Method for treating polyetherimide substrates and articles obtained therefrom

Abstract: The method is provided for treating the surface of a polyetherimide substrate to improve its adhesion characteristics for electrolessly deposited metal, such as copper. There is employed a series of surface treatments including the initial immersion of the polyetherimide substrate in sulfuric acid, contact with aqueous base, such as potassium hydroxide, followed by oxidation with an alkali metal permanganate such as potassium permanganate, and surface treatment with a reducing agent, such as a hydroxylamine salt. Polyetherimide metal composites are also provided.

Description: Background of the Invention The present invention relates to a method for treating polyetherimide surfaces to improve their adhesion characteristics to metal, such as copper, after the metal has been electrolessly deposited onto such molded or extruded polyetherimide surfaces. WO-A-87/00391 discloses a conditioning process for plastics such as polyetherimides including the steps of : treating with an organic swellant composition comprising a polar material in an organic solvent such as a glycol compound, treating with an etchant composition comprising chromium ions and preferably sulfuric acid and an optional treatment with a reductant for removing traces of deposits from the preceding step. Thereafter conventional electroless plating can be carried out. The process disclosed in WO-A 87/00391 does not suggest subsequent etching steps with sulfuric acid and alkaline permanganate. US-A-4515829 discloses a process for treating epoxy circuit boards which process comprises the steps of desmearing with sulfuric acid or plasma etching, immersing in an alkaline solvent solution, immersing in an alkaline permanganate solution and immersing in an acidified reducing solution. The thus treated circuit board may then be plated by any conventional electroless plating process including an activation with tin/palladium. There is no mention in US-A-4515829 of treating a polyetherimide substrate. Summary of the Invention It would be desirable to provide a procedure to treat the surface of an extruded or molded polyetherimide to allow for the production of metal-plastic composites. More particularly, it would be desirable to provide a method for making metal-plastic composites by electroless metal deposition to achieve superior adhesion between the electrolessly deposited metal and the plastic substrate. It also would be desirable to avoid discoloration between copper inserts in molded or extruded polyetherimide plastic substrates resulting from the employment of adhesion promoters, such as thiourea. The present invention is based on the discovery that the non-adherent white film formed on the surface of a molded or extruded polyetherimide resulting from sulfuric acid immersion can be chemically modified by rinsing the treated polyetherimide substrate with water and an aqueous basic solution, such as alkali metal hydroxide solution, followed by immersing the resulting treated plastic substrate into an aqueous alkali metal permanganate solution. The resulting treated substrate, after it has been subjected to a water rinse is found to be completely free of the white residue, normally formed from the sulfuric acid treatment. However, a brown manganese residue remains on the plastic surface. The brown manganese residue is readily removed by immersing the polyetherimide substrate into an aqueous solution of a reducing agent, such as hydroxylamine hydrochloride. Surprisingly, the resulting treated polyetherimide substrate provides excellent adhesion between electrolessly deposited metal, such as copper, and the polyetherimide substrate without the use of an adhesion promoter, such as thiourea. Statement of the Invention There is provided by the present invention, a method for modifying the surface of a polyetherimide substrate to improve its adhesion characteristics toward electrolessly deposited metal, which comprises, (A) treating the surface of the polyetherimide substrate with a degreasing agent, (B) modifying the degreased polyetherimide surface with sulfuric acid, having a concentration of 80 to 99%, (C) treating the modified polyetherimide surface with an aqueous base having a pH of 14 or greater, (D) oxidizing the resulting polyetherimide surface with an alkali metal permanganate resulting in the production of a manganese oxide residue, and (E) effecting the removal of the manganese oxide residue from the polyetherimide surface by treating the polyetherimide surface with a reducing agent. As used thereinafter, the term "degreased" means a polyetherimide substrate having its surface free of oil, molding compounds, finger prints or extraneous material. As used thereinafter the expression "modified with sulfuric acid" means treating the surface of the polyetherimide, by immersion, spraying, painting, or other forms of surface treatment with an aqueous solution of hydrogen sulfate having a concentration of from 80% to 99%, at a temperature of 0°C to 85°C for from 10 seconds to 30 minutes. Among the polyetherimides which are suitable for use in the practice of the present invention are polymers shown in U.S. Patent 3,847,867 and U.S. Patent, 3,803,085. The preferred polyetherimide used in the practice of the method of the present invention can be made by condensing 2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl] propane dianhydride with metaphenylene diamine in an extruder reactor under neat conditions. The resulting polyetherimide is readily injection molded to a variety of complex shapes including substrates for circuit boards or extruded and cut to size in the form of film having a thickness of 0.0013 to 0.064 cm (0.5 to 25 mil). The polyetherimides may contain various amounts of fillers or re-enforcing agents such as talc, mica, aluminum silicate, zinc