Silicones and Silicone-Modified Materials - American Chemical Society

Chapter 2

From Sand to Silicones: An Overview of the Chemistry of Silicones

Downloaded by UNIV OF MISSOURI COLUMBIA on May 31, 2013 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch002

Larry N. Lewis G E Corporate Research and Development Center, 1 Research Circle, Niskayuna, N Y 12309

The chemistry o f silicones is summarized by following the steps necessary to produce a two-part, platinum-cured silicone containing vinyl-stopped polydimethylsiloxane, Si-H-on-chain siloxane, platinum catalyst and catalyst inhibitor. The process begins with silicon dioxide and follows the steps o f conversion to sand to elemental silicon. Silicon is reacted with M e C l to make methylchlorosilanes i n the methylchlorosilane reaction ( M C S ) . The products from the MCS reaction are separated by distillation and then hydrolyzed and condensed to make the various siloxane polymers. Polymers with methyl, v i n y l or Si-H functionality are made as required for the platinum addition-cured silicone product.

The silicones industry got its start in the late 1930's (1,2) and became viable after Rochow's 1940 discovery o f the direct process which reacts elemental silicon with M e C l to produce methylchlorosilanes (3,4). This chapter attempts to summarize some of the steps which take place i n the process o f converting sand into silicones. The "vignette" chosen for this summary is the production o f a platinum-cure, so-called addition cured silicone. This brief review w i l l make use o f the M , D shorthand wherein an M group is M e S i O - and a D group is - M e S i O . Substituents on silicon other than M e are represented with a superscript so that M stands for ( H C = C H ) M e S i O - and D H stands for -(Me)(H)SiO- (5). Figure 1 summarizes the entire process covered i n this review. 3

2

V 1

2

2

Formation of Elemental Silicon Silica (sand) is reduced i n a carbo-electro reduction process to produce chemical grade silicon according to equation 1 (6). The many trace elements present i n silicon are either non-reactive or are required for the M C S reaction. Iron apparently has little

© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

11

12

MeCl Me SiCfe(Di) I MeSCMTri) I < Me SiCl(Mono) > ^MeHSiCl (MH) J Me ClSiH(M H) r

S

io

•Si

2

•MCS

2

.„ d l S l l l L

3

-

2

2

2

Residue Me


Me

M e (

H0

H0 S C , ^ 2

2

0

( )

0

X X X°>r X

Me

m Dl

0

Me / W Me' Me

0H

f

N

N

+

Me/ W Me Me Me

OH

-s'i-o

i \>-Si' Me j N, Me

+

HC,

1 1

OH

(M DxM )

(D ) 4

H0 2

Me SiCl

^

3

Me SiOSiMe 3

(Mono)

(MM)

M

M e H s c ^ (MH)

Me H^.gj Q 0

0

0

>^sr x ^r ^sr

M e

M

* V-Sil

H

2

(D4 )

Pt w. ™:,™r_™, x — —^• Me ClSi(CH=CH ) 2

H °

Me

(MD xM)

Me ClSiH+HC=CH

H

^4

+

H Me / \ H Me / NMe' \i H Me H Me

+ M

+ HC1

3

2

H0

1 1 Z

" >

(H C=CH)Me SiOSiMe (CH=CH )

2

2

2

2

vi

(M H)

2

vi

(M M )

2

V

M W Me

7

7

Me / M e Me Me Me

Me SiNHSiMe 3

Si + HC1

TCS

•

HSiCl + SiCU 3

>800C

Fumed Si0

•

2

D Figure 1.

3

a n d / o r

Overall Scheme for Conversion

„

t r e a t e d

„

fiIler

4

of Sand to Pt-Curable

Silicone

Formulation


13

effect at its normal levels o f 0.5% while aluminum at 0.1 to 0.3% is an essential promoter (3,4). Titanium and calcium are present from 0-200 ppm and may be promoters. Other elements frequently encountered i n the ppm level include C d , C r , C u , N i , P, Pb, Sb, Sn, V , Z n and Z r (3,4). Recent research i n the area o f silicon made for M C S applications can be found in the proceedings to the meeting held every other year i n Norway called, "Silicon for the Chemical Industry" (7-10).

