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Redox Chemistry for Effective Reduction of Free Monomer in Emulsion Polymerization

SpecialChem – Jun 24, 2021

TAGS:   Sustainability / Natural Coatings       

Redox-initiated Emulsion PolymerizationReducing residual monomer contents has become a must for every emulsion polymer manufacturer in order to meet standard quality criteria. Products with no or very low levels of residual monomers simply have a positive commercial appeal as they guarantee:

  • Low VOC
  • No odor, and
  • Enhanced mechanical properties

There are several techniques for reducing residual monomer contents. The most elegant technique is a post-reaction by redox-initiated emulsion polymerization, which is a highly versatile method of polymer production used for the synthesis of more than one-third of the polymers produced in total.


Benefits of Redox Chemistry Over Physical Stripping


Redox polymerization allows manufacturing polymer formulations based on hydrophobic monomers with low free monomer content in the final product. Usually, this is a challenge with traditional process technology especially in terms of monomer conversion which makes it difficult to meet environmental and safety regulations. Physical stripping of the residual monomer from the product is an energy-intensive step requiring installation and use of additional equipment.

The need to lower VOC content is driven by high customer demand for safer products with low odor. Employing a redox pair instead of thermal initiation and/or physical stripping leads to energy savings of the emulsion polymerization process followed by a significant reduction of residual monomer contents and much shorter cycle times.

Redox chemistry is used during the post-polymerization phase to minimize the monomer content. The redox pair is introduced into the system towards the end of monomer conversion which generates a flood of radicals, accelerating the conversion of the remaining monomer molecules into dimers, oligomers and short polymer chains. The post-polymerization step essentially cleans up the product reducing the unreacted monomer content leading to lower VOC and odor.

The use of a suitable redox pair during post polymerization can help to limit the residual monomer content below a critical level. Post polymerization redox chemistry helps increase production capacity by reducing process time and can also help in lowering production costs.

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Redox Optimization for Emulsion Polymer Post Reaction



Power of BRUGGOLITE® FF6 M vs Other Reducing Agents


Sodium Formaldehyde Sulfoxylate (SFS) or Ascorbic Acid (AA) have long been used as reducing agents, along with t-Butyl Hydroperoxide (TBHP) as an oxidizer in emulsion polymerization redox systems. BRUGGOLITE® FF6 M (FF6), made by Brüggemann, has become a standard formaldehyde-free, sulfur-based reducing agent that is more powerful than both SFS and AA. Studies have shown that FF6 can help in:

  • Substantially lowering free monomer content in emulsion polymers.
  • Reducing reaction time and preventing polymer yellowing during the polymerization process and beyond.

The figure below compares the reducing power of FF6 to that of SFS and AA. It shows that FF6 works almost twice as fast as SFS in reducing the dissolved oxygen in water. AA acts much slower than either of the two reducing agents.

Reduction of oxygen in water at 20°C and pH=5, 1 wt% reducing agent
Reduction of oxygen in water at 20°C and pH=5, 1 wt% reducing agent


Advantages of Using BRUGGOLITE® FF6 M


FF6 provides faster reactor throughput and virtually complete conversion of the monomer into a higher quality polymer. During the main polymerization step, an FF6-redox system allows greater control over the reaction. It enables slow, controlled initiation of the polymerization reaction with no consequential retardation later in the cycle. Free radical generation can also be initiated at a lower temperature to avoid side reactions and cross-linking. The redox system flexibility allows tighter control of the molecular weight distribution resulting in improved physical properties.

Comparing Conversion Rate of Reducing Agents During Vinyl Acetate Emulsion Polymerization


The figure below compares the conversion rate during the polymerization of vinyl acetate to PVAC with FF6, SFS and AA as the reducing agents. After 20 minutes, the system with FF6 is approaching more or less complete conversion whereas the processes with SFS and AA both show less than 15% conversion. The use of FF6 leads definitely to a higher conversion percentage and a shorter conversion time.

