Introduction
Surface sizing is an essential step in modern papermaking, directly influencing paper strength, smoothness, water resistance, and printability. Sodium Carboxymethyl Cellulose (CMC) is a high-performance functional additive widely used in surface sizing formulations. With its excellent film-forming ability, viscosity control, and dispersing properties, paper-grade CMC enhances paper quality while improving system stability and reducing overall production costs.
I. Key Functions of CMC in Surface Sizing
1. Improving Surface Strength
Paper-grade CMC provides strong film-forming capability. When incorporated into surface sizing, it forms a uniform and dense protective layer on the sheet surface, improving fiber-to-fiber bonding. This significantly increases surface strength, abrasion resistance, and fold endurance-properties that are particularly important for printing papers, coated grades, and packaging papers.
2. Enhancing Smoothness and Printability
CMC solutions exhibit favorable rheological behavior, allowing precise control of viscosity and penetration during the sizing process. This leads to more uniform coating distribution, resulting in:
smoother sheet surfaces,
more consistent ink absorption,
reduced dusting and linting,
and improved print clarity and appearance.
These advantages make CMC an ideal additive for high-quality printing papers.
3. Increasing Water Resistance and Dimensional Stability
Although CMC contains hydrophilic carboxyl groups, it also demonstrates useful crosslinking capability. When used together with starch, PVA, or other sizing agents, CMC helps:
reduce fiber swelling,
improve resistance to water penetration,
and enhance dimensional stability under humid conditions.
This makes CMC particularly valuable for packaging boards and writing & printing papers that require higher moisture resistance.
4. Stabilizing the Sizing Solution
CMC serves as an effective stabilizer and protective colloid in surface sizing systems. Its presence helps:
prevent particle aggregation and sedimentation,
maintain consistent viscosity throughout storage and operation,
reduce the risk of nozzle clogging,
and ensure uniform film formation.
This contributes to smoother production and more reliable papermaking performance.
II. Synergistic Effects with Other Sizing Agents
Surface sizing formulations often combine CMC with starch, PVA, or emulsion binders. CMC provides synergistic improvements that enhance overall system performance:
With Starch: increases viscosity, improves film formation, and reduces brittleness.
With PVA: improves gloss development and fold endurance.
With Emulsion Systems: enhances wet rub resistance and optimizes surface tension.
Experimental results show that a CMC dosage of 0.2%–0.5% can significantly improve sizing efficiency without affecting runnability.
III. Key Selection Criteria for Paper-Grade CMC
To achieve optimal performance, papermakers should consider the following parameters:
• Degree of Substitution (DS)
A DS between 0.6 and 0.9 ensures good solubility, electrolyte stability, and uniform distribution in sizing formulations.
• Viscosity Range
Typical paper-grade CMC has a viscosity of 400–2,000 mPa·s (2% solution). Selecting the right viscosity depends on machine speed, application method, and target paper grade.
• Purity and Ash Content
High-purity, low-ash CMC minimizes sedimentation, prevents defects such as spots or streaks, and maintains solution clarity.
• Compatibility
The selected CMC must be fully compatible with starch, PVA, and emulsion systems without causing flocculation or instability.
IV. Advantages of Zibo Hondo's Paper-Grade CMC
Zibo Hondo offers a full range of high-quality CMC products designed specifically for papermaking applications. Our products provide:
High purity and low ash content for clean and stable sizing systems
Excellent solubility and solution clarity, suitable for high-speed coating and film-press machines
Superior dispersing and film-forming performance for improved paper quality
Customizable viscosity and DS options to meet different machine conditions and customer requirements
Our CMC products are widely used in coated paper, writing & printing paper, specialty grades, and packaging board, earning strong recognition from papermakers worldwide.
Conclusion
CMC plays a vital role in surface sizing by improving surface strength, smoothness, water resistance, and printability. Through proper selection and formulation design, CMC offers both performance and cost advantages. As the demand for high-performance and environmentally responsible paper products continues to rise, CMC will remain an important additive in the future of papermaking.
