Limitations of Polyurethanes

Polyurethanes can be used over a temperature of -40°C to a maximum of 120°C. The normal working range has a lower maximum temperature of 70°C. 

Hydrolysis is a major problem for polyurethanes, even with the right choice of polyurethane and the use of antihydrolysis agents such as polycar-bodiimides. Care and experience are needed in these conditions. 

Certain chemicals such as the ketones (acetone and MEK), polar solvents, and concentrated acids affect polyurethanes badly. 

A major consideration in all dynamic applications is the buildup of heat. The low conductivity of the polyurethanes does not allow for rapid removal of heat. The design of the part and the grade choice are very important. The part must not be allowed to deflect too much, and a heat sink must be provided. It is desirable to keep the cross-section of the polyurethane at a minimum. 

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When selecting polyurethanes for any application, a full understanding of the working conditions should be obtained to prevent overspecifying the grade and type. These conditions may include the interaction of temperature, hydrolysis, and wear in a dynamic situation. The limitations of polyurethanes also must be taken into account and some redesign carried out if needed. 

Figure 27 The low elastic limit of polyurethane adhesives counteracts the benefit of...

Due to low hydrolytic and chemical resistance and to low melting point, aliphatic polyesters have long been considered to be limited to applications such as plasticizers or macromonomers for the preparation of polyurethane foams, coatings, or... 

RECYCLING AND UTILISATION OF POLYURETHANES - POSSIBILITIES AND LIMITS, PART I... 

The factors which influence pre-gel intramolecular reaction in random polymerisations are shown to influence strongly the moduli of the networks formed at complete reaction. For the polyurethane and polyester networks studied, the moduli are always lower than those expected for no pre-gel intramolecular reaction, indicating the importance of such reaction in determining the number of elastically ineffective loops in the networks. In the limit of the ideal gel point, perfect networks are predicted to be formed. Perfect networks are not realised with bulk reaction systems. At a given extent of pre-gel intramolecular... 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Isocyanates that are produced fi om aliphatic amines are utilized in a limited range of polyurethane products, mainly in weatherable coatings and specialty applications where the yellowing and photodegradation of the aromatic polyurethanes are undesirable [5]. The aliphatic isocyanates are not used more widely in the industry due to the remarkably slow reaction kinetics of aliphatic isocyanates compared to their aromatic counterparts [6]. Due to the slow reactivity of aliphatic isocyanates, it is not practical to use them in the preparation of flexible or rigid foams, which are the main commercial applications for polyurethane chemistry. 

The International Union of Pure and Applied Chemistry [IUPAC, 1994] suggested the term polycondensation instead of step polymerization, but polycondensation is a narrower term than step polymerization since it implies that the reactions are limited to condensations—reactions in which small molecules such as water are expelled during polymerization. The term step polymerization encompasses not only condensations but also polymerizations in which no small molecules are expelled. An example of the latter is the reaction of diols and diisocyantes to yield polyurethanes (Eq. 1-6). The formation of polyurethanes follows the same reaction characteristics as the formation of polyesters, polyamides, and other polymerizations in which small molecules are expelled. 

Although the use of lignin as an additive to polyurethanes is not new (15-20), even the most judicious selection of lignin isolation or modification schemes has not allowed researchers to overcome the incorporation limit of 25 to 40 weight percent of lignin as an active component in polyurethanes. Solvent fractionation allows for the isolation of lignin fractions with well defined solubilities and functionalities (21,22). Both of these features are critical for the practical inclusion of lignin into liquid polyol systems. 

If toughness is not the limiting consideration, then the simple addition of PEG soft segment appears to offer an attractive route to the production of polyurethanes with 25 to 30 wt.% lignin. It is important to note that both of the studies considered here have employed modified lignins. The relative advantage of hydroxypropylation (16) over fractionation (19) would depend on the relative cost of either procedure. 

Hexamethylene diisocyanate is a highly reactive synthetic chemical that is widely used in the production of polyurethane materials. There is no natural somee of HDI. All of the potential exposures to this compound are associated with the production, handling, use, and disposal of HDI and HDI-containing products or materials. Exposures to HDI are often associated with exposures to its prepolymers, especially to a trimeric biinetic prepolymer of HDI (HDI-BT) (see Figure 5-Ia), whieh is widely used as a hardener in automobile and airplane paints, and whieh typieally contains 0.5-1% unreacted HDI (Alexandersson et al. 1987 Hulse 1984 Karol and Hauth 1982). There is evidence that diisocyanate prepolymers may induce asthma at the same or greater frequency as the monomers (Seguin et al. 1987) therefore, there is a need to assess the potential for human exposme to prepolymeric HDI as well as monomeric HDI. Except for limited data on occupational exposures, no information was foimd in the available literature related to the potential for human exposure to prepolymers of HDI. 

Readers, however, should not be prejudiced by these comments. The important consideration is the condensation of any polyalcohol with an isocyanate. Inasmuch as the polyalcohol is the compound that gives us the opportunity to produce a chemically active polymer, a researcher should not be limited by the history of polyurethanes that was guided by the need for a physically strong polymer system. In any case, discussing polyether polyols is a suitable starting point. 

Coagulation is not the only problem with materials intended for implantation, however. Cardiac pacemakers are intended to correct arrhythmias. Insulating materials for a pacemaker lead must be tough and long lasting. The first leads were insulated with polyethylene or silicone rubber. Neither material was considered ideal because of endocardial reactions (polyethylene) and limited durability (silicone rubber). The strength and flexibility of polyurethanes led to their introduction in 1978 as lead insulators. 

TDI has a flash pt of 132° (open cup) and flammable vapor level limits of 05 to 9.5% 2,4-Tolylene diisocyanate is used as a constituent (cross-linking agent) of polyurethane expls, proplnt binders, and as a component of a ballistic modifier (Refs 2a, 8 9). Se in Vol 8, P6 8-Table 3 under PBX Type Explosives-Composition (listed as Polyurethane in the Binder, % column), and P409-R to P415-Table 17 under B. Composite Propellants ... 

For curing compounds, the dependence q((3) can be described by two types of equations. In the first type, it is assumed that, as the gel-point is approached, t— t, the viscosity increases without limit. The second type is an exponential equation, which does not contain any singularities.106 The dependence of the viscosity of polyurethane on molecular weight and temperature can be represented by the following equation ... 

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