Prepolymer nitrogen

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. 

PA-4,6 salt is prepared from adipic acid and 1,4-tetramethylenediamine as described for the PA-6,6 salt (Example la). PA-4,6 salt (20 g), 2 mL water, and 0.2 mL 1,4-tetramethylenediamine (2.1 mol % excess) are added to a 100-mL glass container in an autoclave. The autoclave is flushed with nitrogen, closed, and given a starting nitrogen pressure of 5 bar. The autoclave is heated over a period of 60 min to 180° C and maintained at that temperature for 100 min, when the pressure is increased to about 8 bar. The pressure is then gradually released, the reaction mass cooled, and the material removed from the autoclave. The prepolymer is crushed into small particles (0.1—0.2 mm) (see Example lb). This prepolymer has a relative viscosity (r]rd) of 1.3 as measured in 96% sulfuric acid (1% solution at 25° C). 

Fifteen grams of this prepolymer powder in a wide-bore reaction tube (Fig. 3.18b) which is flushed with nitrogen is placed in a heating block. The heating block is warmed over a period of 1 h to 270°C and maintained at this temperature for 4 h, after which the reaction vessel is removed. The yellow polymer obtained has an r]i h of 1.9. The polymer has a melting temperature of 391°C, a heat of fusion of 148 J/g, a Tg dry at 120°C, and a Tg wet at —15°C. 

A prepolymer is made first by charging Pluracol E2000 [1000.0 g, 1.0 eq., poly(ethylene oxide), 56 OH, BASF] to a suitable container equipped with a mechanical stirrer and a nitrogen gas inlet. Flush the container with dry nitrogen and add Desmodur W (264.0 g, 2.0 eq., 4,4 -methylene-bis(cyclohexyl isocyanate), 31.8% NCO, Bayer). While maintaining a positive N2 pressure on the reaction mixture, stir and heat at 80°C for 2 h. Cool the product to room temperature and check the NCO content (theory = 3.32 %). It might be necessary to warm the highly viscous prepolymer to take samples for titration. To a portion of this prepolymer (250.0 g, 0.2 eq.), add Dabco T-12 (0.25 g, dibutyltin dilaurate,... 

To obtain the polyurethanes, typically a prepolymer was first prepared by reacting the diisocyanate with various diols in dimethylformamide or dimethylacetamide in a two to one molar ratio at 100-110°C for two hours under nitrogen atmosphere. A solution of chain extenders, such as BEP, was then added to the prepolymer reaction mixture and further reacted another three hours. The polymer was isolated by quenching the reaction mixture in cold water. Fine white powder was obtained with a typical yield of around 90%. 

Prepolymer samples were spin-coated onto silicon wafers and fully cured. The cured polymer films were removed from the wafer and tested by TGA. Analyses were carried out to measure the rate of weight loss at various temperatures in air and in nitrogen. From these results the time to a 1% weight loss was determined and these results are depicted graphically in Figure 3.7. 

Microcontact printing (p-CP) is another technique that can be used to place NAs onto different target surfaces. This technique makes use of an elastomeric stamp of polydimethylsiloxane (PDMS) and produces features with lateral resolution in the submicrometer range. The PDMS stamp is topographically structured by casting a PDMS prepolymer against a 3D master. The stamp is then inked with the molecules of interest, rinsed with buffer, blown dry under a stream of nitrogen, and then used to print the material onto the substrate surface (see Fig. 20). 

The reaction was carried out at 100°C for about two hours until the theoretical isocyanate content, as determined by the di-n-butylamine titration method (27), was reached. The PU prepolymer with or without tertiary amine nitrogen groups was dissolved in dry MEK to obtain a prepolymer solution of 30-40% solids. It was then mixed with a mixture of 1,4-BD/TMP (4 1 by equiv. ratio) at an NCO/OH = 1.05/1.0 ratio in the presence of T-12 catalyst (0.05% based on total weight). The reaction mixture was cast in a metal mold treated with a release agent at ambient temperature. After standing 3-5 hours at room temperature, the mold was placed in an oven and post-cured at 100°C for 16 hours. The samples were then conditioned in a desiccator for one week before testing. 

