High Functionality Polyether Polyols Based on Polyglycerol MIHAIL IONESCU* AND ZORAN S. PETROVIC´ Kansas Polymer Research Center, Pittsburg State University 1701 South Broadway, Pittsburg, Kansas, 66762, USA ABSTRACT: Polyglycerol (PGL) is a polyhydroxyl compound obtained by selfcondensation of glycerol in the presence of alkaline catalysts. It is a very attractive polyol as a starter for the synthesis of polyether polyols for rigid polyurethane foams. It is liquid, easy to handle and has a very high average functionality of 4–20 (or more) hydroxyl groups/mol. By propoxylation of PGL or PGL–sucrose mixtures, we obtained new polyether polyols with very high functionalities, which are very difficult or impossible to obtain by other methods. A new technology for PGL-based polyether polyols preparation was investigated. In the first step the self-polycondensation of glycerol to PGL in the presence of potassium hydroxide or potassium methoxide as a catalyst was carried out. In the second step, the crude alkaline PGL was alkoxylated with PO without removing the catalyst, followed by purification of the resulting polyether polyols. Rigid polyurethane foams prepared from the synthesized PGL-based polyether polyols and crude MDI displayed good physical and mechanical properties, excellent dimensional stability, and low friability. KEY WORDS: polyether, polyols, polyglycerol, polyurethanes.
INTRODUCTION
T
he polyether polyols for rigid polyurethane foams are generally obtained by the polyaddition of PO and/or ethylene oxide (EO) to high functionality polyols having 3–8 hydroxyl groups/mol. The chain derived from one hydroxyl group is usually short, having maximum 1–3 PO units [1]. The polyols generally used as starters are glycerol, *Author to whom correspondence should be addressed. E-mail:
[email protected] Figure 1 appears in color online: http://cel.sagepub.com
JOURNAL OF CELLULAR PLASTICS Volume 46 — May 2010 0021-955X/10/03 0223–15 $10.00/0 DOI: 10.1177/0021955X09355887 ß The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav
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trimethylol propane, pentaerythritol, sorbitol, sucrose, a-methyl glucoside xylitol, etc. A usual procedure for making polyether polyols involves alkoxylation of a mixture of two polyols, such as: glycerol–sorbitol, glycerol–sucrose, dipropylene glycol–sorbitol, diethylene glycol–sucrose with PO [1]. The process of preparation of bio-diesel by transesterification of vegetable oils with methanol or ethanol results in huge quantities of glycerol and it is a real challenge to scientists to find new applications for glycerol. The present article describes a new application of glycerol in the area of rigid polyurethane foams, i.e., the transformation of glycerol to PGL, a very high functionality polyol [2–24, 27–31], and the preparation of new polyether polyols by alkoxylation with alkylene oxides (propylene oxide, EO, etc.). The alkoxylation of PGL was made by Sunder and coworkers [25,26] and by Leinweber and coworkers [15], but the synthesized products are high molecular weight polyethers used as deemulsifiers. The present work describes the synthesis of high functionality low molecular weight polyether polyols initiated by PGL. The chains derived from one hydroxyl group were very short (1–2 PO units) resulting in polyols suitable for rigid polyurethane foams. EXPERIMENTAL PART
Raw Materials Glycerol 99.8%, water content 0.1% was purchased from Fisher; PGL3 supplied by Solvay had OH number ¼ 1160 mg KOH/g, average degree of polycondensation n ¼ 3, and average functionality f ¼ 5 OH groups/ mol; propylene oxide 99.8%, water content 0.01% was purchased from Aldrich; potassium methoxide 99.5% was obtained from Aldrich; Desmodur 44 V70L(‘crude’ MDI) obtained from Bayer had NCO content ¼ 31.3%. Methods The GPC chromatograms were acquired on a Waters system consisting of a 510 pump and 410 differential refractometer. Tetrahydrofuran was used as the eluent at a flow rate of 1.00 mL/min at 308C. Four Phenogel columns plus a guard Phenogel column from Phenomenex covering a MW range of 102 to 5 105 were used. Viscosities were measured on a Rheometrics SR-500 dynamic stress rheometer between two parallel plates, 25 mm in diameter with a gap of 1 mm. The hydroxyl values of the polyols were determined according to the ASTM E 1899-97 standard test method for hydroxyl groups, using
