Key Variable in the Long-Term Stability of Waterborne Pigment Concentrates: How Dispersant Molecular Weight Distribution (MWD) Affects Pigment Dispersion, Surface Adsorption-Layer Formation, and Storage Stability

1. The Essence of Long-Term Stability in Waterborne Pigment Concentrates: Stability of the Pigment Surface Adsorption Layer

The long-term storage stability of a waterborne pigment concentrate is outwardly reflected in whether the particle size increases again, whether sedimentation occurs, whether flocculation develops, whether viscosity drifts, and whether color development remains stable. At its core, it depends on whether a stable protective adsorption layer of dispersant can form on the surface of pigment particles.

Grinding a pigment to a fine particle size does not necessarily mean that long-term stability has been achieved. Grinding only opens up pigment agglomerates. What truly determines storage stability is whether the newly exposed pigment surfaces can be covered promptly by the dispersant and whether a firm, continuous, sufficiently thick interfacial protective layer compatible with the waterborne system can be formed. This process can be summarized as three consecutive stages: wetting, dispersion, and stabilization.

1.1 Wetting: Allowing the Liquid Phase to Enter the Pigment Powder Structure

Dry pigment powder is not composed of ideal, individually separated particles. Instead, it is a powder structure made up of primary particles, aggregates, agglomerates, pores, and air interfaces. The essence of wetting is that the aqueous medium and dispersant enter the interior of pigment agglomerates and replace the air interfaces on the pigment surface, converting the pigment surface from a “gas–solid interface” into a “liquid–solid interface.”

Wetting addresses the question of whether the waterborne system can truly access the pigment surface. If wetting is insufficient, air and high-energy interfaces that have not been replaced by liquid may still remain inside the pigment agglomerates. As a result, subsequent grinding efficiency decreases, the particle size distribution tends to broaden, and the system becomes more prone to coarse particles, sedimentation, or particle-size rebound during storage.

1.2 Dispersion: Mechanical Force Opens Agglomerates and Continuously Generates New Surfaces

The dispersion stage relies on mechanical actions such as high-speed dispersion, sand milling, and bead milling to gradually open pigment agglomerates. The key point in this stage is that every newly generated pigment surface needs to be promptly adsorbed and protected by the dispersant.

If the newly exposed surfaces are not covered by dispersant, pigment particles may re-aggregate due to van der Waals attraction, hydrophobic interactions, hydrogen bonding, ionic bridging, or collisions under locally high concentration. In this case, even if the initial particle size test meets the specification, particle-size rebound, flocculation, or viscosity increase may still occur after storage.

1.3 Stabilization: Preventing Pigment Particles from Approaching Each Other Again

The stability of waterborne pigment concentrates mainly depends on electrostatic repulsion, steric hindrance, or the combined action of both. For many waterborne polymeric dispersants, the stabilization mechanism is the combined effect of steric hindrance after anchoring adsorption and electrostatic repulsion provided by ionic groups, namely electrosteric stabilization. Polymeric dispersants usually contain two types of structural components:

Structural Component	Main Function
Anchoring groups	Adsorb onto the pigment surface through acid–base interactions, hydrogen bonding, coordination, π–π interactions, or hydrophobic interactions
Solvated chain segments	Extend into the aqueous phase or resin phase to form a steric protective layer

When two pigment particles approach each other, the polymer chain segments adsorbed on their surfaces begin to compress and overlap. This leads to a decrease in chain conformational entropy and an increase in local osmotic pressure, thereby generating a repulsive force that makes it difficult for the particles to continue approaching each other. This is the core mechanism by which polymeric dispersants provide steric stabilization.

2. Why MWD Matters: A Dispersant Is Not a Single Molecule

In formulation discussions, people often refer to the molecular weight of a dispersant, such as 5,000, 8,000, or 12,000. However, for polymeric dispersants, this description is incomplete.

Polymeric dispersants are usually not composed of molecules with exactly the same chain length. Instead, they consist of a group of chain segments with different molecular weights. A single sample may contain low-molecular-weight short chains, medium-molecular-weight chains, and high-molecular-weight long chains at the same time. What truly affects the stability of waterborne pigment concentrates is not just a single average molecular weight, but the relative proportions of these different chain-length components.

2.1 MWD Is a Map of the Dispersant’s Chain-Length Composition

Molecular Weight Distribution, or MWD, describes the distribution of chain segments with different molecular weights in a polymer sample. It is not a single numerical value, but a distribution curve.

If a dispersant is viewed as a group of polymer chains with different lengths, then MWD describes how many short chains, medium chains, and long chains are present; whether there is an obvious low-molecular-weight tail; whether there is an obvious high-molecular-weight tail; and whether the main distribution is concentrated or broad.

This is more important than a single average molecular weight. Even if two dispersants have the same average molecular weight, as long as their MWDs are different, their adsorption rate on the pigment surface, adsorption-layer thickness, coverage uniformity, system viscosity, and long-term stability may all differ.

