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Efficient Selection of Bio-Based Surfactants for Emulsion Polymerization

Sander van Loon – Mar 24, 2017

TAGS:  Science-based Formulation    

Efficient Selection of Bio-Based Surfactants for Emulsion PolymerizationA proof of concept has been developed at VLCI – Amsterdam to showcase the process of surfactant selection via the HLD theory for emulsion polymerization. This approach, that has already been proved in the fields of personal care, household and EOR, allows a practical and fast selection of the right surfactants for the development of (micro-)emulsions.

The HLD (Hydrophilic Lipophilic Difference) theory is used to make profound predictions about:

  1. The type of emulsion (o/w or w/o)
  2. Efficiency of a surfactant in a defined emulsion system

The most effective surfactant to reach the desired type of (micro-) emulsion can be selected based upon its intrinsic parameter (Cc) and takes the following emulsion parameters into account:

  • Oil
  • Monomer or polymer number (EACN)
  • Temperature
  • Salinity, and
  • Co-solvents

This article will help you understand the practical application of HLD for emulsion polymerization. But before that, let's see how bio-based surfactants are selected.

Selection of Bio-based Surfactants: How is it done in the industry?


Commonly Used Methods: “Trial and Error” and HLB


The “trial and error” approach is still the most common method. In this method, varieties of surfactants are screened at different concentrations without making any prediction. This is obviously time-consuming. HLB values are sometimes given for surfactants, but these values are mainly applicable for EO-based surfactants, and do not give good practical guidance. Furthermore, for bio-based surfactants, the HLB approach is usually not applicable.

New Approach: HLD


HLD is recognized as a powerful and reliable method for effective surfactant selection and formulation of emulsions in various fields. Although little relevant literature on the subject is present, it has also been proven suitable for emulsion polymerization1. Although, there is currently little relevant literature on the subject is present. The ‘HLD’ is an expression of the change in chemical potential of a surfactant molecule (µsw - µso) when it is transferred from the oil phase into the aqueous phase. HLD shifts between negative, neutral and positive values are marked by transitions between emulsions:

The general HLD equation is:
HLD = F(S) - k.EACN - α.∆T + Cc + F(A)

  • F(S) is a function of salinity: as S increases, so does HLD. For non-ionics F(S) = b*S, and for ionics F(S) = ln(S).
  • EACN is the number of carbon atoms in the linear alkane with equivalent behavior to the oil. As EACN and lipophilicity increase, HLD decreases.
  • The value of the coefficient k depends on the type of surfactant used, with the standard value being k=0.17.
  • The effect of the temperature difference with the reference value of 25°C is characterized by the coefficient α. Its value is dependent on the type of surfactant used, such as:

    • +0.01 for ionics
    • -0.06 for ethoxylates
    • 0 for APGs, etc...

  • The Cc value is the characteristic curvature of the surfactant.
  • Finally, F(A) is a contribution of a co-surfactant or alcohol, that may depend on its nature and concentration.

When HLD = 0, the thermodynamically stable state is reached, and all parameters are balanced. This results in a micro-emulsion. From there, slightly negative or positive values of HLD give respectively types I (o/w) or types II (w/o), emulsions that are typically formulated.

Transition Between Different Types of Emulsions


The beauty of this approach relies on its versatility. All kinds of oils, monomers and polymers can be characterized with an EACN, as well as surfactants with Cc values. This allows for accurate ingredient matching, resulting in more stable emulsions.

Once the EACN value of an oil is known, the required Cc value is calculated from the equation to formulate the required emulsion. It is also applicable to blends; the resulting EACN or Cc being the sum of each EACN or Cc multiplied by their respective molar fraction.

Databases of EACN and Cc values already exist in order to properly select suitable surfactants2,3, but they are still very limited. There is therefore a need to extend this to more oils and surfactants. It would be great if the suppliers provide these parameters as well.


Why is HLD effective for emulsion polymerization?


