Characteristics and application methods of film-forming additives for water-based coatings

The film-forming aids of water-based coatings, also known as coagulants, coalescing agents, film-forming aids or co-solvents, can dissolve and swell the polymer particles in the emulsion, so that the particles can also change with moisture at lower temperatures.

Waterborne coating film formation

Water-based coating film-forming additives are widely used in architectural coatings (latex paint), water-based automotive coatings and automotive refinish coatings, water-based electrophoretic coatings, water-based marine coatings, water-based container coatings, water-based anti-corrosion coatings, water-based industrial coatings, water-based adhesives, water-based woodware Coatings, water-based coil and coil coatings, water-based screen printing inks, water-based gravure inks, water-based flexo inks, UV water-based paint inks, and more .

2.

(1) Alcohols (such as benzyl alcohol BA, ethylene glycol, propylene glycol, hexylene glycol);

(2), alcohol esters (such as dodecyl alcohol ester (ie Texanol ester alcohol or alcohol ester-12));

(3), alcohol ethers (ethylene glycol butyl ether EB, propylene glycol methyl ether PM, propylene glycol ethyl ether, propylene glycol butyl ether, dipropylene glycol monomethyl ether DPM, dipropylene glycol monopropyl ether DPnP, dipropylene glycol monobutyl ether DPnB, tripropylene glycol n-butyl ether TPnB, propylene glycol phenyl ether PPH, etc.

(4), alcohol ether esters (such as hexylene glycol butyl ether acetate, 3-ethoxy ethyl propionate EEP), etc.

From the main manufacturers and main products of film-forming additives, the representative ones are Lusolvan FBH of BASF, DBE-IB of Du Pont of the United States, COASOL of Chemoxy of the United Kingdom (diisobutyl adipate, glutaric acid) A mixture of diisobutyl diacid and diisobutyl succinate), Nexcoat 795 (2,2,4-trimethyl-1,2-pentanediol-isobutyl monoester) from Neste perstorp, Belgium, Dowanol pph (Propylene Glycol Phenyl Ether), DAL-PADC, DAL-PADD, DPnB of DowChemical Company of the United States, Texanol, EEH, OE300, OE400 of Eastman Chemical Company, SER-AD of British Hemings Chemical Company FX510, SER-AD FX511, dodecyl alcohol ester (ester alcohol 12) of Jiangsu Runtai Chemical Co.

The state of coalescents in waterborne coatings

Liquid stone water-based paint toning

Coalescents are classified into three categories: A, B, and C according to their location in the polymer

3.

The ideal coalescent should have the following properties:

(1) The film-forming aid must be a strong solvent for the polymer, has excellent film-forming efficiency for a variety of water-based resins, and has good compatibility, and can reduce the minimum film-forming temperature of water-based resins.

(2), low odor, less dosage, excellent effect, good environmental protection, certain volatility, and effectively adjust the drying rate to facilitate construction;

(3) Excellent hydrolytic stability, low solubility in water, and its volatilization rate should be lower than that of water and ethanol.

(4) After adding the latex system, it is adsorbed on the surface of the latex particles, which can be adsorbed by the latex particles and has excellent coalescence performance, and fully dissolve and swell the water-based resin without affecting the stability of the latex particles
.

Fourth, the development direction of film-forming additives

Although the film-forming aid has a great effect on the film-forming of latex paint, the film-forming aid is an organic solvent and has an impact on the environment, so the development direction is an environmentally friendly and effective film-forming aid:

(1) is to reduce the odor
.
Mixtures of Coasol, DBE-IB, OptifilmEnhancer 300, TXIB, TXIB and Texanol all reduce odor
.
Although TXIB is slightly inferior in MFFT reduction and early scrub resistance, these aspects can be improved by mixing with Texanol
.

Liquid stone water-based paint granulation

(two) is to reduce volatile organic compounds (VOC)
.
Most coalescents are an important part of the VOC of coatings, so coalescing should be used as little as possible
.
When choosing coalescents, priority should be given to compounds that are not within the VOC limit, but are not volatile too slowly and have higher film-forming efficiency
.
In Europe, VOCs are those chemicals with a boiling point equal to or lower than 250°C
.
Those substances with boiling points above 250°C are not classified as VOCs, so coalescing aids are developed towards higher boiling points
.
Such as Coasol, LusolvanFBH, DBE-IB, OptifilmEnhancer300, diisopropanol adipate
.

(3) It is low toxicity, safe and acceptable biodegradability
.