2.2 MWD Affects the Functional Roles of Dispersants

Under the same or similar chemical composition, anchoring structure, and solvated chain-segment conditions, dispersants with different chain lengths often show different functional tendencies in waterborne pigment concentrates.

Chain-Segment Type	Main Function	Risk When Excessive	Significance for Long-Term Stability
Short chains	Rapid wetting, fast diffusion, initial coverage	Thin adsorption layer, insufficient steric hindrance, prone to particle-size rebound	Responsible for initiating dispersion
Medium chains	Uniform coverage and formation of the main adsorption layer	Insufficient proportion may lead to discontinuous protection layers	Responsible for the stability foundation
Long chains	Enhance steric hindrance and improve anti-flocculation ability	Viscosity increase, bridging flocculation, uneven adsorption	Responsible for stability enhancement

An ideal dispersant MWD is one in which the proportions of short, medium, and long chains are matched to the pigment surface and the waterborne system. Short chains should be sufficient to assist wetting; medium chains should be sufficient to form a continuous and stable layer; and long chains should be sufficient to enhance steric hindrance, but not so abundant that they cause bridging or viscosity problems.

3. Mn, Mw, and PDI: Three Entry Points for Understanding MWD

Molecular weight distribution is usually characterized by GPC, or Gel Permeation Chromatography, or SEC, or Size Exclusion Chromatography. Common results include Mn, Mw, PDI, and the complete MWD curve.

It should be noted that Mn, Mw, and MWD obtained by conventional GPC/SEC are usually affected by calibration standards, mobile phase, columns, detectors, and polymer conformation. In essence, SEC/GPC primarily separates polymers according to their hydrodynamic volume in solution, and the molecular weight is then calculated through calibration or coupled detectors. Therefore, Mn/Mw values obtained under different testing conditions, with different calibration standards, or from different polymer structures should not be compared directly in a simplistic way.

3.1 Mn: Number-Average Molecular Weight, Reflecting the Influence of Short Chains

Mn, or Number-average Molecular Weight, is the molecular weight obtained by averaging according to the number of molecules. Each polymer chain has the same statistical weight.

Mn is more sensitive to low-molecular-weight short chains. If the number of short chains in a dispersant increases, Mn will decrease significantly even if some long chains are still present. For waterborne pigment concentrates, Mn can help determine whether the proportion of low-molecular-weight components is excessive.

When Mn is relatively low, the dispersant often has a faster diffusion rate, which is beneficial for initial wetting and the start of grinding. However, if Mn is too low, the chain segments extending into the aqueous phase after adsorption are relatively short, resulting in a thinner steric layer and insufficient long-term anti-flocculation ability. The system may show a fine initial particle size, but particle-size rebound may occur after heat aging or long-term storage. Therefore, Mn is not simply “the lower, the better.” Low Mn is beneficial for wetting and diffusion, but excessively low Mn weakens long-term protection.

3.2 Mw: Weight-Average Molecular Weight, Reflecting the Contribution of Long Chains

Mw, or Weight-average Molecular Weight, is the average molecular weight obtained by weighting according to mass contribution. Because high-molecular-weight chains have a larger mass per chain, they have a more pronounced effect on Mw.

Mw is more sensitive to high-molecular-weight long chains. A higher Mw usually indicates a greater contribution from high-molecular-weight chain segments, but the MWD curve must be examined to determine whether the whole distribution has shifted to a higher molecular weight or whether only a small high-molecular-weight tail is present.

For waterborne pigment concentrates, an appropriately higher Mw helps form a thicker steric hindrance layer, increases the repulsive distance between pigment particles, and enhances anti-flocculation ability. However, a higher Mw is not necessarily better. When there are too many long chains, dispersant diffusion slows down and system viscosity increases. If one long chain adsorbs simultaneously onto the surfaces of multiple pigment particles, bridging flocculation may also occur.

3.3 PDI: Polydispersity Index, Reflecting the Breadth of the Distribution

PDI, or Polydispersity Index, is the ratio of Mw to Mn:

PDI = Mw / Mn

IUPAC, the International Union of Pure and Applied Chemistry, recommends using molar-mass dispersity, commonly translated as molecular-weight dispersity or dispersity, with the symbol Đ or ĐM, to represent the concept of Mw/Mn. PDI or Đ is used to describe the breadth of a polymer molecular weight distribution:

PDI / Đ Status	Distribution Meaning	Evaluation Value
Close to 1	Chain lengths are highly concentrated; narrow distribution	Adsorption behavior is more consistent and performance is more controllable
Significantly greater than 1	Chain lengths differ greatly; broad distribution	May combine wetting and stability, but may also introduce instability risks
Relatively high value	Very broad distribution with obvious tails	Low-molecular-weight and high-molecular-weight tails need to be assessed carefully

PDI is an indicator of distribution breadth, not an indicator of stability. PDI itself cannot directly determine whether a dispersant is good or poor. It must be analyzed together with the MWD curve, pigment type, and application system.

