Waterborne coating basics

Waterborne coating formulation system

Waterborne coatings, including waterborne wood lacquers, are water-based dispersions and diluents. Unlike solvent-based coatings, which are not commonly used, their formulation system is a more complex system. Formulation design should not only pay attention to the type of polymer, emulsion and dispersion properties. It also needs reasonable selection of various additives and reasonable matching in consideration of the interaction between the components. Sometimes, special additives are selected for special requirements to form a suitable formula.

Basic composition of the formula

1) Aqueous resin: This is the base material for film formation, which determines the main function of the film.

2) Film-forming aid: After the water is volatilized, the emulsion or the dispersion particles are formed into a uniform and dense film, and the film formation property under low temperature conditions is improved.

3) Antifoaming agent and antifoaming agent: suppressing the bubbles generated in the paint liquid during the production process and causing the generated air bubbles to escape from the liquid surface and break the foam.

4) Leveling agent: Improve the construction performance of the paint to form a smooth, smooth coating.

5) Wetting agent: Improve the wetting property of the paint liquid on the substrate, improve the leveling property, and increase the adhesion of the paint film to the substrate.

6) Dispersing agent: promote the dispersion of pigment and filler in the paint liquid.

7) Rheology additives: provide good flow and leveling properties to paints, and reduce defects in the coating process

8) Thickener: Increase the viscosity of the paint solution, increase the wet film thickness of one coat, and prevent precipitation and delamination of putty and solid paint.

9) Preservative: Prevent the paint from becoming moldy during storage.

10) Fragrance: make the paint have a pleasant smell

11) Colorant: Mainly for the color paint, the water-based paint has the desired color. Colorants include pigments and dyes. Pigments are used in solid paints (coating without revealing wood grain) and dyes are used in clear paints (painting of wood grain).

12) Filler: It is mainly used in putty and solid paint to increase solid content and reduce cost.

13) pH adjuster: adjust the pH of the paint to stabilize the paint.

14) Wax emulsion or wax powder: Improve the scratch resistance of the paint film and improve its hand feeling.

15) Special additives: Additives for special requirements of water-based paints, such as rust inhibitors (iron cans to prevent premature rusting), hardeners (to improve film hardness), matting agents (to reduce paint film gloss), Anti-scratch agent, slip agent (improves paint film feel), anti-blocking agent (prevents coating lamination adhesion), cross-linking agent (made into two-component paint, improves overall performance), water repellent (coating) It has a lotus leaf effect, an anti-wear agent (increasing the abrasion resistance of the coating), an ultraviolet absorber (anti-aging paint for outdoor use, and preventing yellowing).

16) Ionized water: A small amount of deionized water is often added to make the paint.

Waterborne resin

In water-based paint formulations, the binder is a key component in forming the film and determining the film's performance. Formulation should be designed to increase the amount of water-based resin, accounting for 60-70% by volume, so that the effective film-forming content in the paint liquid is as much as possible, so as to ensure that the paint is thicker and plump. High degree.

Waterborne acrylic resin

Acrylic emulsions have been widely used in various fields of the coatings industry due to their versatility, weatherability and diversity. The aqueous acrylic emulsion is formed by emulsion polymerization of a vinyl monomer mainly composed of an acrylate monomer. Various additives such as an emulsifier, a stabilizer, and a pH adjuster are added during the polymerization, and the system is quite complicated. The paint film made of water-based acrylic emulsion has good weather resistance, is not easy to yellow hair, has high hardness and good gloss. In recent years, with the continuous development of water-based acrylic emulsion polymerization technology, multiphase polymerization, core-shell technology, self-crosslinking technology and application of polymer surfactants, and core-shell polymerization technology have further improved and improved the aqueous acrylic emulsion. The nature of the water-based acrylic emulsion is adapted to the needs of different construction and use conditions, and it is continuously expanded. The use of waterborne acrylic emulsions has now expanded to more demanding industrial applications.

Polyurethane dispersion

Polyurethane materials are a general term for a class of macromolecular compounds having a carbamate structure in a molecular structure, and are usually produced by a polyaddition reaction of a diisocyanate and a polyol. The polyurethane polymer has both a polar functional group that forms a physical crosslink and a non-polar and flexible segment. When used properly, its polar tube can also undergo further chemical crosslinking. These molecular properties give polyurethane materials high strength, toughness and resistance to solvents. As a high-quality material with high strength, good weather resistance and strong adhesion, polyurethane has been widely used in the coating field.

