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I. Preface

In the actual production and field application of water-based industrial coatings, lotion, adhesives, and building materials pastes, the quality problems caused by bacteriostatic and anti mold additives are often not caused by the insufficient purity of the product itself, but by system compatibility conflicts, feeding process errors, acid and alkali environment mismatches, excessive addition, and additive system antagonism. The majority of factories in the industry encounter four types of problems with antibacterial agents: discoloration and yellowing of the paint film, ineffective antibacterial and mildew prevention, white spots from additives, and shrinkage and pinholes in the paint film.

This type of problem is highly covert: many faults do not manifest at the moment of discharge, but only occur after 7 days of hot storage, 1 month of storage, film formation and drying, and half a month of outdoor service, which can easily lead to bulk returns, construction site rework, and project customer complaints. This article provides a comprehensive disassembly of micro mechanisms, fault tracing, erroneous operations, hierarchical troubleshooting, and root cause solutions to form a factory fault troubleshooting manual that can be directly implemented.

2、 Disadvantage 1: Antibacterial agents cause discoloration, yellowing, and localized color differences in the paint film

1. Accurate description of the fault phenomenon

The finished coating has a normal discharge and pure color, but gradually turns yellow after heat storage; After the construction film is formed, the overall paint film may turn yellow, with some areas showing dark spots, turning brown, or turning gray; Light colored paint, pure white paint, and transparent varnish are particularly prominent; Some systems may exhibit characteristic issues of colorless storage stage and instant yellowing after drying/high-temperature film formation.

2. Core deep mechanism

The discoloration problem of industrial antibacterial and antifungal agents can be mainly divided into two types of reactions: chemical oxidation discoloration and acid-base hydrolysis discoloration. Under high temperature, strong alkaline, amine, and reducing substance environments, isothiazolinone active compounds undergo molecular ring opening, cleavage oxidation, and generate colored impurities; Inorganic metal ion antibacterial agents will undergo ion displacement and oxidative sulfurization reactions, resulting in dark precipitates.

3. Specific triggers and corresponding scenarios

(1) Reaction between amine curing agent and isothiazolinone (highest occurrence in industrial paint)

In the two-component system of waterborne epoxy and waterborne polyurethane, CMIT/MIT and BIT will undergo nucleophilic reactions with the primary and secondary amines of component B, disrupting the molecular structure of the antibacterial agent and generating yellow brown organic complexes. This is also the most typical root cause of yellowing when preservatives are added to two-component systems. And the reaction is irreversible, once mixed and discolored, it cannot be restored.

(2) Hydrolysis discoloration of CMIT/MIT in high alkaline systems

When the pH of the system is greater than 9.0, the Carcassone antibacterial agent rapidly hydrolyzes and decomposes, producing yellow degradation products, causing the pure white paint and light gray paint to turn yellow as a whole, and the color becomes darker with prolonged storage.

(3) Inorganic silver ion and copper ion antibacterial agents undergo oxidation discoloration

Silver ions react with sulfur and chloride ions in the system to produce black silver sulfide and white silver chloride; The cuprous chloride system is prone to oxidation, resulting in the formation of brown copper oxide impurities, which can cause the paint film to darken, flower, and have uneven color differences. This type of problem often occurs in heavy-duty anti-corrosion, hydraulic coatings, and antibacterial coatings.

(4) High temperature baking oxidation discoloration

Ordinary isothiazolinone has limited temperature tolerance, and baking at temperatures above 120 ℃ can cause thermal decomposition of additives, resulting in burnt yellow residue and yellowing of the paint film.

4. Steps for graded investigation

Step 1: Confirm whether the system is a two-component epoxy/polyurethane, and investigate whether the antibacterial agent was mistakenly added to the B-amine component;

Step 2: Check the pH value of the system to confirm if it exceeds 9.0, and investigate whether CMIT Casone is being used;

Step 3: Conduct a 7-day hot storage test at 50 ℃ to observe whether it turns yellow as the storage time deepens;

Step 4: Check if an inorganic metal ion antibacterial system has been added.

5. Radical solution

Two component system: All antibacterial and anti mold additives are only added to the resin phase of component A, and the mixing of amine curing agents is strictly prohibited;

High alkali system: Eliminate CMIT/MIT and replace with alkali resistant and non yellowing BIT long-term system;

Light colored varnish system: Inorganic metal ion antibacterial agents are prohibited, and pure organic low yellowing compound system is selected;

High temperature baking paint system: Select special antibacterial and mildew resistant models that are resistant to high temperatures and thermal decomposition.

