Prepolymer formation

Catalysts serve a dual purpose in one-component moisture-curing urethanes. The first purpose is to accelerate the prepolymer synthesis. The second purpose is to catalyze the curing reaction of the adhesive with moisture. The most common catalysts used to promote both prepolymer formation (NCO/OH) and later the adhesive curing reaction (NCO/H2O) are dibutyltin dilaurate and DMDEE ((tertiary amine. A stabilizer such as 2,5-pentanedione is sometimes added when tin is used, but this specific stabilizer has fallen from favor in recent years, due to toxicity concerns. DMDEE is commonly used in many one-component moisture-curing urethanes. DMDEE is one of the few tertiary amines with a low alkalinity and a low vapor pressure. The latter... 

Table I. Acid Values of Castor Oil Prepolymer Formation at 180°C.

Acetylene-terminated resins (ATR) bisphenol-based, synthesis, 17-29 thermally curable, 6 Acetylene-terminated sulfone (ATS), thermally curable, 5 Acid values, castor oil prepolymer formation, 241 Addition polymerization... 

Polymerization reactions were carried out via a prepolymer formation followed by chain extension. One equivalent of poly(tetramethylene oxide) [number-average molecular weight (Mn) = 1400g/mol] was injected slowly by a syringe pump into a... 

The function T = T(t) in Eq. (2.42) is determined by the heat of reaction, as discussed above (see, for example Eq. (2.9)). Experimental data confirm that such an equation is correct when there is an inherent source of heat due to an exothermal process of prepolymer formation and when temperature profile is dictated by the surroundings.65... 

Fast pyrolysis of biomass provides a method for the production of phenolics that has the potential to replace at least 50% or more of the phenol in phenol-formaldehyde thermosetting resins. The gel tests indicate that the P/N fractions from pine sawdust pyrolysis with paraformaldehyde have shorter gel times than commercial plywood resins such as Cascophen 313, even without prepolymer formation. A novolak formulation has been prepared using 1 1 by volume of phenol and P/N fraction and about half of the amount of formaldehyde that would be used than if phenol alone were employed. Very promising resols have also been made with a similar substitution of the P/N fraction for phenol. Wood testing and resin formulation development are ongoing activities. The projected economics suggest that additional research and development of this process are fully warranted. 

Resole prepolymer formation is then made to occur by heating above 100 C to evolve water. Above 130-150 C a methylene bridge (to give a novolac by the evolution of formaldehyde, as shown in Scheme 1.13) is formed, rather than an ether link of the resole. 

Polymers of this nature can be polymerized either in solution or in bulk in the latter case they are normally reacted at high temperatures, e.g., 100-150 C. Since our goal was a casting resin, the formulations were reacted in bulk and at lower temperatures to protect heat sensitive electronic components furthermore, low reaction temperatures minimize side reactions that can lead to crosslinking and polymer insolubility. In this process the polyols and diisocyanates were mixed and allowed to react for about 25 minutes at 71 C to form the prepolymer formation while longer times resulted in material too viscous to cast or deaerate. After the indicated time, 1,4-butanediol was added followed by deaeration and subsequent encapsulation of a preheated (71 C) electronic device. A second deaeration of the encapsulated part is usually necessary. Pot life for such a system is about 15 minutes. Final reaction or "cure" was 24 hours at 71 C. 

For the interphase tension change at the boundary with air during prepolymers formation based on individual polyesters and mixtures, the surface tension displayed no change and was 28mN/m for the prepolymer based on L-3003 polyether, 40 mN/m for that based on P-2200 polyester, and 32 mN/m for that based on a 40 1 mixture of polyether and polyester (Table 3.23). For the interphase tension at the boundary with mercury, it was shown that formation of prepolymers based on individual oligoethers and oligoesters and on mixtures is accompanied by significant reduction of interphase tension, this... 

