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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 132-138
Wear Rates of Anodized Aluminum and Polymer Coatings as Model Systems for Lithographic Printing Plates Mlchael C. Hughes, Henry Leldhelser, Jr.,* Shem-Mong Chou, and Wayne Bllder Department of Chemistry and Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania 180 15
The rates of wear of anodically formed oxides were determined under conditions that simulate wear of lithographic printing plates by abrasive inks. The rates were determined using an optical technique based on the interference fringes formed during specular reflection. Minimum wear rates of unsealed coatings were obtained on aluminum anodized in sulfuric acid at 15 V and 20 "C. Wear rates were also determined on coatings that were sealed in different aqueous media. The highest wear rate was obtained with coatings sealed in sodium silicate. The wear rate of all sealed coatings was initially high before falling to a rate typical of the unsealed coating. Superior wettabili of the anodic coating by distilled water during short sealing times was exhiblted by panels sealed in the sodium silicate solution. The rates of wear of polystyrene and poly-n-butylmethacrylate on aluminum anodized under different conditions were determined with optical techniques.
Introduction Previously, a description was given of an apparatus useful in determining the rate of wear of commercial lithographic plates (Noble and Leidheiser, 1981). This apparatus has now been utilized in conjunction with optical techniques to determine the wear rates of aluminum anodized under different conditions and of several polymers applied as a thin coating on the aluminum. The wear rate of thick anodic coatings or of thick polymer coatings is readily studied by gravimetric methods or by the use of various types of thickness gauges. However, when the coatings are of the order of several microns in thickness, these methods are inadequate or difficult. A method is described herein for determining the changes in film thickness as a function of wear for coating thicknesses in the range of 0.5-25 pm. Background Various optical techniques were explored for determining coating thicknesses on anodized aluminum and polymer-coated aluminum, but it soon became apparent that a method based on interference fringes that occur during specular reflectance spectroscopy showed promise. This technique was thus pursued. Wendlandt and Hecht (1966) have discussed the theory of specular reflectance spectroscopy in detail. Several workers have used the interference fringes produced in infrared reflectance spectroscopyto measure the thickness of epitaxial Si02on silicon (Spitzer and Tanenbaum, 1961; Albert and Combs, 1962), and Cheever has used the technique to study the thickness of zinc phosphate coatings on steel (Cheever, 1978). Harrick (1971) has described the use of infrared specular reflectance to measure the thickness of an absorbing polymer film (Mylar), but cautioned against the use of the technique on a film deposited on a metallic substrate because of unpredictable phase changes likely to occur at the substrate/film interface. In the present work it was found that within a given coating/ substrate system the thickness could be calculated from interference fringes using the conventional equations (Wendlandt and Hecht, 1966). Although a 1:l correspondence was not always found between coating thickness determined by this method and by weight loss measurements, a linear relationship was found in all the systems studied. These observations suggested that the coating thickness could be accurately determined above about 2.5 pm by the use of a suitable calibration curve. A t coating 0196-4321/83/1222-0132$01.50/0
thicknesses below about 4 pm, no measurable interference fringes were observed in the infrared region. Usable interference fringes were formed, however, in the near-infrared and visible regions of the spectrum. The thickness of thin polymer coatings was determined, following a suggestion of Hannah (1963), by measurement of the transmittance of the polymer at an absorbing wavelength vs. the transmittance of the bare substrate where Tp = transmittance observed for the polymer coating and T, = transmittance observed for polymer-free substrate. Plots of AA vs. thickness, as determined gravimetrically, were linear to thicknesses of about 0.5 pm, the limit beyond which weight measurements were difficult. The limit of spectroscopic measurement was determined largely by the intensity of the absorption band used. Experimental Section