Base paper is an unprocessed, foundational paper material made from pulp; it is also referred to as raw paper or paper stock. It is primarily composed of plant fibers, with common raw materials including wood, reeds, bamboo, bagasse, rice and wheat straw, cotton, and hemp. Plant fibers consist mainly of cellulose, lignin, and hemicellulose, with cellulose serving as the fundamental structural component of the paper.
The physical properties of base paper directly or indirectly influence print quality. Key physical performance indicators include basis weight, thickness, density (tightness), stiffness, smoothness, and dirt count. Basis weight refers to weight per unit area and affects physical properties such as tensile strength, tear resistance, and stiffness. Thickness is related to basis weight and density, with the tightness of the fiber structure being the determining factor. Density (tightness) refers to the weight per cubic centimeter of base paper, measuring the compactness of its structure. Smoothness refers to the surface topography (degree of unevenness), which affects the reproduction of halftone dots and ink consumption. Dirt count measures the presence of impurity spots on the paper surface. Stiffness indicates the paper's resistance to indentation by another object and reflects the coarseness and rigidity of the fibrous structure.
Water absorbency is a critical property of base paper; measurement methods include capillary absorption rate, surface absorption rate, and Cobb water absorption. For sanitary base paper, beating can enhance the intrinsic water-absorbing capacity of the fibers; however, fiber fibrillation and the generation of fines lead to a denser structure and reduced porosity, thereby diminishing water absorption performance. Creping increases the paper's bulk and absorption capacity but reduces its strength. A pressure-free drying process preserves the bulk of sanitary paper, enhancing its absorption performance. While embossing may adversely affect the absorption of single-ply sanitary base paper, multi-ply lamination creates void spaces between layers, significantly improving absorption performance.
The production of traditional *ciqing* paper (dark blue-dyed paper) requires the mulberry-bark base paper to have a water absorption rate of ≥25 g/m²·s. Base paper for digital thermal transfer printing requires a thickness tolerance of 0.05–0.1 mm and a surface roughness of ≤3 μm. For base paper intended for coating, the ash content is controlled within the 8–15% range, whereas uncoated printing base paper requires a whiteness of ≥82% ISO. Food-grade base paper must comply with the GB 4806.8-2022 food safety standard, with heavy metal migration limits set at ≤3.0 mg/kg for lead and ≤1.0 mg/kg for arsenic.
Base paper is classified into Grade I, Grade II, and Grade III based on the level of processing. Ring crush strength is a key grading criterion for corrugating medium; for instance, the requirements are ≥4.5 kN/m for Grade A, ≥3.5 kN/m for Grade B, and ≥2.5 kN/m for Grade C.