NATIONAL BUREAU OF STANDARDS REPORT

8046

THE EFFECT OF BLOWING VARIABLES ON THE

ACCELERATED WEATHERING DURABILITY OF COAT ING-GRADE ASPHALTS

by

Sidney H, Greenfeld

Hot for publication or

for reference

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1004-12-10141 July 19^ 1963 8046

THE EFFECT OF BLOWING VARIABLES ON THE

ACCELERATED WEATHERING DURABILITY OF COATING-GRADE ASPHALTS

by

Sidney H. Greenfeld

Research Associate

Asphalt Roofing Industry Bureau

Organic Building Materials Section

Building Research Division

Sfofr [£ey ptthlioaii un dr

iTui:. rufDi'jjLiet**

Jointly Sponsored by

Asphalt Roofing Industry Bureau

and

National Bureau of Standards

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authors Bureau of Standards, Washingtoi

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

accounting documents intended

ubjected to additional evaluation

listing of this Report, either in Office of the Director, National he Government agency for which

pies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
* ' ' f
THE EFFECT OF BLOWING VARIABLES ON THE

ACCELERATED WEATHERING DURABILITY OF COATING-GRADE ASPHALTS

Sidney H. Greenfeld

ABSTRACT

The effects of varying temperature, agitation and air-rate during the blowing

of fluxes from California, Kansas, Tia Juana and Talco fields on the properties and accelerated weathering durability of the coating-grade products

were studied. Although the durabilities of asphalts from each source varied

with the blowing conditions, the California and Talco fluxes were more

dependent on the variables studied than were the Tia Juana and Kansas fluxes.

The durabilities of the asphalts produced from the California flux varied from

39 - 68 days inversely with blowing temperature and directly with air rate

and agitation. The durabilities of the Talco asphalts varied in the range

of 32 to 56 days inversely with the blowing temperature, but were relatively

independent of air rate and agitation. The durabilities of the Tia Juana

asphalts were highest when they were blown at the lower temperatures and

higher air rates with little or no agitation. They varied within the range

of 46 to 60 days. The durabilities of the Kansas asphalts ranged from 65 to

87 days. The highest blowing temperatures and air rates produced the least

durable Kansas asphalts. The ranges of variables covered were as follows.

Temperatures 430 - 530°F

Air Rates 38 - 150 cu ft/ton-min.

Agitations 0 - 2200 rpm

All three variables had pronounced effects on the blowing rates of the four

fluxes. However, the final penetrations of the Kansas and Tia Juana

asphalts were insensitive to the blowing conditions. The penetrations of

the California and Talco asphalts varied to a greater extent with blowing

conditions. The California asphalts had penetrations lower than normally

specified for coating-grade materials.

The solubility parameters of the asphaltenes and the asphaltene contents of

the asphalts were related to the flux source, but did not vary appreciably

with the blowing variables.
'

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- 2 -

INTRODUCTION

Controlled oxidation (blowing) has been used to modify the properties of

organic materials for almost a century. The historical development and

general nature of the changes that occur during the blowing

Traxler (2) . These changes consist of hardening of the asphalt, a decrease in its temperature susceptibility and a shift toward a more

gel- like material. While the changes involved are generally similar

among many asphalts the exact nature of the changes that occur in each

asphalt is related to its composition and, therefore, to its source.

The blowing process offers a relatively convenient way for modifying

asphalts

Chelton, Traxler and Romberg (3) have shown that the mechanical properties

of two asphalts were functions of blowing temperature, pressure, air rate

and liquid level. The magnitudes of the effects were different for the

two asphalts studied. The differences between them were attributed to

differences in their viscosities at the blowing temperatures. Gun and

Gurevich (4) have reported that the mechanical properties of blown road

asphalts vary with blowing temperatures.

Rescorla, Forney, Blakey and Frino (5) studied the use of mechanical

agitation to increase the efficiency of oxygen utilization in the

blowing of asphalts. Mechanical agitation (one impeller) decreased the

oxidation time to produce an asphalt with a given softening point. The

asphalt produced with agitation was softer than its non-agitated counterpart. Agitation faster than 880 rpm appeared not to shorten the time

further. The authors recommended additional impellers to improve oxygen

efficiency further.

Eng, Govier and Quon (6), and Muraya et al (7), on the other hand, reported that blowing temperature and air rate had little effect on the

softening point-penetration relationships of Redwater and Arabian

asphalts, respectively, even though the blowing times were nearly halved.

