Foams Structure and Some Properties

The presence of excess surface energy in such gas-liquid disperse system predetermines its nonequilibrium. However, by virtue of the stabilizing effect of surfactants, foam possesses a metastable structure and has a certain lifetime [118], Its properties slowly relax under the action of external factors provided that the latter do not exceed some threshold values beyond which the foam structure is destroyed. 

It has been recently discovered [273] that foam can also exist in steady state, where the phase volume fractions and the bubble size distribution are independent of time. This phenomenon occurs if the foam is a closed (in the terminology of [375]) but not isolated system, that is, there is energy exchange but no mass exchange between the foam and the ambient medium. Such conditions arise in closed vibrating containers under withdrawal of the dissipative heat due to vibrational motion. If a certain threshold of specific kinetic vibrational energy is exceeded, then, after some relaxation time, the foam becomes steady-state (and forms dissipative structures [311]) with Gaussian distribution of bubble sizes. 

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The concept of increasing the solubility was successfully demonstrated for PS-based blends, where CCh-philic components were added [30-32], e.g., fluoroalkyl, fluoroether, carbonyl and siloxane functional materials. Nevertheless, the benefits on the foam structure and properties could be achieved only in some cases. 

Selection of suitable blowing agents for each polymer is very important for producing dimensionally stable foams with the desired cellular structures and physical properties in various end-use applications. Some of the desirable properties of suitable blowing agents are as follows. 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Polyolefins are well adapted to the mono-material concept talc-filled polypropylene and LFRT for structural parts, foamed polyethylene and polypropylene for damping, polypropy-lene/EPDM alloys or copolymers for skins. Some other functions need incompatible polymers with specific characteristics such as optical properties. Without claiming to be exhaustive, the other thermoplastic materials are ... 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

Strandfoam, developed by Dow, is said to be an easy to shape polyolefin foam consisting of thousands of joined strands of foam. The structure is said to improve cushioning and insulation properties and may replace some open cell PU applications. The process is briefly described and utilises ethylene-acrylic acid in a PE carrier. DOW EUROPE SA... 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

The prepolymer used for this study was an MDI-based preparation (Suprasec 1002) from Huntsman PU, Brussels, Belgium. Immobilization of the enzyme was accomplished by coating the inside structure of a reticulated foam with an acetone solution of a hydrophilic polyurethane prepolymer. A 45-pore/in. reticulated foam cut into sheets 0.25 in. thick was used. The coated reticulated foam was immersed in a catalase solution (5 pg/ml) at 4°C and left in the solution for 1 hour to ensure a complete solution. It is known that both the water and some functionality in the catalase react with the isocyanate groups to cause polymerization (the water-isocyanate reaction) and chain termination (the catalase reaction). Controlling the relative concentrations and temperature permits control of the physical properties of the composite and the ability of the foam to function as an enzyme. 

The process of mould filling should not be considered as completely understood and controlled 98>, though some important features of gas-filled system flow were established by Fridman and co-workers 99 100) who have explained the formation of laminated structures in low-pressure moulding of foaming melts and found the correlation between moulding conditions and morphologic macro-structure and properties of foam articles. 

In tEe first four chapters, we have seen that many different kinds of polymers exist and that they have an extremely wide range of properties. Some are stiff, others are soluble, while still others are rubbery. There are plastics, and fibers, and adhesives, and foams. The structure and composition of the macromolecule dictate the ultimate properties. Structure and composition are determined when the macromolecule is synthesized. 

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