Method Of Coating Iron Or Titanium Containing Substrate With Poly(arylene Sulfide)

Abstract

A substrate of iron or titanium or an alloy containing iron and/or titanium is pretreated at a temperature of about 650.degree. F. or higher, and thereafter coated with a composition comprising a poly(arylene sulfide).

Description

BACKGROUND OF THE INVENTION

This invention relates to improved methods of applying poly(arylene sulfide) coatings. In a more specific aspect, it relates to heattreating iron or titanium containing substrates prior to coating with poly(arylene sulfide) compositions such as poly(phenylene sulfide).

Poly(arylene sulfides) such as poly(phenylene sulfide) are well known in the art for their high-temperature stability. While it is generally regarded in the art that these polymers can be adhered to metallic substrates, poly(arylene sulfide) coated articles have not yet achieved significant commercial success. It has been found that, while poly(arylene sulfides) do adhere to metal substrates as broadly alleged in the prior art, the metals which would be of greatest commercial significance if coated with a poly(arylene sulfide), such as steel, titanium, and alloys containing iron and/or titanium, form a very weak bond to the poly(arylene sulfide). Efforts to improve the bond to these particular metals by conventional cleaning treatments, such as degreasing with a solvent or cleaning with acid have proven unsuccessful, indicating that the problem is of a more fundamental nature than simple interference with the bond by impurities on the surface of the metal.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved high-temperature laminate; it is a further object of this invention to provide a coating which is uniformly adhered to a substrate; and it is a still further object of this invention to provide a treating process for iron or titanium containing substrates which will allow improved adhesion to poly(arylene sulfides).

In accordance with this invention, iron or titanium containing substrates are pretreated at a temperature of about 650.degree. F. or higher prior to application of a coating of a poly(arylene sulfide).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Any poly(arylene sulfide) can be used in the practice of this invention. Suitable polymers are disclosed, for instance, in Edmonds et al., U.S. Pat. No. 3,354,129, Nov. 21, 1967. The presently preferred polymer is poly(phenylene sulfide).

The term poly(arylene sulfide) is meant to include not only homopolymers but also arylene sulfide copolymers, terpolymers, and the like. Suitable poly(arylene sulfides) of this invention are those having inherent viscosities in chloronaphthalene at 206.degree. C. of at least 0.1, preferably between 0.1 and 0.3, more preferably between 0.13 and 0.23.

The term iron as used herein is meant to include steel and any alloy containing iron as a major constituent. The instant invention is applicable to steel substrates, titanium substrates, and substrates of alloys containing a major proportion of iron and/or titanium.

The high-temperature pretreating step for the substrate can be carried out by any means known to the art, such as in an oven or in a flame, preferably a gas oxygen flame. The treatment can be carried out at any temperature between about 650.degree. F. and the temperature at which the particular metal in question begins to deform or soften. Preferably, temperatures of 675.degree.-800.degree. are used for the oven treatment. Preferably, the flame treatment is done with a gas oxygen flame having a temperature of about 1,815.degree. C. (natural gas plus oxygen gives a flame of about 2,930.degree. C. and can also be used), and is continued until the metal begins to turn bluish. Preferably, this pretreating step is carried out in the presence of an oxygen-containing gas such as air.

This pretreating step can take from 1 second to 5 hours. Generally, if oven treating is being utilized, the time will vary from about 15 to 90 minutes depending on the temperature. When the gas oxygen flame is used, the treatment is generally continued until a color change is noted in the metal, for instance when the metal begins to turn bluish. This time will vary greatly depending on the intensity of the flame, but generally will be within the range of 1 to 5 minutes.

In a preferred embodiment, the poly(arylene sulfide) coating composition contains titanium dioxide. Any titanium dioxide can be used. The presently preferred form is the rutile form. The titanium dioxide can be present in an amount within the range of 0.5 to 50 weight percent based on the weight of the solids, that is, the weight of the poly(arylene sulfide) and the titanium dioxide. More preferably, the concentration of titanium dioxide will be within the range of 10 to 35 weight percent.

After the substrate has been given the high-temperature pretreatment and has cooled, the coating is preferably applied in the form of a slurry of the poly(arylene sulfide) and titanium dioxide in an inert diluent.

