The Role of Silicone Surfactantsas Surface Tension Reducing Agents

in Polyurethane Foam

The Role of Silicone Surfactantsas Surface Tension Reducing Agents

in Polyurethane Foam

Presented by:Masao Takase

Dow Corning Toray Co., Ltd.Translated by:KH Boo

2Todays presentation Component roles in the foaming process Structure of silicone surfactants Four roles of a silicone surfactant

Emulsification improving compatibility of raw materialsNucleation of bubblesPrevention of coalescence (slow-down of diffusion)Stabilization

Attributes important to surfactant selection Conclusions

3Component roles in the foaming process

Control of physical phenomena:

Static surface tensionDynamic surface tensionInterfacial tension

PolyolIsocyanate

WaterAmine Catalyst

Tin CatalystBlowing Agent

Surfactant

Chemical reaction:Chain extensionGas formationCrosslink

Polymer

Cell

PU Foam

4Types of structures

Branch Type

ABA Type

(AB)n Type

Siloxane Unit

Polyether Unit

Rake Type / Pendant Type

5Surface tension of raw materials Surface Tension with and w ithout Silicone Surfactant

0

10

20

30

40

50

60

70

Water +NEM(0.1%)

TDI-80 PPG(Mw =3000) Blow ing Agent Entire Sytstem

S

u

r

f

a

c

e

T

e

n

s

i

o

n

(

m

N

/

m

) Without Surfactant

With Surfactant

Surface tension of silicone surfactant: 21.4[mN/m]; concentration of silicone surfactant: 1.0pphp.

6The four roles of a silicone surfactant

Control of Interfacial Phenomenon

3. Prevention of coalescence1. Improved compatibility of raw materials

Stabilization of bubblesSupport of mixing

2. Dispersion of inlet gas 4. Stabilization of bubbles

Cream time to gel timeGel Polymer

Cells are formed

Mixing to cream timeLiquid

Cell is not formed

71. Improving compatibility of raw materials

Depending on the specific system, it is possible for thesilicone polyether to improve thecompatibility between materials.

The surfactant acts at the interfaces of the formulation components.

Polyether pendants can be designed to be soluble in different polar components of the system.

The silicone backbone will drive toward hydrophobic/nonpolar materials.

82. Nucleation Many factors affect the rate of nucleation and bubble

growth in the system. It has been shown, in theory and in practice, that lowering

the surface tension of the system leads to an increased number of fine cells in the foam.

92. Nucleation - Dispersion of inlet gas Less surfactant can lead to a higher surface tension, which can cause coarse cells.

1 Part Silicone Surfactant 0.3 Parts Silicone Surfactant

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2. Nucleation of stable bubbles According to classical nucleation theory, for a stable bubble to form, there has to be enough Gibbs free energy to reach a critical radius/nucleus.(1)

2P3 tensionsurface*16radius critical a forming ofEnergy Free sGibb' =

This equation shows that lowering the surface tension alsolowers the amount of energy needed for nucleation!

1% SurfactantNo Surfactant

Carbonated Soda Shaken for 3 Seconds

Number of bubbles increasedSize of bubbles decreased

No Surfactant 1% Surfactant

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The four roles of a silicone surfactant

Control of Interfacial Phenomenon

3. Prevention of coalescence1. Improved compatibility of raw materials

Stabilization of bubblesSupport of mixing

2. Dispersion of inlet gas 4. Stabilization of bubbles

Cream time to gel timeGel Polymer

Cells are formed

Mixing to cream timeLiquid

Cell is not formed

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3. Prevention of coalescence (slowing down diffusion)

Gas will diffuse from areas of high pressure to areas of low pressure.Therefore, gas will flow from smaller bubbles into larger bubbles.

LaPlace and Young theorized the following:

PPP gasgas ==

)2()1((2)(1) radius

1radius

1*Tension Surface*2

The rate at which the gas diffusion takes place is proportional to P.(2)

Therefore, decreasing the surface tension decreases P, which decreases the diffusion rate.

Lower Pressure

Higher Pressure

=rarb

P 112

PBPB r

P =

Both are very stable foams.

Larger Pressure Drop Smaller Pressure Drop

Lower Pressure

Example of coalescence!

3. Prevention of coalescence (slowing down diffusion)

Higher Pressure

15

4. Stabilization There are many theories as to how a surfactant helps

stabilize a foam:GibbsMarangoniEwers & SutherlandPlateau

We will highlight a few.

