Plastic Mortars, Sealants, and Caulking ... - American Chemical Society

10 The Chemistry of Silicone Room Temperature Vulcanizing Sealants JEROME M. KLOSOWSKI and GEORGE A. L. GANT

Downloaded by EAST CAROLINA UNIV on March 22, 2016 | http://pubs.acs.org Publication Date: November 27, 1979 | doi: 10.1021/bk-1979-0113.ch010

Dow Corning Corporation, Midland,MI48640

The first silicone sealants were developed in the early 1940's. These early sealants resembled putties. They contained only a filler and a polydimethylsiloxane fluid; they did not cure to an elastomeric solid. Nevertheless, they offered one main advantage over organic sealants: painting was not required to prevent hardening and weathering. Elastomeric silicone sealants first appeared in the early 1950's. The first of these, patented by J. F. Hyde of Dow Corning Corporation, was a two-part system consisting of an acid-ended polysiloxane and polysilicate. With this system, materials which cured at room temperature were possible and the term "RTV" (Room Temperature Vulcanizing) was coined. These two-part systems required premixing in appropriate ratios before application and curing. In the early 1960's two patents were issued on one-component RTV elastomers, one to Rhone-Poulenc (France) and one to L. B. Bruner, Dow Corning (U.S.A.). These patents described a silicone sealant using hydroxy-ended polydimethylsiloxane and acetoxysilanes. The sealant cured when exposed to moisture in the air. Metal salts of carboxylic acids were added to promote surface cure, eliminating the "tacky" feel of the early materials. This technology set the stage for large-scale commercialization and utilization of silicone sealants. The one-component materials required no mixing before use and could be stored for several months as long as moisture was absent. Although two-component silicone RTV sealants are important in certain applications, one-component sealants account for the greatest share of the silicone sealant market. Silicone sealants are used today in adhesive, encapsulating, impregnating, mold-making, sealing and other applications. Market growth has been rapid. The increasing acceptance of silicone RTV sealants is due to their cost effectiveness and proven performance in building construction and OEM (original equipment manufacturer) applications. This performance has won these sealants a reputation as top-of-the 1

2

3

0-8412-0523-X/79/47-113-113$05.00/0 © 1979 American Chemical Society

Seymour; Plastic Mortars, Sealants, and Caulking Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

114

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S

l i n e m a t e r i a l s i n the eyes of many a r c h i t e c t s , d e s i g n e r s , contract o r s , and equipment engineers. This premium s t a t u s i s due to the unique, but uncomplicated, chemistry of t h e i r components, e s p e c i a l l y the chemistry of the s i l i c o n e polymer backbone. G e n e r a l l y , s i l i c o n e s e a l a n t s possess the best combination o f p h y s i c a l s t r e n g t h , cure r a t e , performance over a wide temperature range, adhesion, s e a l a b i l i t y and w e a t h e r a b i l i t y of any a v a i l a b l e material. Many new s e a l a n t s have surfaced over the past few years w i t h claims of being " s i l i c o n i z e d or being " s i l i c o n e - l i k e " or "has s i l i c o n e added", e t c . U s u a l l y , these " s i l i c o n i z e d " s e a l a n t s cont a i n but a few p a r t s of s i l i c o n i n the form of s i l i c a , s i l i c o n e f l u i d or s i l a n e c o u p l i n g agent; t h e r e f o r e , the "new" " s i l i c o n i z e d " s e a l a n t d i f f e r s , but s l i g h t l y from a r e g u l a r organic sealant and g e n e r a l l y i n only one performance area. T h e i r o v e r a l l performance i s s t i l l i n f e r i o r to t y p i c a l s i l i c o n e s e a l a n t s . The only way known to gain the unique s i l i c o n e s e a l a n t p r o p e r t i e s i s to use a s i l i c o n e sealant. S i l i c o n e s e a l a n t s d e r i v e t h e i r unique p r o p e r t i e s from t h e i r components. S i l i c o n e s e a l a n t s are g e n e r a l l y based on a p o l y d i m e t h y l s i l o x a n e f l u i d or gum. These f l u i d s and gums c o n s i s t of long-chain s i l o x a n e molecules w i t h a l t e r n a t i n g Si-0 bonds, the b a s i c chemical bonds of quartz or sand. These bonds are q u i t e s t r o n g - and the b a r r i e r of r o t a t i o n around the S i - O - S i bond i s low.- - This low b a r r i e r of r o t a t i o n , unmatched i n organic chemist r y , produces the e x c e l l e n t h i g h and low temperature f l e x i b i l i t y of cured s i l i c o n e s e a l a n t s as w e l l as t h e i r unusual cure charact e r i s t i c s . A l k y l s i l o x a n e polymers are g e n e r a l l y transparent to u l t r a - v i o l e t r a d i a t i o n , and t h i s transparency c o n t r i b u t e s to the e x c e l l e n t long-term w e a t h e r a b i l i t y of s i l i c o n e s e a l a n t s . More d e t a i l on a l l of these p r o p e r t i e s i s summarized l a t e r . The v a r i o u s s i l i c o n e s e a l a n t s now a v a i l a b l e provide a broad spectrum of p h y s i c a l p r o p e r t i e s . For example, u l t i m a t e e l o n g a t i o n (as measured by ASTM-D-412) v a r i e s from 50% to 1200%; and one commercially a v a i l a b l e s i l i c o n e s e a l a n t - has a t y p i c a l value of >1200% w i t h almost 100% recovery. T e n s i l e strengths vary from 100 to 1000 p s i , w h i l e moduli at 100% e x t e n s i o n range from 500 p s i . Durometer hardnesses can range from 15 to 80 (Shore A ) . Adhesion c h a r a c t e r i s t i c s depend on the chemistry of the s p e c i f i c s e a l a n t and the s u b s t r a t e to be bonded. For example, a c i d - l i b e r a t i n g (acetoxy) s e a l a n t s bond q u i t e w e l l to g l a s s and to many aluminums, but should not be used against porous cementatious substances because, w i t h time, a t h i n powdery f i l m of 0


1 1

5

II

Ca(0CCH3)2 may form (see Equation 1) and cause adhesion f a i l u r e . N e u t r a l and b a s e - l i b e r a t i n g s e a l a n t s bond w e l l to these cementat i o u s s u b s t r a t e s w i t h no l o s s of adhesion w i t h time. Some of the more recent non-acid-curing s i l i c o n e s e a l a n t s bond e f f e c t i v e l y to most common substances. However, the 100% modulus of these s e a l ants tends to be lower than t y p i c a l a c i d - c u r i n g s i l i c o n e s e a l a n t s .


