- Email: [email protected]
Substrate effects on the mechanochemical degradation of thin polymeric coatings
Substrate Effects on the Mechanochemical Degradation of Thin Polymeric Coatings R. J. N A S H , D. M. JACOBS, AND R. F. SELIG Xerox Corporation, Webster, N e w York 14580
Received July 12, 1978; accepted N o v e m b e r 3, 1978 Ball-milling of a thin polymeric coating (85% poly(methyl methacrylate)/15% polystyrene) on 250-t~m-diameter steel shot not only c a u s e s macroscopic abrasion, but also c a u s e s m a c r o m o l e c u l a r mechanical degradation. Gel permeation c h r o m a t o g r a p h y s h o w s that the weight-average molecular weight o f the coating p o l y m e r d e c r e a s e s from an initial value of 200K to a limiting degraded value of 10K. The degradation can be analyzed as a first-order process, with the rate c o n s t a n t being an increasing function of p o l y m e r molecular weight. T h e rate c o n s t a n t is also a function of the state of the coating substrate, and a change from an untreated stainless-steel substrate to an air-oxidized form, halves the rate constant. F r o m optical microscopy, coating integrity, and gas adsorption measurem e n t s , it appears that the decrease in the rate constant reflects the e n h a n c e d coatability of the airoxidized substrate.
the effect of the substrate on the degradation of thin coatings.
INTRODUCTION
The mechanical degradation of polymers has been extensively studied and is well documented in review articles (1-5) and books (6-10). While most studies have involved polymer solutions, degradation can occur in solid polymers (11-14). In particular, it has been recognized that macromolecular degradation can occur during frictional wear, and there is an increasing interest in the molecular aspects of polymer wear (15-22). In early studies, polymer degradation was chiefly followed using viscometric molecular weight determinations: In recent studies there has been an increase in techniques such as electron spin resonance (23-26), for the observation of the free radicals generated during bond breakage, and gel permeation chromatography, GPC (13, 19, 20-27), for the determination of the changes in molecular weight distribution. In a previous paper (13), we detailed, via a GPC study, the effect of the initial molecular weight on the rate and mode of mechanochemical degradation of thin polystyrene coatings; the present work is a GPC study of
EXPERIMENTAL
As in our previous study (13), spherical stainless-steel shot (250-/~m diameter, 9.10 -z m z g-Z surface area; manufactured by the Nuclear Metals Division of the Whittaker Corporation) was used as the substrate. Acid etching, with 3 min immersion in 10% by volume HCI, was used as one method of substrate pretreatment. A second pretreatment was air oxidization in a Lindburg furnace equipped with a 4-in. id stationary heating barrel, and 1-kg samples of steel shot, held in a stainless-steel boat in the center of the furnace, were fired at several time/temperature combinations. The treated steel shot (and the original shot as a control) was coated with an 85% poly(methyl methacrylate)/15% polystyrene copolymer (216K weight-average, 72.6K number-average molecular weight). The shot and a solution of the polymer were vibrated and gently heated, so as to remove the solvent and leave a free-flowing mass of
366 0021-9797/79/090366-09502.00/0 Copyright © 1979by AcademicPress, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
COATING DEGRADATION 250
367
ured with a Micromeritics 2100 volumetric adsorption apparatus, was used to study the surface changes caused by the substrate pretreatments and b y the coating degradation experiments. The integrity of the degraded coatings was assayed by reacting the accessible substrate with a glacial acetic acid/hydrogen peroxide mixture in the presence of the iron reagent Tiron, with the absorbance o f the resulting blue solution being measured at 650 nm (39). A calibration for each substratetype was made by using the uncoated shot in proportionate amounts.
200
I ~ol50
7i0
I00
RESULTS
50
0.1
0
0.5
P/Po FIG. 1. Kr BET plots ( Vaasin cma STP/g)for various substrate treatments. Q polymer-coated; [] untreated substrate; ~ air oxidized at 800°C for 4 min; A acid etched for 3 min. coated shot. The shot was coated to a nominal 0.3% coating weight, a value which translates to a coating thickness of less than 0.5 ~ m (a precise thickness cannot be given because of the irregularities of the surface of the shot). Coating degradation was created via ballmilling, with a 500-ml glass jar containing 150 g of the coated shot and 230 g of Vain. diameter uncoated stainless-steel shot (added to accelerate the degradation process) being rolled for various timed intervals at 275 rpm. Following ball-milling, 15 g o f the degraded coated shot was added to 10 ml of tetrahydrofuran, and the resultant polymer solution, after filtration through a 0.2-/zm membrane, was analyzed using a Waters Associates Model 200 gel permeation chromatograph. Krypton gas adsorption at 77°K (38), meas-
Preliminary experiments showed that large changes in substrate morphology and chemistry could be readily obtained, and B E T plots (38) of the Kr gas adsorption measurements at 77°K (Fig. 1) were a convenient ] 0 HR
]
r
I I
2HR
8 HR
I
I I I-IR
4 HR
/\
17.5 HR
FIG. 2. Gel permeation chromatograms for the coating on the untreated substrate for the degradation time noted (in hours). The ticks on the abscissa of each chromatogram mark, from left to right, the elution points for molecular weights 104, 105,and 108. Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
368
NASH, JACOBS, AND SELIG
means for monitoring the changes. Subsequent degradation experiments showed that the various substrate pretreatments gave similar results, and, for brevity, therefore, only a representative air-oxidization result will be presented. The substrate to be discussed was achieved by air oxidization at 800°C for 4 rain, a procedure which increased the surface area of the steel shot from 9.10 -3 to 26.10 -3 m 2 g-~, and increased the Kr c-. parameter from 29 to 72. Figures 2 and 3 show the gel permeation chromatograms from the degraded coatings on the untreated and the air-oxidized substrates, respectively. Gas adsorption measurements are shown for the two cases in Figs. 4 and 5, and the coating integrity data are shown in Fig. 6. I
I
0 HR
I
I
I
I
|
I
]
HR
z5
~ zo N-~ -~ ~5 -~ ~ to:
5t ~. 3o kO
2~
~ z~, ~, ~ ~ zc 0
O-
I0 15 M I L L I N G T I M E (HOURS)
40
FIG. 4. Kr adsorption at 77°K. Plot of surface area a n d c-parameter as a function of milling-time for the coating on the untreated substrate.
