Paint Biocides: 7 Types of In-Can Preservatives, Dry-Film Fungicides and Antifouling Biocides, and Their Regulation

Paint biocides are the additives that keep a liquid paint free of bacteria and fungi in the can, keep mold and algae off the dry film and keep fouling off a ship's hull, and formulators choose them from 7 types. Every waterborne paint carries an in-can preservative, exterior and humid-room paints add a dry-film fungicide and algaecide, and hull paints carry antifouling actives. The same additives are sold as paint preservatives, and British texts call the film defect they prevent mould.

Biocides are one of the 15 families of paint additives. This page covers how the 7 types act, which suits which paint, all 26 substances with a page on this site, the 4 failures biocides prevent, the EU and US rules with dates, the tests and sourcing.

How we check: regulations and classifications are read at the body that issues them, standards at the standards body, and every additive level names its basis and its source; the full method is in methodology and fact-checking.

What Are Paint Biocides and Why Does Waterborne Paint Need Them?#

A paint biocide is an active substance, or a product made from one, that controls microorganisms in a paint: EU law sorts it into product type 6 (can), 7 (film), 8 (wood) or 21 (antifouling). Annex V of Regulation (EU) No 528/2012 defines PT6 as preservatives for products during storage, PT7 as film preservatives, naming paints, PT8 as wood preservatives and PT21 as antifouling products for vessels, aquaculture equipment and other structures in water. A paint treated with a biocidal product is a treated article (Article 3(1)(l)), and a treated article with a primary biocidal function is itself a biocidal product (Article 3(1)(a)).

Product type 10, construction material preservatives for masonry, lies outside this page.

Waterborne paint is the paint that in-can actives are registered for: the 1,2-benzisothiazol-3(2H)-one (BIT) label of EPA Reg. No. 6836-423 names interior and exterior emulsion paints and stains. Biocides sit beside binder, pigments, extenders and water, all compared under paint raw materials.

Which Organisms Attack Paint in the Container, on the Film and on the Hull?#

Paint meets 3 groups of organisms: bacteria and fungi in the wet paint, mold and algae on the dry film, and hard and soft fouling on the hull. The 3 groups are listed below.

• Wet paint: bacteria such as Pseudomonas, which bronopol blends target, and fungi; 2-methyl-2H-isothiazol-3-one (MIT) is weaker on fungi, so it is paired with BIT.
• Dry film: mold, which ASTM D3273 grows on interior coatings in a humid chamber, and algae on exterior films.
• Hull: hard fouling such as barnacles, which medetomidine repels, and soft fouling such as algae and slime, the target of copper pyrithione, DCOIT and zineb boosters.

How Do Paint Biocides Work? Thiol Attack, Formaldehyde Release, Copper Ions and Controlled Leaching#

Paint biocides work in 4 ways: thiol attack by isothiazolinones, formaldehyde release by releasers and bronopol, copper ion release at the hull surface, and controlled leaching of dry-film actives from the film. In isothiazolinones, an electrophilic N-S bond reacts with the thiol groups of microbial enzymes. Releasers such as tetramethylol acetylenediurea (TMAD) free formaldehyde, which cross-links microbial proteins. Cuprous oxide releases copper ions at the hull, and a self-polishing copolymer frees its biocides by hydrolysis or ion exchange with seawater (US 2008/0038241 A1, patent background).

Dry-film actives protect only while they stay in the film. In a Zurich facade field study (Burkhardt et al., 2012), runoff reached 62.7 L/m² per year, 6.3 % of precipitation, with biocide concentrations from mg/L down to hundreds of µg/L, highest early. Nichterlein et al. (2025) found that encapsulation improves the UV stability of OIT and DCOIT and lowers leached concentrations, and Bollmann et al. (2017) report soil half-lives under 10 days for isothiazolinones and well over 120 days for terbutryn and the triazoles.

