DIY & Home Improvement

That's my only experience of it. The nuts are usually made of a very hard plastic-like material (originally they'd be made of substances like carved cow bone) and not at all flexible.

The baking soda makes the superglue set almost immediately and also acts as a filler to give it some bulk, that's all. I'd have no faith in it to fix anything flexible like a plastic chair. The superglue most likely wouldn't stick well enough, and the join would be too stiff to be durable on its own.

The best I can offer is to suggest a method using fibreglass as used in car body repairs etc. (either the liquid filler on its own or backed up with fibreglass mat for reinforcement), possibly coupled with the other fixing methods you've tried, though that's also unlkely to be flexible and adherent enough to make anything like a permanent repair unless you're very lucky.

The only other method I can suggest is some type of plastic weld, melting the edges together. I've seen some folks on YouTube use plastic zip ties fed into a hot glue gun as a filler after using a hot blade to try to blend the edges of the crack together beforehand. That seems plausible, but the problem is that any thermoplastic when heated tends to go more rigid than is desirable and is unlikely to match the flexibility of the original material.

I also tend to try to fix stuff rather than buy replacements myself, but I think this is one case where I'd say the chairs have had a useful life and it's time to replace them, hopefully being able to send the old ones to be recycled into something new and useful again.