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    Home > Coatings News > Paints and Coatings Market > Analysis of the factors affecting the coating of electrostitive powder in MDF wood furniture.

    Analysis of the factors affecting the coating of electrostitive powder in MDF wood furniture.

    • Last Update: 2020-10-03
    • Source: Internet
    • Author: User
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    Abstract:
    Explains the difference between medium density fiberboard (MDF) and traditional metal substrate from the angles of surface resistance, moisture content and heat transmission efficiency, and compares the two kinds of plate warm-up modes, infrared and hot air circulation. Experiments show that the medium wave infrared wavelength can be matched with the reaction group in the powder coating, and the low temperature curing powder developed can achieve rapid curing of infrared 130 degrees C/5min on the MDF board, and the coating performance meets national and industry standards.
    Foreword
    Medium Density FiberBoard (MDF) is an artificial platewith a density of 0.5 to 0.8g/cm3, made from wood fibers or other plant cellulose as raw material, by applying acetaldehyde resin or other adhesives under high temperature and heat pressure. MDF plate internal structure is uniform, moderate density, good dimensional stability, small shape variables, large internal bonding strength, good grip, with excellent mechanical properties, making it more suitable for the production of a variety of complex wooden furniture products, is the most widely used materials for indoor wooden furniture.
    the current MDF sheet decoration process mainly has low/ high-pressure melamine patch, plastic film patch, single board patch, paint paper patch, paint spray. During use, free formaldehyde in the surface resin will be released, which will bring greater harm to the living and working environment. The International Agency for Cancer defines formaldehyde as a level 1 carcinogen, and in view of the increasingly stringent laws and regulations on VOC emissions, people's environmental awareness and desire for a healthy living environment, the desire for MDF furniture environmental protection construction is becoming more and more intense.
    recent years, because powder coatings have the advantages of nearly zero VOC emissions, coating, coating industry is a rapid development of the field, in the current national green environment, sustainable development in the context of its rapid development. However, the recommended construction temperature of ordinary thermoo-solid powder coatings is more than 160 to 200 degrees C, if such powder coatings are directly applied to thermal substrates such as MDF, plastics, electrical components, high temperature baking will cause difficult damage to the substrate. If the temperature of 140 degrees C, MDF internal materials will be damaged cracking, which is bound to limit the application of powder coatings. In recent years, with the successful development of low temperature curing (120 to 140 degrees C) powder coating, the innovation of IR curing method, MDF powder coating is in the rapid development stage of product life cycle.
    of MDF sheets
    powder coatings are used in the construction process of traditional metal substrates and have excellent process control levels. MDF is very different from metal substrates. Therefore, before the MDF powder electrostitive spray should be fully understood its characteristics, such as thermal conductivity, conductivity, water content, but also pay attention to the density of the plate, thickness, bonding strength, fiber type, batch stability. See Figure 1.
    1.1 MDF plate thermal conductivity
    Table 1 is a comparison of the thermal conductivity of MDF and other materials, the biggest difference between MDF and metal substrate is thermal conductivity, only 0.07W/m. K, equivalent to asbestos. We know that asbestos is an excellent insulation material and an undesirable conductor of heat. If the MDF is heated by conventional hot air circulation, it can take a considerable amount of time to reach the curing temperature required by the powder. Very low heat transmission coefficient leads to a large temperature gradient inside the plate during the heating process, the central temperature and surface temperature are very different, the heat is not equal, it is easy to cause MDF deformation, expansion, cracking.
    the water content and conductivity of the 1.2 MDF sheet
    resistance is a conductor, 10 to 10 is an insulator, and the conductivity and moisture content of MDF are associated, at room temperature, The surface resistance of the wet MDF plate is a conductor with good conductivity, and the surface resistance of the dry MDF plate is an insulator, which is not conductive, so the MDF is between the insulator and the conductor. In order to realize the electrostitive spraying of MDF board, it is the key to do the charge treatment of the plate. The literature states that if the water content of the plate is too high, the moisture volatilization is too large to cause the coating curing when pinholes, foaming and cracking and other phenomena, if the moisture content is too low, the wood surface resistance is too large, the powder difference, and the plate is susceptible to thermal deformation.
    Cure Mode and IR Matching
    The traditional hot air cycle heating method is to heat the inside of the furnace body slowly and evenly through the flow of hot air, and gradually transfer energy to the work piece, so that the work piece uniform, slowly increase the temperature to complete the powder curing process. The infrared radiation technology is to penetrate the surface of the powder synchronous heating powder inner layer, uniform and rapid internal and external layer synchronous heating powder, can effectively shorten the curing time. The low thermal conductivity characteristics of MDF determine that traditional hot air heating methods are not suitable for curing powder coatings.
    infrared (IR) is a segment of spectral wave that grows up in visible light and is divided into near-infrared, medium-wave infrared and far-infrared bands, as shown in Table 2.