DISCUSSION
2 HR
8 HR
4 HR
17.5 HR
FIG. 3. Gel p e r m e a t i o n c h r o m a t o g r a m s for the coating on the air-oxidized s u b s t r a t e for the degradation time noted (in hours). The ticks on the a b s c i s s a of each c h r o m a t o g r a m mark, from left to right, the elution points for m o l e c u l a r weights 103, 104, 108, and 108. Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
Even from a superficial examination of the GPC chromatograms, it is clear that the abraded coatings undergo well-defined, nonrandom molecular degradation to a limiting low-molecular weight species, and this behavior has been previously discussed in terms of a chain-entanglement (13, 29, 31, 40). Deconvolution of the chromatograms, using a DuPont Curve Resolver, shows that the degradation of the coating on the untreated substrate involves generation of a discrete intermediate molecular weight species (especially evident in the medium degradation samples) in addition to the lowmolecular weight species. However, as seen in our previous study (13), the latter species predominates, and the degradation process appears to follow a simultaneous generation of the two degradation species, rather than a serial progression from high to intermediate, to low molecular weight. The GPC data from the oxidized substrate can be analyzed
369
COATING DEGRADATION
present in the coating is an advantage for a kinetic analysis. For example, from the chromatograms in Figs. 2 and 3, it is clear that the degradation can be viewed as irreversible chain scissions, so that the decrease in the curve height of the initial highmolecular weight peak can be used to follow the loss of high-molecular weight species (the analysis, of course, must be limited to only the highest molecular weight species, since the GPC curve height for other species will reflect the loss by degradation and the gain by degradation of higher species). Thus, the rate of degradation, and its dependence on molecular weight, can be determined from plots of the relative chromatogram curve height at fixed low elution volumes (i.e., fixed high molecular weight species) as a function of time; Fig. 7 shows some representative data. From plots such as Fig. 7, the coating degradation can be expressed as a first-order
35i 3O ~n
E 25
w 20 u re
@ 15 0
I0
)0
ft3O I,re
~< 25 0
0
z 0
gzo I00 - -
I0 15 // MILLING TIME (HOURS)
40
F]6. 5. K r adsorption at 77°K. Plot of surface area and c-parameter as a function of milling-time for the coating on the air-oxidized substrate.
solely in terms of an initial species and a final species, but this apparent difference in the m o d e of degradation between the coatings on the untreated and oxidized substrates, probably only reflects the difference in the respective rates of degradation. For example, since the coating on the oxidized substrate is more resistant to degradation, the medium degree of degradation associated with the maximum generation of an intermediate molecular weight species occurs between the 8 and 17.5 hr degradation times, and is hence not visible in the chromatograms taken at these times. While the breadth of the initial molecular weight distribution hinders the deconvolution of the degradation chromatograms, the fact that many molecular weight species are
[]
8(
--
~
r
§~_o ra []
8
o 40
2C O
I
0
5
O
$
L
I0
15
o 40
MILLING TIME (HOURS)
FIG. 6. Coating integrity (wet-chemical a s s a y using Tiron reagent) as a function o f milling-time. G untreated substrate; [] air-oxidized substrate. Journal o f Colloid and Interface Science, Vol. 70, N o . 2, J u n e 15, 1979
370
NASH, JACOBS, AND SELIG
~o( N, ~L
\
';:.2
\
.
\ \
_
_0.6I ~ ~ 0
5 MILLING TIME (HOURS)
i.e., the volume element of the coating within which the impact force will cause degradation. For the present experiments, the degradation zone size must be comparable to that of the polymer molecules, since the degradation products reflect simultaneous, multiple bond scissions. If the degradation is assumed to occur within a symmetrical area of side b, and extend through the polymer coating, then the probability, P , that a polymer molecule of size I will be at least partially within the degradation region, will be,
(l + b l~ P = k--~0 /
I0
Flo. 7. Plot of In (relative curve height): milling-time for two elution counts on the GPC chromatograms from the coating on the air-oxidized substrate. (3 elution count 24, MW = 694K; [] elution count 26.5, MW = 163K.
where I~ is the surface area of the coating. Now the dimension of a polymer molecule in the solid state is predicted to be the same as the unperturbed dimension measured in solution (41), and experimental studies (4245) are consistent with this view. Thus, if n is the number o f milling impacts per unit time which have sufficient energy to cause bond scission, then,
dNt -- dt
process, i.e., Nt
In - -
No
kt
-
[1]
or, dNt
[3]
nPNt,
[4]
and
Nt=Noexp[--t~-t ((o.64.MUZ)+b) z] -
kNt
dt
L/o
[2]
\
[5]
I
where No and N t are the number of polymer molecules in the initial coating and after degradation time t, respectively, and the rate constant k is an increasing function of molecular weight (since the slope of the plots in Fig. 7 increases with decreasing elution count). Clearly, the rate constant must be a function of the energy of the ball-milling process (fixed in the present experiments), and a function of the probability that any particular polymer molecule within the coating will experience a force sufficient to cause bond breakage. The latter probability will be a function of the size of the polymer molecule and the size of the degradation zone, Journal of Colloid and Interface Science,
o51
-~.-o-~o °4 ~lr,~ _ =
~
~
oz, 400
I
I
600
800
( 1000
1200
(MOLECULAR WEIGHT)V2
FIG. 8. GPC data analyzed according to Eq. [5] (see text). (3 untreated substrate; [] air-oxidized substrate.
Vol 70,N . 2,oJune . 15,1979
COATING DEGRADATION 1
I
I
i
i
i
I
b.
O.
C.
