Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 252-257
252
surfaces indicate that the gaseous glow discharge is still extremely effective in improving surface wettability of PET. Weight losses of the glow discharged PET substrates were higher in air than in nitrogen. Higher weight losses for the glow discharged films than fabrics are probably due to the much smaller surface area per sample of the films. Water retention values of nitrogen-treated fabrics were increased significantly, but not for the air-treated ones. Both wettability and moisture regain data suggest that oxidation of surface, by the glow discharge and/or during storage, make substrates less hydrophilic. The improvement of surface hydrophilicity of PET by glow discharge is likely contributed by changes in both chemical composition and morphology of the surface. The drastic decrease in water contact angle with negligible weight loss and surface morphological change at 30 s exposure to glow discharge strongly suggest that the chemical changes on the surface are predominantly responsible for the initial improvement on hydrophilicity. Further improvement in wettability and additional weight loss at longer exposures indicate the close relationship between better surface hydrophilicity and increasing roughness of the surface. However, gaseous glow discharge does not produce stable chemical modification on the surface. In combining the weight change data with the wettability data, a distinct weight gain in the initial 3 days was accompanied by a decrease in wettability. Further decrease in wettability after the initial 3 days was not associated with any further weight change. This observation, along with the ESCA data, suggests that the decreased wettability could be initially due to the incorporation of atmospheric compounds, i.e., oxygen, in the surface, and later by molecular rear-
rangement at the surface. Registry No. PET (SRU), 25038-59-9.
Literature Cited Blakly, P. R.; Ally, M. 0. J. Text. Inst. 1978, 38. Boening, H. V. "Plasma Sclence and Technology"; Cornell University Press, 1982. Brlggs, D.; Rance, D. G.; Kendall, C. R.; Biythe, A. R. Polymer 1980, 27,895. Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1978, 76,911. Clark, D. T.; Dilks, A.; Shuttleworth, D. I n "Polymer Surface"; Clark, D. T,; Feast, W. J., Ed.; Wiley: Chlchester, UK, 1978; Chapter 9. Hall, J. R.; Westerdahl, C. A. L.; Bodnar, M.; Levi, D. W. J . Appi. Polym. Sci. 1972, 76, 1465. Hollahan, J. R.; Bell, A. T. "Technlques and Applications of Plasma Chemistry"; Wlley-Interscience: New York, 1974. Hollahan, J. R.; Carbon, G. L. J . Appl. Polym. Sci. 1970, 14, 2499. Hudis, M. I n "Techniques and Applications of Plasma Chemlstry"; Hollahan, J. R.; Bell, A. T., Ed.; Wiley-Intersclence: New York, 1974; Chapter 3. Lawton, E. L. J. Appl. Polym. Sci. 1974, 18, 1557. Millard, M. M.; Wlndle, J. J.; Pavlath, A. E. J . Appi. Polym. Sci. 1973, 77, 2501. Moshonov, A.; Avny, Y. J. Appl. Polym. Sci. 1980a, 25, 80. Moshonov, A.; Avny, Y. J. Appl. Polym. Sci. 198Ob, 25, 771. Owens, D. K. J. Appl. Polym. Sci. 1975, 79, 3315. Padhye, M. R.; Bhat, N. V.; Mlttal, P. K. Text. Res. J. 1978, 46, 502. Schonhorn, H.; Denes, F.; Macovaenu, M. M.; Caracu, G.; Totolin, M.; Percec, S.;Balaur, D. Cell. Chem. Techno/. 1970, 2 , 93. Sharma, A. K.; Millich, F.; Hellmuth, E. W. J. Appl. Polym. Sci. 1981, 26, 2205. Shen, M. "Plasma Chemistry of Polymers"; Marcel Dekker: New York, 1976. Shen, M.; Bell, A. T. I n "Plasma Polymerization"; Shen, M.; Bell, A. T., Ed.; American Chemical Society: Washington, DC, 1979; Chapter 1. Vlagiu, 1.; Stannett, V. J . Macromol. Sci., Chem. 1973, A7(8). 1677. Weininger, J. L. J . Phys. Chem. 1981, 65, 941. Wertheiner, M. D.; Shrelder, H. P. J. Appl. Polym. Sci. 1981, 26, 2087. Wrobel, A. M.; Kryszewskl, M.; Rakowskl, W.; Okonlewski, M.; Kubacki, 2. Polymer 1978, 16, 908. Yasuda, H. J. Macromol. Sci., Chem. 1976, 70, 383. Yasuda, H.; Gazicki, M. Biomaterial 1982, 73,68. Yasuda, H.: Lamaze, C. E., Sakaoku, K. J. Appl. Polym. Sci. 1973, 17, 137.
