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A while back, I was made aware of a new ‘rule’ that’s making its way around the virtual hallways of cosmetic school (which one, who said what and why doesn’t matter), the rule being that you should not homogenise Carbomers. Now I’m not sure of the wider context of this truth or how it was originally meant to be taken but it was clear that in this case the statement had been taken as literal truth and that was causing a problem.
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For those to whom the word ‘Carbomer’ is new, Carbomers are not a group of political insurgents intent on causing collateral damage, they are a group of polymeric ingredients used for thickening and suspending things in a formula.
Many grades of Carbomer exist and nearly all of them create crystal clear thickening gels which impart an elegant non-tacky skin feel, have high suspending power/ Yield strength and help stabilise emulsions. These features make them very popular in the world of cosmetic science and as such, it’s very important for a formulator to know how to handle and formulate with them.
In terms of what a polymer is, polymers can be natural or synthetic and are made by creating chemically bonded chains of smaller units called ‘monomers’ together. Natural polymers include starch, a polymer made from glucose monomer units and hyaluronic acid, a polymer with monomers of D-glucuronic acid and N-acetyl-D-glucosamine – two disaccharide sugars.
Carbomer polymers are made from Acrylic Acid monomer, a simple carboxylic acid created through the oxidation of propylene. Propylene is a bi-product of gas production and so in the past and to this day, Carbomers are classed as ‘synthetic’ ingredients – ingredients derived from fossil fuels.
Carbomer History in a Nutshell.
Carbomer polymers were first commercialised in 1958 by the B.F Goodrich company with the trade name Carbopol. Today Cosmetic Chemists, especially those my age and older, may use the terms Carbomer and Carbopol interchangeably to refer to this chemistry. This is mainly because for a long time, the Goodrich polymer range was either the only or the superior option on the market. Over the years the business changed hands, other players came into the market and options opened up. However the blur between the commercial and technical name for this chemical family persists and that’s relevant because in some cases, students of cosmetic science who are learning outside a factory/ commercially scaled-up laboratory setting are showing signs that they are missing the nuance in this little ingredient corner. So before we go on, I want you to appreciate that carbomers/ carbopols are a chemical family rather than just one thing; are a type of chemistry that’s now made by a range of manufacturers in a range of distinct grades; that different grades of carbomer (I’ll use that term from now on) have different features and benefits and that it makes no sense to view this chemistry with a one-size-fits-all lens.
Thinking back to the rule we are exploring: ‘Never homogenise Carbomers’ it’s time to see what we mean by homogenise.
Just as we saw the words Carbomer/ Carbopol can be squished and squooshed up in our heads and taken to be the same, singular thing (reduced & homogenised), so can the term homogenised/ homogenisation.
While I may be pedantic, detailed and precise in my teaching practice, when I’m just being me I can be quite careless with my words and that may be what’s happened here. I’m potentially being more generous in entertaining that idea than my gut wants me to be but hey, life is better if we all think the best so here goes…
Homogenisation/homogenising / Homogenise CAN be terms that are thrown around in a cosmetic lab and taken to mean any of the following:
a) Any method of mixing that achieves a homogenous outcome.
b) Any method of mixing that achieves an optimally-dispersed outcome.
b) The specific process of using a tool that’s called a homogeniser.
c) Slang/ shorthand for mixing that happens at a higher intensity than is required to just blend ingredients together, maybe to achieve outcome a or b OR to aerate the product or just ensure it’s all blended together.
Confusing isn’t it!
Back to the original reason, I’m doing this investigation – the scenario the person speaking to me had found themselves in.
The person I spoke to was struggling to get their powdered carbomer to mix into their water phase without ‘fish eyes’ (blobs of dry polymer with partially hydrated polymer jelly around them) forming. They had a separate homogeniser machine to hand but had not tried it due to what they had been told and instead were using their propeller mixer for this stage.
The formation of fish eyes is a common problem with Carbomers, especially the basic, simple types – later versions are often manufactured to be ‘self-wetting’ or ‘quick-to-disperse’ making this wetting out stage much easier but this grade isn’t so these fish eyes were a problem!
