Electroless metal plating of plastics

Process for plating main group metals on aromatic polymers is carried out by the use of a nonaqueous solution of a salt of an alkali metal in a positive valence state and a main group metal in a negative valence state with contact between the solution and polymer providing a redox reaction causing the deposition of the main group metal and the reduction of the polymer. Products from the process exhibit useful decorative and electrical properties.

BACKGROUND OF THE INVENTION 
This invention relates to a process for metal plating of plastics and more 
particularly to plated plastics where the metal platings are composed of 
main group metals such as Ge, Sn and As. More specifically, the invention 
relates to the formation of metal platings from certain salts of the main 
group metals where the main group metal to be deposited is in the negative 
oxidation state and is oxidized in the process. 
Metallized plastics have been found useful in the electrical, automotive 
and other industries for their electrical, reflective, and decorative 
properties. Previously, electroless deposition processes have been 
developed to chemically deposit a metal on a polymeric substrate by a 
reduction of the metal from a solution of the metal salt. In these 
previous processes, the polymeric substrate is first sensitized by 
treatment with certain other metal salts to cause the desired reduction 
and deposition of metal. While these electroless techniques are useful, 
they have not generally been applied to the deposition of many of the main 
group metals. 
One technique for chemically depositing main group metals has involved the 
decomposition of metal hydrides by exposure to hot substrates to form the 
metal platings. In some instances, the process has been difficult to 
control and explosive conditions have been encountered. Accordingly, one 
object of the invention is a chemical process for plating main group 
metals on a polymeric substrate. A second object is a process for plating 
main group metals under mild conditions of temperature and pressure. An 
additional object is a process for plating a polymeric substrate which 
does not require an initial sensitizing step. Another object is a process 
for plating a polymeric substrate which provides metal platings having a 
varied thickness. 
SUMMARY OF THE INVENTION 
Briefly, the invention is directed to an electroless process for plating a 
main group metal or metals on the surface of a polymeric substrate. The 
process involves forming a solution containing a metal salt of an alkali 
metal in a positive valence state and the main group metal in a negative 
valence state, selecting a polymeric substrate characterized by repeating 
aromatic groups, and conducting a redox reaction by contacting the 
solubilized salt with the surface of the substrate for a sufficient time 
to oxidize and deposit the main group metal in elemental form to produce a 
plated substrate. In the oxidation and deposition step, the alkali metal 
of the metal salt is retained in the plated substrate with the substrate 
being reduced by electrons transferred from the main group during the 
redox reaction. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The inventive electroless process for plating a main group metal on a 
polymeric substrate includes the steps of forming a solution of a salt of 
an alkali metal and a main group metal or metals in a nonaqueous polar 
solvent with the main group metal being in a negative valence state, 
selecting a polymeric substrate characterized by repeating aromatic groups 
and conducting a redox reaction by contacting the salt in solution with 
the surface of the substrate for a sufficient time to oxidize and deposit 
the main group metal in elemental form to produce a plated substrate. In 
the process, the alkali metal is retained in the plated substrate as a 
cation and the substrate is reduced by electrons transferred from the main 
group during the redox reaction. While many of the platings are continuous 
metal films, it is to be understood that the platings include 
polycrystalline and granular deposits which in some instances may be 
discontinuous. 
In the process, a solution is first formed of an alloy of an alkali metal 
in a positive valence state and a main group metal or metal in a negative 
valence state. Alloys of alkali and main group metals such as K.sub.4 
Sn.sub.9, K.sub.4 Ge.sub.9 and K.sub.3 Sb.sub.7 are known to be salt-like 
in structure. These alloys when treated with nonaqueous polar solvents 
such as ethylenediamine or ammonia dissolve to form metallic cations and 
homopolyatomic anions, frequently referred to as clusters as illustrated 
by the following 
##STR1## 
In the cluster, the subscript indicates the number of metal atoms and the 
superscript indicates the number of excess electrons. 