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the reaction with p-toluenesulfonyl isocyanate and potentiometric titration of the resulting carbamate with tetrabutylammonium hydroxide. Synthesis of PGL A stainless steel reactor equipped with a stirrer, nitrogen inlet tube, and condenser was charged with 600 parts of glycerol, 5–6 parts of potassium methoxide or potassium hydroxide. Under a continuous flow of nitrogen the reaction mass was heated at 2508C. Water resulting from the self-polycondensation of glycerol is condensed and collected. The volume of water was a direct measure of the extent of polycondensation reaction. Depending on the desired degree of polycondensation the reaction was carried out around 4–10 h. After the reaction, the resulting alkaline PGL having hydroxyl numbers between 860 and 1200 mg KOH/g, is used without removal of the catalyst for the synthesis of polyether polyols. In this study we used a PGL with OH# ¼ 1114 mg KOH/g corresponding to an average functionality of 5.5 OH groups/mol (calculated). The initial catalyst, potassium methoxide, or potassium hydroxide, is transformed during the reaction to potassium alcoholate of PGL which is the true active species for the anionic ring opening polymerization of PO initiated by the hydroxyl groups [1]. Synthesis of Polyether Polyols Based on PGL A stainless steel pressure reactor equipped with turbine stirrer was charged with 1000 parts of alkaline PGL. Under a protective atmosphere of nitrogen the reaction mass was heated at 115–1258C and around 1000–2000 parts of PO (depending on the desired hydroxyl number) were added stepwise during several hours, with a flow monomer to assure a pressure of 3.5–4 bars at the reaction temperature. After the addition of the calculated quantity of propylene oxide, the reaction mass was maintained under stirring at 115–1258C around 2 h. In this interval of time, major portion of the unreacted PO is consumed and the pressure decreases from 3.5–4 bars to around 0.5–0.8 bars. The last traces of unreacted PO were eliminated under vacuum of 50–65 mmHg. The polyether was purified by the treatment with 1.8–2% disodium acid pyrophosphate in the presence of 1–2% of water, for around 2 h at 85–908C. Water was removed by vacuum distillation at 110–1258C and the solids were removed by filtration under 4–6 bars of nitrogen pressure.
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Synthesis of Polyether Polyols Based on PGL–Sucrose Mixtures The synthesis of polyether polyols based on the PGL–sucrose (PGL-S) mixtures as starters was identical with those for the polyether polyols based on PGL, the single difference being the composition of the initial reaction mass. Thus, instead of PGL alone we used mixtures of 400–600 parts of PGL and 400–800 parts of sucrose, to obtain the desired functionality of the final polyether polyol. Propoxylation, degassing, and purification steps were identical with the corresponding operations described earlier. Characterization of PGL and of Polyether Polyols Based on PGL The synthesized PGLs were characterized by determination of hydroxyl number, viscosity, and alkalinity. The composition of molecular species in PGL was determined by gel permeation chromatography (GPC). PGLs were derivatized with acetic anhydride because the nonderivatized PGLs are insoluble in tetrahydrofuran (the solvent used for GPC). The polyether polyols were characterized by the determination of: hydroxyl number, acid value, viscosity, water content, sodium and potassium content, and unsaturation (Table 1). Table 1. Characteristics of polyether polyols based on PGL and PGL-S. Characteristics PGL/Sucrose (parts/parts (w/w)) Hydroxyl number (mg KOH/g) Functionality (OH groups/mol) Viscosity (mPa. s at 258C) Acid value (mg KOH/g) Water content (%) Sodium/Potassium (ppm/ppm) Unsaturation (mequiv/g) a
PGL-3 (Solvay).
