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The Classification and Interaction of Paper and Ink Part A
I. Paper
Paper, one of the four great inventions of ancient China, is a sheet-like fibrous material primarily used for writing, printing, drawing, or packaging. It is typically manufactured from a water-based suspension of plant fibers that have undergone pulping, followed by forming on a wire mesh, initial dehydration, pressing, and drying.
1. Composition of Paper
Paper consists of plant fibers, fillers, sizing agents, and colorants.
(1) Plant Fibers
Plant fibers constitute the primary structural component of paper. Suitable fibers for papermaking must exhibit favorable characteristics such as ease of separation during pulping, high cellulose content, low lignin content, adequate strength, appropriate length and width, sufficient elasticity, and strong inter-fiber bonding capacity. Additionally, they should be abundant, cost-effective, and suitable for large-scale production.
Commonly used plant fibers in China include rice straw, wheat straw, reeds, bamboo, wood, flax, cotton, and recycled materials such as waste cotton, rags, hemp waste, and waste paper.
During pulping, plant fibers are processed to remove non-cellulosic components such as lignin, pectin, resin, and fats, while retaining desirable constituents like cellulose and hemicellulose. The physical and chemical properties of the selected fibers, along with the pulping method employed, significantly influence the final characteristics of the paper.
(2) Fillers
Due to the porous nature of fiber networks in paper, fillers are added to reduce porosity, enhance smoothness, decrease transparency and absorbency, improve surface uniformity, and meet functional requirements.
Common fillers include kaolin, talc, gypsum, calcium carbonate, and barium sulfate. Talc is typically used in standard printing papers, whereas kaolin and barium sulfate are preferred for high-quality printing grades.
Filler content generally constitutes approximately 20% of the paper's weight. Excessive filler can impair tensile strength and flexibility, hinder ink absorption, and lead to powdering during printing.
(3) Sizing Agents
Sizing compounds are applied to reduce surface porosity and capillary action, thereby enhancing water resistance. They also contribute to improved surface gloss, mechanical strength, and resistance to surface fuzzing.
Common sizing agents include rosin, aluminum sulfate, alum, starch, sodium silicate (water glass), and casein.
Depending on the intended application, various sizing methods-such as internal sizing, surface sizing, heavy sizing, and light sizing-are employed. The dosage typically ranges from 0.25% to 9% of the pulp weight. Over-sizing may negatively affect ink receptivity.
(4) Colorants
Although plant fibers possess inherent coloration, even after bleaching they often retain a slight yellowish or greenish tint, which is insufficient for producing pure white paper. Therefore, colorants are introduced to adjust hue and enhance whiteness.
For white paper production, dyes such as magenta and ultramarine blue are commonly used. High-grade papers may also incorporate fluorescent brightening agents.
In the manufacture of colored papers, both inorganic pigments and organic dyes are utilized depending on the desired shade and performance requirements.
2. Types of Paper
Paper varieties are numerous. According to usage, the former Ministry of Light Industry of China classifies them into 17 categories: 11 types of paper and 6 types of paperboard.
The 11 paper categories include: printing paper, writing paper, drawing and illustration paper, electrical insulating paper, cigarette paper, absorbent paper, instrument paper, photosensitive paper, transfer paper (base paper), industrial technical paper, and packaging paper.
The six paperboard categories are: bookbinding board, boxboard, insulating board, industrial technical board, construction board, and shoeboard.
3. Paper Specifications
Paper specifications are defined by type, dimensions, and weight.
(1) Type
Printing paper is available in two forms: sheet form and roll form. Web (roll) paper is designed for high-speed rotary presses, while sheet paper is more commonly used in conventional printing processes.
(2) Dimensions
According to the original national standard GB147-59, the base paper dimensions for printing, writing, and drawing are specified as follows: Roll widths are standardized at 1575 mm (2×787), 1092 mm, 880 mm, and 787 mm. Sheet sizes include 880×1230 mm, 850×1168 mm, 880×1092 mm, 787×1092 mm, 787×960 mm, and 690×960 mm.
