Drying and curing: A journey to flawless paint finish

According to AS/NZS 2310, drying and curing is defined as a state when the paint film has hardened sufficiently for the object to be moved carefully without marring the film. The drying/curing behavior varies for different paint systems. This can be due to the:
 

• chemistry of paint,
• flow of paint,
• film build-up of paint, and
• nature of solvents and polymers present in the paint systems.
   

Stepwise drying stages of paint film
 

1. Wet paint film: The initial stage of film drying

This is the initial stage of paint which is applied on any substrate. At this stage, the paint film is wet and in workable condition. Leveling and coalescing are at the initial stage. The rate of evaporation of solvents and other volatile compounds is highest in this stage of drying or curing.

 

2. Skinning stage: The emergence of paint film structure

As the volatile compounds start to escape the coating film, the polymer particles tend to come together. They float on the top surface of the applied paint film. This creates thin skinning on the top of the paint film. In this stage, the paint is partially dry-to-touch and the paint film starts to take structure.

 

3. Dry-to-touch stage: Increasing thickness and semi-dry state of paint film

This is the next stage which is very closely related to the skinning stage. In this stage, further evaporation of solvents and additives occurs. During this stage, the thickness of the dried paint film increases. Here, the paint is semi-dry and some of the additives and solvents are still present in the paint film mass which is generally under the dried top layer of the paint film.

 

4. Tack-free time: Achieving dry paint film properties

Tack-free time is the duration in which the paint film is no more tacky. It can be seen to have dry paint film properties. Tackiness is the state where the dry-to-touch paint film still feels sticky and has poor blocking resistance. This is due to the presence of some of the volatile materials and the poorly cured polymer layer.

 

5. Fully cured and dried paint film: The final stage

The fully cured and dry paint film refers to the stage where all the solvents and volatile compounds escape the paint film. This includes:
 

• additives,
• water, and
• co-solvents.

This leaves the film in complete strength and dry state. This is the final stage of the paint film drying process. At this stage, the paint film acquires maximum surface energy. It is characterized by high strength, high blocking resistance, hardness, and complete cure.
 

Stages of Drying in a Paint

Vanderhoff model: Drying mechanism of water-based paints

The Vanderhoff model suggests the process of film formation and drying of water-based paints. As per Vanderhoff's model, drying is categorized into three stages. These are:
 

• the primary dispersion stage,
• the sphere packing stage, and
• finally the polymer packing and curing stage.

Here, the process of drying is dominantly water and solvent-evaporation-based. This model focuses on the lateral drying of the paint, i.e., from edges to center of the paint film.
 

Vanderhoff's Model of Drying of Latex Paints

I. Mastering temperature control

Temperature is the most critical factor in the drying and curing process of the paint film. It controls the dry-to-touch and tack-free time of a paint or coating. For a faster drying time or cure, it is recommended to have a higher temperature. This increases the rate of evaporation of solvents and water from the applied coating.

However, a temperature above a certain limit can induce thermal shock to the coating. This can result in quicker drying but inferior curing. Thus, the paint film may not achieve its desired qualities and performance. Temperatures can be different for different coating types. For example, for epoxy curing or drying the temperature can go up to 80 to 90°C.

 

II. Humidity: The key to optimal paint drying and curing

Humidity is important as it can result in moderating the evaporation of water in the paint film. High humidity can impact the rate of drying. This is because the rate of evaporation of water and solvents decreases due to the presence of moisture. Hence, the drying and curing may get hampered. For latex paint and most paints, it is recommended to have humidity in the range of 50 to 70% of relative humidity.
 

Rate of Drying (Movement of Binder) at Different Temperatures and Humidity with Time

III. The vital role of ventilation and airflow

For better curing and drying it is necessary that there is good ventilation in the surroundings or environment. No or low ventilation can impact the drying. It does so by saturating the internal or the environmental air. One can also increase the rate of drying by increasing the airflow or ventilation in the application area.

 

IV. Exploring the impact of surface types and nature

Drying and curing including the tack-free time and drying-to-touch time depend on the type of substrate surface and its nature.
 

• The drying time for a highly porous substrate is generally less. For e.g., concrete or cement-based substrates.
• Whereas, the drying time for non-porous, rigid surfaces is higher. For e.g., plastics or glass.

Porous surfaces have less drying time because they absorb water and solvents faster. Evaporation also takes out the water and solvents from the film. This dual action leads to faster drying times, dry-to-touch time, tack-free time, and curing time.

 

V. Choosing the right paint characteristic for the substrate

The process of drying is also dependent on the different paint characteristics. These include the sheen level of the paint, the nature of the film build-up of paint, and the type of paint. It is suggested to select the right type of paint for substrates.
 

A shorter or optimum drying time has many advantages in terms of application and performance. Some of them are as follows:
 

• It increases the speed of the coating process. Fast-drying paint reduces the coating time by 4 seconds or 3 coats. Fast-drying paint quickly attains the desired hardness. Thus, it requires less recoating time. Also, the painted surface is ready to use in a short time.
   
• Faster drying is accompanied by better film hardness and s also related to the better-curing properties of a paint film. It means that optimum drying also results in better film properties. These properties include durability, finish, uniformity, block resistance, and toughness.
   
• A faster-drying film will also show less tackiness. The tack-free time and dry-to-touch time are shorter for fast-drying paints.
   
• The fast-drying paints are especially preferred for areas where human beings are residing. These include houses, public places, or gathering areas. This is because the fast-drying paints will not emit fumes and solvents for longer times. Hence, they are more health-friendly as compared to slow-drying alternatives.

• ISO 9117-3 — It determines the surface-drying characteristics of a coating, paint, or varnish. They dry by the action of air or chemical reaction of their components. The reaction takes place with ballotini glass beads.
   

• ISO 9117-4 — It determines the time taken to reach various stages of drying of organic coatings. It uses a mechanical straight-line or circular drying-time recorder.

• ISO 9117-5 — It determines if the coatings have reached various stages of drying. This includes those produced using multi-coat systems. This test is also known as the modified Bandow-Wolff test.
   

• ASTM D 5895 — It determines drying or curing during film formation of organic coating using mechanical recorders.
   
• ASTM D 1640 — It determines the drying, curing, or film formation of organic coatings.