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Journal of Coatings Technology, 50, 641, pp. 27-30, 1978-06
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Device for weathering sealants undergoing cyclic movements
Karpati, K. K.
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National Research Council of Canada
Conseil national de recherches du Canada
DEVICE FOR WEATHERING SEALANTS
UNDERGOING CYCLIC MOVEMENTS
by K.K. Karpati
4'
Reprinted from
Journal of Coatings Technology Vol. 50, No. 641, June 1978 p. 27-30.'
BUILDING
RESEARCH-
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ADBR Paper No. 790
Division of Building Research
On a conCu et fabrique' un dispositif manuel simple qui permet de soumettre des e'chantillons individuels de produits d'e'tanche'ite' a des mouvements cycliques. Le dispositif'peut 2tre employe' dans le cadre d'essais de vieillissement accele're lorsqu'un des facteurs de vieillis- sement correspond aux mouvements cycliques pouvant 2tre acck- lere's par l'augmentation de l'amplitude des cycles. D'autres facteurs de vieillissement peuvent 2tre change's en m2me temps. Le m2me principe d'essai utilisant ce genre d'e'tau peut 2tre appliqui a tout mate'riau sujet des contraintes cycliques, detraction ou de compres- sion en pe'riode d'utilisation.
Device for Weathering Sealants
Undergoing Cyclic Movements
K.K. Karpati
National Research Council
of
Canada*.
manually operated device has been designed and pro- duced that will enable individual sealant specimens to be subjected to cyclical movements. This simple device can be used in accelerated testing where one of the aging factors is cyclical movements that can be accelerated by increasing the amplitude of the cycling. Simultaneously, other aging factors can be varied. The same principle of testing involving this type of device can be applied to any -aterial that is subject to cyclic, tensile, or compressivetresses in service.
INTRODUCTION
Laboratory testing to characterize the mechanical ca- pability of building sealants (one-part silicones and two-part polysulfides) has been developed to a consid- erable degree by the Division of Building Research,
National Research Council of Movements of
various types of joints in buildings have also been measured to provide information regarding the rates of strain and the elongation or compression to which seal-
ants may be subjected in service.', To study the
behavior of sealants undergoing normal aging condi- tions, a weathering rack was constructed6 to simulate service conditions by imposing continuous movement on the specimens attached to it: compression at high temperatures and tension at low temperatures. The movement is imposed as a differential movement be- tween aluminum bars and the steel frame of the rack undergoing outdoor temperature changes. Although the thermal coefficient of linear expansion of aluminum is twice that of steel, sufficiently large differential movements can only be attained with a large size rack.
Because few users or manufacturers can afford the co~istruction of a relatively expensive rack, there is need for a simple, cheap device that can subject sealant specimens to extension and compression while ex- posed outdoors or to various artificially produced con- ditions. The device to be described was developed to meet this requirement.
*Materids Section, Evision of Building Research, Ottawa, Canada.
DESCRIPTION OF DEVICE
The device is designed to impose intermittent move- ment on a sealant specimen by manual adjustment of its width. Each device holds a single specimen (the weath- ering rack cycles more than 200 specimens). Figure 1 shows the device with and without a specimen of stan- dard size (95 x M x 2 in., or 1.25
x
1.25 x 5.08 cm) sealant bead mounted on it. A failed specimen is used to allow a view of the adjusting screw behind the sealant bead. The screw traverses the center of the aluminum blocks and is locked at one end (Figure 1). The distance between the blocks can be adiusted bv means of this screw fromo
to 2 in. (5.08 c k ) with Bn accuracy of k0.005 in. (0.013 cm), measured by means of calipers at the ends of the sealant specimen. The regulating action is similar to that of a vice, and will be referred to as such. A mechanical drawing of the vice is shown in Figure 2. Once a specimen has been tightened to the aluminum blocks of the vice, tension or compression can be im- posed on the sealant bead by varying its width. On a standard one half inch wide specimen, a maximum of 300% extension can be produced. The compression that can be imposed is limited, for at 10Wo compression the sealant bead would be completely extruded. For a specimen with a width of less than one half inch the maximum percentage extension would be greater; cor- respondingly, for a specimen with greater than one half inch width the maximum percentage extension would be less, produced with a vice having a two-inch tbtal travel.The &% movement that a sealant can withstand withoutfailure, i.e., its movement capability, is its most important characteristic. Sealant manufacturers suggest that joints should be designed with not more than 225% (that is 25% extension and 25% compres- sion, starting with a relaxed material) yearly movement for high performance sealants. This value is based on the service performance observed by manufacturers in the last two decades, the period during which such products have been available. (When the products were first marketed, greater movement capability was claimed.)