FIG. 9. GPC chromatograms for (a) entire coating; (b) loose polymer, removable by water washing; (c) coating not removed by water washing. The ticks on the abscissa of each chromatogram mark, from left to right, the elution points for molecular weights I(P, 104, 105, and 10e.
where the factor 0.64. M 1/2 relates the polymer molecular weight, M, to the unperturbed dimension in A (46). Figure 8 shows the high-molecular weight data plotted according to Eq. [5], and the respective expressions are, with the degradation time in hours,
371
produced by the substrate pretreatment reflects the influence of the substrate on the coating process. For example, Fig. 6 shows that the integrity of the coating on the oxidized substrate decreases slowly from an initial high value, while the coating on the untreated substrate has a lesser initial value, and decreases rapidly with roll-milling. Also, low-power optical microscopy shows that small fragments of the coating on the untreated substrate transfer between the coated shot even at short milling times, and after 4 hr of milling the coated shot surface is 10% covered with 50-/xm platelets; the coating on the oxidized substrate, by contrast, retains its initial smooth appearance throughout the early stages of milling and shows very few transferred platelets after 4 hr of milling. [While platelet formation has been theoretically discussed (50) in terms of polymer mechani-
Nt = No e x p ( - 1.49.10 -r x t(0.64" M 1/2 + 600)2),
[6]
for the untreated substrate, and, N t = No e x p ( - 7 . 0 6 . 1 0
-s
X t(0.64" M 1/2 + 670)2),
[7]
for the oxidized substrate. Thus the analysis, given the above assumptions, shows that the two cases have similar geometric dependencies, and differ only in the first term of the rate constant. Since the ball-milling conditions were fixed, it appears that the oxidized substrate halved the number of effective impacts per unit time. Now, the mechanochemical degradation of ball-milled poly(methyl methacrylate) powder has been shown to be increased by the addition of metal powders (11), and frictional wear (of both the polymeric and metallic member of the friction couple) has also been shown to be enhanced by metal contact (11, 15, 16, 20, 21, 47-49). For the present study, however, where the polymer/ substrate interface is remote from the contact zone, it is more likely that the observed decrease in the degradation rate constant
>¢
0
O. I
0.2
03
P/Po
FIG. 10. Kr BET plots (V,ds in cm a STP/g) for the coated untreated substrate, for the milling-times noted (in hours). Journal of Colloid and Interface Science, Vol. 70, N o . 2, J u n e 15, 1979
372
N A S H , JACOBS, A N D S E L I G
cal properties and the dynamics of the contact events, the present experiments show that the substrate can also be a significant factor.] Since platelet transfer will increase the amount of the coating within the impact zone, the difference in platelet generation rate from the coatings on the untreated and oxidized substrates is a possible cause of the difference in the degradation rate constants for these substrates. Thus, with this view, the oxidized substrate delays coating fragmentation and macromolecular degradation, because it promotes the formation of a complete, adherent coating (presumably through the substrate topography and surface chemistry). The net effect is that the degradation of the coating on the oxidized substrate has an apparent induction period, i.e., the time required to decrease the coating integrity and hence produce transferplatelets. (After extended milling, the coating on both types of substrates becomes identically powdered.)
200
~ 150 g
i°,oo 50
0.1
PIP,,
0.2
0.3
Flo. 11. K r BET plots (Vad , in cm a STP/g) for the coated air-oxidized substrate, for the milling-times noted (in hours). Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
ao
~ 60
i
o o
z 40
20 / 1
L
-
J---
-
~
--[
o.z 0.4 0.6 o.e ~.o FRACTION OF SURFACE AREAAS SUBSTRATE
FIG, 12. Kr c-parameter calculated for a surface which is composed of the coating polymer (low cparameter) and the air-oxidized substrate (high cparameter).
As a test of the above hypothesis, the loose surface particles were separated from a partially abraded coating by water washing, and indeed the GPC chromatogram (Fig. 9b) for the free particles shows an enhanced population of highly degraded molecules [compare Fig. 9b with the chromatogram for the entire coating (Fig. 9a)]. However, it is clear from the GPC chromatogram of the washed coating (Fig. 9c) that not all of the degradation is associated with the abraded particles, and certainly degradation (e.g., as seen with the oxidized substrate) can occur without large-scale platelet production. The gas adsorption measurements also give information on the abrasion process, and the BET plots (Figs. 10 and 11) for the abraded coatings clearly show the millinginduced changes in surface area and chemistry. Since the surface energy of the polymer coating is less than that of the substrate (especially the oxidized substrate), it might be thought that the Kr c-parameter would give a direct indication of the change in coating integrity. However, gas adsorption data from widely heterogeneous surfaces require
373
COATING DEGRADATION I001 I
I
REFERENCES
I
I
O "0
so )
O
E O
~
so
8 8 40 I
I
__
I
5 tO 15 MILLING TIME (HOURS)
20
FIG. 13. Coating integrity as a function of millingtime, calculated from the Kr adsorption data (Figs. 4 and 5) and the relationship shown in Figure 12. Q untreated substrate; [] air-oxidized substrate.
careful analysis (51, 52); in particular, the composite c-parameter will be most influenced by the low-energy surface. For example, Fig. 12 shows the predicted c-parameter for a range of polymer/oxidized substrate surface fractions, and this plot, in conjunction with the surface area values for the abraded samples and the uncoated substrate, provides a coating integrity plot (Fig. 13) which agrees well with the wet-chemistry plot (Fig. 6). Similarly, the composite c-parameter can be calculated from the data in Fig. 6 and the surface area data, and the calculation shows that the experimental c-parameter:milling time relationship passes through a maximum because of the increase in the surface area of the abraded coating. In conclusion, while the present work has shown the effect of molecular weight and substrate condition on the mechanochemical degradation of a polymeric coating, it must be noted that the results are specific to the coating considered; coatings of tougher or more compliant polymers might be expected to be more resistant to degradation.
