Isocyanate: hydroxyl ratio

Series II Unfilled PUs using various compatible polyol blends (PB) in which the code numbers in Table I represent in order the weight ratios of polyols T32/75, LHT240 and EG. A mixture of triethylene diamine and dibutyltin dilaurate was used as catalyst throughout. The various polyol-based reactants and derived RIM PUs are summarised in Table I, including isocyanate/hydroxyl ratios (multiplied by 100), expressed as the System Index. Thus, 1001 represents stoichiometric equivalence, 1041 a 4% excess by weighted equivalents of isocyanate and 971 a 3% excess of hydroxyl. 

H.D. Rozman, K.R.A. Hilme, and A. Abubakar, Polyurethane composites based on oil palm empty fruit bunches Effect of isocyanate/hydroxyl ratio and chemical modification of empty fruit bunches with toluene diisocyanate and hexamethylene diisocyanate on mechanical properties. /. Appl. Polym. Sci. 106, 2290-2297 (2007). 

An ingeniously simple screening method was used by Britain and Gemeinhardt [146] to evaluate catalysts for the isocyanate/hydroxyl reaction. To approximate as closely as possible actual polymerization conditions, the 80 20 ratio of 2,4- and 2,6-tolylene diisocyanate (80 20 TDI) isomers and a polyether triol of 3000 molecular weight were mixed at NCO OH ratio of 1.0. A 10% solution of catalyst in dry dioxane was added, the final catalyst concentration being 1% of the weight of polyether. The time for the mixture to gel at 70°C was noted as an indication of catalytic strength. This technique used the same reactants employed in one-shot flexible polyether-based foam systems, almost completely eliminated solvent, and was used to screen quickly hundreds of possible catalysts. 

Polyurethanes are formed when a diisocyanate (or polyisocyanate) is reacted with hydroxyl groups at a molar ratio of 2 or higher (isocyanate hydroxyl). When the polyol and polyisocyanate are combined in the presence of a suitable catalyst, the exothermic polymerization reaction begins spontaneously. This type of synthesis is an addition polymerization. Most polyols and polyisocyanates used for manufacturing PUs are liquid at standard room temperature. Industrially, the PU synthesis reaction is rapid, and the product is a solid polymer. The reaction rate can be varied significantly by changing the catalyst type and concentration, facilitating the use of PUs in a variety of applications. ... 

They are linear block copolymers of the (AB) type in which A is the "soft" segment derived from the polyol and B is the "hard" segment derived from the diisocyanate and chain extender. Theoretically, infinite molecular weight would be achieved at an isocyanate to total hydroxyl ratio of 1.0. Thus the number of moles of diisocyanate is equal to the sum of the number of moles of polyol and chain extender as shown. Hardness of the materials can be varied by altering the molar ratio of chain extender to polyol which in turn affects the weight ratio of hard segment to soft segment in the polymer. 

The importance of monol content in 4,T-MDI/BDO cured elastomer systems was determined by comparing 4000-MW PPG diols prepared via ultra-low monol technology, DMC and potassium hydroxide. They are designated as ultra-low monol, low monol and conventional and have monol contents of 0.005,0.016 and 0.085 meq/g, respectively. This corresponds to functionalities of 1.98, 1.94 and 1.71. We prepared 6% NCO 4,4"-MDI prepolymers and chain extended with BDO at an isocyanate to hydroxyl ratio (NCO OH) of 1.03 [15]. Table 9.3 summarises the monol effect on elastomer processing characteristics (pot life and demould time), and physical properties. 

The BDO was added (isocyanate to hydroxyl ratio (NCO OH) of 1.03) and mixed thoroughly using a Jiffy mixer until the blend was homogeneous. This solution was immediately poured into preheated moulds at 100 °C treated with mould release. After demoulding, the samples were post-cured at 100 °C for 16 hours. The polymer samples were conditioned at 23 °C and 50% relative humidity for at least four weeks prior to testing. 

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Both pigmented and unpigmented polyurethane paints have been prepared using a polyester resin containing hydroxyl functional groups and the biuret trlmer of hexamethylenedllsocyanate as a crosslinker. The molar ratio of hydroxyl/isocyanate has been chosen 1.0 and the pig-ment/binder ratio 0.6. 

All polyurethanes have been obtained by reacting 1 1 molar ratios of isosorbide and diisocyanate. A slight excess of about 5-10% of the diisocyanate or of the diol resulted in the generation of soluble polyurethanes with lower molecular weight and possessing either functional isocyanate or hydroxyl groups, respectively. 

Polyurethanes (PU s). SL and 4,4 -diphenylmethane diisocyanate (MDI) were dissolved in tetrahydrofuran (THF), and the solution was stirred for 1 hr at 60°C. A THF solution of polyethylene glycol (PEG 400) and diethyl bis(2-hydroxyethyl)aminomethylphosphonate (polyol containing phosphorous) was added to the reaction mixture, and the reaction time was extended for 1 hr. In all reactions, the molar ratio of the total amount of isocyanate groups to the total amount of hydroxyl groups (NCO/OH) was maintained at 1.2. The lignin content in PU was 20 wt%. Each solution was drawn on a glass plate, and allowed to dry for 48 hr. The residual solvent in a sample was removed under vacuum and curing of each PU film was carried out at 120°C for 3 hr under a pressure of 50 kg/cm2. 

Kuhn and Balmer (22) have shown that, as the polymer concentration is decreased, the change in viscosity becomes constant. At these concentrations it was assumed that no intermolecular reaction was taking place. A similar effect is demonstrated for the present system in Figures 7 and 8 for a given initial ratio of isocyanate to hydroxyl and a given reaction time, the polymer concentration had no effect on the viscosity change below a certain point. 

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