Daniele Tammaro

A novel lab-scale batch foaming equipment: The mini-batch

In this paper, we report the design of a new experimental apparatus for the study of the foaming process of thermoplastic polymers with physical blowing agents. The novel lab-scale batch foaming equipment is capable of achieving accurate control of the processing variables, namely, the temperature, the saturation pressure and the pressure drop rate and, furthermore, of allowing the achievement of very high pressure drop rates, the observation of the sample while foaming and the very fast extraction of the foamed sample. By recalling the considerations discussed by Muratani et al. (J Cell Plast 2005; 24: 15), the design converged into a simple, cheap, and very small pressure vessel, thereby denoted as mini-batch. We herein describe the overall design path of the mini-batch, its characteristics, configurations, together with some examples of use with polystyrene and CO2 as the blowing agent.

Dynamic fluid-film interferometry as a predictor of bulk foam properties

Understanding and enabling the control of the properties of foams is important for a variety of commercial processes and consumer products. In these systems, the role of surface active compounds has been the subject of many investigations using a wide range of techniques. The study of their influence on simplified geometries such as two bubbles in a liquid or a thin film of solution (such as in the well-known Scheludko cell), has yielded important fundamental understanding. Similarly, in this work an interferometric technique is used to study the dynamic evolution of the film formed by a single bubble being pressed against a planar air-liquid interface. Here interferometry is used to dynamically measure the total volume of liquid contained within the thin-film region between the bubble and the planar interface. Three different small-molecule, surfactant solutions were investigated and the data obtained via interferometry were compared to measurements of the density of bulk foams of the same solutions. The density measurements were collected with a simple, but novel technique using a conical-shaped bubbling apparatus. The results reveal a strong correlation between the measurements on single bubbles and complete foams. This suggests that further investigations using interferometric techniques can be instrumental to building a more detailed mechanistic understanding of how different surface-active compounds influence foam properties. The results also reveal that the commonly used assumption that surfactant-laden interfaces may be modeled as immobile, is too simplistic to accurately model interfaces with small-molecule surfactants.

Elasticity in Bubble Rupture

When a Newtonian bubble ruptures, the film retraction dynamics is controlled by the interplay of surface, inertial, and viscous forces. In case a viscoelastic liquid is considered, the scenario is enriched by the appearance of a new significant contribution, namely, the elastic force. In this paper, we investigate experimentally the retraction of viscoelastic bubbles inflated at different blowing rates, showing that the amount of elastic energy stored by the liquid film enclosing the bubble depends on the inflation history and in turn affects the velocity of film retraction when the bubble is punctured. Several viscoelastic liquids are considered. We also perform direct numerical simulations to support the experimental findings. Finally, we develop a simple heuristic model able to interpret the physical mechanism underlying the process.

Interferometric measurement of film thickness during bubble blowing

In this paper, we propose digital holography in transmission configuration as an effective method to measure the time-dependent thickness of polymeric films during bubble blowing. We designed a complete set of experiments to measure bubble thickness, including the evaluation of the refractive index of the polymer solution. We report the measurement of thickness distribution along the film during the bubble formation process until the bubble‘s rupture. Based on those data, the variation range and variation trend of bubble film thickness are clearly measured during the process of expansion to fracture is indicated.

Insight into bubble nucleation at high-pressure drop rate:

This paper presents insight in bubble nucleation in polymer foaming with physical blowing agent using a batch foaming technique. In our experiments the bubble nucleation is triggered by a sudden pressure drop that causes the supersaturation in the polymer gas solution. In fact, the pressure drop rate is an important process variable since it plays a role in both bubble nucleation and growth. Herein, we investigated very high pressure drop rates, and confirmed the great importance of the pressure drop rate as foaming process variable. The results show that the number of nucleated bubbles increases of one order of magnitude and the foam density is reduced if the pressure drop rate is increased from 50 to 500 MPa/s. Interestingly, the number of nucleated bubble increases linearly in a bi-logarithmic scale as function of pressure drop rate at all the investigated temperatures. Moreover, in the current paper, it is discussed how talc used as nucleating agent plays a role in cooperation with pressure drop rate on bubble nucleation at different foaming temperatures.

Anomalous swelling of molten PCL/scCO2 solutions

The swelling behavior of molten poly(e-caprolactone) in contact with carbon dioxide has been analyzed by using the pendant drop method. At subcritical pressures, the swelling kinetics of the polymer-gas solution follow the typical Fickian diffusion behavior, the volume approaching an equilibrium value after the initial transient. In supercritical conditions, after the first transient, the volume of the solution does not approach an equilibrium value but still, slowly increases. Raman experiments have suggested that this anomalous time-dependent swelling in scCO2 could be due or to slow, secondary diffusion, or to polymer dissolution in scCO2.

PS foams at high pressure drop rates

In this paper, we report data on PS foamed at 100 °C after CO2 saturation at 10 MPa in a new physical foaming batch that achieves pressure drop rates up to 120 MPa/s. Results show how average cell size of the foam nicely fit a linear behavior with the pressure drop rate in a double logarithmic plot. Furthermore, foam density initially decreases with the pressure drop rate, attaining a constant value at pressure drop rates higher than 40 MPa/s. Interestingly, furthermore, we observed that the shape of the pressure release curve has a large effect on the final foam morphology, as observed in tests in which the maximum pressure release rate was kept constant but the shape of the curve changed. These results allow for a fine tuning of the foam density and morphology for specific applications.