WO2001061069A2

WO2001061069A2 - Plasma polymerized primers for metal pretreatment

Info

Publication number: WO2001061069A2
Application number: PCT/US2001/040122
Authority: WO
Inventors: F. James Boerio; Craig M. Bertelsen; Jennifer Kinn; R. Giles Dillingham
Original assignee: University Of Cincinnati
Priority date: 2000-02-18
Filing date: 2001-02-16
Publication date: 2001-08-23
Also published as: AU2001247975A1; WO2001061069A3

Abstract

The plasma-polymerized acetylene coatings described in the present invention provide superior adhesion characteristics between metals and various other substances, such as rubber, adhesives, paints and coatings. The structure and composition of the acetylene coatings are controlled to provide a metal surface having a high degree of unsaturation (i.e., acetylenic character) in the plasma polymer thereby providing a large number of reactive attachment sites to which other substances may bond.

Description

PLASMA POLYMERIZED PRIMERS FOR METAL PRETREATMENT

F. James Boerio, Craig M. Bertelsen, Jennifer R. Karcher, R. Giles Dillingham

This application is based on and claims priority from U.S. Provisional Patent Application Serial No. 60/183,406, F. James Boerio, Craig M. Bertelsen, Jennifer R.

Karcher filed February 18, 2000.

This work was supported in part by NSF Grant CTS-9407809.

Field of Invention

[0001] The present invention relates to methods of surface modification of metals using a plasma polymerization technique. More specifically, this invention relates to the priming of a metal surface with a plasma-polymerized acetylene coating to promote adhesion, prevent corrosion and reduce wear on the metal surface.

Background

[0002] Plasma polymerization is an environmentally compatible technique for depositing thin polymer coatings onto a variety of substrates. Such coatings can be used to promote adhesion, prevent corrosion and reduce wear to polymer and metal surfaces. With greater restrictions being placed on the use of industrial processes that produce hazardous by-products (e.g., chromate conversion coatings), novel processes that are compatible with the environment, such as plasma polymerization, are needed for the pretreatment of materials, especially in the pretreatment of metals. [0003] Many applications using metals require that the surface of the metal first be modified, pretreated, or finished to inhibit corrosion, to improve adhesion or wear resistance, or to enhance the appearance of the metal. The specific treatment depends on the characteristics of the metal to be treated. For example, aluminum is commonly etched in a mixture of chromic and sulfuric acids or anodized in either phosphoric or chromic acids prior to adhesive bonding. Phosphate or chromate conversion coatings may be applied to aluminum, steel, and galvanized steel to inhibit corrosion prior to adhesive bonding or painting.

[0004] In general, these processes work well at providing stable surfaces for painting or adhesive bonding or further treatment. However, these techniques usually involve the use of dangerous and/or toxic materials such as acids, organic solvents, and hexavalent chromium, frequently diluted with large volumes of water. Handling of these solutions is potentially dangerous, and disposing of the effluents after treatment can be a serious environmental problem that significantly increases production costs. Simple, modern, high performance techniques for the surface modification of metals that eliminate these toxic materials and the pollution that they generate are needed.

