Resins and Additives Containing Silicon - ACS Publications

46 Resins and Additives Containing Silicon SHELBY F. THAMES

Downloaded by UNIV QUEENSLAND on April 16, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch046

Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39401

Inductive Effects and ( p - d ) πBonding Bond Strengths Silicon-Halogen Bond Silicon-Carbon Bond Double Bond to Silicon Silicon-Hydrogen Bond Silicon-Oxygen Bond Silicon-Nitrogen Bond Characteristics of Silicone Polymers Silicon-Containing Polymers Silicones as Water Repellents Pigmentation Silicones as Additives The synthesis of tetraethylsilane, the first organosilicon compound in which s i l i c o n was bonded to carbon, was accomplished in 1863. This r e a l i t y , when combined with the fact that the carbon and s i l i c o n atoms are members of the same periodic group, provided impetus for the idea that there might exist a branch of chemistry in which silicon would take the place of carbon. The pursuit of this idea was the cornerstone of the most careful and extensive series of investigations in organosilicon chemistry. These investigations were i n i t i a t e d by F. S. Kipping (1). His series of investigations led him ultimately to conclude that the field of organosilicon chemistry did not match that of carbon and that the differences between the two f i e l d s were greater than the similarities as the following w i l l indicate (2): 1. 2. 3. 4. 5.

Both carbon and silicon have a normal covalency of four. Both carbon and silicon have their normal bonding tetrahedrally arranged. However, silicon is larger (20%) and heavier than carbon. Silicon is less electronegative (more electro positive). Under favorable conditions s i l i c o n may have a coordination number greater than 4; for example, silicon can u t i l i z e its 3d orbitals for bonding.

In the f i r s t two relationships, carbon and s i l i c o n exhibit striking similarities, while the latter comparisons confirm marked 0097-6156/85/0285-1117S07.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1118

APPLIED POLYMER SCIENCE

d i f f e r e n c e s between the two elements. For the most p a r t these d i f f e r e n c e s can be e x p l a i n e d by c o n s i d e r i n g two fundamental properties of the s i l i c o n atom: (1) i t s low e l e c t r o n e g a t i v i t y and (2) i t s vacant 3d o r b i t a l . The e l e c t r o n e g a t i v i t y of s i l i c o n on the P a u l i n g s c a l e i s 1.8 w h i l e t h a t f o r carbon i s 2.5 ( T a b l e I ) . Thus, s i l i c o n i n r e l a t i o n t o carbon i s an e l e c t r o n donor, and t h e r e f o r e s i l i c o n bonds have a greater degree of i o n i c character than do t h e i r carbon analogues (Table I ) . I t must be remembered, t h e r e f o r e , t h a t the s i l i c o n atom i s the most e l e c t r o p o s i t i v e partner of not only the Si-C bond but a l s o the Si-Η bond. Thus, i o n i c c l e a v a g e s of such bonds as Si-O, S i - C , and Si-Η proceed by e l e c t r o p h i l i c attack (E ) on s i l i c o n ' s bonding mate and n u c l e o p h i l i c attack (Nu"~) on s i l i c o n i t s e l f (Scheme I). Downloaded by UNIV QUEENSLAND on April 16, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch046

+

I

I

Si

Si

-Z

·+

Z*

+

E+

>

Nu"

>

Si

I

Nu Scheme I

Inductive E f f e c t s and (p-d)ir Bonding The e l e c t r o p o s i t i v e e f f e c t of s i l i c o n gives r i s e to markedly higher b a s i c i t i e s of t r i m e t h y l s i l y 1 - s u b s t i t u t e d a l i p h a t i c amines (3) as compared to t h e i r carbon analogues. This i s , of course, the r e s u l t of a p o s i t i v e i n d u c t i v e e f f e c t where competing ( p - d ^ bonding i s not possible. This was confirmed by the comparative studies of Sommer and Rockett (4) with s i l i c o n - and non-silicon-containing amines. In each case the s i l i c o n containing amine was the stronger base (Scheme I I ) . This i n d u c t i v e e f f e c t diminishes as the number of carbon atoms i n s u l a t i n g s i l i c o n from n i t r o g e n i n c r e a s e s , as shown by t h e decreases i n the r a t i o of Κκ /Κκ . Amine 1

Amine 2

Kb /K 1 2 b

CH ) -Si-CH -NH2

CH -NH2

1.82

CH )3Si-CH2CH NH2

CH CH2NH2

1.73

CH3) Si-CH2CH CH2NH2

CH CH CH NH

3

3

2

3

2

3

2

3

3

3

2

2

2

1.14

Note decreasing influence on nitrogen b a s i c i t y as the number of methylene groups between s i l i c o n and nitrogen increases

Scheme I I Consider, f o r example, Compound I I I as compared to Compound IV u s i n g I I as a r e f e r e n c e w i t h no i n d u c t i v e or e l e c t r o n i c e f f e c t s (Scheme I I I , with chemical s h i f t s i n parentheses).


46.

THAMES

(6.26)

1119

Resins and Additives Containing Silicon

(6.26)

(6.13)

(6.44)

(5.83)

H

^3 +1

+1

^

H-

4>

+1

+1 , 3

(7.31)

(5.83)

CH.

