Paper sizing controls how readily a sheet wets and takes up liquid. That matters because an ink film does not simply sit on paper: depending on the process, its vehicle, water, solvent, or other mobile components may spread across the surface, enter pores, or remain near the surface while the ink dries by another route.

The practical effect of paper sizing on ink absorption is therefore a balance, not a universal quality ranking. Too little resistance to the relevant liquid can allow excessive penetration, weak colour density, show-through, strike-through, or uneven appearance. Too much surface holdout can leave more liquid at the surface, changing set-off risk, rub resistance, coalescence, adhesion, or the drying energy and time required. The suitable balance depends on the paper construction, ink system, print process, coverage, and finishing sequence.

Sizing is only one part of that system. Formation, porosity, roughness, coating, moisture, fibre furnish, calendering, ink rheology, and press settings can all materially alter the result. A sizing measurement is useful for comparing or controlling paper, but it is not a substitute for qualification on the intended press.

Sizing is controlled resistance to wetting

Cellulosic fibres are naturally hydrophilic: they attract water and can form a connected capillary structure within the sheet. An untreated fibre web can therefore take up liquids quickly. Paper sizing modifies that tendency to achieve a controlled level of liquid resistance appropriate to the intended use.

This is relevant beyond printing. The same broad property affects the behaviour of fountain solution, coatings, adhesives, silicone treatments, and packaging liquids. A paper intended to receive an aqueous coating may need a different absorption profile from a sheet intended for high-speed inkjet printing or conventional sheetfed printing.

Sizing should not be understood as making paper “waterproof” or automatically making it better for printing. It changes wetting and liquid transport. A sheet that resists a water-based ink sufficiently to control lateral spread may behave quite differently with an oil-based ink vehicle, a low-surface-tension coating, or a hot-melt adhesive.

The mechanisms behind wetting and capillary penetration are complex because paper is not a smooth, uniform surface. It contains fibres, fillers, pores, surface irregularities, and often treatments with non-uniform distribution. The table of contents for TAPPI Press’s The Sizing of Paper reflects this scope, covering internal sizing, wetting, dynamic wetting, surface energy, and capillary penetration rather than treating sizing as a single paper attribute.

Internal and surface sizing act in different regions

Paper mills may use internal sizing, surface sizing, or both. The distinction matters when interpreting datasheets or investigating a print problem.

Internal sizing

Internal sizing is introduced into the fibre furnish before sheet formation. Its purpose is to reduce the affinity of the fibre network for liquids and influence how readily liquid can enter the sheet structure.

In production terms, internal sizing is part of the paper’s bulk liquid-resistance profile. It can affect the way water or ink vehicle moves through the fibre matrix, including the potential for deeper penetration and show-through. Its outcome still depends on fibre type, fillers, retention, sheet density, pore structure, drying, and subsequent surface treatments.

A well-sized base sheet may be required before surface treatment or coating, but internal sizing does not by itself define the final print surface. A rough, porous sheet and a dense, smooth sheet can have different print results even when their internal liquid resistance appears similar.

Surface sizing

Surface sizing is applied after the web has formed, typically at a size press or related surface-treatment station. It can modify near-surface chemistry, partially influence pore access, and improve surface strength. Starch-based formulations and polymer-containing treatments are commonly used for this purpose, although the exact formulation and treatment level are grade-specific.

A technical paper on surface sizing agents describes the untreated fibre web as hydrophilic and capillary-active, and explains sizing as controlled reduction of liquid pickup into the fibre matrix. In practice, a surface treatment may reduce rapid liquid entry and excessive spreading while also changing the sheet’s response to picking, dusting, coating, or subsequent converting.

Surface sizing is not identical to a pigment coating. A coating is generally designed to create a more substantial engineered print surface, often using pigments and binders. Surface sizing is thinner and works more directly with the existing paper surface and near-surface pores. Some papers use both a surface size and a coating; others use neither or rely mainly on bulk properties.