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Marangoni effect on stabilization: Fluid flows from low surface tension to high.

Higher-tension area created

Liquid flow pushes fluid back to stabilize the thinned wall.

4. Stabilization by Surfactant Migration Surfactant will flow from an area of high to low concentration!

This will not only drop the surface tension of the newly formed area, but it also will drag with it some of the entangled liquid layer (theorized by Ewers/ Sutherland).(2)

18

Does surface tension control everything??

No, it does not.

Other important variables that impact performance and surfactant selection:

Solubility characteristics Molecular weight differencesReactivity differencesetc.

19

Surface tension is only one of the many variables!

This foam completely collapsed in secondswith an ultra-low surface tension surfactant!

Surfactant 21 Dynes/cm

Surfactant30 Dynes/cm

20

Important roles of silicone surfactant by application

Flexible Slab HR Molded Rigid

Improving Raw Material Compatibility Medium Medium High

Dispersion of Inlet Gas Medium Medium High

Prevention ofCoalescence

Medium Medium Medium

Stabilization High Low Medium

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Structural components of silicone surfactant

1. Type of structure2. Siloxane Mw 3. Modified polyether Mw 4. Number of modified polyether unit5. EO / PO ratio of polyether6. End unit of polyether7. Copolymer Mw8. Silicone content of copolymer

Si O

CH3

O

C3H6O(EO) a(PO)bR

Si

CH3

CH3

m

SiO

CH3

CH3 CH3

Si

CH3

CH3

CH3 n

22

Flexible slab and hot mold

Si O

CH3

O

C3H6O(EO) a(PO)bR

Si

CH3

CH3

m

SiO

CH3

CH3 CH3

Si

CH3

CH3

CH3 n

High Mw of siloxane and polyether O-R capped polyether EO/PO ratio of polyether is approximately 50 to 50 Silicone content of copolymer is lower

23

HR mold

Si O

CH3

O

C3H6O(EO) a(PO)bR

Si

CH3

CH3

m

SiO

CH3

CH3 CH3

Si

CH3

CH3

CH3 n

Low Mw of siloxane and polyether Various kinds of polyether Silicone content is wide range

24

Rigid refrigerator/construction /

Si O

CH3

O

C3H6O(EO) a(PO)bR

Si

CH3

CH3

m

SiO

CH3

CH3 CH3

Si

CH3

CH3

CH3 n

Medium Mw of siloxane and polyether High EO content of polyether

25

Conclusions Physical phenomena such as Emulsification, Nucleation of

bubbles, Prevention of coalescence, and Stabilization are significantly influenced by the selection of the silicone surfactants.

Selection of the right surfactant is a key to your success! Dow Corning knows which is the ideal molecule to

help you meet the requirements of your system. For more information:

Please visit the Dow Corning booth #818Visit our website, www.dowcorning.com/polyurethaneOr e-mail us at specialtychemicals@dowcorning.com

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References 1. Lee, S.T., & Ramesh, N.S. (2004). Polymeric Foams:

Mechanisms and Materials, p 81. New York: CRC Press LLC.2. Herrington, Ron, & Hock, Kathy. (1997). Dow Polyurethanes:

Flexible Foams, p 3.12 and 3.17. USA: The Dow Chemical Company.

Some of your Dow Corning polyurethane foam team DC PUASome of your Dow Corning polyurethane foam team DC PUA

Vicky James, Masao Takase, Stephanie Gathman

Bee Yung, Chris Velasquez Kuang Hoe "KH" Boo

The Role of Silicone Surfactantsas Surface Tension Reducing Agents in Polyurethane FoamTodays presentation Component roles in the foaming processTypes of structures Surface tension of raw materials The four roles of a silicone surfactant Improving compatibility of raw materials2. Nucleation 2. Nucleation - Dispersion of inlet gas 2. Nucleation of stable bubbles Carbonated Soda Shaken for 3 SecondsThe four roles of a silicone surfactant 3. Prevention of coalescence (slowing down diffusion)4. Stabilization 4. Stabilization by Surfactant Migration Does surface tension control everything??Surface tension is only one of the many variables!Important roles of silicone surfactant by applicationStructural components of silicone surfactantFlexible slab and hot moldHR mold Rigid refrigerator/construction / Conclusions References Some of your Dow Corning polyurethane foam team DC PUA