Field of Invention:
[0001] The present invention relates to a pre-treatment solution and to a method of forming
a layer of a coating metal on a substrate, said substrate having a plastics surface,
more specifically to a method of forming a layer of a coating metal on at least one
of a polyamide and a polyoxymethylene plastics part. Such solutions and methods are
applied to plating polyamide or polyoxymethylene plastics substrates especially in
decorative plating industry.
Background Art:
[0002] Polyamide plastics parts have been electroplated for many years. Nevertheless, little
research effort has been made in this field because electroplating polyamide plastics
parts has ever been an application having a narrow application range due to the limited
market thereof. A first method has been described in a publication on the
65th Conference of the American Electroplating Society (AES) in Washington (1978) which achieved a peel strength of the metal layer to the plastics part higher than
about 1 N/mm. However, no indication was made as to specific details of this method,
except for that no chromic acid was used. The influence of the surface texture obtained
after the first method step on the adhesion strength was described to be similar to
that of acrylonitrile-butadiene-styrene-copolymer (ABS) plastics parts. With ABS plastics
parts a clear correlation proves to exist between the microscopic surface texture
and adhesion strength.
[0003] Nowadays two main alternatives are available as to the first method step in pre-treating
polyamide plastics parts, these alternatives comprising pre-treating the parts with
a solution containing chromium(VI), on the one hand, and pre-treating the parts using
a solvent and an acid, on the other hand. Pre-treatment using chromium(VI) has the
main disadvantage of being dangerous to health and to environment, therefore its use
is limited. Further, in contrast to ABS plastics, polyamide plastics exhibit a relatively
open structure of the polymer, thus having a considerable water uptake of up to 6
%. This property will bring also about considerable uptake of chromium(VI) into the
polymer, which cannot be easily removed by rinsing and other chemical treatment.
[0004] Therefore the other alternative method mentioned comprising pre-treating polyamide
plastics parts with an aqueous solution containing mineral acid in combination with
a solvent, mostly a glycol ether, has seemed to be advantageous. In this method, after
pre-treatment has been performed, the plastics part is catalyzed, this method step
being comprised of two or three method steps: A first optional step comprises immersing
the plastics part into a solution of a polar polymer thus providing the surface of
the part with a very thin layer of this polymer. This polymer serves to bond the catalyst
more strongly, and at a higher concentration, to the surface of the polyamide plastics
part. Thus consumption of the expensive catalyst will be reduced and further catalyzation
of the part will be more reliable. After rinsing off adhering solution from the plastics
part, the part is usually catalyzed by contacting same with a - solution containing
colloidal palladium. Such solutions are acidic and contain tin chloride and are perfectly
well suited to catalyze nonpolar olefinic polymers like ABS or polycarbonate. Such
solutions are also described to be used for the catalyzation of polyamide plastics
parts (
EP 0 406 859 B1).
[0005] However, it has proved not to be necessary to use these complicated and expensive
catalysts for the treatment of the microporous polyamide plastics parts. Solutions
containing ionic palladium also prove suitable, such solutions being much easier to
prepare and being less expensive. Depending on the type of the catalyzation method,
after treatment with the catalyst, the plastics parts are further treated in a specific
solution. If, for example a colloidal palladium catalyst has been used, the plastics
part is contacted with a so-called accelerator, which is to remove a tin sheath surrounding
the colloidal particles. All solutions which are capable of solubilizing tin(II) may
serve this purpose, such as diluted hydrochloric acid, sulphuric acid, sodium hydroxide
and organic acids. Ionic palladium being adsorbed to the plastics parts used will
instead be reduced to the elemental palladium metal by means of reducing agents, such
as by boranates (dimethylaminoborane and alkali metal borohydrides) and hypophosphites.
[0006] Nowadays, most applied methods comprise treating a polyamide plastics part using
a swelling and cleansing solution, said solution containing a glycol ether, a mineral
acid and a surface active agent. Residues of this solution adhering to the plastics
part will be rinsed off thoroughly after it has been treated in this solution for
the treatment time required. Thereafter the part is treated in a suitable conditioner
and finally catalyzed in an acidic palladium salt solution. After having been exposed
to a reduction solution the part is nickel plated in an electroless nickel solution,
then electroplated with copper and finally electroplated with a metal desired to be
the final finish, mostly nickel or chromium.
[0007] Further
DE 31 37 587 C2 discloses pre-treatment of polyamide plastics parts for decorative applications with
a solution containing an organic solvent, a glycol ether, for example, and an acid,
such as hydrochloric acid. Subsequently, the parts are plated by electroless metal
coating, the metal adhering well to the parts.
[0008] Further,
EP 0 406 859 B1 and
EP 0 604 131 B1 disclose pre-treating polyamide plastics parts with oxidizing acids, such as chromic
acid/sulfuric acid for example. In addition
EP 0 604 131 B1 discloses etching the polyamide plastics part with mineral acids including hydrochloric
acid, sulfuric acid, phosphoric acid, chromic acid, formic acid and acetic acid. The
method of both documents further comprises contacting polyamide plastics parts with
a solution containing colloidal palladium and tin (II) the tin (II) being removed
after the treatment by acceleration. In addition
EP 0 604 131 B1 discloses treating the polyamide plastics parts applying a sensitizer-activator method
or soaking the parts in a palladium solution-reduction solution.
[0009] Considering that three rinse steps are in general required between each of the steps
of this method described above, the overall sequence, up to the electroless nickel
plating step, involves 16 method steps. "
Kunststoffmetallisierung - Handbuch für Theorie und Praxis" ("Plastics Metallization
- Handbook for the Theory and Practice"), Eugen G. Leuze Verlag, Saulgau, Germany,
1991, pages 47 and 141-156, mentions another treatment method for polyamide plastics parts involving catalyzing
same with a palladium catalyst which comprises palladium complex compounds comprising
organic ligands. Such organic ligands provide the complex compounds with a specific
affinity to the polyamide surface, which are said to influence adhesion of the metal
layer formed thereafter on the plastics part. This document further mentions, that
slight swelling or "roughening" of the polyamide plastics surface would be advantageous.
