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The Classification and Interaction of Paper and Ink Part B
Ink is a colorant used in printing, formulated as a fluid substance with specific viscosity. It consists of pigment particles uniformly dispersed in a binder medium.
1. Composition of Ink
Ink is primarily composed of pigments, binders, fillers, and additives.
(1) Pigment
Pigments are responsible for color development in ink and significantly influence its performance characteristics. They are finely divided, insoluble powders-colored, black, or white-that do not dissolve in water or organic solvents. Based on origin and chemical composition, pigments are classified into two main categories: organic and inorganic.
Inorganic pigments consist of oxides of non-ferrous metals or insoluble metal salts. These can be further subdivided into natural and synthetic inorganic pigments, with the former derived from mineral sources.
Organic pigments are colored organic compounds, also categorized as natural or synthetic. Currently, synthetic organic pigments are predominantly used due to their broad color range, excellent chromatic properties, and superior performance compared to inorganic counterparts.
Dyes are soluble organic compounds that dissolve in water or certain organic solvents. Insoluble colored precipitates known as lake pigments can be produced by treating specific dyes, and these are suitable for use in printing inks.
The selection of pigments for printing inks is subject to stringent requirements regarding color quality, dispersion stability, lightfastness, transparency, and other attributes. Ideal pigments should exhibit hues close to spectral colors with high saturation. Magenta, cyan, and yellow pigments used in three-color process printing must possess high transparency. Additionally, all pigments must demonstrate resistance to water, alkali, acid, alcohol, and various chemicals. Their oil absorption capacity should remain moderate to ensure optimal compatibility with the binder system.
(2) Binder
The binder constitutes the primary component of ink, serving to disperse pigments evenly while imparting appropriate adhesion, rheological behavior, and transfer efficiency. After printing, it fixes the pigment onto the substrate through film formation. The binder is commonly referred to as "varnish" or "ink vehicle."
Binders may be formulated from a variety of materials, including drying vegetable oils, mineral oils, solvents, water, and synthetic resins-all of which can serve as base components for different ink systems.
The rheological properties, viscosity, acidity (acid value), color, water resistance, and overall print performance of ink largely depend on the binder. By varying the binder type, the same pigment can yield distinct ink types. Conversely, using different pigments with the same binder results in inks belonging to the same category, as the fundamental characteristics are dictated by the binder. Therefore, ink quality depends critically on both pigment and binder, with the latter playing a dominant role.
(3) Filler
Fillers are white, transparent, semi-transparent, or opaque powdery substances used primarily to occupy volume within the ink formulation. Strategic use of fillers allows for reduced pigment consumption, cost optimization, and adjustment of ink properties such as thickness and flow behavior. Furthermore, fillers enhance flexibility in formulation design.
(4) Additives
Additives are auxiliary substances incorporated during ink manufacturing or application to enhance specific performance characteristics. When an ink formulated with basic components fails to meet required specifications under changing conditions, small quantities of additives are introduced to address deficiencies.
Common additives include drying agents, anti-drying agents, diluents, viscosity reducers, plasticizers, and others.
2. Classification of Ink
Inks can be categorized in multiple ways based on various criteria:
(1) By Printing Method
i. Based on printing technique: inks for letterpress, offset, intaglio, gravure, screen printing, etc.
ii. Based on printing process: direct printing, indirect printing, etc.
(2) By Substrate
Inks are classified according to the material being printed, such as paper, metal, plastic, fabric, and others.
(3) By Drying Mechanism
i. According to drying principle: penetration drying, oxidative polymerization, evaporation drying, photopolymerization (UV curing), thermal curing, and cooling solidification.
ii. According to drying method: air drying, hot air drying, infrared drying, ultraviolet curing, and cold setting.
(4) By Ink Characteristics
i. By color: yellow, red, blue, white, black, gold, silver, metallic, fluorescent, pearlescent, etc.
ii. By function: magnetic ink, security (anti-counterfeiting) ink, edible ink, foaming ink, aromatic ink, recording ink, etc.
iii. By durability: lightfast, heat-resistant, solvent-resistant, abrasion-resistant, alcohol-resistant, chemically resistant, etc.