(7.16)

(7.31) II

III

IV


Scheme I I I Bond Strengths The strength of the carbon-carbon bond and the s i l i c o n - c a r b o n bond i s 82.6 and 76.0 k c a l / m o l , r e s p e c t i v e l y , and thus the l a t t e r i s s l i g h t l y more r e a c t i v e toward homolytic cleavage. This i s shown by the greater ease of thermal decomposition of the t e t r a a l k y l s i l a n e s (5) as compared to t h e i r carbon analogues. H e t e r o l y t i c cleavage, on the other hand, i s a d i f f e r e n t matter, and the fact that the s i l i c o n atom i s l a r g e r t h a n c a r b o n by a p p r o x i m a t e l y 20%, l e s s e l e c t r o n e g a t i v e , and c a p a b l e of a g r e a t e r maximum c o o r d i n a t i o n number t h a n t h e c a r b o n atom makes t h e s i l i c o n - c a r b o n bond c o n s i d e r a b l y more r e a c t i v e than the carbon-carbon bond toward a number of reagents (6^, 7). Table I . E l e c t r o n e g a t i v i t i e s of Selected Elements and Ionic Character and Ionic Bond Energies of Si-X Bonds

Element

Electro negativity

Ionic Character (%)

Ionic Bond Energy (kcal/mol)

Si

1.8

C

2.5

12

222.9

H

2.1

2

249.8

Ν

3.0

30

0

3.5

50

242.4

S

2.5

12

192.7

F

4.0

70

237.4

Cl

3.0

30

190.3

Br

2.8

22

179.0

J

2.4

8

167.4


1120


Silicon-Halogen Bond The s i l i c o n - h a l o g e n bonds, on the other hand, occupy a p o s i t i o n of g r e a t i m p o r t a n c e i n o r g a n o s i l i c o n c h e m i s t r y as most a l l o r g a n o s i l i c o n compounds must be s y n t h e s i z e d u l t i m a t e l y through h a l o s i l a n e intermediates. The three general methods a v a i l a b l e f o r the preparation of h a l o s i l a n e s from s i l i c o n are l i s t e d i n Scheme IV.


^Cl 1.

2RMgX + S1CI4

2.

S1O2

> R Si

+ 2MgClX

2

> Si +

+ 2C

2C0

Δ Si + C l

3.

R-CH

> S1CI4

2

= CH-R

CI \ + H-Si-R

Λ

cat. / > R-CH-CH

1

1 \

Cl

Η

Si-R CI

Scheme IV They i n c l u d e the G r i g n a r d a l k y l a t i o n process o r i g i n a l l y discovered by F r i e d e l and C r a f t s (1), the d i r e c t method developed by Rochow (8) and h y d r o s i l y l a t i o n (9). Once prepared, the s i l i c o n h a l i d e s are much more r e a c t i v e than t h e i r carbon analogues toward polar reagents. Thus, while carbon t e t r a c h l o r i d e (CCI4) and c h l o r o f o r m (CHCI3) are s t a b l e toward aqueous s o l v e n t s , s i l i c o n t e t r a c h l o r i d e (S1CI4), and t r i c h l o r o s i l a n e (S1HCI3) are hydrolyzed r a p i d l y , even i n moist a i r . Silicon-Carbon Bond The Si-C bond i s most frequently formed v i a the Grignard synthesis r o u t e and was t h e f i r s t i n d u s t r i a l method of p r e p a r i n g organosilanes, Equation 1. S1CI4 + CH

MgBr

3

> (CH )4Si + MgBrCl

(1)

3

S t e r i c f a c t o r s do p l a y a r o l e i n product i d e n t i t y and y i e l d s , as exemplified by the f o l l o w i n g competitive reaction, Equation 2. ( C H ) S i C l + CH MgBr + C H MgBr 2

5

3

3

2

5

( C H ) S i C l + ( C H ) S i C l + C H MgBr 3

3

2

5

3

2

5

> (C H5) Si-CH3 (only) 2

3

> (CH )3SiC H5 + 3

2

(C H )4Si 2

(2)

5

Furthermore, c y c l i c products are prepared by the Grignard route, Equation 3.

synthesis


46.

1121


THAMES

CH

2 \ h „

Mg

.[

] +

M

g

B

r

C l

L

2


'Br

/ Br

CH l

2

(3)

In a d d i t i o n t o Mg, s e v e r a l o t h e r o r g a n o r a e t a l l i e s such as organosodiums, - l i t h i u m s , -aluminums, -mercury, and -zincs can be employed for the formation of organosilicon compounds. The S i - C bond i s , i n g e n e r a l , more p o l a r than the C-C bond i n that a bond of approximately 12% i o n i c character i s formed. Thus, a r y 1 t r i m e t h y l s i l a n e s are c l e a v e d by a c i d s (6) under r a t h e r m i l d c o n d i t i o n s , w h i l e t e r t i a r y b u t y l a r y l s a r e r e s i s t a n t and a r e not cleaved by even strong acids. S i m i l a r l y , the s i l i c o n - c a r b o n bond i s a l s o more susceptible to basic cleavage (7) than the carbon-carbon bond, and any substituent that tends to increase the p o l a r i t y of the bond has a greater e f f e c t on the r e a c t i v i t y of a s i l i c o n - c a r b o n than on i t s carbon-carbon analogue. Furthermore, the m o l e c u l a r weights of s i l i c o n e polymers a r e higher than t h e i r v i s c o s i t i e s indicate. This i s a function of the low intermolecular forces c h a r a c t e r i s t i c of organosilicones. In a d d i t i o n , the c l e a v a g e of a r y l - s i l i c o n bonds by n e u t r a l reagents d e l i n e a t e s even more the v a s t d i f f e r e n c e s between the f i e l d s of organosilicon and carbon chemistry. The c a r b o n - s i l i c o n bond i s a l s o more r e a c t i v e t o w a r d h a l o g e n a t i o n than i s the carbon-carbon bond. T h i s has been demonstrated by attempts t o brominate or i o d i n a t e t r i m e t h y 1 phenylsilane (Scheme V). /

