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The gloss of printed matter is significantly influenced by the physical and chemical properties of paper, particularly its surface smoothness, ink absorbency, and inherent gloss. These characteristics are determined by multiple factors during paper manufacturing, including fiber type, pulping degree, calendering process, filler and sizing agent composition, and coating techniques.
1. Surface Smoothness of Paper
Surface smoothness refers to the time required for a specific volume of air to pass through the gap between a defined area of paper and a glass plate under standardized pressure, measured in seconds (s). This parameter varies across paper types: papers with smoothness below 80 s are classified as low-smoothness; those between 80 s and 300 s as medium-smoothness; and those ranging from 300 s to 600 s as high-smoothness. For example, coated paper typically exhibits a smoothness of approximately 800 s, with an average surface void size of about 2.5 μm. In China, domestically produced 150 g/m² premium double-sided coated paper has a smoothness of around 500 s, whereas imported equivalents of the same basis weight can exceed 1000 s. High-grade domestic art-coated paper achieves smoothness values between 1000 s and 1500 s, representing the highest standard currently attainable in the country.
Smoothness directly correlates with surface micro-topography. Higher smoothness ensures more uniform contact between ink and paper, facilitating complete dot reproduction and enhancing the final print's gloss. Upon ink transfer, a smoother substrate enables the formation of a more planar ink film, which promotes specular reflection when illuminated, thereby increasing perceived brightness. Conversely, rougher surfaces result in uneven ink distribution, leading primarily to diffuse reflection and diminished luster in the finished product.
2. Ink Absorbency of Paper
Ink absorbency denotes the paper's capacity to absorb ink binders. Generally, rougher papers with larger inter-fiber pores exhibit higher absorption rates. The absorbency varies significantly among different paper grades.
Excessive ink absorbency leads to rapid penetration of the binder into the paper matrix, leaving pigment particles concentrated on the surface. This results in diffuse reflection and reduced gloss. In contrast, low absorbency allows the binder to remain near the surface, promoting oxidation-based drying and forming a continuous ink film that enhances color vibrancy and gloss. However, excessively low absorbency may lead to delayed drying, set-off, and ink crystallization.
For coated papers, two primary factors govern ink absorbency: first, the type and quantity of adhesive used in the coating layer. A lower adhesive-to-pigment ratio increases porosity and thus raises ink absorbency. Additionally, if the coating slurry concentration is too low, adhesive and water may penetrate into the base sheet, reducing adhesive content in the coating and further increasing absorbency. Second, the nature and fineness of the pigment particles influence absorbency. Finer pigments form denser coatings, minimizing binder migration into the substrate. Coarser pigments with larger interstitial spaces increase absorbency and impair the gloss of the printed output.
Ink absorbency is quantitatively assessed using a calibrated ink suction apparatus. A standardized amount of test ink is applied to the sample sheet. After a specified interval, excess ink is removed with cotton or a soft lint-free cloth. The resulting ink mark intensity correlates with the paper's absorbency and is measured using a reflectometer. Lower reflectance indicates higher absorbency, while higher reflectance suggests lower absorbency. Results are typically expressed as a percentage (A).
3. Inherent Gloss of Paper
The intrinsic gloss of paper also contributes to the overall gloss of printed materials. In coated papers, this property depends largely on the use of gloss-enhancing additives-such as aluminum stearate and paraffin-and advanced finishing processes like supercalendering. Supercalendering compresses the paper structure, yielding a highly smooth and reflective surface.
The combined effect of paper gloss and ink absorbency is quantified as surface efficiency, defined as the arithmetic mean of gloss and the complement of absorbency:
Surface Efficiency = [(100 − A) + G] / 2
where A represents the ink absorbency (as a percentage), and G denotes the paper's gloss value (also as a percentage). This formula enables comparative evaluation of different paper types. As shown in relevant data tables, coated papers generally exhibit higher surface efficiency than offset papers, and imported coated papers often outperform their domestic counterparts.
Coated papers are designed with relatively low absorbency to promote surface drying via oxidation and polymerization, minimizing binder penetration and preventing defects such as poor gloss, powdering, and image instability. When printing on highly absorbent papers, it is advisable to increase ink film thickness and printing pressure to achieve adequate density. However, this may lead to tonal compression and loss of gradation. Furthermore, prints produced on four-color presses tend to exhibit slightly higher absorbency compared to those from single-color presses due to wet-on-wet printing conditions, which require faster initial penetration to enhance ink viscosity and improve overprint receptivity.
To achieve optimal print quality characterized by vibrant colors and high gloss, it is essential to select paper with high surface efficiency.
