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Characteristics and usage of water-based coating film-forming additive (alcohol ester XII)!
2022/3/24 14:56:27
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I. Overview of film-forming additives
Film-forming additives of waterborne coatings, also called accelerators, film-forming additives, film-forming cosolvents or solvents, can dissolve and swell polymer particles in emulsion, so that the particles can produce plastic flow and elastic deformation to form a film with the evaporation of water at a lower temperature, but they can volatilize and escape in a short time after film formation, without affecting the glass transition temperature of the coating, and the coating will not stick back at high temperature. Film-forming additives are organic solvents with high boiling point, which have strong solubility and molecular weight of hundreds, and most of them are alcohols, alcohol esters and alcohol ether compounds. In fact, the film-forming additive is a solvent for polymers. In the process of film drying, the remaining film-forming additives make polymer droplets dissolve and fuse into a continuous film. Film-forming additives can not only dissolve, but also temporarily plasticize polymers. Film-forming additives are volatile temporary plasticizers, which can promote the plastic flow and elasticity of latex particles.
Waterborne coating film-forming additives are widely used in architectural coatings (latex paint), waterborne automotive coatings and automotive repair coatings, waterborne electrophoretic coatings, waterborne marine coatings, waterborne container coatings, waterborne anticorrosive coatings, waterborne industrial coatings, waterborne adhesives, waterborne wood coatings, waterborne coiled materials and coiled materials coatings, waterborne screen printing inks, waterborne gravure printing inks, waterborne flexographic printing inks and UV waterborne coating inks.
Second, the chemical type and state of film-forming additives
Chemical types of film-forming additives
(1) alcohols (such as benzyl alcohol BA, ethylene glycol, propylene glycol and hexanediol);
(2) alcohol ester (such as dodecanol ester (i.e. 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, propylene glycol monomethyl ether DPM, dipropylene glycol monopropyl ether DPnP, dipropylene glycol monobutyl ether DPnB, tripropylene glycol n-butyl ether TPnB, propylene glycol diphenyl ether PPH, etc.). );
(4) alcohol ether esters (such as hexanediol butyl ether acetate and ethyl 3- ethoxypropionate EEP);
Present situation of film-forming additives in waterborne coatings
According to the position of film-forming additives in the polymer, it can be divided into three categories: A, B and C. The emulsion takes water as the continuous phase and consists of spherical micelles with hydrophobic polymer chains stably formed by emulsifier. The location of film-forming additives added to the emulsion system depends on their hydrophobicity/hydrophilicity. Among them, the type A of emulsion polymer is mainly petroleum ether and other hydrocarbons, such as turpentine, dipentene pine oil, cetyl alcohol and so on. Type AB is mainly diesters and alcohol esters at the interface between emulsion polymer and water, such as Texanol ester alcohol, Lusolvan FBH, DBE-IB and COASOL；; ABC type mainly exists between polymer particles, on the interface and in water, and mainly includes ethylene glycol esters and ethylene glycol ester ethers, ethylene glycol butyl ether (EB), propylene glycol phenyl ether (PPH) and propylene glycol monomethyl ether DPM；. Type C is mainly alcohols and diols in water, such as ethanol, diethylene glycol, dipropylene glycol and triethylene glycol. In the traditional classification, it can be divided into oil solubility and water solubility from the compatibility with polymers.
Third, the selection criteria of film-forming additives
Characteristics of ideal film-forming additives
(1) The film-forming assistant must be a strong solvent of polymer, which has excellent film-forming efficiency and good compatibility with various water-based resins, and can reduce the film-forming temperature of water-based resins without affecting the appearance and luster of the paint film;
(2) low odor, low dosage, excellent effect, good environmental protection and certain volatility, and the drying speed can be effectively adjusted, which is convenient for construction;
(3) Excellent hydrolytic stability, low solubility in water, its volatilization rate should be lower than that of water and ethanol, it should remain in the coating before film formation, and it must be completely volatilized after film formation, which will not affect the coating performance;
(4) After adding the latex system, it can be adsorbed on the surface of latex particles, and it has excellent coalescence performance. Fully dissolving and swelling water-based resin will not affect the stability of latex particles.
Fourth, the development direction of film-forming additives
Although film-forming additives play a great role in the film formation of latex paint, they are organic solvents and have an impact on the environment, so the development direction is environmentally friendly and effective film-forming additives:
(a) is to reduce the smell. Coasol, DBE-IB, OptifilmEnhancer300, TXIB, TXIB and Texanol can all reduce odor. Although TXIB is somewhat poor in reducing MFFT and early washing resistance, it can be improved by mixing with Texanol.