    Based on infrared wavelength functional analysis, long-wave infrared heats the powder coating surface, short-wave infrared penetrates the powder coating to reach the MDF substrate, causing moisture inside the MDF to escape from the coating surface, affecting the appearance of the coating and causing the sheet to crack, and medium-wave infrared provides the energy needed for powder curing without damaging the substrate and matching the powder.
    according to the infrared absorption spectrum of the powder coating (Figure 4, Figure 5), the polyester resin at the base sealing end has a more obvious infrared absorption peak in the wave number range of 4000 to 400cm-1. For example, the feature absorption peak in spectral 4 is the telescopic vibration absorption peak of hydroxyl O-H at 3445.76cm-1, and the telescopic vibration absorption peak of methyl C-H at 2964.1cm-1. At 1508.91cm-1 is the bisphenol A structure, where the pair replaces the ring skeleton vibration absorption peak, and at 827.21cm-1 is the benzene ring on the opposite position to replace the curved vibration absorption peak outside the C-H surface. At 1508.91cm-1 and 827.21cm-1 are the characteristic absorption peaks of BPA epoxy resins.
    map 5, 3435.69cm-1 is the telescopic vibration absorption peak of hydroxyl O-H of end-pyride polyester resin; Upper C-H telescopic vibration absorption peak; 1722.98cm-1 is ester-based C-O telescopic vibration absorption peak; 1410.45cm-1, 13 77.03cm-1 is adjacent to two methyl bending vibration absorption peaks, such as neo-glycol; Shrinking vibration absorption peak; 1016.11cm-1, 727.74cm-1 is the ester-based benzene ring-to-position replacement absorption peak, so there is a benzoic acid structure.
    Figure 6 is an infrared map of the household polyester/epoxy-mixed powder coating, in which 828.44cm-1 and 1509.1cm-1 are the characteristic absorption peaks of the powder coating, when the infrared within the wave number When the radiation spectral frequency corresponds to the absorption band of the atomic group in the powder coating, the radiation is directly acting on the chemical bonds, which will absorb infrared radiation energy, resonance occurs, resonance enables rapid accumulation within the molecule, and accelerates the intersection of epoxy-based fracture and the production of niobine in polyester resin. Table 3 is the absorption wavelength of a common group.
    construction process
    the construction process of powder coating on MDF sheet is roughly As shown in Figure 7. Before the charge treatment should be treated, wood pre-treatment and metal substrate such as phosphation, passivation and other chemical treatment, should choose the appropriate number of sandpaper on the surface and edge of the MDF plate to polish, and then blow clean with an air gun, to ensure that the surface of the plate without sand particles, fiber residue, and focus on the edge of the plate, sharp corner grinding, increase the powder on the edge of the powder.
    According to the reference, the internal moisture of the MDF plate preheat treatment will evaporate to the surface of the plate, reducing the resistance of the surface of the plate, thereby increasing the conductivity of the plate and making the powder easier to spray. The MDF sheet was compared with IR100C/2min and hot air circulation 100C/2min respectively, of which 0 to 2min was the warm-up stage and 2 to 12min was the cooling stage. Figures 8, 9 and 10 are the test data for plate surface resistance (GB/T1410-2006), surface temperature and surface moisture content (GB/T1931-2009) after different warm-up methods.
    From the above test results, IR warm-up heat transfer is faster, the model surface within 2min reached 100 degrees C, but the water content of MDF surface did not change significantly, maintained between 7% to 7.5%, the surface resistance from 10 to 12 to 10 x 10.
    the surface temperature of the MDF sheet was warmed up by hot air convulsive circulation, the surface temperature of the plate gradually decreased from 26.5 to 40.2 degrees C to 28.6 degrees C;
    will be infrared warm-up and hot air cycle preheat 2min after the MDF model to do electrostitectrectral spray testing, in the hook and corner area film thickness is thin, there is a clear shielding powder phenomenon, the surface of the model film thickness is uneven, large fluctuations. The MDF is electrically treated by preheating, and two repeated heatings can cause damage to the MDF sheet and are not recommended.
    MDF powder coating performance detection
    after the use of water-based conductive liquid treatment, MDF plate surface resistance reduced to 10 to 6 s, MDF plate electrostitive spray powder coating, by infrared 130 degrees C /5min curing, according to the standard film performance detection, performance indicators as shown in Table 4, comprehensive performance and paint basically consistent, can meet the general application of the furniture industry.
    In conclusion
    compared with liquid coating or melamine veneer paper, the static spraying of powder coating on MDF board can effectively solve the release of VOC and formaldehyde in the process of furniture coating and finished product use, and is a truly environmentally friendly coating method. Because the medium wave infrared wavelength matches the powder coating, the powder coating can be completely cured without damaging the substrate. Reasonable pre-treatment of the plate to the MDF plate moisture content and electrical control is necessary. MDF powder spraying can be commercial production, powder coating performance is only a small part of it, in MDF electrostitive powder spraying broad market prospects, powder coating manufacturers hope to be able to and sheet suppliers, spray equipment suppliers, furniture production terminal enterprises strong joint, jointly promote the healthy development of furniture coating.
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