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Microphone application density-Determination of open cell ratio of rigid foam (20191213)
2019-12-17 点击255次 McMurritique (Shanghai) Instrument Co., Ltd.
Foam plastic, also known as microcellular plastic, is composed of a polymer film wall and many small holes divided by the film wall. The rigid foam plastic measured in this article is a kind of foam plastic. It refers to foam that has no flexibility, high compressive hardness, and can reach deformation when the stress reaches a certain value. It cannot be restored to its original state after the stress is relieved. People usually use the thermal insulation and mechanical properties of rigid foam plastics as thermal insulation materials and sound insulation materials, heat insulation materials such as pipes and containers, floating materials and shock-absorbing packaging materials.
Rigid foam contains many closed, independent, open, and connected pores. Different pore structures will affect its product performance. The open porosity of rigid foam as an indicator of its insulation quality is a very important variable that needs to be controlled in the production and evaluation process of the product. Therefore, a simple and accurate open and closed porosity test method has practical significance for foaming process research and product quality control. The procedure for determining the porosity by air specific gravity method is provided in ASTM D2856, but the equipment used in this standard is no longer produced. At present, the open-closed porosity of rigid foams can be accurately measured by the AccuPyc series true density meter (Figure 1) of the United States. This article details how to use the AccuPyc series of instruments to perform the porosity test, and each detailed step and calculation process are explained in detail. Based on the ideal gas equation of state, the Accupyc instrument of the American Mike Company uses the gas displacement method to measure the open and closed porosity of rigid foams, which complies with GBT10799 and ASTM D2856 standards . Accupyc's method for testing the open porosity of rigid foams is easy to operate and accurate, with a small amount of test samples. It is only necessary to cut two cubes with a side length of 2.5 cm from the material to be tested, and then measure the volume of the sample. At this time, some closed holes become open due to cutting. You must correct them to obtain the corrected opening. The pore volume can accurately characterize the cell structure of the sample and correctly evaluate the product performance . Therefore, it is necessary to perform a second equal-area cutting on the test sample and measure the volume again to complete the correction calculation of the open-cell volume changed from closed-cell to open-cell. Figure 2 is a schematic diagram of Accupyc's equipment. AccuPyc II uses the principle of gas displacement method, using He as a gas medium, replacing the air in the voids and pores of the material to obtain the actual volume data of the material. As shown in the figure, the sample forms a sealed environment in the sample chamber. One valve is opened, and the gas enters the sample chamber. After a certain pressure is reached, the valve is closed. After the sample and the gas reach equilibrium, the pressure sensor reads the pressure P1 at this time. After the 2 valves are opened, the gas will diffuse from the sample chamber into the expansion chamber. After the gas and sample in the two chambers reach equilibrium, the pressure sensor reads the pressure P2 at this time. Then, based on the ideal gas state equation PV = nRT, the true volume V of the material is converted, and V is further used for the calculation of the porosity.
Figure 1 shows AccuPyc II 1340 full-automatic gas displacement method true density meter / open and closed porosity analyzer
Figure 2 Schematic diagram of AccuPyc's work
In order to obtain accurate test results, the volume of the instrument needs to be calibrated with the actual analysis gas before the sample is analyzed, and then the test is started.
Sample preparation. Use a special cutter to cut the sample into two 2.5 cm cubes, taking care to ensure that the resulting cubes are accurate in size.
Weigh the mass and measure the volume. The mass of the sample was accurately read with a ten-thousandth scale, and the reading was accurate to 0.0001 g. Accurately measure the geometric volume Vg of the two cubes cut out, Vg is a key parameter used to calculate the volume of openings.
Place the two cubes in the AccuPyc sample cell, close the lid, set the test conditions, and click Start to start the test. At this time, the instrument measures the gas replacement volume Vp1. For testing, a 100cc sample cell is usually used. Table 1 lists the recommended test parameters.
Table 1 AccuPyc instrument analysis test parameters
Purge fill pressure
Number of purges
Analysis fill pressure
Number of analyses
0.0050 psig / min
Use run precision?
Percent full scale
After the analysis is completed in step 3, open the cover of the sample compartment, take out the two cubes, and cut the two cubes three times along a line parallel to the side of the cube with a special tool (see Figure 3). After the two cubes are cut, 16 small cubes can be obtained. At this time, the exposed surface and the number of openings due to cutting will double.
Put the cut 16 small cubes into the AccuPyc sample cell, close the lid, and click OK to continue the test. At this time, the instrument measures its volume Vp2 .
Figure 3 is a schematic diagram of cutting
The volume of the open cells in the porous material is Voc, and the volume of the cut cells on the original surface of the sample is Vcc, then Vp1 and Vp2 have the following relationship:
Vp1 = Vg-Voc-Vcc Formula 1)
Vp2 = Vg-Voc-2Vcc Formula (2)
By (1) * 2- (2), we get
式（3） 2Vp1-Vp2 = Vg-Voc formula (3)
Then Voc is
式（4） Voc = Vg-2Vp1 + Vp2 Formula (4)
The aperture ratio is calculated as follows:
Co = Voc / Vg * 100% The porosity of the material can be used as a reference for quality control in the production and research and development of the product. Through the porosity data, it has practical significance for the research of foaming process and judgment of product quality. This article details how to use the AccuPyc II 1340 True Density Meter to perform the porosity test, and details the test principle and calculation process. The test sample has a small amount of use, and the correction of the cut holes on the surface of the sample is extremely simple. The test result has high accuracy and good stability, and can be used as a reference index for scientific research and quality control.
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