Development of Test Procedure for Predicting Performance of

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

Downloaded by EAST CAROLINA UNIV on January 12, 2018 | http://pubs.acs.org Publication Date: November 27, 1979 | doi: 10.1021/bk-1979-0113.ch012

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.


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


12.

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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 .


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



PLASTIC

Figure 1.

MORTARS,

SEALANTS,

AND

CAULKING

Original and extended model specimens (4)


COMPOUNDS

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0 100 200 PERCENT EXTENSION

Journal of Paint Technology

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



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

EXTENSION

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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


164

PLASTIC MORTARS, SEALANTS, AND CAULKING COMPOUNDS ll.OOr

Τ =T =-30 F (-34.40 0

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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


KARPATi


165


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


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 .


0

T


KARPATi

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

T =73 F ( 2 2 . 8 0

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0.394 0.1969 0.0394 0.01969 0.00394

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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

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4 00

I I L 5.00 6.00 7.00

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Determination of shift factors (4)



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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,



KARPATi


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Failure envelope of polysulfide sealant (4)



170


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Figure 10.

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


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