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Hydrophobically modified alkali swelling associative acrylic thickener (HASE) is a mainstream rheological additive in water-based coatings and lotion systems. Different from ordinary cross-linked alkali swelling thickeners (ASE), HASE molecules contain a large number of carboxyl groups in the main chain and hydrophobic monomers in the side linking branches; Constructing a three-dimensional network through a dual mechanism of pH induced chain extension and dynamic association of hydrophobic groups. In this paper, the synthesis process of HASE lotion polymerization and the regulation of molecular structure are systematically described; The interaction between thickener and anionic acrylic lotion (styrene acrylic/pure acrylic) was emphatically analyzed, and the effects of emulsifier type, particle size, acid value, and additives (wetting dispersion, defoamer) on the compatibility of lotion were studied; Analyze the causes of compatibility defects such as water separation, post thickening, foaming, and decreased gloss during storage, and provide molecular design schemes and on-site formula optimization strategies. The results showed that the optimal comprehensive performance was achieved when the molar ratio of hydrophobic monomers was 1.8%~3.2% and the carbon chains were C12~C16; The total amount of anionic/nonionic emulsifiers and free surfactants in the lotion system are the core factors to destroy the association network; The compatibility stability with acrylic lotion can be significantly improved by adjusting the hydrophobic group structure of thickener, optimizing the system pH, and using HASE+HEUR compound.

Acrylic lotion is widely used for emulsion paint, water-based adhesive and paper coating. It is difficult to meet the construction requirements of anti settlement, anti sagging and roll coating flow by relying solely on the viscosity of lotion itself.  

The three mainstream systems of rheological additives:

1) Cellulose ether (HEC): only thickens the aqueous phase, with insufficient high shear viscosity, affecting gloss and water resistance;  

2) ASE ordinary alkali swelling acrylic acid: no binding groups, no physical cross-linking between molecules, weak rheological control ability;  

3) HASE associative acrylic thickener: It combines alkali swelling and hydrophobic association, and can simultaneously connect water phase and latex particles, achieving ideal pseudoplastic rheology of “low shear anti sagging, high shear easy flow”.

Domestic lotion manufacturers generally use anionic/non compounded emulsion system (AES/SDS+fatty alcohol non-ionic, no APEO system). Free emulsifiers on the surface of lotion will compete to adsorb HASE hydrophobic end, which is prone to collapse of thickening efficiency, water separation during storage, thickening after a long time, and increase of foam. Therefore, it is very important for lotion formula and paint mixing process to master the laws of HASE synthesis molecular design and clarify the matching boundary between thickener and acrylic lotion.

Distinguishing concepts:

✅  HASE: Combination type acrylic thickener (research object of this article, acrylic skeleton)

✅  HEUR: Polyurethane associative thickener (non acrylic skeleton, often used in conjunction with HASE)

Principle and Process of Synthesis of HASE Associated Acrylic Thickener

2.1 Monomer system (lotion polymerization formula system)

Acidic hydrophilic main monomers acrylic acid (AA) and methyl acrylic acid (MAA). Provide carboxyl groups to achieve pH responsive swelling; The molar ratio of AA/MAA is 7:3 to 9:1. The carboxyl content determines the degree of molecular chain extension after neutralization: too high carboxyl group → too strong hydrophilicity, weakened binding, and decreased water resistance; Carboxyl group too low → insufficient swelling, low thickening efficiency.

Flexible copolymer monomers (alkyl acrylate) ethyl acrylate EA, methyl methacrylate MMA. Adjust the glass transition temperature Tg of the polymer to avoid excessive rigidity of the polymer chain, which is beneficial for hydrophobic groups to approach each other and form associations.

Core: Hydrophobic binding functional monomer long-chain alkyl polyoxyethylene ether (methyl) acrylate.

Alkyl carbon chain: preferably C12~C16; C<8 has extremely weak binding ability; C ≥ 18, water solubility becomes poor, thickening agent lotion is easy to delaminate when stored;

EO polyoxyethylene chain link n=10~25: The longer the EO is, the more compatible it is with lotion, but the hydrophobic association strength decreases;

Optimal addition range: accounting for 1.8% to 3.2% of the total monomer mole fraction, less than 1.5%: only alkali swelling effect, weak binding network, rheological properties close to ordinary ASE; Above 3.5%: The hydrophobicity of the polymer is too strong, and it cannot be fully dissolved in water after neutralization, resulting in a decrease in thickening efficiency and a tendency for the system to become turbid.

Optional functional monomer small amount AMPS (2-acrylamido-2-methylpropanesulfonic acid): improves electrolyte stability and is suitable for high pigment volume concentration PVC systems; Micro crosslinking monomer (multifunctional acrylic ester): Moderate crosslinking enhances low shear viscosity, while excessive crosslinking directly loses binding ability.

