PREPARATION AND PROPERTIES OF GLYCOL GLYCOSIDE POLYETHERS FOR RIGID URETHANE FOAMS F. H . O T E Y , B O N N I E L . ZAGOREN, FLORENCE L. BENNETT, AND C. L. M E H L T R E T T E R
AVorthernRegional Research Laboratory, U. S.Department of Agriculture, Peoria, 111.
Reproducibility of viscosities and hydroxyl numbers of glycol glycoside polyethers was achieved through control of process variables in the production of glycol glycosides from starch. A series of six glycoside mixtures, prepared at 125" to 130" C., with 0.7% acid catalyst and a reaction time of 30 minutes, was etherified with 7 moles of propylene oxide per anhydroglucose unit. The polyethers had average deviations in hydroxyl numbers of f 2 and in viscosities of f1700 centipoises. Recovery of excess ethylene glycol by distillation from the glycoside mixture was nearly quantitative, and its re-use, without purification, did not affect polyether properties. The preparation and properties of the glycoside-based polyethers are within the limits of commercial acceptability for use in rigid urethane foam production.
s
EARLIER
work (2) we described the preparation of glycol
1 glycoside mixtures by acid-catalyzed transglycosylation of
starch Lvich ethylene glycol. \e also demonstrated that the corresponding mixed polyethers obtained by reaction with propylene oxide can be satisfactorily incorporated into usual formulations for rigid foam. However, the polyethers had variations in color, viscosity, and hydroxyl number which appeared to be related to the fluctuating composition of the glycol glycoside mixtures. Additional research to control the transglycosylation reaction variables has resulted in the preparation of consistently uniform polyethers. Re-use of the recovered unreacted ethylene glycol in the preparation of glycol glycosides and some general properties of the polyethers were also studied.
Improved Method for Making Glycol Glycosides from Starch
Procedure. Use of reduced pressure during the entire reaction led to establishment of the following improved procedure for preparing glycol glycosides' Mix 242 ml. (4.4moles) of ethylene glycol and 1.3 grams of concentrated sulfuric acid at room temperature in a four-necked, 1-liter flask. Equip the flask with a stirrer having a glass sleeve and Teflon paddle, thermometer, downward condenser, and gas inlet tube. Add 178.1 grams, dry basis [1.1 moles; a mole of starch is intended to mean the gram-formula weight of one anhydroglucose unit (AGU)], of air-dried starch with good stirring. Flush the system with a n inert gas and reduce the pressure to about 25 mm. of Hg and then apply heat with a n electric heating mantle. I n about 10 minutes the temperature should be at 105' C., at which time the suspension becomes a thick paste or gel. Adjust pressure to about 130 mm. of Hg to avoid foaming. After 10 minutes as a gel, the mixture suddenly becomes a solution of low viscosity. Continue the reaction at 130 mm. of H g and 125' to 130' C. for 20 minutes from the time of gel formation. Then again reduce the pressure to 25 mm. of Hg for a n additional 10 minutes. The solution will now be dark gray. Adjust the system to atmospheric pressure with nitrogen and add enough powdered barium hydroxide [4.3 grams of Ba(OH)S, 8H20] to neutralize the sulfuric acid. Upon neutralization the color changes to tan or yellow. Add 3.5 grams of powdered potassium hydroxide (2% based on starch) as the etherification catalyst. Remove unreacted ethylene glycol from the glycoside mixture at a reduced pressure of 1 to 5 mm. of H g as the temperature is raised to 150' C. \Veight 224
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PRODUCT RESEARCH A N D DEVELOPMENT
increase of the flask contents indicates the presence of 0.79 to 0.81 mole of ethylene glycol per AGU. Etherify by adding propylene oxide directly to the residue of glycosides (2). Discussion of Procedure. 4 n y pearl starch is satisfactory for the process. Reduced pressure during the reaction removes starch moisture (usually 137,) and allows a minimum of hydrolysis of starch to lower polysaccharides and glucose. Such by-products are alkali-labile and when present in excessive amounts cause darkening of the glycoside mixture upon addition of alkali. Also, the product mixture is more uniform when prepared under reduced pressure. .4lthough considerable leeway in pressure is permissible after gelation, it should be reduced sufficiently to allow complete and rapid removal of any remaining water. Barium hydroxide is preferred for neutralization of the sulfuric acid because insoluble barium sulfate can be removed by filtration. Although other alkaline substances, such as potassium acetate or sodium bicarbonate, may be used to neutralize the glycoside mixture and to serve as the etherification catalyst, the sulfate must then be removed by ion exchange. Hydrochloric acid is not as desirable as sulfuric acid for the transglycosylation catalyst because it is more corrosive and imparts a darker color to the glycoside mixture. Polyether Properties
Deionization. Crude polyethers were dissolved in 90% ethanol, and the precipitated barium sulfate and traces of high-molecular-weight solids were removed by filtration. The filtrate was then deionized with Dowex 50 resin to p H 3.5, followed by Duolite A-4 resin to p H 5 to 6. The solution was decolorized with carbon (Darco G-60) and then comentrated in vacuo (water pump) on a steam bath and finally at a reduced pressure of 1 mm. of Hg for 3 hours.
