Several common drying methods for water-based coatings of furniture!

Coating drying of waterborne coatings is the need to ensure the quality of coating, and is an indispensable process of the entire coating process, which directly affects the production cost and ergonomics

The conditions under which the coating is dried has a great

Film forming process for water-based coatings of furniture

The film formation process of waterborne coatings is a molecular chain agglomeration phenomenon, which can generally be divided into 3 steps: water volatilization, particle deformation and particle merger

After construction, the first is water volatilization, when the latex particles of the water-based coating account for 74% (volume) of the rubber layer, the particles are close to each other and reach a dense filling state; Secondly, the moisture continues to volatilize, the polymer particles are deformed, the capillary pressure is higher than the deformation resistance of the polymer particles, the pressure between the particles is generated, the more the medium volatilizes, the greater the pressure, and then agglomerates and fuses to form a continuous coating film; Finally, the moisture continues to volatilize, when the pressure reaches the point where the molecular chain in each particle can diffuse into another particle molecular chain, the polymer molecular chains gradually diffuse to each other, so that the membrane is homogenized, and the curing and drying process

Study of the hydrophoresis process

Theoretically, the moisture volatilization process of waterborne coatings conforms to the theory of vertical drying by the surface and the inside

Drying method

The drying process of wood is the process of gradually removing moisture from the inside of wood, while the drying of water-based coating coatings for furniture is to a large extent the process of gradually removing water from the inside of the coating, in a certain sense, the two have certain similarities

Therefore, when further studying the drying method of water-based coating of wood furniture, we can learn from the relatively mature wood drying method

The main factors affecting the drying speed of water-based wood coatings are the level of ambient temperature, the size of relative humidity, the thickness of the paint film and the speed of air flow

Different drying methods have obvious differences in the drying speed and film forming quality of waterborne coatings, and the following is an overview of the various drying methods of

natural drying

The latent heat of evaporation of water is very large, reaching 2457.
7KJ/Kg, and the evaporation of moisture in water-based coatings needs to absorb a lot of heat
.
The volatilization of moisture is affected by atmospheric temperature and humidity and wind speed, the volatilization rate increases with the increase of temperature, decreases with the increase of air humidity, and becomes larger with the increase of wind speed, which is because the temperature rises, the molecular thermal movement is more intense, and the liquid water molecules are more likely to get rid of the attraction of other water molecules and become steam molecules
.

However, with the increase of air humidity, the proportion of water vapor in the air increases, and the partial pressure of water vapor increases, so that more water vapor molecules become liquid water molecules in time, so the volatilization rate of water decreases
with the increase of air humidity.

The greater the wind speed, the more water molecules are taken away from the swing surface in a time, so the faster
the volatilization speed.

Natural drying has the advantages of simple method and wide application, but at the same time has the disadvantage
of slow drying speed.
Under natural conditions, temperature and humidity and wind speed are constantly changing, and the drying speed and film forming quality are unstable
.
If the temperature is high and humidity or the humidity is relatively large, the coating is easy to whiten and the drying speed is slower
.
At low temperatures, the drying speed is very slow; Especially at 5 °C, water-based coatings are difficult to form films
.
These are important reasons
that restrict the promotion and application of waterborne coatings.

Hot air drying

Hot air drying is a heating drying method
that adopts the principle of convection, takes hot air with a temperature of 40 ~ 60 ° C as the heat carrier, transfers thermal energy to the coating on the surface of the workpiece, and cures into a film after absorbing energy.

Commonly used as a heat source, the air is heated first, and the heat is transmitted by the hot air to the surface of the coating in the form of convection, so that the coating can be dried
quickly.

When drying with hot air, the hot air around the coating is the heating medium
.
The coating always has a certain thickness, and it takes a certain amount of time
for heat to be transferred from the surface of the coating to the inner boundary.
The speed of heat transfer depends on the thickness of the coating and its thermal conductivity
.
Therefore, when convective heating, the coating surface is always heated
first.
At the beginning of drying, the moisture in the surface layer volatilizes most strongly, and the curing of the coating also begins from the surface layer, and then gradually expands to the bottom layer, resulting in the final drying
of the bottom layer.
The hot air drying process can significantly accelerate the drying speed of the coating, and has the characteristics of strong adaptability, which is a more widely used drying form
.
In large furniture factories, hot air drying line is the main form of forced drying of current solvent-based coatings, which are used as drying water-based coatings, due to the long drying time of water-based coatings, on the one hand, the drying equipment needs to be studied, and the drying process
needs to be studied on the other hand.
Temperature, humidity, operating speed (drying time) of the workpiece and ventilation conditions throughout the drying line are essential
to ensure drying quality and the final quality of the paint film.

Microwave drying

Microwave refers to electromagnetic waves with a wavelength of 1mm-1m and a frequency of 300MHz-300GHz, which is penetrating, and the commonly used microwave frequency is 915-2450MHz
.

Microwave heating uses the principle of dielectric loss, and the dielectric constant of water is much larger than that of dry matter, and most of the energy released by the electromagnetic field is absorbed
by the moisture in the coating.
The microwave field periodically changes the direction of the applied electric field at a high speed of hundreds of millions of times per second, causing the water molecules to swing rapidly, producing a significant thermal effect, so that the temperature inside and on the surface of the coating rises
rapidly at the same time.
The advantage of microwave heating is that the drying speed is particularly fast, and different substances have selective absorption for microwaves; There are no shape requirements for the dried object; For the heating of the coating film is very uniform, there is no temperature gradient, and the thick film can be dried
.
For water molecules, microwaves with a frequency of 2450MHz have the optimal balance
of energy conversion and loss.
Microwaves of this frequency can penetrate a water layer of 30mm thickness and can be used to dry waterborne coatings of various thicknesses
.

UV drying

For water-based UV wood coatings, UV-cured drying can
be used.
The so-called ultraviolet curing refers to the drying method
of water-based UV wood coatings that are cured under ultraviolet irradiation with a wavelength of 300-400 nm.

In the waterborne UV coating contains a small amount of photosensitizer, under the irradiation of ultraviolet rays, the photosensitizer absorbs a specific wavelength of ultraviolet light, decomposes to produce active groups, triggers the polymerization reaction of film-forming substances, forms a mesh structure and solidifies
the coating.
Ultraviolet curing has the characteristics
of fast coating curing speed and good coating quality.
But
.
This method can only be used to dry water-based UV coatings, and only into flat furniture finishing panels
.

Infrared drying

Infrared curing can be used to dry coatings for waterborne wood coatings
.
The so-called infrared curing actually refers to the furniture panels and their coatings that are coated under the illumination of infrared rays, absorbing radiant energy and being converted into thermal energy, thereby realizing the curing
of the coating.

Infrared is an invisible ray, between visible light and microwaves, with a wavelength of 0.
72-1000um
.
According to the wavelength range, it can be divided into "near", "medium" and "far" infrared, and far infrared dry coatings
are commonly used.
Infrared curing has the advantages of fast curing speed, rapid heating, good curing quality, etc.
, but when the coating is dried by infrared heating, there is a significant temperature gradient in the coating, and its drying is extended from the surface to the interior, which makes it unsuitable for drying thicker coating films; Infrared drying can only heat the area where infrared rays can reach, and cannot be used to dry three-dimensional objects
.