[0005] Previous work has focused on evaluating the corrosion protection of plasma polymerized silica on various substrates. For instance, Schreiber et al. investigated the use of plasma polymerized siloxane-like coatings for the corrosion protection of low carbon steel (see Schreiber, H.P., M.R. Wertheimer, A.M. Wrobel, Thin Solid Films, 72, 487 (1980)). Substrates were plasma etched to remove surface contamination before plasma polymerized siloxane-like coatings were deposited on the metal surface. These substrates were then exposed to a corrosive environment of LiOH at 275°C for 3 days with no visible signs of corrosion. Additionally, for approximately 4 weeks, samples were cycled ten times a minute through immersion in a 4% NaCl solution then dried in air. The siloxane-like coated specimens showed no visible evidence of corrosion whereas the non-coated control specimen showed extensive pitting and corrosion damage. Yasuda et al. and other investigators have extensively investigated the use of DC plasmas to obtain thin, corrosion-resistant coatings of SiO₂ on steel surfaces (see TJ. Lin, B.H. Chun, H.K. Yasuda, D.J. Yang, J.A.Antonelli, J. Adhesion Sci. Technol. Vol. 5, No. 10, p. 893 (1991); U.S. Patent No. 4,980,196, Yasuda et al., Dec. 25, 1990; U.S. Patent No. 4,981,713, Yasuda et al., January 1, 1991 ; U.S. Patent No. 5,312,529, Antonelli et al., May 17, 1994; U.S. Patent No. 5,812,000 Kobayashi et al, September 22, 1998). In other literature and patents, Hu and Tou developed a plasma process for creating a protective coating of SiOi ₈-₂ Co₃-ι oHo ₇-₄₀ on various surfaces (see R.J. Dykhouse, J.R. Dykhouse, T. Hu, P.J. O'Connor. Proc. 40^th Ann. Tech. Confi, Society of Vacuum Coaters 40 (1997); U.S. Patent No. 5,298,587, Hu et al., March 29, 1994; U.S. Patent No. 5,320,875, Hu et al, June 14, 1994; U.S. Patent No. 5,433,786, Hu et al., July 18, 1995; U.S. Patent No. 5,494,712, Hu et al, February 27, 1996). While plasma-polymerized silica affords one viable approach for metal pretreatment, this current technology has several drawbacks 1) the deposition rate required to obtain coatings of plasma polymerized silica with the desired structure is low, prohibiting fast cycle times, 2) in addition to coating the desired substrate, plasma deposition of silica produces deposits over most of the interior surface of the reactor which must be removed on a regular basis requiring dismantling of the reactor and mechanical cleaning using techniques such as sandblasting, 3) finally, these processes require the use of large amounts of oxygen in the process gas This necessitates the use of vacuum pumps configured for use with inert pumping fluids, which are expensive These fluids must be replaced at regular intervals when contaminated with powder and other byproducts of the deposition process

Moreover, none of this work addresses the use of organic plasma polymers to optimize adhesion performance or wear resistance of a metal surface What is needed is a non-pollutmg and commercially viable alternative surface pretreatment to both current wet-chemical treatments and silica-based plasma polymers which is based on thm coatings of plasma- deposited acetylene having an optimized structure The process of the present invention provides such an alternative This process utilizes plasma- polymeπzed acetylene coatings for pretreatment (I e , pnmmg) of metal surfaces This is a dry, rapid process, requmng only small amounts of inexpensive gases Further, deposits left in the reactor may be cleaned without dismantling through the use of an etching plasma at regular intervals Summary of the Invention

[0008] The present invention describes a method of applying plasma- polymerized acetylene coatings to the surfaces of metal substrates. Generally the process involves generating a plasma in the presence of a vacuum, preferably using an alternating current microwave-powered reactor. The surface of the metal substrate is then placed in operative association with the plasma. What is meant by operative association is that the sample should be a sufficient distance from the quartz window of the reactor to ensure adequate deposition rates of the plasma on to the metal surface; generally this will be about 110 mm. In many instances the metal surface is etched prior to deposition, and preferably this etching is done with a non-polymerizable gas plasma. The metal surface may also need to be degreased prior to the etching step.

[0009] At this point, a carrier gas, consisting essentially of argon, and acetylene as the monomer gas are introduced into the plasma. The surface of the metal substrate is then maintained in operative association with the plasma for about 30 seconds to about 5 minutes so that a plasma-polymerized acetylene coating is deposited on the surface of the metal substrate. The amount of argon is in a ratio of from about 1 :1 to about 1 :5, by volume, relative to the amount of acetylene and preferably in a ratio of from about 1 :2 to about 1 :4, by volume. The power of the reactor is maintained from about 75W to about 150W and the combined gas pressure for argon and acetylene is from about 0.5 Torr to about 1.5 Torr. [0010] One application of this technology is applying the plasma-polymerized acetylene coatings to the surface of a steel substrates prior to bonding the surface of the steel with rubber. Another application is applying the plasma- polymerized acetylene coatings to the surface of an aluminum substrates prior to bonding the surface of the aluminum to an epoxy resin.

