2-Octyl-2H-isothiazol-3-one (OIT, CAS 26530-20-1) is the member of the isothiazolinone family that protects the dry film rather than the wet product. Its longer alkyl chain makes it strongly lipophilic and only sparingly soluble in water, so it partitions into binders, plasticisers and fibres instead of staying in the aqueous phase where microbes grow. That single property difference explains why OIT is bought for paints, adhesives, leather, plastics and timber, and why it is not the right choice for in-can preservation. This article covers the structural logic of the family, the chemistry and production route, the carrier systems used to deliver a water-insoluble active, film protection practice, comparison with DCOIT, IPBC, zinc pyrithione and BIT, stability, regulation and what a buyer should check on a certificate.
1. Identity and Physical Character
| Property | Value |
|---|---|
| Chemical name | 2-octyl-2H-isothiazol-3-one |
| CAS number | 26530-20-1 |
| Synonyms | OIT; octhilinone; 2-n-octyl-4-isothiazolin-3-one; 2-octyl-1,2-thiazol-3-one |
| Molecular formula | C11H19NOS |
| Molecular weight | 213.34 |
| InChIKey | JPMIIZHYYWMHDT-UHFFFAOYSA-N |
| Canonical SMILES | CCCCCCCCN1C(=O)C=CS1 |
| Water solubility | very low – sparingly soluble, which drives its behaviour in every application |
| Lipophilicity | high, from the eight-carbon alkyl chain |
| Typical commercial form | 45% solution in an organic carrier; also water-based dispersions and technical solid |
OIT is a five-membered ring carrying nitrogen and sulfur adjacent to a ketone, with an n-octyl group on the nitrogen. The ring is the biocidal pharmacophore and is shared with MIT, CMIT and DCOIT; the octyl chain is what makes OIT different. Chain length controls partitioning, and partitioning controls everything else: spectrum, persistence, leaching and even the hazard profile.
2. The Chain-Length Logic of the Isothiazolinone Family
Reading the family by alkyl chain rather than by trade name makes selection much simpler.
| Active | CAS | Alkyl chain | Water solubility | Where it works | Practical role |
|---|---|---|---|---|---|
| MIT | 2682-20-4 | methyl | high | aqueous phase | in-can preservation, alone or blended |
| CMIT | 26172-55-4 | methyl plus 5-chloro | high | aqueous phase | fast knockdown in-can |
| BIT | 2634-33-5 | benzyl-fused, no alkyl chain | low | aqueous phase as a salt/solution | in-can, stable over a wide pH range |
| OIT | 26530-20-1 | n-octyl | very low | binder, film, fibre, plasticiser | dry-film fungicide |
| DCOIT | 64359-81-5 | n-octyl plus 4,5-dichloro | very low | film, especially immersed surfaces | antifouling and severe-exposure film protection |
The pattern is that increasing lipophilicity moves the active out of the water and into the substrate. Microbes in a wet product – bacteria, yeasts and moulds growing in the aqueous phase – are controlled by the water-soluble members. Mould and algae growing on a dry film, drawing moisture from the air and feeding on the binder, are controlled by the lipophilic members. Blending across that divide is how a complete preservation package is built: an in-can active for the bucket, a film active for the wall.
The chlorinated analogue DCOIT is more potent against a broader spectrum, including algae, and is the standard choice for marine antifouling and for coatings exposed to condensation or immersion. OIT is the workhorse where the exposure is atmospheric and the requirement is mould and yeast control at a lower cost and a lower regulatory burden.
3. Mode of Action
Isothiazolinones act at the cell by a two-step mechanism. The ring reacts with nucleophilic thiol groups – cysteine residues of enzymes and coenzyme A – forming a mixed disulfide and opening the ring. The opened species then reacts with further cellular nucleophiles, including iron-sulfur centres and amine groups, disrupting energy metabolism. In practice the effect is a rapid inhibition of growth and, at sufficient concentration, loss of viability.
Because the mechanism depends on nucleophilic attack, the same chemistry that kills the cell also consumes the active in the formulation. Any free thiol, sulfide, sulfite, thiosulphate or unprotonated amine in the product will react with OIT. In a film this matters differently from a liquid: the active is largely protected inside the binder, but any water-soluble nucleophile that migrates into the film will still find it. Copper and iron ions accelerate degradation, which is why coatings formulated with metallic driers or zinc pigments need a compatibility check.
