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Surface chemistry of polyurethane foam formation
Surface Chemistry of Polyurethane Foam Formation III. Effect of Gas Diffusion between Bubbles and Surface Viscosity on Bubble Stability M. J. O W E N AND T. C. K E N D R I C K Research Department, Midland Silicones Ltd., Barry, Glamorgan. United Kingdom
Received September 12, 1967 Gas diffusion between foam bubbles is shown to be an insignificant effect in polyurethane foam formation. The fact that at equivalent surface elasticities linear polysiloxane-potyether block copolymers are less effective polyol foam stabilizers thai1 branched polysiloxane-polyether block copolymers is traced to the differences in surface viscosity exhibited by these copolymers. This difference may be the reason for their different polyurethane foam stabilizing powers. INTRODUCTION
than 22.3 dynes c m - and had a rate of surface tension lowering lying in an opt i m u m range. The details of these studies are given in Parts I and I I , but one of the main conclusions was t h a t ABA linear copolymers with two polyether segments, even when used at selected concentrations where they exhibited o p t i m u m rates and extents of surface tension lowering, did not adequately stabilize polyurethane foam. Furthermore, as will be seen in this paper, the linear ABA copolymers do not stabilize polyol foams as well as do the branched copolymers. The effects of these copolymers on gas diffusion between bubbles and surface viscosity have been investigated as possible reasons for this difference. As we have used the Brown, T h u m a n , and McBain single bubble technique for studying gas diffusion (4) we have used their simple equation relating the bubble radius r to the time t:
The precise role of the cell size control agents, known as foam stabilizers, used in polyurethane foam formation has not yet been fully elucidated. B oudreau (1) has given a review of the theory of surfactant effects in such a system and has reported on surfactant evaluation techniques. This present paper continues our a t t e m p t to determine the practical relevancy of the several effects which are thought to occur. We have already presented our ideas on the roles of equilibrium and dynamic surface tension in surface elasticity (2, 3), and we now report a study of foam drainage, surface viscosity, and gas diffusion between foam bubbles. The type of surfactant normally used to stabilize polyurethane foam during its formation is a polysiloxane-polyether copolymer. I t was our hope that parallels could be drawn between actual foam stabilizing ability and the behavior of these compounds in LG 56, a polyol used in polyurethane foam production. I t was concluded from studies in the surfactant/polyol model system that branched polysiloxane-polyether copolymers (containing more than two polyether segments) functioned adequately as polyurethane foam stabilizers if they lowered the surface tension of the polyol LG 56 to less
where ro is the radius at time t = to, P the atmospheric pressure, ~ the surface tension and k the film permeability constant. A similar equation has been given b y De Vries (5) for any particular bubble in a
Journal of Colloid and Interface ,Science, Vol. 27, No. 1, May 1968
46
r~ -
4a r02 = -- - - k l , P
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION foam. Princen and Mason (6) have shown that the theory used to obtain the MeBain equation was inexact and have presented a modified theory. We have used the simple McBain equation because we are concerned only with comparisons between various surfaetants, not with the detailed nature of the gas transport through the film. Any more sophisticated analysis would require more parameters, particularly film thickness, which are not known in this ease. VISCOSITY EFFECTS The simplest view of the mechanism of drainage of material out of a foam film is that it is the result of two forces, capillary suction and gravity. At the point where two or more foam films between gas bubbles meet the surfaces have negative curvature wth respect to the liquid phase, and hence the pressure there is lower than in the films themselves (on the assumption that these have plane interfaces). There will consequently be a capillary suction drawing material out of the fihn, often against gravity. However, gravity will ensure that the net flow is downwards out of the foam. The work of Mysels, Shinoda, and Frankel (7) has shown that the situation is rather more involved. Not only are films sucked into the borders between them but thinner films are sucked out as well. This phenomenon of m a r s h a l regeneration must involve both the bulk phase between the two faces of the fihn and these surface layers themselves. An increase in either or both of the bulk or surface viscosities will retard the flow of material in the film and help to stabilize the foam. A description of the flow properties of an interface requires two viscosity coefficients. One is the usual two-dimensional analog of bulk shear viscosity at constant surface are,~ and the other is the surface dilationM viscosity (8) associated with changes of area in the interface. This latter quantity is related to the rate of lowering of surface tension. We are concerned here with the surface shear viscosity as we know that the difference between the linear and branched copolymers does not lie in their dynamic
47
surface tension (see Part II). Moreover, we have observed that in polyurethane foams stabilized by our ABA linear copolymers the foam rose in a controlled manner but coarsened rapidly at the top of the foam rise. There is thus a period of time before gelation where the surface viscosity effect at relatively constant area may be important. There is no completely suitable method for measuring the surface viscosities of soluble surface layels (9). The canal method is theoretically sound but cannot handle slow surface aging solutions as we have here, nor can non-Newtonian surface behavior be readily detected--a drawback shared by the comparative viscous traction technique. All oscillating and rotating devices employ equations for the surface viscosity which assume that there is no frictional drag exerted on the surface by underlying layers of liquid. This is a most unlikely supposition for a foaming system. Nevertheless, we have, chosen to use an oscillating surface viscometer because of its ability to detect nonNewtonian surface flow. The surface viscosity u in surface poises is obtained from the usual equation
2303 ( R : where I is the polar moment of inertia of the bob, T the period, R~ the radius of the cup, R~ the radius of the bob, A the logarithmic decrement on the solution surface, and A0 the logarithmic decrement on a clean solvent surface. Tsehoegi (10) has given a good derivation of this equation. EXPERIMENTAL METItODS Except for the surface viscosity work these studies were carried out with the polyol LG 56 as before to give a model system, free from polymerization variables, yet closely related to the real situation. The surfactants were polysiloxane-polyether block copolymers of the type usually used to assist polyurethane foam formation. T h e y have been fully described in a previous paper (2), and a summary of their constitution is given in Table I. Copolymers A, B, and C are all branched copolymers containing more than two polyether chains per molecule, Journal of Colloid and Interface Science, Vol. 27, No. 1, May 1968
48
OWEN AND KENDRICK TABLE I ~:)ROPERTIES OF THE COPOLYMERS
Copoly- Polyetber portion -~n Wt. %
met no.
