To-isocyanate ratio

Figure 7. Elastically effective cross-link density versus bake temperature for an acrylic-urethane coating as a function of polyol to isocyanate ratio.

Accurate metering is less Important with nylon systems as well. Urethane stream ratios must typically be held to within 0.5Z of target in order to achieve the desired finished part physical properties, these being very sensitive to isocyanate ratio. All of the physical properties of nylon RIM materials are essentially determined by the blend of materials made in the stream which contains the initiator and rubber modifier components. The second stream contains only catalyst solution. Thus, being off-ratio will affect only reaction rate, not physical properties. Nylon systems can be formulated so that ratio changes as high as 5% do not show any effect on either kinetics or properties. 

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. 

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 ... 

Urethane gels and ultrasoft elastomers are a more recent development.18 They are made primarily by reacting high-molecular-weight polyether polyols with a stoichiometric deficiency of isocyanate. The low NCO-to-OH ratio allows for a wide latitude in hardness adjustment. These low-hardness elastomers are used for seating applications (such as gel bicycle seats), shoe inserts, and soft padding for orthopedic devices. 

There is another method, called the quasi-prepolymer technique, which is similar to the full-prepolymer process but utilizes prereacted isocyanates in the 10-20% NCO range. This eases processing compared to full-prepolymer systems, requiring lower temperatures and volume ratios typically from 4 1 to nearly 1 1 (polyol to isocyanate). 

The cure kinetics will depend on the initial isocyanate to hydroxy ratio and on the humidity. Assuming that the concentration of water in the coating is constant during cure, it is possible to define the following parameter which determines the effect of humidity on cure ... 

A more complete discussion of the kinetics of isocyanate crosslinking in the presence of humidity along with experimental verification of the rate equations used above has been given by van der Ven, et al. (18). Equations 38 and 39 can be integrated numerically for different values of isocyanate to hydroxy ratio and H. Plots of the consumption of hydroxy and isocyanate functionality are shown in Figure 2 for H-0 and H-0.66 assuming equal Initial Isocyanate and hydroxy levels. High humidity increases the rate of consumption of isocyanate and reduces the consumption of hydroxy. 

In general, coating systems are designed to achieve optimum properties at crosslinking levels short of 100% conversion in order to minimize cure time. The effect of Isocyanate to hydroxy ratio and humidity on crosslink density after a fixed cure time is shown in Figure 4. The fixed cure time has been arbitrarily defined as the time at which a coating with equal Isocyanate and hydroxy functionality reaches 85% conversion in the absence of humidity (H-0). If the initial ratio of isocyanate to hydroxy is less than or equal to 1, the crosslink density drops with increasing humidity. 

Bond formation leading to crosslinking is based on the -NCO + HO— -NHCOO- reaction which belongs to the category of the A + B - AB type. Therefore, a maximum of crosslinking density and minimum of sol fraction is predicted for the stoichiometric system for which the ratio of concentrations of hydroxy to isocyanate groups, rH, is equal to unity ... 

In the Montreal case-control study carried out by Siemiatycki (1991) (see monograph on dichloromethane in this volume), the investigators estimated the associations between 293 workplace substances and several types of cancer. Isocyanates were one of the substances, and it was stated that the most common form in this study was toluene diisocyanates. The main occupations to which isocyanate exposure was attributed in this study were motor vehicle refinishers, motor vehicle mechanics and foundry workers. Only 0.8% of the study subjects had ever been exposed to isocyanates. For most types of cancer examined (oesophagus, stomach, colon, rectum, pancreas, prostate, bladder, kidney, skin melanoma, lymphoma), there was no indication of an excess risk due to isocyanates. For lung cancer, in the population subgroup of French Canadians (the majority ethnic group in this region), based on 10 cases exposed at any level, the odds ratio was 2.2 (90% CI, 0.9-5.3). [The interpretation of the null results has to take into account the small numbers and presumably low exposure levels. Workers had multiple exposures.]... 

The most efficient intermediate layers are polyurethane adhesives or glues.175 In this case, the correct choice of molecular weight of the polyester and its ratio to diisocyanate is very important. The dependence of the adhesive strength on the molecular weight and the isocyanate-to-diol ratio is shown in Fig. 4.13. The best adhesion to metal is achieved when an intermediate layer based on low-molecular-weight polyester and at a ratio of NCO OH = 2.5 is used. 

Dimerization of phenyl isocyanate, catalyzed by lanthanide complexes, has been reported by Deng et al. <2003CHJ574>. A number of lanthanide complexes were tried and Sm(SPh)3(hmpa)3 was found to be the most effective catalyst. Conversion was as high as 96% with 2500 1 of substrate to catalyst ratio (Scheme 47). 

Aniline was converted into its novolak analogue by reacting with formaldehyde and hydrochloric acid in the presence of divalent metal cations such as Ca and Fe. The ratio of aniline/formaldehyde/hydrochloric acid was 7.5 1.0 0.3, respectively, using 0.00025 wt% metal ions. These oligomeric products are designed to be further modified to isocyanates by reacting with phosgene. 

Urethane stoichiometry also plays a key role in influencing the polyurethane pad properties. Stoichiometry refers to the ratio of reactive groups (usually diol or diamine moieties) to isocyanate groups. The presence of excess isocyanates... 

Furthermore, the alcohol itself was found to function as a weak catalyst. Thus the value of the second order rate coefficient was found to increase as the alcohol/isocyanate ratio was increased. 

Both of these mechanisms are compatible with the experimental results. At low alcohol/isocyanate ratios both give a catalytic effect which is proportional to the acid concentration. Also, both predict that at high alcohol excess the catalytic effect disappears. According to mechanism I this would be due to the formation of the complex becoming the rate determining step at high alcohol excess. On the other hand, in the case of mechanism II,... 

Problem 5.38 Calculate the sol fraction and the degree of cross-linking in the urethane, networks formed by stepwise polymerization of 2-hydroxymethyl-2-ethyl-l,3-propanediol and 1,6-hexamethylene diisocyanate to 90% conversion of the hydroxyl groups. Compare these properties for two urethane systems with (a) r = 1 and (b) r = 0.75, where r is the mole ratio of hydroxyl to isocyanate groups. 

In many respects the chemistry of flexible urethane foam manufacture is similar to that of the VulkoUan-type rubbers except that gas evolution reactions are allowed to occur concurrently with chain extension and cross linking (see Figure 4.30). Most flexible foams are made from 80/20 TDI, which refers to the ratio of the isomeric 2,4-tolylendiisocyanate to 2,6-tolylendiisocyanate. Isocyanates for HR foams are about 80% 80/20 TDI and 20% PMDI, and those for semiflexible foams are usually 100% PMDI. 

The water-swelling polyurethane is a prepolymer obtained from polyoxyalk-ene (polyether) polyol reacted with isocyanate (ratio 1.1 to 1.7 of isocyanate groups to reactive hydrogens in polyol). 

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. 

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