Polyurethanes networks

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

Polyurethane networks based on triisocyante and diisocyanate connected by segments consisting of polyisobutylene are rubbery and exhibit high temperature properties, hydrolyic stability, and barrier characteristics. ... 

The PEO-based polyurethane networks can hardly be considered as a practical perspective in agriculture, though in principle this technique for obtaining SAH should not be neglected. 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Storey RF and Hickey TP. Degradable polyurethane networks based on D,L-lacdde, glycohde, e-caprolactone, and trimethylene carbonate homopolyester and copolyester triols. Polymer, 1994, 35, 830-838. 

Gel Point and Shear Modulus. Trifunctional and tetrafunc-tional polyurethane(25,26,28) and trifunctional polyester net-works (32) have been studied. The gelation data for the reaction systems forming the polyurethane networks were those discussed with reference to Figure 6 and Table II. 

Figure 9. Molar mass between elastically effective junction points (Mc) relative to that for the perfect network (Mc°) versus extent of intramolecular reaction at gelatin (pr,c) for polyurethane networks (29).

Figure 10. Tg versus ac for dry, trifunctional polyurethane networks, (26). Reaction systems MDI/LHT240 (system 3 of Figure 9), Mc° is 710 g/mol, v is 30, at various initial dilutions of reactants. - - is MDI/POP diol. Mrepeat = M0°.

Table IV. Shear modulus and Tg of polyurethane networks prepared from bulk reactios (29). G(298K) - shear modulus at 298K, (a) from uniaxial...

The deviations from Gaussian stress-strain behaviour for the tetrafunctional polyurethane networks of Figure 9 are qualitatively similar to these found for the trifunctional polyester networks (Z5), and the error bars on the data points for systems 4 and 5 in Figure 9 indicate the resulting uncertainties in Mc/Mc. It is clear that such uncetainties do not mask the increases in Mc/Mc with amount of pre-gel intramolecular reaction. 

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 this contribution, we report equilibrium modulus and sol fraction measurements on diepoxidet-monoepoxide-diamine networks and polyoxypropylene triol-diisocyanate networks and a comparison with calculated values. A practically zero (epoxides) or low (polyurethanes) Mooney-Rivlin constant C and a low and accounted for wastage of bonds in elastically inactive cycles are the advantages of the systems. Plots of reduced modulus against the gel fraction have been used, because they have been found to minimize the effect of EIC, incompleteness of the reaction, or possible errors in analytical characteristics (16-20). A full account of the work on epoxy and polyurethane networks including the statistical derivation of various structural parameters will be published separately elsewhere. 

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

Polyurethane Networks. Again, for Tqjj. 1 the sol fraction fits well the theoretical curves, if the measured values of the average functionality fn of the polyoxypropylene (POP) triol, the final conversion of isocyanate groups, and the formation °f... 

Figure 3. Sol fraction h>, vj. the molar ratio r0H = [OH]/[NCO] for polyurethane networks from polyoxypropylene triols. Key --------, theoretical curve for...

Figure 4. Theoretical curves for reduced moduli Gd/RT (mol/cm3) of polyurethane networks from polyoxypropylene triols versus the gel fraction w . Value of A indicated. Networks from LHT-240. Continued.

The functionality of precursors varying between/ = 2 and/ = 6 is considered to be low (Figure 5.2). Polyurethane networks prepared from bifunctional telechelics and trifunctional triisocyanates, diepoxide (f = 2)-diamine (f = 4) systems, diepoxide if = 4)-cyclic anhydride (/ = 2) systems, phenol (/ = 3)-formalde-hyde if = 4) resins, or melamine (/ = 6)-formaldehyde (/ = 2) resins are in this category. 

Figure 5.12 Structure of a polyurethane network with dangling chains prepared from F1 + F2 + F3 components...

Another approach was attempted by Seppala and Kylma who reported the synthesis of poly(ester-urethane)s by condensation of hydroxyl terminated tel-echelic poly(CL-co-LA) oligomers with 1,6-hexamethylene diisocyanate (Scheme 33) [94]. The diisocyanate acts as chain extender producing an increase in molecular weight of the preformed oligomers. The authors claim that some of the copolymers present elastomeric properties. Using a similar method. Storey described the synthesis of polyurethane networks based on D,L-LA, GA, eCL,... 

Figure 9. Reduced equilibrium modulus of polyurethane networks from POP trlols and MDI in dependence on the sol fraction. networks from POP triol Mjj - 708, o networks from POP triol Mjj = 2630. C-) calculated dependence using Flory junction fluctuation theory for the value of the front factor A indicated. (Reproduced from Ref. 57. Copyright 1982 American Chemical Society.)...

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

The moduli of model polyurethane networks clearly show reductions below the values expected for perfect networks, with the reductions increasing with pre-gel intramolecular reactlon(5-7). The reductions can be shown to be too large to come solely from pre-gel loop forma-tion( ), some must occur post-gel. In addition, extrapolation to conditions of zero pre-gel intramolecular reaction, by increasing reactant concentrations, molar masses of reactants or chain stiffness, still leaves a residual proportion of inelastic chains due to gel-gel intramolecular reaction. It is basically a law-of-mass-action effect( ). The numbers of reactive groups on gel molecules are unlimited. Intramolecular reaction occurs, and some of this gives Inelastic chains. Only a small amount of such reaction has a marked effect on the modulus. 

Figure 1, Ratio of molar mass between elastically effective junctions to front factor (M(-/A) relative to molar mass between junctions of the perfect network (M ) versus extent of intramolecular reaction at gelation (pj- (.) Polyurethane networks from hexamethylene diisocyanate (HDI) reacted with polyoxpropylene (POP) triols at 80°C in bulk and in nitrobenzene solution(5-7,12). Systems 1 and 2 HDI/POP triols >i= 33, V2= 61. Systems 3-6 ...

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