EDWARD R. NELSON, T. J. KILDUFF, and A. A. BENDERLY Diamond Ordnance Fuze Laboratories, WashiHgton 25, D. C.
Bonding of Teflon B E C A U S E OF SEVERAL extraordinary characteristics-high temperature and chemical stability, low dielectric loss factor, and resistance to the penetration of water and water vapor-polytetrafluoroethylene or Teflon (Du Pont) has found wide usage in chemical and electrical applications. However, in many instances, difficulty in bonding to Teflon has significantly reduced its usefulness. Early attempts to bond to Teflon largely involved formulation of special adhesives (7, 6) and modification of the polymer itself ( 7 7). Recently, however, the polymer was modified so that ordinary adhesives could achieve good bonds (3,8, 70). In this method the polymer surface is turned grayish brown with sodium or other alkali metals in anhydrous liquid ammonia, presumably due to carbon left behind when the sodium-ammonia complex has extracted fluorine atoms from the polymer. Detection of fluoride ions in the bath (8) tends to confirm this mechanism. In I attempts to synthesize certain glycidyl ethers in this laboratory, a sodium-haphthalene complex in tetrahydrofuran was employed, which attacked the surface of a Teflon stirrer contained in the reaction flask. This suggested' that the complex and other metal-aromatic hydrocarbons should be investigated as surface treating agents. Recent work on metal-aromatic hydrocarbons (7, 9, 74) indicates that they are solvated complexes of metal ions and aromatic ion radicals in a mole ratio of 1 to 1. Such complexes can be formed between any of the alkali metals and many aromatic hydrocarbons, or some aromatic nitro compounds in certain ethers or tertiary amines. A review of sodium compounds by Hansley ( 5 ) includes many references to such sodium complexes. Scott and others (72) showed that, in many chemical reactions, behavior of metal-aromatic complexes is similar to that of the sodiumammonia complex. Because the use of the sodium-naphthalene complex for treating Teflon required little or no exhausting of noxious fumes, the system was studied and ~
Editor's note-After this paper was submitted for publication, a patent covering a substantial part of .this treating process was issued -i.e., Rappa ort, G. (to General Motors Corp.1 U. S. Patent 2,809,130 (Oct. 8, 1957).
bondability of treated specimens with an epoxy resin has been determined. T h e work has been extended to include a few experiments on polychlorotrifluoroethylene (Kel-F; Minnesota Mining & Mfg. Co.), and silica-filled Teflon. Experimental Methods Preparation of Treating Baths, Sodium-naphthalene baths were made by two procedures: with relatively large lumps of metallic sodium, and with dispersions of sodium in organic solvents. Following the method of Scott and others (72), 23 grams (1 gram-atom) of sodium metal cubes, 0.25 to 0.5 inch on a side, were added at one time to a solution of 128 grams (1 mole) of naphthalene and 1 liter of commercial grade tetrahydrofuran contained in a 2-liter reaction flask equipped with a stirrer and a drying-tube outlet. The mixture was then stirred for two hours at room temperature, by which time the reaction was virtually complete and a dark brownblack color had developed. The bath was then considered ready for use. When using sodium dispersions, a 1-molar solution of naphthalene in tetrahydrofuran was placed in a 2-liter reaction flask equipped with a stirrer and a drying-tube outlet. Then 76.7 grams of a sodium dispersion containing 7070 xylene and 30% sodium was added. The reaction produced some heat, thereby requiring venting through the drying tube, but the temperature did not reach a point where cooling was required. After very little stirring, the bath turned dark and was ready for use. Also, a semisolid dispersion of 45% sodium in white oil was used. Similar results were obtained but about 15 minutes of stirring were required to complete the reaction. However, the white oil dispersion has an advantage over the solvent dispersion-it is usually not spontaneously flammable. Directions for the preparation and handling of sodium dispersions are well described (73). Another series of baths was also prepared, using 200 ml. of tetrahydrofuran, 0.2 gram-atom of sodium or lithium, and 0.2 mole of an aromatic hydrocarbon. These formed complexes of lithium-naphthalene, lithium-phenanthrene, sodium-phenanthrene, and sodium-anthracene, Reagents were shaken until reaction color became intense. Treatment of Polyfluorocarbons. The solution of metal-aromatic hydrocarbon was kept sealed in a reaction flask except when polymer parts were
