Foam-in-place process

In the foam-in-place process a liquid urethane chemical mixture containing a fluorocarbon blowing agent is simply poured into a cavity or metered in by machine. The liquid flows to the bottom of the cavity and foams up, filling all cracks and comers and forming a strong seamless core with good adhesion to the inside of the walls that form the cavity. The cavity, of course, can be any space, from the space... 

Polyurethane foams used as a core in sandwich structures are usually made by the foam-in-place process. In this process, the skins (cured polyester, epoxy fiberglass, or aluminum) are laid up in a mold, and the urethane foamant is injected into the space between the skins. 

Much effort has been expended to try and produce flat-top foams. In one process polyethylene sheets placed along the side-walls of the trough rise with the foam. In another technique the reactants are metered from the mixing head into a fixed trough in which partial expansion takes place. The foaming material is then drawn over a weir by a moving band of paper and then drawn down a slope so that the top surface maintains a constant level as the material expands. 

There are many ways in which foams can be processed and used as slabs, blocks, boards, sheets, molded shapes, sprayed coatings, extruded profiles, foamed in place in existing cavities, in which the liquid material is poured and allowed to foam, and as structural foams (Chapter 6, STRUCTURAL FOAM). Conventional equipment such as extruders, injection, or compression machines is used. However specially designed machines are available to just produce foamed products. 

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

TDI Prepolymer Process. This process has the major advantage of better flowability in pour-in-place processes, e.g., household-refrigerator insulation, than crude TDI-based or polymeric MDl-based processes. An example of TDI-based prepolymers and a formulation for making rigid urethane foam on a small scale is shown below. 

Foam-In-PIace. The foam-in-place (or pour-in-place) method is used for the production of refrigerators, deep freezers, sandwich panels, and similar applications. This process is also used for field applications, such as indoor- and outdoor-tank insulation (79), LPG (liquefied petroleum gas) tank insulation, heavy oil-tank insulation, chemical-tank-car insulation, and pipe-covering insulation, among others (79). 

Notice 1 (30 July 1987) validated the specification. This is the Air Force foam-in-place specification. It covers the materials required and the preparation of the formulation used in the process. 

A process that somewhat bridges the gap between molded packaging and foam-in-place uses a fixture, rather than the actual product, to produce polyurethane cushions, encapsulated in polyethylene sheet, of the desired size and shape. As mentioned, foams can also be produced in bags rather than directly in the box, so that the product does not need to be surrounded by polyethylene. Polyurethane foams can be produced by extrusion methods, as well, but these are rarely used for packaging applications. 

Obtained forms of foamed plastics are blocks, sheets, slabs, boards, molded products, and extruded shapes. These plastics can also be sprayed onto substrates to form coatings, foamed in place between walls (i.e., poured into the empty space in liquid form and allowed to foam), or used as a core in mechanical structures. It has also become possible to process foamed plastics by conventional processing machines like extruders and injection-molding machines. 

Several manufacturing processes can be used to produce phenolic foams (54, 74) continuous production of free-rising foam for slabs and slab stock similar to that for polyurethane foam (65,75) foam-in-place batch process (56,76) sandwich paneling (58,77,78) and spraying (65,79). 

Applications. Rigid polyurethane foam laminates for residential sheating (1.2-2.5-cm-thick with aluminum skins) and roofing board (2.5-10.0 cm thick with roofing paper skins) are the leading products, with about 45 t of liquid spray systems also in use. Metal doors insulated by a pour-in-place process constitute another substantial use. 

Flexible polyether foams are most commonly produced by a one-shot process. In this procedure, diisocyanate, polyol, water, catalysts and surfactant are all mixed simultaneously. Commonly, the mixture is fed continuously into a moving trough wherein, after a few seconds, the foam begins to rise. After 1—2 minutes the foam reaches its maximum height it is then cut into blocks which are allowed to stand for several hours before being sliced into sheets of the required thickness. Alternatively, the mixture of reactants is fed into heated moulds to give foamed-in-place mouldings. 

Foams differ in the process conditions under which they are generated. A material that requires a high temperature or an external pressure for foam formation cannot be easily foamed in place, as could a system that can be mixed and poured at room temperature. And, of course, the polymer from which the foam is derived can be a thermoplastic or a thermoset, rubbery, or glassy. 

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