(2) Reduce volatile organic compounds (VOC). Most film-forming additives are important components of coating VOC, so the amount of film-forming additives should be as small as possible. When selecting film-forming additives, priority should be given to compounds that are not within the VOC limit, but their volatility should not be too slow and the film-forming efficiency should be higher. In Europe, VOC refers to those chemicals whose boiling point is equal to or lower than 250℃. Those substances whose boiling point exceeds 250℃ do not belong to VOC category, so the film-forming additives develop to high boiling point. Such as Coasol, LusolvanFBH, DBE-IB, OptifilmEnhancer300 and diisopropanol adipate.
(3) It has low toxicity, safety and acceptable biodegradability.
(4) It is an active film-forming assistant. Dicyclopentenoxyethyl acrylate (DPOA) is an unsaturated polymerizable organic compound with homopolymer Tg=33℃ and tasteless. Its structural formula is as follows:
In the latex paint formula with high Tg value, DPOA and a small amount of drier, such as cobalt salt, are added, and no film additives are added. DPOA can reduce the film-forming temperature and make latex paint film at room temperature. However, DPOA is non-volatile, which is not only environmentally friendly, but also improves the hardness, anti-adhesion and brightness of the coating film through oxidative free radical polymerization under the action of drier. Therefore, DPOA is called an active film-forming additive.
Verb (abbreviation of verb) Film-forming Mechanism of Waterborne Coatings
The film-forming process of waterborne coatings is complex, which has experienced a process from dispersion to mutual aggregation of polymer particles. After construction, the water volatilizes, and the spherical particles must fuse with each other to form a continuous coating. The formation of water-based paint film is divided into the following processes.
(1) The particles gradually approach the filling process.
Spherical particles remain dispersed in latex paint through the effect of electric double layer and shielding stability. After the construction of waterborne coatings, water gradually volatilizes, and the dispersed polymer particles and pigments and fillers are kept close by electrostatic repulsion and steric hindrance stability, but they can still move freely. At this stage, the evaporation of water is almost the same as that of pure water, and it is constant-speed evaporation.
(2) Particle fusion process.
With the further evaporation of water, the protective layer adsorbed on the surface of polymer particles is destroyed, and the bare particles contact each other.
The gap is getting smaller and smaller, and the volume of paint film shrinks. When water evaporates, its driving force will also disappear. When the capillary tube passes through a large size, due to the capillary efficiency, the capillary pressure is higher than the deformation resistance of polymer particles, and the particles are destroyed and deformed, and then agglutinate and fuse into a continuous paint film. This process is the key to whether water-based paint can form a film. If the glass transition temperature (Tg) of emulsion polymer is high (in order to make the coating film have good mechanical properties, weather resistance and pollution resistance, the Tg value should not be too low), it is difficult to deform at low ambient temperature, thus hindering the melting process and leading to the failure to form a film. At this time, film-forming additives are needed to assist film formation. Film-forming additives can make latex particles swell and soften, so it is easy to fuse them together to form a continuous film.
(3) The process of mutual diffusion, infiltration and cross-linking of polymer segments to form a film.
With the passage of time, the additives remaining in water gradually diffuse to the coating film, and make the long chain segments of polymer molecules permeate and diffuse with each other, winding to form a uniform coating film with good performance, and gradually evaporate from the coating film with the film-forming additives, forming an ideal coating film with excellent performance.
Use and dosage of film-forming additives for waterborne coatings
Polymers such as emulsions or dispersions usually have a glass transition temperature Tg higher than room temperature. In order to make the latex particles blend well into a uniform paint film, film-forming additives must be used to reduce the film-forming temperature (MFFT). The film-forming temperature (MFFT) of the emulsion has an important influence on the hardness, glass transition temperature and low-temperature coating performance of the coating. The hardness and luster of the film are high when the film forming temperature is high, but it can not be coated at a slightly lower temperature; Although the film forming temperature is too low and the film coating performance is good at low temperature, the film has low glass transition temperature, soft and sticky at high temperature and poor pollution resistance. The basic function of film-forming additives is to reduce the film-forming temperature of emulsion and latex paint, so that water-based coatings can be painted at low temperature.
The amount of film-forming additives depends on the amount of emulsion or water dispersion in the formula and the glass transition temperature Tg. The amount of emulsion or water dispersion in coatings is large, the Tg of polymers is high, and the amount of film-forming additives is also large, and vice versa. When designing the formula, first consider that the film-forming additive accounts for about 3%-5% of the emulsion or water dispersion, or 5%- 15% of the solid content of the emulsion or dispersion.