(5) By Composition
i. By raw materials: drying oil-based, resin-oil-based, solvent-based, water-based, paraffin-based, ethylene glycol-based, etc.
ii. By physical form: paste (gel), liquid, and powder inks.
(6) By Application
Based on end-use: newsprint ink, book ink, packaging ink, construction material ink, trademark ink, etc.
3. Key Properties of Ink
Ink is a colored, viscous fluid with controlled flow characteristics, capable of adhering to substrates and undergoing drying after printing. Its most critical properties include color, rheology, and drying behavior.
(1) Viscosity
Viscosity refers to the internal resistance of a fluid to flow, reflecting molecular interactions that impede relative motion. In printing, ink viscosity influences ink transfer and interacts with paper structure. Excessively high viscosity leads to uneven ink distribution, paper linting, and mottling; excessively low viscosity may result in emulsification and soiling, compromising print quality.
Ink viscosity is influenced by the binder's inherent viscosity, pigment and additive concentration, particle size, and dispersion quality.
Optimal viscosity levels depend on printing speed, paper porosity, and ambient temperature.
(2) Yield Value
The yield value is defined as the minimum shear stress required to initiate flow in a fluid. Inks with high yield values exhibit poor flowability and difficulty in spreading, whereas those with low yield values tend to blur or produce indistinct dots.
This property is closely related to ink structure and is a key indicator for evaluating the performance of offset and gravure inks.
(3) Thixotropy
Thixotropy describes the reversible change in ink consistency when subjected to mechanical agitation: the ink becomes less viscous (thinner) under shear force and regains its original viscosity upon standing.
Due to thixotropic behavior, ink flows more readily on rotating rollers, facilitating transfer. Once transferred to the substrate and free from shear, it thickens again, preventing dot gain and ensuring sharp image reproduction. However, excessive thixotropy hinders ink release from the fountain and disrupts uniform ink distribution.
(4) Fluidity
Fluidity denotes the ability of ink to flow under its own weight. This property affects handling, pumping from storage to the ink fountain, and even distribution across rollers and plates.
Fluidity is determined by viscosity, yield value, thixotropy, and temperature.
(5) Ink Filament Length
Ink filament length refers to the extent to which ink can stretch into continuous filaments without breaking. Short filaments are preferred in offset and letterpress printing to minimize misting or flying ink. Inks with appropriate filament length produce uniform, dense ink films and are often used as indicators of print performance quality.
Filament length correlates with thixotropy, yield value, and plastic viscosity.
(6) Drying Behavior
After deposition on the substrate, ink transitions from a liquid or semi-solid state to a solid film-a process known as drying. This transformation occurs through changes in the binder phase, which may involve penetration, evaporation, oxidation, or polymerization.
Different ink formulations employ distinct drying mechanisms depending on binder composition. Upon transfer to the substrate, part of the binder penetrates the surface, while solvents evaporate. Simultaneously, chemical reactions increase the viscosity and hardness of the ink film until a durable, solid layer forms.
Typically:
- Letterpress inks dry mainly via penetration;
- Offset inks rely on oxidative polymerization;
- Gravure inks, containing highly volatile solvents, dry primarily through evaporation.
4. Characteristics of Specific Ink Types
(1) Letterpress Ink
Letterpress inks include book and periodical inks, rotary press inks, color inks, plastic inks, flexographic (rubber relief) inks, water-based inks, and newspaper inks.
Book and periodical inks are designed for flatbed presses and are characterized by moderate viscosity and yield value. Given the porous nature of typical book papers, these inks utilize penetration drying mechanisms.
Color letterpress inks, often used in copperplate printing, require excellent dot reproduction, rapid setting, and resistance to spreading. They typically employ binders that cure through oxidative film formation combined with partial penetration, ensuring fast fixation and good transfer.
Newsprint rotary inks are formulated for high-speed printing and thus require high fluidity and low viscosity. These inks are predominantly penetration-drying, relying on rapid absorption into fibrous paper substrates.