oxide, titanium dioxide, carbon black, glassy fibers, glass spheres, carbon fibers and mixtures thereof. Polyetherimides also can be melt blended with silicone-polyimide polymers. In addition to the aforementioned fillers, the polyetherimides also can contain additional additives such as pigments, ultraviolet radiation absorbing agents, impact modifiers, platicizers, microwave absorbing agents, stabilizers, processing aids, and anti static agents. The polyetherimide substrate of the present invention may be in the form of a printed circuit board having flat or curved surfaces, which can include cavities, copper inserts such as pins or heat sinks, raised and recessed regions and through-holes. In addition, the polyetherimide may be in the form of a multi-level circuit board, or be shaped in specific designs such as coffee pots, thin films to provide flexible circuitry, or thin films adaptable to being incorporated as part of other molded shapes. The polyetherimide is initially degreased with a degreasing agent such as detergent, or a suitable organic solvent, such as a halo hydrocarbon for example a Freon solvent, such as 1,1,2-trichlorotrifluoroethane. After the initial degreasing step, the polyetherimide is modified with concentrated sulfuric acid as previously defined. A residual film of varying thicknesses can be formed on the polyetherimide surface during the sulfuric acid treatment step. The film may remain on the surface of the polyetherimide even after rinsing. The film is treated with an aqueous base of pH greater than 14, such as an aqueous basic solution of 0.1M to about 10M of an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, In addition to alkali metal hydroxide, there is included tetramethylammonium hydroxide. The aqueous base hydroxide treatment preferably is effected at room temperature. A water rinse can also be used if desired. The surface of the resulting polyetherimide is then immersed into an aqueous solution of an alkali metal hydroxide and alkali metal permanganate at a temperature of 25°C to 85°C for a period of 2 to 20 minutes. The use of an alkali metal hydroxide, as previously defined, with the alkali metal permanganate is optional. The permanganate oxidizing solution contains from 1 to 75 grams of the alkali metal permanganate, per liter of solution for effective results.The substrate can then be removed from the bath for a water rinse. The rinse can be effected by immersing the treated substrate in water, or by spraying or brushing. The alkali metal permanganate preferably includes potassium or sodium permanganate. Following the water rinse, a brown manganese containing film may be present on the surface of the rinsed polyetherimide substrate. It has been found effective at this time to treat the polyetherimide with a reducing agent such as an aqueous solution of hydroxylamine hydrochloride by immersing the polyetherimide substrate into the reducing bath. Additional reducing agents which can be used are, for example, a solution of 1 to 10% of stannous chloride in 5% HCl, an aqueous solution of 1 to 30% by weight of hydrogen peroxide, and a 1 to 30% solution of sodium bisulfite. The resulting polyetherimide substrate is then rinsed and allowed to dry to form a polyetherimide substrate having a chemically altered surface rendering it hydrophilic. In addition, it has been found to have a slightly higher ≡C-O- content, possibly indicating increased formation of carboxylic acid, ether, and/or alcohol groups. Its modified surface is capable of being activated even after an extended period of time with a tin-palladium colloid in a standard manner and metallized with an electrolessly deposited metal such as copper. Superior adhesion between the deposited metal and the polyetherimide surface is achieved. Although such a treatment is not deemed critical, its use often enhances the uniform deposition of electrolessly-applied metals onto the substrate. Activation and plating processes suitable for the present invention are also described in U.S. Patents 3,011,920 and 3,841,881. A water rinse generally follows the activation step. After surface activation and rinsing, electroless plating can be undertaken. The polyetherimide surface can be subjected to a heat treatment after electroless deposition of the metal. Oven heating of the entire article, i.e., substrate with metal thereon, is sufficient, although any heating method is suitable. Typically, this heat treatment is carried out at a temperature ranging from about 50°C to about 170°C for about 5 minutes to about 120 minutes, with higher temperatures within the above range generally compensating for shorter duration, and vice versa. Although the mechanism is not understood, the heat treatment appears to reduce the time required to attain optimal adhesion. An article prepared by the method of this invention usually has an electroless metallization layer which is about 0.25 µm (micron) to about 2 µm (microns) thick; a first electrolytically-applied layer of at least about 5 µm (microns); and a second electrolytically-applied layer having a thickness of at least about 5 µm (microns). Adhesion of the metal to the substrate was evaluated by measuring the force necessary to peel strips of the metal from the substrate surface. In the test, the metal surface of each plated sample is etched into 0.32 cm (1/8 inch) strips. An end of each strip is clipped to an Ametek digital force measuring gauge which is connected to a computer processor. Force values required to lift the metal strips from the substrate are converted by the computer into pounds per inch peel values. Multiple peel values for each strip are obtained and then averaged. In order that those skilled in the art will be better able to practice the present invention, the following examples are given by way of illustration and not by way of limitation. Example 1 A 2.54 cm by 15.2 cm by 0.16cm) (1 