High voltage


Si0

2

+ C

•

SiO + SiC

•

Si + C O

>1200°C

i

The Direct Process or Methyl Chlorosilane Reaction ( M C S )

The M C S reaction is shown i n equation 2. Typically elemental silicon is reacted

Si+MeCl

C u (3-5%) Z n (400-2000 ppm) •

M e S i C l (Di, 75-90%)

Sn (5-30 ppm)

M e S i C l (Mono, 1-5%)

2

2

MeSiCl (Tri,5-10%) 3

3

Al(500-4000 m) P P

^ ^ ^ . S - * % )

290-305°C M e H S i C l ( M H , 0.1-1%) 2

2

other low boilers (0.1-0.5%) residue (0.5-5%)

2

in a fluidized bed reactor in the presence o f copper, tin zinc and other promoters (11). Critical factors for the M C S reaction include selectivity for dimethyldichlorosilane (Di), rate o f methylchlorosilane production, silicon utilization and spent metal loss. The largest volume polymer prodr^t produced is the polydimethylsiloxane polymer ( P D M S ) . P D M S i n turn is made from hydrolysis o f D i thus selectivity for D i is very important. The M C S reaction is a solid/gas reaction that produces a liquid product mixture. Optimum economic performance is achieved when the silicon utilization is high and the amount o f spent metal lost is low. The mechanism o f the M C S reaction has been discussed for over 50 years but detailed understanding is still lacking at the molecular level (3,4). T w o interesting and potentially critical pieces o f the M C S mechanism puzzle are the form o f copper i n the reaction and the type o f intermediates present. Figure 2 illustrates a mechanism for formation o f D i from silicon, copper and methyl chloride.



14

Figure 2. Proposed MCS

Mechanism

C u S i or "eta-phase" has long been proposed as the active copper species in the M C S reaction (12,13). Recent work has suggested that a silylene intermediate is important in the M C S reaction (14). Activated silicon (as eta-phase) reacts with one equivalent o f M e C l to make a copper-bound silylene, S i ( M e ) C l . The silylene can then react with a second equivalent o f M e C l to form M e S i C l . 3

2

2

Siloxane Polymer Formation from Methylchlorosilanes The product mixture from a typical M C S reaction is subjected to several distillation and isolation steps. The product mixture can be roughly divided into monomers and residue. The monomers are separated from the residue stream b y distillation; the residue contains siloxanes and disilanes. Some monomers can be recovered by various redistribution reactions o f the residue mixture (15). The individual monomers are separated by distillation where the separation o f D i from T r i is difficult. W i t h D i as an example, equation 3 shows the hydrolysis and condensation to form linear and cyclic polysiloxanes. Another useful material is hexamethyldisiloxane ( M M ) which forms from hydrolysis/condensation o f M e S i C l (mono), equation 4. 3


15

Me /

-Si-O f „ 0 "Si, -Si 6 "Me Me'O-Sf M e

s

„ S i C

H0

,0, ,0. ,0. ,0 _0H + X J y( J X Me J W M e / ^ e Me

n

M e

2

b

^

( D j )

V

N

%

:

M e

s

M

Me' 0 H

H

C1

e

H

(M DxM° )


+

(Pa)

H 0 2

Me SiCl

^

3

Me SiOSiMe3

+ HC1

3

(Mono)

(MM)

The main cyclic product from equation 3 is octamethylcyclotetrasiloxane ( D ) which can be isolated from the product mixture by distillation. Synthesis o f polydimethylsiloxanes o f virtually any degree o f polymerization is accomplished by either acid or base catalyzed ring opening polymerization o f D with the appropriate concentration o f M M as a chain-stopper, equation 5 (5). 4

