NATIONAL BUREAU OF STANDARDS REPORT

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ASSESSMENT OF PHOTOOXIDATION OF FLEXIBLE PLASTICS

ROOFING BY A COLORIMETRIC METHOD

by

Keith G. Martin

<NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring

maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. Its responsibilities include development and maintenance of the national stand- ards of measurement, and the provisions of means for making measurements consistent with those standards; determination of physical constants and properties of materials; development of methods

for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; assistance

to industry, business, and consumers in the development and acceptance of commercial standards and

simplified trade practice recommendations; administration of programs in cooperation with United

States business groups and standards organizations for the development of international standards of

practice; and maintenance of a clearinghouse for the collection and dissemination of scientific, technical, and engineering information. The scope of the Bureau’s activities is suggested in the following

listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Ref- erence Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory.** Ionosphere Research and Propagation. Troposphere

and Space Telecommunications. Radio Systems. Upper Atmosphere and Space Physics.

Institute for Applied Technology. Textiles and Apparel Technology Center. Building Research.

Industrial Equipment. Information Technology. Performance Test Development. Instrumentation.

Transport Systems. Office of Technical Services. Office of Weights and Measures. Office of Engineering Standards. Office of Industrial Services.

* NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. ** Located at Boulder, Colorado.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

421 ,04“ 12 -42 101 45 1 March 1965 8646

ASSESSMENT OF PHOTOOXIDATION OF FLEXIBLE PLASTICS

ROOFING BY A COLORIMETRIC METHOD

by

Keith Go Martin

Organic Building Materials Section

Building Research Division

Institute for Applied Technology

(Present address t Division of Building Research, Commonwealth

Scientific & Industrial Research Organization, Highett,

Victoria, Australia.)

Sponsored by

National Bureau of Standards

IMPORTANT NOTICE NATIONAL BUREAU OF STAN

for use within the Government. Be and review. For this reason, the p whole or in part, is not authorize. Bureau of Standards, Washington

the Report has been specifically prf

Approved for public release by the

Director of the National Institute of

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

accounting documents intended

jjected to additional evaluation

iting of this Report, either in

iffice of the Director, National

i Government agency for which

es for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

investigation of this aspect has therefore been made and at the same time

the applicability of a recently developed colorimetric method for measuring

photochemical degradation of polyesters [ 1 ]

to the types of plastics used

in these new roof coverings has been explored.

1.2 Colorimetric Assessment of Photooxidation

Methods of evaluating the changes in plastics due to exposure to solar

radiation are usually concerned with changes in visual appearance (color,

gloss light transmittance) or changes in physical properties (tensile

strength, extensibility, stiffness, tear strength). Chemical changes

are extremely difficult to study since these commercial materials are

inher ent ly variable in chemical structure and usually contain a nunb er of additives with high content of functional groups.

It is therefore of considerable interest that a colorimetric method of

assessing photochemical degradation has been developed and applied to

several classes of plastics [2]„ The method is based upon the reaction

of a solution of N, N-dimethyl-p-pheny lenediamine (DMPDA) with the sur- face of the exposed plastic Since photochemical degradation is a sur- face phenomenon the method is more sensitive than methods of evaluation

that concern the complete specimen.

2 , EXPERIMENTAL

2.1 Materials Examined

Five liquid-applied coatings and two prefabricated sheet materials

were examined. Most of the coatings were applied to strips of glass

(3-1/2 X 5/8 X 1/8 in.) by means of a Fisher-Payne dip coater adjusted

to give a thickness of .005 to .007 in. When cured, the coatings were removed from the edges and one face of the glass by means of a razor blade and the thickness of the cured coating was checked. An area of

coating of approximately 2 sq„ in. (3-1/4 X 5/8 in.) was exposed. The

types of coatings examined were:

acrylic emulsion (Ac)

butyl rubber emulsion (BR)

chlorsulphonated polyethylene solution (CSP)

poly(vinyl-chloride-acetate) solution (PVCA)

and silicone rubber solution (SR).

All coatings were commercial products, but only Ac, CSP, and SR were specially formulated for roofing application. The PVCA coating was

clear, and the others were pigmented white.

2 Numbers in brackets refer to the list of references at the end of this report.

- 3

-

The flexible sheets examined were 2

QolQO in. poly (vinyl-chloride) (PVC), light grey

and .002 in. poly (vinyl-fluoride) (PVC), both clear and white.

These were cut into strips 3-1/4 x 5/8 in. and fixed to the glass supports

by means of adhesives. Both materials were commercial products, the

former being designed as a flexible roof flashing and the latter as a sheet roofing being fixed with special adhesive to a neoprene "bonded

asbestos felt.

2.2 Methods of Exposure

The plastics were exposed to both solar and carbon-arc irradiation.

