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Quick-drying is the most frequently heard customer request for water-based coatings.
Due to the uniqueness of its molecular structure, that is, the strong hydrogen bonding between molecules, its characteristics are clearly different from most organic solvents.
In the field of water-based coatings, this feature is concentrated in that, due to the high heat of evaporation of water, the evaporation rate of water is ten times or even dozens of times slower than common coating solvents.
Moreover, because the content of water vapor in the air is significant and changes greatly with the seasons, the evaporation rate of water changes accordingly.
In the most severe case, if the relative humidity of the air reaches 100%, the evaporation of water will stop, and the non-aqueous solvent will not be affected by this factor.
Although water-based coatings face the above-mentioned technical challenges, they will inevitably become an important member of the coatings field due to their environmental protection characteristics.
With the unremitting efforts of water-based paint workers over the past ten years, water-based paint technology has become increasingly mature.
The main factors affecting the drying speed of water-based coatings and the corresponding measures that can be taken when formulating are discussed below.
1.
Resin selection: Like all coatings, the performance of water-based coatings is largely determined by the resin used in the formulation.
Most water-based film-forming resins are emulsion systems, and the film-forming mechanism of this system is different from that of solvent-based coatings.
The solvent-based resin and the solvent form a single-phase system.
As the solvent evaporates, the viscosity of the system increases until it becomes a solid.
In terms of the mechanical properties of the system, it is a continuous process.
The aqueous emulsion is a two-phase system.
As water evaporates, the viscosity of the system does not change much at the beginning, but when the volume of the emulsion particles in the total system volume reaches a critical value, the system suddenly changes from a commercial state to a solid state.
A discontinuous process, this critical point is the beginning of the surface drying of water-based paints, so the surface-drying time of water-based paints is shorter than that of some solvent-based paints.
From the surface drying to the performance of the paint film, it depends on the evaporation rate of the residual water in the system, the interpenetration of the macromolecules in the emulsion particles, and the volatilization rate of other small organic molecules in the system.
In order to optimize the system, when making water-based paint formulations, the resin should be selected from the following aspects: a.
Solid content: Generally, the higher the solid content of the emulsion, the closer it is to the critical value of surface drying and the faster the drying speed.
fast.
However, if the solid content is too high, it will also bring a series of unfavorable factors.
If the surface dries too quickly, the interval between brushing will be shortened, which will cause inconvenience in construction.
Emulsions with high solid content usually have poor rheological properties due to the small resin particle spacing and are not sensitive to thickeners, making it more difficult to adjust the spraying or painting performance of the coating.
b.
Emulsion particle size: The smaller the emulsion particles, the smaller the distance between the particles under the same solid content, the lower the critical value of surface drying, and the faster the drying speed.
The small size of the emulsion also brings other advantages such as good film-forming properties and high gloss.
c.
Resin glass transition temperature (Tg): Generally speaking, the higher the Tg of the resin, the better the final film-forming performance.
However, in terms of drying time, the trend is basically the opposite.
For resins with high Tg, it is usually necessary to add more film-forming aids in the formulation to facilitate the mutual penetration of the polymer between the emulsion particles and promote the quality of the film.
These film-forming additives need enough time to evaporate from the system, which will actually extend the time from surface drying to full drying.
Therefore, from the factor of Tg, the drying time and the film-forming performance are often inconsistent.
d.
Phase structure of emulsion particles: Depending on the preparation process of the emulsion, the same monomer composition may form different particle phase structures.
The well-known core-shell structure is one of these examples.
Although it is impossible for all particles of an emulsion to be made into a core-shell structure, the metaphor of this image is that people can have a popular understanding of the film-forming properties of the emulsion.
If the shell Tg of the particles is low and the core Tg is high, then the system requires less film-forming aids and dries faster.
However, since the continuous phase is a low-Tg resin after film formation, the hardness of the paint film will be affected to a certain extent.
On the contrary, if the shell Tg of the particles is high, a certain amount of additives is required to form the film, and the drying speed of the film will be slower than the former, but the hardness after drying will be higher than the former.
e.
Types and dosage of surfactants: common emulsions Certain surfactants are used in the manufacturing process.
Surfactant has the effect of isolating and protecting the emulsion particles, and has a great influence in the film formation process of the particles fusion, especially in the initial stage, that is, when the surface is dry.
