Moisture permeability test of silicone hip pads: methods and practices

Moisture permeability test of silicone hip pads: methods and practices
In today’s international market, silicone hip pads are favored by many consumers for their unique comfort, durability and functionality. For international wholesale buyers, it is crucial to understand the moisture permeability of silicone hip pads, as this is directly related to the comfort and user experience of the product. Silicone hip pads with good moisture permeability can effectively expel moisture, keep the buttocks dry, and prevent the occurrence of problems such as eczema, especially for people who sit or lie for a long time. This article will introduce the moisture permeability test method of silicone hip pads in detail to help you better evaluate and select high-quality products.

1. Principle of moisture permeability test
Moisture permeability refers to the ability of a material to allow water vapor to pass through its surface. For silicone hip pads, the moisture permeability test is mainly to evaluate its breathability by measuring the rate at which water vapor passes through the silicone material under certain conditions. The basic principle of the test is based on the diffusion of water vapor from the high humidity side to the low humidity side driven by the pressure difference on both sides of the material. By precisely controlling the temperature, humidity and wind speed of the test environment, the actual use scenario can be simulated to accurately determine the moisture permeability of the silicone hip pad.

2. Common moisture permeability test methods
(I) Moisture absorption (desiccant) method
Test preparation
Select a suitable desiccant, usually anhydrous calcium chloride, whose particle size should be between 0.63 – 2.5mm. Place the desiccant in an oven at 160℃ for 3 hours to ensure that it is completely dry so that it can accurately absorb water vapor.
Prepare a clean, dry test cup and place about 35g of cooled desiccant in it. Gently shake the test cup so that the desiccant forms a plane, and its surface should be about 4mm lower than the sample to leave enough space for the sample and ensure good contact between the desiccant and the sample.
Cut the silicone hip pad sample to a suitable size so that it can completely cover the top of the test cup and ensure that the test surface is facing up.
Test process
Put the test cup assembly containing the desiccant and the sample into the test instrument and ensure that the temperature and humidity of the test environment meet the standard requirements, generally 23℃ and 50% relative humidity.
At the initial stage of the test, let the test cup balance in the test environment for 1 hour to allow the sample and desiccant to adapt to the environmental conditions. Then take out the test cup, put it in a desiccator and balance it for half an hour, then weigh it and record the initial weight M1.
Put the test cup back into the test instrument and test it for the time specified in the standard or test protocol, usually 24 hours. After the test, take out the test cup again, put it in a desiccator and balance it for half an hour, then weigh it and record the final weight M2.
Result calculation
The moisture permeability (WVT) can be calculated by the following formula: WVT = (M2 – M1) / (A × t), where A is the area of ​​the sample and t is the test time. This formula shows that the moisture permeability is equal to the mass of water vapor passing through the sample per unit area per unit time. For example, if the test results show that the mass change of the sample after 24 hours is 1.2g, and the sample area is 100cm², then the moisture permeability is 1.2g / (100cm² × 24h) = 0.005g / (cm²・h).

(II) Evaporation (positive cup water) method
Test preparation
Use a measuring cylinder to accurately measure water at the same temperature as the test conditions. The amount of water should be determined according to the requirements of each standard. For example, for some standards, 100 ml of water may need to be measured.
The silicone hip pad sample is carefully installed on the test cup to ensure that the seal between the sample and the test cup is good to prevent water leakage or the entry of external air, which may affect the test results.
Test process
Place the positive cup of the test cup containing water and the sample in the test instrument. The temperature and humidity of the test environment should meet the standard requirements, such as 23℃ and 50% relative humidity.
Let the test cup balance in the test environment for a period of time, such as 1 hour, to ensure that the sample and water adapt to the environmental conditions. Then weigh the initial weight of the test cup M1.
Perform the test for the specified test time, usually 24 hours. After the test, weigh the weight of the test cup M2 again.
Result calculation
The calculation formula of water vapor transmission rate (WVT) is: WVT = (M1 – M2) / (A × t). Unlike the moisture absorption method, the initial weight M1 is greater than the final weight M2 because water evaporates through the sample during the test. For example, if the test results show that the mass of the test cup has decreased by 0.8g after 24 hours and the sample area is 100cm², the moisture permeability is 0.8g/(100cm² × 24h) = 0.0033g/(cm²・h).
(III) Evaporation (inverted cup water) method
Test preparation
Similar to the positive cup water method, use a measuring cylinder to measure water at the same temperature as the test conditions and determine the amount of water according to the standard requirements.
Fix the silicone hip pad sample on the test cup to ensure good sealing.
Test process
Place the inverted test cup containing water and sample in the test instrument so that the sample is in contact with the water surface. The temperature and humidity of the test environment should be kept stable, such as 23℃ and 50% relative humidity.
After balancing, weigh the initial weight M1 of the test cup.
Carry out the test for the specified test time, such as 24 hours, and then weigh the final weight of the test cup M2.
Result calculation
The calculation formula of water vapor transmission rate (WVT) is also: WVT = (M1 – M2) / (A × t). The difference between the inverted cup water method and the normal cup water method is that the water is placed in the test cup in different positions. The inverted cup water method allows the sample to directly contact the water, which may be closer to some actual use scenarios, such as the moisture permeability of hip pads in a humid environment.
(IV) Potassium acetate method
Test preparation
Inject saturated potassium acetate solution into the test cup, and the amount of solution is about 2/3 of the height of the cup. Potassium acetate solution has specific humidity characteristics and can provide a stable humidity environment during the test.
Carefully seal the silicone hip pad sample at the mouth of the test cup to ensure a good seal to prevent evaporation of the solution or intrusion of external moisture.
Test process
Place the test cup with the sample sealed upside down in the test water tank. The test water tank should also contain a certain amount of saturated potassium acetate solution to keep the humidity of the test environment stable.
Weigh the total mass M1 of the test cup before the test, and then weigh the total mass M2 of the test cup again after 15 minutes, and record the data of the two weighings.
Result calculation
The moisture permeability is calculated based on the mass change, but due to the relatively special test time and conditions of the potassium acetate method, its calculation formula may be slightly different, and it is necessary to refer to specific standards, such as JIS L1099 method B-1, JIS L1099 method B-2, ISO 14956, etc.

