The Foaming Process

Disperses Solid Urea Partic. in the Pblymer AAotrix 

In standard foam production, a mixture of liquid materials is dispensed and the gas bubbles within the foam structure are only generated by a rather slow chemical reaction. In the case of liquid carbon dioxide foaming the mixture is already dispensed in a two-phase process. 

In addition to nucleation, there is one further objective to be fulfilled or at least supported by the silicones used. That is the emulsification of the raw materials used in the foam 

Another major point of consideration is obviously the processing latitude of silicone surfactants. This phrase describes the variability of catalyst concentration without running into either closed, dead foam or observing first signs of foam instability indicated by splits in the foam. Not only will a different silicone surfactant result in a different processing window, e.g., amount of stannous octoate variation between the two extremes, but also the position of the processing window might be different. 

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Foam spray dryiag coasists of forcing gas, usuaHy air or nitrogea, iato the product stream at 1.38 MPa (200 psi) ahead of the pump ia the normal spray dryer circuit. This method improves some of the characteristics of dried milk, such as dispersibHity, bulk deasity, and uniformity. The foam—spray dryer can accept a condensed product with 60% total soHds, as compared to 50% without the foam process. The usual neutralization of acid whey is avoided with the foam—spray dryer (see Drying Foams Sprays). 

A rather strange but nevertheless large-scale application of U-F resins is in the manufacture of firelighters, made by a modification of the foam process. The resin solution is blended with a small amount of detergent and then whisked with paraffin. A hardener is added and the resin allowed to set. In effect the product is a U-F foam saturated with paraffin. 

Surfactants used as lubricants are added to polymer resins to improve the flow characteristics of the plastic during processing they also stabilise the cells of polyurethane foams during the foaming process. Surfactants are either nonionic (e.g. fatty amides and alcohols), cationic, anionic (dominating class e.g. alkylbenzene sulfonates), zwitterionic, hetero-element or polymeric (e.g. EO-PO block copolymers). Fluorinated anionic surfactants or super surfactants enable a variety of surfaces normally regarded as difficult to wet. These include PE and PP any product required to wet the surface of these polymers will benefit from inclusion of fluorosurfactants. Surfactants are frequently multicomponent formulations, based on petro- or oleochemicals. 

Most slabstock foams are open-celled, that is, the walls around each cell are incomplete. Towards the end of the foaming process, the polymer migrates from the membranes between cells to the cell struts, which results in a porous structure. In some cases, cells near the surface of the foam collapse to form a continuous skin, which may be trimmed off later. 

To confirm the heterogeneous nucleation and the nanocellular feature in the foam processing, TEM observation of the cell wall in the PLA/MMT-ODA foam was conducted. 

STUDY OF THE FOAMING PROCESS OF POLYETHYLENE WITH HIGH PRESSURE C02 IN A MODIFIED EXTRUSION SYSTEM... 

Demonstrations are given of the importance of extensional or elongational viscosity in the foam process. New polypropylenes are compared in extensional flow and it is shown how rheological differences allow the prodnction of low density foam on tandem extrnsion equipment. 6 refs. 

In industrial practice the foaming processes are sometimes divided into chemical and physical processes. Chemical processes are those where the formation of gas takes place by decomposition of an unstable inorganic or organic compound or by a chemical reaction. Physical processes mean such techniques where the foaming gases are pumped into the polymer or are formed by the evaporation of liquids. 

While we will discuss the control of the various properties presented in the last section, the composition of a device must represent an optimum of all the properties. As we have shown, the quality of a foam is achieved by a complex combination of chemical and physical effects. No unifying model combines them in a sufficiently precise way as to minimize the work involved. Thus, as we discuss ways to control compressive and tensile strength, we must be aware that these properties will affect the foaming process. The design process is made somewhat easier by using composite techniques. In this way, one can separate the physical requirements of a device... 

Figure 9.15 Temperature and relative height evolution during the foaming process. Full curves are the theoretical values predicted by solving the differential equations describing the system s behavior. (Vazquez and Williams, 1986. Reprinted with permission from RAPRA Tech. Ltd)... 

The aim of this section, therefore, is to correlate systematically the compatibilization of PPE/SAN 60/40 blends by SBM triblock terpolymers with the foaming behavior of the resulting blend. The reduction of the blend phase size, the improved phase adhesion, a potentially higher nucleation activity of the nanostructured interfaces, and the possibility to adjust the glass transitional behavior between PPE and SAN, they all promise to enhance the foam processing of PPE/SAN blends. 

The foam processing window of the blend systems is also controlled by the glass transition temperature of the blend phases. With regard to the neat blend system, the addition of PS continuously lowers the glass transition temperature of the PPE/PS phase, as predicted by the Couchman equation [77] (Fig. 25). For the carbon dioxide-laden case, the plastifying effect needs to be taken into account, which lowers the glass transition temperature of both PPE/PS and SAN. 

A special type of injection moulding is the manufacture of foamed articles. The foaming process takes place in the mould, so that mould filling requires considerably lower injection pressures and mould closing forces. Very large mouldings can be made in this way. 

Since melamine is not soluble in the polyol or MDI, it should be very fine dispersed so that it does not interfere with the foaming process. The effect of melamine particle size on properties of flexible PU foams was studied by Kageoka et al.69 They reported that the foam with the finer particles showed higher hardness, better tensile properties, and less flammability than that with the larger ones. A flame-retarded foam with better physical properties can be manufactured by a polyol including melamine particles smaller than the strut thickness of the resultant foam. 

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