Prepolymer Storage

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. 

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Tor [7] developed a new method for the preparation of thin, uniform, self-mounted enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme was dissolved in a solution containing synthetic prepolymers. The electrode was dipped in the solution, dried, and drained carefully. The backbone polymer was then cross-linked under controlled conditions to generate a thin enzyme membrane. The method was demonstrated and characterized by the determination of acetylcholine by an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a penicillinase electrode. Linear response in a wide range of substrate concentrations and high storage and operational stability were recorded for all the enzymes tested. 

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. 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

Dow Chemicals (2001) gives the following storage life data regarding Isonate 125M, which is a pure MDI (Table 3.1). A prepolymer prepared with pure MDI does not have the dimerization problems of the diisocyanate. 

Sample Prepolymer in feeding (weight %) Storage modulus3 (MPa) Tgb (°C)... 

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. 

Fig. 3. Stoichiometric DGEBA/DDS network M, versus prepolymer resin molecular weight. M . (M, calculated from equilibrium rubbery moduli at T = T, -h 45 K). O M, from equihbrium tensile experiments M, from 0.16 hz dynamic mechanical storage modulus measurements (After LeMay >)...

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. 

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