Suitably, the alloy is formed of an alkali metal with a normal valence of 
+1 such as potassium, sodium or lithium and preferably potassium, and a 
main group metal or metals. Suitable main group metals include Ge, Sn, Pb, 
As, Sb, Bi, Si and Te with Sn, Pb and Sb being preferable. The process may 
also be used to deposit alloys of the main group metals that exist as 
heteropolyatomic anions in solution such as Sn.sub.8 Tl, Sb.sub.3 As.sub.4 
and Pb.sub.4 Sn.sub.5. 
The alloy of the alkali and main group metal is dissolved in a nonaqueous, 
polar solvent to provide a medium for the redox reaction in which the main 
group metal is oxidized to its elemental form and the polymeric substrate 
is reduced by acquiring excess electrons. These solvents suitably include 
those known to dissolve these alloys such as primary and secondary alkyl 
(including alkylene) amines containing about 2-4 carbon atoms such as 
ethylamine, propyl amine, butyl amine, ethylenediamine and the like; 
amides such as dimethyl formamide; ammonia and the like. Combinations of 
these solvents with hydrocarbon solvents such as toluene are useful with a 
50/50 combination of ethylenediamine and toluene being particularly 
useful. 
The advantage of the alloy in solution is associated with the main group 
metal being in a negative valence state and capable of being readily 
oxidized back to the elemental state by a substrate redox reaction. In 
this reaction, the main group metal is oxidized to the elemental form 
(Sb.sub.7.sup.-3 .fwdarw.7Sb.sup.0 +3e.sup.-) while the substrate is 
reduced. 
In general, suitable polymers that are reducible by the solubilized salts 
of the main group metals are characterized by repeating aromatic groups. 
For favorable plating results, they also exhibit resistance to degradation 
by the solvents and/or the alkali metal cations. This resistance may be 
exhibited either in the original polymeric form or as in the reduced form. 
As an illustration, treatment of a polyester film (Mylar) with a solution 
of ethylenediamine and a potassium-tin salt results in the reduction of 
the polyester as indicated by a change in color; however, the reduced film 
becomes degraded and partially solubilized. With polystyrene and a similar 
solution, the degradation also occurs but at a lower rate. 
It has been found that aromatic polyimides, polysulfones and copolymers of 
styrene and vinyl pyridine provide favorable results. In view of the 
increased rates of the redox reaction and platings produced on the 
aromatic polyamides and polysulfones that the presence of 
electron-withdrawing groups are preferred adjacent to the aromatic ring 
either in the polymeric backbone or as substituents. Accordingly, suitable 
polymers include aromatic polyimides, polyamides, polysulfones, styrene 
polymers with vinyl pyridine, substituted styrene polymers with 
electron-withdrawing groups and other polymers with the above 
characteristics. The preferred polymers include the polyimides and 
polysulfones. 
Advantageously, the polymers include electron withdrawing groups in the 
backbone or as substituents on the aromatic groups. Illustrative of those 
in the backbone are carbonyl and sulfonyl groups while the groups 
substituted on the aromatic groups may include nitrite, thiocyanide, 
cyanide, ester, amide, carbonyl, halogen and similar groups. 
As is known, aromatic polyimides may be illustrated by the following 
##STR2## 
where R.sub.1 and R.sub.2 are single or multiple aromatic groups. 
Polysulfones may be illustrated by the following: 
##STR3## 
where R.sub.1 and R.sub.2 represent single and multiple aromatic groups as 
in In the copolymer of styrene and vinyl pyridine, the general repeating 
units are 
##STR4## 
In the plating process, the alloy solution is contacted with the substrate 
for a time sufficient to oxidize and deposit the main group metal in 
elemental form on the substrate. In the reaction, the alkali metal is 
retained by the plated substrate and functions as a substrate cation. 
Suitably, conditions include an inert atmosphere, a temperature in the 
range of about 0.degree.-50.degree. C. and preferably about 
20.degree.-30.degree. C., and a time which may vary from a fraction of a 
minute to about 5-10 minutes (although in some instances, the time may be 
in the order of 24 hours). The shorter times are evident for solutions of 
tin salts in ethylenediamine in contact with an aromatic polyimide. 