By ensuring that the HLD is slightly lower than 0 (type I, o/w), the maximum amount of surfactant molecules are present at the monomer droplets interface. That too, with the minimum added surfactant. Therefore, the surfactant is used at its utmost efficiency. The required Cc is calculated by implementing the following variables in the HLD equation:

  • Composition of the monomer phase
  • Salinity
  • Presence of co-solvent, and
  • Temperature profile of the reaction

The emulsion stability is a major concern when performing emulsion polymerizations. Oil separation, coalescence of the droplets and Ostwald ripening all affect the final product by creating:

  • Agglomerates
  • Monomer residues, or
  • Undesired polymerization grade

When the surfactant (or blend of surfactants) is selected to reach the required Cc value, the emulsion is stable and can be prepared with the lowest energy input. Therefore, it’s very effective approach for emulsion polymerization!


High Throughput Preparations


The co-polymerization of a well-known system of Butyl Acrylate (BA) and Vinyl Acetate (VA) (30:70) is chosen to illustrate the surfactant selection via HLD for emulsion polymerization. HLD scans are performed in accordance with the theory to characterize both monomers and surfactants, rationalizing the surfactant selection.

In order to speed up the HLD characterization of these compounds, the High Throughput FORMAX is used. This automatically prepares a large number of samples, in parallel and on small scale. Likewise, the emulsion polymerization reactions are carried out with the FORMAX HT platform in an automated and parallel way.

High Throughput preparations


HLD Scans for the Determination of EACN and Cc Parameters


The scans are performed on both monomers and surfactants. They are used as the only unknown species in an oil/water/surfactant system whose HLD parameters are known. Several samples of such a system are prepared by modifying only one of the parameters of the HLD equation (typically the salinity or the temperature) in a controlled way. This is done in order to cover the different emulsions types (I, II and III).

o/w (Type I, HLD < 0) → w/o (Type II, HLD > 0), via Type III (HLD ≈ 0) gives the point where HLD = 0


The HLD parameter of the unknown species can then be calculated by filling all the other parameters in the HLD equation. The samples are deliberately prepared in such a way that phase separation is obtained quickly, in order to easily visualize the different emulsion types.

Calculating EACN of Monomer Blends


Here, the 2 monomers are characterized separately. Once the EACN values are obtained for these 2 building blocks, the EACN of any monomer blend can be calculated. The scans are made with a surfactant whose Cc is known in a 50:50 (vol) blend of salted water and monomer, the only unknown component.

The picture below displays the scans for EACN’s determination of butyl acrylate (BA), with the phase transitions highlighted:

Calculating EACN of Butyl Acrylate

Determining EACN of butyl acrylate


These scans lead to EACNBA = -1.9±0.2

The picture below displays the scans for EACN’s determination of vinyl acetate (VA), with the phase transitions highlighted:

Determining EACN of Vinyl Acetate

Determining EACN of vinyl acetate


These scans lead to EACNVA = -1.7±0.3

In this case, the polymerization is based on 30:70 - BA:VA blend. The calculation for the EACN of the monomer blend is given below:

Component
 
wi (wt%)
 
xi (mol%)
 
EACN
 
Butyl Acrylate
 
30%
 
22%
 
-1.9
 
Vinyl Acetate
 
70%
 
78%
 
-1.7
 

Calculation for the EACN of the monomer blend 


Knowing that EACN = Σxi . EACNi, this leads to EACNblend = -1.7


Applying the HLD Equation for Surfactant Selection


The HLD value to emulsify the 30:70 BA:VA blend should be slightly lower than 0, because it:

  • Ensures a very stable type I (o/w) pre-emulsion (monomers and surfactants emulsified in an aqueous phase)
  • Offers a narrower particle size distribution, and
  • Lowers emulsification energy requirement

HLD = F(S) - k.EACN - α.∆T + Cc

Knowing that the polymerization reaction is performed at zero salinity, the HLD equation indicates that the surfactant should have a Cc close to -0.6 (for a 50:50 (vol) blend of monomers:water, at 25°C). Then the modelling tool4 gives an estimated Cc value close to -0.2 for a 65:35 - monomers:water pre-emulsion blend.

