Hureaulite conversion coating as a base for the bonding of rubber to metal

Abstract

A metal-mineral system and method for making and using it. The metal-mineral system creates a bond between a metallic substrate and a top layer, preferably of rubber (to be adhered to). The metal-mineral system includes a fine crystalline coating of hureaulite (manganous iron phosphate) that is formed at less than 80° C. as a conversion coating onto the metallic substrate. The conversion coating passivates the metallic substrate and provides an enhanced surface area of contact that anchors the adhesive to the conversion coating. An adhesive is bonded to the conversion coating. A rubber top layer is applied to the adhesive. The metal-mineral system is resistant to breakage under stress and chemical attack. The method for making the metal-mineral system creates a reduced quantity of sludge with a beneficial effect on waste treatment procedures.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to compositions and processes for depositing a manganous iron phosphate (“hureaulite”) conversion coating on a metallic surface. More specifically, the invention relates to such compositions and processes that produce, at a temperature not more than about 80° C., a conversion coating that is applied to a metallic substrate that is suitable for supporting an adhesive to which an organic coating is affixed.

[0003] 2. Background Art

[0004] Conversion coatings form a base for the adhesion of organic coating to metallic substrates. As used herein, the term “conversion coating” refers to the conversion of a metallic surface to a non-metallic, mineral surface. The conversion coating is chemically anchored to a metallic substrate, and provides a corrosion-resistant base.

[0005] The rubber bonding industry utilizes phosphate-based conversion coatings to insure a strong bond between the metal substrate and an adhesive to which a variety of organic compounds, including rubber and paint, may be bonded. However, the standard conversion coatings used in the rubber bonding industry are heavy zinc phosphates or calcium-modified zinc phosphate conversion coatings which impart large crystals of hopeite and phosphophyllite to the steel substrate surface. Both of these types of phosphates require elevated temperatures of 92° C. to 98° C. for operation, and are known to generate large amounts of sludge when used on the typical hardened steel substrates that are used commercially. More specifically, heavy zinc phosphating baths form a heavy crystalline conversion coating which consists of a mixture of hopeite [(Zn3(PO4)2).4H2O] and phosphophyllite [(Zn2Fe(PO4)2.4H2O]. Such phosphating solutions typically require long processing times, high temperatures, and large amounts of zinc. The crystals deposited are large and overlapping.

[0006] Illustrative of the prior art are commonly owned U.S. Pat. Nos. 5,595,611; and 5,728,235; and M. S. Boulos & M. Petschel, “Coatings for Rubber Bonding and Paint Adhesion,” JOURNAL OF MATLS. ENG. & PERF., Vol. 6(4) (August 1997). Each of these is incorporated herein by reference.

SUMMARY OF THE INVENTION

[0007] The invention is a metal-mineral system that creates a bond between a metallic substrate and a top layer, preferably an organic coating, to be adhered thereto. The metal-mineral system includes a conversion coating of manganous iron phosphate (hureaulite) that is formed at about 50° C. to 60° C. The conversion coating is deposited onto the metallic substrate, which it serves to passivate. An adhesive is affixed to the conversion coating. In use, the conversion coating provides an enhanced surface area of contact for affixing the adhesive thereto. Superior performance is achieved by the metal-mineral system having high resistance to breakage and stress and the characteristic of being chemically resistant.

[0008] Thus, the invention describes a conversion coating which imparts a fine crystalline coating of hureaulite and iron-hureaulite to a metallic, preferably, steel substrate. This mineral system results in improved stress performance as compared to currently used processes.