Moreover, these unique chemicals have a certain degree of solubility in water and oil phases, and the part dissolved in the resin actually acts as a film-forming aid.
Different surfactants, due to their different solubility in the resin, have different film-forming agents.
2.
Resin curing mechanism: Water-based resin film formation and curing generally have several levels of mechanism.
First of all, the aggregation and fusion of emulsion particles is a mechanism that all emulsions must experience when they surface dry.
Then, the volatilization of water and other film-forming aids fully manifests the basic properties of the thermoplastic resin itself, which is the second stage of curing.
Finally, some emulsions introduce a cross-linking mechanism during the preparation, or introduce a cross-linking agent when the coating is used, so that the hardness of the film is further improved on the basis of the thermoplastic resin.
The cross-linking mechanism of this last step has a great influence on the final curing speed and degree of the film.
Common crosslinking mechanisms include oxidative crosslinking (such as alkyd resin crosslinking), Michael additive crosslinking (such as some self-crosslinking emulsion systems), and nucleophilic substitution crosslinking (such as epoxy, polyurethane, etc.
) ).
These cross-linking reactions are all affected by temperature, pH and other factors.
The relationship between the curing requirements of the system and other properties should be balanced when formulating.
3.
The amount and type of film-forming aids: In theory, any resin solvent is a film-forming aid.
In practice, considering factors such as safety, cost, speed, etc.
, there are only a dozen common film-forming additives, mainly some high-boiling alcohols, ethers and esters.
These film-forming additives will have their own preferences for different water-based paint engineers.
Generally, an experienced engineer usually uses only two or three kinds of film-forming additives.
The main considerations are the distribution of reagents between water and resin and the distribution inside resin particles.
Especially when the water-based resin is a multi-phase resin, the selection and matching of the film-forming aids becomes particularly important.
4.
Construction environment: At the beginning of this article we discussed the issue of water.
It is precisely because of the characteristics of water that the construction environment of water-based coatings is higher than that of oil-based paints.
The main purpose is to control the temperature and humidity during construction as much as possible.
The construction of general formula should try to avoid high humidity environment.
If construction must be carried out under high humidity, the formula should be adjusted, or a resin with fast film-forming properties should be selected or the site should be isolated.
Due to the uniqueness of its molecular structure, that is, the strong hydrogen bonding between molecules, its characteristics are clearly different from most organic solvents.
In the field of water-based coatings, this feature is concentrated in that, due to the high heat of evaporation of water, the evaporation rate of water is ten times or even dozens of times slower than common coating solvents.
Moreover, because the content of water vapor in the air is significant and changes greatly with the seasons, the evaporation rate of water changes accordingly.
In the most severe case, if the relative humidity of the air reaches 100%, the evaporation of water will stop, and the non-aqueous solvent will not be affected by this factor.
Although water-based coatings face the above-mentioned technical challenges, they will inevitably become an important member of the coatings field due to their environmental protection characteristics.
With the unremitting efforts of water-based paint workers over the past ten years, water-based paint technology has become increasingly mature.
The main factors affecting the drying speed of water-based coatings and the corresponding measures that can be taken when formulating are discussed below.
1.
Resin selection: Like all coatings, the performance of water-based coatings is largely determined by the resin used in the formulation.
Most water-based film-forming resins are emulsion systems, and the film-forming mechanism of this system is different from that of solvent-based coatings.
The solvent-based resin and the solvent form a single-phase system.
As the solvent evaporates, the viscosity of the system increases until it becomes a solid.
In terms of the mechanical properties of the system, it is a continuous process.
The aqueous emulsion is a two-phase system.
As water evaporates, the viscosity of the system does not change much at the beginning, but when the volume of the emulsion particles in the total system volume reaches a critical value, the system suddenly changes from a commercial state to a solid state.
A discontinuous process, this critical point is the beginning of the surface drying of water-based paints, so the surface-drying time of water-based paints is shorter than that of some solvent-based paints.
From the surface drying to the performance of the paint film, it depends on the evaporation rate of the residual water in the system, the interpenetration of the macromolecules in the emulsion particles, and the volatilization rate of other small organic molecules in the system.
In order to optimize the system, when making water-based paint formulations, the resin should be selected from the following aspects: a.
Solid content: Generally, the higher the solid content of the emulsion, the closer it is to the critical value of surface drying and the faster the drying speed.
fast.