All polyethers described were deionized by this procedure and had the following properties: acid number, 0.3; pH, 5 to 6 at 5% concentration in 10 to 1 methanol and water; ash, 0.007 to 0.01%; and Gardner color, 6 to 8. At 25' C. the specific gravity of the polyethers varied from 1.111 at a hydroxyl number of 380 to 1.135 at a hydroxyl number of 450.
Table 1.
Acid Concn.,a yo
0.59 0.75 0.92 0.42 0.59 0.75 0.92 2.0 0.59 0.75 0.92 a
Moles Propylene Oxide per AGV
6 6 6
Effect of Reaction Variables on Polyether Properties
30 Minutes Viscosity, Hydroxyl ,\To.
CP.
89,500 93,500 94,000
451 453 452
43.000 48,500 42,300 44,000 23,000 28 500 20,000
417 41 8 414 424 386 392 391
7
7 7
7 7
8 8 8
Transglycosylation catalyst.
~
Sulfuric acid:
yc,based on dry weight of starch.
Inorganic residues present may be diminished by treating the crude polyether at about 100" C. with tartaric acid and suction-filtering through Celite-coated cloth. Although this method only partially removes the potassium ions, the polyether obtained is satisfactory for use in "one-shot'' foam formulations. \Vhen the calculated amount of tartaric acid is added (0.5 mole per equivalent weight of K O H present), the polyether has a p H of 8 and 0.1% ash. With 0.6 mole of the acid the polyether has a p H of 6 to 7 and contains 0.04 to 0.067, ash. Effects of Reaction Variables. A number of glycol glycoside mixtures were prepared by transglycosylation of starch as described above, except that concentration of sulfuric acid catalyst and reaction time were varied. Acid concentration was expressed as per cent sulfuric acid based on the dry weight of starch. Reaction time was measured from the start of gelation to the time of barium hydroxide addition. Weight increases indicated that 0.78 to 0.82 mole of ethylene glycol had reacted per AGU of starch. By reaction of each glycoside mixture with 6, 7: and 8 ((+0.1) moles of propylene oxide per AGU, polyethers were obtained and their physical properties were determined and examined for uniformity (Table I). Increasing the acid concentration from 0.4 to 2% gave only a slight decrease in viscosity. Increasing the reaction time from 30 to 90 minutes gave a similar effect. But increasing both variables produced a trend toward lower viscosities, particularly at the loivest level of propoxylation. I n the preparations studied deviations in hydroxyl numbers were due entirely to variations of r t 0 . l mole of reacted propylene oxide. With a 0.4% concentration of sulfuric acid more than a 30-minute reaction time was required to yield glycosides suitable for etherification. Thus acid concentrations between 0.5 and 2% and reaction times between 30 and 90 minutes were not critical. Preferred economical process conditions are 0.5 to 0.77, sulfuric acid and 30 minutes of reaction. Reproducibility. Starch is a high polymer composed of anhydroglucose units, and its transglycosylation with glycol by the established procedure yields a mixture of glycosides (7). A corresponding mixture of glycoside pol yethers is produced by reaction of glycol glycosides with propylene oxide. With such a complex mixture it would be expected that uniformity of physical properties would be difficult to obtain. However, by reducing pressure at the beginning of glycoside for mation, rapidly stirring, and strictly regulating other reaction conditions uniformity was achieved. T o determine the degree of uniformity attainable for polyether properties, a series of six
Transglycosylation Reaction T i m e 60 Minutes Vzscositvjc Hydroxyl CP. No. 90,500 447
84,000 456 88,400 460 44.500 418 43 900 408 40,000 413 37,500 424 38,100 422 24: 500 381 21,100 387 22,500 392 AGU, anhydro,ylucose unit.
90 MinzLtes Viscosity,c Hydroxyl CP. No.
89,000 71,000
456 v55 458
41,500 35,000 32,500
414 418 418
25,000 20,500 22,500
391 391 398
77,500
...
..