Treatment with chromic acid, as well as with acids, and caustic, would not be applicable
since these agents would damage the plastics parts irreversibly. More specifically,
the method comprises contacting the polyamide plastics part with an organic palladium
complex first, then intermediate treatment of the part, comprising swelling the amorphous
surface of the part, using an etchant, which contains salts of calcium und aluminium,
and finally electroless nickel plating the part.
[0010] The aforementioned methods prove to be lengthy and complicated. In order to reliably
metal-plate a polyamide plastics substrate with as little effort as possible, a method
will have to be provided which requires considerably less method steps while achieving
metal coatings on the polyamide plastics substrate having strong adhesion to the surface
thereof.
[0011] A method of forming a metal layer on the surface of polyamide plastics parts is described
in
U.S. Patent No. 5,296,020, the method comprising treating the polyamide plastics part with an activator formulation
consisting of organic noble metal, fillers, organic solvent and an aqueous dispersion
of a polyurethane polymer. More specifically, the organic noble metals are provided
by organometallic compounds of palladium with olefins, with α,β-unsaturated carbonyl
compounds, with crown ethers and with nitriles. The activator compounds are then reduced
to metallic form, or are complexed by means of complexing agents and introduced into
the aqueous dispersion of the polyurethane polymer. The solvent may be glycol ethers,
for example ethylene glycol monomethyl ether, diglyme or propylene glycol monomethyl
ether acetate and simply serves to dissolve the organic Pd compound. Therefore, such
solvent may be employed in a small amount. Metallization of the plastics part, once
pre-treated in this formulation, by dipping for example, will be performed by electroless
metal plating, such as by electroless nickel plating. Ionic palladium may be reduced
in the electroless metal plating solution directly with no further method step being
required.
[0012] Further,
U.S. Patent No. 5,300,140 discloses a hydroprimer for metallizing substrate surfaces, polyamide surfaces for
example. This hydroprimer is used to be applied to the substrate surfaces by application
of a thin layer thereof to the substrate surfaces for subsequent electroless metal
plating, nickel plating for example. The hydroprimer contains, in addition to water,
a water-dispersible polymer selected from the group consisting of water-dispersible
polyacrylates, polybutadienes, polyesters, melamine resins, polyurethanes and polyurethane-ureas,
further an ionic noble metal, a colloidal noble metal or a covalent or complex compound
of a noble metal as metallization catalyst and a filler. Possible activators are organometallic
complex compounds of palladium with olefins, with
α,β-unsaturated carbonyl compounds, with crown ethers, with nitriles and with diketones.
Ionic metals in the form of salts, such as halides, may likewise be used. The noble
metal may be reduced in the electroless metal plating solution directly.
[0013] The aforementioned methods are said to yield metal layers coating the substrate surfaces
at high adhesion. The catalyst formulations, however, have been shown not to be very
reliable in terms of metal plating capability (uniformity of metal plating) and adhesion
of a metal layer deposited.
[0014] Another method of electroless plating a polyamide plastics part is disclosed in
U.S. Patent No. 4,554,183. This method comprises pre-treating the polyamide plastics part with a solution of
a mixture of halides of elements of the 1
st and 2
nd main groups of the Periodic Table of Elements with salts of weak inorganic bases
and strong inorganic acids in a non-etching organic swelling agent or solvent for
polyamides and treating the plastics part with a metal-organic complex compound of
palladium, for example. Coordination complexes of olefins with palladium, which contain
functional groups, as indicated in this document, are used as the metal-organic complex
compounds. The metal-organic complex compounds are advantageously employed in the
form of the dispersions thereof, and in particular, the solutions thereof in suitable
organic solvents, these solvents being water-immiscible. Examples are given which
exclusively describe solutions comprising a coordination complex of an olefin with
palladium in an organic solvent. Reduction of the coordination complex compound may
be performed in an electroless nickel plating bath directly.
[0015] The method described in
U.S. Patent No. 4,554,183 suffers from the disadvantage that the solutions contained therein do not contain
water at all and instead contain pure organic solvent. In practice, such solutions
may not be used due to the requirement to reduce total oxygen concentration (TOC).
Moreover, the solution will have to fulfil the requirements, that it shall not have
any detrimental environmental impact, not contain solvents having any halogens being
bonded to carbon, be water-based and, if organic solvents are used, these solvents
not being easily volatile, having a high ignition point and not being toxic, like
mutagenic or teratogenic. Thus the solutions disclosed in this document cannot be
used in industrial practice.
[0016] It is therefore an object of the present invention to provide a method of forming
a layer of a coating metal on a substrate.
[0017] More specifically, it is an object of the present invention to provide a method of
forming a layer of a coating metal on a plastics surface, more specifically on at
least one of a polyamide and a polyoxymethylene plastics surface.
[0018] It is still another object of the present invention to provide a method of forming
a layer of a coating metal on a polyamide or polyoxymethylene plastics surface, this
metal layer firmly adhering to the plastics surface.
[0019] It is still another object of the present invention to provide a method of forming
a layer of a coating metal on a polyamide or polyoxymethylene plastics surface, the
method being easy and low effort and hence performed at low cost.
[0020] It is still another object of the present invention to provide a method of forming
a layer of a coating metal on a polyamide or polyoxymethylene plastics surface, the
method being environmentally acceptable, and acceptable with respect to health,
i.e., with respect to toxicity of the chemical species used.
[0021] It is still another object of the present invention to provide a pre-treatment solution
to be used in a method of forming a layer of a coating metal on a substrate surface,
more specifically on a polyamide or polyoxymethylene plastics surface.
[0022] It is still another object of the present invention to provide a pre-treatment solution
to be used in a method of forming a layer of a coating metal on a polyamide or polyoxymethylene
surface, the solution being cost-effective.
Description of the Invention:
[0023] The aforementioned objects are achieved by the pre-treatment solution of claim 1
and by the method of claim 17. Preferred embodiments of the invention are recited
in the subordinate claims.
[0024] In the following description, numerous specific details are set forth in order to
provide a thorough understanding of the present invention. It will be understood,
however, to one skilled in the art, that the present invention may be practiced without
some or all of these specific details. In other instances, well known process operations
have not been described in detail, in order not to unnecessarily obscure the present
invention.