C

H

3 CH-

CH

+

Br

9

/ \ ^ \0\

Br +

Br~Si(CHj

3

Scheme V Anderson and Webster (10) showed the p y r i d y l - s i l i c o n bond of 2( t r i m e t h y l s i l y l ) p y r i d i n e to be susceptible to cleavage by a l c o h o l and water to form unsubstituted pyridine. Thames and coworkers (11) showed that n e u t r a l reagents, i n c l u d i n g aldehydes, a c i d h a l i d e s , chloroformâtes, and anhydrides, have the a b i l i t y to cleave s e v e r a l s i l i c o n - c a r b o n bonds on s e l e c t h e t e r o c y c l e s (Schemes V I - V I I I ) . However, the s i l i c o n - c a r b o n bond of n o n h e t e r o c y c l e s i s not susceptible to neutral reagent cleavage, and too, the f u l l extent of h e t e r o c y c l i c cleavage reactions i s not known. S i m i l a r contrast i s seen i n the behavior of the two bonds toward anhydrous aluminum c h l o r i d e . In carbon c h e m i s t r y , the F r i e d e l C r a f t s reaction i s one of the most commonly used methods of forming carbon-carbon bonds. In organosilicon chemistry, however, aluminum c h l o r i d e i s a convenient means of c l e a v i n g the s i l i c o n - c a r b o n bond (12). Even anhydrous f e r r i c c h l o r i d e cleaves s i l i c o n - c a r b o n bonds under mild conditions.



OSiMe

O


ΕtOH

° -

P h

C

- -

Ar-CH-Ph

a

Ar=a,b

2

H

3

Ar-CH-Ph

Ar=a,b

ο C 1

Ar-i-Ph

>

A r = a ,b

3.

H Et-O-C-Cl

^

A

r

Ο J .

0

E

t

+

Me SiCl 3

Ar=a,b

Ο 4.

Ph-NCO

Ph(CO)-O

COOSiMe,

Ar-C

Ο SiMe, Il 1 Ar-C-N-Ph 3

>

Ο il Ar-3-NH-Ph

Ar=a,b

Scheme VI


46. THAMES



ο

Scheme V I I

O-Si

,N ^

/ Si

R-

1

R-

Ο II C-OH

Scheme V I I I


1123

1124


Double Bond to S i l i c o n


I t i s a l s o most s i g n i f i c a n t that there was, u n t i l p u b l i c a t i o n of the work by A. G. Brook (13) and Robert West (14), a complete absence of evidence for the existence of any organosilicon compound containing a double bond to s i l i c o n . Kipping (15) had concluded i n the e a r l y years of organosilicon chemistry that an ethylenic bonding between carbon and s i l i c o n was e i t h e r i m p o s s i b l e or i t c o u l d be produced only under exceptional circumstances. Perhaps i t i s the l a t t e r case that makes the work of Brook et a l . and West et a l . possible. Brook and co-workers have synthesized and confirmed the structure of 2 - ( l adamantyl)-2-trimethylsiloxy-l,l-bis(trimethylsilyl)-l-silaethylene, Equation 4.

(4) L i k e w i s e , West and coworkers have r e p o r t e d the s y n t h e s i s of t e t r a m e s i t y l d i s i l e n e by the p h o t o l y s i s of 2 , 2 - b i s ( m e s i t y l ) h e x a m e t h y l t r i s i l a n e . The compound i s bright orange-yellow and e x i s t s as a c r y s t a l l i z i n g s o l i d . I n the absence of a i r , i t i s s t a b l e up t o i t s melting point of 176 °C, Equation 5.

CH

CH

3

3

(5)

Furthermore, v i n y l s i l a n e s a r e a r e a l i t y and undergo an a n t i Markownikoff a d d i t i o n of HBr as a r e s u l t of the (ρ-ά)τϊ bond influence, Equation 6. R Si-CH=CH + HBr 3

2

> R Si-CH -CH -Br 3

2

2


(6)

46.


THAMES

1125


Silicon-Hydrogen Bond In o r g a n i c chemistry the element most commonly a s s o c i a t e d w i t h carbon i s hydrogen, yet i n o r g a n o s i l i c o n chemistry the number of compounds c o n t a i n i n g t h e s i l i c o n - h y d r o g e n bond i s a l m o s t a n e g l i g i b l e portion of the t o t a l . This fact gives r i s e to one of the most important differences between the two branches of chemistry i n which there i s much greater r e a c t i v i t y toward polar reagents of the s i l i c o n - h y d r o g e n bond as compared w i t h the carbon-hydrogen bond. Thus, i n c o n t r a s t to the i n e r t n e s s of methane (CH4), s i l a n e (S1H4) i s spontaneously flammable and i s hydrolyzed r e a d i l y by water and even more r e a d i l y by aqueous a c i d s and bases w h i l e halogen a c i d s react with i t to give mixtures of h a l o s i l a n e s and hydrogen, Equation 7. Cl HS1CI3

2

> S1CI4 *2

R3S1H

> R3S1-X + HX *2

RS1H3

X

2

> RSiH X

> R-SiHX

2

(7)

2

Furthermore, under certain conditions the Si-Η bond i s known to add t o e t h y l e n i c d o u b l e bonds, and s u c h a r e a c t i o n i s termed h y d r o s i l y l a t i o n , Equation 8. R R

R H PtCl ?

