I agree with Bambi.
Today I was winding balls from leftovers of Caron Simply Soft 100% acrylic and also Lily Sugar 'n Cream 100% cotton. The Russian join worked well on both acrylic and cotton. Now I have one large ball of acrylic (from several pieces) and one separate ball of cotton (from two pieces).
I overlapped 2 inches (5 cm) on each side of the join and it doesn't slide out even with the slippery acrylic. Now the cotton was more fiddly in getting the yarn to thread into the splice since it was so grabby. I'm sure it will not unravel, slide, or fray.
I would guess that nylon is somewhere in between the slippery acrylic and the grabby cotton.[/COLOR]