PGL-1a PGL-2 PGL-3 PGL-S-1 PGL-S-2 PGL-S-3 PGL-S-4 PGL-S-5 1/0
1/0
1/0
1/1
1.5/1
2/1
1/2
1/2
448
484
428
420
414
420
386
402
5.0
5.5
5.5
6.3
5.9
5.6
7
7
2910
4550
2730
14,100
7170
6020
10,100
10,400
0.08
0.06
0.082
0.055
0.065
0.063
0.048
0.037
0.088 14 /30
0.07 12/29
0.08 17/ 28
0.065 10/30
0.050 8/38
0.058 10/35
0.062 13/ 30
0.048 18/32
0.02
0.012
0.013
0.022
0.011
0.018
0.021
0.016
Density Compression strength: – parallel – perpendicular Tensile strength Flexural strength Dimensional stability 24h/808C 24h/ 298C Friability, 10 min
Characteristic
134 64 163 240 0.60 0.20 10.96
vol.% vol.% %
27.02
PGL-1
kPa kPa kPa kPa
kg/m
3
U.M.
0.50 0.40 9.18
126 72 152 352
27.53
PGL-2
0.87 þ0.40 4.25
123 57 150 450
25.91
PGL-3
0.33 þ0.40 15.09
136 75 133 225
27.1
PGL-S-1
1.10 0.40 4.04
152 88 170 357
28.4
PGL-S-2
0.33 0.73 6.08
132 77.6 173 240
28.23
PGL-S-3
0.33 0.30 5.58
126 70 161 187
29
PGL-S-4
Table 2. Characteristics of rigid polyurethane foams based on PGL and PGL-S.
0.74 þ0.30 8.1
152 84 164 187.5
28.9
PGL-S-5
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Rigid PU Foams Characterization The rigid polyurethane foams were prepared by usual laboratory procedure consisting in essence of highly efficiency mixing of the PGLbased polyether polyols with crude ‘MDI’ at an isocyanate index of 110, in the presence of blowing agents, catalysts, and silicone surfactants. The foams were characterized by determination of density, compression strength, tensile strength, flexural strength, dimensional stability (at 808C and 278C) and friability. The characteristics of rigid PU foams based on PGL polyols are presented in Table 2. RESULTS AND DISCUSSION
PGL is obtained by self-condensation of glycerol in presence of alkaline catalysts [3,10,12,13,15–23,29] or by catalytic polyaddition of glycidol to glycerol [14,24–27]. PGL obtained by alkaline self-condensation of glycerol is a very high functionality mixture of polyols having an average functionality of 4–20 (or more) hydroxyl groups/mol and it is a complex mixture of linear, branched, and cyclic structures of different polycondensation degrees [2,4,5,8,22,28–31], predominant species being linear PGLs, as it is observed in Schemes 1 and 2. The content of cyclic structures in the composition of PGL obtained by self-condensation of glycerol in alkaline catalysis is lower than 10% [2,4,8] the predominant species being linear PGLs followed by branched PGLs [2,4,8] which becomes more important at higher extent of polycondensation. By analyzing these structures from polyurethane point of view, the presence of cyclic or branched structures are not detrimental. All these structures are high functionality polyols. Alkoxylation of these PGLs with alkylene oxides generates short polyether chains from all the hydroxyl groups in the reaction system resulting in high functionality polyether polyols having ideal structures for rigid polyurethane foams. Figure 1 presents the gel permeation chromatogram of a typical PGL used in the present study, having average functionality f ¼ 5.5 OH groups/mol. The first peak from the right corresponds to the free glycerol followed (from right to left) by the peaks characteristic for diglycerol, triglycerol, tetraglycerol etc. Schemes 3–5 show the most probable structures of polyether polyols based on linear PGL (Scheme 3), branched PGL (Scheme 4), and cyclic PGL (Scheme 5). Of course any polyether polyol based on PGL is a complex mixture of all the species presented in Schemes 3–5. Scheme 6 illustrates the probable structure of polyether polyols based on PGL-S mixture.