The aluminum used in this study was a 1100 alloy expressly anodized for use as a lithographic plate. The anodic coating was removed by immersing the plate in a mixture of 2% Cr03 and 5% H3P04at 95 "C for 5-10 min. The metal was then etched in a NaOH-NaF electrolyte, desmutted in HN03, chemically polished in H3P04-HN03 solution, and thoroughly washed with distilled water immediately before anodizing. The anodization was carried out in 15% H2S04,5% H3P04,2% oxalic acid, and 3% H2Cr04under different conditions of temperature and applied potential. The surface roughness before anodizing was 0.76 pm average height above and below a center line. Polymer coatings were applied from solution using a spinning disk applicator. The thickness was controlled by varying the velocity of rotation and the polymer solution concentration. Polystyrene, poly-n-butylmethacrylate, and polybutadiene were applied from a toluene solution and the Epon 1001-polyaminewas applied from a 65% butyl cellosolve-35% xylene solution. Four different aqueous solutions were utilized for sealing the aluminum oxide coatings: distilled water, nickel acetate solution, potassium dichromate solution, and sodium silicate solution. Distilled water sealing was carried out at the boiling point. The nickel acetate solution contained 6 g/L of nickel acetate and 8 g/L of boric acid: the pH was 5.6 and the sealing temperature was 90 f 5 "C. The potassium dichromate solution contained 15 g/L potassium dichromate and 3 g/L NaOH: the pH was 6.9 and the 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1 , 1983
133
Table I. Rate of Formation of the Anodic Coating on Aluminum as a Function of Anodizing Conditions
electrolyte 15% H,SO,
5% H,PO,
:K t .-
i2t
2% H,C,O, 7
6 5 4 Thickness, pm
3
Figure 1. High molecular weight polystyrene on a commercial aluminum plate anodized in sulfuric acid. The solid line represents the film thickness measured using the multiple fringe technique and the points represent measurements using a single fringe. Solid circles are unworn films and solid triangles are worn films.
3% H,CrO,
sealing temperature was 90 f 5 OC. The sodium silicate solution contained 5 % sodium silicate by weight: the pH was 11.4 and the sealing temperature was 90 f 5 "C. The infrared specular reflectance spectra were obtained using a Perkin-Elmer Model 186-0373 specular reflectance accessory on a Perkin-Elmer Model 283 infrared spectrometer. Apparent coating thicknesses were calculated on the basis of the frequencies of the interference fringe maxima or minima according to the equation n T= (2) 2(vl - vz)($ - sin2b')1/z where n is the number of fringes, u1 and v2 are the initial and final frquencies of the fringes, 7 is the refractive index of the coating, b' is the angel of incidence of the infrared beam, and T i s the coating thickness. In the method used in this study, where b' is SO", eq 2 may be simplified to n T= (3) 2(Vl - V 2 h In a constant polymer/substrate system the change in thickness should be proportional to the reciprocal of the frequency of any individual fringe. The equation can then be further simplified to 1 T a (4) V?
and can be utilized where only a single fringe is identifiable for a direct measure of the relative wear of a polymer film. Figure 1 shows an example of the validity of this simplified method in the case of high molecular weight polystyrene on an anodized aluminum substrate. The coated plates were worn by a procedure outlined previously (Noble and Leidheiser, 1981). The procedure involves a back-and-forth motion of a weighted block against the sample plate immersed in distilled water to which a corundum abrasive was added. The pressure of the block acting on the test plate was 22 g/cm2. Wettability measurements were made by placing a drop of doubly distilled water on the anodized surface and measuring the contact angle by means of an NRL Contact Angle Goniometer Model A-100. A container of doubly distilled water was placed in the environmental chamber in order to establish a high humidity and minimize the rate of evaporation. Contact angle measurements were made 2 min after a water drop of 5 p L was placed on the sub-
0 4000
3000
anodizing voltage, V
20 20 10 30 20 20 20 10 20 30 20 10 20
10 15
15 15 20 80 100 120 120 120 40 60 60 60 80 60 80 80 80 100
30 20 38 30 38 45 38
2000 Y,
coating thickness teomp, as a wt gain, C mg/cm*-min 0.027 0.061 0.032 0.133 0.171 0.012 0.016 0.008 0.022 0.051 0.021 0.024 0.050 0.095
0.106 0.022 0.014 0.025
0.036 0.024
logo
cm-'
Figure 2. Typical interference fringe pattern of unworn and worn anodized aluminum. See text for details.
