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NATIONAL BUREAU OF STANDARDS REPORT

8110

A COMPARISON OF THIN-FILM OXIDATION

WITH CONVENTIONAL WEATHEROMETER EXPOSURES

by

S. H. Greenfeld

and

Jo C, Weeks

U. S. DEPARTMENT OF COMMERCE

NATIONAL DUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1004-12-10141 18 October 1963 8110

A COMPARISON OF THIN-FILM OXIDATION

WITH CONVENTIONAL WEATHEROMETER EXPOSURES

by

So H. Greenfeld and J, C. Weeks

Jointly Sponsored by

Asphalt Roofing Industry Bureau

and

National Bureau of Standards

IMPORTANT NOTICE NATIONAL BUREAU OF S

for use within the Government, and review. For this reason, t whole or in part, is not authi Bureau of Standards, Washing

the Report has been speclflcall

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

ess accounting documents intended

s subjected to additional evaluation

e listing of this Report, either in he Office of the Director, National

y the Government agency for which

copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A COMPARISON OF THIN-FILM OXIDATION

WITH CONVENTIONAL WEATHEROMETER EXPOSURES

********

ABSTRACT

The thin-film oxidation characteristics of eleven asphalts

blown in laboratory stills from four different fluxes were determined and compared with conventional durability data

determined in weatherometers . The more volatile oils in the

asphalts seemed to be the least resistant to oxidation, for

asphalts produced from the fluxes that were reduced the most

before blowing and the asphalts produced in the runs with the

largest blowing losses were the most resistant to oxidation.

However, some of the asphalts that had retained these oils were the more durable.

The time required to produce a carbonyl index of 0.080

for these asphalts correlated with durability and the correlations fit the curve reported by Wright and Campbell (3). However, these data did not fit the other correlations reported

by these authors.

********

A COMPARISON OF THIN-FILM OXIDATION

WITH CONVENTIONAL WEATHEROMETER EXPOSURES

S. H. Greenfeld and J. C. Weeks

1. INTRODUCTION

Exposure of asphalts in the form of films approximately twenty-five

mils thick on aluminum panels (1)1/ in weatherometers has been used for

many years as a means for predicting the life expectancy of finished roofing products. Although this procedure has a number of deficiencies, a sufficient background of information has been developed to permit asphalt

technologists to use the technique with confidence. Thus, a decision involving asphalt durability can be made in a matter of weeks instead of

years

.

As long as asphalt sources were relatively few in number and little

was being done to change processing techniques, the few weeks required for

weatherometer evaluations were not objectionable. However, as more sources

of asphalt have become available and processing technology has advanced, more rapid methods for predicting durability have become desirable. Greenfeld and Wright (2) reported on a comparison of four such procedures. While

all gave results that generally correlated with durability, there was always

sufficient scatter to raise questions concerning the value of these correlations for predicting the durability of individual, unfamiliar asphalts.

One of these four procedures consisted of the exposure of films of asphalt

one mil thick to a carbon arc for seven hours and measuring the change in

absorbance of the film in the 5.88p. region of the infrared spectrum. The

data scattered as in the other procedures, but the method was developed

further by Wright and Campbell (3) to yield improved correlations with

durability. One of the principal reasons for the scatter was the nature

of the oxidation of asphalts. Each asphalt, as it oxidizes, undergoes an induction period followed by a uniform rate period and a changing rate

period and terminates with film rupture. As each of these periods is

variable in length as well as in magnitude of oxidation, the selection of

any arbitrary exposure time can lead only eventually to difficulty. The

improved correlations, therefore, each involved some meaningful part of

the oxidation-rate curve rather than some arbitrary oxidation time. Wright

and Campbell, working with eight asphalts, reported correlations with

durability (51-9c cycle) of the time to film failure, the time to deviation

from the uniform rate period, the time to the end of the induction period,

and the time to produce a given change in absorbance. (aA=0.080).

All of this work was based on commercial asphalts; their physical

properties were within coating-grade roofing asphalt specifications and

they were actually being used by roofing manufacturers when obtained.

—^The numbers in parentheses refer to references listed at the end of the

report

.

- 2 -

It appeared that this th in-film oxidation rate procedure might be used

to yield data that would permit a better understanding of the behavior of

the asphalts produced in the laboratory. Specimen asphalts were selected

to represent the ranges of durability of the asphalts blown from each flux

and the effects of some of the blowing variables on thin-film oxidation.

This brief report covers the results of this study.

2. MATERIALS AND METHODS

The fluxes from which most of the asphalts used in this study were blown and the blowing apparatus and procedure have been described (4)

.

The asphalts produced in runs B217, B213 and B81 were blown from harder

fluxes than those previously described, but coming from the same corres- ponding batches of crude. These were produced by the refineries by con- tinuing to reduce the crude after the softer fluxes had been withdrawn.