Si-H + C=C R R

H0 -!--£—>

fi

fi

R

?

Si

C - C - H R

(8)

R

In the Si-Η bonding the s i l i c o n atom i s more e l e c t r o p o s i t i v e than hydrogen and possesses 2% i o n i c c h a r a c t e r . Thus, a t t a c k of the S i bond takes p l a c e v i a e l e c t r o p h i l i c a t t a c k on hydrogen or by n u c l e o p h i l i c attack on s i l i c o n . The most widely a p p l i c a b l e method f o r the synthesis of s i l i c o n hydrides on a laboratory s c a l e i s the reduction of s i l i c o n - h a l o g e n , -nitrogen, or -oxygen bonds. Silicon-Oxygen Bond Compounds c o n t a i n i n g s i l i c o n - o x y g e n bonds occupy a p o s i t i o n of s p e c i a l importance i n organosilicon chemistry. Many i n d u s t r i a l l y important m a t e r i a l s c o n t a i n c h a i n s of silicon-oxygen bonds, which p r o v i d e f o r m o l e c u l e s of very high m o l e c u l a r weight h a v i n g h i g h chemical and e l e c t r i c a l resistance, low temperature c o e f f i c i e n t of v i s c o s i t y , and strong water-repellent p r o p e r t i e s . Aldehydes g i v e high polymers t h a t are analagous to the p o l y s i l o x a n e s w i t h the e x c e p t i o n t h a t they c o n t a i n an u n s u b s t i t u t e d hydrogen atom i n the chain (Scheme IX). However, these carbon compounds depolymerize so r e a d i l y that they have no i n d u s t r i a l importance.


1126


R

^,οη ->

^ y S i

2x

R

^OH

H

H H H 1 1 1 4c-o-c-o-c-o} I I I χ H H H

^C=0

3x

H Downloaded by UNIV QUEENSLAND on April 16, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch046

R R ! I 40-Si-o-Si} I I χ R R

R Si=0 not possible R Scheme IX A common species containing a silicon-oxygen bond i s that of the s i l a n o l s and s i l a n e d i o l s , which are analogous to a l c o h o l s and gemd i o l s i n the carbon s e r i e s . Both the s i l a n o l s , E q u a t i o n 9, and s i l a n e d i o l s , Equation 10, undergo i n t e r m o l e c u l a r condensation with the e l i m i n a t i o n o f water. T h i s r e a c t i o n i s both a c i d and base c a t a l y z e d and o c c u r s so r e a d i l y w i t h a l k y l - and mixed a l k y l a r y l s i l a n o l s that the uncondensed s i l a n o l s are frequently quite d i f f i c u l t to prepare. 2 R S i - 0 H — > R S i - 0 - S i R + HOH 3

3

OH / X R Si \

(9)

3

R

R R I I I — > 40-Si-O-Si-O-Si} + 3H 0 I I I χ R R R 2

2

OH

(10)

Furthermore, the s t a b i l i t y of two hydroxyl groups on a s i l i c o n atom i s much greater than two hydroxyl groups on a carbon gem-diol, as these carbon compounds dehydrate to g i v e ketones, Equation 11, or aldehydes, Equation 12, w h i l e t h e c o r r e s p o n d i n g s i l a n e d i o l s g i v e r i s e t o t h e f o r m a t i o n o f po 1 y s i 1 o x a n e s v i a i n t e r m o l e c u l a r condensation.

—> R

OH

^ C = 0 + HOH R ketone


(11)

46.

R

OH

\ /

1127


THAMES

R

/

C

\

\ ^

+ HOH

^C=0

H

OH

(12)

H aldehyde

Other Si-0 bonds are produced i n the f o l l o w i n g manner, Equation 13. + R OH — >

R SiX

+

3

2


f

R Si-X 2

f

2R 0H f

S1CI4 + 4R 0H

R Si-0-R

f

+ HX

3

f

—>

R Si - (0R ) 2

f

—>

Si(OR )

2

+ 2HX

+ 4HC1

4

(13)

f

R may be a v a r i e t y of s u b s t i t u e n t s of both the a l i p h a t i c and aromatic types. The Si-0 bond can i t s e l f be cleaved by appropriate reagents, as i n Equation 14. ^Si-OR + R'OH

^SiO-R

f

+ ROH

(14)

This reaction can be s h i f t e d to the r i g h t by either removing the a l c o h o l formed i n the condensation (ROH) or by using a large excess of t h e a l c o h o l (R'OH). Such condensation r e a c t i o n r a t e s a r e greatest with the larger number of hydroxyl groups per s i l i c o n atom; for example R3SI-OH < R Si(OH) 2

χ

k-Cl

+ HOH

Si-OH

—>

«

+

Si-OH

X H-N ^

+ HC1

Scheme X

L i k e w i s e , the S i - N bond can be s p l i t by H S a l t h o u g h w i t h g r e a t e r d i f f i c u l t y than with oxygen-containing reagents, Equation 15 2

1

R Si-N-R + H S — > R S i - S ~ S i R 3

2

3

3

+ R3

SiSH


(15)

