CA1162798A

CA1162798A - Method for preparing adherent layers on polyolefins

Info

Publication number: CA1162798A
Application number: CA000362232A
Authority: CA
Inventors: Boy Cornils; Gert Rehberg; Bela Tihanyi; Jurgen Weber
Original assignee: Ruhrchemie AG
Current assignee: Ruhrchemie AG
Priority date: 1979-10-17
Filing date: 1980-10-10
Publication date: 1984-02-28
Also published as: EP0027625A1; JPS635424B2; ES495953A0; ES8106020A1; DE2941896A1; DE3061044D1; ATE1753T1; JPS5665029A; EP0027625B1

Abstract

ABSTRACT OF THE DISCLOSURE
A method for producing a layer of a metal, metal oxide or metal carbide on a polyolefin, by contacting a surface of said polyolefin with a thermally decomposable metal compound in the gas phase, while said poly-olefin is heated at or above the decomposition temperature of the metal compound, in the presence of a gaseous carrier and/or reactant or reactant mixture, is disclosed..

Description

This invention relates to a method of producing coatings on poly-olefins by decomposing at least one volatile and easily thermally decompos-able metal compound from a gaseous mixture with a carrier gas and optionally other components onto a hot polyolefin substrate. This invention also re-lates to the so produced metal coated polyolefin.
It is known to produce metallic coatings on plastic surfaces by spraying the latter with suitable solutions, vapor deposition with metal vapors in vacuo, and by plastics electroplating ~cf. e.g. Rompp, Chemie Lexikon, 7th Edition 1974; "Kunststoffmetallisierung" (Plastics Electroplat-ing), No. 1902 and ~1. Domininghaus, Kunststoffe II; Processing, Surface Treatment, and Shaping; VDI-Taschenbucher T 8, 1969). However, these methods have considerable disadvantages if used to coat plastics, in particular polyolefins.
According to the spray method, silver coatings for example are ob-tained on plastics surfaces by spraying a silver salt solution and a reduc-ing agent. A shiny silver coating is thereby produced, which, however, is generally of uneven thickness and exhibits a poor degree of adhesion. Parts of the metal coating may flake off in the form of small flat platelets, if the article is subjected to even slight mechanical stress.
Metal coatings on plastics are nowadays mainly produced by high vacuum vapor deposition methods. However, working in a high vacuum of 10 3 -10 5 mbar requires expensive apparatus involving complicated techniques, and accordingly the range of application of plastics articles coated in this man-ner is restricted, for reasons of economy, only to special areas. In addi-tion, coating with difficultly vaporizable metals such as tungsten is pos-sible only with electron beam heaters (see Kirk-Othmer, Encyclopedia of chemical technology, 2nd Edition 1967, Vol. 13, Pages 249-284). Attempts have been made to circumvent further disadvantages of this method by complicated repeated pretreatment of the surface (see the company publication Hostalen PP "Metallisieren im Hochvacuum" (Metallisation in a high vacuum) of Hoechst AG).
Polyolefins can also be coated by the plastics electro-plating method, in which the surface must previously be made electrically conducting. This technique is mainly used in the case of ABS graft copolymers.
It is also known to coat metal plates by thermal decomposition of a readily decomposable metal compound on hot metal surfaces. This method was also applied to plastics having a fairly high thermal load capacity, e.g. polyesters, PTFE and ABS polymers, but not however to polyolefins.
On account of the relatively low crystallite melting points of the polyolefins, the poor adhesion of the metal coating produced by the aforementioned methods on the polyolefin, and the necessary pretreatment of the plastics surface by glow discharge or treatment in a vacuum chamber, the coating of polyolefins has proven to be technically and economically unsatisfactory.
Although the demand for metallized plastics articles and in particular foils (polyester, polyamide, PTFE) has risen sharply, and the use of coated polyolefins in solar technology, for packaging and insulating purposes, for capacitors, and for coating internal and external surfaces of tubes, pipes and other extruded and moulded parts would be advantageous, the use of metallized polyolefins has stagnated.
This invention, therefore, seeks to provide a widely applicable method for producing strongly adherent layers of metals ~'~