7 Types of Paint Biocides Compared#

The 7 types of paint biocides are in-can preservatives, isothiazolinones, formaldehyde releasers, dry-film biocides, mildewcides, antimicrobial additives and antifouling biocides; the first three protect the wet paint, the next three the dry film, the last the hull.

The table compares the 7 types by zone, chemistry, paint types, one registered product level with its basis, strengths and limits.

Isothiazolinones and formaldehyde releasers are chemistries inside in-can preservation, and OIT and DCOIT also serve on the film and hull. Mildewcides are a US class of registered, ready-to-add dry-film products.

Which Types Protect the Wet Paint?#

In-can preservatives, isothiazolinones and formaldehyde releasers are the 3 types that protect the wet paint, all PT6. Among in-can preservatives, DBNPA (2,2-dibromo-2-cyanoacetamide) gives a quick kill that decontaminates raw materials or a spoiled batch before a long-term preservative takes over.

Isothiazolinones are the thiol-reactive actives BIT, MIT, CMIT/MIT (3:1 reaction mass of 5-chloro-2-methyl- and 2-methyl-2H-isothiazol-3-one) and methylbenzisothiazolinone (MBIT). BIT acts more slowly than CMIT but is more stable to heat and pH, while CMIT degrades at high pH, faster with heat and amine bases.

Formaldehyde releasers such as TMAD, MBO and HPT preserve the paint through the formaldehyde they free. MBO (reaction product 3:2) is approved for PT6 from 2027-06-01 to 2032-05-31 as a candidate for substitution (Implementing Regulation (EU) 2026/385), HPT (reaction product 1:1) was refused by Implementing Decision (EU) 2026/599, and the MBM approval ended on 2022-03-31.

Which Types Protect the Dry Film and the Hull?#

Dry-film biocides, mildewcides and antimicrobial additives protect the dry film or its surface, and antifouling biocides protect the hull. Dry-film biocides are PT7 fungicides and algaecides: iodopropynyl butylcarbamate (IPBC) is a broad-spectrum fungicide but weak on algae, and carbendazim is poor on algae, so outdoor paints add algaecides such as terbutryn or diuron.

Antimicrobial additives act on the painted surface. Silver zinc zeolite is approved for PT7 from 2026-03-01 to 2036-02-29, but not for paints on houses, facades, noise barriers or structures above water (Implementing Regulation (EU) 2024/2635). An anti-bacterial paint with a primary biocidal function is a biocidal product in the EU, and in the US the treated-article exemption does not cover public-health claims (EPA PRN 2000-1).

Antifouling biocides are PT21 actives: cuprous oxide is the main copper biocide, copper thiocyanate the off-white one for light-colored paints, and tralopyril a metal-free biocide against hard fouling. Medetomidine, a barnacle repellent, is used at 0.1 % on wet paint weight (producer recommended use), at which it alone makes the paint Aquatic Chronic 2 H411.

How to Choose a Paint Biocide by Paint Type#

Choose a paint biocide by where the paint fails: every waterborne paint needs in-can protection, exterior and humid-room paints add a dry-film fungicide and algaecide, wood stains need actives approved for wood, and hull paints need PT21 actives.

The table matches 5 paint types to their biocide types, a registered product level with its basis, and the point to watch.

The EU Ecolabel allows dry-film actives only in outdoor paints, at up to 0.50 % w/w PT7 actives of the final product, and in indoor paints for high-humidity rooms, at up to 0.10 % w/w (Decision (EU) 2025/2607). Each paint type's full additive package is set out in paint formulations by paint type.

Solventborne paints carry dry-film protection too: the OIT label of EPA Reg. No. 707-100 covers latex and solvent-based paints and stains, with a coupling solvent such as butyl glycol in solventborne coatings.

Which Biocide Combinations Do Paint Makers Use?#

Producers combine 5 sets of actives: CMIT/MIT with bronopol, BIT with bronopol, BIT with zinc pyrithione, diuron with carbendazim and IPBC, and cuprous oxide with zinc pyrithione. The 5 sets are listed below.

