Paper coating quality fundamentals through absorption, drying, and storage humidity

Introduction

Paper surface finishing uses processes such as coating, varnishing, and lamination to add gloss, improve the appearance of printed matter, and protect against dirt and scratches.

A beautiful finish makes a product look more valuable.
Uneven gloss or gloss loss, conversely, can make printed matter or packaging look substantially poorer in quality.

The following questions arise on the shop floor.

“Gloss is unstable even with the same varnish.”
“There are uneven areas resembling roller marks.”
“Gloss drops after drying.”
“It looked good immediately after finishing, but lost gloss after storage.”
“Paper surface quality worsens during the rainy season.”

When such problems occur, the coating material or varnish may be suspected first.

Material selection is, of course, important.
However, materials alone cannot explain the cause in paper finishing.

Paper readily absorbs moisture and is strongly influenced by humidity.
It also absorbs coating liquids.
Surface condition may vary between lots.

Uneven gloss and gloss loss therefore arise from paper absorption, application conditions, drying, storage humidity, and stacking conditions, as well as the coating material.

This article examines why uneven gloss and gloss loss occur from the perspectives of substrate, process, and storage.

1. Paper finishing provides both appearance and protection

Paper surface finishing serves various purposes.

Typical purposes include the following.

• Adding gloss
• Improving appearance
• Protecting printed surfaces
• Reducing dirt adhesion
• Reducing scratching
• Improving wear resistance
• Increasing product value
• Creating a premium impression
• Providing packaging functions

For printed matter, packages, labels, cartons, catalogs, and cards, surface gloss and smoothness directly affect product value.

Even with the same printing, the quality of the surface finish greatly changes the customer’s impression.

Paper finishing must therefore consider appearance, protection, and mass production consistency together, rather than merely applying material.

2. Paper differs from metal and resin

There is a fundamental point to understand when considering paper finishing.

Paper is highly susceptible to moisture and liquids.

Metal generally does not absorb liquids internally.
Some plastics are affected by solvents, but do not absorb liquids in the same way as paper.

Paper, by contrast, consists of fibers.
Its surface has fine irregularities, and its interior contains voids.
Coating material or varnish may therefore be absorbed into its surface and interior.

Uniform absorption produces a relatively consistent finish.
Uneven absorption, however, causes gloss differences or uneven gloss.

Paper finishing requires examining the paper itself, as well as the material applied.

3. What is uneven gloss?

Uneven gloss is a condition in which surface gloss is not uniform.

One area looks shiny.
Another looks dull.
Streaks resemble roller marks.
Only one area of the paper looks less glossy.
Unevenness becomes visible at certain angles.

These conditions constitute uneven gloss.

Uneven gloss may seem to be only an appearance issue.
For printed matter and packaging, however, it is a very important aspect of appearance quality.

It can make the whole product look cheap or poorly printed.
In premium packaging, branded products, appliance packaging, cosmetics boxes, and food packaging in particular, it may lead to serious complaints.

Uneven gloss results from combinations of materials, application conditions, paper absorption, and drying conditions.

4. What is gloss loss?

Gloss loss is a reduction in surface gloss after finishing.

The surface was glossy immediately after finishing.
Gloss dropped after drying.
Gloss disappeared after storage.
The surface became dull after stacking.
Gloss declined during the rainy season or under humid conditions.

These situations may indicate gloss loss.

Gloss loss resembles uneven gloss, but requires a slightly different perspective.

Uneven gloss means gloss differs between locations.
Gloss loss means gloss declines across all or part of the surface with time or storage conditions.

Paper finishing quality must be assessed after storage, after transport, and during customer use, as well as immediately after finishing.

5. Uneven paper absorption causes gloss defects

Paper absorption is one of the most important factors in surface finishing.

If paper receives the coating uniformly, the finish is relatively uniform.
If absorption varies, layer thickness and surface condition become uneven.

Gloss differences result.

Causes of uneven absorption include the following.

• Variation in paper surface smoothness
• Different paper lots
• Differences in paper density
• Paper moisture condition
• Differences in printing ink absorption
• Different substrate treatments
• Dirt on the paper surface
• Adhered paper dust

Even with the same coating material, different paper conditions produce different finishes.

When uneven gloss occurs, check paper type, lot, absorption, and surface smoothness, as well as the material.

6. Unsuitable application viscosity causes roller marks

Paper finishing may use rollers to apply coating material or varnish.

