1 What Is Carbomer: A Crosslinked Polyacrylic Acid Polymer

1.1 Basic Definition

Carbomer is a high-molecular-weight, nonlinear, crosslinked acrylic polymer. It is typically produced by polymerizing acrylic acid monomers and forming a three-dimensional crosslinked structure through a small amount of polyalkenyl polyether crosslinking agent.

In cosmetics and personal care products, carbomer is mainly used for thickening, dispersion, suspension, and auxiliary emulsion stabilization. From a structural perspective, however, its role is that of an aqueous-phase rheology modifier. Rheology refers to the flow and deformation behavior of a material under external force. What carbomer changes is not a single isolated parameter, but the overall way in which the aqueous phase flows.

1.2 Structural Framework

The structure of carbomer can be simplified as:

Polyacrylic acid backbone: [-CH2-CH(COOH)-]n + a small amount of crosslinked structure: a three-dimensional network formed by connection through allyl ethers of sucrose, allyl ethers of pentaerythritol, or other polyalkenyl polyether crosslinkers.

Here, [-CH2-CH(COOH)-]n represents the repeating unit of polyacrylic acid. The —COOH group in each repeating unit is a carboxyl group, which is the key structural feature responsible for the pH response and neutralization-induced thickening of carbomer.

When pH, or potential of hydrogen, changes, the existing form of the carboxyl group also changes:

Before neutralization: —COOH

After neutralization: —COO⁻ M⁺; here, M⁺ may represent a sodium ion, potassium ion, ammonium cation, or a cation formed after neutralization with an organic amine.

This transformation is the key to carbomer functionality. Before neutralization, the carbomer network is relatively contracted, and the system has low viscosity. After neutralization, the carboxyl groups are converted into negatively charged carboxylate groups. Repulsion between these negative charges causes the crosslinked network to swell, increasing the volume of the microgel particles. As a result, the viscosity, elasticity, and yield stress of the system all increase.

1.3 Structural Features and Performance Basis

The performance of carbomer is mainly determined by four structural features.

2 How Carbomer Changes from Powder into a Gel Structure

2.1 Dispersion and Hydration: The Prerequisite for Forming a Uniform Structure

Carbomer is usually a white, fluffy powder. After being added to the aqueous phase, it must first undergo dispersion and hydration. The key purpose of this stage is not immediate thickening, but allowing carbomer particles to fully contact water and form a uniformly dispersed state.

If the powder is added too quickly, the outer layer of the particles rapidly absorbs water and forms a gel-like film, trapping dry powder inside and producing “fish-eye” agglomerates. At this point, the system may become locally viscous, but a uniform polymer structure has not been formed. Even if a neutralizing agent is added later, problems such as particles, reduced transparency, and uneven local viscosity may still occur. Therefore, the first step in a carbomer formulation is sufficient dispersion rather than directly pursuing viscosity.

2.2 Neutralization and Ionization: The True Activation of Carbomer

To form a high-viscosity gel, carbomer usually requires appropriate neutralization. Common neutralizing agents include sodium hydroxide, potassium hydroxide, triethanolamine, and aminomethyl propanol. The specific choice depends on the target pH, product type, skin feel requirements, and formulation compatibility. The neutralization process can be summarized as follows:

2.3 pH Should Be Appropriate: Higher Is Not Always Better

Carbomer needs appropriate neutralization to fully perform its function, but pH is not simply a case of “the higher, the better.” At lower pH, carboxyl group ionization is insufficient, electrostatic repulsion is weak, network swelling is incomplete, and system viscosity remains low. As the degree of neutralization increases, the number of carboxylate groups rises, the microgel particles swell, and viscosity, elasticity, and yield stress increase.

The final rheological behavior is also jointly affected by carbomer grade, dosage, degree of neutralization, ionic strength, solvent composition, surfactants, and other ingredients. For conventional carbomer systems, when the pH is too low, carboxyl group ionization is insufficient and network swelling is incomplete. When the pH is too high, or when the electrolyte, surfactant, or solvent conditions in the system are not compatible, viscosity loss, changes in transparency, poorer skin feel, or reduced stability may also occur. Therefore, in actual formulation design, the target pH should be determined based on the specific carbomer grade and system conditions, rather than assuming that a higher pH will always produce stronger thickening.

