Abstract Carbon-black dispersant screening is often reduced to a convenient rule: the lowest millbase viscosity wins. That shortcut can miss two questions—whether the pigment is genuinely deflocculated and whether its color potential is fully developed. This study compares a comb polyurethane hyperdispersant with an imported comb-structured reference in a 1K air-drying hydroxy acrylic system. With MA100 carbon black, the test dispersant produced slightly higher absolute viscosities, while the 5/50-rpm viscosity ratio decreased from 1.568 to 1.555 and L* decreased from 5.1 to 5.0. PR177 red showed the same direction in viscosity ratio and a small increase in a*. The results demonstrate that a single viscosity reading is not sufficient to rank dispersion quality. Fineness, viscosity at two shear conditions, viscosity ratio, color development and post-storage change should be evaluated together.

 

A narrow formulation question that is often oversimplified

In industrial black coatings, automotive refinish, plastics coatings, metal packaging and premium wood coatings, carbon black provides more than color. Its high surface area and complex aggregate morphology can strongly alter millbase rheology. Particle networks may build at low shear, increasing milling and pumping load, and the system may show viscosity drift, re-agglomeration or color change during storage.

Formulators therefore look first at millbase viscosity. The rule that “lower is better” is meaningful only when formulation, temperature, shear history, solids and dispersant dosage are identical. Even under controlled conditions, one viscosity point describes only one shear state. It does not show whether particles remain stabilized during storage, application and film formation.

 

Why carbon black makes a single viscosity point unreliable

Carbon black is not a collection of independent, ideal spheres. Primary particles are fused into aggregates, and aggregates form larger agglomerates through van der Waals attraction and other interactions. Milling can break the weaker agglomerates, but it does not automatically stabilize newly exposed surface. If anchoring is insufficient, particles can approach again after leaving the high-shear zone and rebuild reversible or irreversible structures.

This is why two millbases can both reach ≤10 μm fineness yet show different low-shear viscosity, jetness, gloss and storage response. A grind gauge is useful for judging whether coarse agglomerates remain that may disrupt film smoothness. It is not a complete particle-size-distribution or surface-coverage measurement.

 

Four limitations of conventional screening

Comparing one spindle speed. A single viscosity cannot separate a generally thicker liquid from a system with a disproportionately strong low-shear network. At least one low and one higher speed are needed, together with their ratio.

Treating fineness as the endpoint. Passing a grind-gauge target does not prove optimum color development or resistance to re-agglomeration during storage.

Correcting viscosity with solvent. Dilution lowers the number but does not strengthen the adsorbed layer on carbon black; it also reduces solids and narrows the application window.

Replacing chemical stabilization with longer milling. Additional energy may further break agglomerates, but weak pigment affinity still permits flocculation after the millbase leaves the mill.

 

What a comb architecture changes

Polymeric dispersants typically combine pigment-affinic groups with compatible solvated segments. The anchoring groups adsorb on the particle surface, while the compatible chains extend into the continuous phase and create a steric barrier. When coated particles approach, chain compression and overlap increase the free energy of the local region and resist further contact.

The benefit of a comb architecture is not simply higher molecular weight. Anchor density, side-chain density and compatibility with the resin-solvent phase can be designed together. Strong adsorption plus well-solvated side chains can help control particle networking, color development and storage stability. Poor compatibility, however, cannot be corrected simply by increasing molecular weight or dosage and may introduce viscosity or film-compatibility problems.

Figure 1. Schematic comparison of re-agglomeration and steric stabilization by a comb polymer.

 

Experimental design: fixed formulation, three response dimensions

The study compared the comb polyurethane hyperdispersant Disuper S9 with an imported comb-structured reference in the same 1K air-drying hydroxy acrylic system. The millbase contained 5.0% dispersant and 15.0% pigment, equal to 33.3% dispersant on pigment. PR177 red and MA100 carbon black were milled to ≤10 μm, then let down under the same formulation and process.

All conclusions are specific to this formulation and test condition. The source record did not state viscosity temperature, spindle geometry, color-difference equation or film thickness. Consequently, the 0.1 decrease in L* is treated as directional numerical evidence, not an exaggerated claim of a universally visible advantage.

Table 1. Base formulation for the 1K hydroxy acrylic evaluation.

Millbase material	Amount / %	Letdown material	Amount / %
Hydroxy acrylic resin	50.00	Hydroxy acrylic resin	60.00
PU mixed solvent	30.00	Millbase	30.00
Dispersant	5.00	Leveling agent	0.30
Pigment	15.00	n-Butyl acetate	4.85
Total	100.00	Xylene	4.85
		Total	100.00

 

 

Result 1: absolute viscosity was slightly higher, so it cannot support a “stronger viscosity reduction” claim

For the MA100 millbase, the reference measured 183.6 cP at 50 rpm and 287.9 cP at 5 rpm. The test dispersant measured 192.8 and 299.9 cP, approximately 5.0% and 4.2% higher. The defensible conclusion is therefore not that the test product reduced viscosity more. The two samples occupied a similar viscosity range, with the test sample slightly higher.

