AOS in Emulsion Polymerisation

Sodium alpha-olefin sulfonate (AOS, CAS 68439-57-6) is used as an anionic emulsifier in emulsion polymerisation, where it stabilises monomer droplets and polymer particles and helps control particle size. Its sulfonate stability and electrolyte tolerance make it useful in latices that sulfates struggle with. This article covers how AOS is used as an emulsifier and what it contributes to the latex.

1. What an emulsifier does in emulsion polymerisation

In emulsion polymerisation, the emulsifier (surfactant) does three jobs: it emulsifies the monomer, stabilises the growing polymer particles, and controls the number and size of the particles. Anionic emulsifiers such as AOS give electrostatic stabilisation; they are often combined with a nonionic emulsifier for a balance of stability and performance.

2. Why AOS

Property	Benefit as an emulsifier
Sulfonate head group	Strong electrostatic stabilisation; stable across pH
Hard-water tolerance	No precipitation with calcium/magnesium in the process water
Electrolyte tolerance	Robust in latices that carry salts
Controlled particle size	Higher emulsifier gives smaller particles / more particles
No ethoxylation	No 1,4-dioxane concern; useful for clean-label latices

3. Typical systems

Latex	Typical use of AOS
Acrylic and styrene-acrylic	Anionic emulsifier for coatings, adhesives and binders
Vinyl acetate / VAE	Co-emulsifier to improve stability and particle size
Styrene-butadiene (SBR latex)	Anionic emulsifier in the polymerisation recipe
Paper and board coatings	Latex binder stability
Textile and non-woven binders	Latex stability against electrolytes in the finish bath

4. What AOS affects in the latex

Property	Effect of emulsifier level / type
Particle size	Higher emulsifier level generally gives smaller particles
Viscosity	Rises as particle size falls (more particles)
Mechanical stability	Good electrostatic stabilisation resists shear
Chemical / electrolyte stability	Sulfonate tolerance helps in salt-bearing systems
Film properties	Emulsifier level affects water sensitivity and adhesion

There is always a balance: more emulsifier gives smaller particles and better stability but can increase water sensitivity and foaming in the finished latex. Optimise the emulsifier type and level against your target particle size, viscosity and film properties.

5. Dosage (starting points)

Parameter	Typical practice
Emulsifier on monomer	About 0.5 – 3% (anionic, often with a nonionic co-emulsifier)
Anionic : nonionic ratio	Set by the target particle size, stability and film properties
Feed	Part in the initial charge, part in the monomer / pre-emulsion feed

These are starting points; the recipe is developed for the specific monomer system and target latex.

6. AOS vs other anionic emulsifiers

Emulsifier	Notes
AOS	Sulfonate; hard-water and electrolyte tolerant; no EO
Sodium lauryl sulfate (SLS)	Sulfate; widely used; can hydrolyse in aggressive conditions; more irritating
Sodium dodecylbenzene sulfonate (SDBS / LAS-Na)	Sulfonate; cost-effective; hard-water tolerance lower than AOS
Nonionic (alcohol ethoxylates)	Often blended with anionic for stability and film properties
Reactive / polymerisable emulsifiers	Bond into the polymer; better water resistance, higher cost

Frequently Asked Questions

Is AOS used as an emulsifier in emulsion polymerisation?

Yes – it is an anionic emulsifier used to stabilise monomer droplets and polymer particles and to control particle size, often with a nonionic co-emulsifier.

How much AOS is used?

Typically about 0.5-3% on monomer as a starting point, split between the initial charge and the feed, optimised for the target particle size and stability.

What does emulsifier level do to the latex?

More emulsifier generally means smaller particles and higher viscosity; too much can raise water sensitivity and foaming. Optimise against your target properties.

Why AOS instead of SLS or SDBS?

AOS tolerates hard water and electrolytes better and is stable across pH; unlike ethoxylated emulsifiers it carries no 1,4-dioxane concern.