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Weathering characteristics of coating-grade asphalts
Jones, P. M.; Gibbons, E. V.
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Weathering Characteristics
of
Coating-Grade Asphalts
Few of t h e residual asphalts produced from crude oil sources are suitable f o r roofing purposes because of their wide variations in quality. Those t h a t a r e selected a r e f u r t h e r processed and segregated to provide the coating grade asphalts used in t h e Canadian roofing industry.
Final processing involves a i r blowing t h e soft residue. The resultant product usually has a softening point between 210 and 230°F and a pene- tration a t 77OF of between 1 5 and 25 dmrn. To provide satisfactory service the asphalt must possess suitable consistency characteristics f o r t h e preparation of shingles and other products. It must also be capable of resisting changes in its' properties upon exposure to t h e weather. The nature of these changes has been studied and discussed by many workers: Abraham' in summarizing the work of several workers describes the changes a s oxidation, dehydrogenation, carbonization, polymerization and evaporation; Beitchman" Kleinschmidt and Snoke3, Galloway4, Pfeiffers and others have described some of t h e changes t h a t occur to asphalts during exposure. I n general, a loss in weight was observed and the asphaltene fraction increased a t the expense of t h e maltene fraction. Greenfelda has made a detailed study of t h e chemical changes two asphalts undergo upon weathering. He concludes t h a t the changes a r e complex and numerous, t h a t oxidation occurs resulting in the formation of volatile products such a s carbon dioxide and water, acidic water soluble products and a product t h a t is insoluble in water a n n-pentane. Stewart7 used infra- red analysis to study chemical changes and noted a n increase in t h e car- bony1 content a s degradation proceeded. Wright and Campbell8 have used this change in carbonyl content as a means of measuring degradation in studies of the parameters in accelerated weathering. A review of these and many other studies on weathering of asphalts has recently been made by Marting.
Although most of these studies have investigated t h e chemical nature of the changes accompanying weathering, the most important effect from a practical point of view is the increase in hardness of the asphalt. It has been shown by Mertens and Greenfeldlu t h a t this increase depends upon the nature of t h e crude from which the asphalt is obtained and upon the method of manufacture. The asphalt hardens progressively until i t is unable to withstand strains imposed by thermal and moisture changes or wind vibrations and cracking o r fracture results. When this occurs t h e waterproofing ability of the asphalt coating is lost and deterioration of t h e underlying fabric may be accelerated. Hardening of the asphalt may also result in t h e loss of protective mineral granules from t h e surface so t h a t more asphalt is exposed to radiation, which f u r t h e r accelerates degrada- tion. The nature of the physical and chemical changes t h a t give rise to these effects is known in general, but little information is available of t h e way in which they are related to the physical properties and chemical composition of t h e materials.
Mertensll has reported a correlation between weatherometer life and t h e dispersiljility of t h e asphaltenes. This concept has been used by Jones12
*P. M. JONES and E. V. GIBBONS, Division of Building Research, National
in studies with coating grade asphalts. A good correlation is found between t h e dispersibility of the asphaltenes and t h e durability of coating grade asphalts produced from straight run, crude blend, oil fluxed residues and residue catalyzed with ferric chloride.. No correlation was found for asphalts catalyzed with phosphoric acid and each of the production methods gave different correlation equations.
H e i t h a u s ' h l a i m s t h a t the range of t h e dispersibility values in Mertens' study is too small t o be of major practical importance; t h a t pene- tration has been correlated with durability and t h a t maltene viscosity is correlated with durability'+. I n a n extensive study Hamadals has obtained correlations between durability and penetration, ductility, softening point and density and also gives the following relationship: if flash point (OC)
-
2 X softening point (OC) 120°C, the probability of having a good weather resistant asphalt is high.At t h e Division of Building Research of t h e National Research Council of Canada a study has been made of 1 5 coating-grade asphalts. This report presents a summary of their properties and t h e influence of these proper- ties upon their performance as weather resistant coatings.
MATERIALS AND METHODS OF TEST
Fifteen asphalts were obtained in 1959 from widely distributed crudes used in t h e production of asphalt shingles and other prepared roofing and siding products. Ten were from Canadian crudes, three from Venezuela and two from the Middle East. The properties and sources of t h e asphalts a r e listed in Table I ; i t may be noted t h a t several are from t h e same source -for example, asphalts No. 3 and 7 a r e both from Lloydminster, with a
three-year time interval, and asphalts No. 8 and 14 a r e from southern Alberta crudes. The original sample No. 14 (durability 22 cycles) was improved by different blending techniques t o produce No. 8, which has a durability of 50 cycles.