Lauryl Glucoside is selected from VLCI database as it exhibits the following advantages:

  • It has Cc ≈ -0.2 which is, within the required range of Cc values
  • It is Bio-based
  • Also, it is APG surfactant. That is, the parameter α=0, meaning that the temperature has no influence on the type of emulsion. This is a great advantage as the reaction takes place at 75°C while the pre-emulsion is being prepared at room temperature


Emulsion Polymerization & Performance of HLD Predicted Surfactant


The performance of lauryl glucoside (LG) is evaluated against sodium nonylphenol ether sulfate 10 EO (NPES). It is a known surfactant for emulsion polymerization. The experiment was performed at different concentrations, with oleyl cetyl ethoxylate 25 EO (OC25) used as a co-surfactant.

Pre-Emulsions Stability


This time-lapse video shows the stability test over 1 hour for the different samples.




Samples on Left-Hand Side:

  • NPES - 2%
  • NPES - 1%
  • NPES - 0.5%
  • NPES - 0.25%
Samples on Right-Hand Side:

  • LG - 2%
  • LG - 1%
  • LG - 0.5%
  • LG - 0.25%


Reaction Process


The reaction is performed by continuous addition of the pre-emulsion into the initial reactor charge, which contains the initiator (potassium persulfate). The reaction steps and temperature profile are displayed on the diagram below:

Emulsion Polymerization process
Emulsion Polymerization Process


The resulting polymer dispersions are evaluated according to the following criteria:

  • Presence of agglomerates or sediment in the polymer dispersion
  • Solid or gelled residues in the reactors
  • Dry film properties, such as:

    • Gloss
    • Hardness
    • Water resistance (150 microns wet film thickness)

These simple evaluations can validate the surfactant selection before committing to deeper analytical determinations, like:

  • Particle size distribution
  • Molecular weight, and
  • Grade of ramification

Results


None of the reactions produces significant residues in the reactors. Also, none of the produced dispersions contain agglomerates or sediments. The picture below displays the different emulsions together with their respective film properties.

Emulsions with their respective film forming properties

Different Emulsions with Their Respective Film Forming Properties


The replacement of NPES by lauryl glucoside in the emulsion polymerization of 30:70 - BA:VA leads to stable polymer dispersions. Also, the corresponding films give higher hardness and better water resistance, while reducing the need for a co-surfactant.


Conclusion


Monomers can be characterized as well as novel and complex surfactants. This allows green chemical specialties to be selected in a more predictive fashion. This R&D strategy saves time and money by:

  • Improving the stability, and
  • Reducing surfactant usage and energy input

Through this example, it was shown that the HLD approach can be practically and efficiently applied to surfactant selection for emulsion polymerization. Therefore, extension of existing databases with HLD parameters would support the work of formulators to select surfactants for emulsions. Also, polymer dispersions and should be seriously considered as an addition or an alternative to conventional development approaches.

Find Suitable Bio-based Surfactants for Your R&D




References

  1. https://www.stevenabbott.co.uk/practical-surfactants/emupol.php
  2. https://www.stevenabbott.co.uk/practical-surfactants/eacn.php
  3. https://www.stevenabbott.co.uk/practical-surfactants/cc.php
  4. https://www.stevenabbott.co.uk/practical-surfactants/fishtail.php


This article was first published in 2017 and is revised in 2020.