10.

KLOSowsKi

115

Chemistry of Silicone Sealants

A N D G A N T

II

II

CaC0 + 2 H0CCH Ca(0CCH ) + C0 + H 0 In both one-component and two-component s i l i c o n e s e a l a n t s , the system i s c r o s s l i n k e d by using molecules w i t h m u l t i - r e a c t i v e s i t e s . E i t h e r displacement/condensation o r a d d i t i o n r e a c t i o n s are used i n commercially important s e a l a n t s . I n a d d i t i o n r e a c t i o n s , (Equation 2 ) , a s i l y l - h y d r i d e , ESiH, r e a c t s w i t h an unsaturated s i t e w i t h no evolved by-product. These r e a c t i o n s are c a t a l y z e d by a t r a n s i t i o n metal complex ( i . e . , H P t C l 6 ) . One-component a d d i t i o n systems are t e c h n i c a l l y f e a s i b l e , but g e n e r a l l y r e q u i r e a cure r e t a r d e r ^ and heat a c t i v a t i o n f o r cure, and are n o t considered RTV s e a l a n t s . I n these sealants a C-C bond i s introduced i n t o the backbone o f the system and t h i s could decrease the thermal o x i d a t i v e s t a b i l i t y . 3

3

3

2

2

2


2

Eq. 2 R R ~Si-0-Si-CH=CH R' R

H PtCl 2

^

6

R R RSiO- SiOR' R CH CH R-Si-R' 0 R-Si-R R, R

are g e n e r a l l y CH , C H , e t c . 3

6

5

In s e a l a n t s using a condensation/displacement cure, a s m a l l molecule such as water, a l c o h o l , amine, amide, o r a c e t i c a c i d i s r e l e a s e d as i n Equation 3. These s m a l l molecules e v e n t u a l l y v a p o r i z e l e a v i n g only the s t a b l e ΞείΟΞχΞ bonds formed from the c r o s s l i n k i n g , r e i n f o r c i n g and p o l y m e r i z a t i o n r e a c t i o n s . Eq. 3 R R' •^v^iOSi-OX R' R

+

R" R YSiOSio R R"

R R R R -SiOSiOSiOSi* R' R R R'

X i s g e n e r a l l y H, CH , CH CH 3

2

0

+ XY

3

CH

CH 0 3

II f It -N Y i s g e n e r a l l y -0CCH , -N C0, OH, R, R' a r e g e n e r a l l y CH , CH CH , 0

3

0

CH

3

C-CH3 , -0C=CH , 2

(^pf

3

3

3

2

I t i s not p r a c t i c a l t o attempt t o d e t a i l the complete chemi-


116

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S

s t r y o f a l l a v a i l a b l e s i l i c o n e RTV s e a l a n t s . Therefore, we w i l l concentrate on the p r o p e r t i e s and chemistry of the highest volume products, the one-component s i l i c o n e s e a l a n t s .


Composition A s i l i c o n e s e a l a n t can be as simple as a mixture of polymer, f i l l e r and c r o s s l i n k e r . I t can be a b i t more complex w i t h polymer v a r i a t i o n s f o r even greater thermal s t a b i l i t y o r even lower Tg (glass t r a n s i t i o n temperature); w i t h a c a t a l y s t added to give a d r i e r surface or speed the cure; w i t h a combination of f i l l e r s added to enhance e l o n g a t i o n o r durometer o r lower the c o s t ; w i t h other a d d i t i v e s f o r r e t a r d i n g fungus growth or i n c r e a s i n g adhesion or making them f i r e r e s i s t a n t , e t c . B u t , r e g a r d l e s s of the a d d i t i v e s , t h e b a s i c performance i s d e r i v e d from the combination of polymer, f i l l e r and c r o s s l i n k e r . Polymer The b a s i c polymer f o r the vast m a j o r i t y of the s i l i c o n e s e a l a n t s i s hydroxy-ended p o l y d i m e t h y l s i l o x a n e . (CH ) t

3

(CH )

2

3

(CH ) 3

2

HO-Si-0- - S i - O ι

2

-Si—OH. »

I t has a low T

g

(-123°C) and a r a t h e r

X

s m a l l v i s c o s i t y change w i t h temperature- as i s seen i n F i g u r e 1. T y p i c a l organic o r carbon backbone polymers are a f f e c t e d more by temperature changes because of the higher b a r r i e r of r o t a t i o n around t h e C-C-bond.— Sealants made w i t h t h i s s i l i c o n e polymer are extrudable over a wide temperature range; they can be a p p l i e d i n w i n t e r on the Alaskan North Slope or i n the heat of an Egyptian summer. When the polymer i s then c r o s s l i n k e d i n t o a three-dimensional network to form t h e cured s e a l a n t , the r u b b e r i n e s s , o r e x t e n s i b i l i t y , does not change s i g n i f i c a n t l y w i t h temperature. A sealant used i n A l a s k a w i l l continue to s t r e t c h e a s i l y and s e a l when temperatures are f a r below f r e e z i n g . The s i l i c o n e polymer backbone i s composed of S i - O - S i bonds. This bond i s very strong and s t a b l e w i t h a bond energy of 87 K c a l / mole. The polymer can t o l e r a t e 250°C t o 300°C without decompos ing.— The f u l l y compounded s i l i c o n e s e a l a n t , when cured to a rubber, can withstand 200°C f o r s u s t a i n e d periods of time w i t h no s p e c i a l a d d i t i v e s ; and even higher temperatures w i t h polymer modi f i c a t i o n s and/or heat s t a b i l i t y a d d i t i v e s . — The S i - O - S i molecu l a r s t r u c t u r e i s a l s o transparent to U.V., so s i l i c o n e s e a l a n t s are v i r t u a l l y unaffected by weather. Samples of s i l i c o n e s e a l a n t s used i n e x t e r i o r c o n s t r u c t i o n a p p l i c a t i o n s have been t e s t e d a f t e r 20 years o f a c t u a l performance. These samples e x h i b i t e d essen t i a l l y no change i n p h y s i c a l p r o p e r t i e s o r adhesion during that time p e r i o d .