3. Factors affecting moisture permeability test
(I) Environmental conditions
Temperature and humidity are key environmental factors affecting the results of moisture permeability tests. Different test standards specify different temperature and humidity conditions. For example, some standards specify a test temperature of 23°C and a relative humidity of 50%, while other standards may require higher temperatures or humidity. Changes in temperature and humidity will directly affect the diffusion rate of water vapor in the silicone hip pad. Generally speaking, as the temperature rises, the molecular motion intensifies, the diffusion rate of water vapor accelerates, and the moisture permeability increases; the greater the humidity difference, the greater the driving force of water vapor, and the higher the moisture permeability.
(II) Test time
The length of the test time also has a certain impact on the test results of moisture permeability. A longer test time can more accurately reflect the moisture permeability of the sample during long-term use, but it may also cause fluctuations in environmental conditions during the test, thereby introducing errors. Therefore, when choosing the test time, it is necessary to make a comprehensive consideration based on the actual use of the product and the requirements of the test standard.
(III) Sample preparation
The sample preparation process includes steps such as cutting, cleaning and installation of the sample. The standardization of these steps will directly affect the accuracy of the test results. The size of the sample should meet the standard requirements, and the edges should be neat, without damage and wrinkles, to avoid leakage or accumulation of local water vapor, which will affect the test results. In addition, when installing the sample, ensure that the seal between the sample and the test cup is good to prevent the entry of external air or the leakage of internal water vapor.
(IV) Test equipment
The accuracy and stability of the test equipment are crucial to the test results of moisture permeability. High-precision weighing equipment can accurately measure the mass change of the test cup, thereby improving the calculation accuracy of the moisture permeability. At the same time, the temperature and humidity control system of the test equipment should be able to stably maintain the set environmental conditions to avoid deviations in test results due to fluctuations in environmental conditions. In addition, the wind speed setting of the equipment will also affect the test results, because the wind speed will change the flow state of the air around the test cup, thereby affecting the diffusion rate of water vapor.
(V) Performance of desiccant
In the moisture absorption test, the performance of the desiccant has a direct impact on the test results. Factors such as the water absorption capacity, particle size distribution and dosage of the desiccant will affect its absorption rate and total amount of water vapor. Anhydrous calcium chloride is a commonly used desiccant with strong water absorption capacity, but if the particle size is too large or too small, it may affect its contact area and reaction rate with water vapor, resulting in deviations in test results. Therefore, when using desiccant, it should be selected and processed in strict accordance with the standard requirements to ensure the consistency and stability of its performance.

4. How to choose a suitable moisture permeability test method
(I) Selection based on product characteristics
Different silicone hip pad products may have different characteristics and usage requirements, so it is necessary to choose a suitable moisture permeability test method. For example, for silicone hip pads with thin thickness and good air permeability, moisture absorption method or evaporation method can be used for testing to accurately evaluate its moisture permeability. For silicone hip pads with thick thickness and high density, it may be necessary to select test methods such as potassium acetate method that can provide a more stable humidity environment to ensure the reliability of the test results.
(II) Consider the test purpose and application scenario
The test purpose and application scenario are also important bases for selecting the moisture permeability test method. If it is to evaluate the moisture permeability of silicone hip pads in ordinary indoor environments, moisture absorption method or evaporation method can be selected to simulate daily use scenarios. If it is to study its performance in special environments, such as high humidity, high temperature and other environments, it may be necessary to select the corresponding test method or adjust the test environment according to specific conditions.
(III) Reference to international standards and industry practices
In the international market, different countries and regions may adopt different moisture permeability test standards. Therefore, when selecting the test method, international standards and industry practices, such as ASTM E96, ISO 14956, etc., should be referred to to ensure the universality and comparability of the test results. In addition, understanding the target market’s requirements and recognized standards for moisture permeability testing will help select appropriate test methods and improve the market competitiveness of products.

5. Summary
The moisture permeability test of silicone hip pads is an important means to evaluate their comfort and functionality. Through the test methods introduced above, such as moisture absorption method, evaporation method and potassium acetate method, the moisture permeability of silicone hip pads can be accurately determined, providing strong support for product research and development, production and sales. In practical applications, factors such as product characteristics, test purpose and application scenarios should be comprehensively considered to select appropriate test methods, and test conditions should be strictly controlled to ensure the accuracy and reliability of test results. For international wholesale buyers, understanding the significance of moisture permeability test methods and results will help better select high-quality products, meet market demand and improve customer satisfaction.