CA1319571C - Treatment method for imparting antimicrobial and hydrophilic properties to aluminum surfaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Aluminum and aluminum based alloy surfaces can be made highly hydrophilic and resistant to the growth of odor causing microbes by treatment with a solution combining a waterglass and a borate or boric acid to form a liquid film, drying and heating the liquid film to form an adherent solid film, covering this solid film with a liquid film of a solution of a salt of hydroxide of a multivalent cation such as calcium or magnesium, and drying this second liquid film.

Description

1 3 1 9 5 7 ~ PATENT
~ocket M 4659 P + A/NI

TREATMENT METHOD FOR IMPARTING ANTIMICROBIAL AND
HYDROPHILIC PROPERTIES TO ALUMINUM SURFACES

Field of the Invention This invention relates to a surface treatment method fox making the surfaces of metals, particularly aluminum and aluminum alloys (both being briefly denoted herein merely as "aluminum"), hydrophilic, resistant to growth of microbes, and corrosion resistant. More particularly, the present invention relates to a surface treat~ent method for aluminum materials for use in heat exchangers.
Statement of Related Art ThPre are many known surface treatments for aluminum heat exchangers and their components such as fins and the like with the objective of preventing white rust. Examples of these surface treatments are anodic oxidation coatings and resin coating treatments, including those which contain hexavalent chromium. These coated surfaces are almost entirely lacking an affinity for water; instead, they are water repellent. ~hromate conversion coating treatments are also used, but such coatings have only a modest water affinity even on freshly formed coatings, and with the ~;,,.

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,' passage of time, particularly under hot, dry conditions, chromate conversion coatings tend to become thoroughly hydrophobic surfaces.
The fin spacing in many heat exchangers has been made very small in order to pack as much cooling surface area as possible into a given volume and thereby improve the radiation or cooling effectiveness. When the cooling surfaces or fins are even moderately hydrophobic, having a water wetting angle substantially greater than zero, water droplets tend to form on the surfaces by condensation of water from the atmosphere. When the cooling surfaces are very closely spaced, such water droplets can easily bridge the spaces between adjacent surfaces, thereby impeding the smooth flow of air through the spaces between the cooling surfaces and reducing the heat exchange efficiency.
Moreover, the water droplets accumulated in the fin gaps, because they are easily dispersed and scattered about by the heat exchanger's blower, are not effectively collected by any conventional droplet collector installed at the bottom of a heat exchanger, so that the vicinity around the heat exchanger becomes wet.
Accordingly, in order to avoid the generation of the mesh-like obstruction by the water droplets remaining in the fin gaps, treatments have been proposed for the purpose of imparting hydrophilicity to aluminum surfaces and improving their wettability by water.
In particular, the treatment of aluminum surfaces by silicates, for example, waterglass, etc., is widely employed as a hydrophilicizing treatment because of the high water affinity and high heat resistance of the surfaces thus produced, as well as low cost~ Proposed methods for this type of treatment include, for example, coating an aqueous silicate solution directly on conversion-treated aluminum or coating the silicate solution on the aluminum after the preliminary formation of an organic polymer film.
For example, Japanese Patent Application Laid Open 131q571 Number 50-38,645 proposes a method for the formation of a hydrophilic coating in which aluminum is first treated with a solution containing alkali metal carbonate and alkali metal chromate or alkali metal dichromate and is then treated with alkali metal silicate.
United States Patent 3,989,550 teaches a hydrophilicizing treatment method which consists of a first treatment with a solution containing fluorine, chromic acid, and phosphoric acid, in each case as ions. This is followed by treatment with an alkali metal silicate and then heating.
Japanese Patent Application Laid Open Number 54-57,264 teaches a heat exchanger that has surfaces with both corrosion resistance and hydrophilicity, as a result of the formation of a conversion coating on the aluminum surfaces.
Here, after the surfaces of an aluminum plate-fin-type heat exchanger have been treated with an aqueous solution of a silicic acid compound, a treatment is then carried out with an aqueous alkaline solution which contains an alkaline earth metal compound.
Japanese Patent Application Laid Open Number 57-82,467 teaches an improvement in physical properties by means of a post-treatment, and concerns not only the prevention of the discoloration of aluminum surfaces by hot water, but also refers to an improvement in corrosion resistance. In this case, the aluminum surface is coated with an aqueous solution of an alkaline silicic acid material. After drying, an anticorrosion coating is then formed by neutralization with an acid such as nitric acid, etc.
Japanese Patent Application Laid Open Number 61-84,383 teaches a method for treating the surface of aluminum for heat-exchanger service whose characteristic feature is that the aluminum surface is first coated with an aqueous solution of an alkali metal silicate to which a borate has been added; after drying, an aqueous solution of an inorganic acid, organic acid, or acidic salt of an inorganic or organic acid is then applied. KBO2, CaB~O7, i~A

