Development of Test Procedure for Predicting Performance of

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12 Development of Test Procedure for Predicting Performance of Sealants Κ.K.KARPATI

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Building Materials Section, Division of Building Research, National Research Council of Canada, Ottawa, Canada K1A 0R6 The outside envelope of a building undergoes cyclic movements in responding to changing temperature and moisture conditions. It may consist of building elements such as panels of various materials with joints between or large wall sections interrupted by expansion joints. These building elements or sections expand and contract as one unit with any change of temperature or moisture content and the resulting movements are accommodated at the j o i n t . To prevent the passage of water, a i r or dust the joints must be sealed. Organic materials, called sealants, applied as viscous liquids are used for this purpose. To regulate the cross-section of the sealant bead, a solid back-up material that is most frequently a flexible closed-cell polyethylene foam rod of circular cross-section is f i r s t placed into the joint. It has to be wider than the joint and i s forced into the opening i n such a way as to allow the application of the sealant to an even depth. The liquid sealant i s applied against the back-up material, to which it has no adhesion, but adheres to the edges of the building elements. The sealant is so formulated that i t keeps i t s shape as applied and hardens through chemical or physical processes to form a viscoelastic rubber-like material that withstands extension or compression. The sealant is extended at low temperatures and compressed at high temperatures because the building elements meeting at the joint contract with decreasing temperature and expand with rising temperature. The demands on sealants are severe because of the large temperature changes imposed on the outside of buildings, especially in cold climates. To investigate their performance, a testing procedure i s needed that provides a connection between laboratory and outdoor behavior. A great many factors influence sealant performance and systematic experimenting is needed to find a way of testing that defines the essential sealant properties and discards those that are secondary. Such a test procedure is based on complex relations between the different factors that govern polymer behavior, but it can be made easy to perform by means of 0-8412-0523-X/79/47-113-157$05.75/0 © 1979 American Chemical Society

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

158

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

various s i m p l i f y i n g steps i n the development of p r a c t i c a l t e s t i n g conditions.

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T e n s i l e Test The f a c t o r s that govern the behavior o f sealants are: s t r e s s , s t r a i n , temperature, r a t e o f deformation, humidity, a i r , l i g h t , type and c o n d i t i o n o f s u b s t r a t e , and presence of water or chemicals. Of these, s t r e s s , s t r a i n , deformation r a t e , and temperature are o f primary importance. They have t o be known simultaneously i n d e s c r i b i n g a m a t e r i a l at any given age, humidity, s u b s t r a t e c o n d i t i o n , e t c . Consequently, these four f a c t o r s must be included i n any t e s t method devised t o measure mechanical p r o p e r t i e s , w h i l e the other v a r i a b l e s must be kept constant a t values considered t o be r e a l i s t i c . C y c l i c t e s t s provide the best r e p r e s e n t a t i o n of the c o n d i t i o n s t o which s e a l a n t s are subjected i n p r a c t i c e . They are v e r y complex t e s t s , however, and can be designed s a t i s f a c t o r i l y o n l y i f the m a t e r i a l p r o p e r t i e s are w e l l known from the r e s u l t s o f t e s t s using simpler loading p a t t e r n s and i f the r a t e s are r e l a t e d t o those of a c t u a l j o i n t s . T e n s i l e extension a t constant r a t e , s t r e s s r e l a x a t i o n under constant s t r a i n , and creep under constant s t r e s s are three o f the simpler t e s t s used t o o b t a i n the m a t e r i a l p r o p e r t i e s o f polymers. T e n s i l e extension i s not the simplest of the three t e s t s (of the f o u r b a s i c v a r i a b l e s o n l y temperature can be kept constant), but i t has been chosen because i t i s t h i s type of l o a d i n g that occurs i n the sealant i n a j o i n t when the chance of f a i l u r e i s most probable. There i s l e s s l i k e l i h o o d o f f a i l u r e when the sealant i s compressed i n summer than when i t i s extended i n w i n t e r . In a d d i t i o n , the t e n s i l e t e s t i s the l e a s t timeconsuming and most l a b o r a t o r i e s are equipped f o r i t . Model Specimens A f t e r s e l e c t i n g the t e n s i l e t e s t as the b a s i c method f o r i n v e s t i g a t i n g sealant behavior, i n both l a b o r a t o r y t e s t s and outdoor performance, the s i z e and shape of the specimen have t o be considered. T e n s i l e t e s t s are u s u a l l y c a r r i e d out on dumbbell o r ring-shaped specimens, s t r e s s f i e l d s o f which remain p a r a l l e l during extension. The true s t r e s s can be c a l c u l a t e d , t h e r e f o r e , through the minimum c r o s s - s e c t i o n a t any time during the experiment. Sealant beads i n b u i l d i n g j o i n t s , however, have an extremely complicated s t r e s s f i e l d because the side of the bead curves i n on extension and the s t r e s s changes d i r e c t i o n , c o n c e n t r a t i n g at the ends and edges as extension progresses. Consequently, the specimen chosen f o r the i n v e s t i g a t i o n i s a model of the s e a l a n t bead used i n b u i l d i n g j o i n t s , i . e . , i t has a s t r e s s f i e l d s i m i l a r t o t h a t of the sealant i n a b u i l d i n g j o i n t . The model can f a i l e i t h e r c o h e s i v e l y o r a d h e s i v e l y , as does a sealant i n a j o i n t . T h i s i s a d i s t i n c t advantage compared with dumbbell

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

12.