8 Comments on "Efficient Selection of Bio-Based Surfactants for Emulsion Polymerization"
Sander van L Aug 30, 2017
Dear Jose C, thanks for your question and nice note! Indeed, we perform the reactions at the same rpm and stirrer in our High Throughput system to obtain consistent results. The next step indeed is the full characterization of the products and film, and optimize the emulsions even further. In October 2017 we will start to continue in this direction, so I hope to publish something again the beginning of 2018! Or let me know if you have an interest to collaborate of course.
Jose C Aug 23, 2017
Did the emulsions and polymerizations were carried out at the same rpm (stirring speed) and using same stirrer paddle?. Note: it should be well received if your experiments should be correlated with the final product properties such as particle diameter , mass molecular...and so for. Congratulations
Titus S May 10, 2017
Dear Sander, thank you for your extensive explanations. When characteristic curvature loses its original meaning 'characteristic' needs a more detailed understanding in future. Otherwise it will be only successful fitting have a sufficient number of variables, unfortunately. Greetings Titus
Sander van L May 9, 2017
Thank you Saleem R for your nice comment! Hope you will use the HLD-NAC approach too!
Sander van L May 9, 2017
Indeed you are right about the APG, thank you for noticing it. Despite having only 1 glucose ring per molecule Lauryl Glucoside will be affected in the same way by the temperature as APGs. So a simplification was made by including it in the “APGs” group. Here the aim was to replace the ionic surfactant. A small amount of OC-25 was kept from the initial formulation, as monomer emulsification is not the only key process parameter to obtain a binder with good properties, such as substrate wetting. Nevertheless, a highly stable emulsion could be obtained using only Lauryl Glucoside. These are the answers, let me know if all is clear now. Thanks, Sander
Sander van L May 9, 2017
Dear Titus, Thanks for your comments, and sorry for my late reply on this! See here the answers to your questions, hope this helps. The salinity S represents the salinity of the salts presents in the water phase (ex. NaCl). For non-ionic surfactants, the effect of the salts presents in the water phase is described by the function: F(S) = b*S, the value of b depends on the type ions (ex. b = 0.13 for NaCl). From Prof. Steven Abbott’s website (https://www.stevenabbott.co.uk/practical-surfactants/cc.php):“But what is Cc? The name was originally Characteristic Curvature which seemed a good idea at the time. There are some thermodynamic reasons why the name might be thought appropriate. But the truth is that the name really has no helpful meaning. Let's just call it Cc (perhaps imagine it meaning Characteristic) because that's what is generally used in the literature. More important than the name are the values.” SDHS is an ionic surfactant, therefore it contributes to some extent to the salinity of the emulsion. To calculate the equivalent salinity brought by an ionic surfactant you can use the formula on Prof. Steven Abbott website: https://www.stevenabbott.co.uk/practical-surfactants/hld-expert.php The monomers used are technical grade and therefore not 100% pure. They contain some moieties (impurities, synthesis by-products, solvent residues…) that might affect the HLD equation. The EACN is independent of the concentration of the surfactant used but the effect of the moieties is dependent on this concentration and is mostly visible at low concentrations of surfactant. Furthermore, the polarity of certain oils (such as monomers) may affect the linearity of the HLD equation with respect to the surfactant concentration. In order to assess these effects, the EACN of the monomers was measured at 2 different surfactant concentrations. Rest in the next reply....
saleem r Apr 2, 2017
A beautiful and informative article regarding bio based coating technology and good effort to make environmentally paints and other products
Titus S Mar 29, 2017
Dear Sander, thank you for interesting article, however, some details are omitted necessary for full understanding What is nonionic salinity? For me salinity is concentration of salts. One would expect different effect of salinity for mono- and multivalent ions, even between sodium and lithium e.g. in case of SDS. 'Cc is known in a 50:50 (vol) blend' my understanding would be 'surfactant curvature' is determined by surfactant molecular architecture and would therefore not depend on mixing ratio oil/water??? EACN - BA and VA, what is SDHS, why do test at 2 concentrations, why is SDHS 4% related to Ssurf 0.18 g/ 100 ml? Lauryl glucoside should be one glucose ring per molecule - what makes it an APG (polyglucoside)? Term co-surfactant is generally used with meaning of added to improve surfactant action, e.g. hexanol to SDS. Then OC-25 is second compound in surfactant mixture 10/1. Why is it added, when lauryl glucoside is selected as ideal?

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