Li Xiang-quan (Chengdu Youpin Coating Co., Ltd., Chengdu 610501, Sichuan, China)
Abstract: According to the flooding phenomenon during the construction process of polyurethane matt white paint, the influencing factors on the flooding were analyzed. The corresponding solutions to improve the efficiency of the coating were proposed.
In the furniture coatings market, matte wood coatings occupy a large market share. With the improvement of people’s material and cultural living standards and the pursuit of fashion personality, consumers’ requirements for the coating effect of furniture have become higher and higher. The matte white coating has a soft and elegant luster, which not only adapts to modern aesthetics, but also makes eyesight not easy to fatigue. It gives a simple, elegant and comfortable feeling. It is quiet, stable, and more decorative. It is well received by consumers. Polyurethane matte white paint is added to the formula with a certain amount of matte powder. The matte powder is entrained to the surface of the coating film with the up and down convection formed when the solvent evaporates during the coating film formation process. Due to the comparison of the viscosity of the coating film surface layer Large, so the matte powder stays on the surface of the coating film and increases the pigment volume concentration of the surface of the coating film, forming a fine roughness surface. When the incident light reaches the surface of the coating film, it scatters and produces a matte appearance. However, due to the construction conditions and the environment, the polyurethane matte white paint is prone to uneven gloss and flooding, especially at low temperatures. This disadvantage is more likely to affect the appearance of furniture finishes. These problems have always troubled furniture manufacturers. And technical staff.The gloss of the matte coating film is closely related to the dispersion state of the matte powder in the coating film, and the dispersion state of the matte powder is affected by a number of factors such as the coating process, the coating environment and the drying conditions of the coating film, which become the influence on the coating. The key factors in the quality of the package, how to maintain the luster of the matte white topcoat and avoid the ills of flowering, are the problems that both paint manufacturers and users need to face. This experiment optimizes all aspects from formula design to construction process and painting conditions to improve the gloss uniformity of the matte white topcoat and avoid flooding, so as to obtain a satisfactory coating film appearance and meet the requirements for high decorativeness of furniture.
1 Experimental part
1.1 Main raw materials
Alkyd resin, rutile titanium dioxide, dispersant, defoamer, leveling agent, anti-settling agent, anti-yellowing additive, CAB, matte powder, wax powder, TDI trimer 1451, HDI trimer 3390 , Solvents, etc., are industrial products.
1.2 Preparation of matte white finish
Stir the resin, solvent and dispersant evenly, then add titanium dioxide (see Table 1 for titanium dioxide slurry formulation), and stir evenly at high speed, and then grind to a fineness of less than 20 μm.
Add alkyd resin, titanium white slurry and additives to the dispersion tank, stir at low speed, add matte powder and wax powder, disperse at high speed for about 20 minutes, and finally add the remaining materials, disperse at low speed for 10 minutes. After passing the inspection, filter The package was given a matte white finish, and the formula is shown in Table 2.
1.3 Testing of coating properties
Press m (main agent): m (curing agent): m (thinner) = 1: 0.5: (0.5 ～ 0.7) spray paint on the white primer board, generally spray 2 “ten” characters, etc. After drying, observe the surface state of the coating film to see if the coating film is flooding and the degree of flooding.
