Polyurethane , cross-linked

In condensation polymerizations involving formation of polyesters or polyurethanes, cross-linking can be achieved by use of some of a trihydroxylic alcohol, such as glycerol, in addition to the dihydroxylic alcohol. 

The same investigators have successfully applied cryogel processing to prepare composite of a nitrocellulose gel (an energetic gel), polyurethane cross-linked nitrocellulose and the energetic material CL-20 [50]. Under those processing conditions, all of the components were initially in solution, which enabled the formation of nanoparticles of the... 

K Polyurethane Cross-linking Ester, urethane, or urea hydrolysis Diisocyanate and diols [47,94-96]... 

In general, the interdomain space of the segmented polyurethanes decreases with an increase of HSC. But at HSC = 45%, the interdomain space increases slightly. This may be attributed to the discontinuity-continuity transition of the hard phase. The decrease of the interdomain space with the increase of HSC suggests the increasing concentration of the hard domains, i.e., the physical cross-linking density of the polyurethane cross-linked networks. 

Two-part, polyurethane One-part, polyurethane (cross-linking, hot melt) ... 

The hydroxyl groups can be esterified normally the interesting diacrylate monomer (80) and the biologicaky active haloacetates (81) have been prepared in this manner. Reactions with dibasic acids have given polymers capable of being cross-linked (82) or suitable for use as soft segments in polyurethanes (83). Polycarbamic esters are obtained by treatment with a diisocyanate (84) or via the bischloroformate (85). 

Superabsorbents. Water-sweUable polymers are used extensively in consumer articles and for industrial appUcations. Most of these polymers are cross-linked acryUc copolymers of metal salts of acryUc acid and acrylamide or other monomers such as 2-acrylamido-2-methylpropanesulfonic acid. These hydrogel forming systems can have high gel strength as measured by the shear modulus (134). Sometimes inorganic water-insoluble powder is blended with the polymer to increase gel strength (135). Patents describe processes for making cross-linked polyurethane foams which contain superabsorbent polymers (136,137). 

The physical properties of polyurethane adhesives result from a special form of phase separation which occurs in the cross-linked polyurethane stmcture. The urethane portions of polyurethanes tend to separate from the polyol portion of the resin, providing good shear strength, good low temperature flexibiUty, and high peel strength. Catalysts such as dibutyltin dilaurate [77-58-7], stannous octoate [1912-83-0], l,4-diazabicyclo[2.2.2]octane... 

One of the first attempts to produce polyurethane was from the reaction of an intermediate polyol of 1,3- and l,4-bis(hydroxyhexa uoroisopropyl)benzene m- and -12F-diols) by reaction with epichlorohydrin. This polyol was subsequentiy allowed to react with a commercial triisocyanate, resulting in a tough, cross-linked polyurethane (129,135,139). ASTM and military specification tests on these polyurethanes for weather resistance, corrosion prevention, bUster resistance, and ease of cleaning showed them to compare quite favorably with standard resin formulations. 

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

Polyurethane foams are formed by reaction with glycerol with poly(propylene oxide), sometimes capped with poly(ethylene oxide) groups with a reaction product of trimethylolpropane and propylene oxide or with other appropriate polyols. A typical reaction sequence is shown below, in which HO—R—OH represents the diol. If a triol is used, a cross-linked product is obtained. 

The Hquid monomers are suitable for bulk polymerization processes. The reaction can be conducted in a mold (casting, reaction injection mol ding), continuously on a conveyor (block and panel foam production), or in an extmder (thermoplastic polyurethane elastomers and engineering thermoplastics). Also, spraying of the monomers onto the surface of suitable substrates provides insulation barriers or cross-linked coatings. 

The physical properties of polyurethanes are derived from their molecular stmcture and deterrnined by the choice of building blocks as weU as the supramolecular stmctures caused by atomic interaction between chains. The abiHty to crystalline, the flexibiHty of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, eg, polyurethane foams, other factors such as density determine the final properties. 

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). 

Some commercial durable antistatic finishes have been Hsted in Table 3 (98). Early patents suggest that amino resins (qv) can impart both antisHp and antistatic properties to nylon, acryUc, and polyester fabrics. CycHc polyurethanes, water-soluble amine salts cross-linked with styrene, and water-soluble amine salts of sulfonated polystyrene have been claimed to confer durable antistatic protection. Later patents included dibydroxyethyl sulfone [2580-77-0] hydroxyalkylated cellulose or starch, poly(vinyl alcohol) [9002-86-2] cross-linked with dimethylolethylene urea, chlorotria2ine derivatives, and epoxy-based products. Other patents claim the use of various acryUc polymers and copolymers. Essentially, durable antistats are polyelectrolytes, and the majority of usehil products involve variations of cross-linked polyamines containing polyethoxy segments (92,99—101). 

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