[0025] As referred to herein, the term "inorganic ligands" means any neutral or ionic inorganic
chemical species which is arranged at a central atom or ion and the number of which
depends on the coordination number of the central atom or ion. In contrast to organic
ligands, the inorganic ligands comprise those species which do not contain C-H-bonds.
[0026] As referred to herein, the term "halogen acids" means hydrofluoric acid, hydrochloric
acid, hydrobromic acid and hydroiodic acid.
[0027] As referred to herein, the term "pKs" means the negative Brigg's logarithm of Ks,
Ks being the dissociation constant of the acid in an aqueous solution, producing hydronium
(H
3O
+) ions.
[0028] As referred to herein, the term "electroless metal plating solution" means a metal
plating solution which contains a chemical species capable of reducing coating metal
ions to elemental coating metal. For example, an electroless nickel plating solution
may contain hypophosphite salts, hypophosphoric acid or dimethyl amine borane as the
reducing agents.
[0029] As referred to herein, the term "substrate" means any workpiece which may be coated
with a metal layer, such as a plate, other moulded device or powder.
[0030] As referred to herein, the term "wt-%" means a fraction of components contained in
a mixture, the term indicating this fraction as parts of the component by weight per
100 parts of the mixture by weight.
[0031] As referred to herein, the term "alkyl" means a chemical species with the general
chemical formula C
nH
2n+1, with n being an integer > 0 and more specifically being an integer from 1 to 8,
more specifically from 1 to 6 and most specifically from 1 to 4. Alkyl species may
be branched or unbranched and may preferably be methyl, ethyl, n-propyl, iso-propyl,
n-butyl, iso-butyl and tert-butyl.
[0032] As referred to herein, the term "etching of plastics surface" means the roughening
and modifying of a plastics surface using chemical species, more specifically solutions
into which the plastics substrates may be immersed. Due to this treatment the bonds
of the polymers treated are either broken and/or oxidized and/or certain chemical
functional groups are modified. Concurrently with chemically modifying the plastics
surface it will also be hydrophilized. Such effects have not only been found out with
polyamide and with polyoxymethylene substrates but also with ABS substrates. If ABS
substrates are treated using a chromic acid etch, carboxylic acid, keto and aldehyde
groups are formed from the polybutadiene moiety of the polymer whereas, in the case
of polyamide and polyoxymethylene substrates, the condensation reaction performed
to form the polyamide or polyoxymethylene polymer is reversed by the action of strong
acids, such as sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid or formic
acid. The amino groups formed due to this reaction in polyamide plastics are believed
to cause hydrophilicity of the polyamide surface. The hydroxyl groups formed due to
this reaction in polyoxymethylene plastics are believed to cause hydrophilicity of
this plastics material.
[0033] As referred to herein, the term "swelling of plastics surface" means swelling such
surfaces with polar organic species, which are believed to be incorporated into the
polymer matrix, thus expanding same and slightly softening the surface thereof. Very
strong polar solvents are capable of completely dissolving the polymer. Such dissolution
will damage the plastics surface considerably. Therefore, for the treatment of polyamide
and polyoxymethylene substrates only organic solvents may be used which cause slight
swelling thereof, glycol ethers for example, or strong solvents the impact thereof
being reduced if they are mixed with water (propylencarbonate, γ-butyrolactone for
example).
[0034] As referred to herein, the term "conditioner" (agent promoting adsorption of noble
metal catalyst nuclei to the polyamide or polyoxymethylene surface) means chemical
species which are capable of influencing the surface properties of plastics substrates
for the metal plating of such substrates. Predominantly the conditioners are organic
molecules which adsorb at the plastics surface and enhance hydrophilicity of the substrate
due to a plurality of identical functional groups, -OH groups or -NH groups for example.
The conditioner may comprise positively or negatively charged groups.
[0035] As referred to herein, the term "activating" means a preparation of the plastics
surface for the subsequent electroless metal plating method step, comprising depositing
a noble metal, being in ionic form or in colloidal form, to the plastics surface.
If the noble metal is in ionic form such noble metal ions are subsequently reduced
to elemental noble metal using a reducing solution, the elemental noble metal forming
noble metal catalyst nuclei. If the noble metal is in colloidal form such colloidal
metal serves the starting catalyst nuclei. In general, any protective colloid surrounding
the colloid particles must be removed after deposition of the particles (acceleration).
In any case electroless metal plating will thereafter be initiated from a solution
containing coating metal salt and reducing agent. In general gold, palladium and platinum
as well as iridium and rhenium can be employed as the noble metal catalyst used for
the oxidation of phosphite or formaldehyde reducing agents (for silver depositing
for example). This can concurrently initiate autocatalytic reduction of metals like
nickel, copper, iron and cobalt.
[0036] The pre-treatment solution and the method serve the formation of a layer of a coating
metal on a substrate, the substrate preferably having at least one of a polyamide
and a polyoxymethylene surface, the substrate more specifically being a polyamide
or polyoxymethylene plastics substrate. Such solution and method may accordingly be
used to plate polyamide or polyoxymethylene plastics substrates for decorative purpose,
for example for sanitary appliances, in automotive industry, as furniture fittings,
for jewelry and for other applications, wherein a polyamide or polyoxymethylene plastics
substrate, which is mainly used because of its mechanical, chemical, processing or
thermal properties, is metal-plated.
[0037] The polyamide plastics substrate may be a polyamide 6, polyamide 6.6, polyamide 11
and polyamide 12 plastics substrate, but may likewise be any other polyamide plastics
substrate. Further, both polyamide or polyoxymethylene plastics material being provided
with filler material (for example mineral powder such as kaolin or glass fibers),
and polyamide or polyoxymethylene plastics material not being provided with filler
material may be treated in accordance with the method of the invention. It has been
found out that polyamide 11 and polyamide 12 plastics substrates cannot be metal plated
using prior art methods. Polyamide and polyoxymethylene plastics substrates are injection
molded to give them a specific shape depending on their intended use.
[0038] The pre-treatment solution of the invention contains at least one organic solvent,
at least one noble metal in ionic form being capable of reducing coating metal ions
to elemental coating metal and at least one acid. The method of the invention comprises:
a) providing the substrate; b) contacting the substrate with the pre-treatment solution;
and c) forming the layer of coating metal on the substrate by contacting the substrate
with an electroless metal plating solution.