1128


hydrogen halides, Equation 16, or f

R3S1-NR2

+ HX

> R3S1-X + HNR 2

R Si(NR )2

+ HX

> R SiX2 + 2H-NR2

RSi(NR )3

+ HX

2

2

2

2

> R-S1X3 + 3HNR

(16)

2

acid halides to give h a l o s i l a n e , Equation 17. (R Si) -N-H + S 0 C 1 Downloaded by UNIV QUEENSLAND on April 16, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch046

3

2

2

> R3S1-CI + (R3SiN) S 0

2

2

2

(17)

S i l i c o n - n i t r o g e n bonds are formed i n the f o l l o w i n g manner, Equation 18. ^SiX

+ 2HNR

> ^Si-NR

2

RH SiCl + NH3

2

+ HNR -HX 2

> ( R H S i ) N + 3HC1

2

2

(18)

3

Such a reaction i s c o n t r o l l e d by the bulk of the c h l o r o s i l a n e . For i n s t a n c e , a t a h i g h e r s u b s t i t u t i o n of the s i l i c o n atom, o n l y s i l a z a n e s are formed, Equation 19. 2 R S i C l + NH3

> R Si-N-SiR

3

3

3

+ 2HC1

(19)

H As w i t h s i l o x a n e s , s i l y l a m i n e s c a n be p r e p a r e d "reamination reaction," E q u a t i o n 20.

1

R3S1-N-R + H-N ;

ι

by t h e

R -Si-N-R" + H-N 3

^

\

\

f

N

f

R R" R" R (20) As t h i s reaction i s r e v e r s i b l e , i t can be used s u c c e s s f u l l y only i f the amine formed can be removed by d i s t i l l a t i o n , thereby s h i f t i n g the reaction to the r i g h t . An important s i l i c o n - n i t r o g e n compound i s hexamethy 1 d i s i l a z a n e or HMDS. T h i s reagent i s an e f f e c t i v e s i l y l a t i n g agent and has found wide a p p l i c a b i l i t y a s s i s t i n g i n t h e v o l a t i l i z a t i o n o f temperature-sensitive a l c o h o l . The s i l y l a t i o n process i s c h a r a c t e r i z e d by the f o l l o w i n g generalized equation (Equation 21): 2 H

(CH ) Si-N-Si(CH3)3 + ° 3

3

R

— > 2(CH ) -Si-0-R

nonvolatile alcohol

3

3

+ NH f 3

(21)

volatile s i l y l ether

The reaction can be run neat, at low temperatures, and provides for high conversion to the s i l y l ethers.


46.

THAMES

1129



C h a r a c t e r i s t i c s of S i l i c o n e Polymers S i l i c o n e resins t r a d i t i o n a l l y possess (1) enhanced s t a b i l i t y to heat and greater resistance to oxidation than t y p i c a l organic polymers, (2) retention of physical properties over a wider temperature range than hydrocarbon polymers and t h e i r properties change more s l o w l y w i t h t e m p e r a t u r e , (3) h i g h e r w a t e r r e p e l l e n c y , and (4) i n c o m p a t i b i l i t y with organic materials and thus do not d i s s o l v e i n or adhere to such materials. These p r o p e r t i e s a l l o w them to be employed i n a myriad of a p p l i c a t i o n s i n a d d i t i o n to s u r f a c e c o a t i n g s , i n c l u d i n g p l a s t i c moldings (16), e l e c t r i c a l i n s u l a t o r s (17), c l e a n i n g substances, shoe-, f u r n i t u r e - , automotive, and f l o o r waxes (18), i n v i r o l o g i c a l practice (19), c l o t h coatings, and mold release agents, biomedical a p p l i c a t i o n s , and hydraulic f l u i d s and l u b r i c a n t s (20). Silicon-Containing Polymers A v a s t m a j o r i t y of the c o m m e r c i a l l y a v a i l a b l e s i l i c o n - c o n t a i n i n g polymers today are poly siloxanes or s i l i c o n e s . In the same way that aldehydes and a l c o h o l s are c h a r a c t e r i z e d by the presence of the -CH0-H and Ξ C-OH m o i e t i e s , r e s p e c t i v e l y , a s i l o x a n e i s a compound c o n t a i n i n g t h e bond = S i - 0 - S i = , and s i l i c o n e s are m a t e r i a l s c o n s i s t i n g of a l t e r n a t i n g s i l i c o n and oxygen atoms i n which most s i l i c o n atoms are bound to a t l e a s t one m o n o f u n c t i o n a l o r g a n i c r a d i c a l . Polymers and copolymers of R\, R2» and R3 monomers are termed poly(siloxanes). Linear poly(siloxanes) CH3

I containing more than 1000

-(Si-0) CH

3

u n i t s make up the c l a s s of s i l i c o n e rubbers. End capping l i n e a r p o l y ( s i l o x a n e s ) of l e s s than 1000 {Si(CH3)2-0} u n i t s make up the category of s i l i c o n e f l u i d s , Me3Si-[0Si(CH3)2] -0SiMe3. S i l i c o n e r e s i n s , on the o t h e r hand, are branched p o l y ( s i l o x a n e s ) , Equation n

2 2

'

