Dispersant Specification: Particle Size Reduction, Zeta Potential and Pigment Wetting Efficiency

Overview #

The specification parameter that most procurement teams get wrong when sourcing industrial dispersants from China is not active content percentage — it’s the combination of minimum zeta potential threshold and particle size reduction efficiency under your specific pigment loading conditions. A dispersant that delivers –35 mV zeta potential at 20% pigment loading in a lab sample can collapse to –18 mV at 45% loading in production, and no standard COA will flag that unless you specify the test conditions explicitly. When we evaluate Chinese dispersant suppliers, the first document we request is not the TDS — it’s the zeta potential vs. concentration curve across the full loading range the buyer intends to use.

Critical Selection Criteria: The Six Parameters That Change the Recommendation #

1. Zeta Potential at Operating Pigment Loading #

Zeta potential is the single most predictive parameter for long-term dispersion stability, and it is also the most commonly misrepresented on Chinese supplier TDS documents. The threshold that matters: a stable dispersion requires zeta potential ≤ –30 mV (or ≥ +30 mV for cationically stabilized systems). Values between –25 mV and –30 mV indicate marginal stability — acceptable for short shelf-life coatings, unacceptable for inks or pigment concentrates with a 12-month storage requirement.

What most buyers do not specify — and should — is the pigment loading at which zeta potential is measured. We have seen Chinese suppliers report –42 mV at 10% TiO₂ loading, which looks excellent, but the same product at 50% loading (the buyer’s actual production condition) measures –22 mV. That is a formulation failure waiting to happen, and it will not appear on any incoming inspection unless you specify the test condition in your purchase order.

Per ASTM International method ASTM E2865, zeta potential measurement should be conducted at the actual use concentration, not at a diluted reference condition. Require this explicitly on your TDS request.

2. Particle Size Reduction Efficiency (D50 and D90) #

Particle size reduction efficiency is reported as D50 (median particle diameter) and D90 (90th percentile diameter) after a defined dispersion protocol. For high-performance coating applications, a well-formulated dispersant should achieve D50 ≤ 300 nm and D90 ≤ 800 nm for organic pigments at 30% loading after 60 minutes of bead milling at 3,000 rpm. For inorganic pigments such as TiO₂ or iron oxides, D50 ≤ 500 nm and D90 ≤ 1,200 nm are the relevant thresholds.

The D90 value is more operationally important than D50 for most coating and ink applications. Coarse particles above 1,500 nm cause gloss reduction, filter clogging, and nozzle blockage in digital printing. Most Chinese supplier TDS documents report only D50 — and often under idealized lab conditions. Always request D90 alongside D50, and specify the milling protocol and pigment type used for the measurement.

3. Pigment Wetting Efficiency: Wetting Time and Contact Angle #

Pigment wetting efficiency determines how quickly and completely the dispersant displaces air from pigment surfaces during the wetting-out phase. The relevant measurement is dynamic contact angle reduction: an effective dispersant should reduce the contact angle of water on a carbon black surface from approximately 120° to below 40° within 60 seconds at 0.5% active dispersant concentration.

Wetting time — measured as the time for a pigment powder to fully wet out in a defined liquid medium — should be ≤ 90 seconds for carbon black at 0.5% dispersant loading in water. Suppliers who cannot provide wetting time data under defined conditions are almost always relying on generic product descriptions rather than application-specific testing. This is a disqualifying gap for buyers sourcing dispersants for water-based ink or coating systems.

4. Molecular Weight and HLB Value #

For polymeric dispersants, molecular weight (Mw) determines the steric stabilization layer thickness and the anchoring efficiency on pigment surfaces. The optimal Mw range for most pigment dispersion applications is 5,000–25,000 g/mol. Below 3,000 g/mol, steric stabilization is insufficient for long-term stability. Above 40,000 g/mol, viscosity contribution becomes problematic in high-solids systems.

