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Official Digest, 35, 465, pp. 992-1002, 1963-10
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Study of the radiation effects on attached paint films by dynamic
modulus of elasticity measurements
Yamasaki, R. S.
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TH1N21r2
no. 201
c.2
BLDG
NATIONAL RESEARCH COUNCIL CANADA
DIVISION OF BUILDING RESEARCH
STUDY OF THE RADIATION EFFECTS O N ATTACHED PAINT FILMS BY DYNAMIC MODULUS OF ELASTICITY MEASUREMENTS
R E P R I N T E D F R O M
O F F I C I A L DIGEST. V O L . 35. NO. 465. OCTOBER 1963. P.992
-
1002R E S E A R C H PAPER NO. 201
O F THE
DIVISION OF BUILDING RESEARCH
P R I C E 2 5 C E N T S
OTTAWA
T h i s p u b l i c a t i o n is being d i s t r i b u t e d b y t h e D i v i s i o n o f B u i l d i n g Research o f t h e N a t i o n a l Research C o u n c i l a s a c o n t r i b u t i o n towards b e t t e r b u i l d i n g i n Canada. I t should not be reproduced i n whole o r i n p a r t , w i t h o u t permission o f t h e o r i - g i n a l p u b l i s h e r . The D i v i s i o n would be g l a d t o b e o f a s s i s t a n c e i n o b t a i n i n g s u c h permission. P u b l i c a t i o n s of t h e D i v i s i o n o f B u i l d i n g Research may be o b t a i n e d by m a i l i n g t h e a p p r o p r i a t e r e m i t t a n c e , ( a Dank, E x p r e s s , o r P o s t O f f i c e Money Order o r a cheque made p a y a b l e a t p a r i n Ottawa, t o t h e Receiver General of Canada, c r e d i t N a t i o n a l R e s e a r c h C o u n c i l ) t o t h e N a t i o r a l Research Council,
Ottawa. Stamps a r e not acceptabl-e.
A coupon system has been i n t r o d u c e d t o make payments f o r p u b l i c a t i o n s r e l a t i v e l y simple. Coupons a r e a v a i l a b l e i n denominations o f
5,
25, and 50 c e n t s , and may b e o b t a i n e d b y making a re- m i t t a n c e a s i n d i c a t e d above, These coupons may b e used f o r t h e purchase of a l l N a t i o n a l Research C o u n c i l p u b l i c a t i o n s i n c l u d i n g s p e c i f i c a t i o n s o f t h e Canadian Government S p e c i f i c a t i o n s Board.ached Pailit Films
y
Dynamic Modulus
Elasticity Measurement
By R. S. YAMASAKI
National Research Council of Canada*
T h e effects of w e a t h e ~ i ~ l g agents o n the lncchanical plopelties of paint films in a film-sub5trate system have been consiclcrecl f o r study by the vibrating-reed method. T h e method used by Van Hoorn ancl Bruin is modified, chiefly as to reed design, in ordel to study the effects of fadeometer radiation on t h e clynanlic modulus of elasticity of paint films attached to stainless steel substrates. T h e measurements disclose cliffercnccs in modulus changes among films ancl two eITccts of racliatio~l which are a t t ~ i b u t e d to gloss thermal and photo-~acliation effects. As such, if modulus changes a l e comparable or glcater, this method call p ~ o b a b l y be ~ ~ s c c l t o
INTRODUCTION
T h e protective properties of a paint film clepencl, among other things, on the ability of the film to expancl or contract in sympa~h) with the surface on which it is applied and remain intact. Hence stress-strain properties of free film under differ en^ exposure conclilions have been studiecl rather extensively. T o obtain a more complete unclerstancling of the behavior of
pain^
films, however, the complete sys~em, film and sub-strate, should be considerecl. T h e free-film behavior is then modifiecl by the adhesive influence of the subs~rate o n the internal stresses and cli- mensional changes of the film. A knowledge of such interactions should further clarify the mechanism of film breakdown.
After a literalure review, the vibrating-reeel method used by Van Hoorn and Bruin1 was chosen for stuclying he mechanical properties
*Org,uiic Materials Section, Division of Bullding Re\eaich, A l o ~ ~ u c a l K d , O r t a ~ v a 2, O n t . , C;~n,id:~.
on the dynamic modulus of elasticity o l paint films attached to a sub- strate. This report presents the preliminary results.
THEORY O N THE VIBRATING-REED METHOD
ssentially, the vibrating-reecl method involves the determination of lges produced by a n attached paint film in the transverse natural ce frequency of a rectangular metal blade. T h e resonance lre- of the blade is measured when it is bare, alter i t hcls been coated ~ a i n t film, and after the film has been subjected to weathering agents such as radiation. I n this way the appropriate dynamic illodulus of elasticity of the film may be obtained.