KARPATi

Testing Plastic Mortars and Sealants

159

or ring-shaped specimens that can f a i l only c o h e s i v e l y . The model also permits the study of adhesive p r o p e r t i e s of sealants on v a r i o u s s u b s t r a t e s . Another advantage of the model specimen i s that most standards c a l l f o r i t and many i n v e s t i g a t o r s use i t , so that data are a v a i l a b l e f o r comparison of m a t e r i a l s or c o n d i t i o n s . The s i z e of the sealant bead i s i χ i χ 2 i n . (1.3 χ 1.3 χ 5.1 cm), as i l l u s t r a t e d i n Figure 1, unextended and extended, the l a t t e r showing curved s i d e s .

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D e r i v i n g the Test Procedure Three-Dimensional Representation. Whatever the c o n f i g u r a t i o n of the specimen, an i n f i n i t e number of t e n s i l e curves can be obtained by changing e i t h e r temperature or t e n s i l e extension r a t e . Figure 2, f o r one-part c h e m i c a l l y curved s i l i c o n e , and Figure 3, for two-part p o l y s u l f i d e model specimens, show a few examples t h a t are obtained by v a r y i n g the t e s t c o n d i t i o n s . The question a r i s e s , which of the t e n s i l e curves d e f i n e s the mechanical p r o p e r t i e s of a s e a l a n t unambiguously? I t i s known t h a t f o r some polymers, and using specimens with p a r a l l e l s t r e s s f i e l d s , a l l t e n s i l e curves can be reduced t o a s i n g l e one at a given temperature i f the s t r e s s and s t r a i n at each point of the curve are d i v i d e d by the s t r a i n r a t e (producing time as abscissa) and i f an e m p i r i c a l c o r r e c t i o n f a c t o r , λ, the extension r a t i o , i s a p p l i e d t o s t r e s s ( 1 ) . This treatment o f r e s u l t s has a l s o been a p p l i e d (2) to t e n s i l e curves obtained w i t h the model specimens. The r e s u l t i n g s i n g l e curves of u n i t s t r a i n r a t e are shown i n Figures 4 and 5, derived from Figures 2 and 3, r e s p e c t i v e l y . As may be seen, the f i t of the i n d i v i d u a l t e n s i l e curves t o the s i n g l e curve of u n i t s t r a i n r a t e i s e x c e l l e n t f o r s i l i c o n e and w i t h i n acceptable l i m i t s f o r p o l y s u l f i d e s e a l a n t s . This proves that the model specimen can be used f o r an i n v e s t i g a t i o n intended to e s t a b l i s h the interdependence of the t e n s i l e curves. I t would permit the d e r i v a t i o n of a simple and r a t i o n a l method f o r presenting the deformation c h a r a c t e r i s t i c s o f sealing materials. The u n i t s t r a i n r a t e curves are not f u l l y s u i t a b l e f o r d e s c r i b i n g the behavior o f s e a l a n t s , f o r they give l i t t l e i n f o r m a t i o n on f a i l u r e behavior because the f a i l u r e p o i n t s f o r given c o n d i t i o n s f a l l at v a r i o u s p o i n t s on them. To i n v e s t i g a t e f a i l u r e p r o p e r t i e s as a f u n c t i o n of the f o u r b a s i c v a r i a b l e s a three-dimensional system has to be used: an example i s shown i n Figure 6 f o r two-part p o l y s u l f i d e s e a l a n t . Each l i n e represents a t e n s i l e curve, c a l c u l a t e d from the o r i g i n a l curves, and the f a i l u r e p o i n t s form the upper curved edge of the three-dimensional representation. There are s e v e r a l steps of c a l c u l a t i o n i n v o l v e d i n a r r i v i n g at the three-dimensional p r e s e n t a t i o n . They are necessary i n order t o reduce the number of v a r i a b l e s from f o u r to t h r e e . The f i r s t step i n o b t a i n i n g the curves i n Figure 6 i s t o r e c a l c u l a t e

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

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PLASTIC

Figure 1.

