Branched and Crosslinked Polyurethanes

Essentially two methods can be used for the preparation of branched and crosslinked polyurethanes. 

The degree of crosslinking here depends essentially on the structure and functionality of the polyhydroxy compound so that the properties of the polyurethane can be altered by variation of this component. This procedure is applied mainly to the preparation of lacquers (reactions with diisocyanates at low temperature in anhydrous solvents such as butyl acetate) or moldings (usually with capped diisocyanates at higher temperatures). 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

Suitable starting compounds are polyesters from poly(ethylene oxide) and adipic acid, also poly(propylene oxide) or poly(oxytetramethylene) with molecular weights around 2,000, whose hydroxy end groups can be reacted with very 

The chain-extended, linear poly(ester urethanes) so obtained can now be crosslinked in a second stage, involving reaction with - water - or glycols - or diamines. 

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N-C-0- in an average molecule, and whether it is a linear, branched, or crosslinked polyurethane. These are more easily said than done. In pure chemical and model compound studies, these quantities can be controlled quite closely, but in the manufacturing kettle, control, though not that precise, will make a real difference in use properties. 

The reactions of urea interaction with isocyanate and m-ethane group with isocyanate are very important for synthesis of polyurethane isoeyanurate materials, because they induce formation of branched and crosslinked structures in polyurethanes. As is known, these structures are hard-wearing. These reactions are slowly proceeding (during 5 - 6 h), but are suitable because of required temperature level, equal to polyisocyanmate synthesis (120 -140°C), also usually applied in the industry. 

Of the two reactions, isocyanurate formation is the most widely utilized. Diisocyanates can be converted via this reaction into trifunctional isocyanurate derivatives and subsequently used to introduce branching and crosslinking into a polyurethane.These crosslinks have greater thermal stability than either allophonate or biuret linkages, and hence they are more useful in elevated temperature applications. 

Generally by increasing the MW between the branching points some properties of crosslinked polyurethanes, such as tensile strength, elongation, modulus and tear strength increase, while the hardness decreases [2],... 

Crosslinked polyurethanes are not soluble and of course swell, the degree of swelling decreases with the increase in crosslink density. For example, for a flexible polyurethane foam in the presence of acetone, the degree of swelling is around 116% at a molecular weight between branch points (Mc) of 1650 and becomes 90% at a Mc of 1070 and 83% at a Mc of 690 [2]. 

Cyclic derivatives of type III include cyclic Mannich bases, such as dihydroben-zoxazines 497, employed as detergents for lubricating oils, - and cyclic urcides 498, precursors of crosslinking agents for fabrics, as well as other cyclic derivatives prepared by conversion of Mannich bases. Macromolecular derivatives of type IV are relatively small in size and have branched (star-shaped) structures they are of considerable importance as, for example, corrosion inhibitors 499, plastics stabilizers 500, - pre-polymers for epoxy-based electrophoretic paints, and polyols in polyurethane synthesis. ... 

Chain branches in stepwise polymerization may occur due to side reactions such as the addition of an -OH to a double bond in unsaturated polyester synthesis. Alternatively, chain branches can be a delibemte addition such as in polyurethane synthesis where a functionality greater than 2 is brought to bear (e.g. by using a triol chain extender) so that chain branching can occur. In systems of stepwise polymerization in which the reagents are polyftinctional these branches will lead to the occurrence of crosslinks and gelation. This is... 

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. 

The preparation of polyurethane lattices is possible in several ways. In one instance special monomers are polymerized in an aqueous medium to produce a thermoplastic polyurethane emulsion. Or thermoplastic polyurethanes made in solution or dissolved in solvents are emulsified in water, then solvent is removed. Or an isocyanate-terminated urethane prepolymer, possibly with hydrophilic branches, is blocked with an appropriate isocyanate blocking agent, and emulsified in water together with a crosslinking agent. 

The network structure of linear and branched TPUs is obviously different. The branched thermoplastics are capable of forming allophanate and/or possibly biuret crosslinks under suitable conditions. These conditions are partly met by the processing temperatures but usually a subsequent hot-air cure is necessary to achieve the optimum set properties. Inevitably, therefore, the branched TPUs have considerably lower compression and tension set properties than the truly all-linear thermoplastic TPUs, and in this respect approximate to the castable polyurethanes. 

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