Polyethylene was first discovered by accident in a laboratory in 1898. This is a running theme throughout the history of the plastics industry. It was not until 35 years later that the first commercially viable polyethylene was synthesized in 1933 by Imperial Chemical Industries (ICI). The long gap between the initial discovery of a polymer and the commercialization is also a recurring theme in the history of the plastics industry.
Polyethylene is the most commonly used plastic globally. According to Wikipedia, annual production is around 80 million metric tons or 176 billion pounds. It is used in many applications such as film that is used for everything from moisture barrier to grocery bags. It is also used for things like pick-up truck bed liners and tanks of all kinds. In recent years, polyethylene has been used for increasingly demanding applications like automotive fuel tanks, potable water pipe, artificial hip joints and even fiber for bulletproof vests.
Polyethylene’s use in these increasingly demanding applications has been made possible by innovations from the material manufacturers. Unlike TPO, these innovations have not happened at compounders but during the reaction of the polymer. These innovations have brought a dizzying array of new polyethylene materials to the market. A quick look at Matweb shows that there are currently 5534 different grades of polyethylene. I want to break down some of the different types that are now available.
Polyethylene is made through the polymerization of ethane. Ethane is derived through steam cracking from constituents of petroleum or natural gas. This polymerization is complex. By altering the conditions, catalysts, and through the addition of co-monomers, the properties of the finished product can be altered dramatically. The tensile strength can vary from 1200 psi for some LDPEs to 7500 psi for some UHMWPEs.
There are currently 8 different types of polyethylene that are on the market.
- High Density Polyethylene (HDPE)
- High Density Polyethylene Copolymer
- Linear Low Density Polyethylene (LLDPE)
- Low Density Polyethylene (LDPE)
- Ultra High Molecular Weight Polyethylene (UHMWPE) (also HMWPE and HDPE-HMW)
- Cross Linked Polyethylene (PEX or XLPE)
- Medium Density Polyethylene (MDPE)
- Very Low Density Polyethylene (VLDPE)
I am going to talk about the first five of these today. Cross linked polyethylene is only used for water supply piping and is a subject in itself that maybe I will cover in a future article. MDPE and VLDPE are not very commonly used and are not worth the time at the moment.
High Density Polyethylene
High density polyethylene or HDPE is defined as having a density of 0.941 g/cm³ or higher. I am referring specifically to homopolymer HDPE (IE: homogeneous HDPE). Copolymers are covered in the next section. It is polymerized in a way that produces long molecular chains with very little branching. This allows the chains to be packed very closely together which increases the density.
HDPE is susceptible to stress cracking. Stress cracking is defined in ASTM D883 as “an external or internal crack in a plastic caused by tensile stresses less than its short term mechanical strength”. Increasing the molecular weight of the material (increasing the lengths of the molecular chains) can improve stress crack resistance but this also lowers the melt flow of the material and makes it more difficult to process. In addition, the molecular weight distribution can also help with stress crack resistance. You will often see materials that are advertised as “narrow molecular weight distribution” or “bimodal”. Here is what these terms mean:
- Broad Molecular Weight Distribution (BMW): The material has a wide variation in the size of the molecular chains. This gives the best stress crack resistance.
- Narrow Molecular Weight Distribution (NMW): The material has a smaller variation in the size of the molecular chains. This will result in parts that will hold their shape better and better creep resistance.
- Bimodal Molecular Weight Distribution: This material has primarily two different lengths of molecular chains mixed together. This gives better physical properties and good processibility.
You would think that higher molecular weights would result in higher tensile strength material but actually the density of the material has more impact on tensile strength. HDPEs with density over 0.960 g/cm³ will typically have very high tensile strengths, some as high as 4600 psi.
Here is a chart that I found at the plastics pipe institute website that shows how density, melt flow rate and molecular weight distribution effect the properties of HDPE²:
HDPE Copolymers
In order to improve the stress crack resistance of polyethylene, copolymers were introduced. According to Ineos, a manufacturer of HDPE copolymer, “Stress cracking is directly influenced by the type, length and complexity of chain branching”¹. Adding a co-monomer to the ethylene produces HDPE copolymer that has increased short chain branching and thus, is more stress crack resistant. Copolymers tend to have lower densities than homopolymers because of the short chain branching. The principle advantage of copolymer HDPE over higher molecular weight homopolymers is that you get the improved stress crack resistance without the corresponding loss of melt flow rate.
