Hydroxyl Content in Acrylic Polyols
Why OH% is crosslink density, how to convert it to hydroxyl number and equivalent weight - and the solids-basis trap that quietly halves or doubles your hardener charge.
💡 The one-paragraph version
Hydroxyl content tells you how many reactive sites the resin offers per unit mass. Each one becomes a urethane bond when the polyisocyanate hardener arrives, so hydroxyl content is a direct proxy for how tightly the cured film will be crosslinked. Two conventions exist for stating it - a weight percentage of OH groups, or a hydroxyl number in mg KOH/g - and they are trivially interconvertible. The genuinely dangerous ambiguity is not which unit is used, but whether the figure refers to the solid resin or to the dispersion as supplied.
🔬 What a Hydroxyl Group Actually Does in the Film
In an uncured acrylic dispersion, the polymer chains are separate objects. They pack together as water leaves and they entangle, but nothing chemically joins one chain to the next. That is why an uncrosslinked acrylic film is thermoplastic: warm it, or put a solvent on it, and the chains slide past each other.
A hydroxyl group hanging off the backbone - put there by copolymerising a monomer such as 2-hydroxyethyl methacrylate - changes that. When an isocyanate group meets it, the two form a urethane linkage. Because the polyisocyanate hardener carries several NCO groups on one molecule, each hardener molecule can bridge two, three or more polymer chains. Do that across the whole film and the separate chains become one continuous covalent network.
This is why hydroxyl content matters so much more than any other single number on the data sheet. Double the hydroxyl content and you roughly double the number of bridges per unit mass. The network becomes tighter, segments between crosslinks become shorter, and the film's behaviour changes across the board:
- ✅ Hardness and scratch resistance rise, because chain segments have less freedom to move
- ✅ Solvent and chemical resistance rise sharply - a solvent can swell a loose network but struggles to penetrate a tight one
- ✅ Effective glass transition temperature rises, so the film stays hard at higher service temperatures
- ⚠️ Elongation, impact resistance and flexibility fall
- ⚠️ Internal stress in the film rises, which can pull at adhesion on difficult substrates
- ⚠️ The reaction consumes more hardener, and it consumes it faster
⚗️ Crosslink density is a trade, not an upgrade. There is no point on the scale that is simply "better". A coating that must flex needs a looser network; a coating that must survive hot pans and cleaning chemicals needs a tighter one. Specifying the highest hydroxyl grade available because it reads best on paper is the most common and most expensive mistake in this product category.
📐 Two Ways of Saying the Same Thing
You will see hydroxyl content expressed in two units, and which one appears depends largely on supplier convention and region.
Hydroxyl content, OH% (weight percent). The mass of hydroxyl groups as a percentage of the resin mass. The hydroxyl group –OH has a molar mass of 17.008 g/mol, so this figure converts directly into moles of reactive site.
Hydroxyl number, or OH value (mg KOH/g). Inherited from classical titration practice: the mass of potassium hydroxide, in milligrams, chemically equivalent to the hydroxyl content of one gram of sample. KOH has a molar mass of 56.106 g/mol.
Because both describe the same underlying quantity of hydroxyl, converting between them is one multiplication:
📐 Conversion
Hydroxyl number (mg KOH/g) = OH% × 56 106 ÷ 17.008 ÷ 100
Hydroxyl number ≈ OH% × 33.0
OH% = Hydroxyl number ÷ 33.0
A third derived quantity is often more useful than either: the hydroxyl equivalent weight, which is the mass of resin containing one mole of hydroxyl. It is what you actually need when you size a hardener charge.
📐 Equivalent weight
OH equivalent weight (g/eq) = 1 700.8 ÷ OH%
OH equivalent weight (g/eq) = 56 106 ÷ Hydroxyl number
Worked across a representative range of solid-resin hydroxyl contents - these are illustrative figures for the arithmetic, not the specification of any particular grade:
| OH% on solids | Hydroxyl number (mg KOH/g) | OH equivalent weight (g/eq) | Typical film character |
|---|---|---|---|
| 1.0% | 33 | 1 701 | Flexible, high gloss, forgiving |
| 1.5% | 50 | 1 134 | Flexible with moderate hardness |
| 2.0% | 66 | 850 | General purpose |
| 2.5% | 83 | 680 | Hardness with usable flexibility |
| 3.0% | 99 | 567 | Hard, chemically resistant |
| 3.5% | 116 | 486 | Hardest, least forgiving |
💡 Read the equivalent weight column as "grams of resin solids per mole of hydroxyl". Halving the equivalent weight doubles the hardener you will need - the relationship is linear and it goes straight to your cost per litre of finished coating.
⚠️ The Basis Trap: On Solids, or As Supplied?
Everything above assumed the hydroxyl figure refers to solid resin. For waterborne acrylics that is the normal convention, and it is the convention we use. But it is not universal - some suppliers quote hydroxyl content on the dispersion as supplied, water included. Both are defensible. Neither is wrong. The problem is that the two numbers describe the same resin and differ by a large factor, and data sheets do not always say which one they mean.