MORTARS,

SEALANTS,

AND

CAULKING

Original and extended model specimens (4)

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

COMPOUNDS

KARPATi

161

Testing Plastic Mortars and Sealants

1

1

35

1

F

-

*

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/

/

/

/

H * /

/ /

/

RATES 0F EXTENSION

/ / i i

I *

/

*

CM/MIN. 5.000 1.000 0.1000 0 05000 0.01000

/

'

0.001 000 0.0005000

Δ Β D Ε F H I

IN./MIN. 1.969 0.394 00394 0.01 969 000394 0 000394 0 0001969

20 ;

/

/ 200

/ Β / / / / * /

/

··/

λ / / / ψ /.y /ν ~ψ

-40

1

1

e

F

1

100 200 300 PERCENT EXTENSION

400

0 100 200 PERCENT EXTENSION

Journal of Paint Technology

Figure 2. Tensile curves of silicone specimens at different extension rates and two temperatures (2)

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

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

100

200

300

ιοο

Figure 3.

1/

Η/

ζ*

; /

/

/

/

..c

0.0005000

300

0.0001969

0.001969

0.01969

400

10

20

200h

100

cm/min

0.1969 0.0394

C 0.5000 D 0.1000 0.00394

F o.oiooo

%

300

Journal of Paint Technology

EXTENSION

200

° INTERMEDIATE POINT

• BREAK POINT

T = 73F (22.80

0.01969

0.05000

Ε

0.394

1.969

in/min

Β 1.000

A 5.000

4C

Tensile curves of polysulfide specimens at different extension rates and two tempera­ tures (4)

EXTENSION

200

Η

G 0.005000

0.05000

0.1969

Ε

in/min

cm/min

C 0.5000

ο INTERMEDIATE POINT

. BREAK POINT

Τ =-30F ( - 3 4 . 4 0

300

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

1.00

78 7 39 37 7 78 3 937 0 787 0 079 0 039 0 016 0 004

0 1969 0 0394 0 01969 0 00394 0000394 0 0001969 00000787 0.00001969

1.000 0 5000 1000

0

0 05000 0 01000 0 001000 0 0005000 0.0002000

C D

F H I J

LOG TIME (MIN)

1 3 00

! 2 00

1 100

Journal of Paint Technology

D

Κ : 0.00005000

4 00

1

393 7 1969 0 394

5.000

E-

extension/mm

in / m i n cm/min

A Β

POINTS

• β

Figure 4. Tensile curves reduced to unit strain rate for silicone specimens at different tempera­ tures (2); λ, extension ratio (empirical correction (2)); S, load; and R, strain rate.

LOG TIME (MIN.)

0.00

POINTS

7 INTERMEDIATE

• BREAK

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164

PLASTIC MORTARS, SEALANTS, AND CAULKING COMPOUNDS ll.OOr

Τ =T =-30 F (-34.40 0

10.00 h

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/y

C Ε 6 H

/

8.00h

/y

•is

cm/min 0.5000 0.05000 0.005000 0 0005000

in/min 0.1969 0.01969 0.001969 0 0001969

• BREAK POINT

7.00h

° INTERMEDIATE POINT

6.00 -2 00

000

1.00 log t(min)

Έ

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

log t (min)

Journal of Paint Technology Figure 5. Tensile curves reduced to unit strain rate for polysulfide specimens at different temperatures (4); λ, extension ratio; R, strain rate; T, test temperature, degree Kelvin; T , reference temperature, degree Kelvin; t, time; and S, load. 0

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

KARPATi

Testing Plastic Mortars and Sealants

165

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12.

Figure 6. The property surface for polysulfide sealant (4); λ, extension ratio; S, load; T, test temperature, degree Kelvin; T , reference temperature, degree Kel­ vin; t, time; and SL , time-temperature shift factor. 0