4. Manufacture
OIT belongs to the same synthetic family as MIT and CMIT: a dithiodipropionamide is closed into the isothiazolinone ring with a chlorinating agent. The difference is the amine used to build the amide.
| Stage | Chemistry | Control point |
|---|---|---|
| 1. Amide formation | n-octylamine is reacted with a 3,3′-dithiodipropionate ester (or an equivalent propionate building block) to give N,N’-dioctyl-3,3′-dithiodipropionamide | residual free octylamine, which carries through as odour and as a nucleophile that consumes the product |
| 2. Cyclisation and chlorination | the diamide is closed with a chlorinating agent such as chlorine or thionyl chloride, forming the isothiazolinone ring | stoichiometry and temperature: these decide yield, colour and the level of over-chlorinated by-products |
| 3. Quench and workup | the reaction mass is quenched, the organic phase separated, washed and dried | residual acidity and water content – the specification limit of 1.0% water is met here |
| 4. Formulation to 45% | the technical is dissolved in the chosen carrier and standardised to 45% active | carrier selection and clarity; the density window 1.02 – 1.05 g/mL follows from this step |
Two consequences follow for buyers. Over-chlorination products are the main organic impurity to control, and they show up as colour and as a lower assay. Residual octylamine is the impurity that matters for odour and for formulation stability, because it is a nucleophile that will slowly consume the active. Neither is visible on a specification sheet that lists only appearance, assay, water and density – which is why the process history behind a supplier matters more here than for a commodity.
5. Delivering a Water-Insoluble Active: Carrier Systems
| Form | Composition | Advantages | Constraints |
|---|---|---|---|
| Solvent-borne 45% | OIT in an organic carrier, water ≤1.0%, density 1.02 – 1.05 | clear liquid, high active per kilogram, easy to incorporate into solvent-borne and high-solids systems | carrier compatibility with the binder; contributes to VOC; viscous, needs weighed addition |
| Water-based dispersion | OIT dispersed with surfactants, typically 20 – 30% active | no solvent burden; suitable for water-borne paints | surfactant load, freeze sensitivity, lower active per kilogram, risk of sedimentation |
| Technical solid | 97 – 99% OIT | lowest freight cost per kilogram of active, maximum formulation freedom | requires dissolution and handling controls; dust exposure is a sensitiser risk |
| Encapsulated / supported | active on a carrier or in a matrix | controlled release, reduced leaching in wet-service coatings | cost, and a different efficacy profile that must be tested rather than assumed |
The 45% solvent-borne grade is the volume product because it delivers the most active per unit of freight and pours easily enough to be metered in a paint plant. Its limitation is not efficacy but compatibility: adding it to a water-borne acrylic requires pre-dilution with a water-miscible solvent and a stability test, because the carrier can shock the latex and produce grit or a viscosity shift.
6. Applications
6.1 Architectural and industrial coatings
Exterior paints, plasters, renders and facade coatings are exposed to rain, condensation and organic soiling, which is enough for mould and algae to establish on the surface. OIT provides mould and yeast control in the film; algae control usually needs a partner active such as DCOIT, IPBC or zinc pyrithione because OIT alone is weaker against algae. In interior paints, OIT covers damp substrates such as kitchens, bathrooms and basements.
6.2 Adhesives, sealants and construction chemicals
Cured adhesive and sealant films based on acrylics, polyurethanes and silane-modified polymers support fungal growth in humid conditions. OIT is added to the wet adhesive and partitions into the polymer as the film forms. Because the film has little mobile water, the required dose is set by film challenge testing rather than by analogy with paints.
6.3 Leather, textiles and plastics
Leather and technical textiles are preserved in the wet state (beamhouse, tanning, finishing) and protected afterwards for tropical storage and shipping. In plastics, OIT protects plasticised PVC and polyurethane articles from fungal attack, where the plasticiser itself is a food source. Both uses rely on the lipophilic character of the molecule: the active must partition into the substrate rather than remain on its surface, or it will be washed off in the first rainfall or laundering.
6.4 Wood and wood-based panels
For timber in service, OIT is used as a fungicide in formulations applied by dipping or pressure treatment. Performance depends on penetration and on the leach resistance of the whole formulation, not on the active alone; a formulation that leaches out in the first wet season has failed regardless of its laboratory result.
