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    Home > Coatings News > Paints and Coatings Market > Discussion on Pretreatment Technology of Wood Product Powder Coating

    Discussion on Pretreatment Technology of Wood Product Powder Coating

    • Last Update: 2021-05-07
    • Source: Internet
    • Author: User
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    Authors: Zhou Guanghao, Zhang Kai, Wang Yijian, Zhong Jinhuan/Wuyuan Science and Technology Academician Workstation;

    Hu Honghui, Zhang Haocong/Wuyuan Technology Group·Hangzhou Wuyuan Technology Industry Co.
    , Ltd.


    Purpose: Applying the principle of functional composite material preparation, discussing the pretreatment interface theory of wood products coating, and designing a water-based two-component infrared reflective thermal insulation based on organic coupling agent according to the protection requirements of the macro and micro interface of wood products.
    The coating is used as the base coating of wood products.
    In order to obtain normal temperature powder electrostatic spraying and ensure the uniformity of the coating, a water-based conductive composite coating is applied on the basis of the base coating.


    Test: Using the microwave rapid drying method, the moisture content of wood products/MDF wood-based panels can be kept constant, and the formaldehyde emission of MDF wood-based panels can be reduced.
    After sanding the composite coating, the surface roughness of the coating is 3-8μm, which is suitable for powder electrostatic spray
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    Use the pull method to determine the adhesion test of the coating and the substrate, and the ultrasonic coating thickness test method to test its thickness.
    The results show that the thermal insulation effect of the water-based primer is 15℃ lower than that of the ordinary water-based paint/back cover powder, and it can keep the substrate surface temperature <100℃, avoiding blistering and cracking of the powder coating on the substrate, and the surface resistance of the conductive coating is 104- 7Ω, the powder coating is evenly coated, which avoids the uneven distribution of surface moisture on the surface of the preheating method that causes changes in ambient temperature and humidity, which affects powder spraying and coating quality problems.
    Coating adhesion experiments show that under the action of 3Mpa tensile force, the coating and the substrate peel off (substrate cohesion failure).


    Conclusion: Using infrared reflective heat insulation water-based primer, the powder is thermally cured within 160℃×8min, the substrate temperature can be ensured <100℃, and the substrate moisture content is constant 6-8%, which can prevent the wood product substrate from evaporating and inducing powder coating The blistering/pinhole/cracking phenomenon during the curing of the layer, the water-based conductive intermediate coating has good performance, and meets the requirements of powder electrostatic spraying.
    Therefore, the use of water-based infrared reflective heat insulation and conductive composite coating can meet the production technical requirements of wood product powder spraying pretreatment .


    Overview 


    The surface powder coating technology of wood products has been developed for more than 20 years.
    Because wood products are heat-sensitive substrates, the dielectric constant of dry wood is affected by various factors such as moisture content, density, texture direction, and frequency.
    The yield rate has a great influence.
    The powder coating of heat-sensitive substrates is a systematic engineering technology, involving materials (substrates/coatings), processes (pre-painting and powder spraying and curing processes), equipment (intelligent control), and ecological environment control (three wastes) Treatment) and other influencing factors.
    In recent years, many companies have invested in MDF wood-based panel furniture powder coating production lines.
    Due to the phenomenon of orange peel, cracks, bubbles, pinholes, uneven coating, thick edges, and exposed bottoms in the coating during powder electrostatic spraying, the overall curing degree is not easy, and Low-temperature powder (<130°C) storage problems have caused the finished product rate of wood product powder coating projects to be 30-60%, and investment enterprises have unstable production and serious losses.
    During the implementation of the project, it was believed that “the future is bright, and the road is too tortuous”.
    After market research, this article believes that the success rate of powder coating of wood products, and the pretreatment factors of powder coating have an impact on the qualification rate of products accounted for more than 60% (see Table 1).
    This article attempts to explore and discuss the pretreatment of wood products powder coating.

     Technical Principle 


    Protection and modification of wood macro interface

    The purpose of powder coating on wood surface is 

    1) Protect wood from environmental changes, pests and diseases, and use surface coating to improve hardness and wear resistance;

    2) The beautification and decoration products prevent the long-term release of formaldehyde in the MDF board, which is harmful to human body and mind health;

    3) Coating with environmentally friendly powder can achieve the goal of zero emission of three wastes.


    Functional composite material preparation and coating system engineering (see Table 2)

    According to the above process technology analysis, the core technology of the two methods lies in the interface problem between the substrate and the surface coating.


    Basic Theoretical Analysis of the Interface Between Wood and Coating

    (1) The macro interface of wood

    According to "Lumber Science", only arbor wood, including coniferous wood (also called soft wood) and hard wood (also called hard wood), a living tree is composed of three parts: crown, trunk, and root.
    The canopy is composed of leaves and branches, which account for 5-25% of the standing timber volume, and the roots also account for 5-25% of the standing timber volume.
    The remaining part is the trunk, which accounts for 50-90%.
    Usually the source of wood is the trunk.
    Therefore, the base material of wood products The three major characteristics of the product are porosity, hygroscopicity and anisotropy.
    MDF wood-based panels are used as a substitute for two kinds of natural wood, but due to the use of adhesive resin in the molding process, it has disadvantages such as formaldehyde release.


    (2) Micro-interface analysis of wood

    When studying the structure of wood, observe through a microscope any part of the wood is composed of many honeycomb-shaped small chambers, these small chambers are called cells, various cells, whether they are needles with tracheids as the main components Wood or hardwood with wood fiber as the main component, all of their properties are related to cells.
    Therefore, only with the understanding of the microstructure of the cell wall can we grasp the nature and changing laws of wooden substrates.
    The influence of moisture in wood plays a very important role in powder coating.


    (3) Moisture of wood

    The moisture in wood is mainly composed of free water, absorbed water and combined water.
    The moisture in the cell cavity is called free water, which affects the weight of the wood, but does not affect the properties of the wood; the moisture present in the cell wall is called absorbed water , Its change causes the wood to shrink and expand; the moisture in the chemical composition of the wood is called compound water, and this moisture rarely does not change during the drying process.
    When the moisture content of the wood reaches below the fiber saturation point, the strength of the wood will increase with the decrease of the moisture content.
    When the moisture content of the wood is above the fiber saturation point (fsp), the conductivity of the wood is constant and the resistance is basically constant; When the moisture content is below fsp, the conductivity of wood is positively correlated with the moisture content.
    The current powder spraying process uses the moisture control of the preheated substrate to determine the surface resistance.


    (4) Principle of interface between substrate and primer

    The surface of wood products is rough and porous.
    It is affected by wood fibers and pores during paint coating, which can easily induce defects such as collapse and blistering.
    Therefore, the surface of wood products must be pretreated before painting.
    The temperature and humidity of the environment in which the wood products are used have a great influence on the moisture content of the wooden substrate.


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    [6]Zhong Jinhuan, Yang Yan, Wang Yijian.
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