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Investigation on photochemical dosimeters for ultraviolet radiation
St,/at E~e'r'.. 3 ~.'~i 42, Nt~ 5 pp 4 0 5 - 4 1 6 ,
I989
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Prinlcd in Ihc I' S A
INVESTIGATION ON PHOTOCHEMICAL DOSIMETERS FOR ULTRAVIOLET RADIATION B. BERRE Department of Physics and Meteorology. Agricultural University of Norway. 1432 Aas-NLH. Norway
and D. LALA Materials and Structures Division. The National Sv~edish Institute for Building Research. G~ivle. Sweden Abstract--The paper presents results from the photochemical measurements of ultraviolet solar radiation (UVR) using films of Polyphenylene oxide (PPO/. The measurements were carried out at ten Nordic stations and on two test buildings. The PPO films darken when exposed to wavelengths shorter than ~.f~'~ nm. This darkening is measured as the increase in optical density of the film at 340 nm (AOD340j and is to a first approximation proportional to the product of time and intensity. Accelerated experiments in Atlas Weather-ometer to study the temperature dependence showed that up to 40°C temperature had no effect on the degradation of the PPO films. A comparison between results from the photochemical measurements of UVR with instrumental measurements using Eppley Pyrradiometer on horizontal surfaces shows the agreement between both types of measurements is better than 20%. Measurements on certain parts of the test buildings show a certain order of UVR values on different facades and eaves. !.
Data for UVR as function of time. site and exposure surface is scarce because of the extensive instrumentation involved. It is important to know UVR in different climates and sites in order to be able to forecast the service-life of different materials in outdoor uses. Thereby it is easier to plan. design, and construct the equipment needed for accelerated studies on durability. There are, in general, three main classes of detectors for measuring UVR, namely biological, chemical, and physical. According to the biological methods, one measures the amount of erythema or pigmentation of the skin in response to UVR. It is also possible to measure the effectiveness of such radiation in killing bacteria or viruses. Among the chemical methods for measuring U V R are dosimeter of IG Farben, Landsberg's Glass Dosimeter. and UV film pyranometer. The physical methods are considered to be of most interest in meteorology and aeronomy. Among the instruments used for measuring the near U V R are the Eppley ultraviolet pyranometer and CSIRO ultraviolet pyranometer[5-9]. The instrumentation used in these methods is, however, expensive. In order to use these methods for continuous measurements of U V R at different sites and exposure surfaces, enormous resources are needed. This is the reason behind the efforts to find simple methods which can give satisfactoD' results in practical times and for reasonable costs. It is well known that photodegradation of certain polymers (PVC and PPO) occurs through a photooxidation mechanism under the influence of UVR. There is a simple correlation between the structural changes in the polymer and the amount of U V R absorbed by the system during photodegradation. Mea-
INTRODUCTION
Ultraviolet radiation (UVR) is the most effective factor for degradation of organic materials through photooxidation. It is the deciding factor with respect to service life of synthetic and biopolymers which are used outdoors. In addition to this, UVR has very important biological effects. UVR causes irreversible chemical changes affecting the mechanical properties of organic materials in what is called photo-oxidative degradation. This degradation manifests itself in the form of embrittlement followed by loss of tensile properties, objectional discoloration, etc. These negative effects appear in different external building materials such as plastics, paints, and wood products. The degradation mechanism and factors affecting degradation are well known, UVR, air oxygen, and internal photoactive groups are the most important factors. Various impurities with photoactive groups are introduced in the material during synthesis, processing and storage. These groups absorb U V R and initiate the photo-oxidative degradation. Quantitative and qualitative analysis of these materials makes it possible to develop and improve effective processes to photostabilize organic materials. Unfortunately, only few research stations have continuous measurements of the most important degradation factor, i.e., UVR. Generally only global and direct solar radiation are measured at various meteorological stations throughout the world[l-4]'. In Sweden, for example, solar radiation is measured at twelve stations, while UVR is only measured at two stations: in G~ivle (SIB's station) and in Norrk6ping ( S M H I ' s station). 405
B. BERRE zmd D LAI..A
4O6
sating the structural changes in the polymer pcriodically by spectrophotometric methods gives enough information on the accun-mlated UVR which has been absorbed during the exposure period. The results arc calibrated against data from UVR measurements by instrumental physical methods. The photochemical methods were developed by Martin et al. (PVC methodll0-13] and Davis er al. tPPO mcthod)t 14-17]. Shimta et al. used other polymer fihns, poly (methyl methacrylate) and polyethylene to measure radiation intensity in certain accelerated light sot, rces (carbon arc lamp and xenon arc lamp) within standardization working group IS0-Sc6/ WG 21181. 2. EXPERIMENTAL PROGRA,MME
2.1 M a t e r i a l s a n d lnstrument.~
PPO films (approx 20 p.m thickness) v,'crc prepared from a solution of PPO in chloroform. The PPO films used were kindly supplied by Mr. J. Lohmeijer, General Electric Plastics BV, The Netherlands. PPO powder was also kindly supplied by Profcssor W. Ring, Chemische Werke Huls AG, Federal Republic of Germany. The spectrophotometric analysis was carried out by the Department of Physics and Meteorology, Agricultural University of Norway, Aas. For UV analysis Beckman spectrophotometer Model 25 was used. Special film holders and exposure elements made of aluminium were made at the institute's mechanical workshop in Gfivle (Fig. 1). For calibration of results from the exposed fihns, the following pyranometers were used:
NLH's station in Aas: Eppley TUV p',rradiometer /295-385 nml, SMHI's station in Norrk6ping: Sol UVA Radiometer t315-400 nml. Brewer Spectrophotometer UVB 1290-315 rim) and Sol DUV Radiometer i306 nmI. Atlas Xenon Weather-ometer type 600 DMC-WRC was used in the study of the temperature dependence of the PPO films. 2.2 U V R m e a s t t r e m e t t t s in N o r d i c coupttries Photochemical methods for the measurement of UVR offer preconditions for carrying out a larger programme of measurements surveying the UVR environment. Such a programme should first include measurements at a number of exposure stations. Secondly, measurements at a number of points on a welldefined building. Measurement results are intended to provide a basis for evaluating the ageing characteristics of various building materials under outdoor use and for developing various methods for accelerated ageing. It is hoped that these measurements will give a well-described standard test method which can be utilized in different application of research on durability of building materials. The project is carried through within the Nordic Building Research Co-operation Group--Materials and Structures--Ageing Service life (NBS-MK) and included continuous exposure of PPO films in various directions (horisontat, south/45 ° and south/90 ° at the several stations within the Nordic countries where the ageing of certain building materials is investigated (Fig. 2). Also a number of PPO films on two buildings at
Fig. 1. PVC and PPO films mounted in aluminium holders.
Investigation on photochemical dosimeters for ultraviolet radiation
407
ICELAND REYKJAVIK
TRONDHEIM
SWEDEN •
FINLAND
NORWAY
p
IO OFINSE BERGEN • ,
G~VLE
OTANIEM
HOFI..,.
BORAS
O NORRKDPING
Fig. 2. Location of the Nordic exposure stations (o). SIB's station in G~ivle and at the Norwegian Building Research Institute (NBI) in Trondheim were exposed.
