A viscosity number travels well and means almost nothing. Two creams can read 5,000 cP on the same spindle and behave nothing alike: one spreads and rebuilds, the other drags and never recovers; one survives three freeze-thaw cycles, the other splits in its first summer. The number matches because three different physical structures can produce it, and which structure you bought decides everything else.
This guide separates the three routes that matter for emulsions and anhydrous systems, gives the process window each one needs, and ends with the lines that belong on the purchase order. All three are usually ordered by product name alone, which is exactly how a formulator ends up with the wrong one.
Three mechanisms, three failure modes
| Route | What actually builds the viscosity | Process | Typical level | Fails when |
|---|---|---|---|---|
| Lamellar gel network | Fatty alcohol crystallites swelling in water, stabilised by an anionic surfactant | Hot, 70-75 °C, controlled cooling | 3-10% of the blend | Cooling too fast; cationic actives; electrolyte |
| Inverse-emulsion polymer | Water-swollen acrylamide copolymer released from an oil carrier | Cold, after emulsification | 0.5-2% | Too little shear; pH below 4; high electrolyte |
| Oil-phase crystallisation | Comb copolymer with C18/C22 side chains forming a yield stress in oil | Hot, 80-90 °C, slow cooling | 2-8% of the gellant | Wrong monomer; shear during crystallisation |
Two of the three are bought as finished materials. The third is bought as a monomer, which is the part most purchasing departments never see.
Route one: the lamellar gel network
The classic self-emulsifying wax is cetearyl alcohol blended with sodium lauryl sulfate, CAS 67762-27-0 and 151-21-3, sold as a flake that melts at roughly 50-55 °C. The alcohol is not there as an emollient and the SLS is not there as a foaming agent. The pair forms lamellar sheets: bilayers of fatty alcohol with the sulfate head groups facing the water, and those sheets stack into a gel phase that gives the finished cream its yield point.
This is why the HLB of the blend is such a poor predictor of how thick the result will be. Viscosity comes from the crystalline network, not from the curvature of an interface, and the network is set by thermal history. A batch cooled from 75 °C to 40 °C in twenty minutes and a batch cooled over two hours can be the same formula and 30% apart in viscosity. If a customer reports that a cream got thinner without a formula change, look at the cooling ramp before you look at the raw material.
- Use 3-5% for lotions and 5-10% for creams. The flake melts at 50-55 °C, so heat both phases to 70-75 °C and hold long enough for the wax to be fully melted, not merely dispersed.
- Hold the finished emulsion at pH 5-7. The sulfate is fully ionised across that window and the gel phase is stable; below pH 4 the anionic charge falls away and the network weakens.
- Expect incompatibility with cationic conditioning agents. The SLS complexes with quaternary ammonium compounds and the structure collapses, so a cationic cream needs a non-ionic or cationic self-emulsifying base instead.
- Watch the electrolyte. Sodium chloride at 1-2% narrows the lamellar region; at higher levels the gel phase gives way and the emulsion thins or splits.
Route two: the inverse-emulsion polymer
Acrylamide/Ammonium Acrylate Copolymer (and) Mineral Oil (and) Laureth-7, CAS 26100-47-0 with 8042-47-5 and 9002-92-0, arrives as a milky white liquid at 60% active content. The polymer is not dissolved in the drum. It sits as swollen aqueous droplets dispersed in cosmetic-grade mineral oil, held there by Laureth-7. Stir the liquid into water, the surfactant inverts the emulsion, the droplets release the polymer, and the polymer uncoils into a thickening network.
That inversion step is the whole trick, and it is where most production problems start. Poured into a hot oil phase, or into water that is not being stirred, the emulsion never inverts cleanly and the batch shows fish eyes that cannot be worked out. Added to the water phase under agitation, after the emulsion is formed and below 40 °C, 1% gives a smooth, glossy gel.
- 0.5-2% is the working range. At 2% the result is a thick cream; at 0.5% it is a flowable lotion that still suspends pigment.
- The useful pH window runs from mildly acidic to alkaline. The thickener is an ammonium carboxylate, so below roughly pH 4 the salt protonates, the charge collapses and viscosity falls with it. Above pH 12 the polymer slowly hydrolyses.