t

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

166

PLASTIC MORTARS, SEALANTS, AND CAULKING COMPOUNDS

each p o i n t of the o r i g i n a l t e n s i l e curves f o r p l o t s of the type shown i n Figure 7. The a b s c i s s a i s the l o g a r i t h m of time, d e r i v e d by d i v i d i n g the s t r a i n by the s t r a i n r a t e , and the o r d i n a t e i s the load c o r r e c t e d f o r " t r u e s t r e s s " by λ, the extension r a t i o , and by T /T, the r e f e r e n c e temperature d i v i d e d by the t e s t temperature, both i n degrees K e l v i n ( 2 ) . The temperature c o r r e c t i o n i s t h a t r e q u i r e d by the theory of r u b b e r l i k e e l a s t i c i t y . F i g u r e 8 i s the p r o j e c t i o n of the o r i g i n a l t e n s i l e curves on the s t r e s s versus time plane. These curves are shown as dotted l i n e s i n F i g u r e 7. The continuous l i n e s are the best f i t s connecting the t e n s i l e curves at 5, 10, 15, 20, 25, 40, 60, 80, 120, 160 and 200 per cent extensions. From t h i s p l o t the i s o c h r o n a l s t r e s s - s t r a i n curves can be d e r i v e d by reading values from the best f i t t i n g l i n e s f o r a given time. The times chosen were those at which f a i l u r e occurred f o r each specimen. The a c t u a l breaking s t r e s s e s and a s s o c i a t e d s c a t t e r were thereby preserved. The i s o c h r o n a l curves are shown i n the background of F i g u r e 6 i n the log s t r e s s - l o g s t r a i n plane. The t h i r d dimension i s added by s h i f t i n g the curves along the time a x i s , which i s p e r p e n d i c u l a r to t h i s plane, according to the time at which each curve was read. T h i s procedure i s f o l l o w e d f o r each temperature, and w i t h the help of the WLF s h i f t f a c t o r , a , curves obtained at a l l temperatures are i n c o r p o r a t e d i n the one three-dimensional r e p r e s e n t a t i o n . I t must be recognized t h a t f o r s i l i c o n e s e a l a n t s the timetemperature s u p e r p o s i t i o n was not necessary because the u n i t s t r a i n r a t e curves f e l l on the same cumulative s i n g l e l i n e at each temperature (Figure 4 ) . In other words, the s i l i c o n e s e a l a n t was i n s e n s i t i v e t o temperature changes w i t h i n the temperature r e g i o n observed and w i t h i n experimental e r r o r . The WLF s h i f t f a c t o r was a l s o i n v e s t i g a t e d and e x p e r i m e n t a l l y d e r i v e d f o r p o l y s u l f i d e s e a l a n t u s i n g the model specimen ( 4 ) . The b e s t - f i t t i n g (continuous) l i n e s of the v a r i o u s extensions (Figure 7) were used as g u i d e l i n e s f o r manually s h i f t i n g the p l o t s obtained at d i f f e r e n t temperatures along the time a x i s u n t i l the l i n e s f o r each e x t e n s i o n formed a smooth curve (Figure 8 ) . From the measured s h i f t s the constants o f the WLF equation, o f t e n r e f e r r e d to as " u n i v e r s a l constants," were c a l c u l a t e d and compared w i t h constants obtained f o r other polymers. The d i f f e r e n c e between those c a l c u l a t e d here and the u n i v e r s a l constants was s m a l l , but i t was l a r g e enough to r e q u i r e use o f the former i n s h i f t i n g the s e a l a n t data. The surface formed by the c a l c u l a t e d curves s h i f t e d along the log t - l o g Βτγ a x i s (Figure 6) i s the property surface of the p o l y s u l f i d e s e a l a n t . A s i m i l a r s u r f a c e could be d e r i v e d f o r the s i l i c o n e s e a l a n t . The three-dimensional system gives a complete and coherent d e s c r i p t i o n o f s e a l a n t p r o p e r t i e s , i . e . , from a s i n g l e t e n s i l e curve any other can be c a l c u l a t e d once t h i s system i s known. I t i s , however, too complex f o r everyday use and i n the next phase of the i n v e s t i g a t i o n steps were taken to d e r i v e a simpler way o f c h a r a c t e r i z a t i o n .

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0

T

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

KARPATi

Testing Plastic Mortars and Sealants 1.00 T =-30F (-34.40

T =73 F ( 2 2 . 8 0

0

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7.00

• BREAK POINT ° INTERMEDIATE POINT

J L 5.00 -4.00 -3.00 -2.00 -1.00

0.00

cm/min A 5.000

in/min 1.969

Β I 000 C 0.5000 D 0.1000 Ε 0.05000 F 0.01000

0.394 0.1969 0.0394 0.01969 0.00394

1.00

J 2.00

L 3.00

4.00

5.00

log t(min)

Journal of Paint Technology Figure 7.

Time dependence of stress at 73°F (4)

T = T = -30F(-34.4C) 0

J -4.00 -3.00 -2.00 -100

0.00 1.00 2.00 3.00

4 00

I I L 5.00 6.00 7.00

8.00

log t - log Qj

Journal of Paint Technology Figure 8.

Determination of shift factors (4)