However, if the solid content is too high, it will also bring a series of unfavorable factors.
If the surface dries too quickly, the interval between brushing will be shortened, which will cause inconvenience in construction.
Emulsions with high solid content usually have poor rheological properties due to the small resin particle spacing and are not sensitive to thickeners, making it more difficult to adjust the spraying or painting performance of the coating.
b.
Emulsion particle size: The smaller the emulsion particles, the smaller the distance between the particles under the same solid content, the lower the critical value of surface drying, and the faster the drying speed.
The small size of the emulsion also brings other advantages such as good film-forming properties and high gloss.
c.
Resin glass transition temperature (Tg): Generally speaking, the higher the Tg of the resin, the better the final film-forming performance.
However, in terms of drying time, the trend is basically the opposite.
For resins with high Tg, it is usually necessary to add more film-forming aids in the formulation to facilitate the mutual penetration of the polymer between the emulsion particles and promote the quality of the film.
These film-forming additives need enough time to evaporate from the system, which will actually extend the time from surface drying to full drying.
Therefore, from the factor of Tg, the drying time and the film-forming performance are often inconsistent.
d.
Phase structure of emulsion particles: Depending on the preparation process of the emulsion, the same monomer composition may form different particle phase structures.
The well-known core-shell structure is one of these examples.
Although it is impossible for all particles of an emulsion to be made into a core-shell structure, the metaphor of this image is that people can have a popular understanding of the film-forming properties of the emulsion.
If the shell Tg of the particles is low and the core Tg is high, then the system requires less film-forming aids and dries faster.
However, since the continuous phase is a low-Tg resin after film formation, the hardness of the paint film will be affected to a certain extent.
On the contrary, if the shell Tg of the particles is high, a certain amount of additives is required to form the film, and the drying speed of the film will be slower than the former, but the hardness after drying will be higher than the former.
e.
Types and dosage of surfactants: common emulsions Certain surfactants are used in the manufacturing process.
Surfactant has the effect of isolating and protecting the emulsion particles, and has a great influence in the film formation process of the particles fusion, especially in the initial stage, that is, when the surface is dry.
Moreover, these unique chemicals have a certain degree of solubility in water and oil phases, and the part dissolved in the resin actually acts as a film-forming aid.
Different surfactants, due to their different solubility in the resin, have different film-forming agents.
2.
Resin curing mechanism: Water-based resin film formation and curing generally have several levels of mechanism.
First of all, the aggregation and fusion of emulsion particles is a mechanism that all emulsions must experience when they surface dry.
Then, the volatilization of water and other film-forming aids fully manifests the basic properties of the thermoplastic resin itself, which is the second stage of curing.
Finally, some emulsions introduce a cross-linking mechanism during the preparation, or introduce a cross-linking agent when the coating is used, so that the hardness of the film is further improved on the basis of the thermoplastic resin.
The cross-linking mechanism of this last step has a great influence on the final curing speed and degree of the film.
Common crosslinking mechanisms include oxidative crosslinking (such as alkyd resin crosslinking), Michael additive crosslinking (such as some self-crosslinking emulsion systems), and nucleophilic substitution crosslinking (such as epoxy, polyurethane, etc.
) ).
These cross-linking reactions are all affected by temperature, pH and other factors.
The relationship between the curing requirements of the system and other properties should be balanced when formulating.
3.
The amount and type of film-forming aids: In theory, any resin solvent is a film-forming aid.
In practice, considering factors such as safety, cost, speed, etc.
, there are only a dozen common film-forming additives, mainly some high-boiling alcohols, ethers and esters.
These film-forming additives will have their own preferences for different water-based paint engineers.
Generally, an experienced engineer usually uses only two or three kinds of film-forming additives.
The main considerations are the distribution of reagents between water and resin and the distribution inside resin particles.
Especially when the water-based resin is a multi-phase resin, the selection and matching of the film-forming aids becomes particularly important.
4.
Construction environment: At the beginning of this article we discussed the issue of water.
It is precisely because of the characteristics of water that the construction environment of water-based coatings is higher than that of oil-based paints.
The main purpose is to control the temperature and humidity during construction as much as possible.
The construction of general formula should try to avoid high humidity environment.
If construction must be carried out under high humidity, the formula should be adjusted, or a resin with fast film-forming properties should be selected or the site should be isolated.
