NCO/OH ratio

The first urethane reaction in Fig. 1 is used in two major ways in adhesives. In one case, a two-component adhesive usually employs a polyol and polyisocyanate with catalyst. This can react at room temperature to form the polyurethane. The second use of this reaction is to make an isocyanate-terminated prepolymer. Reacting a stoichiometric excess of isocyanate with polyol can produce an isocyanate-terminated prepolymer. A prepolymer is often made with an NCO/OH ratio of 2.0, as shown below, but the isocyanate ratio can range from 1.4 to over 8.0, depending upon the application ... 

R" can represent a wide variety of backbones, such as polyethers, polyesters, etc. These backbones will be discussed shortly. In actual practice, most prepolymers are made with NCO/OH ratios as low as 1.4, but the ratios can be as high as 3.0 or greater. 

Most moisture-curing hot-melt adhesives utilize a crystallizable backbone and are based almost exclusively on monomeric MDI at NCO/OH ratios of 1.5 to 2.2. Poly(hexamethylene adipate) polyol is the workhorse of the curing hot-melt adhesives. 

Preliminary Considerations. Table II gives the mole/kg of the components used in preparing the LHT-240 and Tri-NCO elastomers. The values are based on the assumption that the concentrations of hydroxyl and double-bond groups in the PPG are in fact those obtained from the analyses already discussed and that the NCO/OH ratio is 1.00 instead of 1.02, listed in Table I. In reality, this ratio may have been essentially unity because, as mentioned already, the degree of swelling in benzene and the sol fraction were found to be minima when the nominal ratio was 1.02. The results obtained from the calculations discussed subsequently, and summarized in Table HI, do not differ significantly from those obtained if the NCO/OH ratio is equated to 1.02. 

The polyurethane formulation Involved a proprietary crossllnkable system based on poly(propylene glycol) and methylene dllsocyanate (NCO/OH ratio = 1.0). For studies of viscoelastic, energy absorption, and fatigue behavior, the weight fractions of PUMA were 0, 0.25, 0.50, 0.75, and 1.0 for studies of tensile and tear strength, the ratios were 0, 0.10, 0.20, 0.25, 0.30, and 0.40. Reactants were mixed at room temperature, degassed, poured Into a mold, and cured at 60 C for 48 hr. 

The composition of starting materials for the PU s studied in the present investigation are listed in Table I. Five series of PU films were synthesized in which PEG of different molecular weights were applied. The iso-cyanate/hydroxyl group (NCO/OH) ratio was kept constant (1.2) in all samples since variations in the NCO/OH ratio may also lead to variations in crosslink density (8). 

Previous studies of KL-polyether-derived PU s have shown that high contents of KL (> 30 — 35%) result in hard and sometimes brittle PU s regardless of the NCO/OH ratio used (6) and regardless of the molecular weight of KL (Yoshida, H. Morck, R. Kringstad, K. P. Hatakeyama, H.,... 

Materials. The materials are described in detail in the studies summarized in Table I. Most of these results are based on kraft lignin. The NCO/OH ratio of all of these networks were high greater than 1.5. All of the networks were prepared from homogeneous solutions of the lignin-based polyol, added (soft segment) polyol, and diisocyanate. Films were cast and cured under mild conditions with a controlled loss of solvent. The films were post-cured to insure complete reaction (25). 

In several studies the NCO/OH ratio was also varied. To facilitate the comparisons, only networks with NCO/OH ratios between 1.5 and 3.0... 

Values for NCO/OH ratio of 1.5 and 2.1 were averaged to give a single point. 

This structure has superior water-resistant properties in comparison to conventional polyols used for PU synthesis. Room temperature cures are easily obtained with typical urethane catalysts. Short chain diols, fillers and plasticizers may also be used in their formulations in order to vary physical properties. Formulations usually with NCO/OH ratio of 1.05 are used for this purpose. Such urethanes are reported to be flexible down to about -70 °C. HTPB is regarded as a work horse binder for composite propellants and PBXs. HTPB also successfully competes with widely used room temperature vulcanizing (RTV) silicones and special epoxy resins for the encapsulation of electronic components. HTPB-based PUs are superior in this respect as epoxy resins change their mechanical properties widely with temperature. 

Early work published in Saunders and Frisch, Volume II (Saunders 1962) shows how various properties change different diisocyanate-to-polyol ratios. As the NCO/OH ratio increases from 2 to 2.75, the main physical properties of tensile strength, modulus, tear strength, and hardness increase. Other positive improvements are compression set and resilience. The pot life and... 

