Closed cell content in foams
The determination of closed and open cell content in foams is based on the determination of the samples volume by means of gas pycnometry, which is an analytical method for density and volume analysis described separately on this homepage. When investigating foams, the most common challenge is to determine the amount of vesicular polymer cells completely closed, as these cells determine the insulation capacities of rigid foams commonly used in the thermal insulation of housing.
The measurement is carried out as stated per DIN ISO 4590 „Determination of the volume fraction of open and closed cells in rigid foams“. Initially, a geometrically exact sample body will be cut (cube, cuboid or cylinder) and its exact dimension will be determined by means of a micrometer in order to calculate the geometric volume. Afterwards, the sample body will be analyzed in the pycnometer at a low pressure of roughly 0.25 bar. The analytical gas employed here is nitrogen, as helium will penetrate into the walls of the closed cells in the foam. This first measurement includes the volume of the sample body yielding the closed volume, into which the analysis gas cannot penetrate and from which the amount of closed cells will be derived. The so-called uncorrected method terminates here, the quotient from closed volume to geometric volume multiplied by 100% determines the percentage of closed cell content in the sample.
In the so-called corrected method relates to the fact that by cutting through the sample cells previously closed will be opened. This can be corrected by an additional measurement. For this, the sample body will be cut into smaller parts and all parts obtained after cutting will be measured in the pycnometer again. The amount and position of cuts as well as the equation for results depend on the geometry of the original sample body and can either be determined by DIN ISO 4590 or by measuring.
DIN ISO 4590