An Approach to the Characterization of Isocyanate-Terminated Prepolymers Mervyn B. Jackson Division of Protein Chemistry, CSIRO,Parkville (Melbourne), Victoria 3052, Australia
D . H . Solomon Division of Applied Chemistry, CSIRO, Fisherman's Bend (Melbourne), Victoria 3207, Australia
THEVOLUME of polyurethanes produced and their fields of application have increased enormously since the initial industrial development began in Germany in the late 1930's. Polyurethane products have found extensive use as foamed plastics, polyurethane elastomers, and as coatings ( 1 ) . More recently polyurethanes have found commercial application in the textile industry. Isocyanate-terminated prepolymers produce antifelting finishes on wool and stabjlize creases (2-7). The characterization of linear polyurethanes (8) and the analysis of polyurethane foams (9) have been reported. We wish t o report an approach to the characterization of commercially available prepolymers containing reactive isocyanate groups. Such prepolymers usually have a polyether backbone, or, less frequently, a polyester backbone. Hydroxy-terminated polymers of this type are capped with diisocyanates to produce the isocyanate prepolymers. We have found (IO) that such capping reactions may give a prepolymer whose functionality and molecular weight are greater than predicted from the stoichiometry of the reactants used. Thus it may often be desirable to characterize laboratory-prepared prepolymers in addition to commercial products. EXPERIMENTAL
I R spectra were recorded on a Unicam SP1200 Grating Infrared Spectrophotometer on liquid films. NMR spectra were recorded on a Varian HA-100 Spectrometer as dilute solutions in deuterochloroform using tetramethyl silane as an internal standard, The polymeric diols and triols were supplied by the Imperial Chemical Industries of Australia and New Zealand Ltd. Hexamethylene diisocyanate was an Aldrich Chemical Co., Inc. product. The hydroxyl number of hydroxy-terminated polymers was determined by the p-toluenesulfonic acid catalyzed acetylation with acetic anhydride (11). The isocyanate concentra(1) J. H. Saunders and K. C. Frisch, "Polyurethanes: Chemistry and Technology. Part 11: Technology," Interscience Publishers, New York, N.Y., 1964. (2) Netherlands Patent Application Number 6616279 (to Deering Milliken) (1966). (3) British Patent 1,062,185 (to Deering Milliken) (1963). (4) British Patent 1,062,564 (to Deering Milliken) (1963). (5) Australian Patent 288,116 (to Farbenfabriken Bayer Aktiengesellschaft) (1965). (6) I. B. Angliss, J. R. Cook, J. Delmenico, H. D. Feldtman, B. E. Fleischfresser, F. W. Jones, and M. A. White, 4th International Wool Textile Research Conference, Berkeley, 1970. (7) R. 0. Rutley, J . Soc. Dyers Colour., 86, 337 (1970). (8) J. L. Mulder, Anal. Cliitiz. Acta, 38, 563 (1967). (9) B. Dawson, S. Hopkins, and P. R. Sewell, J . Appl. Polym. Sci., 14, 35 (1970). (10) M. B. Jackson and D. H. Solomon, J. Macromol. Sci., in press. (11) R. S. Stetzler and C. F. Smullin, ANAL.CHEM.,34, 194 (1962). 1074
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tion was determined by reaction with n-butylamine (12) or di-n-butylamine (13). Molecular weights were measured using a Mechrolab Model 301A Vapour Pressure Osmometer. The solvent was either tetrachloroethylene or chloroform and benzil was used to calibrate the instrument. Synthesis of Isocyanate Capped Polymers. METHODA. The poly01 was dried by azeotroping with either tetrachloroethylene or toluene under a nitrogen atmosphere. Hexamethylene diisocyanate (HMDI) (2 equivalents) was added to the poly01 (one equivalent of hydroxyl group) and the mixture heated and stirred under a nitrogen atmosphere for 2 hr at 110 OC and a further 11i2hr at 130-135 "C. For capping with toluene diisocyanate (TDI), a mixture of TDI (2 equivalents) and the polyol (1 equivalent) was heated and stirred under a nitrogen atmosphere at 100 "C for 1l/* hr. METHODB. The poly01 was dried as above with toluene and the toluene removed under vacuum. To the poly01 (1 equivalent of hydroxyl group) was added HMDI (4-6 equivalents of isocyanate). The temperature of the contents was increased from 25 to 110 "C over a period of 1 hr with stirring under an atmosphere of nitrogen. The contents were then maintained at 110 "C for 2 hr and at 130 O C for a further hour. After cooling the solution, excess H M D I was removed under vacuum. Gel Point Measurements. An isocyanate-terminated polymer in dry dioxan was mixed with a solution of butan-1,4diol containing triethylene diamine in such ratios that :
[NCO] = [OH] and [triethylene diamine]
=
0.298N
= 0.021M
and the reaction solution was thermostated at 24 "C. The concentration of isocyanate at any given time was determined by reacting an aliquot with an excess of a standard solution of n-butylamine and back-titrating the residual amine with a standard hydrochloric acid solution. Gel point experiments for the diols and triols were determined similarly using HMDI as the other reactant. The gel point in each case was taken as the time when the reaction mixture no longer flowed when the flask was inverted. Hydrolysis of an Isocyanate-Terminated Prepolymer. The prepolymer (2.4 grams) was dissolved in ethanol (25 ml), a solution of 2 0 x aqueous potassium hydroxide (30 ml) was added, and the solution was heated in a steel bomb at 150 "C for 24 hr. After cooling the bomb, the contents were transferred to a separating funnel and the solution was extracted four times with 40-ml portions of ether. The ether was removed from the combined extracts and the residue taken up in 5N hydrochloric acid (25 ml). This solution was extracted three times with 40-ml portions of ether, the ether solution was washed successively with dilute hydrochloric acid, water, dilute sodium bicarbonate solution, and water and dried (magnesium sulfate). After removal of the ether, the residue (12) S. Siggia and J. G. Hanna, ibid., 20, 1084 (1948). (13) ASTM D1638-61T, ASTM Standards, 1961, p 176.