Boltom H40 dendritic molecules were covalently linked in this work to make a network with aliphatic 1,6-hexamethylene diisocyanate (HDI). The molar NCO/OH ratio was varied for the reactants from 10 to 50% to prepare networks with different degrees of connectivity of dendritic units. The network samples are designated as H40/Z where Z stands for NCO/OH ratio expressed as a percentage. The network formation reaction of H40 with HDI was carried out in N,N-dimethylformamide (DMF) of 99.8% purity at 90 C. No catalyst was added. Isocyanates react readily with moisture to form urea linkages, therefore special precautionary measures were implemented to prevent moisture uptake either by HDI or DMF. Observation of the reaction vessel was maintained over the course of the reaction to monitor the viscosity of the solution. As viscosity of the solution increased to the desired level, suggesting that the gelation point was near, the solution was cast onto a glass plate which was immediately placed... 

This development was reported in 1995 by T.-M. Feng and B.A. Waldman (1995) of Osi Specialities Inc (now part of Witco). Silane-endcapped urethane polymers generally consist of urethane backbones, prepared at a low NCO.OH ratio, with reactive organofunctional silane groups at the end. 

Feng and Waldman used 4,4 -diphenylmethane diisocyanate. The polyol is either 2000 or 4000 molecular weight polypropylene glycol (PPG) and the catalyst is dibutylin dilaurate (DBTOL). The NOC-terminated urethane prepolymers are made in the usual way by reacting an excess of methane diisocyanate with PPG in the presence of DBTDL at an NCO OH ratio of 1.4 2.0. The reactions are run at 70°C for about 3 h to reach a constant percentage of NCO. 

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. 

Figure 26 shows a comparison of bum-through time between the two foams based on polymeric isocyanate (polymeric MDI) and a TDI-prepolymer (65). In the case of polymeric isocyanate-based foams, a remarkable increase in bum-through time resulted after increasing the NCO/OH ratio, which reached a maximum value at 3.0. 

Figure 31. Optimum range of NCO/OH ratio, Aliphaticity Index and molecular weight (65).

Acceptable percent firiability in industrial applications is less than 30% weight loss, i.e., the OH/NCO equivalent ratio is more than 0.2 (NCO/OH ratio is less than 5.0). Figure 33 shows the relationship between OH/NCO ratio and oxygen index in terms of ASTM D 2863. Oxygen index relates to flame retardance (16). 

NCO/OH ratio 1.05. DBTDL 0.004203 % Sn on binder solids. Solvent propylene glycol monoethyl ether acetate/xylene... 

NCO/OH ratio and chain extender structure on low temperature and elevated temperature properties, thermal stability during processing, and processability, in particular injection molding behavior. 

Polymer made at an NCO/OH ratio of 1.03 was found to have an optimum rate of hardness build-up. Its hardness build-up curve, shown in Figure 6, was essentially superimposable with that of a commercially acceptable PTMG-based polymer and in field trials it molded at the same cycle time without difficulty. Thus molding behavior in large automotive molds could be related to a simple test which can be carried out on a relatively small sample. 

Control of NCO/OH ratio was demonstrated to be critical in these materials to obtain polymer which can be molded acceptably and reproducibly. 

Figure 6. Effect of NCO/OH ratio on hardness build-up for polymer made from tipped PPG polyol (45% EO) at NCO/OH ratios of 1.01 (0), 1.03 (%), 1.05...

Figure 8. DSC cooling curves of TPU elastomers (based on 2000 M.W. tipped PPG polyol (45% EO) varying in NCO/OH ratio).

Figure 9. Hardness build-up curves of polymers containing 50 wt % polyol made at 1.04 NCO/OH ratio and based on 1000 M.W. (AJ. 2000 M.W. f J, and...

Preparation of Cast Elastomers. The cast elastomers were prepared in a two-step procedure. First prepolymers were made from one polyether polyol (poly(oxy-tetramethylene) glycol of 1000 M.W., (POTMG)) and two polyester polyols (adipate polyester of 2000 M.W. (PAG) and polycaprolactone of 1250 M.W. (PCL)) by reaction with the corresponding diisocyanates (MDI, PPDI, CHDI or NDI) at an NCO/OH ratio of 2/1. The temperature was maintained at 80°C and periodic samples were withdrawn to determined the isocyanate content. When the isocyanate content of the mixture reached within 0.3% of the calculated value, the reaction was stopped by cooling. The prepolymer could be kept for a period of six months in the absence of moisture. The isocyanate-terminated prepolymers were then chain-extended with... 

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