Key words: application of film-forming additives in latex paint; App application
1 preface
Architectural coatings occupy a very important position in the coating industry. At present, the proportion of domestic architectural coatings in coatings is increasing. With the improvement of people's living standards, the quality requirements of architectural latex paint are getting higher and higher, and the quality of coating film directly affects the performance of coatings.
It is generally believed that latex paint can form a film completely in a short time, but the temperature control in each film-forming stage is ignored. Especially, the temperature of external latex paint changes greatly after construction, which makes the performance of the final coating unsatisfactory, such as decreased gloss, poor adhesion, poor scrub resistance, poor stain resistance and unsatisfactory weather resistance. Adding film-forming additives effectively can greatly reduce the film-forming temperature, which is an effective measure to improve the low-temperature construction performance of latex paint. In this paper, a series of experiments were carried out on the application of several film-forming additives in latex paint, which has certain reference function for the formulation design of architectural latex paint.
2 experimental part
Different from solvent-based coatings, the film-forming mechanism of latex paint is generally divided into the following processes:
First, the filling process. After the construction of latex paint, the water volatilizes, and when the latex particles account for 74% (volume) of the film layer, the particles are close to each other, reaching a dense filling state. Emulsifiers and other water-soluble additives in the components remain in the water between particles.
Second, the process of melting Taiwan Province. Water continues to evaporate, and the protective layer adsorbed on the surface of polymer particles is destroyed. The exposed particles contact each other, and the gap becomes smaller and smaller. When the capillary diameter is large, the capillary pressure is higher than the deformation resistance of polymer particles due to the capillary action, and the particles are deformed, and finally coagulate and fuse into a continuous coating. This process is the key to whether the emulsion can form a film. If the glass transition temperature (Tg) of emulsion particles 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), it will be difficult to deform at low ambient temperature, which will hinder the melting process and lead to the failure to form a film. At this time, film-forming additives are often needed to assist film formation.
Third, the diffusion process. The additives remaining in water gradually diffuse to the coating film, and make the long chains of polymer molecules diffuse with each other, forming a uniform coating film with good performance.
Film-forming additive is a volatile temporary plasticizer, which can promote the plastic flow and elastic deformation of latex particles and improve their coalescence. It can form a film in a wide construction temperature range. The ideal film-forming assistant should have the following characteristics: as a good solvent for polymer emulsion, it can reduce the minimum film-forming temperature of polymer emulsion; Small solubility in water towel; It has certain volatility, can stay in the coating and play a role in the film-forming process, and completely volatilizes after film-forming, without affecting the performance of the coating; Does not affect the stability of the emulsion.
There are many kinds of film-forming additives, including alcohols (such as stupid methanol), ester alcohols (such as Texanol ester alcohols), alcohol ethers (such as ethylene glycol butyl ether and propylene glycol diphenyl ether) and alcohol ether esters (such as hexanediol butyl ether acetate). Commonly used film-forming additives are Texaco ester alcohol, benzyl alcohol (BA), ethylene glycol butyl ether (EB) and propylene glycol phenyl ether (PPH). The following is a comparative test of these commonly used film-forming additives.
2. 1 Minimum film-forming temperature instrument for main instruments and equipment (Nisshin Industrial Co., Ltd., IV605)
2.2 Emulsions for test This test adopts representative and widely used emulsions such as pure acrylic, styrene acrylic, acetic acrylic and tertiary vinegar in China, such as Changxing, BASF, Liantan, National Starch, Rohm and Haas, Jiangsu Kouchu Group, Beijing Oriental Chemical J- 1, Beijing Tongzhou Mutual Benefit Chemical Plant, Beijing Xiangzhen Trading Company, Shandong Qingzhou Baoda Chemical Plant, Beijing Kexin Industry and Trade Co., Ltd. Due to space constraints.
2.3 Performance test of film-forming additives in emulsion and compatibility test of coatings: The emulsion was directly mixed with Texanol ester alcohol, benzyl alcohol, ethylene glycol butyl ether and propylene glycol phenyl ether to observe the performance of the emulsion.
Determination of emulsion viscosity: After adding human film-forming additives to the normal compatible emulsion, measure the viscosity and observe the viscosity change.
Determination of low film-forming temperature of emulsion: Mix compatible emulsion with several film-forming additives to determine its minimum film-forming temperature (MFT).
Freeze-thaw stability test of emulsion: add Texanol, BA, EB and PPH film-forming additives with corresponding amount of pore liquid (the minimum film-forming temperature is reduced to 0℃), put them in-10% refrigerator 16h, then put them in standard conditions (room temperature 23 2℃, relative humidity 50 5%) 8h, and so on.