1. Casale, A., Whitlock, L. E., Porter, R. S., and Johnson, J. F., Polym. Prepr. Amer. Chem. Soc., Div. Polym. Chem. 12(2), 496 (1971). 2. Richardson, M. O. W., in "Advances in Polymer Friction and Wear" (L. H. Lee, Ed.), p. 787. Plenum Press, New York, 1974. 3. Regel, V. R., Leksovskii, A. M., Slutsker, A. I., and Tamuzs, V., Mekh. Polim. (4), 597 (1972). 4. Goto, K., Shikizai Kyokaishi 45(2), 723 (1972). 5. Casale, A.,J. Appl. Polym. Sci. 19(5), 1461 (1975). 6. Jellinek, H. H. G., "Degradation of Vinyl Polymers." Academic Press, New York, 1955. 7. Grassie, N., "Chemistry of High Polymer Degradation Processes." Interscience Publishers, New York, 1956. 8. Baramboim, N. K., "Mechanochemistry of Polymers." Maclaren and Sons, London, 1964. 9. Geuskens, G., "Degradation and Stabilization of Polymers." Wiley, New York, 1975. 10. Jellinek, H. H. G., "Aspects of Degradation and Stabilization of Polymers." Elsevier, New York, 1977. 11. Dmitrieva, T. V., and Gorokhovskii, G. A., Sin. Fiz. Khim. Polim. (9), 113 (1971). 12. Kaneniwa, N., and Ikekawa, A., Chem. Pharm. Bull. 20(7), 1536 (1972). 13. Nash, R. J., and Jacobs, D. M., Faraday Spec. Discuss. Chem. Soc. (2), 210 (1972). 14. Shunkevich, A. A., Novak, I. I., and Korsukov, V. E., Mekh. Polim. (3), 541 (1975). 15. Gorokhovskii, G. A., Chernenko, P. A., Dmitrieva, T. V., and Potemkina, O. A., Probl. Treniya Iznashivaniya (1), 84 (1971). 16. Gorokhovskii, G. A., Cherenko, P. A., Vonsyatskii, V. A., and Popov, I. A., Primen. Polim. Kach. Antifrikts. Mater., Respub. Nauch.Tekh. Konf [Mater] 189 (1970). 17. Kompaniets, V. A., Kostetskaya, N. V., and Natanson, M. E., Tekhnol. Organ. Proizvod. (10), 68 (1973). 18. Mustafaev, V. A., Balahanova, T. S., Muradov, V. V., and Mel'nikova, M. A., Konf. Kunstst. Masch.-Fahrzeugbau, Vortr. 3rd., 167 (1972). 19. Richardson, M. O. W., and Pascoe, M. W., in "Advances in Polymer Friction and Wear" (L. H. Lee, Ed.), p. 585. Plenum Press, New York, 1974. 20. Gorokhovskii, G. A., Klyuev, E. A., Martynenko, D. F., Dmitrieva, T. V., and Dudnik, M. I., Probl. Treniya Iznashivaniya g, 128 (1975). 21. Dmitrieva, T. V., Probl. Treniya lznashivaniya 7, 127 (1975). 22. Prikhad'ko, O. G., Tromfimovich, A. N., and Burya, A. I., Probl. Treniya lznashivaniya 8, 133 (1975). Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
374
NASH, JACOBS, AND SELIG
23. Zakrevskii, V. A., and Korsukov, V. E., Vysokotool. Soedin., Ser. A 14(4), 955 (1972). 24. Sakaguchi, M., Kodama, S., Edlund, O., and Sohma, J., J. Polym. Lett. Ed. 12(11), 609 1974. 25. Sohma, J., and Sakaguchi, M., in "Degradation and Stabilization of Polymers" (G. Geuskens, Ed.), p. 157. Wiley, New York, 1975. 26. Sakaguchi, M., and Sohma, J., J. Polym. Sei. Polym. Phys. Ed. 13(6), 1233 (1975). 27. Breitenbach, J. W., Rigler, J. K., and Wolf, B. A., Makromol. Chem. 164, 353 (1963). 28. Green, C. D., Hershey, H. C., Patterson, G. K., and Zakin, J. L., Trans. Soc. Rheol. 10, 489 (1966). 29. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., J. Polym. Sci. Part C 16, 1 (1967). 30. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., J. Polym. Sci. Part C 16, 13 (1%7). 31. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., Polymer 8, 87 (1967). 32. Abernathy, F. H., and Paterson, R. W., J. Fluid Mech. 43, 689 (1970). 33. Morimoto, K., and Suzuki, S. J.,Appl. Polym. Sei. 16(11), 2947 (1972). 34. Fukutomi, T., Tsukada, M., Kakurai, T., and Noguchi, T., Polym. J. (Japan) 3, 717 (1972). 35. AbdeI-Alim, A. H., and Hamielec, A. E.,J. Appl. Polym. Sci. 17, 3769 (1973). 36. Glynn, P. A. R., and Van der Hoff, B. M. E., J. Macromol. Sci., Chem. A7(8), 1695 (1973). 37. Chung, K. H., Kim, K. J., and Kim, S. D., Tachan Hwahak Hoechi 19(5), 386 (1975).
Journal of Colloidand InterfaceScience. Vol.70, No. 2. June 15. 1979
38. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity," p. 84, Academic Press, New York, 1967. 39. Pelognin, L., and Rader, C., Mater. Perform. 13(8), 24 (1974). 40. Abbas, K. B., and Porter, R. S., J. Polym. Sci., Polym. Chem, Ed. 14(3), 553 (1976). 41. Flory, P. J., J. Chem. Phys. 17, 303 (1949). 42. Flory, P. J., Pure Appl. Chem., Macromol. Chem. 8, 1 (1972). 43. Krigbaum, W. R., and Godwin, R. W., J. Chem. Phys. 43, 4523 (1%5). 44. Aiberino, L. M., and Graessley, W. W., J. Phys. Chem. 72, 4229 (1968). 45. Hoffman, M., Makromol. Chem. 144, 309 (1971). 46. Brandrup, J., and Immergut, E. H., "Polymer Handbook," 2nd ed., p. IV-38. Wiley, New York, 1975. 47. Gorokhovskii, G. A., Soviet Mater. Sci. 1(5), 365 (1%5). 48, Gorokhovskii, G. A., and Agulov, I. I., Soviet Mater. Sci. 2(1), 78 (1966). 49. Higham, P. A., Bethune, B., and Stott, F. H., Wear 46, 335 (1978). 50. Ahuja, S. K., J. Colloid Interface Sci. 57, 438 (1976). 51. Joy, A. S., in "Proc. 2nd. Intl. Conf. of Surface Activity" (J. H. Schulman, Ed.), Vol. II, p. 54. Butterworths, London, 1957. 52. Dormant, L. M., and Adamson, A. W.,J. Colloid Interface Sei. 38, 285 (1972).