2 Results and discussion
2.1 Selection of hydroxy resin
White coatings are required to have a certain resistance to yellowing. To avoid yellowing caused by unsaturated double bonds in oils, saturated fatty acid modified short oil alkyd resins such as coconut oil fatty acid modified alkyd resin and lauric acid are often used. Alkyd resin. Although synthetic fatty acid resins and hydroxyacrylic resins commonly used in wood coatings have excellent yellowing resistance, they are generally not used due to difficulty in matting and limited improvement in the mechanical properties of the coating film. Coconut oil fatty acid modified resins have excellent gloss retention, Coconut oil fatty acid modified resins are usually chosen when preparing matte white topcoats because of their excellent color retention and excellent drying properties. The matting of the coating is generally achieved by matte powder. The stability of the matte powder in the coating is the main factor that causes the gloss of the coating film to fluctuate and bloom. Different coconut oil fatty acid modified resins have a relative molecular mass and a relative molecular mass. Different distributions and different types and numbers of active functional groups are included. These differences cause their wettability to matte powders and pigments to be different, which affects the gloss stability of matte white topcoats. Generally, if the relative molecular mass distribution of alkyd resin is too wide, it means that there is a resin component with too small or too large molecular weight, which has poor compatibility with other components, and the surface tension of the coating will not easily reach equilibrium, and the force between particles will be generated. Repulsion or shrinkage movement, phase separation occurs, and it is easy to cause matte powder to agglomerate. During the drying process, the coating film is prone to pitting and flowering. The relative molecular weight of the resin is too large, the viscosity is not easy to wet the matte powder, the dispersion stability is not good, the leveling during spraying is poor, the matte powder is unevenly arranged in the coating film, and there is large flocculation and agglomeration, which causes The coating film is flooding. The relative molecular weight of the resin is too low, although the viscosity is low and the leveling is good, but the drying speed is slow, it is not easy to dull, it is easy to cause the disordered movement of the matte powder, resulting in unstable gloss, and it is also prone to flower defects. Selecting an alkyd resin with an appropriate molecular weight range not only has good leveling properties, but also improves the wettability of pigments and matte powders, reduces flooding, and gives the matte white topcoat good construction performance. Several coconut oil fatty acid modified resins commonly used in the market are used to prepare PU matte white topcoats. The experimental results are shown in Table 3.
It can be known from Table 3 that the alkyd resins B, C, and D have poor flexibility and adhesion, and are easily brittle at low temperatures, especially when used in northern regions. The main reason is that such resins are used for extinction, and a large number of polyfunctional alcohols or acids are used to increase the branching degree and viscosity of the resin, which makes these resins not only have poor adhesion and flexibility, but also have matt powder. The poor wettability of the matte powder causes the matte powder to easily reunite and cause flooding. Secondly, a large amount of titanium dioxide powder and high oil absorption matte powder are added in the paint making process, which further reduces the mechanical properties of the coating film. Resin A selected in this experiment has moderate viscosity, good leveling and easy matting, which can reduce the amount of matte powder. Because the resin structure contains more primary hydroxyl groups, the steric hindrance is small, which is beneficial to the interaction with the curing agent-NCO Combined to form a network structure, improve the coating film performance, and the high glass transition temperature is conducive to the release of the solvent, accelerate the convection of the solvent inside the coating film and increase the viscosity of the surface layer, resulting in more retention of the matte powder, making the matte powder more stable. Good orientation effect, uniform distribution in the coating film, ensuring stable gloss of the coating film, at the same time, its molecular design is reasonable, rigid and flexible, and adhesion and flexibility have been greatly improved. Brittle cracking is not easy to occur at low temperatures. It is completely able to Meet the performance requirements of furniture for PU matte white finish.
2.2 Effect of additives
In order to obtain good coating film appearance and good construction performance, matte white topcoats inevitably need to add additives, but if the selected additives are poorly compatible with the resin, the dispersion is uneven during use or the state changes during storage, It will cause uneven surface tension of the coating film, and it is easy to cause malady. Therefore, try to choose an assistant with better compatibility through experiments. In matte finishes, the better the dispersion effect of the matte powder, the closer it is to the original particle size, the easier it is to distribute the matte powder uniformly and stably in the coating. During the drying process, the matte powder can be better arranged on the surface of the coating film to avoid matte powder. Increased aggregation and stacking lead to flooding. Especially the matte white topcoat, because the amount of matte powder is large, if the dispersant is not selected properly or the amount is insufficient, the matte powder fails to disperse well, resulting in poor stability, flocculation during use, and it is more likely to cause coating during application. Membrane flowering. In this experiment, a modified block structure copolymer dispersant is selected, which has a unique two-group structure. During the powder dispersion process, the anchoring group at one end of the molecule preferentially adsorbs on the surface of the powder particles and is firmly fixed on the surface of the powder particles. Above, a stable adsorption layer is formed, which promotes the resin to wet the powder particles well, thereby playing a role in stably dispersing the matte powder and titanium dioxide. The association group at the other end obtains a certain association structure by combining with hydroxyl and carboxyl groups on the resin molecule, forming a larger form of the overall structure of the macromolecule, making the anchored matte powder and titanium dioxide more stable and evenly distributed on the In the coating system, it can prevent the occurrence of undesirable phenomena such as powder settling and agglomeration, and make the matte white topcoat have good storage stability. Improve the leveling when spraying, promote the matte powder to be evenly arranged in the coating film, avoid the disorderly movement of the matte powder, make the gloss of the matte white topcoat more uniform, and prevent the floating color from flooding.