An unsuitable viscosity can then cause roller marks.

Roller marks are defects in which roller traces appear as streaks.
A uniform film is not formed, causing uneven gloss or appearance.

Excessively high viscosity makes uniform spreading difficult.
Excessively low viscosity makes application quantity unstable and increases susceptibility to absorption and flow.

Check the following items.

• Application viscosity
• Thinning conditions
• Roller pressure
• Roller speed
• Application quantity
• Paper conveying speed
• Coating material temperature
• Variation in operator adjustments

Viscosity control is extremely important in paper finishing.
It is preferable to control measured values rather than rely on the impression that conditions are “as usual.”

7. Residual solvent causes blushing and gloss defects

Residual solvent requires attention when using solventborne coatings or varnishes.

If the surface closes over or products are stacked before solvent has escaped sufficiently, blushing or gloss loss may occur.

Blushing means the film or coating layer appears whitish.
Transparency is lost and appearance quality declines substantially.

Residual solvent problems arise for reasons such as the following.

• Low drying temperature
• Short drying time
• Insufficient exhaust
• Excessive application quantity
• Slow solvent evaporation
• Solvent absorbed into paper is difficult to remove
• Products are stacked immediately after drying

Because paper readily absorbs liquids, solvent may remain inside as well as at the surface.
The surface may look dry while solvent remains internally.

Paper finishing must not be assessed simply by whether the surface looks dry.

8. Insufficient drying causes pressure marks, blocking, and gloss loss

Insufficient drying is an important cause of paper finishing problems.

Stacking insufficiently dried products may cause the following defects.

• Pressure marks
• Blocking
• Gloss loss
• Surface roughness
• Sticking
• Rub marks
• Reduced gloss
• Reduced smoothness

Pressure marks are impressions left by stacking pressure.
Blocking is adhesion between finished surfaces.

To improve productivity, it may be tempting to stack paper products immediately after drying.
However, stacking before sufficient drying substantially reduces surface quality.

The following checks are needed to prevent insufficient drying.

• Drying temperature
• Drying time
• Exhaust capacity
• Application quantity
• Paper type
• Time before stacking
• Cooling time
• Stack height
• Storage humidity

Drying does more than determine line speed.
It is an important process for protecting surface quality.

9. Humid storage expands paper and reduces gloss

Paper is highly susceptible to humidity.

Finished paper stored in humid conditions may absorb moisture and expand.
Expansion may reduce surface smoothness and make gloss appear lower.

This can cause gloss loss.

There was no problem immediately after finishing.
Inspection accepted it.
Gloss dropped after warehouse storage.
The surface became dull after transport.
Complaints increased during the rainy season.

In these cases, check storage humidity and transport conditions.

Paper finishing quality is not completed within the processing stage alone.
Storage and transport must also be considered.

10. The rainy season and warehouse humidity are major risks in Vietnam

Vietnam’s hot, humid environment has a major effect on paper finishing quality.

Warehouse humidity is especially likely to rise during the rainy season.
In warehouses with insufficient air conditioning or dehumidification, paper products may absorb moisture and change in surface quality.

Pay attention to the following points in Vietnamese factories.

• Rising humidity during the rainy season
• Warehouse humidity
• Storage directly on the floor
• Storage beside walls
• Moisture inside packaging
• Time waiting for shipment
• Humidity during transport
• Condensation inside containers
• Storage conditions at the customer’s site

Paper products acceptable inside the factory may become defective after storage or transport.

Paper finishing therefore requires anticipating quality changes after storage, as well as inspecting appearance immediately after processing.

11. Checking procedure for uneven gloss or gloss loss

Checking in the following order makes causes easier to organize when uneven gloss or gloss loss occurs.

11-1. Distinguish the defect type

First distinguish uneven gloss from gloss loss.

• Does gloss differ between locations?
• Has gloss declined over time?
• Has gloss declined overall?
• Does the unevenness resemble roller marks?
• Is blushing present?

Classifying the defect’s appearance helps identify which processes to check.

11-2. Establish when it occurs

• Immediately after finishing?
• After drying?
• After stacking?
• After storage?
• After transport?
• At the customer’s site?

The timing determines whether application, drying, or storage conditions should be examined.

11-3. Check paper information

• Paper type
• Paper lot
• Surface smoothness
• Absorption
• Moisture condition
• Printing ink condition
• Substrate treatment

If defects increased after a paper lot change, check the paper’s absorption and surface condition.