10. KLOsowsKi

117


A N D G A N T

VISCOSITY v s . TEMPERATURE


1,000, OOO-i

1,000 J -20

1

1

» 0

'—' 20

» » 40

• ! 60

(°C) Figure 1. Viscosity vs. temperature Siloxane polymers a r e made by condensation (Equation 4) o r c y c l i c p o l y m e r i z a t i o n (Equation 5 ) . Low v i s c o s i t y f l u i d s , higher molecular weight polymers and even very h i g h molecular weight gums may be produced from these r e a c t i o n s . T h i s range o f r e a c t i o n pro ducts a l l o w s f o r f l e x i b i l i t y i n d e s i g n i n g t h e molecular a r c h i t e c t u r e of the s e a l a n t . Eq. 4 χ H0[Si(CH )20] H η 3

>- H 0 [ S i ( C H ) 0 ] Η cat. nx 3

2

+

(x-l)H 0 2

Eq. 5 χ [Si(CH ) 0] 3

+ HOH

2

η

>> cat.

H0[Si(CH ) 0] Η nx 3

2

The molecular weight of a polymer chosen f o r a s e a l a n t w i l l i n f l u e n c e t h e t e n s i l e and e l o n g a t i o n ( s t r e s s and s t r a i n ) charac t e r i s t i c s of t h e s e a l a n t . Therefore, a s e a l a n t f o r m u l a t o r can


118

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S

match polymer molecular weight to the s t r e s s - s t r a i n performance values he seeks. The molecular weight of a polymer i n a s e a l a n t w i l l not change the s e a l a n t ' s w e a t h e r a b i l i t y or c h a r a c t e r i s t i c s . Those p r o p e r i t e s a r e a consequence of the S i - O - S i backbone s t r u c t u r e and e x i s t r e g a r d l e s s of molecular weight. Fillers


Almost a l l s i l i c o n e sealant compounders use some percentage of a s i l i c a f i l l e r f o r a c h i e v i n g d e s i r e d t e n s i l e s t r e n g t h . Most use a fumed s i l i c a which can be i l l u s t r a t e d as f o l l o w s :

MACRO SCALE

MOLECULAR SURFACE OF SILICA

A s i g n i f i c a n t part of the r e i n f o r c i n g nature of fumed s i l i c a comes from the p a r t i c l e shape and s i z e but some of the r e i n f o r c e ment comes from the molecular r e a c t i o n s on the s u r f a c e . The general molecular s t r u c t u r e i s S1O2, that of sand, w i t h the surface p a r t i a l l y covered w i t h SiOH, the same f u n c t i o n a l group that i s on the ends of the polymer. This means that the f i l l e r can be i n c o r p o r a t e d i n t o the polymer network by the same chemistry that t i e s the polymer ends together. The t e n s i l e s t r e n g t h of a n o n f i l l e d , but c r o s s l i n k e d p o l y d i m e t h y l s i l o x a n e f l u i d i s about 20 p s i . The same p o l y m e r / c r o s s l i n k er mixture w i t h 15% h i g h l y f u n c t i o n a l s i l i c a f i l l e r could have a t e n s i l e s t r e n g t h of >600 p s i or a 3 0 - f o l d i n c r e a s e . Thus, s i l i c a or fumed s i l i c a i s t r u l y the work-horse f i l l e r of the s i l i c o n e i n d u s t r y . One a t t r i b u t e of s i l i c a f i l l e r s , when o f the proper p a r t i c l e s i z e and p r o p e r l y d i s p e r s e d , i s t h e i r transparency i n the polymer. This allows the p r e p a r a t i o n of the c l e a r s i l i c o n e s e a l a n t s commonly used. The s i l i c a s can e i t h e r be untreated or t r e a t e d by any of s e v e r a l methods. ^'- - - This t r e a t 1

1

3


10.

KLOsowsKi

A N D G A N T


119

ment can impart reinforcement w i t h minimum i n c r e a s e i n t h i x o t r o p y . The f i l l e r technology described here i s not r e a l l y unique to s i l i c o n e s e a l a n t s but holds true f o r a v a r i e t y of s e a l a n t s . The s i l i c a , however, serves a dual purpose i n that i n a d d i t i o n to reinforcement i t a l s o imparts t h i x o t r o p y (flow c o n t r o l ) to prevent the s e a l a n t from f l o w i n g out of the j o i n t s before cure. S i l i c a , then, i s an important p a r t of a l l or n e a r l y a l l s i l i cone s e a l a n t s . I f the s e a l a n t i s c l e a r , s i l i c a s probably the only filler. I f the s e a l a n t i s opaque, s i l i c a may or may not be present f o r reinforcement but i t ' s almost always present f o r flow c o n t r o l . The s i n g l e drawback to fumed s i l i c a i s c o s t ; t h e r e f o r e , only a minimum q u a n t i t y i s used and there i s a c o n t i n u a l search for ways to use l e s s . Other s i l i c a f i l l e r s can be used as w e l l . One such f i l l e r i s ground q u a r t z . T h i s a l s o has SiOH on the surface and i t a l s o t i e s i n , but has a much l a r g e r p a r t i c l e s i z e , lower s u r f a c e area and lower r e l a t i v e d e n s i t y of SiOH. Because of t h i s , i t gives much l e s s reinforcement on an equal weight b a s i s when used as a replacement f o r fumed s i l i c a . N o n - r e i n f o r c i n g extending f i l l e r s are used i n many s i l i c o n e RTV s e a l a n t f o r m u l a t i o n s . These i n c l u d e calcium carbonate, carbon b l a c k s , t a l c s and other i n o r g a n i c f i l l e r s . The s e m i - r e i n f o r c i n g f i l l e r s and extending f i l l e r s are o f t e n used to provide b u l k and impart d e s i r a b l e h a n d l i n g c h a r a c t e r i s t i c s . The f i l l e r then c o n t r i b u t e s to reinforcement, t h i x o t r o p y , and b u l k . Since most of the f i l l e r s are m i n e r a l - l i k e i n o r g a n i c m a t e r i a l s , they g e n e r a l l y don't add or d e t r a c t from the i n t r i n s i c s i l i c o n e p r o p e r t i e s f o r which the s e a l a n t s are most o f t e n s o l d , such as good e l e c t r i c a l i n s u l a t i n g p r o p e r t i e s , weather r e s i s t a n c e , heat s t a b i l i t y and low temperature s e r v i c e a b i l i t y . Organic f i l l e r s (polymers, r e s i n s , rubbers) have been added to some s i l i cones and indeed enhance c e r t a i n p r o p e r t i e s or reduce cost but always at the expense of another property. The property most o f t e n s a c r i f i c e d i s thermal s t a b i l i t y . P u t t i n g together the proper p o l y m e r - f i l l e r combination t o achieve the d e s i r e d durometer, t e n s i l e , e l o n g a t i o n and modulus w h i l e keeping the cost as low as p o s s i b l e i s one of the p r o p r i e t a r y f e a t u r e s of each sealant formula.


