Waterborne coating is an effective application system for reducing VOCs and odor.

VOC means volatile organic compound.
In solventborne coating, organic solvents in paints and thinners evaporate, affecting odor, the working environment, fire risks, and environmental compliance.

Waterborne coating replaces some or much of the solvent with water, so VOC reduction can be expected.
It is a strong option for factories considering working environment improvements and environmental requirements.

However, waterborne coating is not a simple change in which solvent is merely replaced with water.

Drying control is particularly difficult in waterborne coating.

In solventborne coating, the coating film forms as organic solvent evaporates.
In waterborne coating, water must be removed from the coating film appropriately.

How this water escapes changes substantially with temperature, humidity, airflow, film thickness, the drying oven, and flash-off time.

Drying control for waterborne coating is a particularly important issue in hot, humid environments such as Vietnam.

1. Waterborne coating is effective for environmental requirements

A major advantage of waterborne coating is the ease of reducing VOCs.

In solventborne coating, organic solvents evaporate during application and drying.
This creates issues involving odor, operator exposure and workload, fire risk, and environmental requirements.

Because waterborne coating uses water as its main medium, it offers the following advantages over solventborne coating.

• VOCs are easier to reduce
• Odor is easier to reduce
• Fire risk is easier to reduce
• It contributes to working environment improvements
• It helps meet environmental regulations
• It demonstrates environmental consideration to customers

Waterborne coating is therefore an effective option for factories addressing environmental requirements.

However, environmental friendliness and easy process control are not the same thing.

Waterborne coating has its own difficulties.
Drying control is the central one.

2. Water dries differently from solvent

In waterborne coating, water must be removed from the coating film.
However, water dries differently from organic solvents.

Water drying is strongly affected by temperature and humidity.

Higher temperature makes water easier to evaporate.
However, higher humidity means more moisture in the air, making water harder to evaporate.

Even at the same temperature, drying in waterborne coating may therefore be slower on humid days.

Vietnam has a rainy season and highly humid days, and humidity inside factories also fluctuates.
Application conditions that worked during the dry season may therefore become unstable in the rainy season.

Simply fixing the drying time is insufficient in waterborne coating.
Drying conditions must be controlled while monitoring temperature and humidity.

3. Water may remain inside even when the surface is dry

A key point in waterborne coating is that moisture may remain inside the coating film even when its surface looks dry.

If only the film surface dries first, moisture inside may have difficulty escaping.
Heating in that state can make internal moisture evaporate suddenly and cause coating film defects.

Common defects are as follows.

• Popping
• Pinholes
• Blisters
• Film blistering
• Rough finish
• Gloss defects
• Poor adhesion
• Drying defects
• Curing defects

In waterborne coating, dry to the touch is not sufficient.
Whether moisture inside the film has escaped appropriately must be considered.

Thick films, recesses, overlapping coats, and shapes that are difficult to dry require particular attention.

4. Flash-off time becomes important

Flash-off is the period after application and before entering the drying oven during which some water or solvent is allowed to escape from the film.

This flash-off period is extremely important in waterborne coating.

Placing a freshly coated item immediately in a hot drying oven can make water inside the film evaporate rapidly and cause popping or pinholes.
Excessively long flash-off, on the other hand, may reduce productivity or increase dirt pickup risk.

Items to check are as follows.

• Flash-off time
• Temperature during flash-off
• Humidity during flash-off
• Airflow exposure
• Coating film thickness
• Substrate shape
• Waiting time before the next process
• Risk of dirt pickup

Flash-off in waterborne coating should not be treated as mere waiting time.
It is an important process for stabilizing coating film quality.

5. The drying oven heating curve also matters

In waterborne coating, drying is not simply made faster by increasing the drying oven temperature.

Rapid heating can make water in the film evaporate suddenly and break through the coating film surface.
This causes popping, pinholes, and blisters.

The drying oven heating curve is therefore important in waterborne coating.

A heating curve describes how the temperature of the product or coating film rises.