strate. The average value of 4 different droplets was taken as the contact angle. Results Rates of Formation of the Anodic Film on Aluminum. Formation rates of the anodic coating as a function of anodizing conditions were determined by gravimetric means. The maximum time of anodization was 30 min in all cases for all the data summarized in Table I. Although the growth rate was not perfectly linear with time of anodization in all cases, the best straight line was drawn through the points in order to obtain the values given in the table. Rates of Wear of the Anodic Coating on Aluminum. An example of the changes in the interference fringe pattern of the anodic coating on aluminum as a consequence of reduced thickness by wear is shown in Figure 2. The aluminum was anodized in 2% oxalic acid at a potential of 40 V for 66 min at 20 O C . The specular reflectance spectrum exhibiting the interference fringes is shown as the solid line in Figure 2. The dotted line is the spectrum obtained after 2000 cycles in the wear apparatus. The shifts in the fringe maxima and minima are readily apparent above approximately 2000 cm-'. Figure 3 shows spectra for an anodic coating formed in a similar manner but sealed in boiling water for 30 min after anodization. The interference fringes at higher frequencies are masked by the intense 0-H stretching band, but sufficient fringes are present for thickness measurements.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983
r
I
IOOb
I
- Unworn 2000 passes
I
-
2 MIN.
55 6
~
'I I I
I Y
4000
3000
2000 Y , cm'
1000
Figure 3. Typical interference fringe pattern of unworn and worn anodized aluminum that had been sealed in boiling water for 30 min. __
1000 CYCLE
Figure 5. Rates of wear of anodized aluminum sealed in nickel acetate solution (NA),potassium dichromate solution (PD), distilled water (DW), and sodium silicate solution (SS) for 2 min.
I
I000 CYCLE
2000
10 MIN.
45c
2000
Figure 4. Rates of wear of anodized aluminum sealed in nickel acetate solution (NA), potassium dichromate solution (PD), distilled water (DW), and sodium silicate solution (SS) for 1 min. Reference curve for an unsealed anodized aluminum is also given (solid squares).
1
I
I
1000
2000
CYCLE
Table II. Rates of Wear of Anodic Coating on Aluminum a s a Function of Preparation Conditions
electrolyte 15% H,SO,
2% HzC,O,
oxide thickness, bm 5 5 5 5 5 10 10 10 10 10 5 5 5 5 10 10
3% H,CrO,
5% H,PO,
10 5 5 5 5 5 5
anodizing temp, voltage, V "C
10 15 15 15 20 10 15 15 15 20 40 60 60 80 60 60 80
60 80 80 100 100 120
20 10 20 30 20 20 10 20 30 20 20 10 30 20 20 30 20 38 38 45 38 20 20
wear rate in pmlcycle ( X 105) 14.4 3.6
Figure 6. Rates of wear of anodized aluminum sealed in nickel acetate solution (NA), potassium dichromate solution (PD), distilled water (DW), and sodium silicate solution (SS) for 10 min.
-551
% I m
30 MIN l
0
P. 0
0.5 1.7 12.8 9.8 0.4 6.7 11.9 8.2 15.3 2.5 6.5 10.3 7.8 11.8 14.0
12.5 28.5 12.0 10.7 1.0
The thickness of the anodic coating was determined after 0, 200, 600, 800, 1200, 1600, and 2000 wear cycles. Over this period of time the rate of wear was linear with wear time so the data may be expressed in terms of the amount of aluminum oxide removed per wear cycle. Such
I -ss
I
4.5
1000
2000
CYCLE
Figure 7. Rates of wear of anodized aluminum sealed in nickel acetate solution (NA), potassium dichromate solution (PD),distilled water (DW), and sodium silicate solution (SS) for 30 min.
data are given in Table I1 for two different film thicknesses and for different electrolytes, anodizing voltages and temperatures. Rates of Wear of Sealed Anodic Coatings on Aluminum. The wear behavior of the sealed coatings was very different from that of the unsealed coatings in that there was a rapid rate of wear during the very early stages, followed by a much lower rate of wear. The data for the four different sealing solutions are summarized in Figures 4-8 for sealing times of 1 , 2 , 10, 30, and 60 min. It will be
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 135
I
'i
60 MIN
0
, I
3500
30002500 2000
I
1400
1000
Wavenumber. cm-
1000
2000
CYCLE
Figure 8. Rates of wear of anodized aluminum sealed in nickel acetate solution (NA),potassium dichromate solution (PD), distilled water (DW), and sodihm silicate solution (SS) for 60 min.
