Comparison of foam materials

[matthuszagh]

I'm in need of replacement foam material for two different contexts: the first is to replace the foam padding in an SMA gage kit (thread about removing the old foam here: https://www.eevblog.com/forum/beginners/remove-disintegrating-foam-from-wooden-case/msg4368976/). The second need is to replace foam lining on the inside bottom cover of an HP 8642B (images attached).

The choice of replacement is important because some foams degrade into a corrosive substance and are therefore not suitable for long-term installations (on the order of a decade or more). I've heard that many (all?) foams are likely to degrade over several decades (true?). But, even if this is so, I'd like to find a foam that degrades gracefully. By that I primarily mean not exuding a corrosive goo. Crumbling would be more acceptable but still not preferred. A graceful failure would be something like losing its elastic quality but mostly remaining intact under minimal abrasion/strain. Since this is geared toward things like the 2 examples I gave above, heat and moisture tolerance are important. UV exposure tolerance is probably also somewhat important. But, I don't need something that can survive harsh outdoor climates etc.

I know very little about different kinds of foams, but I'll start off with the types of foams I've encountered and their attributes.

Polyurethane foam
Polyurethane foam is soft and therefore often used as padding for cameras and other delicate mechanical equipment and for electronic applications. However, as far as I can tell, it is the main foam people complain about when they're left with a corrosive goo. This has also been reported in the scientific literature (eg https://www.jstor.org/stable/20619421).

Polyethylene foam
Also commonly used. Think of a pool noodle. It's harder than polyurethane foam and therefore, when used for padding, more suited to contexts where the item being held can withstand a bit more mechanical shock. I've also read that it doesn't degrade as readily as polyurethane foam. Indeed, searching "polyethylene foam degradation" on google scholar yields fewer relevant results than for polyurethane foam.

Neoprene foam
Think of a mouse pad or wetsuit (note, though, that in each of those examples there is also a fabric lining for durability, which is not part of the actual foam). These are supposedly long-lasting, fairly chemically inert, resistant to oils and moisture, and very heat resistant. But, as far as I can tell, this seems to be less common in relevant contexts than the above two. I'm not sure why, as it seems to be a high-quality choice.

Silicone foam
I don't know anything about this, but silicone products (eg silicone rubber) are often high-quality and maybe that extends to the foam too.

EPDM foam
No knowledge of this either.

What else is there? What part of my analysis is missing/wrong? What do people use for high-quality long-life applications? Studies to back up claims about longevity and failure mechanisms are especially appreciated, but any relevant information is also appreciated.

[Someone]

Additional complication... RF devices have used special "loaded" foams that are lossy at the required frequencies.

[Alex Eisenhut]

https://www.pelican.com/us/en/accessory/cases/pick-n-pluck-foam-insert/1022

?

[David Hess]

I recently replaced the rotted foam in the center console of my pickup with what I suspect is neoprene foam, which I also use to protect the heel of my shoes.  I had considerable difficulty removing the old adhesive and ended up using Goo Gone, but it still took a lot of elbow grease.  If you could find this with sufficient thickness, I think it would work.

Polyethylene foam seems to last forever and is available in any thickness and big blocks.  Its physical characteristics are nothing like the open cell polyurethane foam which rots, but I think it would cushion even better because the open cellular foam tends to compact and crush.  Since polyethylene is basically inert, for an adhesive I might use either the foaming or non-foaming Gorilla glue which seems to work well in such applications.  RTV adhesive might also work; I know there are industrial silicone adhesives that would work.  It is difficult to get adhesives to stick to polyethylene.

[james_s]

There's also Polypropylene foam, it's extremely durable stuff, I have a handful of RC airplanes made out of it and it's practically indestructible. Haven't had one deteriorate yet although the oldest I have is about 8 years old so who knows.

[Black Phoenix]

I would recommend Poron Foam - https://www.stockwell.com/poron-foam-cellular-urethane/

I've been using it extensively in my applications (keyboard case padding between PCB and case, plate and PCB, paddings for boxes, etc) and it has been holding very well in every application I throw at it.

Also this foam is very good at isolating noise when used in such applications. It can be produced with very different thicknesses, it can be compressed to a certain extent without retaining memory (go back to their own non compressed shape) and it is easily cut to different shapes without fraying.