The application of UV paint on furniture
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Wooden furniture was introduced to China in the 1990s, gained popularity among consumers and the market, and the industry has since experienced rapid growth. At the same time, consumers are placing increasing demands for environmentally friendly home decorations, requiring that they be free from harmful substances such as VOCs (volatile solvents like formaldehyde and toluene) and heavy metals. At present, most domestic manufacturers of wooden furniture use traditional solvent-based coatings, which have a slow drying time and result in low production efficiency. The disadvantage of heavy environmental pollution. At this time, a new type of technology—ultraviolet technology (UV)—began to be gradually adopted in this industry. UV products have advantages such as being environmentally friendly, offering vibrant colors, high brightness, a high-gloss finish, scratch resistance, antibacterial properties, acid and alkali resistance, easy cleaning, not attracting dust, preventing condensation, and not fading. It is also easy to store and process, enabling the rapid production of high-quality furniture panels. Moreover, its formaldehyde emission level is much lower than that of ordinary materials, which brings significant benefits to distributors. UV materials are regarded as the green industrial technology of the 21st century. UV coatings refer to ultraviolet-curing coatings, which use ultraviolet light as a curing source to enable rapid cross-linking of resins at room temperature. They represent the next generation of 4E-type green coatings and possess the following characteristics: 1. Fast curing speed (complete drying generally occurs within 3 seconds), high production efficiency, and good energy utilization. (Energy) 2. UV coatings are solvent-free coatings that contain no volatile solvents; they cause no environmental pollution during application and meet ecological and environmental protection requirements. (Ecology) 3. UV-cured coatings have an extremely high curing rate; a thick film layer can be obtained with just one application. Their cross-linking density is **higher than that of thermally cured coatings. The coating exhibits excellent properties such as high hardness, wear resistance, resistance to acids and alkalis, salt spray resistance, and solvent resistance. In particular, the paint film is full and has a particularly prominent luster, demonstrating excellent film quality. (Excellence of finish) 4. UV coatings ensure stable and reliable operation during the coating process; the first-pass qualification rate is generally above 98%, which helps to reduce coating costs. UV coating equipment is simple to use, requires low investment, takes up little space. UV coatings do not clog the equipment or cause corrosion, resulting in a low failure rate; thus, the overall cost of application is low. (Economy) UV technology is most commonly used in furniture and wood products, such as bamboo and wood floors (for pouring or rolling application), cabinets (mainly through spraying), the door industry (also via spraying), as well as for post-curing processes on interior stairs Next, the applications of UV in coating equipment will be introduced one by one. There are three coating methods: curtain coating, roller coating, and spray coating. Spray coating: Also known as flow coating or pouring coating, it is a painting method in which the paint is sprayed or poured over the surface of the workpiece. It represents an improvement over dip coating; although it requires additional equipment, it is suitable for large-scale mass production processes and constitutes a relatively economical and efficient painting method. Roll coating: Similar to the printing process, it involves transferring ink through an ink transfer roller to a anilox roller, and from there to the substrate. Spraying: A coating method in which UV paint is diluted and then sprayed in a mist form onto the surface of the workpiece using a spray gun under pressure. This method is flexible and suitable for coating objects of various shapes. When using this coating technique, it is necessary to employ a water curtain spray booth; otherwise, it can cause significant pollution to both humans and the environment. Coating Method Advantages Disadvantages Spraying: Saves paint, high production efficiency, does not pollute the environment; the paint film can achieve a mirror-like finish. High solvent evaporation; coatings that evaporate quickly in the initial stage are not suitable for this method. It is mainly used for coating flat surfaces; it cannot be used on vertical surfaces, and is limited to painting flat or planar surfaces. Roll coating: Saves paint, high production efficiency, does not pollute the environment. The paint film is thin and smooth. It is limited to painting flat products, and the paint film may have slight marks from the rollers. Spraying is suitable for products of various shapes; it can be used for curing the product. The equipment required is inexpensive, but paint waste can be as high as 40-50%, there is significant pollution of the working