HLB (Hydrophilic-Lipophilic Balance) value governs whether the dispersant is suited for aqueous or solvent-borne systems. HLB 8–12 is the working range for water-based pigment dispersions; HLB 3–6 is appropriate for solvent-borne and UV-cure systems. Specifying HLB without specifying the measurement method is insufficient — require the Griffin method or Davies method and the specific value, not a range.

5. Acid Value and Amine Value #

For polycarboxylate and polyacrylate dispersants, acid value (mg KOH/g) determines the density of anchoring groups available for pigment surface interaction. The effective range for most inorganic pigment applications is 30–80 mg KOH/g. Below 20 mg KOH/g, anchoring is insufficient for basic pigment surfaces such as calcium carbonate or TiO₂. Above 100 mg KOH/g, the dispersant may cause excessive foam in water-based systems and compatibility issues with binder resins.

Amine value is the corresponding parameter for amine-functional dispersants used in solvent-borne and UV systems. Effective amine value range: 15–60 mg KOH/g. Require both acid value and amine value on the COA, with the titration method specified — ASTM International ASTM D974 for acid value and ASTM D2073 for amine value are the standard references.

6. Thermal Stability and pH Stability Range #

For dispersants used in high-temperature processing (ceramic slurries, industrial coatings cured above 150°C, or pigment concentrates processed at elevated temperatures), thermal stability is a critical parameter that is almost never specified on Chinese supplier TDS documents unless explicitly requested. A dispersant that degrades above 120°C will cause viscosity spikes and color shift in production — and the failure mode is intermittent, making root cause analysis difficult.

Require thermal stability data: the dispersant should maintain ≥ 90% of its initial zeta potential value after 4 hours at the maximum processing temperature. pH stability range should be specified as the operating window within which zeta potential remains ≤ –30 mV — for most polyacrylate dispersants, this is pH 7–10. Outside this range, performance degrades rapidly and is not recoverable by dosage adjustment.

Dispersant Type Comparison: Selection by Application #

The table above reflects performance ranges we observe across qualified Chinese suppliers under standardized test conditions. The nonionic surfactant-based category is the one most frequently oversold by Chinese suppliers for applications that require genuine steric or electrostatic stabilization — the zeta potential values simply do not support long-term stability in demanding applications.

Most procurement teams over-specify acid value and under-specify the parameter that actually drives production performance: the zeta potential vs. pigment loading curve across the full concentration range used in their process. A dispersant with a strong acid value on paper but a poorly designed polymer backbone will lose stability above 35% pigment loading, and that failure will not appear in a single-point COA measurement.

Qualification Testing and Lot-to-Lot Consistency #

In our supplier qualification program, we require three consecutive batch COAs before recommending a Chinese dispersant supplier for volume procurement. The parameters we track across batches are: active content (±1.5% tolerance), acid value (±5 mg KOH/g), and zeta potential at the buyer’s specified pigment loading (±3 mV). Suppliers who cannot provide this data across six months of production are not ready for qualification, regardless of how well the initial sample performs.

The incoming inspection protocol we recommend for dispersant procurement includes:

• Active content by non-volatile residue method per ASTM International ASTM D2369: tolerance ±1.5% of specified value
• Zeta potential at buyer-specified pigment loading: minimum –30 mV, measured per ASTM E2865
• Viscosity at 25°C (Brookfield, spindle and speed specified): ±10% of TDS value
• pH: ±0.3 units of specified value
• Particle size D90 of dispersed pigment: ≤ 800 nm for organic pigments, ≤ 1,200 nm for inorganic

The AQL level we apply for dispersant incoming inspection is AQL 1.0 for critical parameters (zeta potential, active content) and AQL 2.5 for secondary parameters (viscosity, pH, color). This is tighter than most buyers apply, but the cost of a dispersant failure in a coating or ink production run — rework, downtime, substrate waste — far exceeds the cost of incoming inspection.

