Determining the Open Cell Content of Cellular Plastics via Gas Displacement

Measuring open-cell content is a crucial aspect when it comes to selecting a suitable foam. This article describes using the BetterPyc 380, an ASTM D6226-compliant gas pycnometer, to obtain accurate and reliable results. The automated process saves time, helping users to select a foam based on specific requirements.

Plastic rigid foam is a durable and lightweight material that is employed in a wide variety of applications, such as packaging, insulation, and construction. The properties of the foam, including its compressive strength and thermal conductivity, depend heavily on its open cell content.

Open cells are interconnected pores within the foam that enable air and other gases to pass through, while closed cells are isolated pockets of gas that are completely enclosed by the solid foam material.

Measurement of the open cell content of the plastic rigid foam is critical for determining its quality and appropriateness for various applications.

One of the most reliable and accurate methods for measuring open cell content is the use of an automatic gas pycnometer.

This article aims to provide engineers and researchers with a complete guide to measuring the open cell content of plastic rigid foam by using the automatic pycnometer BetterPyc 380. The measurement method, experimental process, and measurement results are detailed below.

Determining the Open Cell Content of Cellular Plastics via Gas Displacement

Image Credit: Bettersize Instruments Ltd.

Method

Boyle's Law is utilized in the test method to establish the open-cell volume of a foam specimen, as defined in the ASTM D6226 standard. In a typical automated gas pycnometer, the analysis gas is pressurized in a calibrated sample chamber to target value P1.

The pressure is subsequently released by opening the expansion valve to the reference chamber and is documented as P2. Measuring the pressures both before and after the opening of the expansion valve allows the accessible cellular volume of the foam sample to be established.

A measure of the open-cell volume is given by the difference between the geometric volume of the specimen and the volume of the sample chamber displaced by the specimen.

Determining the Open Cell Content of Cellular Plastics via Gas Displacement

Image Credit: Bettersize Instruments Ltd.

Experimental

The procedure is described as follows:

  1. From the three types of foam samples, cut two 2.5 cm cubes with a blade and a ruler.
  2. Determine the geometric volume of the cubes accurately utilizing a Vernier caliper.
  3. With the BetterPyc 380, using the settings detailed in Table 1, measure the specimen displacement volume (VSPEC).
  4. Cut each cube into eight small foam specimens with the Foam Cutting Tool for trisecting both cubes.
  5. Place all 16 foam specimens into the sample chamber of the BetterPyc 380 and then measure the specimen displacement volume (VSPEC2).

Table 1. Measurement conditions. Source: Bettersize Instruments Ltd.

Parameter Value Parameter Value
Temperature 23 °C Sample cup 100 ml
Gas type Nitrogen Equilibrium 0.0050 psig/minute
Purge cycle 10 Purge pressure 3 psig
Analysis cycle 3 Analysis pressure 3 psig

 

The calculation formula for determining the open cell content (OV) of plastic rigid foam with the correction for cells opened during specimen preparation is as follows:

Where V denotes the geometric volume, VSPEC denotes the specimen displacement volume, and VSPEC2 denotes the specimen displacement volume after specimen trisecting.

Results

Table 2 provides a summary of the open-cell content of the foam samples. The software of the BetterPyc 380 automatically calculated the average open-cell content and standard deviation for each foam sample.

The results suggest that the foam-grey sample has the highest open-cell content, at 73.60%. This is followed by the foam-white sample with 51.43%, and lastly, by the foam-black sample with 48.23%.

For all three samples, the standard deviation is reasonably low, suggesting that the measurements were both repeatable and consistent.

Table 2. The open cell content of foam specimens. Source: Bettersize Instruments Ltd.

Sample Open Cell Content (%) Std. Dev. (%)
1 2 3 Average
Foam-black 48.09 48.39 48.22 48.23 0.12
Foam-grey 73.49 73.51 73.79 73.60 0.14
Foam-white 51.61 51.24 51.45 51.43 0.15

 

Determining the Open Cell Content of Cellular Plastics via Gas Displacement

Image Credit: Bettersize Instruments Ltd.

Conclusion

Employing the BetterPyc 380 for measuring the open-cell content of foam delivers accurate and reliable results for the selection of the most suitable foam material. The instrument adheres to the ASTM D6226 standard, offering a comprehensive and effective method for measuring the open-cell content of foam.

The automatic process of the BetterPyc 380 saves time and removes the requirement for complex calculations, meaning it is an efficient and convenient method.

This information has been sourced, reviewed and adapted from materials provided by Bettersize Instruments Ltd.

For more information on this source, please visit Bettersize Instruments Ltd.

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