Flexible foams crosslinked structure

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

In Figure 1.9 one can see an hypothetical crosslinked structure of a flexible polyurethane foam resulting from an oligo-triol of MW of 3000-6500 daltons and a diisocyanate [3-16, 20]. 

Figure 1.9 Hypothetical crosslinked structure of a flexible polyurethane foam...

The idealised reaction for the hydrolysis of the crosslinked structure of flexible PU foams is shown in reaction 20.16. - - -x... 

Flame retardant flexible foams are very difficult to obtain due to the low crosslink density, low aromaticity, open cell structure and long polyolic aliphatic chains. Generally, flame retardants flexible foams are produced with additive flame retardants, for example with powdered melamine + tris (2-chloropropyl) phosphate [14]. 

A polyol of high MW (3000-6500 daltons) and of low functionality, of around 2-3 hydroxyl groups/mol, if reacted with a diisocyanate, leads to a low crosslinked, flexible polyurethane structure. This structure is characteristic of flexible polyurethane foams. Because the resulting structure is a crosslinked one, the MW of the resulting polyurethane is infinite in value. Only linear polyurethanes have a finite and determinable MW. 

For crosslinked polymers (in this category they are the majority of polyurethanes, for example flexible, semiflexible and rigid PU foams, etc.), which have a MW that is practically infinite [12], the molecular weight between the branching points (Mc) is considered. The value of Mc depends strongly on the oligo-polyol structure. 

In general, the thermoset material was obtained by heating the N-cyanourea terminated monomer at a temperature above 100 C or at its melting temperature. Depending on the structure, the crosslinking reaction occurred within a few seconds to several minutes, and the final product varied from rigid foam to flexible film. Typically,... 

An interesting class of monomers which polymerize into different polymeric materials at different temperatures was discovered. At room temperature, the linear pol3rmer was obtained either from the polymerization of a dicyanourea monomer or directly from the polymerization of a mixture containing a diisocyanate and cyanamide. At elevated temperature (>100 C), the dicyanourea monomer or the mixture of diisocyanate and cyanamide crosslinked to a rigid foam or flexible film depending on the structure of the monomer. IR, DSC, and NMR were used for characterization of the polymers. 

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