508
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 508-514
Literature Cited ..~ __ . . ._ Hara, S.; Mori, K. et at. Deselinatbn 1S77a, 21, 183-194. Hara, S.; Goldsmith, R. et at. Desalination 1S77b, 22, 311-333. Iwata, K.: Murakaml. H. et ai. Roc. 7th Int. Svmo. . . Fresh Wafer Sea lS80, 2 , 143-152. Riley, R. L.; Milstead, C. E. et ai. DesaHnatlOn 1977, 23, 331-355. Catalogs. Cellulose acetate RO membranes: NItto Electric Industrial Co., Ltd.; Toyobo Co., Ltd.; Toray Industries, Inc.; Paterson and Candy Interna-
tional Ltd.: De Danske Sukkerfabrkker. RO Mvision: U. 0. P.. Fluid Svstem Dhrlslon; Desalination Systems, Inc. Polyamide RO membranes: E9, E 10 of Du Pont. Composite RO membranes: TFC of U.O.P., Fluid System Dlvlslon; FT-30 of FiimTec Corporation.
Received for reuiew October 8, 1980 Accepted March 26, 1981
Sealants for Solar Collectors Morrls A. Mendelsohn," Russell M. Luck, Fred A. Yeoman, and Francis W. Navlsh, Jr. Westinghouse Research and Development Center, Pittsburgh, Pennsylvanla 15235
Elastomeric sealants employed in thermal solar collectors are subjected to extremely harsh conditions. In order to function properly for long periods of time, they must exhibit good resistance to environmental factors such as high and low temperature extremes, oxygen, ozone, water, and ultraviolet radiation. Several types of candidate elastomers consisting of fluorocarbon, silicone, acrylic, ethylene-acrylic, ethylene-propylene terpolymer, and butyl and chlorosulfonated polyethylene were subjected to extensive testing. Results are reported on outgassing characteristics during thermal aging and on retention of such physical properties as compression set, elongation, and tensile strength. Relative hydrolytic stabllltles of these materials are also discussed.
Introduction The thermal solar collector business is one of the potentially fastest growing business activities worldwide. Targets for total energy output generated by solar collectors are being developed by the US. Government. Many municipalities are considering or have passed legislation requiring that new housing construction include solar collectors at least for hot water. Polymeric materials are finding immediate use in collectors primarily as thermal insulation and sealants which are in the form of caulking compounds and gaskets (Figure 1). Work is also being performed on application of polymeric compositions for glazings and frames. This paper will discuss some ongoing studies of the endurances of several commercially available materials that could find use as caulks or gaskets. In order for plastics or elastomers to display necessary long-term endurance, they must offer excellent resistance to the harsh environment of the collector. First, they must possess good long-term resistance to air at high temperatures. Temperatures inside the cell may exceed 200 "C during the relatively brief periods, possibly as long as several weeks, while the collector is under no-flow or stagnation conditions. During normal operation, the units attain temperatures between 125 and 200 "C. Depending on the collector design, the sealant can experience prolonged exposure to relatively high temperatures. In addition, the sealants are exposed to such environmental stresses as high humidity, ozone, ultraviolet radiation, etc. Since the solar collector industry is in its infancy, almost no new materials have been developed specifically for it. As a result, this investigation was performed to identify deficiencies of the currently available sealant materials for this application with the expectation that this would lead to development of superior materials. Elastomeric materials evaluated consist of a sampling of the high-temperature elastomers which include fluoro0196-4321/81/ 1220-0508$01.25/0
elastomers and several silicone rubbers, as well as several intermediate temperature elastomers (Table I). The gasket materials prepared from preformed sheets are classified as PS (performed seals), whereas the caulks are designated as SC (sealing compound) materials (ANSI/ ASTM D3667-78). The selection of materials for this study was based upon a combination of an extensive computerized literature search and recommendations from the manufacturers of the elastomers. Materials were excluded from further study for reasons such as poor resistance to heat, oxygen, ozone, water, and ultraviolet radiation. In addition, several high-temperature specialty compounds were eliminated because their extremely high cost made them unattractive for use in solar collectors. Further narrowing of the list to the most suitable materials was accomplished by employing a screening test which utilized the tests developed by Stiehler et al. (1978) (tests are also described in ASTM D-3667-78). However, we made an exception in the case of butyl rubber. Although butyl rubbers have inadequate