3. RESULTSAND DISCUSSIONS 3.1 PPO method
In the case of PPO films, exposure at all Nordic stations were carried out 1985/1986. For films exposed at NLH's station in Aas, we had the possibility to measure the optical density at 340 nm, OD340 nm, for each film before and after exposure and thereby the exact absorption change for each film. For the other stations and for practical reasons, we had to measure the absorption only after exposure. The av° erage absorption before exposure for NHL's films was considered as an approximate value for the absorption of films at the other stations before exposure. This introduces an error. The absorption for the unexposed films depends on many factors related to the quality of the film, e.g., the chemical purity, homogeneity, thickness, and mechanical properties. This implies that the change in optical density, AOD340 nm is not only the result of UVR exposure
but is also affected by the variation in the unexposed films. Figure 3 shows AOD340 nm against UVR values against horisontal surface measured instrumentally in Aas. An appreciable variation in the measurements is noticed in the figure. The variation in optical density at 340 nm (OD340 nm) for the unexposed films is shown in Fig. 4. The average value of OD340 nm for Aas films is 0.166 - 0.001. The standard deviation is 0.054. Figure 4 shows the distribution of this variation and the belonging normal distribution. This unforeseen variation in the unexposed film which is a function of the quality and thickness of the film might explain the variation of ~OD340 nm for a given dose of UVR as demonstrated in Fig. 3. Davis et al.[14] reported that PPO films degrades and darkens when exposed to radiation of wavelengths shorter than 400 nm. This darkening is measured as the increase in optical density of the film at 340 nm (AOD340 nm) and is to a first approximation proportional to the product of time and intensity. Initially, AOD340 nm increases autocatalytically until a limiting dose is reached, thereafter it increases linearly with dose. The nonlinear start interval is noticed for a group of films exposed at Aas in Novem-
408
B. BERREand D. LALA
•
UV
2
E c: c) ¢,.) r'~ o 1 <]
o/ o
26 ( M J i m 2)
UV
Fig. 3. Calibration of PPO films against UV (295-385) nm, Aas.
her 1986 (Fig. 5). A deviation from the linear correlation can also be observed for high UVR doses. For a degraded dark film, the radiation is not as effective through the film. (E.g. when OD340 nm = 3, only 5% of the incident radiation is transmitted through the film and compared to low dose exposure the AOD340 nm is less for an extra UVR dose.) Figure 6 shows results from exposure of a group of films at Aas in July 1986 and can be explained by this point. When using PPO films as UVR dosimeters it is important to take into consideration the results shown in Figs. 5 and 6. 3.2 PPO measurements against horizontal surfaces PPO films directed horizontally, south/45* (S/45)
and south/90 ° (S/90) were exposed monthly at all stations. All films were evaluated after each exposure period. The results from the horisontally exposed films at two stations (Aas and Norrk6ping) were calibrated against data from instrumental measurements at these two stations. Figure 3 shows correlation between AOD340 nm and the measured UVR. Due to the variations in the quality of the films a certain distribution of the results can be noticed. The results in Fig. 3 can be adjusted to a linear correlation. The partially nonlinear tendency discussed in Figs. 5 and 6 can explain the approximate linearity of Fig. 3. In future use of PPO films as dosimeters of UVR, one should establish a procedure to avoid the autocatalytic start area through
%
NUMBER29.0 OF 26.0FILMS 22.019.016.0-
13.09.76.53.2. 0
,
o
,
o
,
,
-
~
~
..~
o
~ OO3COnrn
o
o
o
o
d
Fig. 4. Distribution of variations in optical density at 340 nm for the unexposed PPO films.
Investigation on photochemical dosimeters for ultraviolet radiation
409
0.2.
t:= C:9 O 0.1 (:9
0
0
i UV( M J i m 2 J
Fig. 5. Calibration of a PPO film exposed during November 1986, Aas.
preexposure of the films. The results in Fig. 3 are used to calculate UVR for exposure against inclined surfaces (S/45 and S/90). A calibration equation for a linear correlation in Fig. 3 is UV (MJm -2) = 5.6 x AOD340 nm
(1)
Wavelength interval for eqn (1) is (295-385) nm. In Norrk6ping instrumental measurements were carried out in two bands, UVB (290-315 nm), UVA (315-400 nm) and damaging ultraviolet (DUV) (306 nm). DUV is defined as UVB radiation according to ACGIH's and SSI's damaging effect curve. DUV is a parameter related to the damaging solar effect on skin and eyes. As the proportion of DUV and UVB relative to the radiation which darkens the film varies with the seasons, the PPO films are not suitable as dosimeters of UVB or DUV. AOD340 nm for a given dose of UVB or DUV depends on the time of the year the exposure is done. Figure 7 shows monthly averaged daily values of UVR against horisontal surface determined in two ways. One where an Eppley TUV Pyrradiometer is used (Aas station). In the other way UVR is calculated from AOD340 nm and the relation given in Fig. 3 and eqn (1). The agreement between the two ways is better than 20% and in accordance with the spread shown in Fig. 3. The seasonly variation of radiation is clearly demonstrated. For the months September through December we have observations in 1985 and 1986. It is seen that the monthly averaged daily radiation can vary from one year to the next.
3.3 PPO measurement against inclined surfaces Tables 1-3 demonstrate monthly averaged daily values of UVR (295-385) nm measured photochemically by the PPO method at different exposure stations and in different exposure directions. During 1985 the films were exposed from both sides while in 1986 an absorptive protection plate was mounted behind the film. At Aas station two sets of films were exposed with and without protection. Figures 8 and 9 show monthly averaged daily values of UVR against S/45 and S/90 respectively. In both cases the films were protected on the back side. These figures show values measured photochemically and calculated according to a later described model. The agreement is satisfactory taking into account the variation in the PPO films and the assumption in the calculation. Below we give a short description of the calculation on inclined surfaces from radiation data on a horisontal surface. It is standard theory as found in Duffle and Beckman[27] where also references to original works are given. It is assumed that solar radiation consists of three components. Direct and diffuse radiation from the sun and diffuse radiation reflected from the ground. Both diffuse components are assumed to be isotropic. According to these conditions the monthly averaged daily value on inclined surface,/-t,, is given by:
+/.~p ( 1 -
Icos B)2
(2)
410
B. BERRE and D. LALA
E tO ",,4"
O
<3
I
0
5
10
2O
15
UV(MJ/m 2) Fig. 6. Calibration of a PPO film exposed during July 1986, Aas. measured radiation. The average day for each month used in the calculation, is taken from Table 1.6.1 in[27]. Aas lies at 59°40 ' north. For p the following albedo values were used: 0.25 during the period MayNovember, 0.3 in April, 0.6 in March. 0.7 in February and December and 0.8 in January. The relation /~d//~ is determined according to eqn (3).
where /-1 is the corresponding UVR value on horisontal surface, /~d the diffuse component and /~b is the relation between daily direct radiation on inclined and horisontal surfaces. This parameter is described in detail in[27]. Further in eqn (2) 13 is the angle between the inclined and the horisontal surface, p is the reflectance of the surroundings or the ground for the
Q
e
1.0.
X Q
e
I>,.
X
o 0.5-
x
"o r',4
I E •
x
I[
x
x
,
It ,
SEP
j
,
,
,
,
i
,
,
NOV JAN MAR MAY 1985 -' : ~ 1986
,
,
JUL
,
,
SEP
,
,
NOV
Fig. 7. Monthly averaged daily values of UVR against horisontal surface, (x) measured by Eppley P2,Tradiometer, (o) measured photochemically by PPO method.
Investigation on photochemical dosimeters for ultraviolet radiation
411
Table 1. Monthly averaged daily values of UVR (295-385) nm ( M J / m z) measured photochemically by PPO method at stations: SIB/G/ivle. SMHl/Norrk6ping and N L H / A a s . H, S/45 and S/90 denote exposure against horisontal surface, inclined surface 45°/south and vertical surface 90°/south respectively SIB/Giivle Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
H
S/45
. 0.27 0.17 0.11 0.13 0.27 0.26 -0.69 0.75 -0.39 -0.16 0.05 0.02
.