area, and the paint film formed is relatively thick. Given the different characteristics of the three coating methods, manufacturers can choose the coating equipment that best suits their needs. When choosing equipment, the properties of wooden materials and the performance of paints also need to be taken into consideration. There are four types of paints commonly used in the painting process for wooden products: NC, PU, PE, and UV. Among them, UV paint stands out among wood paints due to its excellent environmental compatibility, high hardness of the paint film, rapid curing time, good scratch resistance, and chemical resistance, making it the preferred brand for improving the painting processes on wooden products. List of the characteristics of the above four types of paints: Paint name, Solid content, Drying time, Hardness. NC: 25-40% solid content, 0.5 hour drying time, hardness level of 1H; PU: 40-60% solid content, 4-6 hours drying time, hardness level of 2H; PE: 90% solid content, 2 hours drying time, hardness level of 3H; UV: 100% solid content, 2-4 seconds drying time, hardness level of 4-5H. In terms of solid content, hardness, and drying time, UV paint has clear advantages. Then why hasn’t it become the dominant product on the market? 1. UV coatings have been developed and used for a short period of time; they have only been in use in the market for 50–60 years. It is only in recent years that people have gained a proper understanding of them. 2. Its development is quite challenging, as technical barriers are difficult to overcome. Since UV coatings are of the photochemical reaction type, their properties are complex, which gives rise to various problems such as the impact of UV light on adhesion and the effect of ingredients on drying speed.
I. The thickness of the paint coating is uneven, with inconsistent application on the left, right, front, and back sides of the workpiece. Causes: 1. Unequal gaps on either side of the knife used in the painting system. 2. Wear of the painting knife or blockage by foreign objects. 3. The paint heating system has not yet reached a stable and uniform state. Suggested solutions: 1. Adjust the knife so that the gaps on both sides are equal. 2. Grind down the blade edges and remove debris, thereby improving the filtering capacity inside the painting tank. 3. Heat the coating tank and start the paint circulation pump of the coating machine to ensure continuous circulation and heating, as well as filtering (preheating and filtering for at least 5–8 hours or more). II. Small pitting on the surface of the coated film: Causes include 1. An inappropriate formulation of the UV paint, or too high a storage temperature, along with prolonged storage that allows ultraviolet light to penetrate; 2. Premature pre-curing, which results in the formation of small gel-like particles. 3. Fine sawdust, wood shavings, and dust contaminate the paint film, resulting from inadequate dust removal. Suggested solution: 1. Choose a UV coating with an appropriate formula and stable performance. 2. Floors stored in high-temperature and strong ultraviolet environments should not be kept there for more than 3 months. 3. Try to keep the area around the painting lines clean, and carry out dust removal in the workshop, production lines, and work surfaces. III. The surface of the UV-coated film has small bubbles, tiny pinholes, as well as circular pits of varying sizes and distributions. Reasons for this: 1. UV coating itself has poor defoaming properties. 2. The first coat of sealant does not fill the capillaries adequately, leaving small gaps; the air in these gaps expands and escapes as a result of the heating from the paint, curing machine, and leveling machine. These small bubbles get trapped within the paint film and solidify, resulting in small bubbles and specks ； Large bubbles heat up and burst, forming pinholes or small circular pits. Suggested solution: 1. Use a UV topcoat with good defoaming properties. 2. The first filling layer at the back cover must be sealed thoroughly, with no gaps left. 3. Try to avoid operating the paint sprayers, curing ovens, and leveling machines at high temperatures, which could cause the substrate temperature to rise too high (above 60°C), leading to air expansion and escape. 4. It is recommended to use a new type of putty filler before applying the first coat of primer. (. Use UV coatings with good compatibility. Try to avoid being mixed with incompatible substances such as oil and water. Thoroughly remove dust and avoid contamination by other substances, such as solvents. ) IV. The painting curtain is unstable, and irregular tearing occurs in the membrane-based painting curtain. Causes: 1. Improper paint formulation, too low viscosity, excessively high paint temperature, resulting in reduced tension in the curtain. 2. The gap at the curtain coater blade is too small, causing the tension to decrease until the film tears. 3. The blade edge is blocked by contaminants (the filter screen has failed); the coating is contaminated, resulting in failed filtering. Suggested solution: 1. Use a coating with an appropriate formula, low viscosity, and low requirements regarding paint temperature. 