A B C D E F G H I J
1,520 2,500 2,500 2,200 2,200 2,200 2,200 2,200 2,200 2,200
29 25 22 34 17 27.4 37.8 39.4 51.5 62.5
Po]ysiloxane portion Mn
1,960 5,950 5,200 2,670 910 1,680 2,690 2,880 4,700 7,400
Mw
M~,
970 1,716 2,550 3,100=t=300 4,1004-500 7,100=E500
See Part I for details of measurements. whereas copolymers D to J are all linear ABA copolymers containing two polyether chains per molecule. With the construction of a sufficiently massive surface viscometer bob to give oscillations in the polyol surface significant differences between pure polyol and solution surfaces could not be observed. Greater changes in the rate of damping of the bob could be produced b y minor variations in the depth of immersion (<0.01 cm) than were apparent between various solutions. This does not mean that surface viscosity effects will not be important in thin films of course. Moreover, in the actual polyurethane system the region of interest is after the initial foam rise before gelation, where temperatures are well in excess of 100°C and the films may be very mobile. We chose to work with a less viscous solvent at room temperature rather than with the polyol at elevated temperature. Consequently the results presented here are for aqueous solution; similar results were obtained in tripropylene glycol. Foams were formed b y blowing compressed air into the polyol and surfactant mixture for a fixed time (30 secs) and then sucking the mixture through a number 4 glass sinter into a graduated cylinder. For the gas diffusion work Brown, T h u m a n and McBain's technique and apparatus were exactly ~dopted. Although it is a single bubble rather than a real foam study it is the most straightforward means of obtaining Journal of Colloid and Interface Science, Vol. 27, No. 1, May 1968
direct comparisons of the effect of the various surfaetant copolymers. Our studies were carried out in a thermostatted bath at 25°C -4- 0.1°C using the polyol L G 56 as the solvent. All solutions were left in the apparatus for an hour to ensure temperature equilibration. The bubble was then formed and measurements of the radius were commenced after a least a further 2 hours. This was sufficiently long for surface tension equilibrium to have been attained in most cases studied. Measurements were continued at intervals for a further 4 hours. In a real polyurethane foam the gas is carbon dioxide and a variety of other blowing agents. Air was used in this case for convenience as quantitative figures for the real situation were not required. The surface viscosity apparatus consisted of a circular knife-edged bob, 3 cm in radius, made of aluminum and suspended from an adjustable head b y a 0.0043 in. diameter phosphor bronze torsion wire 30 cm long. The solution was contained in a brass cup 7 cm in diameter, 1 cm deep, sitting in a recess on a leveling table. Oscillation was started by twisting the adjustable support head, and the amplitude was measured using a lamp and scale in conjunction with ~ small mirror attached to the wire chuck at the center of the bob. The zero rest position was noted on the scale after oscillations had ceased. The logarithmic decrement A, being the slope of the plot of the logarithm of the amplitude (in degrees) against the number of swings, was calculated from each series of amplitudes measured on either side of the zero position and the mean of the two values taken. About twelve measurable swings usually occurred. The apparatus was enclosed in a wood and Perspex box to protect it from draughts and mounted on a concrete pier to minimize interference from vibrations. The cup and bob were coated at various times with silicone rubber or Teflon which was baked to l l 0 ° C overnight in an oven. Tests on pure water surfaces showed that for a period of at least 8 hours no surface viscous material was leached out of these coatings; neither was there any atmospheric contamination
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION during this time. The bob was always lowered to a constant depth of immersion in the liquid by means of the fine adjustment on the support head. Sensitivity to depth of immersion was not critical. A movement of 0.1 cm vertically in the surface made a difference in the logarithmic decrement A of about 0.002, and since the bob could ahvays be placed to an accuracy of 0.01 cm by observation through a cathetometer errors from this source would certainly be less than 0.001, which is the accuracy to which A could be obtained. Measurements were made at room temperature 23°C 4- I°C. For pure water A0 decreases with increasing temperature at approximately 0.001 per °C rise. It was found that as long as both the clean water surface and the solution surface were studied at the same temperature a consistent temperature-invariant quantity A -- A0 was obtained, i.e., surface shear viscosity is not noticeably affected over a 2-degree range. For clean water /% at 23°C is 0.066. This was checked continually on clean water surfaces to ensure consistency of the apparatus, purity of the water, and freedom from later contamination. In all cases solutions were studied until /x - A0 became constant indicating complete surface aging. For the present system, if we insert the values of the constants the surface viscosity is given by u -- (A -- A0) X 1.02 X l0 -~ surface poises. The error in a is 4-0.001, and the error in A0 much less as it is the mean of very many determinations. The errors in T, I, R~, and R~ are insignificant compared to the error in A and thus the over-all lower limit of accuracy of the instrument is 10-4 surface poises. The period T was in fact found to be constant for the solvent and all solutions ( = 9.33 sec) as the equation demands. RESULTS AND DISCUSSION The simple polyol foam forming technique showed gross differences between the drainage of foams stabilized by branched copolymers (A, B, and C) and linear ABA copolymers (D to J).