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High Functionality Polyether Polyols Based on Polyglycerol
HO
Catalyst
OH
230–270°C
OH
HO
OH + H2O
O OH
OH
x
(a)
Scheme 1. General reaction of self-condensation of glycerol to polyglycerol.
(a)
HO
(b)
O
OH
HO
OH
OH
O
OH
OH x
(c)
OH
O
O HO
O
HO
OH
OH
O O
OH O
O
OH
OH
O O
O
HO x
HO
38.812
37.140
35.515
36.205
30.00 28.00 26.00 24.00 22.00 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00
34.267 34.733 34.981
MV
Scheme 2. Possible structures of molecular species existing in polyglycerol: (a) linear polyglycerol, (b) branched polyglycerol, (c) cyclic polyglycerols.
26.00 27.00 28.00 29.00 30.00 31.00 32.00 33.00 34.00 35.00 36.00 37.00 38.00 39.00 40.00 Minutes
Figure 1. GPC of a polyglycerol (OH# ¼ 1114 mg KOH/g) used as a starter in the present study after derivatization with acetic anhydride.
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HO OH
KOH
CH3
+
OH
O
Z. S. PETROVIC´
AND
110oC
O
OH x CH3
H O HC
CH2 y
O
O
O
CH3 CH2 CH O H y
O
O
x CH3 H O HC
CH3 CH2 CH O H
CH2 y
y x = 0,1,2,3,4... y = 0,1,2...
Scheme 3. Synthesis and structure of a polyether polyol derived from linear polyglycerol.
HO
O HO
OH O
5y
OH
KOH
CH3
+
110oC
O
HO H3C H O CH CH2
O y
O
O O
CH2
O
H3C H O CH CH2
O CH2 y
O CH2
CH3 C O H H y
CH3 C O H H y
CH3 C O H H y
Scheme 4. Synthesis and structure of a polyether polyol derived from a branched polyglycerol.
As mentioned earlier the PGL used in this study for the synthesis of polyether polyols had an average functionality of around 5.5 hydroxyl groups/mol and a hydroxyl number of 1114 mg KOH/g. Glycerol has a much higher hydroxyl number of 1829 mg KOH/g. PGLs of higher functionalities are very viscous. For example, a PGL of functionality of f ¼ 6.5 hydroxyl groups/mol has the viscosity of 56,000 mPa. s, and the PGL of functionality of f ¼ 12 hydroxyl groups/mol the viscosity at 258C
231
High Functionality Polyether Polyols Based on Polyglycerol HO
O OH
O HO
KOH
CH3
O
(2+x)y
+
110°C
O
x CH3 CH2
H O HC
O
O
y
O
CH3 CH2 CH O H y
O O
CH3 CH2
H O HC
O y
x
Scheme 5. Synthesis and structure of a polyether polyol derived from a cyclic polyglycerol.
CH3
CH3 H O
HC CH2
CH2 CH O H O
O
O
O
y
y
O x CH3
CH3 H O
HC
CH2 CH O H
CH 2 y
y
+ CH3 H O
HC
CH3 CH 2
O O
y
H2 C O
CH2 CH O H y
O
H O
HC
CH3
O
O
CH3
CH2 O CH2 CH O H
O
CH 2
O
CH3 H O
y
CH3
y
HC
CH2 CH O H y
CH 2 y
CH3 CH2 CH O H y
Scheme 6. Structure of polyether polyols based on mixture polyglycerol–sucrose.