1 3 1 q 5 -I 1 and Na2B407 are taught as optimal examples of the borate, while phosphoric acid or dihydrogenph~sphate, tannic acid, myoinositol hexaphosphate, polyacrylic acid, and their salts are given as optimal examples of the inorganic or organic acids and acidic salts thereof. Benefits taught for this method are an improvement in corrosion resistance and prevention of saponification of the lubricating oil applied in post-treatment of the treated aluminum.
Turning now to a consideration of the operation of automobile air conditioners and heating and/or cooling air conditioners in household service, one encounters the problem that an undesirable odor, which accompanies the fan draft during operation and particularly during start-up, often fills the interior of the conditioned room or passenger compartment. This odor tends to be stronger when more time has elapsed from the installation or last previous use of the air conditioner. The cause of the worsening of the odor with increased time is believed to be proliferation of fungi and/or bacteria in the air conditioner, particularly on and between the fins of the heat exchanger. With regard to preventive measures directed at this, discharging the air to the exterior using a bypass during the initial stage of operations and sterilization using a sterilizing lamp have been proposed.
However, the most effective method involves the application to the heat exchanger itself of a coating which contains an antimicrobial agent.
The ~ollowing are examples of teachings of how to impart an affinity for water to the heat exchanger fin surfaces while at the same time preventing the undesirable odor, through the application of a coating which contains an antimicrobial agent to the heat exchanger, filter, ducts, and the like, in air conditioning service: Japanese Patent Application Laid Open Numbers 58-10,051, 58-10,052, 58-101,717, 58~102,073, 59-124,428, 59-199,339, 60-50,397, and 61-168,675. However, in each case the antibacterial and anti~ungal agents in the preceding are organic f ~

1 31 q571 compounds or their metal salts.
Boric acids, the oxyacids afforded by the hydration of B203 and their salts are known for antiEungal and antibacterial applications.
Busan ll-M1 (sold by Buckman Laboratories, Inc., USA) is an anti-rust pigment which has barium metaborate as its principal component. It is marketed for paint and plastic applications as a pigment which also has antimicrobial properties, and it is registered with the EPA under No.
1448-17-AA. Its particle size is approximately 65% > 2 micrometers, approximately 15% between 1 to 2 micrometers, and approximately 10% < 1 micrometer.
Japanese Patent Application Laid Open Number 56-81,373 discloses an antifungal paint which contains zinc borate, while Japanese Patent Application Laid Open Number 60-146,807 discloses the addition of a thickener, such as an acrylic acid polymer, etc., to an antifungal agent which contains hydrogen peroxide and boric acid. The latter refers to application as a coating on bathroom ceilings, etc. Japanese Patent Application Laid Open Number 60-90,266 relates to a novel pigment which is obtained by a reaction in which a silicic acid compound is added to a phosphoric acid compound plus a boric acid compound plus Ca, Sn, Ba, Zn, Al, or their equivalents. This is described as effective both as an antifungal as well as an anti-rust in paint applications.
As discussed above, a number of methods are known for the treatment of heat exchangers and aluminum used in their fins in order to impart hydrophilicity and antifungal and/or antibacterial properties. Furthermore, treatment agents simultaneously manifesting both of these effects have also been proposed.
However, these known antifungal~antibacterial hydrophilicizing methods suffer from various disadvantages that are particularly notable in heat exchangers used for air conditioning. Surface-treatment agents with a high hydrophilicizing capacity are usually used to ameliorate ! ~

