Detection advantages of the moisture permeability

2022-09-28
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Detection advantages of sensor moisture permeability tester

Abstract: the temperature and humidity of the test environment are the two main factors affecting the moisture permeability test data. Therefore, effective control of these two items can significantly improve the accuracy and repeatability of the test data. This paper compares two kinds of moisture permeability detection methods, and introduces the detection advantages of sensor method with Labthink tsy-w3 electrolytic moisture permeability instrument as an example

key words: moisture permeability, sensor method, weighing method, temperature control

because the water vapor content in the package can change the quality of some products, special attention should be paid to the moisture permeability of materials when selecting packaging materials. The moisture permeability of packaging materials has significant differences due to different materials, so the detection of moisture permeability is very important. There are many factors that affect the moisture permeability test results, among which the stability of the test temperature and the humidity difference on both sides of the sample have the most significant influence on the test results

1. The importance of temperature control in moisture permeability test

the vast majority of crystalline polymers are semi crystalline polymers. Theoretically, it is considered that the crystalline part of the polymer is an impermeable region in the path of the permeation molecules in the internal diffusion process of the polymer, and the diffusion mainly occurs in the amorphous part. The longer the polymer molecular chain, the more its conformation. When the temperature rises, because the thermal movement is not applicable to harder steel or thinner plates, the faster the conformation of the molecular chain changes, the lower the cohesion of the polymer, and the faster the diffusion speed of the permeable molecules in the polymer, that is, when the temperature rises, the barrier property of the material will be reduced

the permeation process of water vapor to polymer is obviously affected by temperature fluctuation (this is similar to the permeation of inorganic gas), and the relationship with temperature obeys Arrhenius equation:

with the increase of temperature, the moisture permeability coefficient increases, but the increase of different polymer membranes is different. Some literatures point out that the moisture permeability of amorphous materials increases faster with the increase of temperature, while the increase of materials with a certain degree of crystallinity will be slower

2. Influence of humidity difference on test data

because water vapor is a polar molecule, some polymers will first absorb water vapor and swell during the penetration of water vapor into polar polymers, which will increase the free volume. The moisture permeability coefficient of materials is obviously dependent on the concentration of water vapor. Accordingly, the moisture permeability of materials is also affected by the change of humidity, which shows that the moisture permeability of some polymers changes nonlinearly with the relative humidity difference on both sides. For example, the relationship between the moisture permeability of hydrophilic polymer cellulol and the relative humidity difference is not linear. This phenomenon of non-linear relationship between permeability and differential pressure (for moisture permeability test, it is the relative humidity difference) is the most significant difference between water vapor and common inorganic gases in the process of polymer penetration

3. Difference between weighing method and sensor method

at present, there are mainly two types of moisture permeability detection: weighing method and sensor method. Their detection principles are different, and the equipment structure has obvious differences

in short, the weighing method is to seal the sample in the moisture permeable cup, then place the moisture permeable cup in a constant temperature and humidity environment, use the constant temperature and humidity environment and the desiccant or saturated salt solution placed in the moisture permeable cup to control the relative humidity on both sides of the sample, and calculate the moisture permeability of the sample by measuring the increase or decrease of the weight of the moisture permeable cup during the test. The test temperature of this test method is provided by the constant temperature and humidity environment, and the humidity is the difference between the humidity in the moisture permeable cup and the ambient humidity. Due to the need to place and shift the moisture permeable cup in a constant temperature and humidity environment before and during the test, the required constant temperature and humidity environment is often large, which is not conducive to the control of the temperature and humidity of the environment

Figure 1 Weight loss method test schematic diagram

sensor method is divided into infrared sensor method, electrolysis method and dynamic relative humidity measurement method according to the different types of sensors used. The detection structure and principle of the infrared sensor method are similar to those of the electrolysis method. Both of them are to seal the sample in the upper and lower test chambers, control one side of the sample to a high humidity state with saturated salt solution, and purge the other side with dry gas to keep it dry, so that a specific humidity difference is formed on both sides of the sample. The water vapor entering the drying side through the sample is carried into the sensor by the dry air flow, and the moisture content is measured to judge the moisture permeability of the sample. The equipment based on these two detection principles has good working independence, and the volume of the required test chamber is very small, which makes it easy to control the temperature and humidity. The detection principle of dynamic relative humidity measurement method is different from that of infrared sensor method and electrolysis method. In this method, the temperature control is also easy to realize, and the humidity difference on both sides of the sample is constantly changing in the whole test process according to the test principle

Figure 2 Schematic diagram of electrolysis method

4 The advantages of temperature and humidity control of sensor moisture permeability tester

volume is one of the main factors affecting the uniformity of temperature and humidity in a space. The smaller the volume, the better the uniformity control effect of temperature and humidity. As mentioned above, the equipment structure of the sensor method is easier to realize the uniform control of temperature and humidity in the test environment than that of the weighing method, and the ideal control effect can be obtained through special technical treatment. Taking Labthink tsy-w3 as an example, this paper illustrates the advantages of electrolysis method in temperature and humidity control under three main categories of a spring testing machine

labthink tsy-w3 electrolytic moisture permeable instrument adopts high-precision electrolytic humidity sensor. Its outstanding structural feature is that it improves on the principle of using water bath to control temperature, uses constant temperature circulating water to maintain the temperature of the test chamber, and the constant temperature circulating water is controlled by the external circulation controller to provide circulating power. In particular, it is worth mentioning that the cycle controller used by tsy-w3 can both heat and cool. The temperature control range is 0 ℃ ~ 100 ℃, and the temperature control accuracy can reach ± 0.1 ℃. It not only fully meets the needs of domestic and foreign inspectors, although they are in the weekend rest time of employees, but also the temperature control is the most ideal in Labthink series moisture permeability tester. Selecting saturated salt solution for humidity control can make the humidity on the high humidity side of the sample uniform and stable, while the air flow on the low humidity side is controlled by the dryer. Therefore, a very stable humidity difference can be obtained on both sides of the sample with very small fluctuations. The more accurate the temperature and humidity control is, the more conducive to the test results. The repeatability of the measured data of tsy-w3 is very good in the whole test range (of course, the uniformity of the material is also a key factor affecting the repeatability of the test data). In addition, the test time of this equipment is relatively shorter than that of weighing equipment, especially when testing materials with low moisture permeability, it can reflect the greater advantage of detection efficiency. Moreover, it is insensitive to the vibration in the test environment, effectively avoiding the impact of environmental interference on the test efficiency

5. Summary

the temperature and humidity of the test environment are the two main factors affecting the moisture permeability test data. Therefore, effective control of these two items can significantly improve the accuracy and repeatability of the test data. Comprehensive comparison shows that Labthink tsy-w3 electrolytic moisture permeable instrument shows strong detection advantages in temperature and humidity detection and control

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