Detailed Description of the Invention

[0011] Many types of silica-based plasma polymer coatings have been reported. However, plasma-polymerized acetylene coatings provide superior adhesion characteristics between metals and various other substances, such as rubber, adhesives, paints and coatings. The structure and composition of the acetylene coating is carefully controlled to provide a metal surface having a high density of reactive attachment sites to which other substances may bond.

[0012] The plasma-polymerized acetylene coatings of the present invention are unique in that they are preferably prepared using a microwave reactor and much of the acetylenic character of the monomer is retained in the polymer coating. This is due in large part to the gentleness of the microwave plasma- polymerization process. The reactor used for these depositions has been fully described in U.S. Patent Number 6,077,567, Boerio et al., June 20, 2000, which is herein incorporated by reference.

[0013] Prior to deposition of the plasma polymerized acetylene coating onto the metal surface, the low-pressure gas plasma is first used to clean the surface of the metal to a level unobtainable by wet cleaning techniques. The freshly cleaned surface is then immediately coated using gas-phase monomer to form a strong thin layer of acetylenic polymer. Because of this cleaning and oxide- conditioning step, the sub-micron polymer coating is chemically bonded to the metal surface. This chemical bonding protects the underlying metal from corrosion.

The chemical nature of the plasma-polymerized acetylene coating is important in forming a quality coating. Specifically, a high degree of unsaturation (i.e., acetylenic character) must be retained in the plasma polymer. This is accomplished through manipulation of the plasma polymerization conditions. The chemical nature of the plasma coating is controlled by the processing parameters used in the polymerization process. Infrared spectroscopy is used to monitor the formation of the plasma- polymerized acetylenic coating. Bands of interest are monitored at 3295 cm^"1, 3050 cm-', and 2800-3000 cm^"1 due to stretching vibrations of C≡CH (acetylenic hydrocarbon), C=CH (olefinic hydrocarbon) and CH and CH (paraffinic hydrocarbons) respectively. By varying the flow ratio of gases (argon and acetylene), pressure and power, different amounts of acetylenic (CH), olefinic (CH₂) and paraffinic (CH₃) carbon are produced in the coatings. Generally, the pressure, flow ratio of gases and power all have an effect on the chemical nature of the coatings produced. For instance, at lower acetylene pressures, more paraffinic carbon and less acetylenic and olefinic carbon is found in the coatings. At higher pressures the amount of paraffinic carbon decreases. By using simple qualitative tape tests (see below) it was found that better adhesion of the coating to the substrate was achieved with a pressure of about 1.5 Torr. [0015] When varying the ratio of flow gases (Ar : acetylene) while maintaining constant pressure, similar changes are observed. When the ratio of argon to acetylene is about 2.5:1, a greater amount of paraffinic carbon is found in the coatings. As the ratio of the argon to acetylene decreases (from about 1 : 1 to about 1 :5) to provide more acetylene gas in the reaction mixture, the amount of paraffinic carbon in the coating decreases and the amount of acetylenic and olefinic carbon increases. A preferred ratio of argon : acetylene is from about 1 :2 to about 1 :4. In simple tape tests, better adhesion of the coating to the substrates is obtained when the ratio of argon to acetylene is low (see below). When the ratio of argon to acetylene is increased to yield more argon in the reaction mixture, the coatings are easily removed with the tape.

[0016] Varying the power also affects the percentages of acetylenic, olefinic and paraffinic carbon in the coating. When the power is increased (ranging from 75 W to 150 W), the amount of acetylenic carbon in the coating decreases. Conversely, when the power is decreased, the amount of acetylenic carbon increases. At low power settings (75 W) and varying gas pressures, the concentration of acetylenic carbon remains approximately constant while the concentration of paraffinic carbon increases with respect to the concentration of olefinic carbon found in the coating. Power is the only variable which does not affect the adhesion of the coating to the substrate. In the tape tests of the coatings prepared at 75 W, 110 W and 150 W, visual examination shows little if any of the coating is removed by the tape. This indicates that flow ratio and pressure are more important than power for producing a coating which adheres to the metal substrate. [0017] Plasma-polymeπzed acetylene coatings are excellent pπmers for rubber-to-metal bonding One application for such coatings is to facilitate adhesion between natural rubber and steel tire cords used m the production of tires Currently, brass plating is used to promote adhesion betw een steel tire cords and natural rubber, but the plating process produces hazardous byproducts such as cyanide Another application for the plasma-polymerized acetylene coatings is to facilitate bonding with epoxy adhesives and also to enhance pamt adherence to the metal surfaces These coatings may also be used in combination with other coatings as a multi-layer coating to enhance properties of the material

