WO2001012379A1

WO2001012379A1 - Method and composition for improved flux slurry wetting in heat exchanger brazing

Info

Publication number: WO2001012379A1
Application number: PCT/US2000/019561
Authority: WO
Inventors: Dennis D. Przybycien; David Childree
Original assignee: Kaiser Aluminum & Chemical Corporation
Priority date: 1999-08-16
Filing date: 2000-08-02
Publication date: 2001-02-22
Also published as: AU6490800A

Abstract

A non-ionic surfactant is added to a fluoridic flux slurry used for preparation of aluminum alloy heat exchanger component surfaces to be brazed. Relatively non-wettable (by a water-flux mixture) surfaces created by braze sheet rolling and part-forming lubricants are made wettable by the addition of a small quantity of the surfactant to the aqueous flux slurry. The surfactant is preferably a non-ionic alcohol based surfactant, such as Antarox BL225, and is added to the aqueous flux slurry in a concentration by volume of 0.01-5.0 %, and most preferably, 0.02-0.10 %. Use of the surfactant containing flux slurry results in a uniform thin coating by the slurry of all parts of the assembly that does not dewet from the aluminum alloy surfaces. The surfaces are thus uniformly treated by the flux to insure that the components can be brazed properly.

Description

METHOD AND COMPOSITION FOR IMPROVED FLUX SLURRY WETTING IN HEAT EXCHANGER BRAZING

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates in general to a method and composition for preparing aluminum alloy heat exchanger components for brazing in which, a surfactant is applied prior to adding cleaning flux to the parts to be brazed to improve uniformity of flux coverage. 2. Description of the Background Art The use of brazing to join aluminum alloy components of aluminum heat exchangers assemblies is the preferred process in the automotive industry. The surface of the alloy is often coated with oxide or thermal decomposition products of residual rolling oil which interfere with the formation of high quality, continuous, strong and defect free braze bonds. High quality bonds are necessary to achieve high mechanical strength, corrosion resistant, and leak-free brazed evaporator units.

In the past, workers in this field removed oxide layers from aluminum surfaces using treatment fluxes containing mixtures of chloride and/or fluoride compounds. Representative examples of patents describing this flux and the processes are U.S. Patent Nos. 3,393,447 to Paul, issued Jul. 23, 1968, 4,224,086 to Stokes, Jr. et al, issued Sep. 23, 1980, 4,556,165 to Yamawaki et al, issued Dec. 3, 1985, and 4,579,605 to Kawase et al, issued Apr. 1, 1986. However, the effectiveness of this process is dependent on the uiiiformity with which the treatment fluxes coat the aluminum surfaces. The residual forming oil on the aluminum surfaces creates a problem in this regard. In the past, alkaline or acid cleaners were often used to clean the oil off of the aluminum surfaces, however, the use of chemical cleaning raises environmental concerns. As a result, more and more heat exchanger manufactures remove forming oil via a process known as "thermal de-oiling" that is not completely effective at removing all of the oil. The residual oil is not solvent degreased or chemically cleaned before brazing. As such, the residual organic surface created by the thermal de-oiling (TDO) process can be hydrophobia If the surface is hydrophobic, uniform application of the fluoridic water-flux slurry is likely to result in inhomogeneous flux coverage that could result in substandard joints that could ultimately leak. Given the expected growth and usage of TDO, some changes are thus needed to improve the wetting of the water-flux slurry in heat exchanger brazing manufacturing process.

Volatile polar solvents, such as alcohols or glycols, have been added in the past to flux slurries in an attempt to improve the uniformity with which the flux slurries can be applied to components to be treated prior to brazing. However, these solvents do not work particularly well and must be used in relatively large quantities, thereby resulting in a substantially increased cost of the brazing process. In addition, their use results in the generation of volatile organic compounds that present environmental and safety concerns, and further increase the expense of using such solvents. As a result, a need therefore remains for an improved process by which aluminum alloy heat exchanger and other components can be effectively cleaned prior to brazing.

SUMMARY OF THE INVENTION The present invention fulfills the foregoing need through provision of a method and composition by which the wetting characteristics of a treatment flux are substantially improved to insure uniform flux coverage of an aluminum alloy surface to be prepared for brazing. More particularly, the inventors have discovered that a particular class of surfactants, namely, non-ionic surfactants, can be added in small amounts to the flux to increasing its wetting characteristics enough that uniform coverage by the flux of the aluminum components to be brazed is insured.

In experiments to test the efficacy of the surfactant modified flux slurries, it was found that the addition a low concentration, e.g., 0.01% to 5.0%, and most preferably 0.02- 0.1%, of a non- ionic alcohol based surfactant, such as Antarox BL225, is sufficient to modify the surface tension of the flux slurry enough that uniform flux slurry wetting of the components to be treated occurs. Preferably, the flux slurry/surfactant solution is continuously recycled, and no volatile organic compounds are emitted. Insignificant amounts of non- volatile surfactant remain on the heat exchanger parts as they go through subsequent process steps. Thus, the subject process does not raise environmental concerns, and does not substantially increase the cost of the flux slurry since only small amounts of the surfactants are necessary to obtain the desired results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES 1-3 To test the effectiveness of using non-ionic surfactants or wetting agents to improve flux slurry wetting, a number of experiments were performed using three example compositions, all of which are based on a 15% potassium fluro-aluminate (NOCOLOK) aqueous flux slurry, and a surfactant added to the slurry in different concentrations. First, an experiment was run to determine the range of surfactant concentrations that are necessary to obtain full flux slurry wetting of an aluminum surface to be fluxed. The surfactant selected for this experiment was a non-ionic alcohol based surfactant known as Antarox BL225 from Rhone-Poulenc/Rodia, although other non-ionic surfactants can also be employed, such as DowFax from Dow Chemical Corp., and BetzSolv 203A from Metchem Betz. BL 225 is specifically composed of ethoxylated propoxylated C8-10 alcohols. Although any surfactant which is capable of reducing the surface tension of water to the point of permitting wetting on brazing sheet surface will work in this application, it is preferred to use non-ionic surfactants because they are not surface reactive and do not leave non-volatile carbon residue on the brazing sheet.

