Put two 25 kg bags of zinc stearate side by side. Both carry CAS 557-05-1, both pass 325 mesh, both soften at 118–125 °C, and both come with a certificate of analysis that agrees on every line. One occupies 114 litres on the pallet. The other occupies 50. Load them into the same 40-foot high cube and the first container ships 16.7 tonnes while the second ships 20 — a 16% loss of payload that appears nowhere on the quotation.
The number responsible is bulk density: the mass of powder per unit of volume as poured. It is a real, specified, supplier-controlled figure, it varies threefold between grades of one CAS, and on most purchase orders it is simply missing.
Three densities, and only one of them is a purchase decision
Chemistry fixes one of these. Processing fixes the other two, which is why two suppliers quoting the same CAS can ship products that behave like different materials.
| Quantity | What it measures | Zinc stearate, typical | Who sets it |
|---|---|---|---|
| True (particle) density | Solid matter per unit volume of the particle itself | 1.095 g/cm³ | Chemistry. CAS 557-05-1 fixes it. |
| Bulk (loose) density | Mass per unit volume of powder as poured, voids included | 170–500 g/L | Particle size distribution, agglomeration, surface treatment, drying route |
| Tapped density | The same, after a defined number of taps | typically 1.2–1.6 × bulk | The same variables, plus flow behaviour |
The ratio between the first two gives the void fraction: voids = 1 − bulk density ÷ true density. At 220 g/L that is 1 − 0.220 ÷ 1.095 = 80%. A bag of light-grade zinc stearate is four-fifths air by volume. That is not a defect — it is the physical consequence of making a very fine, very light, very high-surface-area powder, and it is precisely what makes that powder disperse well.
The trap is that one CAS can be made either way. Producers distinguish a light grade at roughly 200 g/L from a heavy grade at roughly 500 g/L, and commercial spec sheets carry bulk density as a controlled line rather than a footnote: one supplier's grades are listed at 220–300, 230–300, 300–400 and ≥400 g/L, another's at ≤360, ≤450 and ≥400 g/L, with a bead form at ≥400 g/L. Nothing in the CAS, in the INCI or in the mesh number tells you which of those you are buying.
Decision one: how much fits in the container
A 25 kg bag takes 25,000 ÷ bulk density litres. That is the whole calculation.
| Bulk density | Volume of one 25 kg bag | 40' high cube (76 m³) at a 20 t booking | What limits the load |
|---|---|---|---|
| 170 g/L | 147 L | 12.9 t | Volume |
| 220 g/L | 114 L | 16.7 t | Volume |
| 300 g/L | 83 L | 22.8 t by volume → capped at 20 t | Weight |
| 400 g/L | 63 L | 20 t in 50 m³ | Weight |
| 500 g/L | 50 L | 20 t in 40 m³ | Weight |
Below roughly 300 g/L the container cube fills before the weight does, and from that point onward you are paying ocean freight for air. The gap between 220 and 500 g/L is 3.3 tonnes per container at a fixed 20 tonne booking — on a USD 2,000 freight that is a 16% swing in landed cost, before anyone has discussed the price per kilogram.
The same figure decides warehouse economics. 100 tonnes at 220 g/L needs about 455 m³ of racking; at 500 g/L it needs 200 m³. Where storage is charged by the pallet position, that difference is usually larger than the price the buyer negotiated.
Decision two: whether the silo works
Bulk density together with particle shape sets the angle of repose and the flow function, and those decide whether a hopper discharges or bridges. A light, fluffy, high-surface-area powder has strong inter-particle cohesion: it rat-holes, it arches over the outlet, it fluidises unpredictably and then floods. The same chemistry in bead or granular form at ≥400 g/L flows like sand and can be fed gravimetrically.
This is the practical reason a supplier catalogue carries both a powder range and a bead range of the same stearate. The bead costs more per kilogram and is worth it on any line with an automatic dosing system, because the alternative is an operator with a scoop and a variance nobody can measure.