49
Soon after collection of the foam three layers could be discerned, a liquid layer at the bottom, an intermediate "gas emulsion" layer, and a true foam layer at the top. Drainage was followed b y measuring the rise of the liquid/emulsion and emulsion/ foam interfaces with time until the foam/air interface fell, indicating collapse of the foam. This last interface normally started to fall about 40 min after foam formation for all three branched copolymers (A, B, and C) at concentrations between 0.25% and 2.00%. For the linear copolymers at 1.00 % the foam/air interface fell after about 15 rain. Copolymer G in particular was tested at 0.2% concentration, where its rate of lowering of surface tension was the same as copolymers A and B at 1.0 % and 0.4 %, respectively (at which concentrations they are adequate polyurethane foam stabilizers; see P a r t II). Collapse of the foam/ air interface still occurred about three times faster for copolymer G at this concentration than was the case for the branched copolymers. This rapid bursting of bubbles stabilized by the linear copolymers canno' then be due to poor surface elasticity. Moreover no polymerization or temperature variables arise in the model surfactant-polyol system. The discrepancy in bubble stability can be due only to a marked facilitation of gas diffusion between bubbles b y the linear copolymers or a smaller bulk or surface viscosity in comparison to the branched copolymers. In the gas diffusion studies in all cases a straight line could be drawn through the points on the r2/t plot. The permeability constant values ]c are given in Table II. For the branched copolymers k is essentially independent of both concentration and surface tension. The same is true for the linear copolymers, but the k value is much less than it is for the branched copolymers. The simple equation is fitted, and there is clearly an effect on gas diffusion apart from simple surface tension lowering. In fact the linear copolymers F, G, and H permit less gas transfer than the branched copolymers A, B, and C. In the absence of any thickness determinations no mechanism for this difference can be advanced. What can be said Journal of Colloid and Interface ,~ciecne, Vol. 27, No. i, M~y 1968
50
OWEN AND KENDRICK TABLE II BUBBLE PERMEABILITIES(k)
Copolymet no.
(wt. %)
Conc.
Surface tension (dynes
k (¢m sec -1)
A A B B C C F G H
0.28 1.87 0.26 1.87 0.27 1.91 1.00 0.15 0.15
22.5 21.0 22.5 21.2 23.5 22.0 21.0 23.0 21.0
0.0011:E0.0005 0.0011±0.0005 0.0012±0.0005 0.0011 :t:0 . 0005 0.0018±0.0005 0.0017±0.0005 0.00054-0.0005 0.0004±0.0005 0.0007±0.0005
crn_X )
about these results is that if reduced gas diffusion between bubbles was a major factor in preserving bubbles from bursting the linear copolymers would be more effective than the branched copolymers. The reverse is true. Moreover the magnitude of the gas diffusion effect is extremely small. The lifetime of a polyurethane foam before polymerization gels the mixture is about 2 rain. In such a time gas diffusion of the above magnitude for a bubble of radius 1 m m would account for a change in bubble size of the order of 10-4 to 10-5 cm, which is insignificant compared to the over-all rapid growth of the foam. Various solutions of the copolymers in the polyol LG 56 were prepared up to 2 % concentration by weight, and their viscosities measured. In no case was a difference of more than 2 % from the solvent noticed; neither was there any significant difference between the viscosities of the branched and linear copolymers that might explain why polyol surfactant foams using the linear copolymers collapsed three times faster than the branched copolymers. Figure 1 shows the surface viscosities at various concentrations up to 1% b y weight of the copolymers A, B, C, D, F, G, H, and I. Similar results were obtained with tripropylene glycol solutions. As a class the branched copolymers show at all concentrations a surface viscosity about twice that of the linear copolymers. This is a clear difference explaining the more rapid rate of bursting of polyol foams stabilized by linear copolymers. Copolymer B foams polyurethane adeJournal of Colloid and Interface Science, Vol. 27, No. 1, ]~ay 1968
quately down to 0.35% (see Part II of this series (3)), where ~ ~ 1.3 >< 10 -3 surface poises, and copolymer A foams adequately down to 0.6%, where ~ ~ 1.9 X 10-3 surface poises. We might thus suppose that if a copolymer has a surface viscosity of 1.3 X 10-3 surface poises or more, then if suitable surface elasticity properties are presumed it should adequately stabilize polyurethane foam. Copolymer C, which doer not suitably operate at any concentration, has a surface viscosity greater than 1.3 X 10-3 surface poises above 0.45% concentration. However, its suitable dynamic surface tension range (see Part II) is between 0.3 % and 0.4 %. There is no concentration at which optimum surface elasticities and viscosities coincide. The optimum foaming concentrations for the linear copolymers on surface elasticity considerations alone were D at 0.3%, G at 0.2%, and H at 0.15%. The surface viscosities at these concentrations are 0.6 X 10-3 surface poises, 0.5 X 10-3 surface poises, and 0.4 >< 10-3 surface poises, respectively. They are between two and three times too small. Copolymer F at 0.6%, although not at optimum surface elasticity, gave a better foam than the preceding three
2
__
~
_ _ 0.2
I 0.4
_
J . _ _ 0.6
Concentration
I 0.8
I 1.0
weight %
FIG. 1. Surface viscosities of t h e copolymers in aqueous solution as a funct,ion of c o n c e n t r a t i o n .
