Thermosetting foams

A new era of plastics began with the appearance of plastic foams. This period might be called the Plastic Foam Age.  

Plastic foams can be called expanded plastics, cellular plastics or foamed plastics, and include both thermoplastic and thermosetting plastics. 

Thermosetting foams can be defined as foams having no thermoplastic properties. Accordingly, thermosetting foams include not only cross-linked polymer foams, but also some linear polymeric foams having no thermoplastic properties, e.g., carbodiimide foams and polyimide foams. These foams do not melt and turn to char by heating. 

In principle any kind of polymer-forming reactions can be employed for foam preparation. Accordingly, all kinds of thermosetting polymers can theoretically lead to foamed materials. 

Polyvinyl alcohol cellulose Formal formation Flexible 

In this group of products, foaming takes place at the same time with the chemical reaction (polycondensation) between the initial starting materials in view to produce the polymer. The most common are made of PU, PIR, PF, UF, EP, and silicone polymers. The most versatile and most used in the construction industry are the foams based on PU and more recently on PIR. 

PU foam is available in flexible or rigid forms, closed and open cell. Its characteristics 

Though closed cell rigid PU foams are excellent thermal insulators, they suffer from the drawback of unsatisfactory fire resistance even in the presence of phosphorus, and halogen-based flame retardants. From the flammability point of view, PIR, which are also based on isocyanates have greater flame resistance than PU. PIR withstand service temperature up to 149 °C compared with 93 °C for PU [35]. 

PIR foams are produced by using standard PU foaming equipment. Unmodified PIR foams have a highly crosslinked structure, and therefore are extremely brittle. What did prove successful was to lower the crosslinking density of the foams by adding modifiers, which led to, modified polyisocyanurate foams such as [40] urethane-modified PIR foam, amide-modified PIR foam, imide-modified PIR foam, carbodiimide-modified PIR foam and oxazolidone-modified PIR foam. 

Dimensional stability in the case of closed cell foams is dependent on the ability of the foam to resist atmospheric pressnre. Changes in the internal cell pressure are due to the fact that the initial gas in the cells does not diffuse out quickly. The foam, to be stable, must resist the differential pressnre. 

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Additives have the same effect on thermoplastic foaming processes as on thermoset foaming processes. Environmental conditions are important in this case because of the necessity of removing heat from the foamed stmcture in order to stabilize it. The dimensions and size of the foamed stmcture are important for the same reason. 

Polystyrene and polyurethane (thermoset) foams dominate the market, with PVC and polyethylene foams accounting for less than 1%. 

Control of crosslinking is critical for processing thermoset plastics, both the reaction prior to the gel point and that subsequent to the gel point. The period after the gel point is usually referred to as the curing period. Too slow or too rapid crosslinking can be detrimental to the properties of a desired product. Thus, in the production of a thermoset foamed product, the foam structure may collapse if gelation occurs too slowly. On the other hand, for reinforced and laminated products the bond strength of the components may be low if crosslinking occurs too quickly. 

For the preparation of the foam, a solution of 1 g technical sodium diisobutyl naphthalene sulfonate in 50 ml of 3% orthophosphoric acid is prepared. 20 ml of this solution are poured into a 11 beaker and air is stirred in with a fast running mixer until the cream-like dispersion has reached a volume of 300-400 ml.Then,20 ml of the prepared urea/formaldehyde resin are mixed in, whereby the resin must be evenly distributed. After 3-4 min the introduced resin gellifies into a molded article permeated with many water/air pores under the influence of the acidic catalyst. After 24 h,the crosslinking is completed. Drying for 12 h at 40 °C in a circulating air dryer yields a brittle thermoset foam.The foamed plastic obtained is hydrophobic and has a large internal surface. It can take up about 30 times its own weight of petroleum ether. 

The proper balance between viscosity and gas evolution can be controlled by a number of factors such as a suitable type and concentration of catalyst and surfactant, the presence of a nucleating agent (not always necessary) (17,18) and control of reaction temperature (or exotherm). Additional factors that must be considered are the use of a suitable chemical blowing agent, which is especially important for the production of thermoplastic foams, and the formation of oligomers (prepolymers) which exhibit higher viscosities than monomers in the preparation of thermoset foams (e.g. polyurethane foams). 

Most thermosetting foams are prepared by the simultaneous occurrence of polymer formation and gas generation. This is the principle of preparation of thermosetting plastic foams, as shown in Figure 1. 

Table 1 shows a classification of thermosetting foams. Among the foams listed in this table, polyurethane foams have the largest market share in the thermosetting-plastic-foam market. 

Table 1 Classification of Thermosetting Foams Foam Reaction Property...

Thermosetting Foams 15 Table 2 Model Isocyanate Reactions for Foams... 

Thermosetting Foams 63 Table 21 Comparison of Molded Flexible Foams... 

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