The trouble with all synthetic kernmantel rope/cord (sheath and core construction) is that traditional knots don't work as well as they do on the material they were developed on; manila & hemp.
If a knot slips, try another. The basic overhand on the bight works well on kernmantel as does the extra bend "figure of 8 or extra again figure of 9" variations as an end knot. The alpine butterfly works very well as an inline 3 way knot, and all the variations of the barrel knot work well too; single & double fisherman's barrel knot on the bight etc.
All those knots were developed using this type of kernmantel cord and work by having relatively large turns and lots of surface to surface contact with the standing end thus generating an adequate amount of friction. The up shot is these knots generally preserve more of the cord strength than traditional knots when performing the same function.
The rule of thumb is that a knot reduces a rope's strength by 50%, interestingly on kernmantel ropes the knots mentioned above all preserve more, from around 60% for the overhand to the upper 80's on the figure of 8 & 9, so are all good choices. The bowline comes in in the 40's, so below 50% integrity. Of course the bowline is an easy knot to undo and it still works well enough to be a practical choice for many jobs.
The strength of any cord is always given as a "minimum breaking load" and one should be wary of these figures. If using it to secure items one should consider a 10:1 safety margin of the minimum breaking load as a working maximum if the item to be secured is valuable or important, 5:1 if you can afford to lose it.
So an MBL of 450kg, for example, gives us a safe working load limit of 45kg (10:1), thats more than enough for a guy line in any situation but well short of what I'd feel comfortable with, say restraining a branch when cutting from wind-blown timber or lowering myself with.
Knots and strength are also subject to a mass of variables, knot dressing affects strength and contact with abrasive or sharp objects (rocks and stones) can drastically reduce tensile strength, dynamic loading can generate multiples of load that can cause failure, especially when wet. I guess what I'm trying to say is that one should be careful with minimum breaking strength, always work with a margin of safety.
I can understand the fascination with kernmantel cords and it's quite practical for stuff like guys etc, but there's lower cost simple twine available that'll do the same job well but are also more practical for jobs where the cord won't survive, or are more suited to the job; whippings for construction etc. Not as sexy looking but more consumable at an affordable price. Kernmantel is also stretchy, more so before it's been loaded up a few times, so good for guys but unreliable where static security is required, in that regard polyester cord is much better.
I can't see past tarred polyester twine myself
not being picky or anything!!
