Prepolymers

Historically, the adhesives based on reactive condensation prepolymers were the first materials to be extensively studied, developed and introduced to the market. 

Once cleaned, the metal surfaces are coated with a primer prepared by rollmilling 200 g of adhesive composition and 5(X) g of methanol. A first layer of primer is appUed to the metal parts, dried for 10 min at 90°C, covered with a second layer of primer and finally dried at 90°C for 1 h in an air circulating oven. The adhesive film is inserted between the two primed metal surfaces and the assembly is placed between the platens of a hydraulic press preheated to 316°C. A pressure of 0.3-0.5 MPa is applied and the test specimens are cured at 316°C for 1.5 h, before cooling down to 100°C under pressure. The assemblies are removed from the press and postcured in an air circulating oven as follows the temperature is raised from 20 to 300°C in 1 h, then from 300 to 350°C in 5 h and left at 350°C for 5 h. The lap shear strength values determined at different temperatures are plotted in Fig. 29 for TU-2 Ti/li assemblies [curve (a)] and ZIO CNT 1810 stainless steel coupons [curve (b)]. 

Ageing tests were also performed with stainless steel specimens subjected to thermal-oxidative stressing at 3(X)°C for 2000 h. The experimental results are plotted 

The resistance of the TU-2 titanium assemblies to shorter term exposure at high temperature was determined at 340, 377, 400 and 450°C. The lap shear strength data plotted in Fig. 32 were determined at the corresponding ageing temperature. 

4 -carbonylbis(l,2-benzenedicarboxylic acid) dimethyl ester 33 with 4,4 -methylenebisbenzeneamine 34 (Fig. 16) in NMP solution. The FM 34 brand, which contained arsenic derivatives, was withdrawn from the market and replaced by FM 34B-18 without arsenic pentasulphide. 

Historically, the adhesives based on reactive condensation prepolymers were the first materials to he extensively studied, developed, and introduced to the market. One of the best performing polyimides in this category is Nolimid 380, produced by Rhone-Poulenc after an extensive series of tests. The resin binder is the mixture of monomers 30a and 30b, as well as the low-molecular-weight (oligoimide) poiyimide 31 of Fig. 9.15. Typical compositions are prepared as previously reported by mixing the two monomers and 4,4 -oxybisbenzeneamine in NMP and methanol.To this solution, introduced into a roll-mill, are added aluminum powder (400 mesh) 

Once cleaned, the metal surfaces are coated with a primer prepared by roll-milling 200 g of adhesive eomposition and 500 g of methanol. A first layer of 

Aging tests were also performed with stainless steel specimens subjected to thermal-oxidative stressing at 300 °C for 2000 h. The experimental results are plotted in Fig. 9.31 in which curves (a), (b), and (c) show the variation of the lap-shear strength values measured at 25,300, and 350 °C, respectively. 

Specimens of both TU-2 and 6A14V titanium alloys were also subjected to the so-called Mach 3 Fighter test, which consists of 500 h of aging at 300 °C, followed by 24 h at 340 °C and 1 h at 350 °C. At the end of the test, the lap-shear strength was measured at 25, 300, and 350 °C. Before aging, the initial strengths were 20, 15.5, and 12.5 MPa at these temperatures, respectively. Lap-shear strengths of the 

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Stopping the polymer at this point requires the ratio of formaldehyde to phenol to be less than unity. Both methylene and ether bridges are known to be present. The reaction is either acid or base catalyzed, and branching is uncommon at this stage. The products are variously known as A stage resins, novolacs, or resole prepolymers. 

A large number of polymeric compounds have been investigated, but most modem propellants utilize prepolymers that ate hydroxy-functional polybutadienes (HTPB), carboxy-functional polybutadienes (CTPB), or a family of polyethylene oxides (PEGs) to form urethanes. Typical cure reactions... 

To produce a spandex fiber by reaction spinning, a 1000—3500 molecular weight polyester or polyether glycol reacts with a diisocyanate at a molar ratio of about 1 2. The viscosity of this isocyanate-terrninated prepolymer may be adjusted by adding small amounts of an inert solvent, and then extmded into a coagulating bath that contains a diamine so that filament and polymer formation occur simultaneously. Reactions are completed as the filaments are cured and solvent evaporated on a belt dryer. After appHcation of a finish, the fibers are wound on tubes or bobbins and rewound if necessary to reduce interfiber cohesion. 