Received for review July 16, 1984 Accepted D e c e m b e r 10, 1984
Application of Acoustic Emission Analysis on Adhesion and Structural Problems of Organic and Metallic Coatingst H. Hansmann Department of Materials Engineering, University of Duisburg, 0-4 700 Duisburg, West Germany
The adhesion of organic and metallic coatings as well as the formation of their structures is doubly important for
effective corrosion prevention: protection against mechanical deterioration and protection against electrochemical deterioration. In spite of extensive theoretical approaches, In many fields of coatings there are still problems on how to find a method for a reliable and sensitive evaluation of adhesion. A practicable contribution to a critical examination of adhesion as well as to structural examinations of corrosion protective coatings may be given by acoustic emission (AE) analysis. The paper reviews ongoing activities related to the use of AE to the delamination process of organic coatings and the deformation, cracking, and peeling of metallic hotdip and electroplate coatings.
The adhesion of organic coatings is arbitrarily determined by delamination characteristics rather than by a well-defined state. An adequate adhesion theory must take into account the efficiency of intermolecular forces in ad+ D e d i c a t e d to Professor H.G. Mosl6, U n i v e r s i t y
of D u i s b u r g ,
with b e s t wishes for h i s 60th b i r t h d a y . 0196-4321/85/1224-0252$01.50/0
hesion interaction and the viscoelastic and cohesive properties of the f i i as well as the kind of applied loading which leads to failure (Kaelble and Uy, 1970). We think that a combination of the adsorption, polarization, and diffusion theories with the weak boundary theory is a good basis for the interpretation of the observations which can be obtained from the delamination process, the affecting parameters, and the resulting acoustic emission activity 0 1985 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 253
11 Fe subrtrut hgreusad 21 Fe rubrtrot Fe-phor 3) Fe rubstrut Zn-phor
)1
TRANSIENT hYALYS FFT ENERGY MPLITUDE RISETIE
I
I
SPECiKN
Iood/eiongntion
I
I I
””
COHWEVENT
I
i”n ARLITUDE
\
1 6
Figure 1. Block schematic of the AE analysis equipment.
ringdown counts
31
Z
/
I/
I1 21
AI
Figure 3. AE of steel substrate.
tured by a broad-band (AET FAC 500) transducer (sample rate 10 MHz; dynamic range, 8 bit; sample time, 204.8 ps). ‘The energy E of the detected AE signal is defined by
E =
-4
,
-5
-
-6
.
-7
,
-0
’
C = [C(E*P)]’/* T1m.
0
V ( t )dt
(1)
where U(t)= transducer output and T = signal duration. Due to the frequency-dependent attenuation of the acoustic signal we compensate this effect by multiplying each energy E of signal i with the mean frequency F of its Fast Fourier Transform. So, a better approximation of the released acoustic energy is given by the expression
-9 -10
T
‘
1
10
20
30
c
’
’
’
40
I0
60
?E
“
80
a
’
’
’
L
[“.I
“
‘
9 0 I00 I10 120 130 140 118 I80 I ? 0 1 1 1 1 9 0 2 0 0
Figure 2. AE signal (delamination captured by a FAC 500 broadband sensor; 96 dB gain; sensitivity about -70 dB, V/pbar.
during the delamination. The first three theories cover the conditions of adhesion boundary formation. The weak boundary model leads to an understanding of the delamination process. Acoustic Emission Analysis When a critical tension, particularly a critical energy, is applied to a coated specimen, spontaneous relaxation occurs by different deformation processes, i.e., by delamination. The energy released herewith can be detected partly as acoustic emission (Strievens et al., 1980). The kind of acoustic emission which is obtained during this process is a “burst” type signal. The acoustic signals transduced (by a piezocrystal element) can be analyzed after amplification and filtering. The oscillations accumulate as “ring down-counts” in arbitrary units. Other standard acoustic emission signal attributes are the amplitude (maximum and mean value), the energy, and the frequency spectrum. Figure 1shows a block schematic of the acoustic emission analysis equipment that we use for investigations of the burst signals that are emitted by “low acoustic activity” specimens during tensile testing. The transducers work at resonant frequencies in the range of 0.1 to 1 MHz. For frequency analysis purposes it is necessary to use highly damped high-frequency transducers. The signal processing includes statistical analysis: on-line by a hybrid computer (Dunegan Endevco Corp. Model 820) and off-line by digital computer devices (Hewlett-Packard). The intention of our activities in this field is the deduction of adhesion energy from AE signal characterization. Figure 2 shows a typical delamination signal, cap-
(2)
where C = value that approximates the AE energy and 2. = summation of all accepted AE signals. It could be shown earlier (see Hansmann and Mosl6,1982) that this C value involves the most significant information about any change or difference in the adhesion of organic coatings compared with other AE signatures. Influencing factors on the AE of coated sheets during tensile testing are adhesion characteristics, deformation and cracking of the substrate, film properties, and film thickness. Adhesion characteristics of course are the interesting factor. As discussed above, the adhesion practically must be treated as a phenomenon that relates to real circumstances, e.g., the type of loading and the cohesion properties of the film. That means that the remaining reducable influences are substrate deformation,cracking, and the film thickness. In terms of AE amplitude, the delamination of organic coatings in the presence of substrate AE activity and film AE activity can be expressed (according to Sklarcyzk, 1983) by UDel
=
[utotal-
Vfih - u ~ u b s t r a t e112 l
(3)
Figure 3 shows the load/accumulated ringdown counts (in arbitrary units) and rms (root-mean-square of U ( t )of the time signal; integration time, 3 s) vs. elongation of uncoated test sheets in uniaxial tension. The rms curves are shifted vertically against each other for better reading. Degreased (1)and Fe-phosphatized (2) steel St 1403 substrates differ negligibly in AE activity, while Zn-phosphatized (3) substrates emit some burst-type signals which trigger about 20.000 counts. The rms curve even indicates additional burst-type signals by some peaks (in the first quarter of strain hardening) to the continuous AE of mild steel (1). I t is now easily possible to separate AE caused by delamination from AE caused by substrate deformation. Figure 4 shows the AE activity of a deteriorated polyester
254
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 Table I acrylic coatings, d=60pM substrate pretreatment curing temperature
~AI
Figure 4. AE of a deteriorated polyester coating.
