Rubber-Brass Adhesion Improved Through Treatment of the Metal With Amino Carboxylic Acid or Salt Thereof

This invention relates to the adhesion of rubber to brass, and more particularly, it relates to the adhesion of rubber to brass plated steel wire tire cord.

Applicant's copending U.S. patent application filed May 15, 1980, Ser. No. 150,103, and entitled "Adhesion of Brass to Rubber" discloses a process of treating brass with aqueous solutions of ammonium, Na, K and other salts of saturated aliphatic carboxylic acids of 1 to 5 carbon atoms to improve the adhesion of the brass to rubber.

Applicant's copending U.S. patent application filed May 22, 1980, Ser. No. 152,235, and entitled "Adhesion of Rubber to Brass" discloses a process of treating brass with a dilute acidic alcoholic solution followed by H.sub.2 S gas treatment, or with a dilute aqueous ammonia solution, preferably followed by H.sub.2 S gas treatment, to improve the adhesion of brass to rubber.

The bond between brass and rubber is believed to involve the formation of a thin layer of cuprous sulfide. During compounding and fabrication of the tire, the rubber picks up a small amount of water. Moreover, the presence of a layer of zinc hydroxide and/or oxide on the surface of the brass before use or the formation of zinc hydroxide and/or oxide by water in the rubber reacting with metallic zinc, leads to the very rapid growth of the cuprous sulfide layer and/or the formation of more zinc oxide and/or hydroxide. Some cuprous oxide, also, may be in these outer layers of the brass coating. These reactions are accelerated by the heat developed during operation of the tire. Thus, there is formed a relatively thick friable or weak layer of corrosion products where debonding can occur in said layer or between said layer and the brass and/or rubber strata.

Accordingly, it is an object of the present invention to overcome the difficulties alluded to above and to provide a method of treating brass to improve its adhesion to rubber.

A further object is to increase the resistance of the rubber-brass bond to degradation caused by heat and the presence of moisture.

Another object is to provide a brass-rubber composite exhibiting an improved brass-rubber bond.

Yet another object of this invention is to provide brass with a surface which is more amenable to bonding to rubber.

These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description and working examples.

According to the present invention it has been found that the corrosion products on brass plated steel cord in many instances can readily be removed by treatment of the cord with dilute aqueous solutions of amino carboxylic acids or their corresponding ammonium, lithium, sodium and/or potassium salts and salt hydrates. After dipping in the treating solution the cord may be rinsed in water, dried and combined with a vulcanized rubber compound and vulcanized. Adhesion tests of the treated cords in many instances show improvements in the unaged and humid aged strengths of the brass plated steel cord to rubber.

It is believed that the improved results in adhesion are obtained because the dip of the aqueous amino carboxylic acid solution removes an outer surface layer(s) of oxide(s) and/or hydroxide(s) especially of zinc and increases the ratio of copper to zinc in the outer surface layer(s) of the brass plating on the steel cords. Some copper oxide, also, may be removed by the treatment. Thus, a fresh or new brass surface is provided which is more amenable to bonding to rubber.

Brass plated steel tire cords (wire) are well known for use in the belts and carcasses of passenger, truck and off-the-road tires and for other purposes like belts. The wire may be woven or non-woven filaments of steel, and the wire or cords when used in tires is usually called a fabric. The steel may be dipped, electroplated or otherwise coated with the brass as is well known. The brass plating should be complete although some iron may be exposed on commercially brass plated steel cords. Usually the brass is deposited as a thin coating on the steel, usually not over about 1.2% by weight of the steel. The brass may contain from about 60 to 95% by weight of copper, preferably from about 62 to 72% by weight of copper, and even more preferably in the upper portion of this range to obtain higher adhesion, the balance being essentially zinc except for very minor amounts of other elements or compounds as adventitious or alloying materials. For more information on brass please see "Encyclopedia Of Chemical Technology," Kirk-Othmer, 2nd Ed., Vol. 6, pages 183 to 265, 1965, Interscience Publishers, a division of John Wiley & Sons, Inc., New York.