These three harder fluxes are described in Table 1.

TABLE 1.

Properties of Hard Fluxes

Source

Property Kansas L2y Talco 85/100 Tia Juana 611

Softening Point, °F 120 120 111

Penetration @ 32°F, 1/10 mm 19 ___ 25

Penetration @ 77°F, 1/10 mm 83 92 91

Penetration @ 115°F, 1/10 mm TS TS 232

Flash Point (coc) , °F 620 560 620

Specific Gravity @ 77°F 1.0077 — --- Specific Gravity @ 60°F — 1.032 —

Coating Asphalt Designation

^Partially blown at the refinery.

B217 B213 B81

The blowing conditions used to make the asphalts employed in this

study and the properties of the asphalts are summarized in Table 2.

The conventional durability figures were obtained from the exposure

of coatings 25 mils thick on aluminum panels in two weat^erometers. The

specimens were prepared as described in ASTM C 1669-591,— exposed to the

51-9c cycle (Cycle A of ASTM D 529-59T) and removed at the 50% failure

level (ASTM D 1670-59T)

.

— Teflon reinforced with glass was used in place of the dextrin-coated

paper

.

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- 4 -

The thin films were prepared in accordance with a modification described by Wright and Campbell (3) of the procedure reported by Greenfeld

and Wright (2). A further modication in the film holders was made by

Wright and Campbell (5); the new holders were used in this work. They

were approximately 3/8 inch wide and 2 inches high and consisted of two

1/8-inch thick aluminum plates between which the asphalt film was clamped,

A window 1/4-inch wide by 1-inch high with rounded corners permitted the

asphalt to be exposed and scanned.

The thin films were exposed in a Model SMCR Weather-Quieter at 120±2°F

and 40% R.H. The infrared scans were made in a Perkin-Elmer Infracord

Spectrophotometer, Model 137. A special adapter plate, was made to hold

the specimens in the spectrophotometer.

The carbonyl index was calculated from the change in absorbance at

5.88p produced during exposure. A baseline drawn between the 5.5 and 6 . 5p

shoulders was used as the reference as described in (3) and shown in

Figure 1.

3. RESULTS

A typical pair of infrared spectra are shown in Figure 1. Only the

portion of the spectra pertinent to this study were measured in order to save time.

Figure 2 contains oxidation rate curves at 120°F and 40% R.H. of

three asphalts blown from the California flux. Figure 3 contains three

oxidation rate curves of Kansas asphalts exposed under the same conditions.

Figure 4 contains the oxidation rate curves of three Talco asphalts and

Figure 5, two oxidation rate curves of Tia Juana asphalts.

Table 3 is a summary of the conventional durability of the 11 asphalts

studied and a compilation of a number of parameters measured on the curves

in Figures 2 to 5

.

4. DISCUSSION OF RESULTS

Figure 1 contains typical infrared spectra of one thin film of

asphalt before and after exposure for 22 hours to the radiant energy of

a carbon arc at 120°F and 40% R.H. These conditions of exposure were se- lected to produce oxidation results that would be comparable to those re- ported earlier (3)

.

In Table 3 are tabulated parameters that can be

related directly to those Wright and Campbell (3) correlated with durability

for eight asphalts. These results do not fit their correlations involving

length of induction period, slope of the constant rate period, time to

deviation from the constant rate or breaking point. However, except for

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CARBONYL

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FIG. 2- CARBONYL INDEX VS EXPOSURE TIME.

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- 5 -

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- 6

three asphalts, these data do fall on the correlation of durability vs,

time to produce a carbonyl index of 0.080 (Figure 6)

.

There are several good reasons for their correlation in the one instance and lack of correlation in the other four. Looking at the lack of

correlation first, the time to film rupture is the most readily explained,

for a different type of specimen holder was used,, The narrow, slit -type

holder in this study supported the thin films of asphalt better than did

the holder with a round opening used by Wright and Campbell and, thus,

permitted the films to degrade more before film rupture occurred „ However, the stress distribution was more uniform in the holder with the

round opening and the repeatability was better.

Because the plots of carbonyl index vs. time generally produced Sshaped curves, the locating of the end of the induction period and the

second inflection point (end of constant-rate period) were quite difficult and, consequently, not of very high precision. Similarly, there was sufficient scatter in the individual datum points to make the exact location of the straight line, constant rate, portion of the curve somewhat

arbitrary. Thus, the lack of correlation in these latter three cases may

be for purely mechanical reasons. Wright and Campbell, possibly for the same reasons, did not attempt to draw a quantitative correlation between

durability and time to the end of the induction period or with the slope

of the constant-rate portion of the curve.

Thus, only the time to a carbonyl index of 0.080 correlates well and

agrees well with the correlation of Wright and Campbell (3). Their cor- relation with the addition of the data from this study are reproduced in

Figure 6.