3、 Disadvantage 2: Antibacterial and anti mold effects are ineffective, and additives still cause mold, odor, and bloating

1. Accurate description of the fault phenomenon

The formula clearly includes antibacterial/anti mold additives, but the finished coating may become sour, smelly, swollen, and have a sharp drop in viscosity after 1-3 months of storage; The short-term growth of black spots, green algae, and mold spots on outdoor construction paint films, which completely fail to meet the expectations of anti-corrosion and mold prevention, is the most troublesome hidden quality problem for factories.

2. Core Failure Mechanism

90% of the antibacterial and anti mold effects are not due to insufficient dosage, but rather due to activity being neutralized by the system, deactivated by the environment, locked in by the package, and misplaced selection. The active groups of antibacterial agents are inactivated by chemical reactions, encapsulated by resin and unable to free, and the environmental pH temperature exceeds the stable range, resulting in the “existence but ineffectiveness” of adjuvants.

3. Eight real causes of failure

(1) Misalignment of selection: The anti-corrosion agent tube is in liquid form, and the mold inhibitor tube is in solid form, which cannot be replaced by each other

Only adding preservatives inside the can, the paint film has no anti mold ability, and it will inevitably mold outdoors; Only adding dry film mold inhibitor, the aqueous phase has no bactericidal ability, and storage will inevitably cause bloating and odor. This is the industry’s largest fundamental error.

(2) Excessive pH environment leads to hydrolysis deactivation

CMIT/MIT rapidly hydrolyzes and fails at pH>9.0; Partial IPBC and OIT systems degrade slowly in strongly alkaline environments, leading to a significant decline in their anti mold ability. High pH industrial paint is most prone to “still deteriorate even with preservatives added”.

(3) Amine curing agent inactivates antibacterial activity

In a two-component system, the amine group directly encapsulates and destroys the active structure of isothiazolinone, resulting in zero antibacterial activity, even if added normally, it is completely ineffective.

(4) Adding position error, locked by resin cross-linking

Anti mold agents are added in the early stage of grinding, and are wrapped in a large amount of resin and powder inside the cross-linked network of the paint film, which cannot migrate to the surface for antibacterial purposes. Surface mold grows freely, and internal additives are completely wasted.

(5) Secondary pollution in production

The mixing kettle, pipeline, and storage tank have not been cleaned for a long time, and the inner walls are adhered with rotten residue and biofilm. After filling with new materials, they are cross contaminated, far exceeding the sterilization load of the antibacterial agent.

(6) Antagonism of adjuvants and consumption of activity

Strong reducing agents, partial inhibitors, and excessive anionic surfactants within the system can consume antibacterial activity and cause a decrease in effective content.

(7) Misconception of Adding Quantity: Insufficient Critical Adding Quantity

Some factories fix the addition rate at 0.1%, but in high temperature summer, high humidity storage in the south, and high nutrient slurry systems, the basic addition amount cannot cover the microbial pressure, resulting in later bacterial return.

(8) Long term accumulation of drug-resistant bacteria

Long term use of the same single component BIT system results in a small amount of residual tolerant strains, which accumulates over time and leads to a gradual deterioration of the antibacterial effect.

4. Standardized investigation process

Step 1: Check the type of additives and confirm whether the “dual combination of antibacterial and dry film mold prevention in the tank” is achieved;

Step 2: Check the pH of the system to confirm if it exceeds the tolerance range of the adjuvant;

Step 3: Check the feeding position of the two-component and confirm if component B was added incorrectly;

Step 4: Stability test of 50 ℃ thermal storage for 14 days to determine if the activity has declined;

Step 5: Check the cleanliness of equipment pipelines and eliminate cross contamination.

5. Long term cure plan

The high alkali industrial system uniformly uses a compound anti-corrosion system mainly based on alkali resistant BIT, and eliminates alkali resistant Carcassone;

Strictly follow the rules for adding component A to the two-component system to prevent contact with amines;

Outdoor engineering paint mandatory dual system: anti-corrosion inside the tank+dry film anti mold and anti algae compound;

Adopting a complex antibacterial system to avoid the long-term use of a single adjuvant that can lead to drug-resistant bacteria;

Establish a monthly equipment disinfection and sterilization system to remove the pipeline biofilm.

4、 Disadvantage 3: Release of antibacterial/antifungal agents, frost spraying, white spots, and graininess

1. Accurate description of the fault phenomenon

There are no abnormalities in the storage of the paint. After the construction film is dried, the surface of the paint film appears fine white spots, powdery precipitates, misty clouds, and local particles; Initial wiping can remove it, but later infiltration into the paint film cannot be erased, affecting gloss, transparency, and paint film flatness, especially for clear and high gloss paints.

2. Core fault mechanism

The solubility saturation and compatibility imbalance of antibacterial and antifungal agents in resin systems. The amount of additives added exceeds the current solubility limit of the system, or the hydrophilic and lipophilic equilibrium values of the additives do not match the resin. During the film-forming process, as water and solvents evaporate, the additives cannot be uniformly blended and are forced to migrate and precipitate onto the surface of the paint film, forming spray frost, white spots, and fogging.

