The Science Behind Swestep 2

In part 1 of this series we broke down the first bit of jargon, catalytic, to help understand the science that is the basis of Swestep’s business, catalytic pressureless depolymerization or CPD. Today, in part 2 we’ll jump to the “D”. You’ll need to understand the “D” before you can understand why the “P” is worth being in the named.

D is for Depolymerization

Depolymerization is the process of breaking down a complex chain, or polymer, into its smaller individual units, or monomers. Any Greek out there could depolymerize the word, to -mer, meaning “part” or “unit”, and poly-, meaning “many”, or mono-, meaning “one”. Therefore, a polymer, or many unit, is simply made up of a bunch of monomers, or single units. What the monomer is and how they are arranged relative to each other gives rise to a staggering array of possible polymers.

Before going any further, how about a few easy to grasp examples. Consider the classic kid’s toy, Lego. Imagine a castle, rocket ship and racecar all built from Legos. In each case the monomer is the same- Lego building blocks. However, these monomers are combined in different ways relative to each other, creating vastly different structures, or polymers. Of course, Legos have their limitations. For example, if you wanted to build a sturdier castle you should choose a different monomer such as brick. Construct a polymer out of bricks and you’ve got yourself a sturdy wall. Since we are ultimately trying to understand polymers on a chemical level, here is an example from biology. Certainly you’ve heard of protein and probably also amino acids. On its most basic level, a protein is a polymer of amino acids. Depolymerize a protein and you’ll be left with a pile of amino acids. Put another way, amino acids are the monomers of protein.

What about Swestep? What is being depolymerized in its CPD process? The input, or feedstock, is plastic. Most plastics out there, from lotion bottles to plastic bags to kids’ toys, are made from a chemical called polyethylene. Remember that “poly-“ means many, so polyethylene is a polymer made from many molecules of ethylene, which is the monomer. However, when Swestep depolymerizes polyethylene they aren’t going all the way back to the monomer, ethylene. And why would they? Ethylene isn’t renewable diesel, though it is related.

What polyethylene, ethylene, diesel and even other common fuels such as butane and propane, all have in common is that they are all hydrocarbons. Hydro- for the atom hydrogen and -carbon for… well carbon of course. That is to say that molecules that are called hydrocarbons are made entirely of hydrogen and carbon. No oxygen. No sulfur. No nitrogen. Just hydrogen and carbon. The chemical formula for ethylene is C2H4 while common diesel fuel is C12H24, though it can range from C10H20 to C15H28. The chemical formula for polyethylene is CnH2n where “n” is some number in the billions. All Swestep has to do is chop up, or depolymerize, polyethelyene, or any other hydrocarbon polymer, into the right size pieces. If those pieces contain 10-15 atoms of carbon, voila, you’ve got diesel!

The relationship between hydrocarbons has been understood by scientists for decades and the idea that they could interconvert between them through polymerization and depolymerization has been around for just as long. However, until recently that idea has remained in the realm of the theoretical. What is different today is that we are just now beginning to master the practice of the idea. Perhaps Swestep’s mastery of depolymerizing plastic into renewable diesel has something to do with the last letter in that whole CPD thing. Tune in next week to find out how going “pressureless” got the plastic goose to finally start laying eggs.