Prepolymer preparation

Isocyanates must be used strictly in accordance with the government s and supplier s recommendations. The material is in the unreacted form with the potential for the presence of vapors. MDI at temperatures above 50°C has a substantial vapor pressure compared to when it is at ambient temperature. Care must be taken if work is carried out using PPDI, as it has a very high vapor pressure. The fumes will bring tears to the eyes if contact is made. 

If the presence of isocyanates can be smelled, this means that the safe exposure limits have been exceeded. 

Engineering controls must be the first line of safety when using any diisocyanate, followed by the strict use of personal protective equipment (PPE). There also have to be strict engineering protocols so that cross-connection between sources of polyols and diisocyanates cannot be made. Personal and atmospheric monitoring must be carried out. It is important that the operators in this area be under medical screening. 

Isocyanates are also strong skin sensitizers and can cause irritation to the eyes and mucous membranes. The correct protection equipment must be worn and there must be thorough washing of the hands and arms after exposure to isocyanate liquid or its vapors. 

The factory should have the correct hazardous chemical signage at every entrance to the shop floor. Protocols must be set up to handle any spilled isocyanates. Minor spills in-house must be correctly neutralized and the waste disposed of according to local regulations. The local fire and emergency authorities must be made aware of the presence of isocyanates in the factory and have the appropriate documentation of how to handle any emergency. The industry professional body should have this information available for the fire and emergency authorities. 

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This process provides a prepolymer with a narrow molecular weight distribution, but the mean molecular weight can be varied over a wide range, and the process may also be adjusted to achieve various microstructures (cis, trans, and vinyl). On the other hand, prepolymers prepared by the free radical initiation technique generally exhibit a broad molecular weight range and a somewhat branched structure. 

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. 

Viscosity measurements at constant temperature were made using a conventional Shirley—Ferranti cone-and-plate viscometer. In some cases complications arose due to phase inversion or to the fracture of low-molecular-weight polymer. One case was a typical one-shot flexible polyether foam system. On the other hand, a polyether prepolymer prepared from 2000 M.W. poly(oxypropylene)glycol and 80 20-TDI, reacted with water, formed a homogeneous system initially, and was suitable for study with this technique. The rate of viscosity increase at several temperatures is shown in Fig. 15. 

Prepolymer Preparation. A well-defined, low molecular weight polymer is synthesized. This prepolymer must be stable, have the base structure desired in the finished product, and cure in the desired manner. 

Another new and interesting polymer family, the Oxirons, are under development by Food Machinery and Chemical Corp. (13), These, too, are struoto-pendant prepolymers prepared from butadiene to which the epoxy functional group is introduced. Functionality may be of the epoxy or unsatiuration type. A typical prepolymer might be a material of a molecular weight of 1500. These systems are cross-linked with anhydrides, amines, and peroxides. 

Titration indicated that almost all of the methanedisulfonic acid catalyst (or its reaction products) was present in the black material which was deposited on the flask walls during the latter stages of the tetramethylcyclobutanediol prepolymer preparation when hexane was the solvent. Since the catalyst is very soluble in water and insoluble in hydrocarbons, it evidently came out of solution with some decomposition products when no water remained in the system. An analysis of the prepolymer indicated that only 0.002% sulfur (1% of the original catalyst) was present. After solid-phase buildup, less than 0.001% sulfur was found in the polymer. 

Before the prepolymer preparation, it is important to confirm that the polyols used have a low water content, e.g., less than 0.02% and a low pH value in order to prevent gelation in the prepolymer preparation. Table 23 shows rigid urethane-foam formulation by the semi-prepolymer process for obtaining a 30-kg/m density foam by hand mixing. 

Nylon block copolymers were previously synthesized from the anionic polymerization of caprolactam in the presence of polyurethane prepolymers. (11) The prepolymers, prepared from the reaction of diisocyanates with polyether glycols, contained Isocyanate end groups which initiated caprolactam polymerization. Sodium caprolactam was used to catalyze the reaction. This copolymer system is the basis for some current areas of nylon 6 RIM research. (12) NYRIM nylon block copolymers are formed from stoichiometric mixtures of polymeric polyols and caprolactam using poly acyllactam initiation which was described previously. The reactions are as follows ... 

A feature of the slurry polymerization is that the reactivity of the carboxylate ion toward the lactone ring is much higher than that of the phosphine. It has been estimated that under the conditions of prepolymer preparation, the propagation rate is at least 600 times higher than the initiation rate. Another aspect of the system is the stability of the living polymer chain although termination reactions can take place, prepolymer remains active almost indefinitely when stored at room temperature. 

