Chapter 9
Effect of Silicone Surfactant on Air Flow of Flexible Polyurethane Foams
Downloaded by NORTH CAROLINA STATE UNIV on August 20, 2012 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0669.ch009
Xiaodong D. Zhang, Christopher W . Macosko, and H . T. Davis Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, MN 55455
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. A i r 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.
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. 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 130
© 1997 American Chemical Society
In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Air Flow in Flexible Polyurethane Foams
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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.
Downloaded by NORTH CAROLINA STATE UNIV on August 20, 2012 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0669.ch009
Designed Surfactant Series and Resulting Foam A i r Flow 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. Table I. Formulations for air flow test Formulation weight (gram) 100 Voranol 3137 water 4.0 0.30 Dabco33LVc 0.20 Dabco T - 9 1.0 Silicone Surfactants 110 Voranate T-SOHndex
Unit: pphp
a
b
d
f
a
Parts per hundred parts polyol (by weight). 1000 equivalent weight triol containing 13% ethylene oxide, 87% propylene oxide and all secondary hydroxyl groups (Dow Chemical). 33% triethylenediamine in dipropylene glycol (Air Products). Stabilized stannous octoate (Air Products). 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. b
c
e
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. 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
In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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POLYMERIC FOAMS
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