T

Crosslinkers The e x c e p t i o n a l i n t r i n s i c performance of s i l i c o n e s e a l a n t s i s due to the s i l i c o n e polymer, the f i l l e r , and the unique c r o s s l i n k ing systems. In t h i s s e c t i o n we w i l l focus on c r o s s l i n k i n g systems used i n one-part RTV s i l i c o n e s e a l a n t s . Most of these systems i n v o l v e condensation r e a c t i o n s i n which a by-product i s produced and the by-products are important to f i n a l s e a l a n t p r o p e r t i e s . Two-part s i l i c o n e s e a l a n t s which are cured by a d d i t i o n r e a c t i o n s , w i t h no by-products, and two-part s i l i c o n e s e a l a n t s that cure by condens a t i o n are a l s o a v a i l a b l e , but they are not as commercially


PLASTIC MORTARS, SEALANTS, AND CAULKING COMPOUNDS

120

1

important today as the one-part s i l i c o n e RTV s and are g e n e r a l l y more expensive. The most common c r o s s l i n k e r s employed i n s i l i c o n e s e a l a n t s today are these r e a c t i v e s i l a n e s : O H Me 0 MeSi(OC-CH ) ; 3

3

MeSi(N^"T~^)

3

(N—cQ) ,

; MeSi(0Me) ; MeSi(OEt)

2

3

and MeSi(0-N=CEtMe) .


3

A l l of the above c r o s s l i n k e r s are t r i - f u n c t i o n a l (3 r e a c t i v e s i t e s ) monomers. Patent l i t e r a t u r e a l s o d e s c r i b e s s e v e r a l cure systems which employ t e t r a f u n c t i o n a l c r o s s l i n k e r s such as 0

II (EtO)ifSi or ( C H C 0 ) i S i . And f i n a l l y , there are s h o r t polymeric c r o s s l i n k e r s such as ( E t O ) S i O S i ( O E t ) , X S i O S i R X , X R S i O S i X R or X S i O S i X O S i X where R i s g e n e r a l l y an a l k y l and X i s any of the f u n c t i o n a l groups on the c r o s s l i n k e r s mentioned above. 0 3

t

3

2

2

3

3

2

2

2

3

II The f i r s t c r o s s l i n k e r , M e S i ( 0 C C H ) , i s the one most commonly used throughout the w o r l d today, due to i t s a v a i l a b i l i t y and f a s t r e a c t i v i t y . I t ' s a good c r o s s l i n k e r producing h i g h e r modulus s i l i c o n e s e a l a n t s u s e f u l i n s t r u c t u r a l a p p l i c a t i o n s where a c i d induced c o r r o s i o n i s not a problem. (Modulus v a r i a t i o n s w i l l be d i s c u s s e d l a t e r ) . In a l l condensation cures the f u n c t i o n a l group on the c r o s s l i n k e r i s r e l e a s e d i n i t s protonated form. This has been g r o s s l y o v e r s i m p l i f i e d i n Equation 6 to i l l u s t r a t e that point: q3

E

3

6

RSiX

3

+

3 H0Si=

RSi(0SiΞ)3

+

3

HX

(R = a l k y l , a r y l , e t c . ; χ = f u n c t i o n a l group such as 0 0CCH , OMe, 3

etc.).

T h i s c o r r e c t l y i m p l i e s that a l l the acetoxy f u n c t i o n a l i t y present on the c r o s s l i n k e r w i l l be released d u r i n g the cure as a c e t i c a c i d . The a c e t i c a c i d corrodes copper and some other metals, and a l s o r e a c t s w i t h concrete forming a l a y e r of C a ( 0 A c ) , which i n t e r f e r e s w i t h the s t a b i l i t y of the bond between the s e a l a n t and the c a l c i f e r o u s s u r f a c e . A c e t i c a c i d a l s o has a pungent odor which some f i n d o f f e n s i v e . Thus, s e a l a n t s c o n t a i n i n g t h i s most common c r o s s l i n k e r are not d e s i r a b l e f o r a l l a p p l i c a t i o n s . Η The second c r o s s l i n k e r l i s t e d above, MeSi(N-{ s ) ) i s r e l a t i v e l y inexpensive and f a s t to r e a c t . I t releases cyclohexylamine when i t cures. Cyclohexylamine doesn't r e a c t w i t h concrete, 2

3


10.