What I do when working with Carbomers is have my water phase warmed up and moving first, I then sprinkle the Carbomer into the vortex made by my propeller mixer as if I’m dusting a cake with icing sugar. I usually have my mixer on a medium speed (maybe 1500-2000 rpm ish) although I gauge the RPM more by sight than measurement as it depends on the batch size, the vessel I’m using, how much Carbomer I’m trying to hydrate, whether there’s any other chemical in the water and the water temperature. It all matters (detail you see).
Once all of my powdered Carbomer is in the water and mixed around somewhat I have a look at it. If it looks like it is going in nicely and if I have time to let it do its thing I’ll leave it here and give it time enough to fully wet without moving it to my homogeniser, this could take anything from 5 to 20 minutes for a laboratory batch. If I’m in a hurry or just can’t be bothered I’ll pop this under my homogeniser and give it a whirl to speed up the wetting process.
OH NO, DID SHE SAY SHE HOMOGENISES CARBOMER?
Yes I did…
YOU NAUGHTY GIRL, WE WERE TOLD NOT TO DO THAT!
Yes but I think that rule is utter bollocks.
Carbomer Through The Stages.
Many, but not all grades of Carbomer go through a step-by-step metamorphosis process before they become the rheology modifiers of your dreams. Each step of this process is different and should be treated as such.
My question now is this: Is whatever problem Carbomer’s have with homogenising (whatever that means) equal across every stage or more important for some stages? Let’s dig further…
1. The Dry Powder Stage.
Many (but not all) carbomers come supplied as powders. These powders may be described as self-wetting, self-neutralising or they may be just regular powders. In any case, your Carbomer, if it is a powder, is going to be easier for you to handle if it is in a free-flowing state. As with any powdered ingredient, Carbomer powders can become lumpy and/or compressed (solid). Look at your powder before adding it to your formula and if it is either of those things and ESPECIALLY if it is not a self-wetting type, pre-work your powder by breaking up the lumps into free-flowing powder/ particles again.
2. Getting it WET/ Wetting out your powder.
Adding your Carbomer powder to the water phase is not the only way to get Carbomer into a formula but it’s the way I’m talking about here because this is where at least some of the homogenisation need/ confusion comes from. There is a difference between getting your powder wet and fully wetting out your powder, we are aiming for the latter. Wetting out a powder means that each individual particle of Carbomer powder has been touched and penetrated (ooooohhh eerrr Mrs) by the water, just getting it a bit wet is what gives you fish eyes and problems. If you fail to optimise this stage, you will fail to achieve the maximum viscosity and stability for the amount of Carbomer you’ve added, regardless of what you do next. This is a very important step!
3. Neutralising the powder
Not all Carbomer chemistry needs neutralising by the formulator/ manufacturer but many do. For those grades that do need neutralising, as supplied, the Carbomer is in its acid form and we neutralise it with a relevant alkali to complete its metamorphosis and give us our complete gel. It’s common in manufacturing for neutralising to be one of the very last steps before taking the product out of the mixer and into storage. It’s relatively uncommon (although not entirely unheard of) for the Carbomer to be neutralised early in the piece. Once neutralised, the viscosity of the finished product typically increases and any mixing you want to do after this requires more energy input as there’s more resistance. Is it possible that the ‘don’t homogenise Carbomer’ rule is talking about step 3 only or mainly? I feel it may be and even then it may not always be a black-or-white issue.
Important Step Along Your ‘Carbomer’ Journey.
For your Carbomer to reach its maximum viscosity it needs to be fully wetted. For the Carbomer to be fully wetted, it needs to be able to experience the water phase (whenever it is introduced to it) to its full potential. This is more likely when you’ve maximised your Carbomer’s surface area by breaking down any compacted, lumpy powder and you’ve prevented any fish eyes from forming through adequate and optimal mixing. You’ll generally find water that is warmer hydrates Carbomer powder faster than water that is colder and faster mixing avoids fish eyes and fully wets the powder more efficiently than no or slow mixing.
And this is why I feel leaving students with the idea that homogenising carbomer-containing formulations is wrong is very unhelpful.
Nuance, detail and context matter and that’s why I both LOVE cosmetic science and HATE it when people teach it with broad, shallow brush strokes. It may be technically right, at least on one level to use caution when using a homogenisation step with Carbomer in some scenarios but impractical and a potential problem CAUSING if taken too broadly and/or literally.