Received July 12, 1978; accepted N o v e m b e r 3, 1978 Ball-milling of a thin polymeric coating (85% poly(methyl methacrylate)/15% polystyrene) on 250-t~m-diameter steel shot not only c a u s e s macroscopic abrasion, but also c a u s e s m a c r o m o l e c u l a r mechanical degradation. Gel permeation c h r o m a t o g r a p h y s h o w s that the weight-average molecular weight o f the coating p o l y m e r d e c r e a s e s from an initial value of 200K to a limiting degraded value of 10K. The degradation can be analyzed as a first-order process, with the rate c o n s t a n t being an increasing function of p o l y m e r molecular weight. T h e rate c o n s t a n t is also a function of the state of the coating substrate, and a change from an untreated stainless-steel substrate to an air-oxidized form, halves the rate constant. F r o m optical microscopy, coating integrity, and gas adsorption measurem e n t s , it appears that the decrease in the rate constant reflects the e n h a n c e d coatability of the airoxidized substrate.
the effect of the substrate on the degradation of thin coatings.
INTRODUCTION
The mechanical degradation of polymers has been extensively studied and is well documented in review articles (1-5) and books (6-10). While most studies have involved polymer solutions, degradation can occur in solid polymers (11-14). In particular, it has been recognized that macromolecular degradation can occur during frictional wear, and there is an increasing interest in the molecular aspects of polymer wear (15-22). In early studies, polymer degradation was chiefly followed using viscometric molecular weight determinations: In recent studies there has been an increase in techniques such as electron spin resonance (23-26), for the observation of the free radicals generated during bond breakage, and gel permeation chromatography, GPC (13, 19, 20-27), for the determination of the changes in molecular weight distribution. In a previous paper (13), we detailed, via a GPC study, the effect of the initial molecular weight on the rate and mode of mechanochemical degradation of thin polystyrene coatings; the present work is a GPC study of
EXPERIMENTAL
As in our previous study (13), spherical stainless-steel shot (250-/~m diameter, 9.10 -z m z g-Z surface area; manufactured by the Nuclear Metals Division of the Whittaker Corporation) was used as the substrate. Acid etching, with 3 min immersion in 10% by volume HCI, was used as one method of substrate pretreatment. A second pretreatment was air oxidization in a Lindburg furnace equipped with a 4-in. id stationary heating barrel, and 1-kg samples of steel shot, held in a stainless-steel boat in the center of the furnace, were fired at several time/temperature combinations. The treated steel shot (and the original shot as a control) was coated with an 85% poly(methyl methacrylate)/15% polystyrene copolymer (216K weight-average, 72.6K number-average molecular weight). The shot and a solution of the polymer were vibrated and gently heated, so as to remove the solvent and leave a free-flowing mass of
366 0021-9797/79/090366-09502.00/0 Copyright © 1979by AcademicPress, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
COATING DEGRADATION 250
367
ured with a Micromeritics 2100 volumetric adsorption apparatus, was used to study the surface changes caused by the substrate pretreatments and b y the coating degradation experiments. The integrity of the degraded coatings was assayed by reacting the accessible substrate with a glacial acetic acid/hydrogen peroxide mixture in the presence of the iron reagent Tiron, with the absorbance o f the resulting blue solution being measured at 650 nm (39). A calibration for each substratetype was made by using the uncoated shot in proportionate amounts.
200
I ~ol50
7i0
I00
RESULTS
50
0.1
0
0.5
P/Po FIG. 1. Kr BET plots ( Vaasin cma STP/g)for various substrate treatments. Q polymer-coated; [] untreated substrate; ~ air oxidized at 800°C for 4 min; A acid etched for 3 min. coated shot. The shot was coated to a nominal 0.3% coating weight, a value which translates to a coating thickness of less than 0.5 ~ m (a precise thickness cannot be given because of the irregularities of the surface of the shot). Coating degradation was created via ballmilling, with a 500-ml glass jar containing 150 g of the coated shot and 230 g of Vain. diameter uncoated stainless-steel shot (added to accelerate the degradation process) being rolled for various timed intervals at 275 rpm. Following ball-milling, 15 g o f the degraded coated shot was added to 10 ml of tetrahydrofuran, and the resultant polymer solution, after filtration through a 0.2-/zm membrane, was analyzed using a Waters Associates Model 200 gel permeation chromatograph. Krypton gas adsorption at 77°K (38), meas-
Preliminary experiments showed that large changes in substrate morphology and chemistry could be readily obtained, and B E T plots (38) of the Kr gas adsorption measurements at 77°K (Fig. 1) were a convenient ] 0 HR
]
r
I I
2HR
8 HR
I
I I I-IR
4 HR
/\
17.5 HR
FIG. 2. Gel permeation chromatograms for the coating on the untreated substrate for the degradation time noted (in hours). The ticks on the abscissa of each chromatogram mark, from left to right, the elution points for molecular weights 104, 105,and 108. Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
368
NASH, JACOBS, AND SELIG
means for monitoring the changes. Subsequent degradation experiments showed that the various substrate pretreatments gave similar results, and, for brevity, therefore, only a representative air-oxidization result will be presented. The substrate to be discussed was achieved by air oxidization at 800°C for 4 rain, a procedure which increased the surface area of the steel shot from 9.10 -3 to 26.10 -3 m 2 g-~, and increased the Kr c-. parameter from 29 to 72. Figures 2 and 3 show the gel permeation chromatograms from the degraded coatings on the untreated and the air-oxidized substrates, respectively. Gas adsorption measurements are shown for the two cases in Figs. 4 and 5, and the coating integrity data are shown in Fig. 6. I
I
0 HR
I
I
I
I
|
I
]
HR
z5
~ zo N-~ -~ ~5 -~ ~ to:
5t ~. 3o kO
2~
~ z~, ~, ~ ~ zc 0
O-
I0 15 M I L L I N G T I M E (HOURS)
40
FIG. 4. Kr adsorption at 77°K. Plot of surface area a n d c-parameter as a function of milling-time for the coating on the untreated substrate.
