Suspension Concentrates

Herbicides, pesticides, and fertilizers used in agricultural applications are often supplied as suspension concentrates.  These agrochemical concentrates contain active ingredients as minute, finely divided crystal particles suspended in water.  Over time, these crystals tend to agglomerate and form larger crystals, leading to the active ingredient falling out of suspension and losing its potency.  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 a model herbicide suspension concentrate was tested utilizing xanthan gum and METHOCEL™ F4M as stabilizers.  Xanthan gum was chosen for comparison as it is currently used in many commercial suspension concentrate formulations as a cost-effective stabilizer.  The contents and concentration of the herbicide suspension are detailed below.  
 

  • 400g/L Metribuzin
  • 0.2-0.3% Stabilizer
  • Balance:
    • Antifoaming agent
    • Freeze/Thaw stabilizer
    • Surfactant
    • Dispersant
    • Wetting agent
    • Preservatives
    • Water

 

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

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


0.2% Xanthan

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0.2% METHOCEL™ F4M

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


Improving  Suspension Concentrates Stabilized with Xanthan Gum

As mentioned, due to its cost-effectiveness, xanthan gum is commonly used in suspension concentrates as a rheology modifier and stabilizer.  Even though improved stability of the formulation may be desired, complete replacement of xanthan gum in a commercial formulation may be cost prohibitive.  In these cases, METHOCEL™ can be blended with xanthan gum to inhibit crystal growth and improve the suspension stability of the active ingredient while minimizing raw material costs.  Particle size distribution of the same model formulation based on Metribuzin was tested with a blend of xanthan gum and METHOCEL™ F4M used as the stabilizer.  This was compared to the previous formulation stabilized by only xanthan gum to demonstrate how METHOCEL™ improves suspension stability when blended with xanthan gum.


Particle Size Distribution after Storage for Two Weeks at 54°C

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


Selecting the Proper METHOCEL™ Grade for Your Active Ingredient

So far, the data presented have demonstrated that the addition of METHOCEL™ F4M as a stabilizer to a suspension concentrate is more effective than xanthan gum at stabilizing Metribuzin in a suspension.  There are many grades of METHOCEL™ available and many types of active ingredients that can be used to formulate suspension concentrates.  Naturally, there are a couple of questions to consider.  For example, will all grades of METHOCEL™ behave accordingly?  What if the active ingredient isn't Metribuzin? 

To evaluate the grade’s effect on active ingredient stability, the model suspension was stabilized with a blend of xanthan gum and varying grades of METHOCEL™ at a 0.075% use level.  The charts below illustrate differences in the size distribution of particles in the suspension after two weeks at room temperature (RT) and 54°C.


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

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

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

Additionally, the impact of the concentration of METHOCEL™ on active ingredient stability was evaluated.  In this instance, METHOCEL™ F4M was blended with xanthan gum at varying levels for use as the stabilizer.  After storage for two weeks, particle size distribution was measured at room temperature (RT) and 54°C.


Two Weeks at RT - Varying levels of METHOCEL™ F4M

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Two Weeks at 54°C - Varying levels of METHOCEL™ F4M

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

Generally, there is no one-size-fits-all solution for stabilizers used in solution concentrates.  The optimal grade of METHOCEL™ and use level must be explored for each formulation and active ingredient type.  There are many commercially available grades of METHOCEL™ that range in properties of methoxyl and hydroxypropyl substitution as well as molecular weight.  ChemPoint is ready to help you find the proper grade of METHOCEL™ for your formulation.


Conclusion

When used at low levels as a stabilizer in suspension concentrates, METHOCEL™ hydroxypropyl methylcellulose can prevent active ingredient crystal particle growth.  The stability provided by METHOCEL™ allows for improved potency of the active ingredient over longer shelf life.  This improved potency also reduces the amount of active ingredient needed in the formulation, leading to reduced formulation costs.  To summarize, the benefits of using METHOCEL™ as a stabilizer for solution concentrates are listed below.
 

  • Improved and extended product shelf life
  • Improved active ingredient potency
  • Reduced active ingredient concentration
  • Reduced formulation cost

While METHOCEL™ F4M has proved to be the optimal stabilizer for the model Metribuzin suspension presented in this article, the appropriate grade and use levels for your active ingredients and suspension concentrate formulations will vary.  Contact us below to discuss the right grade of METHOCEL™ to stabilize your active ingredients.

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