What happens after ink reaches the sheet

Once a liquid ink contacts paper, several events can overlap:

  1. Wetting: The ink must make contact with the surface. Surface chemistry, roughness, and ink surface tension influence whether it wets readily.
  2. Spreading: The deposited drop or film may spread laterally. Controlled spreading can improve coverage, while excessive spreading can reduce edge sharpness or increase mottling.
  3. Penetration: Mobile components of the ink can enter pores and voids. Pigments or colorants may remain near the surface, move with the liquid, or interact with a receptive treatment.
  4. Setting and drying: Depending on the ink, liquid may absorb into the substrate, evaporate, oxidize, react under ultraviolet or electron-beam energy, or undergo a combination of these mechanisms.

The relationship is important because ink absorption affects visible print properties. The review Detailed Insights to Liquid Absorption and Liquid–Paper Interaction identifies paper topology, surface energy, porosity, and pore-size distribution as important influences on ink absorption, alongside ink viscosity and surface tension. It also links ink-paper interaction to print density, gloss, mottle, and print-through.

When a sheet absorbs a liquid component very quickly, the printed film may lose surface continuity or leave less colorant concentrated near the surface. Depending on the ink system, this can reduce apparent density or gloss and increase penetration through the sheet. Uneven absorption across the web can contribute to uneven print appearance.

However, very high holdout is not automatically desirable. If a wet ink film remains largely at the surface, it may need another effective drying route. On conventional printing systems, the balance between ink setting into the sheet and subsequent drying is especially important for handling, overprinting, coating, and finishing. The relevant question is not whether the paper absorbs “more” or “less,” but whether it accepts the intended ink film at a controlled rate compatible with the press and downstream process.

The printing process changes the sizing requirement

Aqueous inkjet, conventional liquid-ink processes, and electrophotographic printing place different demands on the substrate. Results from one should not be assumed to apply to another.

Water-based inkjet

In aqueous inkjet, the substrate must manage a small, rapidly deposited water-based drop. If wetting and lateral movement are too high, line edges and text can lose sharpness and adjacent colours may bleed. If the surface is too resistant or lacks an appropriate receptive treatment, the ink may remain on the surface too long, dry poorly, or produce weak colour development depending on the colorant and ink chemistry.

The study Wetting and Absorption Characteristics of Ink Jet Paper addresses this process-specific relationship for drop-on-demand aqueous inkjet. Its relevance is not that one absorption result creates a universal inkjet specification, but that wetting and absorption must be managed together.

Surface-treatment work in Effects of Surface-Size Treatment on Performance of Plain Paper in Non-Impact Printing similarly reports that polymeric surface sizes can modify plain-paper surface chemistry. For water-based inkjet, the authors describe a moderately hydrophobic surface as a way to control lateral spreading and support ink holdout. That finding is specific to the studied materials and printing context; it should not be extended automatically to every inkjet ink or paper grade.

Conventional liquid inks

Offset, flexographic, gravure, and related processes use different ink formulations, transfer conditions, film weights, and drying mechanisms. Sizing can influence penetration and set, but the final drying behaviour may also depend on evaporation, oxidation, heat, radiation curing, or a combination of mechanisms.

For example, a paper that takes up part of the ink vehicle can assist setting in some conventional print situations. Yet the same tendency may reduce gloss or increase strike-through if penetration is excessive. A change in ink tack, viscosity, fountain solution balance, anilox volume, impression, drying temperature, or coverage can alter the observed result without any change in sizing.

Electrophotographic toner

Electrophotographic printing does not deposit a liquid ink in the same way. Toner transfer, fusing, and adhesion depend strongly on the substrate surface and thermal response. Surface treatment can still matter, but liquid-absorption tests alone cannot approve toner adhesion or fused-image durability.

The non-impact printing study cited above evaluated both aqueous inkjet performance and electrophotographic toner behaviour, reinforcing a useful qualification principle: a paper treatment that improves one process may not deliver the same advantage in another.

Absorption tests indicate mechanisms, not finished-job approval

Paper test results can help compare lots, diagnose variation, and define a target range. They need interpretation in the context of the test liquid and method.