Experimental Examples

General Procedures

[0018] The formation of a plasma-polymenzed acetylene coating is a three- step process compnsmg

1 Precleanmg of the surface (if necessary) with an aqueous detergent to remove heavy soils

2 Cleaning and oxide conditioning using non-polymeπzable gas plasma

3 Deposition of a thm corrosion resistant pnmer coating using a polymeπzable gas plasma

[0019] This pretreatment process produces a surface ideal suited for adhesive bonding or painting that exhibits a long shelf- life [0020] Plasma-polymerized coatings are preferably deposited using the microwave reactor described in U.S. Patent No. 6,077,567 although other types of low ion kinetic energy reactors, such as RF reactors may be used also. Samples are placed in the center of the reactor on a rotating stage that was positioned below the quartz window. Two monomer gas inlets are positioned approximately 2.5 inches above the stage on both sides.

[0021] Generally, depositions are preceded by a 10 minute argon etch at a pressure of about 0.5 Torr, at a power of about 200 W, and an argon flow rate of about 64 seem (standard cubic centimeters per minute). Infrared spectra of the coatings are obtained using a Nicolet 760 Magna FTIR spectrometer equipped with a Spectra-Tech FT-85 grazing angle reflectance accessory. The grazing angle used for all spectra was 85°. In addition to the infrared analysis, simple tape tests in which double-stick clear tape is pressed to the surface of the coating and is then removed in order to evaluate the adhesion of the coatings to the substrates.

Rubber-Steel Adhesion Studies

[0022] Plasma-polymerized acetylene primer coatings are more effective than brass plating as a pretreatment for bonding rubber-to-steel. Joints prepared from adherends coated with plasma-polymerized acetylene coatings had breaking strengths that were ~6 MPa higher than those prepared from brass substrates. In addition to having better adhesion in dry environments, the plasma-polymerized primer coatings also outperformed the brass in steam- aging durability tests. [0023] Lap joints are prepared to determine the effectiveness of plasma- polymerized primer coatings in promoting adhesion between steel substrates and rubber. Rubber/steel joints are prepared using small polished steel substrates that are bonded with a vulcanized natural rubber. The bonding area for the rubber/steel lap joints is 64 mm (0.1 in ).

[0024] Steel adherends used for the rubber/steel lap joints may be polished before bonding in order to reduce adhesion caused by mechanical interlocking. The polishing process involves degreasing with acetone, rough polishing with 600 grit silicon carbide, intermediate polishing with 6 μ and l μ diamond paste, and then final polishing with 0.3 μ aluminum oxide.

[0025] All of the primer coatings are prepared using an Ar : acetylene ratio of

0.8 and HOW of power. Pressure and deposition time were varied for the primer coatings. For each pressure (0.5 Torr and 1.5 Torr), coatings are deposited on adherends for 30 seconds, 1 minute, and 5 minutes. Both brass and polished steel are used as control sets for this experiment. The breaking strengths from these joints are shown in Table 1. The breaking strengths of the rubber/steel joints prepared from adherends that are primed with plasma- polymerized coatings are equal to or greater than the breaking strengths of the rubber/brass control joints. The joints prepared from the adherends primed with the plasma-polymerized coatings deposited at 1.5 Torr for 1 minute show the best adhesion with a breaking strength of 25.4±6.5MPa. The failure modes of these joints are either cohesive within the rubber or mostly cohesive within the rubber with some areas of tear-out. This behavior is consistent with the brass control joints. [0026] There is a direct relationship between pressure, breaking strength of the lap joints, and intensity of certain bands in the IR spectra. As previously discussed, as pressure increases, the intensity of CH₂ and CH₃ (paraffinic) stretching bands in the IR spectra of plasma-polymerized acetylene coatings decreases. There is a linear relationship between breaking strength of the lap joints and the amount of paraffinic carbon present in the plasma-polymerized coating. Therefore, the lower the concentrations of paraffinic carbon, the greater the adhesion of the primer coating.