- Electrolyte tolerance is better than a carbomer but not unlimited. High-salt scrubs and aluminium-based antiperspirants need a bench trial before the batch sheet is written.
- The carrier is mineral oil. If the brief says oil-free or petrolatum-free, this grade is not eligible, whatever the INCI name suggests.
Route three: crystallise the oil phase
Anhydrous gels, lipstick cores and oil serums are not thickened by water-side polymers. They are thickened by crystallising a small fraction of the oil phase, and the most efficient way to do that is with a comb copolymer whose side chains are long enough to crystallise and whose backbone is long enough to hold a network together. Those copolymers are made from alkyl (meth)acrylate monomers: behenyl methacrylate (CAS 16669-27-5, C26H50O2, MW 394.7) supplies a C22 side chain and stearyl acrylate (CAS 4813-57-4, C21H40O2, MW 324.6) a C18 one.
This is the route that hides in the supply chain. The formulator buys the gellant, the gellant maker buys the monomer, and the properties of the gellant - yield point, clarity in oil, resistance to syneresis at 45 °C - inherit almost everything from the monomer specification. A C22 side chain crystallises at a higher temperature than a C18 one, so the same backbone with more behenyl gives a firmer gel and a higher melting point, and a blend of the two gives formulators a dial rather than a switch.
- Buy the monomer by solidification point and monoester content. Residual diacrylate acts as a crosslinker during polymerisation and turns a pourable gellant into a rubbery one that will not spread.
- Check the inhibitor. These monomers ship with a phenolic inhibitor such as MEHQ to stop them polymerising in the drum. The level is part of the specification, and an under-inhibited drum can gel in a warm warehouse.
- Specify storage below 30 °C and keep the drum sealed. The materials are waxy solids at ambient temperature in most climates and will stratify if allowed to melt and re-solidify repeatedly.
- Ask for the residual acrylic acid figure. It drives odour, and it is the number a customer's safety assessor will ask about.
Matching the route to the process
| Requirement | Best fit | Why |
|---|---|---|
| Cold-processed lotion, high water content | Inverse-emulsion polymer | No heat; viscosity built at ambient temperature |
| Rich cream with a defined melting sensation | Lamellar gel network | The gel phase melts on skin contact, and that is the sensation |
| Transparent anhydrous gel | Oil-phase crystallisation | Refractive index matched to the oil; no water to scatter light |
| High-electrolyte or AHA system | Inverse-emulsion polymer, tested | Carbomer and lamellar gels both lose structure here |
| Cationic conditioner base | Neither of the first two | Sulfate and anionic polymer both complex with quats |
Four lines for the purchase order
- The INCI string in full, not the trade name. "Self-emulsifying wax" describes at least four different blends, and only the INCI tells you whether the surfactant is SLS, sodium cetearyl sulfate or a glucoside.
- The active content and the carrier. For the inverse-emulsion polymer, 60% active in mineral oil is the common grade; a different dilution changes your use level, and the carrier decides whether an oil-free claim is possible.
- The process window you will run. Say cold process, pH 6.5, 1.5% electrolyte, and a supplier can tell you in one email whether the grade will hold. Say nothing, and the answer arrives as a split batch.
- The monomer constants, if you are buying monomers: solidification point, monoester content, inhibitor level, residual acrylic acid.
Supply
Shanghai Better Chemical supplies all three routes from China, with COA, SDS and TDS issued against the application you declare:
- Cetearyl Alcohol (and) Sodium Lauryl Sulfate (CAS 67762-27-0; 151-21-3) - anionic self-emulsifying wax for O/W creams and lotions, 3-10%.
- Acrylamide/Ammonium Acrylate Copolymer (and) Mineral Oil (and) Laureth-7 (CAS 26100-47-0; 8042-47-5; 9002-92-0) - cold-process thickener, 60% active, 0.5-2%.
- Behenyl Methacrylate (CAS 16669-27-5) - C22 monomer for oil-phase gellants and film-formers.
- Stearyl Acrylate (CAS 4813-57-4) - C18 monomer for the same polymers.
The aqueous side of the same problem is covered in our guides to xanthan gum in cosmetics and home care and to cetearyl alcohol as a co-emulsifier. Tell us the process, the pH and the electrolyte load, and we will confirm which route holds before you commit to a drum.