3 From Microstructure to Macroscopic Rheology

3.1 Carbomer Gel Is Not an Ordinary Viscous Liquid

The gel formed by carbomer is a structured system composed of swollen, crosslinked microgel particles. These particles crowd, contact, and interact with one another in the aqueous phase, causing the system to exhibit non-Newtonian fluid behavior. A non-Newtonian fluid is a fluid whose viscosity changes with shear conditions. Carbomer gel behaves differently at rest, during dispensing, and during application precisely because its rheological state changes under external force.

3.2 Yield Stress: The Key to Suspension and Anti-Sagging

Yield stress refers to the minimum stress required for a system to begin continuous flow. When the external force is below the yield stress, the system behaves more like a soft solid and can maintain its shape. When the external force exceeds the yield stress, the system begins to flow noticeably.

Once carbomer gel has yield stress, many common product behaviors can be explained:

3.3 Shear Thinning: Stable at Rest, Easy to Spread During Use

Shear thinning refers to the decrease in apparent viscosity when a system is subjected to shear. Carbomer gel has a relatively strong structure at rest. When subjected to external forces such as pumping, squeezing, stirring, or spreading, its internal structure is partially disrupted or rearranged, viscosity decreases, and the product becomes easier to flow and spread.

This enables carbomer to meet two types of requirements at the same time. Carbomer does not simply create a heavy texture; rather, it provides structured flow that changes during use.

4 How Structure Translates into Functions in Daily Chemical Formulations

4.1 Thickening: Building the Aqueous-Phase Framework

The essence of carbomer thickening lies in building a structural framework in the aqueous phase through the swelling and interaction of crosslinked microgel particles. After carbomer is fully dispersed and appropriately neutralized, the volume of the microgel particles increases. The particles crowd one another, restricting the free flow of the aqueous phase. As a result, the system exhibits higher viscosity and stronger structural character. This effect is commonly used in:

4.2 Suspension: Using Yield Stress to Support Particles

For products containing insoluble particles, long-term stability cannot rely solely on high viscosity. Truly effective suspension requires the system to have sufficient low-shear structural strength at rest. Through its microgel network and yield stress, carbomer provides structural support for particles, pearlescent agents, capsules, or bubbles, making them less likely to migrate under gravity. The key factor determining suspension performance is not a single viscosity value, but whether the system has sufficient structural support under resting conditions.

4.3 Auxiliary Emulsion Stabilization: Restricting Oil Droplet Movement

In emulsion systems, carbomer is usually not the primary emulsifier, nor does it independently complete emulsification. Its main role is to strengthen the structure of the continuous phase, reducing the movement, collision, coalescence, and creaming risk of oil droplets, thereby helping improve emulsion stability.

4.4 Texture Modification: Creating a Perceptible User Experience

The effect of carbomer on product texture comes from its rheological characteristics. At rest, the carbomer network gives the product support, body, and a stable appearance. During application, shear thinning allows the product to spread more easily. After external force is removed, the system structure can recover to a certain extent, helping the product maintain a certain degree of adhesion. These properties are often used to design fresh-feeling gels, gel creams, lightweight emulsions, and water-light serums.

5 Why Carbomer Sometimes Fails

The performance of carbomer depends on whether the crosslinked microgel structure can be correctly formed and remain stable. Many formulation problems can be explained from a structural perspective.

5.1 Insufficient Dispersion: The Structure Has Not Been Uniformly Established

If carbomer powder is not sufficiently dispersed, agglomerates or “fish eyes” may form. In essence, the outer layer of the particles rapidly absorbs water and forms a gel-like barrier, preventing the interior from continuing to hydrate. Common manifestations include:

5.2 Insufficient Neutralization: Incomplete Network Swelling

If neutralization is insufficient, the carboxyl groups mainly remain in the —COOH form and cannot be fully converted into —COO⁻ M⁺. Negative-charge repulsion is insufficient, and the crosslinked microgel cannot fully swell. Possible results include:

5.3 Unsuitable pH: The Structural State Deviates from the Optimal Range

The rheological behavior of carbomer is closely related to pH. When pH is too low, carboxyl group ionization is insufficient. When the pH or overall system conditions are unsuitable, viscosity, transparency, skin feel, and stability may also be affected. A rational pH design needs to consider the following factors at the same time:

5.4 Excessive Electrolytes: Charge Repulsion Is Screened

The swelling of neutralized carbomer depends on the repulsion between —COO⁻ groups. Salts, electrolytes, multivalent ions, or high-ionic-strength systems may screen this charge repulsion, causing the microgel network to contract or lose structural strength. This may lead to:

Certain mineral salts, salt-form active ingredients, botanical extracts, surfactant systems, or high-electrolyte raw materials may all affect the carbomer structure. For traditional carbomer systems, high ionic strength usually weakens charge repulsion and leads to viscosity loss. However, in specially designed salt-tolerant, low-pH, or surfactant-compatible acrylic polymer systems, the effect of salts may differ. Therefore, in high-salt or complex systems, the carbomer grade, applicable system, and formulation compatibility should be evaluated first, and confirmation should be made through small-scale trials and stability testing.

5.5 Mismatched Grade Selection: Structural Advantages Cannot Be Fully Realized

Different carbomer grades vary in crosslinking density, dispersion rate, transparency, viscosity level, electrolyte tolerance, and shear resistance. Carbomer 940, Carbomer 980, Carbomer 934, Carbomer 941, and other grades are not identical. Actual grade selection should be determined according to the product system.

6 Summary of the Structure–Performance–Formulation Function Relationship

The formulation functions of carbomer can be summarized as one complete chain:

Polyacrylic acid backbone → large number of carboxyl groups → formation of —COO⁻ M⁺ after neutralization → enhanced negative-charge repulsion → swelling of crosslinked microgel particles → particle crowding and contact → formation of viscosity, elasticity, yield stress, and shear thinning → thickening, suspension, auxiliary emulsion stabilization, and texture modification

7. Classification Table of Representative Chemicals Related to Carbomer

Note: The products listed in the table are mainly intended for scientific research, formulation research, or method development. Whether they are suitable for cosmetics, pharmaceuticals, medical devices, or other end products should be determined based on the product COA, SDS, quality grade, regulatory requirements, and actual application validation.

Table 1. Carbomer Structural Framework, Acrylic Polymers, and Structure-Related Monomers

Table 2. Neutralizers, pH Adjusters, and Buffer-Related Raw Materials

Table 3. Dispersion, Pre-Wetting, and Hydroalcoholic Gel-Related Raw Materials

Table 4. Electrolytes, Chelating Agents, Surfactants, and Rheological Control Materials

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

References

[1] Cosmetic Ingredient Review Expert Panel. Final Report on the Safety Assessment of Carbomers-934, -910, -934P, -940, -941, and -962. Journal of the American College of Toxicology, 1982, 1(2): 109–141.

[2] CosmeticsInfo. Carbomer. Personal Care Products Council, Ingredient Safety Information.

[3] Lubrizol Advanced Materials. Carbopol® Polymer Excipients - Homopolymers, Copolymers, Interpolymers. Lubrizol Corporation.

[4] Kim J Y, Song J Y, Lee E J, Park S K. Rheological properties and microstructures of Carbopol gel network system. Colloid and Polymer Science, 2003, 281: 614–623.

[5] Gutowski I A, Lee D, de Bruyn J R, Frisken B J. Scaling and mesostructure of Carbopol dispersions. Rheologica Acta, 2012, 51: 441–450.

[6] Piau J M. Carbopol gels: Elastoviscoplastic and slippery glasses made of individual swollen sponges. Journal of Non-Newtonian Fluid Mechanics, 2007, 144(1): 1–29.

[7] Islam M T, Rodríguez-Hornedo N, Ciotti S, Ackermann C. Rheological characterization of topical carbomer gels neutralized to different pH. Pharmaceutical Research, 2004, 21(7): 1192–1199.

[8] Lubrizol Advanced Materials. Neutralizing Carbopol® and Pemulen™ Polymers in Aqueous and Hydroalcoholic Systems. Lubrizol Corporation.

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