DISCUSSION
2 HR
8 HR
4 HR
17.5 HR
FIG. 3. Gel p e r m e a t i o n c h r o m a t o g r a m s for the coating on the air-oxidized s u b s t r a t e for the degradation time noted (in hours). The ticks on the a b s c i s s a of each c h r o m a t o g r a m mark, from left to right, the elution points for m o l e c u l a r weights 103, 104, 108, and 108. Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
Even from a superficial examination of the GPC chromatograms, it is clear that the abraded coatings undergo well-defined, nonrandom molecular degradation to a limiting low-molecular weight species, and this behavior has been previously discussed in terms of a chain-entanglement (13, 29, 31, 40). Deconvolution of the chromatograms, using a DuPont Curve Resolver, shows that the degradation of the coating on the untreated substrate involves generation of a discrete intermediate molecular weight species (especially evident in the medium degradation samples) in addition to the lowmolecular weight species. However, as seen in our previous study (13), the latter species predominates, and the degradation process appears to follow a simultaneous generation of the two degradation species, rather than a serial progression from high to intermediate, to low molecular weight. The GPC data from the oxidized substrate can be analyzed
369
COATING DEGRADATION
present in the coating is an advantage for a kinetic analysis. For example, from the chromatograms in Figs. 2 and 3, it is clear that the degradation can be viewed as irreversible chain scissions, so that the decrease in the curve height of the initial highmolecular weight peak can be used to follow the loss of high-molecular weight species (the analysis, of course, must be limited to only the highest molecular weight species, since the GPC curve height for other species will reflect the loss by degradation and the gain by degradation of higher species). Thus, the rate of degradation, and its dependence on molecular weight, can be determined from plots of the relative chromatogram curve height at fixed low elution volumes (i.e., fixed high molecular weight species) as a function of time; Fig. 7 shows some representative data. From plots such as Fig. 7, the coating degradation can be expressed as a first-order
35i 3O ~n
E 25
w 20 u re
@ 15 0
I0
)0
ft3O I,re
~< 25 0
0
z 0
gzo I00 - -
I0 15 // MILLING TIME (HOURS)
40
F]6. 5. K r adsorption at 77°K. Plot of surface area and c-parameter as a function of milling-time for the coating on the air-oxidized substrate.
solely in terms of an initial species and a final species, but this apparent difference in the m o d e of degradation between the coatings on the untreated and oxidized substrates, probably only reflects the difference in the respective rates of degradation. For example, since the coating on the oxidized substrate is more resistant to degradation, the medium degree of degradation associated with the maximum generation of an intermediate molecular weight species occurs between the 8 and 17.5 hr degradation times, and is hence not visible in the chromatograms taken at these times. While the breadth of the initial molecular weight distribution hinders the deconvolution of the degradation chromatograms, the fact that many molecular weight species are
[]
8(
--
~
r
§~_o ra []
8
o 40
2C O
I
0
5
O
$
L
I0
15
o 40
MILLING TIME (HOURS)
FIG. 6. Coating integrity (wet-chemical a s s a y using Tiron reagent) as a function o f milling-time. G untreated substrate; [] air-oxidized substrate. Journal o f Colloid and Interface Science, Vol. 70, N o . 2, J u n e 15, 1979
370
NASH, JACOBS, AND SELIG
~o( N, ~L
\
';:.2
\
.
\ \
_
_0.6I ~ ~ 0
5 MILLING TIME (HOURS)
i.e., the volume element of the coating within which the impact force will cause degradation. For the present experiments, the degradation zone size must be comparable to that of the polymer molecules, since the degradation products reflect simultaneous, multiple bond scissions. If the degradation is assumed to occur within a symmetrical area of side b, and extend through the polymer coating, then the probability, P , that a polymer molecule of size I will be at least partially within the degradation region, will be,
(l + b l~ P = k--~0 /
I0
Flo. 7. Plot of In (relative curve height): milling-time for two elution counts on the GPC chromatograms from the coating on the air-oxidized substrate. (3 elution count 24, MW = 694K; [] elution count 26.5, MW = 163K.
where I~ is the surface area of the coating. Now the dimension of a polymer molecule in the solid state is predicted to be the same as the unperturbed dimension measured in solution (41), and experimental studies (4245) are consistent with this view. Thus, if n is the number o f milling impacts per unit time which have sufficient energy to cause bond scission, then,
dNt -- dt
process, i.e., Nt
In - -
No
kt
-
[1]
or, dNt
[3]
nPNt,
[4]
and
Nt=Noexp[--t~-t ((o.64.MUZ)+b) z] -
kNt
dt
L/o
[2]
\
[5]
I
where No and N t are the number of polymer molecules in the initial coating and after degradation time t, respectively, and the rate constant k is an increasing function of molecular weight (since the slope of the plots in Fig. 7 increases with decreasing elution count). Clearly, the rate constant must be a function of the energy of the ball-milling process (fixed in the present experiments), and a function of the probability that any particular polymer molecule within the coating will experience a force sufficient to cause bond breakage. The latter probability will be a function of the size of the polymer molecule and the size of the degradation zone, Journal of Colloid and Interface Science,
o51
-~.-o-~o °4 ~lr,~ _ =
~
~
oz, 400
I
I
600
800
( 1000
1200
(MOLECULAR WEIGHT)V2
FIG. 8. GPC data analyzed according to Eq. [5] (see text). (3 untreated substrate; [] air-oxidized substrate.
Vol 70,N . 2,oJune . 15,1979
COATING DEGRADATION 1
I
I
i
i
i
I
b.
O.
C.
