I
R. G. BAKER and A. T. SPENCER Bell Telephone Laboratories, Inc., Murray Hill, N. J.
Structural Bonding of Polyolefins Bonds between polyolefins and copper have been achieved which are at least 10 times stronger than those obtained by present commercial methods. The bonding technique described here may apply to a wide range of uses where the excellent electrical properties of pure olefins cannot be used now because of bonding problems. It is particularly promising for submarine cables
ALTHOUGH approaches have been described for bonding polyethylene MANY
and other polyolefins, the structural use of these polymers is often limited by the difficulty in adhesively bonding to them. The most common method is the application of a layer of very tacky adhesive. This method is quite unsatisfactory for structural bonding, however, because such adhesives flow under stress. Recently, Peters and Lockwood (7) described a method for bonding polyethylene to itself or brass using a layer of partially hydrogenated polyisobutylene as an adhesive. This material is vulcanized to the brass under heat and pressure and a t the same time partially dissolves in the polyethylene. The bonds formed by this method are structurally strong and reproducible. Other methods for bonding polyethylene depend on modification of the polymer surface. These methods use various forms of oxidation by ozone (70), permanganate ( I ) , chromic acid (7, 5), flame treatment ( Z ) , and electrical discharge ( 9 ) to give surfaces bearing many polar groups. Several of these methods are in commercial use for rendering polyethylene printable, but none has been found satisfactory for structural bonding, especially where high peel strengths are required. A further process has been described by Foord ( 4 ) where polyethylene is bonded under heat and pressure to copper bearing a thermally produced cuprous oxide film. Yet another method for improving the adhesion of polyethylene to metals has been described by Taylor and Rutzler ( S ) , who found that the tensile shear
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printed circuits
strength of bonds formed between metals and fused polyethylene is a function of the surface roughness of the metal specimens. Here the increased adhesion to preoxidized metal specimens was laid to a microscopic smoothing of the surface during oxidation that allows an increased number of effective contacts between the polymer molecules and the surface. The bond is formed wherever polymer chain-metal oxide contact is made. The work presented herein makes use of actual surface oxidation of the polymer during the bonding process a t a temperature considerably higher than that used in Taylor and Rutzler's work. The oxygenated groups in the polymer are then able to form strong chemical bonds with the oxides remaining on the metal surface. I n this procedure, cupric oxide films on copper alloys are used for bonding to polyolefins in situ during a simple molding operation without the use of intermediate adhesive material or polyolefin surface treatment. Bonds between polyolefins and alloys containing more than about 80% copper have been accomplished which have reproducibility and strength more than adequate where high physical properties are required. Also, because of the good electrical properties of the polymer, this type of bonding shows promise for electrical applications. Experimental Samples of metal foil for bonding were carefully degreased and acid etched to
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microwave devices
give a clean reproducible surface for subsequent oxidations. After cleaning, copper, phosphor bronze (95% Cu, 4y0 Sn, and 0.470 P) and beryllium copper (9870 Cu and 2y0 Be) specimens were oxidized directly for various times a t 98 2' C. in a 180 grams per liter solution of Ebonol C. Brass surfaces (65y0 Cu and Z) were first pretreated in a 120 gram per liter solution of brass activator for 2.5 minutes a t 98 f 2 O C. and then oxidized for various times in a 180 grams per liter solution of Ebonol C special. The preceding proprietary materials are products of Enthone, Inc., New Haven, Conn. Determination of thickness and composition of the oxide film was done electrochemically by a slight modification of the method of Campbell and Thomas ( 3 ) whereby a small oxidized panel is reduced a t constant current as cathode in 0.1N NHhC1. As current is caused to flow, the potential of the cathode with reference to a silver chloride electrode remains at a constant definite level while any one oxide is being reduced. After its complete reduction the potential changes to that of the other oxide and finally to that of hydrogen. The thickness is computed from the amount of current per unit necessary to reduce a given oxide. Before bonding, the polyolefin was premolded a t 200° C. into sheets 0.080 inch thick. Strips of oxidized copper alloy, 4l/2 X 1 X 0.005 inch, were then laid out on the polyolefin sheet with a half-inch strip of 0.010 inch. Teflon under one end of each to prevent bond VOL. 52, NO. 12
DECEMBER 1960
10 15
formation in that area; this area later served as a gripping tab for the peel strength test. T h e mold was brought up to bonding temperature, the polyolefin sheet inserted with the oxidized strips in place and the mold closed. A constant pressure of 400 p.s.i. was applied for the desired time and temperature. After cooling to room temperature, the mold was disassembled and the resulting laminate of copper and polyethylene cut into individual sections. The final thickness of polyolefin in the test specimens, molded into strips, was 0.063 inch. The peel strength was determined on an Instron testing machine at a constant 2 inch per minute rate of crosshead separation. The peel angle was 180'.