Items to check are as follows.

• Drying oven inlet temperature
• Oven temperature distribution
• Heating rate
• Holding temperature
• Holding time
• Hot-air exposure
• Actual substrate temperature
• Film thickness
• Time for moisture to escape

Waterborne coating may require a design that dries the film while removing moisture in stages, rather than immediately baking at high temperature.

A drying oven does more than raise temperature.
It removes moisture appropriately and completes the development of coating film performance.

6. High humidity makes drying defects more likely

Humidity control is important in waterborne coating.

When air humidity is high, moisture in the coating film has difficulty escaping into the air.
Drying may therefore be delayed on humid days even at the same temperature and drying time.

Vietnam’s rainy season and humid days cause drying conditions to vary with the factory environment.

Common problems under high humidity are as follows.

• Drying defects
• Tackiness
• Popping
• Pinholes
• Blisters
• Gloss defects
• Increased dirt pickup
• Poor adhesion
• Delays in the production cycle

In waterborne coating, it is important to record humidity as well as temperature.

Recording temperature and humidity on days with normal production as well as days with defects makes cause analysis easier.

7. Booth airflow also affects drying

Booth airflow is also important in waterborne coating.

Appropriate airflow helps moisture escape from the coating film surface into the air.
However, excessive airflow may disturb spray mist during application or dry the surface alone too quickly.

Insufficient airflow allows humid air to remain near the coated surface, making moisture harder to remove.
Mist and odor also accumulate and affect the working environment.

Items to check are as follows.

• Booth air velocity
• Balance between supply and exhaust air
• Airflow exposure at the coated surface
• Temperature and humidity distribution
• Flow of coating mist
• Airflow during flash-off
• Conditions along the route to the drying oven inlet

In waterborne coating, booth airflow affects drying quality as well as the working environment.

8. Thicker films make moisture harder to remove

Film thickness also has a major effect on drying in waterborne coating.

The thicker the film, the harder it is for moisture inside to escape.
Applying too much thickness in one pass may dry only the surface while leaving moisture inside.

Common problems in thick films are as follows.

• Drying defects
• Popping
• Pinholes
• Blisters
• Sagging
• Rough finish
• Curing defects
• Poor adhesion

Waterborne coating must be designed through the number of application passes, flash-off, and drying conditions, rather than depositing all the required thickness at once.

Film thickness control is also drying control, as well as appearance control.

9. Substrate shape changes drying behavior

In waterborne coating, the drying condition changes with substrate shape.

Flat areas may dry relatively easily.
Recesses, interiors, closed-pocket areas, joints, and areas around welds, on the other hand, may retain water or moisture.

Shapes requiring attention are as follows.

• Deep recesses
• Box shapes
• Inner pipe surfaces
• Closed-pocket structures
• Shapes where water tends to collect
• Thick parts that heat slowly
• Complex welded structures
• Areas concealed by fixtures

With such shapes, the entire product does not necessarily dry uniformly even inside the drying oven.

In waterborne coating, drying conditions must be based on difficult-to-dry areas as well as flat areas.

10. Wastewater treatment becomes part of the process in waterborne coating

Waterborne coating is effective for VOC reduction, but its use of water creates wastewater treatment issues.

Treatment of equipment wash water, booth water, wastewater, paint residues, and similar materials must be considered.

Items to check are as follows.

• Wastewater quantity
• Paint components in wastewater
• pH
• Water quality parameters such as COD
• Availability of treatment equipment
• Sludge generation
• Control of cleaning water
• Treatment of paint waste liquids
• Regulatory compliance
• Operating costs

While waterborne coating reduces solvent odor and VOCs, wastewater and sludge must be controlled.

Conversion to waterborne coating should therefore be considered as environmental management for the entire factory, rather than simply a paint change.

11. Common problems in waterborne coating

The following problems are more likely in waterborne coating.

These defects relate to temperature and humidity, film thickness, the drying oven, booth, surface preparation, and wastewater control, as well as the waterborne paint itself.

