DE1949683A1 - Method of plating with aluminum - Google Patents

Description

September 30, 1969 H / W (106) 13,668

The Dow Chemical Company, Midland, Michigan, USA

METHOD OF PLATING WITH ALUMINUM

This invention relates to plating or cladding of substrates with aluminum. In particular, this invention is directed to a method at that a substrate with an aluminum coating and this Aluminum coating is provided with a protective top coating.

It is known that metallic aluminum coatings can be produced by using aluminum hydrides with a substrate at a temperature at which the aluminum hydride is decomposed. Such processes, which are also known as plating with aluminum, require the decomposition of the Aluminum hydrides have relatively high temperatures and are therefore unable to deposit or plating of aluminum can be used on a wide variety of heat sensitive substrates. The heat required will be the reaction systems in these working methods by means of convection ovens, heat guns or as radiant heat fed. When such heating means are used, it is difficult to maintain the critical temperature to control closely. Therefore, the platings made in this way are not always of uniform adhesive strength. It * exists-therefore a There is a great need for a method by which one can achieve uniform plating with aluminum at a relatively high level can perform at low temperatures *

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Another difficulty with the known plating with aluminum hydrides is that numerous alurainium hydride compounds are fickle in the presence of water, so plating is necessary essentially in an anhydrous and inert atmosphere perform. This gives rise to the additional wish after a plating process with aluminum, in which hydrides can be used, which are not water-resistant are.

A substrate that has been provided with a thin coating or film of aluminum is usually also preserved nor a top coat made of a resin or a plastic, for example made of a varnish resin, around the soft aluminum to protect against scratches and tarnishing and to maintain the mirror-like surface finish. Until now at least two separate procedural steps are required been to apply these two coatings; a process step for the plating with the aluminum and a further process step for the top coat. What is needed, however, is a method in which at the same time and in one operation uniform plating with aluminum at relatively low levels Temperatures and a top coat is reached. This would reduce the plating of heat sensitive substrates become possible and at the same time the number of process stages and the time required for the production of the End product can be reduced. By the present invention, the deposition of aluminum and one protective top coat at low temperatures possible, with a more precise control of the reaction conditions is possible than before and at the same time the number of process stages is reduced.

This invention relates to a process in which a catalyst composed of a transition metal is deposited on a substrate, which catalyst is capable of decomposing aluminum hydrides with the formation of metallic aluminum coatings at a relatively low temperature; an aluminum hydride compound is deposited on this substrate in the presence of said catalyst; the coated with the catalyst and the aluminum hydride substrate is contacted with a heated inert liquid bath in contact by a substance is dissolved, which can form a top coat; the substrate is kept in contact with the bath, e.g. B. by dipping to decompose the aluminum * hydride compound, with a metallic aluminum coating on the substrate; that with the AIu-

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The minium coated substrate is removed from the bath and the inert liquid is removed, leaving a topcoat for making coatings suitable material, for example a resin or a plastic, on the substrate coated with the aluminum forms.

In practicing the present invention, essentially any solid material will be used Suitable substrate. Examples of substrates that can be used are metals such as iron, magnesium, Brass and copper, polymers such as polyolefins, polyamides, polyesters and polymeric fluorocarbon compounds, Glass, paper, textiles, charcoal and graphite, wood and ceramic materials are all named after can be coated with aluminum using the method of the invention. The nature of the surface to be clad is determined to a large extent the gloss of the obtained aluminum plate ation. In general, the use of smooth, non-porous surfaces, such as those in the given to most metals and some polymer films, a glossier plating than bgi comparatively porous surfaces, such as those found on paper or textiles. On the surface of some polymers like Polyethylene, polytetrafluoroethylene, acrylonitrile-butadiene-styrene terpolymers and polypropylene can be made even better adhesion of the aluminum plating according to the invention can be achieved if the surface has been made polar beforehand, for example by sulfonation or by a corona discharge. With the invention, many shaped bodies such as contact wires, metal tips, reflectors, Car accessories and other decorative items are plated.

The term "aluminum hydride" is used in its broad sense herein and includes any hydride compound containing at least one aluminum atom to which a hydrogen atom is directly bonded, both the solvated and unsolvated forms of those aluminum hydrides being meant occur in both forms. Examples of compounds that belong to this group of compounds include: aluminum trihydride, the substituted aluminum hydrides, such as those of the empirical formula AlHnX3- _n »in which X is halogen, an -OR group or a -R group, where R is an alkyl, sub- ^{f-substituted} alkyl, can be aryl or substituted aryl group, and η has a value of 3 or may have less.