Solar exposures were carried out by taping the glass supports to aluminum

panels which fitted onto outdoor exposure racks. The racks were inclined

at 45® to the horizontal facing south and mounted 3-5 feet above a gravel

surfaced asphalt roof approximately 350 feet above sea level at Washington,

D. C» (lato 38®57 long. 77®05"W)„ A number of replicates of each

specimen were exposed and duplicates were withdrawn after various durations of exposure between August 1963 and June 1964. It is important to

note that these outdoor- exposed specimens were not shielded from rain.

Exposures to the carbon arc were carried out with similar specimen

holders as used, for the outdoor exposures. The aluminum panels were

fitted onto the rotating drum, of an Atlas single-arc weatherometer.

Model SMC-R, so that the specimens faced the arc continuously. The

specimens faced the arc continuously. The panels were inverted daily.

Exposure to the radiant energy from the enclosed arc was made under a constant black body surface temperature of 140 ±5®F<, and the specimens

were not subjected to water spray. A set of replicates was exposed and

duplicates removed from, the weatherometer for examination after various

durations of exposure.

Clock time was used to define the exposure durations to the carbon-arc

irradiation. This is valid, since the intensity of radiant energy on the

specimens is approximately constant. According to previous work [3]

this intensity is estimated to be 6 lang leys/min. compared to a maximum

intensity of solar radiation recorded on a surface normal to the solar

beam at the above site during midsummer of 1.4 langleys/min. which is

equivalent to 1.3 langleys/min. on a surface facing 45 C ’S.

4 -

Durations of outdoor exposures were also expressed in clock time. However,,

since the intensity of solar radiation is continually changing it would

have been of more Interest to have recorded solar radiation intensity

particularly with respect to a series of different ranges of wavelength.

As an item of general interest the total hours of sunshine as recorded at

Washington National Airport (lat„ 38® 51 11 N^, long, 77 Q 02”W) have been

included.

2.3 Methods of Assessment

Treatment of the irradiated plastics with DMPDA and subsequent evaluation

were carried out essentially in accordance with the methods described by

Gray and Wright [1, 2] Solutions used were as follows:

Q„2Q0 g DMPDA, 2HC1 was dissolved in 5,0 ml 0.4N potassium hydroxide

and immediately diluted with 100 ml methyl alcohol to prevent discoloration,, then made up to 200 ml in volumetric flask with benzene

to give a reagent concentration of 1,00 mg/ml„

(b) Buffer

25 ml of Q„2 molar potassium acid phthalate was added to 20 „ 3 ml

of 0, IN hydrochloric acid and diluted with water to 100 ml in a volumetric flask to give a buffer of pH = 3,0,

(c) Reagent Detector

1,000 g benzoyl peroxide was dissolved in 10 ml benzene and diluted

to 100 ml in a volumetric flask with methyl alcohol to give a con- centration of 10,0 mg/ml

o

A Meeco Tristimulus Color Difference Meter, standardized on the green,

red and blue reflectance values of a white glass 90 percent reflectance

standard was used to define the color of the plastics. The color of

each specimen was defined both before and after exposure to irradiation

and before and after treatment with DMPDA, The sample port of the

meter was taped with white polyvinylf luoride to reduce its area to a little less than the exposed surface areas of the specimens of plastics.

The white 90 percent reflectance standard glass was placed behind each

specimen and a heavy black cloth used as a cover to eliminate stray

light. Green, red and blue reflectance values were determined, the

specimen rotated about 180' and the measurements repeated. Average

values were then calculated and corrected to the modified Adams

Coordinate System described by Glasser and Troy [4].

5

After withdrawal from exposure to radiation the specimens had color

measurements taken and were then treated with DMPDA. Each of the

duplicates of the exposed plastic and a matching pair of unexposed

specimens were separately immersed in 15,0 ml of the reagent solution.

Special test tubes were used so that the 15 ml of solution would

completely cover the plastics. The tubes were stoppered, securely

sealed and placed in a mechanical shaker for 2 hours. After removal

of the specimen, the content of unreacted DMPDA in the reagent

solution was determined.

A 1,0 ml aliquot of the reacted DMPDA solution was added to a 100 ml

volumetric flask containing approximately 80 ml of methyl alcohol and

2„Q ml of buffer. After shaking, this solution was reacted with 1.0

ml of the reagent detector and the time that the addition of detector

was made was noted. It was found convenient to handle eight specimens

at a time and add the detector to the eight reagent aliquots at 1 min.

intervals. After shaking and making up to 100 ml the intensity of

pink color developed was measured at fixed time intervals after

commencement of the reaction. This was done with a Beckman DU manual

spectrophotometer by measuring absorbance (A) in 1.0 cm cells at

520jj, against methyl alcohol as reference. Readings of A were usually

made after 15, 25, 35, and 45 minutes for each of the eight solutions.