thermal stabilities and gave poor results on the screening tests, the best of the butyl compounds screened was selected for further evaluation since these elastomers have found widespread use as solar collector sealants. Undoubtedly some good materials may not have been included; however, this sampling will provide an indication of the applicability of several families of elastomers for use as solar collector sealants. This paper will discuss effects of thermal and hydrolytic aging on the sealant materials. In later papers covering the continuation of this study, the effects of low temperature, weathering, and ozone resistance will be reported. Experimental Section Outgassing Measurement. In this test, a sample of the material is placed in the bottom of a glass tube, and a sodium chloride infrared crystal is mounted in the open end of the tube and held in place by a series of dimples 0 1981 American Chemical Society
Ind. Eng. chem. Rod. Res. Dev., Vol. 20. No. 3, 1981 509
Table I. Elastomers Evaluated code supplier's designation A silicone rubber sealant (also known as DC 732) B
790 buildingsealant (also known as DC 790)
C D E F G H I
RTV 103 mono eternal flex bvnalon sealant
J K L
3300-12A, Vamac
Tremco butyisealant SE-7550 Silastic 747 HS-70
type elastomer
class
supplier
SC
silicone
Dow Coming
SC
silicone
Dow Corning
SC SC SC SC
silicone acrylic terpolymmer
General Electric Tremco
PS PS PS
Gibson-Homans
bvnalon bktyl silicone silicone silicone
Tremco
General Electric Dow Coming Dow Corning polymer compounded by North American Reiea DuPont
PS PS PS PS
ethylenelacrylic
M
210-108-35-1,Hycar 4054 31-323-0731A, Viton PLV 1008, Viton
N
3300-11, Nordel
PS
DuPont
0 P
SR,35020 8EX-123 (Butvl 100)
PS
ethylene-propyl1?ne terpolymer (EPDM)
acrylic fluorocarbon fluorocarbon
Goodrich
DuPont DuPont polymer compounded by
Pelmor
butyl butvl
stalwart
NPC80/40
PS PS
silicone
R
...
PS
butyl
T
...
PS
EPDM
Dow Coming polymer compounded by North American Reiss obtained from a used Pittsburth Plate Glass collector sample of gasket compounded supplied by Bio-Energy Systems
Q
Polysar
Figure 2. Outgassing measurement apparatus. Figure 1. Assembly of a typical solar collector.
in the glass wall of the tube. This assembly (Figure 2) is then positioned vertically into a closely fitting hole in the top of an oven so that only the lower twwthirds of the tube is inside the oven. Most of the condensable outgassing products condense on the bottom surface of the sodium chloride crystal, with a small amount condensing on the glass tube area surrounding the crystal. Condensable volatiles are o b ~ e ~ as e dweight increase of the crystal, and noncondensables by the difference between this and the total weight loss of the test sample. The coated sodium chloride crystal is placed directly into the infrared spectrophotometer for determination of the chemical nature of the condensable products. Many sealant compositions were characterized by this teat (Table I). The preformed sealants (PS)were evaluated as received. The seal caulks (SC) compounds were cast in approximately 0.15 cm thick sheets on fluorocarbonfilm. All of the SC compounds were room temperature vulcanized for 4-6 weeks prior to testing. In the standard outgassing test, the oven was operated at 150 "C for a period of 9 days. The sodium chloride crystal reached an equilibrium temperature of 65 & 2 "C during these tests. The compositions of the condensable products were identified by infrared analysis using a Perkin-Elmer Model 700 infrared spectrophotometer. Relative light transmittance values, over the range of
4W950 nm,were obtained using a Coleman spedrophw tometer with a tungsten power supply. Aging of Class PS Tensile Specimens. Dumbbell tensile specimens of preformed seals (Class PS materials) were prepared from sheets of post-cured, vulcanized elastomers approximately 0.18 cm in thickness which had been provided to us by the several rubber companies involved. Specimens were aged in forced air circulating ovens at temperatures ranging from 225 to 125 OC in 25O i n t e ~ s l s . In general, not more than three aging temperatures were used for any one composition, and aging temperature selection was based upon knowledge of the thermal stability range of each material. Testing began after one day of aging and aging exposures were doubled with each successive interval. The tensile specimens were teated using an Instron test machine in accordance with the ASTM D-412-75 procedures. Values reported are averages for three specimens. Hardness measurements were performed using a Shore A durometer. Aging of Class SC Tensile Specimens. Caulking compounds were spread with a razor blade into a rectangular mold 14 cm X 2.5 cm X 0.16 cm and then held at room temperature in the open face mold for 4-6 weeks. Tensile specimens were then die cut from the room temperature castings.