or compounds on polyolefins -that avoids the afore-described disadvantagesO
Thus this invention provides a method for producing a layer of a metal, metal oxide or metal carbide on a polyolefin, which comprises contacting a surface of said polyolefin with a thermally decomposable metal compound in the gas phase, while said polyolefin is heated at or above the decomposition temperature of the metal compound, in the presence of at least one member selected from the group consisting of a gaseous carrier, a gaseous reactant, and a gaseous reactant mixture, the velocity of the gas phase charged with the thermally decomposable metal compound being from 1 cm/sec to 2m/sec.
The method according to the invention enables polyolefins to be provided with strongly adherent metal, metal oxide or metal carbide layers without the surface of the polyolefin having to be pretreated. The process does not require vacuum or the rendering of the surface to be treated conductive.
The composition of the layer on the polyolefin depends on the nature of the thermally decomposable metal compound existing in the gas phase, as well as on the gaseous carrier and/or reactants or reactant mixtures.
For the coating, the polyolefin is heated to temperatures of 50 to 300C and preferably 130 to 250C. The temperature selected depends on the thermal stability of the polyolefin and on the decomposition temperature of the metal compound. Another important factor is what type of coating, i.e. metal, metal oxide or metal carbide, is to be formed.

Polyolefins suitable for the coating according to the J~
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method of the invention are in particular high and low density polyethylenes, ultra high molecular weight polyethylene with a viscosimetrically determined molecular weight of above 1 million, copolymers of ethylene, e.g. with butene, hexene, vinyl acetate, polypropylene, and copolymers of propylene or polyisobutylene.
~ he method according to the invention can be used to produce layers of widely differing metals. As examples, there may be mentioned; vanadium, chromium~ molybdenum, tungsten, manganese, iron, cobalt~ nickel, rhodium, - 3a -.

copper or silver. The metals are used in the form of easily thermally decom-posable compounds. Such compounds include metal carbonyls, and organometallic compounds such as metal alkyls especially Cl 18 alkyls, metal aryls especially C6 ~ Cl~ aryls, metal aralkyls especially those whose aryl groups have 6-12 carbocyclic carbon atoms and whose alkyl group has 1-18 carbon atoms, metal cyclopentadienyls, and metal arenes, especially those arenes having one or two aromatic groups and aliphatic chains of up to 18 carbon atoms; particular-ly contemplated arenes include cycloheptatriene-metal carbonyls, dibenzene metal carbonyls~ and benzene-metal carbonyls.
The decomposable compounds also include the acetylacetonates and the salts of lower carboxylic acids with up to 6 carbon atoms, especially alkanoic acids with one or more carboxylic acid groups.
Examples of metal compounds that can be used within the scope of the method according to the invention are the following (the temperatures within the brackets denote the decomposition temperature);
Cycloheptatriene chrome tricarbonyl C7H8Cr(C0)3 (128 to 130 C) Molybdenum hexacarbonyl Mo(C0)6 (150 to 151C) Tungsten hexacarbonyl W(C0)6 ~approx. 170C) Cobalt(II)-2-methylbutyrate Co(C4HgC00)2 (<170C) Cyclopentadienyl-iron-dicarbonyl iodide C5H5-Fe(C0)2J (approx- 119 C) Phenyl copper (I) C6H5Cu (80 C) ' Succinylo-pentacarbonyl manganese Mn(CO)5 C0CH2-CH2-cooH ~97 to g8 C) Tris(triphenylphospine)rhodiumcarbonyl hydrogen RhH(CO) (Ph3P)3 (~38 C) Cyclopentadienyl-triphenylphosphine-methyl-nickel NiC5H5(Ph3P)Me (115 to 118 C) Vanadium hexacarbonyl V(CO)6 (60 to 70C) An important feature of the new method is that the thermally decom-posable metal compound is used together with a gaseous carrier. This carrier may be inert under the chosen reaction conditions, i.e. it reacts neither with the metal compound nor with the polyolefin. One can also use active carriers~
i.e. one can use carriers that, for example, react with the metal compound and form a metal layer having a particular composition. Examples of gaseous carriers are nitrogen, hydrogen, carbon monoxide, carbon dioxide, inert gases (e.g. helium, argon), ammonia, oxygen, vapors of organic solvents, and water vapor (steam). The gaseous carrier may consist of a single substance, though mixtures such as air can also be used.
To produce purely metallic coatings, metal alkyls, metal aryls or metal compounds that also contain other ligands in addition to alkyl and/or aryl radicals are preferably employed as the thermally decomposable metal compound, and an inert substance is used as gaseous carrier. Layers contain-ing metal carbide are preferably formed from metal carbonyls. Metal oxide layers are obtained from a wide variety of decomposable metal compounds if oxygen or an oxygen-containing gas mixture is used as gaseous carrier.
The adhesion and properties of the layer formed by the method ac-cording to the invention on the polyolefin essentially depend on the partial , pressure of the metal compound in the gas phase, which is governed by two limit-ing conditions. On the one hand the partial pressure must be as low as possible so as to ensure that the metal is deposited in a fine, adherent form. On the other hand the deposition rate should be sufficiently large so that the coating is effected in an economically reasonable time and the polyolefin is not sub-jected to any unnecessary thermal stress. Partial pressures of 1 to 500 mbars and preferably 1 to 100 mbars have proven particularly suitable.
The structure of the layer formed on the polyolefin can also be influ-enced by varying the decomposition temperature and the velocity of the gas stream. At low gas stream velocities the metal coating is preferentially formed, especially in the case of a long coating apparatus, on the part of the article being coated that is closest to the inlet side, whereas the part in the vicinity of the outlet is only insufficiently coated. This disadvantage can be obviated by increasing the velocity of the gas stream to 1.4 meters per second. The velocity of the gas stream is usually l cm/sec. to 2 m/sec., preferably 0.27 to 1.4 m/sec.
Moreover, the coating rate and thus the duration of the coating pro-cedure can be regulated, depending on the strength of adhesion, by the velocity of the gas stream.
Finally, the residence time of the non-metallic decomposition products in the reaction space can be reduced by a higher velocity of the gas stream.
The said non-metallic decomposition products are thereby prevented from splitting into undesirable secondary decomposition products such as carbon on account of the catalytic effect of the deposited metal surface.
In the case of fairly high gas stream velocities it is recommended to recycle the gas mixture in order to avoid substance losses and obviate the necessity of preheating the carrier gas.