It is especially important to note that PDI only indicates how broad the distribution is; it does not indicate where the distribution is broad. Two dispersants may have the same PDI: one may contain too many short chains, while the other may be dominated by medium chains with a small amount of long chains. Their wetting behavior, viscosity, storage stability, and particle-size rebound risk in waterborne pigment concentrates may be completely different.

3.4 The Relationship Between Mn, Mw, PDI, and MWD

Mn, Mw, and PDI are three entry points for understanding MWD, but they cannot replace the complete MWD curve.

Indicator	Main Information Reflected	More Sensitive to Which Chains	Significance in Pigment-Concentrate Stability
Mn	Number-average molecular weight	Short chains	Helps determine whether low-molecular-weight components are excessive
Mw	Weight-average molecular weight	Long chains	Helps determine whether high-molecular-weight chains contribute significantly
PDI / Đ	Ratio of Mw to Mn	Distribution breadth	Helps determine whether chain-length differences are large
MWD curve	Complete molecular weight distribution	Full distribution shape	Helps determine the proportions of short, medium, and long chains

4. How MWD Affects the Quality of the Pigment Surface Adsorption Layer

The core pathway through which MWD affects the long-term stability of waterborne pigment concentrates is as follows:

MWD changes the chain-length composition of the dispersant → chain-length composition changes adsorption rate, adsorption-layer thickness, and coverage uniformity → adsorption-layer quality determines whether pigment particles can remain separated over the long term.

It should be emphasized that MWD affects chain-length composition, adsorption kinetics, and adsorption-layer morphology. However, whether the dispersant can truly adsorb onto the pigment surface also depends on the matching between the anchoring groups and the pigment surface. Without sufficient anchoring strength, even a well-designed chain-length distribution cannot form a stable adsorption layer.

4.1 Short Chains: Faster Wetting but a Thinner Protective Layer

Short-chain dispersants have lower molecular weight and faster diffusion rates. They can enter the pores of pigment agglomerates more quickly and approach newly exposed pigment surfaces. Therefore, an appropriate amount of short chains helps improve initial wetting and the start of dispersion.

However, after adsorption, short chains have a limited extension distance and form a thinner steric layer. When pigment particles collide during storage, the repulsive effect provided by the thin adsorption layer is insufficient, making the particles more likely to approach each other and undergo weak flocculation or particle-size rebound. Typical signs of an excessive short-chain proportion include:

① Fine initial particle size;

② Low initial viscosity;

③ Apparently good grinding efficiency;

④ Increased D90 or D99 after heat aging;

⑤ Particle-size rebound or soft flocculation after long-term storage.

Therefore, short chains are suitable for rapid wetting and initial coverage, but they cannot serve as the main support for long-term stability.

4.2 Medium Chains: The Main Body of a Stable Adsorption Layer

Medium-chain dispersants achieve a better balance among diffusion rate, adsorption coverage, and steric hindrance. They do not form a protective layer as thin as short chains do, nor are they as likely as overly long chains to cause bridging or viscosity abnormalities.

The role of medium chains is mainly reflected in three aspects:

① They can participate in covering newly generated surfaces during grinding;

② They can form a relatively continuous and uniform adsorption layer;

③ They can provide sufficient but not excessive steric hindrance.

Medium-chain components usually constitute the main supporting range for the long-term stability of waterborne pigment concentrates. If the proportion of medium chains is insufficient, even if short chains provide good initial wetting or long chains provide locally thicker protective layers, the overall adsorption layer may still be discontinuous and nonuniform, resulting in insufficient long-term storage stability.

4.3 Long Chains: Enhanced Steric Hindrance, but Excessive Amounts Can Cause Bridging and Viscosity Risks

The main value of long-chain dispersants is that, after adsorption, they can extend into the aqueous phase or resin phase to form a thicker steric layer. When two pigment particles approach each other, compression and overlap of the long-chain layer generate a stronger repulsive effect, which helps improve anti-flocculation ability.

For high-specific-surface-area organic pigments, carbon black, highly transparent pigments, and high-solids pigment concentrates, an appropriate amount of long chains usually helps improve long-term stability. However, excessive long chains can introduce three types of risks.

① Bridging flocculation: Under conditions of insufficient coverage, excessive dispersant, multipoint adsorption, or multiple strong adsorption sites on the pigment surface, if one long-chain dispersant simultaneously adsorbs onto two or more pigment particles, it may connect particles that should remain separated and form a flocculated structure. In this situation, the dispersant no longer only protects particles; it becomes a bridge connecting them.

② Viscosity increase: Long chains increase the viscosity of the continuous phase and may also enhance weak interactions between particles. In high-solids systems, too many long chains can increase low-shear viscosity, reduce grinding efficiency, and impair filterability.

③ Uneven adsorption: Long chains diffuse more slowly and require a longer time for conformational adjustment. If the pigment surface has already been occupied by short chains, subsequent adsorption of long chains may be insufficient, or slow replacement and rearrangement may occur during storage, resulting in changes in particle size and viscosity.

4.4 Adsorption-Layer Rearrangement: An Important Cause of Long-Term Instability

In dispersants with broad distributions, components with different chain lengths compete for adsorption sites on the pigment surface. During the early stage of grinding, short chains may adsorb first. As time passes, medium or long chains may continue to adsorb, replace short chains, or readjust their conformations.