1 31 q57 1 the problem of mesh-like obstruction by condensed water droplets. These agents are generally highly hydrophilic organic polymers or waterglass. These materials are water soluble, and water is generally used as the solvent in these hydrophilicizing treatment solutions, thus avoiding environmental problems and ignition hazards. When an antifun~al is added to such an agent, a homogeneous mixed solution can be easily prepared if a water-soluble antifungal, for example, p-hydroxybenzoic acid, salicylic acid, sorbic acid, salts of the foregoing, borax, and the like, is used.
A problem with these water-soluble substances is their short term of effectiveness due to elution by condensed water during operation of the air conditioner. In a specific response to this retention problem, a poorly water-soluble or water-insoluble organic antifungal can be used ky emulsifying and dispersing it in water after dissolution in a suitable solvent, or an inorganic or organic antifungal can be suspended in water after comminution, followed in either case by addition to the hydrophilicizing treatment solution. However, the hydrophilicizing treatment solutions afforded by these methods again suffer from several problems: in addition to stability problems with the solution itself due to sedimentation and separation of the emulsified antifungal, it is difficult to obtain a uniform coating due to phase separation from the waterglass or hydrophilic polymer when the coating is dried.
Furthermore, due to the narrow fin spacing in heat exchangers~ as well as considerations of the heat conductivity and cost, it is advantageous for the coated hydrophilicizing film to form as thin a film as possiblP, less than 1 micrometer after drying. This places stringent requirements on comminution of particles to be suspended after grinding. Also, due to limitations in this regard, complete coverage cannot usually be obtained by a film produced using a hydrophilicizing treatment agent in which f~, 2 7~g7-~3 antlfungal particles are suspendecl; pinholes occur frequentl~ and when they occur subs~antially recluce anticorrosion performance.
The presen-t applicants have previously discovered tha-t a hydrophilic film, which manifests an excellent ~orrosion resist-ance, excellent hyd~ophilicity, good initial odor characteristics, good film bonding, and good p.rocessing properties, can be obtained by coating and drying a waterglass hydrophilicizin~ treatment agent on aluminum surfaces followed by treatment with a solution containing a multivalent metal salt. This has been the suhject of Japanese Patent Applications 62-179028 and 6~ 17gO29~ No borate or other antimicrobial agent is used in the hydrophilicizing treatment according to these earlier inventions.
According to the present invention there is provided a process for generating an antimicrobial or hydrophilic surface on an aluminum substrate, comprising:
(a~ coating the surface of said substrate with a first liquid film of an aqueous solution comprising (i) an alkali metal salt or a quaternary ammonium salt of silicic acid, said salt having a molar ratio of silica to monovalent cation between about 0.5 and about 8.5, and (ii) a boric acid or a salt thereof;
(b) drying by heating the liquid film produced in step (a) to obtain a first solld film on said surface;
(c) coating the solid film formed in s-tep (b) with a second liquid film of a solution comprising water and at least 0.01 mole per liter of a soluble salt or hydroxide of a cation with a charge of at least two f~

131957 1 ~7587 ~
units; and (d) drying by heating the liquid film formed in step (e~.
In a preferred embodiment the aluminum substrate is prepared by the following additional treatments before the above step (a):
(a') treatment with a chronie acid and ~hromate ion containing treatment solution; and (a'') coating the surfaee procluced by the treatment of part (a') with a film about 0.3 microns thick of a polyurethane elastomer.
Descriptlon of the Invention Except in the operatiny examples or where othe~wise expressly stated, all numbers in this description expressing quantities of materials or reaction cond:ltions are to be understood as modified by ~he word "about".
It has been diseovered that an antimierobial hydro-philicizing surface treatment, which will provide a high hydrophilicity, high corrosion resistance, and high antifungal performance, and which avoids diffieulties with lubrieating oil eoatability in post-treatment, ancl which results in ].ow odor, both initial and lony-term, can be obtained by first eoating aluminum with a hydrophilieizing aqueous treatment solution containing an alkali metal and~or a quaternary a~monium silieate acid, along with borie acid and/or its water-soluble salts, followed by heating and drying to form a homogeneous film, and by then carrying out treatment with a solution containing at least one - 7a -275~7-~3 species of multi~alent metals salt or hydroxide.
While not ~ishi~ to be ho~lnd by theory, the applicants believe that the boric acid and/or ~ater-soluble borate added to the ~7aterglass hydrophilicizing treatment solution may be converted to a poorly ion-exchangeable - 7b -i~