.A
Figure 10. Specular infrared reflectance spectra of high molecular weight polystyrene on a commercial plate anodized in sulfuric acid. Dotted line represents unworn coating and solid line represents spectrum from coating after 100 cycles in wear apparatus.
t
I
0 PD
0 DW A NA
,- 40
I
20M
2
SEALING TIME I min )
Figure 9. Contact angle of water with anodized aluminum surfaces sealed in sodium silicate solution (SS), potassium dichromate solution (PD), distilled water (DW), and nickel acetate solution (NA) for times up to 60 min.
noted that the oxide sealed in sodium silicate exhibited the greatest rate of wear over long time periods and that the wear rate was approximately linear over the entire time period, whereas the other sealants led to a high rate of wear during the early stages of wear which fell off with time of wear. The wettability of the anodized surface by water after sealing was determined for all the aqueous environments at 1, 2,5, 10, 20, 30, and 60 min sealing time. These data are plotted in terms of contact angle of the water droplet with the anodized surface in Figure 9. Coatings sealed in the nickel acetate, potassium dichromate, and distilled water exhibited reduced wettability after short sealing times and good wettability after long sealing times. The sodium silicate sealant was unique in that excellent wettability was exhibited at all sealing times. All sealing treatments resulted in very small contact angles (good wettability) at all sealing times in excess of 10 min. The tendency of an ink (Red GA 80-1421) to wet the anodized and sealed aluminum surface was determined concurrently with the contact angle measurements. Ink, when applied with a hand roller, readily stuck to all the aluminum surfaces when the surface was dry because of the inherent roughness of the surface. When the surface was wetted with distilled water, the results were in agreement with the contact angle measurements. Ink wet the surface to a degree when the water contact angle exceeded approximately 30' and only wet the surface minimally when the contact angle was below this value. The reddening of the aluminum surface as determined with the naked eye was a direct function of the contact angle.
4 6 8 1 0 1 2 Thickness by Wsight,pm
Figure 11. Comparative film thickness of high molecular weight polystyrene on anodized aluminum as determined by interference fringe technique and by weight measurement. Table 111. Least-Squares Parameters for Thickness by Weight vs. Thickness Measured by Infrared Specular Reflectance for Several Polymer-Metal Substrate Systems ~~~~
polymer/substrate
slope
rz
polystyrene/anodized A1 polystyrene/unanodized A1 epoxy-polyamide/cold-rolledsteel epoxy-polyamide/unanodizedA1 polybutadiene/unanodizedA1 polystyrene/cold-rolled steel
1.00 0.905 1.04 0.95 0.68 0.75
0.998 0.995 0.98 0.96 0.96 0.98
~
Wear of Polymer Coatings on Anodized Aluminum. The application of the infrared specular reflectance technique to polymer films requires careful calibration, for reasons detailed by Harrick (1971). Figure 10 shows infrared specular reflectance spectra for high molecular weight polystyrene coated on a commercial anodized aluminum plate, unworn and after only 100 cycles in the wear testing apparatus. The interference fringes are superimposed upon the spectrum of the polystyrene, but a number of fringes are visible which can be used for thickness measurements. Figure 11shows a plot of polystyrene film thickness determined by interference fringes vs. thickness measured by weight. For this particular system, a linear least-squares regression analysis gave a slope of 1.00 and rz value of 0.998. Table I11 gives fit parameters obtained for this and a number of other systems tested. It can be seen that, while the slope of the calibration curve varies considerably, the correlation coefficient is always high enough that the interference fringe measurement may be considered as accurate as the weight measurement. As the polymer coatings become thinner than about 2.5 pm the interference fringes no longer appear in the normal infrared region. The empirical relationship AA = log Ts/ T,
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Ind. Eng. Chem. Prod. Res. O w . , Vol. 22, No. 1, 1983 05-
04-
io
403-
02-
05 a
I
A A. 02
-A A
I
I
I
400 BOO
I2
06
IO Thickness b y W e i g h t , p m
Figure 12. Comparison between the AA function (see text) and the film thickness of high molecular weight polystyrene on anodized aluminum as determined by weight.
1200 1600 Cycles
Figure 14. Wear of low molecular weight polystyrene on a commercial plate anodized in sulfuric acid. Circles represent measurements made by interference fringe technique and triangles by the transmittance technique.
