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We study experimentally the gravity-driven bubbly flow inside a large Hele-Shaw cell. The bubbles and foam were created by a series of upside-down overturns of the half-filled cell about its horizontal axis. When the liquid flows down it entraps a large number of bubbles, which remain stable as the liquid contains surfactant molecules. The total number and sizes of these bubbles slowly asymptote to a steady state after dozens of overturns. It takes longer to reach this asymptote when the viscosity of the liquid is larger. The bubbles also become more monodisperse with more cell over-turns. The number and distribution of the bubbles in turn affects the average motion of the liquid phase, which is characterized by the downwards motion of the liquid center of mass. We use high-resolution 6k video-camera to track the trajectories of thousands of bubbles. This required the development of software codes to identify individual bubbles and follow them between video frames. Successful thresholding algorithm required a machine-learning component, which was integrated into the program. This program also needed to account for possible splitting or coalescence of adjacent bubbles. The program can also find the velocities along the trajectories. In this way we can find the vertical velocity of bubbles as a function of their sizes. The smaller bubbles are sometimes observed to move downwards against their buoyancy. This occurs when the viscous stress from the surrounding liquid phase overcomes the upwards buoyancy force. Bubbles with similar sizes were often found to be stacking together and having worm-like rising movement that is faster than their individual rising velocity. The occurrence of the bubble stacking was dependent on the distance between the bubbles, their sizes and their wakes. Clusters of tiny bubbles that are much smaller than the gap of the Hele-Shaw cell were observed to form layers which can severely hinder the overall liquid motion.

The company Kiviks Musteri AB produces a pear base wine that forms stable foam, which is problematic from a production perspective. The aim of this thesis was to investigate the factors underlying foam stability in the pear base wine and to find means for its reduction. This was done by foam testing wines and varying several variables, such as the fining agents normally used in the wine production (bentonite, gelatin, siliceous earth and activated carbon), enzyme treatment, and by changing the fermenting yeast species. Results: The wine started to form stable foam during fermentation, and foam stability could be reduced by using more bentonite and carbon during the fining process. The other fining agents appeared to have only limited impact on foaming characteristics. No pectin was present according to the pectin test, but protein bands were evident from SDS PAGE analysis, though absent in samples treated with increased doses of bentonite. In conclusion, pectin is not a major foaming agent in the wine, the yeast is most likely the producer of the foaming agents, carbon and bentonite have a reducing effect on foam stability, bentonite also reduces protein content. Proteins are likely to be involved in the foam stabilization but are not the sole contributors to stable foam.

Wood Plastic Composites (WPCs) have experienced a healthy growth during the last decade. However, improvement in properties is necessary to increase their utility for structural applications. The toughness of WPCs can be improved by creating a fine cellular structure while reducing the density. Extrusion processing is one of the most economical methods for profile formation. For our study, rectangular profiles were extruded using a twin-screw extrusion system with different grades of HDPE and with varying wood fibre and lubricant contents together with maleated polyethylene (MAPE) coupling agent to investigate their effects on WPC processing and mechanical properties. Work has been done to redesign the extrusion system setup to achieve smoother and stronger profiles. A guiding shaper, submerged in the water, has been designed to guide the material directly through water immediately after exiting the die; instead of passing it through a water cooled vacuum calibrator and then through water. In this way a skin was formed quickly that facilitated the production of smoother profiles. Later on chemical blowing agent (CBA) was used to generate cellular structure in the profile by the same extrusion system. CBA contents die temperatures, drawdown ratios (DDR) and wood fibre contents (WF) were varied for optimization of mechanical properties and morphology. Cell morphology and fibre alignment was characterized by a scanning electron microscope (SEM). A new compression molding system was developed to help in quick evaluation of different material formulations. This system forces the materials to flow in one direction to achieve higher net alignment of fibres during sample preparation, which is the case during profile extrusion. Operation parameters were optimized and improvements in WPC properties were observed compared to samples prepared by conventional hot press and profile extrusion. / UOIT