The effectiveness of film-forming additives lies in the film-forming efficiency of film-forming additives on emulsion particles. According to the solid content of the emulsion of 5%- 15%, film-forming additives were added, the MFFT decline value was measured, and the relationship between the decline value and the amount of film-forming additives was drawn. The slope of the obtained straight line indicates the film-forming efficiency of the film-forming additive for a given emulsion, and the greater the slope, the higher the film-forming efficiency. Obviously, the film-forming efficiency has the specificity of emulsion resin, that is, it is effective for each resin or each type of resin, and it is related to the initial MFFT of 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, so a relative slope value should be obtained by regression method for comparison.
When the emulsion system is determined, it is very important to choose the appropriate film-forming additives. Coating is a practical science, and it is very important to select film-forming additives through a large number of experiments to ensure the low-temperature workability of waterborne coatings. The practice shows that the MFFT of waterborne coatings should be kept below 10℃ and less than 5℃ until the coatings can form a uniform paint film without cracking and powdering, and the amount of film-forming additives should be found out. If the amount of film-forming additives reaches 65,438+05% or more of the emulsion or dispersion, it is not desirable, so we should consider changing other film-forming additives and try again. In view of the fact that film-forming additives can not reduce MFFT, it is difficult to ensure sufficient low-temperature workability and freeze-thaw stability by adding a large number of film-forming additives to the emulsion with too high Tg. The Tg of common emulsion and water dispersion should not be greater than 40℃, otherwise it is difficult to reduce MFFT below 10℃ with a large number of film-forming additives. It is not appropriate to unilaterally pursue the high hardness of the coating caused by high Tg, not to mention the problem of VOC exceeding the standard caused by excessive film additives.
Seven, some experience of using film-forming additives.
(1) Relationship between coating film-forming additives and film-forming temperature (MFFT)
The film-forming temperature of latex refers to the temperature at which latex can be continuously coated without cracking. Until the film-forming temperature reaches 0℃, the less film-forming additives are used, the better. Film-forming additives act like plasticizers. After adding film-forming additives, the MFFT of emulsion can be reduced, and the reduction range increases with the increase of film-forming additives. However, after the amount of film-forming additives reaches a certain level, MFFT hardly decreases. The degree of MFFT decline is also related to the types of film-forming additives and emulsion types. Film-forming additives that significantly reduce MFFT in one emulsion may have little or no effect on the other emulsion. From this we think:
① The decrease of MFFT increases with the increase of the dosage of film-forming additives, but adding more film-forming additives after the MFFT is reduced to a certain extent is of no help, so effective film-forming additives should be selected and used in a small amount;
(2) Due to the particularity of water system, the film-forming temperature can only be reduced to about 0℃, and then the film-forming temperature will no longer decrease obviously with the increase of the amount of film-forming additives;
③ For the same substrate, the effects of different film-forming additives on reducing MFFT are quite different, some are very obvious, and some can hardly reduce the film-forming temperature;
(4) Different types of substrates have different effects on the same film-forming additive. Adding a small amount of film-forming additives to one emulsion system may not have obvious effect or even be completely ineffective in another emulsion system. Therefore, it is very necessary to screen the variety and dosage of film-forming additives for a specific waterborne coating system.
Film-forming additives are closely related to the types of MFFT. It is an important job for formulators to obtain the same amount of film-forming additives under the same film-forming temperature and film-coating effect.
The film-forming additives can not only reduce the film-forming temperature and increase the film density, but also improve the construction performance, increase the leveling property of the coating, prolong the opening time and improve the storage stability of the coating, especially the low-temperature frost resistance.
(2) Relationship between film-forming additives and film hardness
Film-forming additives are mostly high-boiling organic compounds with large molecular weight, which have slow migration speed and low volatilization rate, thus prolonging the time to reach the final hardness. In traditional coatings, phthalate plasticizers can't volatilize, but they will slowly migrate to the surface of coatings over time, resulting in poor long-term anti-adhesion and scratch resistance of coatings and easy dusting. Therefore, plasticizer should not be used as much as possible in the formulation of waterborne coatings. Generally, alcohol ester film-forming additives volatilize more slowly than alcohol ether film-forming additives, so the film hardness made of alcohol ester film-forming additives is also slow. The influence of film-forming additives on the hardness of waterborne coatings after film formation mainly depends on two factors: the migration speed of film-forming additives in coatings and the volatilization rate in air. We compared the changes of coating hardness with time after adding dibutyl phthalate (DBP) and three film-forming additives, namely alcohol ester-12(Texanol 1), dipropylene glycol n-butyl ether (DPnB) and tripropylene glycol n-butyl ether (TPnB). After adding DBP into the coating for 46 days, the hardness of the coating hardly increased and remained high. The hardness of the coating containing two kinds of alcohol ethers increased rapidly, and that of DPnB increased rapidly.
(3) VOC content of film-forming additives and organic volatiles
The content of volatile organic compounds (VOC) is one of the concerns of coating industry. Every country has a different definition of VOC, so according to the laws and regulations of different countries, the VOC of the same paint will be different.
