Foam Expansion

Initiation and Growth of Cells. The initiation or nucleation of cells is the formation of cells of such size that they are capable of growth under the given conditions of foam expansion. The growth of a hole or cell in a fluid medium at equiUbrium is controlled by the pressure difference (AP) between the inside and the outside of the cell, the surface tension of the fluid phase y, and the radius r of the cell ... 

One such process is the TAP process, the basic patent being held by Dow. It was developed in Japan by Asahi in conjuction with Toshiba. Foam expansion after mould filling is made possible by use of retractable mould cores. Because of the difficulty of allowing expansion in more than one direction this process has been largely limited to the production of flat products. Efficient gas sealing systems are also vital and the process needs close control. For this reason it has not been widely used in either Europe or North America. 

Swell Ratio Parameter for Prediction of Chemically Crosslinked Low Density Polyethylene Foam Expansion Characteristics... 

In the manufacturing of crosslinked polyolefin foam, the foam expansion and cell stabilization highly dependent on the degree of crosslinking. It has been reported that at higher crosslinking level the foam produce have higher density [1, 2]. 

Low-expansion foam—expansion up to 20 times foam to solution volume... 

A textured metallocene polyethylene foam sheet suitable for use in a floor covering is made using a highly coactivated azodicarbonamide package which blows the metallocene polyethylene effectively. The preferred coactivators are zinc oxide and urea. The textured surface of metallocene polyethylene foam is formed by a chemical embossing process which utilises a liquid triazole having an alkyl moiety as a foam-expansion inhibitor. The triazole is dissolved in a non-polar solvent to form the foam inhibitor. The preferred inhibitor is a hydrocarbon which may be halogenated. 

The density of chemicaUy-blown LDPE foam was altered by varying the amount of blowing agent, degree of crosslinking of the polymer, and the foam expansion temperature. A theory was proposed for the equilibrium density, based on the gas pressures in a Kelvin foam structure, and a rubber-elastic analysis of the biaxial stretching of the cell faces. 20 refs. 

Toronto, 27th April-2nd May 1997, p.2000-4. 012 CHARACTERISATION OF CROSSLINKED POLYETHYLENE FOR FOAM EXPANSION USING BUBBLE MACHINE Park C P... 

A simple bubble machine is devised and successfully applied in characterising lightly crosslinked PE resins for foam expansion. The biaxial stress-strain relationship is deduced from the air injection rate and pressure. The effects of strain rate, temperatnre and crosslinker level on the stress-strain behavionr are investigated. Uniaxial extension experiments are also performed and compared with biaxial extension data. 5 refs. 

Foamability Foam volume generated under a fixed set of conditions from a fixed amount of liquid. Endpoint can be based on foaming time or maximum sustainable foam volume. Methods to monitor foam volume and express foamability vary. Volume can be determined in a graduated cylinder. Alternatively, foam expansion is calculated by dividing the weight of foam by the weight of solution at constant volume. 

The foam expansion may be characterized by the expansion ratio, defined as the ratio of the height H at any time, with respect to the initial height, H0 ... 

Enzymatic hydrolysis modifies the foaming properties of casein. Protamex hydrolysates of sodium caseinate (DH 0.5 and 1.0%) displayed increased foam expansion at pH 2, 8 and 10 as compared with unhydrolyzed caseinate (Slattery and FitzGerald, 1998). Hydrophobic peptides resulting from... 

Recently a new technique has been introduced for the study of foam drainage under pressure drop. The especially constructed apparatus allows automated calculation of foam expansion ratio at any instant of time (see Section 5.3.4). 

A great advantage of mixing foam generators is the possibility to regulate both foam expansion ratio and dispersity, though within a narrow range of alterations. For example, at constant ratio of gas and liquid volumes the dispersity of foam increases when the consumption of liquid and gas rises [45]. 

In a monodisperse foam the deformation of spherical bubbles and formation of films at the places of their contact starts when the gas content in the system reaches - 50% (vol.) for simple cubic bubble packing or 74% for close (face-centred) cubic or hexagonal packing (foam expansion ratio - 4). In a polydisperse foam the transition to polyhedral structure starts at expansion ratio n - 10-20, according to [ 10] but, as reported in [51], this can occur at n < 4, the latter being more probable. The structure which corresponds to the transition of bubbles from spherical to polyhedral shape is called occasionally honeycomb structure. 

If the condition for polyhedricity R/r 1 is not fulfilled, the radius of curvature and the area of the cross-section become dependent on the co-ordinates along the length of the Plateau border. Analytical dependence of the radius of curvature on the co-ordinates (the border profile) at different foam expansion ratio is not found. 

The shape of foam films and border profiles in large interval of foam expansion ratio from 10 to 1500 has been experimentally studied in [83], A regular pentagonal dodecahedron made up of transparent organic glass with an elastic rubber balloon inside it which took the shape of a sphere at inflation (Fig. 1.10) was used as a model of foam cell. 

Both Figs. 1.10 and 1.11 indicate that the curvature of the border along its length becomes smaller compared to cross-section curvature even at foam expansion ratios from 40 -80 and rn/a = 0.5-0.35 (r in centimetres, Table 1.1). Hence, at first approximation, the deviation from the border shape typical for a fully polyhedral bubble can be determined by... 

On the other hand a comparison of the ratio between the volumes of real Plateau border and cylindrical border model, obtained by Pertsov et al. [85], with the ratio (rh /rn)2, evaluated from the dodecahedral model [83] indicates that these ratios correlate well only when n > 640 but when n < 300 they differ by almost 30%. However, it should be noted that the method for calculating border volume in [85,86] at low foam expansion ratio differs from the real volume by 10%. Besides, all values of expansion ratio given in the tables of [86] are twice higher. This means that the interpolation equations for calculating the corrections of the volume and hydro- and electrical conductivities in the cylindrical border model must be corrected. The Plateau border cross-section area is determined by Eq. (1.19) when the contact angle between film and border surface is small 6 (or 1 in radians). 

The foam expansion ratio or its reciprocal value the foam liquid volume fraction, called also relative or volume density of a foam, is used as a basic parameter characterizing the liquid to gas ratio in the foam. 

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