1. The demand for additives in the coating industry and the compatibility of CMC
As a decorative and protective material covering the surface of an object, the performance of coatings depends on the synergistic effect of film-forming substances, pigments, fillers, additives and solvents. Among them, additives are crucial to the construction, rheology, storage stability and film-forming quality of coatings. As a water-soluble polymer, sodium carboxymethyl cellulose (CMC) has unique advantages in coating systems due to its thickening, suspension, film-forming and colloid protection properties: in terms of molecular structure characteristics, the carboxyl group (-CH₂COONa) on the CMC molecular chain ionizes it in water to form a negatively charged polymer chain, which combines with water molecules through hydrogen bonds and electrostatic effects, significantly improving the viscosity of the system; in terms of environmental friendliness, it is water-soluble and non-toxic (in line with environmentally friendly coating standards), and can replace some organic solvent-based additives; in terms of cost advantage, compared with synthetic polymer thickeners, CMC uses natural cellulose as raw material, is relatively cheap and has a wide range of sources.
2. Core functions and mechanisms of action of CMC in coatings
2.1 Thickening and rheological regulation
The working principle is that CMC forms a network structure after dissolving in water, increases the viscosity of the coating through intermolecular hydrogen bonds and entanglement effects, and regulates its rheological behavior (such as thixotropy). Specific applications include thick paste coatings (such as fire retardant coatings and textured coatings), where the thickening effect of CMC can prevent the sedimentation of pigment and filler particles and maintain the uniformity of the system; in terms of improving construction properties, it can adjust the "brushability" and "rollerability" of the coating to avoid sagging (drooping) or splashing. For example, in architectural latex paints, adding 0.1%~0.5% of high-viscosity CMC can optimize the construction feel.
2.2 Suspension and stability maintenance
CMC has a colloidal protective effect. Its molecules are adsorbed on the surface of pigment and filler particles to form electrostatic repulsion and steric hindrance, prevent particle agglomeration, and improve the storage stability of the coating. A typical example is that in water-based inorganic coatings (such as silicate coatings), CMC can effectively suspend metal oxide pigments, avoid stratification, and extend the shelf life of the product.
2.3 Optimization of film-forming properties
CMC can be used as a film-forming aid. Its molecules can be embedded in the network structure of the coating film-forming substance (such as polymer emulsion) to improve the flexibility and crack resistance of the film. In terms of application scenarios, in interior and exterior wall coatings, adding CMC can reduce the drying shrinkage of the paint film, reduce the risk of cracking, and improve the adhesion of the paint film to the substrate.
2.4 Improved water resistance and impermeability
By controlling the degree of substitution (DS) and degree of polymerization (DP) of CMC, its water resistance can be adjusted. For example, CMC with a high degree of substitution (DS>0.8) forms a denser network in the paint film, reducing the penetration of water molecules. In industrial coating applications, in anti-corrosion coatings, CMC is compounded with epoxy resin to enhance the salt water resistance and chemical corrosion resistance of the paint film.
3. Specific applications of CMC in different types of coatings
3.1 Water-based architectural coatings
In latex paints, CMC is used as the main thickener or auxiliary thickener to adjust the viscosity to adapt to construction methods such as spraying and brushing, while improving the leveling and hiding power of the paint film. For example, in styrene acrylic latex paint, adding 0.3% medium viscosity CMC can stabilize the Stormer viscosity (KU value) of the paint in the range of 90~100, meeting the construction requirements; in putty and mortar paint, CMC acts as a binder and water retainer to prevent cement-based putty from drying too quickly and causing cracking, while improving the scraping property.
3.2 Industrial coatings
In metal surface treatment coatings and electrophoretic coatings, CMC acts as a colloid protective agent to stabilize pigment dispersions and ensure coating uniformity; in ship coatings, it is used to adjust the viscosity of antifouling paints and control the release rate of antifouling agents; in wood coatings and water-based wood coatings, CMC is used in combination with hydroxyethyl cellulose (HEC) to balance the thickening effect and film transparency of the coating and avoid whitening of the paint film.
3.3 Special functional coatings
In fire retardant coatings, CMC is used as a carbonizing agent for intumescent fire retardant coatings. It decomposes at high temperature to form a carbonized layer, which enhances the fire retardant effect; in waterproof coatings, in cement-based waterproof coatings, the water retention effect of CMC can promote cement hydration and improve the impermeability and compressive strength of the coating.
4. Technical points and optimization strategies for CMC in coating applications
4.1 Selection of CMC types
The viscosity of high-viscosity CMC (2% aqueous solution) is 5000~20000 mPa・s, which is mainly used in thick paste coatings and anti-settling systems. The advantages are strong suspension and good anti-settling effect; the viscosity of low-viscosity CMC (2% aqueous solution) is 50~500 mPa・s, which is mainly used in latex paints and systems with high leveling requirements. The advantages are good fluidity and high film transparency.
4.2 Compounding technology with other additives
When compounded with associative thickener (HASE), CMC provides basic viscosity, HASE enhances shear thinning properties, and improves the anti-sagging and construction properties of the coating; when synergistically acting with dispersants, CMC is used in combination with anionic dispersants (such as sodium polyacrylate) during the pigment grinding stage to improve the dispersion efficiency of the pigment and reduce the viscosity fluctuation of the system.
4.3 Construction process optimization
The dissolution method is to pre-prepare CMC into a 2%~5% aqueous solution (slowly stirred and heated to about 60℃ to avoid agglomeration), and then add it to the coating system; in terms of pH value control, CMC has the best stability under neutral to weakly alkaline conditions (pH 7~9), and the acidic system (pH<5) is prone to viscosity reduction, and a pH regulator (such as ammonia) needs to be added.
5. Trends in the coatings industry and innovative applications of CMC
Environmentally friendly coatings drive the growth of CMC demand: As VOCs (volatile organic compounds) emission restrictions become stricter, the proportion of water-based coatings increases. As a high-efficiency additive for water-based systems, CMC is expected to grow at an average annual rate of 4.5% (data source: Global Market Insights);
Development of functionalized CMC: Through etherification modification (such as the introduction of hydrophobic groups) or compounding with nanomaterials (such as TiO₂), multifunctional coating additives with antibacterial and weather-resistant properties are prepared;
Intelligent production adaptation: CMC with high substitution and low impurities can meet the requirements of automated color matching systems for additive stability and promote the digital transformation of coating production;
Sustainable development orientation: Developing bio-based CMC with regenerated cellulose as raw material to reduce dependence on fossil resources is in line with the "carbon neutrality" goal of the coatings industry.
6. Summary
Due to its unique rheological properties and environmentally friendly properties, sodium carboxymethyl cellulose has become an indispensable functional additive in the coatings industry. From optimizing the construction properties of architectural coatings to upgrading the performance of industrial coatings, the application of CMC runs through the entire chain of coating production and construction. In the future, as coating technology develops towards high performance and greenness, CMC will continue to provide more efficient and sustainable solutions for the coating industry through molecular structure design and compounding technology innovation.
