Typically, when using polymeric rheology modifiers, the viscosity of a formulation decreases with increasing temperature and the polymers can even degrade at higher temperatures. This can cause problems for the manufacturer or user, like instability of the formulation or difficulties in application. Cellulose fibrils and cellulose in general are stable against temperatures up to 200-300 °C, which makes them a good choice when a temperature stable viscosity modifier is needed. Earlier, we have described how you can achieve a stable viscosity in your formulation with cellulose fibrils in the temperature range of 20-90 °C. This time I would like to discuss what happens when we go over 100 °C, either in wet or dry state.
Read more about the temperature stability of cellulose fibrils below 100 °C on our earlier blog post.
Water-based suspensions at over 100 °C
Earlier, we have shown that cellulose fibrils retain their rheological properties at temperature ranges 20-90 °C. This is practical for many applications since it offers stable viscosity and stability properties, regardless if your using the product on a warm summer day or in the cold winter. However, in certain applications, formulations are exposed to even higher temperatures than 100 °C. For example in oil drilling, the temperature typically are higher than that. It is a demanding environment for many rheology modifiers, especially polymeric ones. The viscosity of polymers decreases generally as a function of temperature because the temperature affects the interactions between the polymer chains, between the polymer chains and the surroundings as well as their conformation. In addition, the polymer chains start to degrade when the thermal degradation temperature is achieved. By contrast, cellulose fibrils have stable viscosity with increasing temperature since the fibril network is less affected by the temperature and the thermal degradation starts at higher temperature than for typical polymeric rheology modifiers.
Cellulose fibrils has shown good temperature stability at high temperature, high pressure (HTHP) conditions. Heggset and her colleagues compared cellulose fibrils to guar gum and xanthan gum at HTHP conditions in order to evaluate their suitability for oil drilling applications. Suspensions of cellulose fibrils, xanthan gum and guar gum were kept three days at 140 °C and their viscosity was measured before and after the heat treatment. The suspensions were at 0.8% concentration and contained sodium formate to reduce radical formation. Both polymer suspensions had clearly lower viscosity after the heat treatment compared to non-treated samples whereas the cellulose fibril suspension had similar viscosity before and after the treatment. This shows that cellulose fibrils can tolerate high temperatures for several days without thermal degradation and loss of viscosity.
Did you see our Topic Tuesday video about temperature stability? You can find it here.
Temperature stability at dry state
Even higher temperatures are possible if cellulose fibrils are used in dried products, like coatings or composites. In dry state, the degradation of the cellulose fibrils begins at temperatures over 200 - 300 °C and the degradation temperature is dependent on the composition of the cellulose fibrils. Cellulose fibrils that are made from wood pulp contain normally cellulose, hemicellulose and lignin in varying amounts. These components have different decomposition temperatures: cellulose starts degrading around 315 °C, hemicellulose 220 °C and lignin 160 °C (according to Yang et al.). If the fibrils are highly purified cellulose, they degrade at higher temperature. Horseman and his colleagues studied how the amount of lignin affect the thermal degradation of cellulose fibrils. They compared fibrils made from bleached pulp (no lignin) to fibrils made from thermo-mechanical pulp which contains lignin. They found out that the fibrils without lignin started to degrade at 30 °C higher temperature than the fibrils with lignin.
In addition, chemical modification of cellulose fibrils tends to lower the thermal degradation temperature. Fukuzumi et al. (2009) produced cellulose fibril by TEMPO oxidation. Compared to the original cellulose, the thermal degradation started at much lower temperature, 200 °C instead of 300 °C, when measured in nitrogen atmosphere.
All in all, cellulose fibrils are stable against high temperatures which makes them a practical additive for demanding applications. They have at least the following benefits:
- constant viscosity in water based formulation under atmospheric pressure
- stable viscosity at high temperature, high pressure (HTHP) conditions (140 °C)
- stable up to 300 °C in dry state depending on the chemical composition of the cellulose fibrils