I
I
FRANK J. BOCKHOFF, Fenn College, Cleveland 15, Ohio
E. TIMOTHY McDONEL and JOHN E. RUTZLER, Jr., Case Institute of Technology, Cleveland 6, Ohio
Effects of Oxidation on Adhesion of Polyethylene to Metals Relatively small degrees of oxidation can considerably improve adhesion values commonly encountered commercially in fluidization and flame spraying
THE
mechanism of adhesion of polyethylene to metals is little understood. However, the method of application to an adherend strongly affecrs adhesional strength. Adhesion of compressionmolded polyethylene to metal inserts is usually weak. I n processes such as fluidization coating (ZO), in which powdered polymer is applied to a surface in intimate contact with air, adhesion strengths in the range 800 to 1200 p.s.i. have been obtained consistently under commercial conditions. Polyethylene adheres to steel somewhat more strongly when applied by flame-spraying (79) than in a fluidization coating. The difference probably is due to greater oxidation of the polyethylene in the flame-spray than in the fluidizationcoating process. All previous industrial experience with the bonding of polyethylene to metals seems to indicate that oxidation of the polymer increases adhesion. Pertinent Literature Adhesion of polyethylene to steel was studied. Plots of joint failure stress us. film thickness were made and extrapolated to 0 thickness, extrapolated value representing upper limit of specific adhesion at 25O C. of 183 kg. per sq. cm. Meaning of adhesion value is difficult to interpret without knowledge of oxygen content and configuration of polyethylene used. .4s conventional polymerization of ethylene is possible only in presence of small amounts of oxygen, a small amount of cross linking and combined oxygen are present in polymer. Earlier commercial polyethylenes contained more oxygen than those currently produced, because of less refined polymerization methods and greater thermal abuse caused by extensive interblending of batches. Infrared studies of early polyethylene showed
904
( 73)
(2)
(9)
Extensive previous data indicate that oxygen-containing groups at the surface of polyethylene improve bonding of the polymer to metals; it is virtually impossible to produce polyethylene free of oxygen-containing groups; and fabrication methods involved in handling polyethylene, such as extrusion, milling, blow molding, and coating processes, increase the concentration of oxygencontaining groups at the surface of the polymer. The present investigation was undertaken to study the role of oxidation in the adhesion of polyethylene to stainless steel. Experimental Procedure Adhesion tests were made using standard buttons of Class 6, Type A, Spec. N:46-3-18 stainless steel 1.129 inches in diameter and rods of Republic Steel No. 416 stainless steel 0.5 inch in diam-
considerable variation of oxv~en content from ," sample to sample. Modern polyethylenes show oxygen contents as low as 0.009% upon leaving the reactor and before processing. Oxygenated groups are necessary for strong adhesion of polyethylene to metalsand other surfaces. Adhesion of polyethylene to aluminum film is promoted by oxidation of surface of heated polymer and inhibited completely by lack of oxidizing atmosphere. Oxidation of surface of polyethylene improves retention of printing ink. Presence of carbonyl groups was established by infrared analysis; rubbing of surface of irradiated film with organic solvents destroys retentive power for ink. Even at room temperature, polyethylene is oxidized to a certain extent. 1-mm. sheets of polyethylene on glass plates will absorb 0.2% of oxygen in 23,000 hours at
INDUSTRIAL AND ENGINEERING CHEMISTRY
eter, following ASTM method D 897-49. At least four samples were measured for each value reported. After dry machining, the buttons were placed in a jig (Figure l), which was preset for a 0.002inch glue line, and coated by one of two methods. The first involved the use of polyethylene powder (Agilene), to simulate fluidization coating. Agilene is a conventional high-pressure polyethylene having a melt index of 2 with a numberaverage molecular weight of 22,000 and a medium degree of branching. The polyethylene was powdered by low temperature grinding, using liquid nitrogen. After the buttons had been preheated to the desired bonding tempera. ture, dry polyethylene powder was applied to their surfaces. After a few seconds, the excess powder was blown off and that remaining was allowed to fuse completely. The hot buttons were then assembled in the preheated posi35" C.; at 50" C. a 0.2% oxygen content will be reached in 3000 hours. At 75" C only 300 hours are required to attain 0.2% oxygen content. Films of polyethylene pressed from unstabilized Bakelite DYKH and milled up to 120 minutes at -IIo" C. showed infrared absorption bands corresponding to carbonyl group but no bands characteristic of aldehyde or carboxyl oxygen. Milling polyethylene at 160' C. on heated rolls exposed to atmosphere causes cross linking of polymer and increased viscosity after 6 hours of milling. Oxygen content was 0.3%, present as C=O, as shown by infrared spectra. ,4t higher temperatures, oxidation is autocatalytic. Intermediate product may be hydroperoxide which initiates free-radical oxidation to secondary products such as ketones, aldehydes, acids, and alcohols.
