This article describes methods for determining the absorbency of paper and cardboard inks for white or near-white paper for lithographic, gravure and letterpress printing. The ink absorption performance of paper is evaluated by measuring the change in the reflectance factor after the paper absorbs ink for a specified period of time. The article also covers the calibration methods of related instruments, such as ink absorptivity Testers and reflectance photometers, and describes the steps for determining air permeability.
definition
Ink absorbency: Refers to the ability of paper and cardboard to absorb standard inks for a specific period of time. It is manifested as the difference in the reflectance factor of the surface before and after the ink is applied. The absorption performance of the paper to the ink is quantified by calculating the change in the reflectance factor before and after the ink.
Ink absorption time refers to the time it takes from the time the ink is applied to the time half of the ink area is wiped off. This time reflects the rate at which the paper adsorbs and absorbs the ink.
principle
Ink absorption performance is evaluated by measuring the ink absorption of paper and cardboard over a defined period of time. The specific method is to apply standard ink to the surface of the paper, and after the specified absorption time, the ink is removed with ink polish. Then, the ink absorption performance of the paper is reflected by measuring the change of the reflectance factor on the surface of the sample. The absorption performance of ink is directly related to the pore structure, surface treatment and other factors of the paper, and the diffusion and permeability of the ink on the paper determine its final absorption value.
instrument
The ink absorption Tester is used to evenly coat the ink on the specimen of the specified size, and wipe off the unabsorbed part to ensure the uniformity and thickness consistency of the ink coating on the surface of the specimen.
Reflectance photometers are used to measure the reflectance factor of paper before and after ink is applied to evaluate the surface change after ink absorption.
The chronograph stopwatch is accurate to 0.1 seconds and is used to measure the ink absorption time, ensuring that the time of the test process is accurately controlled.
calibration
Ink table calibration – The speed of wiping is measured using a chronograph with a resolution of 0.1 seconds to ensure that it is in the range of 15.5±1.0 cm/min.
Ink Absorption Time Verification – Checks the accuracy of the automatic control mechanism to ensure that the absorption time meets the set standards.
Inked Platen Thickness Calibration – Measure the thickness of the inked Platen to ensure that it is in the range of 0.10±0.02 mm.
Center Hole Size Verification – Measure the size of the center hole of the inked platen to ensure that the hole area is 20±0.4 cm².
Automatic Timer Verification – Use a chronograph stopwatch to verify the automatic timer to ensure its accuracy, with an average value of 120±5 seconds.
Test procedure
Samples are taken according to standard methods and processed under standard conditions.
A reflectance photometer is used to determine the green light reflectance factor before the surface of the specimen is coated with ink.
Evenly coat the ink with an ink absorption Tester and wipe off the unabsorbed ink at the specified time.
After the ink is applied, the reflectance factor of the ink on the specimen is determined using a reflectance photometer.
Result calculation
Ink Absorption Value Calculation – Calculate the ink absorption value of each specimen and calculate the arithmetic mean and coefficient of variation of the 5 results.
Ink Absorption Index – Calculate the ink absorption index to exclude the effect of paper brightness on ink absorption results.
Air permeability determination
Air permeability refers to the average air flow per unit area of a sample per unit time and unit pressure difference. Usually measured in cubic meters / Pascal · seconds (m³/Pa · s) or millimeters / (square meters · Pascal · seconds) (mm³/(m² · Pa ·)) indicated.
Instrument – The main instrument for measuring air permeability is the Schauber gas permeability meter
Glass container – used to create a vacuum environment with a deflation valve and a water fill valve to control the flow of water.
Specimen Clamping Device – consists of a pressure ring clamp, a pressure ring holder, and an air chamber to hold the specimen and measure air flow.
Differential pressure gauge – used to measure the differential pressure between the two sides of a specimen, usually in a U-shaped tube type.
Stopwatch – accurate to 0.1 second, used to record time.
How it works
Sandwiched between the air chamber and the pressure ring, a vacuum is created by controlling the flow of water inside the glass container, which pushes air through the specimen. The air flow through the specimen is measured to calculate the air permeability. Changes in airflow are recorded by the flow rate of water.
calibration
Tightness calibration, the instrument is leveled, filled with water, and the specimen is replaced with a rubber liner. Adjust the differential pressure to 1.0 kPa (100 mmH₂O) or 2.5 kPa (250 mmH₂O) and check whether the differential pressure is stable within half an hour to ensure a good seal.
Instrument calibration, check the accuracy of the differential pressure gauge and the overall function of the instrument, and ensure that the calibration value meets the standard.
Experiment preparation
Specimens are obtained according to standard methods.
The specimen is treated under standard temperature and humidity conditions to ensure the accuracy of the test.
The specimen is cut into standard dimensions (60 mm x 100 mm) and marked with front and back.
Test procedure
Close the drain valve and needle valve, open the water filling valve and the air release valve, and fill the glass container with water.
The specimen to be tested is placed between the pressure ring and the air chamber and the specimen is clamped.
The differential pressure was adjusted to 1.00±0.01 kPa in 30 seconds, and the test time was recorded using a stopwatch.
Measure the air flow rate of the breathable specimen and select different test times as needed to ensure that the reading deviation does not exceed 2.5%.
For high-tightness paper and cardboard, if the air flow through the specimen is less than the standard value, the differential pressure can be increased to 2.0±0.01 kPa and the corresponding test time can be selected.
For specimens with a sample thickness of more than 0.3 mm, the edges should be sealed to prevent side air intake from affecting the test results.
Result calculation
The air permeability of each specimen is calculated, and the results are derived from the measurement data and formulas. Air permeability is usually calculated by dividing the volume of air flowing through the specimen per unit of time by the specimen area and differential pressure.
summary
This article elaborates on standard methods for the determination of the absorptivity of paper and cardboard inks, including specimen handling, instrument use, and calibration procedures. The importance of determining the absorption performance of inks is reflected in their impact on print quality, especially in different printing processes. With the correct use of ink absorption Testers and reflectance photometers, the ink absorption capacity of the paper can be accurately assessed. In addition, the method of measuring air permeability also provides supplementary information for the evaluation of paper properties, ensuring the stability and reliability of the paper in practical applications.