Rotary book and periodical inks are used at intermediate speeds between flatbed and newsprint presses. To prevent issues such as poor drying, powdering, and smudging when printing on coated papers, thermosetting inks are employed. During printing, the ink is heated to 200–250°C, volatilizing solvents and enabling immediate adhesion.
(2) Offset Ink
Offset inks encompass general-purpose offset inks, tinplate inks, photosensitive tinplate inks, collotype inks, and thermosetting inks.
A key characteristic of offset inks is high tinting strength, necessary because offset printing produces thin ink films due to its indirect transfer mechanism. With increasing press speeds, good fluidity and rapid drying are essential. Moreover, offset inks must exhibit strong water resistance due to the dampening system used in offset lithography. Pigments must resist dissolution or migration in water, and binders should avoid hydrophilic emulsification, which could impair ink transfer and drying. Resin-based binders generally offer better water resistance than oil-based alternatives.
(3) Gravure Ink
Gravure inks include photogravure inks, engraved intaglio inks, and plastic film inks.
Photogravure ink is a typical example of volatile-drying ink, exhibiting the lowest viscosity among all ink types. Its low surface tension and high fluidity allow complete filling of recessed cells on the gravure cylinder. Strong adhesion ensures efficient ink transfer from cell to substrate, followed by rapid and complete drying.
Engraved intaglio ink is thick yet non-tacky, with a loose, short texture and suitable thixotropy. It must easily fill engraved cells, be cleanly wiped from non-image areas, and transfer effectively to the substrate. After printing, the ink should retain sharp detail without dot gain and dry quickly. Due to its application in security printing (e.g., banknotes), this ink demands pigments with exceptional lightfastness, water resistance, heat resistance, and oil resistance. Special additives are often included to deter counterfeiting.
(4) Screen Printing Ink
Screen printing inks include standard screen inks, plastic inks, oil-based transfer inks, and water-based transfer inks.
Since screen printing involves applying ink through a mesh using a squeegee under pressure, resulting in thick deposits, the ink must have a thick, short, and loose consistency with minimal tack. Pigment concentration may be reduced to facilitate passage through fine meshes.
For oxidation-drying screen inks, rapid post-printing drying is desirable. Evaporation-drying types often require heat-assisted drying to accelerate solvent removal.
Transfer inks typically exhibit low fluidity and viscosity and rely primarily on penetration drying.
(5) Functional Inks
Microencapsulated inks contain functional materials enclosed in microcapsules and dispersed in a suitable binder. Printing does not rupture the capsules, preserving functionality until triggered. Examples include:
- Liquid crystal inks, whose color changes with temperature or pressure, used in thermometers and electronic displays;
- Aromatic inks, encapsulating fragrances released upon capsule rupture;
- Foaming inks, incorporating foaming agents that expand when heated, used in Braille printing and textured graphics.
Metallic inks, such as gold and silver inks, substitute metal powders (e.g., brass or aluminum flakes) for conventional pigments. These powders are mixed into the ink shortly before printing to maintain luster and performance.
Fluorescent inks utilize fluorescent pigments made by dispersing fluorescent dyes in synthetic resins. They produce vivid, eye-catching effects and are widely used in advertising, packaging, and exhibitions. However, they have coarse particles and limited lightfastness, achieving maximum brilliance under ultraviolet illumination.
Magnetic ink contains magnetic iron oxide particles. By controlling magnetic properties, characters printed with this ink can be read by specialized detection equipment. It is commonly used for encoding information on credit cards and checks.
Security (anti-counterfeiting) inks are used in the printing of valuable documents and must exhibit outstanding resistance to light, heat, water, and oil. Some variants incorporate compounds that react under specific stimuli (e.g., UV light, solvents), enabling verification or revealing tampering. Other types include disappearing inks, color-changing inks, and thermochromic inks.
Additional functional inks include gas-sensitive inks (color change upon exposure to gases), thermochromic inks (temperature-responsive), photochromic inks (light-activated color development), edible inks (for food labeling), and ignition inks (used on matchbox striking surfaces).