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

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P r o j e c t i o n s o f the Three-Dimensional System. To s i m p l i f y the three-dimensional system, i t s p r o j e c t i o n s i n t o the v a r i o u s planes can be used. In p a r t i c u l a r , the p r o j e c t i o n of the break p o i n t s i s important because they define the l i m i t a t i o n s sealants have i n p r a c t i c e . Their p r o j e c t i o n i n the log s t r e s s - l o g s t r a i n plane i s the f a i l u r e envelope (5) shown f o r p o l y s u l f i d e s e a l a n t i n Figure 9. The outer l i m i t of the envelope i s w e l l d e f i n e d and i s drawn i n w i t h a dashed l i n e , but the inner one disappears i n the s c a t t e r . For the s i l i c o n e s e a l a n t , Figure 10 gives the f a i l u r e envelope where both the upper and lower l i m i t s are w e l l d e f i n e d . The f a i l u r e envelope i s used i n the l i t e r a t u r e to c h a r a c t e r i z e polymers because i t i s independent of time and temperature, but i t s usefulness i s l i m i t e d w i t h s e a l a n t s . From the p o i n t of view of sealant performance, the p r o j e c t i o n o f the f a i l u r e p o i n t s i n t o the l o g s t r a i n - l o g time plane i s the most important c h a r a c t e r i z a t i o n ; i t i s the s t r a i n t h a t i s imposed on the sealant by the movement of the j o i n t and the s t r e s s develops as a consequence of the imposed s t r a i n . Consequently, the design of a sealed j o i n t i s u s u a l l y based on an estimate o f s t r a i n , not of s t r e s s , and the s e a l a n t i s chosen according to i t s movement c a p a b i l i t y , t h a t i s , the ± per cent movement the sealant can take without f a i l u r e i n a y e a r l y movement. S t r e s s has to be considered only i n those r a r e cases where the substrate i s a f r a g i l e , porous m a t e r i a l whose t e n s i l e s t r e n g t h approaches t h a t of s e a l a n t s . In t h i s case, a sealant w i t h the lowest strength p o s s i b l e has to be chosen. P r o j e c t i o n s of the f a i l u r e p o i n t s are shown i n Figures 11 and 12 f o r s i l i c o n e and p o l y s u l f i d e s e a l a n t s , r e s p e c t i v e l y . The p o i n t s p l o t t e d i n the curves represent the s t r a i n at break at the time needed t o reach the break. For s i l i c o n e s e a l a n t s i t was found t h a t the break p o i n t s obtained at room temperature are s u f f i c i e n t f o r a f i n a l a n a l y s i s . For p o l y s u l f i d e , data obtained at seven d i f f e r e n t temperatures are used, reduced to -30°F (-34.4°C) . The time dependence of the s t r a i n at break i s very d i f f e r e n t f o r the two types of s e a l a n t . The break p o i n t s o f the s i l i c o n e data can be f i t t e d by a s t r a i g h t l i n e , and confidence l i m i t s at v a r i o u s l e v e l s can be drawn on the p l o t (Figure 11). The break p o i n t s of the two-part p o l y s u l f i d e sealant form a broad band, the upper and lower l i m i t s o f which are drawn q u a l i t a t i v e l y . The upper l i m i t i s b e t t e r defined than the lower one (as f o r the f a i l u r e envelope). Because of the d i f f e r e n c e between the p l o t s f o r s i l i c o n e and p o l y s u l f i d e s e a l a n t s the f u r t h e r s i m p l i f i c a t i o n o f c h a r a c t e r i z a t i o n i s d i f f e r e n t f o r the two types of s e a l a n t . S i l i c o n e Sealant. F a i l u r e of the s i l i c o n e specimens occurred at i n c r e a s i n g l y longer times w i t h decreasing s t r a i n r a t e s , covering 4 i time decades, measured i n minutes. With an e x t r a p o l a t i o n o f l i time decade, the extension at f a i l u r e at h a l f a year can be estimated. In Figure 11 the value obtained i s 28 per cent,

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

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

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Journal of Paint Technology Failure envelope of silicone sealant (2)

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

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

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

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Testing Plastic Mortars and Sealants

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w i t h 95 per cent confidence l i m i t s of 57 and 14; that i s , 95 out of 100 t e s t s w i l l give f a i l u r e p o i n t s i n s i d e t h i s confidence i n t e r v a l . Although the c h a r a c t e r i z a t i o n could be done at any time value, the choice of h a l f a year has the advantage of being a c r i t i c a l time i n the l i f e of a sealant as, on average, h a l f a year i s reached i n w i n t e r , at maximum e x t e n s i o n . I f a sealant does not f a i l during the f i r s t w i n t e r , there i s a good chance t h a t i t w i l l l a s t f o r many years. I t has t o be pointed out t h a t the s i n g l e value, 28 per cent, i s based on one batch of a one-part chemically-cured white s i l i c o n e s e a l a n t . This s i n g l e value i s presented o n l y as an i l l u s t r a t i o n o f the t e s t i n g procedure. One can expect v a r i a t i o n s from batch to batch and from manufacturer to manufacturer. Another important f a c t o r t h a t may i n f l u e n c e the c h a r a c t e r i s t i c s i n g l e value i s the aging process t o which the sealant i s subjected before t e s t i n g (one month a t 70°C i n t h i s c a s e ) . I t i s o n l y a f t e r i n v e s t i g a t i o n of a l l the above f a c t o r s t h a t a s i n g l e extension value w i t h i t s confidence l i m i t s can be d e r i v e d t o c h a r a c t e r i z e a l l chemically-cured s i l i c o n e sealants a v a i l a b l e on the market. Such work would need cooperation on the p a r t of manufacturers, but i t would remove much of the u n c e r t a i n t y i n v o l v e d i n sealed j o i n t design. P o l y s u l f i d e Sealant. The s i n g l e value and i t s confidence l i m i t s have to be derived i n a d i f f e r e n t manner f o r two-part p o l y s u l f i d e sealants because of the f l a t maximum formed by the break p o i n t s . Another d i f f e r e n c e from the r e s u l t s obtained f o r s i l i c o n e s e a l a n t s i s that the t e n s i l e p r o p e r t i e s change w i t h temperature. To c h a r a c t e r i z e extreme w i n t e r c o n d i t i o n s when f a i l u r e i s most probable the e x t e n s i b i l i t y i s given f o r -30°F (-34.4°C) (Figure 12). According to the lower l i m i t (drawn by dashed l i n e ) , the sealant can l a s t about 20 years at about 100 per cent extension a t t h i s temperature. This estimate i s supported by only a few p o i n t s i n the neighborhood of t h i s reading. A b e t t e r way o f c h a r a c t e r i z i n g the sealant i s to c o n s i d e r that a l l the readings belong to the same " p o p u l a t i o n " and c a l c u l a t e the mean o f the l o g a r i t h m i c s t r a i n ; t h i s can be done because of the f l a t maximum. From the mean and i t s standard d e v i a t i o n one can c a l c u l a t e the e x t e n s i b i l i t y at which o n l y 1 per cent of the t e s t s w i l l f a i l ; t h i s was 119 per cent i n the p a r t i c u l a r batch used. The same a p p l i e s f o r p o l y s u l f i d e as f o r s i l i c o n e s e a l a n t s : s e v e r a l manufacturers products and batches would have to be t e s t e d f o l l o w i n g the method described and the aging process i n v e s t i g a t e d i n order to d e r i v e an e x t e n s i b i l i t y value t h a t a p p l i e s to a l l good q u a l i t y two-part p o l y s u l f i d e s e a l a n t s . 1