coating on substrate 1. The measured burst activity of this system is nearly 100% related to delamination. The obtained burst signals generate high peaks in the rms curve. Film thickness is not easily reducible. It influences the appearing stresses (peel, shear, and tension) in the boundary in an unpredictable manner. The tendency that has been observed with decreasing film thickness is a decreasing AE activity. This can be explained by decreasing released energy at each microdelamination because of lower accelerated masses. In order to undergo these difficulties one has to compare coatings with equal or similar film thickness. Wet Adhesion. The irreversible partition of the boundary deterioration is an essential aspect of adhesion. Under humid climates the bond strength decreases in an unpredictable manner until catastrophical destruction. In order to predict long-time behavior under operational conditions one investigates the behavior of coatings under intensified short time conditions. Due to the weak cr
lo(
substrate preparation __
mTC-
A
a
first indication of de t f r i or a i i on
0
-TCure-
Zn-phos Zn- phos
160'C 1°C
Zn- phos
2OoeC DTA peak
degreased degreased degreased
2°C
N,+~~
r,??/////$
system 2 Zn-phos. 130 "C
boundary theory, deteriorations begin in the weaker boundaries and with increasing exposure time even destruct strong boundary areas (Kinloch, 1980). Due to the weak boundary theory, the microdelamination velocity of a little coated area between strong boundary areas is controlled by the acceleration by residual stresses and deceleration by the adhesion energy of the weak boundary area between them. An increase of C must be obtained, when the adhesion of weak boundary areas is deteriorated, because of the corresponding decreasing deceleration of the microdelamination. On the contrary, if the deterioration affects the strong boundary areas, this model leads to a decrease of the obtained C value because of the decreased residual stressed which can occur between strong boundary areas. In spite of some scatter, it could be concluded that excellent agreement was found between the model and the obtained experimental results (Table I). Compared to usual mechanical test methods (cupping test, cross-cut test, pull-off test etc.), the AE allows an indication of first irreversible deterioration at a very early stage. The time-shortening factor is dependent on the system and the type of exposition in the range of 2 to 20. Figure 5 shows the obtained C values of cathodic polarized polyester powder coatings (d = 60 pm) at distinct stages. The samples have been dried and normalized after exposure to salt solution; this means that the results represent only the irreversible portion of the loss of adhesion. First indications of deterioration have been obtained by AE analysis after 2-8 h exposure time, and by mechanical test methods after 24 h and more. After 24 h exposure time, one can realize the influence of curing temperature and substrate pretreatment on the deterioration process
AE- Burst energy i c c u m i i l o t i a n
c- c,
system 1 degreased 110 o c
'2h
Y////A 24h -by
mechanical t e s t s
Figure 5. Cathodic polarization (-2 V) effect on polyester-powder coatings; d = 60 pm, in 0.1 N NaCl solution,
160" 18O'C
--l
J
temp
Ind. Eng. Chem. ProU. Res. Dev.. VoI. 24. No. 2, 1985
255
4
1W
0
d0 re10-6v
Figure 8. Amplitude (log) distribution.
%?PI nntim
$1
-t
i
Figure 6. $-initiated microdelamination during yielding of AI.
t
,.
~
AI
Figure 'I. Cumulative energy versus strain of B thin coating on tin.
by the obtained C values. The highest C values were obtained from 180 "C cured samples. This temperature corresponds to the DTA peak temperature of the polyester powder, so optimal cross-linking and the best cohesive properties can be expeded at this curing temperature. The C values obtained from 200 "C cured samples are lower; those from 160 'C cured samples are much lower compared with the 180 O C cured samples, independent from the substrate pretreatment. On the other hand, higher C values are obtained from coatings on a Zn-phosphatized substrate compared with coatings on degreased substrates. The interpretation of this effect is easier because equal film properties can be assumed. The higher C values on Zn-phosphatized steel compared to degreased steel means weaker adhesion after 24 h exposure time. Less easy to interprete is the influence of the curing temperature on the C values. The 180 "C cured films moved the binhest residual stresses and therefore the highest C valu&. Thin Coatincs. A soecial test Droblem is often eiven when the adhesion of thin films (