was azeotroped with benzene (30 ml), the benzene removed under vacuum, and the residue dried under vacuum t o give the poly01 (2 grams). The original hydrochloric acid solution was basified with potassium hydroxide solution and extracted repeatedly with ether. The ether solution was dried (potassium hydroxide pellets) and then the ether was removed to leave the diamine. The diamine was identified by comparison with an authentic sample of hexamethylenediamine. The poly01 was characterized by its IR spectrum, molecular weight, and equivalent weight; the latter two properties were determined by vapour pressure osmometry and hydroxyl end group determination, respectively. RESULTS AND DISCUSSION
The characterization procedure will be illustrated by reference to a polymer at present being used commercially. This polymer will be referred to as prepolymer 1. Spectrometric Examination. Infrared and nuclear magnetic resonance spectral investigations can give useful information about the backbone structure of a polymer and infrared spectrometry may also indicate the nature of the reactive functional groups providing the molecular weight is not too high and therefore the concentration of the reactive groups too low. Thus, the commercial prepolymer 1 had a very strong absorption at 1120 cm-I indicating a polyether backbone (8) and intense bands at 2280 cm-1 and 1730 cm-I which are characteristic of the isocyanate and urethane group, respectively. The absence of any bands near 1600 cm-1 suggested an aliphatic isocyanate as the capping reagent. Nuclear magnetic resonance spectral evidence indicated that the
prepolymer 1 was derived almost exclusively from propylene oxide. The ratio of the methyl protons (at 6 = 1.2 ppm) to the methylene plus methine protons (6 = 3.3-3.7 ppm) was found to be 1:1 . Equivalent Weight and Molecular Weight. The properties of a cured prepolymer depend on the nature of the prepolymer itself. It is thus desirable t o know the molecular weight, which is related t o the chain length, of a given prepolymer. For prepolymers with reactive end groups there is also the matter of the number of reactive groups per moleculethe functionality ( f ) . Functionality may be calculated by dividing the molecular weight by the equivalent weight. Although equivalent weights of polymers with hydroxyl or isocyanate end groups may be reasonably accurately measured by titration procedures with acetic anhydride (11) or nbutylamine (12), respectively, considerable uncertainty in the molecular weights (14) may result in large errors in the measured functionality. In spite of the uncertainty in the molecular weight measurements, some useful information about the functionality of a prepolymer may be obtained from molecular weight and equivalent weight measurements. In Table I are listed the molecular weight and equivalent weight of the isocyanate prepolymer 1 which suggests a functionality of about 3. This conclusion is further substantiated by the functionality of the hydroxy-terminated prepolymer 2 which was obtained after hydrolysis of prepolymer 1. See below. (14) J. Van Dam, in “Characterization of Macromolecular Structure,’’ National Academy of Sciences, Washington, D.C., 1968, pp 336--41.
n
Prepolymer 1 n = functionality or the number of isocyanate-terminated polymer chains originating from the branch point RI.
Prepolymer 2
+
n HzNRzNHz
Table I. Functionalities ( f ) of Prepolymers 1 and 2 Polymer Nature of end groups Prepolymer 1 Isocyanate Prepolymer 2 Hydroxyl By end group titration. By vapour phase osmometry.
Equivalent weight0 1120 1023
Molecular weightb 3200 i- 300 3150 f 300
f 2.9 & 0.3 3.1 i 0.3
Table 11. Functionality cf)of Prepolymers by Gel Point Determination Catalyst concentration Initial Prepolymer Nature of Cross-linker (triethyleneprepolymer number end groups Solvent (functionality) diamine) concn ra f 1 Isocyanate Dioxan Butane-1,4-diol(2) 0.298N 1.00 3 . 2 i 0 . 1 0.021M H400* Isocyanate 0.298N 1 .@I 2.3 Dioxan Butane-l,4-diol(2) 0.022M 0,298N 1 .OO 3.5 Hll00~ Isocyanate Dioxan Butane-1,4-diol (2) 0.022M 2.65N 1.15 2.0 PPG400d None HMDI (2) 0,038M Hydroxyl TRIOL1 1OOe Hydroxyl 2.14N 1.03 3.0 None HMDI (2) 0.023M a r = (initial concentration of the reactive groups of the cross-linker)/(initial concentration of the reactive groups of the prepolymer). ICIANZ diol, MW 400, capped with HMDI. 1100, capped with HMDI. ICIANZ triol, MW ICIANZ diol, MW 400. e ICIANZ triol, MW 1100.
