2012 @ 03:14 PM

As with all polymers Linear Low Density Polyethylene LLDPE has a tremendous variation within the species.
This article explains the differences in context of the history and evolution of Linear Low Density PE.

Think of the differences between the grades like the difference in gasoline choices:

Butene    =   Economy

Hexene   =    Mid grade

Premium  =    Octene

These refer to the types of comonomer married up with the ethylene monomer to make linear low density polyethylene.

A quick history lesson might help.

Until the late 1970's, " linear" polyethylene was verbal shorthand for milk jug high density polyethylene HDPE.  It was nicknamed " linear " because it had very little short chain, or amorphous, branching.  The shape of the molecules was more like spaghetti before cooking vis a vis after cooking.

When Carbide shook the plastic world with the unveiling of their Unipol gas phase linear polyethylene in 1978, there was a lacuna, or " gap " in polyethylene language.  PE which was made in densities under 0.930 is by definition LOW density LDPE polyethylene.  To distinguish this new Low Density Polyethylene LDPE from conventional LDPE, it was dubbed LINEAR low density polyethylene or LLDPE for short.

Dow had a different approach with their " solution " phase polymerization process.  This mimicked the old DuPont Canada SCLAIR LLDPE resin which was the true forerunner of all Linear Low resins.

Carbide opted for the cheaper butene and Dow went top shelf with octene.  In the 80's, Dow was easier to process ( less susceptible to melt fracture ) and had better physical properties than the butene gas phase.  As a side note, Carbide tried for several years unsuccessfully to induce extruders to take the resin in the form of a powder ( think laundry detergent consistency ) and pass along the savings of pelletization.  Dow provided old fashioned pellets.

In the 90's, Mobil tried to bridge the gap with hexene which is less expensive than octene and more expensive than butene.  They made LDPE resins on Unipol style reactors.  They chased Octene with  " super " hexene which has always been " not exactly " octene.  The solution phase process allows more versatility and faster changeovers than Unipol.  

Skip ahead to today.  Butene - 1 is the only LLDPE film resin which is sold as a commodity on the London exchange.  It is cheap and weak.  Nova purchased the DuPont Canada PE resin works a few years ago.  Nova offers both butene and octene LLDPE.  "Super" hexene is still around.  Lower cost Unipol resin plants have sprung up all over the planet.  Nova just announced an expansion of another billion pounds of butene capacity.

It's kind of amusing to look back at the predictions of the late 70's with the benefit of hindsight.  Everybody thought LLDPE would make LDPE obsolete like TV was going to be the nemesis of radio.

Blends of LDPE and LLDPE deliver some interesting properties.  

When specifying Linear Low LLDPE, be specific about which kind of LLDPE you want and whether you want it 100% or blended with one of the myriad varieties of conventional LDPE.

 

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Topics: LLDPE, Linear Low Density Polyethylene

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