This process can lead to changes in the structure of the adsorption layer, including changes in surface coverage density, adsorption-layer thickness, repulsive interactions between particles, and the amount of free dispersant. These changes may further cause system viscosity and particle size to drift over time. Some pigment concentrates have a very fine particle size immediately after production but show particle-size rebound after storage. This is not necessarily because the initial dispersion was poor, but because the adsorption layer had not reached a stable state and continued to rearrange during storage.

4.5 Free Dispersant: Unadsorbed Chain Segments Also Affect Stability

After a dispersant is added, not all of it adsorbs onto the pigment surface. The unadsorbed dispersant remains in the continuous phase and can also affect the stability of waterborne pigment concentrates. Free short chains may affect foaming, migration, and water resistance. Free long chains may increase low-shear viscosity and interact with thickeners, emulsions, resins, or electrolytes, causing rheological drift or weak flocculation.

5. Broad Distribution vs. Narrow Distribution: Stability Depends on Compatibility and Matching

MWD can be divided into narrow distribution and broad distribution. A narrow distribution means that chain lengths are relatively concentrated and the PDI is lower. A broad distribution means that chain lengths differ more significantly and the PDI is higher. For waterborne pigment concentrates, neither broad nor narrow distribution is absolutely superior. What truly matters is whether the distribution matches the pigment surface, grinding process, aqueous system, resin compatibility, solids content, and storage requirements.

5.1 The Advantage of a Narrow Distribution Is Controllability

A narrow-distribution dispersant has more concentrated chain lengths, and its molecular diffusion, adsorption, and protective-layer formation behavior are more consistent.

Its main advantages include more uniform adsorption-layer thickness, more predictable dispersion behavior, smaller batch-to-batch fluctuations, lower risk of adsorption-layer rearrangement during storage, and more controllable particle-size and viscosity changes over time.

Narrow distribution is suitable for systems with high requirements for particle-size stability, batch consistency, spray stability, inkjet stability, or long-term storage stability. However, a narrow distribution does not automatically mean high stability. If the molecular weight range is too low, steric hindrance will be insufficient. If the overall molecular weight is too high, wetting and diffusion will be inadequate, and viscosity risks will also increase.

5.2 The Value of a Broad Distribution Is Functional Gradation

A broad-distribution dispersant contains short, medium, and long chains at the same time. If their proportions are reasonable, it can balance wetting, dispersion, and stabilization. Short chains assist rapid wetting, medium chains form the main adsorption layer, and long chains enhance steric hindrance. For pigments with complex surfaces, many pores, or nonuniform adsorption sites, a moderately broad distribution may sometimes provide better adaptability.

However, broad distribution can also become more difficult to control. Excessive low-molecular-weight tails may lead to good initial dispersion but insufficient long-term protection. Excessively heavy high-molecular-weight tails may cause viscosity increase, bridging flocculation, and uneven adsorption. Competition among chains of different lengths for adsorption may also trigger adsorption-layer rearrangement during storage. The key to a broad distribution lies in whether the proportions of short, medium, and long chains are reasonable and whether the low-molecular-weight and high-molecular-weight tails are well controlled.

5.3 PDI Cannot Replace the MWD Curve

PDI can indicate the breadth of the distribution, but it cannot reveal the distribution shape. In practical evaluation, Mn, Mw, PDI, and the MWD curve must be considered together. For example:

Indicator Combination	Possible Meaning	Stability Assessment
Low Mn, high Mw, high PDI	Many short chains, with a high-molecular-weight tail also present	May combine insufficient protection with bridging risk
Moderate Mn, moderate Mw, relatively low PDI	Concentrated distribution and relatively uniform chain length	Better controllability, but steric hindrance must still be confirmed
Moderate Mn, slightly high Mw, medium PDI	Medium chains as the main component, with an appropriate amount of long chains	May balance wetting and stability
High Mn, high Mw, not high PDI	Overall chain length is high and concentrated	Protective layer may be thicker, but wetting, viscosity, and grinding efficiency require attention

6. Why Initial Stability Does Not Equal Long-Term Stability

Many waterborne pigment concentrates have fine particle size, normal viscosity, and good color development immediately after production, but after a period of storage they show particle-size rebound, sedimentation, flocculation, floating color, flooding, or viscosity changes. The fundamental reason is usually that the adsorption layer on the pigment surface has not remained stable over the long term. Initial particle size only indicates that the pigment agglomerates were opened to a certain extent at that time; it does not prove that the adsorption layer already has long-term storage stability. Long-term stability must be verified through heat aging, centrifugation, freeze–thaw testing, dilution, color development, and other tests.

