you must indicate whether it is nylon 6 or nylon 66 in the patent claim. You can even apply for different patents for these two different materials, but if you don't clearly indicate (or indicate the first choice) the specific material and this material is an important part of the patented technology scheme, then it can't be effectively protected.
The reason is simple. Nylon 6 and Nylon 66 are not the same thing.