Improving the Viscosity Of Hot Pours

Dear Valerie: I keep getting various viscosities on my hot pours…—ALL OVER THE PLACE

Dear All Over:
Any product that gets filled hot and then cools in its packaging can be a processing nightmare, especially when you’re trying to create a product consistently. It can be frustrating because not only are your analytical measurements for specifications all over the place, but product performance may also vary. I still have a little PTSD from our product evaluators rejecting something I knew was the same formula. The good news is it’s likely not a formula issue and you have some hard work ahead of you with fine tuning your process and collaborating with the process engineers in manufacturing. This is the fun part!

I’ll never forget when my senior director of R&D, Kathleen, helped me make my first hot pour as a budding chemist while making lab samples of one of her new hair pomades. First, she watched me make the batch my way, loosely paying attention to temperature ranges and mixing it as fast as I wanted to mix. After pouring the batch into jars and cooling, I noticed the product looked similar from afar but, when picked up out of the jar, was not similar at all. Mine was super hard and hers was creamy. They would have failed a side-by-side evaluation. What gives?

Ask the Right Questions

Various pomades, created by the author, cooling on a conveyor.

It was a good lesson in that I should have been asking a lot of detailed questions about the process before getting started and I would bet a few of these questions would help you and your varying viscosities!

Some questions to consider include:

What was the temperature of the batch right before pouring? Even a 5°C difference in temperature at the time of pouring can transform a product’s texture. If the product is clear when hot, did you pour it at its clear point or right when it starts to get cloudy? Try a lab batch where you cease mixing and pour your formula at the hottest temperature it can reach and make subsequent pours in 5°C increments until the product is not at a pourable consistency. How did their textures and viscosities turn out? It’s incredible how sensitive formulations can be to this parameter. This is also a good exercise for understanding temperature variance that may occur in production, and its impact on viscosity. 

How long was it at that temperature? Sometimes manufacturing and filling hot pours can take an extended period of time. Can your batch withstand continuous mixing while it is hot, or will that impact the way the product sets up? Is it shear sensitive? In another lab batch, try pouring the product as you normally would, then extend mixing time at that temperature before pouring your next sample. Note what happened. Do you need to set some limitations on mixing time while filling?

Was there mixing while cooling down to pour temperature? Continuous mixing while filling? In the first exercise, we turned off the mixer while pouring our batches at different temperatures so we could solely evaluate the impact of temperature on product set up. What role does mixing play during cool down? Mixing while cooling may lend to a creamier or softer formulation or not. One good evaluation to add to your specifications is a puncture or penetration test, if your facility allows it. This will be a good indicator of hardness on top of viscosity.

Now that it’s in its final component, what was ambient temperature like for cooling? Or was it in the refrigerator to mimic a chiller? Again, temperature is critical for how a product regains its structure, and it’s no different in pouring than cooling. A product that cools quickly may be hard, and a product that cools slowly may be soft. This can be used to your advantage or disadvantage. It’s also important to make sure the product cools evenly as well so it’s uniform from the edges to the center. I’ll never forget the time that marketing wanted me to work on a 28oz version of our top-selling men’s hair pomade. I still chuckle at this one. It just wasn’t possible to do it consistently!

More Parameters to Consider

Not to pile on the lab work, but one last scenario I added on to the Kathleen Method was reheating a hot pour batch. If something happens in production (which it always does!) and the batch must stop, can your formula be reheated? What would that do to color? Consistency? Will it be in specification for pH, viscosity or hardness? Fragrance? Preservation system? I can’t tell you how grateful I was to have checked this parameter before I headed into production on a project and then needed to pull it out of my pocket, figuratively—that would be a messy lab coat!

All these little nuances are so important to how the final product turns out, and once you nail this in the lab, get ready to take this information to manufacturing. If you are lucky to have process engineers, it will help them set up manufacturing parameters based on the equipment you have. If it is just you flying solo with your compounders, you’ll have excellent information at your fingertips to help make a more successful, consistent production.

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Valerie George
askvalerie@icloud.com
 
Valerie George is a cosmetic chemist, science communicator, educator, leader and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist. Do you have a formulation question you want answered? Email her at the address above.