Antimicrobial preservatives are diverse in nature; based on their molecular structures, they can be broadly classified into inorganic and organic categories.
Inorganic Antimicrobial Preservatives
Based on their mechanisms of action, inorganic antimicrobial preservatives can be further divided into oxidative and reductive types. Reductive antimicrobial agents exert their bactericidal and bleaching effects through their reducing capabilities-examples include sulfurous acid and its salts, which are primarily utilized as bleaching agents. Oxidative antimicrobial agents, conversely, achieve their bactericidal effects through their oxidizing capabilities. While these agents are highly potent, they tend to be chemically unstable, prone to decomposition, and possess a limited duration of action; furthermore, they often emit a distinct, unpleasant odor. Consequently, they are predominantly employed for the disinfection and sterilization of equipment, containers, semi-finished products, and process water; this category primarily encompasses chlorine-based preparations and peroxides.
Hypochlorites
Hypochlorites are traditional bleaching agents wherein the "active chlorine" component exhibits potent bactericidal activity. Chlorine penetrates the interior of microbial cells, disrupting cellular enzyme proteins or inhibiting enzymes that are sensitive to oxidation, thereby leading to microbial death. Hypochlorites are effective in eradicating a wide spectrum of microorganisms, including vegetative bacterial cells, bacterial spores, viruses, yeasts, and molds. Their bactericidal efficacy is significantly enhanced under conditions involving elevated temperatures, high concentrations, prolonged contact times, and low pH levels.
Chloramines
Chloramines are effective antimicrobial agents characterized by their mild oxidizing properties. In paper manufacturing facilities, chloramines are typically generated by directly mixing ammonium sulfate with a bleaching liquor (containing 18–20 g/L of active chlorine). The specific products of this reaction vary depending on the pH of the solution: at a pH of 5, the reaction yields NHCl₂; at a pH of 7 or higher, the reaction yields NH₂Cl. When added to paper pulp, the recommended dosage of chloramine ranges from 0.03% to 0.05% of the pulp's weight; when applied to "white water" (recycled process water), the recommended concentration is 3–5 mg/L. Both the preparation and application of chloramines should be conducted under alkaline or weakly alkaline conditions; conversely, excessively high pH levels will compromise their bactericidal efficacy. The primary advantages of using chloramines include their low cost and high effectiveness; however, their disadvantages include a strong, pungent odor that negatively impacts the operational environment and poses potential risks to human health. To mitigate the development of bacterial resistance, alternating the use of chloramines with copper sulfate-aligned with the facility's cleaning schedule-has proven to be a highly effective strategy.
Organic Biocides and Preservatives
Organic biocides and preservatives offer numerous advantages, including high efficiency, low toxicity, and excellent biodegradability; consequently, they are widely utilized in the papermaking industry. Currently, the primary agents employed in this sector include organosulfur compounds, organobromine compounds, and nitrogen-sulfur-containing heterocyclic compounds.
Domestically, numerous companies are dedicated to the research and sales of these biocides, with concentrations of activity found in East China (e.g., Nanjing Gutian) and South China (e.g., Guangzhou Guyi).
Organosulfur Compounds
A representative product in this category is Methylene Bisthiocyanate (abbreviated as MBT). MBT possesses a broad spectrum of biocidal activity, demonstrating significant killing effects against bacteria, fungi, and algae; it is therefore suitable for use in the preservation of both paper pulp and paper coatings. In practical application, MBT is typically formulated into a 10% (by weight) solution by blending it with solvents and synergistic agents. When added at a dosage of 7.5 mg/L, it can achieve a sterilization rate exceeding 99.8% within 30 minutes. This agent is suitable for use in systems with a pH value of less than 11. Another commonly used organosulfur biocide is Benzothiazol-3-one. This preservative exhibits very low toxicity to humans and other animals; an addition of just 45–170 grams per ton of paper is sufficient to effectively prevent the spoilage of paper pulp. When used for the preservation of paper coatings, the recommended dosage ranges from 0.015% to 0.03% of the total coating weight.
Organobromine Compounds
2,2-Dibromo-2-cyanoacetamide is a quintessential example of an organobromine biocide. It exerts both killing and inhibitory effects against bacteria and molds, making it suitable for the preservation of both paper coatings and pulp. This compound is highly susceptible to decomposition, with its degradation rate accelerating as both pH and temperature increase. For instance, while it remains stable for 155 hours at 25°C and a pH of 6.0, it decomposes completely within just 6 minutes at a pH of 9.7. Consequently, it is best suited for application in mildly acidic to neutral systems.
Heterocyclic Compounds
An example of this class is 1,3,5-Tris(hydroxyethyl)hexahydro-s-triazine. This compound demonstrates potent killing and inhibitory effects against various microorganisms, including *Aerobacter*, *Pseudomonas aeruginosa*, *Bacillus*, and *Escherichia coli*. It is primarily utilized for the preservation of paper coatings, though it can also be applied to paper pulp preservation; notably, it is effective across a wide pH range and maintains its stability even within strongly alkaline systems. 1,2-Benzisothiazolin-3-one (abbreviated as BTT) is effective against a wide range of microorganisms, including bacteria, molds, yeasts, and sulfate-reducing bacteria; it demonstrates particularly significant bactericidal efficacy against Gram-negative bacilli. It is suitable for use as a preservative in paints and slurries; being stable under both acidic and alkaline conditions, it can be utilized across a broad pH range. A recently emerged class of isothiazolinone-based biocides (also known as Kathon)-primarily composed of 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI)-exhibits excellent antimicrobial activity against various bacteria, molds, yeasts, and algae. The high efficiency, broad-spectrum activity, and eco-friendly nature of this class of bactericidal preservatives are widely recognized globally; they are applicable for the microbial control and preservation of pulp, white water, and paints, within a suitable pH range of 4 to 8.
