Solvent-based ink refers to an ink system that uses organic solvents or solvent-based polymers as the carrier to disperse pigments or dyes, forming an ink mixture. These inks offer high image quality durability and relatively low raw material costs. However, the evaporation of solvents can lead to environmental pollution, and solvent-based inks are still in a phase of rapid development. In response to environmental concerns, a new type of ink, known as ECO-solvent ink (low-solvent ink), has been introduced. This ink causes less environmental harm and is considered the future direction of solvent-based inks.
Categories of Ink Based on Print Technology:
Inkjet inks are categorized based on their ink base, which includes water-based inks, solvent-based inks, and UV inks. Solvent-based inks use non-water-soluble solvents as the main component to dissolve colorants, which can either be pigments or dyes. The composition of solvent-based inks typically includes colorants, solvents, surfactants, moisturizers, pH regulators, drying agents, metal ion chelators, preservatives, and other additives. Below is a brief introduction to the key ingredients: colorants and solvents.
1. Colorants
Common colorants for inkjet inks include dyes and pigments. The amount of colorant typically makes up 0.5% to 10% of the total ink volume, with the most suitable range being between 0.5% and 5%. Both dyes and pigments can be non-ionic, cationic, anionic, or mixtures of these. Chemically speaking, dyes exist as single molecules in the ink, while pigments are composed of aggregates of pigment molecules.
Using dyes as colorants offer advantages such as ease of preparation, low cost, vibrant colors, full-color gamut, and reduced risk of clogging the nozzles. Dye-based inks typically perform better in color reproduction and can achieve a print quality comparable to silver salt photography on high-quality substrates. However, dye-based inks have poor stability, particularly in terms of lightfastness, storage stability, water resistance, and anti-oxidation. The individual dye molecules are chemically unstable under exposure to light, moisture, and oxygen, leading to color fading. Additionally, dye inks are often very picky about the printing medium and may require specialized substrates. On regular copier paper, which is made from interwoven paper fibers filled with a gelatinous substance, dye inks tend to spread along the fibers and cause blurring, which severely impacts print quality.
Pigment-based inks, developed to address the shortcomings of dye-based inks, consist of aggregates of pigment molecules. These inks are more stable, offering excellent lightfastness, water resistance, heat resistance, and anti-oxidation properties, making them less susceptible to weather changes. However, pigment inks have poorer liquid stability and may cause nozzle blockages due to pigment dispersion issues during long-term storage. Based on the advantages and disadvantages of both types of colorants, it can be concluded that dye-based inks excel in color vibrancy and uniformity, making them suitable for indoor prints, while pigment-based inks are more durable and better suited for outdoor applications. Solvent-based inks typically use pigments as colorants, due to both the properties of pigments and the market positioning of solvent-based inks, with dye-based solvent inks being relatively rare.
2. Solvents
Solvents are volatile organic liquids and are an essential component of solvent-based inks. They help regulate ink viscosity, surface tension, and drying properties. Once the ink is transferred onto the substrate, the highly volatile solvent evaporates quickly, while less volatile solvents seep into the substrate due to capillary action, solidifying the binder and colorants on the surface, thus drying the ink. Solvents serve as both true solvents for dissolving binders and polymers and as diluents or co-solvents to adjust ink properties.
Common solvents, classified by chemical structure, include the following types:
Aliphatic solvents: These have poor solubility, a mild odor, and are inexpensive (e.g., petroleum ether, gasoline, kerosene, n-hexane).
Aromatic solvents: These have good solubility but are highly toxic and have a strong odor (e.g., benzene, toluene, xylene).
Esters: These have excellent solubility, a strong fruity odor, and are more expensive (e.g., ethyl acetate, butyl acetate).
Alcohols: These dissolve certain resins well, have a pleasant odor, and are moderately priced (e.g., ethanol, isopropanol, butanol, ethylene glycol).
Ketones: These have very strong solubility, but their odor is unpleasant (e.g., acetone, cyclohexanone).
Ethers: These have strong solubility for certain resins, a mild odor, and antifreeze properties but are more expensive (e.g., ethylene glycol ether, butylene glycol ether).
Mixed solvents: These are blends of two or more solvents and can be adjusted to control the drying speed of the ink.
Solvents are further categorized by boiling point: low-boiling (below 100°C), medium-boiling (100–150°C), and high-boiling (150–250°C). Solvents play a crucial role in determining the viscosity, drying speed, and quality of the ink film. By selecting the right solvent, it's possible to balance drying speed and ink performance, as well as reduce production costs. Organic solvents are the primary component of solvent-based inks, with volatile organic solvents making up around 90% of inkjet inks.