This research provides a new insight on various properties, such as rheological, mechanical, and flame-retarding properties, as well as the foaming behaviors of wood flour /plastic composites (WPCs) through the addition of a small amount of nanosized clay particles. Although WPCs have replaced natural wood in many applications, their industrial usage has been limited because of their weak modulus, low impact strength, low screwing-ability/nailing-ability, high density compared to natural wood, as well as their flammability compared to plastics. In this context, the incorporation of nanoclay and foam structure into WPC has been studied to dramatically alleviate these drawbacks. The melt blending method was used to prepare different types of clay-filled wood flour composites such as intercalated and exfoliated clay nanocomposites. The effects of key processing variables such as the mixing time, mixing temperature and screw speed on clay dispersion were investigated from the thermodynamic and kinetic point of view. Their nanostructure was determined by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Accordingly, effective strategies for controlling intercalation and exfoliation of polyolefin/clay nanocomposites were proposed and evaluated. Wood flour composites with high levels of clay dispersion were synthesized successfully using a general new route (i.e., maleated-polyolefin-based clay masterbatch and dilution). The effects of nanoclay particles on the rheological, thermal, and mechanical properties were identified. In addition, it was demonstrated that a small amount of well-dispersed nanoclay in WPC significantly improved flame retardancy of WPCs. The mechanism of improved flame-retarding effects on nanoparticles was elucidated as well. The relationship between the clay dispersion and the material properties were also clarified. Furthermore, the foaming behaviors of HDPE-based and PP-based wood flour/nanoclay composites were investigated using N2 as the blowing agent in an extrusion process. The cell nucleation and growth behaviors of wood flour/polyolefin/clay composite foams were elucidated while varying the temperature, pressure, wood flour content, clay content and dispersion degrees.

The effects of crosslinking on EVA foaming are studied in this thesis. A fundamental approach was applied to describe the influences of crosslinking on EVA/gas viscosities, gas solubility and diffusivity in EVA, EVA foaming nucleation and early stage of bubble growth, which leads to a better understanding of the plastic foaming mechanism. Although crosslinked polyolefin foaming technology has been well applied in industry, more fundamental and thorough studies are demanded to understand the mechanism, which can serve to improve the present technology. The shear and extensional viscosities have been measured for the chemically crosslinked EVA with dissolved gas which could not be found from literature. Furthermore, by controlling the crosslinking agent amount, the polymer melt strength/viscosity can be controlled, so as to obtain optimum foam morphology. The crosslinking also has effects on the diffusivity and solubility of a blowing agent inside EVA. The solubility and the diffusivity of the blowing agent in the EVA decrease with the crosslinking degree increases. The diffusivity decrease makes more gas is utilized for the foaming rather than leak out of the polymer matrix quickly. This thesis also presents the fundamental studies on the effects of crosslinking on cell nucleation and early bubble growth. Theoretical work and in-situ visualization experimental results indicate that partial crosslinking leads to higher cell nucleation density and slower bubble growth, both of which benefit a fine-cell foam morphology generation. Last but not least, an optimized foaming process was conducted to produce chemically crosslinked EVA foams with large expansion ratios in a batch system, using a chemical blowing agent. The results determine that an optimal crosslinking degree is critical for the crosslinked EVA foaming with maximum expansion ratio. Furthermore, all research results not only benefit the foaming of crosslinked EVA, but also serve the better production of other crosslinked polyolefin foams.

The effects of crosslinking on EVA foaming are studied in this thesis. A fundamental approach was applied to describe the influences of crosslinking on EVA/gas viscosities, gas solubility and diffusivity in EVA, EVA foaming nucleation and early stage of bubble growth, which leads to a better understanding of the plastic foaming mechanism. Although crosslinked polyolefin foaming technology has been well applied in industry, more fundamental and thorough studies are demanded to understand the mechanism, which can serve to improve the present technology. The shear and extensional viscosities have been measured for the chemically crosslinked EVA with dissolved gas which could not be found from literature. Furthermore, by controlling the crosslinking agent amount, the polymer melt strength/viscosity can be controlled, so as to obtain optimum foam morphology. The crosslinking also has effects on the diffusivity and solubility of a blowing agent inside EVA. The solubility and the diffusivity of the blowing agent in the EVA decrease with the crosslinking degree increases. The diffusivity decrease makes more gas is utilized for the foaming rather than leak out of the polymer matrix quickly. This thesis also presents the fundamental studies on the effects of crosslinking on cell nucleation and early bubble growth. Theoretical work and in-situ visualization experimental results indicate that partial crosslinking leads to higher cell nucleation density and slower bubble growth, both of which benefit a fine-cell foam morphology generation. Last but not least, an optimized foaming process was conducted to produce chemically crosslinked EVA foams with large expansion ratios in a batch system, using a chemical blowing agent. The results determine that an optimal crosslinking degree is critical for the crosslinked EVA foaming with maximum expansion ratio. Furthermore, all research results not only benefit the foaming of crosslinked EVA, but also serve the better production of other crosslinked polyolefin foams.

