I. Classification and Mechanism of Biocides

1. Biocide Classification in Papermaking

In the papermaking industry, a wide variety of biocides are used. Based on molecular structure, they can be clearly divided into two major categories: inorganic biocides and organic biocides.

(a) Inorganic Biocides
Inorganic biocides are further subdivided according to their mode of action into oxidizing and reducing types.

  • Reducing Biocides: These utilize their inherent reducing properties to exert antimicrobial effects; typical examples include sulfurous acid and its salts.
  • Oxidizing Biocides: These biocides employ powerful oxidation to kill microorganisms. Although their antimicrobial efficacy is extremely strong, their stability is relatively poor—they tend to decompose, which makes their action short-lived. Thus, oxidizing biocides are mainly used for sterilizing equipment, instruments, and water treatment. Common oxidizing biocides include hypochlorite, chlorine dioxide, and chloramines. Additionally, ozone is gaining increasing attention in the papermaking industry.

(b) Organic Biocides
Organic biocides have the widest range of applications in papermaking. Currently, the main organic biocides in use include compounds containing organic sulfur, organic bromine, and nitrogen-containing heterocyclic compounds. These biocides are noted for their high efficiency and low toxicity.

II. Mechanism of Action of Biocides

When biocides act, they first come into contact with and adsorb onto microbial cell membranes. They then penetrate the cell membrane and enter the cytoplasm, where they exert their antimicrobial effects at various target sites.

  • Some biocides cause protein denaturation within microorganisms, leading to loss of cell viability and ultimately cell death—a process defined as sterilization.
  • Others may induce genetic mutations or interfere with intracellular enzyme activity, inhibiting microbial reproduction and growth (a process known as bacteriostasis).

Oxidizing biocides have significant antimicrobial effects and can act on all types of microorganisms, but they are non-selective. This means that while they kill living microorganisms, they may also react with inert substances such as metal pipes, containers, fibers, and additives, which can impact production processes. Non-oxidizing biocides—primarily including isothiazolinone derivatives, organic bromine compounds, amines, and phenolic compounds—offer good selectivity. Currently, the high-efficiency, broad-spectrum biocides sold in the market typically belong to the non-oxidizing category. Their mechanism of action is mainly based on increasing cell membrane permeability, cutting off nutrient supply to the cell, disrupting cellular metabolism, or altering the structure of cell proteins to inhibit energy production and enzyme synthesis. As a result, these biocides are highly effective against planktonic microorganisms and biofilms, not only limiting and eliminating existing biofilms but also penetrating into biofilms to react with the anaerobic microorganisms contained within. It is important to note that certain non-oxidizing biocides may only be effective against specific types of microorganisms, which is a significant difference compared to oxidizing biocides.

Classification and Mechanism of Biocides

III. Selection Strategies for Papermaking Biocides

The selection of biocides for papermaking must consider multiple factors, among which production conditions, pulp properties, and pH are particularly critical.

  • Targeted Selection: Choose biocides that are effective against the specific microorganisms present in the process.
  • Combination Use: Since a single biocide may have a limited spectrum or lead to microbial resistance, alternating between two or more different types of biocides is advisable for long-term use.
  • pH Consideration: Generally, acidic biocides should be selected for alkaliphilic bacteria, while alkaline biocides should be chosen for acidophilic bacteria.
  • Food Safety Considerations: When producing food packaging paper or board, the toxicity and maximum allowable dosage of biocides must be carefully evaluated to ensure food safety.

IV. Determining the Optimal Timing and Location for Biocide Addition

Typically, the higher the concentration of biocide relative to the total pulp or coating solids, the more pronounced the antimicrobial and bacteriostatic effects. To reduce the overall usage of biocides, they should be added at stages where the pulp concentration or coating solid content is high.

  • For Pulp: Biocides can be added at the headbox, high-consistency stock, mixing tank, or white water system. If the primary purpose is bacteriostasis, biocides may also be added to recycled pulp.
  • For Coatings: They can be directly added to the coating formulation during the final stages of preparation.

Selection Strategies for Papermaking Biocides

V. Methods of Biocide Addition

The main methods of adding biocides include:

  • Single-Dose (One-Time) Addition: Commonly used in coating preservation, this method achieves and maintains the highest concentration of biocide, which is beneficial for effective antimicrobial and bacteriostatic effects.
  • Intermittent Addition: Often used in pulp preservation, this method effectively controls microbial proliferation by maintaining microbial counts within a safe range.
  • Continuous Addition: Although this method provides steady dosing, it is less commonly used in practice due to higher costs. In continuous production processes, timed and quantitative intermittent addition is typically employed, given the time-dependent efficacy of biocides.

VI. Key Considerations During Use

  1. Concentration Control
    There is a minimum effective concentration for biocides. If the concentration is below this threshold, the expected antimicrobial or bacteriostatic effect will not be achieved; if it is too high, production costs will increase significantly. In practice, biocide concentration should be maintained slightly above the optimal effective concentration to ensure effective control of microbial growth and reproduction, preventing deterioration of pulp and coatings.
  2. Effect of pH
    Biocides mainly act in their molecular state rather than their ionic state, and pH is a key factor affecting the state of a substance. For example, benzoic acid is effective as a preservative only when it exists in its molecular form in environments with a pH below 4, while phenolic compounds can remain in their molecular form over a wider pH range. As papermaking processes shift from acidic to neutral-alkaline, the pH of the system increases, which requires biocides to maintain stable performance over a broader pH range.
  3. Duration of Action
    A longer duration of action can lead to complete sterilization, whereas a shorter duration may only provide bacteriostatic effects. Generally, biocides should be added in the early stages of production to ensure they have sufficient time to act.
  4. Solubility
    The lower the solubility of a biocide in water, the stronger its activity tends to be. This is because the hydrophilicity of microbial surfaces is typically lower than that of the pulp system, so a lower solubility helps form a higher local concentration of the biocide on the microbial surface, enhancing its antimicrobial effect.

Methods of Biocide Addition

VII. Future Perspectives and Advanced Technologies

Recent advancements in nanotechnology and green chemistry are driving the evolution of papermaking biocides toward:

  • Nano-Biocides: Utilizing nanoparticles to improve dispersion, stability, and penetration into biofilms.
  • Green and Eco-Friendly Biocides: Employing bio-based raw materials and green synthesis methods to reduce environmental impact and toxicity (supported by studies in Chemical Reviews).
  • Smart-Responsive Biocides: Integrating sensor technology for real-time monitoring of microbial levels and dynamic control of biocide release to improve efficiency and cost-effectiveness.

VIII. References

  1. Heinze, T. (2018). Starch Chemistry and Technology. Springer.
  2. TAPPI T569 om-15: Internal bond strength of paperboard.
  3. Zhang, Y. et al. (2021). “Cationic starch-fiber interactions: A combined QCM-D and AFM study”. Carbohydrate Polymers, 256, 117582.
  4. ISO 14855: Determination of the ultimate aerobic biodegradability under controlled composting conditions.