FIG. 9. GPC chromatograms for (a) entire coating; (b) loose polymer, removable by water washing; (c) coating not removed by water washing. The ticks on the abscissa of each chromatogram mark, from left to right, the elution points for molecular weights I(P, 104, 105, and 10e.
where the factor 0.64. M 1/2 relates the polymer molecular weight, M, to the unperturbed dimension in A (46). Figure 8 shows the high-molecular weight data plotted according to Eq. [5], and the respective expressions are, with the degradation time in hours,
371
produced by the substrate pretreatment reflects the influence of the substrate on the coating process. For example, Fig. 6 shows that the integrity of the coating on the oxidized substrate decreases slowly from an initial high value, while the coating on the untreated substrate has a lesser initial value, and decreases rapidly with roll-milling. Also, low-power optical microscopy shows that small fragments of the coating on the untreated substrate transfer between the coated shot even at short milling times, and after 4 hr of milling the coated shot surface is 10% covered with 50-/xm platelets; the coating on the oxidized substrate, by contrast, retains its initial smooth appearance throughout the early stages of milling and shows very few transferred platelets after 4 hr of milling. [While platelet formation has been theoretically discussed (50) in terms of polymer mechani-
Nt = No e x p ( - 1.49.10 -r x t(0.64" M 1/2 + 600)2),
[6]
for the untreated substrate, and, N t = No e x p ( - 7 . 0 6 . 1 0
-s
X t(0.64" M 1/2 + 670)2),
[7]
for the oxidized substrate. Thus the analysis, given the above assumptions, shows that the two cases have similar geometric dependencies, and differ only in the first term of the rate constant. Since the ball-milling conditions were fixed, it appears that the oxidized substrate halved the number of effective impacts per unit time. Now, the mechanochemical degradation of ball-milled poly(methyl methacrylate) powder has been shown to be increased by the addition of metal powders (11), and frictional wear (of both the polymeric and metallic member of the friction couple) has also been shown to be enhanced by metal contact (11, 15, 16, 20, 21, 47-49). For the present study, however, where the polymer/ substrate interface is remote from the contact zone, it is more likely that the observed decrease in the degradation rate constant
>¢
0
O. I
0.2
03
P/Po
FIG. 10. Kr BET plots (V,ds in cm a STP/g) for the coated untreated substrate, for the milling-times noted (in hours). Journal of Colloid and Interface Science, Vol. 70, N o . 2, J u n e 15, 1979
372
N A S H , JACOBS, A N D S E L I G
cal properties and the dynamics of the contact events, the present experiments show that the substrate can also be a significant factor.] Since platelet transfer will increase the amount of the coating within the impact zone, the difference in platelet generation rate from the coatings on the untreated and oxidized substrates is a possible cause of the difference in the degradation rate constants for these substrates. Thus, with this view, the oxidized substrate delays coating fragmentation and macromolecular degradation, because it promotes the formation of a complete, adherent coating (presumably through the substrate topography and surface chemistry). The net effect is that the degradation of the coating on the oxidized substrate has an apparent induction period, i.e., the time required to decrease the coating integrity and hence produce transferplatelets. (After extended milling, the coating on both types of substrates becomes identically powdered.)
200
~ 150 g
i°,oo 50
0.1
PIP,,
0.2
0.3
Flo. 11. K r BET plots (Vad , in cm a STP/g) for the coated air-oxidized substrate, for the milling-times noted (in hours). Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
ao
~ 60
i
o o
z 40
20 / 1
L
-
J---
-
~
--[
o.z 0.4 0.6 o.e ~.o FRACTION OF SURFACE AREAAS SUBSTRATE
FIG, 12. Kr c-parameter calculated for a surface which is composed of the coating polymer (low cparameter) and the air-oxidized substrate (high cparameter).
As a test of the above hypothesis, the loose surface particles were separated from a partially abraded coating by water washing, and indeed the GPC chromatogram (Fig. 9b) for the free particles shows an enhanced population of highly degraded molecules [compare Fig. 9b with the chromatogram for the entire coating (Fig. 9a)]. However, it is clear from the GPC chromatogram of the washed coating (Fig. 9c) that not all of the degradation is associated with the abraded particles, and certainly degradation (e.g., as seen with the oxidized substrate) can occur without large-scale platelet production. The gas adsorption measurements also give information on the abrasion process, and the BET plots (Figs. 10 and 11) for the abraded coatings clearly show the millinginduced changes in surface area and chemistry. Since the surface energy of the polymer coating is less than that of the substrate (especially the oxidized substrate), it might be thought that the Kr c-parameter would give a direct indication of the change in coating integrity. However, gas adsorption data from widely heterogeneous surfaces require
373
COATING DEGRADATION I001 I
I
REFERENCES
I
I
O "0
so )
O
E O
~
so
8 8 40 I
I
__
I
5 tO 15 MILLING TIME (HOURS)
20
FIG. 13. Coating integrity as a function of millingtime, calculated from the Kr adsorption data (Figs. 4 and 5) and the relationship shown in Figure 12. Q untreated substrate; [] air-oxidized substrate.
careful analysis (51, 52); in particular, the composite c-parameter will be most influenced by the low-energy surface. For example, Fig. 12 shows the predicted c-parameter for a range of polymer/oxidized substrate surface fractions, and this plot, in conjunction with the surface area values for the abraded samples and the uncoated substrate, provides a coating integrity plot (Fig. 13) which agrees well with the wet-chemistry plot (Fig. 6). Similarly, the composite c-parameter can be calculated from the data in Fig. 6 and the surface area data, and the calculation shows that the experimental c-parameter:milling time relationship passes through a maximum because of the increase in the surface area of the abraded coating. In conclusion, while the present work has shown the effect of molecular weight and substrate condition on the mechanochemical degradation of a polymeric coating, it must be noted that the results are specific to the coating considered; coatings of tougher or more compliant polymers might be expected to be more resistant to degradation.