. 0.18 -0.13 0.14 0.31 0.26 --0.81 -0.40 -0.33 0.06 0.03
SMHl/Norrk6ping S/90
H
. 0.22 0.16 0.16 0.14 0.29 0.28 0.53 0.77 0.84 0.52 0.42 0.42 0.20 0.07 0.03
. 0.33 -0.15 0.20 ----0.58 ---0.40 0.08 0.02
H~ lid D, 1
S/45
S/90
H
S/45
S/90
0.40 0.19 0.13 0.18 0.43 -0.28 -----0.52 0.09 0.03
0.38 0.19 0.11 0.23 --0.47 -0.61 -0.43 -0.54 0.10 0.05
0.28 0.34 0.12 0.05 0.16 0.43 0.32 0.68 0.63 1.14 1.08 0.72 0.62 0.42 0.05 0.06
0.35 0.41 0.13 0.11 0.17 0.45 0.47 0.51 0.66 1.16 1.08 0.69 0.94 0.47 0.05 0.04
0.21 0.40 0.09 0.08 0.19 0.34 -0.52 0.47 1.30 1.18 0.53 0.88 0.34 0.05 0.04
.
b) T h e relative a m o u n t o f diffuse radiation is ind e p e n d e n t o f w a v e l e n g t h , i.e., l i d / l i = £3,/~. Ex-
(3)
&,dli
NLH/Aas
p r e s s e d by m e a s u r e d quantities w e get
G
li,,Ib, = #71d,
(5)
w h e r e / ) , is the total d i f f u s e radiation and t~ the total Figure 10 s h o w s the resulting values for lia/19,. There are s t r o n g r e a s o n s to believe that this relation varies t h r o u g h the year. H o w e v e r , we have no w a y to det e r m i n e correct values. W e therefore, s o m e w h a t ar-
g l o b a l radiation. U V R and the diffuse and global radiation w e r e r o u t i n e l y m e a s u r e d . T o d e t e r m i n e the d i f f u s e part o f U V R , I2Id/I21, v a l u e s for fld/15, h a v e to be a s s u m e d . F r o m m o d e l e s t i m a t e s it is k n o w n that the latter relation varies w i t h s o l a r e l e v a t i o n [ 2 6 ] . It also varies w i t h h o w c l o u d y the sky is. T o find the
bitrarily, c h o o s e the m e a n v a l u e s o f the e x t r e m e values given by (a) and (b). The values used in the further
variation r a n g e for lid/15, t w o v a l u e s f o r l i a / l i h a v e b e e n a s s u m e d w h e r e f r o m lid/l~, can be calculated. a) A s s u m e that all U V R is diffuse, i . e . , Ha/H =
c a l c u l a t i o n s are g i v e n by the b r o k e n line in Fig. 10. T o g e t h e r with o b s e r v e d values o f U V R , the total global radiation, t~, and the total diffuse radiation. /),, the relative part o f diffuse radiation in the U V range, lid~ l i is calculated by e q n (3). T h e result is s h o w n in Fig. 11. W e find that the diffuse c o m p o n e n t is be-
1.0. T h i s g i v e s
lia//), =/--i'//~,
(4)
Table 2. Monthly averaged daily values of UVR (295-395) nm (MJ/m") measured photochemically by PPO method at stations: SBI/H6rsholm, VTT/Otaniemi and RB/Reykjavik SBI/H6rsholm Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sept 1986 Oct 1986 Nov 1986 Dec 1986
H 0.45 . __ -0.20 0.16 0.49 -. 1.00 . . --~ ~
S/45
VTT/Otaniemi S/90
--
0.51
.
. m -0.24 0.15 0.58 -. 1.18 . 0.63 ----
0.11 0.04 0.12 0.31 0.42 . .
.
.
.
.
. .
. .
. .
. 0.33 0.37 0.07 0.04
S/90
. 0.11 0.05 0.12 ---
.
. . 0.68 0.34 ---
. .
1.19
.
S/45
. .
--0.25 0.17 0.65 0.52
.
.
H
--0.09 0.04
RB/Reykjavik
. . 0.12 0.06 0.13 0.37 0.49 . . . . 0.55 0.46 0.37 0.08 0.04
H . .
S/45
S/90
0.10 0.06 0.11 0.25 --
0.11 0.06 0.14 0.25 --
---0.05 --
---0.05 0.03
. .
0.10 0.07 0.10 0.26 -. .
. .
. .
. .
---0.05 0.03
412
B. BERRE and D. LALA
Table 3. Monthly averaged daily values of UVR (295-385) n m ( M J / m : ) measured photochemically by PPO method at stations: SP/Bor%, N I L U / B e r g e n and NILU/Kjeller SP/Borhs Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
H . . 0.17 0.09 -0.61 -. . 0.72 . . -. 0.16 0.14
NILU/Bergen
S/45
S/90
.
.
.
.
.
. . 0.80 .
.
. 0.53
.
. 0.12 0.13
. . 0.53 . 0.12 0.14
.
H .
. 0.12 -0.11 0.34 -. . -.
.
S/90
.
.
0.88
.
S/45
.
. 0.18 0.13 -0.78 0.55 .
0.17 0.12 --0.52 . .
H
NILU/KjelIer
0.13 0.07 0.14 0.37 -.
0.11 0.09 0.17 0.36 -.
.
.
.
-.
-.
.
. --
--
-0.07
-0.06
0.62 0.47 0.21 0.06 0.06
.
tween 68% and 88% of the total radiation in the UV band. Using the monthly values from Fig. 11 in eqn (2) gives monthly averaged daily values of UVR against inclined surfaces (S/45 and S/90). There is good agreement between calculated values and UVR v a l u e s m e a s u r e d b y t h e d e g r a d a t i o n o f P P O films. For example, both calculation and the measurements g i v e r a d i a t i o n v a l u e s a g a i n s t the vertical s u r f a c e l e s s t h a n a g a i n s t the h o r i s o n t a l s u r f a c e d u r i n g s u m m e r m o n t h s . T h e a g r e e m e n t s u p p o r t s t h e r e s u l t s s h o w n in Fig. 11 that the d i f f u s e part o f U V R is h i g h .
. 0.45 0.17 0.03 0.14 0.40 0.51 . 0.60 . ---0.04 0.06
S/45
S/90
0.48 0.17 0.07 0.15 0.47 0.52 0.47
0.49 0.21 0.09 0.21 0.5l -0.59
--
0.66
---0.04 --
0.45 0.42 -0.03 0.05
3 . 4 P P O m e a s u r e m e n t s on test b u i l d i n g s Corresponding PPO measurements were carried o u t o n c e r t a i n p o i n t s o n t w o b u i l d i n g s at S I B ' s s t a t i o n in G~ivle a n d at N B I ' s s t a t i o n in T r o n d h e i m . E x p o sure of PPO films was carried out on monthly basis at t h e f o l l o w i n g p o i n t s : a) Vertical e x p o s u r e on f a c a d e s facing s o u t h , north, east and west. b) V e r t i c a l e x p o s u r e o n s h a d o w e d e a v e s o n facades facing south and east. A c c o r d i n g to c a l i b r a t i o n c u r v e (Fig. 3) a n d e q n
1.0
o "EJ
~'E 0.5
~t X
x
1985----+---1986 Fig. 8. Monthly averaged daily values of U V R against inclined surface (S/45) protected on back side, (x) measured by PPO method, (o) calculated from instrumental measurements against horisontal surface.
Investigation on photochemical dosimeters for ultraviolet radiation
413
1.0.
T
>,, o -D i',w
'E
"'1
x
x
0.5
x
x
x x
x
x
o s6~16J
kM,g, i q j
j k&
6Nb
"-
19 85 - ' - ~ - ~ 1 9 8 6 Fig. 9. Monthly averaged daily values of UVR against vertical surface (S/90) protected on back side, (x) measured by PPO method, (o) calculated from instrumental measurements against horisontal surface.
(1), U V R values for each exposure period and point were estimated. Table 4 shows the results for both buildings. From these results, a certain order of U V R values is observed for the different facades and eaves. In general, the following order can be considered:
South > East > West > North > Eaves/south > Eaves/east The difference in values for south and north facades is relatively small and emphasize the fact that the diffuse component of U V R is high. We can fur-
O.I5x
X x
Dt
x
x
x.
0.10-
x
. \
\
/ ~ ,
~
\
f l j
t
f
I
,
\ \
\
0.05-
0
s bgl b3 1985 =
'FM$, M~ ] k ~S b ~ :
b "
=1986
Fig. 10. Monthly averaged daily values of 1:1,~/1),, (x) calculated according to eqn (4), (o) calculated according to eqn (5), (---) used in further calculation.
414
B. BERP.Eand D. LALA
10 •
•
f
H
05
1985 ~
1986
Fig. 11. Monthly averaged daily values of Hd/H, (o) calculated according to eqn (3), (---) used in further calculation.
ther conclude from the order East > West that there is more U V R before noon than after. In other words, the afternoons are generally more cloudy both in G~ivle and in Trondheim. The higher U V R level observed in G~ivle is consistent with Trondheim lying farther up north.