GAP is synthesized by replacing C-Cl bonds of polyepichlorohydrin with C-N3 bonds.The three nitrogen atoms of the N3 moiety are attached linearly with ionic and covalent bonds in every GAP monomer unit, as shown in Fig. 4.6. The bond energy of N3 is reported to be 378 kj mol per azide group. Since GAP is a liquid at room temperature, it is polymerized by allowing the terminal -OH groups to react with hexamethylene diisocyanate (HMDl) so as to formulate GAP copolymer, as shown in Fig. 4.7, and crosslinked with trimethylolpropane (TMP) as shown in Fig. 4.8. The physicochemical properhes of GAP prepolymer and GAP copolymer are shown in Table 4.4 and Table 4.5, respectively.I ]... 

As described in Sections 4.2.4.1 and 5.2.2, GAP is a unique energetic material that burns very rapidly without any oxidation reaction. When the azide bond is cleaved to produce nitrogen gas, a significant amount of heat is released by the thermal decomposition. Glycidyl azide prepolymer is polymerized with HMDI to form GAP copolymer, which is crosslinked with TMP. The physicochemical properties of the GAP pyrolants used in VFDR are shown in Table 15.3.PI The major fuel components are H2, GO, and G(g), which are combustible fragments when mixed with air in the ramburner. The remaining products consist mainly of Nj with minor amounts of GOj and HjO. 

Much the same happens if the prepolymers contain basic (tertiary) amino groups—e.g., diols based on primary amines extended with propylene and ethylene oxide and similar materials. The amine nitrogen reacts with the oxidizer, releasing ammonia, and is itself converted to the ammonium ion. Ensuing ionic interaction raises the viscosity of the batch to the point where it becomes unmixable (see also later section on moisture embrittlement). 

Under a protective nitrogen blanket, the step 1 product was coupled with methoxy polyethylene glycol prepolymers (Mn 1820 and 4740 Da) using 1,3-dicyclohexylcarbodiimide dissolved in 2 1 . The product was isolated after being purified by precipitation and fractionation from mixed solvents of chloroform/diethyl ether or methanol/diethyl ether. 

On completion of the reaction, the material (if not immediately used) must be stored in a clean, dry container. The material of construction must be able to withstand the heat of the material plus any further reheating required. If metal containers are used, they must be lined with an epoxy or similar material that is nonreactive to the polyurethane prepolymer. Any remaining air in the container must be replaced with dry nitrogen gas. All mating surfaces of the closure must be free of the polyurethane prepolymer. 

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. 

The prepolymer also may be discharged by gravity into the container. Nitrogen gas pressure may be used to assist the flow of prepolymer. 

The reactor needs to be kept clean to keep the heat transfer optimal and to prevent solid material in the prepolymer. The method employed is to use an appropriate solvent such as methyl ethyl ketone (MEK), methylene chloride, or m-pyrol (NMP). To prevent an explosive vapor mixture from being formed when the solvent is added to the reactor, the air must be replaced by nitrogen gas. The solvent needs to be heated to just above its boiling point and kept there until the solid material has been softened and removed from the metal. A second rinse with clean solvent may be needed. 

When the prepolymer is melted, it and any gas in the container will expand. Care must be taken to prevent excessive pressure buildup. The container must be vented to reduce the pressure buildup otherwise, it may rupture. After use, the can must be flushed with nitrogen to remove any moisture-containing air. 

After dispensing a run of prepolymer from a container, the prepolymer must be resealed in a state that will prevent attack by moisture in the air. A blanket of dry nitrogen gas must be used to displace any air present in the container. Any bungs or taps must be properly cleaned using a dry solvent such as MEK or MIBK to prevent traces of polyurethane from reacting with the moisture in the air and sealing the container. 

Nitrogen is often used to blanket prepolymers and curatives after the container has been partially used. The blanketing system, if left in place permanently, must be so engineered that the container cannot build up a level of pressure in it that renders the situation unsafe. 

In production units where polyurethane prepolymers are used, liquid nitrogen is an option to provide an inert atmosphere in the reactors. The unit should be set up professionally by a supplier and the liquid nitrogen unit properly maintained. 

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... 

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