🚨 At 45% solids the two conventions differ by a factor of about 2.2
Because only the solid fraction carries hydroxyl groups:
OH% as supplied = OH% on solids × solids fraction
At 45% solids: OH 2.0% on solids = 0.90% as supplied
Error factor if confused: 1 ÷ 0.45 = 2.22×
Take the on-solids figure and use it as though it were on-supply, and you will overcharge hardener by roughly 120%. Make the reverse error and you will undercharge by more than half. In neither case does anything look wrong at the spray gun. The error surfaces days later, when a panel fails a hardness or resistance test and nobody can explain why.
The consequences are asymmetric, and worth understanding separately.
Undercharging hardener gives you an undercrosslinked film. It looks acceptable, it may even gloss well, and it fails on chemical resistance, hardness and water resistance. This is the failure that gets blamed on the resin.
Overcharging hardener is more insidious. The surplus isocyanate does not simply sit inert - in a waterborne system it reacts with water, producing amine, then urea, and releasing carbon dioxide. You pay for hardener you do not need, you introduce more urea into the film than you intended, and you generate more gas that has to escape before the film closes. In thicker coats that gas is trapped and you get bubbles and pinholes. It is a genuinely common root cause of defects that get misdiagnosed as application faults.
The remedy is procedural rather than technical. Ask every supplier - including us - to state the basis explicitly in writing on the technical data sheet, and re-check it whenever you change source or grade. Our figures are stated on solid resin, and the acrylic polyol with the hydroxyl basis stated on the data sheet is supplied with that convention named on the document rather than assumed. Once the basis is confirmed, the full hardener sizing procedure is set out in our NCO:OH ratio and hardener demand guide.
🔗 Hydroxyl Content Is Not the Whole Story: Functionality
Two resins can share a hydroxyl content and still build different networks, because hydroxyl content is a bulk average and network formation depends on how those hydroxyls are distributed among individual chains.
The relevant quantity is functionality - the average number of hydroxyl groups per polymer chain:
Functionality ≈ Number-average molecular weight (Mn) ÷ OH equivalent weight
A chain carrying zero or one hydroxyl cannot contribute to a network - with one it becomes a dangling end, attached at one point and doing nothing structural. Only chains with two or more participate. Because hydroxyl distribution in a free-radical copolymer is statistical rather than uniform, a low-functionality resin will always contain some fraction of unusable chains.
Worked through: a resin with an Mn of 10 000 and 2.0% hydroxyl on solids has an equivalent weight of 850, so functionality is roughly 12 - comfortably enough. A resin with an Mn of 3 000 and 1.0% hydroxyl has an equivalent weight of 1 701, so functionality is under 2, and a meaningful proportion of its chains carry no hydroxyl at all. Same chemistry, entirely different network.
This is why molecular weight belongs on your comparison sheet alongside hydroxyl content, and why a low-hydroxyl grade from one supplier can outperform a nominally identical grade from another. Published work in Progress in Organic Coatings and comparable journals covers the relationship between functionality distribution, network defects and mechanical performance in detail if you want the underlying theory.
⚖️ Why Acid Value Sits Next to Hydroxyl Content
Waterborne acrylics carry carboxylic acid groups as well as hydroxyls, because neutralised acid groups are what make the resin dispersible without a permanent emulsifier. Acid value and hydroxyl content are therefore designed together, and they pull in opposite directions.
Raise the acid value and dispersion stability improves, but the cured film retains more hydrophilic character and water resistance suffers. Lower it and water resistance improves, but the dispersion becomes harder to stabilise and more sensitive to shear, pH drift and hard water. A resin's acid value is not a quality indicator you should try to minimise - it is a designed parameter, and comparing it across suppliers without also comparing stability behaviour tells you very little.
⚗️ One practical consequence worth flagging: carboxylic acid groups are also reactive toward isocyanate, though much more slowly than hydroxyls. At high acid values a small share of your hardener is consumed by that side reaction rather than by the crosslinks you paid for.
🧪 How the Number Is Actually Measured
Classical hydroxyl determination is a wet chemical procedure: the sample is acetylated or phthalated so that the reagent reacts with every hydroxyl present, the unreacted excess is back-titrated, and the difference gives the hydroxyl content. The established methods are ASTM D4274, published by ASTM International, and ISO 14900 and ISO 4629, available through the ISO Online Browsing Platform.
Two practical points follow from how these methods work.
Water interferes. Acetylating reagents react with water as readily as with hydroxyl. A waterborne dispersion therefore cannot be titrated as supplied - the solid resin has to be isolated and dried first, or the value has to be derived from the monomer charge used in the synthesis. That is one practical reason the on-solids convention dominates for this product family: it is the basis on which the measurement is actually made.