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The practical implementation of the method according to the inven-tion is not subject to fixed conditions. The thermally decomposable metal compounds are conveniently brought into the gas phase with the help of the carrier substance. In addition, the carrier substance is passed into or over the metal compound, which is preferably heated at 40 to 200C. The polyolefin may be heated in various ways, e.g. by means of a hot gas stream or by induction heat-ing. It has proved particularly suitable to utilize the heat present in the polyolefin and resulting from the thermal treatment of the polyolefins, e.g.
by compression mouldingj extruding, injection moulding or film blowing, to decompose the metal compounds, with the result that an additional supply of heat is not necessary.
A suitable coating apparatus consists of an evaporator part and a decomposition part. The decomposable metal compound is vaporized with the aid of a heating bath in an apparatus provided with an inlet pipe for the carrier gas. The evaporation apparatus is insulated up to the decomposition part. The article is coated in a cylinder connected to the evaporation equipment and heat-ed by an electric furnace.
The method according to the invention is described in more detail here-inafter with the aid of some examples.
Example 1 An article of ultra-high molecular weight polyethylene (mol. wt.: > 1.0 million) that has been cleaned with benzene is heated in a coating apparatus to 170C in a hot stream of nitrogen containing tungsten hexacarbonyl (partial pressure: 1.74 mbars). The velocity of the gas stream is 4.5 cm/sec. After a period of time a thin transparent film forms on the surface of the article, giving rise to a uniformly deep black metallic covering after a coating period of several hours. The coating produced adheres firmly to the substrate.

Example 2 -An article with a smooth surface of ultra-high molecular weight polyethylene (mol. wt.: > 1.0 million) that has been cleaned with benzene is heated in a coating apparatus to 200C in a hot stream of nitrogen and is then exposed to a stream of nitrogen containing tungsten hexacarbonyl ~partial pressure: 14.1 mbars). The velocity of the gas stream is 4.5 cm/sec. Already after 1 hour an adherent black metallic layer has formed.
Example 3 An article having a rough surace of ultra-high molecular weight polyethylene ~mol. wt: > 1.0 million) that has been cleaned with benzene is heated in a coating apparatus to 150C in a hot stream of nitrogen and is then exposed to a stream of nitrogen containing molybdenum hexacarbonyl ~par~ial pressure: 1.43 mbars). The velocity of the gas stream is 4.5 cm/sec. A
metallic grey coating of molybdenum which also uniformly covers the uneven-nesses is formed.
Example 4 An article having a rough surface that has been cleaned with benzene is heated in the coating apparatus to 180C in a hot stream of nitrogen and is th~n exposed to a stream of nitrogen containing molybdenum hexacarbonyl ~partial pressure: 6.26 mbars). The velocity of the gas stream, which is recycled, is 27 cm/sec. A shiny metallic molybdenum coating that adheres firmly to the surface is formed.
Example 5 An untreated article of ultra-high molecular weight polyethylene is heated in a coating apparatus to 170C in a stream of nitrogen at a pressure of about 10 mbars, and a stream of nitrogen containing copper 2-methyl buty-rate is led thereover. A firmly adhering metallic layer of copper is deposit-~L~62~ `

