INCI number not a good indicator of shampoo free suitability.
Polyquats are cationic polymers, meaning they have positive charges along their backbone or pendant to the chain. They are highly effective as both hair conditioning agents as well as styling agents. These types of molecules can be quite complex in their behavior, and are a subject of extensive study in various fields, including cosmetic science. For this reason (and because I'm a polymer nerd), I find them sufficiently intriguing to revisit the topic fairly often, examining different aspects.
Here at NaturallyCurly.com, we get a lot of questions about polyquaternium ingredients, particularly regarding their removability from hair. Most recently, I read a discussion where the theory was put forth that as long as the polyquaternium number was sufficiently low (say, below -40), then the polymer is fine for use in a shampoo-free or mild-surfactant hair cleansing routine.
Figure 1. Illustration of cationic polymers. a.) linear polymer with charges along the backbone, such as Polyquaternium-10; b.) a comb-shaped polymer, with positive charges pendant to the backbone, such as Polyquaternium-4.
Actually (and perhaps unfortunately,) it is a misconception that one can determine the compatibility of a polyquat and a low-poo method based upon its INCI (International Nomenclature of Cosmetic Ingredients) designation number. This is a logical misunderstanding of how these materials are named. INCI assigns a number to a polymer as it is presented for inclusion into the materials database, in order of submission.
The naming is sequential and is unrelated to molecular structure, molecular weight or charge density. One has to look up the specific polymer in order to obtain its structure. Many of them are structurally quite similar, with various modifications along the backbone, such as the multiple variations of modified cellulose polymer. There are also polyquats made of guar gum, and many others made of various synthetic polymers and copolymers.
Another point of confusion is that the numeric INCI designation does not provide any information regarding the molecular weight of the polymer (the size of the polymer). This property can be quite influential in the performance of the cationic polymer, particularly when it comes to deposition onto the surface of the hair. (Gruber et al, 2001). But the tricky thing for label-readers is that polyquaternium-4 (and all the other cationic polymers) can come in a variety of molecular weights, and there is no way to know which polymer is present without consulting the manufacturer (presuming they would even disclose such information). This could explain why people often observe that they get inconsistent results in very similar products (according to their labels) containing polyquaternium-4 (or -10, or -32, etc.). The polymers may be of substantially different molecular weights, resulting in very different product performance.
Water solubility and build-up:
Cationic polymers can be completely water soluble, or they can be merely water miscible, depending upon things like polymer structure, molecular weight, and charge density. The critical thing to remember is that these polymers perform well as conditioning and styling agents because they are attracted to and adhere to the surface of the hair due to electrostatic interactions (the positively-charged polymer is attracted to the negatively charged portions of the cuticle). The complex that is formed is often very strong, and the interactive forces can be difficult to overcome using conventional methods. A small anionic surfactant such as sodium lauryl sulfate may not be sufficient to overcome that attraction and remove the polyquat.
Polystyrene sulfonate has been found to aid in removal of these polymers from hair, but it is not an ingredient often seen in shampoos. Clarifying shampoos may also not be effective in removing all polyquats from hair. However, some polyquats seem to respond well to traditional shampoos. For instance, researchers at BASF found that polyquaternium-10 is removable with an anionic surfactant.
My personal hunch is that removability is dependent upon the charge density of the polymer (how many positive charges exist along the backbone), its overall structure, and also the state of the hair to which it is applied (more damaged hair has a higher negative charge density, and so would provide a cationic polymer more sites with which to bind tightly). For instance, PQ-10 is a cellulosic derivative, which is very water soluble due to its ether (oxygen-containing) components, which allows for fairly easy removal.
Build-up is also affected by polymer structure. Polyquaternium materials based upon guar gum are more difficult to remove from hair and also display a greater tendency to accumulate on the hair with repeated usage. Polyquaternium-4 is a cellulosic derivative, but with a fairly high charge density. It is extremely substantive to hair and very useful as a styling agent, but has been found to exhibit very little tendency to build up.
I have this other hypothesis that mechanical forces—brushing, washing, combing and styling—are eventually sufficient to detach some of these polymers from the hair. This may be what BASF researchers were seeing when they noticed their new polymer PQ-44 performed very well as a detangler, but did not demonstrate any tendency toward build up. The authors believe that the polymer adheres to the surface of the hair at a few points of the polymer, while the uncharged ends of the polymer form loops that are not flat on the polymer surface. These loops reduce friction between adjacent strands, but also may provide the means for ease of removal later when the hair is washed.
To Polyquat or Not?
In closing, I hope it is more clear now that one cannot rely upon the number of a polyquaternium to determine its removability with any degree of certainty. One has to refer to the INCI database or another source to obtain the exact structure of the polymer in question. Once the structure is known, one can then make certain assessments of its properties.
Although I have seen a few sweeping denunciations of polyquats by some curly hair experts, my personal opinion is that there are certain polyquats that can be advantageous to use in the care and styling of curly hair.
The angled hexagonal structure in combination with the oxygen bonds in the center of the molecule make PQ-10 good for your hair, obviously.
The data indicates that PQ-4, PQ-10, and PQ-44 do not create problems with buildup if one uses a mild shampoo, and each can provide good benefits either in conditioning or styling. There are others that cause more problems with build-up, such as PQ-11 and guar hydroxypropyltrimonium chloride. One recent question was about PQ-37, and while I cannot find specific data on its removability at this time, it looks potentially problematic to me based upon its structure.
As always, choose your products carefully, get samples when you can, and see what works for your hair.