Reactive foaming

Foams are one of the most important outlets for polyurethanes. All polyurethane foams are produced by the process of reactive foaming, during which polymerization and expansion proceed simultaneously. There are two basic variants of this process. The first produces large slabs of foam that are subsequently cut or otherwise shaped to meet end use reqiiirements. During the second variant foams expand within a mold of some type that determines their final shape. 

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 bet veen 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. 

Once fully cured the foam slabs are further sliced to the desired size. Thin slices can be shaved lengthwise from long flexible foam slabs and rolled up to provide foam roll stock for use in upholstery and other industries. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction vith isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated %vithin the mixing head. 

The hot curing process normally uses polyether diol precursors with molecular weights of 3,000 to 5,000 g/mole. We can control the stif iess of the foam by adjusting the average number of isocyanate groups on the chain extender molecules. The higher the functionality of the isocyanate molecules, the more crosslinked, and hence stiffer, will be the product. 

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Polyurethanes differ from most other polymers in that polymerization frequently takes place at the same time that we are molding or forming them into a usable shape. The three most common processes of this type are reactive foaming, reactive injection molding (RIM), and reactive spray coating. 

The investigations (26—29) illustrated the difference between reactive and nonreactive foams. During the tests, core permeabilities ranged from 0.5 to 5.0 md. Fluid loss of the nonreactive foam was approximately half of the reactive foam, although the stability of each did not show a significant difference. This result suggests two possible scenarios. The first is that the foamed acid is destabilized in its reaction with limestone, and this destablization causes greater fluid loss of the gas phase. The second is that the permeability is increased as the add dissolves the limestone. 

Field results are inconclusive as to the mechanism that controls fluid loss of the reactive foams. In certain situations, improvement in productivity has been gained by using foamed add instead of conventional add treatments. However, many field applications of foamed acid show no stimulation benefit over conventional acid treatments. 

The reactive foam exhibited good fluid-loss properties however, the gas phase of the foam was lost to the rock matrix at an extremely rapid rate. In an actual treatment stimulation scenario, the rapid loss of gas would result in the foam quality being depleted at a short distance into the fracture. The treatment would then essentially be reduced to a conventional add treatment with no real gain in production being accomplished. 

Air-supported structures of both t3q)es have been used as forms on which to cast concrete structures. In addition, some developmental work has been done to rigidize such enclosures by the use of reactive resin systems. In addition to the catalyzed setting up of the resin in the membrane, as in the space structure approach, urethane foam materials (and other reactive foams) have been introduced into the beam elements to replace the gas. 

One-step reactive foaming is typical for thermoset polymers. A good example is PU/cIay nanocomposite foams [14,15], where a physical blowing agent, such as pentane, is mixed with monomers and clay nanoparticles. Reaction exotherm leads to a tanperature jump and foaming. Most thermoplastic nanocomposite foams, to date are synthesized via a two-step process the nanocomposite is synthesized first and followed by foaming. 

Nonreactive additive flame retardants dominate the flexible urethane foam field. However, auto seating appHcations exist, particularly in Europe, for a reactive polyol for flexible foams, Hoechst-Celanese ExoHt 413, a polyol mixture containing 13% P and 19.5% Cl. The patent beHeved to describe it (114) shows a reaction of ethylene oxide and a prereacted product of tris(2-chloroethyl) phosphate and polyphosphoric acid. An advantage of the reactive flame retardant is avoidance of windshield fogging, which can be caused by vapors from the more volatile additive flame retardants. 

The isocyanates used with rigid foam systems are either polymeric MDI or specialty types of TDI. Both contain various levels of polymerized isocyanate groups which contribute to molecular weight per cross-link and also may affect reactivity due to steric hindrance of some isocyanate positions. 

Spray. In spray-on appHcations the reactive iagredients are impingement mixed at the spray head. Thickness of the foam is controlled by the amount appHed per unit area and additional coats are used if greater than 2.5 cm (1.0 ia.) thickness is required. This method is commonly used for coating iadustrial roofs or iasulatiag tanks and pipes. 

The next step is to apply a number of loss control credit factors such as process control (emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operating procedures, reactive chemical reviews), material isolation (remote control valves, blowdown, drainage, interlocks) and fire protection (leak detection, buried tanks, fire water supply, sprinkler systems, water curtains, foam, cable protection). The credit factors are combined and appHed to the fire and explosion index value to result in a net index. 

Flame retardants (qv) are incorporated into the formulations in amounts necessary to satisfy existing requirements. Reactive-type diols, such as A/ A/-bis(2-hydroxyethyl)aminomethylphosphonate (Fyrol 6), are preferred, but nonreactive phosphates (Fyrol CEF, Fyrol PCF) are also used. Often, the necessary results are achieved using mineral fillers, such as alumina trihydrate or melamine. Melamine melts away from the flame and forms both a nonflammable gaseous environment and a molten barrier that helps to isolate the combustible polyurethane foam from the flame. Alumina trihydrate releases water of hydration to cool the flame, forming a noncombustible inorganic protective char at the flame front. Flame-resistant upholstery fabric or liners are also used (27). 

Provide adequate fixed fire protection for tanks and vessels containing flammable, unstable or reactive materials. This can include fire loops with hydrants and monitors in the storage area, foam systems for individual tanks, and deluge spray systems to keep the exposed surfaces of tanks cool in case of fire in an adjacent tank. 

As will be discussed later, flexible polyester foams are not altogether satisfactory for upholstery applications and in the 1950s the attention of American chemists turned to the use of polyethers. These materials could be obtained more cheaply than the polyesters but the products were less reactive and with the catalyst... 

Polymeric MDIs, which are also used in polyurethane foams, usually have a lower reactivity than the monomeric material but are also less volatile. The polyisocyanurate produced from this material will be of the type shown in Figure 27.11. 

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