In the process of coating film formation, as the solvent is continuously volatilized, the surface viscosity of the coating film increases, the temperature decreases, the surface density and tension increase, and it sinks to the bottom. Because there are many solvents in the lower part of the coating film and the surface tension is low, a driving force will be generated, which will cause the bottom coating to move to the upper layer and spread on the surface to promote the evaporation of the solvent. This up-and-down flow is repeated, and a vortex appears. Matte powder naturally participates in this flow. When the fluidity difference of the matte powder occurs, the matte powder separates with it, and flooding occurs. The leveling agent generally reduces the surface tension of the system through a controllable incompatibility with the coating system, reduces the disadvantages of the coating film caused by the difference in surface tension gradient, promotes the leveling of the coating film, and improves the defects such as flowering. In this experiment, a polyester-modified silane leveling agent is selected, which has a lower surface tension, which can greatly reduce the surface tension of the coating system, significantly improve the coating’s wettability to the substrate, and help increase leveling and solve matt. The powder is aligned to eliminate surface defects and improve the coating effect. In addition, the polyester-modified silicone-based leveling agent has more or less immiscibility with the coating system, which can produce creep action, easily migrate to the surface of the coating film to form a single molecular layer to change the fluidity of the coating, and ensure Uniform surface tension of the coating film, improve the anti-cratering ability, improve the Bernard vortex of the coating, reduce the sensitivity to air flow, greatly improve the long-wave leveling of the coating film, and eliminate the “big wave” phenomenon after the coating film is dry, so that The coating film has a mirror effect.Adding cellulose acetate butyrate (CAB) can improve the coating performance and physical properties. It is a self-drying resin, has a high glass transition temperature, has good release properties for solvents, and can improve the coating’s properties. Drying speed, on the one hand, reduces the influence time of eddy current during the film formation process of the coating, increases the resistance to the movement of the matte powder during turbulence and convection in the wet film, so that the matte powder has a certain degree of freedom of movement, which is beneficial to the The alignment of the light powder. On the other hand, with the volatilization of the solvent, the volume concentration of cellulose acetate butyrate and the curing agent continue to increase. The incompatibility between the two will inevitably increase the viscosity of the coating film, thereby increasing the retention of the matte powder on the surface. , Improve the distribution of matte powder in the coating film, and reduce the occurrence of flooding accordingly.After the paint is mixed with the paint, due to the low viscosity, the matte powder and pigment are easily precipitated, forming uneven gloss and flooding. Therefore, the white matte topcoat should do a good job of anti-settling, and add a suitable anti-settling agent to the whole system. The formation of a hydrogen bond network structure with microthixotropy, which stabilizes the matte powder and pigment, helps to reduce sedimentation and flocculation during the construction process, and improves the coating workability. Make the coating both have good fluidity and fullness, so that during the drying of the coating film, ensure that the matte powder will not move with turbulence, have a good arrangement, maintain gloss uniformity, avoid flooding, and Prevent sagging and improve construction effect.