There are three different types of HDPE copolymer on the market. Hexene copolymer, Butene copolymer and Octene copolymer. In addition to the three different co-monomers that are used, there are also different catalyst technologies that are used such as Metallocene and Ziegler-Natta. Catalysts are chemicals that are used to increase the rate of a chemical reaction. Manufacturers have also developed methods to alter the distribution of the co-monomer within the finished polymer. All of these things affect the properties in different ways but the intricacies of the different technologies are beyond the scope of this article and beyond the knowledge of the author as well.
I wish that I could tell you that one combination of co-monomer and catalyst is superior to all of the others but after reviewing properties on a number of data sheets, I cannot see any clear advantage or disadvantage to any of these technologies over any other. They are really different methods of achieving the same goal. Each manufacturer will tout the benefits of the technology that they use. If company A makes HDPE hexene copolymer, they will explain to you why it is the best. Company B, on the other hand, which makes butene copolymer will tell you that it is superior to the hexene copolymer. You may need to sample a few different varieties and perform some testing to see which one works best for your application. Many of the manufacturers have excellent technical support people that can help as well.
Linear Low Density Polyethylene (LLDPE)
LLDPE is basically HDPE copolymer with more of the co-monomer in it. As you add more and more of the co-monomer, you get more and more short chain branching. Eventually it starts to lower the density of the material until it is not HDPE anymore. The result of this is referred to as linear low density polyethylene.
LLDPE has less tensile strength than HDPE but more than LDPE. It has excellent stress crack resistance. Like HDPE copolymer, LLDPE can be made with Hexene, Butene or Octene. Sometimes the manufacturers do not tell you what the co-monomer is or they may refer to it a alpha-olefin (or α-olefin) which is a general term that describes any of the three.
Low Density Polyethylene
Low Density Polyethylene has a high degree of short and long chain branching. This is caused by the absence of a catalyst during reaction that would limit the branching. This branching results in less efficient packing of the molecules which results in lower density. It is defined as having density from 0.910-0.940 g/cm³. LDPE has much lower tensile strength and is much softer than HDPE. LDPE can have hardness as low as 90 Shore A, which means that in some cases, it can replace TPE materials.
The biggest application for LDPE is film. The film is in turn used for things like food storage bags, garbage bags and moisture barrier that is used in residential construction. Also some rigid containers like milk jugs are made of LDPE.
Here is a representation of what the molecular chains look like for HDPE, LLDPE and LDPE. Maybe this will help give you an idea of the differences.
Ultra High Molecular Weight Polyethylene (UHMWPE)
UHMWPE (sometimes refered to as HMWPE or HDPE-HMW) is made by allowing the molecular chains in the material to grow to very long lengths, as much as ten times the length of normal HDPE. These long chains do not pack as efficiently as shorter chains so the density of UHMWPE is actually lower than HDPE. The density is usually in the 0.930-0.935 g/cm³ range.
The result is an extremely tough material with very high tensile strength and impact and surface lubricity as well. The downside is that the melt flow of UHMWPE is very low, usually below 0.10 g/10 minutes.
In fact, the melt flow is sometimes so low that the traditional melt flow test does not yield a result. Manufacturers have introduced a new test method to better express the viscosity of these materials. They call it High Load Melt Index or HLMI. It uses the same test equipment as the traditional melt flow rate test, but uses a much bigger weight to push the material through the orifice.
One way to watch for this to look at the conditions listed after the melt flow test on data sheets. Melt flow results are always supposed to be listed with the conditions that the test was performed at. For instance, you will typically see something like this:
Melt Flow Rate (190°C/2.16 kg) 20 g/10 minutes
The “190°C/2.16 kg” is the temperature that the test is performed at and the weight that is used to push the melted material through the orifice.
Materials that are tested at high load melt index will typically look like this:
Melt Flow Rate (190°C/21.6 kg) 20 g/10 minutes
At first glance, this material looks like a 20 melt, but if you notice in the conditions, the weight used has changed from 2.16 kg to 21.6 kg. At the normal conditions, this material is probably something like 0.10 g/10 minutes, however it should be noted that there is no way of directly converting the melt flow rate at one condition to a melt flow rate an another condition.
UHMWPE is available in fibers, shapes and profiles for machining and in powder form for compression molding and extrusion. The melt flow is just too low for injection molding.
Conclusion
I hope this helps clear up some of the differences between the various grades of polyethylene that are on the market today. I wanted the primary focus of this article to be the wide variety of different materials sold under the HDPE moniker today which I think is especially confusing. Picking out a polyethylene used to be a matter of deciding on LDPE or HDPE and then deciding what melt flow you needed. There are many more choices today which is good in a way but makes choosing the right one more difficult.
References:
weeklypellet.com 