Depending on the type of isocyanate used to prepare the polyurethane, the polyurethane emulsion and the corresponding lacquer can be classified into two major categories, aliphatic and aromatic. The aliphatic paint film has excellent weather resistance and yellowing resistance; the aromatic waterborne polyurethane is mostly used for interior decoration paint. The particle size obtained by polymerization is a polyurethane emulsion and a polyurethane dispersion (PUD). The aqueous polyurethane dispersion uses a unique process to disperse the polyurethane particles in water to achieve a water-based film formation. Similar to other emulsion film forming mechanisms, the effect of film formation depends on the ability of the interpolymer to interpenetrate. In order to increase the penetration, on the one hand, the polyurethane molecular chain should be soft enough and the fluidity is good; on the other hand, the emulsion particles should be as small as possible, which can increase the contact area between the particles and reduce the moving distance of the polymer. Aqueous polyurethane dispersions typically have a nanometer size and are translucent or even completely transparent, making them one of the best waterborne paint bases. Aqueous polyurethane dispersions are sometimes referred to as nanoemulsions to distinguish them from ordinary emulsions that are white in appearance.

Waterborne acrylic emulsion

Waterborne Acrylic Emulsion Formulation Considerations: Waterborne Acrylic Resin Composition

The choice of emulsion raw materials is the most important decision for engineers when formulating aqueous paint formulations. The main properties of the coating are determined by the resin. A wide variety of water-based acrylate emulsions, including polystyrene-acrylate emulsions, are on the market. Different aqueous acrylate resin compositions have a great influence on the film-forming properties and cost of the water-based paint. Here are a few simple discussion of the influencing factors.

1) Styrene

The main reason for the introduction of the polyacrylate emulsion into the monomer is that it provides a higher glass transition temperature (Tg), which increases the hardness of the film formation; another reason is cost. Styrene is used in the packaging industry and is one of the most important and mature downstream products in the petroleum industry. It is low in price but highly affected by the macro economy. However, the introduction of styrene, especially when it is more than 15% of the total monomer amount, also has many negative effects on the film formability of the aqueous paint. First, the styrene-containing polymer segment lacks flexibility. Macroscopically, the water-based paint film is hard and not tough; and the benzene ring contained in styrene is an ultraviolet light absorber. When the monomer content is high, The ultraviolet light passes through the adjacent benzene ring to shear the polymer chain, so that the water-based paint film is brittle, the gloss is decreased, and the life is reduced; the structure of styrene also determines its affinity for many organic solvents, water-based paint. The solvent resistance of the paint film is not significantly different from other resins.

2) Acrylate monomer

There are many types of acrylate monomers, and the performance and price vary greatly. The most commonly used in China is butyl acrylate, which has the lowest cost. It is the monomer used by most emulsion manufacturers to match the styrene and make the emulsion of cheap wall paint. Its function is to provide the flexibility of the water-based paint film. Since the acrylate polymer main chain does not contain a group other than the ester group, the main chain and the ester group side chain are excellent in flexibility, so that the water-based paint film lacks hardness. Although this type of polyacrylate emulsion is the choice of interior and exterior wall coatings, performance is not enough as a wood lacquer with high hardness and toughness requirements.

3) Methacrylate monomers

The methacrylate monomer has a step in performance over the acrylate monomer. Since the methyl group has a methyl group on the main chain, the activity of the main chain is restricted, the main chain is strengthened, and the ester side chain provides flexibility, so that the methacrylate type The body has the best balance of hardness and toughness, such as polymethyl methacrylate (methyl methyl methacrylate) is commonly known as plexiglass; methacrylate monomer, because it does not contain benzene ring, it is compared to styrene Monomers have significant advantages over styrene in terms of UV resistance, durability, and solvent resistance.

In summary, water-based acrylate resin manufacturers have a large selection of monomers, which can not only change their types, but also change the relative proportions and components. When choosing water-based acrylate emulsions, water-based paint manufacturers should not only consider the concept of “pure C” or “styrene-acrylic”, which should be based on the actual performance of water-based acrylate resins and their formulations.

When choosing a water-based acrylate emulsion, you should not only consider the concept of voids such as “pure C” or “styrene-acrylic”, which should be measured by the actual performance of the aqueous acrylate resin and its formulation!

Filming aid

The polymer constituting the emulsion or dispersion typically has a glass transition temperature above room temperature. In order for the emulsion particles to fuse well into a uniform paint film, a film forming aid must be used to reduce the minimum film formation temperature (MFFT). The film-forming aid is a kind of small-molecule organic compound, and the film-forming auxiliary agent existing in the paint film will eventually escape and volatilize.