KLOsowsKi

A N D

121


G A N T

but i t does r e a c t w i t h and corrode many metals, and some people f i n d the heavy, musty amine odor o b j e c t i o n a b l e . The t h i r d c r o s s l i n k e r l i s t e d above, MeSi(0Me) , i s a l s o r e a d i l y a v a i l a b l e . I t r e l e a s e s methanol as a by-product, which i s non-corrosive to most substrates and i s not p a r t i c u l a r l y o f f e n s i v e i n odor. T h i s c r o s s l i n k e r i s , however, r e l a t i v e l y slow to r e a c t , and sealants u s i n g i t as a c r o s s l i n k e r r e q u i r e high c o n c e n t r a t i o n s of a s p e c i a l c a t a l y s t to hasten cure. F i n a l l y , the l a s t two c r o s s l i n k e r s have been r e c e n t l y developed. They present fewer problems, but they are q u i t e expensive. Each of the above c r o s s l i n k e r s o f f e r s c e r t a i n advantages and disadvantages, and these must be weighed when choosing a cure system. In the s i l i c o n e sealant market, w i t h i t s d i v e r s i t y of a p p l i c a t i o n s , a s i n g l e , u n i v e r s a l c r o s s l i n k e r i s u n l i k e l y . Instead a knowledge of market a p p l i c a b i l i t y and acceptance, along w i t h cost c o n s i d e r a t i o n s , determines the cure system f o r each sealant i n t r o d u c t i o n . C r o s s l i n k e r r e a c t i v i t y i s a f u n c t i o n of the s i z e of the R group on the s i l a n e and i t s e l e c t r o n withdrawing nature. A few acetoxy-based c r o s s l i n k e r s are l i s t e d below i n i n c r e a s i n g order of r e a c t i v i t y w i t h water or s i l a n o l s . This r e a c t i v i t y c o r r e l a t e s w i t h the e l e c t r o n e g a t i v i t y of the s u b s t i t u e n t s .


3

EtSi(0Ac) 699.1

3

< MeSi(0Ac) 705.9

3

< ViSi(0Ac) 709.2

3

< F CCH CH Si(0Ac) 3

2

2

3

709.7 E l e c t r o n e g a t i v i t y or Ε values —

The

same order e x i s t s when acetoxy i s r e p l a c e d by -0N=CEtMe or

Me » -OMe or - N — C — y ^ y . While the data are not a v a i l a b l e , i t can be assumed that the other l e a v i n g groups would f o l l o w a s i m i l a r trend. Other than cure r a t e , the e f f e c t on the sealant performance of changes i n the R group on the s i l a n e c r o s s l i n k e r i s q u i t e smalL When R i s changed f o r a 4th f u n c t i o n a l group [e.g., M e S i ( 0 A c ) replaced w i t h Si(0Ac)i* (both commonly used)] the r e a c t i v i t y i n c r e a s e s d r a m a t i c a l l y but so does the c r o s s l i n k d e n s i t y . The c r o s s l i n k d e n s i t y i s dependent on the c r o s s l i n k e r func t i o n a l i t y and c o n c e n t r a t i o n , the c h a i n length of the polymer and the c o n c e n t r a t i o n of f u n c t i o n a l , c r o s s l i n k a b l e f i l l e r ( s i l i c a ) . The key f e a t u r e i n these r e a c t i o n s i s that s i l a n e c r o s s l i n k e r s form s i l o x a n e bonds, the same as the polymer backbone. Thus, the s t a b i l i t y and uniqueness of the s i l i c o n e polymer has not been s i g n i f i c a n t l y d i s t u r b e d by the c r o s s l i n k i n g system. The c r o s s l i n k density of a sealant system i s o f t e n m o d i f i e d to achieve a d e s i r e d s t r e s s - s t r a i n performance p r o f i l e . 3


122

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S


Catalyst A l l c h e m i c a l l y c u r i n g s e a l a n t s employ c a t a l y s t s , and s i l i c o n e s e a l a n t s are no e x c e p t i o n . Common c a t a l y s t s f o r s i l i c o n e s e a l a n t s are metal carboxylates, a l k y l m e t a l c a r b o x y l a t e s , and a l k y l m e t a l a l k o x i d e s . More s p e c i f i c a l l y : stannous o c t o a t e , d i b u t y l t i n d i a c e t a t e , d i b u t y l t i n d i l a u r a t e , t e t r a b u t y l t i t a n a t e , d i b u t y l t i n d i m e t h o x i d e and t e t r a i s o p r o p y l t i t a nate are most commonly used. In a l l cases, i n c r e a s i n g the c a t a l y s t c o n c e n t r a t i o n i n c r e a s e s the r e a c t i o n r a t e , but there are l i m i t s unique to each c a t a l y s t . Above these c o n c e n t r a t i o n l i m i t s the i n t r i n s i c s t a b i l i t y of the s i l i c o n e s e a l a n t , and e s p e c i a l l y the thermal s t a b i l i t y , i s l e s s e n ed. C a t a l y s t s provide an e f f e c t i v e means of c o n t r o l l i n g cure r a t e , but they must be used j u d i c i o u s l y . Special Property Additives A d d i t i v e s can be used to improve h a n d l i n g , to augment flame r e t a r d a n t p r o p e r t i e s , to a i d manufacturing processes, and f o r a v a r i e t y of other needs. Such a d d i t i v e s are always p r o p r i e t a r y and, w h i l e they are important, they g e n e r a l l y don't add or d e t r a c t from the i n t r i n s i c p r o p e r t i e s mentioned e a r l i e r . High and Medium Modulus Sealant V u l c a n i z a t i o n Now that the i n g r e d i e n t s are d e f i n e d , the nature of v u l c a n i z a t i o n can be d i s c u s s e d . V u l c a n i z a t i o n occurs through the c r o s s l i n k i n g system. The system shown below i s the system used by many major s i l i c o n e s e a l a n t compounders around the w o r l d , to make high and medium modulus s e a l a n t s . I t c o n s i s t s of hydroxy1-ended p o l y 0

II

d i m e t h y l s i l o x a n e , m e t h y l t r i a c e t o x y s i l a n e [MeSi(0CCH3)3] c r o s s l i n k e r , and a t i n c a t a l y s t . Fumed s i l i c a i s used f o r r e i n f o r c e ment . A system l i k e that described above, when mixed p r o p e r l y (with c o r r e c t concentrations and without moisture) w i l l not change i n v i s c o s i t y during t h e mixing process. In appearance i t w i l l resemble a simple mixture of polymer and f i l l e r , and w i l l extrude from the tube as e a s i l y as toothpaste. A f t e r the s e a l a n t i s d i s charged from the tube, the system i s exposed to the moisture i n the a i r . The r e a c t i v e s i t e s , which are now on the polymer or the f i l l e r surface or p a r t i a l l y reacted c r o s s l i n k e r , w i l l s t a r t to r e a c t w i t h water. (Equation 7) Eq. 7 Me ^ 2~-W)Si(OCCH )2 3

+

H0 2

Me Me ~^Si_0-Si"~+ 0 0 C=0 C=0 CH CH 3

M

2

HOCCH3

3


10.