The reaction of the DMPDA reagent with the benzoyl peroxide detector

was carried out immediately after the 2-hour shaking period (contact

between the reagent solution and the plastics specimen). It was observed that the change in color of the reagent solution itself was

quite marked when irradiated plastics were in contact with the reagent

but not with unexposed specimens. These changes in color of the

reagent solutions were not stable and changed with time from yellow

brown to green brown, A direct reading of A at 41 Op was made on these

solutions approximately 1 hour after completion of the shaking.

Preliminary experiments showed that the Ac and BR coatings were swollen and detached from the glass supports by immersion in the DMPDA

solution. Gray and Wright [2] used a complete solution technique to overcome this problem, but since this method could not be applied to

all the plastics a different approach was used in the present work.

The solvent power of the DMPDA solution was reduced by using methyl

alcohol only to dissolve the DMPDA for reaction with Ac and BR

specimens

.

6

3. RESULTS

3.1 Amount of BMPDA Reagent: Reacted

Absorbance of 52Qp, due to the pink color developed between residual DMPDA

reagent solution and detector, gave a similar plot against time in all

cases; it rapidly increased to a maximum at 25 to 30 minutes after

commencement of reaction, then decreased slowly. A typical set of plots is

shown in Figure 1 . The procedure adopted was to estimate maximum A at

520p as the intercept between the straight line joining the first two readings

with that joining the final two readings.

Although the reaction between BMPDA and benzoyl peroxide is not: completely

understood, Gray and Wright [ 1 j have shown that A at 520p is directly

proportional to DMPDA concentration over the range of concentrations of

interest. The difference between maximum A at 52Qp for the reagent after

contact with an unexposed plastics specimen and after contact with an exposed specimen may therefore be taken as an indication of the amount of

reagent that reacted with products of photooxidation on the surface of the

exposed plastics.

Table 1 gives the mean, maximum A at 520fi for the reagent solutions reacted

with the detector after contact with specimens exposed outdoors and indicates the equivalent amounts of DMPDA depleted from the reagent solution.

Duplicate results were in excellent agreement for these and other measurements made and only means are given in this report.

Table 2 gives similar data for the specimens exposed to carbon-arc

irradiation.

It is of interest to note that the maximum. A at 52Q[d values corresponding

to DMPDA reagent content after contact with the unexposed specimens varies

from 0.428 to 0.464. The concentration of the fresh DMPDA 1

' 2HC1 reagent

solution was 1 mg per ml and according to Gray and Wright [1] when this

is immediately diluted to 1 mg per 100 ml and reacted with benzoyl peroxide

a maximum A at 520ji of 0.433 is obtained. It is known that the DMPDA

solution decomposes on standing and fresh solutions are prepared each

day that the reagent is used However,, it would appear worthwhile to

obtain quantitative information on this decomposition rate.

CSP SPECIMENS

0.38-

o UNEXPOSED

© EXPOSED OUTDOORS 163 DAYS

€> EXPOSED TO CARBON ARC 1789 HR.

0 36 1 1 1 1 1—

10 20 30 40 50

TIME, MIN.

FIGURE 1 . TYPICAL ABSORBANCE AT 520^ VERSUS TIME PLOTS FOR RESIDUAL

DMPDA REAGENT SOLUTION REACTED WITH BENZOYL PEROXIDE.

TABLE 1. AMOUNT OP Di PDA. REACTED WITH SPECIMENS EXPOSED OUTDOORS

1 ^*DT?PTMHv

i\T L EXPOSURE DURATION ! MEAN A

max

AT 520^4

Aa

DI.iPDA

L REACTED

TYPE

i (SUNSHINE I (DAYS) HR) UNEXPOSED EXPOSED

. - . .

- - - (no.)

! Ac

( White) 63 420 .437 .436 .001 .025

125 9L7

I .429 . 424 .005 .114

SR ! (Off-white) 352

114-5

.440

.429

.405

.420

0 035

.009

.798

.205

235 1614 .440 .452 .008 .182

295 2141 .436 .430 .006 .137

CSP 51 352 .456 .453 .003 . 06 8

(White) 163 1145 0 45 2 .445 .007 .160

233 1614 ,456 .448 ,008 .182

295 2141 ,454 .439 .015 ,342

PVCA 51 352 .446 .442 .004 .091

(Clear) 165 1145 . 43 9 .429 .010 .228

3R

(White) 63

125

: 420

: 947

.450

.449

.450

.460

Nil Nil

PVC 63 i 420 .428 .410 .018 ,410

(Grey) 125 ; 947 ,428 .420 .008 .182

PVF 255 ! 1614 » 442 .439 .003 . 068

(White) 295 2141 ,442 .437 .005 .114

PVP 235 1614 .436 .435 .001 .023

(Clear) 295 2141

i

.442 .436 .006 .137

TABLE 2 o AIviOUNT O'? D..PDA REACTED WITH CARBON -ARC IRRADIATED SPjSCIKENS

7

3.2 Color Developed In DMPDA Reagent Solution

Proper attention was not given to the measurement of color developed in

the DMPDA reagent solution due to contact with exposed plastics because

the determination of DMPDA content by reaction with benzoyl peroxide was believed to be the better method of assessing the depletion of DMPDA.