Most film-forming auxiliaries are an important part of coating organic volatiles (VOC), so the less the film-forming auxiliaries should be used, the better. The use of film-forming auxiliaries should give priority to compounds that are not within the VOC limits, but which are not too slow in volatility and have high film forming efficiencies. The amount of coalescent aid depends on the amount of emulsion or aqueous dispersion in the formulation and the glass transition temperature. The amount of the emulsion or aqueous dispersion is large and the Tg of the polymer is high, and the amount of the film-forming auxiliary agent is also large. On the contrary, when the dosage is small, the formulation aid is first considered to account for about 3 parts of the emulsion or the aqueous dispersion. %-5%, or 5% to 15% of the emulsion or dispersion solids. However, for polymer emulsions with Tg over 35 °C, it may be necessary to increase the amount of coalescent to ensure the reliability of low-temperature film formation. At this time, the amount of film-forming auxiliary should be gradually increased until low temperature (about 10 ° C or lower). The coating can form a uniform paint film without cracking or powdering, and find the minimum amount of filming aid.

It is not advisable to use a filming aid in an amount of 15% or more of the emulsion or dispersion. It should be considered to replace the other filming aids and try again. In addition to lowering the minimum film forming temperature and increasing the density of the film, the filming aid can improve the workability, increase the leveling of the paint, prolong the opening time, and improve the storage stability of the paint, especially the low temperature freeze resistance.

The film-forming auxiliary of water-based paints is generally an alcohol ether type solvent, and the most commonly used film-forming aids such as diethanol ethers, propylene glycol ethers and N-methylpyrrolidone have high and low boiling points. When the water-based paint is applied in summer, the water volatilizes faster, that is, the surface is relatively fast, and some of the moisture may be enclosed in the coating film before it is dried out, resulting in the drawback of white or poor leveling of the coating film. Therefore, it is possible to delay the drying speed of the coating film by adding a small amount of a suitable and suitable high boiling point solvent, to extend the opening time of the coating film, and to improve the workability and the appearance of the coating film. In the winter, due to the lower temperature, the drying speed of the water-based paint is slower, that is, the water volatilization is slower, but the film-forming auxiliary is relatively faster than the water, and some of the water-free coating may not evaporate, so that the water-based paint cannot form a dense The coating causes whitening and cracking of the coating film of the aqueous coating. Therefore, when adding a film-forming auxiliary, it is necessary to consider overcoming the workability of the water-based paint in different seasons, and also consider that it is as fast as possible, which is the most difficult technical point of the water-based paint, especially the acrylic with higher Tg. Dispersions are even more difficult to solve. Therefore, a variety of filming aids to match is the best choice.

Film-forming auxiliaries have a compatibility problem with non-resin systems. A good film-forming auxiliaries in one system may cause system instability, or seriousness, or poor recoatability in another water-based wood lacquer. The formula should be designed with full consideration of this point, and the best filming aid and its dosage should be selected through experiments.

Defoamer and suds suppressor

Defoamer is one of the most difficult additives in waterborne coatings to control defoaming and unbalance. The paint liquid will generate bubbles during the application due to agitation. If the bubbles present in the paint are not removed in time, the film will dry and form an unacceptable flaw. The paint also produces air bubbles during production, pumping and filling.

Defoaming is a non-negligible factor in the formulation design process. The defoaming mechanism of the defoaming agent is to penetrate the foam film produced by the system by wetting and continuously spread on the film, so that the surface tension of the foam film is unbalanced and broken. bubble.

In general, defoamers are classified into two types: suds suppressors and defoamers. The role of suds suppressors is to inhibit the foaming of aqueous coatings under the action of external forces, while the defoaming agents are mainly on the surface of the foamed membrane. Disperse, destroy the elastic membrane of the bubble, and make it rupture, but often the effect of the two combinations is better, otherwise at least one effective defoamer should be selected. Most defoamers, especially silicone defoamers, will cause shrinkage of the wet paint film when the dosage is too large. Therefore, the amount of defoamer can be basically eliminated by the principle of eliminating bubbles, so that the defoaming effect should not be excessively pursued. Side effects such as shrinkage holes. For water-based wood lacquers, mineral oil defoamers are more tolerant than silicone defoamers, and adding a little more is less prone to severe shrinkage, which can be preferred.

There are many defoamers on the market, and the choices are very cautious and can be noted in several ways:

1. Controlled compatibility, that is, to achieve the balance between defoaming and surface effects, both defoaming and prevention of defects such as shrinkage cavities and poor leveling.