Storage stability test of emulsion: add the corresponding amount of Texaco ester alcohol, benzyl alcohol, EB and PPH film-forming additives into the normal compatible emulsion and leave it under standard conditions (room temperature 23 2℃, relative humidity 50 5%) for 3 months. Regularly observe the state of emulsion, and determine its viscosity and pH value.
2.4 Performance test of coating
Different film-forming additives (Texanol, EB, PPH and BA) were selected to compare their effects on coating properties. PPH and BA can't be added directly, but should be mixed with alcohol solvent and added slowly in the process of paint making to prevent flocculation. If EB is added slowly with stirring, it does not need to be mixed with alcohol solvent, but the adding speed should be slow.
According to the national standard GB/T 9755-95, Texanol ester alcohol has outstanding advantages in scrubbing resistance, which may be because other film-forming additives are incompatible with pure acrylic emulsion, which affects the film formation of emulsion and thus affects the coating performance. In the production process, it should be noted that the film-forming additives should not be added too quickly to avoid flocculation and affect the performance of the coating.
3 Results and discussion
3. 1 Compatibility of Film-forming Additives with Emulsion
Compatibility test results of film-forming additives and emulsion: BA, EB and PPH have good compatibility in 65 12 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 it is easy to cause flocculation. For pure acrylic emulsion, adding these three film-forming additives will cause flocculation, and sometimes these film-forming additives can be mixed with alcohol solvents to avoid demulsification. Triamcinolone ester alcohol has good compatibility with any emulsion we collected, and the addition method is simple, which is not easy to cause demulsification and has universality for emulsion.
3.2 Influence on the viscosity of emulsion After adding film-forming additives, the viscosity of emulsion basically increased. This is because the film-forming additives will soften the latex particles and expand the latex particles. As long as the dosage is appropriate, it will not affect the use of emulsion. However, due to the swelling of emulsion particles, the concentration of surfactant and protective colloid on the surface of emulsion particles decreased correspondingly, and even was replaced by a large number of film-forming additives, which made the emulsion unstable.
3.3 Influence on MFT of emulsion
Effect of film-forming additives on MFT of emulsion: For emulsion with good compatibility, the dosage of film-forming additives should reach the minimum film-forming temperature of 0℃, and for styrene-acrylic emulsion, the dosage ratio of benzyl alcohol should be added. Texaco ester alcohol is small, which may be due to the principle of similar compatibility. Benzyl alcohol can soften styrene-acrylic emulsion particles to the greatest extent, and reduce the minimum film-forming temperature of emulsion to 0℃. The dosage is small, but it is toxic and has poor compatibility with other types of emulsions, so it must be used in conjunction with alcohol solvents. EB is easily soluble in water, and it is not easy to contact with emulsion particles after adding emulsion, so the dosage is correspondingly large, but the volatilization speed is equivalent to that of water, or even faster, which is not conducive to film formation. Thus affecting the performance of the coating. PPH has a good effect on styrene-acrylic emulsion, but it is not easy to contact with emulsion particles because of its slightly higher solubility in water. Therefore, the effect of PPH compatible emulsion is similar to that of Texan0l ester alcohol, but Texan 0L ester alcohol is simpler than other film-forming additives in the way of adding decapropyl emulsion or other types of emulsion, and the dosage is not large.
3.4 Influence on Freeze-thaw Stability of Emulsion Adding film-forming additives has certain influence on the freeze-thaw stability of emulsion. After five cycles, the emulsion coagulates, because the film-forming additives swell the emulsion particles and reduce the concentration of protective colloid accordingly. Therefore, if film-forming additives and emulsion are used as base materials, it should be noted that they cannot be stored at low temperature.
3.5 Effect on Storage Stability of Emulsion
Film-forming additives have no effect on the storage stability of emulsion (only suitable for emulsion with good compatibility). With the delay of time, the pH value of some emulsions decreased slightly, which was caused by the volatilization of ammonia water used to neutralize the emulsions.
Film forming mechanism of latex paint;
The film formation of latex paint is divided into three processes: dense accumulation of latex particles, fusion of latex particles and mutual diffusion of polymer chain ends.
When latex paint is painted on the substrate, the volatiles (mainly water) in the paint are steamed outside and absorbed inside, and the solid content of the paint is increasing, and the particles are close to each other, finally reaching the densest accumulation.
Continue drying, the adsorption layer covering the surface of latex particles is destroyed, and the exposed polymer particles are in direct contact. Under the swelling and dissolution of the coalescence solvent, the particles soften, deform and fuse to form a continuous film.
At the same time and after the latex particles are fused, the chain-end molecules on the polymer surface penetrate and diffuse with each other, and the coating is further homogenized.
The film formation of latex paint has a great relationship with weather conditions. High temperature, strong wind, low humidity, low temperature and excessive humidity will all lead to poor film formation of latex particles and affect the film performance.