2. Adjust the blade gap to the appropriate position. 3. Remove debris from the blade and filter screen to restore functionality. 4. Remove pollutants and restore filtering function. V. The painted film exhibits a \"orange peel\" appearance, with wrinkles in the paint surface. Causes: 1. Poor precision in the processing of the bare workpiece, along with incomplete sanding. 2. The workpiece is bent and deformed, with an uneven surface, making it difficult to sand or paint the entire surface evenly. 3. The primer treatment was insufficient. 4. The coating has high viscosity and poor leveling properties. 5. The heating during painting has not yet achieved uniformity and stability. 6. The thickness of the coating layer is too great, making it difficult to level it out. Suggested solution: 1. Select qualified blank workpieces and re-sand them to make the surface smooth. 2. Remove workpieces with severe deformation or uneven surfaces. 3. Add 1–2 additional priming coats (including sanding and curing). 4. Use a UV topcoat with low viscosity and good leveling properties. 5. Extend the heating and circulation time to achieve uniform and stable viscosity. 6. It is advisable to keep the paint thickness between 90-100 g/m2. VI. Uneven gloss: Uneven gloss on floors within the same piece or across different pieces. Causes: 1. Inconsistent mixing of matting powder and ultra-wear-resistant powder in the coating. 2. The coating amount is uneven. 3. Inadequate leveling and uneven curing of the coating. Suggested solution: 1. Stir thoroughly until well mixed. 2. Control the coating amount to be consistent and uniform. 3. Prolong the leveling time and control the uniform curing of the coating.
Generally, UV paint exhibits good adhesion to ABS, but its adhesion to PC and BMC materials is relatively poor, which presents some challenging issues. (1) The edges of the irregular-shaped products have cracks and white spots. It is likely that the UV paint does not have enough energy and time during the IR pre-baking stage to allow the solvents contained in it to evaporate; if these solvents remain undevaporated in the paint film, white spots will appear. If they evaporate during the photopolymerization stage, it will cause cracks. (2) The surface of the product’s UV paint is not dry and lacks sufficient hardness. This indicates that the UV light energy is insufficient, preventing the polymer chains from fully cross-linking. It is necessary to check the illuminance; the power of the UV lamps should be at least 80 W/cm. Additionally, attention should be paid to adjusting the irradiation angle so that all areas coated with UV paint can reach a solid state. In the case of products that require an aluminum coating, insufficient curing of the UV paint will result in a darker surface on the aluminum layer, thereby reducing the quality of the product. (3) On the contrary, if the UV paint film becomes brittle, it is often due to excessive light intensity, which causes aging of the polymer chain structure; this can be resolved by adjusting the printing speed or reducing the light intensity. Another factor is that the UV paint film is too thick. (4) Low adhesion. One factor is that the UV paint used is not compatible with the substrate, and another is that the UV paint applied to the substrate is too thick, as the adhesion test is related to the thickness of the paint film. (5) Local scorching and pitting. UV paint contains solvents that cause swelling in thermoplastic materials; it is necessary to immediately expose it to the IR and UV ranges after spraying, unlike other paints which require a period of cooling before drying. (6) Small bubbles are generated. Visually, the painted surface features large transparent spots that float on the top of the paint film. This phenomenon is more noticeable during cold and humid winter periods; we believe that low temperatures may cause the components within the UV paint to separate. Raising the temperature or stirring can help eliminate some of these spots, while changing the paint generally resolves the issue. It seems that the causes are complex, including the presence of water vapor. (7) The product after leaving the UV section emits an unpleasant odor, clearly indicating that the UV paint has not cured completely; this can be felt by pressing it with the hand. One way to reduce this odor is to decrease the spraying thickness; if that doesn’t work, then the formulation of the prepolymer, initiator, reactive monomers, and additives needs to be adjusted. Although UV paint has developed rapidly, due to its wide adoption, once it cures, defects on the painted surface are difficult to rectify by using sandpaper. Moreover, unlike traditional primers, intermediate coats, and topcoats, the adhesion between layers in UV paint cannot be improved through sanding. UV paint is usually applied in a single coat, while some applications use a composite coating method that involves UV paint + vacuum coating + UV paint. At this point, UV paint often acts like a magnifying glass. It causes coating defects (such as pitting) to worsen further, making them difficult to overcome.