The amino carboxylic acids have from 2 to 24 carbon atoms, from 1 to 4 nitrogen atoms and from 1 to 6 acid or carboxylic acid groups. The corresponding ammonium, lithium, sodium and/or potassium salts and their salt hydrates may be used. Examples of such amino carboxylic acids are glycine (preferred), amino butyric acid, gamma amino butyric acid, sodium lauroyl sarcosinate, sarcosine, ethylene diamine tetraacetic acid, ethylene diamine tetraacetic acid disodium salt dihydrate, iminodiacetic acid, nitrilotriacetic acid, the disodium salt of nitrilotriacetic acid, sodium para-aminobenzoate, diethylene triamine pentaacetic acid, hydroxyethyl ethylene diamine triacetic acid, triethylene tetraamine hexaacetic acid, 1,3-diamino-2-propanol tetraacetic acid, 1,2-propylene diamine tetraacetic acid, ethylene diamine diacetic acid, symmetrical ethylene diamine diacetic acid disodium salt, hydroxyethyl glycine sodium salt, iminodiacetic acid disodium salt, N-methyliminoacetic acid disodium salt, oleyl sarcosine, ethylene diamine tetraacetic acid tri sodium salt monohydrate and ethylene diamine tetraacetic acid tetra sodium salt dihydrate. Mixtures of the amino carboxylic acids, their salts and salt hydrates can be used. Aliphatic amino carboxylic acids are preferred. These amino carboxylic acids have been known as chelating agents and surfactants.

The amino carboxylic acid is used in an amount of from about 0.5 to 6.0% by weight in water. Ammonium hydroxide, lithium hydroxide, sodium hydroxide and/or potassium hydroxide may be used to increase the solubility of the amino carboxylic acids in the water and/or to change the pH of the dip solution. Also, it is preferred that the pH of the aqueous solution of the amino carboxylic acids be from about 7 to 11.5.

The brass plated steel cords are dipped in the dipping bath or solution for a period of time and at a temperature sufficient to remove all or at least an appreciable amount of the corrosion products, such as oxides and hydroxides of copper and zinc and so forth, on the surface of or in the outer surface layers of the brass to improve the adhesion of the brass to the rubber. The time may vary from a few seconds to several minutes or more depending on the temperature, concentration and so forth. It is preferred to operate at about room temperature (ca 25.degree. C.) to avoid loss of water, treating agent and so forth although in a properly sealed or other apparatus designed to limit loss of water and treating agent or to permit its recovery, higher temperatures may be used.

After dipping the cords are desirably rinsed (dipped or washed) in water for a time and at a temperature sufficient to remove the treating solution and by-products or the products of the reaction of the treating solution with the surface of the brass coated cord. Temperatures should desirably be at about room temperature to prevent loss of the rinsing medium but higher temperatures can be utilized as set forth above if suitable precautions are taken.

The cords after rinsing are dried in nitrogen or other inert gas or in air provided the moisture content is low, preferably the gas or air should be dry, at temperatures sufficient to remove the solvents and other treating solution without formation of any appreciable amount of Cu or Zn oxides and/or hydroxides. Drying temperatures preferably may vary from about 25.degree. to 200.degree. C.

The treated cord can be combined (calendered, coated, laminated and so forth) with a curable rubber compound and cured (vulcanized). Alternatively, the cord may be stored such as in an inert or dry atmosphere until needed for use.

The rubber to which the treated cord is to be bound is preferably natural rubber or polyisoprene. However, blends thereof with other rubbery polymers like rubbery butadiene-styrene copolymers and/or polybutadiene can be used. For products other than tires there can be used nitrile rubber, polychloroprene and so forth. These rubbers can be suitably compounded with carbon black, silica (preferably dried), zinc oxide, stearic acid, antioxidants, accelerators, resins, sulfur and so forth.

