How does the percentage of phenolic affect the coefficient of restitution of pool balls and how many times has this changed over the years? To possibly have an answer just count good quality ball sets from Sulac or Brunswick.To narrow the number of years lets say from 1970 to present.I read some place the Cyclop balls are 85% phenolic and how does this differ from Aramith balls?
For a material property like that, I think the only way you are going to find out is to do some testing. I think it will depend on exactly what is added, how much is added, how it is mixed, the exact proportions of the parts of the resin itself, the curing additives, ....
I have never seen a ball company publish coefficients of restitution (efficiency of bouncing, abbreviated COR and usually indicated by "e" in equations).
I have heard that Aramith does tests for COR and that for some tournaments they reserve the balls from a production run that bounce the highest off a steel block.
So far as I know, Brunswick never made phenolic billiard balls.