Polyurethane curing reaction

First, attention will concentrate on the polyurethane curing reaction itself that is, the polyaddition of isocyanate and hydroxyl groups. Degree of cure and isocyanate conversion rates (as deduced from FTIR) will be discussed for the bulk and compared with thin films on Au, Al, and Cu (Section 6.3.1). Secondly, the morphology of cured films (OM, SFM) will be treated in Section 6.3.2. 

To ensure a strong bond between liner and insulation as well as propellant to liner, it is necessary that liner as well as propellant cure well at the interfaces. This means that in many cases the rubber insulation must undergo some treatment to remove substances which may interfere with the liner cure. Such substances are usually low molecular weight compounds and can often be removed by heating—e.g., water, which would otherwise react with isocyanate in a polyurethane liner. In addition the insulation and/or the cured liner surface may be washcoated with a cure catalyst which will increase the reaction rate of alcoholic hydroxyl groups over the rate of reaction of water with isocyanate to such an extent that the latter reaction can no longer compete with the cure reaction. 

Physical Stabilization Process. Cellular polystyrene, the outstanding example polytvinyl chloride) copolymers of styrene and acrylonitrile (SAN copolymers) and polyethylene can be manufactured by this process, Chemical Stabilization Processes. This method is more versatile and thus has been used successfully for more materials than the physical stabilization process. Chemical stabilization is more adaptable for condensation polymers than for vinyl polymers because of the fast yet controllable curing reactions and the absence of atmospheric inhibition. Foamed plastics produced by these processes include polyurethane foams, polyisocyanurates. and polyphenols. 

Heat plays an important part in the curing of polyurethanes. The reaction itself gives out heat, so this must be taken into account in determining the temperature of the mold. The mold should be at the maximum temperature the curing prepolymer will reach. An MDI-based system will release heat more rapidly than a TDI-based system therefore the mold has to be hotter than when using a TDI-based material. 

Once the prepolymer and catalyst are added together, the chain extension (curing) reactions will commence. The time taken to mix must be carefully monitored. It must be sufficient to allow complete mixing, but there must be enough pot life left to allow pouring into the mold while the material is at the lowest viscosity possible. This is to allow the polyurethane to fill the mold completely and any entrapped bubbles to reach the surface. 

Most polyurethane is foamed during the polymerization/cure reaction. 

Once RIM had been developed for polyurethane molding, the industry began asking whether it could be applied to other fast polymerization/cure reactions. It was quite readily applied to nylon 6 monomer casting, but the cost of the caprolactam monomer appeared non-competitive. It was also suggested for epoxy cure and possibly other fast reactions, but none of these have yet been developed commercially. 

This type of polyurethane cures irreversibly by chemical reaction at room temperature. Branched or cross-linked thermoset polymers are made with higher functional monomers. Thermoset polyurethanes have a higher... 

The molecule arrangement arising due to the chemical reaction of A and B is called the urethane group. If several of these groups are contained in a polymer molecule, macromolecules, called polyurethanes, will develop, which, after the curing reaction, will finally represent the adhesive layer. 

Regarding their curing reaction these adhesive systems differ considerably from the described epoxy and polyurethane adhesives that are characterized by the principle of polyaddition. 

At this point, an important difference compared to moisture curing, one-component polyurethane adhesives described in Section 4.2.2 has to be mentioned. While for cyanoacrylates already small traces of moisture are sufficient for a quick polymerization, polyurethanes require a considerably higher moislure amount for complete curing, because in such bonds, water in chemically bound form becomes a component of the adhesive layer. In the case of cyanoacrylates, water is merely the starter of the curing reaction. 

Fig. 6.3 IR spectra for the curing reaction of t/pu = l pm polyurethane film on Au at room temperature.

Experimental data concerning the effect of hydrodynamics of the flow in a tube (Reynolds Number) on the MMD of a polyurethane have shown [64] that the lower the flow velocity, the wider is the MMD and the greater is the avera molecular mass of polyurethane. Obviously, in the case, the form of MMD is determined by the effect of the curing reaction. 

Isocyanates. Whereas polyamines and thiols cure epoxy compounds via their epoxy groups, isocyanates cross-link high molecular mass epoxy resins via their hydroxyl groups to form polyurethanes. The reaction takes place at ambient temperature. These combinations cure more rapidly and at lower temperature than epoxy resins cured with polyamines. 

The reduced equilibrium moisture content of heat-treated wood can influence the hardening of adhesives that need water for the curing reaction. This is significant for 1-component polyurethane (PUR) adhesive, which cures slower when bonding heat-treated wood with a smaller content of hydroxyl groups than non-treated wood... 

Isocyanate + Water. Flexible polyurethane foam is made primarily by the reaction of excess isocyanate with a stoichiometric amount of water during the polymeriza-tion/cure reaction. This releases CO2 and foams the polymer as it forms. 

Polyurethanes. The two major types of catalysts, for the polyol-polyisocyanate reaction to form polyurethanes, are tertiary amines and/or organotin compounds such as dibutyl tin dUaurate. For delayed reactions, the amines and/or the isocyanates can be temporarily blocked by adducts, which are removed and liberated during the cure reaction. 

Cast polyurethanes. Polyurethanes are reaction products of an isocyanate, a polyol, and a curing agent. Because of the hazards involved in handling free isocyanate, prepolymers of the isocyanate and the polyol are generally used. 

Reaction Injection Moulding.— By contrast with the epoxy resins, polyurethanes cure at much lower temperatures, 40 °C upwards. The reactants are usually polyether or polyester triols and diols (polyols) and isocyanates or isocyanate-tipped polyester piepolymers. The process problems are essentially those of mixing the reactants sufficiently rapidly to achieve composition uniformity and to control the temperature in the mould. Where foams are produced, control of bubble nucleation is an additional complicating factor e.g. Menges and Schwesig ). 

The final type of polyurethane available is known as the two-part polyol system. One component contains the isocyanate containing compound such as a prepolymer or adduct and the second component is a hydroxy group terminated resin which may or may not contain a catalyst. The most commonly used hydroxy-terminated components are polyols, castor oil, hydroxy-terminated polyesters and some epoxy resins. By varying the components of such a system a wide range of cured properties can be achieved ranging from high flexibility to very hard or brittle. The curing reactions and properties of the different polyurethanes are summarised in Table 10.9. 

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