CA1125630A

CA1125630A - Conditioning of caprolactam polymers for electroless plating

Info

Publication number: CA1125630A
Application number: CA342,111A
Authority: CA
Inventors: David A. Dillard; Eileen Maguire; Lawrence P. Donovan
Original assignee: Crown City Plating Co
Current assignee: Crown City Plating Co
Priority date: 1978-12-19
Filing date: 1979-12-12
Publication date: 1982-06-15
Also published as: DE2948133A1; JPS639019B2; IT1193252B; GB2040970B; GB2040970A; JPS55100972A; FR2444692A1; IT7928066A0; FR2444692B1

Abstract

Abstract of the Invention Caprolactam polymeric substrates are conditioned for electroless plating by etching with an aqueous solution of organic acid containing from 2 to about 10 carbon atoms, preferably an acetic acid compound, such as trichloroacetic acid.

Description

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FOR ELECTROLESS PLATING
Background of the Invention The present invention relates to electroless plating of caprolactam polymers in particular, to provide a surface uniformly receptive to electrolessly deposited nickel a~d 15 coppar.
The beneits of electroless plated, non-conductive articles, particularly plastic articles, are well known.
In the finished product, the desirable characteristics of the plastic and the metal are combined to offer thereby ~; 20 the technical and aesthetic advantages of each.
Polymeric substrates are conventionally plated by pre-etching the surface by contact with an aqueous ~ solution of at least one organic co~,pound active for : conditioning the surface of the plastic, then etching with

2 a strong oxidizing acid or base, seeding the surface with a noble metal catalyst, e.g., a palladium chloride solution, then immersing the seeded surface in an auto-: catalytic electroless solution where an initial coating of a .

.

conductive metal, e.g., coppeX o~ nickel~ ~s established by chemical depo~ition. The metal deposit acts as a buss to allow a thicker coating of metal to be huilt up electrolytically.
Attempts to adapt conventional procedures for electroless plating of polymers to polyamides by us have resulted in a failure to achieve a uniform adherent coat of metal. Having found conventional electroless plating procedures ineffective, a quest was initiated to discover a procudure to enable uniform coating of polyamides with electroless deposited metals.

SUMMARY OF THE INVENTION
It has now been found that electroless plating of caprolactam polymers, particularly filled caprolactam polymers, can be realized by contacting a caprolactam polymer substrate with an aqueous solution comprising at least one acetic acid compound of the formula:

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, X3C-C ~
OH

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wherein each X is independently selected from the group consisting of hydrogen~ hydroxyl and halogen and in which the acetic acid ; compound is present in a concentration sufficient to tender the caprolactam substrate receptive to a noble metal catalyst.

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1 This is followed by seeding the surface with a metal electroless plating catalyst selected from solutions of noble metal ions and colloidal suspansions or dispersions of noble or non-noble metal particles.
Following seeding and activation of the metal catalyst by conventional means, the surface can be uniorml~ plated with electroless copper or nickel.
Contact time in the organic acid etch is normally from about 10 seconds to 15 minutes at room temperatures.
10 Concentration of the acetic acid compound is normally at least about 3 percent ~y weight of solution up to solution saturation, preferab~y from about 5 to about 60 percent by weight.
In carrying out the process, an alkali solution having ~5 a pH of at least about~ 10, may be used to condition the substrate and if used, is pre~erably employed at a temper-ature from about 1~0F to the boiling point of the solutio~
or the softening point of the plastic, whichever is less, preferably 170 to about 200F at contact times r~nging 20 from about O.S to about 20 minutes or more, depending on temperature and alkali concentra~ion. Conc~n~ration is normally from about 2 percent by weight of solution to solution saturation, preferably from about 10 to about 50 percent by weight. It is preferred to employ a solution 25 of an alkali metal hydroxide such as sodium or potassium hydroxide.
In conductin~ the process of the invention, the article may be pre~conditioned by contacting the article in an aqueous solution of an organic solvent, preferably `'` l ~2~i~3 1~39~:JPG
1 an alkali soluble organic solvent for the caprolactam polymer in a concentration up to about 2 percent by weight, preferably up to about I percent by weight. Ethylene glycol, phenolic compounds such as chlorophenol, cresols and salts 5 thereof are presently preferred. The so~vent may be contained in its own bath or included in the alkali solution so long as the alkali does not hydrolyze or otherwise degrade the organic solvent. The solvent serve~ to soften the plastic to acid etch. Contact with the solvent solution is prior 10 to contact with the acid etch and may be, if used, be prior to contact with the alkali solu~ion.
In addition, the substrate after contact with the solution of the acetic acid compound may be further conditioned to remove surface debris of the etching lS by contact with acidic to basic wash.