H12MDI exists in three isomeric forms, the trans-trans, trans-cis, and cis-cis forms. Due to the fact the cis isomer is more sterically hindered, the trans isomer reacts considerably faster with a hydroxyl group than the cis isomer (27). As with other aliphatic isocyanates, catalysis (generally organotln catalysts) is usually used for both the prepolymer preparation and the cure reaction (cross-linking). 

Prepolymer Preparation reprinted from S. R. Sandler, J. Appl. Polym. Sci. 11, 811 (1967). Copyright 1967 by the Journal of Applied Polymer Science. Reprinted by permission of the copyright owner. 

Such a resin of the alkaline novolak-resol type can be prepared by coreacting a prepolymer, prepared by reacting formaldehyde and phenol in the molar ratio of 2.6 1.0, and a prepolymer obtained by reacting formaldehyde and phenol in the molar ratio of 1 1. The two prepolymers are then mixed in 50 50 proportions by mass and coreacted. 

Amino resins-. A related family of polymers is made from random prepolymers prepared by reacting either urea or melamine (shown below) with HCHO. The products are known as aminoplasts. 

Calculated Weight of Each N-Mer Population as a Function of the Methanal/Phenol Ratio and the Chain Length for a Novolac Prepolymer Prepared from a Chaise Ratio 3-Methylphenoi/4-Methylphenol = 50 50 Using Oxalic Acid Catalyst at 90°C. (copyright SPIE-nie International Society for Optical Engineering and with permission from the author [64])... 

The three polyester prepolymers prepared having varying viscosity (different di/tri acid ratio) with essentially the same hydroxyl content were used to define optimum reaction conditions for introduction of unsaturated groups via addition of glycidyl methacrylate to the liable hydroxyls. 

After completion of this prepolymer preparation the contents are degassed and cooled to 100°C. The chain extender(s) previously dried under... 

The preferred polyesters are adipates, ethylene butylene adipate being favoured. The isocyanate used is MDI, and pure MDI rather than crude (mixed isomers) is found to give a product with best performance. The toxic risk from MDI is much lower than from TDI it is reduced further because it is contained in the quasi-prepolymer prepared by the chemical manufacturer and presents no appreciable health hazard at moulding. Chain extenders are usually low molecular weight glycols, with butane glycol being especially used. Other additives are silicone surfactants to improve cell structure, catalyst, a hydrolytic stabilizer, to prevent hydrolysis of the polyester, and colourant. 

The resulting dispersion is blended with an appropriate amount of an unsaturated polyester prepolymer prepared from orthophthalic acid, maleic anhydride, and neopentyl glycol, with a 5% excess of the last component, P3. Molding polymerizes the styrene component, P4, to form C34. The overall composition is 20% rubber for a total of 40% by weight graded-rubber particles and 60% by weight of polyester-styrene present in weight ratio 65 35. 

Results are presented of a studies of the ageing of PU adhesives for leather, the adhesives used being based on hydroxyl-terminated PU prepolymers prepared from suitable adipate polyesters and aromatic isocyanates. Particular attention is paid to the yellowing of white leather in contact with the adhesive. 35 refs. YUGOSLAVIA EASTERN EUROPE... 

Ex. 2. Liquid, isocyanate-terminated prepolymers prepared from diisocyanates such as TDI (I) and Poly B-D glycol (XXVII), a polybutadiene glycol, may be placed on an adherend directly or from solution and permitted to cure via the action of moisture in the air. Useful as rubber-fabric adhesive. 

Instead of the diols above, Maji et al. [56] evaluated a hydroxyl functional polyester-polyurethane prepolymer prepared by the ester formation of diethylene glycol with adipic acid and subsequent urethane formation with the 2,4-toluene diisocyanate (TDI) reaction with the excess hydroxyls on the polyester (Urepan 600 prepared by M/s Rhein... 

Epoxy resins are formed from low molar mass prepolymers containing epoxide end-groups. The most important are the diglycidyl ether prepolymers prepared by reaction of excess epichlorohydrin with bisphenol-A in the presence of a base... 

The tower reactors similar to the ethylene polymerization reactors are used in other free-radical vinyl polymerization processes. Figure 4a shows a schematic of the tower reactor for bulk styrene polymerization developed by Farben in the 1930s [4]. The prepolymers prepared in batch prepolymerization reactors to about 33-35% conversion are transferred to a tower reactor whose temperature profile is controlled from 100°C to 200°C by jackets and internal cooling coils. There is no agitation device in the tower reactor. The product is then discharged from the bottom of the tower by an extruder, cooled, and pelletized. 

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