泡沫塑料又称微孔塑料，是由聚合物构成的膜壁和由膜壁分割成的许多小孔组成。本文中测量的硬质泡沫塑料是泡沫塑料的一种，它是指无柔韧性，压缩硬度大，应力达到一定值能产生形变，解除应力后不能恢复原状的泡沫塑料。人们通常利用硬质泡沫塑料的隔热性和力学特性将其用作绝热绝缘材料和隔音材料，管道和容器等的保温材料，漂浮材料及减震包装材料等。
硬质泡沫塑料中包含很多封闭的、独立的、开放的和连通的孔，不同的孔结构会影响其产品性能。而硬质泡沫塑料的开孔率作为其绝缘质量的指标，是产品生产和评估过程中非常重要的一个需要控制的变量。因此，一个简便易行且精确的开闭孔率测试方法，对于发泡工艺研究及产品质量控制都有实际意义。在 ASTM D2856 中提供了空气比重法测定开孔率的步骤，但是在此标准中所使用的设备已经不再生产。目前，通过美国麦克仪器公司 AccuPyc 系列仪器（图 1）可以精确测定硬质泡沫塑料的开闭孔率。本文详述了如何使用 AccuPyc 系列仪器进行开孔率测试，并对每一个详细步骤和计算过程都进行了详细的说明。

美国麦克公司的 Accupyc 仪器基于理想气体状态方程，采用气体置换法，测量硬质泡沫的开闭孔率，符合 GBT10799 和 ASTM D2856 标准。Accupyc 测试硬质泡沫塑料的开孔率方法，操作简便，结果准确，测试样品用量少。仅需要从待测材料中切下两个边长为 2.5 cm 的正方体，接着测定样品的体积，此时一些闭孔由于切割而变成开孔，必须对其进行校正，求出校正后的开孔体积，才能如实表征样品的泡孔结构并正确评价产品性能。因此，需要在测试样品上进行第二次等面积切割，并再次测定体积，完成由闭孔变成的开孔体积的校正计算。

Prior to analysis, calibrate the AccuPyc using the actual analysis gas and filling pressure.

Preparing and Analyzing the Sample

1. Slice two 1-in. (2.5 cm) cubes from the sample. Use a sharp cutting device such as a razor blade and a ruler to make sure the cubes are cut to precise dimensions.
2. Accurately determine the geometric volume of both sample cubes (V_g). V_g is a crucial number in computing the open cell volume.
3. Place both cubes in the sample container of the AccuPyc and determine the gas displacement volume (V_p1). The sample container holds 100 cm³. The two cubes occupy about 30% of the container, providing enough volume for precise measurements with the AccuPyc. Note that V_p1 is less than volume V_g. This is because open cells are excluded from the AccuPyc measurement. Some of these open cells were created while cutting the sample to size.
4. Cut both cubes three times with a razor blade along lines parallel to the sides of the cubes (refer to the figure below). Trisecting both cubes creates a total of sixteen smaller cubes and doubles the exposed surface and the number of cells opened by cutting.
5. Place all sixteen sample cubes in the sample container of the AccuPyc again and determine the volume (V_p2).

Determining the Percentage of Open Cells

If the volume of open cells in the cellular material is V_oc , the quantity measured in the third step above
(V_p1) is given by

V_p1 = V_g – V_oc – V_cc

The fifth step, which measures V_p2, is expressed by

V_p2 = V_g – V_oc – 2V_cc

Multiplying equation 1 by a factor of two and subtracting equation 2 from the result gives

2V_p1 – V_p2 = V_g – V_oc

which rearranges to

V_oc = V_g – 2V_p1 + V_p2

The open cell percentage (C_o) is then calculated as

C_o = V_oc/V_g x 100