2.3 Effect of matte powder
Matte powder is the key to the matting effect in matte white topcoat, which directly affects the matting effect of the coating film. At present, the commonly used matting powders are mainly silica and organic wax powder. Among them, silica has the most obvious matting effect, and its performance is mainly determined by the porosity, particle size and surface treatment. The experiment proves that under the same conditions, the larger the particle size of silica, the better the matting property; the larger the porosity, the better the matting; the wax-free treatment is better than the wax-treatment; the more the amount of matting, the better the matting effect. The matting effect of the wax powder is obviously inferior to that of the matte powder, but it can obviously improve the surface properties of the coating film, increase the scratch resistance and surface smoothness of the coating film, and is usually used in combination with the matte powder. However, the wax layer on the surface of the coating film will prevent the volatilization of the solvent and affect the drying of the coating film. If the amount used in the formula is too large, the flooding will be more serious. Therefore, in order to reduce flooding and ensure the scratch resistance and surface smoothness of the coating film, the amount of wax powder can be appropriately reduced and the amount of hand feel additives can be increased. The appropriate matte powder system should be selected through experiments to ensure matte Glossy white finishStability, reduce hair ills.
2.4 Effect of curing agent
The effect of the curing agent on the performance of the coating film is very important. In this experiment, the dual requirements of the hardness of the coating film and the resistance to yellowing were considered, so the choice was made between TDI trimer and HDI trimer. According to Table 4, it can be seen that the yellowing resistance of the curing agent of Experimental Scheme A is the best, but it is difficult to dull, the coating film dries slowly, and the hardness is low. It is easy to cause the matte powder to move during the drying process, causing unstable gloss and serious flooding. Difficult to meet the requirements for use. With the increase of the amount of TDI trimer, the matting property is improved, and the flooding phenomenon is significantly improved. The main reason is that the TDI trimer has a high glass transition temperature and poor compatibility with solvents, and it reacts with the hydroxyl groups in the main paint components. It is also much faster than HDI curing agents. During the drying process, the system containing many TDI trimers has a fast reaction speed, a rapid increase in molecular weight, and the release of the solvent, which makes the wet film thicken quickly, and sets the matte powder in a short time, which is good for matte. The powder is evenly distributed in the coating film to stabilize the gloss. At the same time, as the solvent evaporates, the incompatibility of the curing agent will also increase the viscosity of the coating film, help to dull, and reduce flooding. However, with the increase of the amount of TDI trimer, the yellowing resistance of the coating film is reduced, at the same time the surface drying speed is accelerated, the leveling property is deteriorated, and it is easy to cause flooding. The result of scheme D is the most ideal from the perspective of performance and price, which can meet the requirements for use. At the same time, due to the good matting effect, the amount of matte powder can be reduced accordingly, and the chance of hair loss is reduced.
2.5 Effects of solvents
Polyurethane matte white topcoats have higher requirements for solvents, and alcohols and alcohol ether solvents that are reactive with isocyanates should be avoided. They must also meet the urethane-grade standards that do not contain water. If the solvent contains water, it is easy for the pigment and matte powder to return to coarseness and sedimentation, accelerate the flocculation of the matte powder and pigment, and is not conducive to the uniform arrangement of the matte powder. At the same time, water reacts with the curing agent, it is easy to produce pinholes, and its volatilization is slow, which adversely affects the solubility and surface tension of the system, resulting in haze on the surface of the coating and causing flower defects.The role of solvents in coatings is mainly to adjust the coating viscosity and volatilization speed. Not only does it have an important effect on the coating workability and film performance, but its drying speed and volatilization gradient also have a great impact on the gloss stability of the matte white topcoat. Due to the different dissolving power and volatilization rate of various solvents and the reaction rate of -NCO group and -OH group in various solvent systems, different diluents have different effects on the matte white surface coating film. The effect of the solvent on the matting effect of the matte white topcoat mainly depends on the volatility of the solvent and its solubility in the coating system. Generally, the faster the solvent volatilization rate, the better the matting. With the increase of solvent volatilization rate, the decrease of solvent content in the wet film accelerates, the concentration of reactants increases, the reaction speed