UV Ink and Its Formulation Analysis

UV ink, short for ultraviolet curing ink, is a type of ink that utilizes ultraviolet (UV) light to cure. The term "UV" refers to ultraviolet light, which is electromagnetic radiation with wavelengths ranging from 100 to 400 nanometers. Since the discovery of ultraviolet light by German scientist Ritter, UV light has found widespread applications in various industrial fields. Particularly in the printing industry, UV ink is rapidly cured upon exposure to UV light, making it suitable for a variety of printing technologies, especially in piezoelectric ceramic DOD inkjet printing, where it performs exceptionally well.

 

Main Components of UV Ink

UV ink is composed of several components, each playing a critical role in the curing process and final performance. The first key component is the polymerizable prepolymer, which is a film-forming substance that significantly affects the performance of UV coatings. It is the most fundamental ingredient in UV ink. The physical properties of the prepolymer influence both the curing process and the final properties of the cured film. Reactive monomers and oligomers form the foundation of the inkjet formulation, providing the fluidity necessary for smooth ejection through the inkjet printhead. The curing process of UV ink is primarily driven by free radical polymerization, which includes four steps: initiation, propagation, and termination.

 

Next, photoinitiators are essential in UV ink formulations. They absorb ultraviolet light and generate active intermediates that initiate the polymerization reaction. Photoinitiators are classified into hydrogen abstraction type and cleavage type. Hydrogen abstraction type photoinitiators generate free radicals by reacting with compounds containing active hydrogen, while cleavage type photoinitiators decompose into free radicals when exposed to UV light. To ensure optimal curing performance, the selection of photoinitiators must consider their light absorption efficiency at the UV light wavelengths and their stability.

 

Additionally, reactive diluents or photosensitive monomers play a crucial role in adjusting the viscosity, curing speed, and cured film properties of UV ink. These monomers typically have unsaturated double bonds, such as acrylate and methacrylate groups, which contribute to faster curing. The degree of functionality (single-functional or multi-functional groups) influences the curing speed and the final properties of the ink. While traditional acrylate monomers cure rapidly, they can be irritating to the skin, so modern formulations aim to reduce skin irritation through various methods.

 

Colorants in UV ink mainly consist of pigments and dyes. Pigments not only determine the color but also affect other properties of the ink. They are fine powder materials that are insoluble in water and organic solvents. Depending on their origin and chemical composition, pigments can be classified as organic or inorganic. Inorganic pigments are usually metal oxides or insoluble metal salts, while organic pigments are colored organic compounds. The particle size of the pigment is critical to the ink’s weather resistance, color range, gloss, and opacity. For inkjet printing, pigments must have a small particle size, typically less than 1 micron, to ensure smooth flow through the printhead and avoid clogging.

 

Furthermore, additives are essential in UV ink to improve its performance. Common additives include dispersants, defoamers, stabilizers, surfactants, and waxes. Dispersants ensure good wetting of pigments and prevent clumping or settling of pigment particles. Defoamers and surfactants improve the leveling properties of the ink, prevent pinholing, and enhance the ink's gloss. Waxes are used to modify the ink’s rheological properties, improve its water resistance, and enhance printability, while also forming a protective wax layer on the surface after curing to improve abrasion resistance. Additives also help reduce the oxygen inhibition effect, promoting better surface curing.

 

Common UV Ink Formulations

UV ink formulations are typically adjusted according to specific application requirements. Below are two common basic formulations:

 

Formulation A:

This formulation combines dipropylene glycol diacrylate, pentaerythritol tetrakis(3-mercaptopropionate), and pigment dispersants. Dipropylene glycol diacrylate (36.5%) and pentaerythritol tetrakis(3-mercaptopropionate) (35.5%) provide fluidity and proper viscosity while ensuring rapid curing. The pigment dispersant constitutes 21%, ensuring uniform pigment dispersion in the ink. Photoinitiators (7%) are added to ensure effective curing when exposed to UV light.

 

Formulation B:

Formulation B differs from A by using a multi-functional acrylate to ensure the cured film is more durable and rigid. This formulation also includes neopentyl glycol propoxylated diacrylate, methyl acrylate-based oligomers, and liquid photoinitiator mixtures to enhance curing performance. The formulation also contains pigment dispersants and other additives to ensure stability and adhesion during use.

 

Conclusion

UV ink, with its rapid curing properties, is widely used in modern printing technologies. Its formulation consists of several components, including polymerizable prepolymers, photoinitiators, reactive diluents, colorants, and additives, each of which plays a vital role in fluidity, curing speed, adhesion, and surface properties. Depending on specific requirements, the formulation can be adjusted to ensure the best performance, providing high-quality printed products with excellent durability and surface finish.