The difference between drying and curing

In coating operations, people often say that the paint has dried.

The surface does not transfer paint to the hand when touched. The workpiece appears ready for the next process. It appears ready to be packed.

When these conditions are reached, operators may consider the paint dry.

However, there is an important point to bear in mind when considering coating quality.

Paint being dry and a coating film having developed its full performance are not the same thing.

Even when the surface is dry, solvent may remain inside the coating film. Even when its appearance is acceptable, the curing reaction may not have progressed sufficiently. This can subsequently cause poor adhesion, inadequate drying, insufficient chemical resistance, blistering, blocking, insufficient hardness, and other problems.

To stabilize coating quality, it is important to understand how paint becomes a coating film.

Paint becomes a coating film after application

Paint in its can is not yet a finished product.

Paint is applied to the substrate surface and becomes a coating film through drying or curing. That film provides the required appearance, protects the substrate, and delivers the necessary performance.

In other words, coating is the process of converting liquid or powder paint into a stable coating film on the substrate surface.

The mechanism of film formation differs according to the type of paint.

Broadly, there are two types.

The first is paint that hardens through drying. The second is paint that hardens through a reaction.

Understanding this difference makes it easier to identify the causes of inadequate drying and insufficient curing.

What is paint that hardens through drying?

Paint that hardens through drying forms a coating film primarily through the evaporation of solvent or water.

Paint contains resin, pigments, additives, solvents, and other components. After application, solvent or water escapes, leaving the resin, pigments, and other components to form a continuous coating film.

For this type, evaporation is the main mechanism of film formation.

In other words, the coating film forms as solvent or water escapes properly.

The following conditions are important for these paints.

• Thinner used
• Dilution ratio
• Film thickness
• Temperature
• Humidity
• Ventilation
• Drying time
• Number of coats
• Flash-off time

For example, if the film is too thick, solvent inside it may have difficulty escaping even when the surface is dry. Proceeding to the next process in this condition can subsequently lead to tackiness, impressions, blistering, blocking, and poor adhesion.

A thinner that evaporates too quickly can also cause the surface to dry first, making it difficult for solvent inside the film to escape. Conversely, a thinner that evaporates too slowly can delay drying and lead to sagging, dust contamination, and inadequate drying.

For paints that harden through drying, faster drying is not necessarily better. Solvent or water must escape at an appropriate rate.

What is paint that hardens through a reaction?

Paint that hardens through a reaction forms a coating film as a chemical reaction progresses after application.

For this type, solvent evaporation alone does not develop the film’s full performance. A strong coating film forms through a reaction between resin and hardener, a reaction promoted by heat, or curing induced by ultraviolet light.

Typical examples include the following paints.

• Two-component polyurethane paint
• Two-component epoxy paint
• Melamine baking enamel
• Thermosetting powder coating
• UV-curable paint
• Some alkyd paints

With these paints, even if the surface appears dry, the coating film will not deliver adequate performance if the reaction has not progressed sufficiently.

For example, an incorrect mixing ratio between the base and hardener in a two-component paint can cause inadequate curing even when the surface is dry. With a baking paint, curing may be insufficient if the product itself has not become hot enough, even when the oven temperature setting is correct.

For paints that harden through a reaction, it is necessary to verify that the required reaction conditions are met rather than judging drying by appearance.

Surface drying and full development of coating film performance are different

In coating operations, it is particularly important not to confuse surface drying with full development of coating film performance.

Surface drying means that paint is unlikely to transfer to the hand when the surface is touched. At this stage, however, drying or curing inside the coating film may still be insufficient.

Full development of coating film performance means that the film can provide the required hardness, adhesion, chemical resistance, water resistance, durability, and other properties.

These two states are not the same.

Stacking, packing, assembling, or shipping products immediately because their surfaces are dry can cause the following problems.

• Blocking
• Impressions
• Poor adhesion
• Blistering
• Blushing
• Insufficient hardness
• Insufficient chemical resistance
• Insufficient water resistance
• Peeling that develops later

Particular care is needed when the film is thick, humidity is high, temperature is low, or ventilation is poor.

Important controls for two-component paints

Two-component paints are used by mixing the base and hardener.

Three points are important.

The first is the mixing ratio. If the base-to-hardener ratio is incorrect, the reaction may be insufficient, resulting in inadequate curing and coating film performance.

The second is mixing. Even with the correct mixing ratio, insufficient mixing can cause localized undercuring.

The third is pot life. Pot life is the period during which the paint can be used after mixing.

The reaction in a two-component paint begins as soon as it is mixed. As time passes, the reaction can progress internally and make the paint unsuitable for application, even if it still appears usable.

Using paint beyond its pot life can make adhesion, hardness, chemical resistance, water resistance, and other properties inconsistent, even when the appearance initially seems acceptable.

For this reason, the mixing ratio, mixing, and pot life must always be controlled for two-component paints.

Oven air temperature is not the only important factor for baking paints

For baking paints, heat promotes the curing reaction.

Operators often check the drying oven’s temperature setting. However, the oven temperature setting alone is not sufficient.

What matters is the actual workpiece temperature.

Actual workpiece temperature is the temperature of the product itself.

Even if the temperature shown on the drying oven meets the specification, the product may not become hot enough because of its material, thickness, shape, hanging arrangement, or the load placed in the oven.

Particular care is needed with thick metal parts, large workpieces, and products with complex shapes.

Baking conditions must be assessed by whether the product reached the required temperature and remained at that temperature for the required time, rather than simply by the oven’s temperature setting.