6.1 Typical Instability Phenomena and Possible MWD Causes

Storage Problem	Possible MWD Cause	Mechanistic Explanation	Suggested Verification
Particle-size rebound after heat aging	Excessive low-molecular-weight tail; insufficient medium-chain proportion	Thin adsorption layer and insufficient steric hindrance; particles re-aggregate after collisions	Compare D50, D90, and D99 before and after heat aging
Continuous viscosity increase	Excessively heavy high-molecular-weight tail	Long-chain bridging or enhanced weak particle network	Low-shear viscosity, thixotropic recovery, rheological curve
Soft sedimentation	Uneven adsorption coverage or long-chain bridging	Particles form a loose flocculated structure; sediment can be partially redispersed	Centrifugation, static sedimentation, redispersibility
Flocculation after dilution	Adsorption layer is not strong enough, or solvated chain segments have insufficient compatibility	Interfacial equilibrium is disrupted after water or resin is added	Dilution particle size, color development, gloss
Poor color development, floating color, or flooding	Poor compatibility between dispersant and resin, or unstable pigment adsorption layer	Dispersion states of different pigments change, causing different migration rates	Drawdown panel, scraper test, letdown stability
Poor filterability	Increase in high-molecular-weight tail or coarse-particle tail	Flocculated particles or weak networks increase filtration resistance	Filtration test, particle-size tail analysis

D50, D90, and D99 refer to the particle sizes corresponding to 50%, 90%, and 99% of the cumulative particle size distribution, respectively. In long-term stability evaluation, D90 and D99 often reflect the coarse-particle tail and particle-size rebound trend better than D50. However, the detection method, distribution type, and testing conditions must be specified. Among them, D99 is more sensitive to sampling, dilution, and small amounts of coarse particles, so its repeatability usually needs to be carefully confirmed.

6.2 Fine Initial Particle Size but Particle-Size Rebound After Heat Aging

If the dispersant contains too many low-molecular-weight components, initial wetting and grinding may perform well, but the adsorption layer formed will be relatively thin. During storage, pigment particles are continuously affected by Brownian motion, temperature changes, and concentration fluctuations. A thin protective layer cannot provide sufficient repulsion, so the particles gradually form weak flocs.

6.3 Viscosity Increase and Bridging Flocculation

If the high-molecular-weight tail in the MWD is too heavy, the system may initially show strong suspension ability, but viscosity may gradually increase after storage. This is usually related to long-chain bridging, strengthening of weak networks between particles, or interactions between free long chains and the thickening system.

6.4 Flocculation After Dilution Indicates an Insufficiently Robust Adsorption Layer

Some pigment concentrates are stable in their original concentrated state, but flocculate after water is added, resin is added, or they are incorporated into a coating system. This usually indicates that the dispersant adsorption layer is not sufficiently robust, or that the solvated chain segments are not compatible enough with the subsequent system. If the dispersant only relies on the viscosity of the continuous phase to maintain apparent stability, rather than on a firmly adsorbed layer on the pigment surface to provide true stability, the system is prone to instability after dilution, letdown, or environmental changes.

7. Practical Selection and Verification: How to Determine Whether a Dispersant MWD Is Appropriate

For waterborne pigment concentrates requiring long-term storage stability, MWD, pigment surface characteristics, resin compatibility, and storage validation should be evaluated together.

7.1 First Confirm the Meaning of the Molecular Weight Data

“Molecular weight” may refer to Mn, Mw, peak molecular weight Mp, or an estimated value. Different indicators have different meanings and cannot be compared directly. During practical selection, the following information should be confirmed as much as possible:

Data	Question to Be Evaluated
Mn	Whether low-molecular-weight short chains are excessive
Mw	Whether high-molecular-weight long chains contribute significantly
PDI / Đ	Overall breadth of the distribution
MWD curve	Whether a low-molecular-weight tail or high-molecular-weight tail exists
Main peak position	Whether the main chain-length range is appropriate

If only one average molecular weight is available and no MWD curve is provided, the assessment of long-term storage stability is insufficient.

7.2 Focus on Low-Molecular-Weight and High-Molecular-Weight Tails

Two regions of the MWD curve need special attention.

The first is the low-molecular-weight tail. An excessive low-molecular-weight tail is usually beneficial for initial wetting, but it provides insufficient long-term protection and can easily lead to particle-size rebound after heat aging and instability after dilution.

The second is the high-molecular-weight tail. An excessively heavy high-molecular-weight tail may enhance steric hindrance, but it may also cause bridging flocculation, viscosity increase, poor filterability, and structural strengthening after storage.

An appropriate MWD should have a stable main chain-length range while controlling abnormal low-molecular-weight and high-molecular-weight components.

7.3 Evaluate Chain-Length Requirements According to Pigment Type

Different pigments have different requirements for dispersant MWD. Pigment specific surface area, surface polarity, oil absorption, structural complexity, and target particle size all affect the selection of dispersant chain length.