KLOsowsKi

A N D G A N T

123



As t h i s r e a c t i o n proceeds, the system begins to c r o s s l i n k . Subsequent s i t e s r e a c t and are t i e d together as more water from the a i r enters the system. Deep s e c t i o n cure i s r e l a t i v e l y slow because i t i s dependent on the d i f f u s i o n r a t e of water, and a h a l f i n c h bead of s e a l a n t w i l l take s e v e r a l days to develop f u l l rubber properties. During the cure (or v u l c a n i z a t i o n ) process, S i - O - S i bonds are formed, and these are the same s t a b l e bonds present i n the s i l i c a f i l l e r and polymer backbone. The r e s u l t i s a 3-dimensional network of polymer and s i l i c a c o n s i s t i n g e x c l u s i v e l y of s i l o x a n e bonds. 0 Thus, the s t a b i l i t y of the s i l i c o n e i s assured. The MeSi(OCCH )3 c r o s s l i n k e r i n t h i s example can be r e p l a c e d w i t h any of the c r o s s l i n k e r s mentioned e a r l i e r . The r e s u l t i s the same c r o s s l i n k e d network. The s e a l a n t s cure from the o u t s i d e i n as the moisture i n the a i r c o n t a c t s the s e a l a n t and promotes c r o s s l i n k i n g . A " s k i n " w i l l form on the s e a l a n t s u r f a c e , and t h i s s k i n w i l l proceed to become t a c k - f r e e w i t h s o f t , p l i a b l e , uncured s e a l a n t below i t . The cure r a t e w i l l then slow, s i n c e i t i s dependent on moisture permeation through the e x t e r i o r s k i n . The s k i n w i l l i n c r e a s e i n t h i c k n e s s u n t i l the e n t i r e s e a l a n t bead has cured. S i l i c o n e polymers have a n a t u r a l l y h i g h degree of f r e e r o t a t i o n and a l a r g e v o i d volume. This a l l o w s f o r a cured elastomer m a t e r i a l which i s v e r y permeable to water i n the vapor form, yet completely hydrophobic to l i q u i d water. Cure proceeds r a p i d l y under most c o n d i t i o n s . Water vapor w i l l r e a d i l y penetrate the s k i n so that a h i n c h bead develops approximately 80% of i t s p r o p e r t i e s ( i n i t s cured s t a t e ) i n 24 hours. The statements made above apply r e g a r d l e s s of the c r o s s l i n k ing system used. When the c r o s s l i n k e r i s changed, the s k i n format i o n time and the tack f r e e time may change but the e f f e c t on the deep s e c t i o n cure r a t e i s minimal, s i n c e the r a t e of water permeat i o n through the cured s k i n i s g e n e r a l l y the r a t e - c o n t r o l l i n g f a c t o r i n the cure of one-component RTV s i l i c o n e s e a l a n t s . In terms of environmental c o n d i t i o n s , the cure r a t e i s dependent on the temperature and r e l a t i v e humidity d u r i n g the cure process. A s i l i c o n e s e a l a n t used at -45°C w i l l take s e v e r a l days t o become tack f r e e , w h i l e the same s e a l a n t takes 30 minutes at 23°C, 50% r e l a t i v e humidity and 8 minutes at 38°C, 100% r e l a t i v e humidity. The time i t takes to develop a t a c k - f r e e s u r f a c e w i l l a l s o vary w i t h the type of c r o s s l i n k e r u t i l i z e d . With other c r o s s l i n k e r s mentioned e a r l i e r , s e a l a n t s may develop a tack f r e e s u r f a c e i n as l i t t l e as s i x minutes or as long as one hour, given s i m i l a r cure conditions. An i n t e r e s t i n g note i n the development of s i l i c o n e s e a l a n t s i s that t h e i r h i g h r a t e of water vapor t r a n s m i s s i o n has been exp l o i t e d to make a vapor permeable (breathable) c o a t i n g f o r r o o f s and other s u r f a c e s . T h i s s e a l a n t i s a p p l i e d i n the form of a s o l vent d i s p e r s i o n to a t t a i n the necessary c o n s i s t e n c y f o r easy application. 3