De~ailed Description According to the present invention, there is provided a process which enables uniform electroless deposition of ~0 metals onto substrates formed from caprolactam polymers, typically filled substrates.
The process of the invention requires contacting the caprolactam article with an etch solution comprising an organic acid compound containing from 2 to about 10 carbon 2 atoms and having a solubility in waker of at leas~ 3 percent by weight of the solution, and con~act is for a time sufficient to render the surface uniformly receptive to a me~al ca~alyst~ It is preferred to use a solution of an acetic acid compound as def ined ~elow~

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1 As part of the process measures, water rinsing with deionized water is performed for good housekeeping between each s~ep. The substrate may be pre-treated with a dilute solution of solvent for the polyamide, preferably an alkali 5 soluble solvent, to soften the -~urface of the substrate to promote etch. The solvent for the polyamid~ may be . contained in a ~eparate bath or part of the alkaline solution, as herein defined.
In addition, the substrates may be contacted with an acidic or basic solution subsequent to etch to remove debris ]?resent on the surface of the substrate.
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The caprolactam polymers to be conditioned are known as Nylon 6~
The essential step of the process is contact of the caprolactam substance with an organic etch containing preferably at least one acetic acid compound. The presently preferred etch is one comprisi.ng at least about 3 percent by weight of solution, preferably 5 ~o about 65 percent by weight, and more preferably, from about 10 to about 25 percent by weight of solution o~ at least one acetic acid compound of the formula:
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. X3C-~ ~

OH
, .
where each X is independently hydroxyl, hydrogen or halogen9 with halogen pre~erred~ Contact is at room 79-9 . 5 : ` ~5i63(~

1 temperature, although elevated temperatures may be employed. Contact times range from about 10 seconds to 15 minutes or more and are for a time sufficient to etch the surface of the amide to a degree that it becomes 5 uniformly receptive to seeding by a metal electroless plating catalyst.
Among the acetic acid compounds which may be used there may be mentioned trichloracetic acid, acetic acid, hydroxyacetic acid, dichloracetic acid, chloracetic acid, 10 fluoracetic acid, difluoracetic acid, trifluoracetic acid, L bromacetic acid, dibromacetic acid and the like. Tri-chloroacetic acid is presently preferred.
An optional step o~ the prncess is contacting the caprolactam substrate with an aqueous alkaline solution having 1 a pH of at least 10 to condition the surface prior to contact with the acid etch. It is pre~;ently preferred that the a1kaline solution contain at least one alkali metal hydroxide, such as sodium and/or potassium hydroxide. The alkali employed should be present in a concentration from about 2 2 percent to solution saturation, preferably from about 10 to about 50 percent by weight. Solut~on temperature is maintained from about 150~F to the lesser of the boiling ~j point of the solution and the softening point of the caprolactam substrate, preferably from a temperature of ¦ 2 about 170F to 200F~ Contact time may vary from about ¦ 0.5 to 20 minutes or more, depending upon temperature and the alkali concentration, although it has been found I that prolonged immersions will not damage the substrate.

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1 Besides the alkali metal hydroxides, there may be employed alkaline compounds such as sodium metasilicate, trisodium phosphate, sodium carbonate and the like, used alone and/or in combination with an alkali metal hydroxide.
There may also be employed as part of the process, a separate bath used alone; preceding or following the alkaline conditioner or included in the alkaline conditioner, of a solvent for the surface of th~ caprolactam to soften the surface to aid attack by the alkali and~or the etch.
Typically, concentration of the solvent is up to about 2 percent by weight, preferably up to about 1 percent by weight. A wide variety of solvents for polyamides may be used. The presently preferred solvents are ethylene glycol, phenols such as chlorophenol, cresols and the like, and salts thereof. It is presently preferred to precede the acid etch by contact with the solution of the solvent whether or not an alkaline conditioner is used.
The solvent erves to soften the surface of the caprolactam substrate to promote conditioning and/or etching.

In addition, subsequent to etch the caprolactam substrate may be brought into contact with an acid or alkaline solution of either an organic or inorganic base to cleanse ~he surface of debris, i.e., filler and/or degradated resin~
Such solations are normally maintained at room temperature, although elevated temperatures may be employed.
Concentrations are in the range up to about 20 percent by weight, preferably up to about 10 percent by weight.

Functional acids include hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, formic acid, acetic acid 7~-9 7 i3~, and the like. There may also be employed the alkaline solution used for conditioni,ng as well as salt solutions~
such as solutions of borax and ammonium bifluoride. The concentration is that which,will enhance surface finish without de-tracting from the ability of etched surface to accept a metal catalyst.
Whether or not the cleansing treatment is employed, ; the article is rinsed in water, usually deionized water, and seeded with a metal electroless plating catalyst contained in an ~U~QU~ medium. The catalyst may be noble or non-noble metal based. The use of a noble metal catalyst contained ;' in an aqueous medium is preferred. By a noble metal catalyst contained in an aqueous mediu,m,there is meant an ionic solution or colloidal suspension of the free metals. Colloidal suspensions are preferred. The noble metals include gold, platinum and palladium, with palladium preferred. Suitable non-noble metal catalysts are described in 'U.S. Patent 3l958,~48 assigned to the same assignee as this application.
A suitable ionic bath is one containing palladium chloride in a concentration of about 0.2 grams per liter solution and 3 ml of concentrated hydrochloric acid per ; liter of solution. Following seeding, the palladium can be reduced to the free metal state by immersion in a bath of , a reducing agent such as dimethyl amine borane.
Colloidal suspensions of noble metals are described in U.S. Patent 3,011,~20 to C. R. Shipleyl Jr. The presently preferred colloidal suspension is one which is about 1.7 molar in hydrochloric acid ~LCl~. Such suspensions ` are colloidal in nature in which the noble metal colloid ~ '~!, 3~

