Low Density Poly vs. Linear Low Density
How it’s made & different applications.
You’ll notice that the immediate difference between Low Density and Linear Low Density is that one is linear and the other is not. Both refer to low density. We’ll review what polyethylene is; how it is made and some of the terminology. By understanding terms like what is meant by linear and density, the differences between the two fall into place pretty easily.
Let’s start with the basics:
What is polyethylene? (Simplified)
Polyethylene is a combination of words in a way lending a clue as to what it is. “Poly” coming from the Greek word for “many”; and “ethylene” from the gas that is used to “polymerize” the raw material into resin; “mer” = parts. The “poly” or “many” refers to many chemically bonded chains that are created in the process of making polyethylene (a polymer or linking together) from ethylene gas.
The raw material, ethylene gas (a petroleum product) consists of two hydrogen atoms and a carbon or C2H4. What happens in the manufacture of polyethylene is that these carbon and hydrogen atoms link together and line up into chains of CH2’s; they “polymerize”.
Some of the chains are long; some are short and side branching chains form to various degrees in the different polyethylenes. The more chains there are and particularly the more relatively straight chains there are lining up in a linear fashion is what adds to the density, tighter weave and hence, greater mass (meaning it actually weighs more) and it is most crystalline. Molecular Weight and weight distribution of a resin or film also plays a part in the physical properties of a film. Molecular weight is the average weight or size of these chemical chains in the film or resin, but a little too complex to get into right now. It is this chain density and weight that characterizes polyethylene as either Low Density, High Density or Linear Low Density. In Low Density, these molecules are lining up all over the place, sometimes in a “linear” fashion but also with a lot of random loose and open side branching. These are called “amorphous” regions. This structure is typical of regular Low Density poly. In films where most of the chains are lining up straight next to each other, these are called linear films as in Linear Low Density. When chains line up next to each other, the resin becomes of higher density and crystallinity as in LLDPE. It is the branching of LDPE or lack of it in LLDPE that effects the films physical properties, including hardness, flexibility, strength and clarity, among others.
Knowing more about the differences in how the resins are formed and films produced from them, you can now consider the differences regarding package design and choosing the best film for the application. The customer’s application may call for a package where they don’t want something that is super strong, but instead, want something that feels softer and has higher clarity. In this case you would be wise to choose LDPE. Low density does not have the tight, stronger, higher density, crystalline linear pattern as in Linear Low Density. Is it beginning to make some sense? If the customer cares less about clarity, but needs strength, barrier, chemical resistance, puncture resistance, choose the Linear Low Density Polyethylene. It is this linear character and packed, parallel grouping of chemical chains that makes it different than Low Density.
The resin producers (Exxon, Mobil, Equistar, Fina) make these resins under different processes in order to obtain the different properties. They sell the resin to people like DanaFilms, Brentwood, or Converter’s and many others. Our film suppliers make films for us from these resins that have these different properties. The film people simply take the plastic pellets supplied by the resin guys, re-melt them and make film or sheeting out of it. The film contains the properties of the original resin and the film people can process it in a way to enhance or add to these properties based on your needs.
(We may discuss Blown Film Extrusion in the next session).
Resin producers “control” the polymerization as follows:
While each are produced from ethylene gas:
Low Density resin is made in high pressure, high temperature reactors.
Linear Low Density resin is made at low pressure, low temp, combined with butene gas, hexene gas, or octene gas (your choice) each gives other properties.
High Density resin is also made at lower pressure and temperature than LDPE with different catalysts in a different style reactor than the other two.
One thing very interesting about High Density film is that, the reason they call it high density is because the chains we spoke of are parallel and highly packed together, causing it to be “very dense”. I like to picture density and film orientation (which direction the chains lined up) like threads in a bed sheet or the screen in a window. While HD is very strong, it tears easily in the machine direction. This is because most of the chains (fibers) are lined up parallel to one another and with little cross branching, the tear occurs in between and down the length of these polymer chains. This is bad for packing sharp objects that can easily start the tearing action like metal parts or plastic boxes, but good for applications with tear notch and packages requiring an easy open feature such as in garments and cereal bags. The proper film choice all depends on what the customer wants the film to do. There is also a Medium Density poly which has most of the features of HD, but is clear, by comparison. Because it is less dense, you can see through the “threads” or crystallinity.
As density increases:
Chemical resistance; heat resistance; barrier; strength; cloudiness; flex cracking; increase.
Stretch; flexibility; blocking decrease. As density decreases, as in Low Density, the reverse is true.
This discussion was intended to give a simplified view and some general insight into why polyethylene can be “linear”, of different “densities” and what this means in terms of film performance and choosing the right film for the application.