CHEMICAL ANALYSIS
A chromatographic procedure (Kleinschmidtlc) was used to separate t h e asphalts into components. The asphaltene fraction was separated as t h a t portion insoluble in n-pentane, and the remaining maltene fraction was further subdivided by liquid chromatography. An activated fullers earth column was used, white oils being eluted with n-pentane, dark oils with methylene chloride, and resins with water-saturated methyl ethyl ketone. The final fraction, removed with a mixture of acetone and chloro- form, was included with the resin fraction for purposes of reporting. The composition of the asphalts before and after weathering a s determined in this manner is shown in Table 11. The elemental composition of t h e asphalts was kindly determined by Mr. W. J. Montgomery of the Fuels and Mining Practice Division of the Department of Mines and Technical Surveys and is summarized in Table 111.
PREPARATION AND EXPOSURE OF TEST PANELS
The asphalts were exposed for natural and accelerated studies a s films 2% in. by 5y2 in. by 25 mils on aluminum panels prepared by the hydraulic press method17 ; a teflon-coated glass fabric is used instead of dextrin paper to keep the asphalts from adhering to the heated plates. I n the accelerated durability test four panels were exposed in a single-arc weatherometer operated in accordance with ASTM D529-59T, Cycle A ; the panels a r e subjected to 51 minutes of carbon-arc radiation, producing a black panel temperature of 140°F, followed by nine minutes of radiation with water a t 45 -
+
5OF sprayed onto t h e panels. The hourly cycle is repeated 22 timesto produce a single daily cycle, and daily cycles are repeated five days a week. To overcome variations in t h e day-to-day performance of t h e weatherometer the panels were exposed a t different times. This method of exposure has been found to be statistically the equivalent of exposing one panel of each asphalt in four different machines a t t h e same timels.
During exposure t h e panels were rotated according to t h e plan describ- ed in ASTM D529-59T and were tested for the development of flaws every five cycles with a high voltage probe1" A photograph of t h e flaws was examined with a 60-square grid and t h e panel was considered t o have failed when 50 per cent of t h e squares showed a t least one flaw.
The operation of the weatherometer equipment produced results com- parable with those reported by Mertensll who reported a correlation of
Y = 223.8X - 86.1, where Y is t h e durability and X is t h e dispersibility of the least soluble asphaltenes. A correlation using t h e fifteen asphalts described previously and exposed in the equipment at NRC produced a correlation of Y = 211.9X - 84.9. This indicates t h a t one asphalt tested in t h e two different laboratories would differ by about six cycles.
Natural durability studies were made on eleven of t h e asphalts. Four panels of each asphalt were exposed at a n angle of 45 deg. to t h e vertical, facing south, at three exposure sites operated by the Division of Building Research2". The panels were examined a t regular intervals in a manner similar to t h a t used f o r t h e accelerated durability tests.
DISCUSSION OF RESULTS FROM ACCELERATED DURABILITY STUDIES
Although the crudes were from widely different sources, they were processed t o meet specifications so t h a t they present a very narrow range of physical characteristics. Some studies by Wikinson, Striker and Trax1er21 have resulted in suggested criteria f o r establishing t h e quality of asphalts with respect t o durability. These limits a r e shown in Table IV, and if applied to the asphalts under study would eliminate all but No. 24, and 10. Greenfeldz2 reports t h a t all but one of t h e 1 5 asphalts he studied would be excluded by these limits.
In a n effort t o obtain the relationship between physical properties and durability, studies have been made on t h e correlation coefficients. This is a measure of the degree of correlation between t h e measured values and the equation of a straight line t h a t best fits these values. The correlation co- efficient r cannot exceed +1 nor be less than -1 in value. A value of +1 denotes perfect functional relationship between x and y, an increase in x
being associated with an increase in y. A value of -1 is also a perfect functional relationship, with increase in x associated with a decrease in y. When r is zero there is no relation between x and y and values of r between +0.5 and -0.5 a r e considered to indicate no significant relationship between x and y.