2. Defoaming according to the construction conditions and methods, such as brushing, spraying and roller coating, face different degrees of foaming, and also means different defoamers.

3. The balance between foam suppression and foam breaking can be reasonably matched according to different processing and construction methods.

4. Long-term defoaming performance. Many defoamers will gradually reduce their defoaming performance during storage of the paint, and the defoamer content in the paint should be higher when designing the formula.

The amount of defoaming agent is 0.05% to 0.5% of the whole formulation, preferably about 0.1%. If the defoaming agent used exceeds 0.5% to have a good defoaming effect, the defoaming agent should be considered. Different resin systems are sensitive to defoamers. The type and amount of aqueous defoamer selected must be determined experimentally, depending on the system.

Wetting leveling agent

The wetting leveling agent can effectively reduce the surface tension of the system and significantly improve the construction effect of the waterborne wood lacquer. After the addition of the wetting leveling agent, the wetting property and permeability of the paint increase, the leveling property of the paint liquid is improved, and sometimes the shrinkage (frame effect) problem can be overcome. More importantly, the leveling agent can solve common shrinkage problems, especially those caused by excessive use of defoamers. Excessive leveling agent will offset the defoaming effect of the defoaming agent, so that the paint liquid will generate bubbles during construction, and some will have obvious foam stabilization effect. Therefore, it should try to use good leveling property, low foaming property and stable foaming. Small wetting leveling agent. The combination of leveling agent and defoamer, including the selection of varieties and the control of dosage, is the focus of research on waterborne wood coatings.

The ideal wetting agent should contain the following aspects:

1. Effectively reduce the surface tension of the system and improve its wettability and permeability to the substrate.

2. As low as possible foaming and stable foaming.

3. As low a water sensitivity as possible.

4. Does not affect the adhesion between layers.

5. Good balance of surface tension can not only improve the wetting of the substrate, but also affect the leveling or defects due to excessive reduction of surface tension.

The leveling agent is generally used in an amount of 0.1% to 1.0%, preferably about 0.3%. When the defoaming agent is excessive, the amount of the leveling agent may exceed 1% in order to overcome the shrinkage. No leveling agent can be used in the putty formula.

Rheology additive

The rheological additive provides a leveling effect under low shear conditions, allowing the painted liquid to level as quickly as possible, reducing brush marks and avoiding splashing, but this leveling effect is diminished with drying time, film Fixed to prevent the paint from sag in the vertical plane.

Rheological additives can be divided into high, medium and low shear viscosity types: low shear viscosity rheological additives can provide viscosity inside the tank, prevent pigment settling, and effectively control the paint liquid in the standing Snails generated during surface operations; rheological additives that provide medium to high shear viscosity can improve the fullness and leveling of waterborne coatings, while rheology additives with improved high shear viscosity help leveling, Prevent splashing and improve roller and brush application performance. Therefore, the type and amount of rheological additives must be selected according to the requirements and system of the construction. Generally, the type of rheological additive is selected according to the different construction methods, and is often matched by three rheological additives of shear viscosity. In order to achieve the required construction results.

Polyurethane associative thickeners are currently the most widely used in waterborne coatings, but such thickeners are closely related to the selection of other components in the aqueous coating system. For example, the thickening effect is not related to the polymer particle size and hydrophobicity of the water-based paint and the co-solvent. If the polymer particles in the water-based paint are smaller, the more hydrophobic, and the more the partition coefficient of the co-solvent in the polymer. High, its thickening efficiency is high, otherwise a large amount of such thickeners do not reach the required viscosity. At the same time, it is recommended to dilute with water and add it under reasonable shearing force. Otherwise, it may not be flocculated and particles will be produced.

Surface control additive

The types and functions of surface control auxiliaries include:

1. Improve flow and leveling;

2. Reduce the friction coefficient and increase the slip;

3. Enhance the scratch resistance;

4. Anti-blocking;

5. Enhance the arrangement of matte powder;

6. Special hand feeling, such as soft feeling;

7. Others, such as improving the water resistance and stain resistance of the coating film.

On the one hand, they balance the surface tension of the drying process to achieve uniform drying and flow, and reduce surface roughness; on the other hand, they concentrate on the surface during the drying process to form a smooth coating film, which increases the slip and other special properties.

There are two main types of such additives:

1. Organosilicon: This kind of surface control additive can effectively improve the scratch resistance and abrasion resistance of the coating film when it is added in a small amount. Generally, it has poor compatibility with the system. If it is not properly selected, it will affect the coating film. Gloss and adhesion between coating layers.

