Floyd540 is 100% spot on. I have dealt with plastics professionally for the last 45 years and have done hundreds of post mortems on cracked plastic parts for clients and for lawsuits and Floyd540's explanation is concise and accurate. In addition to what he mentioned never ever use thread locker products near plastics and never use flat head screws to assemble plastic pieces together. Use pan head screws that put the plastic in compression with a washer to spread out the load.

 

This is not going to be a chemistry class and it should not be. Since we have people here that have no idea what to do with a hydroxyl group and the fact that methyl has only one C, we should continue to just help each other. The fact that alcohols, all varieties attack some plastics, it is used for determining stress levels , types and concentrations in the laboratory. Different concentrations at different temperatures from different alcohols can be used to quantitatively measure the actual stress. It is a destructive test. It is used all the time in the lab.
For the group in general:
denatured alcohol is not an actual chemical compound. It is a material that is a mixture of several things. Just like lacquer thinner and salad dressing, we don't actually know what all is in it , but we have a god idea about it by the name.
Typically denatured alcohol is a majority (50% to 90 % ) ethanol (the stuff you drink) and the remainder of a material that renders it poisonous. This is typically methanol but can be other materials. These two alcohol groups have completely different properties. So when we pick up a can of "denatured alcohol" we really don't know what is in it ,but we do know that it is BAD. In any event, don't drink it and don't use it to clean plastics unless you know more about the material is.
Be safe- Happy New Year!

 

It's good to be aware of the details, but the fundamental takeaway for me in all of this is that I'll be limiting any use of alcohol to other activities...

 

The absolute best application for the material. At least with that we know what we are doing and we all have success with it. (ethanol only!)

 

I add to that Old Port, XO Cognac and 25-year Single Malt Whiskey.

 

I had one crack between the red and clear plastic.
I removed the light assembly and carefully resealed the crack.
I thought it looked great.
Until I tripped on something one of my children left on the floor while I was working and dropped the whole thing. It totally broke then.
Fortunately, I found a replacement assembly (I think at Suncoast) and it wasn't stupid expensive.

 

997.1 with 80k miles almost cheaper than 25 yo single malt these days, it’s gotten ridiculous

 

I just learned this on my 2018 F150. Had a taillight getting water inside. These are over $100 so I wanted to save it by sealing the edges with clear caulk. Then, I wiped the excess with a dry towel but just to make sure it was perfect I used denatured alcohol. BIG MISTAKE. Before I'm even done I hear a crackling sound. The cracks grew and spiderwebbed across the entire assembly. Result: Not only do I have to buy a new lens now, but I'm also out the cost of the caulk. I should have quit with the dry cloth.

 

Amorphous plastics such as polycarbonate, acrylic, polystyrene which often can have good impact properties, have relatively poor chemical resistance. These transparent plastics will crack in seconds, particularly when under stress, when exposed to alcohols, particularly IPA. Semi crystalline plastics such as nylons ( Nylon 6, Nylon 66, Nylon 12 have superior chemical resistance properties. Headlamp lenses are generally polycarbonate to meet pedestrian impact requirements but must be factory hard-coated to increase life, being exposed to UV and other environmental stresses. The insides of these lenses are not hard-coated. I have over 35 years experience in the engineering plastics industry. There are numerous plastics out there and they all have their strengths and weaknesses. One needs to design for those characteristics.

By the way, denatured alcohol is generally ethanol with some poisonous ( therefore, denatured) alcohol added such as methanol or IPA and a bitterant added so you don't try to drink it.

 

Yeah acetone will completely dissolve polycarbonate which is what most head/tail lights are made of. Keep harsh chemicals like acetone, alcohol, etc. away.

 

PSA that IPA can also wreak havoc on PPF films. I made the mistake of doing the usual 50% IPA wipe down on my freshly polished ppf to remove residue before applying a ceramic coating, standard procedure I’ve done on paint many times. But within 30 seconds I started seeing all these white splotches appear on the film. I quickly wiped it all off, but my 10 seconds of spritzing cost me hours of pretty aggressive polishing to get all the stains off, probably taking most of the film’s clear coat with it.

 

Also dont rub on them with a statically charged cloth else you will get a frunk latch fault and have to get it reset.

You're welcome.

 

That sucks, at least its an easy fix. Not sure how much a new taillight is, but here is a used one that would look perfect with a light buff:

https://www.ebay.ca/itm/264931587279...4AAOSwSpNfqVOO

Hopefully this will ease the pain a bit. Cheers.

 

This is interesting. I introduced rubbing alcohol (either 70% or 90% - don't remember which I had at the time) into a Cayenne 957 headlight, swished it around to remove the fogging that had happened on the inside of the lens, tipped it toward the opening to get it out and left the headlight in that position so any residual could evaporate out. The lens was spotless inside and looked brand new with no spider cracking or other cracks.

Was I lucky or is that type of alcohol compatible with Porsche headlight plastics.

Also, when I applied clear PPF film to both my 997 headlights and the aftermarket LED fog lights, I cleaned them with this same alcohol and then spritzed the film and the lens with it so I could move the plastic around without messing up the adhesive, then squeegeed out the alcohol to let the adhesive bond and cure. Worked perfectly and not cracks there either. Was I lucky 3x (have 2 997s)?