The flux slurry was applied to aluminum brazing sheet, and the following observations were made with percent (v/v) concentrations of BL225 surfactant in 15% NOCOLOK aqueous flux slurry varying from 0.0% (no surfactant) to 0.5%:

TABLE 1

As illustrated in Table 1, the two examples having concentration levels of BL225 of

0.05% or greater resulted in a stable, fully wet condition. The example with 0.01% BL225 resulted in a partial dewetting condition when exposed to an immersion test. The conclusion is that a surfactant concentration of 0.01% to 5.0 % (by volume) can provide the described wetting benefit, a concentration of 0.01% to 1.0% surfactant provides wetting and economic benefit, and a concentration of 0.02% to 0.1% non-ionic surfactant provides optimum wetting and economy. In another test of effectiveness of surfactant wetting enhancement, various brazing materials with a range of surface tension were rated for wettability using 15% NOCOLOK flux slurry with and without surfactant addition. Low surface tension materials, at 23 Dyne/cm, with relatively oily surfaces were fully non- et with aqueous flux slurry. When 0.05% BL225 surfactant was added to the slurry, the braze sheet surfaces were fully wet after immersion.

Moderate surface tension materials, at 35 to 38 Dyne/cm, that were partially cleaned of residual oil through thermal processing were still not water wettable with surfactant-free flux slurry. When slurry was formulated with 0.05% BL225 surfactant the braze sheets surfaces were fully wet after immersion. These test results are summarized in Table 2:

TABLE 2

In yet another test, flux slurry wetting of brazing sheet was evaluated by two variations of a slurry-wetting test. One procedure was to place a single drop of slurry on the braze sheet surface and observe the drop's flow-out and spreading characteristics or contact angle. A relatively low surface tension brazing sheet, 23 Dyne/cm, was used for this test. A drop of aqueous flux slurry did not wet or spread on the sheet and contact angle was near 90°. A second procedure was to immerse the braze sheet into the flux slurry, withdraw the sheet, and observe slurry wetting. When the sheet was immersed in the non- surfactant flux slurry, the slurry dewet and drained from the surface leaving only a drop of slurry remaining. In tests performed with flux slurry formulated with 0.05% BL225 surfactant, the drop test resulted in slurry drop flow- out and flattened drop had a contact angle of approximately 15°. In an immersion test, the sheet fully wet resulting in an even coating of the slurry. The present invention thus provides a number of potential benefits. First, addition of surfactant surface tension modifier to the flux slurry permits complete flux coverage for more complete braze joining and a potential decrease in leak/scrap rate. Addition of the surfactant to flux slurry also results in an even and thin flux coating that can reduce flux use rate, and hence, reduce process cost. Further, surfactant use at the described low concentration does not significantly increase cost of the process. Finally, use of non-reactive, non-ionic surfactant assures that surfactant is removed during flux drying, braze preheat, or prebraze thermal steps, so it does not have any negative impact on the brazing, and does not present any environmental concerns. Although the invention has been disclosed in terms of a number of preferred embodiments, it will be understood that numerous additional variations and modifications could be made thereto without departing from the scope of the invention as defined in the following claims.

Claims

1. A method for applying brazing flux to one or more aluminum alloy components to be brazed comprising the steps of: a) providing an aqueous fluoridic brazing flux slurry; b) adding a non- ionic surfactant in a concentration by volume of between 0.01 and

5.0% to said flux slurry; and c) applying said flux slurry to said aluminum components to be brazed.

2. The method of claim 1, wherein said non-ionic surfactant is added to said slurry in a concentration of 0.02 to 0.1 % by volume.

3. The method of claim 1, wherein said aluminum components to be brazed are selected to be components for a heat exchanger.

4. The method of claim 1, wherein said surfactant is selected to be an alcohol based surfactant.

5. The method of claim 4, wherein said surfactant is selected to contain ethoxylated propoxylated C8-10 alcohols.

6. The method of claim 1, wherein said fluoridic brazing flux is selected to be a potassium fluro-aluminate brazing flux.

7. The method of claim 1, wherein said non- ionic surfactant is added to said slurry in a concentration of 0.02 to 0.1% by volume and is selected to contain ethoxylated propoxylated C8-10 alcohols, and said fluoridic brazing flux is selected to be a potassium fluro-aluminate brazing flux.

8. A flux composition for use in pre-treating aluminum alloy components to be brazed comprising: a) an aqueous fluoridic flux slurry; and b) a non-ionic surfactant in said slurry in a concentration by volume of between 0.01 and 5.0%.

9. The composition of claim 8, wherein said non-ionic surfactant is provided in said slurry in a concentration of 0.02 to 0.1% by volume.

10. The composition of claim 8, wherein said surfactant is an alcohol based surfactant.

11. The composition of claim 10, wherein said surfactant contains ethoxylated propoxylated C8-10 alcohols.

12. The composition of claim 7, wherein said fluoridic brazing flux contains potassium fluro-aluminate.

13. The composition of claim 7, wherein said non-ionic surfactant is provided in said slurry in a concentration of 0.02 to 0.1% by volume and contains ethoxylated propoxylated C8-10 alcohols, and said fluoridic brazing flux contains potassium fluro-aluminate.