Table 5 shows AOD340 nm for each film at each temperature. Also s h o w n in the relative difference in change compared to the change at 25°C. According to these experiments no temperature effect is noticed for the interval 2 5 - 4 0 ° C . The variations seen in Table 5 ( 1 - 6 % ) fall well within the experimental uncertainty.
3.5 Temperature dependence of PPO measurements
4. CONCLUSIONS
To examine a possible temperature effect on PPO films used in these measurements some laboratory experiments were carried out. Several PPO films were exposed in Atlas X e n o n Weather-ometer at 50% RH and for 65 hours at each temperature. OD340 nm was measured for each film before and after exposure.
Knowledge o f aging factors affecting building materials is of great importance for the prediction of service life for materials and structures. Ultraviolet solar radiation (UVR) causes irreversible chemical changes affecting the mechanical properties o f materials. Data for U V R intensity as a function o f time,
Table 4. Monthly averaged daily values of UVR (295-385) nm (MJ/m 2) photochemically by PPO method at certain points on two test buildings in G,~ivle and Trondheim. S, N, W, and E denote vertical exposure on facades facing south, north, west, and east respectively, TS and TE denote vertical exposure on shadowed eaves facing south and east respectively SIB/G~ivle Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
NBI/Trondheim
S
N
W
E
TS
TE
S
N
W
E
TS
TE
0.52 0.21 0.17 -0.16 0.36 0.33 -0.77 0.85 0.54 0.42 0.40 0.21 0.06 0.02
0.22 0.12 0.12 0.11 0.13 0.15 0.15 0.47 0.59 0.59 0.46 0.43 0.27 0.16 0.04 0.02
-0.06 0.14 -0.14 0.17 0.23 0.37 0.46 0.71 -0.37 0.23 0.13 0.03 0.02
-0.21 0.21 0.13 0.15 0.27 0.24 0.30 0.71 0.58 0.59 0.41 -0.15 0.05 0.02
0.08 0.06 0.06 0.05 0.07 0.10 0.06 0.10 0.09 0.14 0.09 0.08 0.04 0.05 0.01 0.02
0.12 0.04 0.09 0.05 0.06 0.09 0.07 0.10 0.06 0.15 0.15 0.08 0.11 0.04 0.03 0.01
0.42 0.16 0.07 0.06 0.07 0.24 0.52 0.51 -0.79 ---0.10 0.03 0.02
0.22 0.09 0.02 0.04 -0.18 0.29 0.38 0.41 0.48 0.33 0.46 0.14 0.06 0.02 0.02
0.30 0.10 0.03 0.04 0.05 0.18 -0.41 0.34 ---0.16 0.08 0.04 0.02
-0.12 0.04 0.03 0.05 0.23 0.39 --0.61 0."51 0.22 0.21 0.09 0.03 0.02
0.06 0.03 0.02 -0.03 0.06 0.11 0.08 0.08 0.16 0.10 0.14 0.04 0.03 0.02 0.02
0.03 0.01 0.02 -0.03 0.06 0.03 0.03 0.05 0.07 0.05 0.02 0.01 0.03 0.01 0.02
Investigation on photochemical dosimeters for ultraviolet radiation Table 5. The temperature dependence of PPO films measured by exposure in Atlas Weather-ometer for 65 hours, 50% relative humidity for each temperature
Temperature (°C) AOD340 nm 25 30 35 40
Temperature effect (%) compared to 25°C
1.789 1.674 1.806 1.680
0 -6 +1 -6
site and exposure surface by physical instrumental methods is scarce because o f the extensive instrumentation involved. It is demonstrated that by photochemical methods one can measure U V R at different sites and exposure surface in practical times and for reasonable costs. Results from photochemical measurements o f U V R using films of Polyphenylene oxide (PPO) are discussed. The m e a s u r e m e n t s were carried out at ten Nordic stations and on two test buildings. A comparison between results from the photochemical measurements of U V R with m e a s u r e m e n t s using Eppley Pyrradiometer gives an agreement better than 20%. M e a s u r e m e n t s on certain parts of the test buildings shows a certain order of the U V R values on different facades and eaves. Accelerated experiments in Atlas W e a t h e r - o m e t e r to study the temperature dependence o f PPO films showed that up to 40°C temperature has no effect on the degradation of the films within experimental uncertainty. The increase of the optical density of the PPO films at 340 nm (AOD340) is to a first approximation proportionai to the product of time and intensity. Initially, AOD340 n m increases autocatalytically until a limiting dose in reached, thereafter it increases linearly with dose. W h e n using PPO films as dosimeters of U V R , one can avoid the autocatalytic area through preexposure of the films. The PPO method shows promise in measuring U V R at reasonable costs. The technique as described, yields an accuracy of 20%. For improvements, investigations on the autocatalytic area, and on the reasons for and implications of the variations o f the film quality, are needed.
Acknowledgments--The authors gratefully acknowledge the support of the Nordic Building Research Co-operation Group--Materials and StructuresmAging and Service Life (NBS-MK). The help of the following project group is gratefully appreciated: Odd Anda, NILU, Norway, Erik Brandth, SBI, Denmark, Lars Dahlgren, SMHI, Sweden, Tore Gjelsvik, NBI, Norway, Leif Vidar Jakobsen, NLH, Norway, Weine Josefsson, SMHI, Sweden, Lars Karvohen, SP, Sweden, Bj6m Marteinsson, RB, Iceland and Liisa Rautiainen, VTT, Finland. REFERENCES
1. D. Lala, Monitoring of ultraviolet radiation. Nordic Seminar on Durability and Service Life of Surface Coatings, Technical Research Centre of Finland, Turku (May 1982).
415
2. H. E. Ashton and P. J. Sereda, Environment, microenvironment and durability of building materials. Durability of Building Materials, 1, 49-65 (1982). 3. R. S. Yamasaki, Intensity variation of ultraviolet, visible near infrared bands of terrestrial solar radiation, Journal of Paint Technology, 43(555), 75-83 (197l). 4. R. S. Yamasaki, Solar ultraviolet radiation on horisontal, south/45* and south/vertical surfaces, Durability of Building Materials, 2, 17-26 (1983). 5. K. L. Coulson, The measurement of ultraviolet radiation. Solar and Terrestrial Radiation, Methods of Measurements, Academic Press, New York (1975). 6. H. Landsberg. The ultraviolet dosimeter, Bull. Amer. Meteor. Soc., 18, 161-167 (1937). 7. H. Landsberg and W. Weyl, Measurements of ultraviolet sums with photosensitive glass, Bull. Amer. Meteor. Soc., 20, 254-256 (1939). 8. J. Sherrod, H. Neuberger, and D. Yeng, New standards for Landsbergs glass rod method of integrating ultraviolet radiation, Trans. Amer. Geophys. Union, 31, 696-698 (1950). 9, J. N. Pitts, G. W. Cowell, and D. R. Burley, Film actinometer for measurement of solar ultraviolet radiation intensities in urban atmospheres. Env. Sci. Tech., 2, 435-437 (1968). 10. K. G. Martin, Monitoring ultraviolet radiation with polyvinyl-chloride. Br. Polym. J., 5, 443-450 (1973). 11. K. G. Martin and R. I. Tilly, Influence of radiation intensity upon photo-oxidation of unstabilized polyvinyl-chloride. Br. Polym. J., 1, 213-216 (1969). 12. K. G. Martin and R. I. Tilly, Influence of radiation wavelength on photo-oxidation of unstabilized polyvinyl-chloride. Br. Polym. J., 3, 36-40 (1971). 13. K. G. Martin, Solar weathering indices for australian sites. CSIRO Austr. Div. Bldg. Res. Tech. Pap. (Second ser.)No. 18, 1-33, 1977. 14. A. Davis, G. W. Deane, D. Gordon, G. V. Howel, and K. J. Ledbury, A world-wide programme for the continuous monitoring of solar ultraviolet radiation using poly(phenylene oxide) films, and a consideration of results, J. Appl. Polym. Sci., 20, 1165-I 174 (1976). 15. A. Davis and G. W. Deane, Possible dosimeter for ultraviolet radiation, Nature, 261, 169-170 (1976). 16. A. V. J. Challoner, M. F. Corbett, A. Davis, B. L. Diffy, J. F. Leach, and I. A. Magnus, Description of application of a personal ultraviolet dosimeter: A review of preliminary studies. National Cancer Institute Monograph, 50, 97-100 (1978). 17. A. Davis, D. Gordon, and G. V. Howel, Continuous world wide UV monitoring programme, Explosives Research and Development Establishment Technical Report No. 141 (July 1973). 18. T. Shirota, Y. Watanabe, and K. Yoshikawa, Report on round robin test on PMMA and PE standard referens materials for testing light exposure. Unpublished work report from the Standardization Working Group ISO-Sc6/WG 2 (1983). 19. D. Lala, J. F. Rabek, and B. Rhnby, Photodegradation and photostabilization of the two phase system poly(vinyl chloride)-polybutadiene, Polymer Degradation and Stability, 3, 307-321 (1981). 20. B. R~nby and J. F. Rabek, Free radicals formed during photo-oxidation of polymers. ESR Spectroscopy in Polymer Research, Springer Verlag, Berlin (1977). 21. B. R~nby and J. F. Rabek, Photo-oxidative degradation of polydienes. Appl. Polym. Syrup., 35, 234-240 (1979). 22. B. RCmby and J. F. Rabek, Photodegradation and photooxidation of poly(phenylene oxide) (PPO). Photodegradation, Photo-oxidation and Photostabilization of Polymers, Wiley, London (1975). 23. P. J. Keller, L. B. Jassie, and P. D. Gesner, Primary processes in UV-photolysis of poly(phenylene oxide). J. App. Polym. Sci., 11, 137-150 (1967).
