Polymerist Basics: Polyethylene
Polymer basics for those not so chemically inclined
Welcome to another edition of Polymerist Basics where I attempt to explain a particular polymer without going to in-depth into the chemistry. This is meant to be a 1-pager explanation so brevity is part of the goal, but if you are a professional chemist or someone with deep knowledge in a particular topic and feel that I am missing something essential let me know. If you are just a regular person looking to learn more about chemistry and how it plays a pivotal role in your life then you are at the right place.
When I write the word polyethylene it might mean different things to different people. For some it means disposable shopping bags, to others it might mean vapor barriers for crawl spaces, while to others they scratch their head in confusion. The production and application of polyethylene in everyday goods is critical to modern western life, but why and how?
Chemically, polyethylene is not too dissimilar from something like a high grade candle wax or coconut oil (professional chemists, give me some room). What makes a fat resistant to water is that it’s primarily a bunch of carbon and hydrogen atoms bonded together in long linear chains otherwise known as hydrocarbons. Polyethylene is for the most part similar, but on a much much longer scale and this is what makes it an excellent barrier to stop water and as an insulator of electricity. This also means that polyethylene can also be burned and it’s why special grades are also used as an additive in candles.
You might be thinking polyethylene is polyethylene right? Wrong. There are a wide variety of polyethylene grades out there ranging from low density (LDPE), linear low density (LLDPE), high density (HDPE), to name just a few. In reading those names it might appear that density plays an important role in how polyethylene functions and its properties. We can think of the different grades of polyethylene on a slide scale of density from low density at one end and high density at the other and the density is what enables the properties.Â
The density of polyethylene all comes down to how the polyethylene polymers pack together, which is also dependent on how polyethylene is synthesized. Low density polyethylene has that low density primarily due to how much branching is in the polymer. Think about stacking a bunch of large tree branches and how much free space there is between the small branches and you’ve got an analogous situation with low density polyethylene.Â
If you were to cut those branches so that each stick was unbranched and then stacked them they would take up much less space and that is essentially what high density polyethylene is in comparison to low density. Linear low density polyethylene is if you then glued on sticks of a very specific length at very specific positions along the tree branch. When chemists (Zieglar, Natta, Hogan, and Banks) figured out how to make polyethylene without branches they also set the stage for being able to insert very specific branches at certain points in polyethylene which enabled a tremendous amount of various grades of polyethylene.Â
The way that polyethylene polymers fit together combined with their length ultimately determines the end physical properties. This is why there are incredibly light and soft non-woven fabrics made from polyethylene while there are harder and more durable sheets. This gets us further away from polymer chemistry and more towards polymer physics, which I will save for later.
The different grades of polyethylene also make it more difficult to recycle. Low density polyethylene (grocery bags) does not recycle well with high density polyethylene (milk jugs) because the properties that gave the initial materials value are not reproduced in a mixed recycled material. In an ideal world we would seperate all of our polyethylene products based on their type, but for now we put in the blue bins next to aluminum, glass, paper, and all of the other plastics.
If you liked this check out prior issues of Polymerist Basics with Epoxy Resins and Nylon.
Unfortunately discussing the basics of plastics is not enough and the conversation in my opinion needs to be elevated to address more pressing issues in the global market today. With regard to HDPE why not discuss the problems associated with milk bottle HDPE like blow out plugs and the recycling of HDPE bottles and the problems associated with contents of plastic bottles that degrade prematurely in stores and become rancid or the effects of poor process stability on molecular properties of the final molded container on shelf life and end of life upcycling issues;etc.
I worked at a site that manufactured ZN catalysts and was amazed at the value they added to the Polyethylene manufacturing process. When the catalyst to unlink and re-use plastics is developed, it will be a great piece of IP.