inserted or removed. This was necessary to exclude air and water vapor, both of which were deleterious to the life of the bath because the complex reacts with both oxygen and water. Mostly 15-minute immersion in the sodium-naphthalenetetrahydrofuran bath was employed. After treatment, -the specimen was removed from the bath with tongs, washed with acetone, and subsequently with water. The acetone wash removed excess organic materials, whereas the water removed traces of metallic salts from the surface. When a bath was expended because constituents were consumed or sludge formed, it was poured into alcohol which decomposed the remaining complex and reacted with unconsumed metal. Treated Teflon, both filled and unfilled, appeared almost black while wet and lightened to a uniform gray-brown color upon drying in the air. Treated Kel-F appeared to be similar to the treated tetrafluorocarbon except that its surface was somewhat spotted. Some specimens were cut from a sheet of Teflon that had been commercially treated on one side by the sodiumammonia process. These were exposed for 15 minutes on their untreated side only to the sodium-naphthalene-tetrahydrofuran bath and the bondability of the two sides was compared, using the disk-tensile test and a loading rate of 600 pounds per square inch: Bond Str., Side of Bond Str., Side of Lb./Sq. In. Failure Lb./Sq. In. Failure C 1400 1730 c, L 1000 C 1950 L 1710 c, L 1150 L 1870 L C = commercial; L = lab. treatment. Bonding of Treated Teflon and Bond Testing. The adhesives employed throughout the investigation consisted of 100 parts by weight of a commercial epoxy resin, viscosity 5 to 9 poises at 25 O C., and 5 parts by weight of a curing agent, dimethylaminopropylamine. A thin film of the adhesive was brushed onto the surfaces to be bonded. The parts were assembled, and heated for 1 hour in a forced draft oven at 100' C. The bondability of the treated Teflon was determined in three ways by modification of three standard te?t procedures. For the butt-tensile test, a modification of Method 1011 of Federal Specification LP-406b, standard Type 1 tensile specimens were machined from sheets of polymer, cut in half across the center of the constriction, treated, and bonded. These specimens were exVOL. 50,
NO. 3
0
MARCH 1958
329
3
Table I.
Bond Strength in Tension and Shear of Treated Teflon Bond Str., Lb./Sq. In. No. Samples
Bond Test Tested Av . Butt-tensile, bonded halves 6 1340 Standard-tensile, 1 piece 3 1530 Disk-tensile, 600 Ib./sq. in./min. 4 1790 Lap-shear 6 1600 Sodium-naphthalene-tetrahydrofuranbath: immersion time, 15 min.
tended to rupture on a constant-extension-rate tester at a head travel of 0.05 inch per minute (Table I). Teflon Bond Strength after Treatment
(Sodium-naphthalene-tetrahydrofuranbath) In.a Treat. ~ i Bond ~ Str., ~ Lb./Sq. , Min. Av. Range 0.5 1
5 15 60 480 1440
1810 2010 1810 1800 2160 1940 1750
1500-2 100 1850-2150 1600-1950 1650-1950 1800-2450 1600-2300 1200-2550
1480 1750
1100-1850 1400-2100
la 6Sb
Disk-tensile test, 2500 pounds per square * Age of bath days; treatment time is 60 min.
a
inch per minute loading rate. in
For the disk-tensile test, a modification of Method 1011.1 of Federal Test Method Standard No. 175, a disk of treated Teflon, 0.020 inch in thickness and 1.25 inches in diameter, was bonded between two standard brass adhesion test plugs. Several such assemblies were subjected to tension on a constant-rateof-loading tester at a rate of loading of 600 pounds per square inch of bonded are aper minute (Table I). The majority of such assemblies, however, was tested on a small laboratory-automatized tensile-tester a t a loading rate of approximately 2500 pounds per square inch. For the lap-shear test, a modification of Method 1033 of Federal Test Method StandardNo. 175,astrip oftreatedTeflon, 1 X 1 X 0.020 inch was bonded between two brass lap-shear plates. These assemblies were subjected to tension on a constant-rate-of-loading tester at a rate of loading of 600 pounds per square inch per minute (Table I). Other Measurements on Treated Teflon. Dielectric constant, K , and dissipation factor, tan 6, were measured at 23’ C. and 40% relative humidity over the frequency range lo2 to IO5 cycles per second on a commercial capacitance bridge and surface resistivity was measured on a slide-back electrometer ( 2 ) : Polymer Treateda Untreated
K
-
Tan 6
2.01 0.0001 2.001 0.0001* to 2. 003*
Surf. Resist ., p S OhmsQ
210’6 2 10’6
a Sodium naphthalene - tetrahydrofuran bath; immersion time, 15 min. (4).