1 3 1 ~57 1 multivalent metal salt by the subsequent aqueous multivalent metal salt solution. Elution by condensation water generated by operation of the heat exchanger is thus made more difficult. Furthermore, the multivalent metal ion may form a cross link between borate anions and the anionic silicate network characteristic of waterglasses by bonding to each kind of negatively charged entity. This would serve to strengthen the hydrophilic film.
The film formed in the present invention, besides preventing initial odor from heat exchangers that utilize it, also prevents the longer-term development of odor originating in the growth of bacteria and fungi.
The first treating solution used in this invention is an aqueous solution containing quaternary amine silicate(s), for example, QAS - 40 or QAS 25 from Nissan Chemical Industries, Limited, or alkali silicat(es), either of these types of silicates being alternatively referred to below as waterglass(es). Waterglasses do not have a fixed composition as do most salts, but can have greater or lesser amounts of silicon oxide for a given amount of cations. The solution for this invention preferably has a value of 0.5 to 8.5 for the (SiO2)/(monovalent cation) molar ratio, with Li, Na, and K preferred as monovalent cations. More preferably, the cations are Na or K and the molar ratio is 1 to 2.
The preferred types of boric acids for the present invention are those with molecular formulas that can be matched by mixing B2O3 with varying numbers of water molecules, for example, orthoboric acid ~H3B03), metaboric acid (HB02), and tetraboric acid (H2B4O7).
Waterglass, and particularly high SiO2 waterglass, is prone to coagulation upon the addition of acidic substances. Thus instead of the free boric acids noted above, it is generally preferred to use neutral water~soluble salts, ~or example, sodium, potassium, or lithium borate. Furthermore, ammonium borate can also be admixed in part.

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1 31 q57 1 It is often useful to add water soluble organic polymers to the solutions of silicates and borates described above. suitable polymers include polyvinyl pyrrolidone, the water-soluble salts of polyacrylic acid or polymethacrylic acid, the polymers and copolymers of unsaturated acids such as maleic acid, or itaconic acid, the polymers and copolymers of acrylamide, methacrylamide, and their derivatives, the partial hydrolyzates of polyacrylamide and polymethacrylamide, polyvinyl alcohol, poly(2-hydroxyethyl acrylate), the alkali metal salts of polystyrenesulfonic acid, and pullulan.
The weight ratios in the aqueous solution preferably is 5 to 100 parts, more preferably 20 to 60 parts, of borate for each 100 parts of waterglass, and 0 to 100 parts, more preferably 0 to 60 parts organic polymer for each lO0 parts of the sum of the waterglass and borate.
Of course, in addition to the preceding, surfactants, leveling agents, non-boric acid antimicrobials, fragrances, colorants, defoamers, rust inhibitors, and the like can be present. Also, in order to increase the hydrophilicity, hydrophilic metal oxide sols, for example, silica sols or alumina sols can also be added.
The hydrophilicizing treatment solution obtained as described above can be coated and then dried on a bare aluminum surface, or it can be coated and dried after a conversion treatment or priming with an organic polymer has been used on the aluminum.
In the present context, aluminum also comprises aluminum alloys, and no specific restriction is placed on the application methodology: coating can be carried out either on the sheet-form fin material or after installation in a heat exchanger. In other words, one may freely select ~rom among brush application, immersion, spraying, roll coating, or flow coating, among others, according to the form of the substrate to be coated, quantity of application, and similar factors. Sheet-form fin material is typically roll coated, while the installed product is . ~ 3 typically imm~rsion coated.
After application of the hydrophilicizing treatment agent, the aluminum heat exchanger is dried by heating, without a water rinse, to form a hydrophilic coating film.
While the thickness of this film is not specifically restricted in the practice of this invention, thicknesses of approximately 0.02 to 2 microns are normally preferred and 0.05 to 1 micron will usually be most preferable.
Drying is typically conducted in a hot-air drying oven, but infrared heating or any other appropriate means can also be used. The usual hot-air drying oven has orifices through which hot air is ejected, and the aluminum heat exchanger or other object is dried by passage through the chamber while hot air is ejected from these holes.
While the drying conditions will vary substantially according to the part to be dried, 80 to 300 degrees Centigrade for 5 seconds to 30 minutes will usually be suitable, with the longer times at the lower temperatures, and the shorter times at the higher temperatures.
The dried hydrophilic film is then treated with an aqueous solution of the salt of a multivalent metal. Both flat fin sheet and assembled heat exchangers can be suitably treated by immersion, and flat fin sheet can also be treated by various other methods such as spraying, roll coating, and curtain coating, among others.
Useable as the multivalent metal are magnesium, calcium, strontium, barium, aluminum, titanium, zirconium, chromium, molybdenum, manganese, iron, cobalt, ~opper, zinc, tin, and any other cation with at least two charges that forms a water soluble salt. Magnesium, calcium, barium, and zinc are preferred. Water soluble solvents such as alcohol can also bs present in the treatment solutions. There is no limitation on the anion(s~ present, so long as water solubility is obtained, the nitrates, nitrites, fluorides, carbonates, bicarbonates, and hydroxides are advantageous from the standpoint of corrosion resistance.

