Polyurethane paint finishing process analysis
Ningbo Smile tools Co.,ltd , https://www.nbsmiletools.com
1. Overview of domestic furniture paints
China's furniture lacquer has a long history. The lacquer, tung oil and its finishing process have been used for thousands of years, and it has continued to this day. Now some of the mahogany furniture produced by some enterprises in the south still use the lacquer and its production process. In the past 100 years, China's furniture has experienced shellac, phenolic paint, alkyd paint, nitrocellulose paint, polyurethane paint, unsaturated polyester paint, water-based paint and photosensitive paint. In the 50s and 80s of the last century, oil-based paints such as phenolic paints and alkyd paints were often used for the decoration of ordinary furniture, and nitrocellulose and polyurethane paints were used for medium and high-grade furniture. In the early 1980s, many furniture companies in China used the most advanced photosensitive paint (ultraviolet light-curing coating) at that time. So far, some panel furniture manufacturers still use photosensitive paint, which is also widely used on the surface of some solid wood flooring. Finished. In 1984, Shanghai Furniture Research Institute developed the water-based paint used in wooden furniture and passed the appraisal. Its products have also been used in furniture enterprises in some provinces and cities for many years, but due to its performance, it is no longer used in furniture. It has been widely used in industries such as metals, plastics and cement. In recent years, with the increasing number of paints, the number of paints and finishing methods available for people to choose is increasing, but in actual production, polyurethane paint should occupy a dominant position.
The special two-component polyurethane paint for furniture (referred to as polyester or PU paint) is rich in variety, color, gloss and feel, and the physical and chemical properties of the film drying speed, adhesion, hardness, heat resistance, wear resistance and chemical resistance. The decorative performance is also very superior. After the 1990s, it became the main paint for the surface decoration of furniture in China. However, PU paint is also a typical solvent-based paint. Although the solid content of the two-component PU paint on the market is 40%-50%, the solid content after adding the thinner during the paint adjustment can only reach about 30%, and nearly 70%. The organic solvent is largely volatilized into the air during the spraying process, and only a small part remains in the coating. These volatile organic solvents will seriously pollute the environment. The technical requirements of China's furniture environmental labeling products stipulate that the paint used in the product must be water-based paint or UV-curable paint. Therefore, from the perspective of environmental protection, the paint species has yet to be further studied.
2, PU paint finishing process
The coating process of PU paint can be classified according to transparency (transparent and opaque), gloss (bright and matt), filling holes (filling and boring), whether polishing (polishing and original light), and transparent coloring. This article mainly introduces the transparent coating process of PU paint.
The transparent coating is divided into two processes of bottom coloring and surface coloring according to the coloring method. The bottom coloring is directly coated on the surface of the substrate with a coloring agent, and then the transparent primer and the topcoat are coated. The coloring effect of the process is good, the color is uniform, the layering is clear, and the wood grain of the substrate is clear, which is a commonly used coating process in actual production; Coloring is a coloring transparent topcoat for coloring when applying a topcoat, eliminating the need for a bottom coloring process and no underlying coloring. However, this process requires higher spray coating technology. If the topcoat has poor transparency, it is coated. The grain definition in the layer will be affected. The basic finishing process of the two finishing processes is now listed.
2.1 bottom coloring process
Substrate sanding: Grinding the substrate with 120-240 grit paper and removing the grinding dust;
Filling coloring: coloring the substrate with a coloring agent (or adding a special pore-filling agent to the coloring agent) to color the substrate, and the surface of the substrate is not ground or lightly ground after drying;
Primer: use a low-viscosity PU primer (commonly known as Depot), spray it once, and then dry the paint film;
Grinding: Lightly grind or not grind with 240-320 sandpaper to remove grinding dust;
Primer: The primer can be applied in two ways. One can spray 1-2 times of unsaturated polyester (PE) clear primer, and the other is spray 2-3 times PU clear primer (secondary primer). The former can achieve a fuller mirror effect, followed by the drying of the paint film after spraying;
Grinding: Finely sanding with 240-320 sandpaper to remove grinding dust;
Color correction: use suitable coloring agent (or adjust the coloring agent into the powder-free primer) to spray on the uneven color or damage of the coating and dry it;
Topcoat: Use bright or matt PU clear topcoat as required, finely spray once, and dry the paint film.