Η

Me

1

Si-O-Si-O-Si-0 Me

Me

y

0 (22) χ Such polymers are produced v i a the h y d r o l y s i s and subsequent condensation of c h l o r o s i l a n e s . The monomers t y p i c a l l y employed are mono-, d i - and/or t r i c h l o r o s i l a n e s c o n t a i n i n g a l k y l and/or a r y l groups, such as the methyl and phenyl moieties. I t was Ladenburg who, i n the l a t e 1800s, prepared p o l y ( d i e t h y l s i l o x a n e ) , the f i r s t s i l i c o n e polymer, v i a the h y d r o l y s i s of d i e t h y l d i e t h o x y s i l a n e (21). I t was not u n t i l 1927, however, t h a t K i p p i n g (22) r e p o r t e d t h a t the m a t e r i a l s were macromolecular i n structure. Such polymers were not made commercially a v a i l a b l e on a


1130


large s c a l e u n t i l the 1940s when Dow Corning Corp., General E l e c t r i c Co., and the P l a s k o n D i v i s i o n o f Libby-Owens-Ford G l a s s Co. began producing polysiloxanes. Generally, the properties obtained with a given s i l i c o n e polymer depend upon i t s molecular weight, the organic substituents attached, and the c h l o r i n e content of the o r g a n o c h l o r o s i l a n e p r e c u r s o r ( s ) . The s i l i c o n e o i l s a r e t y p i c a l l y l i n e a r p o l y ( d i m e t h y l s i l o x a n e ) o b t a i n e d by h y d r o l y s i s of d i m e t h y l d i c h l o r o s i l a n e containing some t r i m e t h y l c h l o r o s i l a n e , such that the s i l a n o l ends are capped with nonreactive t r i m e t h y l s i l y l groups, Equation 23. CH CI CH \ / / X Si + 2Y CH3-S1-CI


3

/ \

CH

CH CH CH I 1 \ > CH -Si-0-Si-0-Si-CH

3

2

CH

3

3

3

\ 1

CI

3

3

H0

1

CH

3

3

Y

CH X

(23)

3

1 3

CH

3

Y

C r o s s - l i n k i n g of l i n e a r polymers can be e f f e c t e d as w e l l as c o n t r o l l e d by the j u d i c i o u s use of t r i c h l o r o s i l a n e s , Equation 24. CH

Cl

3

\

/

CH3

+ 2Y CH -Si-Cl 3

CI

3

CH

I I I

3

R

CH

CH

3

CH

I I

3

I

CH

3

CH

3

I I

CH

3

CH

3

I

I

0

CH

3

CH

3

CH

3

CH

3

I

3

+

H0 2

2RSÎ-C1

>

1 CI

3

CH -Si-0-Si-0-Si-0-Si-0-Si-CH

I

I

I

Si CH

CI

CH3

3

I

I

(24)

-Si-0-Si-0-Si-0-Si-0-Si-

1 CH

I 3

CH

3

I

I

R

CH

1 3

CH

3

During the reaction sequence c y c l i c trimers may be produced and are stripped o f f v i a d i s t i l l a t i o n or are p u r i f i e d and subsequently p o l y m e r i z e d (23) t o a h i g h m o l e c u l a r weight polymer v i a the c a t a l y t i c action of s u l f u r i c a c i d , Equation 25.


46.


THAMES

R 3R Si(OH) 2

2

—>

0

\ / Si

1131

R

\

/

(25)

Si

R

R 0

0

R

R


R = a l k y l or a r y l In f a c t , these c y c l i c t r i m e r s may be i n c o r p o r a t e d i n t o s i l i c o n e resins of the type employed i n surface coatings. In such products, a s i l a n e t r i o l i s incorporated to c r o s s - l i n k the r e s i n ( t h i s crossl i n k i n g r e a c t i o n has been p r e v i o u s l y d e s c r i b e d ) and to a l l o w the presence of excess hydroxyl groups a f t e r r e s i n preparation. These materials containing excess hydroxyl groups are then applied to a substrate as a surface coating and subsequently cured v i a a thermal c y c l e of up t o 60 min at 500 °F. When c a t a l y z e d , t h e i r cure c y c l e may be reduced to 30-60 min at 400 °F. Some t y p i c a l c a t a l y s t s and the l e v e l s at which they are u s u a l l y employed are l i s t e d i n Table II. The importance of substituent i d e n t i t y with respect to thermal l i f e of a number of c o m p l e t e l y condensed polymers of a l k y l - o r a r y l t r i c h l o r o s i l a n e has been accomplished by Brown (24) as shown i n Table I I I . C o l o r e d enamels based on unmodified s i l i c o n e resins withstand continuous exposure to 260-370 °C. Black enamels are s e r v i c e a b l e to 430 °C, and aluminum combinations a r e s e r v i c e a b l e up t o 650 °C. Ceramic f r i t formulations w i l l operate s u c c e s s f u l l y at 760 °C, and l i k e aluminum f i n i s h e s , the s i l i c o n e r e s i n d i s i n t e g r a t i o n i s followed by - S i - 0 - f r i t (aluminum) bond formation (25). The enhanced thermal s t a b i l i t y of the s i l i c o n e s has been p o s t u l a t e d by Andrianov and S o k o l o v (26) t o be due t o (1) the high energy of the S i O S i bond, (2) the s t a b i l i t y of o r g a n i c r a d i c a l s t o o x i d a t i o n a t t h e s i l i c o n atom, (3) s t a b l e S i O S i bond f o r m a t i o n within the polymer a f t e r p a r t i a l degradation, and (4) the formation of a l a y e r of s i l i c o n - o x y g e n bonds on the s u r f a c e of t h e polymer during thermal oxidation. A b r i e f summary of the s i l i c o n e s i n high-temperature c o a t i n g applications follows: Outstanding Advantage