Aluminum silver paste is a highly unique effect pigment, which can be classified into floating type, non-floating type, electroplated silver, simulated electroplated silver, soft silver, delicate sparkling silver, medium sparkling silver, coarse sparkling silver, high sparkling silver, and strong sparkling silver. It is composed of very smooth and extremely thin scale-like structures; particles of small size offer high coverage, strong coloring power, and clear color intensity. By combining high density of fusion with high whiteness, it enables the surface coating to achieve an elegant, soft, mirror-like optical metallic effect. Classification of aluminum silver pastes: All aluminum pigments can be divided into two main categories: floating and non-floating. In the grinding process, replacing one fatty acid with another will result in aluminum pigments having completely different properties and appearances. Since the appearance of each type of aluminum sheet differs from that of other types, the technical parameters that affect the product formulation and end use also change depending on the desired effects. I. Floating type: In floating aluminum pigments, aluminum flakes are arranged in an orderly manner near the surface of the paint film, forming an opaque silver-colored layer. This opacity is the key factor that makes floating aluminum pigments important functional raw materials. They are generally used in roof coatings that require high levels of protection, as well as in maintenance coatings, anti-corrosion coatings, reflective coatings, coatings with a plated appearance, and commercial coatings. The appearance of floating aluminum pigments varies depending on the size of the particles, ranging from a rough, white appearance to a dull look, high reflectivity, or a mirror-like effect. Coarse-grade floating aluminum pigments have low coverage and a rough texture, but they enable the creation of the whitest and brightest coatings. On the contrary, fine-grained floating aluminum pigments have the highest coverage, the smoothest texture, and a highly reflective coating. II. Non-floating type Non-floating aluminum paste possesses excellent coloring properties and strong covering power, and is widely used in industrial coatings, automotive parts coatings, and decorative coatings ； Non-floating pigments are distributed evenly and parallel throughout the entire paint film, resulting in a coating appearance that differs from that of floating aluminum pigments. Since they can be mixed with various pigments and used in different systems, they offer unique advantages for industrial painting. However, any pigment or dye used with non-floating aluminum pigments must be transparent in order to achieve a realistic metallic effect. There are a wide variety of non-floating aluminum pigments, with varying levels of coloring power, particle sizes ranging from coarse to fine, and colors ranging from bright white to dull gray. Non-floating aluminum pigments come in various acid resistance levels, which can reduce the impact of adverse environmental factors such as industrial smoke and acid rain on coated surfaces.
I. Adhesion theory and mechanism: Adhesion arises when two objects are placed together such that there is close molecular contact at the interface, resulting in the formation of a new interfacial layer. Adhesion is a complex phenomenon that involves physical effects and chemical reactions at the “interface”. Since usually each observable surface is associated with several layers of physically or chemically adsorbed molecules, the exact number of interfaces is not known; the question is where to draw the boundary between the two surfaces and where the actual adhesion takes place. When the coating is applied to the substrate, adhesion is formed during the drying and curing process. The magnitude of these forces depends on the properties of the surface and the binder (resin, polymer, matrix). Broadly, these forces can be divided into two categories: primary valence forces and secondary valence forces (Table 1). Chemical bonds are covalent forces, which possess a much stronger adhesive strength than secondary forces; secondary forces are based on much weaker physical forces such as hydrogen bonds. These forces are more common on substrates with polar groups (such as carboxyl groups), while they are less frequent on non-polar surfaces such as polyethylene. Table 1: Strength of bonds and bond energy – Strength/Type/Energy (kcal/mol)/Examples. Covalent bond: Primary force, 15–170; applies to the vast majority of organic compounds. Hydrogen bond: Secondary force