Author: Unionchem Publish Time: 2026-02-04 Origin: Qingdao Unionchem Co.,Ltd.
In the paper industry, the margin between a premium sheet and a reject often lies in the additives. Papermakers are constantly balancing the need for higher machine speeds with the demand for stronger, smoother paper.
Carboxymethyl Cellulose (CMC) is often referred to as "liquid fiber" because of its chemical similarity to cellulose pulp. This makes it one of the most effective additives for increasing dry strength and modifying coating rheology.
However, the CMC used in the wet end is vastly different from the CMC used in the size press. Choosing the wrong viscosity or substitution degree can lead to foaming, poor retention, or coating streaks. Here is a guide to selecting the right CMC for your paper mill.
CMC is versatile because it can be applied at two different stages of the paper machine.
The Goal: To bond fibers together within the sheet.
The Mechanism: CMC is anionic. When used with a cationic wet-strength resin or starch, it forms a complex that bridges fibers.
The Benefit: It significantly improves Tensile Strength and Burst Strength. It also acts as a dispersion agent, helping to distribute fibers evenly for a more uniform sheet formation.
The Goal: To seal the surface and control water/oil absorption.
The Mechanism: CMC forms a continuous, tough film on the paper surface.
The Benefit: It improves Pick Resistance (preventing surface fibers from being pulled off during printing) and reduces porosity. In coating colors, it acts as a Water Retention Agent, preventing the binder from migrating too quickly into the base paper.
(Explore our specialized paper grades in our Carboxymethyl Cellulose (CMC) Product List.)
To optimize performance, you must match the CMC grade to the specific application point.
For Wet End: Medium to High Viscosity is often preferred to maximize fiber bonding.
For Surface Sizing: Low Viscosity is critical. You need the solution to penetrate the paper surface slightly and run at high solids content without splashing at the size press.
For Coating: A specific rheology is needed to ensure the coating blade runs smoothly (high shear stability).
The Standard: While Technical Grade CMC is cheaper, Refined CMC (min 98% or 99.5%) is becoming the standard for high-speed machines.
Why: Impurities (like salts) in lower grades can cause corrosion in the machine or affect the efficiency of other chemical additives. High purity ensures consistent "Runnability."
The Impact: DS affects solubility and film formation.
Recommendation: A DS of 0.7 to 0.9 is standard. However, for surface sizing where excellent film formation is required to block oil or grease (e.g., in food packaging), a higher DS might be beneficial for better uniformity.
Challenge: A manufacturer of recycled linerboard was struggling with low Burst Strength (Mullen) due to the poor quality of recycled fibers.
Solution: They added a Medium-Viscosity Unionchem CMC to the wet end.
Result: The CMC improved fiber-to-fiber bonding. Burst strength increased by 15%, allowing the mill to use a higher percentage of cheaper, lower-grade waste paper without sacrificing quality.
Challenge: A coated paper producer faced issues with "mottling" (uneven print density) and dusting during offset printing.
Solution: Incorporated a Low-Viscosity Coating Grade CMC into the coating formulation.
Result: The CMC improved the water retention of the coating color. This prevented the binder from soaking into the paper too fast, leaving more binder on the surface. The result was a smoother surface with excellent pick resistance and sharp print quality.
Saving Energy: By improving drainage in the wet end (when used with retention aids), CMC can help the paper dry faster, reducing steam consumption in the dryer section.
Eco-Friendly: As the industry moves away from plastic coatings, CMC offers a biodegradable, renewable alternative for creating barrier properties in food packaging.
Carboxymethyl Cellulose is a powerful tool for the modern papermaker. It allows for the use of lower-quality fibers while maintaining strength, and it creates the premium surface finish required by today's printers.
At Unionchem, we understand the hydrodynamics of the paper machine. We offer tailored CMC grades that balance viscosity, purity, and cost.
Need to improve your paper parameters?Review our Paper Grade CMC Specifications or contact our technical team for a trial.
Q1: How does CMC improve paper strength?
A: CMC improves "Dry Strength." Because CMC has a similar chemical structure to cellulose, it adsorbs onto the fibers and creates additional hydrogen bonds between them. This makes the fiber network stronger, increasing tensile and burst strength.
Q2: Can CMC be used for food packaging paper?
A: Yes. High-purity, refined CMC is non-toxic and is widely used in food contact paper and board. It is an excellent film-former for creating grease-resistant barriers.
Q3: What is the difference between "Wet End" and "Surface Sizing" CMC?
A: "Wet End" CMC is added to the pulp slurry before the paper is formed (usually higher viscosity). "Surface Sizing" CMC is applied to the dried paper sheet at the size press (usually low viscosity) to seal the surface.
Q4: Does CMC help with retention?
A: Yes, but indirectly. While CMC is anionic (like the fibers), when used in a system with cationic additives (like cationic starch or PAE), it helps form flocs that retain fines and fillers, improving the overall First Pass Retention (FPR).
