The theory of the shampoo method is to generate a lot of foam in the carpet, allow this foam to dry, have the resulting residue attract the soil, and vacuum up the residue and soil the next day.
Carpet shampoo products must have the following characteristics which dictate their ingredients:
Very high foam levels to reduce wetting.
Very stable foam.
High lubricity to reduce damage to the carpet fibers from the brushes.
Dry to a non-sticky residue.
Therefore, a carpet shampoo must contain a very foamy chemical. The most common ingredient is sodium lauryl sulfate or one of its relatives. Sometime when you're taking a shower or bath, look at the ingredient statement on your hair shampoo bottle. It will say "Contains sodium lauryl sulfate" or some similar wording. Sodium Lauryl Sulfate and its relatives exhibit a very high and very stable foam and are only fair detergents. The problem is that they dry to a soft, sticky residue which will cause resoiling.
Because of the resoiling problem, carpet shampoos will frequently also have an anti-resoiling additive such as a resin to reduce this resoiling tendency.
Because shampoos are actually very poor detergents and basically simply bury the dirt, they frequently also contain high levels of optical brighteners which take invisible ultraviolet light and convert it to visible light, thus making the carpet appear cleaner and brighter than it really is, for a while. It will eventually give the carpet a yellow cast and the yellow cannot be removed.
Carpet manufacturers STRONGLY discourage the use of optical brighteners in carpet cleaners because of the resultant irreversible yellowing.
Two primary types of machines are used for this process:
The Cylindrical Foam Shampoo machine uses an air compressor to create dry foam before the foam is applied to the carpet and the carpet is then agitated with a revolving cylindrical brush which combs the foam through carpet pile. This method will leave dirt trapped in the carpet pile. Carpet must be thoroughly vacuumed before and after cleaning.
The Rotary Shampoo method uses an ordinary rotary floor machine (the same kind used for stripping wax), sprays shampoo onto the carpet from a dispensing tank, and a rotary brush whips the detergent to a foam. Most carpet mills and carpet fiber producers discourage the use of rotary brushes on carpet because of the potential damage that can occur. Overwetting is common with this method which can cause jute straining, shrinkage, and odor.
Shampoo methods are inferior due to poor cleaning plus resoiling problems. The Rotary Shampoo method can damage the carpet, especially cut pile (which is what most residential carpet is).