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Other aluminum hydride compounds that can also be used according to the invention are "complex aluminum hydrides such as L1A1H4, NaAltfy, Mg (AIH4) 2 and complex substituted aluminum hydrides such as those with the empirical formula M (AlHnjX4- _ra ) _a> where X the has the meaning given above , m has a value of 4 or less and M is a metal or a mixture of metals, preferably an alkali or alkaline earth metal * and a has a value which corresponds to the valency of M ent. The verbally simple aluminum hydrides , which contain at least two hydrogen atoms bonded to the aluminum, for example AlH3, AlH2Br and LiAlH4, are particularly useful. Mixtures of these different aluminum hydrides can also be used.

In order to facilitate the application of the aluminum hydride compounds, it is usually advantageous to use them in solvated form To use form. As a solvating agent or as an agent that complexes with the aluminum hydrides form, be ethers and other oxygen-containing organic Compounds called and compounds that have a functional group such as a divalent sulfur atom or a trivalent Contain nitrogen atom or a trivalent phosphorus atom, causing the solvation of the aluminum hydride with such connections becomes possible. An etherate is usually used as the solvate, and there are a large number of Ethers with about 2 to about 20 carbon atoms to form this etherate well suited. Usually lower aliphatic ethers such as ethyl, propyl or butyl ethers are used; However, it is also possible to use aromatic ether Groups such as methyl phenyl ether, ethyl phenyl ether, propyl ~ phenyl ethers or alicyclic ethers such as tetrahydrofuran too use. ,

In order to facilitate the application of the alurainium hydride compound and to obtain a uniform application, in generally any solvent or mixture of solvents or suspending agents for the aluminum hydride can be used, these means with the aluminum hydride should not react further than to form a complex or a solvate »Suitable solvents include aromatic hydrocarbons such as benzene, toluene and xylene, aliphatic hydrocarbons such as hexane, ethers and tertiary amines.

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Optionally, such aluminum hydrides can be prepared in situ simultaneously during the plating step, starting materials are used which form alurainium hydrides, for example mixtures of lithium aluminum hydride and aluminum chloride or sodium aluminum hydride and alurainium bromide. The presence of a metal halide such as LiCl, MgCl2 or AlCl3 along with the aluminum hydride is for the plating reaction is not detrimental.

In order to decompose the aluminum hydrides below their normal decomposition temperatures and thereby cause the aluminum formed to form a coating or plating on the substrate, it is necessary to bring the aluminum hydride into contact with certain decomposition catalysts based on transition metals. Such transition metal-based decomposition catalysts which can be used in the invention are compounds of metals of groups IVA and VA of the periodic table. If the catalyst is applied to the substrate in a solvent, it is advantageous that the metal is in the form of a compound which is soluble to an extent of at least 1 10 ""% by weight of the solvent used. Such compounds that have proven themselves are, for example, ZrCl4, NbCl5, VOCl ₂ , VOCI3, TiCl4 * 2 (C ₂ H ₅ X ₂ O, TiCU, TiBr4, VCl ₄ , Ti (OC ₂ H5) ₂ Cl ₂ , TiCl ₂ (i-OC ₃ H7) ₂ , TiCl ₂ «2 (C ₂ Hs) ₂ O, Ti (BH4) ₃ -2

(C2H5) ₂ O. Some of the catalysts based on transition metals listed here have a more pronounced effect than others in reducing the decomposition temperature of the aluminum hydride. The chlorides, bromides and oxychlorides of titanium, niobium, vanadium and zirconium appear to be more effective than other compounds of Groups IVA and VA of the transition metals. It has been found that TiCl _{4 is} particularly effective in lowering the decomposition temperature of the aluminum hydrides and the resulting aluminum plating. If the catalysts defined above are not used, undesirably high temperatures have to be used for the decomposition of the aluminum hydride. However, at such temperatures, even if decomposition is achieved, no coating or cladding of aluminum will typically be formed.

The decomposition catalyst based on a transition metal is preferably applied to the substrate prior to contact with the aluminum hydride. Such a decomposition catalyst can be applied directly to the substrate as a finely divided solid, or as a liquid solution or suspension or can also be applied as steam, if this »nature allow the catalyst and the substrate used.