(3,28)
(5,70, 7779)
tioning jig, and excess polyethylene was squeezed from the joint. The second method was designed to reduce the amount of polymer oxidation. A 0.004-inch film was scarfed on a lathe from the center of an extrusion-molded polyethylene (Agilene) rod, to eliminate the oxidized surface layer commonly encountered in extruded and blown film (7). The film was stored under nitrogen until used. The positioning jig was set for 0.002-inch glue line thickness: The film was inserted, and the assembly heated a t 200' C. for 20 minutes, or to 300' C. for 10 minutes. After fusion of the polymer, the jig was closed. When cool, the bonded buttons were removed from the jig, and the polyethylene crown was removed. Commercial blown film (Bakelite DYNH-3) was also evaluated by the film method to determine the effect of surface oxidation on its adhesion to stainless steel. Specimens were tested at a crosshead speed of 0.2 inch per minute. Percentage of adhesional failure was estimated, from microscopic examination, as that percentage of the total area which failed at the metal-polymer interface. In another set of experiments, scarfed film was subjected to controlled oxidation in oxygen at 95' C. before bonding to stainless steel buttons. The experimental apparatus and procedure were essentially the same as those used by Shelton and Winn (25) and Blum, Shelton, and Winn (4): except that all specimens were also in contact with mercury vapor. As expected, oxygen uptake was too small to measure, because of sensitivity limitations (28). Infrared absorption spectra of films were obtained using a Perkin-Elmer Model 21 double-beam recording spectrophotometer.
,-
HOLDING PLATES BUTTON
I+
i POSITION PLATFORM Figure 1. After dry machining, adhesion buttons were placed in a positioning iig. Dimensions in inches around the edges of the buttons. Commercial blown film applied to buttons a t 300' C. by the film method produced bonds having an average tensile strength of 1426 p.s.i., which was significantly higher than the value for the scarfed film specimens. Bonds to steel rods made in presence of nitrogen showed lower strengths than corresponding bonds made in air. The roughness of the surface of the adherends apparently affected bond strength and when the surface roughness was comparable, the bond strengths to the two samples of steel were comparable.
Table 1.
Results Tensile strengths of bonds made with powder and bonds made with film are shown in Tables I and 11. Figure 2 shows results for scarfed polyethylene subjected to controlled oxidation. Bonds made from powder were generally much stronger than those made from film. A maximum strength of 2650 p s i . was obtained from bonds formed between powder and steel buttons at 200' C. Bonds formed from the powder a t 300' C. exhibited a tensile strength of only 1365 P A . ; they also showed more cohesive failure than the lower-temperature bonds. A marked discoloration was noted in the 300' C. bonds made between powder and buttons. The same general type of result was found for steel rods. Scarfed polyethylene, applied to buttons by the film method a t 300' C., showed a tensile strength of 100 p.s.i.; at 200' C. the value rose to 256 p.s.i. with signs of cohesive failure, especially
,-TEST
Material Agilene powder
Agilene scarfed film Commercial blown film
Data from bonds made from scarfed film subjected to controlled oxidation (Figure 2) fall into two groups, which are rather sharply separated by differences in physical characteristics of the ruptured films and in carbonyl group content as shown by infrared spectra. Observed characteristics of the two types of strength behavior are noted in Figure 2. Infrared absorption curves were obtained for each of the preoxidized scarfed films (Figure 3). Curves A , B, and E refer to the films used to obtain the tensile strength data for oxidimeter
Adhesion of Polyethylene to Stainless Steel Buttons (0.002-inch bond thickness) % Coating Tensile AdheApplication Temp., Adhesion, sional Remarks Failure Method O c. P.S.I. Powder Powder (0.004 inch) Powder
300 300
1365 1750
26 46
Films discolored Films discolored
200
2650
97
No film discoloration
Film
300 200
100 256
100 100
300
1426
100
Film
Considerable edge adhesion
Table 11. Adhesion of Polyethylene to Stainless Steel Rods (Bond thickness, 0.002 inch. Bonding temp., 225' C.) Tensile Mean % Application Adhesion, Deviation, Adhesional Material Method P.S.I. % Failure Agilene powdera Agilene scarfed filmb
Powder in air Film in air Film in NZ Film in airC
2170 1123 808 285
25 17 12 27
16 97 100 97
No. of
Samples 7 9 7 6
0.67% oxygen by microanalysis before bonding. 0.43% oxygen by microanalysis before bonding. Surfaces about equal in roughness to those to which Table I refers and about twice as rough as other 23 adherends in this table.
