Crosslinking elastic polyurethanes

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. 

Thus, from high MW diols (MW = 1000-4000 daltons) polyethers (polyalkyleneoxides, polytetrahydrofuran (PTHF)), polyesters, polycarbonates (PC), polybutadienes, etc., by the reaction with diisocyanates [toluene diisocyanate (TDI), or pure diphenyl methane diisocyanate (MDI)], high MW linear polyurethanes are obtained (no crosslinking), with high elasticity (polyurethane elastomers, spandex fibres, some adhesives and sealants, etc). 

Crosslinking in polyurethanes leads to a decrease in the molecular mobility and flexibility and causes an increase of rigidity, softening points and modulus of elasticity and reduces elongation and swelling by solvents (only linear polymers are soluble, crosslinked polymers are only swelled by the organic solvents). 

The crosslinking of polyurethane resin composed of diisocyanate derived from 4,4 -diisocyanate diphenylmethane and a low-viscosity polyethertriol was also investigated under micro-wave conditions. The reactions were carried out without a catalyst and led to final networks with mechanical properties least equivalent to those prepared under conventional conditions. For example, the average elasticity modulus determined from uniaxial compression with samples (25 mm of height and 12.5 mm of diameter) was equal to 3120 MPa for curing under... 

The finite size effects in the contact between a spherical lens of polyurethane and a soft flat sheet of crosslinked polyfdimethyl siloxane) (PDMS) has been addressed by Falsafi et al. [37]. They showed that for deformations corresponding to contact diameters larger than the sheet thickness, the compliance of the system was affected by the glass substrate supporting the soft sheet. In order to minimize the finite size effects in the adhesion measurement of small elastomeric lenses, Falsafi et al. [38] and Deruelle et al. [39] used relatively thick elastic sheets to support their samples. 

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. 

Figure 5.5 Calculated dependence of weight fractions of various substructures in the crosslinking system of H3 + I2 type on the initial molar ratio of H-groups to l-groups, ah (a polyurethane system) DC - dangling chains, BC - backbone chains, S - sol (backbone chains are elastically active network chains without dangling chains)...

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... 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

Polyurethane Networks. Andrady and Sefcik (1983) have applied the same relationship as Rietsch et al. (1976), to the glass transition temperature of networks based on poly(propylene oxide) diols with a controlled molar mass distribution, crosslinked by aromatic triisocyanates. They obtained a Kr value of 25 K kg mol-1, about twice that for PS networks. They showed that the length distribution of elastically active chain lengths, directly related to the molar mass distribution of the starting poly(propylene oxide), has practically no effect on Tg. 

Modified polyurethanes as shown in Fig. 4.6 are used for valuable hand variations and as additives for cellulose crosslinkers. Their flexible and elastic films on the fibre surface improve elastic resilience, which is, for example, important for pile fabrics. The principle of their elasticity is, as in elastan fibres, the molecular combination of so-called weak and hard segments. The large weak... 

Creep behavior of a single sample at three different crosslink densities is depicted in Figure 4, vq being the virgin network sample which was crossllnked by irradiating for 30 minutes to yield which was further Irradiated 30 minutes to yield V2. In the previous work on polyurethane samples, values obtained from equation 3 were used to determine the rubber elasticity vertical... 

Values of Au qQ 3 calculated from stress relaxation of the swollen networks agreed fairly well with those derived from swelling of polyurethane networks. The anomalous behavior of polyurethanes has been reported (12). Swelling at different solvent activity and stress relaxation of swollen networks are valuable techniques for network characterization. Other networks such as crosslinked polystyrene will be examined by these methods. The role of the Gaussian approximation in rubber elasticity will be evaluated in calculating Mc for highly swollen networks. 

The extent of crosslink decrease is determined by the network knots concerrtration. Such knots usually have a functionality of 3 or 4. Frmctiorrality depends on the type of curing agent. Crosslinked polyurethanes cured by polyols with three OH-groups are the examples of a three-functional network. Rubbers cured via double bonds are the examples of four-funetional networks. Eq. [6.4] is also used in other forms, depending on form of elasticity potential and concentration dependence of the %j parameter used. 

The cured silane crosslinking spray foams exhibit very good mechanical properties. They possess a high hardness and a very good elasticity. The compressive strength is comparable to common polyurethane (PU) spray foams. Foam densities < 40 kg/m, i.e. the densities of common PU foams,... 

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... 

It will thus be obvious that both covalent and polar crosslinks contribute to the crosslink network and hence modulus of polyurethane elastomers. The dependence of this modulus or temperature can be divided into contributions from a covalently linked network conforming to the statistical theory of rubber elasticity, and contributions from secondary crosslinks which are assumed to have a temperature dependence governed by the Arrhenius law in which the modulus of elasticity is represented by the equation ... 

Polyurethane rubbers have now been used as seal materials for some time on account of their unique ability to combine resistance to swelling in oil with high strengths and high stiffnesses. Their ability, in some classes, to be processed as thermoplastics, is also considered useful as manufacture of the seal can then be automated and hence quality is more reproducible. A limitation in their use has been that they depend upon physical types of crosslinking for their elastic and strength properties and when certain specific temperatures are reached these crosslinks rapidly weaken and the polyurethane elastomer melts and fails. At present most rubber seals are made from vulcanized covalently crosslinked rubbers where crosslinks are based on sulphur or carbon, and these do not melt at elevated temperatures, but instead decompose. 

The possession of these unique properties can, in part, be explained by the way the polyurethane elastomer structure differs from that of conventional elastomers. An elastomer is usually described as a material characterized by the property of high elasticity. This means it has the ability to stretch to a great extent under load and to recover almost completely when the load is released. This property of elasticity is a result of a particular molecular structure, the general features of which are exhibited by all elastomers, although there are many variations in detail. In simple terms this structure consists of long linear flexible molecular chains which are joined together at intervals by crosslinks which may be in the form of... 

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