Prepolymer vacuum

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

The PET prepolymer utilized in the solid-state polymerization can be in the form of pellets or chips. However, it will preferably be crushed into powder (particle size 125-250 pm)428 and then vacuum dried overnight at 110°C. 

Wang and coworkers first reported the use of these monomers as a novel elastomeric material for potential application in soft tissue engineering in 2002. The molar ratio of glycerol to sebacic acid they used was 1 1. The equimolar amounts of the two monomers were synthesized by polycondensation at 120°C for three days. The reaction scheme is shown in Scheme 8.1. To obtain the elastomers, they first synthesized a prepolymer and then poured an anhydrous 1,3-dioxolane solution of the prepolymer into a mold for curing and shaping under a high vacuum. 

When this prepolymer was cured in a vacuum oven for 12 h at 60°C, the resulting material also exhibited a strong broad absorption at 2245 cm-l. Likewise, a similar prepolymer allowed to cure at ca. 

Scheme 2. Polyanhydride synthesis via melt polycondensation involves first the formation of oligomeric acetylated prepolymers, followed by condensation under vacuum. Acetic acid is formed as a byproduct of the second reaction.

The basic poly(phenylene vinylene) (PPV) polymer is commonly prepared by the sufonium prepolymer route developed by WessUng and Zimmerman in 1968 but much modified by subsequent workers. The synthesis starts from 1,4-bis(chloromethyl)benzene, via the bis-sulfonium salt formed by reaction with tetrahydrothiophene, and then polymerisation is effected to give the prepolymer by reaction with lithium hydroxide (Figure 3.39). Because of the inherent insolubility of PPV it is this prepolymer that is used to form the film coating on the substrate, for example by using a doctor blade technique. The prepolymer is converted into PPV on the substrate by heating in an oven under vacuum at 200 °C for 8-10 h. 

On completion of the reaction, the material may be carefully degassed to remove any entrapped gasses and unreacted diisocyanate. Care must be taken to prevent foaming. Applying vacuum to the glassware does create the potential for it to implode. Suitable safety precautions must be taken. Table 3.2 indicates some typical time/temperature reaction conditions for some simple prepolymers. 

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 preparation of the isocyanate side is similar to that of a standard prepolymer, with the difference being that only some of the polyol is reacted with isocyanate. All handling procedures are the same as for the normal prepolymer except the application of a vacuum at the end of the reaction. As there is an excess of isocyanate in this side, the exotherm is generally not as large as for a full prepolymer. Viscosity and NCO checks need to be carried out. 

Vessels used for the degassing of prepolymers must be designed to withstand the 250 to 280 Pa gauge vacuum applied. The exhaust fumes from the... 

There are two main times during the processing of polyurethane where vacuum degassing may be required. The first is the prepolymer as supplied by the manufacturer. The second is after mixing the prepolymer with the rest of the system (curative, pigment, extenders, and fillers). 

There are two main methods to "de-gas" the prepolymer. The first is a simple batch system composed of a vacuum pump and a pot designed to... 

In parts where amounts of entrapped air must be kept to an absolute minimum, the process may be carried out under full vacuum. The unit consists of two chambers, one for mixing the prepolymer, curative, and other ingredients, and the lower chamber for the mold. 

The devolatilizer (7) is held under a very high vacuum to remove unreacted monomer and solvent from the polymer melt. The monomer is distilled in the styrene recovery unit (8) and recycled back to the prepolymizer. The polymer melt is then pumped through a die head (9) to form strands, a waterbath (10) to cool the strands, a pelletizer (11) to form pellets and is screened to remove large pellets and fines. The resultant product is air-conveyed to bulk storage and packaging facilities. 

Physical Properties of RIM Elastomers. The elastomers were compounded from demolsturized and purified polyols and quas1-prepolymer. The well blended components were vacuumized and molded In steel molds measuring 6"x6"x0.070". The samples were cured for 1 hour at 120 C, conditioned for 1 week and submitted for determination of physical properties by Instron. The permanent sets were determined at an extension of 300%. The results are complied in Tables I and III. 

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