Per the updated national standard GB786-87, untrimmed single-sheet formats of 880×1230 mm, 900×1280 mm, and 1000×1400 mm are adopted to align with international standards and facilitate cultural exchange. However, due to existing equipment and supply constraints, the 787×1092 mm format remains permissible as a transitional non-standard size until the year 2000, after which it will be gradually phased out. This transition reflects the shift toward harmonization with international paper size conventions.
(3) Weight
Paper weight is expressed in terms of basis weight and ream weight, most commonly as grammage.
Basis weight, or grammage, refers to the mass per unit area, measured in grams per square meter (g/m²). For example, 60 g/m² denotes a paper weighing 60 grams per square meter. Papers with a basis weight of 200 g/m² or less are classified as paper; those exceeding this threshold are categorized as paperboard.
Ream weight indicates the total weight of 500 sheets of a given size. For instance, a ream of 52 g/m² paper measuring 880×1230 mm weighs approximately 28 kilograms.
4. Printing Papers
Printing papers encompass a range of types tailored to specific applications, including newsprint, letterpress paper, offset paper, coated offset paper, dictionary paper, map paper, gravure paper, art paper, white board paper, and synthetic paper.
(1) Newsprint
Primarily used for newspapers and letterpress-printed periodicals, newsprint is soft, elastic, and highly absorbent, allowing rapid ink adhesion. Calendering produces a smooth surface resistant to linting, ensuring clear and solid double-sided prints. It possesses adequate mechanical strength for high-speed rotary printing and good opacity. However, due to its high mechanical wood pulp content and residual impurities, it is susceptible to yellowing and embrittlement over time. Its high absorbency and poor water resistance make it vulnerable to damage.
(2) Letterpress Printing Paper
Used for printing anthologies, classics, textbooks, and magazines via letterpress, this paper shares similarities with newsprint but offers superior quality. It exhibits higher smoothness, water resistance, and whiteness. While its ink absorption is slightly lower than newsprint, it is more uniform.
(3) Offset Printing Paper
Designed for offset printing of color picture books, albums, posters, trademarks, and premium publications, offset paper is available in single-sided and double-sided variants. Single-sided paper is used for items like cigarette boxes and labels, while double-sided paper suits illustrations, maps, and similar outputs.
Offset paper must exhibit low elasticity and high water resistance to prevent dimensional changes and misregistration during multi-color printing. It should be free of lint and dust, have a compact structure to resist picking under repeated impression, and moderate ink absorbency to allow binder retention on the surface for proper drying and gloss development. A smooth, contaminant-free surface ensures print integrity and protects printing blankets and plates.
(4) Coated Offset Printing Paper
Also known as coated paper, this is produced by applying an inorganic coating (e.g., clay or calcium carbonate) onto base paper, followed by super-calendering. The resulting surface is exceptionally smooth and highly reflective, ideal for reproducing fine details in art prints, catalogs, calendars, and product samples.
Coated paper enables precise dot reproduction and excellent tonal gradation. The surface must resist powdering and delamination. Ink absorption should not be excessive; otherwise, prints may lack luster or develop powdering issues.
(5) Dictionary Paper
Used for printing dictionaries, pocketbooks, reference works, and scientific literature, dictionary paper is lightweight, ranging from 25 to 40 g/m². It requires high opacity to prevent show-through, uniform fiber distribution, smoothness, consistent thickness, and soft texture. However, its edges are prone to curling.
(6) Map and Nautical Chart Paper
Topographic map paper is used for multi-color offset printing of maps and atlases, classified into Special Grade and No. 1 Grade-Special for topographic maps, No. 1 for general maps and atlases. Nautical chart paper is similarly used for maritime charts.
Both types are white, low in dust (especially free of black particles longer than 1 mm), smooth on both sides, tightly structured, dimensionally stable, and capable of reproducing fine details accurately. They exhibit high folding endurance and, in the case of nautical chart paper, exceptional tensile strength.
(7) Gravure Printing Paper
Used for intaglio printing of art books, illustrations, and picture magazines, gravure paper must be strong, white, and highly smooth with excellent water resistance. It should resist linting, powdering, and show-through during printing.
