When most people were talking about the brutal polar vortex that hit Chicago earlier this year, I am sure many were looking for ways to best protect the products they are producing, transporting, storing and using from being destroyed by freezing. In this blog post, I will briefly mention a few tips on how to make products freeze-thaw stable such that they can be used in winter harsh areas.
Every product development is evaluated successful when at least the performance desired is obtained and when the stability over the intended time and at the intended temperatures are confirmed.
When planning stability tests, it is important to consider that the product might be manufactured in one area with a given climate and transported to the consumer in another area of the world with a different climate. One should also consider possible temperature fluctuations during transport and storage. In some regions, freezing cold temperatures are part of every winter. The stability tests help the producer confirm that the product get to the consumer with the same properties and characteristics as was intended by the formulator. These tests either confirm that the product is freeze thaw stable or allow the producer to indicate that the product should be protected from freezing temperatures.
What happens when a product is not freeze thaw stable?
When a product is not freeze thaw stable, it irreversibly loses one or all of its initial characteristics and performance when it freezes. Thawing it lead to a product that appears different (possible phase separation), smells different or even flows differently than before freezing. These might not only be cosmetic changes of the product, they might also mean loss in performance.
Which products are susceptible to be damaged after freezing?
Freeze thaw stability can be a major issue in liquid based products. This, as we can easily imagine, might be more of an issue when the product contains a lot of water. Such products can range from Cosmetic lotions, creams and skin cleansers to water-based paints and coatings, to cleaning product to agrochemical products.
A few tips on how to make your products freeze-thaw stable.
The ingredients you use in your formulation will define how stable the formulation will be to freezing and thawing. Following are a few tips on how to design a freeze thaw stable formulation.
- Use materials that has the ability to bind water. The stronger the water is bind to an ingredient, the longer it might take the formulation to freeze. This increases the formulation’s resistance to freezing.
- Use efficient stabilizers that can keep the formulation ingredients in a dispersed and homogeneous position throughout the formulation and reduce structural changes that can occur after thawing. Stabilizers with high yield stress will prevent the fast growth of crystals formed during freezing and reduce their aggregation avoiding phase separation after thawing.
- Use, when possible, solvents or anti-freeze agents such as glycols. Depending on the use concentration of such solvents, they will lower/depress the freezing point of the whole formula making the product more resistant to freezing hence avoiding the freeze thaw stability problem.
- Use additives that can control the ice crystal growth. A homogeneous dispersion of ice crystals in an emulsion during freezing and during thawing might avoid an eventual creaming of the emulsion.
Of course these are only some simple tips to try and in no way an overview of what to do. Designing a formulation to fit both the performance and stability requirements can be very challenging. It involves a lot of optimization and testing.
What about Freeze thaw stability of Cellulose fibrils?
Cellulose fibrils are most often available dispersed as water suspensions. These are not freeze thaw stable when not formulated since during freezing, the water crystallizes and breaks the hydrogen bonds formed with the hydroxyl groups on the surface of the cellulose fibrils. The available hydroxyl groups now can form Hydrogen bonds with the neighboring cellulose fibrils. In addition, when the water is freezing, the concentration of the fibrils in the part not yet frozen is quite high and the fibril to fibril contact points are increased making this phenomenon more prominent.
The resulting cellulose to cellulose Hydrogen bonds (hornification) are quite strong. It is therefore almost impossible to re-disperse the fibrils in the water phase afterwards (after thawing) using general laboratory dispersing equipment.
Freezing pure cellulose fibril water suspensions will collapse the fibrillar network, leading to loss in the initial high surface area of the material and its water holding capacity. This impacts the material properties and performance negatively.
Store the material at temperatures above water freezing temperatures to avoid damaging the product properties.
Will a formulation using Cellulose fibrils be freeze thaw stable?
Cellulose fibrils are not freeze thaw stable (when not in a formulation) due to hornification. This does not necessarily imply that a formulation with cellulose fibrils will not be freeze thaw stable. In a formulation, the fibrils are more dilute and also have reduced contact with each other due to the presence of other ingredients.
Cellulose fibrils might even help improve the freeze thaw stability since the material is a good stabilizer and has a high water binding capacity. As I have written above, both these characteristics might be helpful in obtaining a freeze thaw stable formulation.
Rebecca Blell first started working with microfibrillated cellulose MFC in 2009. During her studies, she was part of the SustainComp project and her task was to understand and incorporate MFC into thin layers for improved film properties. As a research scientist at Borregaard today, she focuses on the Exilva product and its performance in personal care and home care applications. Rebecca has a PhD in physical chemistry from the University of Strasbourg, France, and experience from international locations.
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