This research provides a new insight on various properties, such as rheological, mechanical, and flame-retarding properties, as well as the foaming behaviors of wood flour /plastic composites (WPCs) through the addition of a small amount of nanosized clay particles. Although WPCs have replaced natural wood in many applications, their industrial usage has been limited because of their weak modulus, low impact strength, low screwing-ability/nailing-ability, high density compared to natural wood, as well as their flammability compared to plastics. In this context, the incorporation of nanoclay and foam structure into WPC has been studied to dramatically alleviate these drawbacks. The melt blending method was used to prepare different types of clay-filled wood flour composites such as intercalated and exfoliated clay nanocomposites. The effects of key processing variables such as the mixing time, mixing temperature and screw speed on clay dispersion were investigated from the thermodynamic and kinetic point of view. Their nanostructure was determined by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Accordingly, effective strategies for controlling intercalation and exfoliation of polyolefin/clay nanocomposites were proposed and evaluated. Wood flour composites with high levels of clay dispersion were synthesized successfully using a general new route (i.e., maleated-polyolefin-based clay masterbatch and dilution). The effects of nanoclay particles on the rheological, thermal, and mechanical properties were identified. In addition, it was demonstrated that a small amount of well-dispersed nanoclay in WPC significantly improved flame retardancy of WPCs. The mechanism of improved flame-retarding effects on nanoparticles was elucidated as well. The relationship between the clay dispersion and the material properties were also clarified. Furthermore, the foaming behaviors of HDPE-based and PP-based wood flour/nanoclay composites were investigated using N2 as the blowing agent in an extrusion process. The cell nucleation and growth behaviors of wood flour/polyolefin/clay composite foams were elucidated while varying the temperature, pressure, wood flour content, clay content and dispersion degrees.

Surface tension of a polymer melt in a supercritical fluid is a principal factor in determining cell nucleation and growth in polymer microcellular foaming. Previous work has presented the surface tension of the amorphous polymer, polystyrene (PS), in supercritical CO2 determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), together with theoretical calculations for a corresponding system. The dependences of the surface tension on temperature, pressure and polymer molecular weight were discussed and the physical mechanisms for three main experimental trends were explained using Self Consistent Field Theory (SCFT). This thesis introduces recent work on the surface tension measurement of the crystalline polymer, high density polyethylene (HDPE), in supercritical N2 under various temperatures and pressures. The surface tension was determined by ADSA-P and the results were compared with those of the amorphous polymer PS. The dependence of the surface tension on temperature and pressure, at temperatures above the HDPE melting point, ~125°C, was found to be similar to that of PS; that is, the surface tension decreased with increasing temperature and pressure. Below 125°C and above 100°C, HDPE underwent a process of crystallization, where the surface tension dependence on temperature was different from that above the melting point, i.e., decreased with decreasing temperature. Differential Scanning Calorimetry (DSC) characterization of the polymer was carried out to reveal the process of HDPE crystallization and relate this to the surface tension behavior. It was found that the amount of the decrease in surface tension was related to the rate of temperature change and hence the extent of polymer crystallization. In the second part of the thesis, surface tension dependences on temperature, pressure and clay concentrations were studied for HDPE nano-clay composites (HNC) and compared with pure HDPE. It was found the trends with temperature and pressure were the same with PS in CO2 and HDPE in N2; that is, the surface tension decreased with increasing temperature and pressure. In all nanocomposite samples, the surface tension decreased compared with pure HDPE. This could be a good explanation for the better polymer foaming quality with the addition of clay in the polymer. A minimum surface tension was found with the sample at ~3% concentration of clay. The degree of crystallinity of HNC was analyzed by Differential Scanning Calorimetry (DSC) at different clay concentrations. A minimumz crystallinity was also found at the clay concentration of 3%. The possible relationship between surface tension and polymer crystallinity was discussed.