Rubber is a kind of elastic polymer, which can produce large deformation under the action of small external force. It can be restored to its original state after the external force is removed, so it is one of the best choices for sealing and protective materials. Rubber is widely used in various industries including food, pharmaceuticals and others, such as gasket or O-rings used in packaging caps of food or pharmaceutical products or machinery to provide sealing, shock absorption and cushion functions. Sometimes, it can be seen in the wires as the insulation case to protect the wires.
Analysis on Water Barrier Property of Rubber
Rubber comes in different types and manufacturing processes to meet various requirements. However, the enterprises have the similar requirements for the quality control items of physical properties of rubber materials. Physical properties are mainly tensile strength, elongation rate at break, deformation at break, hardness and change in performance before and after aging test. The above mechanical properties may influence the sealing performance, barrier property to oxygen and water vapor. However, in addition to the mechanical properties, we believe that the barrier property (to oxygen and water vapor) of the rubber material itself is another factor that is required to achieve a good seal performance.
The water barrier property of the rubber material refers to the barrier property to water vapor and liquid water. To be specific, it refers to the ability of the rubber material to prevent the water vapor or liquid water permeate from the higher concentration side to the lower concentration side. The whole permeation process can be divided into several steps i.e. adsorption, dissolution, diffusion and desorption. The water molecules on the higher concentration side are adsorbed on the surface of the material, dissolved into the material and diffused inside and then desorbed from the lower concentration side (See Figure 1). The duration of permeation process is determined by two factors. One is the speed at which the water molecules permeate and dissolve inside the rubber material, which is represented by the solubility parameter. The other one is the speed at which the water molecules move in the rubber matrix, which is represented by the diffusion coefficient. Rubber materials of different types, processing techniques and thickness may have quite different solubility coefficients and diffusion coefficients of water molecules. Therefore, it is necessary to obtain the accurate water transmission rate to study the moisture resistance of the rubber materials.
Water Barrier Property Test Method of Rubber
In practical applications, the water permeation has a great impact on the use of rubber materials. When rubber is used as the insulating coating of the wire and cable, it must have strong water resistance and low water vapor permeability, otherwise water molecules will seriously damage the wire or cable's electrical properties. When it is used as sealing packaging material, it must have qualified water vapor barrier property to avoid deterioration of contents inside the package due to excessive moisture.
Among all the current standards for rubber materials, there is no clear test method for water barrier property of the rubber materials. The most commonly used methods include GB/T 1037: Test method for water transmission of plastic film and sheet-Cup method, ASTM E96: Standard Test Methods for Water Vapor Transmission of Materials and ISO 1420-2001: Rubber- or plastics-coated fabrics - Determination of resistance to penetration. The former two standards focus on the water vapor permeation of the material, while the latter one mainly focuses on the study of the permeability of the material under a certain water pressure. There are similarities as well as differences between those test methods therefore it is necessary to select proper test method according to the actual test needs.
Test Method 1: GB/T 1037-88
The test method is based on the principle of "measuring the amount of water vapor that permeates through the specimen at a certain water vapor pressure difference between the two sides of the specimen and under specific temperature and relative humidity, and calculating the water vapor permeance and permeability coefficient". By weighing the test dish with desiccant to calculate the water vapor transmission rate, this test method is also known as "desiccant method". To be specific, the desiccant is placed in a clean vessel to about 3 mm below the surface of the specimen. Then the vessel is placed in the test dish for WVTR testing and moved on the dish support.
The specimen is clamped on the test dish with clamping ring and lid and then sealed with wax. The test dish sealed with specimen is weighed and placed in a chamber at constant temperature of 23±0.6° C and constant relative humidity of 90±2%. The test dish is removed from the chamber and weighed again after 16 hours. Then, the test dish is put into the chamber again and weighed again 24 hours, 48 hours and 96 hours after the test starts, respectively until the difference between the adjacent two weighing operations is less than 5%, the test can be stopped. It should be noted that, the test dishes should be weighed in a consistent order each time. The test dishes should be shaken gently after weighing to mix the desiccant in the test dish. The weight of moisture absorbed by the desiccant should not exceed 10% of the desiccant weight, otherwise, the test results may be inaccurate.
This standard was revised in 1988, mainly revising definitions of the permeability parameters and the test principle. However, it is difficult to seal wax in practical operations to achieve a good seal performance. The requirement for unique relative humidity is not conducive to the moisture permeability test under non-standard conditions.
Test Method 2: ASTM E96-2012
ASTM E96 was formulated in 1953 and was the earliest standard for moisture permeability testing. ASTM E96 includes two methods, i.e. desiccant method and water method, which is the biggest difference from GB/T 1037-88. Water method has totally opposite higher humidity side and lower humidity side. For water method, distilled water is added into the test dish. Specimen is m ounted in the test dish to form a humidified side and dry side between the two sides of the specimen. Water vapor transmission rate will be calculated according to the weight change of the test dish (see Figure 2). Both desiccant method and water method are the certified test methods of ASTM E96. There must be difference between the test results obtained by using the two test methods, which are incomparable. In actual tests, test method should be selected according to the actual application environment of the material.
We selected four kinds of rubber materials of the same thickness and named them as 1# silicone rubber, 2# EPR (ethylene propylene rubber), 3# butyl rubber, 4# neoprene. Then we used Labthink's C360H water vapor transmission rate tester to test the water vapor transmission rate of the above specimens with water method of ASTM E96. The test results are listed in Table 1.
It can be seen from the test data, 1# silicone rubber has the worst water vapor barrier property, 2# ethylene propylene rubber and 3# butyl rubber have the best water vapor barrier property since their water vapor transmission rate is 1.86% and 1.24% of that of the silicone rubber. The water vapor barrier property of 4# neoprene is in the middle level among all the four specimens. According to the test results, products that are sensitive to water vapor are recommended to be sealed or protected with 2# or 3# rubber materials, while silicone rubber can be used in the products that do not require high water vapor barrier property of rubber products.
Method 3: ISO 1420-2001
This standard provides a test method for the water barrier property of rubber materials when a hydrostatic pressure is applied to a rubber material over a fixed period. Compared with other two methods, this method is mainly used in the barrier property test of liquid water.
The test instrument consists of an open-mouth container with a sample clamping device and a pressure gauge. The test area of the upper part of the open-mouth container is 100cm2, and the bottom is designed with a water inlet. The top of the container has a hole (metal mesh) which has perimeter not more than 3cm and is made of wire with a diameter of 0.1cm-0.12cm, used to prevent deformation of the specimen.
Open the inlet valve and inject the water until it overflows. Hold the test surface facing downward on the container and install the metal mesh. Open the inlet valve again and increase the pressure in the vessel to the specified pressure within 1 min ± 10 s (this pressure should be ≤ 30 kPa) and maintain this pressure for 2 min ± 10 s.
Observe whether the part of the specimen exposed to the air has a water leakage point. Test all the five specimens with the same method. If no water leakage point is observed, the specimen material passes the test. In addition, the final hydrostatic pressure value of the rubber material can also be determined with this method. To be specific, increase the pressure continuously at certain rate until the water leakage point of the specimen appears and the hydrostatic pressure value at this time is recorded. The average value of the results of five specimens will be taken as the final result.
In summary, when rubber materials are used as sealing and protective materials, the water barrier property has an extremely important impact on the permeation of water vapor and liquid water. Currently, the water barrier property of rubber materials is mainly tested using the three test methods mentioned in this article, including the water vapor permeability and liquid water permeability. Those test methods cover a wide range of WVTR testing of various rubber materials and all easy to operate, which may be helpful to the quality control and improvement on the performance of rubber materials.