[0039] By using the pre-treatment solution and the method of the invention the problems
encountered with the prior art solutions and methods will be overcome:
[0040] Due to the fact that the solution contains an acid, adhesion of the coating metal
applied to the polyamide or polyoxymethylene plastics substrate will be optimized
without being urged to use those organic solvents in the pre-treatment solution, which
are problematic, with regard to toxicity, environmetal impact, water-miscibility,
volatility, ignition point, stability against decomposition by acid attack, cost and
the necessity of having halogens bonded to carbon.
[0041] Further, as adhesion of the coating metal to the polyamide or polyoxymethylene plastics
substrate will be achieved without any problem, the solution of the invention may
also be aqueous, such that the content of any organic solvent contained therein may
be minimized. Therefore, TOC will be minimized, too.
[0042] Further, due to the fact that only four method steps, including three rinse steps,
are required for pre-treatment prior to metal plating the plastics substrate, the
method of the invention can be easily performed with low effort. This again will also
assure application of the method at low cost.
[0043] Further, due to the fact that no film of a polymer is formed on the plastics substrate,
if a pre-treatment solution is used which contains a binder, in which method the catalyst
metal would be incorporated in such film, metal plating may be effected without any
problem at all surface sites on the plastics substrate, even if catalyst metal is
used at a low concentration. As concentration of the expensive catalyst metal may
be low therefore, the pre-treatment solution may likewise be cost-effective.
[0044] The at least one acid may preferably have a pKs at or below 5, more preferred at
or below 3 and most preferred at or below 2. The at least one acid is able to attack
the surface of the polyamide or polyoxymethylene plastics material, thereby rendering
same wettable by further treatment agents on the one side and imparts a certain roughening
to the surface thereof on the other side, because acids are in general capable of
dissolving polyamide and polyoxymethylene plastics material. Unlike with pre-treatment
of ABS substrates, roughening of the plastics surface will, however, not be required
to achieve sufficient adhesion of a metal layer formed on the plastics surface and
will be minimized to avoid too much damage of the surface. It seems that surface texture
does not significantly influence adhesion of the metal layer formed. It is rather
believed that the acid contained in the pre-treatment solution dissolves a thin surface
film of the polyamide or polyoxymethylene plastics material, thus creating a clean
and polar surface. However, acid attack was not observed to dissolve a considerable
amount of polyamide or polyoxymethylene plastics material. The impact of the acid
may therefore be based on another mechanism. Though not be bound to any theory, the
inventors assume that the acid breaks hydrogen bonds present at the plastics surface.
This would be in contrast to the breakdown (etching) mechanism of chromic acid which
seems to involve breaking carbon-carbon bonds, thus chemically modifying the plastics
surface.
[0045] It has been known that the history of the substrate to be treated will have considerable
influence on the final product being metal plated. This will, in particular, be true
for polyamide and polyoxymethylene plastics substrates. In this respect an important
parameter is the temperature of the moulding tool applied during the moulding process
and which is used to form a plastics part. In general, moulding temperature is not
controlled. If polyamide or polyoxymethylene plastics material is processed, however,
a low tool temperature will be responsible for a slightly corrugated surface being
formed. However, adhesion of the metal layer deposited on the plastics surface will
be very high. For this reason as low a tool temperature as possible, of 25 to 35°C
for example, will be favourable. If a higher tool temperature of, for example, above
60°C is applied, surface quality will be better. However, adhesion will then be lower.
Such lower adhesion values may be compensated for, to a certain extent, by using in
the pre-treatment solution a suitable acid at a suitable concentration. For example
hydrochloric acid proves to be favourable at a concentration of up to about 200 ml/l
(37 wt.-% hydrochloric acid solution), whereas sulfuric acid, phosphoric acid (without
hydrochloric acid) or methane sulfonic acid were not suitable. C
1- to C
4-carboxylic acids, such as formic acid and acetic acid, formic acid being preferred,
likewise cause considerable adhesion of the metal layer to the plastics surface. Optimum
concentration of these other acids is 200 - 250 g/l. It has to be taken into consideration,
however, that formic acid may reduce noble metal, more specifically Pd
2+, to elemental noble metal, more specifically palladium.
[0046] However, rendering the surface wettable by contacting it with the at least one acid
is believed to be due to a chemical functionalization of the surface: Polyamide plastics
contain amide functional groups which will be broken with the aid of the acid, thereby
forming carboxylic acid groups and amine groups. These functional groups render the
plastics substrate hydrophilic and thus wettable by aqueous solutions. Similar mechanisms
provide for wettability of polyoxymethylene plastics surfaces.
[0047] As indicated before, the at least one acid is even more preferably selected from
the group comprising halogen acids, formic acid, acetic acid and lactic acid. Most
preferably the halogen acid is hydrochloric acid. Using these acids a very good adhesion
of the metal layer formed on the surface of the polyamide or polyoxymethylene plastics
substrate may be achieved.
[0048] Particularly good adhesion of the metal layer formed on the polyamide or polyoxymethylene
plastics surface was found to be realized, if the pre-treatment solution contains
phosphoric acid in addition to another strong acid such as hydrochloric acid. Most
preferably the pre-treatment solution contains in one litre about 150 ml of an aqueous
solution of 37 wt.-% hydrochloric acid, about 100 ml phosphoric acid and about 50
ml organic solvent. Best adhesion values are obtained if the concentration of hydrochloric
acid in the pre-treatment solution is 150 ml/l.
[0049] The at least one organic solvent contained in the pre-treatment solution may preferably
be selected from the group comprising glycol ethers, more specifically glycol ethers
having chemical formula R
1-O-gly
x-R
2, wherein R
1 is C
1- to C
6-alkyl, gly is -CH
2-CH
2-O- or -CH
2-CH(CH
3)-O-, wherein further x is 1, 2, 3 or 4 and wherein R
2 is H, C
1- to C
6-alkyl or R
3-CO-, with R
3 being C
1- to C
6-alkyl. More preferably the organic solvent is selected from the group comprising
diethylene glycol monoethyl ether and diethylene glycol monoethyl ether acetate. The
concentration thereof in the pre-treatment solution may be in the range of from 1
to 990 ml/l, preferably from 15 ml/l to 150 ml/l. If concentration of the organic
solvent is in a concentration range of from 50 to 200 ml/l adhesion of a metal layer
on the polyamide or polyoxymethylene plastics surface drops if the acids are a mixture
of hydrochloric and phosphoric acid.