[0027] Overall, the joints prepared from adherends that had plasma- polymerized acetylene coatings deposited at 1.5 Torr outperform the joints with coatings prepared at 0.5 Torr. There is a difference of -2MPa in breaking strengths of the joints prepared from adherends that are primed with plasma- polymerized coatings deposited at 1.5 Torr and 0.5 Torr. Nonetheless, joints prepared from adherends primed with plasma-polymerized acetylene coatings deposited at pressures of 1.5 Torr and 0.5 Torr, outperform the brass control joints. Joints prepared from adherends primed with coatings deposited at 1.5 Torr for 1 minute have breaking strengths that are -6 MPa larger than the brass control joints, while those prepared with coatings deposited at 0.5 Torr for 1 minute have breaking strengths of -4 MPa larger than the brass control joints.

[0028] Although the joints with coatings deposited at 1.5 Torr for 1 minute show the greatest amount of adhesion, joints prepared with coatings deposited at 1.5 Torr for 30 seconds perform almost as well. From Table 1, it can be seen that the joints prepared from the adherends with coatings deposited at 1.5 Torr for 1 minute and those deposited for 30 seconds show superior breaking strengths when compared to all of the other joints. The failure mode for all of

these joints is cohesive within the rubber. Although there are still areas of

tear-out on some of the failure surfaces, the areas are small and less apparent then with joints primed with coatings prepared at 0.5 Torr. In both cases, the

failure mode is similar to that of the brass joints. By decreasing the deposition time to 30 seconds, a thinner coating is prepared. Short deposition times are

required in a process suitable for manufacturing.

Table 1. Breaking strengths of miniature lap joints prepared from polished steel substrates coated with plasma-polymerized acetylene films deposited at HOW and a ratio of 0.8 Aπacetylene.

[0029] In addition to developing primer coatings that promote adhesion

between natural rubber and steel, the coatings also need to be corrosion resistant. Joints for durability testing are prepared from adherends coated with

plasma-polymerized acetylene coatings deposited at 1.5 Torr, 110 W, 0.8 Ar:C2H2 ratio for 1 minute. Five joints are submitted to steam aging and five joints are submitted to salt water aging. The joints prepared from plasma-

polymerized acetylene coatings on steel adherends are compared with joints

prepared from brass and polished steel. After exposure to the corrosive

environments, the joints are pulled to determine the breaking strengths. The results of the durability tests are shown in Table 2. As shown in Table 2, the joints prepared from adherends coated with plasma-polymerized acetylene

outperform the brass joints by ~7.3 MPa. The control joints prepared from the

polished steel do not perform well at all, displaying less than half of the

strength of the brass joints.

Table 2. Breaking strengths of miniature lap joints after three-day steam aging and three-day salt aging corrosion tests. Epoxy- Aluminum Adhesion Studies

[0030] Surface Preparation. Metal surfaces are degreased in acetone prior to use. Aluminum is etched to remove the thick, weak native oxide prior to primer deposition using a non-chromate acid etch (Aldeox 171, Abrite Chemical Co.).

[0031] Deposition of Plasma Polymers. After the acid etching process, samples are introduced into a microwave plasma reactor (the process has also been demonstrated in an RF plasma reactor). In the reactor and prior to deposition, the samples are first pre-treated using plasma pretreatment conditions and a non-polymerizable gas. This is followed by deposition with plasma-polymerized acetylene. The pretreatment takes place at 200 W, with an argon flow rate of 64 ml/min., a process time of 600 seconds and at a pressure of 0.5 Torr. The deposition with plasma-polymerized acetylene takes place at HOW, with an acetylene flow rate of 80 ml/min, an argon flow rate of 64 ml/min., a process time of about 60 to about 600 seconds and at a pressure of about 1.5 Torr.