The ranges of temperature and air rate covered were 465 - 525 £ F and

55 - 75 ft^/ton-min, respectively.

Others have reported on the effects of blowing on the composition of

coating-grade asphalts (8, 9, 10, 11, 12, 13, 14). Lockwood (15) has

shown with his own data and the data of Rescorla et al (5) that except

at high temperatures and low air rates blowing seems to conform to a first order reaction rate.

The numbers in parentheses refer to references at the end of this

report.

have been described in standard references such as Abraham
j
- 3

-

In all these studies^, no attention was given to the effect of any of the

blowing variables on durability. In this study,, however^ durability!/

will be of primary importance and the physical properties will be of

secondary consideration. However,, comparisons of the "specification

properties" among the various asphalts will be made and the sensitivity

of these properties to variations in blowing conditions will be discussed.

2. EQUIPMENT

Two 2-liter stainless steel blowing stills were used^ as shown in Figure 1.

The one on the left contained an agitator^ the one on the right an x*member

air distributor. Figure 2 is a view of the disassembled stills,, showing

the 4-inch OD x 21 -inch still housing,, the four 500-watt strip heaters

fastened to the 1/4- inch heat distributing jackets., the air distribution

systems and the agitator design. Air was introduced into both stills

through 3/8-inch stainless steel tubes,, which enter through the tops and

carry the air to within one inch of the bottom of the stills. In the

agitated still the air was delivered to the center of the lowest impeller^

where it was broken into extremely fine bubbles as it was thrown outward

by the three impeller blades. The ascending air-asphalt mixture was

drawn successively into the second and third impellers,, where the air was

again subdivided. In the non-agitated still the air was distributed

through twelve holes equally spaced in the four arms of the X, The total

area of the holes was 1.2 times that of the 3/8-inch tube. This ratio is

frequently used in commercial stills.

The asphalt flux was introduced through the 1/2- inch stainless steel 45°

elbows on the tops of the stills and was sampled and removed through the

stainless steel valves at the bases of the stills.

Compressed air was filtered through glass wool and metered to each still

through a rotameter,, the meter closest to each still in Figure 1. The

other two rotameters were not used in this study.

The heat input was controlled manually through the two variable transformers at the bottom of Figure 1

All of the asphalts in this study were blown at atmospheric pressure.

When a positive pressure appeared on one of the gages a blockage in that

air line was indicated and that still was dismantled for cleaning. Gleaning was accomplished by heating the dismantled still and air tubes to about

600° C in a muffle furnace and completely oxidizing the residues in them.

2 / Durability as used in this report is as determined in a Weather -Ometer.
' i- ' |

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FIGURE 2, COMPONENTS OF BLOWING STILLS

- 4 -

3. MATERIALS

Based on a survey of crudes being used for the production of coatings for

shingle and roll roofing products in the United States, four fluxes were

selected o These asphalts represented a large fraction of the tonnage of

asphalt used for roofing manufacture. The properties of these fluxes are

listed in Table 1.

4. PROCEDURE

4. 1 Blowing

Two thousand grams of flux were heated in a closed gallon can in an air oven

to about 300°F while the still was being preheated to about 400°F o The flux

was poured into the still through a funnel. The quantity put into the still

was determined by difference, i.e., the funnel and gallon can were weighed

before and after the flux was poured into the still. The temperature of

the still was raised to the blowing temperature desired and the air was

turned on. Periodically, the air was turned off and samples were taken for softening

point determinations. The air was turned on immediately afterwards. When

the expected softening point was over 200°F, the air was kept off during the

softening-point determination to avoid over-blowing. The still was drained

into two penetration cans and five pint cans (partially full) when the run

was complete.

The asphalt in all of the cans was weighed (the intermediate samples before

softening point rings were poured) and the losses determined by difference.

Penetrations and asphaltene contents were determined on the final products.