[0050] The organic solvent proves to significantly influence the adhesion of a metal layer
formed on the polyamide or polyoxymethylene plastics surface. Glycol ethers have been
found out to be particularly suitable. The compounds mentioned explicitly prove to
fulfil the requirements of not having any detrimental environmental impact, not comprising
any halogens being bonded to carbon, being water-miscible, not being easily volatile,
not having a low ignition point, being stable against decomposition by acid attack,
having low cost and not being toxic, like mutagenic or teratogenic.
[0051] Both, the at least one acid and the at least one organic solvent further serve the
wetting of the plastics surface in order to assure complete and reliable contacting
of the plastics surface with the treatment liquids. Further, the plastics surface
must be pre-treated prior to metal plating such that no contamination thereof may
interfere with the electroless metal plating method. As pre-treatment according to
the invention only comprises one method step and in addition three rinse steps, such
pre-treatment step must be able to remove all contaminants in addition to wet (hydrophilize)
the surface and allow the catalyst to sufficiently adsorb to the plastics surface.
Experiments have shown that some cleansing effect takes place if the pre-treatment
solution of the invention is used. However, severe impurities adsorbed at the plastics
surfaces as well as finger prints have to be avoided thoroughly.
[0052] In a preferred embodiment the noble metal is palladium. Such metal proves to have
superior catalyst properties, once it has been reduced to metallic nuclei. Its concentration
in the pre-treatment solution may be in the range of from 10 mg/l to 2000 mg/l, preferably
from 20 mg/l to 100 mg/l.
[0053] The noble metal being in ionic form may in a preferred embodiment be formed not to
comprise any ligand or to exclusively comprise inorganic ligands, such as halide ligands,
more specifically chloride ligands, hence not comprising any organic ligand. Most
preferably the noble metal in ionic form is Pd
2+. If nickel is subsequently electroless plated, any other metal, having an electrochemical
standard potential of a few hundred mV more negative than nickel, may be used and
will hence be able to initiate nickel deposition, such as iron or nickel. In an aqueous
solution palladium ions will have water molecules be coordinated to the central ion.
Pd
2+ may be used in the form of palladium chloride, palladium nitrate or palladium sulfate.
The concentration thereof will, in general, not be critical as to the ability of the
activated plastics substrate to be metal plated. Neither too high a concentration
thereof, nor too low a concentration thereof has revealed any negative effects. The
noble metal concentration may be selected to be at or below 100 mg/l, more specifically
at or below 80 mg/l, even more specifically at or below 60 mg/l, even more preferably
at or below 40 mg/l and most preferably from 20 to 40 mg/l (or at about 30 mg/l).
Of course, if noble metal concentration is reduced more and more, initiation of electroless
metal deposition will be delayed more and more. However, such delay does not have
any detrimental implication, all the more electroless metal plating is more slowly
during the first plating period than in subsequent periods.
[0054] Most preferably, the pre-treatment solution is an aqueous solution. Again, such embodiment
fulfils the requirement that the pre-treatment solution shall have reduced TOC and
no detrimental environmental impact.
[0055] Further the pre-treatment solution may advantageously be free of any binder. In contrast
to the embodiments disclosed in
U.S. Patents No. 5,296,020 and
5,300,140, the present invention will preferably do without such binder, such that no film
will be formed on the surface of the polyamide or polyoxymethylene plastics substrate,
such film incorporating the catalyst nuclei required for initiating electroless metal
plating. Thus, such films would require much more catalyst metal to initiate electroless
metal plating than the present method, because many catalyst nuclei in the film would
be sheathed from the electroless plating solution.
[0056] A very favourable embodiment of the present invention comprises using an agent promoting
adsorption of noble metal catalyst nuclei to the polyamide or polyoxymethylene surface
(conditioner). Such agent will preferably be contained in the pre-treatment solution.
More specifically, the agent promoting absorption of noble metal catalyst nuclei to
the polyamide or polyoxymethylene surface may be a water-soluble polymer having polar
groups, for example positively or negatively charged groups, such that they are able
to mediate adhesion between the plastics surface and chemical species contained in
the solution. It may, for example, be a polyelectrolyte compound. In one most preferred
embodiment such polyelectrolyte compound may be a polyimidazolium compound, such as
polyimidazolium methosulfate. Using the adsorption promoting agent, adsorption of
a metal layer formed on the polyamide or polyoxymethylene plastics surface will be
considerably enhanced. Concentration of this agent in the pre-treatment solution may
be in the range of from 1 mg/l to 5000 mg/l, preferably from 10 mg/l to 100 mg/l.
[0057] The agent promoting adsorption promotes adsorption of the noble metal catalyst nuclei
to the plastics substrate but also promotes adsorption of the catalyst to any holding
elements for plastics parts, a rack, for example. This adsorption will increase noble
metal consumption on the one hand and further initiate coating metal deposition on
the holding elements on the other hand. Thus coating metal must be stripped off either
frequently. However, as the agent promoting adsorption of the noble metal catalyst
nuclei will enhance adsorption of the noble metal, the concentration thereof may be
minimized, to 20 mg Pd
2+ per litre pre-treatment solution for example. In addition, if the agent promoting
adsorption of the noble metal catalyst nuclei is used, a couple of polyamide or polyoxymethylene
plastics substrates may be metal plated, which would otherwise not be accessible to
plating.
[0058] In addition the agent promoting adsorption of the noble metal catalyst seems to some
extent enhance adhesion of a metal layer deposited on the plastics substrate if no
phosphoric acid is used in the pre-treatment solution but only hydrochloric acid.
[0059] Further the pre-treatment solution of the invention may contain a surface active
agent. This agent will reduce surface tension of the solution such that the plastics
substrate may be more easily wetted. No requirements are posed to its chemical structure
and identity. Apart from an as low as possible surface tension of the pre-treatment
solution to be achieved, this agent will preferably simply be easily rinsed off and
be harmless, especially with respect to animals. For this reason nonylphenol derivates
are not preferred.