510
Ind. Eng. Chem. Prod. Res.
Dev., Vol. 20, No. 3,
1981
Prior to aging, the specimens were post-cured for 24 h at the aging temperature in order to minimize the very large changes that occur a t the outset of elevated temperature exposure as volatiles (present in large quantities in caulking formulations) are driven off. Aging was performed as described for the PS compounds. Hydrolytic Stability Tests. Hydrolytic stability testing was carried out by immersion of dumbell specimens of elastomers in water a t 125, 100, 83, and 67 "C for selected test intervals and monitoring deterioration of mechanical properties as the test exposures proceeded. Tensile specimens (ASTM D-412) of all Class PS compositions and of as many of the Class SC materials as possible were aged hydrolytically and deterioration of tensile properties was monitored. Tensile strength, ultimate elongation, and tensile modulus a t 100% extension of triplicate specimens were monitored as the hydrolytic aging progressed. Prior to these measurements and subsequent to removal from water immersion, samples were air dryed 16 h at 80 "C. Unfortunately, the nonsilicone caulking compounds could not be monitored because they were too weak and tacky for testing. One of the silicone SC materials, B, was not evaluated since its extensibility exceeded the capability of the testing apparatus. Compression Set Aging Tests. Compression set measurements were carried out by the method of ASTM D-395-69, Method B. This procedure involves aging of the sample under constant deflection. Aging times and temperatures selected for these tests are not those suggested in the ASTM procedure, but rather periods and temperatures chosen to provide Arrhenius data which can be used to predict durability in a variety of thermal environments. Depending upon the type of elastomer, test temperatures employed ranged from 100 to 250 "C. Samples used in these studies consisted of 1.27 cm thick stacks of 2.54 cm diameter disks die cut from elastomer sheets having a thickness of 0.16 to 0.20 cm. Compression set values reported are average values obtained from testing of four specimens. Results and Discussion Aging Phenomena. Since the literature contains many excellent articles and reviews on polymer degradation, this paper will not discuss detailed mechanisms. However, a summary of the more salient effects of the degradation processes is presented below. As specimens of the sealants are aged in air or water a t elevated temperatures, initially they may show an improvement in physical properties because of enhancement of their cure; however, shortly afterwards, they generally begin to display a dimunition of tensile strength, modulus, and elongation. On the other hand, some materials exhibit increased strength and rigidity, eventually embrittling during aging. Much of the degradation that results in softening characteristics is caused by scission of the polymer chains with consequent reduction in molecular weight. Conversely, an increase in rigidity of the polymer matrix is attributed to reactions that provide additional cross-links. Further complications result when some polymers, expecially certain copolymers, simultaneously undergo chain scission and cross-linking reactions. Frequently, this causes a material to develop a cheesy consistency in which both tensile strength and elongation are severely reduced. Physical properties are also affected by the outward diffusion of the relatively lower molecular weight compounds. In air at elevated temperatures the sealants exhibit loss of volatile material. Immersion in hot water
results in additional loss of material because of leaching and erosion. The low molecular weight mobile compounds present may either be products of chain or branch scission or additives incorporated into the sealants. The latter includes processing aids, plasticizers, antioxidants, ultraviolet stabilizers, etc. As expected, the loss of processing aids and plasticizers contributes to a reduction in the extensibility and an increase in the rigidity of the polymeric material. Loss of stabilizers, such as antioxidants, antiozonants, and ultraviolet absorbers through chemical reaction or diffusion causes the polymer to become more susceptible to degradation. In