inch by 6 inch by 0.062 inch) molded polyetherimide slab having a specific gravity of 1.27 was treated as follows: 2 minutes in 1,1,2-trichlorotrifluoroethane Dry 0.5 minute in concentrated sulfuric acid, 23°C 2 minute water rinse 5 minutes in KOH(5M), 23°C 5 minutes in KMnO₄(15/g/L), KOH(1.2N), 75°C 2 minute water rinse 5 minutes in NH₂OH·HCl(2.5%), 23°C 2 minute water rinse The polyetherimide slab was examined by X-ray photoelectron spectroscopy to a depth of about 5 nm (50 Å) as shown by the following table: The polyetherimide slab was then immersed for 5 minutes in Shipley 1175A at a temperature of 65°C. There was utilized about 2 1/2% by volume of the agent in the conditioning mixture. The substrate was then rinsed for 2 minutes in water, then immersed for 1 minute in a bath containing 270 grams of Shipley Cataprep® 404, per liter of water. The substrate was then immersed in a bath for 3 minutes at 44°C containing 1.5 volumes of Shipley Cataposit® 44, per 100 volumes of Shipley Cataposit® 404 solution. The substrate was then rinsed for 2 minutes. The treated substrate was then immersed for 3 minutes in a bath of 20 volumes of Shipley Cuposit® Accelerator 19, per 100 volumes of water. The substrate was then rinsed for 2 minutes. The substrate was then immersed in a bath at 48°C for 30 minutes of Shipley Cuposit® 250 which consists of distilled or deionized water - 81.2% by volume, CUPOSIT 250M - 12.0% by volume, CUPOSIT 250A - 3.0% by volume, CUPOSIT Z - 2.3% by volume, and CUPOSIT Y - 1.5% by volume . There was obtained a composite of the polyetherimide substrate and electrolessly deposited copper having an average thickness of about 1.27 µm (0.05 mils). The composite was then removed from the electroless plating bath and rinsed in water for 2 minutes and then heated in water 95°C for 2 hours. The same procedure was repeated to produce additional composites of various polyetherimide and electrolessly deposited copper. The polyetherimide slabs were reinforced with various filler. The different composites were then separately immersed in an electroplating bath for 1 hour at 25°C. The electroplating bath consisted of an aqueous solution of 125 g/liter of CuSO₄·5H₂O, 60 g/liter of H₂SO₄ and 50 ppm of chloride in the form of HCl. It was run at a current density of 36A/sq.ft. The resulting composites had about 42.4 µm (1.67 mil) of electrolytically deposited copper. The composites were then heat treated for 16 hours at 95°C. Adhesion of the copper metal to the substrate was evaluated by measuring the force necessary to peel 0.32 cm (1/8th inch) strips of metal from the substrate surface. The 0.32 cm (1/8th inch) strips were prepared by a patterning technique using nitric acid as an etchant. The end of each metal strip was clipped to an AMETEK digital force measuring gauge connected to a computer processor. The force values to lift the metal strips from the polyetherimide substrate were converted by the computer to pounds per inch peel values. Multiple peel values were obtained for each strip. In Table II below, there is shown average adhesion values for the respective polyetherimides blended with various fillers. Example 2 The procedure of Example 1 was repeated using a polyetherimide reinforced with 30% by weight of glass fiber. Prior to the electroless deposition of copper to form the polyetherimide-copper composite, a variety of reducing agents were used to effect the removal of the brown manganese containing film after the potassium permanganate oxidation step. The following results were obtained with the polyetherimide-copper composite using the same adhesion measuring procedure as Example 1: The above results show that a variety of reducing agents can be used in addition to the hydroxlamine hydrochloride to achieve optimum adhesion results. Example 3 Polyetherimide composites made in accordance with example 2, using the NH₂OH·HCl manganese remover were initially rinsed with a 10% by weight solution of sulfuric acid in water. The polyetherimide composites were then electroplated. The composites were initially immersed in an electrolytic copper bath as described in example 1, (Non Additives) at 25°C for a period of up to 60 minutes (Additives). A current density of 36 ASF was used. Composites were also electroplated following the same procedure in an electrolytic copper bath containing Lea Ronal Copper Gleam PCM Plus, a brightener, for a total electroplating time of 60 minutes.. The following adhesion results were obtained following the same adhesion test procedure of example 1:

Claims: A method for modifying the surface of a polyetherimide substrate to improve its adhesion characteristics for electrolessly deposited metal which comprises (A) treating the polyetherimide surface with a degreasing agent, (B) modifying the degreased polyetherimide surface with sulfuric acid, having a concentration of 80 to 99%, (C) treating the modified polyetherimide surface with an aqueous base to provide a pH of 14 or above, (D) oxidizing the resulting polyetherimide surface with an alkali metal permanganate resulting in the production of a manganese oxide residue, and (E) effecting the removal of the manganese oxide residue from the polyetherimide surface by treating the polyetherimide surface with a reducing agent A method in accordance with claim 1, where the polyetherimide is reinforced with glass fiber. A method in accordance with claim 1, where the reducing agent is hydroxylamine hydrochloride. A method in accordance with claim 1, where the alkali metal permanganate is potassium permanganate. A method in accordance with claim 1, where the degreasing agent is 1,1-2-trichlorotrifluoroethane.

Inventors: DUMAS WILLIAM VINCENT, FOUST DONALD FRANKLIN, DUMAS, WILLIAM VINCENT, FOUST, DONALD FRANKLIN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1990
Main classification: C23C18
IPC codes: C23C18, H05K3, C08J7, B32B15