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Preferably, however, the substrate is contacted with a sufficient amount of a relatively dilute solution of the decomposition catalyst to wet the surface of the substrate. The solvent for the catalyst is then removed, for example by evaporation, leaving the catalyst substantially uniformly dispersed on the surface to be plated. Catalyst solutions with at least about 1 χ 10% by weight of decomposition catalyst and preferably in concentrations of about 5 χ 10% to about 100% by weight catalyst produce, when applied to the substrate, sufficient catalyst to plating the aluminum from an aluminum hydride at a considerably lower temperature than this in the absence of decomposition! " catalyst is possible. It has been found that the uniformity of the distribution also has the catalyst on the substrate has a significant effect on both the uniformity than the thickness of the aluminum plating. It is therefore advantageous to apply the catalyst to the substrate in a manner which ensures a relatively uniform distribution.

For substrates where the surface properties the uniform distribution of a catalyst solution or of aluminum hydride difficult, for example metallic Magnesium, and certain solid polymers, have been found to be beneficial to such To add a small amount, for example 0.0001 to about 5.0% by weight, of a wetting agent to solutions. Suitable wetting agents include, for example, stearates such as sodium or aluminum stearate or aluminum alkoxides such as aluminum isopropoxide a.

Solvents that are suitable for the decomposition catalyst based on a transition metal are those normally liquid materials in which the catalyst is soluble to an extent of at least 1 χ 10 -6% by weight and which do not adversely affect the substrate and which the anion of the Do not change the catalyst so much that it becomes insoluble. Suitable solvents include non- reactive solvents such as benzene, hexane and halogenated hydrocarbons, reactive solvents such as alcohols, aldehydes, ketones, mercaptans, carboxylic acids and mineral acids, and coordination solvents such as ethers, nitriles, amides and amines.

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By applying the decomposition catalyst to only selected areas of the substrate, it is possible to obtain aluminum plating only in such selected areas. Decorative patterns, instructions or printed circuit boards can be attached in this way. Similarly, all or part of a selected substrate can be coated or plated with aluminum to increase the ability of such a surface to adhere to other materials. Coatings made of aluminum on surfaces of glass, ceramic materials, metals or polymers are of particular interest in order to increase their adhesion to adhesive polymers or adhesion polymers or copolymers, for example to copolymers of ethylene and acrylic acid.

As soon as the desired shape and amount of Zersetzungka catalyst has been applied to the substrate, the catalyst-containing surface of the substrate is brought into contact with a suitable form of the aluminum hydride. In general, it is advantageous to use the aluminum hydride as a 0.1 molar to 1.0 molar solution or suspension of the aluminum hydride, it being possible for these solutions or suspensions to be applied by dipping, spraying or other suitable means. However, good results are also obtained if the catalyst-containing surface of the substrate is brought into contact with finely divided solid aluminum hydride.

The solvent for the aluminum hydride compound is evaporated in the control and the so treated substrate is then contacted with a hot inert liquid bath containing the dissolved material for the top coat in contact, for example by dipping.

The inert liquid bath can consist of any substance which is liquid at elevated temperatures, typically at temperatures between 50 and about 200 ° C., and which is essentially inert to aluminum hydride and the particular substrate used. The bath should be essentially anhydrous, that is, it should typically contain less than about 20 parts per million of water because of the sensitivity of most aluminum hydrides to moisture. Similarly, application of the aluminum hydride compound to the substrate material should be carried out in a substantially anhydrous atmosphere for best results. Suitable materials for the bath are, for example, naphtha, ethylbutylbenzene, ethylbenzene, diphenyl ether, octadecane, xylene and other similar materials. Other inert aliphatic

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or aromatic compounds and ethers can be used that are liquid in the specified temperature range are. However, the bath should not be made using compounds with reactive groups as with alcohols, ketones, aldehydes, esters, acids and unsaturated aliphatic compounds, as in presence these connections result in aluminum claddings are dark and uneven and the substrate also contains unplated areas.

It is advantageous, but not necessary, to use substances which boil in the range from about 100 to about 140.degree. C. for the bath. Such Bad enables the transfer of the heat required for the production of aluminum plating and evaporated ausserdera also easy, thereby forming a smooth coating, the only ψ a minimum of flow marks has. Inert liquids with a higher boiling point can be removed after the top coat has been applied, for example by wiping them off or letting them run off.

