Cross-linking polyurethane

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. 

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. 

Properties and Applications of Cross-linked Polyurethane Rubbers... 

Properties and applications of cross-linked polyurethane rubbers... 

The undesirable properties of thermoplastic polyurethane elastomer, i.e., softening at high temperatures and flow under pressure, which limit their use at elevated temperamres have been reduced by cross-linking with EB radiation. The cross-linked polyurethane shows good mechanical properties and also displays good resistance to aggressive chemicals, e.g., brake fluid [432 35]. 

Traditionally carpet tile backings have been made from bitumen, PVC or polyurethane. There are problems with the environmental impact of these materials and they are difficult to recycle. Shaw industries developed their EcoWorx carpet tile backing using a polyolefin resin. This has superior environmental profile to PVC and is easier to recycle than bitumen and the cross-linked polyurethane backings. EcoWorx has lower toxicity, better adhesion and good dimensional stability. It is easy to recycle and can be separated from any fibre type used in the carpet tile. It works particularly well with nylon-6 fibres. EcoWorx tiles contain 40% recycled content and Shaw have established a collection and return scheme to close the loop. 

One way of obtaining the more useful cross-linked polyurethanes is by using a trifunctional reagent. Thus either the TDI can react with a triol or the propylene oxide can be polymerized in the presence of a triol. Then the isocyanate-alcohol reaction would of course give a cross-linked urethane. 

Eluorinated cross-linked polyurethanes can be obtained by the right combination of fluoropolyether macromers with the cyclic trimers of HDI and IPDI. 

In cross-linked polyurethanes, there are actual chemical bonds formed in a three-dimensional manner. The main difference is that with the cast-able polyurethanes the actual chemical structure consists of two major zones, a hard zone and a soft zone. This is not as pronounced in the cross-linked polyurethanes. A typical example is the addition of TMP (trimethy-lol propane) to polyurethane to make it softer. This curative will make the material much softer as it breaks the hard segment zoning up to a certain degree. They do, however, give a material with improved compression set properties. 

P-Mannich bases arc prepared from phosphonic esters possessing hydroxy groups. -" Representative compounds of this type are 420 (Fig. 160), which give cross-linked polyurethanes of structure 421, by reaction with diisocyanates. 

Figure 3.8 Storage and loss moduli versus reduced frequency in small-amplitude oscillatory deformation for a cross-linked polyurethane rubber, Sorbothane 70, at 20°C. The imposed deformation was uniaxial extension, yielding tensile moduli E and E" which were converted to shear moduli using G = E /3 and G" = E"/3. The data at 16 different temperatures ranging from —81°C to 80°C were collapsed to a master curve using methods described in Section 3.5.2, (From Larson et al. 1996, reprinted with permission from Steinkopff Publishers.)...

Polyurethanes are formed by polyaddition of diisocyanates and dihydroxy components. When, instead of diols, polyhydric alcohols are used, more rigid cross linked polyurethanes are formed. The more OH-functionalities present in the hydroxyl... 

Problem 5.25 A cross-linked polyurethane is to be made from a diisocyanate, 1,4-frfl/ji-cyclohexane diisocyanate (CHDI), and a polymeric tetrol, R(OH)4, without addition of any water. Let the stoichiometric ratio of initial concentrations of NCO to OH groups be denoted by r. 

Schollenberger (Goodrich)—Virtually cross-linked polyurethane elastomers. 

Polyurethane Enamels. Small volumes of this enamel type are used in applications for which the temperature does not rise above approximately 105 C. These enamels are made from hydroxyl-bearing polyesters and blocked isocyanates. During the heating cycle in the enameling process the isocyanates become unblocked and react with hydroxyl groups of the polyesters to form a cross-linked polyurethane film. Wires so insulated can be directly soldered, without prior removal of the polyurethane insulation. 

By proper choice of either the isocyanate or the polyol, actual chemical cross-links can be introduced in either the hard or soft segments that may be beneficial to some properties. The effectiveness of these cross-links is offset by a disruption of the hydrogen bonding between polymer chains. Highly cross-linked polyurethanes are essentially amorphous in character exhibiting high modulus, hardness, and few elastomeric properties. Many adhesives fall into this category. 

A multistep reaction, polyaddition, is a polyreaction of at least two bifunctional or higher functional compounds. Polyaddition can result in either linear polymers (thermoplastics) or cross-linked plastics (duroplastics), depending on the specific functionality. Cross-linked products are obtained by means of a reaction of a bifunctional reactant with a trifunctional one. The more polyfunctional the reactant, the more closely meshed the cross-linking will be. That is why the polyols in polyurethane or epoxy resin production are frequently replaced by polyesters and polyethers containing large numbers of OH groups. Polyaddition, like polycondensation, is a multistep reaction. Fig. 4. Important polyadducts include linear and cross-linked polyurethanes as well as epoxy resins (see [2]). 

The two-photon absorption technique was applied for investigating cross-linked polyurethane containing modifled stilbene as potential frequency doubling or electro-optical controlling devices [38], properties of some donor-acceptor molecules with stilbene and azobenzene molecules as backbone [39], 4-fluorophenylethynyl-and 4-nitro-( )-stilbenylethynylruthenium complexes [40], series donor-acceptor stilbene analogues [41], and so on. 

Polyurethanes have a wide range of applications. Polymers with relatively little cross-linking produce stretchable fibers (Spandex and Lycra) used for bathing suits. Polyurethane foams are used in furniture, mattresses, and car seats and as insulation in portable ice chests. Cross-linked polyurethanes form very tough surface coatings in paints and varnishes. The major component of the Jarvik-7 artificial heart is also a polyurethane. 

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