Paper smoothness and roughness

Definition

Paper smoothness and roughness are the key parameters to describe the geometric structure of paper surfaces, and they complement each other. Smoothness usually refers to the degree to which the paper surface is close to the ideal plane under certain conditions, while roughness characterizes the amplitude and frequency of microscopic unevenness on the surface. These characteristics directly affect the printing quality, coating uniformity, writing feel and packaging performance, and are important indicators for papermaking process control and terminal application evaluation.

Measurement principles and methods

The measurement of smoothness and roughness is mainly based on the principle of contact and non-contact. In the contact method, the commonly used air leakage method is used to determine smoothness: the paper specimen is placed on a glass anvil under a specific pressure and the time it takes for a certain volume of air to pass through the gap between the paper surface and the glass. For roughness measurement, a stylus profiler is mostly used, which records the profile curve through the probe across the surface, and then calculates parameters such as arithmetic average roughness (Ra). Non-contact methods such as optical interferometry and laser scattering are used to invert surface topography by analyzing light signals and are suitable for fragile or high-precision samples.

Technical standards

The International Organization for Standardization (ISO), the American Society for Testing and Materials (ASTM), and the Chinese National Standard (GB) have developed relevant test standards to ensure the comparability and accuracy of measurement results. Here are some common criteria:

Influencing factors

The smoothness and roughness of the paper are affected by a combination of factors. The type of fiber raw material, the degree of beating, the amount and distribution of fillers, the chemical conditions of the wet section, and the calendering process parameters will all significantly change the surface structure. For example, high-beating fibers can improve fiber bonding and reduce surface porosity; Uniform filling of fillers such as calcium carbonate helps reduce roughness; The linear pressure, roll surface temperature and speed of the calender directly determine the flatness of the final surface. In addition, changes in ambient temperature and humidity may cause the paper to absorb moisture and expand, affecting the measurement stability.

Data processing and characterization

In data processing, roughness parameters are usually calculated based on contour curves. Arithmetic mean roughness (Ra) is the most commonly used characterization quantity, defined as the arithmetic mean of the absolute distance from each point to the midline on the contour curve, expressed as:

Ra = (1/L) ∫₀^L |y(x)| dx

where L is the sampling length, and y(x) is the deviation of the contour curve from the midline. Smoothness data is often directly reported in seconds (s) or converted to a specific smoothness index. Mean and standard deviation are calculated for multiple measurements to assess repeatability.

In the printing industry, insufficient smoothness can lead to uneven ink transfer, loss of dots, or dull colors. If the roughness is too high, it is easy to cause transmission or loss of gloss. In the packaging sector, smooth surfaces facilitate uniform coverage of barrier coatings and improve barrier performance. Therefore, during the production process, it is necessary to set the smoothness and roughness control range according to the end use, combined with online monitoring and laboratory sampling to ensure batch consistency. It is recommended to establish an internal quality control chart to track the change trend of parameters after process adjustment.

At present, measurement technology is developing in the direction of high-throughput, non-contact, and online real-time monitoring. The multi-sensor fusion system can simultaneously obtain multi-dimensional data such as smoothness, roughness and gloss. Surface defect detection and topography reconstruction technology based on machine vision is gradually applied to the production line. However, the correlation of data between different measurement principles, the standardization of characterization methods for complex structured papers (e.g., coated paper), and environmental factor compensation algorithms are still topics that need to be studied. In the future, data analysis models combined with artificial intelligence are expected to improve prediction accuracy and process regulation efficiency.

References

ISO 5627:1995, Paper and board — Determination of smoothness.

ISO 8791-4:2007, Paper and board — Determination of roughness/smoothness — Part 4: Print-surf method.

ASTM D3285, Standard Test Method for Smoothness of Paper.

GB/T 456-2002, Determination of smoothness of paper and cardboard.

Paulapuro, H. (2000). Paper and Board Grades. In Papermaking Science and Technology.