METHOCEL™ Stabilizes Suspension Concentrates

Suspension Concentrates

Herbicides, pesticides, and fertilizers used in agricultural applications are often supplied as suspension concentrates.  These agrochemical suspension concentrates contain the active ingredient as minute, finely divided crystals suspended in water.  Over time these crystals have a tendency to agglomerate and form larger crystals.  This leads to the active ingredient falling out of suspension and a loss of active ingredient potency.  In order to maximize the shelf life and efficiency of the suspension concentrate, active ingredient crystal growth must be inhibited.
 

METHOCEL™ Stabilizes Active Ingredients

METHOCEL™ hydroxypropyl methylcellulose can be used to improve the stability of active ingredients in suspension concentrates.  To demonstrate this, the particle size distribution of the model herbicide suspension concentrate below was tested utilizing xanthan gum and METHOCEL™ F4M as stabilizers.  Xanthan was chosen for comparison as it is currently used in many commercial suspension concentrate formulations as a stabilizer.
 
  • 400g/L Metribuzin
  • 0.2-0.3% Stabilizer
  • Balance:
    • Antifoaming agent
    • Freeze/Thaw stabilizer
    • Surfactant
    • Dispersant
    • Wetting agent
    • Preservatives
    • Water

Testing was performed after initial suspension of the active ingredient (Metribuzin) and after two weeks at elevated temperature (54°C) to determine the amount of crystal growth that had occurred during storage.

The charts below show the results of the particle size distribution testing:


0.2% Xanthan


0.2% METHOCEL F4M

When analyzing the charts above we can see that the particle size distribution of the suspension stabilized with xanthan gum shifts dramatically after storage for two weeks at elevated temperature. This shift in particle size distribution over time is attributed to active ingredient crystal growth.   The particle size of the suspension stabilized with METHOCEL™ F4M remains relatively consistent after two weeks at elevated temperature showing that METHOCEL™ F4M inhibits the formation of crystals and thus improves the stability of the active ingredient (AI) and thus the overall potency of the formulation.


Improving Xanthan Gum Stabilized Suspension Concentrates

Xanthan gum is commonly used in suspension concentrates as a rheology modifier and stabilizer due to its cost effective nature.  Complete replacement of xanthan gum in a commercial formulation may not be feasible from a cost perspective even though improved stability of the formulation is desired.  In these cases METHOCEL™ can be blended with xanthan gum to inhibit crystal growth and improve suspension stability of the active ingredient while minimizing the effect to raw material costs.  Particle size distribution of the same model formulation based on Metribuzin was tested with a blend of xanthan and METHOCEL™ F4M used as the stabilizer and compared to the formulation stabilized by only xanthan gum  to demonstrate how METHOCEL™ improves suspension stability when blended with xanthan gum.


Particle Size Distribution after Storage for 2-weeks at 54°C

The particle size distribution data above shows that by replacing 0.1% xanthan gum with 0.1% METHOCEL™ F4M crystal growth of active ingredient can be inhibited and the suspension made much more stable during storage.  It is also worth noting that only test formulations containing METHOCEL™ either entirely or as part of a stabilizer blend exhibited the crystal growth inhibition property.


Selecting the Proper METHOCEL™ Grade for Your AI

So far we have demonstrated that the addition of METHOCEL™ as a stabilizer to a suspension concentrate is more effective than xanthan gum at stabilizing Metribuzin in a suspension.  However, will all grades of METHOCEL™ behave accordingly?  What if the AI isn't Metribuzin?  To evaluate the effect of grade on AI stability, the suspension was stabilized with a blend of xanthan gum and varying grades of METHOCEL™ at 0.075%. The charts below illustrates the difference in particle size distribution of the model suspension after two weeks at room temperature and 54°C.


2 Weeks at RT - 0.075% use level, varying METHOCEL™ grades

2 Weeks at 54°C - 0.075% use level, varying METHOCEL™ grades

The results of the particle size distribution testing show that for storage at room temperature, METHOCEL™ F4M outperforms METHOCEL™ F50 and K4M as an AI stabilizer.  However, at elevated temperatures both METHOCEL™ F4M and F50 worked well to stabilize the Metribuzin.

Additionally, the influence of use level of METHOCEL™ on AI stability was evaluated.  In this instance METHOCEL™ F4M was blended with Xanthan at varying levels for use as the stabilizer and particle size distribution was measured after storage for two weeks at room temperature and 54°C.


2 Weeks at RT - Varying levels of METHOCEL™ F4M

2 weeks at 54°C - Varying levels of METHOCEL™ F4M

The data above shows that during storage at room temperature and at elevated temperature, increasing the level of METHOCEL™ F4M in the suspension did not benefit AI stability.  Crystal growth inhibition of Metribuzin is optimized using METHOCEL™ F4M at very low use rates.

As can be seen by the data above, there is no one size fits all solution for stabilizers used in solution concentrates.  The optimal grade of METHOCEL™ and use level needs to be explored for each formulation and active ingredient.  There are many commercially available grades of METHOCEL™ ranging in methoxyl and hydroxypropyl substitution as well as molecular weight.  ChemPoint is ready to help work with you to find the proper grade of METHOCEL™ for your formulation.


Conclusion

When used in low levels as a stabilizer in suspension concentrates, METHOCEL™ hydroxypropyl methylcellulose demonstrates the ability to prevent active ingredient crystal growth.  The stability provided by METHOCEL™ allows for improved potency of the active ingredient over a longer shelf life.  In addition, the improved potency often allows for a reduction in the amount of active ingredient needed in the formulation and ultimately a reduction in formulation costs.  To summarize, the benefits of using METHOCEL™ as a stabilizer for solution concentrates are listed below.
 
  • Improved & extended product shelf life
  • Improved active ingredient (AI) potency
  • Reduce AI concentration
  • Reduce formulation cost

While METHOCEL™ F4M has proved to be the optimal stabilizer for our model Metribuzin suspension, care should be taken to explore the appropriate grade and use levels for your active ingredients and suspension concentrate formulations.  Contact us today to discuss the right grade of METHOCEL™ to stabilize your active ingredients.

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