C y c l i c Tests Sealants undergo d a i l y c y c l e s superimposed on the y e a r l y one as b u i l d i n g movements f o l l o w weather c o n d i t i o n s . To i n v e s t i g a t e

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the connection between t e n s i l e and c y c l i c a l behavior, model specimens of one-part chemically-cured s i l i c o n e sealant were s t r a i n c y c l e d on a t e n s i l e t e s t e r ( 6 ) . The amplitude and r a t e of the c y c l e s o c c u r r i n g i n p r a c t i c e were known from j o i n t movement i n v e s t i g a t i o n s (7_, S_, 9_, 10) . The average d a i l y movement i s about ±4 per cent of j o i n t width f o r a j o i n t that moves ±25 per cent a year. The s i l i c o n e sealant model specimen, the o n l y sealant type used f o r the c y c l i c a l experiments so f a r , showed rubbery behavior when c y c l e d at ±4 per cent. To cause f a i l u r e , the d a i l y c y c l e s had to be superimposed on the maximum y e a r l y extension. For example, specimens were c y c l e d between 25+4 and 25-4 per cent extensions. By choosing the c y c l e s i n t h i s manner the l a b o r a t o r y s t r a i n r a t e s were brought as c l o s e as p r a c t i c a l l y p o s s i b l e to a c t u a l j o i n t movement r a t e s . I t was a l s o necessary to d e f i n e the number of d a i l y c y c l e s to be imposed. Various c o n s i d e r a t i o n s led to the s e l e c t i o n of 120 c y c l e s . I t became c l e a r from the experiments t h a t a number of c y c l i c a l t e s t s at i d e n t i c a l c o n d i t i o n s were needed i n order to e s t a b l i s h the p r o b a b i l i t y of 50 per cent f a i l u r e . F a i l u r e s occur with a s c a t t e r , and i f a l l t e s t s pass or f a i l one does not know what the f a i l u r e l i m i t i s . In a s e r i e s of experiments the i n i t i a l extension, the superimposed c y c l e s , and the r a t e of movement were v a r i e d . For the p a r t i c u l a r batch of s i l i c o n e sealant used i t was found that f o r a 25 per cent extension a c y c l i c a l movement of ±8 per cent had to be superimposed at a r a t e of movement of 1.0 cm/min i n order t o achieve c l o s e to (but l e s s than) 50 per cent f a i l u r e i n the number of specimens t e s t e d . Lower r a t e s or higher extensions a t the same r a t e of movement produced more f a i l u r e s . For example, the number of f a i l u r e s increased to f o u r - f i f t h s of the t o t a l number of t e s t s when e i t h e r the above ±8 per cent to ±12 per cent were increased or when the r a t e from 1.0 to 0.1 cm/min was decreased. These r e s u l t s can be explained i f they are examined i n conjunction w i t h Figure 13, which i s s i m i l a r to Figure 11 except that another batch of sealant was used. The s t r a i n s and r a t e s used i n the c y c l i c a l t e s t s can be l o c a t e d i n t h i s p l o t . The f a i l u r e p o i n t s at each r a t e form a l i n e almost perpendicular to the b e s t - f i t l i n e , the alignment of the points being i n d i c a t e d by arrows. Close to the i n t e r s e c t i o n of a l i n e formed by the break p o i n t s and the lower confidence l i m i t one may read the extension on which the d a i l y c y c l e should be superimposed t o o b t a i n approximately 50 per cent f a i l u r e i n a c y c l i c a l experiment. The i n i t i a l l y imposed s t r a i n w i t h the superimposed d a i l y c y c l e s should reach l e s s than h a l f way t o the b e s t - f i t l i n e . The f u r t h e r one enters i n t o the confidence l i m i t s by changing any of the c y c l i n g c o n d i t i o n s , i . e . , proceeding v e r t i c a l l y on the p l o t by i n c r e a s i n g the s t r a i n or h o r i z o n t a l l y toward decreasing s t r a i n r a t e , the greater the chances of f a i l u r e I f t h i s c o r r e l a t i o n of t e n s i l e breaks and f a i l u r e s obtained from the c y c l i n g t e s t s i s v a l i d at r a t e s of the n a t u r a l l y o c c u r r i n g average d a i l y c y c l e s (0.00014 cm/min ( 6 ) ) , then f a i l u r e i n the j o i n t can be p r e d i c t e d from the t e n s i l e data. From Figure