The colored solutions were unstable,, some changing more rapidly than

others. However , some absorbances at 41 Op determined approximately one hour after completion of the 2-hour shaking period are given in Table 3.

Phenylenedi amines are well known as reagents to detect the presence of

peroxides which; if present; give a characteristic pink color to the

solution. No such color was observed when most of the exposed plastics

were immersed in the DMPDA solution. However; part way through the

project the technique was slightly altered and several drops of the DMPDA

solution were placed on the surfaces of the exposed plastics before

immersing the specimen completely in DMPDA. In many cases the characteristic pink color developed immediately indicating the presence of

peroxides on the surface of the plastics. It generally persisted for a few minutes only. The plastics surfaces on which peroxides were thus

detected are indicated in Table 3.

3.3 Color Developed on Specimen Surface

The color that developed on the exposed plastics surfaces after the speci- mens were shaken in the DMPDA reagent solution were very stable and

changed little in storage after several weeks. Tristimulus reflection

measurements were usually taken on dry specimens 1 day after the immersion

in the DMPDA reagent

.

Some changes in color occurred when unexposed specimens were immersed; but

were usually much less than that induced with the exposed specimens.

Table 4 gives the Adams coordinates for the unexposed untreated specimens

and the unexposed DM?DA“immersed specimens together with the differences

which may be attributed to reaction between the reagent and the unexposed

plastics. Where clear plastics were measured; the determinations depend

upon the white 90 percent reflectance standard glass used as a backing.

Table 5 gives the changes in Adams coordinates due to the outdoor exposure

both for the untreated surface (differences between exposed and unexposed

specimens) and the DMPDA"treated surfaces (differences between exposed

DMPDA"treated and unexposed DMPDA -treated specimens).

TABLE 3. AMOUNT OF COLOR DEVELOPED IN DiVPDA REAGENT SOLUTION

SPECIMEN OUTDOOR EXPOSURE CARBON -ARC IRRADIATION

TYPE DURATION AA at DURATION AA at 410|i

(DAYS) (SUNSHINE

410n (hr.)

Hr.)

Ac 63 420 O 028p 1517 .175

P

( white) T 25 947 .08^

BR 51 352 o007

p

723 .094

(off white) 163 1145 • log 1789 a54p

233 1614 .115p 2742 .2954

295 2141 .o6cr 5649 0645

CSP

(white) 51

163

352

1145

.052

* 210

106

245

.210

233 1614 449 o250

295 2141 723 .215

1789 C82p

2742 .53CT

PVCA

( clear) 51 552 .133 106 .148

163 1145 .249 449 •431

725 o602

1083 .696

SR 63 420 il P

320 Nil

( white) 125 947 .075 610 * 002

n 1517 .014

PVC 63 420 . .045 320 .160

( grey) 125

j 947 .280 1517 .300

PVF

(white) 233 i 1614 .°°7_ t 2742 .480

P

295 2141 .012

N 3649 • 470

P

FVF

( clear) 233

295

1614

2141

•O1^

.033

1084

2742

• 310p

•495;

j

3649 .43R

P = Peroxides detected

N = Peroxides not detected.

TABLE 4. COLOR CHANGES DUE TO TREATMENT OF UNEXPOSED SPECIMENS WITH

DMPDA REAGENT

SPECIMEN

MEAN FOR

UNTREATED

MEAN FOR

TREATED

CHANGE

L a b L a b AL . 1 Aa

Ac

( white) 93.93 -0.5

1

j +Co 1 95.00 -1.0 + 5.9 -0.93 -0.5 + 2.8

BR

( off-white)

85.28

'

-0 o 7 + ) o 7 83.28 -1.3 + 7.9 -2.00 - 0.6

j

+ 4.2

( white) 95.56 +0.5 +1.8 94=15 +0.3 + 2.5 -1.25 i o + 0.7

PVCA

(cl ear)

88 c 99 - 2.9 + 1.2 87.58 -5.2 +4.2 -1.61 +5.0

SR

(white) 97.24 -0.2 + 0.2 92.89 -0.2 +0.1 -4.35 0 -0 . 1

PVC

( sre,y) 81. 51 -o . 6 + 2.0 78.58 +0.3 + 5.3 - 2.93 +0.7 + 3 * 3

PVF

( wiiite) 89.50 +0.4 -1.3 83.77 +0.2 -0.5 -0.73 -0.2 + 0.8

PVF’