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Figure 2. Tensile of film adhesion specimens subjected to controlled oxidation in oxygen a t
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95O
c.
I 1
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40
00
120
160
200
HOURS IN OXIDIMETER
times of 0, 12.5, and 60.7 hours, respectively.
Discussion Increase in the oxidation of polyethylene increased the strength of adhesion, whether the polyethylene was applied to the adherends as powder or film. When powder was used, the fused polyethylene was exposed to the atmosphere prior to joining the adherends; there was much less exposure of the polymer to air when bonding was done with film, and still less when the atmosphere was nitrogen. Oxidation of the film prepared from powder appeared to be appreciable, particularly at 300' C., as the films showed a slight brownish discoloration. At 200' to 225' C., however, no discoloration was observed and specimens exhibited twice the strength obtained with bonds in which the polyethylene was discolored. Furthermore, failure in discolored films was primarily cohesive; in colorless films, primarily adhesive. I t appears that extensive oxidation a t 300' C. lowered the cohesive strength of the polyethylene. The strength of 2650 p.s.i. obtained for bonds made from polyethylene powder a t 200' C. is well above the tensile strength of the relatively less oxidized parent material. Controlled oxidation can apparently affect both the adhesive and cohesive properties of polyethylene. I t is possible that limited oxidation, accompanied by cross linking, was responsible for the abnormally high value of 2650 p.s.i. I n bonds formed a t 300' C., however, the amount of oxidation was apparently enough to lower the cohesive strength of the polyethylene.
906
Blown film gave an average strength of 1426 p.s.i. in tensile rupture, while scarfed film on buttons gave as low as 100 p.s.i. At the temperature of extrusion of polyethylene, one would expect a rapid surface oxidation in the presence of air (7). This oxidation was evidenced in the comparatively high tensile strengths of bonds made from blown film. Three groups of bond strengths appear to be established by the data in Table I. The first of these (around 2650 p s i ) apparently can be taken as the approximate tensile strength to be expected from polyethylene that is fairly strongly oxidized on the surface. This value is slightly above that obtained in work (73) on early polyethylenes, which probably contained considerably more oxygen than modern polymers. The second group of values, from 1365 to 1750 p d . , results from the use of blown film or overoxidized powder; the third group, for scarfed film, may be regarded as representing approximately the tensile adhesion of commercially available polyethylene. The data in Table I are supported by the results obtained using rods of a slightly different stainless steel and polyethylene of known initial oxygen content (Table 11). Table I1 also shows that even use of scarfed film, which prevents extensive oxidation during bonding, by no means eliminates such oxidation, as can be seen from the results obtained from films bonded to the smoother adherend surfaces in air and nitrogen. The curves of Figure 2 show a steeply rising strength for the shorter times of preoxidation, followed by a drop and
INDUSTRlAL AND ENGINEERING CHEMISTRY
then by a slowly rising strength with longer times of oxidation. Oxidation of the scarfed film failed to produce tensile strengths as high as for blown film. Figure 3,A, for scarfed film not preoxidized, shows no bands a t 5.85 microns corresponding to the carbonyl group, After exposure to oxygen for 12.5 kours, bands appear a t 7.94, 9.85, and 12.6 microns, as shown in curve B. These same bands appear in E for film that was oxidized for 60.7 hours, but they are either extremely weak or missing in C, D,and F. C and F were obtained from samples of film oxidized for 12.5 and 60.7 hours, respectively, and then rubbed with paper toweling; D was obtained from a film that had been oxidized for 12.5 hours and removed from one of the buttons after rupture of the adhesive bond. Comparison of the peaks of 7.94, 9.85, and 12.6 microns in Figure 3 leads to the conclusion that some bands in the infrared absorption spectrum of oxidized polyethylene are associated with some substance that can be rubbed off the surface and disappears when the film is bonded to stainless steel buttons. Infrared spectra of other samples of film oxidized