In the coatings and printing ink industries, viscosity is a critical parameter. It not only reflects the liquid’s thickness but also governs application, surface finish, and adhesion. A proper understanding of this helps control production processes and ensures the final product’s quality.
WHAT IS VISCOSITY?
Viscosity is the resistance of a liquid to flow under force. In fluid mechanics, it is defined as the ratio of shear stress to shear rate. The unit is poise, more commonly centipoise. Water has about 1 cP, while printing inks can reach thousands of cP.
It is not simply a physical value. Viscosity shows the flow behavior and spreading ability of a liquid in different conditions. In coatings and inks, even a small change can alter application results and surface appearance completely.
VISCOSITY TYPES AND FLOW BEHAVIOR
Newtonian and pseudoplastic
Newtonian fluids have viscosity that does not change with force. Water and organic solvents are typical examples. In contrast, many inks and coatings are pseudoplastic. Their viscosity decreases under shear, making them easier to apply and disperse. This explains why solvents flow easily, while inks and coatings need force to spread. When designing formulations, balance is required to allow easy handling and stable film structure. This directly affects the film’s durability.
Thixotropic
Thixotropy describes viscosity decreasing under long shear and recovering when at rest. This property is crucial in coatings. It prevents sagging on vertical walls while still allowing smooth application. Some specialty inks also require thixotropy to maintain sharpness. Thixotropy further helps improve pigment suspension during storage. Under stirring, the system becomes fluid for easy use, then thickens again when idle. This ensures coatings and inks remain stable for extended periods.
IMPACT OF VISCOSITY ON COATING & PRINTING QUALITY
For coatings
Viscosity defines application performance. If too high, coatings are difficult to spray or roll, leaving rough surfaces. If too low, coatings may drip or run. A proper level ensures smooth, even films. Viscosity also affects adhesion. Good wetting allows coatings to bond firmly to substrates. Moreover, thixotropic behavior prevents sagging on walls or ceilings.
For printing inks
In inks, viscosity directly influences ink flow and image clarity. If viscosity is not appropriate, prints may blur or smear. Higher viscosity helps maintain detail but can make printing difficult. Lower viscosity allows fast spreading but reduces sharpness. Therefore, proper control of viscosity is vital to ensure print quality. In practice, each printing method requires tailored viscosity to optimize speed and precision.
VISCOSITY EXAMPLES AND RECOMMENDED LEVELS
Spray coatings require 50–200 cP for fine atomization. In roll coating, suitable viscosity is 500–2000 cP for even spreading. For anti-drip wall coatings, higher levels are needed, about 3000–15000 cP, with strong thixotropy.
Screen printing inks usually have very high viscosity, from 5000 to 50000 cP. This maintains ink layer thickness, image sharpness, and durability. In offset or flexographic printing, viscosity is adjusted according to ink type and solvent system for optimal results.
VICHEM – AUTHORIZED DISTRIBUTOR OF CERTIFIED CHEMICAL ADDITIVES IN VIETNAM
Vichem is an authorized distributor of chemical additives with full legal compliance in Vietnam. We not only provide high-quality products but also deliver international-standard after-sales services. Our services include:
- Technical consulting: Supporting customers in choosing the right product samples for their intended use.
- Color matching tests: Our experts conduct color matching trials to ensure results meet design standards.
- Application support: Assisting customers during production integration to ensure optimal performance.
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