addition to chemical degradation, loss of properties occurs as a result of essentially physical phenomena. For example, in some cases, the increase of compression set on aging at elevated temperatures can be attributed primarily to chemical reactions; however, in other cases, this effect results from an essentially viscoelastic phenomenon. Another effect involves alteration of attractive forces between the polymer chains and fdler particles. As water permeates into the polymer matrix, it not only behaves as a plasticizer but competes for sites on the surface of the filler. The concentration of water at the polymer-filler interface increases with increasing hydrophilic character of the filler. This reduces the reinforcing ability of the filler, thereby lowering the modulus of the composition. Thus the overall degradation process can be quite complex for many materials, since it involves a combination of several simultaneously interacting mechanisms. Outgassing. The outgassing of preformed seals (PS) and seal caulks (SC) is of high concern to solar collector manufacturers. The evolved materials usually condense on the cooler glass glaze surfaces, either in the form of a liquid or a low-melting solid. These deposits significantly reduce the transmittance of solar light through the glass and thereby reduce the efficiency of the collector. Inside each solar collector two types of dew (condensation) points exist. One is a moisture dew point and the other an organic hydrocarbon vapor dew point. We are all familiar with water vapor dew points as a function of temperature. Organic dew points are analogous to the moisture dew point. The amount of a given organic vapor present in the air a t any specific pressure/temperature condition is a function of the volatility of that particular organic material at those conditions. Naturally, air will hold more vapor, whether it be moisture or organic, at a high temperature than it will a t a low temperature. As the temperature decreases to a point below the organic vapor dew point, condensation will occur on the coolest surfaces in the collector which usually include the glass glaze. As the temperature of the collector rises above the dew point, the condensate on the glass will evaporate. The evaporation of moisture is clean and complete, and no residues or deposits are left on the glass. However, over long periods of time, moisture condensates can leach out sodium or other metal salts from the glass and a white salt deposit will slowly form. Organic condensates result from the diffusive loss of low molecular weight compounds originally present in the compounded elastomer seal and of products formed when the sealant or coating is degraded or depolymerized as a result of exposure to elevated temperatures, water, ultraviolet light, oxygen, ozone, etc. In many cases, these organic condensates do not evaporate from the glass in a clean manner and usually leave either a colored oily residue, a white powdery deposit, or a continuous solid film. This solid film often results from further chemical reac-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 3, 1981 511
Table 11. Outgassing of Sealants
compound
%
%
condensable
noncondensables
composition of condensable
0.4 0.3 2.4 4.0 15.4 6.7 0.03 0.2 0.2 2.1 0.09 -0.01 0.1 0.3 0.2 0.3 -0.05 1.0 0.4
cyclic or linear alkyl polysiloxane and an oil cyclic or linear alkyl polysiloxane cyclic or linear alkyl polysiloxane acrylic fragments or oxidized oil alkyl sulfonic acid ester oxidized butyl or oxidized oil cyclic or linear alkyl polysiloxane alkyl polysiloxane and an aromatic ester cyclic or linear alkyl polysiloxane acrylate and ethylene fragments insufficient sample for analysis insufficient sample for analysis insufficient sample for analysis naphthenic processing oil oxidized butyl fragments or oxidized oil stearic acid cyclic or linear alkyl polysiloxane oxidized butyl fragments or oxidized oil oxidized EPDM fragments or oxidized oil
A B
2.7 1.0 1.6 0.75
C
D E F G H
13.0 7.0 0.01 0.9 0.3 1.75 0.3 -0.01
-
I J K L M N 0 P
- 0.01 0.5 -
Q R
T
0.6 2.3 0.01 1.9 3.2
1 3.0
0
1
2
3
4
5
6
7
8
9
9
l i m e (days1
Figure 3. Sealant aging at 150 OC-percent condensables and noncondensables (percents are based on origninal weight of specimen).