124

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S


The chemistry j u s t o u t l i n e d produces s i l i c o n e s e a l a n t s which cure to a f a i r l y tough, r e s i l i e n t rubber. G e n e r a l l y , s e a l a n t s made by the above route are the higher modulus, strong s i l i c o n e s e a l a n t s used i n products l i k e s i l i c o n e g l a z i n g s e a l a n t s , s i l i c o n e adhesives and s i l i c o n e bath tub caulk. For many a p p l i c a t i o n s these s e a l a n t s are too high i n modulus. For places sealed w i t h s i l i c o n e s e a l a n t s that see tremendous movement (>25% expansion and c o n t r a c t i o n ) , l i k e expansion j o i n t s i n highways or b u i l d i n g s , the toughness of the above sealant r e s u l t s i n undue s t r e s s on the bond l i n e at h i g h e l o n g a t i o n . In these a p p l i c a t i o n s , e i t h e r the sealant w i l l f a i l a d h e s i v e l y or the sealant w i l l p u l l the concrete surface (or s i m i l a r surface) apart. Low and Very Low Modulus Sealants The way chosen by most compounders to produce low modulus s e a l a n t s i s to i n c r e a s e the l e n g t h of the polymer c h a i n , but t h i s approach presents problems. When the f l u i d polymer i s changed to a v i s c o u s polymer and again mixed w i t h a f i l l e r , the sealant i s very t h i c k and s t i f f and doesn't extrude e a s i l y from the tube. This problem i s remedied through the a d d i t i o n of low molecular weight, nonreactive f l u i d s o f t e n e r s . This works q u i t e w e l l and the r e s u l t a n t s e a l a n t has a lower c r o s s l i n k d e n s i t y and s t r e t c h e s e a s i l y , f a i l i n g c o h e s i v e l y when the s t r e s s i s great. U l t i m a t e e l o n g a t i o n i n an ASTM t e n s i l e bar increases from 400% or 500% to 600% or 800%. This s o l u t i o n has a s e r i o u s drawback, however. No s o f t e n e r has yet been invented f o r s i l i c o n e s that doesn't bleed and cause s t a i n i n g on some b u i l d i n g s u r f a c e s . Recently Dow Corning C o r p o r a t i o n developed a one-package sealant (the f a m i l i a r c a r t r i d g e ) and General E l e c t r i c developed a two-package s e a l a n t (mix i n p a i l s on s i t e ) which s k i r t s these problems. The approach i s b a s i c : compound w i t h r e l a t i v e l y low molecular weight polymer, so that the e x t r u s i o n r a t e i s s a t i s f a c t o r y and add a chain extender to the system. The chain extender chosen must be one which b u i l d s the polymer chain length during the cure and before the c r o s s l i n k e r t i e s i n t o the system. The r e s u l t i s a s e a l a n t w i t h very long polymer chains between c r o s s l i n k s and no s o f t e n e r . The chemistry of such a chain extended system i s very complex. The key to success i s u t i l i z a t i o n of a chain extender which c l o s e l y matches the r e a c t i v i t y of the c r o s s l i n k e r . Both must be s u f f i c i e n t l y s e n s i t i v e to r e a c t w i t h water from the a i r and then condense s i l a n o l s so t h a t a dry surface cure r e s u l t s . I f the chain e x t e n s i o n i s f a s t e r than the c r o s s l i n k i n g a c t i v i t y , the system b u i l d s to a gum w i t h very low c r o s s l i n k d e n s i t y and a gummy, s t i c k y product r e s u l t s . I f the c r o s s l i n k i n g i s f a s t e r than the chain e x t e n s i o n , then a tough, high modulus, low elongat i o n sealant r e s u l t s . The d e t a i l s of these systems are s t i l l q u i t e p r o p r i e t a r y but i t ' s s u f f i c i e n t to say that c o n t r o l l e d c r o s s l i n k i n g i s p o s s i b l e . Very low modulus, high e l o n g a t i n g s e a l a n t s are a v a i l a b l e which



10.

KLOsowsKi

A N D G A N T


125

have a l l the performance expected of a s i l i c o n e i n terms o f weathering and s t a b i l i t y . The previous 800 to 1000% extension l i m i t f o r s e a l a n t s has been surpassed. The s e a l a n t s made by chain extension have a t y p i c a l elonga t i o n i n the ASTM t e n s i l e bar of 1600%. This t r a n s l a t e s to 50% compression and >50% e x t e n s i o n i n an expansion j o i n t w i t h no bleed. This o f f e r s tremendous design f l e x i b i l i t y to the a r c h i t e c t o r c o n s t r u c t i o n engineer. The bottom l i n e i s t h i s : the b u i l d i n g owner or other sealant user can now choose the modulus of s i l i c o n e he needs. He can have high modulus f o r an adhesive a p p l i c a t i o n , medium modulus f o r gen e r a l s e a l i n g and g l a z i n g , and low modulus w i t h very high elonga t i o n f o r a p p l i c a t i o n s such as expansion j o i n t s and highway j o i n t s . Each c l a s s of s i l i c o n e s e a l a n t has the t y p i c a l d u r a b i l i t y charact e r i s t i c s of s i l i c o n e s s i n c e the c r o s s l i n k i n g and/or chain exten s i o n forms only s t a b l e s i l o x a n e bonds. Long Term Value of S i l i c o n e Sealants Thus f a r we have addressed many of the i n i t i a l p r o p e r t i e s o f s i l i c o n e s e a l a n t s and the chemistry that leads to those p r o p e r t i e s . Many of the long term p r o p e r t i e s of s i l i c o n e s e a l a n t s can be summed up i n the general term, w e a t h e r a b i l i t y . Weatherability implies: Resistance to O x i d a t i o n , E s p e c i a l l y Ozone A t t a c k . S i l i c o n e s e a l a n t s maintain t h e i r performance a f t e r 50,000 hours i n an ozone r i c h atmosphere, surpassing a l l other s e a l a n t s . — This property r e s u l t s from the i n o r g a n i c c h a r a c t e r of the b a s i c chemical bonds. This inherent i n e r t n e s s i s one reason that s i l i c o n e s e a l a n t s e x c e l i n outdoor a p p l i c a t i o n s . Resistance to Water Damage. Water r e p e l l e n c y i s not unique to s i l i c o n e s e a l a n t s . Many organic m a t e r i a l s e x h i b i t s i m i l a r con t a c t angles w i t h water; but, s i l i c o n e s have the a b i l i t y to main t a i n t h i s property longer due to a molecular o r i e n t a t i o n of the s i l o x a n e molecule to a p a r t i c u l a r s u b s t r a t e . E s s e n t i a l l y , the hydroxy groups on the s u b s t r a t e e i t h e r bond to the r e s i d u a l hydroxy-ends of the s i l o x a n e system or hydrogen-bonds are formed w i t h the s i l o x a n e u n i t s . This can cause the molecule to o r i e n t i t s e l f such t h a t the organic s i d e groups ( i . e . , methyl) are d i s p l a c e d toward the outer s u r f a c e , thereby exposing the hydrophobic p o r t i o n . This o r i e n t a t i o n i s made p o s s i b l e by the low b a r r i e r of r o t a t i o n around the s i l o x a n e , Ξ8ί-0-8ίΞ bond. Resistance to Photochemical Reaction I n i t i a t e d by U.V. L i g h t (the sun). With the i n t r o d u c t i o n and acceptance of s i l i c o n e s e a l a n t s the s t r e s s t e s t s have had to be made more s t r i n g e n t . F i l t e r s are removed from the Weather-O-Meter t e s t s so some change can be noted i n 6000 hours. P r e v i o u s l y , s e a l a n t s were considered good i f performance changes were not s i g n i f i c a n t a f t e r 250 hours. In f a c t , s i l i c o n e s e a l a n t s are now used to s e a l the t e s t chamber of weather-simula t i n g m a c h i n e s . — Resistance to Hardening or Cracking. F u l l y cured s e a l a n t s



