Prepolymer manufacture

The NCO level is normally specified by the prepolymer manufacturer but can slowly be reduced if the material has not been properly stabilized or if it has been stored for a long period at elevated temperatures. This will effectively give off ratio mixes, the consequence of which could be low Durometer readings, poor tear strength, or low tensile strengths. 

Sodium and potassium content in polyethers is determined by flame photometry from aqueous solutions of polyethers disaggregated before, or directly from solutions of polyethers in ethanol. The determinations are based on calibration curves made with solutions having known amounts of sodium and potassium ions. The maximum content of Na and K ions in polyethers was around a maximum of 5-10 ppm. In the polyether polyols used for prepolymer manufacture, the maximum limit for Na and K content is accepted as a maximum of 2 ppm in order to avoid the trimerisation and gelation of the prepolymer during storage. 

Melan, Melamin n Melamine/formaldehyde prepolymers. Manufactured by Henkel, Germany. 

Most general-purpose release agents have been developed for this market in part because of their low toxicity and chemical inertness and do not usually present health and safety problems. Some of the solvent dispersions require appropriate care in handling volatile solvents, and many supphers are offering water-based alternatives. Some of the sohds, particularly finely divided hydrophobic sohds, can also present inhalation problems. Some of the metallic soaps are toxic, although there is a trend away from the heavier, more toxic metals such as lead. The reactive type of release coating with monomers, prepolymers, and catalysts often presents specific handling difficulties. The potential user with health and safety questions is advised to consult the manufacturer directly. 

In more recent years, molded flexible foam products are becoming more popular. The bulk of the molded flexible urethane foam is employed in the transportation industry, where it is highly suitable for the manufacture of seat cushions, back cushions, and bucket-seat padding. TDI prepolymers were used in flexible foam mol ding ia conjunction with polyether polyols. The introduction of organotin catalysts and efficient siHcone surfactants faciHtates one-shot foam mol ding, which is the most economical production method. 

Semiflexible molded polyurethane foams are used in other automotive appHcations, such as instmment panels, dashboards, arm rests, head rests, door liners, and vibrational control devices. An important property of semiflexible foam is low resiHency and low elasticity, which results in a slow rate of recovery after deflection. The isocyanate used in the manufacture of semiflexible foams is PMDI, sometimes used in combination with TDI or TDI prepolymers. Both polyester as well as polyether polyols are used in the production of these water-blown foams. Sometimes integral skin molded foams are produced. 

In these systems the prepolymer is prepared with an excess of isocyanate to give an isocyanate-terminated molecule. This is then reacted with unreacted glycol and other ingredients. The disadvantage of this system is that component streams are of similar volume and viscosity, this facilitating both metering and mixing. The system is used mainly for the manufacture of microcellular products. 

The same dibutyltin compounds are used in the industrial manufacture of poly(urethane) foams, the first step in which involves the addition of a polyether glycol to 2,4-diisocyanotoluene, to produce the urethane prepolymer having isocyanate end-groups. 

These subgel prepolymers have been manufactured by industry for some time and used in two-component adhesives or coating materials. For one-component materials, storage below the actual Tg, whenever applicable, is the most efficient method for blocking the reactivities. This kind of blocking is used in powder coatings. 

BMI or aminobismaleimides or addition polyimides from the reaction of a diamine and a bismaleimide to make a prepolymer that is then cured by an excess of diamine. This type is particularly convenient for the manufacture of thick parts. 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

It is in the technique of solidifying the mass that plastisol propellants differ so markedly from composite propellants. In composite propellants, the nonvolatile liquid is comprised of monomers or low molecular weight prepolymers. Solidification is accomplished by completion of the polymerization reactions. Much attention must be given to the degree of completion of these reactions during manufacture so as to minimize changes in physical properties as a consequence of continued slow polymerization, or so-called post-cure, following manufacture. 

Since these polymers vary in average functionability, primary and secondary carboxyl groups, 1-2 and 1-4 addition product, and chainbranching products, it is not surprising that excellent control must be exercised during manufacture of the prepolymers and during propellant production. 

Information regarding the chemical identity of HDI is located in Table 3-1. Most of the HDI manufactured in, or imported into, the United States is converted into HDI prepolymers (polyisocyanates). These prepolymers are biurets and trimers. Information for those prepolymers is shown in Table 3-2. 

Except for occupational settings, no information was formd in the available literature on eoncentrations of HDl or HDl prepolymers in air. Because of the relatively short atmospheric half-life (approximately 2 days) from reaction with hydroxyl radicals (see Section 5.3.2.1), significant atmospheric concentrations of HDl would be expected to be found only near sources of this substance (e.g., waste streams from manufacturing or processing facilities, hazardous waste sites, occupational settings). Atmospherie eoneentrations of HDl and HDI-BT found in occupational settings are siunmarized in Section 5.5. 

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