4

The ring-opening polymerization reaction can be used to prepare polymers with other functional groups. A n addition-cured formulation uses vinyl-containing and Si-H-containing polymers. Preparation o f an Si-H-on-chain polymer from ringopening polymerization o f teramethylcyclotetrasiloxane ( D ) and M M is shown i n equation 6 and formation o f a vinyl-stopped polydimethylsiloxane by ring-opening polymerization o f divinyltetramethyldisiloxane and D is shown in equation 7. H

4

4

Me M

e

-Si-O f o' Si Si ,6'Me s

X

N

fi Me

O U -" S" i? ' ^ . Merl ^ M e 1

+

Me

3SiOSiMe

3

(MM)

(Pa) Me. Me

((T Me Me

)p^

J

\ Me

Me Me

Me J

\vie

Me

MDnM

Dm 5


16

Me H

1

^Si-(> J O Si

^Si

O "Me N

H

° " N Me

+ M M

M D % M

+

0%

H

H

(D ) Downloaded by UNIV OF MISSOURI COLUMBIA on May 31, 2013 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch002

4

(H C=CH)Me SiOSMe (CH=CH ) + 2

2

2

v i

2

D

4

^

-

v i

M

DmM

v i

+

Dm

v i

(M M )

Fillers The physical properties o f cured polysiloxane materials are dramatically influenced b y fillers (1,2). So-called non-reinforcing (extenders) and reinforcing fillers are typically used; the most common reinforcing filler is silica. H i g h surface area silica, called fumed silica, is formed by burning the product mixture obtained from the trichlorosilane ( T C S ) reaction o f equation 8. Only a small amount o f untreated

TCS Si + HC1

H S i C l + SiCU

8

3

fumed silica can be added to polydimethylsiloxane polymers. Surface treating is thus carried out in order to improve the blendability o f silica with siloxanes. Typical treating agents include M e S i C l , M e S i N H S i M e , and D . The treatment step is shown i n equation 9 where the silanol surface o f the fumed silica is modified with trimethylsiloxy groups. 3

OH

f

3

3

4

OH SMe

OH - ^ ^ ^ O H

+

SMe I 0H 3 J

3

Me Si3

SMe —(

l-SiMe

3

OH

OH

3

OH SiMe

3

OH

S i M e

3 9


17

Platinum-Catalyzed Addition Cure


The hydrosilylation reaction (16,17) is a well known reaction for formation o f silicon carbon bonds. When vinyl-containing polysiloxane is reacted with m u l t i - S i - H containing polysiloxane in the presence o f a platinum catalyst, a crosslinked network forms, equation 10. Platinum is so active for the hydrosilylation reaction that inhibitors are added to moderate the rate o f crosslinking (18). A typical platinum catalyst used by industry is the so-called Karstedt catalyst (19,20). A reaction o f Karstedt's catalyst with a ligand, L (an inhibitor perhaps), is shown in equation 11.

v i

v i

Pt (M M ) 2

3

11


18

A typical low temperature cure inhibitor is dimethyl fumarate which reacts with Karstedt's catalyst to form a platinum-fumarate complex as shown i n equation 12 (20). 0

Pt(M " M " ) *

+4

Me

K

0.


1

0

Me 6^6

12

Other common inhibitors employed for platinum-cured siloxanes include maleates, fumarates, acetylenic alcohols, phosphines and tetramethyltetravinyltetrasiloxane (D ). Other key variables for platinum-cured siloxanes include: The molecular weight o f the vinyl-stopped polymer, M D n 3 V r ; the amount o f S i - H i n the Si-H-onchain polymer, M D n M and the ratio o f v i n y l to S i - H ; the amount o f filler and degree o f surface treatment, the amount o f platinum catalyst and the amount and type o f inhibitor. V1

4

v,

H

Summary and Conclusion W i t h all o f the aforementioned variables, hundreds o f Pt-curable silicone products are available including: heat-cured rubber, liquid injection moldable products and release coatings. There are also many curable products based on chemistry other than platinum: condensation cure R T V , peroxide-cured and U V - c u r e d epoxy silicones. W i t h other functional groups and/or molecular weights, even more products are


19

possible including: low viscosity fluids/high viscosity gums, alkoxy and acetoxy end groups for condensation cure, amino and epoxy functional groups used for coupling agents, phenyl groups for high temperature applications and fluoro-substituted groups to impart solvent resistance.