(4) It is an active film-forming aid
.
Dicyclopentenyl oxyethyl acrylate (DPOA) is an unsaturated polymerizable organic compound, homopolymer Tg=33℃, odorless
.
Its structural formula is as follows:

In the formulation of latex paint with higher Tg value, no coalescence aid is needed, but DPOA is added, and a small amount of drier, such as cobalt salt, is added
.
DPOA can reduce the film forming temperature, so that the latex paint can form a film at room temperature
.
However, DPOA is not volatile, which is not only environmentally friendly, but also undergoes oxidative radical polymerization under the action of a drier, which increases the hardness, anti-adhesion and brightness of the coating film
.
Therefore, DPOA is known as an active coalescent
.

5.
Film-forming mechanism of water-based coatings

The film-forming process of water-based paint is more complicated, and it has to go through a process from dispersed polymer particles to agglomeration of each other into a whole
.
After its construction, the water evaporates, and the spherical particles must fuse with each other to form a continuous coating film
.
Water-based paint film is divided into the following processes

(1) The particles gradually move closer to the filling process
.

The spherical particles maintain the dispersed state in the latex paint with the effect of double electric layer and shielding stabilization.
After the water-based paint is applied, the water gradually evaporates, and the polymer particles and pigments that were originally dispersed by electrostatic repulsion and steric hindrance stabilization , The filler particles gradually get closer, but they can still move freely
.
At this stage, the volatilization of water is similar to the volatilization of pure water, and it is a constant rate of volatilization
.

(2) The particle fusion process
.

With the further volatilization of the water, the protective layer adsorbed on the surface of the polymer particles is destroyed, and the exposed particles are in contact with each other.

The gap is getting smaller and smaller, and the volume of the paint film shrinks.
When the water volatilizes to the end, its driving force will also disappear.
When the capillary diameter is reached, the capillary pressure is higher than the anti-deformation force of the polymer particles due to the capillary effect.
, the particle stability is destroyed and deformed, and finally agglomerates and fuses into a continuous coating film
.
This process is the key to whether the water-based paint can form a film.
If the glass transition temperature (Tg) of the emulsion polymer is high (in order to make the coating film have good mechanical properties, weather resistance and stain resistance, the Tg value should not be too low.
), at a lower ambient temperature, it is difficult to deform, which will hinder the fusion process, resulting in the inability to form a film
.
Coalescents can swell and soften the latex particles so that they can be easily fused together to form a continuous film
.

(3) The process of polymer segment interdiffusion, permeation, cross-linking and film formation
.

Water-based stone-like paint configuration

With the passage of time, the additives remaining in the water gradually diffuse to the coating film, and the long chain segments of the polymer molecules penetrate and diffuse into each other, and are wound to form a uniform coating film with good performance.
It gradually volatilizes and finally forms an ideal coating film with excellent performance
.

6.
Usage and dosage of film-forming aids for water-based coatings

Polymers such as emulsions or dispersions generally have a glass transition temperature Tg above room temperature
.
In order for the emulsion particles to fuse well into a uniform film, coalescents must be used to lower the minimum film forming temperature (MFFT)
.
The minimum film forming temperature (MFFT) of the emulsion has an important effect on the hardness, glass transition temperature and low temperature coating performance of the coating film
.
The minimum film-forming temperature is high, and the hardness and gloss of the coating film are high, but it cannot be painted at a slightly lower temperature; the minimum film-forming temperature is too low, and the low-temperature coating performance is good, but the glass transition temperature of the coating film is low, and the high temperature is soft.
Stick back, poor stain resistance
.
The basic function of film-forming additives is to reduce the minimum film-forming temperature of emulsions and latex paints, so that water-based paints can be painted at low temperatures
.

The amount of coalescent agent added depends on the amount of emulsion or aqueous dispersion in the formulation and the glass transition temperature Tg
.
When the amount of emulsion or water dispersion in the coating is large and the Tg of the polymer is high, the amount of film-forming aid is also large, and vice versa
.
When formulating, first consider that the coalescent agent accounts for about 3%-5% of the emulsion or aqueous dispersion, or 5%-15% of the solid content of the emulsion or dispersion
.

The effectiveness of film-forming aids is shown in the film-forming efficiency of film-forming aids on emulsion particles
.
Add the film-forming aid according to 5%-15% of the solid content of the emulsion, measure the drop value of MFFT, and plot the drop value against the amount of film-forming aid.
The film forming efficiency, the larger the slope, the higher the film forming efficiency
.
Obviously, the film-forming efficiency is specific to the emulsion resin, that is, it appears to be effective for each resin or each type of resin, and is related to the initial MFFT of the emulsion
.
It is worth noting that when the amount of film-forming additives is large, the decline of MFFT often deviates from the linear relationship.
At this time, a relatively best slope value should be obtained by regression method for comparison
.