1-tol 80
T
i
f
I
//e
I
I O
* 0
A
A
A
A A
A
A
A I
400
IS00
1000
500
Wavenumber, cm-’
Figure 13. Specular infrared reflectance spectra of high molecular weight polystyrene on a commercial plate anodized in sulfuric acid as a function of the number of wear cycles: (a) 1800 cycles, (b) 2100 cycles, (c) 2300 cycles, (d) 2500 cycles, (e) 2700 cycles, and (0 uncoated and unworn.
permits the determination of polymer film thickness to smaller values, provided that an absorption band of sufficient intensity is present in the spectrum of the polymer coating. The “ring-breathing” mode bands of polystyrene appearing in the 700-800-cm-’ region are excellent for this purpose. Figure 12 shows a calibration curve of the AA function vs. thickness by weight in the case of high molecular weight polystyrene on anodized aluminum. This calibration curve, and ones similar to it for the other polymer coatings, were used to determine coating thickness directly from the AA measurements. Figure 13 shows an absorption band for polystyrene as a function of the number of wear cycles. The decrease in the intensity of the absorption band as a function of the number of wear cycles is striking. Plots of the rates of wear of low molecular weight polystyrene, high molecular weight polystyrene, and poly-n-butylmethacrylate on a commercial plate anodized in sulfuric acid and a laboratory plate anodized in 5% phosphoric acid are given in Figures 14-16. Effect of Nature of Substrate on Wear of Polystyrene Coatings. A series of experiments was carried out to determine if the rate of wear of a polymeric coating is appreciably influenced by the manner in which the anodized aluminum substrate is prepared. A range of conditions was used with 15% H2S04,5% H3P04,and 2% H2C204anodizing baths in preparing the anodic oxide and the anodized substrate was used in both the sealed and unsealed conditions. Sealing was carried out for 30 min in boiling distilled water. The thickness of the anodic oxide
800
I
1
I
1200 1600 Cycles
4
2000 2 4 0 0
Figure 15. Wear of high molecular weight polystyrene on a commercial plate anodized in sulfuric acid (open points) and in 5% phosphoric acid (solid points). Circles represent measurements made by interference fringe technique and triangles by the transmittance technique. I 0
0
I
A
3.
A
I .
I
,
I
800
1
I
1600 cycles
I
I
2400
I
I
3200
Figure 16. Wear of poly-n-butylmethacrylate on a commercial plate anodized in sulfuric acid (open points) and in 5% phosphoric acid (solid points). Circles represent measurements made by interference fringe technique and triangles by the transmittance technique.
was approximately 2 pm in all cases and the thickness of the polystyrene coating was 12-22 pm. The results of these experiments are summarized in Table IV. Discussion The wear apparatus (Noble and Leidheiser, 1981) was originally developed in order to compare the ability of commercial lithographic plates to withstand the abrasive nature of many printing inks. The method has proven to be adequate to this task and satisfactory correlations were obtained between commercial experience and data generated in the accelerated test. In the present study, the emphasis has been changed and the wear apparatus has been examined as a tool for determining the wear resistance of anodic coatings on aluminum prepared in different
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 137 Table IV. Rates of Wear of Polystyrene Coatings o n Sealed and Unsealed Anodized Aluminum, 2 pm in Thickness rate of wear in pm/cycle electrolyte
15% H,SO, . -
5% H,PO,
2% H,C,O,
anodizing, voltage, V
anodizing temp, "C
10 15 15 15 20 80 100 120 120 120 40 60 60 60 80
20 10 20 30 20 20 20 10 20 30 20 10 20 30 20
(x 104)
polystyrene coating thickness unsealed
sealed
unsealed
sealed
12.1 12.8 13.1 12.1 11.9 14.0 13.5 13.1 12.7 12.4 15.3 18.7
16.5 15.8
9.6 11.1 13.9 11.5 14.1 19.2 20.3 20.8 21.2 18.4 21.1 18.8
27.4 17.2