Stabilizers and pigments are normally slurried with macroglycol and added to the polymeric glycol charge, prior to diisocyanate addition. Therefore, care must be taken to avoid additives that react significantly with diisocyanates or diamines under processing conditions. Also, stabilizers should be chosen that have no adverse catalytic effect on the prepolymer or chain-extension reactions. 

Following this work, the y -12F-diol was used for the direct reaction with hexamethylene-1,6-diisocyanate in the presence of dibutyltin dilaurate to produce a cross-linked elastomer or a reactive prepolymer which was terminated with either isocyanate or hydroxyl groups, depending on which reactant was in excess (142,143). 

Titrations with dibutylamine [111-92-2] can also be used to determine the NCO content of isocyanates and prepolymers. Generally, an excess of amine in a suitable solvent such as chlorobenzene [108-90-7] is added to the sample. The resulting solution is allowed to react and the unreacted amine is back- titrated with dilute hydrochloric acid. For low NCO content levels, a colorimetric method is often used. The isocyanate-containing species is titrated with amine and the unreacted amine is deterrnined using malachite green [569-64-2]. 

Emulsion—Suspension Polymerized Pigment Ink. Polymerization of a polar prepolymer as the internal phase in an oil-based external phase (24) gives a fluorescent ink base in which spherical fluorescent particles are dispersed. This base is suitable for Htho and letterpress inks (qv). An... 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

In the first step of the polymerization process, a prepolymer is prepared as a slurry in water. Excess diamine is added to control the degree of polymerization, eg, degree of polymerization = 6-14 (158). This prepolymerization step is conducted at approximately 200°C under autogenous pressure for less than 90 min. 

The prepolymer is separated from the water by spray drying and then formed into cylindrical pellets of uniform size (159). At this point additives can be added to the porous pellets from solution or suspension. These pellets are then placed in a soHd-phase condensation reactor where they are heated to 260°C for up to 4 h under nitrogen, with a small amount of water added. The pressure is maintained close to atmospheric pressure. At the end, x > n. 

Copper-based thermal stabilizers are also effective photostabilizers for nylon. They can be added before polymerization, or the soluble salts (eg, CuSO can be appHed to fibers as part of the finish or to fabrics as post-treatments. The effectiveness of the copper salt—alkah haUde system added to prepolymer in retarding phototendering and photoyeUowing of the resulting spun yam is illustrated in Figure 5. 

Reaction Injection Molding. RIM uses the anionic polymeri2ation of nylon-6 to carry out polymeri2ation in the mold. A commercial process involves the production of block copolymers of nylon-6 and a polyether by mixing molten caprolactam, catalyst, and polyether prepolymer, and reacting in a mold (27,28). 

Polymerization of the dianhydride and diamine proceeds through an intermediate poly(amide acid) stage before ring closure converts the adjacent acid and amide groups to the polyetherimide (94). The polymerization can be carried directiy to the polyetherimide as a single-step process, or first to an ainide—acid-containing prepolymer, which can be isolated, and then to the polyetherimide. 

G. E. Manser, "Nitrate Ester Polyether Glycol Prepolymer," JANNAF Propulsion Committee Meeting, New Orleans, La., Chemical Information Agency, John Hopkins University, Baltimore, Md., 1984. 

Fig. 3. The key steps of the Kureha process, as disclosed in the patent Hterature (48), are (/) dehydration of aqueous feedstocks (sodium sulfide or its functional equivalent) in the presence of A/-methyl-2-pyrrohdinone (2) polymerization of the dehydrated sodium sulfide with -dichlorobenzene at alow temperature to form a prepolymer (J) addition of water to the prepolymer (4) a second, higher temperature polymerization step and (5) polymer recovery.

Another significant use of 3-methylphenol is in the production of herbicides and insecticides. 2-/ f2 -Butyl-5-methylphenol is converted to the dinitro acetate derivative, 2-/ f2 -butyl-5-methyl-4,6-dinitrophenyl acetate [2487-01 -6] which is used as both a pre- and postemergent herbicide to control broad leaf weeds (42). Carbamate derivatives of 3-methylphenol based compounds are used as insecticides. The condensation of 3-methylphenol with formaldehyde yields a curable phenoHc resin. Since 3-methylphenol is trifunctional with respect to its reaction with formaldehyde, it is possible to form a thermosetting resin by the reaction of a prepolymer with paraformaldehyde or other suitable formaldehyde sources. 3-Methylphenol is also used in the production of fragrances and flavors. It is reduced with hydrogen under nickel catalysis and the corresponding esters are used as synthetic musk (see Table 3). 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

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