4.2 Exposure and Evaluation

A pint can of each asphalt was heated on a thermostated electric hot plate

to just over 400°F (205°C) and maintained at that temperature until the

asphalt surface was essentially free of air. (Those asphalts blown with

agitation usually contained more finely divided air than those blown without

agitation.) Four exposure specimens were made of each asphalt by the

hydraulic press method (ASTM D1669-59T). However, Teflon reinforced with

glass fibers was used as the parting material in place of the dextrin-coated

paper. The Teflon was stripped from each specimen by a fiTm pull when the

specimen had cooled to room temperature.
1 -

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TABLE 1 Properties of Fluxes

California

Source Coastal Kansas Talco Tia Juana

Softening Point ,°F 122 101 104 104

Penetration @ 32°F,1/10 mm 20 42 44 44

Penetration @ 77°F,1/10 mm 64

1/ TS—

249 190 152

Penetration (3 115°F,l/10mm TS TS 222

Specific Gravity at 77°F 1.030 0.994 1.020 1.004

Flash Point (C.O.C.) ,°F 460 635 555 575

1 / too soft
-

V'
- 5

-

The specimens were exposed the following morning, two in each of two

Weather-Ometers, to the 51-9C cycle (ASTM D529-59T Cycle A) 0 All

specimens were made on Mondays and exposed on Tuesdays in order to keep

the weekends, periods during which the Weather-Ometers were not operated,

in the same relative places in the exposures of all specimens.

Failure was determined by a high-voltage probe (ASTM D1670-59T) at the

end of a dry part of a cycle,, The durabilities reported are the average

of the four specimens at the 50% failure level as measured with a 60-square

grid.

4.3 Properties

Softening points were measured by a slight modification of ASTM Method

D36-26. The penetrations, by ASTM D5-61. The softening point determination was modified in three respects. (1) A shouldered ring was used,

as described in ASTM E28-58T. (2) The ring holder assembly was similar

to the one described in ASTM E28-58T, but contained holes for four rings

instead of two. (3) A gentle stream of air was bubbled through the

glycerine during the higher softening point determinations to provide the

agitation required for a uniform temperature rise.

The asphaltene-content and solubility parameter determinations were as described by Greenfeld and Wright (16).

5. RESULTS

The data obtained in this study may conveniently be summarized under the

three principal independent variables investigated t (1) Temperature,

(2) agitation and (3) air rate. These summaries are presented as Tables

2, 3, and 4, respectively.

Data are presented on the changes of softening points with blowing time

for each of the asphalts under the various blowing conditions. Figures 3,

4, 5, and 6 show the effect of blowing temperature on the change of

softening point with blowing time (blowing rate) for California, Talco,

Tia Juana and Kansas asphalts, respectively.

Figures 7 and 8 contain data showing the effect of air rate on blowing

rate for California and Talco and Kansas and Tia Juana asphalts, respectively.

Figure 9 is a summary of the effect of agitation on blowing rate for all

four asphalts.
!

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FIGURE 4

TIME, MINUTES

FIGURE 5

TIME .MINUTES

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FIGURE 7

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k

SOFTENING

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0 20 40 60 80 100 120 140

TIME, MINUTES FIGURE 8

0 ICO 200 300

o

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TIME , MINUTES

FIGURE 9

Figures 10^, 11, and 12 are graphical presentations of the time required to

blow each of the fluxes to coating grade softening points as functions of

temperature, air rate and agitation 0

6 o DISCUSSION OF RESULTS

No general specification for coating-grade roofing asphalts exists. Each

company that purchases these asphalts has its own specif ications Q However,, these cover narrow ranges of softening points and penetrations.

For purposes of discussion, a broad specification will be assumed which

will encompass the majority of the company specifications. Such a hypothetical specification is reproduced in Table 5.

The softening point-penetration relationships of the asphalts will be

discussed relative to this specification. As seen in Tables 2, 3, and 4,

all of the Kansas and Tia Juana asphalts complied with the specification;

their penetrations were essentially independent of blowing conditions.

Only the low- temperature penetration of the two Talco asphalts blown at

about 430 °F failed to meet this specification, and they failed by only

one unit. None of the California asphalts met the penetration requirements.

The necessity of a flash point in excess of 450®F precluded the production

of a flux from this source which could be blown to grade without modification.

Unlike the Kansas and Tia Juana fluxes, those from California and Talco

fields produced products the penetrations of which varied with the conditions

under which they were blown. As the blowing temperature was increased

from 430®F to 530®F the penetrations of the California products blown to

approximately a 220°F softening point decreased. Those blown without

agitation had lower penetrations than the corresponding products blown

with agitation. These lower values were probably related to the much

longer residence times in the stills during the unagitated runs and the

correspondingly higher weight losses. As seen in Tables 2, 3, and 4^

the weight losses for the unagitated runs with the California flux were

©f the order of 107o , while the California asphalts blown with agitation

lost 1.5% or less.

