The answer depends on the way of washing, which is one of two ways:
- washing in a container (with hands)
- washing in running water (includes washing machine) or pouring water on the cloth.
In washing in a container, you must wash three times and squeeze in every time.
In washing in running water, you have two cases depending on the sort of najasa, whether it's seen or not:
If the najasa is seen, the mass (Aein) of it must be removed, even in one time. The taste of it must be removed as well. If the colour or smell of it were hard to be removed that will be OK.
If the najasa is unseen, water must get in and out of the cloth three times (this will be fulfilled in the washing machine).
Ahmad bin Mohamad Al-Tahtawi, Hashia on Maraqi Alfalah, p. 159
So you can conclude that chemicals wont make a difference except in the case of removing the mass of najasa because the process will be faster.