Going back to what I said about homogenisers and how the term ‘homogenise’ is used by cosmetic chemists in a practical, factory setting.
I would describe how I ‘wet’ my Carbomer powder as ‘homogenising’ on my formula write-up meaning I’d typically use my homogeniser at a relatively low speed (3000-5000 RPM) to help reduce the potential for fish-eyes, speed up wetting and minimise the time taken on the plant (time is money in cosmetic scale-up). I would then typically note the end point as the point at which the powder appears fully wetted (partially swelled), with no visible fish-eyes. I may then suggest returning to the propeller mixer to complete the wetting if needs be, depending on how much Carbomer was in the formula, how much free-water was there for it to ‘wet’ into (remembering that not all wet stuff is water, glycerin and other humectants can make water unavailable for the swelling), and what temperature constraints I had in the water phase. However, it may be more accurate of me to swap out the word ‘homogenise’ for ‘disperse’. Now this would be pedantic as in my laboratory setting the mixer I call my homogeniser is actually sold as a dispersing tool (just to confuse everyone) but the way it mixes is the type of force that the manufacturers of Carbomer avoid caution with so with that in mind I’m going to be careful not to disperse/ homogenise the crap out of my Carbomer at any stage. As with all tools one can use them gently or otherwise and the outcome you achieve will vary accordingly.
Is There Any Evidence To Suggest Homogenising Carbomer Is Bad?
Yes there is.
If we set aside what I’ve said about the semantics of language cosmetic chemists use around the term ‘homogenising’, using a mixer capable of creating high levels of friction and/ or a mixer that has a very fine chopping and/or grinding motion can damage polymeric thickeners such as Carbomers. Indeed, the manufacturers of Carbomer do caution against all-out homogenisation. This advice is typically directed at the neutralised rather than pre-wetted Carbomer but it isn’t always made crystal clear and maybe that, plus a lack of experience of how factories operate, has contributed to the level of confusion we now find ourselves in. Well that and the fact, pre-neutralised Carbomer and/or other acrylate thickeners may be more suceptible to homogenisation viscosity loss than the old-fashioned and more common types.
So What Does Homogenising Do To Carbomers?
As the shear force and/or temperature the polymer is exposed to increases, the chances of damaging the polymer chains rises. Based on experimental data I’ve seen to date, the exact ‘break point’ varies, depending on whether the polymer has been neutralised or not, how concentrated it is, what grade of polymer it is and the exact type of mixer it is being exposed to. This damage manifests as a non-recoverable loss of viscosity – so the end gel or product is thinner than it otherwise could be.
In a laboratory setting, it is possible to test all manner of different variables to whatever limits you can achieve. Not every scenario that can be created in a lab is likely or even possibly experienced in real-life. Most factory equipment and time constraints mean that maximum speeds and mixing durations are hardly even reached in practice. What is relevant and what needs to be managed is what’s likely and then what’s possible.
Who is Right? What Rule Do We Adopt?
I would answer this by urging you to think about the Carbomer/ Homogeniser in the context of your specific formula and go from there.
If you are making an emulsion and using Carbomer as your water-phase thickener/ rheology modifier it probably makes sense to use a homogeniser to ensure fast and full wetting of the Carbomer prior to neutralisation. In this case, you are saying Homogenisation but meaning ‘dispersing’ – tool will likely be the same but the outcome is visually different.
Homogenisation isn’t an all-or-nothing mixing situation so in an emulsion setting you could experiment with different speeds and mixing times to get the maximum benefit with minimum risk (remembering risk is not just damage to the Carbomer but (more likely) failure to fully wet out the Carbomer).
