The viscosity of the surface sizing starch is one of the key parameters that affect the performance of paper. It directly influences the physical and chemical properties of the paper by regulating the distribution, penetration and film-forming characteristics of the sizing solution on the paper surface. The specific impacts and mechanisms are as follows:

The influence on the surface structure of paper

Coating uniformity and smoothness:：

Starch-based low-viscosity sizing: High fluidity, capable of forming a thin and uniform coating, reducing surface defects (such as spots and stripes), and enhancing smoothness;

Improve smoothness; Starch-based high-viscosity sizing: The coating is relatively thick, which may result in an uneven film layer, leading to an increase in surface roughness and affecting the printing suitability.

Penetration depth and fiber bonding​

Low-viscosity starch gel has strong permeability and can penetrate deep into the fiber network, enhancing hydrogen bond connections between fibers and increasing internal strength.

High-viscosity starch gel mainly forms a film on the surface, reducing internal penetration, but through hydrophobic groups (such as styrene) it forms a protective layer, enhancing surface strength.

The impact on the functional performance of paper

​Water resistance: High-viscosity starch sizing (such as cationic starch or modified starch or denatured starch, etc.) forms a dense film, effectively blocking water penetration and prolonging the water resistance of the paper.

Low-viscosity starch sizing (such as oxidized starch) forms a loose network after penetration, with relatively weak water resistance, but is conducive to the absorption of ink by the subsequent coating process.

Printing suitability: Ink absorption: Low viscosity sizing agents reduce surface pores and decrease ink penetration.

Wear resistance: High viscosity sizing agents form a dense film layer that can reduce feathering and powder shedding during the printing process.

Mechanical strength: Surface sizing agents bridge the fibers, enhancing the paper's folding resistance and tear energy absorption.

Impact on production process​

​Applying glue efficiency and energy consumption:

Low-viscosity glue has good fluidity, which can reduce the energy consumption during the gluing and pressing process.

High-viscosity sizing agents require heating or dilution, which increases the complexity of the process.

Glue Formulation Ratio and Cost​

Low-viscosity sizings can reduce usage while enhancing environmental sustainability and lowering costs. High-viscosity synthetic sizings often require combination with starch, which may increase raw material expenses.

Adaptability to the application scenarios of paper

Packaging Paper: High-viscosity sizing agents (such as AKD) are required to form a waterproof layer, preventing grease penetration.

​Cultural Paper: Low-viscosity sizing agents (such as oxidized starch) optimize printability by balancing strength and ink absorbency.

​Specialty Paper: Medium-viscosity sizing agents (such as chitosan) offer a balance between strength and breathability, making them suitable for label paper or medical packaging.

Techniques for controlling viscosity​

Chemical modification: Adjust viscosity through oxidation, cationization or graft copolymerization.

Temperature: Heating reduces the viscosity of the starch slurry.

Dilution: Add hot water (80-90℃) to adjust the viscosity of the starch adhesive and thereby regulate the fluidity of the coating liquid.

Equipment compatibility: High-viscosity adhesives require a high-pressure spraying system, while low-viscosity agents are suitable for immersion coating or roller coating.

The viscosity of surface sizing should be designed comprehensively based on the purpose of the paper, production process, and performance requirements.Summary: The viscosity of surface sizing should be designed comprehensively based on the purpose of the paper, production process, and performance requirements. Low viscosity focuses on uniformity and printing suitability, high viscosity enhances water resistance and mechanical strength, while medium viscosity balances both aspects.