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On the subject of the solution concentration, while practice of the invention will be possible as long as the cation's solubility is at least 0.01 mol/L-H2O, treatment is more effective at concentrations of at least 0.1 mol/L-H2O. With regard to the upper limit on ~he concentration, preferably no more than 3 moles of cations /L-H2O and particularly preferably no more than 2 mol metal ion/L-H2O is used from the standpoint of economics and because an excessive add-on in the absence of a water rinse can cause a deterioration in the heat-exchange performance. It is possible to use a slurry, for example, milk of lime, as long as some degree of solubility exists.
While no rigorous restriction applies to pH of the solution, one should avoid strongly acidic solutions capable of decomposing the silicic acid salts of multivalent metals, and the use of aqueous solutions with a pH of 3 to 13 is preferred, while a pH of 4 to 12 is even more preferable.
As long as the effect of treatment according to the application under consideration is not hindered, the aqueous multivalent metal salt solution may contain various additives. For example, in addition to surfactants, defoamers, antimicrobials, fragrances, colorants, and the like, when treating a surface bearing a chromic acid/chromate conversion treatment, one may find it effective to add a Cr6 reductant, for example, hydrazine salts, to reduce chromium elution. Furthermore, when the aqueous multivalsnt metal salt solution is to be roll coated on flat aluminum sheet, the addition of a thickener, leveling agent, surfactant, or the like may be useful.
When a slurry or aqueous solution of calcium hydroxide is used, a hydroxyl group-containing organic compound can also usefully be added in order to increase the solubility.
The time and temperature of treatment may be varied within wide limits. Short times at high temperatures may be used, but generally treatment at ambient temperature is preferred because of its lower costO