Chief L i m i t a t i o n s

1. Air-dry tack free 1. High cost (some formulations) 2. Service tempera2. Low f i l m b u i l d ture up to 1000 °F 3. I n t e r i o r or ex3. Limited solvent t e r i o r service resistance

Typical A p p l i c a t i o n 1. High-temperature stacks 2. B o i l e r and b o i l e r breeder 3. Exhaust l i n e s , manifolds, and mufflers

4. Limited chemical resistance



1132 Table I I . S i l i c o n Curing Catalysts Suggested Metal Content Based on Nonvolatile Material (%)


Metal

Comments

Zinc

0.3-0.5

Generally considered universal c a t a l y s t

Manganese, cobalt

0.1-0.5

Good top hardness, often d i s c o l o r s

Iron

0.1-0.03

Good top hardness, Fast, give poor shelf s t a b i l i t y and heat resistance

Lead, t i n

Poor shelf l i f e , cause g e l l a t i o n and poor heat resistance

Table I I I . Thermal L i f e as a Function of A l k y l or A r y l Substituent on S i l i c o n Group Bonded to S i l i c o n Phenyl Methyl Ethyl Propyl Butyl Pentyl Decyl Octadecyl Vinyl

a

Approximate H a l f - l i f e at 250 °C (h) >100,000 >100,000 6 2 2 4 12 26 101

l

Time at which h a l f of the groups are replaced by oxygen.

T r i f u n c t i o n a 1 m a t e r i a l s , such as m e t h y l t r i c h l o r o s i l a n e , g e n e r a l l y p r o v i d e f o r a hard, sometimes b r i t t l e ( i f CH3-S1CI3 i s employed) r e s i n and enhanced cure rate f o r organosilicones. Highpheny 1 - c o n t a i n i n g r e s i n s seem t o be more s t a b l e t o o x i d a t i o n (see T a b l e I I I ) , t o have a b e t t e r s h e l f l i f e , to have b e t t e r heat r e s i s t a n c e , to be tough, t o be more s o l u b l e , and to a i r - d r y but t o be l e s s t h e r m o p l a s t i c than polymers c o n t a i n i n g a high l e v e l of (CH3)2Si0 r e p e a t i n g u n i t s (2, 31_, 32). The r e s i n s c o n t a i n i n g the pheny 1-methy 1 f-CH3Si(C5H5)0} u n i t a r e tough and f l e x i b l e w i t h a moderate modulus (24). Resins containing a large proportion of f(CH3)2Si04 r e p e a t i n g u n i t s , on the other hand, a r e s u p e r i o r i n hardness at high temperatures (27), f l e x i b i l i t y , water repellency, low-temperature p r o p e r t i e s , c h e m i c a l r e s i s t a n c e , cure r a t e and In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


46.

THAMES


1133

a b i l i t y to withstand thermal shock, g l o s s retention, arc resistance, and u l t r a v i o l e t and i n f r a r e d s t a b i l i t y . Although organic groups other than phenyl and methyl have been employed i n s i l i c o n e s , t h e i r high price l i m i t s t h e i r m a r k e t a b i l i t y (24). Indeed, s i l i c o n e s are v i r t u a l l y t r a n s p a r e n t to u l t r a v i o l e t energy and are thus not s u b j e c t t o the u s u a l d e g r a d a t i o n and subsequent f a i l u r e of conventional c o a t i n g s . Unmodified s i l i c o n e c o a t i n g s on e x t e r i o r exposure f o r 12 years have been shown to e x h i b i t l i t t l e l o s s of g l o s s , c o l o r change, c h a l k i n g , or any other sign of f a i l u r e . In f a c t , s i l i c o n e s are so s t a b l e that nonchalking coatings can be made from f r e e l y chalking pigments such as titanium dioxide. S i l i c o n e coatings are often modified with organic r e s i n s i n order to a c h i e v e the d e s i r e d hardness, adhesion, and a b r a s i o n resistance and to reduce t h e r m o p l a s t i c i t y and produce a f a s t e r cure coating. However, modification of coatings r e s u l t s i n a compromise or blend of properties, and t h i s should be taken i n t o consideration when formulations are developed. Blending s i l i c o n e s with organic r e s i n s can be affected i n two ways. (1) The f i r s t i s f o r m a t i o n of a c o l d b l e n d i n which the s i l i c o n e r e s i n i s added to the o r g a n i c r e s i n . The u s u a l maximum l e v e l of s i l i c o n e s used i n c o l d blends i s 50% although maintenance paints t y p i c a l l y contain 30%. Cold blending i s an e f f e c t i v e method for i m p r o v i n g heat s t a b i l i t y or e x t e r i o r d u r a b i l i t y of o r g a n i c coatings. I t i s e s s e n t i a l , however, that such blends possess good c o m p a t i b i l i t y . There are disadvantages to c o l d blending i n that the types of s i l i c o n e r e s i n s u t i l i z e d i n c o l d b l e n d i n g are h i g h e r i n cost than r e a c t i v e s i l i c o n e s and, of course, p h y s i c a l blending does not c h e m i c a l l y combine the s i l i c o n e and o r g a n i c r e s i n i n t o one homogeneous p o l y m e r i c system. Thus, r e a c t i v e groups on both the s i l i c o n e r e s i n and o r g a n i c r e s i n are s u b j e c t to h y d r o l y s i s and contribute l i t t l e i f any to chemical and s o l v e n t resistance. (2) S i l i c o n e s can a l s o be combined i n such a manner t h a t a chemical bond i s formed between the s i l i c o n e r e a c t i v e intermediate and the organic polymer. The major f u n c t i o n a l i t i e s p r o v i d i n g f o r bond formation of r e a c t i v e intermediates are the methoxy (=Si-0CH ) and the s i l a n o l (=SiOH) groups, Equation 26. 3