12. A practical shop-floor checking sequence

When drying defects or popping occur in waterborne coating, checking in the following order makes causes easier to organize.

1. Identify the defect type

Is it popping?
Pinholes?
Blisters?
A drying defect?
A gloss defect?
First distinguish how the defect appears.

2. Check film thickness

Check for excessive thickness.
Check for thickness differences between areas.

3. Check temperature and humidity

Check temperature and humidity during application, flash-off, and drying.

4. Check flash-off time

Check whether enough time is provided for moisture to escape before entering the drying oven.

5. Check drying oven conditions

Check actual product temperature, heating rate, and holding time, as well as the oven setpoint.

6. Examine substrate shape

Check for difficult-to-dry areas such as recesses, interiors, and thick parts.

7. Check booth air velocity

Check whether airflow is too strong or too weak and whether it reaches the coated surface appropriately.

8. Review surface preparation and underlying surface moisture

Check whether water or contamination remains on the underlying surface.

In waterborne coating, it is therefore necessary to check where and how moisture escapes, as well as the paint.

13. Directions for stabilizing waterborne coating

The following improvements are effective for stabilizing waterborne coating drying.

Record temperature and humidity

Record temperature and humidity during application, flash-off, and drying.

Standardize flash-off conditions

Define standards for time, temperature, humidity, and airflow exposure.

Review drying oven heating

Avoid rapid heating and check for a temperature curve that allows appropriate moisture removal.

Control film thickness

Control application passes, delivery rate, and film thickness measurements to avoid excessive thickness.

Check difficult-to-dry areas

Focus on recesses, inner surfaces, thick parts, and areas concealed by fixtures.

Review booth airflow

Check supply and exhaust air, air velocity, and accumulation of humid air.

Include wastewater treatment in the review

Check wastewater and sludge control in advance when converting to waterborne coating.

Train operators

Share with the shop floor that waterborne coating dries differently from solventborne coating.

14. How CoatureVietnam can help

CoatureVietnam does not view waterborne coating solely as a paint-change issue.
We review drying, temperature and humidity, booths, wastewater, safety, and color approval together.

We can provide the following support.

Checking waterborne paint suitability

We check waterborne paints against the application, substrate, required performance, and drying conditions.

Checking drying conditions

We review flash-off, the drying oven, actual product temperature, and the heating curve.

Making temperature and humidity control visible

We clarify the relationship between shop-floor conditions and defects, including rainy and dry seasons.

Reviewing booths and drying ovens

We check supply and exhaust air, air velocity, oven temperature distribution, and drying capacity.

Film thickness control

We establish film thickness control to prevent popping, pinholes, and drying defects caused by excessive thickness.

Reviewing wastewater treatment

We organize the wastewater and sludge treatment considerations required for introducing waterborne coating.

In-house training

We explain the difficulties of drying waterborne coatings clearly to shop-floor personnel as well.

15. Summary: waterborne coating becomes stable only when drying conditions are designed as well

Waterborne coating is effective for reducing VOCs and odor and improving the working environment.
It is a strong option for factories pursuing environmental measures.

However, waterborne coating involves more than replacing solvent with water.

What matters in waterborne coating is removing moisture from the coating film appropriately.

This requires control of the following conditions.

• Temperature
• Humidity
• Flash-off time
• Booth air velocity
• Film thickness
• Substrate shape
• Drying oven heating curve
• Actual product temperature
• Wastewater treatment
• Underlying surface moisture

Waterborne coating is environmentally friendly.
However, incorrect drying control is not friendly to quality.

Coating the Future with Science and Care

Science means understanding and controlling moisture removal through temperature and humidity, film thickness, and drying conditions.
Care means establishing a process that allows operators to produce acceptable products consistently without being troubled by unstable drying conditions.

CoatureVietnam reviews waterborne coating as a process encompassing booths, drying ovens, temperature and humidity, wastewater, and operating standards as well as paint, and supports improvements suited to the shop floor.

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