In the case of creating an emulsion, it’s all very well everyone talking about the way high shear homogenisation results in a permanent reduction (up to 50%) in Carbomer viscosity but if you don’t consider the fact that such high shear is unlikely in most cosmetic manufacturing settings, a shear rate that is too low is almost guaranteed to leave at least some of the Carbomer powder either un-wetted of sub-optimally wetted and that will cause more than just viscosity loss. It is also worth mentioning that most Carbomer grades we use in cosmetic formulating achieve their maximum viscosity as a function of pH. Most cosmetic products are set to a pH of the skin rather than the maximum pH for the Carbomer so you are unlikely to be needing the maximum viscosity possible from your Carbomer anyway. So risking a little Carbomer breakage and then rectifying it (if necessary) with a slight pH adjustment is more sensible than the alternative which is not recoverable once the wetting stage is past. Finally keep in mind that you can homogenise a cosmetic emulsion BEFORE neutralising your Carbomer, in fact, this is most likely the norm.
In the case that you are making a gel, especially one that stretches the limits of the Carbomers salt, solvent, surfactant or free-water tolerance, it may well be more detrimental to homogenise the neutralised gel, especially at high speed. However, in the vast majority of formula scenarios this isn’t going to be necessary or aesthetically pleasing. Homogenising a fully neutralised gel may trap air causing a visual bubbling effect, change in your specific gravity and higher potential for negative micro and oxidation results. While all of these things (once known) could be mitigated against and some level of homomgenisation achieved, it is less likely in general that one would homogenise (especially at high speed) this type of formula.
The Bottom Line – Summary.
Anyone who is involved in sharing, teaching or passing on information about Cosmetic Science (including me – and I know I’m not above making my own mistakes) should keep in mind the APPLIED nature of this science and the complexities and nuance involved in each formula, grade of ingredient and manufacturing setting. I typically caution against reducing things to ‘always’ and ‘never’ statements, especially when they are left unqualified as this can lead to misunderstandings and confusion.
Sure, the world of nuance, multiple meanings and vagueness is also confusing but at least it gives you more to question and test and THAT’S what it is all about.
For those that skip straight to the last line the answer is as follows:
Homogeniser style mixing can damage Carbomers in a way that permanently reduces their capacity to reach maximum viscosity. Whether using a homogeniser does this or not depends on many things including the stage in which you homogenise (wetting, pre neutralisation or post neutralisation), the style of homogeniser you use (machine), the time/ speed settings employed and the overall character of the formula. In a practical setting, there is plenty of evidence and many scenarios where the extra speed and power a homogenising step brings improves overall formula outcome and reduces the chances of stability, aesthetic and microbial problems. Therefore it is unhelpful to hold the belief that homogenisers should never be used with Carbomers.
I really like this article on Carbomers written by Mike J.Fevola of Johnson and Johnson even though he also states that you shouldn’t homogenise Carbomers. He’s definitely with the mainstream in using that throw-away line and I won’t judge him too harshly for it as he’s not all wrong and it’s not the main take-home from the article.
This report from 2014 by Lyndel Speedy from Monash laboratory in partnership with Ensign Laboratories (hello chaps) is also a good read and one that seems to confirm the opposite of what I’m saying (i.e: they are showing why you shouldn’t homogenise Carbomers). However, this primarily investigates Carbomers after neutralisation so it is very useful for people using Carbomers but doesn’t really talk about the wetting step in any detail. When it does I don’t actually think their method was that fair as the homogenised samples got way less time to wet -and wetting still takes some time. Anyway, it’s still good in terms of getting you thinking.
This paper on mdpi is quite a good overview of Carbomer rheology. It’s very detailed and good for those wanting to understand the chemistry and rheological flow to a greater degree.
Here is a link to Lubrizol who own the Carbomer range now. This gives you an entry point into rheology, shear and the different polymers in their range. They have a lot of resources that are helpful.
I also found this useful from Silverson. Silverson are manufacturers of the industry standard ‘homogeniser’ so it was good to hear from them with regards to mixing, wetting and neutralising Carbomer. It was through reading this that I realised that I had to talk about equipment as well as the action of homogenisation.
One last paper I’ll share is this one which goes into more detail about the rheology of carbomers. One thing that interests me still is this idea of shear ‘breaking’ the polymers. The rheology of Carbomers as a family is quite complex and is also linked to the concentration of the Carbomer in solution. I am still not sure about size and shape of the safe zone for using high shear mixing with Carbomer in general and have found it hard to find a paper that really explores viscosity changes over time and different pH after different shear. It’s often you get one or two variables but not all three.