2.2 Synthesis process (semi continuous lotion polymerization, general for industrialization)

System emulsifier: low free anionic/non compounded emulsifier is used (to avoid a large amount of free surface activity carried by the finished HASE and interfere with the compatibility of downstream lotion);

Reaction temperature: 75-83 ℃; Initiator APS (ammonium persulfate), dosage 0.4% to 1.0%;

operating procedure

① Pour bottom water into the reaction kettle, heat up to the preset temperature, and add some initiator;  

② The monomer, emulsifier and deionized water are pre emulsified at high speed to prepare stable pre lotion;  

③ The pre lotion+initiator aqueous solution is added at a constant speed synchronously, and the dropping time is 3h;

④ After the dropwise addition is completed, it should be kept warm and matured for 90 minutes to eliminate residual monomers;  

⑤ Cool down, filter, and obtain a milky white HASE thickener solution (solid content of 28% to 32%).

2.3 Impact of Key Synthesis Parameters on Product Performance

1) The optimal range for molecular weight is between 50000 and 150000; Low molecular weight → short molecular chain, unable to form a continuous binding network; High molecular weight → the system is prone to gel, difficult to feed, and poor water separation resistance; It can be regulated by a small amount of thiol chain transfer agents. 2) Pre emulsification and uniform dropwise addition of hydrophobic monomers to achieve random copolymerization of hydrophobic units; If hydrophobic monomer is locally enriched, thickener itself is easy to agglomerate, and compatibility stability with the lotion is greatly reduced.  

3) Control the synthesis stage with free emulsifiers to minimize free surface activity as much as possible; HASE finished products come with a large amount of surfactants, which compete for binding sites between the coating system and latex particles, making them the most common compatibility killer.

3 Interaction and adaptation mechanism of HASE and acrylic lotion

3.1 Dual binding mode (pH 7.5-9.5 after neutralization)

HASE undergoes neutralization with ammonia/organic amine, ionization of carboxyl groups, and electrostatic repulsion to fully stretch the coiled polymer chains, resulting in two types of associations:

Intermolecular association: Different HASE macromolecules aggregate at hydrophobic ends to form physical crosslinking points;

Particle association (most important): HASE hydrophobic groups are adsorbed on the surface of acrylic emulsion particles, connecting a large number of dispersed lotion particles into an overall three-dimensional network.

Rheological essence: network → low shear high viscosity (anti settling, anti sagging); Shear external force destroys hydrophobic association → viscosity decreases (construction flow); Shear elimination, rapid reconstruction of association.

3.2 How the parameters of lotion affect the adaptability

(1) Lotion emulsification system (core influencing factors)

Anionic lotion (styrene acrylic/pure acrylic, industry mainstream)

① The lotion contains a large amount of free anionic emulsifier (SDS/AES): free surface activity will wrap the hydrophobic side chain of HASE, shielding association, which directly shows that the viscosity still cannot rise when a large amount of thickener is added;  

② The content of non-ionic emulsifier in lotion is high: fatty alcohol polyoxyethylene ether can form competitive association with HASE hydrophobic group;

Phenomenon comparison: high-quality lotion with low free emulsifier → HASE thickening efficiency is high, and the addition amount is 0.3%~0.8%; Ordinary low-cost styrene acrylic lotion (high residual emulsifier) → the same thickening target, the dosage is increased by more than 50%, and it is easy to separate water.

(2) Latex particle size

Small particle size lotion (80~140 nm): large specific surface area, can provide a large number of adsorption sites, and has strong association with HASE;

Large particle size lotion (>220 nm): low specific surface area, insufficient association point, relying solely on intermolecular association, poor storage stability, easy to delaminate.

(3) Acid value of lotion

The content of polyacrylic acid (AA) in lotion is high, and the negative charge density on the surface of latex particles is large; Electrostatic repulsion hinders the HASE hydrophobic groups from approaching the latex surface, weakening particle binding.

(4) Comparison of lotion types

Pure acrylic lotion: the soft and hard monomers are balanced, the particle surface polarity is moderate, and the HASE adaptability is optimal;

Styrene acrylic lotion: Styrene chain segment has strong hydrophobicity, which is more conducive to adsorption with HASE hydrophobic groups; However, styrene acrylic lotion often uses more emulsifiers, so the risk of free surface activity is higher;

Self crosslinking lotion (including NMA, etc.): the surface polarity increases, and some models are prone to slow thickening after compatibility.

3.3 Synergistic/conflicting effects of other additives in coating formulations

Wetting agents (non-ionic surfactants) strongly compete for binding! High HLB non-ionic wetting agents (such as X-405) preferentially bind to HASE hydrophobic tails, disrupting the network; Solution: Wetting agent should be added during the grinding stage as much as possible, while HASE should be slowly added during the later stage of paint adjustment.

Mineral oil defoamers contain hydrophobic silica, which can interact with HASE hydrophobic groups, leading to system thickening and flocculation in severe cases; Organic silicone defoamers have a relatively smaller impact.

Film forming aids, propylene glycol and other co solvents, with a small amount of co solvent, moderately soften hydrophobic micro areas and improve leveling; Excessive addition can disrupt hydrophobic binding and significantly decrease viscosity.