Brookjeld oiscosity, 25' C.
glycol glycoside mixtures was prepared lvith 0.77, sulfuric acid as catalyst and 30 minutes' reaction time. Each glycoside was made to react with 7 moles of propylene oxide per AGU, and the polyethers were isolated as described above. In this study (Table 11) viscosities were duplicated within a n average deviation of +1700 cp., and hydroxyl numbers were repeated within the accuracy of the analytical method. Effect of Recycling Ethylene Glycol. Since a n excess of 3.2 moles of glycol is added per mole of starch during transglycosylation, for an economical process it is essential that this excess be recovered for re-use. A study was rherefore conducted to determine efficiency of recovery and recycling of the excess glycol in several preparations. Equipment, described above, was modified so as to replace the condenser with two 250-ml. vacuum pump traps connected in series. T h e first trap, A, was kept at 25" C. and the second. B , was cooled with solid carbon dioxide until transglycosylation was completed. Then both A and B were cooled with solid carbon dioxide as unreacted glycol was removed at 1 to 5 mm. of Hg. Small condensers, cooled with solid carbon dioxide, were employed for convenience ; however, water-cooled condensers would suffice for large-scale operations. The amounts of glycol and water collected in each trap were determined by refractive index measurements. A rapid and relatively accurate method of analysis results from the linear relationship which exists between per cent glycol in water and refractive index of the solution. T h e first glycoside mixture of the series (Table 111) was prepared from 262.4 grams of glycol. Upon removal of unreacted glycol and water, 21 1.4grams of distillate were collected in A . Refractive index measurement showed this distillate to contain 98% or 207.2 grams of glycol. I n B, were collected 21.5 grams of distiliate that contained 99% water. Conden-
Table II. Reproducibility of Polyether Properties Moles Propylene Mole Oxid? per Glycol Hydroxyl No. Viscosity, AGU" per AGU Found Calcd. Cp., 25' C.
Av. Av. dev. a
7.01 6.97 7.01 6.95 6.93 7.07 6 , 99
0.81 0.81 0.81 0.80 0.79 0.83 0.81
410 410 413 418 412 412 412.5 *2
419 420 419 41 9 419 419 419
41,500 44,300 44,700 42,800 48,700 43,300 44,200 il,700
AGU, Anhydroglucose unit.
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Table 111.
Recycling of Recovered Ethylene Glycol and Effect on Polyether Properties
Polvether Moly n .
Ethviene Glvcol. Grams
1 2 3 4 5 6
262.4 56.5 57.4 56.1 57.9 59.8
52.8 56.4 52.3 55.7 56.7 55.5
207.2 205.6 206.6 204.0 203.0 203.4
209.6 207.3 210.7 207.0 205.2 207.3
2.4 1.7 4.1 3.0 2.2 3.9
25.5 29.0 29.5 26.9 27 . a 29.7
23.7 27.9 27.9 26.0 26.0 27.0
6.9 6.88 6.87 6.91 6.90 6.93 Av. 6 . 9 0
Av. dev.
41 2 422 416 422 41 a 422 419 zk3.3
41 9 420 420 41 9 41 9 418 41 9
45,600 45,100 49,000 45,500 43,500 47,600 46,000 f1,500
Total ethylene glycol used = 346.7 grams. Total ethylene glycol lost = 17.3 grams or 5%. Starch moijture.
a
AGV.
Anhydroglucose unit.
sate from A was combined with 56.5 grams of pure glycol for use in the second glycoside preparation. This procedure was repeated for a total of six trials, in each of which the fraction in A contained 98 to 99y0pure glycol, with no indication of color or contaminant buildup. Total material balance was within =k1 gram for each preparation, although slightly more water (2 grams) was recovered than expected and about 2 grams of glycol were unaccounted for. Apparently fraction B contained more glycol than was indicated by analysis. Total loss of glycol, including that in fraction B, was 570 based on the total amount of glycol used in the six preparative experiments. Each glycoside mixture \vas made to react with a total of 6.9 moles of propylene oxide to determine the effect of glycol recycling on polyether properties. The polyethers had an average viscosity of 46,OOO cp. with an average deviation of ?C 1500. An average viscosity of 43>0@@ cp. is obtained by interpolation when 6 . 9 9 moles of propylene oxide react per AGU. These values are in excellent agreement Lyith thoseobtained for the series reported in Table 11, in Ivhich pure glycol was used. Recycling of the
glycol apparently had no adverse effect on hydroxyl numbers or color of the polyethers. Viscosity. Measurements of viscosity of the polyethers were made in 25-mm. diameter vials with a Brookfield viscometer, Model LVF. Figure 1 shows the effect of moles of propylene oxide reacting per AGU on viscosity of the polyethers. Each point represents an average of six separate preparations. Polyethers with 10 moles of propylene oxide per AGU have viscosities ranging from 7000 to 10,000 cp. Sufficient data were not obtained to establish accurately a point for this level of propoxylation. Polyether viscosities are greatly affected by temperature, as demonstrated in Figure 2. Several commercial polyethers were observed to have a temperature viscosity relationship similar to the glycoside based polyethers. Hydroxyl Numbers. Predicted hydroxyl numbers were
c
100,000, !O,OOO
70,000
60,000 E 0
.r” 50,000 0 0
.-
40,000
>
1
30,000
10,000
tc 6
7
Moles of propylene Gxide/AGU
a
Figure 1. Effect of moles of propylene oxide reacted per anhydroglucose unit (AGU) on viscosity of polyethers 226
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PRODUCT RESEARCH A N D DEVELOPMENT
I
I
40 50 Temperature, “ C Figure 2.
