I got two types of plastic pants this year and the side seam became undone.
I do not know if there was to much air in them and when I sat up it popped out a weak seam or they just became undone as the body heat warmed the seam.
Anyway I attempted to repair them and it worked.
Using a technic to seal the "skin" on radio control air planes worked.
Set down a towel on a flat surface (table top).
Set the Iron to silk setting.
Turn pants inside out.
Carefully position seam so that it is flat and both sides are laying over each other.
Use the tip of the iron to "tack" the seam in 2 or 3 places.
Place the tip of the iron on the seam for about 5 seconds then move it to the next spot.
Now use the side of the iron to seal the seam for a count of 10 to 15 seconds.
The plastic should weld into each other and a good seam is made.
Just be careful not to get the iron to hot. You want to melt weld the two sides together on the seam, not melt it to a point it sticks to the iron.
I repaired three pairs and the seams look OK not as neat as the original, but when I tested the seam it held and did not tear.
I hope this is useful to someone.