The penetrations of the Talco asphalts increased as the speed of agitation

was increased. Except for the 900 rpm run; the weight losses decreased

correspondingly. Temperature, in the absence of agitation, had little

effect on penetration; but with agitation, the penetrations at 77®F

dropped appreciably as the temperature was increased.
I 'I
12x10

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CO

LxJ

8

LxJ

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o

_i

CD

0

400

\

SrO

tt- s \

AIR BLOWING

BLOWING TIME

vs

TEMPERATURE

N

\ V

\

\

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• CALIFORNIA

TALCO

° TIA JUANA

^ KANSAS — 1400 RPM , 75cu.ft./ton-min. — 0 RPM , 75cu.ft./ton-min.

\

\

450 500 550 600

BLOWING TEMPERATURE °F

FIGURE 10
'
BLOWING

TIME,

MINUTES

I4xl0

2

12

10

8

6

4

2

0

0 25 50 75 100 125 150 175

AIR RATE ,cu.ft./ton- min.

AIR BLOWING

BLOWING TIME

vs

AIR RATE

n

\

\ • CALIFORNIA

TALCO

o TIA JUANA

* KANSAS — 1400 RPM 475 °F - - 0 RPM 475 °F

\

V.

3

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\

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\

V \

s

T

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k \

4

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= 1

FIGURE II

BLOWING

TIME,

MINUTES

TABLE 5.

Hypothetical Coating - Grade Aiphalt

Specification

Property Range

Softening Point, °F 215 - 225

Penetration @ 32°F, 1/10 mm 10 min

Penetration @ 77°F, 1/10 mm 15-25

Penetration @ 115°F, 1/10 mm 25-45
-
- 7

-

The weight losses for all of the runs except those involving California

asphalt without agitation were reasonably low. These losses were a little

erratic because of the variable holdup in the stills,, which was largely

dependent on the temperature at which the asphalts were discharged. The

first run in a completely clean still always had a high holdup; this

situation was corrected in the approximate loss figures reported. In two

of the runs some undeterminable transfer loss occurred and^ consequently^

no loss figures are reported.

In 1 959 P Greenfeld_j Her tens P and John (17) published an article pertaining

to the statistical considerations involved in the comparison of the

durabilities of asphalts exposed in Weather-Qmeters. Since that time s modifications in the evaluation procedure have occurred. The statistics,,

therefore^ are not strictly applicable to the results obtained in this

study. However,, for discussion purposes,, it will be assumed the variability

of these results is not greater than for the results analyzed and P thus,, the

average of one set of four specimens must be at least 12 percent greater

than that of another set of four specimens in order for the first material

to be truly more durable than the second.

The durabilities of the blown asphalts produced from the four fluxes

covered a range of 32 to 87 days. The Kansas flux produced the more

durable asphalts while the Talco flux produced the least durable ones.

However,, there was an overlapping of the California,, Talco and Tia Juana

products. From the California flux,, asphalts were produced which were more

durable than the best Tia Juana product and others which were less durable

than the least durable Tia Juana.

The durabilities of all of the asphalts varied with the temperatures at

which they were blown. In genera l s the durability was lowest for the

asphalts blown at about 525®F and highest for those blown at about 435 S F.

There was usually less difference between the asphalts blown from a single

source at 435 and 475 °F than between those blown at 475 and 525®F.

The largest effect of blowing temperature was observed in the California

asphalts (Table 2). The durability was highest for the product produced

at the low blowing temperature with agitation and lowest for the asphalt

produced at the high temperature without agitation. The former,, with a durability of 68 days,, was two-thirds more durable than the latter

(41 days).

A similar set of results was obtained for the asphalts blown from the

Talco source. The most durable,, 50 days,, was produced at the lowest

temperature with agitation^ and the least durable was produced at the

highest temperature without agitation (32 days).
'

i .yj
- 8

The Tia Juana and Kansas fluxes were less sensitive to blowing temperature

than the California and Talco asphalts 0 In both of these the most durable

asphalts were produced by blowing at the intermediate (475°F) temperature

without agitation and the least durable products were produced at the highest

blowing temperature without agitation. All of the products blown with

agitation were of intermediate durabilities. When agitation was used^ the

durabilities of asphalts made from either of these fluxes were independent

of blowing temperature and in the middle of the durability ranges of the

respective products blown without agitation G

As seen in Table 3^ durability was independent of the degree of agitation

for three of the four asphalts; only for California asphalt was durability

found to vary with the degree of agitation during blowing. Durability

increased from 44 days when blown without agitation to 67 days when blown

with the agitators rotating at 2200 rpm. While none of the California

products had penetrations high enough to meet the minimum requirements

of a coating-grade material,, the most durable product had the highest

penetrations.

