Foamed structure-properties relation

The use of AOS and other surfactants as steam-foaming agents has been studied by several oil companies in laboratories and in the field [55-62]. In the next section we will view olefinsulfonate structure-property relations [40] that have helped design optimum surfactants for enhanced oil recovery applications. 

This survey deals with the fundamental morphological parameters of foamed polymers including size, shape and number of cells, closeness of cells, cellular structure anisotropy, cell size distribution, surface area etc. The methods of measurement and calculation of these parameters are discussed. Attempts are made to evaluate the effect and the contribution of each of these parameters to the main physical properties of foamed polymers namely apparent density, strength and thermoconductivity. The cellular structure of foamed polymers is considered as a particular case of porous statistical systems. Future trends and tasks in the study of the morphology and cellular structure-properties relations are discussed. 

The applicability of the Gibson and Ashby approach to structure-property relationships, particularly deformation mechanisms, is discussed in relation to a series of thermoplastic foams. The foams examined were based on LDPE, EVA, and a propylene copolymer. A full range of foam mechanical properties is discussed. 8 refs. EUROPEAN COMMUNITY UK WESTERN EUROPE... 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

FOAM STRUCTURAL PARAMETERS AND RELATED PROPERTIES TECHNIQUES FOR DETERMINATION... 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

Chapter 4 Foam Structural Parameters and Related Properties Techniques 345... 

A clearer understanding of the relationship between foam structure and mechanical properties of solid foams has been developed by Gibson and Ashby (1988). They related the mechanical properties (e.g., strength, modulus, yield stress, fracture toughness) of idealised cellular solids to their relative density. This work considered the cell walls of solid foams as a three-dimensional network of beams (Figure 20.18) and treated their deformation in terms of classical solid mechanics, with strength and modulus related to beam thickness and length by the equations ... 

It is then possible to generate a Finite Element Mesh of the foam microstructure to enhance understanding of the physical properties and how the relate to the structure. A sample of a foam structure with an FE mesh is shown in Figure 18. 

There are significant differences in properties between PS foams and PE foams. How do these relate to the chemical structure/property relationships of the base polymers ... 

Before discussing the various forms of polymers which may be encountered it is pertinent to describe briefly the basic structure of water-based polymer dispersions(ll). The starting point is a monomer which forms droplets in water. Aqueous surfactants are adsorbed at the droplet surface to stabilise the emulsion before an initiator is added to cause polymerisation under controlled conditions of pressure, temperature and stirring rate. The latex is thus an aqueous dispersion of small discrete polymer particles to which is added a range of additives, such as coalescents, anti-foaming agents, bacteriocide and anti-oxidants, to improve shelf life and properties related to its end use. 

Polymer Composition. The properties of foamed plastics are influenced both by the foam structure and, to a greater extent, by the properties of the parent poljuner. The poljuner phase description must include the additives present in that phase as well. The condition or state of the pol5nner phase (orientation, crystallinity, previous thermal history), as well as its chemical composition, determines the properties of that phase. The pol5uner state and cell geometry are intimately related because they are determined by common forces exerted during the expansion and stabilization of the foam. 

The mechanical properties of structural foams and their variation with polymer composition and density has been reviewed (100). The variation of structural foam mechanical properties with density as a fimction of polymer properties is extracted from stress-strain curves. However, because of possible anisotropy of the foam, the data must be considered as apparent data. These relations can provide valuable guidance toward arriving at an optimum structural foam, however. 

Structural Variables. The properties of a foamed plastic can be related to several variables of composition and geometry often referred to as stmctural variables. 

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. 

Expansion and crosslinking processes and blowing agents used in the preparation of low density polyolefin foams are examined. Properties of these foams are reviewed and related to structure, and major application areas are described. 148 refs. 

Caessens, P.W.J.R., de Jongh, H.H.J., Norde, W., Gruppen, H. (1999). The adsorption-induced secondary structure of p-casein and of distinct parts of its sequence in relation to foam and emulsion properties. Biochimica et Biophysica Acta, 1430, 73-83. 

While the Bingham plastic model is an adequate approximate description of foam rheology, it is by no means exact, especially at low strain rates. More detailed models attempl to relate the rheological properties of foams to the structure and behavior of the bubbles. 

The properties of a foamed plastic can be related to several variables of composition and geometry often referred to as structural variables. These variables include polymer composition, density, cell structure (i.e., cell size, cell geometry, and the fraction of open cells), and gas composition. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Surfactant foaming properties are related to surfactant chemical structure parameters such as hydrophobe size, ethylene oxide chain length, and hydrophile functional group. 

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