Originally, intaglio paper included substrates for high-security prints like banknotes and stamps. These have since been classified separately as banknote paper and stamp paper.
(8) White Board Paper
Used for packaging boxes, educational charts, and decorative backing, white board paper features a uniform fiber structure. The surface layer contains fillers and sizing agents, and is coated with pigment and calendered. This results in a dense, smooth, white surface with consistent ink absorption, minimal linting, and high fold durability.
(9) Synthetic Paper
Used for high-end art prints, maps, and reference books, synthetic paper is made from hydrocarbon-based chemical raw materials with additives. It exhibits excellent tensile strength, water resistance, thermal stability (-60°C to 60°C), resistance to light, mold, and chemical corrosion.
Synthetic paper is non-toxic, pollution-free, and dust-free, making it suitable for cleanroom environments. It does not shed fibers or powder, rendering it ideal for office use and computer printing in sensitive settings. It is increasingly replacing traditional paper in information technology applications.
5. Printing Performance of Paper
The printing performance of paper determines its suitability for smooth operation during printing and its ability to produce high-quality output. This performance is influenced not only by paper characteristics but also by printing conditions and ink properties.
General quality requirements for printing paper include:
- High and uniform whiteness within the same batch
- Minimal dust content
- Consistent translucency and gloss
- Adequate mechanical strength for processing
- Uniform thickness, density, and structure across batches
- Moisture content between 6% and 8%
- Square edges for sheet paper, with edge deviation no greater than ±3 mm
Key printing properties include:
(1) Smoothness
Smoothness is a critical printing property. Higher smoothness ensures better contact between the paper and printing plate, enabling uniform ink transfer and sharp image definition. In contrast, rough surfaces result in uneven contact and inconsistent ink deposition.
While offset printing uses an elastic blanket that mitigates some surface irregularities, high-quality reproduction still demands smoother paper. For lower-smoothness papers, increased printing pressure can partially compensate for surface unevenness.
Surface smoothness also directly influences gloss: smoother surfaces reflect more light and appear glossy, while rough surfaces scatter light and appear matte.
(2) Ink Absorbency
Ink absorbency refers to the extent to which paper absorbs ink, particularly the penetration of the ink's binder into the fiber network. It depends on pore size and paper compactness.
Papers with small inter-fiber gaps exhibit limited capillary action and thus lower absorbency. Conversely, overly porous papers absorb excessive binder and pigment, leading to ink penetration and translucent prints.
Ink absorbency is affected not only by paper structure but also by ink viscosity, printing pressure, and dwell time.
(3) Elasticity and Plasticity
During printing, paper undergoes deformation under mechanical stress. In letterpress, the raised image areas compress the paper, causing localized deformation.
Paper may exhibit three types of deformation under load:
- Instantaneous elastic deformation: immediate recovery upon removal of force.
- Hysteresis elastic deformation: gradual return to original shape after force removal.
- Plastic deformation: permanent change in shape after force removal.
The first two are reversible; the last is irreversible. Fiber elasticity, especially instantaneous elasticity, supports effective printing performance.
These behaviors depend on fiber composition, moisture content, calendering, and compaction.
(4) Surface Strength
Surface strength determines resistance to wear, powdering, and fiber picking during printing. High-viscosity inks require strong paper surfaces to avoid shedding, which can foul printing plates. In offset printing, weak surfaces may lead to emulsification of ink with dampening solution, causing scumming on non-image areas.
(5) Moisture Content
Moisture content is defined as the percentage of water weight relative to the total paper weight.
It significantly affects print quality. Excess moisture reduces strength, increases plasticity, and slows drying. Insufficient moisture makes paper brittle, prone to breakage, and susceptible to static electricity.
Cellulosic fibers are hygroscopic, so paper moisture equilibrates with ambient temperature and humidity. A 10% change in relative humidity typically alters paper moisture by about 1%. To maintain stability, printing environments should be controlled at 18–24°C and 60–65% relative humidity.