2. Wax emulsions, such as palm wax and polyethylene wax: affect the gloss of the coating to a certain extent, which is not related to the particle size and distribution. The wax emulsion should be added to 2% to 8% of the total amount of the formulation to have obvious effect. Excessive wax emulsion affects the strength of the film and reduces the adhesion between the layers. The recommended optimum dosage is 3% to 5%. Putty does not use wax emulsion unless there is a special surface effect. In general, there is no need to add a wax emulsion in the solid color paint. Sometimes wax emulsions can be replaced with wax powder.

It is better to use two types of surface control agents, but the following aspects should be very careful:

1. Whether it seriously affects the gloss of the film;

2. The effect on adhesion between layers;

3. Stabilization evaluation;

4. Storage stability of the system.

Matting agent

Matting agents are additives used to control the gloss of aqueous coatings to meet the broad needs of mainstream matt paints in today's market. The extinction mechanism of the matting agent can be arranged on the surface of the coating film in an orderly manner, so that the surface of the coating film is uneven, causing the light reflected on the surface of the coating film to diffusely reflect, thereby achieving the purpose of extinction.

The resin used in waterborne coatings has a relatively low solid content, and the active ingredient of the waterborne coating is not high. In addition, it is a dispersion system, so the matting of the waterborne coating is relatively easy, but the matting of different aqueous resin systems is difficult, generally acrylic. Emulsions are relatively well matte than aqueous polyurethane dispersions, but are better at anti-settling properties than aqueous polyurethane dispersions. Therefore, the selection of matting powder must be considered in different systems and uses. The matting agent selects its refractive index as much as possible. The refractive index of the non-aqueous resin is similar, which reduces the influence on the transparency of the coating film. At the same time, the matting effect, dispersibility, anti-sinking property and hand feeling in the system are also considered.

preservative

Water-based coatings are water-based systems that are easier to bring into bacteria during production and are more susceptible to microbial degradation during storage. Therefore, the type and amount of preservatives should be properly added during formulation to prevent system and film coating. corruption. From the perspective of environmental protection , it should use a less toxic preservative, and also pay attention to the compatibility with the system, and must be diluted with water when adding, otherwise, it will flocculate and the system will be bad.

In general, the use of isothiazolinone preservatives in an amount of 0.1% is sufficient to prevent the paint from becoming moldy during storage. After adding 0.1% preservative, sealing storage for two or three years is not a problem.

pH regulator

Water-based paints may also need to adjust the pH if necessary. Most emulsions or aqueous dispersions have been adjusted to a pH of 8 to 9 when they are produced, eliminating the need for pH adjusters when making paint. If the pH value is different, add 0.05% to 0.1% of the pH adjuster during the paint making process, and adjust the pH of the paint to 7-9. Many waterborne wood lacquers can only be stabilized under neutral to slightly alkaline conditions. When the pH is too high or too low, the lacquer may cause flocculation, sedimentation, coarsening, and deterioration of construction performance.

The pH requirements of different systems are different. Therefore, the pH of the aqueous coating is adjusted according to the system, and the matching of the preservative in the formulation is considered. It should also be noted that the addition must be diluted with water in multiples to avoid causing the system to produce particles.

Pigments and fillers

In the solid paint, white paint is taken as an example. The amount of titanium dioxide should be sufficient to ensure that the paint film has sufficient hiding power. The amount of titanium dioxide should not be lower than 13%, but it is not necessary to be higher than 22%. The situation is not the same as solvent-based white paint.

Fillers may be added or not added. However, a small amount of filler must be added to the water-based putty, such as talc, heavy calcium and zinc stearate, and the total amount can be between 15% and 30%. The more filler, the worse the transparency of the putty, but the better the interstitial.

Colorants include pigments and dyes. Pigments are used in solid paints (coating without revealing wood grain) and dyes are used in clear paints (painting of wood grain).

When there is a pigment filler in the formulation, a wetting dispersing agent of 2% to 10% of the total amount of the pigment and filler is added to help disperse the pigment filler.

Other additives

1. Fragrance: As long as the dosage of the essence can improve the odor of the paint liquid, the dosage is about 0.05%, and the individual case can be as high as 0.1%. Of course, it can be used.

2. Ultraviolet absorber: indoor water-based wood lacquer seldom uses ultraviolet absorbing agent, varnish produced by emulsion or water dispersion made of TDI, and outdoor lacquer, especially outdoor lacquer, it is best to add ultraviolet absorbing agent to Prevents photodegradation and reduces the rate of photochromism.