Coatings World - March 2017

Scott A. Brown, Technical Manager, Global TS MPP, Lonza Inc., Alpharetta, GA

Introduction

Increasing regulatory requirements have put pressure on biocide manufacturers and have slowed the pace of new preservative introduction. The emerging challenge for formulators of water-based coatings is that fewer preservatives are available, and many of those that remain contain biocidal active substances which attract hazard communication phrases if present above a threshold trigger level.

This is particularly true for European
coating formulators. Against this back-
drop we have formulation trends in the
coatings industry, such as the removal
of co-solvents, which have resulted in
coatings with an increased susceptibil-
ity to in-can spoilage. These formulation
changes have created an upward trend on
the preservative concentrations required
for robust preservation. The paint in-
dustry is facing an intensifying clash
between the formulation-driven trend to-
ward higher preservative concentrations,
and the regulatory driven trend toward
lower preservative concentration trig-
ger thresholds for hazard communica-
tion. Given the headwinds impacting the
historically popular in-can preservative
active agents, there is likely to be a re-
evaluation of antimicrobial active agents
from other use patterns. For example,
the pyrithione active agents, which were
traditionally used as fungicides, are find-
ing increased use as co-biocides for in-
can preservation. These and other blends
will be welcome new preservation tools
for the coating formulator’s toolbox as
the industry faces an increasingly com-
plex regulatory environment.

Preservation – Past

Prior to the 1970s, organo-mercurial compounds were commonly used as biocides for industrial products such as paints. While they were very effective in controlling micro-organisms, they were hazardous to humans and persistent in the environment. Consequently, legislation was introduced that resulted in their phase-out and eventual ban. Organo-mercurials and other heavy metal-based biocides were removed from decorative paints in Europe in the mid-1970s, and in the United States heavy metal-based preservatives followed a similar trajectory. Organo-mercurial compounds were once extensively used in the United States, both as in-can preservatives and as film fungicides for water-based paints; however, mercury-containing biocides eventually came under regulatory pressure and the U.S. EPA cancelled all registrations for mercury-based compounds in interior line paints in 1990.

Due to the extreme safety and regulatory pressure placed on mercury-based preservatives, many synthetic organic antimicrobials were researched and developed. Among these were several different types of formaldehyde condensates, which rely on the release of free formaldehyde from the hydrolysis of a parent nitrogen-based structure. Many of these formaldehyde-condensate compounds found wide use in material preservation.

While employing a common anti-micro-bial mechanism, these compounds represented a diverse range of both cyclic and acyclic structures. Differences in structure were associated with differences in preservation properties, with variation in features such as the rate of formaldehyde release and the sensitivity to hydrolysis at alkaline pH.

During the heyday of these compounds, a coatings formulator could select from a wide range of formaldehyde mechanism preservatives the one that gave the best fit with the needs of his or her particular coatings application. However, over time, the number of available formaldehyde-mechanism active agents has declined. There are several drivers behind this trend. First, and most obvious, are the direct regulatory headwinds faced by compounds of this type. It has long been acknowledged that exposure to formaldehyde gas is detrimental to human health. Formaldehyde gas has both acute and chronic hazards, and in 2004 formaldehyde was formally classified by the International Agency for Research on Cancer (IARC) as a human carcinogen. Given that formaldehyde-condensate compounds are very different in structure and properties from formaldehyde gas, the assessment of the risk to humans posed by the potential release of formaldehyde by these compounds is not straightforward. Nevertheless, different regulatory agencies around the world have gone in the direction of including a carcinogenicity risk assessment as part of their overall risk assessment process when they