When an emulsion system is determined, it is very important to select the appropriate coalescent
.
Coating is a practical science.
It is very important to select the best film-forming auxiliary through a large number of experiments to ensure the low temperature construction of water-based coatings
.
Practice has shown that the MFFT of water-based coatings should be kept below 10 °C, preferably less than 5 °C, until the coating can form a uniform paint film without cracking and pulverization, and find the minimum amount of coalescing aids
.
If it is not advisable to use 15% or more of the emulsion or dispersion, consider changing other coalescents and try again
.
In view of the fact that the effect of film-forming aids in reducing MFFT is not omnipotent, it is difficult to add a large amount of film-forming aids to the emulsion with too high Tg to ensure sufficient low-temperature workability and freeze-thaw stability
.
The Tg of common emulsions and water dispersions should not be greater than 40°C, otherwise it is difficult to reduce MFFT below 10°C with a large amount of film-forming aids
.
It is not suitable to one-sidedly pursue a coating film with high Tg and high hardness, not to mention that doing so will cause the problem of excessive VOC due to the excessive amount of film-forming additives
.

7.
Several experience in using film-forming aids

(1) The relationship between coating film-forming aids and the minimum film-forming temperature (MFFT)

The minimum film-forming temperature of the latex refers to the lowest temperature at which the latex forms a continuous coating film without cracking, and the less the amount of the film-forming aid is, the better it is until the minimum film-forming temperature of 0°C is reached
.
The effect of film-forming aids is similar to that of plasticizers.
The addition of film-forming aids can reduce the MFFT of the emulsion, and the magnitude of the reduction increases with the increase of the amount of film-forming aids
.
However, MFFT almost no longer decreases when the amount of coalescent agent reaches a certain level
.
The magnitude of MFFT decline is also related to the type of coalescent and the type of emulsion.
Coalescents that significantly reduce MFFT for one emulsion may have little or no effect on the other
.
From this we believe that:

① The magnitude of MFFT reduction increases with the increase of the amount of coalescent additives, but it is useless to add more coalescent additives after the MFFT is reduced to a certain level, so the best coalescent additives should be selected and the minimum dosage should be used;

②Due to the particularity of the water-based system, the film-forming temperature can only be lowered to about 0°C at the lowest level, and the film-forming temperature will no longer drop significantly after increasing the amount of film-forming aids;

③ For the same base material, the effects of different film-forming additives in reducing MFFT are quite different, some are very obvious, and some can hardly reduce the minimum film-forming temperature;

④Different types of base materials have completely different effects of the same film-forming aid.
Adding a small amount of film-forming aid to one emulsion system may have a significant effect in another emulsion system.
The effect may not be obvious, or even completely.
invalid
.
Therefore, it is very necessary to screen the variety and dosage of the best coalescent for a specific waterborne coating system
.

The efficiency of film-forming aids to reduce MFFT has a great relationship with its types.
Obtaining the film-forming aids with the lowest film-forming temperature and the best coating effect under the same dosage is an important task for formulators
.

In addition to reducing the minimum film-forming temperature and increasing the density of the paint film, the film-forming aid can also improve the construction performance, increase the leveling of the paint, prolong the opening time, and improve the storage stability of the paint, especially the low-temperature antifreeze
.

(2) The relationship between film-forming aids and film hardness

The film-forming aids are mostly high-boiling organic compounds with larger molecular weights, which have slow migration speed and low volatilization rate, which prolongs the time to reach final hardness
.
In traditional coatings, phthalate plasticizers cannot be volatilized, but slowly migrate to the surface of the paint over time.
The problem is that the coating film exhibits poor blocking resistance and scratch resistance for a long time.
And easy to get dusty
.
Therefore, plasticizers should be avoided as much as possible in waterborne coating formulations
.
Alcohol ester-based film-forming aids generally volatilize slower than alcohol-ether-based film-forming aids, so the hardness of the coating film using alcohol esters as film-forming aids is also slower
.
The influence of the coalescent on the hardness of the coating film after water-based paint film formation mainly depends on two factors: the migration speed of the coalescent in the coating film and the volatilization rate in the air
.
We compared the addition of dibutyl phthalate (DBP) and three coalescents, alcohol ester-12 (Texano1), dipropylene glycol n-butyl ether (DPnB) and tripropylene glycol n-butyl ether ( The hardness of the coating film after TPnB) changes with time.
The hardness of the coating film with DBP is almost no increase after 46 days, and the hardness is always very low.
The hardness of the coating film with Texanol after 46 days is lower than that of the alcohol ether; The hardness of the coating film increased relatively rapidly, and the hardness of DPnB exhibited the fastest
.