=Si-0-Me + H0-organic polymer —> =Si-0H + H0-organic polymer —>

ESi-O-organic polymer + CH3OH =Si-0-organic polymer + HOH

(26)

The r e s u l t i n g s i l i c o n e - a l k y d copolymers are thus f o r m u l a t e d i n t o coatings that have the same general and p h y s i c a l properties as alkyd r e s i n s and that can be a p p l i e d i n the usual manner. S i l i c o n e - a l k y d copolymers g e n e r a l l y require no s p e c i a l primer types. While there are numerous s i l i c o n e intermediates that can be and are employed i n the formation of o r g a n i c - s i l i c o n e copolymers, a few are shown (19) i n F i g u r e 1. These m a t e r i a l s are g e n e r a l l y reacted with hydroxy1-terminated o r g a n i c polymers, w i t h the r e s u l t a n t c o p o l y m e r s p o s s e s s i n g d r a m a t i c a l l y improved weathering r e s i s t a n c e , a p r o p e r t y whose performance i s e s s e n t i a l l y proportional to the s i l i c o n e content (24) as shown i n F i g u r e 2. Likewise, s i l i c o n e incorporation d r a m a t i c a l l y improves e x t e r i o r performance over non-silicone-containing counterparts as i l l u s t r a t e d i n F i g u r e 3 (24). In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


1134


Further confirmation of the extended weatherability of s i l i c o n e c o n t a i n i n g c o a t i n g s has been r e p o r t e d by F i n z e l (25). Coatings c o n t a i n i n g s i l i c o n e s had lower f i l m e r o s i o n r a t e s than c o n t r o l coatings, and the rates were i n v e r s e l y proportional to the s i l i c o n e content (see Figures 4 and 5). Recently I have confirmed reports from Dow-Corning Corp. that a Dow-Corning methoxy-terminated intermediate reacts e f f e c t i v e l y with hydroxyl-terminated a c r y l i c latexes to produce a c o v a l e n t l y bonded a c r y l i c - s i l i c o n e possessing properties expected of s i l i c o n - m o d i f i e d polymers. Our work has i n v o l v e d the incorporation of the s i l i c o n e i n t e r m e d i a t e onto a h y d r o x y l - t e r m i n a t e d a c r y l i c , which i s subsequently combined with a melamine c r o s s - l i n k i n g agent f o r the p r o d u c t i o n of a t h e r m o s e t t i n g s i l i c o n e - a c r y l i e . T h i s s i l i c o n e m o d i f i c a t i o n has a l l o w e d f o r the p r o d u c t i o n of c o n v e n t i o n a l l y pigmented TSA f i l m s that r e t a i n the base polymer t e n s i l e strength w h i l e exceeding i t s e l o n g a t i o n , g l o s s r e t e n t i o n , and water resistance. S i l i c o n e s as Water Repellents S i l i c o n e s a r e well-known as water r e p e l l e n t s . They have been employed on masonry above grade l e v e l t o r e p e l l i q u i d water not under pressure. They a r e used above grade l e v e l and not under hydrostatic pressure. For instance, they l i n e the masonry pores but do not f i l l the pores. Thus, they are not a moisture b a r r i e r but a moisture r e p e l l e n t . The c l e a r - c o l o r l e s s nature of s i l i c o n e r e p e l l e n t s a l l o w s preservation of the o r i g i n a l appearance of the concrete. Their lack of f i l m formation leaves the masonry pores open, thereby a l l o w i n g t r a n s p o r t of m o i s t u r e outward to the atmosphere. They are h i g h l y durable to environmental conditions and w i l l r e t a i n t h e i r r e p e l l e n t p r o p e r t i e s f o r s e v e r a l years. There a r e two types of waterr e p e l l e n t s i l i c o n e s : (1) one i s the r e s i n - s o l v e n t type t h a t i s c h a r a c t e r i z e d as a p a r t i a l l y h y d r o l y z e d , p a r t i a l l y a l c o h o l i z e d s i l i c o n e r e s i n d i s s o l v e d i n a hydrocarbon solvent. Generally, such products approach the 5% n o n v o l a t i l e l e v e l . (2) The second i s a water-soluble a l k a l i n e s i l i c o n e r e p e l l e n t that i s recommended f o r use on limestone and other nonsiliceous stone and masonry. R

OH

Si' ^OH

Figure 1. Structures of some s i l i c o n intermediates employed i n the formation of s i l i c o n copolymers. In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

46. THAMES

1135


100 %

Silicone 50


m ui

.40 -30

o ιΗ ϋ Ο Chalking _J

\

1

1

L 1

J_

JL

2

Years Exposure i n F l o r i d a Figure 2.

Improvement i n g l o s s s t a b i l i t y as the s i l i c o n content i s increased.

Pure

Silicone Alkyd

Alkyd-Melamine B a k i n g Enamel

Air-Drying 0

6

12

18

Organic-Alky

Resins and Additives Containing Silicon - ACS Publications

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