Chemistry: caustic soda too strong for PET containers

[klugesmith]

A strong aqueous solution of sodium hydroxide (NaOH) was too much for some molded PET plastic containers salvaged from the recycling bin.
I'd mixed the drain cleaner in an empty peanut butter jar, being mindful of the temperature rise, and used it to remove aluminum foil residue from some empty medicine containers.
A few weeks later I found the container leaking, having become brittle and cracked where plastic touched the caustic liquid.
Transferred what was left to a water bottle, made of the same stuff, so it was no surprise that the same failure happened again.


Last time I'd heard of inorganic stuff attacking plastic, was an anecdote on 4hv forum.  The poster had got a little sulfuric acid or nitric acid from a friend, and took it home (barely made it) in some small plastic container.

[T3sl4co1l]

Yup, it's a polyester, so you're doing an acid/base catalyzed reaction, hydrolyzing it into its components (alcohols and acids).  Specifically, ethylene glycol (the major component of antifreeze) and terephthalic acid (which is much more fun to pronounce than it is to type, I must admit).

If done in a shortage of water -- say in concentrated sulfuric acid, a reasonably strong dehydrating agent and also a strong acid -- the reaction goes the other way, condensing alcohols and acids into esters (Fischer ester synthesis reaction).  An O-chem class staple being amyl alcohol + acetic acid ==> amyl acetate (a major component of banana scent).

As for storage, prefer nonreactive polymers like hydrocarbons -- polyethylene (HDPE) and polypropylene most common.

Glass is perfectly fine too, if less rugged; but very strong NaOH should not be stored in glass as it will be attacked (forming some water glass in solution).

NaOH also reacts with atmospheric CO2 (forming the much less aggressive, but still basic, sodium carbonate aka washing soda), so it needs to be stored tightly, preferably dry.

Tim

[Mads Barnkob]

I find it interesting that you see the attack in the middle where the plastic is actually thickest. And I think I know why.

I worked with PET blow moulding machines for breweries and other liquid product manufacturers for 3 years and this was the kind of machines I did service/upgrades on: https://www.krones.com/en/products/machines/krones-contiform-3-pro.php

The PET preform is heated in a oven before entering the blow mold station, but it is the sides of the preform and not the top/bottom that gets the most heat, so the bottom middle is actually pretty brittle and it makes sense when you see the proces of the blow. It is pressurized low first and then bang up to some 25-40 Bar at the end, but you have a very uniform stretch and in the perfect world you have the "tap" of the preform dead centered as the bottom of the bottle.



So my theory is that you have a stronger and more uniform plastic structure in the thin walls than the thicker bottom middle that was transformed at a lower temperature from preform to bottle, as the preform cools off fast from oven to final blow stage, the blow air is not pre-heated.

[profdc9]

It is possible that there is increased residual stress in the bottom of the container which makes it more vulnerable to hydrolysis.  Perhaps by viewing the bottom of the container between crossed polarizers, birefringence due to internal stresses can be visualized as compared to the container walls.

I find it interesting that you see the attack in the middle where the plastic is actually thickest. And I think I know why.

I worked with PET blow moulding machines for breweries and other liquid product manufacturers for 3 years and this was the kind of machines I did service/upgrades on: https://www.krones.com/en/products/machines/krones-contiform-3-pro.php

The PET preform is heated in a oven before entering the blow mold station, but it is the sides of the preform and not the top/bottom that gets the most heat, so the bottom middle is actually pretty brittle and it makes sense when you see the proces of the blow. It is pressurized low first and then bang up to some 25-40 Bar at the end, but you have a very uniform stretch and in the perfect world you have the "tap" of the preform dead centered as the bottom of the bottle.



So my theory is that you have a stronger and more uniform plastic structure in the thin walls than the thicker bottom middle that was transformed at a lower temperature from preform to bottle, as the preform cools off fast from oven to final blow stage, the blow air is not pre-heated.