In the prepolymer-ionomer process , the compound which contains the moieties which are the precursors of ionic groups is incorporated in the polymer chain already at the stage of urethane prepolymer formation. Then they are neutralised, which results in the formation of a prepolymer-ionomer which is emulsified in water and eventually crosslinked. In this process, the prepolymer-ionomer is usually dissolved in a small amount of water-miscible solvent of high boiling point, e.g., N-methylpyrolidone, which plays the role of coalescing agent in the process of film formation. It is then possible to obtain DPUR which contain either cationic DPUR with a pH of less than 7 (cationic moieties are attached to the polyurethane or polyurethane-urea chain) or anionic DPUR with a pH of greater than 7 (anionic moieties are attached to the polyurethane or polyurethane-urea chain). If non ionic hydrophilic moieties are attached to or incorporated in the polyurethane or polyurethane-urea chain, then a nonionic DPUR may be obtained. 

Water and carbonated beverage bottles are made predominantly from poly(ethylene terephthalate) (PET). The polymer is made by condensation reaction of ethylene glycol with either terephthalic acid or its dimethyl ester. The process in practice includes three steps prepolymer formation, melt condensation to increase viscosity, and solid-state polymerization at 180-230°C to yield a resin with an average molecular weight that is high enough for use as bottle resins. Antimony trioxide is used as a catalyst in polymerization (Duh, 2002). 

The reaction conditions, particularly the acidity and the temperature, have a profound effect on the character of the resultant products. In general, three stages can be identified formaldehyde addition to phenol, prepolymer formation below 100°C, and crosslinking at temperatures above 100°C. The reactions between phenols and aldehydes were first described by Bayer, who also obtained the first patent on the preparation of methylolphenols. The first kinetic studies in alkaline media were reported by Lederer and Manasse. The first patents on resin produc s were issued to Smith, for an ebonite and wood substitute, and to Blumer, for a shellac substitute. However, not until Baekeland s famous heat and pressure patent did phenolic resins become prominent and widely used polymers. 

In the presence of alcohols, the reactive species is a carbenium ion (4), formed from the hemiacetal (5) (Eq. 7). Efforts to synthesize model compounds continue the work started by Bender and coworkers. A series of new dimeric and trimeric compounds, derived from o-cresol, have been synthesized and their structures determined by IR, NMR, and elemental analysis. Additional homologues of p-cresol have been prepared by Kammerer and Niemann, to study the melting behavior of a wide range of methylene-bridged phenol oligomers. Prepolymer formation, namely novolac production, has been studied in batch and continuous flow reac-... 

In -this paper, we will shortly discuss our approach to prepare hydroxy-terminated MDl/BDO-based urethane oligomers and their 4-diphenylmethane carbamates, 18 and describe in more detail the synthesis of monodisperse hydroxy-terminated oligo(oxy-tetramethylenes), their use for the investigation of the prepolymer formation, and the synthesis of linear PU-elastomers with narrow (monodisperse) soft and/or hard segment length distribution by using the hydroxy-terminated segment precursors. The chemical structures of the model systems studied in this... 

Fig. 7. Reaction scheme for the prepolymer formation and conversion of the macrodiisocyanate into non-reactive methylurethane.

GPC analysis of the prepolymer formation and purification (starting molar ratio P0TM-T8/MDI=1 10, Fig. 10a), and of the chain extension reaction (extender Diol-2, Fig. 10b) the log M scale on the upper abscissa is based on polystyrene (—) Rl-detector (...) UV-detector. 

The principles of the prepolymer formation and of the analysis of the composition of the prepolymer have already been described in the Experimental Part. The prepolymer formation in the melt was investigated with monodisperse polyols of different chain lengths and for molar ratios of MDI/polyol varying between 2 and 35. In order to facilitate the chromatogrhic analysis and to avoid side reactions of the macrodiisocyanate samples during the analytic procedure the isocyanate groups were converted into inactive methylurethanes (Figure 7) prior to the HPCL analysis. 

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