Emulsion polymerization of Vinyl acetate at 20°C and pH=5
Emulsion polymerization of Vinyl acetate at 20°C and pH=5


Comparing Residual Monomer Levels in Emulsion Polymerization of Vinyl Acetate & VeoVa™


When used in the post-polymerization step, FF6 helps to reduce the monomer content below the critical level without the need for physical stripping of the residual monomer. This applies to both homophilic and homophobic monomers. The figure below shows the change on the residual monomer level over time of an FF6 redox pair in a vinyl acetate VeoVa™ copolymer. H2O2 was substituted for TBHP to ensure a VOC-free process. The figure also shows that the residual monomer level rapidly declines below the 1000ppm level for both vinyl acetate and VeoVa™.

Emulsion polymerization of Vinyl acetate and VeoVa™
Emulsion polymerization of Vinyl acetate and VeoVa™ using H2O2 as oxidizer

The figure below describes the different residual monomer levels of vinyl acetate in a post-polymerization run of 60 minutes in total. The curves show that the addition of FF6 and H2O2 during polymerization leads to a rapid decline of the residual monomer levels, which finally underrun the threshold of 1000 ppm.

Reduction of residual levels of vinyl acetate
Reduction of residual levels of vinyl acetate, redox pair: FF6 and H202


High Reactivity Whilst Reducing Polymer Yellowing & Oxidation


The higher reactivity of FF6 allows adding fewer amounts of reducing agents needed for efficient emulsion post-polymerization compared to conventional reducing agents, such as SFS.

The higher reactivity of FF6 can also yield a higher concentration of free radicals as with other reducing agents formed under the same conditions. This situation can lead to termination of radicals; so that the effective number of radicals is reduced and the performance of the system is decreased. To overcome this situation the concentration of free radicals has to be lowered. This can be done by lower dosage rates of the redox initiator system, which can be seen in the figure below.

The higher reactivity of FF6 allows adding less amount of redox pair
The higher reactivity of FF6 allows adding less amount of redox pair

In addition, it is important to add the redox initiator or at least one component (reducing agent) not as a one-shot, but at a lower rate, continuously over time. Moreover, FF6 also reduces polymer yellowing and oxidation.

Due to its reactivity FF6 provides acceptable reaction rates without the need for adding an iron salt (promoter). Omitting Iron in the formulation further contributes to polymer color retention both during polymerization and with aging. The figure below shows that polymers produced with FF6 do not show a yellowing tendency, as compared to polymers produced with AA, even at elevated temperatures.

FF6 reduces polymer yellowing during polymerization and aging
FF6 reduces polymer yellowing during polymerization and aging


Important Variables Impacting Residual Monomer Content, Conversion and Process Time


The dosage level, the feed rate, and the ratio between reducing agent and oxidizing agent are important variables that impact the residual monomer content. In addition, the pH and the temperature also impact monomer conversion and the process time. The table below summarizes the recommended process parameters for the application of FF6:

 Parameter Recommended Value
 pH  4 - 6.5
 Temperature  60 – 65°C
 Ratio (Reducing Agent: Oxidizing Agent; (wt%)  1 : 1
 Addition  Continuous
 Dosage (referring to formulation)  0.1 – 0.2 wt%


Conclusion


Brüggemann´s reducing agents have high reactivity leading to improved performance and shorter reaction time even at low dosage rates.

  • The use of BRUGGOLITE® FF6 M as a reducing agent during post-polymerization allows significant reduction of the residual monomer content to meet low VOC requirements.
  • BRUGGOLITE® FF6 M is formaldehyde and VOC free. It has non-yellowing properties which help retain the original coloration of the emulsion polymer during production and aging.
  • BRUGGOLITE® FF6 M has all relevant FDA food contact notifications and can be used for production of emulsion polymers which go into food contact applications.


Other Derivatives for Redox-initiated Emulsion Polymerization


View all the commercially available additives supplied by Brüggemann, analyze technical data of each product, get technical assistance or request samples.



References

  1. https://en.wikipedia.org/wiki/Radical_polymerization
  2. https://www.brueggemann.com/en/reducing-agents
  3. https://www.coatingsworld.com/issues/2020-04-01/view_technical-papers/redox-optimization-for-emulsion-polymer-chase-new-approach-to-reduce-vocs-cycle-time-and-cost

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