Triol-crosslinked polyurethanes

Buckley, C. P, Piisacariu, C., Caraculacu, A. (2007), Novel triol-crosslinked polyurethanes and their thermo-rheological characterization as shape-memory materials. Polymer, 48,1388-96. 

In addition, from thermal and thermomechanical measurements, it is found that typical epoxy-amine networks exhibit one glass transition temperature, Tg, and one sharp well-defined relaxation peak. The same techniques were used for crosslinked polyurethanes based on triol and diisocyanate or diol and triisocyanate (Andrady and Sefcik, 1983). Similar conclusions to those found for epoxy-amine networks were attained. 

In this study, the use of a PM polyol as a rubber modifier for a highly crosslinked, polyurethane resin (T = 150 °C) was assessed again in comparison with an oil-based PB polyol. The polyurethane resin matrix was formed from pure MDI and a polyol blend comprising a polyoxypropylene triol, LHT240 (Union Carbide) of equivalent weight 227.6 g-mol"1, and trimethylol propane,... 

If a prepolymer derived from an oligo-triol or an oligo-polyol, having three or more terminal -NCO groups is used, if it is in contact with atmospheric humidity, crosslinked polyurethanes are obtained. 

For imperfect epoxy-amine or polyoxypropylene-urethane networks (Mc=103-10 ), the front factor, A, in the rubber elasticity theories was always higher than the phantom value which may be due to a contribution by trapped entanglements. The crosslinking density of the networks was controlled by excess amine or hydroxyl groups, respectively, or by addition of monoepoxide. The reduced equilibrium moduli (equal to the concentration of elastically active network chains) of epoxy networks were the same in dry and swollen states and fitted equally well the theory with chemical contribution and A 1 or the phantom network value of A and a trapped entanglement contribution due to the similar shape of both contributions. For polyurethane networks from polyoxypro-pylene triol (M=2700), A 2 if only the chemical contribution was considered which could be explained by a trapped entanglement contribution. 

In a three-component polyurethane system with OH groups, wherein crosslinker is a triol, one finds them substantially more reactive than those of the macro-molecular diol. The hard clusters grow substantially larger, at relatively low conversions. Their size remains almost constant because all triol units have been used up in the reaction. In the opposite case, the lower reactivity of OH groups found in macromolecular diol compared with that of triol, the clusters remains small throughout the reaction and grows larger only at its end. 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

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... 

The product is called l,l,l—Trimethylolpropane (TMP) and is used mainly in the manufacture of triols by adduction with propylene oxide (PO), for flexible polyurethane foams and of synthetic lubricants by esterification with fatty acids. The allyl ethers of trimethylolpropane are used as crosslinking agents, for example, in acrylic resin systems. The world-wide capacity for TMP production is in an increasing trend. 

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]. 

In the recent study made by us [63], the usual diol chain extender was replaced by a triol (TMP), producing crosslinked PU networks without phase segregation. The aim was to ensure high degrees of strain recoverability, to produce candidate thermally-triggered shape-memory polyurethanes. 

There is another group of polyurethanes that is chemically crosslinked with a crosslinker, either triol or polyamine or polyisocyanate. They are single-phase elastomers, and they display lower strengths than the thermoplastic urethanes. However, their properties are less temperature sensitive, and elastic recovery is generally considerably better (permanent set is smaller) than in TPUs. Their strength can be improved by adding proper fillers. Such systems are called cast systems since they are processed by casting... 

Polyurethanes are crosslinked in different ways depending on the choice and stoichiometry of reactants and reaction conditions. For example, an isocyanate-terminated trifunctional prepolymer is synthesized from a polyol and diisocyanate. The polyol may be a polyester prepolymer synthesized from a triol or a hydroxyl-terminated polyether. Further polymerization and crosslinking of the isocyanate-terminated prepolymer is achieved by reaction with a diamine... 

The high reactivity of the isocyanate moiety has been exploited to yield polyurethane/urea hydrogels. A polyisocyanate prepolymer obtained by fiinctiontdizing a moderate molecular weight polyether diol or triol with a diisocyanate will crosslink on exposure to an aqueous environment due to partial hydrolysis and urea formation (55). A similar hydrogel results by reacting a polyisocyanate... 

Many polyurethane systems commonly are based on reacting either a polyester or polyether polyol with a diisocyanate. Then this isocyanate-terminated prepolymer is later crosslinked with a curative or chain extender, which is typically a diol, triol, or diamine. 

Trimethylolpropane is a trifunctional monomer that can provide a unique crosslink density when used as a chain extender and curative with cast polyurethane systems. Usually, TMP (a triol) is used in combination with diols for curing MDHermi-nated prepolymers. Also, TMP is sometimes used to cure TDI ester prepolymers for low-hardness applications such as printing rolls. 

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