In our qualification program, we have seen suppliers pass initial sample approval and then deliver out-of-spec material at production volume. The trigger is almost always a raw material substitution at the monomer or surfactant level — something that a standard COA will not catch without incoming zeta potential and particle size spot-testing. One case involved a polyacrylate dispersant where the supplier switched to a lower-purity acrylic acid source at volume, reducing acid value from 52 to 31 mg KOH/g. The buyer’s coating line ran for three weeks before the stability failure became visible as settling in the storage tanks. By that point, 40,000 liters of pigment concentrate were affected.

The SAC China Standards GB/T standard governing dispersant active content measurement (GB/T 6753.3) allows a ±2.5% tolerance on active content — wider than the ±1.5% we recommend for critical applications. Buyers who accept GB/T compliance as sufficient without specifying tighter tolerances in their purchase order are accepting a wider performance window than most coating and ink formulations can tolerate.

Compliance and Regulatory Considerations #

For dispersants used in food-contact coatings, packaging inks, or any application where ECHA REACH compliance is required, the SVHC (Substances of Very High Concern) status of the dispersant’s surfactant and monomer components must be verified. Several alkylphenol ethoxylate (APEO)-based dispersants that remain in production in China are restricted under REACH Annex XVII for textile and leather applications and are under increasing regulatory pressure for coatings applications in the EU market.

Require a full REACH SVHC declaration — not just a statement of compliance — listing all components above 0.1% w/w. Chinese suppliers frequently provide a generic “REACH compliant” statement that covers only the finished product without disclosing individual component SVHC status. This is legally insufficient for EU import and will not satisfy a customer audit.

For water-based dispersants used in food-contact paper coatings or direct food-contact applications, FDA Guidelines 21 CFR 176.170 (components of paper and paperboard in contact with aqueous and fatty foods) governs the permitted substances. Not all Chinese dispersant suppliers maintain FDA 21 CFR documentation — and many who claim compliance have not had the formulation reviewed against the positive list. Require the specific CFR section cited and the supporting documentation, not a self-declaration.

The English technical content available for Chinese-manufactured dispersants is almost entirely produced by Western brand owners (BASF, Lubrizol, Evonik) for their own products. Chinese domestic suppliers rarely produce English-language application data or compliance documentation at the same depth. That gap is precisely why specification errors happen at the sourcing stage — buyers assume that a Chinese supplier’s “equivalent” product has been tested to the same application standards as the Western reference product. It has not, unless you require the data explicitly.

Practical Guidance for Buyers #

When sourcing dispersants from China, the first parameter to request from suppliers is not active content or viscosity — it is the zeta potential vs. pigment loading curve at your specific pigment type and loading range. This single document will tell you more about real-world performance than any other item on a TDS. Most Chinese suppliers will not have this data ready; the ones who do are the ones worth qualifying.

The sourcing mistake we see most often is accepting a single-point zeta potential measurement (typically at 10–20% pigment loading) as evidence of performance at production loading (often 40–65%). A dispersant that measures –42 mV at 10% loading but –22 mV at 50% loading will cause stability failures in production — and the failure will be intermittent and difficult to trace back to the dispersant.

Before committing to volume order, require three things: (1) zeta potential data at your actual pigment loading, measured per ASTM E2865; (2) three consecutive batch COAs showing acid value within ±5 mg KOH/g of the specified value; and (3) a REACH SVHC declaration listing all components above 0.1% w/w if the product is destined for EU markets. Suppliers who cannot provide all three within two weeks of qualification request are not operationally ready for your supply chain.

For related sealing and fluid-handling applications where dispersant compatibility with elastomers is a concern, see our category on pump and valve seals and specialty polymer additives.

What to Specify on Your TDS Request: Buyer Checklist #

Use this checklist when issuing a TDS or qualification data request to a Chinese dispersant supplier. Every item should have a specific numeric value or test condition attached — not a range, not a qualitative descriptor.