is maintained in suspension by a protective colloid,- i~e.
stannic acid colloids. Follo~ing seeding, the colloid is removed by immersion in an acidic or alkaline accelerator solution to remove the protective colloid and expose the absorbed noble metal.
Although less preferred, there may also be employed a seeding method which involves contacting the etched article with a sensitizing solution containing stannous chloride followed by immersion in an activator solution, such as a palladlum chloride solution, where the ionic palladium is reduced to the free metal on the surface of the substrate.
Suitably activated, the article may be electrolessly ; plated by conventional means. Electroless copper and nickel formulatlons, such as those described in UOS~
Patents 3,011,920 and 3,874,072 may be employed. Electro-less copper solutions and typically based on a Soluble copper salt, such as copper sulfate, a complexing agent for the cupric ion such as Rochelle salt, an alkali hydroxide for adjustment of pH, a carbonate radical as a buffer and a reducing agent for the cupric ion such as formaldehyde.
Following electroless plating, the substrate may be electrolytically plated by conventional means, with copper, nickel, gold, silver, chromium and the like to provide the ~5 desired finish on the article.
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1 Example An article molded of Capron(TM) XPM-1030, a filled Nylon 6, manufactured and sold by ~llied Chemical Corpora-tion, was contacted with a 15 percent by weight to volume aqueous solution of trichloracetic acid solution maintained at room temperature for 5 minutes. After rinsing in deionized water, the article was seeded by contact with a propriet~ry colloidal tin-palladium catalyst, as described in U.S.
Patent 3,011,920, at an acid molarity of about 1.7. An alkaline accelerator was used to expose the palladium metal.
The catalyst was maintained at about 120F and the accelerator at about 105F. The seeded article was electrolessly plated using CupositlTM) PM-990, manufactured and sold by the Shipley Company. The electroLess plating solution was maintained at room temperaturle.
Following alectroless plating, the article was rinsed in deionized water, soaked in an alkaline cleaner, passed to a reverse current cleaner, an acid dip, ~right acid `l copper and nickel electrolytic plating solutions, and finally ¦ 20 to a chromium plating solution.
¦ The plated article was subjected to a cycle test to ~i determine adhesion performance under thermal stress conditions~ In this test, the plated article was maintained at a temperature of 180F for 1 hour, then cooled to and maintàined at room temperature for 30 minutes then cooled to -20F and maintainad at that temperature for 1 hour.
The cycle test was passed.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows;

1. A process for etching the surface of a caprolactam polymer substrate for electroless plating which comprises contacting the caprolactam polymer substrate with an aqueous etching solution comprising at least one acetic acid compound having the formula:

wherein each X is independently selected from the group consisting of hydrogen, hydroxyl and halogen and in which the acetic acid compound is present in a concentration sufficient to render the caprolactam polymer substrate directly receptive to a noble metal catalyst.

2. A process as claimed in claim 1, in which the caprolactam polymer substrate is formed of a filled caprolactam polymer.

3. A process as claimed in claim 1, in which the total acetic acid compound concentration of the conditioning solution is from about 3 percent by weight of the solution to solution saturation.

4. A process as claimed in claim 1, in which the total acetic acid compound concentration of the conditioning solution is from about 5 to about 60 percent by weight of the solution.

5. A process as claimed in claim 1, in which the caprolactam polymer substrate is contacted with the conditioning solution for a period of from about 10 seconds to about 15 minutes.

6. A process as claimed in claim 1, in which the acetic acid compound is trichloracetic acid.

7. A process as claimed in claim 1 in which the caprolactam polymer substrate is contacted, prior to contact with the aqueous etch solution, with a dilute aqueous solution of an organic solvent for caprolactam polymer for a time sufficient to soften the surface of the caprolactam polymer substrate.

8. A process for etching the surface of caprolactam polymer substrates for electroless plating which comprises contacting the caprolactam polymer substrate with an aqueous etch solution of trichloracetic acid in which the trichloracetic acid is present in a concentration of from about 5 percent by weight to solution saturation for a time of from about 10 seconds to 15 minutes to render the caprolactam polymer substrate directly receptive to a metal electroless plating catalyst.

9. A process as claimed in claim 8, in which the caprolactam polymer substrate is formed of a filled caprolactam polymer.

10. A process as claimed in claim 8, in which the concentration of trichloracetic acid in solution is from about 5 to about 60 percent by weight of the solution.

11. A process as claimed in claim 8 in which the caprolactam polymer substrate is contacted, prior to contact with the aqueous etch solution, with a dilute aqueous solution of an organic solvent for caprolactam polymer for a time sufficient to soften the surface of the caprolactam polymer substrate.