CONTENTS Hydroxyl Number

If it is difficult to obtain consistent results with certain polyesters, an alternative method is where the acetic anhydride is mixed with perchloric acid and 1,2-dichloroethane. The polyester sample is added to this solution and the mixture warmed gently. A second mixture of A, A -dimethyl-formamide, pyridine and water is then added and the excess acid titrated with standard potassium hydroxide using thymol blue as indicator. 

In many cases, where the colour of the material prevents sharp visual colour changes at the pH end-point, potentiometric titrations must be used. Sometimes a different pH end-point results using phenolphthalein indicator, as opposed to a potentiometric end-point, preventing absolute comparisons between techniques. However, the visual end-point using a suitable indicator such as phenolphthalein usually provides good accuracy and reproducible results on samples analysed by the same technique, provided the colour of the sample does not interfere with end-point detection. 

A typical analytical procedure for determining the hydroxyl number of a polyol is now described. The hydroxyl group is esterified with a station of phthalic anhydride in pyridine. The excess reagent is back-titrated with standard sodium hydroxide solution and a blank is run on the reagents to determine the amount of anhydride consumed. 

Excessive amounts of water in the sample interfere by destruction of the esterification reagent and contribution to a high unreal hydroxyl number value. If the water content of the sample exceeds 0-2%, it is recommended that the sample be dried. Primary and secondary amines and higher fatty acids also interfere since they react with the reagent to form stable compounds and would be included in the analysis. 

Neither the phthalic anhydride or pyridine and/or the phthalic anhydride-pyridine reagent should be allowed to stand open to the air or used when there is evidence of discoloration. Pyridine and phthalating reagent should always be handled in the hood. If these reagents come in contact with the skin, wash immediately with soap and water. 

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There are no generally accepted specifications manufacturers set their own specifications for hydroxyl number range, melt viscosity, water content, etc. Typical properties of Du Pont Terathane PTMEG are Hsted in Table 2, and selected thermal properties are given in Table 3 (260). For all grades, water content is <0.015 wt % ash, <0.001 wt % iron, <1 ppm peroxide, <5 ppm as H2O2 and flash point TOC is >163°C. 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

Thus, a polyester sample (1-3 g, exactly weighed) is dissolved in 25 mL of a titrated solution of acetic anhydride in dry pyridine (10% mass). The solution is heated to reflux for 1 h. After cooling, 50 mL pyridine and 10 mL water are added. The excess acetic acid present in the resulting solution is titrated by aqueous potassium hydroxide (0.5 mol/L) using a potentiometric titrator. The determination must be carried out in duplicate and a blank titration must be performed under the same conditions. The mass of polyester and the concentration of reactants should be adjusted to ensure that at least a fourfold excess of acetic anhydride is used. The final result (OH content) is expressed in mmol OH/g polyester or as the hydroxyl number, defined as the number of milligrams of KOH required to neutralize the acetic acid consumed per gram of polyester. [Hydroxyl number = (number of mmol OH/g polyester) x 56.106.]... 

Specification sheets will often give the equivalent weight of the polyol in terms of the hydroxyl number ("OH" value). The hydroxyl number is defined as "the number of milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl content of 1.0 grams of polyol." Manipulation of this definition gives the follow equation ... 

The chemical loading increases with increasing relative number of hydroxyls. The curve levels off at a maximum of 1.16 mmol/g, for I > 1. This shows that the chemical bonding of the silane molecules is controlled by the hydroxyl content, as long as the aminosilane is in excess of the number of hydroxyls. As the hydroxyl number is further increased, with constant concentration of the reagent solution, the chemical loading remains constant. Reaction will result in a higher coordination of the silane molecules. 

Isocyanate index (index number) = I (X)(actual amount of isocyanate used)/ (equivalent amount of isocyanate required). An excess of isocyanate groups is used in some applications like flexible foam. The analytical values required for isocyanate formulas are the isocyanate value, hydroxyl number, residual acid value (acid number), and water content. The last two parameters reflect the following reactions ... 

Hydroxyl number (OHN). This number reflects the content of hydroxy fatty acids, fatty alcohols, mono- and diacylglycerols, and free glycerol. 

The hydroxyl number is very important for the synthesis of polyurethane. It is defined as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of one gram of the sample. The principle of the analytical method is that the hydroxyl group is esterified with a solution of phthalic anhydride in pyridine. The excess reagent is back-titrated with standard sodium hydroxide solution, and a blank is run on the reagents to determine the amount of anhydride consumed. The phenolphthalein is an indicator for the titrate. Because the color of recycled polyol is dark brown, the faint pink endpoint is not clear, so a potentiometric titrate can be applied. 

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). 

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] ...

Tables 4.9-4.14 show some general characteristics of polyether polyol PO-EO block copolymers, such as MW, hydroxyl number, functionality, viscosity and colour, but also some other characteristics such as unsaturation, EO content, and potassium and sodium content which are specific for polyether polyols.

The hydroxyl number of a polymer polyol is lower than the hydroxyl number of the initial polyether polyol used for grafting. The hydroxyl number decrease is a function of the polymer polyol solid content (generally the solid part has no hydroxyl groups). For the estimated hydroxyl number calculation at a known solid content, equation 6.17 is used ... 

The etherification reaction (18.23) shows that the number of PO units per hydroxyl group is higher than one (the value of x is in the range 1 < x < 2). This reaction leads to the decrease of the hydroxyl number and of the phosphorus content. 

The same substances mentioned before are used as acid acceptors. The phosphorus polyol shown in 18.28 has an hydroxyl number of 490-495 mg KOH/g, a phosphorus content of 13-13.6% and a nitrogen content of 6.1-6.2%. The acidity of all phosphorus polyols presented in Section 18.2 is a maximum of 2 mg KOH/g. 

These oligomers are prepared by polymerization of THF with protonic acid initiators, and according to the patent literature and other available information, fluosulfonic acid has been used on a technical scale. A recent paper describes the successful application of H2S04/S03 mixtures 11). The diol of Mn = 1000 is a white opalescent solid at 24 C it melts at 32 °C giving a syrupy liquid with a hydroxyl number of 112-116, an acid number less than 0.05, a carbonyl number less than 4.0 and a Ca content le>s than 5 ppm Na and K are less than 10 ppm 8). 

Analysis. Solvent purity is assessed by means of gas chromatography [14.175]-[14.181], physical properties, water content, evaporation residue, and acid, saponification, and hydroxyl numbers [14.182]. Color and smell are also evaluated. 

Fluorine content Formula weight Hydroxyl number Melting point Moisture content Purity... 

This is a measure of the hydroxyl content (hydroxyl group present) of the oil and is expressed as the number of milligrams of KOH equivalent to the hydroxyl content of 1 g of oil. It is usually determined by the acetylation reactions of oil with acetic anhydride in the presence of pyridine, followed... 

Since 2-9% excess isocyanate was used to prepare the polymers in this study, the polymers undoubtedly contained some allophanate groups. The effect of excess -NCO content compared to hydroxyl number of the polybutadiene on tensile properties of the polyurethane are shown in Figure 1. As the -NCO content increased, the ultimate tensile strength, a f passed through a maximum at about 6% excess -NCO and then stayed constant. As shown in Table 3, Shore A Hardness increased continuously with % free-NCO content. 

Polyols are identified by their hydroxyl numbers. If the polyol is initiated using ethylene glycol, and the polyol can be identified as a diol, with a functionality of 2, the average molecular weight can be determined. Hydroxyl number is defined as the number of milliweights (or milligrams) of potassium hydroxide equivalent to the active functions (hydroxyl content) of 1 g of the compound or polymer. 

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