11-4. Check application conditions

• Coating material viscosity
• Thinning conditions
• Application quantity
• Roller pressure
• Roller speed
• Conveying speed
• Operating temperature

For roller marks or streak-like unevenness, focus on application conditions.

11-5. Check drying and storage conditions

• Drying temperature
• Drying time
• Exhaust capacity
• Time before stacking
• Stack height
• Storage humidity
• Transport conditions

Drying and storage conditions are important for gloss loss or pressure marks.

12. Basic items to control in paper finishing

The following items are important to control in paper surface finishing.

Paper

• Paper type
• Paper lot
• Surface smoothness
• Absorption
• Moisture condition
• Presence of paper dust
• Printed surface condition
• Substrate treatment

Coating material

• Type
• Viscosity
• Thinning conditions
• Solids content
• Drying properties
• Residual solvent
• Compatibility with paper
• Gloss performance

Application conditions

• Roller conditions
• Application quantity
• Conveying speed
• Roller pressure
• Application unevenness
• Operating temperature
• Operator adjustments

Drying conditions

• Drying temperature
• Drying time
• Exhaust
• Solvent removal
• Cooling time
• Time until stacking is possible

Storage and transport

• Storage humidity
• Storage temperature
• Stack height
• Packaging condition
• Time waiting for shipment
• Container condensation
• Storage environment at the customer’s site

13. Questions for customer visits

For factory managers

• Are there complaints about uneven gloss or gloss loss in paper products?
• Do defects appear immediately after finishing or after storage?
• Do defects increase during the rainy season or humid periods?
• Is the burden of rework or disposal substantial?

For production engineering personnel

• Do you check paper absorption?
• Do you control coating material viscosity?
• Are roller pressure and application quantity standardized?
• Are drying temperature, time, and exhaust controlled?
• Is the time before stacking defined?

For quality assurance personnel

• Do you record uneven gloss and gloss loss separately?
• Do you measure gloss values?
• Do you link paper lots to defect rates?
• Do you check storage humidity and gloss changes?
• Do you consider residual solvent when blushing occurs?

For purchasing personnel

• Do you share paper type and lot information with production?
• Do you check absorption and gloss effects when changing suppliers?
• Are paper grades being changed solely for cost?
• Do you confirm storage and transport conditions with suppliers?

For finishing supervisors

• Do viscosity adjustments vary between operators?
• When roller marks occur, do you check viscosity and roller conditions?
• Are products being stacked before sufficient drying?
• Are finished products stored in humid locations?
• Is the conditioning time after finishing observed?

14. How CoatureVietnam can help

CoatureVietnam can support improvements in paper finishing problems through application, drying, and storage conditions, as well as material selection.

Specific support includes the following.

• Organizing causes of uneven gloss and gloss loss
• Classifying defect timing
• Organizing paper lot and defect rate information
• Reviewing coating material viscosity control
• Checking roller conditions
• Checking drying conditions
• Identifying residual solvent risks
• Checking storage humidity control
• Preparing operator checklists
• Preparing improvement proposals for customers

Paper finishing requires an integrated view of paper, application, drying, and storage, as well as materials.

Summary

Coating materials alone cannot explain uneven gloss and gloss loss in paper finishing.

Paper readily absorbs liquids and is susceptible to humidity.
Even with the same material, paper type, lot, absorption, application viscosity, roller conditions, insufficient drying, residual solvent, and humid storage change the finish.

Check the following in paper finishing.

• Paper absorption
• Paper lot
• Coating material viscosity
• Roller conditions
• Application quantity
• Drying temperature
• Drying time
• Exhaust
• Time before stacking
• Storage humidity
• Transport conditions

Even an acceptable product immediately after finishing may lose gloss after storage or transport.
Quality control must therefore cover storage and transport as well as processing.

CoatureVietnam’s principle of “Coating the Future with Science and Care” means separating these substrate and process relationships and turning them into controls reproducible on site.

Science means considering uneven gloss and gloss loss through absorption, viscosity, drying, residual solvent, and humidity.
Care means establishing criteria that let the shop floor reproduce the same quality without relying on impressions.

Paper finishing does not end with application.
Review paper condition, drying, and storage as well.
This is fundamental to preventing uneven gloss and gloss loss and stabilizing paper finishing quality.