tions of the liquid condensate through a thermal, oxidative, or ultraviolet induced mechanism. During the many condensation/evaporation cycles that a solar collector experiences, these condensate coatings slowly build up, producing an adverse effect upon light transmittance. Other sources of outgassing consist of the thermal insulation, organic coatings, and organic polymeric materials used in structural applications. Organic coatings on a glass glaze are frequently observed, in reflective light, as a multicolored transparent pattern similar to that formed by a thin film of oil on water. A test was developed (Figure 2) to study the degradation and outgassing of seals and coatings. This test method provides data on the rate and degree of outgassing of a material and also provides an analysis of the composition of the evolved products. Table I1 and Figures 3 and 4 show the results of these outgassing studies. In general, the preformed seal materials produce a smaller amount of outgassing products than the caulk seal materials. Infrared spectrophotometric analyses of the outgassing products indicated that the silicone sealants, with the exception of compound Q, evolved a significant quantity of either a low molecular weight linear or a cyclic alkyl polysiloxane. Many of the preformed seals (K, N, 0, P) evolved mixed processing and/or extender oils of the naphthenic type as well as stearic acid and metal stearate additives. The butyl sealant (R) evolved low molecular weight chain scission fragments which slowly oxidized during the test as evidenced by an increasing infrared
Sealants
-3-
30
%T
0
1
2
3
4
5
6
7
8
9
9
Time ldaysl
Figure 4. Sealant aging at 150 OC-percent condensables and noncondensables (percents are based on original weight of specimen).
adsorption band at 1730-1740 cm-'. The extruded EPDM rubber (T)also evolved low molecular weight chain scission fragments, which oxidized during the test. The volatility rate (slope) for this material has remained almost constant since the beginning of the test and indicates depolymerization. The quantity of very low molecular weight noncondensables evolved is less than 0.4% for most of the test specimens. It is believed that a large portion of the noncondensable fraction is moisture, which had been adsorbed by the specimens. Silicone C produced 2.4% noncondensables which consisted primarily of low molecular weight alkyl siloxanes and the acetic acid formed during room temperature vulcanization. Butyl sealant, R, also produced a substantial quantity of noncondensables which were identified as low molecular weight butyl polymer, organic solvents, and moisture. Retention of Physical Properties. Thermal aging in air of tensile specimens of the superior PS (preformed seal or gasket) compounds (Table 111) has shown that among the materials having an intermediate thermal rating, the ethylene-acrylic copolymer, J (Vamac), and the polyacrylate, K (Hycar 4054), display the best retentions of tensile strength. These are followed closely by the EPDM ethylene-propylene terpolymer, N (Nordel), and then the butyl rubber, P (Polysar). After 32 days at 150 "C, the ethylene-acrylic copolymer showed a slight increase in tensile strength and its elongation had nearly doubled. However, since the elongation increased by slightly more than a factor of 2 after aging
512
Ind. Eng. Chem. Prod.Res. Dev., Vol. 20, No. 3, 1981
Table 111. Comparative Property Retention of Thermally Aged PS Compounds aging for 32 days original 1 2 5 "C 150 "C compound type T E T E T E N ethylene-propylene 2280 440 2150 180 1050 110 terpolymer (EPDM) 2080 170 2240 J ethylene-acrylic 390 2350 330 380" 530" P butyl 1470 500 620 1150 K
1570
acrylic
170
180
1300
1240
175 "C
160
T 690"
E
1030 40" 200
20 10" -0
5"
aging for 32 days 1 7 5 'C 200 "C 225 "C __-_ 210 -.1600 fluorocarbon 1700 190 1750 210 silicone 680 940 210 770 220 670 100 1320 silicone 1030 250 980 180 420 -0 510 -0 G silicone 950 700 870 510 600 260 390 80 Data are reported for 1 6 days aging. Original material cured by suppliers. T = tensile strength (lb/in.'); E = elongation to break (%).
L I Q
Table IV. Comparative Effects of Thermal Aging on Compression Set 150 "C
125 "C
compound
1 7 5 "C
type
20 days
40 days
20 days
40 days
20 days
P J K N I"
butyl ethylene acrylic acrylic EPDM silicone
48 44 32 28
54 51 40 38
72 72 57 47
Q"
silicone silicone
64 63 45 38 67 49 68
76 73 60 41 97 84 99 24
G"
__ __ __
-_ -__
__
__
40 days
__
-74 60
_-
__ __
__ fluorocarbon __ -_ -32 Compression set of the silicones exceeded 90% in less than 5 days at 200 "C. b Viton displayed compression sets of 49% and 65% after 40 days at 200 and 225 "C, respectively. Materials were aged under compression in the test fixture. Lb
t 0
2
0
4 0 6 Time (days1
0
Figure 6. Effect of thermal aging on compression set of a silicone elastomer.
