All paper grades possess their own specific technical specifications, which are primarily determined by factors such as the raw materials, manufacturing processes, and equipment used to produce the base paper. For instance, the technical specifications for kraft paper include: basis weight, thickness, cross-direction thickness variation, whiteness, opacity, surface absorbency, breaking length, cross-direction folding endurance, smoothness, cross-direction dimensional stability, print surface strength, moisture content, density (bulk), bursting strength, ring crush strength (index), folding endurance, tearing strength, and so forth.
Generally, while paper manufacturers establish technical specifications for their kraft paper, few enterprises in the actual printing industry are able to effectively correlate these measured data points with their specific printing requirements. Instead, they blindly prioritize price and the final printed result, completely overlooking the fundamental nature and inherent properties of the product itself. Consequently, sales representatives in the market are often unable to provide printing manufacturers with comprehensive solutions tailored to their specific needs. It is crucial to recognize that kraft base paper is defined by a distinct set of technical specifications and physical performance parameters.
Basis Weight: This refers to the weight of the paper per square meter, typically measured in grams per square meter (g/m²). The basis weight must be uniform; otherwise, the paper stack will tilt or warp, preventing proper feeding and printing. Furthermore, non-uniform basis weight directly compromises the paper's smoothness.
Stiffness: The fibers within the paper determine its density and porosity (inter-fiber spacing). Bagasse pulp offers good toughness and medium-to-long fibers; bamboo pulp provides excellent stiffness with relatively long fibers; wheat straw pulp features high porosity with medium-to-long fibers; and acacia wood pulp consists of fine, short fibers. Coniferous pulp is characterized by high porosity and long fibers. The formation of paper can be likened to the construction of a building: coniferous pulp serves as the steel reinforcement, while other pulps act as the cement and sand. The cohesion between the "cement and sand" depends on the degree of porosity between them; otherwise, this cohesion must be achieved through sizing agents and pulp refining processes. Consequently, adequate stiffness allows the paper to perform optimally on high-speed printing presses-provided, of course, that the paper's overall uniformity is consistent.
Whiteness: Manufacturers adjust the whiteness-specifically its hue-to meet the varying requirements of their clients; however, a higher whiteness level is not necessarily always better. Whiteness does not have a significant direct impact on the mechanical printing performance of the paper itself. From a printing perspective, however, the paper's whiteness directly influences the color reproduction and aesthetic quality of the final printed product. Therefore, whiteness is regarded as one of the most critical parameters among the various properties of paper. As the name implies, "whiteness" simply refers to the degree of purity and brightness of the paper's white appearance. It represents a material's ability to reflect light waves across the entire visible spectrum. Currently, my country predominantly assesses the whiteness of paper by measuring its "brightness" (also referred to simply as "whiteness").
However, this standard brightness metric relies solely on the paper's light reflectance values and does not take into account the visual characteristics of the human eye; this is significant because perceived whiteness-how white the paper *looks* to an observer-is a physiological combination of color purity and true reflectance. Due to the addition of fillers and dyes during the manufacturing process, the standard brightness metric can no longer accurately reflect the whiteness as it is visually perceived. Consequently, the international community is increasingly adopting the concept of "visual whiteness" to characterize paper's degree of whiteness; since the measurement of visual whiteness is grounded in the visual characteristics of the human eye, it provides a more accurate representation of how white the paper actually appears. For instance, consider two sheets of paper: Sheet A measures a standard brightness of 70, while Sheet B measures 68. Theoretically, Sheet A should appear whiter than Sheet B; however, it is quite possible that Sheet B will *look* whiter than Sheet A. This discrepancy is often attributed to the addition of chemical additives-specifically "optical brighteners" (such as ultramarine blue)-during the paper-making process; these additives can *enhance* the visual perception of whiteness without actually *increasing* the paper's intrinsic reflectance value. Paper with high brightness levels reflects nearly all incident light, resulting in sharper, more vivid color reproduction on printed materials. For "cultural papers" (such as writing and printing papers), a certain level of brightness is required; however, it is not the case that "the brighter, the better." Paper with excessively high brightness can appear glaring and harsh to the eyes, potentially causing visual strain.
Thickness: Paper thickness refers to the caliper measurement relative to the paper's basis weight (weight per square meter). It is defined as the distance measured between two parallel plates-under a specific, standardized pressure-with the paper sample placed between them. (Testing Instrument: Model PY-H606A Paper Thickness Tester). Many printing plants, when purchasing kraft paper, tend to use thickness as their primary criterion. For example, they might assume that if a sheet of 70-gram paper measures a thickness of 85 microns (µm), then any paper measuring 85 microns during the printing process must necessarily be 70-gram paper. This assumption, however, is incorrect. Manufacturers often adjust the paper's "bulk" (the ratio of thickness to weight) to suit the specific type of printed product being produced; consequently, a 65-gram paper can sometimes be manufactured to possess the same thickness as a 70-gram paper. Therefore, the appropriate specifications must be determined through effective communication and collaboration between the end-user (the printer) and the supplier. Provided that the paper's basis weight (grammage) remains consistent across the web width, variations in thickness generally do not have a detrimental impact on printing performance. Thickness Cross-Directional Variation: This refers to the variation in paper thickness across its width (in the cross-direction). If this value is excessively high, it indicates uneven paper thickness; this may lead to creasing or folding during printing, or in extreme cases, prevent the paper from passing through the printing press at all.
Density: The weight of the paper per cubic centimeter. Increasing paper density enhances its tensile strength and burst strength; however, excessive density leads to two drawbacks: first, it reduces the paper's opacity; and second, it compromises the paper's bulkiness, resulting in a poor tactile feel and reduced compressibility. Since paper thickness is rarely perfectly uniform, and the contact pressure between the printing plate and the paper varies during the printing process, print quality is inevitably affected. Consequently, paper that is relatively soft, elastic, and highly compressible consistently produces printed results with sharp impressions and distinct tonal gradations.
Smoothness: Smoothness is primarily the result of surface treatment processes. Paper that has undergone surface sizing and subsequent soft calendering typically achieves a smoothness rating of 35 seconds or higher. A smooth surface also helps to minimize surface dusting or linting during printing. The degree of smoothness significantly impacts the fidelity of halftone dot reproduction; higher smoothness results in finished prints with vivid, lifelike colors. Conversely, paper with a surface smoothness rating below 20 seconds is prone to printing defects such as dot gain (spreading), ink bleeding, and show-through (ink penetrating to the reverse side).
Opacity: Opacity is defined as the ratio of the reflectance of a single paper specimen placed over a "completely absorbing" black backing to the reflectance of a stack of specimens thick enough to be completely opaque. Simply put, it measures the degree to which ink "shows through" the paper. For printing paper, high opacity is essential to prevent show-through-ensuring that ink applied to one side does not penetrate to the reverse side-thereby preserving the clarity of the text or images printed on the opposite side. Writing paper also requires a certain level of opacity to facilitate writing on both sides of the sheet. For both printing and writing papers, the general requirement is: the higher the opacity, the better.
Surface Absorbency: This refers to the paper's capacity to absorb water or other liquid substances. The surface absorbency should fall within a specific range to facilitate the transfer and absorption of ink; if it is excessively high, the paper's water absorption capacity becomes too great, making it prone to deformation during printing.
Breaking Length: The length at which a sheet of paper or cardboard ruptures under its own weight; this metric indicates the paper's resistance to tensile breakage. This parameter is of critical importance for paper intended for web printing, as a higher breaking length helps the paper withstand the tensile forces exerted by the printing press.