up to 160 hours showed that this phenomenon was not confined to the two samples studied. The anomalous bands in the infrared spectrum at 7.94, 9.85, and 12.6 microns correspond to three of the four bands associated, according to Colthup (8) with the covalent carbonate group, O=C(O-R),. As these peaks have not been reported in previous work on polyethylene oxidation, it is possible that oxidation in the presence of mercury vapor follows a different mechanism, leading to loosely held, perhaps adsorbed, oxidation products on the surface. I n fact, the 7.94- and 12.6-micron peaks did not appear when oxidation was carried out in the absence of mercury vapor in subsequent experiments, but they were found whenever mercury vapor was present. Mercury vapor, under these experimental conditions, decelerates the rate of oxidation of rubber (24)* If the failure to attain as great a tensile strength with film that had been exposed to oxygen in the oxidimeter as with blown film is attributed to the presence of loosely held oxidation products on the surface of the polyethylene, then bonds should be stronger if the polyethylene is rubbed prior to application to the adherends. The average tensile strength of bonds to stainless steel buttons made from film that was in the oxidimeter for 95.25 hours was 337 p s i ; a piece of the same film that was rubbed with paper toweling prior to bonding showed a tensile strength of 975 p.s.i. This implies that the loosely held oxidation product on the surface of polyethylene, oxidized in the presence of mercury, acts as an antiadhesive. The blown film was oxi-
A D V A N C E S IN A D H E S I V E S dized in the absence of mercu5y and showed no bands a t 7.94, 9.85,or 12.6 microns, although a band a t 5.85 microns corresponding to the carbonyl group, was found. It appears that the infrared spectra account for the low adhesion of the preoxidized scarfed film and the much greater adhesion of the blown film. All specimens whose strengths fell on the steep portion of the curve of Figure 2 showed cohesional failure around the edges of the buttons; this type of failure was not observed for specimens whose strengths fell on the low-slope portion of the curve. “Squeeze-out” during bonding may have caused considerable mixing of the loosely held oxidation product into the bulk of the polyethylene, thus reducing its surface concentration, particularly a t the edges, where maximum mixing occurred. Increased edge adhesion was observed as far as l/32 inch into the bond around the circumference of the buttons. In the case of strongly opidized films, there was perhaps too much of the loose oxidized layer to be appreciably mixed into the bulk of the polymer. Another cause of cohesional failure a t the edges of the joint is excessive oxygen absorption and reaction a t the bonding temperature to produce carbonyl groups. Rods bonded in presence o$ air showed this same type of cohesional failure around the edges; those bonded in nitrogen did not. When polyethylene is oxidized in the absence of mercury, carbonyl groups are the primary oxidation products and these are an integral part of the polymer. This results in high bond strengths, achieved with blown film and powder bonds. However, oxidation in the presence of mercury leads to loosely held oxidation products which counteract the increase in adhesion resulting from ordinary oxidation. Mechanism of Bonding Polyethylene apparently can bond to a metal oxide surface-practically all metals have oxide films on them-as a result of London dispersion forces, dipole-ion interactions between group dipole moments of C-H, 0-H, or C=O groups of the polymer chain and oxygen or metal ions of the surface oxide, and hydrogen bonding, or electric charge transfer (26). All these mechanisms probably are involved to varying degrees in the total bonding force. However, as the strength of adhesion increased markedly with oxidation of polyethylene under the proper conditions, the main portion of the enhanced bond strength of the oxidized material may best be accounted for by dipole-ion interactions involving the 0-H and C=O groups of the chain and surface oxygen and m,etaI ions, respectively, and dipole-ion interactions
between C-H groups adjacent to oxygenated qarbon atoms and the oxide ions in the metal surface. Acknowledgment T h e authors are indebted to the American Agile Gorp. for assistance in supplying the materials used and for machining services. They thank the Esso Education Foundation for financial support of the experimental work.