416
B. BERR~ and D. LALA
24. D. Lala, Photochemical measurements of ultraviolet radiation. Symposium Proceedings: Recent Advances in Pyranometry, lEA Solar R & D, SMHI, NorrkOping, Sweden (1984). 25. D. Lala, Ultraviolet radiation measurements by photochemical methods. The National Swedish Institute for Building Research Bulletin M85:12, G~ivle, Sweden (1985).
26. V. Hansen, Spectral distribution of solar radiation on clear days: a comparison between measurements and model estimates. Journal of Climate and Applied Meteorology, 23, 772-780 (1984). 27. J. A. Duffle and W. A. Beckman, Ratio of total radiation on a tilted surface to that on a horisontal surface. Solar Engineering of Thermal Processes, John Wiley and Sons, London (1980).
I989
OiQ~:_Fi,#'X-.,;i -~'~ 1"2~i " ~3 Cop?right ¢: Igr,:, Per_'~'~on Pre,~s pie
Prinlcd in Ihc I' S A
INVESTIGATION ON PHOTOCHEMICAL DOSIMETERS FOR ULTRAVIOLET RADIATION B. BERRE Department of Physics and Meteorology. Agricultural University of Norway. 1432 Aas-NLH. Norway
and D. LALA Materials and Structures Division. The National Sv~edish Institute for Building Research. G~ivle. Sweden Abstract--The paper presents results from the photochemical measurements of ultraviolet solar radiation (UVR) using films of Polyphenylene oxide (PPO/. The measurements were carried out at ten Nordic stations and on two test buildings. The PPO films darken when exposed to wavelengths shorter than ~.f~'~ nm. This darkening is measured as the increase in optical density of the film at 340 nm (AOD340j and is to a first approximation proportional to the product of time and intensity. Accelerated experiments in Atlas Weather-ometer to study the temperature dependence showed that up to 40°C temperature had no effect on the degradation of the PPO films. A comparison between results from the photochemical measurements of UVR with instrumental measurements using Eppley Pyrradiometer on horizontal surfaces shows the agreement between both types of measurements is better than 20%. Measurements on certain parts of the test buildings show a certain order of UVR values on different facades and eaves. !.
Data for UVR as function of time. site and exposure surface is scarce because of the extensive instrumentation involved. It is important to know UVR in different climates and sites in order to be able to forecast the service-life of different materials in outdoor uses. Thereby it is easier to plan. design, and construct the equipment needed for accelerated studies on durability. There are, in general, three main classes of detectors for measuring UVR, namely biological, chemical, and physical. According to the biological methods, one measures the amount of erythema or pigmentation of the skin in response to UVR. It is also possible to measure the effectiveness of such radiation in killing bacteria or viruses. Among the chemical methods for measuring U V R are dosimeter of IG Farben, Landsberg's Glass Dosimeter. and UV film pyranometer. The physical methods are considered to be of most interest in meteorology and aeronomy. Among the instruments used for measuring the near U V R are the Eppley ultraviolet pyranometer and CSIRO ultraviolet pyranometer[5-9]. The instrumentation used in these methods is, however, expensive. In order to use these methods for continuous measurements of U V R at different sites and exposure surfaces, enormous resources are needed. This is the reason behind the efforts to find simple methods which can give satisfactoD' results in practical times and for reasonable costs. It is well known that photodegradation of certain polymers (PVC and PPO) occurs through a photooxidation mechanism under the influence of UVR. There is a simple correlation between the structural changes in the polymer and the amount of U V R absorbed by the system during photodegradation. Mea-
INTRODUCTION
Ultraviolet radiation (UVR) is the most effective factor for degradation of organic materials through photooxidation. It is the deciding factor with respect to service life of synthetic and biopolymers which are used outdoors. In addition to this, UVR has very important biological effects. UVR causes irreversible chemical changes affecting the mechanical properties of organic materials in what is called photo-oxidative degradation. This degradation manifests itself in the form of embrittlement followed by loss of tensile properties, objectional discoloration, etc. These negative effects appear in different external building materials such as plastics, paints, and wood products. The degradation mechanism and factors affecting degradation are well known, UVR, air oxygen, and internal photoactive groups are the most important factors. Various impurities with photoactive groups are introduced in the material during synthesis, processing and storage. These groups absorb U V R and initiate the photo-oxidative degradation. Quantitative and qualitative analysis of these materials makes it possible to develop and improve effective processes to photostabilize organic materials. Unfortunately, only few research stations have continuous measurements of the most important degradation factor, i.e., UVR. Generally only global and direct solar radiation are measured at various meteorological stations throughout the world[l-4]'. In Sweden, for example, solar radiation is measured at twelve stations, while UVR is only measured at two stations: in G~ivle (SIB's station) and in Norrk6ping ( S M H I ' s station). 405
B. BERRE zmd D LAI..A
4O6
sating the structural changes in the polymer pcriodically by spectrophotometric methods gives enough information on the accun-mlated UVR which has been absorbed during the exposure period. The results arc calibrated against data from UVR measurements by instrumental physical methods. The photochemical methods were developed by Martin et al. (PVC methodll0-13] and Davis er al. tPPO mcthod)t 14-17]. Shimta et al. used other polymer fihns, poly (methyl methacrylate) and polyethylene to measure radiation intensity in certain accelerated light sot, rces (carbon arc lamp and xenon arc lamp) within standardization working group IS0-Sc6/ WG 21181. 2. EXPERIMENTAL PROGRA,MME
2.1 M a t e r i a l s a n d lnstrument.~
PPO films (approx 20 p.m thickness) v,'crc prepared from a solution of PPO in chloroform. The PPO films used were kindly supplied by Mr. J. Lohmeijer, General Electric Plastics BV, The Netherlands. PPO powder was also kindly supplied by Profcssor W. Ring, Chemische Werke Huls AG, Federal Republic of Germany. The spectrophotometric analysis was carried out by the Department of Physics and Meteorology, Agricultural University of Norway, Aas. For UV analysis Beckman spectrophotometer Model 25 was used. Special film holders and exposure elements made of aluminium were made at the institute's mechanical workshop in Gfivle (Fig. 1). For calibration of results from the exposed fihns, the following pyranometers were used:
NLH's station in Aas: Eppley TUV p',rradiometer /295-385 nml, SMHI's station in Norrk6ping: Sol UVA Radiometer t315-400 nml. Brewer Spectrophotometer UVB 1290-315 rim) and Sol DUV Radiometer i306 nmI. Atlas Xenon Weather-ometer type 600 DMC-WRC was used in the study of the temperature dependence of the PPO films. 2.2 U V R m e a s t t r e m e t t t s in N o r d i c coupttries Photochemical methods for the measurement of UVR offer preconditions for carrying out a larger programme of measurements surveying the UVR environment. Such a programme should first include measurements at a number of exposure stations. Secondly, measurements at a number of points on a welldefined building. Measurement results are intended to provide a basis for evaluating the ageing characteristics of various building materials under outdoor use and for developing various methods for accelerated ageing. It is hoped that these measurements will give a well-described standard test method which can be utilized in different application of research on durability of building materials. The project is carried through within the Nordic Building Research Co-operation Group--Materials and Structures--Ageing Service life (NBS-MK) and included continuous exposure of PPO films in various directions (horisontat, south/45 ° and south/90 ° at the several stations within the Nordic countries where the ageing of certain building materials is investigated (Fig. 2). Also a number of PPO films on two buildings at
Fig. 1. PVC and PPO films mounted in aluminium holders.