Any substance which does not react essentially with the aluminum hydride compound and which gives a clear, flexible and adhesive film which is soluble or dispersible in the heated, inert bath liquids can be used as the coating material. The coating materials are generally organic materials and include, for example, film-forming resins and film-forming resins such as polyethylene, polypropylene, polyvinylidene fluoride, hexafluoroacetone, polyformaldehyde, polyparaxylene, polyethers, such as polyethylene oxide and polypropylene oxide, copolymers of epoxides and other similar Materi-) alien a that dry to a non-drying topcoat. The actual material used for the top coat will depend on the toughness desired and other properties of the coat being made. The concentration of the coating material in the liquid bath depends to some extent on the desired thickness of the finished topcoat. The bath is typically maintained at a temperature of about 50 to about 200 ° C, but a temperature of about 100 to about 150 ° is most preferred. The deposition temperature for the aluminum coating depends on the particular aluminum hydride compound used, the decomposition catalyst used and, to a certain extent, also on the catalyst concentration. Such deposition temperatures, however, are much lower than those required in the absence of a catalyst.

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The present process can also be used as a continuous operation for coating various shaped articles, for example films, webs of fabric or paper. It can also be used to coat other objects such as contact wires, metal tips, beads, powders, various irregularly shaped objects for light reflectors or tubes.

The invention is explained in more detail in the following example:

EXAMPLE

A sample of a commercially available polyester film was first coated with a metal catalyst, said catalyst of TiCl4 dissolved in the inert solvent was ethyl ether. The solvent was evaporated. Then an AIH3 solution consisting of 0.25 molar AIH3 in diethyl ether was applied to the sample and the solvent was evaporated again.

An organic bath was then prepared from xylene containing 5% by weight of polystyrene as material for the top coat, this bath being inert to aluminum hydride. The liquid bath was heated to 130 ° C and the sample of the film was immersed in the bath for 5 minutes, during which time an aluminum plating was formed on the film. The sample was removed from the bath and the xylene allowed to evaporate from the surface of the sample. A polystyrene topcoat formed on the metallic aluminum coating. The top coat adhered very well and gave the coated film a high degree of stiffness.

Similarly, various other substrate materials such as magnesium, aluminum, textiles, glass became ceramic Treated materials and paper to obtain glossy and uniform aluminum films that a cover made of a plastic material such as polyethylene, Polypropylene or polytetrafluoroethylene possessed.

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Claims (7)

Patent claims: 1. A process for the production of an aluminum coating on a substrate and a protective top coating of a film-forming material on this aluminum coating, characterized in that (a) the substrate with an aluminum hydride compound 1 »presence of a decomposition catalyst for the aluminum hydride based on a compound of an over gangsme- is coated tallcs, (b), the coated substrate said bath is maintained at a temperature is brought with a heated inert liquid bath containing a film-forming material for forming the protective overlay coating in an inert liquid in contact, the is sufficient to decompose the aluminum hydride. Compound so that metallic aluminum is deposited on the substrate, and (c) the aluminum coated substrate is removed from the bath and the inert liquid is removed from the coated substrate so that a coating of the film-forming material on the with alumin ium coated substrate arises. 2. The method according to claim 1, characterized in that as a decomposition catalyst for the aluminum hydride compound a compound of the transition metals of groups IVA and VA of the periodic table is used. 3. The method according to any one of claims 1 or 2, characterized in that the decomposition catalyst and the aluminum hydride compound are applied to the substrate as liquid dispersions or solutions which contain the decomposition catalyst and the aluminum hydride compound. 4. The method according to claim 3, characterized in that the liquid dispersions or solutions contain a wetting agent. 5. The method according to claim 1 or 2, characterized in that the material forming the top coat is an in oil is soluble film-forming material that dries to a non-stick topcoat. 009818/1208 6. The method according to any one of claims 1 or 2, characterized characterized in that the inert liquid bath consists essentially of an anhydrous material that is at a temperature from 50 to 200 ° C is liquid and is further characterized in that it is with the aluminum hydride compound, does not react with the decomposition catalyst and the substrate. 7. The method according to claim 1 or 2, characterized in that the inert bath at a temperature between 50 and 200 ° C is maintained. 009818/1208