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is around 220,000 mPa. s. Generally nonalkoxylated PGLs are too viscous to be used directly in rigid polyurethanes (viscosity is around 30,000–50,000 mPa. s at 258C at functionality 5–6 hydroxyl groups/mol), but also are incompatible with aromatic isocyanates (‘crude’ MDI or PAPI). By alkoxylation with PO and especially with EO the high viscosity of PGLs decreased more than 10–40 times. Alkoxylated PGLs, at only 1–2 alkylene oxides/OH groups, are perfectly compatible and soluble in aromatic isocyanates and perfectly compatible with all the conventional polyether polyols. PGLs and all polyether polyols based on PGL are totally soluble in water. In Figure 2, are presented the viscosities of propoxylated PGLs as function of hydroxyl numbers and functionality. As in the case of all polyether polyols the viscosity increases with OH number and functionality. In the particular case of propoxylated PGLs, even at very high functionality, the viscosities of the polyols with OH numbers under 400 mg KOH/g are in a very convenient range between 5000 and 6000 mPa. s at 258C. Thus, a polyether polyol based on a PGL of functionality 12 OH groups/mol has OH number of 370 mg KOH/g a viscosity of 5330 mPa. s at 258C. It is difficult to obtain a polyether polyol of such high functionality with such low viscosity by other methods. This low viscosity of polyether polyols based on PGL probably is explained by
Viscosity, 25°C (mPa.s)
25,000
20,000
15,000
10,000 f=3 f = 5.5 f = 6.5 f = 7.5 f = 12
5000
0 200
300
400
500
600
700
800
900
Hydroxyl number (mg KOH/g) Figure 2. Variation of viscosity of polyether polyols based on propoxylated polyglycerol as function of functionalities (f ¼ 5.5–12 hydroxyl groups/mol) and hydroxyl numbers in comparison with propoxylated glycerol (f ¼ 3 hydroxyl groups/mol).
High Functionality Polyether Polyols Based on Polyglycerol
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the presence of the high mobility ether groups existing in the PGL structure (Scheme 2). Properties of synthesized polyether polyols based on PGL and PGL– sucrose are presented in Table 1. Hydroxyl numbers of PGL-based polyether polyols varied between 380 and 490 mg KOH. These hydroxyl numbers are typical for polyols for rigid PU foams [1]. Of course it is possible to obtain any hydroxyl number between 300 and 600 mg KOH/g by controlling the ratio [PO]/[PGL] or [PO]/[PGLþsucrose]. Remarkable is the relatively low viscosity of high functionality polyols based on PGL–sucrose. Polyols based on PGL–sucrose having average functionality of f ¼ 7 hydroxyl groups/mol have viscosities in the range of 10,000–10,400 mPa. s at 258C. A similar polyol based on sucrose–glycerol of functionality f ¼ 7 hydroxyl groups/mol and hydroxyl number 360– 380 mg KOH/g has a viscosity of 30,000–38,000 mPa. s at 258C. In Figure 3, are presented comparatively the viscosities of polyether polyols based on sucrose–glycerol and sucrose–PGL at the same functionalities and hydroxyl numbers. At a functionality of f ¼ 5.3 hydroxyl groups/mol, the polyols based on PGL–S has a little lower viscosity than the corresponding polyols based on sucrose–glycerol mixture. However, at higher functionality (f ¼ 7 hydroxyl groups/mol) the polyether polyols based on PGL-S mixtures have a viscosity 3–4 times lower than those of similar polyols based on sucrose–glycerol mixture. 40,000 Sucrose-G
Viscosity, 25°C (mPa.s)
35,000
Sucrose-PGL
30,000 25,000 20,000 15,000 10,000 5000 0
f = 5.3
f=7
Functionality (OH group/mol) Figure 3. Comparison between the viscosities of polyether polyols based on sucrose– glycerol and of polyether polyols based on sucrose–polyglycerol mixtures as function of functionality at the same hydroxyl number (OH# ¼ 430 mg KOH/g for f ¼ 5.3 hydroxyl groups/mol and OH# ¼ 370 mg KOH/g for f ¼ 7 hydroxyl groups/mol).
