Polyethylene: From Top Secret to the world’s number one thermoplastic
If you're a processor, whether it is IM, thermoforming, extrusion, rotational or any of the many variations, consider this: of the total number of your staff, how many know what polymers are? I don't ...
If you’re a processor, whether it is IM, thermoforming, extrusion, rotational or any of the many variations, consider this: of the total number of your staff, how many know what polymers are? I don’t mean expensive plastic kitty litter in a Gaylord, but what the stuff really is and why it behaves like it does? If you’re typical, the answer is either “few” or “none”.
That’s actually a tribute to the maturity of our industry–you don’t need to be a polymer chemist to break the swizzle stick market wide open, but knowing the basics can’t hurt as resins become more complex and material choices more critical to your bottom line. And if you can read this page, you’re ready for the basic chemistry behind the business.
What’s the simplest commodity polymer? It’s polyethylene. Like other thermoplastics (but not thermosets…more on these in a future column) they’re made mainly from carbon arranged in very long snakelike chains forming huge molecules. How huge? For the atom-sized world in which PE is formed, they’re composed of thousands of carbon atoms coiled in a twisted and convoluted mass.
So what? Carbon black just sits there in a pile and it’s carbon too. The difference is that by linking the carbon atoms together like beads on a string their motion is constrained in two ways. One is by their bonding, the way a freight car has to follow the train it’s coupled to, and secondly (and more interestingly if you’re making money with it) by the way the tangled long chains interact and pack. Simple “straight chain” molecules pack tightly like a box of rubber bands; for you blow molders, that’s high-density polyethylene. If the chains aren’t too neat and contain many branches like a tree, they can’t pack into that box tightly. The result is low density polyethylene, so the blown film industry can rejoice too.
If you make the chains using modern catalysts (chemicals that make chemical reactions go faster) it’s possible to make the chains very straight, producing linear low-density polyethylene, which, because it packs well, can behave like high-density polyethylene. Confusing, isn’t it?
Packing and density aren’t the only factors that determine why PE behaves the way it does. The length of each chain, meaning how many carbon atoms, is very important. There’s a name for it: molecular weight. When you see the term “high molecular weight” think “long chains”.
Why is this important? Because the bent and convoluted chains bump into each other and themselves, like the rubber bands in the box, and the individual carbon atoms interact with their neighbours. “Stick” is another word for “interact” and although the attraction isn’t strong, multiply it by a couple of billion carbon atoms and you have a tough, useful solid resin.
So why does it melt when you heat it? Because as the heat energy makes the atoms in the chains vibrate, they move apart, reducing the attraction. As you can imagine, both processability and physical properties depend on the number of atoms interacting, so molecular weight is important enough to be specified when you order specific polyethylenes like UHMWPE, which stands for ultra high molecular weight polyethylene. Properties not affected by the molecular weight are color, refractive index, hardness and resistance to chemical attack, as well as another of great historical significance, dielectric constant.
Polyethylene was discovered as a laboratory accident in a British ICI lab in 1935. When tested, it was found to have excellent electrical properties, specifically a very high dielectric constant. PE was a top secret technology during World War II, and a good thing too, because its then-unique electrical properties allowed the development of airborne radar, eliminating the threat of German submarines in the Atlantic Ocean. PE is the world’s number one resin, and has a fascinating history, from WW II to Hula-Hoops and the Nobel Prize. And it’s the simplest thermoplastic.
Next Month: Polypropylene