Comparison of Oxygen and Nitrogen Permeability of PE, BOPP and PET
Gas barrier performance is an important research field of thin film materials. According to different applications, gas barrier performance of thin film materials can be divided into barrier or preservation ability of oxygen, nitrogen, carbon dioxide, helium, water vapor, sulfur hexafluoride and other gases, due to the molecular diameter of gases, critical temperature, gas molecules and the different interaction between polymers in materials, the permeability of different materials to same gas is different, the permeability of the same materials to different gases is also different. Therefore, it is necessary to select suitable film materials according to the requirements of applications. For packaging materials, the primary task is to prevent oxidation of products. Nitrogen-filled packaging is an important measure to achieve this function. Therefore, the barrier performance of packaging materials to oxygen and nitrogen is important.
In this experiment, three commonly used film materials – polyethylene (PE), biaxially oriented polypropylene (BOPP), polyethylene terephthalate (PET) were used as tested for their oxygen and nitrogen permeability. Differential pressure method is used to measure the permeability of different gases. The test process is based on ASTM D1434 Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting.
In order to test the gas permeability of three samples and improve the test efficiency, VAC-V2 differential pressure gas permeability tester with three independent chambers is used. The instrument is developed and produced by Jinan Labthink Instruments.
The principle of differential pressure method assumes that when the volume and temperature are constant, the gas pressure is proportional to the gas volume. The test chamber is divided into upper and lower parts by the clamped sample. The upper chamber is filled with test gas under certain pressure. The volume of the lower chamber is fixed and known, and a low pressure environment is formed by vacuum pumping. During the test, the gas in upper chamber permeates into the lower chamber through the sample, results in the increase of the pressure in the lower chamber. The pressure sensor in the lower chamber monitors the increase of pressure in real time, and calculates the amount of gas permeated through the certain area of the sample in certain time.
Figure 1 VAC-V2 Differential pressure gas permeability test system
(1) Cut three specimens with 97 mm diameter from the samples. Apply a layer of vacuum grease around the three test chambers, place the filter paper and sample, press the contact area between the sample and the test chamber gently to remove bubbles, clamp the upper chamber cover.
(2) The equipment is connected with oxygen source. Se the test parameters such as sample name, sample thickness, test temperature, humidity and test mode in the operation software. Turn on the vacuum pump and click start button to start a test. The equipment runs the test according to the set parameters, and the test results are displayed after the test.
(3) When the oxygen permeability test is completed, connect the equipment with the nitrogen gas source, and set parameters to start the nitrogen permeability test.
Test results
The oxygen transmission rate of PE, BOPP and PET were 1603.578 cm3/(m2·24h·0.1MPa)、625.068 cm3/(m2·24h·0.1MPa)、16.953 cm3/(m2·24h·0.1MPa) respectively, the nitrogen transmission rate of PE, BOPP and PET were 403.658 cm3/(m2·24h·0.1MPa)、120.697 cm3/(m2·24h·0.1MPa)、2.487 cm3/(m2·24h·0.1MPa)respectively.
In this paper, three kinds of samples and two kinds of gas permeability were tested by one test. The test efficiency and the cost performance ratio of the test instrument is high, and the test results are accurate. From the test results, the order of permeability to oxygen and nitrogen from high to low is PE, BOPP and PET, shows that PET samples have the highest barrier to gases, followed by BOPP and PE. In addition, the nitrogen permeability of all three samples is lower than their oxygen permeability, which is due to the larger size of nitrogen molecular diameter than oxygen molecular.