This approach was used on the best straight lines t h a t could be plotted between durability and the following: softening point, penetration a t 32, 77 and 115OF, ductility, density, weight loss at 10 cycles, the ratio of resin to asphaltenes, resins plus asphaltenes to total oils, carbon to hydrogen, and the sum of carbon plus hydrogen. The values of t h e various equations and r are shown in Table V. The results show t h a t no significant correlation exists between durability and most of the physical and chemical parameters measured. There is some significant correlation with penetra- tion a t 77 and 115OF. A plot of these values in Figure 1 indicates a wide scatter so t h a t the correlation may be one of coincidence.
An excellent correlation exists between ductility at 77OF and dura- bility. This is shown in Figure 2 in which records for 1 3 of the 1 5 asphalts
a r e plotted. Asphalts No. 1 and 14 were omitted a s showing extreme results, so t h a t the value of r was increased from 0.81 to 0.95. The equation for the line Y = 23.1X
+
8.4, where Y equals durability in days, and X equals ductility in cm. Shown also in Figure 2 a r e the lines of two times the standard error of estimate, which f o r durability is 4.9 cycles. With t h e above equation a n estimate of durability can be made within two standard errors a t the 99 per cent confidence level.This correlation may encourage more study of the ductility test, pos- sibly including microductility as a method of producing a rapid evaluation of the durability of asphalts. A high degree of precision is required, however, because the range of ductility is only 3 cm for these coating-grade asphalts.
RESULTS OF NATURAL DURABILITY TESTS
The panels have now been exposed for about 18 months. A t two of the sites, Halifax and Ottawa, very few failures have been detected, in contrast with those a t Saskatoon where there has been a high degree of failure. Table VI records t h e degree of failure a t t h e three sites.
From these results i t is evident t h a t Halifax does not present a very severe exposure for asphalts probably t h e result of the presence a t t h e roof site of a large amount of fly ash, which coats t h e panels and reduces degradation. It is of interest to note t h a t failures a t Ottawa and Halifax a r e of the same order, a s is predicted by accelerated durability results. The asphalts t h a t fail are those with poor accelerated durability per- formance. The rapid degradation a t Saskatoon is particularly surprising. These panels were exposed in August and first examined in November, when very little failure was detected. In February, however, most had failed badly, indicating the severe effect of cold weather upon asphalt. Only those showing good durability in the accelerated test withstood this cold test, with the single exception of the Pembina specimen, which also failed.
The wide variation in performance a t the three exposure sites makes i t apparent t h a t any attempt to obtain direct correlation between natural and accelerated durability is complex. Certain general statements can, however, be made. There appears to be some relationship between weight losses on natural and accelerated weathering, irrespective of the type of asphalt, and i t also appears t h a t failure is not always due to t h e attain- ment of a certain weight loss. This was shown in the panels from Halifax where weight losses in excess of those recorded in accelerated tests a r e found without failure. The rate of weight loss at t h e Ottawa site appears to be such t h a t 5 cycles in the weatherometer is the same a s 3 months' exposure a t the test site (after the initial loss and for the particular time of exposure).
A word of caution should be added a t this point to prevent any gross extrapolation of results. The accelerated test is a laboratory test t h a t divides asphalts into groups, based on their ability to withstand t h e test. I t does not follow t h a t a n asphalt with a rating of 60 cycles is three times better than one having a rating of 20 cycles. Only actual performance of the shingles can indicate this. The natural weathering test is an unrealistic one; i t uses asphalt on a metal panel for purposes of testing, and introduces thermal strain into the asphalt t h a t would not normally be present with a
prepared roofing product.
CONCLUSION
The complex nature of asphalt and the conditions under which it
degrades makes any correlation study very difficult. Performance of the materials on natural and accelerated weathering indicates t h a t a wide
variation exists between various asphalts. Most physical and chemical tests show very small differences, but because of t h e narrow range of properties these are not suitable for durability ratings. The ductility of asphalts, although covering a very small range, has a n excellent correlation with durability rating. It is suggested t h a t some consideration should be given to the use of the ductility test or a modified, more sensitive, ductility type test a s a measure of the durability of coating grade asphalts.
REFERENCES
1. Abraham, H. Asphalts and allied substances, 6th Ed. V. 5, p. 298-304, New York, Van Nostrand, 1960.
2. Beitchman, B. D. Infra-red spectra of asphalts. J. Res., N.B.S., V. 63A, n. 2, 1959, p. 189-193.
3. Kleinschmidt, L. R. and H. R. Snoke. Effect of light and water on the degradation of asphalt. J. Res., N.B.S., V. 63C, n. 1, 1959, p. 31-36. 4. Galloway, R. M. Chemical composition of asphalt related
to
road service,A.C.S. Symposium on Chemistry and Composition of Asphalts, V. 3, n. 2, April 1958, p. A53-A59.
