Crosslinking polyurethanes

Polyurethanes are used for the fabrication of fibers, while crosslinked polyurethanes are employed as lacquers and adhesives, as coatings for textiles and paper, and as elastomers and foams. 

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

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

Fig. 21. Mooney-Rivlin plots (Eq. IV-1) for a chemically crosslinked polyurethane rubber (from polyoxypropyleneglycol, toluene diisocyanate and trimethylol propane). Note the differences in Cx and C2 values in extension and compression...

Fig. 28. Stress-strain data of a chemically crosslinked polyurethane rubber plotted according to Jackson, Shen and McQuarrie (95), see Eq. (IV-25). Data of Blok-...

Fig. 30. Electron micrographs of freshly cleaved chemically crosslinked polyurethane rubber, before (.4) and after (B) oxygen discharge etching (Spit 161a). Replication with polystyrene in chloroform (gelatin in water yields similar results). Rodlike structural elements of a few tenths of a micron are detectable in B. Reported...

Fig. 32. Stress-strain and birefringence-strain data of a chemically crosslinked polyurethane rubber in Mooney-Rivlin type representations (Eq. IV-26a and Eq. IV-29). Data reported by Blokland (14)...

Fig. 33. Experimental curves of the angular dependence of anisotropic light-scattering of some chemically crosslinked polyurethane rubbers. The first number in the code refers to the average number of oxypropylene units between crosslinks and the second number to the average number of urethane links per chain. More such links seem to yield more anisotropic scattering and thus more "bundle like"...

However, an increase in both Tt min and in the temperature at which it occurs, observed in crosslinked polyurethanes with increasing crosslink density, does not links which provide more space for segmental motion 68,69). 

Fig. 45. Temperature dependence of 13C T, for the propylene oxide CHCH2 resonance in the linear (O, ) and crosslinked polyurethanes (A, 4k). The values are determined from the pulse sequences with cross-polarization ( , A) and the direct application of a carbon 90° degree pulse (O, A) (adapted from Ref.301 )...

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. 

A few papers dealing with silicone latex IPNs have appeared. Frisch et al. [ 111 ] patented a process whereby two emulsions were prepared the first contained a hydroxyl-terminated PDMS and stannous octoate, the second a crosslinked polyurethane, poly(urethaneurea) or polyacrylate latex. The two lattices were blended and a film was cast and cured at 120 °C. The mechanical properties were found to be enhanced, especially in the case of the polyacrylate/PDMS IPN. Noteworthy is the fact that semi-IPNs such as the thermally stable... 

This plastic includes a large group of polyaddition polymers which are formed through the reaction of bifunctional or trifunctional alcohols with di- or polyisocyanates. By varying the starting materials, linear as well as crosslinked macromolecules with correspondingly different properties are formed. Alcohols with three functional groups and/or triisocyanate are used to make crosslinked polyurethane elastomers (PUR). 

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,... 

Figure 3. Scanning electron micrograph of a typical fracture surface of a highly crosslinked polyurethane resin containing 8% w/w of dispersed polymyrcene-based rubber particles.

Figure 4. Variation of relative fracture energy, Gc/G , and tensile toughness, Uy, with rubber content for rubber-modified, highly crosslinked polyurethane resins. Gc/G (-0- polymyrcene, polybutadiene) Uy ( -A- polymyrcene).

Fig. 160. Crosslinked polyurethanes from polyol P-Mannich bases.

Although epoxies dominate the thermoset fracture literature, work has been reported on other systems, e.g., polyester resins, phenol-formaldehyde compounds, peroxide cured polystyrene, and highly crosslinked polyurethanes. In general, these materials exhibit fracture behaviors similar to epoxies, and suggest that thermosets, as a class of materials, display characteristic crack growth properties. 

Table 3. Thermal stability data of coefficients for crosslinked polyurethanes with different...

Bonding of rigid, weakly deformable materials with plastic, flexible materials, for example, metal-rubber In this case, choose adhesives that develop flexible, deformable adhesive layers, for example, low crosslinked polyurethanes, contact adhesives. 

Solubility in organic solvents Tetrahydrofuran (THF is regarded as a solvent with very universal properties, however it is combustible and its vapors must not be inhaled (see Section 7.5.2). In addition, suitable solvents are acetone and methyl-ethyl ketone. Thermoset materials are generally insoluble, polyvinyl chloride, Plexiglas, polystyrol, rubbers and poorly crosslinked polyurethane are swellable thermoplastics. 

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. 

A polyol of low functionality, having around 2-3 hydroxyl groups/mol and with a high MW of 2000-10000 daltons, leads to an elastic polyurethane and on the contrary, a low MW oligo-polyol of 300-1000 daltons, with a high functionality of around 3-8 hydroxyl groups/mol leads to a rigid crosslinked polyurethane. 

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] ... 

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. 

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],... 

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