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The present invention is explained further in the following illustrative but not limiting examples, in which the ~ollowing measurement methods were used:
Contact anqle (hydrophilicityl. The contact angle for small (l to 2 mm diameter) water droplets placed on the solid surface was measured using a FACE
contact angle instrument (model CA-P from Kyowa Kaimen Kagaku). The contact angle was measured shortly after treatment of the samples according to this invention (designated in the Table below as llinitial") and after five cycles of immersion for eight hours in running water at room temperature followed by drying for 16 hours at 80 C (designated in the following Table as "after water wash").
Corrosion resistance. This was based on the salt-spray test method of JIS Z-2371. The reported value is the spray test period until the white rust sur~ace area reached 5%.
Antimicrobial ac.tivitv. The treated aluminum to be tested was cut into a 30 mm x 30 mm square and after immersion in running water for 1 week, was heat sterilized. This was fixed in the center of agar culture medium in a 90 mm petri dish. One mL of a microbial suspension, previously obtained by culturing the isolate from the adhered material on the surface of a heat exchan~er, was uniformly spread on the test specimen and the culture medium surface. Scoring was performed, aft~r incubation for 3 weeks at 28 C, according to the following table:
score rowth 0 no growth l very little growth 2 small amount of growth 3 moderate growth 4 heavy growth Initial Odor. This odor, which is believed due to volatilization of chemicals used in treatment, was evaluated on a 5 point-scale by ten individuals who smelled humid air directed onto the treated sheet. A
maximum score of 5 was allowed for the minimal amount of odor produced by a completely untreated aluminum sheet, while a minimum score of 1 was assigned to the unpleasant odor produced with a sheet subject~d to a chromate treatment, then waterglass treatment (without any boron containing chemical mixed with the waterglass), then nitric acid treatment.
Bondinq. Cellophane adhesive tape was adhered on the hydrophilic film and then quickly stripped off, and the ~xternal appearance was then ~valuated.
Lubricating oil_ coatability (processability).
The status of the oil coating was visually evaluated after immersion in Panting Oil AF-8F (from Idemitsu Sekiyu Kabushiki Kaisha) and stan~ing for 1 minute.
ExamPle 1 An aqueous mixed solution (7% nonvolatiles) of 100 parts (based on solids) potassium waterglass tsio2/x2o molar ratio = 3~ and 35 parts potassium borate was coated, in a quantity affording a dry film thickness of O.4 micrometers, on the surface of A-1100 aluminum sheet (0.1 mm thick). This was dried Eor 3 minutes in a 200 degrees Centigrade oven.
- 25 This sheet was then dipped for 30 seconds in a 3 %
aqueous solukion of calcium nitrate, rinsed, and finally dried for 2 minutes in a 200 degrees Centiyrade oven.
Example 2 This was the same as Example 1, except that before beginning the treatments described in ~xample 1, the aluminum surface was treated with chromic acid/chromate and then coated with a water-soluble polyurethane ~Elastron~
A-42 from Daiichi Kogyo Seiyaku Company, Limited). Drying for 3 minutes at 200 degrees Centigrade gave a film thickness of approximately 0.3 microns. This then received the same hydrophilicizing treatment solution as in Example 1, followed again by the same calcium nitrate solution dip k ~ 3 1 957 1 as above.
Examples 3 throuqh 6 These were treated in the same general way as Examples 1 and 2, but with the variations in materials used as shown in Tabla 1.
Example 7 A chromic acid/chromate-treated aluminum heat exchan~er was dipped into an aqueous mixed solution having 5 % total non volatiles, the non volatiles consisting of 100 parts waterglass no. 3, 40 parts sodium metaborate, and 60 parts sodium polyacrylate. A good solution uptake was found upon withdrawal. The treated heat exchanger was then dried for 5 minutes at 200 degrees Centigrade, immersed for 1 minute in a 3% solution of calcium nitrate, withdrawn, and finally dried for 2 minutes in a 200 degrees Centigrade oven.
Comparison Examples 1 and 2 Treatment followed the procedure of Example 1 or 2, respectively, but with the omission of the potassium borate.
Comparison Example 3 Treatment followed the procedure of Example 1, but with the omission of treatment with the aqueous calcium nitrate solution.
Comparison Example 4 Treatment followed the procedure of Example 3, but using phosphoric acid in place of the magnesium nitrate.
Comparison Example 5 Treatment followed the procedure of Example 4, with the exceptions that the addition of potassium borate was omitted and a nitric acid rinse was executed in place of - the aqueous barium nitrate rinse.
Comparison Example 6 Treatment followed the procedure of Example 4, with the exceptions that the aqueous barium nitrate rinse was omitted and an aqueous slurry of barium metahorate was used in place of the potassium borate of Example 1.

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1 3i q57 1 Comparison Example 7 Treatment followed the procedure of Example 7 with the exceptions that the addition of sodium metaborate was - omitted and a phosphoric acid rinse was executed in place of the aqueous calcium nitrate rinse.
The heat exchangers used in Example 7 and the present comparison example were respectively installed in air conditioners, each then received 2 mL of the microbial - suspension described above for the antimicrobial test, and each was then sprayed with approximately 10 mL of warm culture solution (100 mL water, 4 g glucose, 1 g peptone, and 0.5 g agar). After standing for 24 hours, each was then run through 10 cycles where 1 cycle comprised 1 hour of air cooling, 6 hours standing at room temperature, 1 hour of air cooling, and 16 hours standing at room temperature. The odor at the start of air-conditioner ~ operation was then evaluated by 10 people, on a scale running from 1 point for a highly objectionable odor to a maximum of 5 points for minimal odor, similar to that used for scoring initial odor as described above. The average odor score for Comparison Example 7 was 2.3 points, against an average of 3.8 for Example 7 according to the inventlon.

Some results for Examples 1 through 7 and Comparison Examples 1 through 7 are collectively reported in the second part of Table 1. As a comparison of Examples 1 through 7 with Comparison Examples 1 through 7 makes cl~ar, the invention of the pr~sent application not only affords an excellent hydrophilicity, corrosion resista~ce, initial odor resistance, bonding, and lubricating oil coatability, but also has longer lasting hydrophilicity and an excsllent antimicrobial performance.

