Bronopol was invented by The Boots Company PLC, Nottingham, England in the early 1960s and first applications were as a preservative for pharmaceuticals. Bronopol's low mammalian toxicity (at in-use levels) and exceptional activity against bacteria (especially the troublesome Gram-negative species) ensured that it became popular as a preservative in many consumer products such as shampoos and cosmetics.

Bronopol was subsequently taken up as an effective antimicrobial in many industrial environments such as paper manufacturing, oil exploration and production and cooling water disinfection.

World production soared from the tens of tonnes in the late 1970s to current estimates in excess of 5,000 tonnes. This is quite something considering the effective use-concentration which can be as low as 0.0025% (25 parts per million). Manufacturing today is the business of low cost producers, mainly in China.

Pharmaceutical products containing Bronopol are registered in a number of countries and Bronopol is the subject of a specific monograph in the British Pharmacopoeia 1998.

Applications

Today, Bronopol is used in consumer products as an effective preservative agent, as well as a wide variety of industrial applications (almost any industrial water system is a potential environment for bacterial growth, leading to slime and corrosion problems - in many of these systems Bronopol can be a highly effective treatment).

The use of Bronopol in personal care products (cosmetics, toiletries) has declined since the late 1980s due to the recognised potential for nitrosamine formation.

The Nitrosamine Problem

Although fairly ubiquitous in our diet and the environment, and even produced within the stomach from various foodstuffs, many nitrosamines are known or suspect carcinogens and therefore should be avoided in manufactured goods.

Nitrosamines are relatively easily produced from secondary amines and amides in the presence of nitrite ions (this is why they are formed in-vivo from foodstuffs).

While Bronopol is not in itself a nitrosating agent, under conditions where it decomposes (alkaline solution and/or elevated temperatures) it can liberate nitrite and low levels of formaldehyde and these decomposition products can react with any contaminant secondary amines or amides in a personal care formulation to produce significant levels of nitrosamines (due to the toxicity of thes substances, the term 'significant' means levels as low as 10s of parts per billion).

Manufacturers of personal care products are therefore instructed by regulatory authorities to 'avoid the formation of nitrosamines' which might mean removing amines or amides from the formulation, removing Bronopol from a formulation, or using nitrosamine inhibitors.

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Kaolin preserves: A recipe for success
From Pulp & Paper, 6/1/05 by Wallace, Shari

WHEN I WAS A LITTLE GIRL, my grandmother cooked "up a storm" as we say in south Georgia. She made preserves each year from all kinds of fresh fruit: pear, blackberry, plum, strawberry, and even fig, and there was always enough for family and friends to enjoy. Everybody loved to eat her homemade jellies - it was a family tradition.

At Thiele Kaolin Company, we also believe in tradition and that includes producing kaolin "preserves", that is, achieving long-term preservation of our kaolin slurry products. This has come through conducting numerous lab studies, listening to customer needs and continually striving for improvements, as well as a process of trial and error at times.

Preservation from bacteria contamination is a costly process in nearly every industry and kaolin is no exception. Spoilage occurs when aerobic bacteria deplete the oxygen content and the slurry becomes anaerobic. Then, facultative anaerobes like sulfate-reducing bacteria form hydrogen sulfide, which imparts a foul smell to the clay and eventually form iron sulfide that turns the clay gray to black. Early control of aerobic bacteria will avert this.

FDA certification needed

Because the majority of Thiele's products are used by the paper industry, any biocide we use must have special FDA certification for food contact applications. That narrows the field of choices considerably, to only about five biocides, including Glutaraldehyde, Thione, Benzisothiazolin, Kathon and Bronopol, each of which has certain chemical incompatibilities that limit their application. Significant thought goes into determining the best biocide: is it chemically compatible with our system and our customer's, is it effective against a broad spectrum of microorganisms, how long does it reside in the product, what are the chemical breakdown products?

One might be tempted just simply to overdose the slurry with biocide. However, the truth is that overdosing leads to several serious issues. One, it increases the cost since the biocides are among the most expensive chemicals added to our product. second, we want to provide an environmentally friendly product in terms of total biocide present. Most of the FDA-approved biocides available for use have upper use limits specified. This leads to a third reason for not overdosing: it could limit our customers from cleaning their system and tanks with adequate levels of biocide. Finally and perhaps most importantly, we do not want to develop chemical resistance, so that later when higher levels are really needed, suddenly they do not work.

A simple approach

In light of this, Thiele has developed a simple, but effective, approach to slurry preservation that, while it requires some time and effort to complete, provides a preserved kaolin product. This is done through ongoing research, conducting 30-day lab studies to evaluate biocide levels and their residual effects on contamination and also on slurry properties.

There are a number of advantages to this approach. It allows us to pinpoint the most effective dosage level so we can insure that we are not over or under-dosing. This along with 14-day microbial challenge testing, allows us to determine the efficacy of residual biocide on further contamination. This further allows us to determine the lowest level of biocide that will result in preservation failure. The test period mimics the longest transit time on average to customer destinations all over the country. Customized sample sizes allow testing on a more representative bacterial population that leads to enhanced plant scale-up. The small sample size typically used in biocide supplier test work can lead to false positive results. Our test process allows us to analyze not only our final product, but further, upstream to help reduce biocide demand. This can be accomplished through periodic plant profiling in which samples are tested at specific points throughout the process for microbial activity. This approach allows us to perform microbial identifications, evaluate effects of seasonal weather patterns on bacteria growth, and determine the presence of resistant bacterial species. Finally, the test approach also allows us to determine chemical effects on other slurry properties such as viscosity, pH, and optical characteristics.

Slurry preservation is integral to our success. In some ways it is just like my grandmother's cooking, with just the right ingredients at the right amounts, you can achieve the desired final product.

SHARI WALLACE is a research microbiologist working with the materials research group, Thiele Kaolin Company.

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