Application of rubber permeability testing on automotive inner tubes

Application of rubber gas permeability test on automobile inner tube: Introduce the measurement principle of rubber gas permeability test standard GB/T 7755, ventilation chamber structure, operation procedure, and how to test the inner tube of the car according to the standard, and test the test The method was compared with the current car inner tube permeability test method.
Key words: standard; gas permeability; automobile inner tube; detection;

I. Overview At present, automotive inner tires are mostly made of butyl rubber, or butyl rubber and other rubber. The rubber materials used for inner tubes are required to have high strength tensile and tear resistance. The air tightness is to ensure that the car does not need to be inflated for a long time. To understand which kind of rubber produced by the production process has better airtightness, it is necessary to test the airtightness of related products.
The general test method in the industry is to fill the entire inner tube with a gas and connect it to the pressure gauge to observe the pressure drop of the entire inner tube over a period of time. This holistic test method currently has no other ideal implementation than to observe the pressure drop of the pressure gauge.
This test method is low in accuracy, time consuming, and inconvenient to operate. GB/T 7755-2003 (Determination of gas permeability of vulcanized rubber or thermoplastic rubber) gives a method for testing the airtightness of rubber materials, which can quickly and accurately check which rubber has better air tightness, so that It is further calculated which wheel inner tube made of rubber has a better sealing property.
Second, GB/T 7755 test principle is as follows: Put a circular sample into the cavity of the gas chamber with constant temperature. The sample divides the cavity into upper and lower parts, and the upper cavity applies a certain pressure to make the gas The sample is penetrated by the pressure difference, so it is also called the high pressure side, while the lower chamber is generally called the low pressure side. Will be high

The pressure side is connected to a constant pressure gas reservoir to ensure that the high pressure side maintains a constant pressure throughout the test. The gas permeates through the sample to the low pressure side, and the low pressure side volume must be V, so the pressure will increase by ΔP from P. The volume of gas permeating to the low pressure side is ΔV, and the volume of the original gas is compressed to V-ΔV. Using a high-precision pressure sensor to monitor the pressure change ΔP on the low pressure side, the gas permeation amount can be calculated from the following formula. V:
P·V=(P+△P)·(V-△V) (1)
Ventilation chamber structure diagram III, test process
1. Carefully select the sample that is flat, scratch-free, non-perforated, surface-free, and other defects from the material to be tested, and cut the circular sample of the required size and the thickness of the sample. Should exceed 10% of the average thickness;
2. Wipe the dust on the surface of the lower cavity with a soft material, or the vacuum grease left in the previous test;
3. Evenly apply a layer of vacuum grease to the sealing area of ​​the lower chamber;
4. Place the cut porous gasket on the low pressure side as the support of the sample to resist the high pressure side gas pressure load.

The sample will not be significantly deformed throughout the test, but the porous gasket should not be exposed to the vacuum grease;
5, the sample is placed flat, as far as possible to make the porous gasket and the sealing rubber ring in a concentric position;
6. Put the high pressure chamber and the low pressure chamber together and keep the sealed state;
7. Connect the high pressure side of the venting chamber to the gas storage bottle. According to the gas permeability of the sample, the test pressure can be set to 0.2 MPa.
0.5MPa;
8. Detect the pressure rise on the low pressure side.
Fourth, the application of GB/T 7755 test method on the inner tube of a car
GB/T 7755 test method is applied to the gas permeability test of automobile inner tube, which can greatly improve the detection accuracy and shorten the test time.
To perform this standard test instrument, the volume on the low pressure side should be made small according to the standard requirements. In this paper, 12 ml is temporarily taken, the permeation area of ​​the sample is 36 cm2, and the instrument test accuracy is 0.3 Pa. If the instrument monitors the low pressure side 0.3 The pressure change of Pa can be applied to the formula (1)
Calculate the amount of gas permeation.
In the formula, the pressure on the low pressure side is P=0.1 MPa, V=12 ml, ΔP=0.3 Pa, and ΔV=3.6×10-5 ml can be obtained, that is, the volume on the low pressure side is increased by 3.6×10-5 ml. The pressure on the high pressure side is calculated as 0.2 MPa, and the gas loss on the high pressure side is 1.8 × 10 -5 ml. This value is the gas permeation amount on the high pressure side in the area of ​​36 cm 2 of the sample.
The outer surface area of ​​a car inner tube is about 0.6m2. According to the above calculation, the permeation amount is 0.003 ml. How much pressure drop will cause a pressure drop of a 22L tire? Calculated as follows:
P1·(V1-△V)=(P1-△P)·V1
Where P1=0.2MPa is the pressure inside the car inner tube;
V1=22L is the volume of the inner tube of the car;
â–³V=0.003 ml is the leakage amount of the inner tube of the automobile.
It can be found that ΔP=0.0272Pa,
That is to say, under the same test conditions, when the pressure change of 0.3Pa can be detected by the GB/T 7755 test method, the traditional detection method can only detect the pressure change of 0.0272 Pa, if you want to use the traditional method. Monitoring the tire pressure change of 0.3Pa requires 11 times the time taken by the GB/T 7755 test method. If the pressure gauge resolution of the conventional method is generally above 100 Pa, then when the pressure change of the tire reaches the small resolution of 100 Pa of the pressure gauge,
The test time will be more than 3,600 times the GB/T 7755 test method.

In addition, the GB/T 7755 test method achieves specific detection of specific materials, and the test results are determined only by the material itself, and have nothing to do with other factors, and have good guiding significance for the selection and improvement of the inner tube material.
V. Conclusion In order to obtain high-quality products, or to speed up the upgrading of products, in addition to continuous improvement in technology, technology, etc., there must be ideal testing methods to ensure fast, accurate data detection for scientific research or production. It often plays a decisive role.

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