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Table 1.
, ..
No. alurNnum substrate for borate post-treatment hydrophilicizing aclditive rinse trea tment Example ~ no pretreatment sheet potassium borate calcium nitrate xample 2 chromic acid/chromate sheet potassium borate calcium nitrate + primer Example 3 phosphoric acid/chromate sheet potassium borate magnesium nitrate Example 4 chrornic acid/chrornate shees potassium borate barium nitrate Example 5 chromic acid/chromate sheet potassium borate calcium hydroxide Example 6 chromic acid/chromate sheet potassium borate zinc - acetate xample 7 chrornic acid/chromate heat sodium metaborate calcium nitrate exchang~
Comparison Examples no pretreatment sheet _ calcium nitrate 2 chromic acid/chromate sheet _ calcium nitrate + primer 3 no pretreatment sheet potassium borate 4 phosphoric acid/chromate sheet potassium borate phosphoric acid chrornicacid/chrornatesheet _ nitric acid 6 chromic acid/chromate sheet barium metaborate 7 chromic acid/chromate heat _ phosphoric acid . _ _ exchanger (continued on the next page) A`

Tablel. (continued from rcvious a No. hydrophilicity conoslonini tial antimicrobial bond- lubricating (contact angle) resistance c,dor performance ability oil coatability ini tial wash (hours) Example I < 5o< 10 -15 192 4 ~ 5 I ~ +
Example 2 < 5o< 5O -10 > 480 5 I +
Example 3 < 5o< 5O ~ IOO 384 3 ~ 4 I ~ +
Example 4 < 510 ~ 15 > 4804 ~ 5 I + +
Example 5 ~ 5o< 5 ~ 10 > 480 5 I + +
Example 6 < 5o10 ~ 15 > 480 4 0~1 + +
Example 7 < 5~~ 5 ~ 10 > 480 4 (3.8j~

Comparison Examples <5<10~15 t68 3~4 3~~ +
2 <5< 5~15 >480 4 3-4 .~ +
3 < 515 ~20 120 3 2~3 x x 4 <5 10~15 360 2 4 ~
< 515 ~ 20> 480 I 3~4 x +
6 < 515 ~ 20> 480 3 2 x x ~ 510 ~ 15> 480 :2 -3(2.3)" _ (note) ~The evaluation method consisted of evaluation by odor and thus differed ~rom the other examples and comparison examples.

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Claims (9)

1. A process for generating an antimicrobial or hydrophilic surface on an aluminum substrate, comprising:
(a) coating the surface of said substrate with a first liquid film of an aqueous solution comprising (i) an alkali metal salt or a quaternary ammonium salt of silicic acid, said salt having a molar ratio of silica to monovalent cation between about 0.5 and about 8.5, and (ii) a boric acid or a salt thereof;
(b) drying by heating the liquid film produced in step (a) to obtain a first solid film on said surface;
(c) coating the solid film formed in step (b) with a second liquid film of a solution comprising water and at least 0.01 mole per liter of a soluble salt or hydroxide of a cation with a charge of at least two units; and (d) drying by heating the liquid film formed in step (c).

2. A process according to claim 1, wherein said boric acid is orthoboric acid, metaboric acid, or tetraboric acid.

3. A process according to claim 2, wherein said cation with a charge of at least 2 units is magnesium, barium, calcium or zinc.

4. A process according to claim 1, wherein said cation with a charge of at least two units is magnesium, barium, calcium or zinc.

5. A process according to any one of claims 1 to 4 wherein the dried film formed in step (b) has a thickness of about 0.02 to about 2 microns.

6. A process according to claim 5, wherein the salt of step (a) has sodium or potassium cations and a silica to cation molar ratio of about 1 to about 2.

7. A process according to claim 1, wherein the salt of step (a) has sodium or potassium cations and a silica to cation molar ratio of about 1 to about 2.

8. A process according to any one of claims 1 to 4 and 6 or 7, comprising the following additional treatments of the aluminum substrate before step (a):
(a') treatment with a chronic acid and chromate ion containing treatment solution; and (a'') coating the surface produced by the treatment of part (a') with a film about 0.3 microns thick of a polyurethane elastomer.

9. A process according to claim 5, comprising the following additional treatments of the aluminum substrate before step (a):
(a') treatment with a conventional chromic acid and chromate ion containing treatment solution; and (a'') coating the surface produce by the treatment of part (a') with a film about 0.3 microns thick of a polyurethane elastomer.