1. Casale, A., Whitlock, L. E., Porter, R. S., and Johnson, J. F., Polym. Prepr. Amer. Chem. Soc., Div. Polym. Chem. 12(2), 496 (1971). 2. Richardson, M. O. W., in "Advances in Polymer Friction and Wear" (L. H. Lee, Ed.), p. 787. Plenum Press, New York, 1974. 3. Regel, V. R., Leksovskii, A. M., Slutsker, A. I., and Tamuzs, V., Mekh. Polim. (4), 597 (1972). 4. Goto, K., Shikizai Kyokaishi 45(2), 723 (1972). 5. Casale, A.,J. Appl. Polym. Sci. 19(5), 1461 (1975). 6. Jellinek, H. H. G., "Degradation of Vinyl Polymers." Academic Press, New York, 1955. 7. Grassie, N., "Chemistry of High Polymer Degradation Processes." Interscience Publishers, New York, 1956. 8. Baramboim, N. K., "Mechanochemistry of Polymers." Maclaren and Sons, London, 1964. 9. Geuskens, G., "Degradation and Stabilization of Polymers." Wiley, New York, 1975. 10. Jellinek, H. H. G., "Aspects of Degradation and Stabilization of Polymers." Elsevier, New York, 1977. 11. Dmitrieva, T. V., and Gorokhovskii, G. A., Sin. Fiz. Khim. Polim. (9), 113 (1971). 12. Kaneniwa, N., and Ikekawa, A., Chem. Pharm. Bull. 20(7), 1536 (1972). 13. Nash, R. J., and Jacobs, D. M., Faraday Spec. Discuss. Chem. Soc. (2), 210 (1972). 14. Shunkevich, A. A., Novak, I. I., and Korsukov, V. E., Mekh. Polim. (3), 541 (1975). 15. Gorokhovskii, G. A., Chernenko, P. A., Dmitrieva, T. V., and Potemkina, O. A., Probl. Treniya Iznashivaniya (1), 84 (1971). 16. Gorokhovskii, G. A., Cherenko, P. A., Vonsyatskii, V. A., and Popov, I. A., Primen. Polim. Kach. Antifrikts. Mater., Respub. Nauch.Tekh. Konf [Mater] 189 (1970). 17. Kompaniets, V. A., Kostetskaya, N. V., and Natanson, M. E., Tekhnol. Organ. Proizvod. (10), 68 (1973). 18. Mustafaev, V. A., Balahanova, T. S., Muradov, V. V., and Mel'nikova, M. A., Konf. Kunstst. Masch.-Fahrzeugbau, Vortr. 3rd., 167 (1972). 19. Richardson, M. O. W., and Pascoe, M. W., in "Advances in Polymer Friction and Wear" (L. H. Lee, Ed.), p. 585. Plenum Press, New York, 1974. 20. Gorokhovskii, G. A., Klyuev, E. A., Martynenko, D. F., Dmitrieva, T. V., and Dudnik, M. I., Probl. Treniya Iznashivaniya g, 128 (1975). 21. Dmitrieva, T. V., Probl. Treniya lznashivaniya 7, 127 (1975). 22. Prikhad'ko, O. G., Tromfimovich, A. N., and Burya, A. I., Probl. Treniya lznashivaniya 8, 133 (1975). Journal of Colloid and Interface Science, Vol. 70, No. 2, June 15, 1979
374
NASH, JACOBS, AND SELIG
23. Zakrevskii, V. A., and Korsukov, V. E., Vysokotool. Soedin., Ser. A 14(4), 955 (1972). 24. Sakaguchi, M., Kodama, S., Edlund, O., and Sohma, J., J. Polym. Lett. Ed. 12(11), 609 1974. 25. Sohma, J., and Sakaguchi, M., in "Degradation and Stabilization of Polymers" (G. Geuskens, Ed.), p. 157. Wiley, New York, 1975. 26. Sakaguchi, M., and Sohma, J., J. Polym. Sei. Polym. Phys. Ed. 13(6), 1233 (1975). 27. Breitenbach, J. W., Rigler, J. K., and Wolf, B. A., Makromol. Chem. 164, 353 (1963). 28. Green, C. D., Hershey, H. C., Patterson, G. K., and Zakin, J. L., Trans. Soc. Rheol. 10, 489 (1966). 29. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., J. Polym. Sci. Part C 16, 1 (1967). 30. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., J. Polym. Sci. Part C 16, 13 (1%7). 31. Cantow, M. J. R., Johnson, J. F., and Porter, R. S., Polymer 8, 87 (1967). 32. Abernathy, F. H., and Paterson, R. W., J. Fluid Mech. 43, 689 (1970). 33. Morimoto, K., and Suzuki, S. J.,Appl. Polym. Sei. 16(11), 2947 (1972). 34. Fukutomi, T., Tsukada, M., Kakurai, T., and Noguchi, T., Polym. J. (Japan) 3, 717 (1972). 35. AbdeI-Alim, A. H., and Hamielec, A. E.,J. Appl. Polym. Sci. 17, 3769 (1973). 36. Glynn, P. A. R., and Van der Hoff, B. M. E., J. Macromol. Sci., Chem. A7(8), 1695 (1973). 37. Chung, K. H., Kim, K. J., and Kim, S. D., Tachan Hwahak Hoechi 19(5), 386 (1975).
Journal of Colloidand InterfaceScience. Vol.70, No. 2. June 15. 1979
38. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity," p. 84, Academic Press, New York, 1967. 39. Pelognin, L., and Rader, C., Mater. Perform. 13(8), 24 (1974). 40. Abbas, K. B., and Porter, R. S., J. Polym. Sci., Polym. Chem, Ed. 14(3), 553 (1976). 41. Flory, P. J., J. Chem. Phys. 17, 303 (1949). 42. Flory, P. J., Pure Appl. Chem., Macromol. Chem. 8, 1 (1972). 43. Krigbaum, W. R., and Godwin, R. W., J. Chem. Phys. 43, 4523 (1%5). 44. Aiberino, L. M., and Graessley, W. W., J. Phys. Chem. 72, 4229 (1968). 45. Hoffman, M., Makromol. Chem. 144, 309 (1971). 46. Brandrup, J., and Immergut, E. H., "Polymer Handbook," 2nd ed., p. IV-38. Wiley, New York, 1975. 47. Gorokhovskii, G. A., Soviet Mater. Sci. 1(5), 365 (1%5). 48, Gorokhovskii, G. A., and Agulov, I. I., Soviet Mater. Sci. 2(1), 78 (1966). 49. Higham, P. A., Bethune, B., and Stott, F. H., Wear 46, 335 (1978). 50. Ahuja, S. K., J. Colloid Interface Sci. 57, 438 (1976). 51. Joy, A. S., in "Proc. 2nd. Intl. Conf. of Surface Activity" (J. H. Schulman, Ed.), Vol. II, p. 54. Butterworths, London, 1957. 52. Dormant, L. M., and Adamson, A. W.,J. Colloid Interface Sei. 38, 285 (1972).