* covered by several patents

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Rise Height and Rise Profile Measurement

The classic method for characterizing foams is to determine the rise height or rise profile by measuring the change in height due to the expansion of the foam sample in a cup, a cardboard box or a cylindrical container. The start time is generally accepted to be the start of the reaction between the mixed components A (polyol + additives) and B (isocyanate) after mixing. The rise time is the time which elapses until maximum expansion has occurred. The patented ultrasonic fan sensor PFT of the FOAMAT? system (Figs. 1, 2) can be used for all types of foams, including rigid foams with large heat release. The rise profile is the fingerprint of a foam. During quality assurance testing, it is compared with given master curves. A master curve (Fig. 3) is a tolerance band showing the margins of the rise profile for a “good” foam sample. Rise height measurement is still the standard test in foam qualification. With the FOAMAT? system new measurement techniques have become available, revealing more detailed information of the foam generation process.

Reaction Temperature

The exothermal cross linking reaction causes the temperature increase in the foam sample. Thin thermocouples are ideal for measuring the temperature inside the foam because they have a low heat capacity and are easy to handle. They interfere little with the foam formation and can be used repeatedly.

Foam Pressure Measurement

Pressure builds up in the foam after the components have set. Stable cell walls are formed that prevent the foam from expanding further and to release blowing agents. Thus, wall elements, panels or sheet metal are stressed at right angles to the direction of foam flow if they are foam backed for the purpose of providing insulation or rigidity. As high pressure forces are generated, the production equipment has to be reinforced or supported. The forces are measured as the rise pressure, so called because the local stress present inside the foam after setting is critically dependent on the foam rise height. The rise pressure is measured with the patented FPM (Foam Pressure Measurement) device. The FPM replaces usual test cups. Whereas the rise curve reflects the dynamics of blowing agent generation, the rise pressure mirrors the cell properties, which are affected by the polymerization reaction. Measuring the pressure can yield important information about the effects of catalysts and stabilizers on the setting reaction. For production purposes, the pressure curve determines the gel point and the pressure decay, which indicates when to open the mould. Since the foam is free to expand upwardly while the pressure is being measured, the ultrasonic fan sensor PFT can measure the rise height simultaneously.

Fig. 4. FPM 2 on the stand of the Foam Qualification System FOAMAT?.

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Viscosity Determination

A particular advantage of measuring the pressure by the FPM is that it allows the viscosity of the foam to be calculated directly from the experimental data provided by FOAMAT?. This is achieved by using the Hagen-Poisseuille viscosity model. The model starts on the assumption that the viscosity is determined by the force necessary to move a longitudinal element of foam at a specified speed through a tube, which is the FPM in this case. The reaction force is obtained directly from the rise pressure. The pressure data and the rise height curve measured in the FPM are sufficient for calculating the viscosity vs. time. For producers, the viscosity gives additional information for optimizing the process control in moulded foam production.

Dielectric Polarization Measurement

Dielectric polarization is a new measurement parameter that gives insight into the electrochemical processes occurring during foam formation. Dielectric polarization is essentially determined by chain-like molecules with a large dipole moment due to their polar ends (OH, NCO). Chain formation precedes the crosslinking reaction that ultimately suppresses all dipole mobility during curing. The dielectric polarization sensor CMD (Curing Monitor Device) is located in the bottom section of the FPM (Fig. 5). The dielectric polarization shows the formation of intermediates like polyurea and displays the final curing of the foam giving a constant signal after the chemical reaction is completed. CMD is provided in combination with the pressure measurement device FPM. In order to simulate the production conditions in a mould, the CMD sensor can be heated with closed loop control.

Undefined test container temperatures spoil the correlation between the test result and the production situation. PIR foams require external heat to react and cure properly. Otherwise these formulations remain sticky and voids appear. To overcome this, Format Messtechnik GmbH has introduced the temperature controlled Advanced Test Container ATC (Fig. 6). Another advantage of the new precise temperature controlled test container is the better reproducibility of the measurement data compared to non temperature controlled systems.

Fig. 6. The Advanced Test Container ATC is a temperature controlled test container giving most reproducible and production near test results.

System Configuration

The complete Foam Qualification System FOAMAT? and its periphery is shown in Fig. 8. The mixer operation is controlled by the software FOAM according to the user’s input. The controller unit and the balance are connected to the PC. The foot switch (pedal) is used to start a measurement cycle and to operate the mixer off-line.