K.K. KARPATI
The vices provide a test facility for determining the movement capability of existing sealants and should be more reliable than observations of buildings where such irregularities as joint width, off-plane twisting of the sealant, and poor workmanship may influence per- formance. These factors are difficult to observe objec- tively, partly because their occurrence may be erratic and partly because when failure is noticed the failure- causing joint configuration may not still be present. In addition, during the time lag between the occurrence and notice of failure, any clear evidence of poor adhe- sion due to application on a dirty surface may be obscured by more recent dust or deposits from soiled water from adjacent areas. Vices are also helpful in developing sealants based on new types of polymers or modified formulations of old ones. For studies of sealantlpaint systems, the test sealant can be painted at various times during its life.
In addition to the factors usually investigated in con- nection with aging of polymers-heat, light, water- cyclical movement, which imposes alternating tensile and compressive stresses, can be studied through the use of vices. A polymer under stress and a relaxed one may react quite differently to the usual factors of aging. In addition, relaxation of stress occurs in varying de- grees, depending on type of polymer, value of stress, temperature, and length of time under stress. As a result of stress relaxation, the interaction of stress with the aging factors changes continuously. Another aspect of the effect of relaxation associated with cyclical stress is that it may result in permanent deformation, causing unwanted local thinning of the sealant bead.
EXAMPLE OF USE OF VICE
Consider the exposure to outdoor weathering of a sealant that has undergone no movement and is, there- fore, in a relaxed state. Exposure could be initiated at a time when average air temperature is equal to the mean annual value for the region. At arbitrarily chosen time intervals the width of the specimen should be changed in proportion to the total yearly movement planned for the specimen, and to the time interval during which the yearly maximum temperature difference will occur, that is, half a year. With +25% yearly change, a one half
Flgure 1--Cyclic movement
I
KLARA K. KARPATI has been associated with the Division of Building Research of the National Research Council of Canada since 1965. Previously, she was employed in paint research and development by I.C.I., England, and by Trimetal Paint Co., Belgium. She obtained the License Speciale en Chimie lndustrielle from the Universitv of Brussels and araduated in chemist6 from the university of Sciences of Budapest. In Hungary, Mrs. Karpati worked in the Central Paint Research Laboratories.
inch wide specimen requires 0.5 x 0.2513 = 0.042 in. (0.107 cm) width change each time if changes are made at monthly intervals. As the maximum error of the adjustment (0.005 in.) is not cumulative, shorter inter- vals and correspondingly smaller width changes are quite practical. Upper limits of ?80% were successfully used without affecting the stability of the vice.
Cycles can also be started with an initial tensile or compressive stress already imposed. A test of this type is designed to simulate situations arising for various reasons on the outside of buildings. One possibility is that the temperature at installation is close to one of the yearly extremes instead of the preferred annual8 mean temperature. If the temperature is close to the yearly maximum, the sealant will work mainly in extension, and about double the amount it would if installed at the mean temperature. If it is close to the yearly minimum (although this is less likely), the sealant will work mainly in compression.