[0009] Since hureaulite crystals are harder than hopeite-phosphophyllite crystals, the hureaulite conversion coating forms a base which is less susceptible to breakage under stress, and hence improves performance of the rubber-to-metal bond. Hureaulite crystals are also more chemically resistant than hopeite-phosphophyllite crystals, which also enhances performance under a variety of water-based and solvent-based adhesives.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a schematic, cross-sectional view (not to scale) of a metal-mineral system of the present invention; and

[0011] FIG. 2 is a flow chart of the main and optional steps followed in practicing the method of the disclosed invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0012] Turning first to FIG. 1 of the drawings, there is depicted a metal-mineral system 10 for creating a bond between a metallic substrate 12 and a top layer 14 to be adhered to. A fine (about 5-25 microns) crystalline conversion coating 16, preferably of hureaulite or of iron hureaulite is applied to the substrate 12. The manganous iron phosphate conversion coating of the present invention has a formula (Mn,Fe)5H2(PO4)4.4H2O (a manganous iron hureaulite). In use, the conversion coating 16 passivates the metallic substrate 12 and provides an enhanced surface area of contact for affixing an adhesive 18 to the conversion coating. The top layer 14 is applied to the adhesive 18. The resulting metal-mineral system 10 has the characteristics of resistance to breakage under stress and chemical attack.

[0013] Representative process steps and optional process steps are depicted in FIG. 2. As shown, the metal substrate is first cleaned in an alkaline solution and then rinsed in warm water. Optionally, depending on the surface type and condition of the substrate, an acid pickling step may be used, followed by a cold water rinse and a subsequent alkaline cleaning step. Following rinsing in warm water, the metal surface becomes clean and is oil-free. Next, the oil-free, clean metal surface is immersed in a nucleating solution of manganese-iron, and a manganese-iron phosphating solution from which the hureaulite crystals are formed on the metal substrate. This is then followed by cold water rinse step. Optionally, a corrosion-protective seal can be applied. This is followed (optionally) by exposure in a dry-off oven before the adhesive is applied. Following another dry-off exposure step in an oven, the adhesive-coated part may be stored. The part is subsequently bonded to rubber by application of pressure and heat, and then allowed to cool before use.

[0014] The hureaulite conversion coating forms at relatively low temperatures of about 60° C., which represents a substantial savings in energy when compared to currently used chemistries that operate at about 92° C. to 98° C. An added benefit is the relatively lower amount of sludge generated, which represents savings in waste-treatment.

[0015] The concentrate chemistry includes that found in the processing bath (Henkel Surface Technologies' commercial product Parco Lubrite (R) LT-10) and its replenishment (Parco Lubrite (R) LT-10 Replenisher). These products have compositions in accordance with the disclosures of U.S. Pat. Nos. 5,595,611 and 5,728,235 which were referenced earlier herein.

[0016] The principal advantages of the invention are:

[0017] Improved stress performance of the metal-to-rubber bond;

[0018] Lower operating temperature;

[0019] Lower sludge generation;

[0020] Internally accelerated; and

[0021] Provides corrosion protection to the treated work piece.

[0022] Preferably, the processing solution operates at a Q-Value=3.5, with a free acid=2 to 6; and a total acid=35 to 60. As used herein, the term “Q-value” is defined as the ratio of nitrate concentration to phosphate concentration, or [NO3]/[PO4].

[0023] The total acid content, consistent with general practice in the art, is measured in “points”, which are defined for the purposes of this description to be equal to the milliliters (“ml”) of 0.1 N NaOH required to titrate a 10 ml aliquot sample of the composition to a pH of 8.2 (e.g., with a phenolphthalein indicator). The content of “free acid” of compositions according to the invention is defined in the same way as points of total acid, except at the titration is to a pH of 3.8 (e.g., with bromophenol blue indicator). Also, as used herein, the term “rubber” refers to any number of natural or synthetic high polymers that are characterized by elastic recovery after vulcanization with sulfur or another cross-linking agent.

[0024] It is reported that phosphate baths based primarily on manganese as coating-forming cations usually form a crystalline species on ferrous surfaces comprised of (Mn,Fe)5H2(PO4)4.4H2O. This is a Mn, Fe hureaulite mixed crystal with variable amounts of the interchangeable Mn and Fe. W. Rausch, THE PHOSPHATING OF METALS, ASM, p. 103, which is incorporated herein by reference.