Decision three: the dust, and the number that is not on the SDS
Light powder is not only a handling nuisance. Zinc stearate is a combustible dust, and its lower explosive limit is quoted at 11.6 g/m³ in Chinese reference sources and at 20 g/m³ on the IPCS International Chemical Safety Card 0987. Take the conservative figure. A 6 × 6 × 3 m charging room holds 108 m³, so 1.3 kg of powder fully suspended in that room is already at the lower explosive limit; at the 20 g/m³ reading it is 2.2 kg. One dropped and burst 25 kg bag in a closed room is an order of magnitude past either number.
The discrepancy is itself worth carrying into procurement. The same safety card lists auto-ignition at 420 °C while widely circulated Chinese entries quote 900 °C, and neither is necessarily wrong: a dust cloud and a dust layer ignite at very different temperatures, and a number without the test method behind it is not a specification. If a safety case rests on it, ask for EN 14034 or VDI 2263 data rather than a figure copied from a web page.
The engineering answer is not a bigger extractor. It is to buy the dust out of the process — bead or granular form, a 40% aqueous dispersion, or a liquid.
Decision four: how accurately you can dose it
A typical stearate dose is 0.2–1.0 phr. On a 500 kg batch, 0.2 phr is 1 kg and 0.1 phr is 500 g; a bench scale reading to ±5 g is ±1% relative, which is fine. Run the same scale on a 20 kg trial batch, where 0.1 phr is 20 g, and the error is ±25%. Small batches, not large ones, are where powder weighing fails — and they are exactly where development, colour matching and specialty runs happen.
Liquids remove the problem by moving dosing from a balance to a pump. That is the entire commercial case for the liquid antioxidant family, and it is a more honest argument than the chemistry.
The form ladder
Every step down this table trades a handling cost for a formulation cost. The right rung is the one your process is already on.
| Form | Bulk density | What it solves | What it costs |
|---|---|---|---|
| Light powder | 170–250 g/L | Fastest wetting and dispersion, highest surface area, best matting and anti-blocking | Dust, volume, poor flow, bridging |
| Heavy powder | 350–500 g/L | Freight and storage volume, visibly less dust | Slower wetting; can leave specks where shear is low |
| Bead / granule | ≥400 g/L | Free-flowing, gravimetric feeding, low dust | Needs enough shear or heat to break down |
| Aqueous dispersion | ≈1,000 g/L as supplied | No dust at all; blends directly into water-based systems | Water enters the formula; freeze sensitivity; finite shelf life |
| Liquid, 100% active | 0.95–1.00 g/cm³ | Pumpable, meterable into a stream, no dust | Carrier compatibility; often higher cost per active |
The liquid antioxidants, and why they exist
Antioxidant 1135 — the grade we list as liquid hindered phenolic 535 — is the cleanest example. It melts below 10 °C, sits at 0.95–1.00 g/cm³, has a flash point near 152 °C and a viscosity quoted anywhere from 70 to 600 mPa·s depending on grade and temperature, and is dosed at 0.15–0.5%. In a polyether polyol it dissolves; on a flexible foam line it can be metered into the stream; from a drum it can be pumped. Its function is not exotic — it stops peroxide build-up during polyol storage and scorching during foaming — but the reason it exists as a liquid at all is that a solid cannot be added to a liquid polyol stream at 0.2% without a melting step and a dispersion step.
The same logic explains the rest of the liquid line: TNPP (CAS 26523-78-4) for polyolefin processing, its phenol-free replacement TTDP (CAS 77745-66-5), and the high-molecular-weight phenol-free blend L-20, which has no single CAS precisely because it is a blend. Two procurement notes follow. First, a liquid blend must be bought on active content or on phosphorus content, because the CAS tells you nothing — L-20 is the case in point. Second, TNPP was added to the ECHA SVHC candidate list on 21 January 2025 over the environmental endocrine-disrupting properties of the nonylphenol it can contain, so the same CAS can carry different regulatory risk depending on the free nonylphenol figure on the COA. The substitution to TTDP is a purchasing decision as much as a technical one.