[0032] Mechanical Testing: Lap-joints are constructed from the treated substrates following ASTM D 1002 using a commercial one-part epoxy resin (Teroson). A half inch joint overlap was used, giving a bonded area of 0.5 in². A bondline thickness of 0.1" was maintained by using copper wire of this diameter as a shim. The lap joints are clamped with spring clips during the curing cycle. The strengths quoted are an average of several lap-joints. Testing is performed on an Instron universal testing machine, using a crosshead speed of 0.05 inches/min. [0033] Adhesive joint durability is evaluated by comparing the strength of lap joints before and after hot water immersion. The immersion test is performed by suspending lap joints in a distilled water bath maintained at 60° C for 3

weeks. Table 3 shows the results of mechanical testing. The strengths of the

lap joints did not decrease significantly during the 3-week immersion.

Table 3. Strengths of lap joints prepared from adherends pretreated with plasma polymerized acetylene before and after immersion in 60° C water for 3 weeks. Quoted as average +/- two standard deviations (95% confidence interval).

[0034] Samples of ferrotype plate were included in each run as witness

coupons for ellipsometric thickness measurement and RAIR spectroscopy. A

spectrum obtained from one of these coatings shows a band near 3325 cm^"1,

indicating the presence of acetylide groups. The presence of this band is an

indicator that the appropriate coating structure has been obtained.

Claims

What is claimed is:

1) A method of applying plasma-polymerized acetylene coatings to metal substrates comprising the steps of: a) generating a plasma; b) placing a surface of a metal substrate in operative association with the plasma; c) introducing a carrier gas, consisting essentially of argon, to the plasma; d) introducing acetylene as a monomer gas to the plasma; e) maintaining said metal substrate surface in operative association with the plasma for a sufficient period of time so that a plasma-polymerized acetylene coating is deposited on the substrate surface.

2) The method of claim 1 wherein the plasma is generated in a microwave-powered reactor.

3) The method of claim 2 wherein the metal surface is etched with a non- polymerizable gas plasma prior to deposition.

4) The method of claim 3 wherein the amount of argon is in a ratio of from about 1 : 1 to about 1 :5 relative to the amount of acetylene.

5) The method of claim 4 wherein the amount of argon is in a ratio of from about 1 :2 to about 1 :4 relative to the amount of acetylene. 6) The method of claim 5 wherein the plasma is generated with an alternating current powered reactor.

7) The method of claim 6 wherein the power of the reactor is from about 75 W to about 150W.

8) The method of claim 7 wherein the gas pressure is from about 0.5 Torr to about 1.5 Torr.

9) The method of claim 8 wherein deposition time is from about 30 seconds to about 5 minutes.

10) The method of claim 9 wherein the metal surface is degreased prior to surface etching.

11) The method of claim 1 wherein the plasma is generated in the presence of a vacuum.

12) The method of claim 3 wherein the plasma-polymerized coating is applied to a surface of a steel substrate prior to bonding the surface of the steel substrate to rubber. 13) The method of claim 3 wherein the plasma-polymerized coating is applied to a surface of an aluminum substrate prior to bonding the surface of the aluminum substrate to an epoxy resin.

14) A process for priming a surface of a metal comprising the steps of: a) pretreating the surface of the metal by etching said surface; b) placing the surface of the metal in operative association with a polymerizable plasma; c) generating the polymerizable plasma in a microwave-powered reactor; d) introducing a carrier gas, consisting essentially of argon, to the plasma; e) introducing acetylene as a monomer gas to the plasma; f) maintaining said metal surface in operative association with the plasma from about 30 seconds to about 5 minutes so that an acetylene polymer coating is deposited on the metal surface.

15) The process of claim 14 wherein the amount of argon is in a ratio of from about 1 :2 to about 1 :4 relative to the amount of acetylene.

16) The process of claim 15 wherein the plasma is generated with an alternating current powered reactor.

17) The process of claim 16 wherein the power of the reactor is from 75 W to about 150W. 18) The process of claim 17 wherein the combined gas pressure is from about 0.5 Torr to about 1.5 Torr.

19) The process of claim 18 wherein the metal surface is degreased prior to surface etching.

20) The process of claim 19 wherein the plasma is generated in the presence of a vacuum.

21) The process of claim 15 wherein the acetylene polymer coating is applied to a surface of a steel substrate prior to bonding the surface of the steel substrate to rubber.

22) The process of claim 15 wherein the acetylene polymer coating is applied to a surface of an aluminum substrate prior to bonding the surface of the aluminum substrate to an epoxy resin.