z literature Cited (1) Arbit, H. A., Griesser, E. E., Haine, W. A., Tappi40,161 (1957). (2) Barnes, C. E., J. Am. Chem. SOC.67, 217 (1945). (3) Biggs, B. S.,Hawkins, W. L., Modern Plastics 31, 121, 122, 124, 126, 203 (1 9’53) (4) Blum, G. W., Shelton, J. R., Winn, H., IND.ENG. CHEM. 43, 464 (1951). (5) Boiland; J. L., Proc. Roy. Sac. (London) A186,230 (1946). (6) Brown,-A. C., Angel, T. H., Brit. Patent 713,634 (Aug. 18, 1954). (7) Chapman, R. N., Columbo, P., Nucleonics 13,No. 10, 13 (1955). (8) Colthup, N. B., J. Ofit. Sac. Am. 40, No. 6,397 (1950). (9) Cross, L. H., Richard, R. B., Willis, H. A., Discussions Faraday SOG. 9,235-45 (1950). (10) Henderson, J. N., “Study and Report on the Conservation of Critical and Strategic Materials for the Armed Services Medical Procurement Agency,” Case Institute of Technology, Cleveland, Ohio, Monthly Progress Rept. 16, 1 (1955). (11) Horton, P. V.,U. S. Patent 2,668,134 (Feb. 2, 1954), 24,062 (reissue, Sept. 20, 1955). (12) Imperial Chemical Industries, Australian Patent 7295/55 (March 3, 1955). (13) Kraus, G., Manson, J. E., J . Polymer Sci. 6, 625-31 (1951). (14) Kriedl, W.H., U. S. Patent 2,632,921 (March 31, 1953); Ger. Patent 844,348 (July 21,1952). (15) Kriedl, W. H., Hartmann, F., Plastics Technol. 1, (1) 31 (1955). (16) Lasak, F. J., U. S. Patent 2,697,058 (Dec. 14,1954). (17) Mulcahy, M. F. R., Trans. Faraday Sod. 45. 576 (19491. . , (18) Myers, C. S., IND.END. CHEM.44, 1095-8 (1952). (19) Neumann, J. A., Modern Plastics 27, 85 (1950). (20) Neumann, J. A,, Bockhoff, F. J., Product Eng. 28, 140-3 (1957). (21) Raff, R. A., Allison, J. B., “Polyethylene,” p. 99, Interscience, New York, 1956. (22) Rossman, K.,J. Polymer Sci. 19, 141 (1956). (23) Rugg, F. M.,Smith, J. J., Wartman, L. H., Zbid., 11, 1 (1953); Ann. N . Y. Acad. Sei. 57. 398-416 (1953). (24) Shelton, J. R., Wickham, W. T., McDonel, E. T., “Study of Some Factors Involved in Deterioration of Rubber Polymers and Vulcani-
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0
t-
o. K
0 v)
m U
8
14
12 IO 8 6 WAVE L E N G T H ( U )
Figure 3. A.
Infrared spectra
Freshly scarfed 0.004-inch polyethylene film
B.
0.004-inch polyethylene fllm after 12.5 hours in oxidimeter C. B after surface rubbing with paper iowel D. B after adhesion testing E. 0.004-inch polyeihylene film after 60.7 [hours in oxidimeter F. E after surface rubbing with paper towel
zates,” Tech. Rept. 1, Dept. of Army, Proj. 5B99-01-004,1956. (25) Shelton, J. R., Winn, H., IND. ENG.CHEM.38,71 (1946). (26) Skinner, S. M., Savage, R. L., Rutzler, J. E., Jr., J . Apfil. Phys. 24,438 (1953). (27) WHittaker, D., Forsyth, J. S. A,, Brit. Patent 581,279 , .(Oct. 7, 1946). (28) Wilson, J. E., IND.ENG. CHEM.47, 2201 (1955). (29) Wolinski, L. E., U. S. Patent 2,715,075, 2,715,076, 2,715,077 (Aug. 9, 1955). RECEIVED for review September 12,1957 ACCEPTEDMarch 31, 1958 Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Recent Advances in Adhesives, 132nd Meeting, ACS, New York, N. Y.,September 1957. VOL. 5 0 , NO. 6
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