The emulsion trap: dry basis and the water you forgot
A 40% aqueous zinc stearate dispersion is the right answer for latex, water-based coatings and paper finishing, and its arithmetic is unforgiving. If the formula calls for 0.5 phr of zinc stearate on a dry basis, you add 0.5 ÷ 0.40 = 1.25 phr of dispersion — and you have just introduced 0.75 phr of water into a system that may already be at its water limit. On a 100 kg latex batch that is 0.75 kg of water: small, but it moves total solids, viscosity and the film-formation temperature, and it is the most common reason a first trial at replacing powder with dispersion fails.
The second trap is storage. An aqueous dispersion is not a drum of powder with water added; it is a colloidally stabilised system with a shelf life, a temperature window and a sedimentation behaviour. Buy it against a date code, not against a price.
Waxes: form and molecular weight move together
Waxes are the case where form and chemistry cannot be separated. A Fischer-Tropsch wax (CAS 8002-74-2) is a straight-chain saturated paraffin of roughly 500–1,000 molecular weight with a narrow melting range, a dropping point around 100–115 °C, a 25 °C needle penetration of 5 or below and a melt viscosity of only 5–10 mPa·s at 140 °C. A polyethylene wax (CAS 9002-88-4) runs 2,000–5,000 molecular weight, with a wider melting range and 20–60 mPa·s at the same 140 °C. The dosing consequence, quoted consistently across suppliers: the FT wax delivers the same lubrication at 70–80% of the PE wax dose.
Both are sold as flake, granule or powder — three forms with different bulk densities, different feeding behaviour and different melting rates in the hopper. A flake that bridges in a small hopper will feed perfectly as a granule, and neither change appears on the certificate of analysis.
What to put on the purchase order
- Bulk density with the method: "bulk density 250–300 g/L, ISO 3923-1 / ASTM B527". Without the method the number is not comparable between suppliers.
- Form and particle size: powder, bead, granule or flake, plus the mesh or micron cut — and remember that mesh is a sieve opening, not a dispersed particle size.
- True density if you are calculating compound density or cost per litre; it is fixed by chemistry and should not vary between suppliers of the same CAS.
- Active content for liquids and dispersions, stated as a percentage or as the elemental figure, with the carrier named.
- Melting point or dropping point with the test method, because it decides whether the additive is solid or liquid at your process temperature — which decides where in the line it can be added.
- Dust data if the material is handled open: lower explosive limit, minimum ignition energy, and the standard used.
- Shelf life and storage window for dispersions and liquids, as a date code rather than "store in a cool, dry place".
For how to choose between the zinc and calcium soaps and what mesh size means for each application, see our metallic stearate selection guide; for why molecular weight, not chemistry alone, sets how long a stabiliser survives, the antioxidant selection by failure mode; and for the container and drum side of the same problem, our chemical packaging compatibility guide.
Supply
On the stearate side we supply zinc stearate 200 mesh, 325 mesh, 600 mesh and 800 mesh (all CAS 557-05-1), calcium stearate 100 mesh and 325 mesh (CAS 1592-23-0), barium stearate (CAS 6865-35-6), zinc laurate (CAS 2452-01-9) and the dust-free option, zinc stearate emulsion 40% (CAS 557-05-1).
On the liquid side: liquid hindered phenolic 535 (CAS 125643-61-0), TNPP (26523-78-4), the phenol-free TTDP (77745-66-5) and L-20. On the wax side: Fischer-Tropsch wax (CAS 8002-74-2) and PE wax 9038A (CAS 9002-88-4), both available as flake and as powder.
All grades are available for B2B supply with sample quantities shipped worldwide. Bulk density test reports, tapped density data and dust hazard documentation can be requested through the enquiry form on any product page, and we will quote against a stated bulk density band rather than a single line if that is what your silo or your freight calculation needs.