Prepolymer stable

For convenience of application it is usual to utilise the two-stage preparation shown above. Initially the soluble polymer (I) is formed which is then converted into the insoluble thermally stable polyimide (II) Figure 18.35). Suitable solvents for the high molecular weight prepolymer (I) include dimethylformamide and dimethylacetamide. 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

Polydithiazoles Polyoxadiazoles Polyamidines Pyrolyzed polyacrylonitrile Polyvinyl isocyanate ladder polymer Polyamide-imide Polysulfone Decompose at 525°C (977°F) soluble in concentrated sulfuric acid. Decompose at 450-500°C (842-932°F) can be made into fiber or film. Stable to oxidation up to 500°C (932°F) can make flexible elastomer. Stable above 900°C (1625°F) fiber resists abrasion with low tenacity. Soluble polymer that decomposes at 385°C (725°F) prepolymer melts above 405° C (76l.°F). Service temperatures up to 288° C (550°F) amenable to fabrication. Thermoplastic use temperature —102°C (—152°F) to greater than 150° C (302°F) acid and base resistant. 

The partially aromatic PAs are exclusively made of die diamine-diacid type and not die amine-acid type. The aromatic diamines, similar to phenylene diamines, color easily and dieir polymers are conjugated, having a golden brown color. The aromatic diacids used in the formation of partially aromatic PAs are mainly terephthalic and isophthalic acids. Starting with the diacids, the PA salt is made first and with this the salt prepolymers are prepared. The prepolymerization is usually carried out in an autoclave to prevent die sublimation of the reactants. In a laboratory synthesis it would be preferable to avoid this autoclave step as one is not always available. It is possible to start with the more reactive esters, such as diphenyl isophtiialate, or with the acid chlorides starting with the reactive isocyanates is, in principle, also possible. The terephthalic and isophthalic acids are also used to modify PA-6,6 and PA-4,6 to more dimensionally stable copolymers.6,18... 

On the whole, curing procedures appear a promising way to obtain very stable polymer films. Thus, the structure of already mentioned polylysine has been revised as a block polymer involving either the a or e amino groups of lysine Vitamin Bj2 modified carbon electrodes were prepared by thermal curing of a mixture of a diamino functionalized derivative 5 and an epoxy prepolymer 6 of the araldite... 

In order to bring about crosslinking of polyesters with styrene one of two types of initiator systems is used, which differ in the temperature at which they are effective. For curing at elevated temperatures, peroxides are used which decompose thermally to yield free radicals. Among those peroxides employed are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-butyl peroxide, and dodecyl peroxide. Mixtures of polyester prepolymer, styrene, and such initiators are reasonably stable at room temperatures but undergo fairly rapid crosslinking at temperatures between 70 °C and 150 °C, depending on which particular peroxide is used. 

Polyimides are thermally stable, heterocyclic aromatic materials of desirable engineering properties. They are, however, insoluble. A typical mode of preparation1 18 11is given in Fig. 29 where reactants (a) as well as the polyamic acid or pyrrone prepolymers (b) are maintained in solution. 

Baekeland found that a relatively stable resole prepolymer could be obtained by the controlled condensation of phenol and formaldehyde under alkaline conditions. These linear polymers of phenol-formaldehyde (PF) may be converted to infusible cross-linked polymers called resites by heating or by the addition of mineral acids. As shown in structure 4.80, the initial products obtained when formaldehyde is condensed with phenol are hydroxybenzyl alcohols. The linear resole polymer is called an A-stage resin, and the cross-linked resite is called a C-stage resin. 

Monomer 12 is a crystalline solid which, when homopolymerized, affords a high Tg, thermally stable polymer that has potential application in both the microelectronic and aerospace areas. Monomer 13 (mixed isomers) is a liquid at room temperature and, when homopolymerized, also provides a high Tg, thermally stable polymer that has a low moisture uptake and a low dielectric constant. This polymer has been targeted into the microelectronics area because of this interesting set of properties, combined with the prepolymer s unique ability to planarize over underlying topography. 

For r < 1/3, no gel can be produced by epoxy-amine reactions. However, the homopolymerization of the epoxy excess (Eq. 3.37) may take place, finally leading to gelation. So, it is not convenient to use an epoxy excess to synthesize stable epoxy-amine prepolymers. Commercial epoxy-amine adducts are based on an amine excess. 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

Additional fexible and oxidatively high-temperature stable elastomers were prepared by the author [1,2] by thermal curing modified linear poly(silar-ylene-siloxane-acetylene) prepolymers, (I), and (II), respectively. 

A polyfunctional cation-exchange resin prepared by heating a mixture of furfural and /)-phenolsulphonic acid with a prepolymer from furylacrylic add at 60 °C for 5-6 h is stable up to 100 °C, swells 135 % in water (in H form), and possesses static exchange capadty of 4.9-5.0, 2.3, 2.7, and 3.5 meq/g for O.IN NaOH, O.IN CaQj, —COOH groups, and O.IN salsolidine, respectively. 

Monomers and prepolymers that are useful to form shellwalls were reacted with interface modifying molecules to create novel surfactants in-situ leading to stable emulsions [23], The reactions are designed to create polymers with surfactant properties rather than encapsulating shellwalls at the OAV interface. The emulsions were formed without conventional emulsifiers, and were shown to have long term stability without creaming. Absence of classical emulsifiers reduces the tendency toward foaming. 

Such monomers with separated bifunctional sides for polymer formation diow better flexibility of the polymer by the linkage between Pc-units. The heating of (97) produces a prepolymer gel it can be processed into any desired form by additional heating. In air such polymers are stable over several thousand hours at 523-573 K. 

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