Investigation on photochemical dosimeters for ultraviolet radiation
407
ICELAND REYKJAVIK
TRONDHEIM
SWEDEN •
FINLAND
NORWAY
p
IO OFINSE BERGEN • ,
G~VLE
OTANIEM
HOFI..,.
BORAS
O NORRKDPING
Fig. 2. Location of the Nordic exposure stations (o). SIB's station in G~ivle and at the Norwegian Building Research Institute (NBI) in Trondheim were exposed.
3. RESULTSAND DISCUSSIONS 3.1 PPO method
In the case of PPO films, exposure at all Nordic stations were carried out 1985/1986. For films exposed at NLH's station in Aas, we had the possibility to measure the optical density at 340 nm, OD340 nm, for each film before and after exposure and thereby the exact absorption change for each film. For the other stations and for practical reasons, we had to measure the absorption only after exposure. The av° erage absorption before exposure for NHL's films was considered as an approximate value for the absorption of films at the other stations before exposure. This introduces an error. The absorption for the unexposed films depends on many factors related to the quality of the film, e.g., the chemical purity, homogeneity, thickness, and mechanical properties. This implies that the change in optical density, AOD340 nm is not only the result of UVR exposure
but is also affected by the variation in the unexposed films. Figure 3 shows AOD340 nm against UVR values against horisontal surface measured instrumentally in Aas. An appreciable variation in the measurements is noticed in the figure. The variation in optical density at 340 nm (OD340 nm) for the unexposed films is shown in Fig. 4. The average value of OD340 nm for Aas films is 0.166 - 0.001. The standard deviation is 0.054. Figure 4 shows the distribution of this variation and the belonging normal distribution. This unforeseen variation in the unexposed film which is a function of the quality and thickness of the film might explain the variation of ~OD340 nm for a given dose of UVR as demonstrated in Fig. 3. Davis et al.[14] reported that PPO films degrades and darkens when exposed to radiation of wavelengths shorter than 400 nm. This darkening is measured as the increase in optical density of the film at 340 nm (AOD340 nm) and is to a first approximation proportional to the product of time and intensity. Initially, AOD340 nm increases autocatalytically until a limiting dose is reached, thereafter it increases linearly with dose. The nonlinear start interval is noticed for a group of films exposed at Aas in Novem-
408
B. BERREand D. LALA
•
UV
2
E c: c) ¢,.) r'~ o 1 <]
o/ o
26 ( M J i m 2)
UV
Fig. 3. Calibration of PPO films against UV (295-385) nm, Aas.
her 1986 (Fig. 5). A deviation from the linear correlation can also be observed for high UVR doses. For a degraded dark film, the radiation is not as effective through the film. (E.g. when OD340 nm = 3, only 5% of the incident radiation is transmitted through the film and compared to low dose exposure the AOD340 nm is less for an extra UVR dose.) Figure 6 shows results from exposure of a group of films at Aas in July 1986 and can be explained by this point. When using PPO films as UVR dosimeters it is important to take into consideration the results shown in Figs. 5 and 6. 3.2 PPO measurements against horizontal surfaces PPO films directed horizontally, south/45* (S/45)
and south/90 ° (S/90) were exposed monthly at all stations. All films were evaluated after each exposure period. The results from the horisontally exposed films at two stations (Aas and Norrk6ping) were calibrated against data from instrumental measurements at these two stations. Figure 3 shows correlation between AOD340 nm and the measured UVR. Due to the variations in the quality of the films a certain distribution of the results can be noticed. The results in Fig. 3 can be adjusted to a linear correlation. The partially nonlinear tendency discussed in Figs. 5 and 6 can explain the approximate linearity of Fig. 3. In future use of PPO films as dosimeters of UVR, one should establish a procedure to avoid the autocatalytic start area through
%
NUMBER29.0 OF 26.0FILMS 22.019.016.0-
13.09.76.53.2. 0
,
o
,
o
,
,
-
~
~
..~
o
~ OO3COnrn
o
o
o
o
d
Fig. 4. Distribution of variations in optical density at 340 nm for the unexposed PPO films.
Investigation on photochemical dosimeters for ultraviolet radiation
409
0.2.
t:= C:9 O 0.1 (:9
0
0
i UV( M J i m 2 J
Fig. 5. Calibration of a PPO film exposed during November 1986, Aas.
preexposure of the films. The results in Fig. 3 are used to calculate UVR for exposure against inclined surfaces (S/45 and S/90). A calibration equation for a linear correlation in Fig. 3 is UV (MJm -2) = 5.6 x AOD340 nm
(1)
Wavelength interval for eqn (1) is (295-385) nm. In Norrk6ping instrumental measurements were carried out in two bands, UVB (290-315 nm), UVA (315-400 nm) and damaging ultraviolet (DUV) (306 nm). DUV is defined as UVB radiation according to ACGIH's and SSI's damaging effect curve. DUV is a parameter related to the damaging solar effect on skin and eyes. As the proportion of DUV and UVB relative to the radiation which darkens the film varies with the seasons, the PPO films are not suitable as dosimeters of UVB or DUV. AOD340 nm for a given dose of UVB or DUV depends on the time of the year the exposure is done. Figure 7 shows monthly averaged daily values of UVR against horisontal surface determined in two ways. One where an Eppley TUV Pyrradiometer is used (Aas station). In the other way UVR is calculated from AOD340 nm and the relation given in Fig. 3 and eqn (1). The agreement between the two ways is better than 20% and in accordance with the spread shown in Fig. 3. The seasonly variation of radiation is clearly demonstrated. For the months September through December we have observations in 1985 and 1986. It is seen that the monthly averaged daily radiation can vary from one year to the next.
3.3 PPO measurement against inclined surfaces Tables 1-3 demonstrate monthly averaged daily values of UVR (295-385) nm measured photochemically by the PPO method at different exposure stations and in different exposure directions. During 1985 the films were exposed from both sides while in 1986 an absorptive protection plate was mounted behind the film. At Aas station two sets of films were exposed with and without protection. Figures 8 and 9 show monthly averaged daily values of UVR against S/45 and S/90 respectively. In both cases the films were protected on the back side. These figures show values measured photochemically and calculated according to a later described model. The agreement is satisfactory taking into account the variation in the PPO films and the assumption in the calculation. Below we give a short description of the calculation on inclined surfaces from radiation data on a horisontal surface. It is standard theory as found in Duffle and Beckman[27] where also references to original works are given. It is assumed that solar radiation consists of three components. Direct and diffuse radiation from the sun and diffuse radiation reflected from the ground. Both diffuse components are assumed to be isotropic. According to these conditions the monthly averaged daily value on inclined surface,/-t,, is given by:
+/.~p ( 1 -
Icos B)2
(2)
410
B. BERRE and D. LALA
E tO ",,4"
O
<3
I
0
5
10
2O
15
UV(MJ/m 2) Fig. 6. Calibration of a PPO film exposed during July 1986, Aas. measured radiation. The average day for each month used in the calculation, is taken from Table 1.6.1 in[27]. Aas lies at 59°40 ' north. For p the following albedo values were used: 0.25 during the period MayNovember, 0.3 in April, 0.6 in March. 0.7 in February and December and 0.8 in January. The relation /~d//~ is determined according to eqn (3).
where /-1 is the corresponding UVR value on horisontal surface, /~d the diffuse component and /~b is the relation between daily direct radiation on inclined and horisontal surfaces. This parameter is described in detail in[27]. Further in eqn (2) 13 is the angle between the inclined and the horisontal surface, p is the reflectance of the surroundings or the ground for the
Q
e
1.0.