  • Contact angle indicates how a liquid drop wets a surface over a defined time. It can help assess relative wetting behaviour, but paper roughness, absorbency, drop volume, timing, and liquid chemistry affect the value.
  • Bristow-type liquid penetration testing examines dynamic uptake during brief contact with a liquid. It can be useful for studying short-time absorption and pore access, particularly where rapid liquid transfer matters.
  • Cobb-type water absorption testing measures water uptake over a defined exposure. It is useful for some paper and board control purposes, but it does not reproduce every ink chemistry, transfer condition, or drying mechanism.
  • Ink penetration and print trials bring the evaluation closer to the intended process. They can reveal density, show-through, mottle, edge definition, set-off, rub resistance, or other job-specific outcomes that a single absorption number cannot predict.

The surface-size study used contact angle, ink penetration, and Bristow Wheel measurements alongside image-analysis assessment of print quality. This combination is more informative than treating one measurement as a complete description of printability.

Do not compare values from different laboratories or datasheets without confirming the method, liquid, sheet side, conditioning environment, contact time, units, and calculation basis. A result obtained with water at a longer exposure is not interchangeable with a short-contact ink-vehicle test. Exact acceptance criteria should come from the applicable controlled method, grade specification, and process qualification plan.

Specify the print outcome, then qualify the sheet

A robust specification begins with the result required on press and in use. “Good sizing” is too vague to guide a mill, converter, or print buyer.

Use a controlled qualification workflow:

  1. Identify the process and ink system. State whether the application uses aqueous inkjet, offset, flexography, gravure, electrophotographic toner, or another process. Record the ink or toner family, expected coverage, and drying or curing route.
  2. Define the paper construction. Specify grade, basis weight, coating status, intended print side, bulk or caliper where relevant, and any surface treatment. Similar basis weights do not guarantee similar pore structure or liquid response.
  3. State the failure to avoid. Examples include feathering, low density, excessive gloss loss, mottle, strike-through, slow handling, set-off, poor adhesion, or inconsistent colour.
  4. Request relevant data with conditions. Ask which sizing approach is used, which absorption or wetting test was applied, the test liquid, exposure time, conditioning conditions, and whether results apply to the print side, reverse side, or both.
  5. Run a representative press trial. Use the intended artwork, coverage range, ink, press settings, drying or curing conditions, and finishing sequence. Include solid areas, fine type, reverses, and demanding image regions where relevant.
  6. Approve against production criteria. Inspect immediately after printing and after the time needed for downstream operations. Check appearance, handling, rub performance, blocking or set-off, show-through, and finishing behaviour.

Paper sizing can establish a valuable absorption balance, but it cannot guarantee press performance by itself. Select and approve the sheet for the specific interaction between paper structure, treatment, ink, press, drying route, and converting conditions—not from a generic claim of high or low absorbency.

Frequently asked questions

Does more paper sizing always make print dry more slowly?

No. More sizing may reduce uptake of certain liquids, leaving more of a deposited ink film at the surface. Whether that slows drying depends on the ink’s drying mechanism, film weight, coating or surface treatment, press conditions, and available drying or curing energy. A sheet should be assessed with the intended process rather than by sizing level alone.

What is the difference between internal sizing and surface sizing?

Internal sizing is added to the fibre furnish and influences liquid interaction throughout the sheet structure. Surface sizing is applied to the formed sheet and mainly modifies the surface and near-surface region. Both may be present in the same grade, and both interact with formation, porosity, roughness, and other paper properties.

Can a Cobb or liquid-absorption result predict inkjet print quality?

It can provide useful comparative information, but it cannot independently predict inkjet quality. Inkjet output also depends on short-time wetting, lateral spread, ink chemistry, colorant behaviour, receptive treatment, image content, and printer settings. Use relevant test data to narrow options, then verify with a representative print trial.

Why can two papers with similar basis weight absorb ink differently?

Basis weight describes mass per unit area, not pore structure or surface chemistry. Two sheets of similar basis weight can differ in fibre furnish, filler content, formation, density, calendering, coating, internal sizing, surface sizing, roughness, and moisture. Those differences can change both the rate and path of liquid movement through the sheet.