Brass plated steel tire cords treated according to the present invention can be used in the belts and carcasses of passenger, truck and off-the-road tires, in belts and hose and for other purposes.

The following examples will serve to illustrate the present invention with more particularly to those skilled in the art.

In these examples to test the treated cords, the specimen cords were cut and without a solvent wash were embedded in a rubber compound. The rubber compound was 5/8" thick and backed with metal strips to reduce rubber deformation. The single end pullout force was measured on an Instron tester at 5 inches per minute using a special slotted jig to hold the sample (Coates and Lauer, "Rubber Chemistry & Technology," 1972, Vol. 45, No. 1, page 16). Each sample contained 16 cords. Also, humid aging tests were carried out on the sample after every other wire had been pulled (in other words after half the wires were pulled, the sample was humidity aged and then the remaining wires were tested), or on alternative samples. In any event, the reported values for adhesion are the average of 16 pulls and are reported in Newtons (N). The samples of the rubber compounds containing the wire cords embedded therein were cured in a mold at a temperature and for a period of time as indicated.

For humidity aging the rubber samples containing the embedded cords were placed in aluminum cylinders containng 100 cc of liquid water at the bottom of the cylinder, purged with nitrogen 3 times and sealed. The samples were on a screen in the cylinder above the liquid water. The sealed cylinders containing the samples, water and nitrogen were then aged for 72 hours in a hot air oven at 121.degree. C. After this period of time, the samples were removed from the oven and from the cylinders and stored wet in bags until testing which was conducted within a few hours after removal from the oven.

Adhesion tests for the unaged and humid aged samples were conducted at room temperature (about 25.degree. C.).

The rubber compound or stock used to make the samples for the adhesion tests comprised the following ingredients:

Bekaert and Enka Z brass plated steel wire tire cords (both 7.times.4.times.0.20 mm, 6.+-.2 g brass/kg of wire, 67.5%.+-.3.5% copper in the brass) were treated for 48 seconds in the dipping solution at room temperature which was followed by a 40 seconds washing step in water at room temperature and further followed by a 110 seconds drying step in air at 177.degree. C. The treated cords were allowed to cool and then were combined with the ruber compounds as shown above and cured at 153.degree. C. for 35 minutes. The details of the runs and results obtained are shown in Table I, below:

Enka Z brass plated steel wire tire cords (7.times.4.times.0.20 mm, 6.+-.2 g brass/kg of wire, 67.5%.+-.3.5% Cu in the brass) were treated for 48 seconds in the dipping solution at toom temperature which was followed by a 40 seconds washing or rinsing step in water at room temperature and further followed by a 110 seconds drying step in air at 177.degree. C. The treated cords were cooled and then combined with the rubber compound as shown above, except that 3.75 parts by weight of "Crystex" (80% sulfur in mineral oil, Stauffer Chem.) were used in place of 3 parts by weight of Rubbermakers Sulfur. Curing was conducted at 153.degree. C. for 35 minutes. The details of the runs and the results obtained are shown in Table II, below:

Bekaert brass plated steel wire tire cords (7.times.4.times.0.20 mm, 6.+-.2 g brass/kg of wire, 67.5%.+-.3.5% Cu in the brass) were treated for 48 seconds in the dipping solution at room temperature which was followed by a 40 seconds washing or rinsing step in water at room temperature and further followed by a 110 seconds drying step in air at 177.degree. C. The treated cords were cooled and then combined with the rubber compound as shown above, except that 3.75 parts by weight of "Crystex" (80% sulfur in mineral oil, Stauffer Chem.) were used in place of 3 parts by weight of Rubbermakers Sulfur). Curing was conducted at 153.degree. C. for 35 minutes. The details of the runs and the results obtained are shown in Table III, below:

The treatments were generally not as effective with the Bekaert cord as with the Enka cord which may be due to different inhibitor and/or lubricant residues on the Bekaert cord.