Paper Sizing, Tissue Auxiliaries and Surfactant Roles

Why sizing and wet-end chemistry matter

Paper is a porous network of cellulose fibres. Without sizing, water, ink, and coating colour penetrate freely into the sheet — causing feathered printing, poor opacity, and dimensional instability. Sizing creates hydrophobic character at fibre surfaces, controlling liquid penetration while maintaining breathability where needed (tissue, filter paper) or blocking it entirely (cup stock, liquid packaging board).

Tissue grades add further complexity: consumers expect softness, bulk, absorbency for toilet tissue, and wet strength for towel grades. Achieving these properties requires wet strength resins, creping chemistry, and machine cleanliness — all areas where surfactants and process chemicals interact with the main functional additives.

Internal sizing systems

Internal sizing treats the pulp before sheet formation so hydrophobic agent distributes within the fibre wall and on fibre surfaces. The three dominant chemistries are rosin-alum, alkyl ketene dimer (AKD), and alkyl succinic anhydride (ASA).

Rosin-alum sizing: The historical standard for acidic paper making. Rosin size is emulsified and precipitated onto fibres with alum (papermakers alum, aluminium sulfate). Works best below pH 5.5. Declining in alkaline systems but still used in some board and specialty grades.

AKD (alkyl ketene dimer): The leading internal size for alkaline and neutral paper (pH 7–8.5). AKD is a waxy solid delivered as a cationic emulsion (typically 6–30% active). It reacts with cellulose hydroxyl groups to form beta-ketoester bonds, creating durable hydrophobicity. Cure time is 24–72 hours after sheet formation.

ASA (alkyl succinic anhydride): Applied as an on-machine emulsion because it hydrolyzes rapidly in water. Faster cure than AKD — minutes to hours. Preferred for machines running at high speed where sizing must develop before the reel. Requires careful emulsification at the size press or in-line emulsifier.

Surfactant role in AKD and ASA emulsification

AKD and ASA are hydrophobic and must be emulsified before addition to the pulp slurry. Cationic starch or synthetic polymer emulsifiers create fine droplets (1–5 µm) that survive shear in the fan pump and headbox. Nonionic ethoxylates at low dose improve emulsion stability and droplet uniformity — typically below 0.1% on fibre to avoid over-wetting that defeats sizing purpose.

Poor sizing emulsion quality causes visible problems on the machine and in the finished sheet:

• Sizing spots and flecks visible in the sheet
• Uneven Cobb values (water absorption) across the reel width
• Dusting of AKD particles on calender rolls
• Variable HST (Hercules sizing test) leading to printability complaints

Venus nonionic wetting agents assist AKD emulsion preparation when dosed carefully. Overdosing any surfactant in the wet end competes with sizing deposition and increases water absorption — a critical balance for papermakers.

Internal sizing comparison table

See paper sizing products and paper chemicals hub for Venus portfolio.

Surface sizing at the size press

Surface sizing applies starch, PVA, or latex to the formed sheet at the size press (pond or film applicator) to improve surface strength, printability, and barrier properties. Starch is the dominant surface size — oxidized, hydroxyethylated, or cationic starches at 3–12% concentration in the size press bath.

Surfactants in surface sizing serve different functions than in internal sizing:

• Penetration aid: Low-dose nonionic ethoxylate helps starch penetrate the sheet z-direction for internal bond strength
• Defoaming: Size press foam causes spotty application and sheet breaks — silicone and oil-based defoamers are standard
• Antistatic and lubrication: Some formulations include surfactants to reduce static on high-speed coaters

Defoamer overdose can deposit hydrophobic spots that resist printing ink — a common troubleshooting issue on offset paper grades. Venus defoamers are formulated for pulp washing, coating kitchen, and size press applications with minimal interference on sizing performance.

Tissue paper: softness, bulk, and wet strength

Tissue manufacturing on crescent-former or conventional Yankee machines prioritizes consumer-perceived properties over printability. Key chemical systems include:

Wet strength resins: Polyamide-epichlorohydrin (PAE) and glyoxalated polyacrylamide (GPAM) bond fibres when wet, preventing towel disintegration during use. Dosed at 0.2–1.0% on fibre depending on target wet tensile strength.

