[ACS Symposium Series] Polymeric Foams Volume 669 (Science and Technology) || Effect of Silicone Surfactant on Air Flow of Flexible Polyurethane Foams

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<ul><li><p>Chapter 9 </p><p>Effect of Silicone Surfactant on Air Flow of Flexible Polyurethane Foams </p><p>Xiaodong D. Zhang, Christopher W. Macosko, and H . T. Davis </p><p>Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, MN 55455 </p><p>Grafted copolymers which consist of a polydimethylsiloxane backbone and poly(ethylene oxide-co-propylene oxide) pendant groups are used as surfactant to stablize the bubbles in flexible polyurethane foam. The final properties such as air flow through the foam are significantly affected by the structure of the silicone surfactant used. Foams in which only the surfactant was changed provide a relationship between surfactant structure and foam air flow. Air flow of the foam increased as the polysiloxane backbone length of the surfactant decreased. Air flow of the foam increased as the polyether branch frequency of the surfactant increased. The drainage rate of cell windows and the foam top skin breaking time are shown to have significant effect on percentage of open cell windows. Both cell window drainage rate and top skin blow-off time are affected by silicone surfactant structure. Basic surface chemistry measurements such as surface tension, dynamic surface tension and dynamic light scattering method are used to characterize the surfactant properties. </p><p>The addition polymerization reaction of diisocyanates with alcohols was discovered by Professor Dr. Otto Bayer and co-workers in 1937 (V). This discovery set up the fundamental chemistry for the polyurethane industry. Polyurethanes are characterized by their repeating urethane linkages (-NH-CO-0) formed by the reaction between an isocyanate group (-N=C=0) and a hydroxyl group (-OH). Depending on the starting material, a range of products from soft flexible foam through semirigid and rigid foam to molded foam or elastomers can be achieved. Of all the polyurethane products, flexible polyurethane foam has the largest production and is most widely used. </p><p>Silicone-polyether graft block copolymers are used as surfactants in flexible polyurethane foaming systems (2, 3). In the absence of these surfactants, a foaming urethane will experience catastrophic coalescence and eventually cause foam collapse. These surfactants are efficiently adsorbed at the air/liquid interface and thus may have great impact on cell opening/window rupture in polyurethane foam. Percentage of ruptured cell windows in cured foam is shown to be affected by the silicone surfactant structure in the foam formulation. In other words, the silicone surfactant structure has </p><p>130 1997 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>9. Z H A N G E T A L . Air Flow in Flexible Polyurethane Foams 131 </p><p>a great impact on openness of the final foam product, which is directly related to the air flow of the foam. Many physical properties of a flexible polyurethane foam are highly affected by its air flow (4). However, air flow is a hard property to control in industry because the effect of surfactant is not elucidated. Understanding the role of silicone surfactant during foaming, especially its effect on air flow of the final foam product can be helpful in designing new surfactant and obtaining flexible polyurethane foams with desired physical properties. </p><p>Designed Surfactant Series and Resulting Foam A i r Flow </p><p>A series of surfactants was synthesized to vary surfactant backbone length and polyether branch percentage. The structure of the surfactants is shown in Figure 1. The nomenclature of the structure is also shown in Figure 1. (D+D') represents the length of the siloxane backbone, DV(D+D') represents the percentage of polyether branches. Foams were made with these surfactants. Formulation of the foams is listed in Table I. </p><p>Table I. Formulations for air flow test Unit: pphpa </p><p>Formulation weight (gram) Voranol 3137b 100 water 4.0 Dabco33LVc 0.30 Dabco T-9 d 0.20 Silicone Surfactants f 1.0 Voranate T-SOHndex 110 </p><p>aParts per hundred parts polyol (by weight). b1000 equivalent weight triol containing 13% ethylene oxide, 87% propylene oxide and all secondary hydroxyl groups (Dow Chemical). c33% triethylenediamine in dipropylene glycol (Air Products). Stabilized stannous octoate (Air Products). e A n 80:20 mixture of 2,4- and 2,6-toluene diisocyanate (Dow Chemical) Structures of the surfactants are described in Figure 1. SOURCE: Adapted from ref. 6. </p><p>Air flow measurement is used to evaluate the openness of the foam, since a linear relationship between the two has been found (Figure 2). A cell window is considered open if the window is totally missing. If there are there pin-holes on a window, then it is considered partially open. Air flow of the foams was measured according to A S T M D 3574. Foam specimens 50x50x25 mm in size were cut out from the top and bottom part of the buns in the buckets with a 50x50 surface perpendicular to blow direction. The specimens were prepared so that the upper surface of the top specimens was leveled with the shoulders of the buns and the lower surface of the bottom specimen was 25 mm distant from the bottom. In Table II, foam air flow as well as cell opening time are listed for foams made with the designed surfactants. </p><p>An increase in polyether branch percentage tends to increase air flow (Figure 3a). The surfactants with the smallest polyether branch percentage D7(D+D') made the foam collapse or produced foam with lower air flow. Silicone surfactants with very low percentage polyether branches act as defoamers. There is an important balance balance between the defoamer and the silicone foam stabilizer with a higher percentage of polyether branches. Actually, the foam did not collapse when the concentration of </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>132 POLYMERIC FOAMS </p><p>p i t </p></li><li><p>Z H A N G E T A L . Air Flow in Flexible Polyurethane Foams </p><p>(a) 2 1 1 1 l 1 1 1 l 1 1 1 1 1 1 l 1 1 1 </p><p>1.5 Air flow (ftVrain) </p><p>1 </p><p>0.5 l </p><p>M r -I I I , 1 l </p><p>0.06 0.08 0.1 0.12 0.14 0.16 D7(D+D') polyether branch frequecy </p><p>(b) 2 </p><p>1.5 Air flow (ftVmin) </p><p>0.5 U </p><p> 11"" I 1 1" 1" I" 1 i 1 1 I ' I " '"" I I I </p><p>0 1 I I I I I t I I I 50 100 </p><p>D+D' (siloxane backbone length) 150 </p><p>Figure 3(a). Effect of polyether percentage of surfactant on foam air flow. D+Df=60 3(b). Effect of siloxane backbone length of surfactant on foam air flow. D7(D+D*)=0.015 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>134 POLYMERIC FOAMS </p><p>MDsD'sM type surfactant was increased to 2 pphp. The reason the foam had lower air flow is not clear. </p><p>In the series of D7(D+D') = 0.15, larger D+D' (longer siloxane chain) produced lower air flow (Figure 3b). This result may be caused by the slower drainage of film containing longer siloxane chain surfactant, since a large surfactant tends to give a higher surface viscosity (5). Slower drainage causes thicker films and will lead to fewer open windows (6). </p><p>Table II. Time to internal cell opening (to), blow-off time (ti, time at hole formation on top skin of the foam bun) final foam bun height and airflow with different surfactant structures </p><p>Surfactant to (sec) ti(sec) Foam height Air Flow (CFM) (mm) Top / Bottom </p><p>MD54D'6M - 79 202 1.3 / 0.37 M D 5 1 D 9 M 75 77 177 2.2 / 1.9 MD95D' 5 M - 75 191 0.33 / 0.33 MD9oD'ifjM 75 79 215 1.8/0.4 M D 8 5 D ' 1 5 M - 80 209 1.8/.S8 M D 1 1 9 D 2 1 M 76 85 220 1.5/0.28 </p><p>SOURCE: Adapted from ref. 6. </p><p>The silicone surfactant structure has a great impact on many factors in foaming urethane. Some of these factors such as drainage rate of the cell windows and top skin strength of the foam bun are believed to have a significant effect on the cell opening event. Drainage rate will determine the thickness of cell windows at onset of cell opening. Top skin strength can control the visual blow-off time. Cell opening event is shown to be a continuous process between the onset of cell opening and the visual blow-off rather than a sudden event. Both drainage and top skin strength are affected by the surfactants used. These effects will be discussed in the next two sections. </p><p>Effect of Cell Window Drainage </p><p>During foaming, the bubbles initially introduced by mechanical mixing will grow. As the volume fraction of the gas bubbles exceeds 74%, the spherical bubbles will begin to distort into multisided polyhedra and cell windows are formed. As the reaction proceeds, the cell windows will get thinner by drainage and by the biaxial extension caused by bubble expansion. Due to capillary forces, the liquid pressure inside the Plateau borders will be lower than in the cell windows (Figure 4). This pressure difference wiilxause cell window thinning by sucking liquid from the windows into the struts. The silicone surfactant added can retard cell window drainage. Different surfactant structures will have different abilities to retard drainage, resulting in a distribution of different cell window thickness at time of cell opening. </p><p>Theory of thin liquid film drainage Liquid in thin films will drain into the Plateau border by the Plateau border suction (Figure 4). For films with a partially mobile surface, drainage will cause movement of the surface layer and a surfactant surface concentration gradient will be generated. The surfactant surface concentration gradient will cause a surface tension gradient along the surface that will retard the film drainage. This effect is called Gibbs-Marangoni effect and is reviewed by Scriven and Sternling (7). The surface tension gradient can be affected by the diffusion of the surfactant along the surface as well as the diffusion of the surfactant from the bulk. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>9. ZHANG ET AL. Air Flow in Flexible Polyurethane Foams 135 </p><p>Thus the surface rheological properties of the film such as surface viscosity becomes important since they will affect the surface diffusion of the surfactant. The drainage can be obtained by solving the Navier-Stokes equations under the lubrication approximation accounting for the mobile surface (8, 9). </p><p>dh h*Ap dt 24rj/r </p><p>(6n+ntk2HMh)JM tl(6rj + 6 M + nsk?Mh)% Jx(4) </p><p>(1) </p><p>where h is film thickness, is bulk viscosity, R is radius of the film and is the capillary pressure; J 0 , J, and J 2 are Bessel functions; eigenvalues are solutions of equations J 0 (^), and k n =Xn/R. The surface mobility of the film is expressed by surface viscosity 8 and the mobility parameter M . The mobility parameter M compares the contribution of bulk and surface diffusion to the Marangoni effect and is defined as the following (8, 9): </p><p>where D and D s are the bulk and surface diffusion coefficients of the surfactant respectively. C is the surfactant concentration in the bulk; is the surface tension and is the surface excess concentration of the surfactant on the air/liquid interface. </p><p>The analysis for drainage rate in a thin liquid film with constant area is shown in the above section. In polyurethane foaming system, the area of the cell window increases as bubbles grow. Radius and thickness of the film are changing due to the bubble expansion, making drainage calculation even more complicated. Although drainage rate at any instant can be estimated by equation 1 and this relation can be integrated with respect to time to predict film thickness at cell opening time, the parameters in equation 1 are difficult to obtain during foam expansion. </p><p>However, equation 1 does tell us what factors contribute to film drainage. In general, the surfactant changes the drainage rate of thin liquid film by affecting the surface condition of the air/liquid interface. Surfactants with different structure will give different surface conditions. Dynamic surface tension, surfactant diffusivity in bulk solution and on surface of a thin film, surface coverage of surfactant as weU as surface rheology of the thin film will be affected by the surfactant structure. Our previous work showed that the silicone surfactant does not affect reaction kinetics (6). So the bulk viscosity of the liquid material during foaming should be the same for foams made with different surfactants. Since liquid viscosity profiles are the same for formulations with different surfactants, drainage rate should be mainly affected by surfactant surface coverage and surface viscoelasticity (10, 11). Surfactant surface coverage is important since it can determine surface tension of the films during foaming and change surface viscosity as well. Surface viscoelasticity is important for surface flow of surfactant and boundary conditions for drainage flow. As foam expands, cell windows will expand and more surface is generated. Surfactant wil l move to the liquid/gas interface during the expansion. Surfactants can move from the surface along the struts into surface of cell window driven by the surface tension gradient. Surface </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>136 POLYMERIC FOAMS </p><p>Plateau's Border (strut) </p><p>Cell Window </p><p>Figure 4. Diagram of drainage in a cell window. </p><p>32-</p><p>B 30-</p><p>1 I * 28-B </p><p>wm </p><p>5 </p><p>fi </p><p> U S3 m </p><p>26-</p><p>24-</p><p>22-</p><p>20-</p><p> 1% MD 5 1 D f g M </p><p>1% M D n 9 D f 2 1 M </p><p>+ 1% MD D1 M 54 6 </p><p> V 1% MD 5 7 D'3 M </p><p>+ " 1% MD o e D ' M </p><p>95 5 </p><p>V + + </p><p>V </p><p>+ </p><p>V 4, </p><p>+ + + </p><p>^ V A V </p><p> 5 10 5 2 0 2 5 time(s) </p><p>Figure 5. Dynamic surface tension of different surfactant solutions. </p><p>3 0 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>9</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>9. Z H A N G E T A L . Air Flow in Flexible Polyurethane Foams 137 </p><p>viscoelasticity can affect the surface flow rate of the surfactant. Both surface flow rate of surfactant and drainage flow rate will be aff...</p></li></ul>

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