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PGL has a strong capacity to solvate solid sucrose and as an immediate consequence by propoxylation of mixtures PGL-S are obtained polyols without any unreacted solid sucrose. The mixture of PGL–sucrose, even at 60–70% sucrose, is easily stirrable and requires no additional solvent to improve the stirrability of the suspension of sucrose in liquid PGL. These are very important advantages, especially for synthesis of high functionality sucrose polyols. PGL-3 produced by Solvay by a different technology using epichlorohydrin was successfully used for polyether polyols synthesis (PGL-1, Table 1). Of course any other PGL can be used, but an alkoxylation catalyst (KOH or potassium methoxide) must be added. The acid values, water content, and sodium þ potassium content depend on the purification conditions. Thus, acid values50.1 mg KOH/g, water content50.1% and sodium þ potassium content550 ppm (Table 1) are obtained currently by using the adopted purification procedure described before. The characteristics of rigid polyurethane foams based on PGL and PGL-S and ‘crude’ MDI are presented in Table 2. The formulation used for all PU foams based on PGL-based polyether polyols was the following (pph ¼ part by weight per hundred parts of polyol): (1) (2) (3) (4) (5) (6)
PGL polyether polyol: 100 pph Water: 2 pph Silicon BF8461: 1.5 pph Dimethylcyclohexylamine: 1.3 pph HFC 365mfc/227ea: 26 pph Desmodur 44V70L at isocyanate index 110.
The physical and mechanical properties of rigid PU foams based on PGL-based polyether polyols (density, compression strength, tensile strength, flexural strength) are convenient for many applications, but the recommended application will be for thermal insulation of freezers and low temperatures storage tanks, pipes etc. due to the excellent dimensional stability at low temperatures (Table 2). Other remarkable characteristic of rigid PU foams-based on PGL polyether polyols is relatively low friability (the lowest friability have the rigid PU foams based on polyether polyols derived from PGL-S mixtures, Table 2). CONCLUSIONS
PGL was shown to be an excellent starter for the synthesis of high functionality polyether polyols. PGL is a liquid compound, and thus
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easily handled compared with other high functionality polyols used currently as starters for rigid polyols, such as sucrose, pentaerythritol, dipentaerythritol, methyl D-glucoside, which are usually solid with very high melting points. The adopted technology for synthesis of PGL-based polyether polyols has the advantage of utilization of the same catalyst for both polycondensation of glycerol to PGL and alkoxylation of PGL with propylene oxide. The PGL synthesis can be carried out in any polycondensation plant, while alkoxylation of PGL can easily be achieved in any plant for polyether polyols, without modifications. The rigid polyurethane foams based on alkoxylated PGL and alkoxylated PGL–polyols mixtures have good physical and mechanical properties, especially very good dimensional stability and low friability. ACKNOWLEDGMENT
The authors would like to thank Mrs Stanca Capitanu, from National Research Institute for Chemistry and Petrochemistry – Bucharest, Romania, for the preparation and characterization of rigid polyurethane foams based on PGL-derived polyols. REFERENCES 1. Ionescu, M. (2005). Chemistry and Technology of Polyols for Polyurethanes, Shrewsburty, Shropshire, UK. Rapra Technology Ltd. Shawbury. 2. Naweenkumar, T., Sastry, Y.S.R. and Lakshiminarayana, G. Analysis of Polyglycerols by High-performance Liquid Chromatography, J. Chromatogr., 1984: 298: 360–365. 3. Ruppert, A.M., Meeldijk, J.D., Kuipers, B.W.M., Ern, B.H. and Weckhuysen, B.M. Glycerol Etherification over Highly Active CaO-based Materials: New Mechanistic Aspects and Related Colloidal Particle Formation, Chem. Eur. J., 2008: 14: 2016–2024. 4. De Meulenaer, V.B. and Huyghebaert, A. Development of Chromatographic Method for the Determination of Degree of Polymerisation of Polyglycerols and Polyglycerol Fatty Acid Esters, Chromatographia, 2000: 51: 44–52. 5. Jakobson, G. Diglycerin und hohere Oligomere des Glycerins als Synthesebausteine, Fette, Seifen Anstrichmittel, 1986: 88: 101–106. 6. Dolhaine, H., Preuss, W. and Wollmann, K. Strukturen im Polyglycerin, Fette, Seifen Anstrichmittel, 1984: 86: 339–343. 7. Francoois, J., Pouilloux, Y. and Barrault, J. Rational Design of Solid Catalysts for the Selective Use of Glycerol as a Natural Organic Building Block, ChemSusChem., 2008: 1: 586–613.
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