Not every supplier titrates. Many state hydroxyl content as a theoretical figure calculated from the monomer recipe. That is a legitimate and usually accurate approach, but it is a nominal value rather than a measured one, and it will not capture batch-to-batch variation in monomer conversion. If hydroxyl content is critical to your formulation window, ask whether the figure on your COA is calculated or determined - our guide to reading a resin TDS and COA covers what else to check on each batch.
📋 Five Questions Before You Formulate
- ✅ Is the hydroxyl content stated on solids or as supplied? Get this in writing. Everything else depends on it.
- ✅ Is it calculated or determined? Calculated is normal; you just need to know which you have.
- ✅ What is the tolerance? A hydroxyl content with no stated range is not a specification.
- ✅ What is the molecular weight? Without it you cannot judge functionality, and two resins at the same OH% may behave differently.
- ✅ What is the acid value, and which amine neutralises it? This governs both dispersion stability and the water resistance of the cured film.
❓ Frequently Asked Questions
Q1. Is hydroxyl number the same as hydroxyl value?
Yes. "Hydroxyl number", "hydroxyl value", "OH value" and "OH number" all mean the same thing: milligrams of KOH equivalent per gram of sample. Only "hydroxyl content" expressed as a percentage is a different unit - and it converts by multiplying by about 33.
Q2. My supplier quotes hydroxyl number and I need OH%. Is dividing by 33 accurate enough?
Yes, for formulation purposes. The exact factor is 32.989, and the difference from 33.0 is well inside the tolerance band of any commercial hydroxyl specification. Where precision does matter is the basis question, not the rounding.
Q3. Why would anyone choose a low hydroxyl grade?
Because for a large share of real applications it is the correct engineering answer, not a compromise. Flexible substrates crack under a tight network. Decorative topcoats gain nothing from chemical resistance they will never meet. And the low grade consumes substantially less hardener, which is usually the most expensive component in the mixed system. Higher hydroxyl is more resistance, more cost and less flexibility - you should only buy it when the service condition demands it.
Q4. Can I blend two grades to hit an intermediate hydroxyl content?
Usually yes, and it is a legitimate way to fine-tune within a range. Hydroxyl content blends approximately linearly on a solids-weighted basis. Two cautions: the resins must be genuinely compatible, which needs checking at your intended ratio rather than assuming, and blending averages the hydroxyl content without averaging the functionality - the low-functionality fraction of the lower grade does not disappear. Test panels, do not just calculate.
Q5. My film is too brittle. Should I drop to a lower hydroxyl grade?
It is one option, but check the cheaper explanations first. An excessive NCO:OH index over-crosslinks a film just as effectively as a higher hydroxyl grade does, and coalescent that has fully evaporated from an already-tight network leaves it more brittle than the test panel suggested at 24 hours. Verify your index and your cure schedule before changing resin - a grade change is the most expensive way to solve a problem that may be a ratio error.
Q6. Does hydroxyl content change during storage?
The hydroxyl groups themselves are stable - they do not decay on the shelf. What can change is everything around them: pH drifts as the neutralising amine slowly escapes, viscosity moves, and in the worst case the dispersion destabilises. So a resin near the end of its shelf life may formulate differently even though its hydroxyl content is unchanged. Store within the stated temperature range in sealed original containers, and never allow a waterborne dispersion to freeze.
📚 Continue Reading
What Is Waterborne Hydroxyl Acrylic Resin?
The complete guide to acrylic polyols for 2K waterborne polyurethane - chemistry, grades and what water changes.
Read the guide →NCO:OH Ratio and Hardener Demand
Turning the hydroxyl content you have just confirmed into a hardener weight, with worked examples.
Read the guide →When a Film Fails Its Resistance Tests
A diagnostic order for soft films, pinholing, foam and haze - starting with the causes that explain most failures.
Read the guide →Also see: Waterborne Hydroxyl Acrylic Resin (2K Polyol) · Waterborne Polyurethane Dispersion (PUD) · All Coatings & Inks Chemicals
📩 Data Sheets With the Hydroxyl Basis Stated Explicitly
Tell us the substrate, the service condition the finished surface has to survive, your application method, and whether mixing will be done on a line or by hand. We will recommend one or two hydroxyl grades and the hardener type to pair them with. Documentation states the hydroxyl basis on the face of the sheet rather than leaving you to assume it, and includes SDS, co-solvent content, VOC figures for your market and laboratory samples. We reply within 24 hours.
🔗 View the full Waterborne Hydroxyl Acrylic Resin product page →
Xiamen Sinolook Oil Co., Ltd. - Waterborne Hydroxyl Acrylic Resin (WAR). Hydroxyl content is stated on solid resin. Conversion factors and worked figures in this article are general chemistry and are provided for guidance; the hydroxyl contents shown in the conversion table are illustrative values chosen to demonstrate the arithmetic and are not the specification of any particular grade. Confirm all figures against the technical data sheet and certificate of analysis for the grade supplied before formulating. Polyisocyanate hardeners are respiratory sensitisers - follow the hardener manufacturer's safety data sheet and applicable national regulations. Do not allow waterborne dispersions to freeze.
