Introduction
Gas permeability is one of the key indicators of film materials, which directly affects their application effect in packaging, protection and other fields. Among them, the determination of oxygen permeability is particularly important. As a classic gas permeability test method, the differential pressure method is widely used in relevant standards because of its clear principle and relatively simple operation. This article will systematically explain the working principle, test process, data processing and considerations in practical application of differential pressure gas permeation instrument.
How it works:
The differential pressure method is based on Fick's law of diffusion. During testing, the film specimen is sealed in the test chamber and separated into two separate chambers, upper and lower. The upper chamber is the high-pressure side, filled with test gas at a certain pressure; The lower cavity is on the low-pressure side, and the initial vacuum state is the state. Due to the pressure difference between the two sides, the gas will penetrate from the high pressure side through the film to the low pressure side, causing the pressure on the low pressure side to gradually increase over time. By monitoring the rate of change of pressure on the low pressure side over time, the permeability coefficient, solubility coefficient and diffusion coefficient of the gas can be calculated, and then the oxygen permeability can be obtained.
The basic relationship of gas permeability (Q) can be expressed as:
Q = (ΔP * V) / (A * t * Δp)
Among them, ΔP is the pressure change on the low pressure side, V is the volume on the low pressure side, A is the effective area of the specimen, t is the test time, and Δp is the pressure difference between the two sides of the film.
Testing process
The standard testing process typically involves several key steps: First, the specimen is prepared to the specified size, ensuring it is flat, wrinkle-free, and free of pinholes. Secondly, the specimen is carefully installed in the sealing fixture of the test chamber to ensure good airtightness. Subsequently, the low-pressure side is vacuumed to achieve the specified vacuum level. The high-pressure side is then filled with dry test gas (e.g. oxygen) and a constant test pressure is maintained. Finally, the system automatically monitors and records the change of pressure on the low pressure side over time until it reaches a stable osmosis state.
Influencing factors
The accuracy and repeatability of test results are affected by many factors and need to be controlled during testing. The main influencing factors are shown in the table below:
| Film material properties | The chemical structure, crystallinity, thickness and presence of pinhole defects of the material. |
| Test conditions | Test temperature, humidity, and pressure differential setpoints on both sides of the film. |
| Specimen status | The flatness of the specimen, the tension during clamping, and the sealing effect. |
| Instrument status | The vacuum level of the system, the accuracy and calibration status of the sensor, and the cleanliness of the chamber. |
Data processing
Once the pressure-time curve is obtained, the data needs to be processed. After the penetration reaches a steady state, the curve will show a linear upward trend. The slope of the straight segment (dp/dt) is the change of pressure on the low pressure side per unit time. Substituting this slope into the corresponding calculation formula, combined with the known test chamber volume, specimen area and other parameters, the gas permeability can be calculated. For multilayer composites, the total penetration resistance can be approximated as the sum of the resistance of each layer.
Applications and prospects
Differential pressure gas permeation meters are widely used in many fields such as food packaging, electronic component protection, agricultural films, etc., to evaluate the barrier properties of materials to oxygen. With the development of materials science, the demand for high-barrier, multi-functional films is increasing, which places higher demands on the accuracy and efficiency of testing technology. In the future, testing technology may be further developed in terms of automated control, real-time data analysis, and linkage with material structure characterization.
References
1. Working principle: refers to the basic theoretical literature on gas diffusion in polymers.
2. Test process: A number of domestic and foreign standard methods for gas permeability testing of plastic film are integrated.
3. Influencing factors: compiled from a number of technical research reports on the impact of test conditions on gas permeability.