As seen in Table 4, the durabilities of the asphalts blown with agitation

were independent of the air rate in the range of 38 to 150 f tVton-min.

In the absence of agitation the durabilities of the California and Talco

asphalt products were again independent of air rate. However, the

durabilities of the products blown from Tia Juana asphalt were higher

at the higher air rate. The durability of the Kansas product blown at

the highest air rate., conversely,, was lowest.

Each flux possessed an optimum set of blowing conditions with which asphalts

of higher durability could be produced., For some, the ranges of variables

were quite large; for others, they were rather narrow. The conditions under

which the more durable products were made from each of the four fluxes

studied are summarized in Table 6.

There was no general correlation of either solubility parameter or asphaltene content with durability. Both of these variables seemed to be

related to the asphalt source and little affected by the processing.

Even the loss of as much as 1 of the flux through volatilization that

occurred during the blowing of the California asphalt did not noticeably

affect these variables. Mertens (18) and Greenfeld and Wright (16)

reported correlations of durability with solubility parameter of the

asphaltenes and Wilkinson,, Striker, and Traxler (19), and Greenfeld and

Wright (16) with asphaltene content. Greenfeld and Wright (16) pointed

out the tendency of the various asphalts from the same area to group

together in these correlations. Apparent ly, the variations in the

processing of the commercial samples used in these studies contributed

to the differences in durability,, but the inherent solubility parameters

and asphaltene contents remained essentially unmodified.
<1

.

:

.

u

.

.• .

.

I
TABLE 6

BLOWING CONDITIONS PRODUCING MORE DURABLE ASPHALTS

Durability,

(51-9c), Temperature Agitation Air Rate

Source Days °F rpm f t~V ton-min

California 62-68 439-483 1400-2200 75-150

Kansas 80-87 437-475 0-2200 38-75

Talco 50-56 432 0 150

Tia Juana 58-60 435-470 0-900 75-150

- 9

-

7. SUMMARY AND CONCLUSIONS

The effects of varying the temperature,, air rate and amount of agitation

during blowing on the physical properties and durability of asphalts

produced from four fluxes were studied. Each flux responded differently.

The California flux was most sensitive to changes in these parametefs.

The durability of its products decreased as the blowing temperature was

increased from 430 to 530®F^ was independent of the air rate in the

range of 38 to 75 cubic feet per ton minute and increased as the rate

of mechanical agitation was increased from zero to 2200 rpm by three,,

three-bladed turbo-mixer type impellers.

The Talco flux was next most sensitive to the variables. The durability

of its asphalts decreased as the blowing temperature was increased and

was independent of air rate and agitation.

The Tia Juana flux was relatively unresponsive to blowing temperature,,

but the blown products did suffer a loss of durability at highest

blowing temperatures (520®F) in the absence of agitation. Tia Juana

produced the more durable products at the lower rates of agitation

and^ in the absence of agitation^ at the higher air rates.

The durabilities of the Kansas asphalts were relatively independent

of all three variables in the lower two-thirds of their ranges^ but the

asphalts produced at the highest blowing temperature and the highest air

rate were less durable than the others.

The ranges of durability of the asphalts produced from the four fluxes

were as follows.

The blowing time to produce asphalts of approximately 220 ®F softening

point varied from 42 to 125 minutes with agitation and 225 to 1200

minutes without agitation. The blowing times decreased as the temperatiar e^

air rate or agitation was increased.

The Kansas and Tia Juana asphalts produced under all conditions had penetrations within the normal coating-grade ranges. The penetrations of the

Talco asphalts were within the coating-grade ranges under most conditions^

but tended toward the lower limits when blown at the highest and lowest

California

Kansas

Talco

Tia Juana

39 - 68 days

65 - 87 days

32 - 56 days

46 - 60 days
“

,
- 10 -

temperatures. The California flux always produced asphalts with penetrations

lower than desirable. When agitation was used, however, the penetrations

were higher than for the asphalts produced under corresponding conditions

without agitation.