T h e natural resonance lrequency, f,, for a reecl of rectangular ross-section permanently clamped at one end is given by:
where 2 i s a constant ancl equal to 3.5156 for the fundamental fre-
I
quellcy of the system, h,, L, El, and ,,,.are the thickness, length of the unclamped portion, modulus of elasticity a n d density of the reed, respectively.
Now, if the entire length, L, of the ieed is coated on both sicles with a layer of paint of thickness h,, cleilsity p, and modulus of
11
elasticity E,, and letting x = , the resonance frequency is changed t o 11,
f, so that the film modulus, E,,* is given by:
T h e assumption is made here that the fill11 adheres uniformly to the substrate, that it is isotropic, ancl that it has the same thickness on each side of the blade.
Any treatment that alters the n~odulus, the mass of the film, or the film-substrate boncl causes a corresponding change in resonance fre- quency. T h u s by knowing the film thickness a n d density also, the effect o l the exposure agents directly o n the film, as well as indirectly on the film-substrate boncl, may be studied. T h i s method has a number of
+For d c t a ~ l s a5 ta mathemat~cdl d e r ~ \ . a t l o n sec rcIerence (1) artlclc by W. P. van Oolr. 3
eness. Thus, m
amp so as to leave the desired
which drives the reed transversely. T h e free encl of erved edgewise uncler the microscope. T h e oscillator
THE VIBRATING-REED METHOD
he stability and the sensitivity of the reed were improved. From n (1) it may be seen that the resonance frequency of a reed is an function of the length of the vibrating blade. T h e change in produces a change i n frequency. For the long {est periods nticipated there was some fear that the contact area of the clamp on the reed would alter with time. This would change the vibrating length ancl hence contribute to the instability of the resonance frequency of the reed. Thus, reeds of a design insensitive to clamping conditions were Equation (2) shows that the response of the reed is a function of the magnitude of its resonance frequency. T h e higher the absolute resonance frequency, the greater is the frequency change per given modulus change. Thus, for greater sensitivity reeds with relatively high frequencies (about 500 cps) were considered.
T h e new reed was made from one piece of metal. I t con~is~ecl of a relatively heavy non-vibrating shank and a centrally locatecl ~ i b r a t i n g blade. T h e climensions were
4.8
x 1.3 x 0.32 cm and 2 x 1 x (0.025 t 0.0002 cm) , respectively. Ferromag-nctic, corrosion-rebistant stainlessel 446 was used aild the reed blades were given similar matte
TUDY OF RADIATION EFFECTS ON ATTACHED PAINT F1
L,
bare, high ?requencyAreeds. (1) Blade, (2) ~ h a i k '
Figure 1B-Vibrating-reed apparatus. Reed in position for N. R. F. determination. (1) Reed, (2) Clamp, (3) Electromagnet,
a paint with the new ree
ness was lneasured with a
EST PROCEDURE, AND DISCUSSIONS
modulus of four white exterior house paints. These \zr
Oil-Alkyd Paint Film
For this preliminary study four reeds a n d 20 density panels
T a b l e 1-Main NOII-volatile C o m p o n e n t s of W h i t e E x t e r i o r H o u s e P a i n t s
Pigment Paint Designation Pigment Non-volatile Vehicle Volume, %
- . -.
1. NRP 787 TiO,, ZnO, Linseed oil, extra
50% Oil-alkyd paint MgSil long oil alkyd (1:l) 34.7 2. NRP 937 TiO,, ZnO, Raw and bodied
Oil Paint BCWL, MgSil linsccd oils 33.8
3. N R P 941 TiO, LIcdiu~n oil
Alkyd cnalncl paint alkyd 19.8
4. 11423
-
--
Colnmelcial acrylic latex csterior house paint
5 0 % OIL-ALKYD PAINT F l L M
o--o-O -O-+O-O
*/a-.-.+.-•
0. ACRYLIC LATEX PAINT F I L M
- - - x - - - - " - - - - C. ALKYD PAINT F I L M A 27 ,& F I L M * N O R A D I A T I O N 0 28,U F I L M A N O R A D I A T I O N
+
29 ,U F I L M-
W I T H L O W 'd R E E D S----
W I T H H I G H d R E E D SFigure 2-Modulus changes of films on exposure to fadeometer radiation
ometer radiation for ten 24-hour cycles on each face at 44OC and 36% rela~ive humidity. T h e air temperature was 30°C. One reed and eigl density panels were shielded from the radiation to act as co~ltrol Resonance frequencies, film thicknesses, ancl clensities in duplicate were cleterminecl at f r e q u e n ~ intervals, and corresponding clynami moduli ol elasticity of the films were calculated. All measurements wer carried out in a concli~ionecl room kept at 23
+
2OC ancl 50 2 relative humidity.T h e relationship between modulus ancl exposure period was rough1 exponential. See Figure
2
A. T h e nlodulus increased rapidly in tinitial stages, then levelled off. Average increase in mocl~~lus of expos film was 6.4 x 1010 dynes/cm"ronl an initial value of 2.8 x 1010 dynes cm2. I n terms of resonance frequency the increase was from 480 to 48 cps. Aging had very little influence on the moclulus of the sl~ielded film; modulus increased by only 10%. T h e difference between the curves for the exposecl ancl the shielded films gives the measure of the effect of
---
E X P O S U R E P E R I O D , 24-HR C Y C L E
I T H LOW
d
REEDS ---- W I T H HIGHd
R E E D SPhoto-radiation effect on modulus of films
.--*----.
