1. Overview of film-forming additives
Film-forming additives for water-based coatings are also called coagulants , coalescing agent, film-forming co-solvent or co-solvent, which can dissolve and swell the polymer particles in the emulsion, so that the particles can produce plastic flow and elastic deformation with the evaporation of water at lower temperatures and coalesce to form film, but it can evaporate and escape within a short period of time after film formation without affecting the glass transition temperature of the coating film, and the coating film will not stick back at high temperatures. Film-forming additives are high-boiling organic solvents with a molecular weight of several hundred and extremely strong dissolving power. They are mostly alcohols, alcohol esters, and alcohol ether compounds. In fact, film-forming additives are a solvent for polymers. They are used in coating films. During the drying process, the remaining film-forming aids after the water evaporates dissolve and fuse the polymer droplets into a continuous film. In addition to dissolving, the film-forming aids also have a temporary plasticizing effect on the polymer, forming a film. The additive is a volatile temporary plasticizer that can promote the plastic flow and elastic deformation of latex particles, improve their coalescence, and can form films in a wide range of construction temperatures.
Water-based paint Film-forming additives are widely used in architectural coatings (latex paint), water-based automotive coatings and automotive repair coatings, water-based electrophoretic coatings, water-based ship coatings, water-based container coatings, water-based anti-corrosion coatings, water-based industrial coatings, water-based adhesives, water-based wood coatings, Water-based coil and coil steel coatings, water-based screen printing inks, water-based gravure inks, water-based flexo printing inks, UV water-based coating inks, etc.
2. Chemical types and status of film-forming additives
Chemical type of film-forming additives
(1) Alcohols (such as benzyl alcohol BA, ethylene glycol, propylene glycol, hexylene glycol);
(2) Alcohol esters (such as dodecyl alcohol 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, 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 hexanediol butyl ether acetate, ethyl 3-ethoxypropionate EEP), etc.;
The status of film-forming additives in water-based coatings
Based on the position of the film-forming additive in the polymer, it is divided into A , B and C categories. Emulsion uses water as the continuous phase and is composed of hydrophobic polymer chain spherical micelles stabilized by emulsifiers. The position of the film-forming additive added to the emulsion system depends on its hydrophobicity/hydrophilicity. Among them, type A is mainly hydrocarbons such as petroleum ether in the emulsion polymer, such as turpentine, dipentene pine oil, decahydrocai, etc.; type AB is at the interface between the emulsion polymer and water, which is mainly diesters and Alcohol esters, such as Texanol ester alcohol, Lusolvan FBH, DBE-IB, COASOL; ABC type is mainly found between polymer particles, on the boundaries and in water, mainly ethylene glycol esters and ethylene glycol ester ethers, ethylene glycol butyl ether (EB), propylene glycol phenyl ether (PPH), dipropylene glycol monomethyl ether DPM; type C is in water, mainly alcohols and glycols, such as ethanol, diethylene glycol, dipropylene glycol, and triethylene glycol. In traditional classification, it can be divided into oil-soluble and water-soluble in terms of compatibility with polymers.
3. Selection criteria of film-forming additives
Characteristics of ideal film-forming additives
(1) Film-forming additives�It is often used to reduce the viscosity of the resin before hydration during resin synthesis. It can improve the rheological properties and opening time of water-based coatings during construction. It can promote the formation of high-quality coatings during the film-forming process and plays a role in water-based coatings. It plays a very important role, especially for resin systems with high glass transition temperature and minimum film-forming temperature. Its dosage is quite large, accounting for 10%-100% of the solid content of the resin, and sometimes even higher. The boiling points of most commonly used film-forming additives are below 250°C, and VOC must be considered. 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 speed is faster than the first two and faster than alcohol ester-12. It can be said that it has good film-forming effect and the hardness of the coating film is also displayed quickly. It is not within the scope of VOC restrictions, and the use of this film-forming additive has multiple advantages.
Now synthesis Advances in technology have made it possible to completely eliminate the need for co-solvents to reduce viscosity during the synthesis of water-based resins, resulting in emulsions and dispersions that do not contain any VOC solvents. However, film-forming additives often have to be added during the coating preparation process, especially for resins with high glass transition temperatures, which cannot form films at low temperatures without film-forming additives. At this time, it is possible to use a film-forming additive that is not within the VOC range to make a "zero" VOC water-based coating.
American Environmental Protection The Environmental Protection Agency (EPA) lists 50 (category) organic compounds, which are considered to have negligible photochemical reactions and are VOC exempt compounds. Some compounds are still applying for exemptions, and some have been approved. For example, tert-butyl acetate (t-BuoAc) has been excluded from VOCs. It should be pointed out in particular that ethylene glycol, which can be used as a film-forming additive for water-based coatings, avoids environmental pollution.
（四 ) Hydrophilic and lipophilic values HLB of film-forming additives
When designing coating formulations, it is important to consider the activity of film-forming additives and their delamination in the emulsion. The solubility parameter affects the activity of the film-forming aid, because the aqueous emulsion is a two-phase system, mainly the water phase and the polymer phase. The concentration of the film-forming aid in these two phases mainly depends on the hydrophilicity and lipophilicity of the polymer and solvent. balance. The distribution method of film-forming aids in the two phases is the main factor in the activity of film-forming aids. The main fraction of lipophilic film-forming aids is in lipophilic polymers, and only a small amount exists in water; hydrophilic film-forming aids The main fraction of the agent is in the water phase, and only a small amount is present in the polymer. Film-forming additives are closely involved in the film-forming process, so they have an impact on the gloss, scrub resistance, corrosion resistance, water resistance, and drying time of the paint film.
Film formation Additives are one of the most important ingredients in water-based wood coating additives. They directly affect factors such as the drying speed, initial water resistance, low-temperature construction film-forming properties, and storage stability of water-based wood coatings. Film-forming additives directly determine the polymer The degree to which latex particles coalesce into a complete and continuous coating film affects the long-term anti-corrosion effect of the coating film. The combination of film-forming additives and water-based resins is the cornerstone of the formulation of water-based wood coatings. A good film-forming additive must have the characteristics of environmental safety, low addition amount, fast drying and the most effective reduction of the minimum film forming temperature (MFFT) of water-based resin. In actual use, two or more film-forming additives are often used. The low-temperature composition of the system can also be achieved based on the different ratios of the distribution coefficient of the film-forming additive in the water phase and the distribution coefficient in the polymer. The balance between film properties and stability, so the type and dosage of film-forming additives must be determined based on the type of water-based resin in the system, the minimum film-forming temperature, the compatibility with each component of the system, and the tolerance of construction. Formula design This should be fully taken into consideration and the best film-forming additives and their dosage should be selected through experiments. The compatibility test results between film-forming additives and emulsions 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 outside pure acrylic emulsion. It has good compatibility with other emulsions, but these film-forming additives must be added slowly, otherwise it will easily cause flocculation. For pure acrylic emulsion, adding the above three film-forming additives will cause flocculation and easily cause demulsification. The compatibility of dodecyl alcohol ester in any kind of emulsion is very good, and the addition method is simple, it is not easy to cause demulsification, and it is universal for emulsions.
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