Prepolymer, epoxy resins and

Preeclampsia, Viagra and, 164 Prelog, Vladimir, 181 Prepolymer, epoxy resins and, 673 Priestley, Joseph, 245 Primary alcohol, 600 Primary amine, 916 Primary carbon. 84 Primary hydrogen, 85 Primary structure (protein), 1038 Primer strand (DNA), 1108 pro-R prochiralitv center, 316 pro-S prochirality center, 316 Problems, how to work, 27 Procaine, structure of, 32 Prochirality, 315-317 assignment of, 315-316 naturally occurring molecules and, 316-317... 

A large exothermal effect resulting from chemical reactions is typical of these processes. When using oligomeric initial components, for example, formulations based on urethane prepolymers, epoxy resins, and lactams, self-heating may cause thermal decomposition. For this reason the correct choice of the initial solidification temperature is very important. 

This is a low viscosity liquid, which dissolves many amorphous plastic resins, with high TgS enabling their ready incorporation into hot melt systems. It is compatible with cyanate ester prepolymers, epoxy resins and bismaleimides. 

Few non.chemists know exactly what an epoxide is. but practically everyone has used an "epoxy glue for household repairs or an epoxy resin for a protective coating. Epoxy resins and adhesives generally consist of two components that are mixed just prior to use. One component is a liquid "prepolymer/ and the second is a "curing agent" that reacts with the prepolymer and causes it to solidify. 

As in the cyanate/epoxide compositions, polysulfone is used as an additive in cyanate/epoxide/maleimide systems. As an example, a composition for carbon-fiber composites should be mentioned. It contains BPA/DC-BMI prepolymer, epoxide resin and polyethersulfone [111-113] Zn acetate and dicumyl peroxide are added. Polyethersulfone powder was added to the three-component system with 4,4 -diamino-diphenylmethane-based tetraepoxide as one of the epoxy resins used [114]. 

Blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines.18,19 These blocked prepolymers will initially react with the amines to form amine-terminated prepolymers that crosslink the epoxy resin. Urethane amines are also offered commercially for use with epoxy resins to develop hybrid adhesive systems.20... 

The most widely used epoxy resins and adhesives are based on a prepolymer made from bisphenol A and epichlorohydrin. On treatment with base under carefully controlled conditions, bisphenol A is converted into its anion, which acts as a nucleophile in an S142 reaction with epichlorohydrin. Each epichlorohydrin molecule can react with two molecules of bisphenol A, once by S 2 displacement of chloride ion and once by opening of the epoxide ring. At the same time, each bisphenol A molecule can react with two epichlorohydrins, leading to a long polymer chain. Each end of a prepolymer chain has an unreacted epoxy group, and each chain has numerous secondary alcohol groups. 

Crosslinked cyanate ester systems typically exhibit higher glass transition temperatures (T S 250°C), lower moisture absorption and lower dielectric constants than conventional epoxy thermosets. Hence by mixing dicyanates or their corresponding prepolymers with epoxy resins, and then co-curing, the desired level of property improvements can be achieved. Such hybrid thermosets have been used in printed circuit board manufacture. 

More recent efforts have focused on developments that create true hybrids. For example, blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines [68-70]. These blocked prepolymers will react initially with the amines to form amine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanates are commercially available. The DESMOCAP (Bayer) llA and 12A products are isocyanates (believed to be blocked with nonylphenol) used as flexibilizing agents for epoxy resins. ANCAREZ (trademark. Pacific Anchor, Inc.) 2150 is a blocked isocyanate epoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin, and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71]. Urethane amines are offered commercially that can be used with epoxy resins to develop hybrid adhesive systems [72]. 

Precipitate can also form fiom a step reaction. Precipitation polymerizations of prepolymers to form crosslinked, thermoset polymers is a very common commercial reaction of the epoxy resins and phenol-methanal, network polymers covered in this book. These reactions work best when the reaction is slow, mildly exothermic, and undergoes a viscous to solid transformation late in the synthesis. 

These hydroxy-terminated polyesters may have applications as prepolymers for other high molecular weight polymers. These could condense with diisocyanates to produce polyurethane foams [44], either flexible or rigid. They also may have compatibility with epoxy resins and polyester to give rise to polyester epoxy resins, polyester-polyamide [41], and polyester-polyether copolymers. The further condensation with some unsaturated monomer may result in thermoset film formers. They can be used as a mildness additive in metal-working lubricants. 

The addition-reaction product of bisphenol A and glycidyl methacrylate or an epoxy resin and methacrylic acid is bis-GMA 2,2- 7Z5 4-(2-hydroxy-3-methacryloxypropoxy)phenyl propane (Fig. 5). Bis-GMA can therefore be classified as a dimethacrylated epoxy, although it does not contain a reactive epoxy group. Bis-GMA is the most commonly used prepolymer in dental composite restorative materials. Several similar compounds have also appeared as substitutes for Bis-GMA or in addition to it in dental resins. Such dimethacrylates based on bisphenol A with various chain lengths are bis-MA 2,2- fs(4-(metha-cryloxy)phenyl)-propane, bis-EMA 2,2-i7is(4-(2-met-hacryloxyethoxy)phenyl)-propane and bis-PMA 2,2- s(4-(3-methacryloxypropoxy)phenyl)-propane. 

Mukoyama and Mori [42] investigated the elution behaviour of alkylbenzenes, phthalate esters, oligostyrenes and prepolymers of epoxy resins and methylated melamine-formaldehyde resins on a column packed with hydrophilic poly-6-hydroxy-ethyl-methacrylate. 

The isocyanate group is more reactive than the epoxy group in that it will react at room temperature with water and hydroxyl groups as well as with amine groups. However, the latter reaction is too fast to be practicable so the standard two-pack coatings are based on isocyanate and polyhydroxyl prepolymers such as hydroxyl terminated polyesters or polyethers as in the last example given in the section on epoxy resins. 

As is usually characteristic of crosslinked polymers of commercial importance, epoxy resins are prepared in two stages, with the initial reaction leading to a linear prepolymer and the subsequent reaction introducing the crosslinks between the molecules. The prepolymers from which epoxy resins are prepared are diglycidyl ethers with the structure shown in Figure 4.2. 

Reactive prepolymers used as binders are produced by acrylation of oligomers, such as epoxy resins, urethanes, polyesters, silicones, oligo-buta-diene, melamine derivatives, cellulose, and starches. Prepolymers are the principal ingredients of coating formulations and largely determine the basic properties of the coating. Examples of industrially important acrylated prepolymers are in Table 5.9. 

The drums must be clean and dry before being filled. The inside of the drums must be coated with either a phenolic or epoxy resin to provide resistance against attack from the prepolymer. The wall thickness must be sufficient to prevent collapse due to a vacuum being formed as the material cools and contracts. Prior to sealing the drum, nitrogen should be introduced into the free space in the drum. 

Flame-retardant epoxy resins with different silicon contents were prepared using silicon-containing epoxides or silicon-containing prepolymers. The thermal stability and flame-retardant properties of the produced epoxide systems were evaluated and related to the silicon content. The char yields under nitrogen and air atmospheres increased with increase in silicon content. The authors pointed out that the silicon-containing resin has improved flame retardancy over the silicon-free resin as evidenced by the LOI. LOI values increased from 24 for a standard commercial resin to 36 for silicon-containing resins.35... 

Epoxy-urethane adhesives provide properties when cured that are similar to those of epoxynylon adhesives except they offer a major improvement in moisture resistance. Isocyanate monomers and prepolymer react with the hydroxyl groups on epoxy resins to give tough,... 

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