ed on the surface o~ the article within 2 hours.
Example 6 Ultra-high molecular weight polyethylene is coated corresponding to Example 5 with cobalt, and using cobalt 2-methyl butyrate as the de-composable metal compound. The amount of nitrogen is 80 - 100 l/hour~ the velocity of the gas stream is 4.5 cm/sec., and the coating duration is 1 hour. A brownish-black layer is obtained.
Example 7 An extruded article is coated, before cooling, with copper in the coating apparatus in a stream of nitrogen at a gas velocity of 27 cm/sec.
using phenyl copper (I) as decomposable meta] compound. The coating temp-erature is initially about 130C and gradually decreases as the article cools.
le 8 A compression moulded article is coated with vanadium at the con-clusion of the compression moulding stage and before cooling to room temper-ature, in the coating apparatus at about 130C in a stream of nitrogen hav-ing a velocity of 81 cm/sec., using vanadium hexacarbonyl as decomposable metal compound. The coating temperature falls gradually as the article cools. The coating is complete after 10 minutes. The surface is covered with a thin, strongly adherent metal film.
Example ~ (comparison) This example shows the different adhesion strength of a molybdenum layer on glass compared with polyolefins. For this purpose, a glass disc is cleaned with benzene and heated as in the previous examples in the coating apparatus, and then exposed to a stream of nitrogen containing molybdenum hexacarbonyl (partial pressure 6.26 mbars). The velocity of the gas stream :
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which is recycled, is 27 cm/sec. The metallic film formed can be removed from the glass surface by applying and then tearing off an adhesive strip.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing a layer of a metal, metal oxide or metal carbide on a polyolefin, which comprises contacting a surface of said poly-olefin with a thermally decomposable metal compound in the gas phase, while said polyolefin is heated at or above the decomposition temperature of the metal compound, in the presence of at least one member selected from the group consisting of a gaseous carrier, a gaseous reactant, and a gaseous reactant mixture, the velocity of the gas phase charged with the thermally decomposable metal compound being from 1 cm/sec to 2m/sec.

2. A method according to claim 1, wherein the readily decomposable metal compound is a metal alkyl, metal aryl, metal aralkyl, metal carbonyl or a metal salt of a lower carboxylic acid.

3. A method according to claim 1, wherein the readily thermally decompos-able compound is a compound of vanadium chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, rhodium, copper or silver.

4. A method according to claim 1, wherein nitrogen, hydrogen, carbon monoxide, carbon dioxide, an inert gas, ammonia, oxygen, vapors of organic solvents or steam or a mixture thereof is used as gaseous carriers.

5. A method according to claim 1, wherein the polyolefin to be coated is polyethylene or polypropylene.

6. A method according to claim 5, wherein said polyolefin is ultra-high molecular weight polyethylene.

7. A method according to claim 1, wherein the polyolefin to be coated is a copolymer.

8. A method according to claim 1, wherein the surface of the polyolefin to be coated is at a temperature of between 50 and 300°C during the coating operation.

9. A method according to claim 8, wherein the temperature is 130 to 250°C .

10. A method according to claim 1, wherein the coating of the polyolefin is carried out during or immediately after its thermal processing and before it has cooled below the decomposition temperature of the metal compound.

11. A method according to claim 10, wherein said thermal processing is extrusion, compression moulding or injection moulding.

12. A method according to claim 1, wherein the partial pressure of the readily decomposable metal compound admixed with the gaseous carrier is 0.010 to 1.00 bar.

13. A method according to claim 1, wherein the gaseous carrier charged with the readily decomposable metal compound is recycled.