Check out this paper: Article New Solvent for Polyamide 66 and Its Use for Preparing a Sin...

It tells that polyamides can be dissolved in formic acid, or cresols, or certain fluorinated solvents. All are very nasty. The paper teaches a way to reduce amount of formic acid needed to dissolve PA66 (very similar to PA6, I would think) by adding 40% total of urea and calcium chloride. A literature search using Copilot(TM) brought up other papers too.

Good luck!

Dear Steve Nazar ,

Thanks a lot. Why do you say formic acid and cresols are very nasty?

Any significant mistake in handling those, and maybe the fluorinated isopropanol too (I would guess) will cause injury. Check the material data safety sheets (MSDS) before working with these. If they can dissolve polyamides, they can dissolve you too.

Steve Nazar If I use low amount of formic acid (5-10%) for just softening of polyamides, it can be dangerous? is it more dangerous than sulfuric acid or HCl?

Mahjoobeh hajitabar firouzjaie Please be clear that I'm not a toxicologist. But I've worked and taught in a lot of labs full of nasty things and lived to tell of it. Read up on formic acid online. Read up on any of the substances you are using. Insist on proper protective equipment and lab facilities.

I've not worked with formic acid myself, but I read that the vapor becomes a problem at 10 ppm, so you will need a good fume hood. Any contact with skin or eyes will cause severe and lasting damage, so you need a full face shield, impermeable gloves and apron.

It is quite likely more dangerous than concentrated sulfuric or hydrochloric acids. I've worked with both of those, and with reasonable care had no damage to my person.

I did ruin some clothes (holes!) and a nice leather jacket in a car battery explosion, not a lab incident. But if you wash e.g. sulfuric acid off quickly you get maybe a superficial skin burn that heals. It doesn't evaporate, so little spills can sit around waiting to get you. In contrast, little HCl spills evaporate, and you will know its presence by the sharp odor.

But formic acid is worse: both a flesh solvent and a toxic volatile. Any on your skin is going to go deep and cause lasting damage, according to what I read. It is also a very strong lung irritant. With a boiling point of about 100C a lot can go into the air, particularly if hot. It can sensitize some people so that even very low air exposures later cause allergy-like reactions.

Go online and read! Insist on proper equipment for handling dangerous materials. Plan ahead for accidents. Be wide awake when working with nasty things. Never get complacent.

Steve Nazar Thank you for your comprehensive, complete and accurate explanations.