X Q
e
I>,.
X
o 0.5-
x
"o r',4
I E •
x
I[
x
x
,
It ,
SEP
j
,
,
,
,
i
,
,
NOV JAN MAR MAY 1985 -' : ~ 1986
,
,
JUL
,
,
SEP
,
,
NOV
Fig. 7. Monthly averaged daily values of UVR against horisontal surface, (x) measured by Eppley P2,Tradiometer, (o) measured photochemically by PPO method.
Investigation on photochemical dosimeters for ultraviolet radiation
411
Table 1. Monthly averaged daily values of UVR (295-385) nm ( M J / m z) measured photochemically by PPO method at stations: SIB/G/ivle. SMHl/Norrk6ping and N L H / A a s . H, S/45 and S/90 denote exposure against horisontal surface, inclined surface 45°/south and vertical surface 90°/south respectively SIB/Giivle Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
H
S/45
. 0.27 0.17 0.11 0.13 0.27 0.26 -0.69 0.75 -0.39 -0.16 0.05 0.02
.
. 0.18 -0.13 0.14 0.31 0.26 --0.81 -0.40 -0.33 0.06 0.03
SMHl/Norrk6ping S/90
H
. 0.22 0.16 0.16 0.14 0.29 0.28 0.53 0.77 0.84 0.52 0.42 0.42 0.20 0.07 0.03
. 0.33 -0.15 0.20 ----0.58 ---0.40 0.08 0.02
H~ lid D, 1
S/45
S/90
H
S/45
S/90
0.40 0.19 0.13 0.18 0.43 -0.28 -----0.52 0.09 0.03
0.38 0.19 0.11 0.23 --0.47 -0.61 -0.43 -0.54 0.10 0.05
0.28 0.34 0.12 0.05 0.16 0.43 0.32 0.68 0.63 1.14 1.08 0.72 0.62 0.42 0.05 0.06
0.35 0.41 0.13 0.11 0.17 0.45 0.47 0.51 0.66 1.16 1.08 0.69 0.94 0.47 0.05 0.04
0.21 0.40 0.09 0.08 0.19 0.34 -0.52 0.47 1.30 1.18 0.53 0.88 0.34 0.05 0.04
.
b) T h e relative a m o u n t o f diffuse radiation is ind e p e n d e n t o f w a v e l e n g t h , i.e., l i d / l i = £3,/~. Ex-
(3)
&,dli
NLH/Aas
p r e s s e d by m e a s u r e d quantities w e get
G
li,,Ib, = #71d,
(5)
w h e r e / ) , is the total d i f f u s e radiation and t~ the total Figure 10 s h o w s the resulting values for lia/19,. There are s t r o n g r e a s o n s to believe that this relation varies t h r o u g h the year. H o w e v e r , we have no w a y to det e r m i n e correct values. W e therefore, s o m e w h a t ar-
g l o b a l radiation. U V R and the diffuse and global radiation w e r e r o u t i n e l y m e a s u r e d . T o d e t e r m i n e the d i f f u s e part o f U V R , I2Id/I21, v a l u e s for fld/15, h a v e to be a s s u m e d . F r o m m o d e l e s t i m a t e s it is k n o w n that the latter relation varies w i t h s o l a r e l e v a t i o n [ 2 6 ] . It also varies w i t h h o w c l o u d y the sky is. T o find the
bitrarily, c h o o s e the m e a n v a l u e s o f the e x t r e m e values given by (a) and (b). The values used in the further
variation r a n g e for lid/15, t w o v a l u e s f o r l i a / l i h a v e b e e n a s s u m e d w h e r e f r o m lid/l~, can be calculated. a) A s s u m e that all U V R is diffuse, i . e . , Ha/H =
c a l c u l a t i o n s are g i v e n by the b r o k e n line in Fig. 10. T o g e t h e r with o b s e r v e d values o f U V R , the total global radiation, t~, and the total diffuse radiation. /),, the relative part o f diffuse radiation in the U V range, lid~ l i is calculated by e q n (3). T h e result is s h o w n in Fig. 11. W e find that the diffuse c o m p o n e n t is be-
1.0. T h i s g i v e s
lia//), =/--i'//~,
(4)
Table 2. Monthly averaged daily values of UVR (295-395) nm (MJ/m") measured photochemically by PPO method at stations: SBI/H6rsholm, VTT/Otaniemi and RB/Reykjavik SBI/H6rsholm Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sept 1986 Oct 1986 Nov 1986 Dec 1986
H 0.45 . __ -0.20 0.16 0.49 -. 1.00 . . --~ ~
S/45
VTT/Otaniemi S/90
--
0.51
.
. m -0.24 0.15 0.58 -. 1.18 . 0.63 ----
0.11 0.04 0.12 0.31 0.42 . .
.
.
.
.
. .
. .
. .
. 0.33 0.37 0.07 0.04
S/90
. 0.11 0.05 0.12 ---
.
. . 0.68 0.34 ---
. .
1.19
.
S/45
. .
--0.25 0.17 0.65 0.52
.
.
H
--0.09 0.04
RB/Reykjavik
. . 0.12 0.06 0.13 0.37 0.49 . . . . 0.55 0.46 0.37 0.08 0.04
H . .
S/45
S/90
0.10 0.06 0.11 0.25 --
0.11 0.06 0.14 0.25 --
---0.05 --
---0.05 0.03
. .
0.10 0.07 0.10 0.26 -. .
. .
. .
. .
---0.05 0.03
412
B. BERRE and D. LALA
Table 3. Monthly averaged daily values of UVR (295-385) n m ( M J / m : ) measured photochemically by PPO method at stations: SP/Bor%, N I L U / B e r g e n and NILU/Kjeller SP/Borhs Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
H . . 0.17 0.09 -0.61 -. . 0.72 . . -. 0.16 0.14
NILU/Bergen
S/45
S/90
.
.
.
.
.
. . 0.80 .
.
. 0.53
.
. 0.12 0.13
. . 0.53 . 0.12 0.14
.
H .
. 0.12 -0.11 0.34 -. . -.
.
S/90
.
.
0.88
.
S/45
.
. 0.18 0.13 -0.78 0.55 .
0.17 0.12 --0.52 . .
H
NILU/KjelIer
0.13 0.07 0.14 0.37 -.
0.11 0.09 0.17 0.36 -.
.
.
.
-.
-.
.
. --
--
-0.07
-0.06
0.62 0.47 0.21 0.06 0.06
.
tween 68% and 88% of the total radiation in the UV band. Using the monthly values from Fig. 11 in eqn (2) gives monthly averaged daily values of UVR against inclined surfaces (S/45 and S/90). There is good agreement between calculated values and UVR v a l u e s m e a s u r e d b y t h e d e g r a d a t i o n o f P P O films. For example, both calculation and the measurements g i v e r a d i a t i o n v a l u e s a g a i n s t the vertical s u r f a c e l e s s t h a n a g a i n s t the h o r i s o n t a l s u r f a c e d u r i n g s u m m e r m o n t h s . T h e a g r e e m e n t s u p p o r t s t h e r e s u l t s s h o w n in Fig. 11 that the d i f f u s e part o f U V R is h i g h .