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13 one reads approximately 13 per cent extension a t 0.00014 cm/min r a t e at the lower confidence l i m i t . This p r e d i c t s a f a i l u r e f o r t h i s p a r t i c u l a r batch o f s i l i c o n e sealant a t a much lower y e a r l y extension than the ±20 t o ±25 per cent claimed by manufacturers as movement c a p a b i l i t y . The batch used f o r o b t a i n i n g the data p u b l i s h e d by K a r p a t i (2) i s , however, w i t h i n the claimed v a l u e s . Apart from the large v a r i a t i o n from batch to batch, t h i s shows t h a t the c o r r e l a t i o n o f t e n s i l e data w i t h c y c l i n g t e s t s should be e x p e r i m e n t a l l y v e r i f i e d . Laboratory c y c l i n g t e s t s a r e i m p r a c t i c a l at t h i s r a t e o f one c y c l e per day and the c y c l i n g f a c i l i t i e s used f o r exposure o f s e a l a n t s have t o be u t i l i z e d . Weathering Sealants Once a simple but e f f e c t i v e means o f c h a r a c t e r i z i n g t h e sealant by a l a b o r a t o r y t e s t method was found, outdoor exposures were s t a r t e d . The r e s u l t i n g changes i n sealant p r o p e r t i e s a r e being f o l l o w e d by t h i s t e s t procedure as w e l l as by v i s u a l assessment of the degree o f f a i l u r e . D i f f e r e n t methods o f exposing s e a l a n t specimens to outdoor weathering are used: a s t r a i n c y c l i n g weathering rack, a rack t h a t imposes no movement on the specimens, and v i c e - t y p e devices where the movement i s produced by manual adjustments. A v e r t i c a l weathering rack 12.5 m (41 f t ) long and 1.1m (3.5 f t ) wide, accommodating 216 model specimens (11), was e r e c t e d i n a p o s i t i o n f a c i n g south (Figure 14). The rack u t i l i z e s the d i f f e r e n c e i n the thermal c o e f f i c i e n t s o f expansion o f s t e e l and aluminum t o produce c y c l i c movements i n response to temperature changes. I t has a r i g i d s t e e l frame t o which aluminum bars are attached a t one end, l e a v i n g them f r e e t o move on the other end. D i f f e r e n t i a l movement between the b a r and the frame i s t r a n s f e r r e d to the sealant specimens w i t h the help o f v e r t i c a l aluminum p l a t e s attached t o the s t e e l o r t o the aluminum b a r s . Figure 15 shows the b o l t i n g o f the specimens t o the v e r t i c a l p l a t e s . The frame i s so c o n s t r u c t e d t h a t on three q u a r t e r s o f the t o t a l area the y e a r l y movement increases from about ±9 t o ±30 per cent i n 36 increments, the amount o f movement being dependent on the weather of each year. On the remaining one q u a r t e r o f the area movement i s the same on a l l specimens, being about ±13 per cent per year. With t h i s arrangement the l i m i t i n g movement above which a sealant s t a r t s t o f a i l can be determined from a s e r i e s o f specimens exposed to v a r i o u s amounts o f y e a r l y movement. Specimens t h a t do not f a i l can be subjected t o the t e s t i n g procedure d e s c r i b e d above at chosen time i n t e r v a l s . V i s u a l assessment of the specimens p r o v i d e s i n f o r mation on t h e i r performance c h a r a c t e r i s t i c s ; the t e n s i l e t e s t s made on the specimens provide a b a s i s f o r the design of a c c e l e r a t e d aging t e s t s r e l a t e d to performance. The e v a l u a t i o n o f the performance o f v a r i o u s types o f s e a l a n t during a three-year p e r i o d i s i n progress and w i l l be p u b l i s h e d i n the near f u t u r e .