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION linear copolymers. At 0.6% its surface viscosity is 1.0 X 10-3 surface poises, whereas at 1%, where it gave a better foam again, though still not satisfactory, its surface viscosity is 1.2 X 10-3 surface poises-very close to the presumed minimum acceptable value. There is no difference in surface elasticity between 0.6% and 1.0% for copolymer F, and the improvement in foam performance can be directly ascribed to the increase in surface viscosity. The only surface viscosity studies on polysiloxanes reported are the work of Jarvis (11). He finds no detectable surface viscosity with either an oscillating knifeedged bob or a canal viscometer for various ethoxy and trimethyl end-blocked linear polydimethylsiloxanes varying from a fourunit polymer to a fluid of over 100,000 molecular weight. This is as expected as the silicones are known to have low cohesive interactions. The surface viscosity of the eopolymers must come from the polyether portion. It probably arises from the polyether chains being associated together and with the solvent through hydrogen bonding, i.e., between the hydroxyl groups of water and the ether linkages in the polyether
15
%
!/i
" 10
,,,n 0.5~-
J, ~ ,~
o Li~e,3r c0P01ymers X Bronched copolymers
I
I OI
I [ I I ~__1 0.2 0.5 0 4 05 06 0.7 Concentration weight % of polyether
I Q8
FIG. 2. Surface viscosities of the copolymers as a function of actual amount of polyether present.
51
chain. In the polyurethane foam mix such hydrogen bonding will occur between the terminal hydroxyl groups of the polyol and the ether linkages in the polyether portion of the copolymer. Of course these hydroxyl groups react with the isocyanate in the polyurethane forming reaction but until gelation occurs, when the foam is sufficiently solid not to need surface viscosity stabilization, there will be hydroxyl groups available for hydrogen bonding with the additive. For the linear copolymers at each given concentration those with the greater amount of polyether have the higher surface viscosity. In Fig. 2 we see that a plot of surface viscosity against the actual amount of polyether present gives a single curve embracing all the points within their margin of error with the exception of just two points. This supports the view that the surface viscosity arises from the polyether portion alone. The branched copolymers do not lie on this curve at all. The polyether chains are chemically linked together through the polysiloxane chain as well as being held physically together by cohesive interaction and hydrogen bonding through the solvent. The greater degree of branching means more than two polyether chains per molecule as is the case for the linear copolymers, and this increase in number of polyether chains chemically bound together in the branched structures must account for their extra resistance to two-dimensional flow. It is unlikely that branched polysiloxanes have any surface viscosity in their own right. We consider the variations in polyol foams at room temperature to be due to surface viscosity differences, and suggest that this is a likely reason for the differences we have observed in polyurethane foam stabilizing effects. This is of course a large extrapolation, and other variables may be involved, particularly differences in temperature coefficients of the various surface properties of linear and branched polysiloxane-polyether copolymers. REFERENCES 1. BOUDREAU, R. J., Mod. Plastics 44, 133 (1967). 2. KENDRICK, T. C., KINGSTON, B. M., LLOYD, Journal of CoUoid and Interface Science. Vol. 27, No. I, May 1968
52
3. 4. 5. 6.
OWEN AND K E N D R I C K N. C., AND OWEN, M. J., Part I, J. Colloid and Interface Sci. 24, 135 (1967). OWEN, M. J., KENDRICK, T. C., KINGSTON, B. M., AND LLOYD,N. C., Part II, J. Colloid and Interface Sci. 24, 141 (1967). BaOWN, A. G., TI-IUMAN,W. C., AND McBAIN, J. W., J. Colloid Sci. 8, 508 (1953). DE VnIES, A. J., Rubber Chem. Technol. 31, 1142 (1958). PRINCEN, H. M., AND MASON, S. G., J. Colloid Sei. 20, 353 (1965). PRINCES, H. M., OVERBEEK, J. TH. G., AND
Journal ef Colloidand Interface Science, %ol.27, No. 1, May 1968
7.
8. 9. 10. 11.
MASON, S. G., J. Colloid and Interface Sci. 24, 125 (1967). MYSELS, K. J., SHINODA, K., AND FI~ANKEL, S. "Soap Films." Pergamon Press, London, 1959. VAN DEN TEMPEL, IV[., "Surfuce Chemistry," p. 306. Munksgaard, Copenhagen, 1965. JOLY, M., Recent Progr. Surface Sci. 1, 1 (1964). TSCHOEGL, T. W., Kolloid Z. 181, 19 (1962). JAaVlS, N. L., J. Phys. Chem. 70, 3027 (1966).