[0060] For forming an electrically conductive layer on the polyamide or polyoxymethylene
plastics substrate any electroless metal plating solution may be employed. In a preferred
embodiment of the invention the coating metal is nickel. Therefore, to deposit the
first metal layer an electroless nickel plating solution may be used, since electroless
nickel plating is cost-effective and offers the advantage of easily being performed
because the nickel plating solution is essentially stable to decomposition. Further,
use of any noxious compounds such as formaldehyde may be dispensed with.
[0061] In general an electroless metal plating solution, more specifically an electroless
nickel plating solution, contains at least one metal ion source, more specifically
at least one nickel ion source, such as nickel sulfate or nickel chloride, further
a reducing agent, such as at least one hypophosphite salt, hypophosphoric acid, a
borane compound, such as dimethyl amine borane or sodium borohydride or, if copper
is to be deposited, formaldehyde, further at least one complexing agent, such as carboxylic
acids, like succinic acid, citric acid, lactic acid, malic acid, ethylene tetraacetic
acid as well as the salts thereof, at least one buffer or other pH adjusting agent,
like acetic acid/acetate salt or sodium hydroxide. Hypophosphite salts and hypophosphoric
acid are preferred over the other reducing agents due to the cost thereof. In addition,
the plating solution may contain additives which act as a stabilizer to prevent spontaneous
decomposition of the solution.
[0062] The method of the invention makes use of the fact that the reducing agent of the
metal plating solution may also be employed to reduce palladium or other noble metal
ions to elemental palladium or another elemental noble metal. As the same reducing
agent may be used both for the reduction of the noble metal ions and for the deposition
of the coating metal, the noble metal ions may be reduced using the reducing agent
contained in the electroless metal plating solution. Thus the method may in one embodiment
comprise only four method steps (pre-treatment step and three rinse steps) prior to
electroless metal plating the plastics substrate. Hypophosphite salts and hypophosphoric
acid prove to react spontaneously with Pd
2+ ions adsorbed to the polyamide or polyoxymethylene plastics surface, forming palladium
catalyst nuclei which may be large enough to store hydrogen atoms to initiate nickel
plating. If the temperature of the electroless nickel plating solution is about 50°C
reduction of Pd
2+ is complete after less than 30 seconds. Surprisingly, there is practically no delay
in reduction of Pd
2+, compared with methods, in which Pd
2+ is reduced in a separate method step. In general, electroless nickel plating takes
about 6 to 12 minutes; therefore any minor delay in nickel plating initiation cannot
be detected. Electroless nickel plating proves to be reliable irrespective of which
electroless nickel plating solution is employed.
[0063] The method of the invention comprising reducing the noble metal ions in the electroless
metal plating solution, has been found out to be reliable as to metal plating capability
(uniform metal plating), though treatment of the pre-treated plastics substrate with
the electroless metal plating solution was assumed to strip off, by rinsing, at least
partly, the noble metal ions adsorbed to the plastics surface before reduction can
take place. In such event part of the noble metal adsorbed could not serve initiation
of electroless metal plating, but would render the electroless coating metal plating
solution unstable, since coating metal would be deposited at the stripped off noble
metal. Such assumption did not turn out to take place, however. Otherwise noble metal
being stripped off the plastics surface would have tended to destabilize the metal
plating solution. However, such behaviour could not be verified and, likewise, increased
consumption of the reducing agent contained in the metal plating solution could not
be observed.
[0064] Preliminary tests have shown that the previously separately applied method steps
of etching, conditioning and activating, can be merged in one method step (organic
solvent, acid, conditioner and catalyst contained in pre-treatment solution) and that
the previous method steps of reducing the activator species and electroless metal
plating may be merged into another method step, three rinse steps being performed
in between. No precipitation or not negative interference occurs when the pre-treatment
solution of the invention is formulated. Further, the conditioner species is not stripped
off the plastics surface due to the action of the pre-treatment chemicals. Moreover,
simultaneous etching and activating of the plastics surface does not result in uneven
coverage of the surface with noble metal nuclei.
[0065] In another embodiment of the present invention, the substrate may be contacted additionally
with a reducing solution, after method step b), and prior to method step c). Such
reducing solution preferably contains at least one reducing agent capable of reducing
the at least one noble metal in ionic form to elemental noble metal. The reducing
agent may preferably be selected from the group comprising hypophosphite salts and
hypophosphoric acid. By additionally contacting the polyamide or polyoxymethylene
plastics substrates with the reducing solution metal plating may be even more reliable
in that metal plating may, at a given noble metal concentration in the pre-treatment
solution, be performed to yield quicker metal coverage of the surface of the plastics
substrates or the noble metal concentration may even be reduced to a very low value,
20 mg palladium per litre solution for example. Thus, the method may be rendered very
economic since consumption of noble metal may be minimized due to less loss of noble
metal as a consequence of drag-out of the pre-treatment solution from the pre-treatment
bath. Furthermore, the electroless metal plating solution will be protected by removing
any species from the pre-treatment solution, eventually adhering to the plastics surface
from dragging same into the electroless metal plating solution.
[0066] If such separate reducing solution is used for initiating electroless nickel plating
from Pd
2+ activation the reducing solution will preferably be operated at a pH not below 3.
Further, the temperature of this solution is preferably not below 35°C. The concentration
of hypophosphite salt in this solution is recommended to be below 30 g/l, about 20
g/l being sufficient (sodium hypophosphite; other salts or hypophosphoric acid being
at the same molar concentration).
[0067] Rinsing between the pre-treatment step and the electroless metal plating step proves
not to be critical as to metal-plastics adhesion, uniform metal plating and other
requirements. However, the period of time used between the pre-treatment and the first
rinse step has to be as short as possible, to prevent formation of draining structures
on the plastics surface. The first rinse should take at least 1 minute and should
be performed with efficient agitation of the plastics substrate in the rinse, in order
to remove any organic solvent from the substrate matrix.
[0068] Experiments have shown that adhesion seems to significantly depend on the extent
water is absorbed into a polyamide plastics part and also being adsorbed on the surface
thereof. Such assumption may, for example, be derived from the fact that caustic will
not be able to pre-treat the surface of the polyamide or polyoxymethylene plastics
substrate such that sufficient adhesion of a metal layer thereon will be attained.