( clear; 91.19 + 0 . 1 +1.6 90.80 ———

-0.3 + 2.6 -0.39 -0.4 +1.0

TABLE 5. SPECIMEN COLOR CHANGES* DUE TO OUTDOOR EXPOSURE

j EXPOSURE DURATION UNTREATED

IT

n DMPDA TREATED

SPECIMEN I (DAYS) (SUNSHINE) AL Aa 4b | AL Aa Ab

fi (hr.) 1

Ac

j l 63 420 -1.43

H 3

\ -0.7

1 -0.5 -4.00 +0.4 + 1.6

(white) ! 125

t___—

947 -3.71 | +1.2 1

h

,! -0.6 ,, -4.18 +0.2 +3,9

BR 51 352 -0.39

!

j +0.4 i + 1.8 -10.65 -0.5 -1.0

(off-white) l: 163 1145 -2.73 + 1.0 | +2 . 1 j -12.26 + 1.0 -3.3

233 1614 -4.97 3+1.1 I +2.5 i -11.95 + 1.8 -1.4

295 2141 -5.59 + 1.4 | +2.0 ; -14.41 +0.3 -4.1

CSP 51 352 -2.14 -1.1

1 +5.4 -5.48 -1.3 + 13.5

(white) f 163 1145 -1.00 -0.7 +5.2 -5.54 -0.6 + 15|.3

233 1614 -0.86 !i -1.3 +5.2 -6.30 +1.1 + 12.4

295 2141 -0.38 jj -0.8 , +4.6 -10.24 + 1.5 + 10.3

PVCA 51 352 -3.22 +0.2 i +2.0 -4.68 +0.5 +6.5

(clear) 163 1145 -4.53 -1.0 1 +2.8 -10.59 +2.3 +26.0

SR 63 420 -0.45 -0.1 +0.2 -1.17 -0.3 +0.7

(whit e ) \ 125

l' 947 -1.47 +0.3 + 1.0 -5.62 +0.3 +0.7

PVC 63 420 -1.21 +0.1 + 1.3 -4,90 +0 .

2

+5.7

(grey) 125 947 -1.91 -0,3 +3.0 -7.61 -1.0 +9 .0

PVF ;i 233 1614 -0.61 +0.2 -10.1 -1,09 +0.9 +0.1

(white) 295 2141 -1.08 +0.1 +0.3 -1.53 +0.3 + 1.0

PVF

(clear) 233 1614 -0.36

| -0.6 + 1.5 -1.45 +0.5

j 295

J. _4

2141 -1.86 * +1.1 j -4 J- + 1.0 1 - _J_

-1.93 +0 .

3

+3.3

*Color changes expressed as changes in Adams Coordinate System with respect to

untreated and treated unexposed replicate specimens.

AL; + = lighter, - = darker

Aa; + = redder, - = greener

Ab ; + = yellower = bluer

8

Table 6 gives similar results for the carbon-arc irradiated specimens.

As mentioned previously, the specimens were smaller than the exposure port

of the reflectance meter, so that even a black specimen would register an L value. A matt black surface was measured and gave coordinates of L = 31.85, a = 1.7, and b = 2.2. Considered in relation to the results in

Tables 4, 5, and 6, this means that the maximum AL value possible is about

-60 .

4. DISCUSSION

4,1 Outdoor Exposures; Assessment of BMPDA Reaction

Considering the specimens exposed outdoors, the results in Tables 1, 3,

and 5 show that three parameters, the amount of DMPDA reacted, the AA at

41 Op of the reagent solution after DMPDA reaction, and the darkening (-AL)

of the specimen surface after DMPDA reaction, generally increase steadily

with increase of duration of exposure. The increases are not as smooth

as those found for polyesters [1,2] and the amount of DMPDA reacted does

not increase with exposure time for all specimens, anomalous results being

obtained for BR, SR, and PVC„ In each of these matexials considerable

reaction had taken place between DMPDA reagent and the unexposed plastics

as shown in Table 4. This reaction is possibly an indication of residual

free radical polymerization agents in the plastics.

The two synthetic rubbers after exposure gave pronounced peroxide reactions.

Gray and Wright [5] indicate that the DMPDA reagent reacts with certain oc,|0-

dicarbonyl compounds such as ct-keto acids, glyoxal and pyruvic aldehyde.

Peroxides are generally considered to be precursors of the carbonyl com- pounds but due to their rapid decomposition little evidence of their

presence has been reported or expected. The fact that certain plastics do

develop comparatively stable peroxides on their surface is of considerable

interest and may explain the greater than usual reaction with DMPDA for

these specimens.

