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 fourth installment in the Did You Know series to highlight best practices for redox application in emulsion polymers, including reaction kinetic data and guidelines for best practices. This issue focuses on the impact of redox on biocides.

Previous issues in this series highlighted the fundamentals of redox systems typically used for emulsion polymer chase, discussing dosage, rate of addition, rate of reduction of residual monomers, initial screening tests for selecting the best reducing agent or oxidizer for the polymer recipe, and redox optimization for the chase. Less often considered is the effect a chosen reducer or oxidizer can have on a given biocide. With increased regulatory restrictions and the general demand for products with minimal biocide content, any degradation from reducers or oxidizers can be costly.

First, a study on the impact of various reducers on chloromethylisothiazolinone (CMIT) was performed using a model system with a 1:1 weight ratio with tertiary-butyl hydroperoxide. The redox pairs were fed into an aqueous solution with a pH of 4 at 60 °C under agitation over a period of 1 hour to a final concentration of 0.1%. The reaction was cooled to 30 °C, pH adjusted to 8.5 with NaOH, and biocide charged and mixed for 30 minutes.

Given that each redox pair was used at a 1:1 weight ratio, the vastly different molecular weights of each reducer hadn’t been considered. However, as shown in Table 1, the molar redox ratio, calculated from the molecular weights of each redox pair, was an impactful variable on CMIT stability. The trend in Table 1 shows that the more oxidative the environment, the greater the degradation of biocide, highlighting the importance of understanding the stoichiometric levels of redox in your formulation. Data also suggest CMIT is best preserved in acidic environments, and the pH shift noted in the isoascorbic acid sample likely had a positive impact as well.

One might first investigate the redox package used in post-polymerization (chase) as a first step. However, the source of radical generation for initiation and main polymerization should also be considered. Often, thermal initiation with persulfate is used, but when relying strictly on temperature for the cleavage and degradation of such oxidizers, time must be considered due to the reliance on half-life. If this method is used for initiation, it should be understood that residual, unreacted oxidizer may be present when entering the chase step. While the redox package used for chase may be stoichiometrically balanced, the residual oxidizer from main polymerization causes an imbalance in overall redox. This is not a concern for balanced reactions that use chemically generated radicals via redox for initiation and main polymerization.

A separate, but similar study looked at the effects on benzisothiazolinone (BIT) and isothiazolinone blends (CIT/MIT). In this second study, a styrene-acrylic latex with a pH of about 4 was used to test different redox pairs and dosages. All of the oxidizer was charged at once, and the reducer was fed over 30 minutes at 60 °C. Biocide was charged after cooling to 35 °C, and the latex was kept at room temperature for 14 days. Biocidal active ingredient content was tested via HPLC after this period. Table 2 shows the results of testing various reducers at different weight ratios with tBHP, as well as tBHP alone. The final column shows the experimental value of biocide after the 14-day period, while the initial value was theoretical based on the biocide dosage.

A redox ratio of 2:1 between tBHP (oxidizing agent) and Bruggolite® FF6M (reducing agent) resulted in strong degradation of BIT from 200 ppm to 82 ppm (Trial 3), whereas an adjusted ratio of 1:1 (Ox) allowed for the most complete preservation of BIT from Sanitized® BIT20D (Trial 5), similar to the findings with CMIT. The use of either the reducing agent (Trial 7) or the oxidizing agent (Trial 8) alone resulted in significant degradation of BIT. When the reducing agent was changed from Bruggolite FF6M to TP 1646, a consistently high active ingredient content was analyzed for both redox ratios (Trials 11 and 13).

In contrast to the findings with BIT, a 2:1 ratio between tBHP and Bruggolite FF6M did not lead to a significant decrease in active ingredient content when Sanitized CI15 (1.5% CIT/MIT) was used (Trial 4), but degradation occurred with a 1:1 ratio (Trial 6). The sole use of a reducing agent also led to a strong decrease in CIT/MIT content (Trial 9), but this was not the case with the sole use of the oxidizing agent (Trial 10). When the reducing agent was changed from Bruggolite FF6M to TP 1646, a consistently high active ingredient content was again analyzed for both redox ratios (Trials 12 and 14).

In summary, the redox components had less influence on CIT/MIT stability than on BIT stability. Combined with the study on CMIT, we see there is no single solution for all systems. It is essential to use a suitable redox ratio and package for the selected biocide. Furthermore, as noted previously, residual oxidizer from a thermally initiated system that is not accounted for can have potentially devastating results on biocide stability.

As always, we invite your questions and comments at info@epced.com. Additionally, we encourage you to visit Brüggemann’s Institute of Redox Chemistry (see the link at the end) — a collection of tutorials on redox chemistry for emulsion polymerization, providing greater detail on redox combinations and process optimization techniques. For more information, contact Brüggemann.

The “Did You Know….?” series is a bi-monthly note from Emulsion Polymers Consulting and Education (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 those newer to the field, will benefit from the answers and discussion. Experienced practitioners may also find new insights in the discussion. Paint & Coatings Industry magazine has partnered with EPCEd to share the “Did You Know” notes with our readers throughout the year.