Primary hydroxyl content

As an immediate consequence, the oligo-polyols containing terminal primary hydroxyl groups are more reactive in the reaction with isocyanates than the oligo-polyols having only secondary hydroxyl groups. The primary hydroxyl content of an oligo-polyol is an important characteristic, because it is possible to determine the potential reactivity with isocyanates. 

The value of time, tl5 determined graphically, corresponds to bl mols of hydroxyl groups reacted, which represents the total quantity of primary hydroxyl. The primary hydroxyl content is expressed as the ratio between the molar quantity of primary hydroxyl and the total quantity of hydroxyl groups as percentage  

Generally, in any oligo polyol there are the following relationships  

A specific reagent for the primary hydroxyl group determination is triphenylchloromethane [22], which has a very reactive chlorine atom and a bulky substituent (triphenylmethyl). Due to the high steric hindrance of triphenylchloromethane, a selective reaction with primary hydroxyl groups takes place. Unfortunately, the precision is not very high because the secondary hydroxyl groups react only to a very small extent (8-10%). In order to make it a more precise method, it is necessary that before the determination, a calibration curve should be done and the real primary hydroxyl content is corrected by the decrease in the quantity of secondary hydroxyl reacted. 

The method is extremely simple and needs only the neutralisation of the resulting hydrochloric acid with a strong base solution of a known concentration. Unfortunately, as mentioned previously, this method is not very accurate. The most accurate and usual methods for primary hydroxyl determination are two NMR spectroscopic methods 19fluorine NMR and 13carbon NMR. Both methods are described in detail in ASTM D4273 [34]. 

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This method relies on the fact that the isotopes of certain atoms have electrons that will flip under certain conditions. This change in state can be detected, and how they are connected can be shown. The H and 13C isotopes are able to produce resonance spectra for polyurethane raw materials and cured samples. ASTM method D4273 details a method to determine the primary hydroxyl content of polyether polyols. 

ASTM D 4273-83 (1988) Standard Methods for Testing Polyurethane Raw Materials Determination of Primary Hydroxyl Content of Poly ether Polyols, 9 pp (Comm D-20)... 

There is strong evidence that thermal stability is dependent primarily on oxyethylene group content rather than primary hydroxyl content. Table 3 shows that polymer prepared from a 2000 molecular weight polyol with 45% by weight oxyethylene group content but only 46% primary hydroxyl content is more stable than one prepared from a polyol with 30% by weight oxyethylene groups but with 82% primary hydroxyl. 

Figure 3.11 Graphical representation of second order kinetics of the oligo-polyols reaction with different primary hydroxyl contents with phthalic anhydride, a initial concentration of phthalic anhydride b initial concentration of hydroxyl groups t time for total consumption of primary hydroxyl x concentration of a or b reacted at time t. Temperature 30 °C Solvent pyridine...

All the oligo-polyols are used to build the polyurethane high MW structure in a reactive process, as a consequence of the oligo-polyols terminal hydroxyl group reaction with polyisocyanates. The reactivity of oligo-polyols in polyurethane fabrication is a very important practical characteristic. Reactivity is a measure of the reaction rate of an oligo-polyol with an isocyanate in order to make the final polyurethane polymer. One practical method is the measurement of viscosity, in time, by Brookfield Viscosity Test (BVT), especially used to determine the reactivity of ethylene oxide capped polyether polyols. Figure 3.12 shows the effect of the primary hydroxyl content upon the reactivity of ethylene-oxide capped polyether triols of MW of 5,000 daltons. 

It is observed that the oligo-polyols with low reactivity (0% primary hydroxyl, i.e., having only secondary hydroxyls) have the lowest viscosity increase over time. By contrast, the very high reactivity polyols, having 85-100% primary hydroxyl content, have the highest viscosity increase over time. As a consequence, this method of evaluation of viscosity increase in time is a very simple and useful practical method to ascertain the reactivity of oligo-polyols. 

Figure 3.12 Effect of primary hydroxyl content on oligo-polyols reactivity. THF tetrahydrofuran, EO ethylene oxide...

Figure 3.13 Kinetic curves of the phenyl isocyanate reaction with oligo-polyols having various primary hydroxyl contents. [Phenyl isocyanate] = [OH] = 0.5 mol/l. Solvent toluene Catalyst triethylamine Temperature 30 °C...

The marked increase of the reactivity with the primary hydroxyl content increase is obvious. 

To conclude, the common physico-chemical characteristics of oligo-polyols for polyurethanes determined by standard analytical methods are hydroxyl number, hydroxyl percentage, primary hydroxyl content, molecular weight, equivalent weight, molecular weight distribution, viscosity, specific gravity, acidity and colour (See Chapters 3.1-3.11). 

The main characteristics of random PO-EO copolyethers with high EO content are presented in Table 4.8. One observes that the PO-EO copolyether has a medium primary hydroxyl content, of around 50%, and that the EO units are not only internal groups but some of them are terminal groups too. 

Figure 4.23 shows the variation of primary hydroxyl content of a polyether triol with a MW of 5000 daltons against the EO content [52]. 

Based on kinetic considerations, an equation (4.16) was proposed which represents the variation of the primary hydroxyl content as function of the quantity of EO reacted [52]. This equation is very similar to the equation of Weybull and Nicander [80, 81] used to measure the distribution of EO sequences per hydroxyl group in ethoxylated nonionic surfactants (for example, ethoxylated fatty alcohols). 

Figure 4.23 Variation of primary hydroxyl content as a function of EO content in polyether triols block copolymers [PO-EO] with terminal poly[EO] block MW = 5000 daltons catalyst KOH - 0.0056 mol%...