Any low-boiling liquid can be used as the diluent. Preferred materials include ethylene glycol, methyl alcohol, water, and toluene, width ethylene glycol being the most preferred diluent. Certain high-boiling materials such as chlorinated biphenyl and dimethyl phthalate have been found to be less satisfactory. The coating slurries can be applied by any conventional means such as spraying, smoothing with a doctor blade, and the like. In addition, the compositions can be blended with conventional additives such as stabilizers, softeners, extenders, other polymers, other pigments, specific curing agents, and the like.

During the fusing step after the coating has been applied, the solvent is first evaporated off, and then at about 500.degree. F., the polymer melts and fuses together into a continuous coating. While cross-linking agents can be added, it is preferred to allow the cross-linking to take place without specific additives for that purpose. The exact nature of the cross-linking reaction is not known, but is is known that it occurs much faster in the presence of air or oxygen than in an inert atmosphere. Preferably, the curing is done in air at a temperature of 550.degree.-800.degree. F., preferably 650.degree.-750.degree. F., the preferred time within the range of 1 minute to 5 hours, preferably 15 minutes to 2 hours. Of course, there is a relationship between time, temperature, and oxygen, and at higher temperatures and/or higher oxygen contents, the time can be reduced.

EXAMPLE I

A slurry of 60 grams of poly(phenylene sulfide) and 20 grams of rutile titanium dioxide in 180 grams of ethylene glycol was stirred in a Waring blender for 15 minutes. The poly(phenylene sulfide) was an uncross-linked product having an inherent viscosity in chloronaphthalene of 0.2 at 206.degree. C. (In runs 1 and 3, the titanium dioxide was omitted from the formulation). Steel panels measuring 3.times.6 inches were coated with this formulation; three separate coats were applied, the composition being cured at about 700.degree. F. for about one-half hour after each coating. The following sequential steps were carried out on said steel panels. All of the steel panels were first cleaned by wiping with acetone. The control panel was then coated directly, while in runs 1 and 2 the panel was pretreated at 700.degree. F. in an oven under air atmosphere and run 3 was flame treated for 2 minutes prior to coating. All of the pretreated panels were cooled prior to applying the coating. The results were as follows: ---------------------------------------------------------------------------

Temperature Titanium Reverse of dioxide, Impact, Run Pretreatment Pretreatment weight % in.-lb.* __________________________________________________________________________ Control No -- 25 <20 1 Yes 700 0 <100(>20) 2 Yes 700 25 >160 3 Yes Gas Flame 0 >160 __________________________________________________________________________ *Gardner Laboratories Reverse Impact Tester. This equipment comprises a rounded tip which rests above the sample on the reverse side from the coating. A ram is raised a calibrated distance and dropped against the tip which then impinges on the back side of the coated slab. The side opposite that which comes in contact with the tip, that is, the side having the coating, is examined for cracks and looseness of the coating.

A comparison of runs 1 and 2 with the control reveals that at 700.degree. F. pretreatment, neither the pretreatment nor the presence of titanium dioxide gave a coating that was resistant to reverse impact test at 160 in.-lb. However, a combination of pretreating the panels at 700.degree. and the presence of the titanium dioxide in the poly(arylene sulfide) sulfide) composition does give such a bond. Thus, surprisingly, while heat treatment at 700.degree. F. alone will not effect the exceptionally tough bond as exemplified by a >160 value for reverse impact, nor will the presence of the titanium dioxide alone, the combination of the two coact to give the unexpected result, namely a coating so tough that it does not fail at the limiting of the test machine.

It is further noted that the heat treatment at 700.degree. F. does effect some improvement in the quality of the coating and further that heat treatment at higher temperatures, either in the oven or by means of a gas flame as noted by run 3, will effect the production of an unexpectedly high bond without the presence of titanium dioxide.

EXAMPLE II

The following conventional cleaning techniques were employed on steel panels in lieu of the high-temperature pretreatment, and the panels were thereafter coated and otherwise treated in a manner similar to that in example I. All of the following treatments resulted in either no improvement in adhesion or else detracted from the quality of the bond.