Insufficient actual workpiece temperature can lead to undercuring, poor adhesion, insufficient chemical resistance, insufficient hardness, and other problems.

Baking conditions are also important for powder coatings

Powder coatings use little or no solvent. Powder paint is deposited onto the substrate by electrostatic attraction or other means, then heated to melt and cure it.

Powder coatings can offer environmental advantages. However, powder coating is not complete simply because the powder has been applied.

Inadequate baking conditions prevent the coating film from developing sufficient performance.

The following conditions are important for powder coatings.

• Amount of powder deposited
• Film thickness
• Baking temperature
• Baking time
• Actual workpiece temperature
• Temperature distribution inside the oven
• Product shape
• Hanging arrangement
• Powder storage conditions

With powder coatings, excessive film thickness can cause appearance defects and inadequate curing. Insufficient baking can also prevent adequate adhesion and durability from developing.

Powder coating also requires the paint, film thickness, baking conditions, and equipment conditions to be considered together.

Flash-off time also affects coating film quality

Some processes allow a workpiece to stand for a specified period after application instead of immediately placing it in the drying oven. This period is sometimes called flash-off time.

Flash-off time can be important for film formation.

Solvent-borne paints, in particular, require some solvent to escape before baking. Rapid heating before sufficient solvent has escaped can cause solvent popping, blistering, pinholes, surface roughness, and other defects.

Conversely, excessive flash-off time can lead to dust contamination and longer process times.

Flash-off time therefore needs to be controlled according to the paint, film thickness, thinner, temperature and humidity, and drying conditions.

Distinguish inadequate drying from insufficient curing

Inadequate drying and insufficient curing are easily confused among coating problems.

Inadequate drying primarily means that solvent or water has not escaped sufficiently.

Possible causes include the following.

• Excessive film thickness
• Thinner evaporation that is too slow
• Low temperature
• High humidity
• Poor ventilation
• Insufficient drying time
• Insufficient flash-off

Insufficient curing, on the other hand, means that the required reaction has not progressed sufficiently.

Possible causes include the following.

• Incorrect mixing ratio for a two-component paint
• Insufficient mixing
• Use beyond the pot life
• Insufficient baking temperature
• Insufficient baking time
• Insufficient actual workpiece temperature
• Deteriorated hardener
• Insufficient baking of powder coating

Inadequate drying and insufficient curing have different causes and require different corrective measures.

When a defect occurs, it is therefore important to distinguish between solvent or water failing to escape and a reaction failing to progress sufficiently.

Common misunderstandings in coating operations

The following misunderstandings about drying and curing can arise in coating operations.

The first is the belief that a coating is complete if it feels dry to the touch. In fact, internal drying or the curing reaction may still be insufficient.

The second is the belief that baking conditions are satisfactory if the oven temperature setting is correct. In fact, without checking the actual workpiece temperature, it is impossible to know whether the product has reached the required temperature.

The third is the belief that a two-component paint can be used immediately once mixed. In fact, coating film performance will not be stable unless the mixing ratio, mixing, and pot life requirements are followed.

The fourth is the belief that powder coatings are easy to use because they are environmentally friendly. In fact, baking conditions, film thickness, equipment conditions, color changes, and powder handling must all be managed.

Eliminating these misunderstandings helps stabilize coating quality.

Points CoatureVietnam checks

When inadequate drying or insufficient curing is suspected, CoatureVietnam checks the following items.

• Is the paint a one-component, two-component, baking, or powder coating type?
• Does film formation depend primarily on solvent evaporation or a chemical reaction?
• Is the specified thinner being used?
• Is the dilution ratio standardized?
• Is film thickness within the appropriate range?
• For a two-component paint, is the mixing ratio correct?
• Is mixing sufficient?
• Is the pot life observed?
• Is the flash-off time appropriate?
• Is actual workpiece temperature checked in addition to the drying oven’s temperature setting?
• Is baking time controlled as the time after the actual workpiece reaches the required temperature?
• Are products being packed or stacked immediately after drying?
• Does the defect occur immediately after application, after drying, or after use?

Checking these items makes it easier to identify the causes.

How CoatureVietnam can help

CoatureVietnam helps verify drying and curing conditions based on the type of paint and its film formation mechanism.

For example, we can provide the following support.

Paint Optimizer We review the drying conditions, curing conditions, dilution ratio, film thickness, and thinner compatibility of existing paints to help stabilize quality.

Thinner Streamer We help select a thinner suited to the paint and process, considering drying characteristics, solvency, and evaporation rate.

Work standards and process control We establish SOPs for mixing ratios, pot life, flash-off time, actual workpiece temperature, baking conditions, and other factors, translating them into control methods that can be reproduced in daily operations.

Coat Systemizer We support coating process design that includes equipment conditions for drying ovens, baking ovens, powder coating equipment, and other systems.

Summary

Paint drying and a coating film curing are not the same thing.

For paints that harden through drying, it is important for solvent or water to escape properly. For paints that harden through a reaction, it is important to control the mixing ratio, pot life, baking temperature, baking time, actual workpiece temperature, and other factors.

A surface that appears dry does not necessarily mean that the coating film has developed its full performance.

To stabilize coating quality, it is necessary to understand how the paint becomes a coating film and control the process in a way suited to that mechanism.

Distinguish drying from curing. Do not judge solely by apparent dryness. Check actual workpiece temperature, pot life, film thickness, thinner, and flash-off time as well.

Understanding these fundamentals is the first step toward preventing inadequate drying and insufficient curing.

Coating the Future with Science and Care. CoatureVietnam supports both paint selection and the development of processes that form coating films correctly.