For example, the film-forming additives with boiling point above 250℃ may belong to VOC according to the regulations of the US Environmental Protection Agency (EPA), but obviously they cannot be regarded as VOC according to the regulations of China and Australia; Another example: acetone, a commonly used solvent in the synthesis of polyurethane dispersion, is an exempted compound according to the laws of the United States and is not considered as VOC, while it is considered as VOC according to the laws and regulations of China and the European Union.
The cosolvent or film-forming assistant is often used to reduce the viscosity of water-based resin before hydration, improve the rheological properties of water-based coatings and increase the opening time during construction, and promote the formation of high-quality coatings in the film-forming process, which plays a very important role in water-based coatings, especially for resin systems with high glass transition temperature and high film-forming temperature, and its dosage is quite large, accounting for 10%- 100% of the resin solid content. The boiling point of the most commonly used film-forming additives is below 250℃, which belongs to VOC that must be considered. However, triethylene glycol monoethyl ether (256℃), triethylene glycol monobutyl ether (283℃) and tripropylene glycol monobutyl ether TPnB(276℃) have been excluded from the VOC range according to Chinese standards, so special attention should be paid to them as film-forming AIDS, especially tripropylene glycol monobutyl ether, whose volatilization speed is faster than the first two, and faster than alcohol ester-12.
At present, with the progress of synthesis technology, the viscosity of waterborne resin can be reduced completely without cosolvent, and emulsions and dispersions without any VOC solvent can be obtained. However, film-forming additives are often added in the preparation of coatings, especially resins with high glass transition temperature. Without film-forming additives, it is impossible to form at low temperature. At this time, it is possible to make "zero" VOC waterborne coatings by selecting film-forming additives that are not within the VOC range.
The United States Environmental Protection Agency (EPA) lists 50 kinds of organic compounds, which are considered as VOC exempt compounds, and their photochemical reactions can be ignored. Some compounds are still applying for exemption, while others have been approved. For example, tert-butoxyacetic acid has been excluded from volatile organic compounds. In particular, ethylene glycol can be used as a film-forming additive for waterborne coatings, which avoids environmental pollution.
(4) Hydrophilic and lipophilic value HLB of film-forming additives
When designing coating formulation, it is very important to consider the activity of film-forming additives and their delamination in emulsion. Solubility parameters affect the activity of film-forming additives, because aqueous emulsion is a two-phase system, mainly water phase and polymer phase, and the concentration of film-forming additives in these two phases mainly depends on the hydrophilic-lipophilic balance of polymer and solvent. The distribution of film-forming additives in two phases is the main factor of the activity of film-forming additives. The main fraction of lipophilic film-forming additives is in lipophilic polymers, and only a small amount exists in water. The main part of hydrophilic film-forming additives is in the water phase, and only a small amount exists in the polymer. Film-forming additives are closely related to the film-forming process, so they have an impact on the gloss, scrub resistance, corrosion resistance, water resistance and drying time of the film.
Film-forming additives are one of the most important components in waterborne wood coatings, which directly affect the drying speed, initial water resistance, film-forming performance in low-temperature construction and storage stability of waterborne wood coatings. Film-forming additives directly determine the degree of aggregation of polymer latex particles into a complete and continuous coating, and then affect the long-term anticorrosion effect of the coating. The combination of film-forming additives and waterborne resin is the cornerstone of waterborne wood coatings formula. A good film-forming additive must have the characteristics of environmental safety, low dosage, fast drying and effective reduction of film-forming temperature (MFFT) of waterborne resin. In practical use, two or more film-forming additives are often used for matching. According to the different proportions of the distribution coefficients of film-forming additives in water phase and polymer, the balance between low-temperature film-forming performance and system stability can also be achieved. Therefore, the type and dosage of film-forming additives must be determined according to the type of water-based resin in the system, film-forming temperature, compatibility with system components and construction tolerance. This point should be fully considered in the formulation design, and the film-forming additives and their dosage should be selected through experiments. The compatibility test results of film-forming additives and emulsion show that benzyl alcohol (BA), ethylene glycol butyl ether (EB) and propylene glycol phenyl ether (PPH) have good compatibility in styrene-acrylic emulsion, and PPH has good compatibility in other emulsions except pure acrylic emulsion, but these film-forming additives should be added slowly, otherwise flocculation will easily occur. For pure acrylic emulsion, adding the above three film-forming additives will produce flocculation and easily cause demulsification. Dodecyl ester has good compatibility in any kind of emulsion, simple addition method, difficult demulsification and universality for emulsion.
Label: Characteristics and usage of water-based coating film-forming additive (alcohol ester XII)!
Multifunctional auxiliary AMP-95
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