ways and the wear resistance of polymer coatings to lateral abrasion. The method explored in this study should have applicability to the selection of the optimum surface preparation method for the substrate and the selection of a polymeric coating with satisfactory resistance to abrasive wear in any system in which resistance to abrasive wear is important. The infrared specular reflection technique adopted for determining the coating thickness of both the anodized coating on aluminum and the thickness of the polymer coating was useful in the range of approximately 4 to 25 pm in the case of the anodized coating and in the range of approximately 2.5 to 25 pm in the case of the polymer coatings. Fortunately, the relative intensity of a specific absorption band in the polymer allowed the technique to be extended to the range of 0.5 to 2.5 pm for the polymers studied. It is probable that the cited range might be extended to higher and lower values, but our studies did not require this extension and no effort was made to expand the range. The optical technique has the restriction that the coatings must be sufficiently transparent to develop the interference fringes. Also, calibration curves are required for those polymer systems in which the intensity of the absorption band is used to determine the coating thickness. The technique has the major advantage that the thickness measurement is nondestructive and measurements on the same sample may be readily made as a function of time. The rates of wear of the anodic coating on aluminum as a function of anodizing conditions are summarized in Table 11. The rates of wear vary widely and for each system there is an optimum set of conditions. In the case of 15% H2SO4, minimum wear rates were obtained at an anodizing voltage of 15 V and a temperature of 20-30 "C. The optimum conditions for 2% H2C204were an anodizing voltage of 60 V and a temperature of 30 "C. The optimum conditions for 3% H2Cr04and 5 % H3P04were not determined since an insufficient number of conditions was explored. One of the interesting observations was the much greater wear of coatings 10 pm thick as compared to those 5 pm thick under similar preparation conditions. Seven comparisons are made in Table V in which it will be noted that with one exception the wear rate of the 10-pm coating was significantly greater than that of the 5-pm coating. In the exceptional case the wear rate of both the 5 hm and 10 pm thick coatings was high. This result is in accord with the known morphology of anodic films on aluminum. As a general rule the pore diameters are larger for the thicker coatings and the coating has been subjected to the solvent
__
15.6 15.7 13.2 13.0 13.0 12.6
__
20.7 22.1 19.7 21.2 21.6
__
16.7 15.6
__
23.3 19.2
__
26.6 19.3 15.8 11.1 11.9 18.0
__
6.8 8.0 8.7 6.2 8.0
Table V. The Rates of Wear of the Anodic Coating as a Function of Original Oxide Thickness wear rate in pm/cycle ( X los) for anodic coatings of thickness electrolyte
15% H,SO,
2% H,C,O,
v
anodizing anodizing voltage, V temp, "C
10 15 15 15 20 60 80
20
20 10 20 30 20 30 20
40
10 5 pm 14.4 3.6 0.5 1.7 2.5 6.5
pm
12.8 9.8 0.4 6.7 11.9 7.8 11.8
60
SEALING TIME (min.)
Figure 17. The wear after 1500 cycles in the wear apparatus of anodized aluminum sealed in nickel acetate solution (NA), sodium silicate solution (SS),potassium dichromate solution (PD),and distilled water (DW)as a function of sealing time.
powers of the electrolyte for a longer period of time with the consequent development of a less dense coating, especially in the outer layers. The effect of sealing treatment in hot aqueous solutions on the rate of wear in the early stages is strikingly shown in Figures 4-8. Rapid wear occurs during the first 500-1000 wear cycles after sealing coatings formed in sulfuric acid in distilled water, nickel acetate solution, potassium dichromate solution, or sodium silicate solution. Figure 4 shows data for a similar coating that was not sealed. In this case the wear is essentially nil over a wear period of 2000 cycles. In many cases shown in Figures 4-8 this low wear rate again obtains after the outer part of the coating has been worn away. It is apparent that the external part of the sealed coating has a much lower wear resistance under abrasive conditions, but once this is worn away the wear rate achieves its value for the unsealed coating in many cases. Coatings sealed in sodium silicate
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 138-141
138