If the reaction conditions are maintained at constant, the distribution constant K is an important characteristic of the ethoxylation process. If for a given ethoxylation reaction the distribution constant K is determined at any moment, it is possible to appreciate the quantity of EO necessary to obtain the desired primary hydroxyl content for the synthesised EO capped polyether polyol. 

An important aspect of the ethoxylation reaction is that the primary hydroxyl content depends strongly on the hydroxyl number of the intermediate propoxylated polyether polyol. If a polyol is ethoxylated, an intermediate propoxylated polyether with an high hydroxyl number is obtained if the ethoxylation is done with a lower primary hydroxyl content with the same quantity of EO, an intermediate propoxylated polyether with a lower hydroxyl number is obtained (see Table 4.9). 

Table 4.9 The effect of the hydroxyl number of the intermediate propoxylated polyether polyol on the primary hydroxyl content (EO concentration was around 10% against final polyol) [50] ...

No. OH of intermediary propoxylated polyether, mg KOH/g OH of ethoxylated polyether, mg KOH/g Primary hydroxyl content of ethoxylated polyether, %... 

In practice, it is very important to obtain a high primary hydroxyl content with minimum EO quantity. A high EO content leads to turbid polyether polyols because longer poly[EO] chains are insoluble in liquid polypropylene oxide. The flexible PU foams made with highly ethoxylated polyols have poor humidity/ageing/degradation characteristics and a lower compression strength. 

The Effect of the Catalyst Concentration on the Primary Hydroxyl Content... 

Figure 4.25 The effect of catalyst concentration on the primary hydroxyl content MW = 4700-5000 daltons Ethoxylation temperature 130 °C...

If all or the majority of hydroxyl groups are transformed in alcoholate groups, the alcohol-alcoholate equilibrium does not takes place any more and the difference in acidity is not important. In conclusion, a convenient method to increase the percentage of primary hydroxyl groups is to increase the catalyst concentration. In practice, especially for high MW capped polyether polyols, in order to obtain a higher primary hydroxyl content, the concentration of catalyst used is 0.3-0.35%, higher than the usual catalyst concentration of 0.2-0.25% (as KOH) used for PO homopolymers or random PO-EO copolyethers [107]. 

It was observed experimentally that if the EO addition rate is high (in fact a high ethoxylation pressure), cloudy polyols and low primary hydroxyl content are formed [51]. The explanation of this behaviour is that if the rate of EO addition is too high, the equilibrium of alcohol - alcoholate does not have enough time to get established and the polyethylene oxide chains grow on a limited number of hydroxyl groups. As an immediate... 

A low EO addition rate (in fact a low ethoxylation pressure), leads to perfectly clear capped polyols, because the alcohol - alcoholate equilibrium has enough time to get established and, as an immediate consequence, the same quantity of EO is distributed on a high number of hydroxyl groups. Shorter poly[EO] chains derived from a high number of hydroxyl groups are formed and the primary hydroxyl content is higher. To conclude, in order to obtain a high primary hydroxyl content, the rate of EO addition has to be low [51]. For example, for a polyol of MW of 4700-5000 daltons and 15% EO as terminal block, an addition of EO in around three-five hours at 130 °C is a convenient way to obtain a primary hydroxyl content of 70-75%. 

One observes that with temperature increase, the ratio between the reaction rate of EO with the primary hydroxyl, as per the reaction rate of EO with secondary hydroxyl, decreases. If at 70 °C, EO is three times more reactive in the reaction with the primary hydroxyl than with the secondary hydroxyl at 120 °C, EO is only 1.6 times more reactive in the reaction with the primary hydroxyl than in the reaction with the secondary hydroxyl. As an immediate consequence, by ethoxylation of propoxylated polyethers at higher temperatures (125-130 °C), a more uniform distribution of EO units per hydroxyl group takes place and the resulting primary hydroxyl content is higher than that resulting from ethoxylation at lower temperatures (90-105 °C). Of course, another beneficial effect of a higher ethoxylation temperature is that the equilibrium of alcohol - alcoholate is established more rapidly. 

An extremely high primary hydroxyl content is obtained by reacting a purified propoxylated polyether with a cyclic anhydride (for example succinic anhydride), followed by the reaction with EO (addition of EO to the carboxyl groups formed). A primary hydroxyl content of 60% is obtained, with only 5% EO as terminal block [85] ... 

Figure 4.27 The effect of the catalyst nature on the primary hydroxyl content...

The potassium cation is retained, by complexation, at the same chain end, the alcohol -alcoholate equilibrium is perturbed, and EO reacts preferentially with this template structure and the resulting primary hydroxyl content decreases. In the case of bivalent cations with two positive charges, the alcoholate anion is linked more strongly by electrostatic forces and the coordination of the cation with the formed poly[EO] chains takes place to a much smaller extent. 

In conclusion, the ethoxylation catalyst nature has an important influence on the primary hydroxyl content. A higher primary hydroxyl percentage than in the classical reaction catalysed by KOH is obtained by the ethoxylation of the intermediate polyether polyols in acidic catalysis or with alkaline-earth alkoxides or carboxylates [25-29]. 

The last step in the synthesis of the intermediary propoxylated polyether, before ethoxylation, is the degassing step, the elimination of the unreacted PO by vacuum distillation. It was observed experimentally that if the PO is not efficiently removed in the degassing step, the resulting primary hydroxyl content is lower. The explanation is very simple EO is much more reactive than PO and reacts first. After the addition of the majority of EO, the remaining PO (the less reactive monomer), reacts at the end of chain, transforming part of the primary hydroxyls into secondary hydroxyls. 

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