Prepolymer final properties

The production of solid polyurethane parts of the correct engineering quality requires the conversion of either the prepolymer or quasiprepolymer to a solid material. The grade and chemistry of the material must be carefully considered in order to obtain a material that can be reproducibly processed and that has the correct final properties. The correct application of heat also must be used to obtain the best product. 

Both the ratios in the prepolymer production and the curing ratios will affect the final properties of the polyurethane. In the initial prepolymer production, the properties vary according to the molar ratios of the prepolymer. This is illustrated by the graphs compiled in Saunders and Frisch (Saunders, 1962). 

Additive A material combined with the prepolymer and curative to modify the final properties of the casting. This includes plasticizers, fillers, and stabilizers. 

The structures of the oligomers or prepolymers are just as important in determining the final properties of the coatings that are cured by cationic mechanisms as are those that are cured free-radical mechanism. As a result, considerable effort has gone into preparations and studies of prepolymers and oligomers. 

There are several types of polyamide block copoljmiers (15,48). Their synthesis is similar to that of the polynrethane and polyester equivalents. In some cases, only the soft segments are prepolymers whereas in others, prepolymers are used to give both segmental types. Both polyesters and polyethers are used for the soft segments, and this choice affects the final properties of the product. Various... 

In the time interval between phase inversion and gelation of the polystyrene continuous phase, the final morphological features such as size average and size distribution of elastomer domains become fixed. Since these morphological changes affect properties such as modulus and impact resistance, the characteristics of the system at and just after phase inversion and before gelation demand the closest scrutiny. The open time interval was found to decrease as the polyester prepolymer content increases, probably because higher polystyrene conversions are required for the system to reach suitable phase inversion conditions. 

The key to acetylene terminated polyimides is the availability of the end-capper which carries the acetylene group. Hergenrother (130) published a series of ATI resins based on 4-ethynylphthalic anhydride as endcapping agent. This approach first requires the synthesis of an amine-terminated amide acid prepolymer, by reacting 1 mole of tetracarboxylic dianhydride with 2 moles of diamine, which subsequently is endcapped with 4-ethynylphthalic anhydride. The imide oligomer is finally obtained via chemical cyclodehydration. The properties of the ATI resin prepared via this route are not too different from those prepared from 3-ethynylaniline as an endcapper. When l,3-bis(3-aminophenox)benzene was used as diamine, the prepolymer is completely soluble in DMAc or NMP at room temperature, whereas 4,4 -methylene dianiline and 4,4 -oxydianiline based ATIs were only partially soluble. The chemical structure of ATIs based on 4-ethynylphthalic anhydride endcapper is shown in Fig. 45. 

One further phenomena observed In all 73 systems Is the decrease In opacity on curing at elevated temperatures. Above about 60 C the poly(tetramethylene glycol) and excess Isocyanate become miscible. This miscibility may be assisted by the fact that the MDI-BDO hard segments are above their glass transition temperature. To an extent which has not been quantified as yet, llquld-llquld phase separation of MDI and MDI terminated polyol In the prepolymer at low temperatures preslsts Into the final adhesive. The dynamic mechanical behavior of transparent or opaque adhesive, l.e., cured at 60-100 C compared to room temperature are virtually Identical. Similar Immlsclblllty has been observed In other prepolymers (20). This does not appear to adversely affect the adhesive properties of these Halthanes. 

In order to obtain a clearer understanding of prepolymer technology, the theory of gelation is first discussed. A classification of prepolymers is then introduced and finally the available quantitative data on stmcture-property relationships are reviewed. References cited are usually to the most complete and general source and not necessarily to the original publication. 

The excellent properties which can be designed into the final molecule, as well as low application temperatures, make epoxies extremely useful in many electrical applications. Structoset coatings, based on a great variety of prepolymers, are widely used because one is able to tailor coatings for a variety of end uses and to obtain satisfactory properties combined with good economics. 

This prepolymer (Scheme 1.10) is then reacted with a chain-extender diol or diamine to give the final polyurethane or polyurethane-polyurea block copolymer, respectively. The diversity in properties that the final urethane block copolymers exhibit is governed by the nature of the three building blocks the diol-terminated polyester, the di-isocyanate-terminated polyurethane and the chain extender (Hepburn, 1982). As shown in Figure 1.17, these form flexible blocks (from the polyester) that undergo phase segregation as structures of size 1000-2000 nm. 

It will be realized from Chapter 1 that in urethane elastomer formation from liquid components there is the possibility of several reactions occurring simultaneously during a prepolymer or one-shot process, and that the relative proportion of one to the other will affect the overall properties of the final polymer. Thermoplastic and millable urethanes are not, during their processing and fabrication stages, subjected to the type of catalysis discussed in this chapter during their polymer synthesis operations, however, reaction rate-structure considerations will apply. 

C for Adiprene L315. The effect of heated storage on Adiprene LI00 prepolymer on the final product properties is shown in Table 5.2. 

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