[0025] Although the adhesion mechanism between the conversion coating and the substrate is not known with precision, and without being bound by any specific theory, it is reported that the adhesion of manganous iron phosphate coating to metallic substrates stems from a mechanical “keying” of the phosphate crystals into the roughness of a metal substrate. Additionally, the structure and epitaxial orientation between the crystals of the phosphate and the grain structure of the base metal also play a significant role in the adhesion mechanism. W. Raush, supra.

[0026] In the examples below, the rubber bondings studies were conducted on post vulcanized rubber systems where curing adhesives were used to create the rubber to metal bond. If desired, water-based adhesives may be used because of the favorable environmental impact when compared with solvent-based types of adhesive. Solvent-based adhesives may perform better than water-based adhesives because the former had the capability of solubilizing residual soils on the metal surface. In contrast, water-based adhesives tend not to contribute to surface cleaning.

EXAMPLES

[0027] Examples of rubber bonding adhesion testing using the disclosed hureaulite coating are given in the Tables 1 and 2. A number of steel coupons were exposed to different surface treatment conversion coatings and then rubber bonded.

[0028] Table 1 gives the rubber bonding “failure interface” results obtained for these coupons when tested by the ASTM-D429 “B” method using a solvent-based single coat adhesive, 252X. The disclosed hureaulite coating (“Mn-Phosphate”) was compared with two conventional conversion coatings normally used in the rubber bonding industry, namely: calcium-modified zinc phosphate (“Zn—Ca Phosphate”) and heavy zinc phosphate (“Heavy Zn-Phosphate”). The results of Table 1 show a superior performance by the disclosed hureaulite coating. The hureaulite conversion coating could withstand up to 88.2 lb. in−2 with no failure at the coating-to-metal bond (C/M) or the coating-to-adhesive bond (A/C) interfaces, whereas the other two conversion coatings failed at either the C/M or A/C bond, and at a lower applied force. Preferably, first failure occurs at the adhesive-to-rubber (A/R) bond or within the rubber itself (R). 1 TABLE 1 Rubber Bonding Performance of Various Conversion Coatings on Steel Coupons: Failure Using Solvent-Based Single Coat Adhesive 252X1 Applied Conversion Coating Force Percent Failures By Type (Crystalline) (lb. in−2) C/M A/C A/R R Mn-Phosphate/Hureaulite 88.2 100  Mn-Phosphate/Hureaulite 65.1 100  Zn—Ca Phosphate 67.4 15 85 Zn—Ca Phosphate 60.7 25 75 Heavy Zn-Phosphate 72 20 80 Heavy Zn-Phosphate 71 20 80 Grit Blasted — Grit Blasted — Untreated 51.7 90 10 Untreated 51.9 75 25 1CM = failure at coating-to-metal interface; A/C = failure at adhesive-to-coating interface; A/R = failure at adhesive-to-rubber interface; R = failure of rubber

[0029] Table 2 gives the rubber bonding “failure interface” results obtained for these coupons when tested by the ASTM-D429 “B” method using a solvent-based double coat adhesive, 205/252X. The disclosed hureaulite coating “(Mn-Phosphate)” was again compared with two conventional conversion coatings normally used in the rubber bonding industry: calcium-modified zinc phosphate “(Zn—Ca Phosphate)” and heavy zinc phosphate (“Heavy Zn-Phosphate)”. The results of Table 2 show favorable performance by the disclosed hureaulite coating when compared with the two conversion coatings. 2 TABLE 2 Rubber Bonding Performance of Various Conversion Coatings on Steel Coupons: Failure Interface Using Solvent-Based Double Coat Adhesive 205/252X Applied Force Percent Failures By Type Conversion Coating (lb. in−2) C/M A/C A/R R Mn-Phosphate/Hureaulite 92.9 100 Mn-Phosphate/Hureaulite 89.9 100 Zn—Ca Phosphate 85.1 100 Zn—Ca Phosphate 70.0 100 Heavy Zn-Phosphate 95 100 Heavy Zn-Phosphate 113 100 Grit Blasted 96 100 Grit Blasted 100 100 Untreated 71.5 100 Untreated Substrate 64.9 100 CM = failure at coating-to-metal interface; A/C = failure at adhesive-to-coating interface; A/R = failure at adhesive to rubber interface; R = failure of rubber

