Polyurethane is hydrophobic - it does not want to mix with water. Yet a waterborne polyurethane dispersion (PUD) stays as fine, stable particles suspended in water for months. The secret is the internal emulsifier: a water-loving group chemically bonded into the polymer chain. We introduced this idea when explaining how PUD is made; here we go deeper into the ionomer chemistry that makes a polyurethane self-emulsifying.
🧲 Internal vs External Emulsifier: Why "Built-In" Wins
You could, in principle, stabilise polyurethane particles by adding a conventional surfactant (an external emulsifier) - but that comes at a cost. External surfactant molecules are not bonded to the polymer, so they stay mobile in the dried film, where they tend to bloom to the surface and attract water, hurting water resistance and gloss.
An internal emulsifier is different: the hydrophilic group is a permanent part of the polymer backbone. It cannot migrate out, so the film keeps its water resistance while the wet dispersion stays stable. ✅ This is why virtually all high-performance PUD grades rely on internal emulsification.
⚗️ What Is an Ionomer?
When the built-in hydrophilic group is ionic (carries a charge), the resulting polymer is called an ionomer - a polymer with a small fraction of ionic groups along an otherwise non-ionic chain. In PUD, those ionic sites do two jobs at once: they make the polymer disperse in water, and they generate the electrostatic repulsion that keeps particles from clumping together. The type and amount of ionic group is one of the main levers a chemist uses to control particle size and stability.
🧱 Carboxylic Emulsifiers: DMPA and DMBA
By far the most common internal emulsifiers are carboxylic-acid diols. These molecules carry two hydroxyl (–OH) groups that react into the polyurethane backbone, plus a pendant carboxylic-acid (–COOH) group that becomes the water-loving anchor:
- 🔹 DMPA (dimethylolpropionic acid) - the industry workhorse. Its two –OH groups build it firmly into the chain while the –COOH provides hydrophilicity. Reference data: DMPA on PubChem.
- 🔹 DMBA (dimethylolbutanoic acid) - a close relative with an extra carbon. It is more soluble and reactive, often allowing lower use levels and lower processing viscosity, at a higher raw-material cost.
Both are added during prepolymer formation so the acid groups end up evenly distributed along the chain, ready to be activated.
🔓 Neutralisation: Switching the Emulsifier "On"
A bare –COOH group is only weakly hydrophilic. To activate it, the acid is neutralised with a base - commonly a tertiary amine such as triethylamine (TEA), or ammonia - converting –COOH into a charged carboxylate (–COO⁻). Those negative charges are what truly drive dispersion and repulsion.
💡 The degree of neutralisation is a key formulation dial: too little and the polymer disperses poorly; too much and you can over-thin or compromise water resistance. Because the base is often a volatile amine, it can also affect the final VOC and odor profile - one more reason technical data sheets should be read honestly rather than assumed to be solvent-free.
⚡ Sulfonate Emulsifiers: A Stronger Alternative
Instead of carboxylate groups, some grades use sulfonate (–SO₃⁻) diols as the internal emulsifier. Sulfonate groups are more strongly ionic and stay charged across a wider pH range, which can give:
- ✅ Finer particle size and better colloidal stability;
- ✅ Less dependence on volatile neutralising amine;
- ⚠️ Usually a higher raw-material cost than DMPA.
Sulfonate and carboxylate types are both anionic. PUD can also be made cationic or nonionic - a classification we cover fully in anionic vs cationic vs nonionic PUD.
🌐 Nonionic Hydrophilic Segments
A third route uses nonionic hydrophilic segments - typically polyethylene-oxide (PEO) chains built into the backbone. Instead of charge repulsion, these provide steric stabilisation: the water-swollen PEO chains physically prevent particles from touching. Nonionic groups are less sensitive to electrolytes and pH, and are often combined with ionic groups to make robust, dual-stabilised dispersions. The trade-off is that too much PEO can leave the dried film more water-sensitive.
🔬 How the Emulsifier Keeps Particles Apart
Once dispersed, each particle carries hydrophilic groups on its outer surface. Two mechanisms then keep the dispersion from collapsing:
- ⚡ Electrostatic stabilisation - like-charged particles (from carboxylate or sulfonate groups) repel each other.
- 🛡️ Steric stabilisation - bulky nonionic chains form a physical barrier.
The balance of these forces sets the particle size and shelf stability - which then feed directly into film formation, explored in particle size & MFFT in PUD.
It is tempting to think adding more hydrophilic groups makes a "more stable" PUD. In reality it is a trade-off: higher hydrophilic content improves dispersion and stability but increases the dried film's water sensitivity. Good grades use the minimum internal emulsifier needed for stability, then rely on backbone choice and crosslinking for performance - not on drowning the polymer in ionic groups.
💡 Frequently Asked Questions
🔹 What does DMPA do in a polyurethane dispersion?
DMPA is an internal emulsifier. Its two –OH groups bond it into the polyurethane chain, while its –COOH group - once neutralised - becomes the water-loving carboxylate that lets the polymer disperse and stay stable in water.
🔹 What is the difference between DMPA and DMBA?
DMBA has one more carbon than DMPA, giving it better solubility and reactivity. That can mean lower use levels and easier processing, usually at a higher price.
🔹 Why not just use a normal surfactant?
External surfactants are not bonded to the polymer, so they migrate in the dried film and attract water, hurting water resistance. Internal emulsifiers stay locked in the chain and avoid this.
🔹 What is neutralisation and why is it needed?
Neutralisation uses a base (often TEA or ammonia) to convert the weak –COOH acid group into a charged carboxylate, which is what actually drives dispersion and particle repulsion.
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