4 Common Compatibility Defects, Causes, and Solutions (Practical Focus)

4.1 Defect 1: Low thickening efficiency, slow increase in viscosity due to the addition of multiple thickeners

Cause of formation: lotion free emulsifier and non-ionic wetting agent compete for adsorption, blocking the association between HASE and latex particles.  

Countermeasure:

① Low free emulsifier lotion is preferred;  

② Reduce excessive wetting agents in the system;  

③ Select modified HASE with longer hydrophobic carbon chains to enhance binding competitiveness;

④ HASE is compounded with a small amount of HEUR polyurethane thickener.

4.2 Defect 2: water separation/lotion (supernatant) occurs when the coating is stored at room temperature

Cause: The binding force between HASE and latex particles is relatively weak, and the binding network dissociates under static conditions; The aqueous phase is separated from the lotion phase.  

Optimization plan:

1) Synthesis end: Moderately increase the proportion of HASE hydrophobic monomers;  

2) Formula side: Do not use HASE alone; HASE combined with a small amount of HEUR to construct a dual network;  

3) Maintain the pH of the control system at 8.0-8.8 to avoid pH fluctuations that could disrupt the ionization state.

4.3 Defect 3: The initial viscosity is normal after the paint mixing is completed, but it slowly and continuously thickens after being left for a week (later thickens)

Mechanism: Slow migration and redistribution of free surfactants within the system; HASE hydrophobic groups slowly and gradually complete their association, and the network continues to strengthen.  

solve:

① Lotion screening: mature styrene acrylic/pure acrylic lotion with low thickening after priority;

② Choose HASE with moderate hydrophobicity to avoid high hydrophobicity models;  

③ The pH of the system should not be too high (>9.0, thickening will occur after intensification).

4.4 Defect 4: The system foam increases and defoaming is difficult

Reason: HASE itself belongs to surface active polymers, and its molecular structure has amphiphilicity, which can enhance the foam stability of the system; In particular, foam increases significantly when it is mixed with high free emulsifier lotion. Countermeasure: Add HASE during the paint mixing stage, do not add it during the grinding stage; Compatible with shear resistant silicone defoamers.

4.5 Defect 5: Decreased gloss and increased haze of high gloss paint film

Cause: Excessive HASE enrichment occurs at the interface of latex particles, and the film-forming process migrates to the surface of the paint film; Hydrophobic groups form microphase separation. Countermeasure: Adopt the minimum effective addition amount; High gloss topcoat system reduces the proportion of HASE and increases the proportion of HEUR.

5 General guidelines for selection and formula adaptation

5.1 HASE selection ideas corresponding to different lotion

Interior wall emulsion paint, ordinary styrene acrylic lotion (high free emulsifier): medium hydrophobic general purpose HASE, cost-effective priority; Can be paired with a small amount of HEC;

Exterior wall emulsion paint, pure acrylic lotion, semi gloss finish: medium and high hydrophobic HASE, improving water separation resistance and rheological balance;

High gloss water-based varnish, wood lotion: low hydrophobic HASE+HEUR compound to avoid gloss loss caused by a single HASE;

Adhesive, high solid content lotion system: electrolyte resistant modified HASE (introduction of sulfonic acid monomer) is selected.

5.2 Recommended addition range (based on the total mass of the coating)

Single HASE system: 0.25% to 0.9%; Composite system (HASE: HEUR=3:1~5:1): Total thickening agent 0.2%~0.7%.

5.3 Standard addition process (reducing compatibility issues)

Grinding stage: Input dispersants, wetting agents, and fillers;

Paint mixing stage: add lotion, film forming agent and defoamer;

Finally, slowly add the diluted HASE thickener and stir evenly at low speed;  

❌  Do not pour concentrated liquid directly and quickly, as it may cause local excessive aggregation and produce flocculent particles.

6 Conclusion

The optimal synthesis window for HASE associated acrylic thickening agent: hydrophobic monomer molar ratio of 1.8%~3.2%, alkyl carbon chain of C12~C16; By semi continuous lotion polymerization, controlling the molecular weight of 50000~150000 and reducing the free emulsifier of the finished product, products with excellent compatibility with acrylic lotion can be obtained.

The compatibility of HASE with styrene acrylic and pure acrylic anionic lotion mainly depends on the content of free emulsifier in lotion; Free anionic/nonionic surfactants compete for hydrophobic binding sites, significantly reducing thickening efficiency.

The three common compatibility defects of storage separation, post thickening, and foaming are essentially the disruption of the dynamic equilibrium of the hydrophobic binding network; The most economical and effective improvement method at the formula level is to use HASE and HEUR polyurethane thickening compound, stabilize pH, and standardize feeding sequence.

Lotion manufacturers and coating formulation engineers should not only look at the initial thickening capacity when selecting matching models, but must carry out storage stability test at room temperature for 30 days to evaluate the long-term association equilibrium state.