Dry strength resins: Cationic starch and GPAM improve tensile without wet strength contribution — used when softness (low stiffness) is the priority.

Creping chemistry: Adhesion between the sheet and Yankee cylinder dryer is controlled by creping adhesives (polyvinyl alcohol, polyamide) and release agents. Surfactants in Yankee coating packages affect crepe count, bulk, and machine runnability.

Yankee cylinder cleaning: Organic deposits (stickies, pitch, coating adhesive buildup) reduce heat transfer and cause sheet breaks. Alkaline cleaners with surfactants and solvents are applied on scheduled washes. Venus felt and cylinder cleaning chemistries support tissue machine hygiene.

Tissue portfolio: tissue paper chemicals.

Surfactant applications across the paper mill

Deinking and recycled fibre processing

Recycled fibre now constitutes a major share of furnish in printing/writing paper and board. Deinking removes printing ink from recovered paper through pulping, flotation, and washing stages. Nonionic surfactants and fatty acid soaps detach ink particles from fibre surfaces and stabilize them in foam for removal.

Deinking chemistry must balance ink removal against fibre loss and sizing interference in the final sheet. Over-aggressive surfactant dosing in the deink plant can carry over to the paper machine and reduce internal sizing efficiency. Closed water loops amplify this risk. Our paper deinking guide covers surfactant selection for flotation and washing lines.

Bleaching aids and wetting in pulp bleaching

Elemental chlorine-free (ECF) and total chlorine-free (TCF) bleaching sequences use oxidative stages (chlorine dioxide, hydrogen peroxide, ozone) to brighten pulp. Surfactants as bleaching aids improve chemical penetration into fibre walls, giving more uniform brightness and reducing chemical consumption.

Wetting agents must be stable under alkaline peroxide conditions and compatible with chelating agents that sequester transition metals (iron, manganese) which catalyze peroxide decomposition. See bleaching aids and chelating agents guide for integrated bleaching chemistry.

Worked examples: sizing and tissue chemical packages

Woodfree copier paper (AKD internal size):

• AKD emulsion: 0.15–0.25% on dry fibre (as 100% AKD)
• Retention aid: cationic PAM at 0.03–0.05%
• Nonionic wetting aid in emulsion prep: trace dose only
• Alkaline sizing starch at size press: 5–8% in bath
• Target HST > 120 seconds; Cobb 60 < 25 g/m²

Premium bath tissue (wet strength):

• PAE wet strength resin: 0.4–0.8% on fibre
• Creping adhesive on Yankee: PVA / polyamide blend
• Yankee cleaner: alkaline surfactant wash weekly
• No internal sizing — absorbency is a positive attribute
• Target wet tensile > 0.15 kN/m

Recycled linerboard (rosin sizing, closed loop):

• Rosin size emulsion: 1.5–2.5% on fibre
• Alum for precipitation: pH controlled to 4.5–5.0
• Defoamer in machine chest: silicone emulsion at 50–200 ppm
• Deink surfactant carryover monitored — reduce if sizing drops

Process troubleshooting

Environmental and regulatory considerations

Paper mills operate under strict effluent discharge limits for COD, BOD, and surfactant content. Biodegradable nonionic ethoxylates with linear alcohol bases are preferred over persistent chemistries. APE-free surfactant packages align with EU Ecolabel criteria for tissue and graphic paper where applicable.

Venus supplies paper chemicals with documentation supporting customer environmental audits. Closed-loop water systems benefit from low-foam, readily biodegradable surfactants that do not accumulate across process cycles.

Venus paper and tissue chemical supply

Venus Ethoxyethers manufactures defoamers, deinking agents, bleaching aids, felt cleaners, and wetting agents for pulp and paper mills in India and export markets. Technical support covers AKD emulsion optimization, deink plant audits, and tissue machine cleaning protocols.

Contact Venus for samples and mill trials via reach us. Portfolio hub: paper chemicals.