The pigment dependency was clearer with PR177 red: the test sample was about 35.7% higher at 50 rpm and 32.0% higher at 5 rpm. One dispersant can interact differently with different pigment surfaces, so “universal” performance must still be verified pigment by pigment.

 

Result 2: the viscosity ratio adds information about particle networking

The low-/high-shear ratio was calculated as τ = η(5 rpm)/η(50 rpm). A higher value generally indicates a stronger low-shear structure relative to the higher-shear state. With PR177, τ decreased from 1.730 to 1.683; with MA100, it decreased from 1.568 to 1.555. The changes were modest but directionally consistent: despite slightly higher absolute viscosity, the test sample did not show a stronger relative low-shear structure.

This ratio is an internal screening indicator valid only with the same instrument and procedure. It does not replace a full flow curve, yield-stress measurement or three-interval thixotropy test.

Figure 2. MA100 viscosity, two-pigment viscosity ratios and MA100 L* response.

 

Result 3: color must be read together with rheology

In the MA100 coating, L* decreased from 5.1 to 5.0 and ΔE was 0.2. With PR177 red, a* increased from 54.9 to 55.2 and ΔE was 0.4. Both differences were small, but they show that the sample with the lower single-point viscosity did not automatically produce the deeper black or the higher red coordinate.

The useful decision is not to turn one number into a winner. It is to determine whether the formulation balances processable viscosity, limited low-shear networking, color development and storage stability.

Table 2. Original test results. τ = η(5 rpm)/η(50 rpm).

Pigment	Sample	50 rpm / cP	5 rpm / cP	τ	Key color result
PR177 red	Imported reference	1207	2088	1.730	L* 26.2; a* 54.9; b* 44.3
PR177 red	S9	1638	2756	1.683	L* 26.4; a* 55.2; b* 44.6; ΔE 0.4
MA100 black	Imported reference	183.6	287.9	1.568	L* 5.1; a* 0.5; b* -0.7
MA100 black	S9	192.8	299.9	1.555	L* 5.0; a* 0.4; b* -0.5; ΔE 0.2

 

 

A practical screening sequence

First, fix resin solids, solvent ratio, pigment lot, temperature, milling media and energy input. Second, run a dosage ladder based on pigment weight: 30%, 40% and 50% are reasonable starting points for carbon black, while 10%, 20% and 30% can be used for organic pigments. Third, after the same equilibration time, measure viscosity at 5 and 50 rpm and retain a complete flow curve where possible. Fourth, record fineness, L*a*b*, gloss, flooding/floating and rub-out. Fifth, repeat viscosity, fineness and color after ambient and accelerated storage.

Only when these responses remain aligned can a dispersant be said to do more than make the fresh millbase flow—it must preserve the particle state required for downstream processing and film formation.

 

Where Disuper S9 fits

This product is a high-molecular-weight comb polyurethane hyperdispersant containing pigment-affinic groups, intended for organic pigments and carbon black in solventborne and solvent-free resin systems. In the present 1K hydroxy acrylic test, it provided a processable viscosity close to the imported reference, a slightly lower viscosity ratio and a slightly lower L* with MA100. With PR177 it gave higher absolute viscosity, while the viscosity ratio and red-coordinate data remained directionally useful.

It should therefore be positioned as a candidate to be optimized through a dosage ladder and a complete evaluation matrix—not summarized by a generic “lower viscosity” claim. For formulations targeting high jetness, high gloss, transparent color or increased pigment loading, a three-point dosage screen in the actual resin and solvent package is recommended.

 

Conclusion

Low millbase viscosity is an important processing indicator in a 1K hydroxy acrylic carbon-black dispersion, but it is not a sufficient condition for dispersion quality. Fineness answers whether coarse agglomerates remain. Two-speed viscosity and their ratio describe how the particle network responds to shear. L*a*b* and film appearance indicate whether color is developed. Repeating the measurements after storage shows whether that state persists.

The most useful conclusion from this dataset is not that one sample wins on one number. It is that dispersant screening should move from a single-point decision to a multidimensional verification. That change prevents formulators from using dilution, excessive milling or overdosing to mask a surface-stabilization problem.

References

1. J.D. Schofield. Extending the boundaries of dispersant technology. Progress in Organic Coatings 45 (2002) 249–257. DOI

2. F. Nsib, N. Ayed, Y. Chevalier. Selection of dispersants for the dispersion of carbon black in organic medium. Progress in Organic Coatings 55 (2006) 303–310. DOI

3. C.L. Barrie et al. Rheology of aqueous carbon black dispersions. Journal of Colloid and Interface Science 272 (2004) 210–217. DOI

4. ASTM D1210-05(2022), Standard Test Method for Fineness of Dispersion of Pigment-Vehicle Systems by Hegman-Type Gage. Source

5. Corechem Application Laboratory. Comparative evaluation of comb dispersants in a 1K hydroxy acrylic system. Internal test data supplied for this article.

Data note: Formulation and comparative results in this article are taken from Corechem internal technical materials supplied for this work. External references are used only to explain dispersion mechanisms and test boundaries.