The plating formed in the process may vary in thickness and may range 
between a few angstroms to a considerable thickness. Advantageously, the 
range of thicknesses may be 50 .ANG. to 5000 .ANG.. Based on tests with 
tin plating, the metal plating exhibits electrical and other properties 
identifiable with bulk tin. In addition, the plating exhibits good 
adhesion to a polyimide substrate. Preferably, the following general 
instructions are used in carrying out the process. In the preparation of 
the alloy, the constituents of a typical alloy such as K.sub.4 Sn.sub.9, 
are weighed in an argon atmosphere and combined in a quartz tube fitted 
with an air-tight adaptor. The tube and its contents are removed from the 
argon atmosphere and attached to a glass vacuum system. The tube and its 
contents are then exposed to a flow of high purity argon while the quartz 
tube and its contents are heated by an oxygenated flame. After fusion of 
the elements has occurred, and the contents of the tube have liquefied, 
the tube is allowed to cool to room temperature. It is then evacuated and 
returned to the argon atmosphere glove box. The prepared alloy ingot of 
nominal composition K.sub.4 Sn.sub.9 is easily reduced to a powder with 
the aid of a pestle and mortar. 
All the alloys with the exception of K.sub.4 Si.sub.9 can be prepared in 
this manner. K.sub.4 Si.sub.9 is best prepared by combining the elements 
in a sealed tantalum tube, then sealing this tube inside an evacuated 
quartz tube, then heating in an oven for several days at 900.degree. C., 
followed by quenching. 
Prior to the formation of the solution, the solvent must be carefully dried 
and deoxygenated because of the high reactivity of the metal cluster and 
particularly those with Group 3, 4 and 5 metals as opposed to Group 6 
chain species such as Te.sub.3.sup.2-. 
Purification of ethylenediamine is carried out by refluxing the 
ethylenediamine over CaH.sub.2 for approximately 24 hours, then 
distilling. For more reactive alloys such as K.sub.4 Si.sub.9 and K.sub.4 
Ge.sub.9, additional drying may be necessary. This is accomplished by 
stirring the ethylenediamine over sodium in the argon atmosphere box, then 
vacuum distilling the ethylenediamine. 
Anhydrous ammonia can be purified (dried) by passing NH.sub.3 (g) through a 
trap held at -45.degree. C. or by stirring over sodium, then vacuum 
distilling. Dimethylformamide and toluene (used in combination with 
ethylenediamine) can be dried by refluxing over calcium hydride. 
In preparing the solution, the alloys K.sub.4 Sn.sub.9, K.sub.4 Ge.sub.9, 
K.sub.4 Pb.sub.9 readily dissolve in ethylenediamine yielding intensely 
colored red solutions, K.sub.3 Sb.sub.7 dissolves slowly in 
ethylenediamine yielding Sb.sub.7.sup.3- clusters of sufficient 
concentration to affect metal plating only after approximately 24 hours. 
(K.sub.4 Si.sub.9 or KSi dissolve in ethylenediamine, yielding typical 
red-brown solutions but at insufficient concentrations to effect visible 
plating on polyimide, although some plating occurred around the periphery 
of the sample. Concentrations of the alloy in solution are typically in 
the millimolar range (per liter). 
Many of the alloys will also dissolve in DMF, such as K.sub.4 Sn.sub.9 and 
K.sub.4 Pb.sub.9, KBi, for instance. In general, better plating is 
obtained from ethylenediamine solutions rather than DMF. 
Typically, 250 mg of K.sub.4 Sn.sub.9 is added to dried ethylenediamine, 
20-30 minutes is allowed for total (maximum) dissolution of the alloy (the 
total 250 mg does not dissolve owing to nonhomogeneity of the alloy) and 
the solution is now ready for metal plating of polyimide. 
In the preparation of a polymeric substrate such as an aromatic polyimide 
(available under the Kapton trademark), the most effective procedure for 
preparing polyimide involves a simple bakeout procedure of the polymer at 
approximately 200.degree. C. in air. No solvents or cleaning agents are 
used. 