Received September 12, 1967 Gas diffusion between foam bubbles is shown to be an insignificant effect in polyurethane foam formation. The fact that at equivalent surface elasticities linear polysiloxane-potyether block copolymers are less effective polyol foam stabilizers thai1 branched polysiloxane-polyether block copolymers is traced to the differences in surface viscosity exhibited by these copolymers. This difference may be the reason for their different polyurethane foam stabilizing powers. INTRODUCTION
than 22.3 dynes c m - and had a rate of surface tension lowering lying in an opt i m u m range. The details of these studies are given in Parts I and I I , but one of the main conclusions was t h a t ABA linear copolymers with two polyether segments, even when used at selected concentrations where they exhibited o p t i m u m rates and extents of surface tension lowering, did not adequately stabilize polyurethane foam. Furthermore, as will be seen in this paper, the linear ABA copolymers do not stabilize polyol foams as well as do the branched copolymers. The effects of these copolymers on gas diffusion between bubbles and surface viscosity have been investigated as possible reasons for this difference. As we have used the Brown, T h u m a n , and McBain single bubble technique for studying gas diffusion (4) we have used their simple equation relating the bubble radius r to the time t:
The precise role of the cell size control agents, known as foam stabilizers, used in polyurethane foam formation has not yet been fully elucidated. B oudreau (1) has given a review of the theory of surfactant effects in such a system and has reported on surfactant evaluation techniques. This present paper continues our a t t e m p t to determine the practical relevancy of the several effects which are thought to occur. We have already presented our ideas on the roles of equilibrium and dynamic surface tension in surface elasticity (2, 3), and we now report a study of foam drainage, surface viscosity, and gas diffusion between foam bubbles. The type of surfactant normally used to stabilize polyurethane foam during its formation is a polysiloxane-polyether copolymer. I t was our hope that parallels could be drawn between actual foam stabilizing ability and the behavior of these compounds in LG 56, a polyol used in polyurethane foam production. I t was concluded from studies in the surfactant/polyol model system that branched polysiloxane-polyether copolymers (containing more than two polyether segments) functioned adequately as polyurethane foam stabilizers if they lowered the surface tension of the polyol LG 56 to less
where ro is the radius at time t = to, P the atmospheric pressure, ~ the surface tension and k the film permeability constant. A similar equation has been given b y De Vries (5) for any particular bubble in a
Journal of Colloid and Interface ,Science, Vol. 27, No. 1, May 1968
46
r~ -
4a r02 = -- - - k l , P
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION foam. Princen and Mason (6) have shown that the theory used to obtain the MeBain equation was inexact and have presented a modified theory. We have used the simple McBain equation because we are concerned only with comparisons between various surfaetants, not with the detailed nature of the gas transport through the film. Any more sophisticated analysis would require more parameters, particularly film thickness, which are not known in this ease. VISCOSITY EFFECTS The simplest view of the mechanism of drainage of material out of a foam film is that it is the result of two forces, capillary suction and gravity. At the point where two or more foam films between gas bubbles meet the surfaces have negative curvature wth respect to the liquid phase, and hence the pressure there is lower than in the films themselves (on the assumption that these have plane interfaces). There will consequently be a capillary suction drawing material out of the fihn, often against gravity. However, gravity will ensure that the net flow is downwards out of the foam. The work of Mysels, Shinoda, and Frankel (7) has shown that the situation is rather more involved. Not only are films sucked into the borders between them but thinner films are sucked out as well. This phenomenon of m a r s h a l regeneration must involve both the bulk phase between the two faces of the fihn and these surface layers themselves. An increase in either or both of the bulk or surface viscosities will retard the flow of material in the film and help to stabilize the foam. A description of the flow properties of an interface requires two viscosity coefficients. One is the usual two-dimensional analog of bulk shear viscosity at constant surface are,~ and the other is the surface dilationM viscosity (8) associated with changes of area in the interface. This latter quantity is related to the rate of lowering of surface tension. We are concerned here with the surface shear viscosity as we know that the difference between the linear and branched copolymers does not lie in their dynamic
47
surface tension (see Part II). Moreover, we have observed that in polyurethane foams stabilized by our ABA linear copolymers the foam rose in a controlled manner but coarsened rapidly at the top of the foam rise. There is thus a period of time before gelation where the surface viscosity effect at relatively constant area may be important. There is no completely suitable method for measuring the surface viscosities of soluble surface layels (9). The canal method is theoretically sound but cannot handle slow surface aging solutions as we have here, nor can non-Newtonian surface behavior be readily detected--a drawback shared by the comparative viscous traction technique. All oscillating and rotating devices employ equations for the surface viscosity which assume that there is no frictional drag exerted on the surface by underlying layers of liquid. This is a most unlikely supposition for a foaming system. Nevertheless, we have, chosen to use an oscillating surface viscometer because of its ability to detect nonNewtonian surface flow. The surface viscosity u in surface poises is obtained from the usual equation
2303 ( R : where I is the polar moment of inertia of the bob, T the period, R~ the radius of the cup, R~ the radius of the bob, A the logarithmic decrement on the solution surface, and A0 the logarithmic decrement on a clean solvent surface. Tsehoegi (10) has given a good derivation of this equation. EXPERIMENTAL METItODS Except for the surface viscosity work these studies were carried out with the polyol LG 56 as before to give a model system, free from polymerization variables, yet closely related to the real situation. The surfactants were polysiloxane-polyether block copolymers of the type usually used to assist polyurethane foam formation. T h e y have been fully described in a previous paper (2), and a summary of their constitution is given in Table I. Copolymers A, B, and C are all branched copolymers containing more than two polyether chains per molecule, Journal of Colloid and Interface Science, Vol. 27, No. 1, May 1968
48
OWEN AND KENDRICK TABLE I ~:)ROPERTIES OF THE COPOLYMERS
Copoly- Polyetber portion -~n Wt. %
met no.
