Paper performance is judged long after it leaves the paper machine—during printing, packaging, converting, and final use. When paper absorbs liquid too quickly, ink spreads, colors look dull, edges weaken, and surface damage increases. When it resists liquid too much, ink drying slows and coating adhesion suffers.
This balance is controlled by sizing agents.
Sizing agents are not optional additives. They are functional tools that decide how paper reacts to water, ink, glue, and coatings. Understanding how sizing agents work, where they are used, and why different paper grades require different sizing strategies helps mills produce consistent, high-quality paper while reducing waste and customer complaints.
Why Paper Needs Sizing in the First Place
Raw paper fibers are naturally absorbent. Without sizing agents, paper behaves like a sponge. Liquid penetrates uncontrollably, causing:
- Ink feathering and poor print sharpness
- Excessive water absorption and edge wicking
- Loss of strength when exposed to moisture
- Dusting and surface picking during printing
- Inconsistent coating or lamination results
Sizing agents regulate this absorption. They slow down liquid penetration just enough to improve performance without blocking ink or coating interaction completely.
Understanding the Two Ways Paper Is Sized
Sizing in papermaking happens at two different stages, and each stage solves a different problem.
Internal sizing agents: controlling absorption from within
Internal sizing agents are added during the early stages of papermaking, before the sheet is formed. Their role is to reduce how quickly liquid moves through the body of the paper.
These sizing agents help paper:
- Maintain strength when exposed to moisture
- Reduce overall water absorption
- Perform consistently during converting
- Handle humidity changes better
Internal sizing agents become part of the paper structure itself. Because of this, their effectiveness depends heavily on machine conditions, furnish quality, and process stability rather than just dosage.
Surface Sizing Agents: refining the paper surface
Surface Sizing Agents are applied after the sheet is formed, usually at the size press. Their role is not bulk resistance, but surface control.
Surface sizing helps paper:
- Improve print sharpness and ink holdout
- Reduce linting and surface picking
- Increase surface strength and smoothness
- Create uniform print and coating behavior
This is why many printing and writing papers rely on Surface Sizing Agents even when internal sizing agents are already in use.
Common Types of Sizing Agents Used in Paper Making
Different paper grades need different sizing solutions. Over time, mills have adopted sizing systems that match their production goals and operating conditions.
Rosin-based sizing systems
Rosin sizing is one of the oldest sizing technologies still in use. It is commonly associated with traditional acidic papermaking systems.
Rosin-based sizing agents are typically used where:
- Paper grades are cost-sensitive
- Systems operate under controlled acidic conditions
- High filler alkalinity is not required
While modern mills often move toward neutral or alkaline processes, rosin sizing still plays a role in specific grades and setups.
Internal sizing agents for neutral and alkaline paper
Most modern paper machines operate in neutral or alkaline conditions. In these systems, internal sizing agents are designed to work efficiently without interfering with fillers like calcium carbonate.
These internal sizing agents are valued because they:
- Support higher filler usage
- Improve paper brightness and stability
- Reduce corrosion and equipment wear
- Offer long-term sizing performance
Their effectiveness depends less on chemistry knowledge and more on process discipline—proper addition points, retention control, and clean systems.
Surface Sizing Agents used at the size press
Surface Sizing Agents are often used together with starch. While starch strengthens the surface, Surface Sizing Agents improve resistance to liquid penetration and ink spread.
This combination is especially useful when:
- Print quality is critical
- Machines run at high speed
- Lightweight or recycled fiber furnishes are used
- Customers demand consistent visual appearance
Surface sizing allows mills to fine-tune paper performance without changing wet-end chemistry.
Where Different Sizing Agents Are Commonly Used
Sizing strategy changes based on paper grade and end use.
Printing and writing paper
These grades demand clean print, sharp edges, and minimal dusting. Internal sizing agents control water penetration, while Surface Sizing Agents enhance surface strength and ink holdout.
Packaging paper and board
Here, moisture resistance and strength retention matter more than visual perfection. Internal sizing agents help prevent weakening during storage and transport. Surface sizing may be added if printing is required.
Corrugating and kraft grades
Sizing must balance water resistance with bonding performance. Excessive sizing can weaken starch adhesion, so controlled internal sizing is preferred.
Specialty and coated base papers
These papers rely on precise surface behavior. Surface Sizing Agents help ensure coatings stay on the surface instead of soaking into the sheet, improving coating efficiency and appearance.
Why Sizing Problems Are Rarely About “Not Enough Chemical”
Many mills increase sizing agent dosage when facing poor performance, only to see limited improvement. In most cases, the real issue lies elsewhere.
Common hidden causes include:
- Poor retention of sizing agents
- High dissolved and colloidal material interfering with fixation
- Inconsistent moisture profiles
- Low surface starch quality or pickup
- Unstable machine conditions
Effective sizing starts with stable processes. Once the system is controlled, sizing agents perform as intended.
How Mills Achieve Consistent Sizing Performance
High-performing mills treat sizing as a system, not a product. They focus on:
- Stable wet-end chemistry
- Clean systems with low contamination
- Controlled size press operation
- Consistent drying and moisture profiles
- Regular performance testing rather than reactive adjustments
This approach leads to fewer customer complaints, lower chemical waste, and more predictable paper behavior.
Why Sizing Agents Directly Affect Customer Satisfaction
Customers may never ask about sizing agents, but they notice the results immediately. Poor sizing shows up as:
- Blurry prints
- Weak packaging edges
- Dust in printing presses
- Inconsistent coating coverage
Well-sized paper runs smoothly through printing and converting lines, building trust between mills and end users.
Sizing Agents as a Performance Tool, Not Just an Additive
Sizing agents define how paper behaves in the real world. Internal sizing agents protect the sheet from within, while Surface Sizing Agents refine how the surface interacts with ink, water, and coatings.
When used thoughtfully, sizing agents solve real production and customer problems. When treated as a simple chemical addition, they often create more issues than they fix.
For paper makers aiming to improve quality, reduce complaints, and meet modern performance expectations, understanding and optimizing sizing agents is no longer optional—it is essential.