A. 10 percent sulfuric acid, followed by water rinse.

B. 15 percent phosphoric acid.

C. Zinc phosphate (Panels dipped in a solution of 300 grams, 85 percent H.sub.3 PO.sub.4 and 250 grams Zn.sub.3 (PO.sub.4).sub.2, in 1,200 grams deionized water.

D. Commercial iron phosphate treatment.

E. Degreased with methylene chloride, scrubbed with steel wool, rinsed, dipped in 10 percent sodium hydroxide at room temperature for 10 minutes, then dried in oven.

F. Steel panel wiped with methylene chloride, steel wool treated, rinsed and dried. Then treated with 10 percent sulfuric acid--10 percent nitric acid--80 percent water for 5 minutes. Thereafter rinsed for a few seconds and treated with 60 ml. concentrated HC1, 40 ml. water, and 30 percent hydrogen peroxide. Again rinsed, dried, and again polished with steel wool, rinsed and dried.

EXAMPLE III

A formulation similar to that of example I was applied to titanium coupons. One group of the coupons has been pretreated in a gas oxygen flame until the metal turned blue. The other was simply degreased with a solvent. The coating was then cut with a knife blade in the area to be tested. The coating on the laminate with the substrate which has been pretreated in a flame passed the 160 in.-lb. reverse impact test which was the limit of the machine and the coatings on the coupons which had not received the flame treatment failed this test at 160 in.-lb.

EXAMPLE IV

Aluminum coupons were coated with a formulation similar to that of example I. One group of the aluminum coupons was given a high-temperature pretreatment and the other was not. In both instances, the metal failed in the reverse impact test with the bonds still intact.

The data in example II reveals that conventional cleaning techniques do not improve the bond whereas, surprisingly, the heat treatment of the instant invention does. Example IV shows that no improvement is effected by heat treatment of aluminum, as evidenced by the reverse impact test.

It is noted that even the samples which were not treated in accordance with this invention do adhere to the poly(arylene sulfide) coating compositions as is clearly shown by the prior art. However, the treatments of the instant invention bring about such an improvement in the bond that the bond does not fail at the upper limits of the conventional testing machinery.

While this invention has been described in detail for the purpose of illustration, it is not to be construed as limited thereby, but is intended to cover all changes and modifications within the spirit and scope thereof.

Claims

I claim:

1. A coating process comprising the following sequential steps: pretreating a substrate selected from the group consisting of iron, titanium, and alloys containing a major proportion of at least one of iron and titanium, which substrate exhibits a weak bond to poly(arylene sulfide), by heating said substrate to a temperature of at least 650.degree. F.; cooling said substrate; and thereafter applying to said pretreated substrate a coating composition comprising said poly(arylene sulfide).

2. A process according to claim 1 wherein said substrate is heated in an oven at a temperature between 675.degree.- 800.degree. F. for between 15 and 90 minutes.

3. A process according to claim 1 wherein said pretreatment comprises flame treatment.

4. A process according to claim 1 wherein said pretreatment comprises flame treating with a gas oxygen flame until said substrate turns bluish.

5. A process according to claim 1 wherein said coating composition contains 0.5 to 50 weight percent TiO.sub.2 based on the total weight of the poly(arylene sulfide) and TiO.sub.2.

6. A process according to claim 1 wherein said poly(arylene sulfide) is poly(phenylene sulfide).

7. A process according to claim 1 wherein said coating composition is applied in the form a slurry in a diluent.

8. A process according to claim 7 wherein said poly(arylene sulfide) is poly(phenylene sulfide), said process comprising in addition heating said composition, after said composition has been applied to said substrate, at a temperature within the range of 550.degree. to 800.degree. F. to evaporate said diluent and fuse said poly(arylene sulfide) into a continuous coating.

9. A process according to claim 8 wherein said poly(phenylene sulfide) contains between 0.5 and 50 weight percent Ti0.sub.2 based on the total weight of the poly(arylene sulfide) and TiO.sub.2.

10. A process according to claim 9 wherein said substrate is steel.

11. A process according to claim 9 wherein said substrate is titanium.