• CMIT/MIT with bronopol: broad spectrum including Pseudomonas, in a producer blend.
• BIT with bronopol: the same spectrum, in a producer blend.
• BIT with zinc pyrithione: a producer blend for thermal stability.
• Diuron with carbendazim and IPBC: one registered dispersion (EPA Reg. No. 5383-109), for film-forming uses only.
• Cuprous oxide with zinc pyrithione: a registered self-polishing composition of 40.41 % Cu2O and 3.8 % zinc pyrithione on total wet paint (EPA Reg. No. 2693-187).

When to Add Biocides: pH, Temperature and Stage of Addition#

Each paint biocide goes in at the stage its label names, with pH and temperature setting the window. The 4 label rules are listed below.

1. Add CMIT/MIT in the let-down or as a post-add below 45 °C (113 °F) (EPA Reg. No. 707-134).
2. Add bronopol as the final step before packing, after any heating stage, below 40 °C (104 °F) (EPA Reg. No. 464-675).
3. Premix OIT with propylene glycol and coalescent and add it in the let-down before the thickener (EPA Reg. No. 707-100).
4. Add the diuron, carbendazim and IPBC dispersion at the end of the manufacturing cycle, after final pH adjustment (EPA Reg. No. 5383-109).

Bronopol decomposes in alkaline media and releases formaldehyde, with a hydrolysis half-life of 2 months at pH 8 and 20 °C (68 °F) (EPA Reregistration Eligibility Decision, 1995). The batch stages themselves are explained on how paint is manufactured.

Complete Table of Paint Biocide Substances#

The table lists all 26 paint biocide substances with a page on this site, with CAS number, type, function, an example level with its basis and source, EU BPR status for paint and the CLP hazard line.

Four further biocide substances are rows of other pages: dichlofluanid on the antifouling page, diiodomethyl p-tolyl sulfone on the dry-film page, silver zinc zeolite on the antimicrobial page and phenylmercury acetate, a legacy preservative, on the restricted-substances page. Tributyltin oxide (TBTO) has been on the REACH Candidate List since 2008-10-28.

Which Paint Failures Do Biocides Prevent?#

Biocides prevent 4 paint failures: microbial spoilage in the container, viscosity loss when enzymes break down cellulose thickeners, mold and algae defacement of the film, and fouling of hulls.

The table links each failure to its zone, the biocide type and the test or measure.

Two failures start in the container and two on the film or hull. Every other failure and its additive fix is collected under paint defects.

How Are Paint Biocides Regulated? EU Approvals, Sensitizer Labels and US Registration#

In the EU, a preserved paint is a treated article and reaches the market only if every active is approved for that product type and use (Article 58(2) of Regulation (EU) No 528/2012). A paint not classified as sensitizing carries EUH208 once a sensitizer reaches one tenth of its specific concentration limit (CLP Annex I Table 3.4.6 Note 1): from 1.5 ppm for CMIT/MIT, MIT, MBIT, OIT and DCOIT (H317 from 15 ppm), from 36 ppm for BIT (H317 from 360 ppm). Consumer paints with BIT or MIT above their sensitizer classification come with EN 374 gloves.

The table lists the instruments that govern paint biocides, with their dates.

US label rates can exceed EU classification limits. Every use rate on the diuron, carbendazim and IPBC dispersion label gives 1,050 ppm (0.105 %) diuron or more, so the paint would be classified Carc. 1B H350 in the EU, and zinc pyrithione above 3,000 ppm (0.3 %), the upper part of its label range, would make a paint Repr. 1B H360D (calculated). From 2027-02-01, IPBC moves from Acute Tox. 3 H331 to Acute Tox. 2 H330 (Delegated Regulation (EU) 2025/1222).

In the US, EPA announced draft risk assessments for MIT/CMIT, OIT, BIT, BBIT and DCOIT on 2020-05-14 (85 FR 28944), and Proposition 65 lists diuron (cancer, 2002-05-31) and formaldehyde gas (cancer, 1988-01-01). Rules for every family are collected under paint additive regulations.

