1. Introduction of whiteboard paper

Coated whiteboard paper is a type of whiteboard paper with a multi-layer structure and at least one side coated. It generally consists of five layers: coating layer, surface layer, lining layer, core layer, and bottom layer.

Coated whiteboard paper is divided into single-sided coated gray background whiteboard paper, single-sided coated white background whiteboard paper (also divided into yellow core and white core), and double-sided coated whiteboard paper. The main focus of this article is on the coatings of gray based whiteboard paper - duplex board paper.

※ Paper pulp: Duplex board paper is a white board paper that has been coated and processed on one side, and is also a 5-layer structure. The surface layer pulp is generally made of bleached commercial pulp or white paper edges; The lining pulp is generally made of deinked waste paper pulp; The core layer pulp generally uses mixed waste pulp; The bottom pulp is generally made of deinked waste paper pulp or a mixture of corrugated cardboard and newsprint pulp.

2. Introduction of coatings for duplex board paper

Duplex board paper coating is composed of pigments, adhesives, additives, and water. The proportion of pigments in dry coatings is approximately 70% to 85% by mass. Next is adhesive, and the amount of adhesive used in the coating is about 10% to 25% (mass percentage) of the pigment content. Additives, also known as coating auxiliary agents, are used in small amounts in coatings but have a significant effect. They are used to improve the performance of coatings or to meet various characteristic requirements during coating process.

2.1 Pigments

There are many types of pigments used for paper coating, but the most commonly used pigments for gray coated whiteboard paper are kaolin and calcium carbonate. Below is a brief introduction to the performance and requirements of these two raw materials.

2.2.1 Kaolin - Flaky kaolin with particle size less than 2μm

Kaolin is divided into two crystal forms based on the number of crystalline water. One is a hexagonal sheet containing two crystalline water; Another type contains 4 crystalline water and is tubular in shape. For coating paper coatings, the coating made of flake kaolin has good fluidity. After being applied to the paper surface and calendered, it can be arranged parallel to the paper surface, resulting in coated paper with high smoothness, glossiness, and opacity.

The particle size of kaolin is very important for the production of coated paper, with a particle size less than 2 μm, the more of it, the better the smoothness, whiteness, glossiness, and opacity of the coated paper. Fine particle size will produce higher glossiness than coarse particle size after calendering. Therefore, the quantity of the coated grade kaolin has a particle size less than 2 μm should reach around 80%.

2.2.2 Calcium carbonate - Ground Calcium Carbonate

The CaCO₃ used in pigment coated paper includes ground calcium carbonate (GCC) and light calcium carbonate (PCC). The calcium carbonate used for duplex board paper is GCC, but different layers use GCC with different quality requirements. GCC is generally used for coating the base layer(pre-coating) and middle layer of whiteboard paper. The closer to the top layer, the higher the quality requirements for GCC. Results indicate that when the proportion of the GCC particle size between 0 and 2 μm is over 60% (quantity ratio), and the dosage of GCC is below 30%, the effect on glossiness is very small. When the component of GCC particle size is between 0-2m accounts for more than 95% (quantity ratio), even when used at a relatively high ratio, heavy calcium carbonate will not significantly reduce the glossiness of the coated paper.

2.2 Adhesives

The commonly used adhesives for duplex board paper include carboxylated butadiene styrene latex (SBR Latex) and modified starch. SBR latex has little effect on the viscosity of coatings, and can produce coatings with high solid content and good fluidity. SBR latex has good film-forming properties, and the printed coating paper produced has high smoothness and glossiness. Therefore, the adhesives currently used for coating white board paper are mainly carboxyl SBR latex or SBR latex used in combination with modified starch. The use of modified starch is twofold: firstly, to improve the water retention of the coating; secondly, to reduce the cost of the coating.

2.3 Additives

Additives used for pigment coatings include dispersants, water repellents, lubricants, water retention agents, pH regulators, and whitening agents, etc.

2.3.1 Dispersant - Sodium polyacrylate(PAAS)

The main function of adding dispersants to coatings is to prevent coating condensation and sedimentation, improve the fluidity of coatings, and enhance the mixing of pigments and adhesives. The commonly used dispersant in duplex board coatings is sodium polyacrylate.

2.3.2 Water repellent - Polyamide polyurea resin (PAPU).

Water resistant agent is an important coating additive that can reduce the water solubility of pigments and adhesives after drying into films, improve the wet friction and pulling strength of coated paper, improve the printing suitability of coated paper, and reduce quality problems such as shedding of fur and powder. Currently, the most commonly used water resistant agent is polyamide polyurea resin (PAPU).

2.3.3 Lubricant - Calcium stearate

The main function of lubricants is to improve the smoothness and fluidity of wet coating surfaces by reducing interfacial tension, making them easy to flow and spread on the surface of paper, improving the coating adaptability, and improving the coating surface by endowing dry coatings with plasticity and slip properties to better adapt to calendering. They also improve the finished performance of coatings by reducing or eliminating fuzzing and powder shedding (such as improving the smoothness and glossiness of finished paper). The most commonly used lubricant in current coatings is calcium stearate.

2.3.4 Water retaining agent - CMC

When the water retention of the coating cannot meet the process requirements, a water retention agent can be added to improve the water retention of the coating. Currently, the most commonly used water retention agent is carboxymethyl cellulose (CMC).

4. Required chemicals for each coating procedure of duplex board paper

Here share with you the coating chemicals preparation for three coating procedures of duplex board paper: pre-coating,middle coating, and top coating.

Please refer to the following diagram.

For paper coating equipment solutions or any issues during coating process, please feel free to consult Qingdao Jieruixin Machinery and Technology Co., Ltd