Polyurethane crystallizable

Control of enantiomorphic selectivity in polymerization of the substituted oxiranes can lead to controlled-structure polymers, the properties of which will range from crystalline thermoplastics to amorphous elastomer precursors such as are used as soft segments in polyurethanes. Crystallizable sequence distributions in highly controlled-structure polymers can lead to thermoplastic elastomers and/or to elastomers that will stress-crystallize that is, crystallize on stretching as does natural rubber (79). 

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

In unfilled rubbers, which are not capable of strain-induced crystallization, the upturns on Mooney-Rivlin curves have shown to be absent 92 95). They disappear also in crystallizable rubbers at elevated temperatures and in the presence of solvents. On the other hand, the upturns do not appear for butadiene, nitrile and polyurethane rubbers if the limited chain extensibility function is introduced in the Mooney-Rivlin expression 97). Mark 92) has concluded that in the absence of selfreinforcement due to strain-induced crystallization or domains the rupture of the networks occurs long before the limited chain extensibility can be reached. 

Busfield, W.K. (1982) Dynamic mechanical properties of some polycaprolactone-based, crosslinked, crystallizable polyurethanes. J. Macro-mol. Set, Chem., A17 (2), 297-309. 

Structural studies on polyurethane elastomers with crystallizable hard segments... 

There is no filler reinforcement of the polyurethane akin to the reinforcement which takes place when reactive or particulate fine particle size fillers are added to non-strain-crystallizable millable elastomers. Fillers in polyurethane reduce strength approximately in proportion to their volume they too, however, increase stiffness and hardness and are cheaper to use for this purpose than additional diisocyanate which gives the same result through hard-segment increase. 

At the present time, nothing is known about the mechanisms (tf cold crystallization, even though the process is observed in such important commercial materials as polyethylene terejdithalate and polyurethanes (38). It is generally assumed that cold crystallization occurs without massive molecular reairangonents and leads to the production of imperfectly formed crystalline regions (38). In contrast, the author has, as previously mentioned, evidence that for crystallizable aromatic polymers, cold crystallization leads to the formation large and relatively thick single polymer crystals (102)... 

The following examples show thermally induced shape-memory. The first three examples are exclusively physically cross-linked. These examples are two polyurethanes representing thermoplastic shape-memory polymers with Ttrans = Tm or Tg, and a high molecular weight, amorphous polynorbornene. Examples of covalently cross-linked shape-memory networks are so-called heat-shrinkable materials and a shape-memory network with a crystallizable switching segment (Ttrans = Tm) that has been developed for biomedical application. 

The company - classing itself as a specialty materials manufacturer - offers a range of temperature activated polymer products (epoxy, polyurethane and unsaturated polyester based), using a proprietary side chain crystallizable polymer technology. 

Crystallizable Block Copolymer Morphologies While the largest part of the block copolymer literature describes totally amorphous materials, one or more of the blocks may form semicrystalline regions. Examples include polyester-polyether block copolymers (39), where the poly(tetramethylene terephthalate) polyester blocks crystallize, and the thermoplastic polyurethane elastomers, where the polyurethane hard blocks crystallize (40). 

Some recent literature has been published dealing with moisture-reactive hot-melt polyurethane adhesives (Frisch Jr 2002). Most moisture-curing hot-melt adhesives are obtained by reacting a crystallizable polyol such as poly(hexamethylene adipate) and monomeric MDl (NCO/OH ratios = 1.5-2.2). A catalyst such as dimorpholinediethyl ether is also necessary. The polyester polyol plays a key role because open time, viscosity, and glass transition temperature can be adequately tailored. A mixture of hydroxyl-terminated polyesters having different characteristics allows to control the adhesion and hardening time of the adhesive. 

Kyritsis A, Pissis P, Grigoryeva O, Sergeeva L, Brovko A, Zimich O, Privalko E, Shtompel V and Privalko V (1999) Structure-property relationships in thermoplastic apparent interpenetrating polymer networks based on crystallizable polyurethane and st5rrene-acrylic acid copol3uner, J Appl Polym Sci 73 385-397. 

Thermoplastic AIPNs of several compositions prepared by mechanical blending in a roll mill of crystallizable polyurethane (CPU) and styrene-acrylic acid random copolymer (S-co-AA) have been investigated using different techniques by VataUs et al. [212,213]. The CPU was based on TDI (mixture of 2,4- and 2,6-isomers, molar ratio 65 35) and oligomeric buty-... 

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