330
Range
1120-1480 1500-1570 1400-2060 13 10-1840
When the cut edges of treated samples were examined microscopically, the depth of attack by the treating agents in each case was found to be 5 0.00004 inch. When subjected to electron diffraction, structure of the treated and untreated materials was indistinguishable.
Discussion Some bond tests were conducted on a small laboratory-automatized tensile tester a t a rate of loading of 2500 pounds per square inch, whereas others were run on another tensile tester at a standard rate of 600. The higher loading rate was limited to single studies that were comparative within themselves. For all practical purposes bond strength was virtually independent of treatment times in the range 0.5 minute to 24 hours. Recent qualitative tests showed that satisfactory bond strengths were achieved as a result of treatment times as short as 2 or 3 seconds. The treating bath was at least as effective after 2 months of storage as it was when freshly prepared. This aging study was conducted with a bath made from lumps of sodium. I t was used intermittently during the storage period, The sodium-naphthalene-tetrahydrofuran bath was as effective as the commercial sodium-ammonia process in rendering Teflon bondable. No bond tests were conducted on specimens treated with metal-aromatic complexes other than sodium-naphthalene. Hoivever, specimens treated with other such complexes acquired the same dark surface and were presumed to be equally bondable. Bonds achieved with an epoxy adhesive on treated Teflon ranged between 1100 and 2000 pounds per square inch in tension or in shear and approached the ultimate strength of the polymer itself. I n some instances, the polymer tore before rupture of the bond occurred. The bond strengths determined from the dumbell-shaped, butt-tensile specimens, as well as the tensile strengths of similarly shaped uncut specimens, were consistently lower than those determined from the assemblies of two adhesion test plugs and one disk of polymer. These apparently lower strengths are attributed to stretching in the neck of the dumbbell-shaped specimens. Electrical properties of treated Teflon
INDUSTRIAL AND ENGINEERING CHEMISTRY
were virtually identical with those reported in the literature for untreated polymer, probably because of the extreme thinness of the treated surface layer. I t is apparent, however, that a treated surface would wet more readily than an untreated one. Consequently, at extremely high humidities, such as 90 or 9570, a measurement of surface resistivity of the treated polymer would probably indicate the presence of a conducting film of water. That the electron diffraction patterns of treated and untreated polymer were indistinguishable may also be attributed to the thinness of the treated surface layer, or to the treated surface having the same structure as the bulk material. Although no thorough evaluation was made, the few specimens of Rel-F darkened in the usual fashion during treatment and, when bonded to brass with an epoxy resin, yielded bond strengths well in excess of 2500 pounds per square inch in tension. This strength is two to three times that normally achieved with untreated Kel-F. This new process uses a stable bath which does not require refrigeration and would require the use of an exhaust system onlv in a continuous operation where a closed system could not be employed. Therefore, this process lends itself particularly to small-scale or laboratory operations where a small number or a great variety of parts, as well as intermittent service, may be involved. literature Cited
s
(1) British Thomson-Houston Co., Ltd., Brit. Patent 659,439(Oct. 24,1951). (‘2) Doctor, N. J., Franklin, P. J., Instr. and Automation 29, 1131 (June
1956).
(3) E. I. duPont de Nemours & Co., Belgian Patent 548,516. 14’1 Ehrlich. P.. J. Research N u t . Bur. 5 Ha 4
A. R., J . Chem. Sac. 1954,p. 720. (8) iMinnesota Mining & Mfg. Co., British Patent 265,284 (Jan. 9,1957). ( 9 ) Paul, D. E., Lipkin, D., Weissman, S. I., J . Am. Chem. Soc. 78, 116 (1956). (IO) Purvis, R. J., Beck, W. R. (to Minnesota Mining & Mfg. Co.), U. S. Patent 2,789,063(April 16, 1957). (1 1) Rudner, M. A. (to U. S. Gasket Co.), U. S. Patent 2,728,698 (Dec. 27, 1955). (12) Scott, N. D., Walker, J. F., Hansley, V. L,. J. Am. Chem. SOC. 58, 2442 (1936). (13) Sittig, M., “Sodium, Its Manufacture, Properties and Uses,” Reinhold, New York, 1956. 114) Ting Li Chu, Shan Chi Yu, J . Am. Chem. Sot. 76,3367 (1954).
RECEIVED for review July 24, 1957 ACCEPTED November 13, 1957
