Discussion on Pretreatment Technology of Wood Product Powder Coating

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.

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.

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.

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.

Overview 

The surface powder coating technology of wood products has been developed for more than 20 years.

 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.

(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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