Plating of the substrate is accomplished by immersion of polyimide into the 
cluster solution. The depth of coverage (determined gravimetrically) is 
seen to vary from a few hundred angstroms to approximately 5000 .ANG.. 
Immersion time vs depth of coverage is generally dependent on solution 
concentration but typical times are 10-100 sec for K.sub.4 Sn.sub.9 (e.g., 
Sn.sub.9.sup.4-) although metallization is apparent after shorter times. 
In the case of Sn.sub.9.sup.4- and Sb.sub.7.sup.3-, mixed solvent systems 
of 50% v/v toluene and ethylenediamine have been employed with generally 
favorable results with respect to electrical properties and appearance. 
Plated substrates are washed immediately after plating, with liquids such 
as toluene. With mixed solvent systems (ethylenediamine and toluene, this 
wash is generally not necessary. 
For electrical properties (e.g., Sn) a two probe method is employed to 
measure the D.C. resistivity. Annealing the plated substrate at 
approximately 120.degree. C. for Sn, improves the resistivity (in the 
direction of bulk metal). 
Characterization of the metal plating may be carried out by x-ray powder 
techniques and scanning electron microscopy (SEM). Metals identified (as 
plated) by x-ray powder analysis include Ge, Sn, Pb, Sb, and an alloy of 
As.sub.x Sb.sub.y (exact composition being unknown). SEM also confirms the 
presence of the metal as determined through x-ray fluorescence. The 
surface is grainy, and microcrystalline. Discontinuous films can be grown 
from very dilute cluster solutions.

The following examples are provided for illustrative purposes and are not 
intended to be restrictive as to the scope of the invention: 
EXAMPLE I 
In an inert atmosphere (argon) potassium and tin were combined in a quartz 
tube (12 mm O.D.) fitted with a Swagelok adaptor equipped with a glass 
stopcock. The glass stopcock was closed and the quartz tube with its 
contents were removed from the argon atmosphere. The tube was connected to 
a vacuum system, and the space above the closed stopcock evacuated to 
10.sup.-3 torr, then filled with high purity argon gas. Backfilling with 
argon was accomplished through a dynamic flow of gas. Pressure 
equilibration was accomplished by a mineral oil bubbler vented to the 
ambient atmosphere. The stopcock was then opened and the contents of the 
tube heated in an oxygenated flame until liquefication of its contents. 
The quartz tube was then allowed to cool to room temperature and returned 
to the argon glove box. 
The alloy ingot (nominal composition K.sub.4 Sn.sub.9) was pulverized to a 
powder by the use of a pestle and mortar. About 250 mg of the powdered 
alloy was added to about 15 ml of dried ethylenediamine resulting in an 
intensely red colored solution. Not all the alloy dissolved. 
Metal plating of an aromatic polyimide (Kapton) was accomplished by 
immersion of the polymer (previously baked at 200.degree. C. for 
approximately 2 hours in air) into the solution followed by a wash with 
toluene. The depth of coverage varied with the time of immersion. 
EXAMPLE II 
In a second test, 5 ml of the above solution (K.sub.4 Sn.sub.9 in ethylene 
diamine) was diluted to 10 ml with dried toluene. Immersion of the 
polyimide in this solution produced a metal coating with good adherence to 
the polymer. Based on the resistivity of the metal coating after a short 
annealing period, the coating exhibited properties of bulk tin. 
EXAMPLE III 
Similar plating tests were carried out with solutions of Ge.sub.9.sup.4-, 
Sb.sub.7.sup.3- and Pb.sub.9.sup.4-. Deposits of the elemental metal were 
confirmed by the use of x-ray analysis and scanning electron microscopy. 
In another test with a conventional nylon (nonaromatic), it was noted that 
contact between the solution of a potassium-tin salt and the nylon did not 
result in the desired redox reaction although the nylon appeared to be 
resistant to the solution. 
As revealed by the above disclosure, the invention provides a useful 
process for plating main group metals on polymeric substrates 
characterized by repeating aromatic groups. Products of the process also 
exhibit properties useful for decorative, electrical and other purposes.