---•____----.
C. A L K Y O P A I N T F I L M D A C R Y L I C L A T E X PAINT FIL;'
.ch Council of Canada, Division of Building , and is currently engaged in the sludy of che d physical kinetics of degradation of p a i n t filnls
to 6.9 ancl 1.5
%
for the exposed ancl shielded filins, respeOil, Alkyd Enamel, and Acrylic Latex Paint Films
Difficulties in film preparation and moduli measurements were encountered in the preliminary runs with the oil paint. T o improve the sensitivity of the method further and to determine whether modulus is a function of stress frequency, reeds with shorter blades and about twice the resonance frequency were fabricated. See Figtire lA, bottom reed. I n addition, more low frequency reeds were used. T o obtain filins of more uniform thickness, xylene was used to replace mineral spirits as the solvent in the paint. This was also clone with the alkyd paint. A number of reeds and density panels were covered with improvecl radiation shields in order to maintain the films at the same gross temperature as those exposed and thus to determine the baking effect. Exposure* and measure- ment procedures were similar to those of the 50% oil-alkyd paint film, except that alkyd enamel films were exposed longer-for 20 cycles.
T h e sample data for oil paint film are given in T a b l e
2;
the corre- ponding curves describing the changes in moclulus of the film as well as the illustrative results for the other films are shown in Figtire 2. For the most part they are similar to those of the 50% oil-alkyd paint. Low and high frequency reed results were obtained with both irradiated and non- irradiated films.I n interpreting the results emphasis was placed more on the dlanges in modulus than on absolute values, because the foriner gave better pre-
'I'adcometrr railiation as culittcd by a single cnclosrd carbon-arc source ( A S T M E188-G1T. bletllod A) a n d g ~ r ~ i r l g 1 G ? 2 standaid fadlrlg hours of iadiation in a 24-hour 1~111. Illack panel
t c r n p c r a t u ~ c was 43 i 4°C. Sprcimcn t r m p e l a t u r c was 44 i- 4°C and relative l i u m ~ d i t y was
5 N N 0 a * 0
c?'? '? '? 0 -
ij r - m G? 4 ,N
L? I 9 1 0 $ $ $ $ m m m
for the oil paint film and 5 % for the others. As lor de
observed previously, have now disappeared, except for those rylic latex which still persist. T h e res~ilts at 10 cycles, with 1 equency ones for acrylic latex, show that the oil paint film underwent the greatest change, 3.3 x 10'0 dynes/cmQr 80%; acrylic latex, 2.0
x
1010 clynes/cmz or 50%; and alkyd enamel, 1.0x
1010 dynes/cm%r SOO/O. Comparison of the results for gross radiation and photo-radiation effects show that increase in modul~is of oil paint film was mainly clue to photo-effect, that of acrylic latex entirely to photo-effect, and that of alkyd enamel the result of both thermal and photo-effects, the losmer contribution being greater.SUMMARY A N D CONCLUSIONS
A vibrating-reed method has been considered for studying the effects of weathering agents on the elastic property of a paint film in the presence of a substrate. T h e method used by Van Hoorn and Bruin was modified, chiefly as to reed design, for improved sensitivity and stability during relatively long exposures. It was used to study the effect of the fadeometer radiation on the dynamic modulus of elasticity of paint films attached to stainless steel substrates. T h e gross thermal and the photo- radiation effects of the radiation on the films were isolated. For ten 24- hour cycles ol exposure at 44OC and 43% relative humidity the gross radiation raised the moduli of oil, alkyd enamel and acrylic latex films (25 microns in thickness) by 110, 80 and 50%, respectively. Of these the photo-radiation effect contributed moclulus increases of 80, 30 and 50y0, respectively. T h e increase in modulus of the oil paint film, therefore,
obtained here the method should
Reprinted from the October 1963 issue 01 Official DIGEST Volutne 35; N ~ ~ t n b e r 465; Pages 992-1002