Literature Cited (1) Liebhafsky, H. A.; Liebhafsky, S. S.; Wise, G . Silicones Under the Monogram; W i l e y Interscience: N e w Y o r k , 1978. (2) Warrick, E . L., Forty Tears o f Firsts; M c G r a w Hill N e w Y o r k , 1990. (3) Kanner, B.; Lewis, K. M. In Catalyzed Direct Reactions of Silicones; Lewis, K . M.; Rethwisch, D . G . , Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1993; pp. 1-49. (4) Lewis, L.N., The Chemistry of Organic Silicon Compounds, V o l . 2, Rappoport, Z . ; Apeloig, Y., Eds, John Wiley, 1998, p. 1581. (5) R i c h , J.; Cella, J.; Lewis, L.; Stein. J.; Singh, N.; Rubinsztajn S.; Wengrovius, J., Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, John W i l e y : N e w Y o r k , 1997, Vol. 22, pp. 82-142. (6) Downing, J. H.; Kaiser, R . H.; Wells, J. E . In Catalyzed Direct Reactions of Silicon, Lewis, K. M.; Rethwisch, D . G . , Eds.; Elsevier: Amsterdam, 1993, p. 67. (7) Silicon for the Chemical Industry, Oye, H. A.; Rong, H., Eds.; Geiranger, Norway, June 16-18,1992, Institute o f Inorganic Chemistry, NTH: Trondheim, Norway, 1992. (8) Silicon for the Chemical Industry II, Oye, H. A.; Rong, H.; Nygaard, L.; Schussler, G . ; Tuset, J. K r . , Eds.; Loen, Norway, June 8-10, 1994, Tapir Publishers: Norway, 1994. (9) Silicon for the Chemical Industry III, Oye, H. A.; Rong, H.; Ceccarolli, B.; Nygaard, L.; Tuset, J., Kr., Sandefjord, Norway, June 18-20, 1996, Eds., The Norwegian University of Science and Technology, Trondheim, Norway, 1996. (10) Lewis, L. N.; Gao, Y.; Bolon, R.; Ravikumar, V.; D'Evelyn, M. In Silicon for the Chemical Industry IV, Oye, H. A.; Rong, H. M.; Nygaard, L.; Schussler, G . ; Tuset, J. Kr., Eds., Norwegian U n i v . o f Science & Technology, Trondheim, Norway, 1998, p. 157. (11) Ward, W . J.; Ritzer, A.; Carroll, K. M.; Flock, J. W . J. Catal. 1986, Vol. 100, p. 240. (12) Voorhoeve, R . J. H. Organohalosilane: Precursors to Silicones, Elsevier: N e w Y o r k , 1967. (13) Floquet, N.; Y i l m a z , S.; Falconer, J. L. J. Catal 1994, Vol. 148, p. 348. (14) Okamoto, M.; Onodera, S.; Okano, T.; Susuki, E . ; Ono, Y. J. Orgnaomet. Chem. 1997, Vol. 531, pp. 67-71. (15) Ritzer, A. In Silicon for the Chemical Industry II, Oye, H. A.; Rong, H.; Nygaard, L.; Schussler, G . ; Tuset, J., Jr., Eds., Loen, Norway, June 8-10, 1994, Tapir Publishers: Norway, 1994, pp. 242-249. (16) Comprehensive Handbook on Hydrosilylation; Marciniec, B., E d . ; Pergamon Press: Oxford, England, 1992.


Silicones and Silicone-Modified Materials - American Chemical Society

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