5. Pfeiffer, J. P. The properties of asphaltic bitumen, p. 172, New York, Elsevier, 1950.
6. Greenfeld, S. H. Chemical changes occurring during the weathering of two coating-grade asphalts. A.C.S. Symposium on Fundamental Nature of Asphalt, V. 5, n. 4A, Sept. 1960, p. A89-A106.
7. Stewart, J. E. Infra-red spectra of chromatographically fractionated asphalts. J. Res., N.B.S., V. 58, n. 5, 1957, p. 265-269.
8. Wright, J. R. and P, G. Cambell, Determination of oxidation rates of air blown asphalts by infra-red spectroscopy. J. Appl. Chem., V. 12, 1962, p. 256-266.
9. Martin, K. G. A review of the durability of roofing bitumens. J. Inst. Petrol, V. 47, n. 454, 1961, p. 321-328.
10. Mertens, E. W. and S. Greenfeld. Some qualitative effects of com- position and processing on weatherability of coating-grade asphalts. A.S.T.M., S.T.P. NO. 280, 1959, p. 20-28.
11. Mertens, E. W. Predicting weatherability of coating grade asphalts from asphaltene characteristics. A.S.T.M. Bulletin No. 250, 1960, p. 40-44.
12. Jones, T. K. The prediction of coating grade asphalt weatherability from asphaltene dispersibility data. Paper presented
to
A.S.T.M. Annual Meeting, Symposium on Recent Research on Bituminous Materials, June, 1963.13. Heithaus, J. J. Measurement and significance of asphaltene peptiza- tion. J. Inst. Petroleum, V. 48, n. 458, 1962, p. 45-53.
14. Heithaus, J. J. Physical factors affecting the weather resistance of asphalt coatings. Industrial and Engineering Chemistry, V. 1, n. 3, Sept. 1962, p. 149-152.
15. Hamada, M. Durability of asphaltic materials. Proceedings, Arch. Inst. of Japan, V. 69(1-5), 1961, p. 53-72.
16. Kleinschmidt, L. R. Chromatographic method for refractionation of asphalt into distinctive groups of components. J. Res., N.B.S., V. 154, n. 3, 1955, p. 163-166.
17. Greenfeld, S. H. A method f o r preparing uniform films of bituminous materials. A.S.T.M. Bulletin No. 193, 1953, p. 50-53.
18. Greenfeld, S. H., E. W. Mertens and P. W. John. A statistical study of weatherometer data on coating grade asphalts. A.S.T.M., S.T.P. No. 280, 1959, p. 29-37.
19. Jones, P. 21 ompa par is on of two spark-gap testers for asphalt films. Materials Res. and Standards, V. 3, n. 4, 1963, p. 293-295.
20. Gibbons, E. V. Outdoor exposure sites of the national research council. Chemistry in Canada, V. 12, n. 3, 1960, p. 44-48.
21. Wilkinson, C. E., R. N. Traxler and L. Striker. Criterion f o r the evaluation of roof coating asphalts. A.S.T.M., Bulletin No. 230, 1958, p. 42-44.