Pigment Type	Main Characteristics	MWD Focus
Inorganic pigments	High density; some have strongly polar surfaces	Anchoring strength, anti-sedimentation, moderate steric hindrance
Organic pigments	High specific surface area, strong surface hydrophobicity, fine particle-size requirements	Wetting efficiency, adsorption-layer thickness, short-chain/long-chain balance
Carbon black	Complex structure, high specific surface area, high oil absorption	Strong anchoring, sufficient steric hindrance, viscosity control
Highly transparent pigments	Sensitive to coarse particles and particle-size rebound	Adsorption-layer uniformity, control of low-molecular-weight tail
High-solids pigment concentrates	Small interparticle distance and high collision frequency	Prevention of bridging, viscosity control, sufficient stable-layer thickness

7.4 Use Storage Tests to Verify Whether MWD Is Appropriate

The suitability of MWD ultimately needs to be verified through application testing. Looking only at initial particle size is not enough. Special attention should be paid to the particle-size tail, viscosity changes, and stability after letdown into a coating system.

Test Item	Observation	Issue Reflected
Initial D50, D90, D99	Particle size and coarse-particle tail	Initial dispersion efficiency
Particle size after heat aging	Whether particle-size rebound occurs	Long-term stability of the adsorption layer
Viscosity change over time	Whether thickening, viscosity reduction, or enhanced thixotropy occurs	Bridging, flocculation, or rheological structure change
Centrifugal stability	Whether sedimentation, phase separation, or soft flocculation occurs	Resistance to aggregation and sedimentation
Freeze–thaw stability	Whether particle-size rebound or flocculation occurs	Resistance of the adsorption layer to environmental disturbance
Dilution stability	Whether flocculation occurs after water or resin addition	Adsorption robustness and system compatibility
Color development, floating color, and flooding	Changes in hue, tinting strength, and uniformity	Pigment dispersion state and letdown compatibility
Filterability	Whether filter clogging or slower filtration occurs	Coarse particles, bridging, or weak network structure

8. Classification Table of Model Compounds and Experimental Materials Related to Waterborne Pigment Dispersion Stability and Molecular Weight Distribution Research

The following materials are mainly intended for research on dispersion-stabilization mechanisms, control experiments, reference in dispersant structure design, or evaluation of model systems. They do not imply that all of these materials can be directly used as industrial dispersants for waterborne pigment concentrates. In practical formulation applications, comprehensive verification is still required based on pigment type, dispersant structure, dosage, pH, neutralization method, resin system, solids content, and storage-stability testing.

Table 1. Polymeric Dispersants, Protective Colloids, and Steric-Hindrance Chain Segments

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Polymeric protective colloid	9002-89-5	P139541	Poly(vinyl alcohol) (PVA)	Degree of hydrolysis: 87.0–89.0 mol%; viscosity: 80.0–110.0 mPa·s	Forms a hydrophilic protective layer; used for studies on steric stabilization, film-forming compatibility, and particle redispersibility
Hydrophilic polyether chain segment	25322-68-3	P615493	Poly(ethylene oxide)	Viscosity: 65–115 cps	Provides hydrophilic chain segments; used for studies on adsorption-layer thickness, chain-segment solvation, and viscosity effects
Anionic polymeric dispersant	9003-01-4	P661414	Poly(acrylic acid) (PAA)	Viscosity ≤2000 cP (25°C)	Carboxylic-acid-type polymer backbone; used for studies on anionic adsorption, salt-form variation, and the influence of molecular weight distribution on viscosity reduction and stability
Nonionic protective colloid	9003-39-8	P110607	Polyvinylpyrrolidone (PVP)	Average molecular weight 58,000; K29–32	Nonionic water-soluble polymer; used for studies on protective colloids, steric hindrance, and adsorption-layer rearrangement
Polyether solvated chain segment	25322-69-4	P103212	Polypropylene glycol (PPG)	Average molecular weight 4,000	Flexible polyether chain segment; used for studies on hydrophilic–hydrophobic balance, chain flexibility, and continuous-phase viscosity
Anionic polymeric dispersant	9003-04-7	P434409	Sodium polyacrylate (PAAS)	Average Mw ~8,000; 45% in H₂O	Carboxylate-type dispersant model; used for studies on low-molecular-weight tails, pigment surface adsorption, and particle-size retention
Cationic polymeric dispersant	9002-98-6	P434400	Branched polyethyleneimine (PEI)	Average Mw ~25,000 by LS; average Mn ~10,000 by GPC; branched	Amine-functional polymer structure; used for studies on cationic adsorption, bridging flocculation, and surface-charge regulation
Strong anionic polyelectrolyte	25704-18-1	P432468	Sodium poly(4-styrenesulfonate) (PSS)	Average Mw ~1,000,000; powder	Strong anionic polyelectrolyte; used for studies on charge stabilization, electrical double-layer effects, and the influence of high-molecular-weight tails
Anionic polymeric dispersant	9003-03-6	P304873	Ammonium polyacrylate	40% in water	Ammonium-salt-type carboxylate dispersant; used for evaluation of waterborne pigment dispersion, acid–base response, and particle-size rebound after heat aging
Aromatic anhydride copolymer resin	9011-13-6	S1509052	SMA base resin	—	Aromatic anhydride copolymer; used for studies on organic-pigment affinity, resin compatibility, and anchoring-structure design

