Polyethylene
The inside lingo of polyethylene film terms such as low density polyethylene (LDPE), medium density polyethylene
(MDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE), metallocene, fractional melt and barefoot can be confusing.
There is no such thing as “regular poly”. There are literally hundreds of variations on polyethylene resins for film. Each is designed to address certain end uses. Any grocery store has a multitude of different polyethylene resins. The resins used for bags of salt, gallon milk jugs, motor oil, bird seed bags, bread bags, produce rolls, paper towel overwrap and shrink film are all distinctly different.
While you don't need to be a polymer scientist to fill out an RFQ, a basic understanding helps. This 90 second course in polyethylene resin will give you a rudimentary grasp of polyethylene film terminology.
First, the basics:
What is polyethylene ?
Polyethylene is comprised of long chains of the building block ethylene monomer. Ethylene monomer looks like this in English:
By the way, there is no correlation between polyethylene plastic and crude oil. The feedstocks are either natural gas or the naptha section of crude oil. Gasoline and motor oils cannot be made from naptha, or “higher alpha olefins”.
Polyethylene looks like this:
Variations in how these chains are put together determine the wide variety of resin properties. The fancy name for this is molecular weight distribution.
The 2 main properties which determine a resins's personality are density and melt index.
Specific gravity, or density expressed in grams per cubic centimeter (gm/cc) influences the properties the most. Resins are described by the density ranges as explained in the following table:
Melt index (MI) measures viscosity by weighing the amount of plastic resin which extrudes from a cylinder with a hole in the bottom at 300 degrees for 10 minutes. The test method is ASTM D1238. The number of grams is the melt index. A fractional melt has less than a gram, hence the name fractional, or less than 1 gram. It's common sense that resins with high viscosity (resistance to flow) are inherently strong. In film applications, fractional melt is suitable for shipping sacks and shrink film. There is a trade off between melt index and clarity. The higher the melt index, the clearer the resin.
Myths and Facts
Myth: Polyethylene film and bags start as oil and use up hundreds of thousands of barrels of oil every year.
Fact: Polyethylene is condensed natural gas. Ethylene, propylene and many other MONOmers (building blocks) are extracted from the natural gas pipeline to make POLYmers which are longer chains of MONOmers.
Myth: Polyethylene, like all other plastics, never breaks down.
Fact: Polyethylene has a half-life of arguably several hundred years. This is a desirable characteristic for applications such as landfill liners.
Myth: Coextruded films cannot be replaced by monolayer (single layer) films.
Fact: It depends on the application. Many “coex” films are more elaborate and inherently expensive than necessary.
Read some myths / facts about plastics as reviewed by SPI, The Plastics Industry Trade Association.
(MDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE), metallocene, fractional melt and barefoot can be confusing.
There is no such thing as “regular poly”. There are literally hundreds of variations on polyethylene resins for film. Each is designed to address certain end uses. Any grocery store has a multitude of different polyethylene resins. The resins used for bags of salt, gallon milk jugs, motor oil, bird seed bags, bread bags, produce rolls, paper towel overwrap and shrink film are all distinctly different.
While you don't need to be a polymer scientist to fill out an RFQ, a basic understanding helps. This 90 second course in polyethylene resin will give you a rudimentary grasp of polyethylene film terminology.
First, the basics:
What is polyethylene ?
Polyethylene is comprised of long chains of the building block ethylene monomer. Ethylene monomer looks like this in English:
By the way, there is no correlation between polyethylene plastic and crude oil. The feedstocks are either natural gas or the naptha section of crude oil. Gasoline and motor oils cannot be made from naptha, or “higher alpha olefins”.
Polyethylene looks like this:
Variations in how these chains are put together determine the wide variety of resin properties. The fancy name for this is molecular weight distribution.
The 2 main properties which determine a resins's personality are density and melt index.
Specific gravity, or density expressed in grams per cubic centimeter (gm/cc) influences the properties the most. Resins are described by the density ranges as explained in the following table:
Density (gm/cc)
Name
Recycle Symbol
Texture
Heat Resistance
Examples
.940 to .965
High density
polyethylene
HDPE
stiff
high
Milk crates
Gallon milk
Motor oil bottle
Grocery bags
Gallon milk
Motor oil bottle
Grocery bags
.926 to .939
Medium density polyethylene
MDPE
Paper towel overwrap
.915 to .925
Low density
polyethylene LDPE and linear low density LLDPE
Bread bags
Trash bags
Trash bags
.900 to .914
Very Low density polyethylene VLDPE
Dry cleaner bags
.867 to .899
Ultra Low density polyethylene ULDPE
soft
low
Melt index (MI) measures viscosity by weighing the amount of plastic resin which extrudes from a cylinder with a hole in the bottom at 300 degrees for 10 minutes. The test method is ASTM D1238. The number of grams is the melt index. A fractional melt has less than a gram, hence the name fractional, or less than 1 gram. It's common sense that resins with high viscosity (resistance to flow) are inherently strong. In film applications, fractional melt is suitable for shipping sacks and shrink film. There is a trade off between melt index and clarity. The higher the melt index, the clearer the resin.
Myths and Facts
Myth: Polyethylene film and bags start as oil and use up hundreds of thousands of barrels of oil every year.
Fact: Polyethylene is condensed natural gas. Ethylene, propylene and many other MONOmers (building blocks) are extracted from the natural gas pipeline to make POLYmers which are longer chains of MONOmers.
Myth: Polyethylene, like all other plastics, never breaks down.
Fact: Polyethylene has a half-life of arguably several hundred years. This is a desirable characteristic for applications such as landfill liners.
Myth: Coextruded films cannot be replaced by monolayer (single layer) films.
Fact: It depends on the application. Many “coex” films are more elaborate and inherently expensive than necessary.
Read some myths / facts about plastics as reviewed by SPI, The Plastics Industry Trade Association.