How can cellulose ether improve the leveling and anti sagging performance of decorative coatings? ​

cellulose etherAs key additives in decorative coatings such as latex paint, interior wall paint, wood paint, etc., hydroxyethyl cellulose HEC, methyl cellulose MC, carboxymethyl cellulose sodium CMC, etc., regulate the rheological properties (viscosity, thixotropy, etc.) of the coating system, optimize the interfacial behavior during coating formation, and achieve improved leveling and anti sagging performance. The specific functions and mechanisms are as follows:

1、 Improvement effect and mechanism of convection flatness

Flattening refers to the ability of the wet film to automatically flow and develop into a flat and smooth surface after coating application (such as brushing, rolling, spraying). Its core is the flow and repair ability of the wet film under the action of gravity and surface tension. Cellulose ether promotes leveling through the following mechanisms:

Adjust low shear viscosity to promote wet film flow

Cellulose ether molecular chains form hydrogen bonds with water molecules through hydroxyl groups (- OH), creating a three-dimensional network structure that ensures moderate viscosity of the coating under low shear forces (such as wet film standing after construction)

If the viscosity is too high, the wet film is difficult to flow due to gravity, and it is prone to residual brush marks and roller marks;

cellulose etherBy controlling the molecular chain length (degree of polymerization) and degree of substitution, the low shear viscosity is maintained at 500~5000 mPa · s (depending on the type of coating), ensuring that the wet film has sufficient fluidity to fill construction defects (such as brush mark gaps) and avoiding uneven coating caused by excessive flow.

For example, adding 0.2%~0.5% HEC to interior latex paint reduces low shear viscosity by 30%~50% compared to the system without additives. The brush marks after brushing can automatically level within 5-10 minutes, and the smoothness of the coating film is significantly improved.

Reduce surface tension gradient and minimize the phenomenon of "retraction"

After coating application, if there is a local thickness difference on the wet film surface (such as thinner edges), a surface tension gradient will be formed due to the different evaporation rates of water, causing the liquid to flow towards the low surface tension area, resulting in "shrinkage" (such as thickening of the coating edge and depression in the middle). Cellulose ether:

The hydrophilic groups (- OH, - OCH, etc.) in its molecular chain can be uniformly distributed on the surface of the wet film, reducing the unevenness of surface tension;

At the same time, cellulose ether cooperates with film-forming agents (such as ethylene glycol butyl ether) in coatings to slow down the rate of water evaporation, giving the wet film enough time to balance surface tension through flow and reduce shrinkage.

Compatibility with pigments/fillers to avoid particles obstructing leveling

Decorative coatings contain pigments/fillers such as titanium dioxide and calcium carbonate. If they are unevenly dispersed, they can form particle aggregates, hindering the flow of wet films. Cellulose ether molecular chains can be adsorbed onto the surface of pigment particles

Form a spatial barrier to prevent particle aggregation and ensure system uniformity;

Make the particles move synchronously with the liquid phase during wet film flow, avoiding unevenness caused by particle accumulation.

2、 Improvement effect and mechanism of anti sagging performance

Anti sagging property refers to the ability of a coating to avoid excessive sagging and flow due to gravity when applied on vertical surfaces such as walls and facades. The core is that the wet film can quickly restore its viscosity and prevent flow after the high shear force (brushing/spraying force during construction) disappears. Cellulose ether achieves this function through "thixotropic effect" and "structural viscosity":

Thixotropic properties: high shear viscosity reduction, low shear viscosity increase

The three-dimensional network structure formed by cellulose ether exhibits thixotropy (shear thinning):

During construction (under high shear force), the mesh structure is damaged, the molecular chains are disentangled, and the viscosity of the coating is temporarily reduced (to facilitate painting/spraying and ensure smooth construction);

After construction (shear force disappears), the molecular chains quickly re entangle through hydrogen bonds, and the network structure is reconstructed. The viscosity significantly increases within seconds to minutes (usually returning to more than 80% of the initial viscosity), forming a "rigid" wet film that resists gravity sagging.

For example, after adding 0.3% MC to the exterior wall coating, the viscosity during construction decreases from 5000 mPa · s to 1000 mPa · s (easy to apply), and within 5 minutes after construction, the viscosity returns to 4000 mPa · s, which can effectively prevent "tear like" sagging on vertical walls.

Enhanced structural viscosity to increase wet film yield value

cellulose etherThe structural viscosity formed by physical cross-linking (hydrogen bonding, van der Waals forces) of long-chain molecules gives the coating a yield value (the critical value at which a certain external force is required to promote flow). The anti sagging requirement is that the yield value of the wet film should be higher than the shear stress generated by its own gravity:

Cellulose ether can increase the yield value (usually controlled between 5-20 Pa) by increasing the molecular chain density (by increasing the amount added or selecting a high degree of polymerization type);

For thick coating materials (such as textured coatings), a higher yield value can ensure that the wet film thickness does not flow even when it reaches 100-500 μ m.

Collaborate with other components to strengthen the anti sagging network

Cellulose ether can interact with lotion particles (such as acrylate lotion) and thickeners (such as associative polyurethane) in coatings:

The molecular chains are adsorbed on the surface of lotion particles, forming a "polymer particle" composite network to enhance the structural stability;

Combining with the hydrophobic groups of the associative thickener further enhances the viscosity recovery rate and anti sagging strength of the system.

3、 Balanced regulation of leveling and anti sagging properties

There is a certain contradiction between leveling and anti sagging (excessive pursuit of anti sagging may lead to leveling errors, and vice versa), and cellulose ether achieves balance through the following methods:

Molecular Structure Design

Choose cellulose ethers with moderate degrees of substitution (such as HEC with a degree of substitution of 1.8-2.5): Ensure that the molecular chain has sufficient hydrophilicity to form a hydrogen bond network (anti sagging), while not being excessively rigid (conducive to leveling);

Control molecular weight: Low molecular weight cellulose ether (such as MC with a molecular weight of 100000 to 200000) has good leveling but weak resistance to sagging, while high molecular weight (300000 to 500000) is the opposite. In practice, mixed molecular weight models are often used for adjustment.

Add quantity optimization

Interior wall latex paint: addition amount of 0.1%~0.3%, focusing on leveling (avoiding brush marks);

Exterior wall thick coating: Add 0.3% to 0.8%, with a focus on anti sagging properties (to prevent the facade from flowing).

Collaborate with other additives

Combined with a small amount of rheological additives (such as gas-phase silica): silica provides a rigid skeleton to enhance anti sagging, while cellulose ether ensures leveling. The synergy between the two can prevent sagging and eliminate roll marks when the coating is applied in a vertical plane.

summary

The improvement of leveling and anti sagging properties of decorative coatings by cellulose ether is essentially achieved by regulating the rheological properties of the system (viscosity, thixotropy, yield value):

Leveling depends on its moderate viscosity and molecular chain fluidity under low shear, promoting wet film repair;

The anti sagging property relies on its rapid recovery of structural viscosity and thixotropic effect after shearing, preventing the sagging of wet film.

By selecting the appropriate type, molecular weight, and dosage, and coordinating with other additives, a better balance between the two can be achieved, ensuring convenient coating application, smooth coating, and no sagging defects.