--
--
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Hydrolysis of Isocyanate-Terminated Prepolymers. Although linear polyurethanes (8) and polyurethane foams (9) have been successfully hydrolyzed with aqueous alkali, it was necessary t o dissolve the isocyanate-terminated prepolymers in ethanol. The polymer could then be hydrolyzed with alkali in a steel bomb a t 150 'C. Thus, the hydrolysis of prepolymer 1 yielded the corresponding hydroxy-terminated prepolymer 2. If the prepolymer backbone had been of the less common polyester type, it would have been hydrolyzed and the components could then have been separated and identified by the usual methods (8, 9). The equivalent weight and molecular weight of the hydroxy-terminated prepolymer 2 are listed in Table I. Functionality by Gel Point Measurements. Many of the properties of cured elastomeric polymers depend on the cross-link density (15). The cross-link density is determined by the functionality of the prepolymer and the equivalent weight. In view of the uncertainty involved in the calculation of functionality from the molecular weight and equivalent weight, it is desirable t o have a n alternative method of measuring the functionality of a prepolymer. For many years, equations relating the extent of reaction between two reacting groups at the gel point t o the functionality of the reactants have been used. Recently, these equations have been used t o calculate the functionality of a reactant from the extent of reaction at the gel point (16, 17). Strecker and French (16) have derived the expression:
where P A and PB are the fractions of A and B groups initially present that have reacted at the gel point, r is the ratio of the total number of B groups initially present t o the total number of A groups initially present and,fE and gE are the weighted average functionalities of all molecules bearing the reactive groups A or B, respectively. Thus, to determine , f E , it is necessary to know the functionality of the cross-linking agent (or chain-extending agent if the functionality is such that gelation does not occur even at 100% reaction), the ratio of the reactants, and the extent of reaction of either of the reactants at the gel point. Equation 1 was used t o determine the functionality of the unknown prepolymer 1. The cross-linking agent chosen for the isocyanate-terminated polymer was butane-1,Cdiol (gE = 2) and the conditions were such that the initial reactant ratio was 1.00 so that Equation 1 then reduced t o Equation 2. fE
=
1
+ PA*- 1
(15) D. M. French, Rubber Chem. Technol., 42, 71 (1969). (16) R. A. H. Strecker and D. M. French, J . Appl. Polym. Sci., 12, 1697 (1968) and references therein. (17) J. P. Consaga, ibid.,14,2157 (1970).
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The isocyanate concentration was determined by reacting an aliquot with n-butylamine and back-titrating the unchanged n-butylamine with a standard hydrochloric acid solution (12). The reaction was so slow at 24 "C that it was found advantageous t o use triethylenediamine as a catalyst. The functionality of prepolymer 1 calculated by the gel point procedure (Table 11) is consistent with the prepolymer being trifunctional. The reaction between'an isocyanate group of the prepolymer 1 and a n hydroxyl group of butane-1,Cdiol followed second-order kinetics up t o the gel point. The rate constant was 0.30 X 1 g-equiv-l sec-' at 24 "C in the presence of triethylenediamine (0.021M). It would be expected that dilution of the solution would increase the chance of intramolecular reactions which would result in lower measured functionality values. Since the prepolymer 1 has a relatively high molecular weight and the reactive groups are located at the ends of the chains, the probability of intramolecular reactions is low. Furthermore, moderately concentrated solutions were used t o measure the gel point of prepolymer l . Strecker and French ( 1 6 ) have shown that the measured functionality is independent of dilution for an analogous system under similar conditions. In order to check further the validity of the conclusion that prepolymer 1 is trifunctional, the functionality of a diol and triol as well as the isocyanate capped derivatives were measured and the results are shown in Table 11. The value of 3.2 for the functionality of prepolymer 1 is consistent with a side reaction occurring during the capping reaction of the polymeric triol from which it was made. This competing reaction, which may involve the trimerization of isocyanate groups or the formation of an allophanate, is responsible for the considerably higher than theoretical values obtained for the functionality of the capped OCN-R-NHCOO-w-NCO
+ OCN--
NCO *
OCN-R-NCOO--*--NCO CONH------NCO an allophanate diol and triol prepolymers (Table 11). H 400 and H 1100 were prepared by method A. Even higher functionalities were obtained when the corresponding diol and triol were capped by method B. The functionality values obtained for the diol and triol indicate that side reactions such as allophanate formation are not important in the gel point measurements but only at the higher temperatures used in the capping reaction. In order to obtain an isocyanateterminated prepolymer of specified functionality it is therefore important that the conditions of the capping reaction be carefully controlled.
RECEIVED for review June 14, 1971. Accepted October 12, 1971.