represent a significant exception. These coatings exhibited high wear rates even after sealing for only 1 min and the wear rate continued to increase, as shown in Figure 17, as the sealing time was increased. The reason for the high wear rate of the coatings sealed in sodium silicate was not explored, although the high pH of the sealing solution is suspect. It is noteworthy, as discussed below, that coatings sealed in sodium silicate solution exhibited the best wettability. Many commercial lithographic plates receive a silicate treatment of a proprietary nature. The probable reason for this treatment is dramatically shown in Figure 9. Anodized aluminum sealed in sodium silicate solution shows superior wettability under all sealing treatments. Coatings treated in sodium silicate did not exhibit reduced wettability (high contact angle with water) at short sealing times as occurs with anodized aluminum sealed in boiling water, nickel acetate solution, or potassium dichromate solution. Of course, commercial lithographic printing utilizes a “fountain solution” with optimum water wetting conditions. The data summarized in Table IV indicate that the rates of wear of polystyrene coatings are influenced by the anodizing and sealing treatment before the application of the polystyrene coating. Sealing increased the wear rate of anodic coatings formed in 15% sulfuric acid but decreased
the wear rate of anodic coatings formed in 5% H3P04and 2 % H2CZO4. The highest wear rate was obtained with substrates formed in 15% H2S04 and sealed in boilng water and the lowest wear rates were obtained with substrates anodized in 2 % HzC2O4 and sealed in boiling water. Acknowledgment
We are deeply grateful to the Photo Products Division of the du Pont Company for providing support for this work. The encouragement of Dr. C. H. Arrington and Dr. Peter Walker is especially appreciated. Registry No. H2S04,7664-93-9; H3P04,7664-38-2; HzC204, 144-62-7; H2Cr04,7738-94-5; Al, 7429-90-5; NA, 373-02-4; PD, 7778-50-9; SS, 1344-09-8; polystyrene, 9003-53-6; poly-n-butylmethacrylate, 9003-63-8; polybutadiene, 9003-17-2.
L i t e r a t u r e Cited Albert, M. P.:Combs, J. F. J. Electrochem. SOC. 1962, 109, 709-13. Cheever, G. D. J. Coat. Technol. 1978, 5 0 , 78-85. Hannah, R. W. Appl. Spectrosc. 1963, 17, 23-4. Harrick, N. J. Appl. Opt. 1971, 10, 2344-49. Noble, J. W.. 111; Leldheiser, H., Jr. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20,344-50. Spitzer, W. G.; Tanenbaum, M. J . Appl. Phys. 1961, 32. 744-45. Wendlandt. W.; Hecht, H. I n “Chemical Analysis”; Elving, P. J.; Kolthoff, I. M., Ed.; Wiley-Interscience: New York, 1966; Vol. 21, Chapter 2.
Receiued for review July 26, 1982 Accepted September 24, 1982
Effects of Temperature and Pressure on the Viscoelastic Behavior of a Polyurethane Elastomer David L. Questad’ Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania 16802
Kook D. Pae Department of Mechanics and Materials Science, Rutgers University, New Brunswick, New Jersey 08854
The longitudinal ultrasonic velocity, V c , at 5 MHz has been measured for a polyurethane elastomer at various temperatures and pressures. These results are compared with stress-strain measurements made over a range of temperatures and pressures. The elastomer goes through a glass transition if the temperature is lowered or if the pressure is raised. Although the elastic modulus changes markedly at the temperature-induced glass transition or at the pressure-induced glass transition, the bulk modulus changes only slightly and the change decreases as the pressure increases. Large plastic deformations in the pressure-induced glassy state are found to relax out with time, and the relaxation process is very similar to those encountered in creep measurements on polymers.
Introduction
Hydrostatic pressure can significantly change the mechanical properties of polymeric materials (Pae and Bhateja, 1975; Sauer, 1977). Usually an increase in pressure results in a higher modulus, yield stress, and fracture stress-similar results to those obtained by reducing the temperature. The strain at failure, however, may increase or decrease depending on the polymer (Holiday et al., 1964: Sauer et al., 1970). Hydrostatic pressure causes an increase in glass transition temperature, Tg (Bhateja and Pae, 1975), so for polymers with Tg’sbelow the test temperature a pressure-induced glass transition, Pg,can occur. The tre0196-4321/83/1222-0138$01.50/0
mendous increase in relaxation times at Pgresults in very large changes in mechanical properties similar to those which occur at Tg(Questad et al., 1980). Properties which are sensitive to shear undergo large changes a t the glass transition. For instance, the shear modulus, G, or the elastic modulus, E , change by about three orders of magnitude. By comparison, the isothermal bulk modulus changes very little in the vicinity of the glass transition. This is due to the fact that volume changes are small and nonlinear when pressure is raised through Pg. The empirical Tait equation has been used to eliminate the nonlinearity in pressure-volume data (Quach and Simha, 1971) making the transition more noticeable. Such 0 1983
American Chemical
Society