Neither the asphaltene content nor the solubility parameter of the

asphaltenes correlated with durability^ they seemed to be more characteristic of the source than of the processing variations.

8. ACKNOWLEDGMENT

The author is grateful to J. C. Weeks for his help in collecting the

laboratory data,, and to a number of members of the Organic Building

Materials Section of the National Bureau of Standards for their advice

and encouragement. This work was sponsored by the Asphalt Roofing

Industry Bureau.

9 0 REFERENCES

(1) H. Abraham,, ’’Asphalt and Allied Substances 38 , D. Van Nostrand,, II , 165-179, 1961.

(2) R. N. Traxler, ’’Asphalt, Its Composition, Properties and Uses98 , Reinhold, 121-129, 1961.

(3) A. M. Chelton, R. N. Traxler and J 0 W. Romberg, "Oxidized Asphalts’ 8 , Ind. Eng. Chem. 5J, 1353 (1959)

.

(4) R„ Bo Gun and I. L. Gurevich, "Oxidation Temperature in the Production of Asphalt from Petroleum Stocks", Novostx Nefiyanoi Tekh,

Neftepererabotka 1958 . 12-17.

(5) A„ R. Rescorla, W. E. Forney, A. R. Blakey and M, J. Frino,

"Asphalt Oxidation with Agitation", Ind. Eng. Chem. 48, 378 (1956).

(6) J. Eng, Go W. Govier and D. Quon, "The Air Blowing of Redwater

Asphalt Base Stock", Can. J. Techno L 33, 360-77 (1955).

(7) K0 Mnraya, T a Fukushima, Y. Fukuda and A. Shimada, "Studies of

Asphalt Blowing in a Vertical Pilot Converter", Bull. Japan Petrol.

Inst. 2, 63-9 (1960).

(8) M. Katz, "The Composition of Blown Alberta Bitumen", Can. J. Res. JO,

435-51 (1934).

(9) J. M. Goppel and J. Knotnerus, "Fundamentals of Bitumen Blowing' 1 Fourth World Pet. Cong. Proc., Sec. III/G, 399 (1955).

9
. 7’ : I

i

. \ w

.-i

-

; tn

-

•- - *

. yj, .

••

•H i , . •

' -

’

-

.
- 11

(10) L. R, Kleinschmidt and H. R. Snoke, "Ghanges in the Properties of

an Asphalt During the Blowing Operation", J, Res. NBS ^0, 169

(1958) (RP2835)

(11) So Ho Greenfeld, "Chemical Changes Occurring During the Weathering

of Two Coating-Grade Asphalts", J e Res. NBS 64C , 287 (I960).

(12) E. K 0 Liiv, "Character of Oxidized Asphaltenes and the Kinetics

of Their Formation", Izvest. Akad. Nauk c Eston. SSSR, Ser.

Tekh. i Fiz.-Mat. Nauk., 1 957 . 70-89.

(13) A. Rossi, "Manufacture of Blown Asphalts", Bol. Informationis

Petroleras (Buenos Aires), 1 9

,

37-47 (1942).

(14) S. R. Sergienko, V. I. Korchagina, P. N. Galich, L. I. Rutman,

Bo E. Davydov and M. I. Krasavchenko, "Influence of the Nature

of the Raw Materials and the Time of Oxidation on the Composition

and Properties of Oxidized Asphalts", Trudy. Inst. Nefti. Akad.

Nauk. SSSR T2, 187-99 (I960).

(15) D. Co Lockwood, "Determine Asphalt Blowing Kinetics", Petrol.

Refiner 38, 197-200 (1959).

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

the Durability of Roofing Asphalts", Materials Research and

Standards 2, 738-745 (1962).

(17) S. H. Greenfeld, E. W, Mertens and P. W. M. John, "A Statistical

Study of Weather-Ometer Data on Coating-Grade Asphalts", ASTM

Spec, Tech. Pub. No. 280, 29 (1959).

(18) E. W. Mertens, "Predicting Weatherability of Coating-Grade

Asphalts from Asphaltene Characteristics", ASTM Bull. No. 250,

40 (I960).

(19) Co E. Wilkinson, L. Striker and R. N. Traxler, "Criterion for

the Evaluation of Roof Coating Asphalts", ASTM Bull. No. 230,

42-44 (1958).

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