Another question often raised by designers is whether joint width should be designed on the basis of temperature at the time of installation of the sealant or on the basis of temperature at the time the curing pro- cess of the sealant is completed. The curing process takes time, and the sealant may have an initial stress imposed on it even if it is installed at the mean temper- ature. Sealants cure at various rates depending on chemical composition and formulation. (Being tack free on the surface does not necessarily mean that the bulk of the sealant bead has cured to the same degree.) If a sealant is applied early in the morning, most of the curing process may be completed by the afternoon after a temperature increase of perhaps 10 to 30°F (5.6 to 16.7"C). If applied in the afternoon, the same temper- ature change, but now decreasing, may occur during the night, slowing the curing process in comparison with the rate during the day. On a large building, where caulking of joints may take several days, curing will take place at various temperatures in joints of different widths, resulting in a variable stress condition at any given time of year. The effect of the initial stress state has not yet been investigated, and the vice could be helpful in elucidating the problem and providing an answer to the designer's question, "How should var- iations in curing time be taken into consideration when designing joint width?"
K.K. KARPATI
SUMMARY ACKNOWLEDGMEF
A vice has been designed and produced that will
enable sealant specimens to be subjected to cyclical movements. Such movements are intermittent, and are imposed on sealant specimens through manual adjust- ment at arbitrarily chosen intervals. The simplicity of the vice enables not only the sealant manufacturers but also designers and users to make quick tests. The seal- ant manufacturer can use it as a tool in accelerated testing where one of the aging factors is cyclical move- ment, and this can be accelerated by increasing the amplitude and frequency of the cycling. Simultane- ously, other aging factors can be varied. It can be used for comparative testing of possible products for a spe- cific job or to check batch-to-batch variations.
The same principle of testing involving this type of
vice can be applied to any material that is subjected to cyclic. tensile, or compressive stresses in service. Examples are roofing materials. various mbber and plastic products, and paint applied to sealant. The size and shape of the sample to be tested can vary without limitation provided appropriate adjustments are made to the size of the vice. It can also be used to investigate the most practical shape for the prototype of a product or the desirable size for a sample.
The author acknow~eages tne contnDutions of
H.F.
Slade and R. Tetu who realized the concept of the cycling device and produced it.
This paper is a contribution from the Division of Building Research, National Research Council of Canada and is published with the approval of the Di- rector of the Division.
References
(1) Karpati, K.K., "Mechanical Properties of Sealants: 11. Be-
haviour of a Silicone Sealant as a Function of Rate of Move-
ment," JOURNAL OF P A ~ N T T E C H N O L O G Y . ~ ~ , NO. 569.58 (1972).
(2) Karpati, K.K., "Mechanical Properties of Sealants: 111. Per-
formance Testing of Silicone Sealants," JOURNAL OF PAINT
TECHNOLOGY.~~, NO. 571, 75 (1972).
(3) Karpati, K.K., "Mechanical Properties of Sealants: IV. Per-
formance Testing of Two-part Polysulfide Sealants," JOURNALOF
PAINT TECHNOLOGY.~~, NO. 580, 49 (1973).
(4) Karpati, K.K. and Handegord, G.O., "A Rational Approach to
Building Sealant Testing," Adhesives Age, 16, No. 1 l , 2 7 (1973).
(5) Karpati, K.K. and Sereda, P.J., "Joint Movement in Precast
Concrete Panel Cladding," ASTM, Journal of Testing and
Evaluation, 4 , No. 2, 151 (1976).
(6) Karpati, K.K. and Sereda, P.J., "Weathering Rack for Seal-
ants," JOURNAL OF COATINGS TECHNOLOGY. 49, NO. 626, 44
(1977).
Reprinted from the June 1978 issue of the JOURNAL OF COATINGS TECHNOLOGY
Volume 50; Number 641; Pages 27-30.
Copyright 1978 by the Federation of Societies for Coatings Technology, Philadelphia, Pennsylvania, U.S.A.
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