Table 2. Dispersant Synthesis Monomers, Anchoring Monomers, and Hydrophilic Functional Monomers

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Hydroxy-functional monomer	868-77-9	H140643	2-Hydroxyethyl methacrylate (HEMA)	Anhydrous grade, ≥99%; contains 200 ppm MEHQ stabilizer; water ≤0.1%	Introduces hydroxyl interaction sites; used for studies on hydrogen-bond adsorption, hydrophilicity adjustment, and resin compatibility
Carboxylic-acid-functional monomer	79-10-7	A397753	Acrylic acid	Anhydrous grade, ≥99%; contains 200 ppm MEHQ stabilizer	Provides carboxylic acid anchoring and ionizable sites; used for synthesis studies of polyacrylic-acid-type dispersants
Carboxylic-acid-functional monomer	79-41-4	M102642	Methacrylic acid	≥98%; contains 250 ppm MEHQ stabilizer	Provides carboxylic acid structures; used for studies on dispersant acid value, dissociation behavior, and pigment surface adsorption
Polyether methacrylate macromonomer	26915-72-0	P432529	Poly(ethylene glycol) methyl ether methacrylate (PEGMA)	Average molecular weight ~300; contains MEHQ and BHT stabilizers; contains an unspecified amount of residual methacrylic acid	Introduces polyether side chains; used for studies on comb-type dispersants, steric-hindrance chain segments, and molecular weight distribution control
Dicarboxylic-acid-functional monomer	97-65-4	I106140	Itaconic acid	Chemically pure (CP), ≥99%	Provides a dicarboxylic acid structure; used for studies on multipoint anchoring, hydrophilicity, and acid-value adjustment
Flexible acrylate monomer	141-32-2	B100036	Butyl acrylate (BA)	Chemically pure (CP), ≥98%; contains 50 ppm MEHQ stabilizer	Adjusts polymer flexibility and hydrophobicity; used for studies on adsorption-layer conformation and resin compatibility
Rigid methacrylate monomer	80-62-6	M109623	Methyl methacrylate (MMA)	AR, ≥99%; contains 30 ppm DMBP stabilizer	Adjusts polymer hardness and glass-transition behavior; used for studies on dispersant backbone composition and compatibility with film-forming systems
Aromatic hydrophobic monomer	100-42-5	S110374	Styrene	Standard for GC, ≥99.5% (GC); contains 10–15 ppm TBC stabilizer	Provides aromatic structures; used for studies on affinity toward organic pigments and carbon materials, as well as hydrophobic anchoring
Anhydride-functional monomer	108-31-6	M116389	Maleic anhydride	AR, ≥99% (GC)	Provides carboxylic acid structures after hydrolysis; used for studies on anhydride copolymers, pigment affinity, and anionic stabilization
Nonionic hydrophilic monomer	88-12-0	V106155	N-Vinylpyrrolidone (NVP)	≥99%; contains 100 ppm NaOH stabilizer	Provides a hydrophilic lactam structure; used for studies on nonionic stabilization, adsorption-layer solvation, and protective colloids
Flexible hydrophobic acrylate monomer	103-11-7	E108592	2-Ethylhexyl acrylate (2-EHA)	≥99% (GC); contains 10–1100 ppm MEHQ as stabilizer	Introduces flexible hydrophobic side chains; used for studies on chain-segment flexibility, wetting compatibility, and low-temperature flow behavior
Aromatic anchoring monomer	2495-37-6	B102356	Benzyl methacrylate	≥98%; stabilized with 50 ppm MEHQ	Provides a benzyl aromatic structure; used for studies on aromatic-pigment affinity, hydrophobic anchoring, and adsorption-layer construction
Sulfonic-acid-functional monomer	15214-89-8	A106798	2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS)	≥98%	Provides strongly hydrophilic sulfonic acid groups; used for studies on electrolyte resistance, strong anionic stabilization, and aqueous solubility
Long-alkyl methacrylate monomer	142-90-5	L113574	Lauryl methacrylate (LMA)	≥96%; contains 500 ppm MEHQ as inhibitor	Provides long-alkyl hydrophobic chain segments; used for studies on pigment surface affinity, chain-segment association, and rheological effects
Aromatic sulfonate monomer	2695-37-6	S169174	Sodium 4-styrenesulfonate	≥90% (T)	Introduces aromatic sulfonate structures; used for studies on charge stabilization, hydrophilic units, and copolymeric dispersants

Table 3. Wetting Agents, Penetrants, and Surfactants

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Anionic wetting agent	151-21-3	S108350	Sodium dodecyl sulfate (SDS)	ACS, ≥99%	Reduces liquid–solid interfacial tension; used for comparative experiments on pigment wetting, charge stabilization, and surface adsorption
Nonionic wetting agent	9016-45-9	N1372295	Nonylphenol ethoxylate (Tergitol NP-40)	Isomer mixture, white flakes	Nonionic emulsifying and wetting structure; used for evaluation of organic-pigment wetting, surfactant adsorption, and compatibility
Anionic penetrating wetting agent	577-11-7	A106730	Sodium bis(2-ethylhexyl) sulfosuccinate (AOT)	Moligand™, ≥96%	Sulfosuccinate structure; used for studies on rapid penetrating wetting, agglomerate infiltration, and initial dispersion
Low-foam dynamic wetting agent	126-86-3	T301607	2,4,7,9-Tetramethyl-5-decyne-4,7-diol (DL-, meso-mixture)	≥98%	Acetylenic diol structure; used for studies on low-foam wetting, dynamic surface-tension adjustment, and spreading in waterborne systems

