Definition and basic concept of ink absorbency
Ink absorption refers to the ability of the surface of the substrate material to absorb a certain amount of ink within a specified time, which is one of the important physical indicators to evaluate the printing suitability of paper, cardboard and other printing materials. During the printing process, after the ink is transferred to the surface of the paper, its connector will penetrate into the paper, and the pigment will mainly remain on the surface to form an ink film. Ink absorption directly determines the drying speed of the ink on paper, the gloss of the printed matter, the wear resistance of the ink layer, and whether there will be quality problems such as transmission printing or powdering. The ink absorption value is usually expressed as a percentage, which reflects how quickly the material absorbs the ink connector. Excessive ink absorption may lead to reduced gloss of printed products, transparent printing and even powdering of ink film. Too low absorbency may cause slow ink drying, dirty back, or sticking of prints.
Physicochemical principles of ink absorbency
The essence of ink absorbency is the infiltration and adsorption process of complex multiphase fluids in porous media. From a physical point of view, paper is a porous mesh structure material made of intertwined plant fibers, and there are a large number of capillary channels of different shapes and sizes inside. When the ink is transferred to the surface of the paper, the low-viscosity connector in the ink begins to penetrate the paper along these capillaries, driven by capillary pressure. This process can be described using the classical capillary permeation model.
The penetration depth of the connecting material inside the paper is directly proportional to the square root of the penetration time, and its mathematical relationship can be expressed as:
L = (r·γ·cosθ·t / 2η)1/2
In the formula, L represents the penetration depth of the connector, r represents the capillary radius, γ represents the surface tension of the ink, θ represents the contact angle between the ink and paper, t represents the penetration time, and η represents the ink viscosity. This formula reveals the core mechanism of ink absorbency: the capillary structure of paper is the channel for absorption, the physical properties of ink are the driving force for absorption, and time is the regulating factor of absorption.
From a chemical perspective, the polarity and chemical composition of the surface of the paper fibers also affect the wetting and adsorption behavior of the ink. The hydroxyl group on the cellulose molecule gives the paper a certain lipophilicity, but the sizing agent added to the paper changes its surface energy, thereby regulating the penetration rate of the ink. Ink absorption is not just physical penetration, but also includes physicochemical interactions between the components of the connecting material and the fiber, such as hydrogen bonding and van der Waals force adsorption. It is this combination of physical and chemical factors that determines the value of the final ink absorbency test result.
Measurement method of ink absorbency
Laboratory measurement of ink absorbency is usually done using standardized test methods, and the most widely used method is the ink absorbency tester. According to GB/T 12911, the core principle of this method is to print a certain area of ink layer on the surface of the paper with standard ink under specified conditions, and then measure the absorption degree of the paper to the ink connector within the specified time.
Before testing, the paper to be tested needs to be conditioned in a standard temperature and humidity environment, usually at 23±1 degree Celsius and 50%±2% relative humidity conditions for at least 4 hours. The standard ink used for testing has a fixed viscosity and flowability, and the test instrument consists of a uniform ink system consisting of a metal roller and a rubber roller, as well as a printing device that allows precise control of the printing pressure.
The testing process is divided into the following key steps. First, the standard ink is evenly applied to the metal roller, and the ink reaches the specified thickness through the uniform ink system. Then, the paper to be tested is fixed on the printing platform, and the printing mechanism is activated so that the rubber roller comes into contact with the paper and finishes printing at a constant speed. Record the time immediately after printing and allow the ink to stay on the surface of the paper for a specified amount of time (usually 2 minutes). When the specified time is reached, the ink erasing device in the absorbency tester is used to wipe off the unabsorbed ink on the surface at standard pressure. Finally, the reflectance of the printed and unprinted areas is measured separately using a reflectometer.
The formula for calculating the ink absorption value is as follows:
X = (R∞ - Rp) / R∞ × 100% + K
where X represents the ink absorption value, R∞Represents the reflectivity of the paper in the unprinted area, Rprepresents the reflectivity of the area after printing and erasing the surface ink, and K is the correction factor, which is used to eliminate the effect of the difference in whiteness of the paper itself. The K value is determined by a standard whiteboard calibration and usually ranges from 0 to 5. The test results are taken from the arithmetic average of multiple measurements, accurate to one decimal place.
In addition to the reflection method, there is also the weighing method used to measure ink absorbency, which is to calculate the amount of ink absorbed by measuring the change in paper quality before and after printing. However, this method requires high balance accuracy and is cumbersome to operate, so it is not as widely used as the reflection method.
Key factors that influence ink absorbency measurements
Ink absorbency measurements are influenced by a combination of factors, and understanding these factors is crucial for accurately evaluating material print performance and controlling print quality effectively.
The structural properties of the paper itself are intrinsic determinants of ink absorption. The tightness of the paper directly affects the size and number of capillaries, and the paper fibers with high tightness are tightly bonded, with low porosity, high ink penetration resistance, and low absorbency. On the contrary, loose paper has higher absorbency. The degree of pulping determines the degree of filament brooming of the fiber, and the paper fiber with high pulping degree has strong fiber bonding and relatively low absorption. The uniformity of the paper is equally important, and paper with poor uniformity will have local absorption unevenness, resulting in spots on the print. The content of fillers and compounds also significantly affects absorbency, with fillers filling some pores to reduce absorbency, and sizing agents to inhibit ink penetration by reducing surface energy.
The properties of inks also have an important impact on absorbency measurements. The viscosity of standard test inks must be strictly controlled, with high viscosity slowing down and low viscosity accelerating penetration. The thixotropy of the ink also affects its flow behavior during the printing process, which in turn affects the amount of transfer and penetration. There is a coupled relationship between the drying speed and absorbency of inks, and inks that oxidize conjunctiva dry quickly may form an ink film on the surface, blocking further penetration.