6.5 Metalworking fluids and industrial fluids
In metalworking fluids, fungal contamination produces slime, blockages and odour even when bacterial control is adequate. OIT is used as a fungal partner to an in-can bactericide. Because the fluid is an emulsion exposed to air, the active must be dosed into the oil phase with a suitable emulsifier, and the concentration must be monitored rather than assumed.
7. Film Protection Versus In-Can Preservation
| Problem | Sign | Correct active type | Wrong choice and its consequence |
|---|---|---|---|
| Spoilage in the can | gas, pH drop, off-odour, viscosity loss, visible growth | water-soluble in-can active (CMIT/MIT, BIT, bronopol) | a film fungicide: does not reach the aqueous phase, so spoilage continues |
| Mould on the dried film | dark spots, surface growth, customer complaints after a season | lipophilic film active (OIT, DCOIT, IPBC) | an in-can active: it is water-soluble and either stays in the liquid or leaches out |
| Algae on exterior surfaces | green or black staining on north-facing walls | DCOIT, IPBC or zinc pyrithione, usually combined | OIT alone: weak algae spectrum at practical doses |
| Both problems at once | spoilage during storage and mould after application | a two-active package, one of each type | a single high dose of either: costs more and solves half the problem |
This table resolves most field complaints before any laboratory work. A customer who reports “the preservative does not work” is usually reporting either a film problem treated with an in-can active, or a can problem treated with a film active. Determining which phase the growth is in is the first diagnostic step.
8. Comparison with Alternative Film Actives
| Active | CAS | Spectrum | Water solubility | Strengths | Weaknesses |
|---|---|---|---|---|---|
| OIT | 26530-20-1 | fungi, yeasts; weak algae | very low | cost-effective mould control, wide regulatory acceptance in industrial uses | strong sensitiser; needs a partner for algae |
| DCOIT | 64359-81-5 | broad, including algae | very low | best broad-spectrum film protection; marine antifouling standard | higher cost; more restricted in some markets; formulation demands |
| IPBC | 55406-53-6 | strong fungi, weak bacteria | low | excellent against mould and blue stain; widely used in wood and coatings | narrow bacterial spectrum; cost; colour and odour constraints at high dose |
| Zinc pyrithione | 13463-41-7 | fungi and algae | insoluble | good exterior durability; familiar to formulators | zinc discharge concerns; restricted in some regions; opaque at higher doses |
| BIT | 2634-33-5 | bacteria, some fungi | low (used as solution) | stable across pH, milder hazard profile | primarily an in-can active; not a substitute for a film fungicide |
Selection in practice is driven by exposure rather than by spectrum tables. Atmospheric interior exposure with a mould problem: OIT or IPBC. Exterior facade with algae: DCOIT or a zinc pyrithione combination. Wet or immersed service: DCOIT. Wood in ground contact or with blue-stain risk: IPBC. Several markets restrict one or another of these actives for specific product types, so the regulatory filter runs before the technical one.
9. Stability in the Film
| Factor | Effect | Control |
|---|---|---|
| UV exposure | photodegradation at the film surface over months | formulate with UV absorbers or a light stabiliser where exposure is severe; accept a surface-active loss and dose accordingly |
| Alkaline binders | ring opening and loss of activity at high pH | check compatibility with silicate and high-pH binders; consider a different active |
| Nucleophiles in the formula | thiols, sulfites and free amines consume the active | avoid, or dose after their addition and verify by film challenge testing |
| Metal ions | iron and copper accelerate degradation and cause discolouration | stainless contact; check driers and zinc pigments for interaction |
| Leaching by rain or washing | progressive loss of protection on exterior and washable surfaces | test leach resistance of the whole film; encapsulation helps in wet service |
| Storage of the concentrate | moisture pick-up, phase separation, hydrolysis | keep containers closed; water below 1.0%; store below 35 °C and above 5 °C |
10. Regulation and Hazard
OIT is regulated as a biocide by use category, not by substance alone, and the categories differ between markets. In the European Union it is assessed under the Biocidal Products Regulation through product-type-specific authorisations – principally wood preservation and the preservation of fibres, leather, rubber and polymerised materials – and a supplier’s technical material does not carry an authorisation for a customer’s finished product. In the United States, OIT is a registered pesticide active ingredient under FIFRA, and the formulated product must be registered and labelled accordingly. Other markets have their own notification or registration regimes.