Figure 5. Effect of aging in air on tensile strength and elongation of a silicone elastomer. for only 1 day and then decreased very slowly over the ensuing month, we feel that the overall increase in extensibility is a consequence of additional curing during the early aging rather than degradation. Over the same period, the polyacrylate displayed a slight reduction in tensile strength but maintained an essentially constant elongation. Meanwhile, the EPDM showed a decrease of approximately 50% in tensile strength and 75% in elongation. While the elongation of the butyl remained essentially constant, its tensile strength dropped catastrophically after only 16 days at 150 "C. Only the ethylene-acrylic copolymer and polyacrylate elastomers retained over 50% of their original tensile strength on aging for 16 days at 175 "C. After 32 days, only the ethylene-acrylic copolymer has retained close to 50% of its origninal strength, and the
other four compounds have lost essentially all of their extensibility. Among the high-temperature materials evaluated, the fluoroelastomer Viton displayed by far the best retention of properties on aging. After 32 days at 225 "C,it appeared not to have suffered loss of either strength or extensibility. Among the silicones, compound I appeared to display the best retention of tensile properties. Its tensile strength dropped by about 50% and its elongation by about 85% (from 680 to 100) after aging for 32 days a t 225 "C. The silicone, G, experienced a slightly greater decline of properties. While Compound Q has retained about 50% of its original tensile strength, its elongation has dropped to essentially zero. However, its compression set was slightly less adversely affected than that of the other silicones (Table IV). The silicones experience significantly less degradation at 200 and 175 "C.At these temperatures,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 513
Table V. Comparative Property Retention of Thermally Aged SC Compoundsa aging for 32 days compound F
D E
100 "C
16 h a t 150 "C T E 450 -0
T 400
E
acrylic hypalon
100 110
170 200
silicone silicone silicone
16 h a t 175 "C 470 350 100 780 210 270
280 80
150 "C
125 "C
type butyl
E
0
T 230
T
0
. specimens
290 90
290 270
170 60
cracked 710 specimens cracked
E
50
aging for 32 days C
B A
480 120 210
225 "C
200 "C
175 "C 370 610 410
480
400
___ 180
300 170 110
___
360
270 30 170
See footnotes for Table I.
1 125'c
Tensile Strenglh o Eiongtaion
1W 80
s
560
120
s
.L
so2
u"
1w
140
g 120
$
:
4
3- 1m
bo
c
gso
$40
e
a, 0
o
20
40
60
80
im
1x1
Time ldaysl
2
Figure 9. Effect of thermal aging on compression set of an acrylic copolymer.
w 40
Tensile
20 0
o Elanoation ~.>-. -
Tensile Elongation Tensile o Elongation Tensile D Eiongation
0 1 0 2 0 3 0 4 0 0 5 0 6 0 0 Time ldaysl
Figure 7. Effect of water immersion on tensile strength and elongation of a silicone elastomer.
140
3
A A
I
Tensile e Elongation
Tenslie Eiongation m Tensile I] Eiongation A A
1w c 8PC 67'C
1 l7'OC 1 1~~
1 lEoC
3
\
6 0 I l
6
l
l
0
0
~
~
4
c
Time ldayrl
Figure 10. Effect of water immersion on tensile strength and elongation of an acrylic copolymer.