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

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S u f f i c i e n t l y large d i f f e r e n t i a l movements can o n l y be produced w i t h a l a r g e s i z e rack. Because few users o r manufact u r e r s can a f f o r d the c o n s t r u c t i o n o f t h i s r e l a t i v e l y expensive rack, there i s need f o r a simple, inexpensive device that can subject sealant specimens to extension and compression w h i l e exposed outdoors o r t o v a r i o u s imposed c o n d i t i o n s . The device shown i n Figure 16 was developed t o meet these requirements ( 1 2 ) . The device i s designed t o hold a s i n g l e specimen and to impose i n t e r m i t t e n t movement on the sealant by manual adjustment of width, i n c o n t r a s t w i t h the c y c l i c a l movement on the weathering rack where i t i s continuous and automatic. The device resembles a v i c e ; the d i s t a n c e between two aluminum b l o c k s can be adjusted by a screw t r a v e r s i n g the center of both b l o c k s so that the d i s t a n c e between them can be v a r i e d between 0 and 5 cm (2 i n . ) . The specimen i s attached t o the aluminum b l o c k s . Tension o r compression can be imposed on the sealant bead by v a r y i n g the distance between the b l o c k s . Up t o 300 per cent extension can be produced on a standard 1.3 cm (0.50 i n . ) specimen. Exposure and e v a l u a t i o n o f v a r i o u s types o f s e a l a n t u s i n g t h i s device are i n progress. Summary The p r o p e r t i e s o f high performance b u i l d i n g s e a l a n t s have been s t u d i e d as a f u n c t i o n o f f o u r v a r i a b l e s : s t r e s s , s t r a i n , time and temperature, u s i n g t e n s i l e t e s t s . By time-temperature s u p e r p o s i t i o n the number of v a r i a b l e s can be reduced t o three and the m a t e r i a l p r o p e r t i e s c h a r a c t e r i z e d i n a three-dimensional coordinate system. For s e a l a n t s the p r o j e c t i o n o f the f a i l u r e p o i n t s to t h i s system i n the l o g s t r a i n versus l o g time plane i s s u f f i c i e n t f o r c h a r a c t e r i z a t i o n . Further s i m p l i f i c a t i o n can be made depending on the p r o p e r t i e s o f the v a r i o u s types o f s e a l a n t . S i l i c o n e sealant t e n s i l e f a i l u r e data can be f i t t e d by a s t r a i g h t l i n e i n the l o g s t r a i n versus l o g time p l o t and the equat i o n o f the l i n e c h a r a c t e r i z e s the s e a l a n t . A p o s s i b l e f u r t h e r s i m p l i f i c a t i o n i s t o s p e c i f y as a c h a r a c t e r i s t i c o f the sealant the s t r a i n of f a i l u r e , w i t h i t s confidence l i m i t s , f o r an a r b i t r a r i l y chosen time. The f a i l u r e p o i n t s o f p o l y s u l f i d e sealants f a l l on a broad band w i t h a f l a t maximum. This f a m i l y o f sealants i s best c h a r a c t e r i z e d by the average of the l o g s t r a i n at f a i l u r e and the a s s o c i a t e d confidence l i m i t s f o r a s u f f i c i e n t number o f t e s t s . C y c l i c a l movements a r e more d i f f i c u l t t o analyse than t e n s i l e behavior and have been i n v e s t i g a t e d f o r s i l i c o n e s e a l a n t s o n l y . A connection between c y c l i c a l and t e n s i l e t e s t s has been found by comparing the s t r a i n at f a i l u r e f o r c y c l e t e s t i n g on a l o g s t r a i n versus log time p l o t w i t h t h a t f o r f a i l u r e i n simple t e n s i o n . This connection enables some degree of p r e d i c t i o n o f performance, i . e . , behavior f o r c y c l i n g t h a t occurs under n a t u r a l c o n d i t i o n s . The c y c l i c a l t e s t s have shown that lowering the c y c l i n g r a t e o r i n c r e a s i n g the s t r a i n at a given r a t e increases the p r o b a b i l i t y o f

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Figure 15. Bolting the specimens to the strain cycling weathering rack

Figure 16.

Cyclic movement device (12)

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f a i l u r e . I f i t can be shown that the r e l a t i o n between s t r a i n t o f a i l u r e f o r c y c l i c a l and t e n s i l e t e s t s i s a l s o v a l i d f o r c y c l i n g a t a d a i l y r a t e , then p r e d i c t i o n o f performance can be made based on t e n s i l e t e s t s . The r e s u l t s suggest t h a t f a i l u r e o f s i l i c o n e s e a l a n t might occur under n a t u r a l c y c l i n g c o n d i t i o n s a t a lower extension than the claimed e x t e n s i b i l i t y . A t slow r a t e s (one c y c l e per day) l a b o r a t o r y c y c l i n g i s not p r a c t i c a l . An exposure rack and a hand-adjusted device have been developed f o r imposing d a i l y c y c l e s o f s t r a i n s on s e a l a n t specimens. This i n v e s t i g a t i o n of the e f f e c t o f weathering and s t r a i n c y c l e s on sealants i s c o n t i n u i n g and r e s u l t s w i l l be p u b l i s h e d i n the near f u t u r e . This paper i s a c o n t r i b u t i o n from the D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council of Canada and i s published with the approval o f the D i r e c t o r of the D i v i s i o n .

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Received June 5, 1979. Seymour; Plastic Mortars, Sealants, and Caulking Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1979.