Ask the Alchemist #187

Level: Apprentice

Reading time: 8 minutes

Have you seen any stone grinders (of similar size) with more metal than plastic in the center bearing shaft?    We are trying to shy away from as much plastic as possible. The Delrin (= polyformaldehyde) does not seem so friendly either. 

 There is nothing unfriendly about Delrin.  The poly makes ALL the difference. 

I am speaking as a retired chemist here. 

There is a measure that I personally find useful for evaluating the toxicity of chemicals.  It’s called the LD50.  It stands for the 50% lethal dose.  Meaning if 100 animals (usually rats) are given the chemical at a given amount (based on their weight) 50% will die.  Yeah, kind of gruesome.  This does not translate 100% to human consumption but does give relative toxicity to each other.

If one chemical has an LD50 of 1 mg/kg and another has an LD50 of 100 mg/kg, then the later can be said to be 100 times less toxic.

Let’s have a look at PVC since we are all familiar with it.  You know, that stuff that much of your water is carried in.   Poly vinyl chloride. 

Vinyl chloride has an LD50 (orally in rats) of 500 mg/kg.  That is moderately toxic.  I mean, if you could translate it to human doses, a 100 kg healthy male would have to consume 50000 mg or 50 grams to hit the LD50.

If you link up all those vinyl chlorides together into a chain, (poly just meaning many) you have poly vinyl chloride and the LD50 > 10,000 mg/kg.  More than 20 times less harmful.  And notice the greater than sign.  It’s because it becomes impractical to consume enough to be a problem.  That same 100 kg human would need to eat 1 kg of PVC pipe.  That is ludicrous.  No one is going to do that or even could do that. 

But again, it’s worth pointing out this is just a simple way to talk about toxicity.

Delrin is polyformaldehyde.  

The LD50 of formaldehyde is harder to nail down as it varies so much with animal species but it ranges from about 50 mg/kg on up to 250 mg/kg.

Regardless, Delrin’s LD50 follows the same pattern.  >11,000 mg/kg.  This falls into the non-toxic category.  You would end up with intestinal blockages before it hurt you chemically.

What makes these base chemicals so toxic is because they are reactive.  But polymers almost by definition negate that reactivity.  You take all those toxic ends and link them together, making them non-reactive.

And the LD50 demonstrates what happens if you eat them.  They pass through.  If you body could digest them, then they could become reactive again, but biological systems generally don’t know how to digest polymers. 

We eat and get lots of energy from glucose.   If you put a pair of glucose molecules together you get what is called a disaccharide and it goes by the name of maltose.  That lovely stuff in beer and ale.  And we clearly can eat that.  If you link up a few more you get what we call starch.  It’s a little harder to digest yet we can still do it.  But if you go so far as to link up thousands of them you have a polysaccharide and this is another name for cellulose.  Which we can’t eat.  Well, you can eat it, but you can’t digest it and more importantly it isn’t toxic.  What is cellulose?  Wood fiber basically.  Like that pencil some of you are chewing on?  Or that blade of grass you have eaten.

Your body really doesn’t know whether that polymer it’s trying and failing to digest is a polysaccaride, polyvinylchloride or  polyformaldehyde.  They are all indigestible polymers.

Really.  The LD50 for cellulose is listed (when I could find it) >10,200 mg/kg.  Sound familiar?

Now, none of this touches on the manufacturing of Delrin, how that impacts the environment, etc,   Just what is going on in your Melanger.  At the end of the day don’t freak out because something looks or sounds like a chemical.   Or is made up of something that is toxic by itself.  Chemistry just isn’t that simple.   

As a parting example take salt.  Sodium chloride.  Its two component parts are Sodium which is a toxic metal that explodes in water and chlorine that is a deadly gas.  Yet you eat it and need it every day.

That all said, I'm exploring metal bearing.  But bearing need lubricants.  Many of which are polymer.  And have to be held in.  Which requires polymers.  It's a difficult problem to solve.  But we are looking.

With that, I’m going to top up my extraction of 1,3,7-Trimethylpurine-2,6-dione in dihydrogen oxide.