Discussion
Bonding Reaction. The difficulty with most bonding techniques for polyolefins is the lack of polarity at the polymer surface. In bonding this allows the operation of only the weakest interfacial forces in adhesive bonding. When the polyolefin surface has been oxidized. however, it becomes much more susceptible to bonding due to the presence of a certain number of oxygenated polar sites that are capable of substantial interactions with adhesive molecules. I n this work a cupric oxide film tightly bound to the base metal is used as the oxidizing agent. Under the influence of moderately high temperatures. the cupric oxide oxidizes the polyolefin surface and is itself partially reduced to cuprous oxide. The polyolefin surface now bearing a high concentration of polar groups can effectively serve as its own adhesive by hydrogen bonding and other polar interactions with the oxide film. The fact that cupric oxide is the active agent in bond formation is shown by bond strengths obtained to surfaces that were either all cupric or all cuprous oxide. When either thermally or chemically produced cuprous oxide films were used under a wide variety of molding conditions, the strongest bonds achieved measured less than 5 pounds per inch of width in peel strength. When chemically produced cupric oxide films were used under the same conditions, however, the bond strength was increased to 20 pounds per inch or more. Clearly, then, cupric oxide is the active bonding agent. Quantitative evidence for the oxidation reaction was also obtained. Here electrochemical reduction was carried out on oxide films on copper before bonding and on corresponding pieces of copper and polyolefin after bonding. For the analysis of the oxide film on the polyolefin in failed bonds, it was necessary to make the entire surface electrically conductive. This was done by
10 1 6
The conventional wave guide as it appears in the ground guidance system of Nike Ajox and Nike Hercules missiles i s shown at the right. On the left i s a single-splitter device with two imputs and one output which will replace the conventional wave guide. Polyolefins are used in making printed wave guide
evaporating a thin (approximately 300
A.) porous layer of gold on the surface. Electrical contact was made to the gold film by a conductive silver paste and reduction was carried out as above. In all cases there was a very marked increase in the amount of cuprous oxide present in the failed bonds with a corresponding decrease in the cupric oxide. As the bonding takes place me11 below the dissociation temperature of cupric oxide [about 800' C. ( S ) ] , the observed reduction of this oxide must be the result of oxidation of the polymer. Oxide Film Formation. Early tests showed that both thermal and chemical oxide films on copper alloys treated under apparently identical conditions were subject to unexpected variability. The thermal films yielded largely cuprous oxide and nonadherent cupric oxide and were not considered further. Solutionformed films on the other hand could be made to contain largely cuprous or cupric oxide depending on the rate of stirring of the solution. \Yhen the oxidizing solution is stirred either by mechanical rotation of the panel being oxidized or by bubbling air through it, the result is formation of a relatively thin cuprous oxide coating which may or may not contain some of the desired cupric oxide. Formation of cupric oxide seems to require relative stagnation of the solution a t the metal surface. When stirring is rapid, little or no cupric oxide is formed and it may
INDUSTRIAL AND ENGINEERING CHEMISTRY
be nonadherent in spots. When the oxide is formed in a still solution-i.e., with only thermal convection below the boil, it is heavier and contains several times as much cupric as cuprous oxide. In unstirred solutions, the rate of oxide film formation and the maximum coating thickness obtained are different for each alloy studied. Maximum thicknesses are obtained in about 1 hour. The thinnest film, 13,000 A,, is formed on beryllium copper and the thickest, 35,000 A., on copper; phosphor bronze and yellow brass have intermediate thicknesses. The minimum film thickness for satisfactory bonding-Le., a
Polyolefin insulation is shown here bonded to a copper core similar to that used in transoceanic cables
BONDING P O L Y O L E F I N S
*
total of 10,000 A.-is formed on all these alloys by immersion in the treating solution for 10 to 20 minutes. Molding Process. A satisfactory bond is essentially dependent on three molding temperature, parameters : molding time, and thickness of the oxide film. T o o low a molding temperature may not supply enough energy to oxidize the polymer within a reasonable time. Too high a temperature, beside being possibly harmful to the polymer, may consume too much of the oxide resulting in weak bonds. Too thin an oxide coating has essentially the same effect as molding at too high a temperature; it is consumed very rapidly with loss of most bond strength. From the data shown in Table I, it appears that optimum bond strength is achieved if the oxide treatment time is a t least 10 minutes and the molding time 1 to 10 minutes at 288’ C. I t should be noted that the upper limit of molding time at this temperature is dependent on the amount of oxide present. If a
Table I. Best Bond Strength i s Achieved If Oxide Treatment Is for at Least 10 Minutes and Molding Time Is 1 to 10 Minutes at 288’ C. Mold- TreatMolding Time, Min. ment ing i Peel ~ Strength, ~ , Lb./In. Temp. ~ Min. 1 5 10 20 30 O c. 204
232
3
5
7
5
10 14 17 20
4 3
7 6 6
3 7 10 14
11 11 10 9
17 20 260
5 7 4 6 6
3
7 10 14 17 20 288
3
7 10 14
17 20
Treatment
7
19 18 16 18 19 18
16 17 18 17 24 24
1-2 >25 17 20 >25 19
7 10 14 17 20
19 19 18
17 21 21
<
25 >25
17