Table 4. Neutralizers, Acid–Base Regulators, and Inorganic Dispersion Aids

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Volatile alkali neutralizer	1336-21-6	A112077	Ammonia solution	Guaranteed reagent, 25–28%	Used for neutralization of carboxylic-acid-type dispersants, formation of ammonium salts, and evaluation of pH stability
Inorganic alkali neutralizer	1310-73-2	S111498	Sodium hydroxide	Guaranteed reagent, ≥96%	Used for sodium salt formation of carboxylic dispersants, charge-density adjustment, and alkaline storage evaluation
Inorganic alkali neutralizer	1310-58-3	P431767	Potassium hydroxide	Anhydrous grade, ≥99.95% metals basis	Used for potassium salt formation of carboxylic dispersants, evaluation of ionic-strength effects, and alkaline-system adjustment
Organic amine neutralizer	102-71-6	T478536	Triethanolamine	Reagent grade, ≥98%	Used for neutralization of acidic polymers, adjustment of pigment surface charge, and stability studies in waterborne systems
Organic amine neutralizer	108-01-0	D109080	N,N-Dimethylethanolamine (DMEA)	Refined grade, ≥99.5%	Used for neutralization of waterborne resins and dispersion systems, volatile-amine adjustment, and storage-stability evaluation
Inorganic phosphate dispersion aid	7758-29-4	S433949	Sodium tripolyphosphate	Industrial grade, ≥85%	Used for dispersion of inorganic pigments and fillers, complexation of hard-water ions, and studies on electrolyte effects
Organic amine neutralizer	124-68-5	A755868	2-Amino-2-methyl-1-propanol	BioReagent, ≥95%	Used for pH adjustment in waterborne systems, neutralization of carboxylic dispersants, and evaluation of pigment dispersion stability
Inorganic phosphate dispersion aid	7722-88-5	S108847	Sodium pyrophosphate	AR, ≥99%	Used for studies on inorganic-particle dispersion, ion complexation, and salt-sensitive stability
Inorganic phosphate dispersion aid	10124-56-8	S108858	Sodium hexametaphosphate (SHMP)	AR	Used for studies on dispersion stability in mineral pigments, oxide particles, and filler systems

Table 5. Representative Pigments, Inorganic Particles, and Dispersion-Evaluation Models

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Inorganic white pigment	13463-67-7	T431947	Titanium dioxide (IV)	Premium grade, ≥99%	Used for studies on white waterborne pigment concentrates, inorganic-pigment wetting, anti-sedimentation, and particle-size retention
Carbon-material dispersion model	1333-86-4	C431911	Carbon, mesoporous	≥99.95% metals basis; average pore diameter 100 ± 10 Å (typical)	Porous carbon-material model; used for evaluation of wetting, adsorption, and dispersant anchoring on high-specific-surface-area surfaces
Organic green pigment	1328-53-6	P342026	Pigment Green 7	Biological stain	Used for evaluation of phthalocyanine-type organic pigment dispersion, color-development stability, and particle-size rebound
Phthalocyanine organic pigment model	147-14-8	C432135	Copper(II) phthalocyanine	Sublimed grade, ≥99.95% metals basis, triple-sublimed	Used for studies on phthalocyanine pigment affinity, aromatic interactions, anchoring-group adsorption, and dispersant screening
Inorganic oxide dispersion	1309-37-1	I431739	Iron(III) oxide, dispersion	Nanoparticles, ≤110 nm particle size, 15 wt.% in ethanol	Used for evaluation of iron oxide nanoparticle dispersion, particle-size stability, and medium replacement
High-performance organic red pigment	980-26-7	P341832	Pigment Red 122	Industrial grade	Used for evaluation of quinacridone-type pigment dispersion, tinting-strength retention, and resin compatibility
High-tinting-strength organic violet pigment	6358-30-1	P665180	Pigment Violet 23	Tinctorial strength 95–105%	Used for evaluation of dioxazine-type pigment wetting, dispersion stability, and particle-size rebound during storage
Organic yellow pigment	6358-31-2	P730466	Pigment Yellow 74	Dye content 85%	Used for evaluation of azo-type organic pigment dispersion, color-development stability, and dispersant adsorption
High-performance organic red pigment	84632-65-5	P343316	Pigment Red 254	—	Used for evaluation of high-performance red pigment concentrates, steric stabilization, and long-term particle-size retention

Note: The above are representative Aladdin products related to scientific research and formulation studies. For more information on product specifications, grades, and COA details, search by “product name / CAS / item number” on the Aladdin official website.

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