. 0.45 0.17 0.03 0.14 0.40 0.51 . 0.60 . ---0.04 0.06
S/45
S/90
0.48 0.17 0.07 0.15 0.47 0.52 0.47
0.49 0.21 0.09 0.21 0.5l -0.59
--
0.66
---0.04 --
0.45 0.42 -0.03 0.05
3 . 4 P P O m e a s u r e m e n t s on test b u i l d i n g s Corresponding PPO measurements were carried o u t o n c e r t a i n p o i n t s o n t w o b u i l d i n g s at S I B ' s s t a t i o n in G~ivle a n d at N B I ' s s t a t i o n in T r o n d h e i m . E x p o sure of PPO films was carried out on monthly basis at t h e f o l l o w i n g p o i n t s : a) Vertical e x p o s u r e on f a c a d e s facing s o u t h , north, east and west. b) V e r t i c a l e x p o s u r e o n s h a d o w e d e a v e s o n facades facing south and east. A c c o r d i n g to c a l i b r a t i o n c u r v e (Fig. 3) a n d e q n
1.0
o "EJ
~'E 0.5
~t X
x
1985----+---1986 Fig. 8. Monthly averaged daily values of U V R against inclined surface (S/45) protected on back side, (x) measured by PPO method, (o) calculated from instrumental measurements against horisontal surface.
Investigation on photochemical dosimeters for ultraviolet radiation
413
1.0.
T
>,, o -D i',w
'E
"'1
x
x
0.5
x
x
x x
x
x
o s6~16J
kM,g, i q j
j k&
6Nb
"-
19 85 - ' - ~ - ~ 1 9 8 6 Fig. 9. Monthly averaged daily values of UVR against vertical surface (S/90) protected on back side, (x) measured by PPO method, (o) calculated from instrumental measurements against horisontal surface.
(1), U V R values for each exposure period and point were estimated. Table 4 shows the results for both buildings. From these results, a certain order of U V R values is observed for the different facades and eaves. In general, the following order can be considered:
South > East > West > North > Eaves/south > Eaves/east The difference in values for south and north facades is relatively small and emphasize the fact that the diffuse component of U V R is high. We can fur-
O.I5x
X x
Dt
x
x
x.
0.10-
x
. \
\
/ ~ ,
~
\
f l j
t
f
I
,
\ \
\
0.05-
0
s bgl b3 1985 =
'FM$, M~ ] k ~S b ~ :
b "
=1986
Fig. 10. Monthly averaged daily values of 1:1,~/1),, (x) calculated according to eqn (4), (o) calculated according to eqn (5), (---) used in further calculation.
414
B. BERP.Eand D. LALA
10 •
•
f
H
05
1985 ~
1986
Fig. 11. Monthly averaged daily values of Hd/H, (o) calculated according to eqn (3), (---) used in further calculation.
ther conclude from the order East > West that there is more U V R before noon than after. In other words, the afternoons are generally more cloudy both in G~ivle and in Trondheim. The higher U V R level observed in G~ivle is consistent with Trondheim lying farther up north.
Table 5 shows AOD340 nm for each film at each temperature. Also s h o w n in the relative difference in change compared to the change at 25°C. According to these experiments no temperature effect is noticed for the interval 2 5 - 4 0 ° C . The variations seen in Table 5 ( 1 - 6 % ) fall well within the experimental uncertainty.
3.5 Temperature dependence of PPO measurements
4. CONCLUSIONS
To examine a possible temperature effect on PPO films used in these measurements some laboratory experiments were carried out. Several PPO films were exposed in Atlas X e n o n Weather-ometer at 50% RH and for 65 hours at each temperature. OD340 nm was measured for each film before and after exposure.
Knowledge o f aging factors affecting building materials is of great importance for the prediction of service life for materials and structures. Ultraviolet solar radiation (UVR) causes irreversible chemical changes affecting the mechanical properties o f materials. Data for U V R intensity as a function o f time,
Table 4. Monthly averaged daily values of UVR (295-385) nm (MJ/m 2) photochemically by PPO method at certain points on two test buildings in G,~ivle and Trondheim. S, N, W, and E denote vertical exposure on facades facing south, north, west, and east respectively, TS and TE denote vertical exposure on shadowed eaves facing south and east respectively SIB/G~ivle Exposure period Sep 1985 Oct 1985 Nov 1985 Dec 1985 Jan 1986 Feb 1986 Mar 1986 Apr 1986 May 1986 June 1986 July 1986 Aug 1986 Sep 1986 Oct 1986 Nov 1986 Dec 1986
NBI/Trondheim
S
N
W
E
TS
TE
S
N
W
E
TS
TE
0.52 0.21 0.17 -0.16 0.36 0.33 -0.77 0.85 0.54 0.42 0.40 0.21 0.06 0.02
0.22 0.12 0.12 0.11 0.13 0.15 0.15 0.47 0.59 0.59 0.46 0.43 0.27 0.16 0.04 0.02
-0.06 0.14 -0.14 0.17 0.23 0.37 0.46 0.71 -0.37 0.23 0.13 0.03 0.02
-0.21 0.21 0.13 0.15 0.27 0.24 0.30 0.71 0.58 0.59 0.41 -0.15 0.05 0.02
0.08 0.06 0.06 0.05 0.07 0.10 0.06 0.10 0.09 0.14 0.09 0.08 0.04 0.05 0.01 0.02
0.12 0.04 0.09 0.05 0.06 0.09 0.07 0.10 0.06 0.15 0.15 0.08 0.11 0.04 0.03 0.01
0.42 0.16 0.07 0.06 0.07 0.24 0.52 0.51 -0.79 ---0.10 0.03 0.02
0.22 0.09 0.02 0.04 -0.18 0.29 0.38 0.41 0.48 0.33 0.46 0.14 0.06 0.02 0.02
0.30 0.10 0.03 0.04 0.05 0.18 -0.41 0.34 ---0.16 0.08 0.04 0.02
-0.12 0.04 0.03 0.05 0.23 0.39 --0.61 0."51 0.22 0.21 0.09 0.03 0.02
0.06 0.03 0.02 -0.03 0.06 0.11 0.08 0.08 0.16 0.10 0.14 0.04 0.03 0.02 0.02
0.03 0.01 0.02 -0.03 0.06 0.03 0.03 0.05 0.07 0.05 0.02 0.01 0.03 0.01 0.02
Investigation on photochemical dosimeters for ultraviolet radiation Table 5. The temperature dependence of PPO films measured by exposure in Atlas Weather-ometer for 65 hours, 50% relative humidity for each temperature
Temperature (°C) AOD340 nm 25 30 35 40
Temperature effect (%) compared to 25°C
1.789 1.674 1.806 1.680
0 -6 +1 -6
site and exposure surface by physical instrumental methods is scarce because o f the extensive instrumentation involved. It is demonstrated that by photochemical methods one can measure U V R at different sites and exposure surface in practical times and for reasonable costs. Results from photochemical measurements o f U V R using films of Polyphenylene oxide (PPO) are discussed. The m e a s u r e m e n t s were carried out at ten Nordic stations and on two test buildings. A comparison between results from the photochemical measurements of U V R with m e a s u r e m e n t s using Eppley Pyrradiometer gives an agreement better than 20%. M e a s u r e m e n t s on certain parts of the test buildings shows a certain order of the U V R values on different facades and eaves. Accelerated experiments in Atlas W e a t h e r - o m e t e r to study the temperature dependence o f PPO films showed that up to 40°C temperature has no effect on the degradation of the films within experimental uncertainty. The increase of the optical density of the PPO films at 340 nm (AOD340) is to a first approximation proportionai to the product of time and intensity. Initially, AOD340 n m increases autocatalytically until a limiting dose in reached, thereafter it increases linearly with dose. W h e n using PPO films as dosimeters of U V R , one can avoid the autocatalytic area through preexposure of the films. The PPO method shows promise in measuring U V R at reasonable costs. The technique as described, yields an accuracy of 20%. For improvements, investigations on the autocatalytic area, and on the reasons for and implications of the variations o f the film quality, are needed.
Acknowledgments--The authors gratefully acknowledge the support of the Nordic Building Research Co-operation Group--Materials and StructuresmAging and Service Life (NBS-MK). The help of the following project group is gratefully appreciated: Odd Anda, NILU, Norway, Erik Brandth, SBI, Denmark, Lars Dahlgren, SMHI, Sweden, Tore Gjelsvik, NBI, Norway, Leif Vidar Jakobsen, NLH, Norway, Weine Josefsson, SMHI, Sweden, Lars Karvohen, SP, Sweden, Bj6m Marteinsson, RB, Iceland and Liisa Rautiainen, VTT, Finland. REFERENCES
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415
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