The choice of test conditions is directly related to the accuracy and comparability of the measurement results. The size of the printing pressure determines the intensity of the ink squeezing into the inside of the paper, and too high pressure will force the ink to penetrate, making the measured absorption value high. The printing speed affects the contact time between the ink and the paper, and the faster the speed, the shorter the contact time and the reduced absorption. The residence time after printing is a key parameter, and the longer the residence time, the greater the absorption. The influence of ambient temperature and humidity cannot be ignored, and the increase in temperature will reduce the viscosity of the ink and accelerate the penetration. The increase in humidity will cause the fibers to swell and change the capillary structure, which will affect the absorption rate.
Instrument status and operating techniques also introduce measurement errors. The cleanliness of the ink homogenization system directly affects the uniformity of the ink layer thickness. The flatness and pressure stability of the ink erasing device determine the consistency of surface ink removal. The calibration status of the reflectometer affects the accuracy of the reflectance readings. If the operator's technique and force when performing the ink erasing action deviate from the standard requirements, it will also lead to deviation of the result.
Ink absorbency in the printing industry
Ink absorbency index plays an irreplaceable role in the production of printing materials, printing process control and quality evaluation of printed products.
In the paper industry, ink absorbency is an important quality indicator for the factory inspection of paper products. Papermaking enterprises monitor the stability of the sizing process by testing the ink absorption of paper on the production line, adjust the compound formula and sizing amount, and ensure that the paper has moderate absorption. For different applications of paper, the target absorption values vary: newsprint requires high absorbency to accommodate high-speed printing and fast drying; Offset paper needs moderate absorbency to ensure gloss and color reproduction of prints; Coated paper is coated and requires low absorbency for a high gloss finish.
In printing enterprises, ink absorbency data is an important basis for formulating printing process parameters. The incoming material inspection process conducts ink absorption tests on the purchased paper, which can predict possible problems during the printing process. When the paper is highly absorbent, the process personnel will adjust the ink viscosity accordingly, choose an ink with a slower drying rate, or reduce the printing pressure to reduce the risk of transmission. When the paper absorbency is low, it is necessary to speed up the ink drying speed to prevent the back from rubbing, or appropriately increase the printing pressure to ensure the ink sticking. The absorbency data also provides a reference for ink balance control, paper stacking height setting, etc.
In the ink manufacturing industry, R&D personnel use ink absorbency concepts to optimize product formulations. By testing the absorbency performance of inks on different papers, the resin type, solvent ratio, and viscoelasticity of the binders in the inks are adjusted to adapt the product to a wider range of substrates. The development of specialty inks relies on precise control of absorbency, such as UV inks require very low absorbency to ensure curing results, while water-based inks require moderate absorbency for rapid drying.
In the field of packaging materials, ink absorbency is related to the shelf effect of the final product. The printing quality of the carton packaging surface directly affects the first impression of the product by consumers, and the moderate absorbency ensures clear patterns, saturated colors, and suitable gloss. Food and drug packaging also needs to consider the relationship between ink absorption and food safety to prevent excessive penetration of ink components into the package. The ink absorption test of composite materials such as paper-plastic composites and paper-aluminum composites is more complex, and the effect of surface treatment agents on absorbency needs to be considered.
In the analysis of printed quality failures, ink absorbency detection is an effective means to find the root cause of the problem. When the printed matter is powdered, the detection found that the paper absorbency is too high as a common reason. When the ink film is not dry or the back is dirty, the paper is often the problem. By comparing the actual inspection data with the standard values, the source of the problem can be quickly identified and direction can be provided for process improvement.
Summary and outlook
As a core indicator for evaluating paper printability, ink absorption profoundly affects the quality and production efficiency of printed materials. This index is based on the physicochemical principle of capillary permeation, which can be quantified by standardized measurement methods, and is constrained by multiple factors such as paper structure, ink characteristics, and test conditions. In the complete industrial chain from papermaking, ink manufacturing to printing processing, ink absorbency data plays an important role in quality monitoring, process adjustment and fault diagnosis, and is the bridge between printing materials and printing processes.
Looking forward to the future, ink absorbency detection technology is developing in the direction of more precision, intelligence and microscopicity. The new test instrument combines high-resolution camera technology and image analysis algorithm to record the dynamic process of ink penetration in real time, obtain multi-dimensional parameters such as penetration rate, diffusion area, and absorption uniformity, breaking through the limitation of traditional methods that can only obtain a single absorption value. The application of near-infrared spectroscopy technology makes it possible to detect ink absorption non-destructively, and realize rapid online detection by establishing a mathematical model of spectral characteristics and absorbance.
With the rapid development of digital printing technology, the scope of application of ink absorbency concept is also expanding. The interaction between ink and substrate material in inkjet printing puts forward new requirements for absorbency, and the transfer and fixation mechanism of electronic ink is different from that of traditional ink, which promotes the extension of absorbency testing methods to a wider range of material systems and more complex printing methods. In the context of green printing, the popularity of environmentally friendly inks such as water-based inks and UV inks has prompted researchers to re-examine the applicability between absorbency and traditional inks, and explore the establishment of a more universal evaluation system.
Advances in materials science offer more possibilities for regulating ink absorbency. The application of nanomaterials in paper coating can achieve precise regulation of surface pore structure, so that the absorbency can be controlled within a narrower range. The development of new bio-based sizing agents offers a solution to maintain ideal absorption while reducing dependence on petroleum resources. It is foreseeable that with the deepening of understanding of the interaction mechanism between ink and printing materials, the traditional concept of ink absorbency will continue to rejuvenate, providing continuous support for the improvement of printing quality and the progress of printing technology.