Prepolymer Technique

Prepolymers are formed by the reaction of a diisocyanate with an oligo-polyol, at the molar ratio [diisocyanate]/[OH group] of 1/1, in fact only one group of diisocyanate reacts with one hydroxyl group of the polyol. A structure with free terminal -NCO groups called prepolymer is produced (see Equation 2.4)  

By the reaction of a prepolymer with a chain extender such as ethylene glycol, diethylene glycol, 1,4 butane diol or a diamine, the high molecular weight polyurethanes are formed (see Equation 2.5). 

This prepolymer technique is frequently used in the manufacture of polyurethane elastomers, coatings, sealants, flexible foams, monocomponent polyurethanes, etc. 

In the special case of monocomponent polyurethanes, the single partner of the reaction is the prepolymer. The prepolymer is extended to a high MW polymer by reaction with water present in the atmosphere. Water, is in fact, a chain extender and the resulting high MW polymer has both bonds urethane and urea bonds  

If a prepolymer derived from an oligo-triol or an oligo-polyol, having three or more terminal -NCO groups is used, if it is in contact with atmospheric humidity, crosslinked polyurethanes are obtained. 

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There is another method, called the quasi-prepolymer technique, which is similar to the full-prepolymer process but utilizes prereacted isocyanates in the 10-20% NCO range. This eases processing compared to full-prepolymer systems, requiring lower temperatures and volume ratios typically from 4 1 to nearly 1 1 (polyol to isocyanate). 

MDI- or NDI-based PUs usually have better mechanical properties than PUs obtained from aliphatic polyisocyanates. The effect of various diisocyanates on the thermal stability of their PU elastomers is given below for a polycap-rolactone (mol. wt. 2000)/diisocyanate/BDO type elastomer of molar ratio 1 2.6 1 and synthesized using the prepolymer technique. 

The chemistry function is produced by the prepolymer technique. Examples will follow in later chapters. The prepolymer is mixed with a reactant (polyol and/or water) and immediately pressed into a reticulated foam using a nip roller arrangement as shown in Figure 2.16. 

Whether the one-shot process or prepolymer technique is used, the development of a foam involves the juxtaposition of gas generation and the development of tensile strength within the developing foam. The evolution of gas can be via the use of blowing agents or the in situ generation of CO2 from the reaction of water with an isocyanate to produce a water-blown foam. In any case, the gas evolution creates an internal pressure that must be resisted by the development of a gel structure via polymerization reactions as shown in Figure 3.13. 

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

Stengard, R.A., "Chemistry and Prepolymer Techniques," Part 1 in Section IX - Rigid Urethane Foam, Handbook of Foamed Plastics, Ed., Bender, R.J., Lake Publishing Corp., Libertyville, Illinois (1985). 

In polyurethane fabrication, some special techniques are used, such as prepolymer technique, quasiprepolymer technique and one shot technique. 

For example, in practice, polyethers with an alkaline ion content of 50-400 ppm are used successfully. This is possible because in rigid PU foam production the one shot technique is used predominantly. The prepolymer technique is used to a small extent for one component rigid PU foams, used as sealants or in coatings. In this case the polyol needs less than 2 ppm potassium ion (for example propoxylated glycerol), in order to avoid the gellification of the prepolymers, due to the trimerisation of -NCO groups catalysed by K+ ions. 

The AFM images of fracture surfaces revealed that the PUs obtained by the two stage prepolymer poliaddition prove dure, have a more uneven relief than the those of the material obtained by the one step prepolymer technique. In addition, DBDI increases this difference, but leads to more uniform relief forms than does MDI. 

To avoid the appearance of terminal blocks of the type -[I-MD]-, the prepolymer technique is usually used in most of the cases. In this type of synthesis, the MD is first reacted with an excess of I so that the whole amount of MD is transformed in a macrodiisocyanate prepolymer ... 

Table 4.27 Stress-strain data of a PU based on the SS macrodiol PBA with/without hydrogen bonding, obtained by the prepolymer technique...

Rigid polyurethane foams were first used as the core of sandwich structures in aircraft. Polyester polyols were used in combination with TDI, employing the prepolymer technique described for elastomers in Sections IV. A and IV.B. Major developments in manufacturing rigid polyurethane foams came with the introduction of MDI as the isocyanate eomponent and the use of chlorofluorocarbon blowing agents, especially... 

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