Paper folding resistance is an important indicator to measure the resistance of paper to fracture under repeated folding, and is widely used in printing, packaging, banknote paper and archival paper. The testing process is usually carried out under specific conditions using a bending resistance tester according to standard methods, such as ISO 5626 or GB/T 457. As a key variable, ambient temperature and humidity have a significant impact on the fiber structure, internal stress and mechanical behavior of paper, which in turn affects the accuracy and comparability of the test results. The purpose of this paper is to systematically analyze the mechanism and influence of temperature and humidity factors in paper bending resistance test.
Paper is mainly composed of cellulose fibers, and its mechanical properties are highly dependent on the hydrogen bonding between fibers and the flexibility of the fibers themselves. Changes in ambient humidity directly affect the moisture content of paper. When the humidity increases, the paper absorbs moisture, and the water seeps into the fibers, weakening the hydrogen bonding and making the fibers more likely to slide. Temperature changes affect the rate of molecular thermal motion: increasing temperature can promote fiber plasticization, reduce the stiffness of the glass transition region, and may accelerate water migration. The coupling effect of the two jointly determines the response behavior of paper under folding stress. The relationship can be approximated by the following empirical formula:
D ∝ (ηf · Eb) / σf
where D represents bending resistance performance, ηf is the fiber flexibility factor, Eb It is the binding energy between fibers and σf It is the folding stress concentration coefficient. Temperature and humidity change by changing the ηf and Eb and affect D.
The effect of humidity on folding resistance
Under constant temperature conditions, the effect of humidity changes on flexural resistance is usually non-linear. In a low humidity environment, the paper has low moisture content, high fiber brittleness, and the stress is not easy to disperse when folded, and the number of folding resistance is often low. As the humidity rises to a moderate range (such as about 50% relative humidity), the paper will be plasticized to a certain extent, the flexibility of the fibers will improve, and the bending resistance may reach a high level. However, when the humidity is too high, the fiber bonding force is excessively weakened, and the structure of the paper is relaxed. The response curves of different paper grades are different due to the difference in slurry, filler and sizing.
The effect of temperature on bending resistance
The effect of temperature is often synergized with humidity. Under constant humidity, the increase in temperature generally increases the movement of the polymer chain segments inside the paper, making the fibers more prone to deformation and helping to improve flexibility. However, too high a temperature can cause moisture evaporation (especially in low-humidity environments), making the paper dry and brittle, or causing the compound to soften and migrate, altering the surface properties. Therefore, the influence of temperature needs to be comprehensively evaluated in conjunction with the humidity equilibrium state. Most standard test methods specify a range of temperature and humidity conditions to control this coupling effect.
Standard test conditions and deviations
To ensure comparability of test results, international and national standards usually specify laboratory atmospheric conditions. For example, ISO standards often recommend sample equilibration and testing in environments with temperatures of 23°C±1°C and relative humidity of 50%±2%. If the actual conditions deviate from this range, significant errors may be introduced. The following summarizes the impact trends of common deviation directions:
| Conditional bias | Typical trends affecting bending resistance |
| Humidity is too low | The number of folds is reduced |
| Excessive humidity | The number of folds may increase first and then decrease |
| The temperature is too low | Flexibility decreases and the number of folds is reduced |
| The temperature is too high | The effects are complex and depend on the humidity balance |
In actual testing, the sample must be fully balanced at the specified temperature and humidity (usually not less than 24 hours) to evenly distribute the moisture, otherwise the test results will be unrepresentative.
The laboratory should be equipped with reliable temperature and humidity monitoring and regulation equipment to ensure that the testing environment is stable within the standard range. It is good practice to record environmental data on a daily basis. For special paper applications or research purposes, serial tests can be performed at different temperatures and humidity to plot a performance response curve, which helps to understand how the paper behaves in real-world storage and use environments. The test report should clearly indicate the balance and the temperature and humidity conditions at the time of the test to facilitate the interpretation and comparison of the results.
Conclusion
Temperature and humidity are key environmental factors affecting the results of paper bending resistance tests, which play a role by changing the physical state and binding force of paper fibers. Sample balancing and testing in strict accordance with the temperature and humidity conditions specified by the standard is the basis for obtaining reliable and comparable data. Understanding the influence mechanism of temperature and humidity can also help predict the durability of paper in different application environments, providing a reference for material selection and process improvement.
References
ISO 5626: Paper and board - Determination of folding endurance.
GB/T 457: Determination of folding resistance of paper and cardboard.
Britt, K.W. Handbook of Pulp and Paper Technology.
Przybysz, P., et al. Influence of ambient conditions on mechanical properties of paper.