For cosmetics and personal care, OIT is not permitted in the European Union and is restricted or prohibited in several other markets; a technical grade of any isothiazolinone is not a cosmetic ingredient. The hazards that drive these restrictions are consistent across markets:
- Skin sensitisation – OIT is a strong sensitiser; sensitisation is permanent, and it is the reason for gloves, closed transfer and the exclusion from leave-on consumer products.
- Aquatic toxicity – very toxic to aquatic life with long-lasting effects; concentrates and rinsings must never reach drains or surface water.
- Combustibility of the carrier – the 45% solvent-borne grade is a flammable or combustible liquid depending on the carrier; check the SDS before bulk storage and transfer.
11. Purchasing Checklist
| Check | Acceptance criterion | Why |
|---|---|---|
| Total active | ≥45.0%, method stated (HPLC with UV detection is standard) | defines value per kilogram |
| Water content | ≤1.0% | confirms a solvent carrier and flags moisture pick-up |
| Density | 1.02 – 1.05 g/mL at 20 °C | volume-to-mass conversion for dosing |
| Appearance | clear, yellow, viscous; no haze, no sediment | haze suggests water pick-up; sediment suggests phase separation |
| Carrier identity | stated explicitly on the TDS | compatibility with the binder depends on it |
| Residual amine / odour | low | residual octylamine consumes the active and carries odour |
| Documentation | COA per batch, SDS, registration status for the intended use category | customs and product registration |
Two questions worth asking: which carrier is used, and what is the maximum storage time before re-assay. The first determines compatibility with the customer’s system; the second matters because a solvent-borne concentrate that has picked up water over a hot summer will not be the same material at the end of the season.
12. Packaging, Storage and Supply
Standard packaging is 25 kg, 200 kg and 1,000 kg HDPE drums and IBCs, with 5 kg and 20 kg packs for trials. Store between 5 °C and 35 °C in the closed original container, out of sunlight and away from oxidisers, food and feed. Do not allow to freeze. Typical shelf life is 12 – 24 months in unopened packaging depending on the carrier; opened containers should be resealed and used within the campaign.
13. FAQ
Is OIT an in-can or a film preservative?
A film preservative. It is only sparingly soluble in water, so it partitions into the binder rather than staying in the aqueous phase where in-can spoilage happens. In-can protection needs a water-soluble active such as CMIT/MIT, BIT or bronopol.
OIT or DCOIT?
OIT for atmospheric exposure where mould and yeast control at moderate cost is the requirement. DCOIT where the film meets water, condensation or immersion, and where algae control matters – at a higher cost and with a stricter regulatory position.
Why is the product 45% and not 98%?
Because the active is a solid that must be delivered as a pourable liquid. A solvent-borne 45% solution gives a clear, meterable product with the most active per unit of freight that still handles easily in a paint plant.
Can OIT be added to a water-borne paint?
Yes, with a pre-dilution in a water-miscible solvent and a stability test. Adding the solvent-borne grade directly to a latex can shock it and produce grit or a viscosity shift.
Does OIT handle algae?
Weakly. For exterior surfaces with algae staining, pair OIT with DCOIT, IPBC or zinc pyrithione; increasing the OIT dose is a more expensive way to get a worse result.
What is the shelf life of a treated paint?
It depends on the binder, the exposure, the pigment system and the film thickness, and it should be established by exterior exposure testing rather than extrapolated from an accelerated test alone. Accelerated tests predict chemical loss well and predict real weathering less well.
References and Further Reading
- Identity data (formula, molecular weight, InChIKey, canonical SMILES) verified against PubChem CID 33528.
- Dosing arithmetic on this page uses the active fraction of the grade: a 45% product delivers 0.45 g of OIT per gram, so 200 ppm in 1,000 kg requires 200 / 0.45 = 444 g of product.
- Family comparisons are based on the CAS numbers and chain structures listed in section 2; the water-solubility ordering follows from alkyl chain length and is qualitative on this page by design.
- Regulatory summaries are for orientation. Authorisations, permitted product types and labelling must be confirmed for the destination market and the specific use category before commercial use.
This article is technical orientation for industrial buyers. It is not a registration opinion, a formulation approval or a safety assessment for any specific product. Work from the SDS of the actual batch and from efficacy testing of the actual formulation.
For the specification and packaging of our 45% grade see the OIT-45 product page. For film active selection see dry-film preservative selection, for carrier practice see solvent-borne versus water-borne OIT, and for in-can preservation see industrial preservative selection.