According to reports, the microorganisms capable of degrading paint primarily include bacteria, molds, yeasts, and algae. The primary culprits behind paint spoilage are Gram-negative bacteria-specifically members of the genera *Enterobacter* and *Pseudomonas*-rather than molds, as is commonly believed. Conversely, the microorganisms that proliferate on the paint film *after* application are predominantly molds and algae. Therefore, the most critical step in preventing paint spoilage is controlling Gram-negative bacteria-particularly *Enterobacter* and *Pseudomonas* species; this implies that the biocides and preservatives added to the paint should be optimally effective against these specific Gram-negative bacteria. If the objective is to prevent mold growth on the dried paint film as well, a "dry-film" mildewcide should be added in conjunction with the preservative; using these two agents in combination ensures continuous antimicrobial protection throughout the entire lifecycle of the paint. In the case of marine bottom paints, an algicide must also be incorporated to inhibit algal growth.
The selection of biocides, preservatives, and mildewcides for paint applications requires careful consideration of the following criteria:
(1) Broad-spectrum antimicrobial activity, high efficacy, long-lasting potency, and stability within a pH range of 6 to 10.
(2) Safety: non-toxic or low-toxicity to humans, coupled with good biodegradability.
(3) Chemical inertness: it must not react with other paint components and, once the paint film has formed, must not compromise the physical or chemical properties of the film.
(4) Low volatility, excellent compatibility with the paint formulation, and ease of dispersion.
(5) Good overall stability, including resistance to UV radiation, heat, and oxidation.
(6) Ease of application and reasonable cost-effectiveness.
Historically, compounds such as organomercurials, organotins, organoarsenicals, phenolic derivatives (including halogenated phenols), and formaldehyde were commonly employed. The former categories-organomercurials, organotins, and organoarsenicals-have largely been phased out due to their high toxicity and environmental impact; their use is now restricted to limited applications, such as in marine antifouling bottom paints. Formaldehyde, meanwhile, faces usage limitations due to its high volatility, high chemical reactivity, and inherent toxicity. Since the 1970s, the primary biocides and preservatives utilized in the industry have included sodium pentachlorophenate, carbendazim, chlorothalonil, thiram, vinclozolin, and dithiocyanomethane. Sodium pentachlorophenate, due to its high toxicity, has been banned from use. Several subsequent alternatives exhibit poor water solubility and suboptimal antimicrobial and preservative efficacy; they retain a limited market presence solely due to their low cost. Currently, both domestic and international trends favor the use of antimicrobial preservatives that are highly efficient, broad-spectrum, low-toxicity, and water-soluble. This class of biocides demonstrates high antimicrobial potency (typically, the Minimum Inhibitory Concentration-MIC-of the active ingredient against microorganisms falls within the range of a few tens of parts per million) and a broad spectrum of activity (effective against bacteria, molds, yeasts, and algae). Furthermore, they are safe and reliable (generally possessing an oral LD50 in rats exceeding 1000 mg/kg). Isothiazolinone compounds represent a newly emerging and excellent class of antimicrobial agents, characterized by their safety, broad-spectrum activity, high efficiency, and low toxicity.
