Polyurethane systems

Polyurethane systems for paints and coatings are summarized in Table 2.5. 

System desig- nation Hardening principle Application form Film-forming temperature 

One-Pack Systems 1 Oxidation with atmospheric oxygen solvent-containing up to 80 C 

2 Reaction of NCO groups with atmospheric moisture solvent-containing ambient temperature 

4 After activation of microencapsulated polyisocyanate with OH or NHj groups solvent-free 100-160 C 

Because the final polymer is formed actually inside the mould, the background polymer chemistry is especially important to an understanding of how RIM and RRIM systems work. An outline of the main reactions is given in PST 3, together with an account of how they are manipulated for use in this process. 

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Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Step polymerization is used to synthesize multiblock copolymeric elastomers (referred to as segmented elastomers). An example is the polyester-polyurethane system produced by the reaction of a diisocyanate with a mixture of macro diol and smallsized diol (Eq. 14). The macro diol (usually referred to as a... 

As the reaction proceeds beyond the gel point, the molecular weight of EANCs decreases and the fraction of material in the EANCs increases. The fraction of material in dangling chains passes through a maximum but their molecular weight decreases. Figure 5.5 characterizes the behavior of simple polyurethane systems. 

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)...

Covalently bonded substructures having compositions distinguishable from their surroundings are formed in multicomponent systems they are called chemical clusters. The adjective chemical defines covalency of bonds between units in the cluster. To be a part of a cluster, the units must have a common property. For example, hard clusters are composed of units yielding Tg domains. Hard chemical clusters are formed in three-component polyurethane systems composed of a macromolecular diol (soft component), a low-molecular-weight triol (hard component) and diisocyanate (hard component). Hard clusters consist of two hard... 

In a three-component polyurethane system with OH groups, wherein crosslinker is a triol, one finds them substantially more reactive than those of the macro-molecular diol. The hard clusters grow substantially larger, at relatively low conversions. Their size remains almost constant because all triol units have been used up in the reaction. In the opposite case, the lower reactivity of OH groups found in macromolecular diol compared with that of triol, the clusters remains small throughout the reaction and grows larger only at its end. 

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... 

The result is that the extent of reaction, p (cf. Section 3.3.1.1), can be related to the weight average molecular weight, for a polyurethane system under equal stoichiometry ... 

The most advanced technology is the extracorporeal hollow fiber reactor. It is currently in Phase III trial and achieved a good Phase II record to support it. Other techniques including a polyurethane system devised in Japan and encapsulated hepatocytes from UCLA are or were in large animal trials. Whether a device is extracorporeal or is intended for implantation, clinical significance requires a suitable scaffold to support a sufficiently large colony of hepatic cells. For both extracorporeal and implant use, the physical structure of the scaffold must meet certain requirements of strength, void volume, biocompatibility, and other parameters. 

The major problem is moisture that is absorbed into the polyurethane system or into the curative and auxiliary materials. Free water will liberate carbon dioxide when the chain extension is carried out. It is important to keep the reactants dry, as any moisture that may have come in contact with the prepolymer will react to give an amine and carbon dioxide. This amine reacts with more isocyanates to form a disubstituted diamine. The reaction is outlined in Figure 2.9. 

Using a straight polyurethane system, for example, a PTMEG/MDI prepolymer, there will initially be an absorption band at approximately 1733 to 1725 cm1 (Seymour et al., 1970). As the cure starts, this band will initially... 

Catalysts are commonly used to control the pouring and gel times of polyurethane systems. As catalysts are used in small quantities, the addition must be carried out very carefully otherwise, very erratic results may be obtained. Catalysts may be made into a concentrated solution with an inert carrier such as a plasticizer. This reduces a potential dispensing error. They also may be added to a liquid chain extender. 

Most polyurethane systems tend to cling to the glass surface, so an anchor-style stirrer is the best to use. This provides a sweeping motion that helps prevent a very viscous layer from forming on the surface of the vessel. 

Polyurethane systems can be produced to have near-equal or easy mixing component ratios (by either weight or volume). The prepolymers in this type of system are called quasiprepolymers. Quasiprepolymers are normally taken as having at least a fourfold molar excess of isocyanate in the isocyanate side of the system. The other side contains the remainder of the polyol, curative, and any catalysts. 

When adding reactive plasticizers to polyurethane systems, the reactivity of the plasticizer must also be taken into account when calculating the amount to be added. The formula to calculate the equivalent weight of the blend is given in Appendix 5. 

The addition of phosphorous-based plasticizers will provide a degree of fire retardancy to the polyurethane. Fryol PCF and CEF have been used in polyurethane systems. Most phosphorus-based fire retardants are thin liquids that are compatible with polyurethanes. Because of the very varied nature of fire-resistant tests, the material made must be tested to the appropriate standard. 

Because there is a large variety of different materials that are excellent for specific applications, the right polyurethane for the particular application has to be selected. The base chemistry used in producing the polyurethane system can have a profound influence on the final properties of the part produced. The mechanical, thermal, and chemical properties of a cured system have to be selected for each application. A typical example is that certain amines cannot be used in applications where there is contact with food. 

Shao, C. H., Huang, J. J., Chen, G. N., Yeh, J. T., and Chen, K. N., Thermal and combustion behaviors of aqueous-based polyurethane system with phosphorus and nitrogen containing curing agent, Polym. Degrad. Stab., 1999, 65, 359-371. 

Wang, T. Z. and Chen, K. N., Introduction of covalently bonded phosphorus into aqueous-based polyurethane system via postcuring reaction, J. Appl. Polym. Sci., 1999, 74, 2499-2509. 

The substantial work on polystyrene/polybutadiene and polystyrene/ polyisoprene blends and diblock and triblock copolymer systems has lead to a general understanding of the nature of phase separation in regular block copolymer systems (5,6). The additional complexities of multiblocks with variable block length as well as possible hard- and/or soft-phase crystallinity makes the morphological characterization of polyurethane systems a challenge. 

In addition to microphase structures, MDI/BDO-based polyurethane systems have exhibited spherulitic superstructure. Characterization of the birefringence of the spherulites was used to determine the orientation of the hard-segment domains (7). However, because of the sensi-... 

The above analysis takes the synthesis methods, the performance affected by the dispersion of CNTs, enhanced physical properties and the latest applications of carbon nanotube/polyurethane composites described in literature reports as the reference point. In the interest of brevity, this is not a comprehensive review, however, it goes through numerous research reports and applications which have been learned and described in the recent years. Despite that, there are still many opportunities to synthesize new carbon nano-tube/polyurethane systems and to modify carbon nanotubes with new functional groups. The possibility of producing modern biomedical and shape memory materials in that way makes the challenge of the near future. 

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