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The “Did you know….?” series is a bi-monthly note from EPCEd that is intended to present simple questions about topics that are important to those working in the emulsion polymers area. Short and concise answers to those questions are presented to educate readers and to elicit comments and further discussion. Some readers will already know the answers and be familiar with the topic while others, especially newer to the field, will benefit from the answers and discussion. We welcome feedback from readers of this series through our contact website www.epced.com.

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“Did you know…….?.” Five Part Series - January/February: Impact of Redox on Biocides

  Did you know……….there is valuable information available about the use of redox systems for effective reduction of residual monomers/VOCs used in emulsion polymerization? EPCEd is partnering with Brüggemann for this fifth in a series of Did You Know issues to highlight best practices for redox application in emulsion polymers, including reaction kinetic data and guidelines for best practices.

  Previous issues in this series highlighted:
    1) Fundamentals of redox systems typically used for emulsion polymer chase
    2) How redox dosage and rate of addition affect residual monomer removal
    3) Initial screening tests for redox and best practices for redox optimization
    4) Influence of redox balance on biocide retention

  Not as commonly used for production of emulsion polymers, redox use during the main polymerization can provide for significant reductions of energy consumption and cycle times. Notably, the reactor pre-heat step and the hold period after the monomer feed (semi-batch reaction) can be reduced or eliminated. Combined, these account for significant portions of the overall turn-around time for the reactor.
  Unsaturated monomers rely on a free radical to react with the carbon=carbon double bond in the initiation step leading to monomer propagation and polymerization. Typically, this free radical is generated by thermal initiation via cleaving a persulfate at elevated temperature. Radicals can also be generated at low temperature, ~20 °C, via redox combination of a reducer (e.g. Bruggolite® FF6) with an oxidizer (persulfate or peroxide) to provide radical initiation during the main polymerization.
  In the chart below, we show lab scale (3-liter) reactor temperature profile comparisons of main (seeded) latex polymerizations initiated thermally with ammonium persulfate (APS) & three variants of redox initiation with Bruggolite® FF6/APS redox combinations, the latter starting at both 20 °C and 40 °C, with polymerization starting at time 0. In all cases, the monomer feed time was 4 hours. Note the time required to heat up the batch to 80 °C in the Thermal Initiation Run before polymerization was started. The heating time for the thermally initiated reaction shows a reaction + cooling time that is significantly longer than the 20 and 40 °C reactions for this reactor setup.

  In the lab scale experiments, two of the low temperature reactions never exceed 65 °C despite being well insulated. In a production scale run, constant cooling would be expected.
  However, limitations of reactor cooling capability would likely require lower coolant temperature than is commonly available, thereby necessitating a chiller. In an alternative redox profile, as shown in the “20 °C Redox Initiation - Hybrid” curve, the reactor temperature was allowed to climb to ~80 °C. This would enable a sufficiently high temperature difference between the jacket and reactor to provide adequate heat removal. One can imagine a number of other temperature profiles to adjust to the particular characteristics of the available equipment.
  For all runs shown in the graph above, a post-reaction redox addition with Bruggolite® FF6/tBHP followed the main polymerization. Similar low residual monomer levels were obtained in each reaction. The following table shows comparisons of latex properties for the above experiments. The properties are generally quite similar, with the exception of molecular weight which increased significantly as the initiation temperature was decreased.

  Using redox initiation for the entire polymerization could enable significant cost reductions, including:
    - Energy savings.
    - Cycle time savings by eliminating all or most of the initial heating step.
    - Elimination of the lag/hold time between the main polymerization and the chase, further reducing cycle time.
    - Reactor is constantly in cooling mode (perhaps reducing dried polymer on the reactor walls).

  As always, we invite your questions and comments by going to [email protected] . Additionally, we invite you to go to Brüggemann’s Institute of Redox Chemistry (see link at end) – a collection of tutorials on redox chemistry for emulsion polymerization, providing greater detail of redox combinations and process optimization techniques. For more information, contact Brüggemann, [email protected]
  We invite your questions and comments via our website, epced.com

Previous issues of "Did You Know...?" for the current year, will accumulate here:
March-April 2023
May-June 2023
July-August 2023