increases, the molecular weight increases, the viscosity thickens, and the thickening of the matte powder is significantly slowed, which promotes more The matte powder stays on the surface of the coating film, reducing the gloss of the coating film, and at the same time quickly promoting the uniform distribution of the matte powder in the coating film to stabilize the gloss, and the coating film is not prone to bloom. However, the surface dries too quickly. When spraying on a large area, the coating will fly away from the spray gun to the substrate, most of the solvent will volatilize, and the viscosity of the coating on the surface of the substrate will be greatly improved. The level is not good, not only the surface of the coating film is rough, but also the solvent balance. Part of the resin precipitates with the matte powder adsorbed due to the reduction of strong solvents, which causes the matte powder to not be aligned on the surface of the coating film well, causing some areas of the coating film surface to have more matte powder and some areas to have more resin. The coating film has uneven gloss, which causes flower defects. If the solvent is too slow to dry, the matte powder in the coating film will randomly and disorderly move, resulting in large gloss fluctuations, and flooding will also occur. In particular, when a high-boiling-point solvent with poor solubility is used, its volatilization rate is slower than that of a solvent with good solubility, and there are many solvents with poor solubility in the coating film, which results in insufficient solubility and shrinks the resin molecules into clusters or even precipitates. , The coating film surface is easy to wrinkle locally. Or it is sensitive to the airflow, which is affected by the local airflow, and it is more likely to cause the matte powder to be disordered and bloom. Therefore, the solvent must have a suitable volatilization speed to ensure that the matte white topcoat does not bloom.Diluents are usually composed of aromatic hydrocarbons such as ketones, esters, and xylene. Ketones and esters are polar solvents, have good solubility, can form hydrogen bonds with hydroxyl groups, produce associations, and slow down —NCO / —OH The reaction speed is conducive to leveling the coating film and extending the construction period. Aromatic hydrocarbons such as xylene are non-polar solvents, and have poor solubility to the main agent and curing agent, but can promote the -NCO / -OH reaction, accelerate the increase of the molecular weight of the coating film during the drying process, and increase the viscosity of the coating film. Slow down the settling speed of the matte powder, increase the amount of matte powder remaining on the surface of the coating film, and reduce gloss. However, with the increase of the amount of xylene, the dissolving power becomes weaker, resulting in poor leveling of the coating film and easy flooding. Therefore, it is necessary to select the aromatic solvent such as esters, ketones and xylenes with different volatilization rates to form an ideal mixed solvent through experiments. It must have a suitable volatilization rate and good solubility, increase the solubility of the product, and make the resin The polymer fully stretches and interacts, evenly dispersed in the system, fully wets the matte powder and pigment particles, prevents the occurrence of agglomeration, and at the same time prolongs the open state of the coating film surface, ensures sufficient leveling, and is beneficial to the coating film The medium solvent smoothly volatilizes, promotes the directional arrangement of the matte powder, and ensures that the matte white surface has a good coating film state. When formulating the diluent, it is necessary to consider that the temperature of the year varies greatly, and the amount of fast-volatile solvents should be increased in winter, and the proportion of high-boiling solvents should be increased in summer.
2.6 Impact of construction process
In order to obtain good coating film quality and eliminate flooding, good construction conditions are also required to guarantee. According to the construction site, the factors that may affect the gloss and flooding of the coating film are: substrate, temperature, humidity, ventilation conditions and construction. the way.
The smoothness of the substrate has a great influence on the appearance of the coating film. If the substrate is too rough, it will absorb part of the coating, which will directly cause the matt white topcoat to have different partial base-to-base ratios and different drying speeds, resulting in uneven gloss. At the same time, the visual perception of the coating film depends not only on the lighting conditions, but also on the viewing angle. If the substrate is not flat, there will be a light-dark interval for the reflection of light, causing different reflections of light and causing different people’s visual sense, making people feel that the color and gloss of the coating film are different and the coating film is faint. Especially when the gloss is low, the difference is a little, and the change will be particularly obvious from the visual point of view. Feeling of flooding, so it is necessary to smooth the bottom layer as much as possible, which can effectively reduce the long and short wave orange peel of the coating film and obtain a good leveling coating. Film effect.
