Question: I have been discussing rotomolding resin made from butene versus hexene and have received mixed feedback. Is there a real difference between the two and are there applications where one is preferred over the other? Or is it just a question of price and availability, when it comes to using prime material in natural or color?
This is a question that I’m asked quite frequently. There isn’t a simple “bad / good / better” answer, it’s a bit more complicated than that. In the description below, I’ll try to concentrate on what molders really need to know, so my description could be viewed by some polymer chemists as a bit superficial. Others may think it’s over-complicated… I’ve tried to steer a middle way!
Butene and hexene (strictly speaking: butene-1 and hexene-1) are examples of comonomers that are used during the production of different polyethylene (PE) grades, including roto grades. They do essentially the same thing for PE, but their presence can cause some differences in grade performance.
The first thing to say is that PE is a semi-crystalline material. The individual molecules in a piece of solid PE may exist in one of three different configurations.
- Some molecules will be folded up in regular structures; these are in the crystalline phase.
- Some molecules will not be folded up and are present in a more irregular arrangement; these are in the amorphous phase.
- A few molecules may stretch between different crystalline regions, with their ends embedded into two different regions; these are called tie molecules: see the picture below. Remember these, you’ll hear more about them later….
Why does any of this matter?
The proportion of crystalline versus amorphous phases will have a big effect on the PE characteristics. High crystallinity grades are stiff and relatively brittle, low crystallinity grades are more flexible and are tougher.
Crystalline PE is denser than amorphous PE, so the density of the overall material is a good indication of the relative proportions of crystalline and amorphous regions. Roto grades, in the density region 0.935-0.945 g/cm3, represent a roughly equal balance between crystalline and amorphous phases. In other words, reasonably stiff and reasonably tough. Injection molding HDPE grades have a higher density (>0.950 g/cm3 density) and are stiffer but generally more brittle. Film grade LLDPE’s have a lower density (<0.925 g/cm3 mostly) and are less stiff and more tough.
Comonomers are introduced into polyethylene reactors in order to reduce and control the density of the PE grade being made; in other words, to control the balance between crystalline and amorphous phases. The more comonomer that’s added, the lower the density of the product. The comonomer molecules form branches on the main PE backbone chains; these branches prevent the PE chains from folding into regular structures, which lowers the crystallinity and also the density. If no copolymer is introduced, you will make PE with a high density (approx 0.960-0.963 g/cm3), aka true HDPE (which you can’t rotomold successfully; it’s for injection molding).
Having given you that background, let’s get back to the question!
The inferiority / superiority of PE grades made with different comonomers is the subject of discussion amongst the various technical authorities in rotomolding. My own viewpoint has been informed by working in a variety of different geographical areas, where practice can vary. This has been quite instructive.
In the European roto market, you see rotograde PE’s offered that utilise all of the three main comonomer types, viz:
Butene-1 4 C atoms present
Hexene-1 6 C atoms present
Octene-1 8 C atoms present
In contrast, most of the PE roto grades offered in North America utilize hexene-1. Similarly, roto markets in Australasia and South Africa predominantly use hexene-1 grades, because their local suppliers happen to utilize this particular comonomer in all their production. Markets in South Asia, India and China tend to use butene-1 grades; once again, the practice of local suppliers sets the tone.
So – what’s the actual story here?
My comments are specific to the linear medium density polyethylene (LMDPE) grades used for rotomolding; my assessment would be different if we were looking at extruded and cast film applications (which mainly use LLDPE’s; much lower density) or injection molding applications (use traditional HDPE; much higher density).
Comonomer choice is likely to make much more of a difference with LLDPE grades, simply because you need to add so much more of it to achieve the required density (normally down at the <0.925 g/cm3 level). With HDPE (0.950-0.965 g/cm3), there may be zero comonomer, or there may be a small addition.
Roto grades are roughly in the range; 0.930 – 0.945 g/cm3. Therefore the choice of comonomer is likely to have some effect on properties. This is what we see, but it depends on other factors also.
The main area where we may see differences between similar (Melt Index / Density) combinations is long-term properties. The normal measure we use for this is ESCR – environmental stress crack resistance. The ESCR test itself has its faults, the main one being that it can really only provide a rough differentiation between grades; basically the test enables you to describe ESCR performance as “poor”, “good” or “excellent”.
LMDPE grades with a Melt Index (MI) of 5 and above will have “poor” ESCR, whatever the comonomer choice. This is because the MI has a greater effect on ESCR than anything else.
LMDPE grades with a lower MI (2-3.5) will have “good” to “excellent” ESCR properties and here you can observe differences due to comonomer choice. For LMDPE’s in this MI range, hexene-1 grades tend to exhibit higher ESCR values than butene-1 grades. The further benefit of octene-1 compared to hexene-1 is harder to see in the LMDPE range. Differences between hexene and octene are more pronounced when testing in the LLDPE range, for film grades.
The explanation for the differences noticed in ESCR for lower MI grades mainly comes down to the existence of more tie molecules; remember them? The occurrence of tie molecules increases as you go from butene-1 to hexene-1 to octene-1 (see chart below). More tie molecules means that there is an increased tendency to bind areas of crystallinity together in a network structure. As well as improving ESCR, there may be some small improvement in other properties, including low temperature impact strength.
The graph below illustrates this effect in general terms; actually the data is from a study of film grades, so the actual probabilities may not be strictly applicable to roto. However, the trends are the same.
So, my bottom line is as follows:
- If you’re using typical General Purpose roto grades, 5MI and above, you won’t see a lot of difference in performance between similar MI / Density combinations that use butene versus hexene as a comonomer.
- If you’re using typical Tank Grades, 3 MI and below, you may well see discernible differences in performance between similar MI / Density combinations that use butene versus hexene as a comonomer.
- The main difference in performance will relate to long-term factors, as exemplified by properties such as ESCR.
So… whether or not you should worry unduly depends on whether your application demands long-term performance. This is a matter of design judgement but, as a general rule, if your product is subjected to significant continuous or cyclic loading over it’s lifetime, you should be thinking about the material’s performance long term. Examples: underground chambers, overground storage tanks, chemical containers etc
If you’re in any doubt about material selection, your resin supplier will be a good source of initial information.
Some useful extra background can be found in a webinar that I gave in 2019 entitled “Selecting the Right PE Grade for Your Part”. You can find it in the “Webinars” section in the Members-only Area on the ARM website.
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has more than 30 years of experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.