A B C D E F G H I J
1,520 2,500 2,500 2,200 2,200 2,200 2,200 2,200 2,200 2,200
29 25 22 34 17 27.4 37.8 39.4 51.5 62.5
Po]ysiloxane portion Mn
1,960 5,950 5,200 2,670 910 1,680 2,690 2,880 4,700 7,400
Mw
M~,
970 1,716 2,550 3,100=t=300 4,1004-500 7,100=E500
See Part I for details of measurements. whereas copolymers D to J are all linear ABA copolymers containing two polyether chains per molecule. With the construction of a sufficiently massive surface viscometer bob to give oscillations in the polyol surface significant differences between pure polyol and solution surfaces could not be observed. Greater changes in the rate of damping of the bob could be produced b y minor variations in the depth of immersion (<0.01 cm) than were apparent between various solutions. This does not mean that surface viscosity effects will not be important in thin films of course. Moreover, in the actual polyurethane system the region of interest is after the initial foam rise before gelation, where temperatures are well in excess of 100°C and the films may be very mobile. We chose to work with a less viscous solvent at room temperature rather than with the polyol at elevated temperature. Consequently the results presented here are for aqueous solution; similar results were obtained in tripropylene glycol. Foams were formed b y blowing compressed air into the polyol and surfactant mixture for a fixed time (30 secs) and then sucking the mixture through a number 4 glass sinter into a graduated cylinder. For the gas diffusion work Brown, T h u m a n and McBain's technique and apparatus were exactly ~dopted. Although it is a single bubble rather than a real foam study it is the most straightforward means of obtaining Journal of Colloid and Interface Science, Vol. 27, No. 1, May 1968
direct comparisons of the effect of the various surfaetant copolymers. Our studies were carried out in a thermostatted bath at 25°C -4- 0.1°C using the polyol L G 56 as the solvent. All solutions were left in the apparatus for an hour to ensure temperature equilibration. The bubble was then formed and measurements of the radius were commenced after a least a further 2 hours. This was sufficiently long for surface tension equilibrium to have been attained in most cases studied. Measurements were continued at intervals for a further 4 hours. In a real polyurethane foam the gas is carbon dioxide and a variety of other blowing agents. Air was used in this case for convenience as quantitative figures for the real situation were not required. The surface viscosity apparatus consisted of a circular knife-edged bob, 3 cm in radius, made of aluminum and suspended from an adjustable head b y a 0.0043 in. diameter phosphor bronze torsion wire 30 cm long. The solution was contained in a brass cup 7 cm in diameter, 1 cm deep, sitting in a recess on a leveling table. Oscillation was started by twisting the adjustable support head, and the amplitude was measured using a lamp and scale in conjunction with ~ small mirror attached to the wire chuck at the center of the bob. The zero rest position was noted on the scale after oscillations had ceased. The logarithmic decrement A, being the slope of the plot of the logarithm of the amplitude (in degrees) against the number of swings, was calculated from each series of amplitudes measured on either side of the zero position and the mean of the two values taken. About twelve measurable swings usually occurred. The apparatus was enclosed in a wood and Perspex box to protect it from draughts and mounted on a concrete pier to minimize interference from vibrations. The cup and bob were coated at various times with silicone rubber or Teflon which was baked to l l 0 ° C overnight in an oven. Tests on pure water surfaces showed that for a period of at least 8 hours no surface viscous material was leached out of these coatings; neither was there any atmospheric contamination
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION during this time. The bob was always lowered to a constant depth of immersion in the liquid by means of the fine adjustment on the support head. Sensitivity to depth of immersion was not critical. A movement of 0.1 cm vertically in the surface made a difference in the logarithmic decrement A of about 0.002, and since the bob could ahvays be placed to an accuracy of 0.01 cm by observation through a cathetometer errors from this source would certainly be less than 0.001, which is the accuracy to which A could be obtained. Measurements were made at room temperature 23°C 4- I°C. For pure water A0 decreases with increasing temperature at approximately 0.001 per °C rise. It was found that as long as both the clean water surface and the solution surface were studied at the same temperature a consistent temperature-invariant quantity A -- A0 was obtained, i.e., surface shear viscosity is not noticeably affected over a 2-degree range. For clean water /% at 23°C is 0.066. This was checked continually on clean water surfaces to ensure consistency of the apparatus, purity of the water, and freedom from later contamination. In all cases solutions were studied until /x - A0 became constant indicating complete surface aging. For the present system, if we insert the values of the constants the surface viscosity is given by u -- (A -- A0) X 1.02 X l0 -~ surface poises. The error in a is 4-0.001, and the error in A0 much less as it is the mean of very many determinations. The errors in T, I, R~, and R~ are insignificant compared to the error in A and thus the over-all lower limit of accuracy of the instrument is 10-4 surface poises. The period T was in fact found to be constant for the solvent and all solutions ( = 9.33 sec) as the equation demands. RESULTS AND DISCUSSION The simple polyol foam forming technique showed gross differences between the drainage of foams stabilized by branched copolymers (A, B, and C) and linear ABA copolymers (D to J).