(3) Coalescent additives and organic volatile matter content VOC

The content of volatile organic compounds (VOC) is one of the biggest concerns in the coatings industry
.
Different countries have different definitions of VOC, therefore, the VOC determined by the same paint according to the regulations of different countries will be different
.

For example: Coalescents with a boiling point of 250°C or higher may be classified as VOCs according to the regulations of the US Environmental Protection Agency (EPA), but obviously cannot be regarded as VOCs according to the regulations of China and Australia; another example: when synthesizing polyurethane dispersions Acetone, a commonly used solvent, is an exempt compound according to the laws of the United States, and is not considered a VOC, but is considered a VOC according to the laws and regulations of China and the European Union
.

Co-solvents or film-forming additives are often used to reduce the viscosity of the resin before hydration during the synthesis of water-based resins.
During construction, it can improve the rheological properties of water-based coatings and improve the opening time.
During the film-forming process, it can promote high-quality coatings.
It plays a very important role in the formation of water-based coatings, especially for resin systems with high glass transition temperature and high minimum film-forming temperature.
higher
.
Most of the commonly used coalescents have boiling points below 250°C, which must be considered as VOCs
.
However, triethylene glycol monoethyl ether (256°C), triethylene glycol monobutyl ether (283°C), and tripropylene glycol monobutyl ether TPnB (276°C) have been excluded from the VOC range according to Chinese standards.
Special attention should be paid to additives, especially tripropylene glycol monobutyl ether.
Its volatilization rate is faster than the former two, and it is also faster than alcohol ester-12.
It can be said that it has a good film-forming effect, and the hardness of the coating film is also fast.
Not yet within the VOC limits, the use of this coalescent has many advantages
.

The progress of today's synthesis technology has made it possible to completely reduce viscosity without co-solvents during the synthesis of water-based resins, and obtain emulsions and dispersions that do not contain any VOC solvents
.
However, it is often necessary to add film-forming aids in the coating formulation process, especially for resins with high glass transition temperatures, which cannot form films at low temperatures without film-forming aids
.
At this time, it is possible to make "zero" VOC water-based coatings by using coalescents that are not within the VOC range
.

The U.
S.
Environmental Protection Agency (EPA) lists 50 (classes) of organic compounds that are considered VOC exempt compounds with negligible photochemical reactions
.
Some compounds are still applying for exemptions, some have been approved, such as tert-butyl acetate (t-BuoAc) is now excluded from the VOC
.
In particular, ethylene glycol, which can be used as a coalescent for water-based coatings, avoids environmental pollution
.

(4) Hydrophilic and lipophilic value HLB of film-forming aids

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Coal-forming agent is one of the most important components in water-based wood coating additives, which directly affects the drying speed of water-based wood coatings, initial water resistance, low-temperature construction film-forming properties and storage stability.
The degree to which the polymer latex particles coalesce into a complete continuous coating film affects the long-term anti-corrosion effect of the coating film
.
The combination of coalescents and waterborne resins is the cornerstone of the formulation of waterborne wood coatings
.
A good coalescent must have the characteristics of environmental safety, low addition, fast drying and the most effective reduction of the minimum film forming temperature (MFFT) of water-based resins
.
In actual use, two or more film-forming auxiliaries are often matched, and the low-temperature formation of the system can also be achieved according to the different ratios of the distribution coefficient of the film-forming auxiliaries in the water phase and the distribution coefficient in the polymer.
The balance between film properties and stability, so the type and amount of film-forming additives must be determined according to the type of water-based resin in the system, the minimum film-forming temperature, the compatibility with the components of the system, and the tolerance of construction.
Formula design It is necessary to fully consider this point, and select the best film-forming aid and its dosage through experiments
.
The results of the compatibility test of film-forming aids and emulsions show that: benzyl alcohol (BA), ethylene glycol butyl ether (EB), propylene glycol phenyl ether (PPH) have good compatibility in styrene-acrylic emulsions, and PPH is in addition to pure acrylic emulsions.
It has good compatibility with other emulsions, but these film-forming additives should be slowly added dropwise, otherwise flocculation will be easily caused
.
For pure acrylic emulsion, the addition of the above three film-forming auxiliaries will cause flocculation, which is easy to cause demulsification
.
Dodecyl alcohol ester has good compatibility in any kind of emulsion, and the addition method is simple, it is not easy to cause demulsification, and it is universal to emulsions
.