Stability Parameters
– [ ] Zeta potential at [your pigment type] at [your loading %], measured per ASTM E2865: minimum –30 mV
– [ ] Zeta potential vs. loading curve from 10% to [your maximum loading %]
– [ ] pH stability range: specify the window within which zeta potential remains ≤ –30 mV
– [ ] Thermal stability: zeta potential retention ≥ 90% after 4 hours at [your processing temperature]°C

Particle Size Performance
– [ ] D50 after [your milling protocol] at [your pigment loading]: ≤ 300 nm (organic) or ≤ 500 nm (inorganic)
– [ ] D90 after [your milling protocol]: ≤ 800 nm (organic) or ≤ 1,200 nm (inorganic)
– [ ] Milling protocol used for measurement (bead size, speed, duration, pigment type)

Chemical Characterization
– [ ] Active content by ASTM D2369: specify value ± 1.5%
– [ ] Acid value by ASTM D974: specify value ± 5 mg KOH/g
– [ ] Amine value by ASTM D2073 (if applicable): specify value ± 5 mg KOH/g
– [ ] Molecular weight (Mw and Mn) by GPC: specify range
– [ ] HLB value: specify method (Griffin or Davies) and value

Physical Properties
– [ ] Viscosity at 25°C: Brookfield, spindle and speed specified, ± 10% tolerance
– [ ] pH at 25°C: ± 0.3 units
– [ ] Density at 25°C: ± 0.02 g/cm³

Lot-to-Lot Consistency
– [ ] Three consecutive batch COAs for acid value, active content, and zeta potential
– [ ] Shelf life and storage conditions with supporting stability data

Compliance Documentation
– [ ] REACH SVHC declaration listing all components ≥ 0.1% w/w (not a generic compliance statement)
– [ ] FDA 21 CFR section cited (if food-contact application)
– [ ] GB/T standard reference with tolerance class specified

Frequently Asked Questions #

Q1: What is the minimum zeta potential threshold for a stable pigment dispersion?
A: –30 mV is the accepted minimum for electrostatic stabilization. Values between –25 mV and –30 mV indicate marginal stability — acceptable for short shelf-life products, not for concentrates or inks with 12-month storage requirements.

Q2: How do I choose between a low-MW polyacrylate and a high-MW polymeric dispersant?
A: The decision point is pigment loading. Below 40% loading, a low-MW polyacrylate (Mw 3,000–8,000 g/mol) is typically sufficient and lower cost. Above 40% loading, you need the steric stabilization layer of a high-MW polymeric dispersant (Mw 10,000–25,000 g/mol) — electrostatic stabilization alone is insufficient at high solids. See the comparison table above for application-specific guidance. Relevant test method: ASTM International ASTM E2865 for zeta potential verification.

Q3: What is the most common quality failure when sourcing dispersants from China at volume?
A: Raw material substitution at the monomer level — specifically, a switch to lower-purity acrylic acid or surfactant feedstock that reduces acid value below the specified threshold. The failure is not visible on a standard COA unless you are spot-testing acid value and zeta potential on incoming batches. The threshold to watch: acid value dropping more than 10 mg KOH/g below the qualified value is a disqualifying deviation.

Q4: What compliance documentation should I require for dispersants sold into the EU market?
A: A full ECHA REACH SVHC declaration listing all components above 0.1% w/w — not a generic compliance statement. For food-contact applications, require the specific FDA Guidelines 21 CFR section (typically 176.170 for paper coatings) with supporting documentation. Self-declarations without component-level disclosure are legally insufficient for EU import.

Q5: Is a higher acid value always better for dispersant performance?
A: No. Above 100 mg KOH/g, acid value causes foam problems in water-based systems and compatibility issues with binder resins. The effective range is 30–80 mg KOH/g for most inorganic pigment applications. Higher is not better — matched to the pigment surface chemistry is better.

Published by sinoraw.com Technical Team | Request a sourcing consultation