Two items are pending. A draft decision not to renew medetomidine for PT21, after an ECHA committee opinion of 2024-05-28 on endocrine disruption, was notified to the WTO on 2026-07-08 and was not in the Official Journal on 2026-11-03. EPA is considering label instructions on treated paint containers for professional spray painters, proposed so far only for diuron (docket EPA-HQ-OPP-2023-0420).

How Is Paint Biocide Performance Tested? In-Container, Mold and Algae Methods#

Paint biocide performance is measured with 2 test groups: in-container resistance to microorganisms (ASTM D2574-16(2024)) and film resistance to mold and algae (ASTM D3273-21, ASTM D5590-26, ASTM D5589-25, rated by ASTM D3274-09(2026), and EN 15457:2022).

The table gives each test group its current designation and what it shows.

ASTM D5589-25 ranks films in the laboratory and does not replace exterior exposure; antifouling release-rate tests sit with the antifouling biocides page. Every other method is described under paint test methods.

Sourcing Paint Biocides: Active Content, Grades and Quote Requests#

Paint biocides are bought as dispersions, solutions and powders of stated active content, from 1.5 % for a CMIT/MIT grade to 98 % for an IPBC grade. Registered contents include 19-20 % BIT, 45 % OIT, 97-97.15 % technical cuprous oxide and 99 % tralopyril, so a request names the active, its content, the target ppm active, the paint type, the annual volume, the packaging and the delivery country.

Cynthia Challener (CoatingsTech, October 2013) put formulated paint and coatings biocides at USD 500 million in 2012, 70 % of it architectural, and reports that biocide producers qualify biocides in customer formulations and audit plant hygiene.

Cybutryne and tributyltin are banned on ships and not quoted; copper compositions with boosters and copper-free tralopyril compositions replace them. Producer and distributor qualification is covered on paint additive suppliers and manufacturers.

Two related pages cover regional sourcing and the market.

• Producers and distributors in India: paint additive manufacturers in India
• Market size, segments and leading producers: paint additives market

How Biocides Interact With Other Additive Families#

Biocides interact with 3 other paint additive families, because cellulose thickeners need in-can protection against enzymes, amine neutralizers destabilize CMIT, and OIT is premixed with the coalescent. The 3 interactions are listed below.

• Rheology modifiers: see paint thickeners and rheology modifiers.
• Neutralizers: see neutralizing agents for waterborne paint.
• Coalescents: see coalescing agents for latex paint.

Is a Wood Preservative a Biocidal Product?#

A wood preservative paint is a PT8 biocidal product when it protects the wood itself, while a stain whose biocide protects only its own film uses PT7 actives. Annex V defines PT8 as the preservation of wood, from the saw-mill stage on, or of wood products against wood-destroying or wood-disfiguring organisms, including insects. The 20 % IPBC product of EPA Reg. No. 5383-74 draws the same line on its label: it protects the applied film and paint, not the substrate.

Five actives on this page hold EU PT8 approvals: OIT to 2027-12-31 (Implementing Regulation (EU) 2017/1277), IPBC postponed to 2027-12-31 (Implementing Decision (EU) 2026/1129), tebuconazole postponed to 2027-06-30, DCOIT postponed to 2028-06-30 and propiconazole renewed to 2030-11-30 (Implementing Regulation (EU) 2023/2596).

How Much Biocide Does a Paint Contain?#

Biocide levels depend on type and product: in-can labels run from 6.25 ppm active (CMIT/MIT) to 483 ppm active (BIT), dry-film labels reach 4,800 ppm active (IPBC) and antifouling compositions hold up to 60.9 % Cu2O on total wet paint.

Labels state the product, so ppm active equals % product times active fraction times 10,000: 0.10 % of a 19.3 % BIT product equals 193 ppm active, and the Ecolabel PT6 cap of 0.080 % equals 800 ppm. Ladder studies around such levels are explained under paint additive dosage.