Polyurethane rigid, foamed crosslinked

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. 

These compounds can be copolymerized with styrene to form crosslinked copolymers. When polyisocyanates are incorporated in these reaction systems, the resulting polymers are hybrid polymers containing polyurethane linkages. Rigid foams can be obtained by using the above reaction in the presence of a blowing agent. 

Polyurethane materials exist in a variety of forms including flexible or rigid foams, chemical resistant coatings, specialty adhesives and sealants, and elastomers. Most polyurethanes are thermoset materials they cannot be melted and reshaped as thermoplastic materials. Once the reactions have ceased the thermoset polyurethanes are cured and cannot be heat shaped without degradation. The thermal stability results from the croslinking degree of polymer chains (the crosslink density) and from the nature and frequency of repeating units within the polymer chains. 

Rigid polyurethane foams can be made from either polyester or polyether prepolymers, which are crosslinked with polyfunctional isocyanates. The resulting foams are largely closed cell, with only about 5 to 10% of cells being open. Rigid polyurethane foams are widely used as insulation in commercial, residential, and industrial settings. 

Crude MDI and PAPI are especially used in highly crosslinked polyurethanes, such as rigid polyurethane foams. Polymeric MDI have lower vapour pressures than TDI. Mixtures of TDI with polymeric MDI are also used in many applications, (e.g., in high resilience flexible foams). Aliphatic diisocyanates have a much lower reactivity than aromatic isocyanates. The most important aliphatic diisocyanates are presented in Figure 2.5 [1-3, 6, 23-25] ... 

Both structures (13.2 and 13.3) are used successfully in rigid PU foams. Structure 13.2, having a high hydroxyl number, is used more frequently as crosslinker in many polyurethane applications (rigid PU foams, coatings). 

A partial esterification reaction takes places (19.5). This side reaction does not have a negative effect on rigid PU foam fabrication because it gives a very convenient tetrafunctional compound, which participates, together with the amidic polyol, to build the crosslinked structure of polyurethane. These compounds containing ester groups are present in low concentrations, maximum 5-7% [1,2]. 

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. 

However, until recently, the principal application of polyurethanes has been as in situ-polymerized rigid and flexible foams used for insulation and cushioning, respectively.— — Unlike the phenolic, urea and furan mortars, in which the high polymer formation is the result of the chain extension and crosslinking of a prepolymer, the final polyurethane product is the result of a reaction between hydroxyl and isocyanate groups as shown by the following equation ... 

Plastics — rigid polyurethane foam, expanded polyst3u-ene, honeycomb, crosslinked PVC foam (one of the most important core materials), poljnnethacrylimides, other plastics foams (excellent thermal insulation, weaker and more expensive than paper). 

Figure 39. TG curves of unpyrolysed trimethylolpropane trimethacrylate (SR 350) (crosslinked), foamed semi-rigid polyurethane, and unpyrolysed trimethylolpropane trimethacrylate/polyurethane foamed blends (SR 350/PU weight ratio = 5.25/1, low-PU and 1/1, high-PU) under nitrogen...

MARISTICK 1750 Subfloor polyurethane adhesive from M-D Co Semi-rigid, two -component, without solvents Premium grade quick curing PUR foam adhesive Adhesion of metal, wood and ceramic tiles on horizontal and vertical surfaces Subfloor, plywood, gypsum, foamboards, block, fibreboard, drywaU, wallboard, brick, hardboard Provides elasticity of hard rubber, excellent adhesion without shrinkage, cures by crosslinking even at very low temperatures... 

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