The time to onset of cell opening and the modulus around the event were measured using a parallel-plate rheometer for flexible polyurethane foams. Cell opening time was compared with the visual blow-off time. The onset of cell opening occurs several seconds after phase separation, and the visual blow-off occurs several seconds (sometimes more than ten seconds) after the onset. Two series of formulations were studied, one in which the factors other than surfactant (tin catalyst level, TDI index, polyol type) were changed, and the second series in which only the surfactant was changed. In the first series, the modulus at cell opening did not change much with different formulations. However, the modulus development rate during the cell opening period (between the onset and the visual blow-off) changed significantly. Air flow was lower with higher modulus development rate. In the second series, the modulus development rate or the timing of onset of cell opening did not change even when the air flow changed considerably. The fact that the timing did not change and it is several seconds after the phase separation implies that the cell opening is triggered by sudden urea precipitation. Light interferometry showed that the average value of the remaining cell window thicknesses in low air flow foam is higher than that of the high air flow foam. Scanning electron microscopy (SEM) of the final foam products showed that the cell windows are in a variety of stages. The windows were classified into four stages-fully open, partially open, pin holes and closed. Air flow is directly proportional to the effective fraction of open windows. SEM also showed that the cell windows are in several different states. Some window films were totally missing and others have left torn-off films. The possible mechanism of cell window rupture is discussed with respect to the results.

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202101500is known for its near room temperature melting point of 29.76 °C and presents much lower toxicity comparing to liquid metal mercury. In recent decades, Ga has attracted growing research interests due to its fascinating properties including fluidity, metallic electrical and thermal conductivities. [1] Gallium and its alloys have shown applications in microfluidics, [2] sensing, [3] catalysis, [4] self-healing materials, [5] soft composites for biomedical devices, wearable electronics, [6] and electromagnetic wave shielding materials. [7] Gallium and its alloys are attractive materials for establishing soft composites for flexible and malleable electronics and sensors due to their variable electrical resistance under the modulation by a mechanical load. Especially, Ga and Ga-based alloy droplets, embedded into polymeric matrices, can deform alongside the polymeric materials, unlike solid particles. [8] In other words, by combining fluidic behaviors and metallic nature, Ga and its alloys offer distinct advantages that no other traditional fillers can offer. However, these Ga-based liquid metals also present a native thin passivating oxide layer on their surfaces, which drastically reduces the electrical conductivity of the composites. The insulating properties of Ga-based droplets can be overcome by mechanical rupturing of the oxide layer via pressing, [9] stretching, [10] expansion, [11] and twisting. [12] Therefore, Ga and its alloys have been commonly investigated as electrically conductive fillers for the formation of pressure and motion sensing elastomer composites.A variety of polymers, such as silicone (including polydimethylsiloxane (PDMS)), [13] polyvinyl alcohol (PVA), [6a,14] and poly(methyl methacrylate) (PMMA), [15] have been utilized as matrices for the inclusion of Ga-based fillers. However, a large volume fraction of liquid metal additives and mechanical activation are commonly required to achieve high sensitivity for the elastomer composites. [5a,6a,16] Additionally, such elastomers have intrinsic stretchability limitations. As compared to elastomers, sponge materials allow for much higher degrees of compressibility and reversibility, which present new opportunities for the synthesis of highly sensitive pressure-sensing devices and other electrical components.The distribution of the liquid metal additives into polymeric matrices was shown to affect the resulting thermal and electrical properties. [16][17] Typical strategies to disperse liquid Liquid metal droplets of gallium (Ga) and Ga-based alloys are traditionally incorporated as deformable additives into soft elastomers to make them conductive. However, such a strategy has not been implemented to develop conductive sponges with real sponge-like characteristics. Herein, polyurethanebased sponges with Ga microdroplets embedded inside the polyurethane walls are developed. The liquid phase (at 45 °C) and solid phase (at room temperature) transition of the Ga fillers shows the temperature-dependent functional variations in the mec...

An optical fiber evanescent wave fluorescence cure sensor described in a previous paper (1) has been interfaced with a fast detector and computer software to provide real-time monitoring of fast-reacting systems. Advantages to the new system include a reduction in data acquisition time (to 200 ms from 3 min), improved time resolution (to 5 sec from 10 min), and improved signal quality. Isothermal cure monitoring measurements were conducted in both distal and evanescent wave sampling modes for an epoxy-amine system. Following a second order background correction, the results for the two sampling modes were identical. Thus the evanescent wave measurement was unable to detect differences between the chemistry in the bulk and at the fiber-resin interface (if there were any differences to observe).The sensor system was also demonstrated for a fast curing ( < 3 min gel time) polyurethane-isocyanurate system designed for high volume (short cycle time) automotive applications. The system is currently being modified for implementation in a "real world" manufacturing environment.