2.2 surface coloring process
Substrate sanding: Grinding the substrate with 120-240 grit paper and removing the grinding dust;
Filling coloring: Scrape two-component PU paint or one-component lacquer (NC) transparent putty, the coating should be thin and dry;
Grinding: Lightly grind or not grind with 240-320 sandpaper to remove grinding dust;
Priming: spraying with a low viscosity PU primer, and then drying the paint film;
Grinding: Lightly grind or not grind with 240-320 sandpaper to remove grinding dust;
Priming: Spray 2-3 times PU clear primer or PE clear primer 2 times, then dry the paint film;
Grinding: Finely sanding with 240-320 sandpaper to remove grinding dust;
Topcoat: Apply 1-2 times of colored transparent topcoat that requires color tones, then dry the paint film;
Cover light: spray PU clear topcoat, paint film dried for more than 24h.
3. Factors affecting the quality of PU paint finishes
3.1 substrate moisture content
Before the wood is finished, the ideal moisture content of the substrate should be lower than the equilibrium moisture content corresponding to the relative humidity of the use area by 1%-2%; after the moisture content of the substrate is determined, the finishing work should be carried out immediately to avoid the moisture regain of the substrate. PU paint is sensitive to moisture, too high or too low is not good for finishing. Therefore, when defects occur on the surface of the paint film, the first consideration is whether the moisture content of the substrate meets the requirements of finishing.
3.2 substrate contamination
When applying PU paint, the surface of the substrate should be specially cleaned to maintain no dust and other pollution. For example, the dust generated during the grinding process by the construction workers, the oil and sweat stains caused by the hand touching the surface of the workpiece during handling, the wax dust emitted to the air after waxing, the oil and gas contained in the unfiltered compressed air, etc. , all directly affect the quality of the finishing.
3.3 Environmental factors
When the PU paint is applied, if the humidity of the air is large, the surface of the substrate is highly damp and the coating becomes white. At this time, the construction should be suspended, or the doors and windows should be closed, the wet air should be blocked from contacting the surface of the substrate, or the PU paint whitening agent can be added to the paint to reduce the whitening phenomenon; at the same time, special attention should be paid to the storage of the two-component PU paint hardener. After opening the can, it should be sealed to avoid moisture infiltration and affect the quality of the coating.
3.4 paint mixing ratio
The ratio of the two-component PU paint main agent to the hardener should be weighed according to the instructions provided by the paint manufacturer and the standard gauge. If the ratio is not correct, the coating will not dry or the drying will be too slow (the hardener is too small). The paint film becomes brittle and foaming (too much hardener) and other defects. In addition, if the moisture content of the diluent of PU paint is too high or the solubility is insufficient, it will directly affect the quality of the coating.
3.5 coating material for a long time
According to the process steps and process conditions specified in the PU paint instruction manual, the time of drying, recoating, grinding and curing of the coated coating after each application shall be calculated, and the coating shall be carried out within the working time limit of the paint. When the finish is interrupted for more than 3 days, the coated surface is damp or contaminated due to the long interval between the coatings, thus affecting the adhesion between the coatings and other defects caused by pollution (such as Shrinkage, etc.), so the film should be sanded before painting.
3.6 coating foaming
The foaming of the coating is caused by the discharge of gas, which is caused by the evaporation of the solvent, the expansion of the air contained in the pores of the substrate, and the gas generated by the reaction in the PU paint without smoothly forming bubbles through the coating; The other reason is related to the interval between the continuous finishing operation being too short, the static time before the coating is forced to dry at a high temperature, or the coating is exposed to direct sunlight before and after the coating. When the temperature difference between the coating and the coating is too large, It is easy to cause the occurrence of coating blistering.
In short, there are many types of defects that may occur after coating with PU bottom and topcoat, such as bubbles, pinholes, shrinkage, whitening, wrinkles, discoloration, cracking, poor drying, poor adhesion and leveling, etc. The reason is also related to the type of PU paint, the material of the substrate, the surface quality of the substrate, the coating method and the finishing process.