[0030] These examples demonstrate superior adhesion by the disclosed polycrystalline conversion coating, which imparts a fine crystalline morphology to the metallic surface and markedly improves the adhesion of the top layer.

[0031] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims

1. A metal-mineral system for creating a bond between a metallic substrate and an adherent top layer, the metal-mineral system comprising:

a fine crystalline conversion coating of manganous iron phosphate formed at less than 80° C. onto the metallic substrate, the conversion coating serving to passivate the metallic substrate and provide an enhanced surface area of contact for anchoring an adhesive to the conversion coating;

an adhesive bonded to the conversion coating; and

a rubber top layer applied to the adhesive, the metal-mineral system being resistant to breakage under stress and chemical attack.

2. The metal-mineral system of claim 1, wherein the manganous iron phosphate coating comprises:

an average crystal size of about 5 to about 25 microns.

3. The metal-mineral system of claim 2, wherein the manganous iron phosphate coating comprises:

an average crystal size of about 10 to about 20 microns.

4. The metal-mineral system of claim 1 wherein the rubber is selected from the group consisting of natural and synthetic high polymers.

5. The metal-mineral system of claim 1, wherein the metallic substrate comprises steel.

6. The metal-mineral system of claim 1, wherein the metallic substrate comprises zinc.

7. The metal-mineral system of claim 1, wherein the metallic substrate comprises a ferrous metal

8. The metal-mineral system of claim 1, wherein the metallic substrate comprises a galvanized steel.

9. The metal-mineral system of claim 1, wherein the fine crystalline conversion coating is formed at a temperature between 50° C.-80° C.

10. The metal-mineral system of claim 1, wherein the fine crystalline conversion coating is formed at a temperature between 50° C.-60° C.

11. The metal-mineral system of claim 1, wherein the conversion coating of manganous iron phosphate comprises iron hureaulite.

12. A method of making a metal-mineral system for creating a bond between a metallic substrate and an adherent top layer, comprising the steps of:

preparing an oil-free, clean metal surface;

exposing the oil-free, clean metal surface to a nucleating solution to form a modified metal surface;

immersing the modified metal surface in a manganese-iron phosphating bath at less than 80° C. to form a fine crystalline conversion coating of manganese iron phosphate upon the modified metal surface;

bonding an adhesive to the conversion coating; and

applying a rubber top layer to the adhesive, the metal-mineral system have the characteristics of resistance to breakage under stress and chemical attack.

13. The method of claim 12 wherein the temperature of the manganese-iron phosphating bath is at an average temperature between 50° C.-80° C.

14. The method of claim 12 wherein the temperature of the manganese-iron phosphating bath is at an average temperature between 50° C. -60° C.

15. The method of claim 12 wherein the step of bonding the adhesive includes bonding the adhesive relatively uniformly over the entire metallic substrate.

16. The method of claim 12, wherein the processing solution operates at a Q-value of about 3.5, with a free acid amount between 2 and 6 points, and a total acid amount between 35 and 60 points.

17. A method of using the metal-mineral system of claim 1 comprising the steps of:

providing a metal-mineral system according to the method of claim 12; and

deploying the system in a rubber bonding environment where the system has characteristics of resistance to breakage, stress and chemical attack, while generating relatively low amounts of sludge during its preparation.