CMC in Papermaking: Wet End, Surface Sizing and Coating Colours — Choosing Viscosity from 7 to 600 mPa·s

On a paper machine, CMC does a different job at each addition point. At the wet end it raises fibre charge and bonding for dry strength and helps retain cationic additives such as AKD size and wet-strength resin; in the size press it forms a continuous film that improves surface strength, smoothness, ink holdout and grease resistance; and in coating colours it controls water retention and rheology and boosts optical brighteners. Each job needs a different viscosity, from about 7 mPa·s for high-solids coating colours to several hundred mPa·s (2 % solution) for film and strength applications.

Why CMC works in paper

CMC shares its β-1,4 glucose backbone with the fibres it is added to, carries an anionic charge from its carboxymethyl groups, and forms strong, flexible films when it dries. Those three properties explain its uses: it can be attached to fibre surfaces, it adds charge sites that anchor cationic chemicals, and it creates a coherent film on the sheet surface or within a coating layer. Because it binds water strongly, it also controls how quickly water leaves a coating colour or size film.

Wet end: strength, retention and sizing efficiency

Added to the stock, CMC increases the anionic charge of the fibres and improves fibre-to-fibre bonding, which raises dry strength. The extra charge gives cationic additives more places to attach: CMC is the classic anionic partner for polyamide-epichlorohydrin (PAE) wet-strength resin in tissue and towel, and it improves the retention and efficiency of AKD and rosin sizing. Typical additions are 0.2–0.5 % on dry fibre.

Charge balance is the main risk. Unretained CMC adds to the anionic trash in the white water, which consumes cationic retention aids and can upset drainage. Monitor cationic demand or zeta potential when you introduce CMC, and trial the addition order relative to the cationic partner, because adding CMC before or after the PAE changes the result. Lower-DS CMC tends to attach to fibres more readily, while higher DS dissolves more cleanly; paper grades usually sit at DS 0.7–1.0.

Surface sizing: film, smoothness and grease resistance

In the size press or film press, CMC is applied alone or blended with surface-sizing starch. Its film raises surface strength and reduces picking and dusting in printing, smooths the surface, closes pores for better ink holdout and air resistance, and provides a useful barrier to oil and grease. The size-press liquor must stay runnable at its application temperature, so the CMC viscosity is chosen together with the liquor solids: a higher molecular weight gives a stronger film per gram but limits the solids you can apply.

Coating colours: water retention and rheology

In pigment coating colours, CMC acts as a water-retention aid and rheology modifier. It stops water and binder from migrating into the base sheet too quickly, which would immobilise the coating early, cause binder migration and mottle, and disturb runnability at the blade. It also raises low-shear viscosity to stabilise the colour, and it is an effective carrier for optical brightening agents, increasing their whitening efficiency. Water retention is commonly evaluated with a gravimetric method such as the ÅA-GWR test.

At the blade, shear rates are extremely high and coating solids are often in the region of 60–70 %. A low-viscosity CMC delivers water retention without driving up high-shear viscosity, which is why low-viscosity grades of roughly 7–50 mPa·s (2 % solution) are the norm. Typical additions are 0.3–1.5 parts per 100 parts pigment.

Choosing viscosity from 7 to 600 mPa·s

Viscosity falls as temperature rises, and size presses and coating kitchens rarely run at 25 °C, so check the viscosity of the finished liquor or colour at its working temperature as well as the CMC specification. For coating colours, measure both low-shear (Brookfield) and high-shear viscosity.

Preparation and handling tips

• Prepare a stock solution separately rather than adding dry powder to a coating kitchen or size liquor.
• Disperse the powder into a vortex or use instant CMC, and allow full hydration before use.
• Filter stock solutions to remove any gels that could cause coating streaks.
• Preserve stored solutions, because CMC can be degraded by microbes and cellulase enzymes in warm mill water.
• Check compatibility with cationic additives in the same circuit to avoid deposits.

CMS as a cost option

Where cost dominates, paper-grade carboxymethyl starch can take over part of the work. Sentai CMS-P (10–300 mPa·s at 2 %, DS 0.30–0.85) is used as a filler stabiliser, retention aid, surface-sizing agent and coating binder, and it can be blended with CMC to balance cost and performance.

How Sentai can help

Sentai is a vice-chair unit of the Papermaking Chemicals Committee of the China Technical Association of Paper Industry and supplies paper-grade CMC across the paper machine. The PC-Series viscosity is matched to the application from 7 to 600 mPa·s (2 % solution). Send your furnish, coating formulation or size-press conditions and the paper team will recommend a starting window, with CMS-P as an alternative or blend partner. Free 100–300 g samples are available.