Advantages of Sensor-Based Water Permeability Tester

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

1. The importance of temperature control in moisture permeability testing

Most crystalline polymers are semi-crystalline polymers. In theory, the crystalline part of the polymer is the impenetrable area in the path of the diffusion process of the permeate molecules in the polymer, and the diffusion mainly occurs in the amorphous part. The longer the polymer molecular chain, the more conformations it has. When the temperature rises, due to thermal motion, the molecular chain conformation changes faster, the polymer cohesion decreases, and the diffusion rate of the permeate molecules in the polymer accelerates, that is, when the temperature rises, the barrier property of the material will decrease. The

permeation process of water vapor into the polymer is significantly affected by temperature fluctuations (this is similar to the permeation of inorganic gases), and the relationship with temperature obeys the Arrhenius equation:

As the temperature rises, the moisture permeability coefficient increases, but the increase varies for different polymer films. Some literature points out that the moisture permeability coefficient of amorphous materials increases faster with the increase of temperature, while the increase of materials with a certain degree of crystallinity is slower.

2. The influence of humidity difference on test data

Since water vapor is a polar molecule, during the penetration of water vapor into polar polymers, some polymers will first absorb water vapor and swell, increasing the free volume therein. The moisture permeability coefficient of the material has a significant dependence on water vapor concentration. Accordingly, the moisture permeability of the material is also affected by humidity changes, which is manifested as a nonlinear change in the moisture permeability of some polymers and the relative humidity difference on both sides. For example, the relationship between the moisture permeability of the hydrophilic polymer celluloid and the relative humidity difference is not linear. This phenomenon that the permeability and partial pressure difference (that is, the relative humidity difference for moisture permeability testing) are not linear is the most significant difference between water vapor and common inorganic gases in the polymer penetration process.

3. Differences between weighing method and sensor method

There are two main types of moisture permeability tests currently used: weighing method and sensor method. They have different detection principles and equipment structures.

Simply put, the weighing method is to seal the sample in a moisture permeable cup, and then place the moisture permeable cup in a constant temperature and humidity environment. The relative humidity on both sides of the sample is controlled by the constant temperature and humidity environment and the desiccant or saturated salt solution placed in the moisture permeable cup. The moisture permeability of the sample is calculated by measuring the increase or decrease in the weight of the moisture permeable cup during the test. The test temperature of this test method is provided by a constant temperature and humidity environment, and the humidity is the difference between the humidity in the moisture permeable cup and the ambient humidity. Since the moisture permeable cup needs to be placed and moved in a constant temperature and humidity environment before and during the test, the required constant temperature and humidity environment is often large in size, which is not conducive to controlling the temperature and humidity of the environment.

Figure 1. Schematic diagram of the weight loss test method

The sensor method is divided into infrared sensor method, electrolysis method and dynamic relative humidity determination method according to the type of sensor used. Among them, the infrared sensor method and the electrolysis method have similar detection structures and principles. The sample is sealed in the upper and lower test chambers, and a saturated salt solution is used to control one side of the sample to a high humidity state, while the other side is purged with dry gas to keep it dry, so that a specific humidity difference is formed on both sides of the sample. The water vapor that passes through the sample and enters the dry side is carried by the dry air flow into the sensor to measure the moisture content therein to determine the moisture permeability of the sample. The equipment based on these two detection principles has good working independence, and the required test chamber volume is very small, making it easy to control temperature and humidity. The detection principle of the dynamic relative humidity determination method is somewhat different from that of the infrared sensor method and the electrolysis method. In this method, temperature control is also easy to achieve, and the humidity difference on both sides of the sample is constantly changing during the entire test process according to the test principle.

Figure 2. Schematic diagram of electrolysis

4. Advantages of temperature and humidity control of sensor-based moisture permeability tester

Volume is one of the main factors affecting the uniformity of temperature and humidity in a certain space. The smaller the volume, the better the uniform control effect of temperature and humidity. As mentioned above, the equipment structure of the sensor method is easier to achieve uniform temperature and humidity control in the test environment than the equipment structure of the weighing method, and the ideal control effect can be obtained through special technical processing. Take Labthink TSY-W3 as an example to illustrate the advantages of the electrolysis method in temperature and humidity control.

Labthink TSY-W3 electrolysis moisture permeability tester adopts high-precision electrolysis humidity sensor. Its outstanding structural feature is that it improves on the principle of water bath temperature control and uses constant temperature circulating water to maintain the temperature of the test chamber. The constant temperature circulating water is controlled by an external circulation controller to provide circulation power. It is particularly worth mentioning that the circulation controller used by TSY-W3 can be used for both heating and cooling. 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 testing standards, but also has the most ideal temperature control among Labthink series moisture permeability testers. Selecting saturated salt solution for humidity control can make the humidity on the high humidity side of the sample uniform and stable, while the airflow on the low humidity side is controlled by the dryer, so 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, the more favorable the test results. The repeatability of the measured data of TSY-W3 is very good in its entire 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 the weighing method equipment, especially when testing materials with low moisture permeability, it can show a greater advantage in detection efficiency. It is also insensitive to vibration in the test environment, effectively avoiding the influence of environmental interference on 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 permeability instrument has a strong detection advantage in temperature and humidity detection control.