Polyurethane, structure

Figure 3. Continued. NBS Smoke Chamber results (non-flaming) for modifled-polyphenylene oxide and RIM polyurethane structural foam.

It is impossible to list representative properties for the Reaction-Injection-Molded polyurethane structural foams, because their properties can be varied over a wide range depending on the specific application. The foams can be very tough, resilient, blown elastomers or highly rigid structural foams. Furthermore, very recent develop-... 

Atomic force microscopy has been used to evaluate the nature and size of the polyurethane structure at the surface of a sample. In this method, the surface of the sample is coated with a layer of gold and a probe scans over the surface. As the nature of the chemistry below the probe changes, the force field acting on the probe changes. The hard segment clusters can be visualized. 

Xia et al. found that the incorporation of MWNTs and SWNTs did not improve the first degradation temperature, but did improve the second degradation temperature (21). This indicates that carbon nanotubes may preferably interact with the soft segment in polyurethane structure. There were no significant differences between... 

A recent example of XPS analysis of polyurethane surfaces has been provided by Yoon et al. [30]. In these studies polyurethanes were prepared with 4.4 -methylenebisfphenyl isocyanate) (MDI)and hexafluoro- l.5-pentancdiol (FP) extenders as the hard segment, and poly(tetramethylene glycol) (PTMO) as the soft segment. The authors introduced hydrophobic soft segments, either poly-(dimethylsiloxan) (PDMS) or polyisobutylene (PIB) into this polyurethane structure. The concept underlying this polymer design was the desire to modify sur-... 

The physical properties of polyurethane adhesives result from a special form of phase separation which occurs in the cross-linked polyurethane structure. The urethane portions of polyurethanes tend to separate from the polyol portion of the resin, providing good shear strength, good low temperature flexibility, and high peel strength. Catalysts such as dibutyltin dilaurate [77-58-71, stannous octoate [1912-83-0], l,4-diazabicyclo[2.2.2]octane... 

For the work presented here, the polymers considered are in the general class of materials known as polyurethanes. Polyurethanes are particularly attractive for a study of the effect of chemical structure on damping since it is possible to change their Tg s over a wide range of temperatures (>100"C). This corresponds to a damping peak location that spans more than 10 decades of frequency. In addition, changes in polyurethane structure can be used to produce a transition that can vary from narrow to broad. To take advantage of these desirable properties, one must understand the dependence of Tg on the chemical structure of polyurethane polymers. 

A polyol of high MW (3000-6500 daltons) and of low functionality, of around 2-3 hydroxyl groups/mol, if reacted with a diisocyanate, leads to a low crosslinked, flexible polyurethane structure. This structure is characteristic of flexible polyurethane foams. Because the resulting structure is a crosslinked one, the MW of the resulting polyurethane is infinite in value. Only linear polyurethanes have a finite and determinable MW. 

All the previous information regarding the general chemistry of polyurethanes and the structure of isocyanates have a role in the better understanding of how the oligo-polyols get chemically inserted in the high MW polyurethane structure and to understand the role played by the polyol structure in the properties of the resulting polyurethanes. 

Mannich polyols, aromatic polyester polyols, novolak-based polyols) lead, by the reaction with crude MDI, to very rigid polyurethane structures [2] (see Chapter 15). 

The utilisation of hydroxy acids as neutralising agents is based on the following principle if the acid used for neutralisation has a minimum of 1-2 hydroxyl groups, the potassium salt is integrated in the rigid polyurethane structure and the mobility of potassium ions decreases markedly as does the possibility of catalysis. Hydroxy acids, such as, lactic... 

Several general properties, characteristic to classical macromolecular chemistry, are strongly linked to the polyurethane structure, as a direct consequence of the oligo-polyol structure - these are [1, 2, 5, 9, 11] ... 

From low MW oligo-polyols (400-1000 daltons polyethers, polyesters, Mannich polyols, aromatic polyesters, oleochemical polyols, etc.), are obtained rigid, hard polyurethane structures (rigid PU foams, wood substitutes, etc). 

Based on the values of cohesive energies and dissociation energies of the bonds involved in the polyurethane structure, it may be possible to establish the following relative order regarding the thermal stability of polyurethanes function of the oligo-polyol structure ... 

As a general rule the thermal stability of polyurethanes is directly linked with the mobility of polymeric chains. Low mobility, crosslinked polyurethane structures, based on high functionality polyols are more thermostable than the high mobility, low crosslinked... 

The fire resistance of polyurethanes is based on the introduction of flame retardant compounds including polyols, containing chlorine, bromine or phosphorus in their structure. The polyols containing chlorine, bromine or phosphorus are linked chemically in the polyurethane structure and lead to self-extinguishing polyurethanes, with a permanent flame retardancy. 

Baydur . [Bayer Miles] Polyurethane structural foam (RIM). 

Although the name polyurethane might be taken as implying that these materials contain urethane groups (—NHCOO—) in the backbone of the macromolecule, for those polyurethanes in m or commercial use this is not true. For such materials the initial macromolecule tends to be a polyester or polyether it is the crosslinks that involve the formation of a polyurethane structure. 

Place the following polyurethane structures in order of decreasing melting temperature and explain your choice. 

Synthetic-Natural Hybrid Polymers Based on Polyurethane Structures, Properties, and Applications... 

G.M. Bernacca, T.G. Mackay, R. Wilkinson, D.J. Wheatley, Polyurethane heart valves fatigue failure, calcification, and polyurethane structure, J. Biomed. Mater. Res. 34 (1997) 371-379. 

TOF-SIMS spectrum of polyurethane[4,4,l]- Polyurethane structure is shown below the spectrum. (Reprinted with permission from Ref. 48, Copyright 1995 American Chemical Society)... 

These calculations were based on the reaction scheme (8.1) shown opposite and an idealised polyurethane structure from this scheme and the details are given in Table 8.4. 

As shown above, introduction of filler into pol5Uirethane results in the growth of adhesion strength, most intensively for small amounts of filler. This change in adhesion is certainly caused by reconstruction of polyurethane structure under the influence of the introduced filler surface (demonstrated by the data on molecular mobihty variation), and in particular of the polymer adhesive layer structure. In this con-... 

L. H. Lee (Xerox Corporation) What is the nature of the primer for polyurethane structural adhesives ... 

Alan G. McKown obtained a Bachelor of Chemical Engineering degree from Cleveland State University in 1963. He joined the Adhesives, Coatings and Sealers Division of 3M in 1964. At 3M he has been involved in epoxy and polyurethane structural adhesive development. 

A more important trend has been to replace part of the polycarboxylic acid with a polyisocyanate, usually toluene di-isocyanate, thereby introducing a polyurethane structure into the alkyds. These products which are known as urethane alkyds form the basis of the most widely used gloss paints in the UK at the present time, although the straight alkyds are the more common in the USA and other parts of Europe. 

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