Figure 2 illustrates the progressive photooxidation of the surface of the

plastics due to outdoor exposure according to the three methods of assessment. The anomalous values for the amount of DMPDA reacted have not been

included and the separate results for clear and white PVF have not been

distinguished. Each of the three methods of assessment of the reaction

between DMPDA and the photooxidi zed plastics surface has given similar

relative results for PVF, Ac, CSP, and PVCA. This indicates that the

determination of DMPDA in the reagent solution after the reaction may be

unnecessary and that the simpler measurements AA at 41 Op of reagent solution and tristimulus reflectance of specimen surface may be adequate. However, further studies should be made to determine the influence of plastics

TABLE 6. SPECIMEN COLOUR CHANGES* DUE TO CARBON-ARC IRRADIATION

SPECIMEN

EXPOSURE

DURATION

UNTREATED TREATED

(hr.) AL A

a

Ab AL Aa Ab

Ac

(white) 320 -0.68 -2.3 -0.6

610 -1.19 +0.5 -0.7 -3.77 + 1.1 -0.8

1517 -7.09 | -1.5 +2.2 -12.64 +1.7 +0.9

BR

(off-white) 723 +3.33 -1.5 + 1.2 -8.96 +0.4 -0.9

1789 +2.16 0.0 +0.7 -9.77 +0.7 -2.7

2742 + 1.31 0.0 +4.5 -11.31 +0.7 +0.2

3649 +0.83 -0.5 +3.9 -12.08 + 1.6 -1.2

CSP 106 -5.64 -1.9 +22.8 -16.35 +3.0 +24.0

(white) 245 -7.20 +0.6 +28.4 -17.70 +6.1 + 17.2

449 -6.54 -0.6 + 19.2 ; -17.01 +4 .6 + 11.0

723 -6.72 +0.5 + 18.5 -16.51 +2.3 +8.0

1789 -6.82 +0.9 + 14.7 -16.60 +2.5 +6.4

2742 -9.48 0.0 + 15 .5 -19.46 +3.9 +6.0

PVCA 106 -2.20 +0.5 +2.8 -10.26 +3.0 +21.5

(clear) 245 -4.75 + 1.5 +6.1 -21.89 + 11.8 +23.4

449 -8.83 +3.9 +7.4 -23.72 + 13.0 +25 .5 723 -42.60 +17.3 1 +9.2 -43.92 + 17.9 +8.9

1083 -53.80 + 11.0 +3.5 -50.22 + 15.2 +4.7

SR 320 -0.06 -0.2 0 -0.44 +0.2 +0.4

(white) 610 -1.17 -0.3 +0.6 -1.85 +0.2 +0.6

1517 -1.35 +0.3 + 1.0 -5 .08 +0.9 +0.1

PVC 320 -1.10 -1.0 +2.7 -2.95 -0.5 +4 .

3

(grey) 1517 -2.23 -0.3 +3.9 -5.87 -1.3 +4.4

PVF 1084 1 - - _ - - (white) 2742 -1.21 -0.3 + 1.7 -2.38 +0.1 +2.0

3649 -2.47 -1.5 + 1.7 -3.89 -0.8 +5.0

PVF | (clear) 1084 -1.34 -1.5 +0.2 -2.23 -0.8 +0.4

2742 -1.76 -0.5 + 1.9 -2.53 + 1.1 +2.2

3649 -1.97 +0.1 +2.5 -3.82 -0.3 +2.1

*Color changes expressed as changes in Adams Coordinate System with respect to

untreated and treated unexposed specimens.

AL; + - lighter, - = darker

Aa; + = redder, - — greener

Ab; + = yellower, - = bluer

DM

PDA

REACTED,

mg.

A

A^

REAGENT

SOLUTION

A

L

SPECIMEN

SURFACE

EXPOSURE TIME, SUNSHINE HOURS

FIGURE 2. PROGRESSIVE PHOTOCOLIDATIQN Of SPECIMENS EXPOSED OUTDOORS, ACCORDING

TO THE DMPDA REACTION.

9

type upon the absorption spectra in the visible region of the reagent

solution after reaction and also to obtain quantitative data on the

stability of the colored solutions and repeatability of the method.

The SR and PVC specimens have given similar results for both darkening

(-AL) of treated plastic surface and AA at 41 Op of reagent solution

after reaction of exposed samples with DMPDA, Only the BR results are

not in agreement for these two methods of assessment,

4.2 Outdoor Exposure; Changes in Color of Untreated Specimens

Darkening of the plastics due to outdoor exposure without any DMPDA treatment generally increases regularly with increase of exposure time and is

illustrated in Figure 3. Comparison to Figure 2 shows a similar relative

assessment for specimens PVF, SR^ he, CSP^ and PVCA except that the CSP

gets lighter in color after about 50 days of exposure and this is not

paralleled by assessment of DMPDA reaction with the surface. It is of

interest to note that the darkening of the untreated surface ranks the BR

in the same position as does the AA at 41 Op of the reagent solution after

DMPDA reaction with the exposed surface. However } this may fortuitous

since the BR surface picked up considerable grime due to outdoor exposure

and could not be completely washed clean. Darkening of the untreated PVC

is less than expected on the basis of a correlation between results in

Figures 2 and 3, This may be explained by the initial grey color of the

PVC flexible sheet tending the mask the color change due to exposure.

