Prepolymer other additives

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30-50% of the prepolymer the remainder consists of pigments, tillers, plasticizers, adhesion promoters, and other additives. 

If required, pigment and any other additive may be added and predispersed. The pigments used must be in a dry, nonreactive medium. If the pigments are dispersed in a polyol, the paste may absorb moisture and will react with some of the isocyanate groups. Some yellow pigments need to be added to the prepolymer prior to its final heating, as they disperse poorly. 

In the one-shot process, all of the ingredients—isocyanate, polyol, blowing agent, catalyst, additives, etc.—are mixed simultaneously, and the mixture is allowed to foam. In the prepolymer method (Figure 1.29), a portion of the polyol is reacted with an excess of isocyanate to yield a prepolymer having isocyanate end groups. The prepolymer is then mixed with additional polyol, catalyst, and other additives to cause foaming. The quasi-prepolymer process is intermediate between the prepolymer and one-shot processes. 

Definition Prepolymer molding compds. blended with peroxide catalyst, fillers, and other additives cured by molding 150-160 C molded prods, have good temp, and elec, props. 

The preferred polyesters are adipates, ethylene butylene adipate being favoured. The isocyanate used is MDI, and pure MDI rather than crude (mixed isomers) is found to give a product with best performance. The toxic risk from MDI is much lower than from TDI it is reduced further because it is contained in the quasi-prepolymer prepared by the chemical manufacturer and presents no appreciable health hazard at moulding. Chain extenders are usually low molecular weight glycols, with butane glycol being especially used. Other additives are silicone surfactants to improve cell structure, catalyst, a hydrolytic stabilizer, to prevent hydrolysis of the polyester, and colourant. 

Urethane/urea chemistry has evolved and transformed to offer convenient and effective tools for the modification of biomaterials, providing them with desirable properties for biomedical applications. The efforts described above have produced an array of multifunctional urethane/urea chemistry-based biomaterials to meet the specific requirements of each application. A collection of citric acid-based PUs has been developed with tunable mechanical, degradation, photoluminescent, and biomedical properties. The CABE platform technology enables easy modulation of their unique properties by simply altering the ratios of diols, prepolymers, and other additive(s) and... 

Acrylate-based dielectric elastomers could also be synthesized from commercial commodity monomers. Thus, dielectric elastomer precursor solutions were formulated containing a prepolymer (ohgomer), reactive diluents, photoinitiators, and other additives (Niu et al. 2013). The ohgomer, or a blend of a few ohgomers, was selected as the framework responsible for the basic mechanical properties of the... 

Presence and type of photoreactive prepolymers, oligomers, and diluents Type of additionable photoinitiators or unsaturated Presence and type of thermal crosslinking agents and other additives Presence and type of conventional crosslinkers. 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

Typical liquid systems require doses between 10 and 50 kGy. They consist of binders (prepolymers) with acrylic (H2C=CH-CO-0-) double bonds in the main chain (polymaleates and polyfumarates) and of monomers, usually acrylates used as reactive thinners. Other ingredients added to the formulation may be pigments, dyes, fillers, flatting agents, and additives to improve film and surface properties and attain the required performance criferia. ... 

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