49
Soon after collection of the foam three layers could be discerned, a liquid layer at the bottom, an intermediate "gas emulsion" layer, and a true foam layer at the top. Drainage was followed b y measuring the rise of the liquid/emulsion and emulsion/ foam interfaces with time until the foam/air interface fell, indicating collapse of the foam. This last interface normally started to fall about 40 min after foam formation for all three branched copolymers (A, B, and C) at concentrations between 0.25% and 2.00%. For the linear copolymers at 1.00 % the foam/air interface fell after about 15 rain. Copolymer G in particular was tested at 0.2% concentration, where its rate of lowering of surface tension was the same as copolymers A and B at 1.0 % and 0.4 %, respectively (at which concentrations they are adequate polyurethane foam stabilizers; see P a r t II). Collapse of the foam/ air interface still occurred about three times faster for copolymer G at this concentration than was the case for the branched copolymers. This rapid bursting of bubbles stabilized by the linear copolymers canno' then be due to poor surface elasticity. Moreover no polymerization or temperature variables arise in the model surfactant-polyol system. The discrepancy in bubble stability can be due only to a marked facilitation of gas diffusion between bubbles b y the linear copolymers or a smaller bulk or surface viscosity in comparison to the branched copolymers. In the gas diffusion studies in all cases a straight line could be drawn through the points on the r2/t plot. The permeability constant values ]c are given in Table II. For the branched copolymers k is essentially independent of both concentration and surface tension. The same is true for the linear copolymers, but the k value is much less than it is for the branched copolymers. The simple equation is fitted, and there is clearly an effect on gas diffusion apart from simple surface tension lowering. In fact the linear copolymers F, G, and H permit less gas transfer than the branched copolymers A, B, and C. In the absence of any thickness determinations no mechanism for this difference can be advanced. What can be said Journal of Colloid and Interface ,~ciecne, Vol. 27, No. i, M~y 1968
50
OWEN AND KENDRICK TABLE II BUBBLE PERMEABILITIES(k)
Copolymet no.
(wt. %)
Conc.
Surface tension (dynes
k (¢m sec -1)
A A B B C C F G H
0.28 1.87 0.26 1.87 0.27 1.91 1.00 0.15 0.15
22.5 21.0 22.5 21.2 23.5 22.0 21.0 23.0 21.0
0.0011:E0.0005 0.0011±0.0005 0.0012±0.0005 0.0011 :t:0 . 0005 0.0018±0.0005 0.0017±0.0005 0.00054-0.0005 0.0004±0.0005 0.0007±0.0005
crn_X )
about these results is that if reduced gas diffusion between bubbles was a major factor in preserving bubbles from bursting the linear copolymers would be more effective than the branched copolymers. The reverse is true. Moreover the magnitude of the gas diffusion effect is extremely small. The lifetime of a polyurethane foam before polymerization gels the mixture is about 2 rain. In such a time gas diffusion of the above magnitude for a bubble of radius 1 m m would account for a change in bubble size of the order of 10-4 to 10-5 cm, which is insignificant compared to the over-all rapid growth of the foam. Various solutions of the copolymers in the polyol LG 56 were prepared up to 2 % concentration by weight, and their viscosities measured. In no case was a difference of more than 2 % from the solvent noticed; neither was there any significant difference between the viscosities of the branched and linear copolymers that might explain why polyol surfactant foams using the linear copolymers collapsed three times faster than the branched copolymers. Figure 1 shows the surface viscosities at various concentrations up to 1% b y weight of the copolymers A, B, C, D, F, G, H, and I. Similar results were obtained with tripropylene glycol solutions. As a class the branched copolymers show at all concentrations a surface viscosity about twice that of the linear copolymers. This is a clear difference explaining the more rapid rate of bursting of polyol foams stabilized by linear copolymers. Copolymer B foams polyurethane adeJournal of Colloid and Interface Science, Vol. 27, No. 1, ]~ay 1968
quately down to 0.35% (see Part II of this series (3)), where ~ ~ 1.3 >< 10 -3 surface poises, and copolymer A foams adequately down to 0.6%, where ~ ~ 1.9 X 10-3 surface poises. We might thus suppose that if a copolymer has a surface viscosity of 1.3 X 10-3 surface poises or more, then if suitable surface elasticity properties are presumed it should adequately stabilize polyurethane foam. Copolymer C, which doer not suitably operate at any concentration, has a surface viscosity greater than 1.3 X 10-3 surface poises above 0.45% concentration. However, its suitable dynamic surface tension range (see Part II) is between 0.3 % and 0.4 %. There is no concentration at which optimum surface elasticities and viscosities coincide. The optimum foaming concentrations for the linear copolymers on surface elasticity considerations alone were D at 0.3%, G at 0.2%, and H at 0.15%. The surface viscosities at these concentrations are 0.6 X 10-3 surface poises, 0.5 X 10-3 surface poises, and 0.4 >< 10-3 surface poises, respectively. They are between two and three times too small. Copolymer F at 0.6%, although not at optimum surface elasticity, gave a better foam than the preceding three
2
__
~
_ _ 0.2
I 0.4
_
J . _ _ 0.6
Concentration
I 0.8
I 1.0
weight %
FIG. 1. Surface viscosities of t h e copolymers in aqueous solution as a funct,ion of c o n c e n t r a t i o n .