3. Disassemble the main causes

(1) Excessive addition of dry film mold inhibitor

OIT and IPBC belong to oil-based hydrophobic additives, and their addition amounts exceeding 0.6-0.8% can easily break through the solubility of the system. They quickly precipitate and turn white in high light systems, which is the number one reason for varnish fogging.

(2) Incompatible mixture of water-based and oil-based additives

Add a large amount of water-soluble preservatives into solvent based and solvent-free epoxy systems, which are completely insoluble and directly precipitate suspended particles.

(3) Precipitation in low-temperature environment

Low temperature production and construction in winter lead to a decrease in resin holding capacity and solubility of additives, resulting in low-temperature frosting and whitening of the originally stable formula.

(4) Rapid and aggressive application of additives, with excessively high local concentrations

During production, if not diluted and poured at high speed in one go, the concentration of local additives will instantly exceed the standard, forming micro precipitated particles. Uneven dispersion in the later stage will lead to defects in the paint film particles.

(5) Polarity mismatch in resin system

Some high hydrophobic mildew inhibitors and high hydrophilic acrylic emulsion have great difference in polarity. After long-term storage, they slowly delaminate and precipitate, and defects appear after film formation.

4. Investigation and Solution

Strictly control the upper limit of dry film mold inhibitor addition, and it is recommended to control the gloss varnish system within the range of 0.3% -0.5%;

Solvent based and solvent-free systems are strictly prohibited from using water-based antibacterial solutions. Replace with oil soluble specialized anti mold models;

Dilute 1-2 times with water/solvent before feeding, slowly and uniformly add to avoid local over concentration;

Fine tune the low-temperature formula in winter, and replace the dispersed mould proof lotion with better compatibility;

Severe precipitation system, replace the low precipitation, high compatibility anti mold system with a compound type.

5、 Disadvantage 4: Antibacterial agents cause shrinkage, pinholes, poor leveling, and pitting on the paint film

1. Accurate description of the fault phenomenon

When no antibacterial agent is added, the paint film is smooth and full. However, after adding antibacterial/anti mold additives, irregular shrinkage, small pinholes, pockmarks, increased orange peel, and poor spreading may occur; Uneven thickness of construction film, partial exposure of the substrate, and significant decrease in leveling, commonly seen in high gloss industrial paint and topcoat varnish systems.

2. Core fault mechanism

The vast majority of industrial antibacterial and anti mold additives belong to surface active substances, which directly participate in the surface tension balance of the system. When the compatibility of additives is poor, locally enriched, or added excessively, it can cause uneven surface tension at the paint film interface, resulting in local uneven spreading, interface shrinkage, and ultimately the formation of shrinkage pores and pinholes; At the same time, the additive has its own microbubble properties, and bubbles escape during the drying process, leaving pinholes and pockmarks.

3. Specific incentives

(1) Excessive use of highly active surface type antibacterial agents

Some compounded antibacterial lotion contain wetting and emulsification components, and excessive addition will disturb the original wetting leveling system, break the balance of coating surface tension, and induce shrinkage.

(2) Incompatibility of additives causes interface contamination

Low quality anti mildew and bacteriostatic additives contain impurities and residual solvents, which are incompatible with the lotion system, forming pollution points at the paint film interface and becoming the center of shrinkage.

(3) Additive stabilization leads to micro bubble pinholes

Some antibacterial lotion have their own foam stabilizing effects, which are antagonistic to defoamers. The system has microbubble residues, which form dense pinholes and pockmarks after drying.

(4) Disrupting the original balance of additive compatibility

The wetting, dispersing, defoaming, and thickening systems are originally balanced, and antibacterial agents act as external surfactants to disrupt the interfacial balance of the system, directly leading to a decrease in leveling and an increase in paint film defects.

4. Investigation and cure plan

Small scale gradient test: Confirm the safe addition range without causing shrinkage or pinholes by adding gradients of 0.1%, 0.2%, and 0.3%;

Prioritize the use of low surface activity, low foaming industrial specific antibacterial and anti mold systems to reduce interference with interfacial tension;

After adding antibacterial agents, fine tune the defoaming ability of the system to counteract the side effects of microbubble stabilization;

Eliminate the use of miscellaneous high impurity antibacterial agents to avoid interface contamination and shrinkage;

The high gloss topcoat system prioritizes the use of BIT based low interference systems to reduce the paint film defects caused by complex surface activities.

The stable core of industrial antibacterial and anti mold system lies not in blindly increasing the amount of additives, but in precise selection, threshold control, standardized process, and matching system. Only by comprehensively controlling the formula, process, equipment, and storage can the four high-frequency problems be completely eliminated, and stable coating storage and long-term protection of paint film be achieved.