126

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S

made s o f t and p l i a b l e by the a d d i t i o n of s o l v e n t - l i k e p l a s t i c i z e r s crack and harden as the solvent evaporates. This hardening and c r a c k i n g can a l s o be a consequence of U.V.-induced f r e e - r a d i c a l p o l y m e r i z a t i o n at the s u r f a c e . More c r o s s l i n k s are introduced and the s e a l a n t hardens, becomes l e s s rubbery and cracks when s t r e s s i s a p p l i e d . S i l i c o n e s c o n t a i n l i t t l e or no v o l a t i l e p l a s t i c i z e r and don't e a s i l y r e a c t to U.V. r a d i a t i o n . Twenty years of outdoor t e s t s i n F l o r i d a shows no hardening or c r a c k i n g . Usable i n Cold Temperatures. S i l i c o n e s are extrudable from the p a i l or c a r t r i d g e at temperatures down to -45°C w i t h no speci a l h e a t i n g r e q u i r e d . A f t e r i t cures, the rubber s e a l formed i s f l e x i b l e , w i l l expand and c o n t r a c t and continue to s e a l at -45°C. Most other s e a l a n t s become s t i f f and b r i t t l e before -30°C i s reached. This means when j o i n t s have moved to t h e i r u l t i m a t e dimensions and s t r e s s i s maximum, s i l i c o n e s e a l a n t s w i l l s t i l l perform. Usable i n Hot Climates. Most organic s e a l a n t s perform s a t i s f a c t o r i l y when temperatures are warm. But s i l i c o n e s remain e f f e c t i v e i n such hot l o c a t i o n s as the sandwich panel of a s o l a r c o l l e c t o r or around heat ducts or hot p i p e s . However, the designer must choose the r i g h t s i l i c o n e , s i n c e some perform up to 150°C, some to 200°C and 250°C. These temperatures can be contrasted to the 70°C to 120°C o p e r a t i n g maximum of n o n s i l i c o n e s . In a d d i t i o n to t h e i r e x c e l l e n t w e a t h e r a b i l i t y and the benef i t s that i m p l i e s , s i l i c o n e s are sometimes the sealant of choice because of t h e i r c o n t r o l l a b l e cure r a t e ( i t can be f a s t or s l o w ) , or adhesion q u a l i t i e s ( s i l i c o n e s can bond g l a s s together to form an aquarium w i t h no other support or reinforcement necessary or bond g l a s s i n a b u i l d i n g w i t h no other supports n e c e s s a r y ) , or i t s n o n t o x i c i t y (many s i l i c o n e s have FDA, USDA c l e a r a n c e ) , or i t s nonconductive e l e c t r i c a l q u a l i t i e s . S i l i c o n e s e a l a n t s r e t a i n t h e i r e x c e l l e n t adhesion p r o p e r t i e s and t h e i r e x c e l l e n t nonconductive or i n s u l a t i o n p r o p e r t i e s upon aging under heat and i n harsh environments, again because of the s t a b l e bonds formed i n t e r n a l l y and to the s u b s t r a t e . The l i m i t s have not been f u l l y e s t a b l i s h e d , but a c t u a l in-use a p p l i c a t i o n s have surpassed 15-20 years without a s i g n i f i c a n t c h a n g e . — This r e s u l t s i n a confidence f a c t o r that i s very important to the q u a l i t y s p e c i f i e r , engineer or owner who i s i n t e r e s t e d i n c o s t - e f f e c t i v e c o n s t r u c t i o n w i t h long-term u s a b i l i t y . A l l the p r o p e r t i e s mentioned above are the r e s u l t of the i n t r i n s i c q u a l i t i e s of the s i l i c o n e or s i l a n e components of the sealant and the chemistry of the f o r m u l a t i o n . These i n t r i n s i c q u a l i t i e s cannot be d u p l i c a t e d i n organic s e a l a n t s w i t h or without s i l i c o n - c o n t a i n i n g a d d i t i v e s . The chemistry of s i l i c o n e s e a l a n t s i s , to a l a r g e degree, a balance between the chemistry of sand or quartz and the chemistry of hydrocarbon s i d e groups. This unique balance of c h e m i s t r i e s has r e s u l t e d i n a well-deserved "top-ofthe-line" reputation for s i l i c o n e sealants.


10. KLOSOWSKI AND GANT


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Literature Cited 1. 2. 3. 4.


5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Hyde, J. F., U.S. Patent No. 2,571,039 (1951). France Patent No. 1,188,495 (1959). Bruner, L . B . , U.S. Patent No. 3,077,465 (to Dow Corning Corp.) (1963). N o l l , W., "Chemistry and Technology of Silicones", Academic Press, New York, New York, 2nd Edition 1968 p. 305. Roth, W. L., J. Am. Chem. Soc., 1947, 69 p. 474. Information About Silicone Sealants - Dow Corning 888 Highway Joint Sealant. Bulletin 61-408-78 (1978). British Patent No. 1,054,658 (1967). Rochow, E . G . , LeClair, H. G . , J. Inorg. Nucl. Chem. (1955), 1, p. 92. N o l l , W., "Chemistry and Technology of Silicones", Academic Press, New York, New York, 2nd Edition, 1968, p. 464. N o l l , W., i b i d , p. 459. Crossan, I . D . , J. Appl. Pol. Sci.: Appl. Pol. Symp., (1977) 32, p. 421. Brown, E . D . , Hyde, J.F., U.S. Patent No. 3,334,062 (1967). Smith, A . H . , U.S. Patent No. 3,635,743 (1972). Smith, A.L., Angelotti, N . C . , Spectrochimica Acta. (1959), 412-420. [Smith, A . L . ; Value for CF CH CH Si(OAc) , private communication.] "Weather Tester 'Climate' Doesn't Faze Silicone Sealant". Materials News, Dow Corning Corp., Nov.-Dec. 1978, p. 5. C a h i l l , J. M . , Building Research, Building Research Institute Inc., May-June, 1966, 38-39. Unpublished studies, Dow Corning Corporation. 3

15. 16. 17.

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RECEIVED April 2, 1979.


Plastic Mortars, Sealants, and Caulking ... - American Chemical Society

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