’.
-’\ -\
60
Effect of temperature on viscosity of polyethers
obtained from the following equations:
I
o Average h y d r o x y l number, f o u n d
460-
0'
A
AV= 3
Hydroxyl number; number, c a l c d . f o r 0.80 m o l e glycol/AGU
450-
Hydroxyl number =
+
2(G) 56,100 ,V ~
MW
where -V = average number of hydroxyls per AGU G = mole glycol per AGU MW = molecular weight of polyethers = 162 62G f 58.1 (moies of propylene oxide per AGU) The first equation is derived from addition of the three hydroxyls per AGU in starch to the two hydroxyls of ethylene glycol. T h e average number of hydroxyls per AGU does not depend upon extent of reaction of glycol with starch. Glycol glycosides contain a n average of 0.80 mole equivalent of glycol per A G U ; therefore, ,Vis usually 4.6. Figure 3 is the curve obtained by plotting calculated hydroxyl numbers of the polyethers against propylene oxide reacted per AGU. Each point of the curve is a n average of calculated values for six separate preparations. O n this graph points are also located for the average hydroxyl numbers found by isocyanate analysis (3) for the same preparations. Relatively good agreement is shown between calculated and analytical values. More accurate predictions are obtained by adding 7 to the calculated values in the range of 8 to 10 moles of propylene oxide per AGC' and by subtracting 7 from those calculated for 6 to 7 moles of propylene oxide per AGU. Hygrbscopicity, Polyethers exposed to air moisture show a rapid initial weight increase but slowly reach an equilibrium. .4t 50% relative humidity and 20' C., the per cent weight increase shown in Figure 4 was observed with 20-gram samples in a n open Petri dish. Each polyether was dried at 85 O C. and at a reduced pressure of 1 to 5 mm. of Hg before the study. One sorbitol polyether, hydroxyl number 480, was prepared in the laboratory, and the other was a commercial grade with a hydroxyl number of 492. Glycoside-based polyethers appear slightly less hygroscopic than sorbitol-based polyethers.
+
440L
W
g 430E
*
E 420-
P
*
I
410-
400390-
\; 0
QQ
3 8 0 1-
A '
7
M o l e s of propylene oxide/AGU
8
Figure 3. Effect of moles of propylene oxide reacted per anhydroglucose unit (AGU) on hydroxyl number of polyethers
Polyether Commercial S o r b i t o l 0 Sorbitol Gly,coside Glycoside
.
OH No. 492 480 382 458
Conclusions
Polyethers with consistently uniform properties are obtained by propoxylating acid-catalyzed reaction products of starch and ethylene glycol. The starch-glycol reaction is preferably run at 125' to 130' C. for 30 minutes under reduced pressure of 30 to 150 mm. of Hg, with vigorous stirring and with 0.5 to 0.77, sulfuric acid catalyst. Application of these select conditions gives polyethers with viscosities, hydroxyl numbers, and hygroscopicities that are well within the limits of commercial acceptability for use in rigid urethane foams. Acknowledgment
The authors thank 1%'. F. Kwolek, Biometrician, Biometrical Services, Agricdltural Research Service, U. S. Department of Agriculture, stationed at the Northern Laboratory, for assistance in designing some of the experiments. literature Cited
(1) Otey, F. H.: Bennett, F. L., Zagorefi, B. L., Mehltretter, C. L., IND.ENG.CHEM.PROD.RES.DEVELOP. 4, 228 (1965). (2) Otey, F. H., Zagoren, B. L., Mehltretter, C. L., Zbid., 2, 256 (1963). (3)' Ote;, F. H., Zagoren, B. L., Mehltretter, C. L., J . A$$. Polymer Scz. 8, 1985 (1964). RECEIVED for review April 30, 1965
Figure 4.
20°
c.
Hygroscopicity of polyethers a t 50% RH and
ACCEPTED July 26, 1965 Division of Carbohydrate Chemistry, 149th Meeting, ACS, Detroit, Mich.. April 1965. The Northern Laboratory is part of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of .4griculture. Mention of firm names or commercial products does not constitute an endorsement by the U. S. Department of Agriculture. VOL.
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