The California asphalt had essentially no induction period, as seen

in Figure 2; the initial oxidation occurred at a constant rate. After

about 6 to 8 hours, the rate of increase of carbonyl index began to slow

and continued at a decreasing rate until film rupture. It is interesting

to note that after three hours of oxidation the curves for the. three

California asphalts separated, with the most durable oxidizing at the

most rapid rate and the least durable at the least rapid rate. This type

of performance is contrary to what would be anticipated from earlier re- ported work. However, the California flux is relatively rich in lowmolecular-weight, volatile oils. Apparently, these oils were the more readily oxidized components of the asphalt, for the weight losses during

blowing were also inversely related to the oxidation rates. In service,

however, these oils are distributed throughout the bulk of the asphalt

and serve to plasticize it. Their diffusion from the deep interior to

the surface apparently is slow enough to permit the plasticizing effect

of the. oils to contribute more to the. durability than their ease of

oxidation detracts from it, and the durability is effectively increased.

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FIG.

6

-DURABILITY

VS

TIME

TO

CARBONYL

INDEX

OF

0.080.

7

Thus, the asphalt that shows the most rapid oxidation in the thin-film

exposure is the most durable of the three and the one with the least

readily oxidizable oils is also the least durable.

While the California asphalts oxidized most rapidly, the Kansas

asphalts were most resistant to oxidation. All three of the Kansas

asphalts in Figure 3 had induction periods of about 12 hours. However,

the least durable of these asphalts had the lowest oxidation rate during

the constant-rate period and the films of this asphalt remained intact

the longest. All three Kansas asphalts had very low blowing losses and,

again contrary to expectations, the one with the lowest losses had the

lowest penetrations. While these low penetrations may tie in with the

low oxidation rate and low durability, no basic reasons for this performance are apparent.

The three Talco asphalts in Figure 4 performed more in line with

the previous work. The induction periods were directly proportional to

the durability and the slope of the constant-rate period inversely

proportional. The times to film rupture were also proportional to

durability. All of the weight losses were low. These asphalts lined

up well despite the fact that the most durable one was blown from the

85/100 penetration flux and the other two from the 175/200 flux. As

in the case of the California asphalts, the lighter, more volatile oils were removed (this time by distillation) before exposure; these also

could well have been the more easily oxidized oils.

The two Tia Juana asphalts in Figure. 5 were blown from different

grades of flux under the same conditions, B81, blown from the harder

flux, was the more durable, and also oxidized least rapidly. Its blowing losses were about two-thirds of those of asphalt B78.

Thus, in all instances except for the Kansas asphalts the asphalts

that had lost the most oils before exposure oxidized the least rapidly

when exposed as thin films. However, the durability when exposed in the

conventional manner was not related to this loss of oils. Only the time

to produce a carbonyl index of 0.080 in the thin films correlated with

conventional durability.

5. SUMMARY AND CONCLUSIONS

The thin-film oxidation characteristics of eleven laboratory-blown

asphalts were investigated and compared with results on "similar" asphalts

published by Wright and Campbell (3)

.

Only the correlation of time to

attain a carbonyl index of 0.080 with durability agreed with their cor- relation; length of induction period, slope of constant-rate period, end

of constant-rate period, and film rupture did not correlate with durability.

8

The more volatile oils in the asphalts seemed to be the less resistant

to oxidation,, for the asphalts that had the highest blowing losses or were

reduced the most before blowing were usually the ones that resisted oxidation the most (for each flux stock). However, these readily oxidized

oils are necessary to plasticize the asphalt and, despite the fact that

the asphalts with the higher percentages of these oils oxidized more rapidly than those from the same source with fewer oils, they tended to

be more durable.

6 . REFERENCES

(1) ASTM D 1669-59T; Preparation of Test Panels for Accelerated and

Outdoor Weathering of Bituminous Coatings; ASTM D 529-59T,

Accelerated Weathering Test of Bituminous Materials.

(2) Greenfeld, S. H. and J. R„ Wright, "Four Methods for Predicting

the Durability of Roofing Asphalts," Materials Research and

Standards 2 , 738-745, 1962.

(3) Wright, J. R. and P. G. Campbell, "Determination of Oxidation

Rates of Air-Blown Asphalts by Infra-Red Spectroscopy," J. Appl.

Chem. 12, 256-266, 1962.

(4) Greenfeld, S. H. , "The Effect of Blowing Variables on the

Durability of Coating Grade Asphalts," Ind . and Eng. Chem. Part

3, in press.

(5) Private communication. To be presented as "Oxidation of Asphalt

in the Presence of Ozone" by J. R. Wright and P. G. Campbell at

the April 1964 ACS Symposium on Asphalt.

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