22. Greenfeld, S. H. Characteristics of fifteen coating-grade asphalts. J. Res., N.B.S., V. 64C, n. 4, p. 299-305, 1960.
F e n e t r a t i o n a t ( c ) S.G. a t D u c t i l i t y F l a s h F i r e
~ s ~ h a l t s ( a ) Source S..Pt.OF 32OF 77OP 115OF 77OF a t 77OF P o i n t P o i n t D u r a b i l i t y ( b dmm dmm dnm ( a ) ( e ) " 7 OF ( f ) ( g )
Peub i n a 225 9 1 1 . 7 16.2 25.7 0.9993 1 . 7 535 T i a j u a n a 2nd sample 220.5 15.7 20.5 32.5 1.0142 2.5 570 Lloydnins t e r 209.1 9.7 13.0 27.0 1.0382 2.4 490 211d sample
Tia juana 1st sample 217.5 13.7 18.8 29.7 1.0184 2 . 8 555 Marlago 228.9 9 . 3 13.2 21.5 1.0182 2.4 575 A r a b i a n 223.7 1 1 . 3 14.5 23.7 1.0250 2 . 5 605 Lloydrninster 233.2 9.0 1 2 . 0 21.7 1.0342 1 . 7 460 1 s t s a q p l e S e l e c t e d S o u t h e r n 231.4 10.5 1 5 . 3 24.0 1.0259 1 . 6 520 A l b e r t a Crudes South Saskatchenan P i p e l i n e 1st sam2le 230.9 1 2 . 0 14.7 25.5 1.0268 1.8 5 30 R e s t Spur P i p l i n e 227.3 1 2 . 0 18.2 28.5 1.0000 1 . 7 555 C o l e v i l l e 2nd sample 224.1 9.5 12.5 22.8 1.0349 1 . 5 520 South Saskatche!van 231.4 8 . 7 1 2 . 8 23.0 1.0350 1 . 2 5 15 2nd sample C o l e v i l l e 1st sample 230.7 9.5 11.7 23.0 1.0300 1.1 520 M i s c e l l a n e o u s South- 221.2 1 2 . 2 1 5 . 3 25.2 1.0180 1 . 6 540 e r n A l b e r t a Zrudes Ku:va i t 227.6 7 . 3 3.8 16.2 1.0520 0.5 570 ( a ) 11sphalts numbered i n o r d e r of d e c r e a s i n g d u r a b i l i t y . ( b ) A.S.T.X. 935-26
-
T e s t f o r s o f t e n i n g p o i n t of b i t u m i n o u s m a t e r i a l s . ( 3 ) A . S. T.I.1. D5-5911-
T e n e t r a t i o n of b i t u m i n o u s m a t e r i a l s .( d ) A.S.T.Id. D71-52
-
T e s t f o r S p c i f i c GraviCy of a s p h a l t s and t a r p i t c h e s . ( e ) 11. S. T.ZI. D113-44-
T e s t f o r d u c t i l i t y of b i t u m i n o u s m a t e r i a l s .T.Itl. D92-57
-
T e s t f o r f l a s h a n d f i r e Cleveland Open Cup.TABLE I1
CHANGES NI COMPONENTS WHEN MPOSED TO ACCELSRATED WEATHERING
Water P e r c e n t A s p h a l t e n e s R e s i n s Dark O i l s White O i l s W t . Loss A s p h a l t and Bef. ( a ) Bef. ( a ) Bef. ( a ) Bef. ( a a t k r a b i l i t y Zxp. F a i l u r e Xxp. F a i l u r e Exp. F a i l u r e Exp. F a i l u r e F a i l u r e
TABLE I11
CHEMICAL ANALYSES OF ASPHALTS BEFON;: EXPOSURE
( a 1 ( b 1 ( C 1
A s p h a l t D u r a b i l i t y Carbon Hydrogen Oxyeen S u l p h u r N i t r o g e n C/H C+H p e r c e n t p e r c e n t p e r c e n t p e r c e n t p e r c e n t
( a ) 51-9 c y c l e 50 p e r c e n t f a i l u r e u s i n g 6 0 - s q u a r e grid ( b ) Oxygen b y d i f f e r e n c e from a v e r a e e v a l u e s
TABLE l[V
CRITERIA OF WILKINSON CONTROL FOR DURABILITY Property
Softening point
Limits
210 - 230°F Penetration a t 32OF 10 minimum dmm Penetration a t 77OF 18 - 25 dmm Penetration a t 115OF 24 - 40 dmm Flash Point
(Cleveland Open Cup) 500 minimum O F
TABLE V
EQUATIOBS AND CORFIZIATIONS OP ASLSFALT CHARACTSRISTIC M?D DURABILITY
C h a r a c t e r i s t i c S o f t e n i n g P o i n t P e n e t r a t i o n a t 32OP P e n e t r a t i o n a t 77OF P c n e t r a t i o n a t 115OF D u c t i l i t y a t 77OP Density W t . l o s s a t 1 0 c y c l e s Resins Asphaltenes
Resins and Asphaltenes T o t a l o i l s Carbon Hydrogen Equation Y = 221.3
-
0.8X Y = 3.98+
7.6 Y = 3.2X + 3.5 Y = 1.2x + 22.0 P = 2 5 . X + 5.7 Y = 662.4-
602.0X Y = 66.7-
14.6X C o r r e l a t i o n C o e f f i c i e n t g Carbon+
hydrogen Y = 6.0X-
529.5 +0.23 Note : I n a l l c a s e s Y = d u r a b i l i t y X = c h a r a c t e r i s t i cG, 0 Fr. Fr e