01
0
I
1
I
0
a
I
I
a
4
0
I
Time ldaysi
Figure 8. Effect of aging in air on tensile strength and elongation of an acrylic copolymer.
just as at 225 "C,the tensile properties of I appear to have suffered the least, followed by those of Compounds G and Q. Aging characteristics of silicone I and the ethyleneacrylic copolymer, J, are shown in Figures 5-10. Evaluation of three silicone SC (caulk) compounds, A, B, and C, has shown that thus far C has the best retention of properties (Table V). After 32 days at 225 OC,it dis-
played drops in tensile strength and elongation of about 35% and 25%, respectively. Compound A, a close second, lost about 50% of its tensile strength and 35% of its elongation. While sealant B exhibited good retention of its strength, its elongation dropped dramatically from an original value of 780% to 30%. Evaluation of the intermediate temperature caulk compounds showed that the acrylic, D, and Hypalon, E, experienced sharp increases in tensile strength after moderate aging. This is attributed to the loss of volatiles, comprising plasticizers and solvents, that continued to occur after the post-cure. As a result of the opposing effects of reversion (degradation of the polymer chains into small segments which reduces strength and rigidity) and loss of volatiles, the butyl caulk, F, did not exhibit much
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981
514
Table VI. Comparative Property Retention of Materials Exposed to Water
compound
G
Q
I L
N
P J
K A C B
original type silicone silicone silicone fluorocarbon EPDM butyl ethylene-acrylic acrylic silicone silicone silicone I
64 day immersion 83 "C E T
67 "C
class
T
E
T
E
PS
880 1060 1280 1690 2010 1600 2080 1570 160 340 60
750 280 700 220 180 600 420 170 210 290 830
1130 1110 1420 1800 2510 1830 2040" 1360" 21 0 490 60
720 220 480 240 200 540 420" 170a 280 370 800
PS PS PS
PS PS PS
PS
sc
sc sc
1080 1050 1230 1450 2030 1370 2050a 1350" 180 380 40
660 200 460 240 180 520 370a 170" 310 340 800
32 day immersion 125 "C
100 "C T
E
T
E
940 970 810 1480 2000 1440 2200" 1440" 150 340 31
530 190 220 210 170 490 380" 160" 300 350 700
700 710 610 1280 1910 1250 1140
510 180 180 210 180 470 -0
.___
___
21" 150" 150 440 too tacky t o measure
PS compounds were cured by their suppliers. a Material immersed for half the duration shown in column heading. SC compounds were permitted t o age 2 months in room environment before testing. See footnotes for Table I. /
1 Life Corresponding to
/
/'
Aging Temperature, Ti
, / /
Reciprocal Absolute Temperature
(OK-')
Figure 11. Typical Arrhenius plot of aging data.
change in tensile strength early in the aging process. After 32 days at 125 "C, only the acrylic displayed elastomeric characteristics. The initial elongation of the Hypalon was quite low and that of the butyl essentially nonexistent. The three caulking compounds embrittled at 150 "C. After 32 days, only the acrylic displayed significant elongation, whereas cracking of the Hypalon and butyl specimens precluded measurement of their tensile strengths. Accelerated hydrolytic stability tests (Table VI) are still in progress. All of the PS materials are exhibiting good resistance to hydrolysis. Even the polyacrylate, which has hydrolyzable ester groups, is displaying good retention of its properties after 32 days of continuous immersion in boiling water. (This material was not exposed to pressurized water at 125 "C,since a screening test indicated
that it could not endure these conditions.) The ethylene-acrylic copolymer, which also contains ester groups, has shown poor stability to 125 "C water; however, it is performing well after 32 days in water a t 100 "C. Since the SC compounds lack the cross-link density of the PS materials, they are at a great disadvantage in this type of test. Only the silicone caulk, C, has survived 32 days in 125 "C water. At 100 "C, silicones A and C appear to have withstood two months of continuous immersion. The three lower temperature caulking compounds, butyl, acrylic, and Hypalon, were extremely soft and tacky and lacked the physical integrity for this test. At the conclusion of the hydrolytic and thermal aging studies, the data will be subjected to an Arrhenius treatment (Figure 11)so as to provide a better estimate of long term durability of the various materials. Acknowledgment This work has been supported by the Solar Heating and Cooling Research and Development Branch, Office of Conservation and Solar Applications, U.S.Department of Energy. Literature Cited Stiehler, R. D.; Hockman, A.; Embree, E. J.; Masters, L. W. "Solar Energy Systems-Standards for Rubber Seals", NBSIR 77-1437, National Bureau of Standards Report, 1978.
Received for review October 20, 1980 Accepted April 10, 1981 Presented at the 179th National Meeting of the American Chemical Society, Houston, Texas, March 1980,Division of Industrial and Engineering Chemistry.
