Polyurethane networks preparation

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). 

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

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. 

In the case of Fig. 7.6a the cluster formation and the size distribution can be influenced not only by chemical reactions but also by partial miscibility of the substructures during reaction. Polyurethane networks prepared from polyolefin instead of polyester or polyether as macrodiol, can serve as an example. In this particular case an agglomeration of hard domains takes place in the pregel stage, produced by a thermodynamic driving force. 

T. W. Pechar, G. L. Wilkes, B. Zhou and N. Luo, Characterization of soy-based polyurethane networks prepared with different diisocyanates and their blends with petroleum-based polyols , J Appl Polym Sci, 2007,106,2350-62. 

Polyaddition or polycondensation reactions can also be used for the synthesis of covalently crosslinked polymer networks [72, 73]. An example are polyurethane networks prepared by the prepolymer method using poly(tetrahydrofuran), which provided the switching segment, and a diisocyanate and 1,1,1-trimethylol propane, which provided the covalent crosslinks. In these polymer networks the elastic... 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

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. 

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

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. 

Network Synthesis (4) Solid MDI was weighed into a flask and an equivalent amount of polyol added. The mixture was heated to about 40°C to dissolve the MDI. The mixture was then cooled to room temperature and degassed for several minutes under vacuum in order to remove dissolved air. Catalyst was then added and the contents of the flask mixed under vacuum to ensure uniformity and then poured into a mold. All operations were carried out in a dry glove bag to minimize reaction with atmospheric water. The cross-linking process was also carried out in dioxane solution at 70% volume fraction of solids. Polyurethane networks with different crosslink densities were prepared by varying the ratio of difunctional and trifunctional polyols. All samples were extracted with dioxane to remove unreacted and uncrosslinked materialbefore swelling. 

Figure 5. Mooney-Rivlin plots of stress-strain data(lO) for two tetrol-based polyurethane networks from system 5 of Figures 2 and 4, prepared at various dilutions in nitrobenzene as solvent. Networks swollen in nitrobenzene at 4QOC...

H. Yeganeh and P. Hojati-Talemi, Preparation and properties of novel biodegradable polyurethane networks based on castor oil and poly(ethylene glycol) ,... 

Using this technique, a large variety of polyurethanes have been prepared from different vegetable oils. Natural polyols like castor oil (generally trifunctional) are directly reacted with diisocyanates to obtain branched polyurethanes, although it is difficult to control the reactivity. However, bifunctional castor oil can be polymerised with diisocyanates in the presence of suitable chain extenders and catalysts to produce polyurethanes in a more controlled manner (Fig. 6.4). A castor oil polyol-based polyurethane network can also be prepared from epoxy terminated polyurethane pre-polymer with 1,6-hexamethylene diamine. Epoxy terminated pre-polymer is obtained by the reaction of glycidol and isocyanate terminated polyurethane pre-polymer of castor oil polyol, poly(ethylene glycol) (PEG) and 1,6-hexamethylene diisocyanate. ... 

The chemo-enzymatic synthesis of polyurethanes has been reported through the inter-esterification of castor oil and linseed oil at ambient temperature, using lipase as a catalyst and foUowed by treatment of the inter-esterified product with TDI. In the first step, partial esters are prepared by transesterification of soybean and linseed oils with n-butanol in the presence of lipozyme (a lipase) as the catalyst. The partial esters are then reacted with different diisocyanates to obtain a series of polyurethanes. The reaction of polyhydroxy compounds (transesterification reaction between different compositions of castor oil and glycolysed poly(ethylene terephthalate)) with diisocyanates offers a polyurethane network for new insulating coating applications. ... 

B. Tamami, S. Sohn, G. L. Wilkes and G. L. B. Tamami, Incorporation of carbon dioxide into soybean oil and subsequent preparation and studies of nonisocyanate polyurethane networks ,/AppZ Polym Sci, 2004,92,883-9. 

Yuan et al. (1998) synthesized semi-interpenetrating pol50ire-thane/PLA networks. The polyurethane was prepared using PCL diols and triols and toluene-2,4-diisocyanate. The optimum was found to be 5 wt% of crossUnked polyurethane network blended with PLA. The elongation at break increased to 60% and the tensile toughness increased to 18 MJ/m compared to 1.6 MJm for neat PLA. 

Flexible networks are most commonly prepared by one of two methods polymerizing an appropriate mixture of monomers in one step, as in the case of styrene-divinylbenzene or polyurethane networks, or by first preparing long... 

Polyurethane networks were prepared by crosslinking ethylene oxide/propylene oxide block copolymers, ethylene oxide/propylene oxide/dimethyl siloxane graft copolymers, and poly(ethylene oxide) with various isocyanates (179). Complexation with lithium perchlorate led to network shrinkage and increases in Tg. The data suggested regular chain partitioning by the salt. 

Tamami B, Sohn S, Wilkes GL. 2003. Incorporation of Ctubon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks. J Appl Polym Sci 92(2) 883-891. 

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