Caustic is assumed to break the amide bonds or polyamide plastics material, too, but
it will also introduce much water into the polymer matrix due to its strong hydrophilization
properties and thus impedes sufficient adhesion of the metal layer. Similar effects
will be brought about with polyoxymethylene. For this reason, using aqueous solutions
to process the polyamide or polyoxymethylene plastics substrates seems to be of major
disadvantage. However, since the method consists of but four method steps prior to
metal plating the plastics substrate, and the first metal layer will then seal the
plastics surface, thus preventing any further contact of the surface with water, excessive
water uptake is avoided. It moreover proves advantageous to store a metal plated plastics
part in order to enhance adhesion of the metal layer. In general, such increase in
adhesion will be complete after about 3 days at room temperature, but may be complete
after only a few hours if the storage temperature is raised to 60°C for example.
[0069] Adhesion of the metal layer to the polyamide or polyoxymethylene plastics substrate
may be higher than a metal layer to an ABS substrate, though roughness of the polyamide
and polyoxymethylene plastics surfaces is much lower than that of an ordinarily pre-treated
ABS plastics surface. This observation may be attributed to electrostatic interactions
between the metal layer and the polyamide or polyoxymethylene plastics surface, which
is more polar than the ABS plastics surface. Such higher adhesion will be obtained
with polyamide or polyoxymethylene plastics surfaces after any water being adsorbed
to the interface between the metal layer and the plastics surface and absorbed into
the plastics material has been distributed all over the plastics material. Such distribution
may be brought forward by the porosity of the polyamide and polyoxymethylene plastics
material.
[0070] To treat the plastics substrates in accordance with the invention they are brought
into contact with the pre-treatment solution, optionally the reducing solution, the
electroless metal plating solution and the rinse liquids by immersing them into these
liquids or by spraying or splashing the solutions to the plastics substrate surfaces.
For this purpose, the plastics parts may either be secured to racks, or contained
in a drum, or be conveyed through a treatment apparatus, which may for holding the
parts be equipped with baskets on which the parts are deposited.
[0071] The invention will be described more clearly with reference to the following Figure
and Examples. The embodiments shown in the Figures and Examples are not intended to
limit the scope of the invention.
- Fig. 1
- shows a graph of adhesion of a nickel/copper layer on a polyamide plastics strip using
different acids and acid combinations at different organic solvent concentrations
in the pre-treatment solution.
Example 1:
[0072] To prepare a pre-treatment solution, to 1 litre of an aqueous solution containing
200 ml/l diethylene glycol monomethyl ether and 40 ml/l 37 wt.-% hydrochloric acid
250 µl of a solution of palladium dichloride, which contained the palladium salt at
a concentration of 10 wt.-%, referring to the palladium content, 0.5 ml of a 30 wt.-%
solution of the conditioner Lugalvan
® (trade mark of BASF, Germany) PVI and 0.1 g sodium lauryl sulfonate were added. Thus,
the concentration of Pd
2+ was 25 mg/l in the pre-treatment solution. The pre-treatment solution formed was
clear and slightly yellowish. This solution was heated and held at a temperature of
40°C.
[0073] 3 mm thick plates made from Durethan
® (trade mark of Lanxess, Germany) BM240 (polyamide 6, filler: kaoline) which had a
width of 5 cm and a length of 7 cm were treated in the pre-treatment solution for
10 minutes while slightly agitating the plate in the solution. Thereafter the plate
was rinsed with cold tap water and afterwards held in the water for another 2 minutes
while slightly agitating same. Then the plate was nickel-plated for 10 minutes at
a temperature of 45°C in a commercial electroless nickel-plating bath which contained
3 g/l nickel and 20 g/I sodium hypophosphite. The nickel layer deposited was subsequently
copper electroplated (Cupracid
® (trade mark of Atotech, Germany) Ultra), with 2.5 A/dm
2 cathodic current density, until a metal layer thickness of about 30 µm was attained.
After rinsing, the plate was stored for 1 hour at 70°C. Then a stripe having a width
of 1 cm was cut from each plate and copper peeled off the strip using a tensile testing
machine (Instron
® (trade mark of Instron Corp., USA)). The adhesion of metal to the polyamide plastics
part was measured to be 1.10 N/mm.
Example 1.1:
[0074] Repeating Example 1, the concentration of Pd
2+ in the pre-treatment solution was varied and the amount of Pd [mg/m
2] measured depending on the Pd
2+ concentration (dissolution of Pd from a plate having a defined surface area using
nitric acid and determination of Pd amount in the nitric acid solution by AAS (atomic
absorption spectrometry)). The amount of Pd adsorbed on the polyamide plastics parts
is given in Table 1.
Table 1: Amount of Pd on polyamide plastics plates
Palladium Concentration [mg/l] |
Palladium Amount Adsorbed [mg/m2] |
40 |
12.7 |
60 |
18.0 |
80 |
23.2 |
100 |
25.2 |
120 |
30.7 |
Example 1.2:
[0075] Further experiments have been performed to evaluate maximum temperature of the pre-treatment
solution. For this purpose Example 1 was repeated. An upper temperature of 40°C was
considered appropriate to assure that no streaks formed if the polyamide plastics
parts treated were transferred from a container containing the pre-treatment solution
to a first rinse tank. If temperature was selected to be higher than 40°C too much
solvent proved to be evaporated during the transfer so that the components of the
pre-treatment solution contained in the liquid film adhering to the plastics surface
concentrated, further reacted with the plastics surface and thus uneven conditions
occurred at the plastics surface (formation of relief-like structure).
Example 2:
[0076] The procedure of Example 1 was repeated. Instead of hydrochloric acid 40 ml/l formic
acid (about 98 wt.-%) were used. After copper electroplating adhesion of the metal
layer on the plastics strip was found to amount to 1.85 N/mm.
Example 3:
[0077] The procedure of Example 1 was repeated. Instead of Durethan
® BM240 Minlon
® (trade mark of E.I. Du Pont de Nemours, USA) 73M40 (polyamide 6.6) was used. The
dimensions of the plastics part were the same as before. Adhesion of the metal layer
to the plastics surface was determined to be 0.97 N/mm.
