The specific requirements differ for each of the distinct dye processing procedures and end uses. For example, the reduction of dye particle size during cake grinding does not require the dispersing agent to adsorb as strongly to the particles as is required when batch dyeing.
What is the subject of auxiliary verb "is" in "as is required when batch dyeing."? How to recognise this subject? Please tell me when this type of omission of subject occurs usually.
Here's what is implied:
the reduction of dye particle size during cake grinding does not require the dispersing agent to adsorb as strongly to the particles as [the level of adsorbing strength that] is required when batch dyeing.
The central idea in the dispersing agent to adsorb as strongly is how much or what level of adsorption the dispersing agent is facilitating - it's the "object" of require.
As clues us in that we are comparing something.
Since a subject was not specified on the second require it's assumed that the subject and "object" of require are the same.
It's common in comparison expressions with as to only specify what has "changed" in the second phrase:
I can't walk as far as he can (no need to specify walk, it's understood).
This drink doesn't taste as good as yours. (no need to specify taste or it's object, drink.)
The assembling process doesn't need as much ventilation as the painting process. (verb is need and object is ventilation, that remains understood over into the second as phrase.)
