Catalysts prepolymer stability

The polyaddition reaction is influenced by the stmcture and functionaHty of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxyl group (primary or secondary). Impurities also influence the reactivity of the system for example, acid impurities in PMDI require partial neutralization or larger amounts of the basic catalysts. The acidity in PMDI can be reduced by heat or epoxy treatment, which is best conducted in the plant. Addition of small amounts of carboxyHc acid chlorides lowers the reactivity of PMDI or stabilizes isocyanate terrninated prepolymers. 

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... 

The activity of Ti catalysts in SSP depends on the kind of stabilizer fed into the reactor. In the production of PET, phosphorous-containing chemicals are commonly added as stabilizers. These products improve the thermal stability, particularly in processing, which results in reduced degradation and discoloration and are therefore of importance with respect to quality. Such materials are added during the production of the prepolymer. These stabilizers are mainly based on phosphoric or phosphonic (phosphorous) acids or their esters. 

For the best effect, the stabilizers must be added to the reactants before the point of acid/base equilibrium is reached. Catalysts are normally not added to the prepolymer, as they may be rather basic and may cause the reactions to proceed too swiftly. 

Thermal Stabilization of Polyformals. Polyformals, built up in solution, were neutralized with a little ammonium hydroxide when they were to be isolated by concentration under reduced pressure on the steam bath. This prevented breakdown of the polymer because of the acidic catalyst. The degradation was particularly bad with polyformals obtained from primary diols (cyclohexanedimeth-anol and decanediol). In one experiment, the polyformal of decanediol was obtained with an inherent viscosity of 0.9 when the catalyst had been neutralized and 0.3 when it had not been neutralized. Breakdown under these conditions was not observed with the prepolymer of tetramethylcyclobutanediol. 

A foamable compound is produced by compounding the obtained prepolymer mixture with a physical blowing agent, a foam stabilizer, and further additives (e.g., adhesion promoters, catalysts, etc.). 

Catalysts are essential components in the liquid injection moulding (LIM) or reaction injection moulding (RIM) processes. Prepolymer systems for solid polyurethane elastomers usually do not require catalysts, and if long-term hydrolytic stability is required in the final polymer their use is undesirable. 

The major problem arising from the use of HPj s in acid catalysis is their deactivation over time and the requited regeneration. It is obvious that HPAs cannot be regenerated by combustion, since their stability does not go beyond —300"C. However, there are two ways to tackle the problem. One is to decrease the rate of decay by using, when possible, bifunctional catalysts in the presence of llj and/or to work under supercritical conditions for which the prepolymer-polymer type of products responsible for deactivation could be washed out. The other possibility deals with the regeneration of the catalyst. For example, since at low reaction temperatures one should not obtain coke products and polymer species should predominate, it should also be possible to remove the poisonous products by washing with adequate solvent under the optimum conditions, or by mild hydrogenation. 

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. 

Polyurethane/lmide Modified Foam n A polyaryl polyisocyanate (PAPI) is reacted with a 3,3 ,4, -benzophe-none tetracarboxylic dianhydride (BTDA) to form an isocyanate prepolymer. This prepolymer can be compounded with a polyol, a blowing agent, a catalyst, and a ceU stabilizer to form the modified foam. Such a foam containing 5% BTDA in the prepolymer has better thermal properties than conventional polyurethane foams. 

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