SURFACE CHEMISTRY OF POLYURETHANE FOAM FORMATION linear copolymers. At 0.6% its surface viscosity is 1.0 X 10-3 surface poises, whereas at 1%, where it gave a better foam again, though still not satisfactory, its surface viscosity is 1.2 X 10-3 surface poises-very close to the presumed minimum acceptable value. There is no difference in surface elasticity between 0.6% and 1.0% for copolymer F, and the improvement in foam performance can be directly ascribed to the increase in surface viscosity. The only surface viscosity studies on polysiloxanes reported are the work of Jarvis (11). He finds no detectable surface viscosity with either an oscillating knifeedged bob or a canal viscometer for various ethoxy and trimethyl end-blocked linear polydimethylsiloxanes varying from a fourunit polymer to a fluid of over 100,000 molecular weight. This is as expected as the silicones are known to have low cohesive interactions. The surface viscosity of the eopolymers must come from the polyether portion. It probably arises from the polyether chains being associated together and with the solvent through hydrogen bonding, i.e., between the hydroxyl groups of water and the ether linkages in the polyether
15
%
!/i
" 10
,,,n 0.5~-
J, ~ ,~
o Li~e,3r c0P01ymers X Bronched copolymers
I
I OI
I [ I I ~__1 0.2 0.5 0 4 05 06 0.7 Concentration weight % of polyether
I Q8
FIG. 2. Surface viscosities of the copolymers as a function of actual amount of polyether present.
51
chain. In the polyurethane foam mix such hydrogen bonding will occur between the terminal hydroxyl groups of the polyol and the ether linkages in the polyether portion of the copolymer. Of course these hydroxyl groups react with the isocyanate in the polyurethane forming reaction but until gelation occurs, when the foam is sufficiently solid not to need surface viscosity stabilization, there will be hydroxyl groups available for hydrogen bonding with the additive. For the linear copolymers at each given concentration those with the greater amount of polyether have the higher surface viscosity. In Fig. 2 we see that a plot of surface viscosity against the actual amount of polyether present gives a single curve embracing all the points within their margin of error with the exception of just two points. This supports the view that the surface viscosity arises from the polyether portion alone. The branched copolymers do not lie on this curve at all. The polyether chains are chemically linked together through the polysiloxane chain as well as being held physically together by cohesive interaction and hydrogen bonding through the solvent. The greater degree of branching means more than two polyether chains per molecule as is the case for the linear copolymers, and this increase in number of polyether chains chemically bound together in the branched structures must account for their extra resistance to two-dimensional flow. It is unlikely that branched polysiloxanes have any surface viscosity in their own right. We consider the variations in polyol foams at room temperature to be due to surface viscosity differences, and suggest that this is a likely reason for the differences we have observed in polyurethane foam stabilizing effects. This is of course a large extrapolation, and other variables may be involved, particularly differences in temperature coefficients of the various surface properties of linear and branched polysiloxane-polyether copolymers. REFERENCES 1. BOUDREAU, R. J., Mod. Plastics 44, 133 (1967). 2. KENDRICK, T. C., KINGSTON, B. M., LLOYD, Journal of CoUoid and Interface Science. Vol. 27, No. I, May 1968
52
3. 4. 5. 6.
OWEN AND K E N D R I C K N. C., AND OWEN, M. J., Part I, J. Colloid and Interface Sci. 24, 135 (1967). OWEN, M. J., KENDRICK, T. C., KINGSTON, B. M., AND LLOYD,N. C., Part II, J. Colloid and Interface Sci. 24, 141 (1967). BaOWN, A. G., TI-IUMAN,W. C., AND McBAIN, J. W., J. Colloid Sci. 8, 508 (1953). DE VnIES, A. J., Rubber Chem. Technol. 31, 1142 (1958). PRINCEN, H. M., AND MASON, S. G., J. Colloid Sei. 20, 353 (1965). PRINCES, H. M., OVERBEEK, J. TH. G., AND
Journal ef Colloidand Interface Science, %ol.27, No. 1, May 1968
7.
8. 9. 10. 11.
MASON, S. G., J. Colloid and Interface Sci. 24, 125 (1967). MYSELS, K. J., SHINODA, K., AND FI~ANKEL, S. "Soap Films." Pergamon Press, London, 1959. VAN DEN TEMPEL, IV[., "Surfuce Chemistry," p. 306. Munksgaard, Copenhagen, 1965. JOLY, M., Recent Progr. Surface Sci. 1, 1 (1964). TSCHOEGL, T. W., Kolloid Z. 181, 19 (1962). JAaVlS, N. L., J. Phys. Chem. 70, 3027 (1966).