Yellowing (+b coordinate in table 5) has not increased regularly with

duration of outdoor exposure for the untreated surfaces and only darkening

(-AL) is a useful parameter for evaluating the visual changes in color of

the plastics due to outdoor exposure.

4,3 Carbon-arc Exposures; Assessment of DMPDA Reaction

Each of the assessments of the DMPDA reaction with the carbon-arc irradiated plastics (Tables 2, 3 f and 6) has increased with increase of

exposure duration as illustrated in Figure 4, The ranking of the various

plastics surfaces is similar for the three modes of assessment except in

two cases. The AA values at 41 Op of reagent solutions for the PVF speci= mens were abnormally high;, but at present an explanation of this result

cannot be made. The other result that is not in general agreement is the

darkening (-AL) of the treated PVC surface which has been discussed

previously.

-6

LJO

2

or

z>

if)

uj

u

Ui

Q.

if)

<

EXPOSURE TIME, DAYS

0 350 420 7 950 il5CT 1610 2140

EXPOSURE TIME, SUNSHINE HOURS

FIGURE 3. PROGRESSIVE DARKENING OF SPECIMENS DUE TO OUTDOOR EXPOSURE

ONLY.

FIGURE 4. PROGRESSIVE PHOTOGKHUTION OP SPECUCNS EXPNKD TO PHMi IT

IRRADIATION, ACCORDING TO IS DHPM REACTION.

10

It is worthy of note that none of the determinations of DMPDA reacted

has given apparent anomalous results despite the fact that peroxides were detected on most of the surfaces irradiated.

4.4 Carbon-arc Exposures; Changes in Color of Untreated Specimens

Figure 5 illustrates the progressive darkening (-AL) (table 6) of the plastics irradiated with the carbon-arc. Results for BR are unusual since a short duration of exposure has caused the surface to become lighter and

further subsequent exposure has then induced darkening. Comparison of

Figure 5 to Figure 4 shows a similar ranking of the different plastics to

that given by the DMPDA reaction.

Yellowing of these plastics (+Ab, table 6) shows a regular change for most

specimens. In several cases (BR, CSP, and PV€A) yellowing passes through

a maximum,. As found for outdoor exposures, only darkening (=AL) is a useful

parameter for evaluating the visual color changes of the different plastics

due to irradiation.

4.5 Comparison of Performance of Materials

Although further work needs to be done concerning the validity of using the

DMPDA reaction as a method for comparing the relative photochemical stabilities of different types of plastics, an apparent applicability to a wide

range of plastics has been established in this work. On the basis of the

four methods of assessment, the general rankings found for the plastics are indicated in Table 7. The general order for outdoor exposure beginning with

the least stable is

;

PVCA

PVC

CSP

Ac

SR

PVF

the relative ranking of BR being not. clear.

The general order for stability to carbon-arc irradiation beginning with

the least stable is:

PVCA

CSP

PVC

BR

Ac

PVF

SR

FIGURE 5. PROGRESSIVE DARKENING OF SPECIMENS DUE TO CARBON-ARC

IRRADIATION ONLY.

TABLE 7o INCREASING ORDER OP PIIOTOGHEI.JGAL STABILITY OF THE

FLEXIBLE PLASTICS

METHOD OF ASSESSMENT

Dr.FDA TREATED UNTREATED

Dr, PDA

REACTED

A A 410M

REAGENT SOLUTION

-AL SPECIMEN

SURFACE

-A L SPECIMEN

SURF,ACE

Out doer Exposures

BR

FVCA PVCA F/CA PVCA

PVC PVC

CSP CSP CSP CSP

Ac Ac Ac Ac

PVC

BR BR

oK SR SR

PVF FVF PVF FTP

Carbon-arc Exposures

PVCA PVCA PVCA PVCA

CSP CSP CSP CSP

PVF' PVC PVC

BR BR BR

Ac Ac Ac Ac

PVC PVC

SR

PVF PVF PVF

SR SR SR

- 11

4.6 Comparison of Effects of Different Exposures

Apart from the anomalies previously discussed, there are a few differences

between the effects induced in the plastics by the different exposure con- ditions. A comparison of the performance of the different plastics as shown

in Figures 2 and 4 does indicate that PVCA is more labile when irradiated

with the carbon arc than with natural sunlight. The GSP specimens also

show during initial carbon-arc irradiation, a relatively rapid reaction.