Painting environment temperature lower than 10 ℃ or higher than 30 ℃ will affect the painting effect. The increase of the general temperature aggravates the up and down convection of the solvent inside the coating film and the volatilization of the surface, causing more matte powder to migrate to the surface layer of the coating film as the solvent diffuses. At the same time, because the viscosity of the surface layer of the coating film rises faster, the more the matte powder stays on the surface layer, the greater the roughness of the coating film surface, the lower the gloss, and the gloss is more stable and avoids flooding. However, the temperature is too high, which causes the solvent to evaporate too quickly, which makes the coating level worse, and it is also easy to bloom. When the temperature is too low, the coating reaction is slow, the crosslinking is not good, and the solvent is not easy to volatilize, which causes the coating to dry incompletely. The matte powder is in a wet state for a long time, and it is easy to diffuse or flow freely, resulting in uneven distribution of the matte powder. Phenomenon, causing uneven gloss and flowering. Therefore, in order to obtain a stable gloss at different temperatures and avoid coating film flooding, the volatilization rate of the solvent in the system must be adjusted and a suitable diluent must be selected.
When the humidity exceeds 80%, the drying performance of the coating film is obviously affected. This is mainly due to the partial reaction between the moisture in the air and the curing agent on the coating film surface, which causes the resin and powder in the coating film to phase separate, and the coating produces haze. So that the gloss is uneven. At the same time, it has an adverse effect on the solubility and surface tension of the system, which makes the solvent difficult to volatilize, which causes the coating film to dry slowly, affects the matte powder arrangement, and causes flower defects.
2.6.4 Ventilation environment of dry room
The dry room has good ventilation and fast air circulation, which can promote the diffusion and volatilization of the solvent inside the coating film, ensure the surface gloss of the coating film is stable and reduce flooding. On the other hand, the ventilation environment of the drying room is poor, and the solvent vapor fills the entire site, which seriously hinders the further evaporation of the solvent from the coating. Some slow-drying solvents will also re-dissolve some coating film-forming substances, causing the matte powder to precipitate and cause uneven gloss. If the workpiece is placed too densely, flooding will be more serious.2.6.5 Construction methodIn the construction on site, the operator’s different construction habits and construction methods will affect the thickness and uniformity of the coating, causing instability in gloss, and then flooding. In particular, it is necessary to consider whether it will sag during the construction of the facade, so the coating amount is often less than the construction of the plane. In addition, the spraying of the facade is usually easy to uneven, and the overlap between each gun is not uniform, which is not easy. A smooth coating film was obtained. Therefore, the gloss of the facade construction is lower than that of the flat construction, and it is more likely to cause uneven gloss. The gloss is higher in the thicker parts and lower in the thinner parts, which causes the coating film to bloom. Therefore, in order to obtain a stable gloss, the influence of the amount of coating and the application method on the matting effect must be considered. Only by balancing the relationship between the two can achieve a good coating effect. For large-area spraying, use a slow-drying thinner to make the spray thicker. At the same time, the spray gun should be uniform to make the coating film uniform in thickness to make the gloss uniform. When spraying on the facade, the construction viscosity can be increased, and the gun can be moved slowly to have a better flatness of the coating film to avoid flooding.
The factors that affect the matting of polyurethane matte white topcoat are various. In the formulation design, the influence of various materials on the matting effect should be considered, and the appropriate materials should be selected to prepare matte matt that is not prone to flooding. White finish. At the same time, strengthening coating management and properly adjusting thinners and construction methods can effectively prevent and improve the coating film flower defects, improve furniture coating quality, and obtain a satisfactory coating film appearance.