Example 4:
[0078] An interior door handle as typically used in automotive industry and made from Durethan®
BM240, which had been moulded into a mould having a tool temperature of 40°C, was
treated according to the method described in Example 2. After a storage time period
of 3 days adhesion could no longer be measured because it was so high that either
the metal strip ruptured or the plastics material ruptured; no separation at the interface
between the metal layer and the plastics surface could be attained during the peel
testing.
Example 5:
[0079] To prepare a pre-treatment solution, to 1 litre of an aqueous solution containing
150 ml/l diethylene glycol monoethyl ether acetate and 100 ml/l 98 wt.-% formic acid
0.50 ml of a palladium sulfate solution, which contained the palladium salt at a concentration
of 4 wt.-%, referring to the palladium content, and 0.1 g sodium lauryl sulfonate
were added. Thus concentration of Pd
2+ was 20 mg/l in the pre-treatment solution. The pre-treatment solution formed was
heated and held at a temperature of 40°C.
[0080] A 3 mm thick plate made from Durethan
® BM 420 which had a width of 5 cm and a length of 7 cm was treated in the pre-treatment
solution for 10 minutes while slightly agitating the plate in the solution. Thereafter,
the plate was rinsed with cold tap water and afterwards held in the water for another
2 minutes while slightly agitating same. Then the plate was nickel-plated for 10 minutes
at a temperature of 45°C in a commercial electroless nickel-plating bath which contained
3 g/l nickel and 20 g/l sodium hypophosphite. The nickel layer deposited was subsequently
copper electroplated. After rinsing, the plate was stored for 1 hour at 70°C. As described
previously, adhesion between the plastics surface and the metal layer was tested and
measured to be 1.64 N/mm.
Example 6:
[0081] Example 1 was repeated with the palladium ion concentration in the pre-treatment
solution being, in this case, 50 mg/l, different acids and acid combinations as well
as the organic solvent (diethylene glycol monoethyl ether) being used at different
concentrations. The temperature of the pre-treatment solution was 30°C. Duration of
treatment in the pre-treatment solution was 6 minutes. Adhesion values measured are
shown in a graph in Fig. 1.
[0082] Fig. 1 shows that best results with respect to adhesion of the nickel/copper layer
on the polyamide strip were obtained using 150 ml/l hydrochloric acid (37 wt.-%) and
100 ml/l phosphoric acid. Best adhesion was achieved if the organic solvent concentration
was set to a relatively low value (50 ml/l for example). Further, if the concentration
of the acids used is rather low, concentration of the solvent has preferably to be
increased to a concentration higher than 50 ml/l. Some difference in adhesion values
was in fact observed on either side of the plates. This was attributed to different
moulding conditions on the two sides of the plate.
1. Pre-treatment solution, containing at least one organic solvent, at least one noble
metal in ionic form being capable of reducing coating metal ions to elemental coating
metal and at least one acid.
2. Pre-treatment solution according to claim 1, wherein the at least one acid has a pKs
at or below 5.
3. Pre-treatment solution according to any one of the preceding claims, wherein the at
least one acid is selected from the group comprising halogen acids, formic acid, acetic
acid and lactic acid.
4. Pre-treatment solution according to claim 3, wherein the halogen acid is hydrochloric
acid.
5. Pre-treatment solution according to any one of the preceding claims, wherein one of
the at least one acid is phosphoric acid.
6. Pre-treatment solution according to any one of the preceding claims, wherein the pre-treatment solution contains in one litre about 150 ml of an aqueous
solution of 37 wt.-% hydrochloric acid, about 100 ml phosphoric acid and about 50
ml organic solvent.
7. Pre-treatment solution according to any one of the preceding claims, wherein the noble
metal is palladium.
8. Pre-treatment solution according to any one of the preceding claims, wherein the noble
metal in ionic form does not comprise any ligand or exclusively comprises inorganic
ligands.
9. Pre-treatment solution according to any one of the preceding claims, wherein the noble
metal in ionic form is Pd2+.
10. Pre-treatment solution according to any one of the preceding claims, wherein the at
least one organic solvent is selected from the group comprising glycol ethers having
chemical formula R1-O-glyx-R2, wherein R1 is C1- to C6-alkyl, gly is -CH2-CH2-O- or -CH2CH(CH3)-O-, wherein further x is 1, 2, 3 or 4 and wherein R2 is H, C1- to C6-alkyl or R3-CO-, with R3 being C1- to C6-alkyl.
11. Pre-treatment solution according to any one of the preceding claims, wherein the organic
solvent is selected from the group comprising diethylene glycol monoethyl ether and
diethylene glycol monoethyl ether acetate.
12. Pre-treatment solution according to any one of the preceding claims, wherein the pre-treatment
solution is an aqueous solution.
13. Pre-treatment solution according to any one of the preceding claims, wherein the pre-treatment
solution is free of any binder.
14. Pre-treatment solution according to any one of the preceding claims, wherein the pre-treatment
solution further contains an agent promoting adsorption of noble metal catalyst nuclei
to a plastics surface.
15. Pre-treatment solution according to claim 14, wherein the agent promoting adsorption
of noble metal catalyst nuclei to the plastics surface is a polyelectrolyte compound.
16. Pre-treatment solution according to claim 15, wherein the polyelectrolyte compound
is a polyimidazolium compound.
17. Method of forming a layer of a coating metal on a substrate, the substrate having
a plastics surface, the method comprising:
a) providing the substrate;
b) contacting the substrate with the pre-treatment solution according to any one of
clams 1 to 16;
c) forming the layer of coating metal on the substrate by contacting the substrate
with an electroless metal plating solution.
18. Method according to any one of the preceding claims, wherein the substrate is contacted
with a reducing solution after method step b) and prior to method step c), said reducing
solution containing at least one reducing agent capable of reducing the at least one
noble metal in ionic form to elemental coating metal.
19. Method according to claim 18, wherein the at least one noble metal is palladium and
the at least one reducing agent is selected from the group comprising hypophosphite
salts and hypophosphoric acid.
20. Method according to any one of claims 17 to 19, wherein the coating metal is nickel.
21. Method according to any one of claims 17 to 20, wherein the electroless metal plating
solution is an electroless nickel solution.
22. Method according to any one of claims 17 to 21, wherein the plastics surface is at
least one of a polyamide and a polyoxymethylene surface.