On the other hand,, the SR specimens appear to be relatively more stable

to the carbon arc than to outdoor exposure. It is not possible to discuss

these observations at this stage since there are at least four important

differences between the methods of exposure, vizs

(a) the spectral energy characteristics of the sources of

radiant energy;

(b) the intensities of the radiant energy falling on the specimens;

(c) the surface temperatures of the specimens;

(d) the presence of water on the specimens.

It may be noted however that the carbon-arc irradiated specimens have degraded only about twice as fast as the specimens exposed outdoors when

considering the hours of irradiation received. As discussed previously,

the average intensity of solar radiation would be at least ten times less

than the radiation impinging on the specimens in the carbon arc.

Another point of interest is that peroxides were detected on the surfaces

of CSP and PVF exposed to carbon-arc irradiation, but not when exposed

outdoors (Table 3). The opposite behavior was observed for SR, peroxides

being detected on an outdoor exposed specimen, but not on a specimen

irradiated with the carbon arc. These observations appear to be associated

with the extent of photooxidation of the surfaces. Both the CSP and PVF

specimens were degraded to a greater extent in the carbon arc than when

exposed outdoors; whereas the SR was degraded further in the outdoors

exposure according to the methods of assessment employed.

- 12

5 . CONCLUS IONS

1 „ A wide range of types of organic polymers and elastomers form photooxidation products on their surfaces which react with DMPDA. Samples,

mostly commercial products of the following roofing materials, have shown

little reaction with DMPDA when treated as unexposed specimens and con- siderable reaction when specimens that had been exposed outdoors or irradiated with the carbon arc were treated:

acrylic, butyl-rubber, chlorsulphonated-polyethylene,

poly (vinyl-chloride-acetate) and silicone rubber coatings,

poly (vinyl-chloride) and poly (vinyl fluoride) flexible sheets.

Other workers [2] had shown similar reactions with rigid polyesters,

poly (vinyl-chloride) , poly (methyl-methacrylate) and cellulose-acetatebutyrate.

2. Quantitatively, the reaction with the flexible materials does not

appear to be as repeatable as when applied to the rigid materials. Three

methods were found to be useful for assessing the extent of the DMPDA

reaction with a photooxidized surface, viz:

(a) determination of depletion of DMPDA by colorimetric detection

with benzoyl peroxide;

(b) measurement of color developed in the DMPDA reagent solution

(A at 41 Op.) ;

(c) measurement of darkening of specimen surface (Adams coordinate

-AL) .

Apart from some anomalies which necessitate further investigation, good

agreement was found between the ranking of the different materials by

the different methods of assessment.

3. The assessments of the reaction with DMPDA gave results that generally

agreed with the amount of darkening (Adams coordinate -AL) caused by

exposure only.

4. The DMPDA reaction offers considerable promise as a means of assessing

the relative stability to photooxidation of different types of organic

polymers and elastomers after comparatively short periods of exposure.

The roofings examined in this work were generally rated in the following

- 13

order beginning with the least stable”

poly (vinyl- ch lor ide-acetate) coating

poly (vinyl-chloride) sheet

chlorsulphonated-polyethylene coating

butyl-rubber coating

acrylic coating

silicone-rubber coating

poly (vinyl-fluoride) sheet

Some differences in this order were found for the different methods of

assessment and according to whether the materials were exposed outdoors

or to the irradiation of the carbon arc.

5.

Peroxides may be detected on the surfaces of photooxidi zed polymers

and elastomers by the application of several drops of DMPDA solution to

the surface and observing the development of a pink color.

6

.

ACKNOWLEDGMENT

The author wishes to express his appreciation to Mr. V. E. Gray for his

assistance and interest in the work,, and to Mr, W. C. Cullen who provided

the samples of roof coatings and sheetings.

7.

REFERENCES

[1] V. E. Gray and J. R. Wright , "'Colorimetric Method for Measuring

Polyester Degradation Due to Weathering" , Journal of Applied

Polymer Science, Vol. 7_, pp. 2161-2174 (1963).

[2] E e Gray and J. R. Wright, "Measurement of Photochemical Degradation in Certain Plastics by Color Reactions with Phenylenediamines",

Journal of Applied Polymer Science, Vol. pp. 1505-1519 (1964).

[3] K. Go Martin, "Influence of Radiant Energy Source on Bitumen Oxidation",

Journal of Applied Chemistry (London), Vol. j[4, p. 514 (1964).

[4] L. B. Glasser and D. J. Troy, "Color Coordinate Tables (Modified

Adams Chromatic Value System)", Journal of the Optical Society of

.America, Vol. 42, p. 652 (1952).

[5] V. E. Gray and J. R. Wright, "The Use of Visible and Ultraviolet

Spectroscopy to Identify Carbonyl Compounds in Photodegraded

Plastics" (to be published).