US 3567649 A
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United States Patent 3,567,649 AQUEOUS EMULSION FOR PLATING PRETREAT- MENT OF MOLDED POLYOLEFINS John J. Grunwald, New Haven, and Eugene D. DOttavio,
Thomaston, C0nn., assignors to MacDermid Incorporated, Waterbury, Conn. No Drawing. Filed Mar. 28, 1968, Ser. No. 717,006
Int. Cl. B01g 13/00; B44d 1/092 [1.5. C]. 252--310 6 Claims ABSTRACT OF THE DISCLOSURE The present invention relates generally to the pretreatment of surfaces of molded polyolefins for subsequent chemical plating of a metal on such surfaces, and is concerned more especially with novel pretreatment bath compositions, and processes of using such compositions, to enhance the adhesive strength of the resulting metal-t0- plastic bond in the plated substrates.
The invention affords important practical advantages over compositions and processes known heretofore which have been used for similar purposes.
The successful application to commercial and industrial use of metal plated polymer articles hinges, to a great extent, on the strength of the metal-to-polymer bond. Properties such as the ability of the resulting composite of metal plate and polymer substrate to withstand extreme temperature changes and mechanical impact are directly related to adhesion of the metal to the polymer substrate. It is commonly known that the as-molded surfaces of most polymers do no ordinarily provide a satisfactory base for achieving a strong bond to a subsequently deposited film or plate of copper, nickel or cobalt, for example. Many procedures are known for modifying the surface of the substrate in one way or another to effect greater bonding strength between the substrate and metal plate. One of such known methods involves the use of white phosphorus dissolved in trichloroethylene. The plastic substrate surface is brought into contact with the phosphorus-trichloroethylene bath, usually by immersing the substrate in a suitable tank containing the solution, and is held in contact with the solution at an appropriate temperature for a sufficient period of time to effect the desired modification of the surface resulting in greater adherence of subsequently-deposited metal film. The aforesaid process further involves careful rinsing of the pretreated substrate to remove excess phosphorus-trichloroethylene solution adhering to the surface of the substrate before chemical plating, and for this purpose a water solution of dimethylformamide is used as a first rinse, which is then followed by a hot water rinse. The substrate is next treated in a nickel sulfate-ammonium hydroxide bath to form a nickel phosphide film which serves as a preliminary deposit for the subsequent electroless deposition of a heavier film of metal such as copper, nickel, cobalt, etc. The latter is usually supplemented by a further, still heavier, metal plate produced electrolytically.
The phosphorus-trichloroethylene system just described is quite effective in providing good bond strength of the resulting metal deposit on the substrate surface. The sys- 3,567,649 Patented Mar. 2, l971 term, however, has disadvantages in that the phosphorustrichloroethylene bath must be operated at temperatures of l50l60 F. to be effective, and at such temperatures there is substantial evaporation loss as well as a problem of providing adequate ventilation to eliminate the danger of toxicity from fumes emanating from the bath. This can be and is controlled to some extent by the use of a water seal on the surface of the bath, but where automated plating operations are involved, the continuous entry of the articles into the treatment bath, and their subsequent removal, substantially interferes with the effectiveness of the water seal. Along with the toxicity problem there is also one of fire hazard due to the phosph0rus-trichloroethylene fumes. Also, because of the low solubility of white phosphorus in trichloroethylene at normal ambient temperatures, bath compositions having sufficient high phosphorus concentrations to be effective cannot be manufactured and shipped in ready-to-use form but must be prepared at the use site. This is not only in convenient but because of the extreme activity of white phosphorus, especially in oxidizing environments, there is a hazard to personnel in handling it. In addition, excess phosphorus-trichloroethylene solution remaining on the surface of treated articles upon removal from the pretreatment bath is not easily removed by ordinary water rinses, yet it is essential that this excess be removed in order to secure improved bond strength in the subsequently applied metal deposit. Accordingly it is necessary in the prior process described above to use a solvent, such as dimethylformamide, to eliminate most of the excess phosphorus-trichloroethylene solution, and then to follow that organic solvent rinse with water rinses. It has also been found that there is an appreciable corrosive effect of the phosphorus-trichloroethylene mixture upon most metals of the type that can be used practically in providing a tank or container for the pretreatment bath. The difliculty is not eliminated by resort to plastic coated treatment tanks, such as Plastisol lined tanks, as there is a tendency of the lining to soften. Since most of the present electroless plating operations are carried out in plasticcoated tanks, the carry-over of excess phosphorus-trichloroethylene into those tanks, if adequate rinsing procedures are not employed, causes excessive plating out of metal on the surface of the tank itself from the electroless plating solution.
It has now been found and it is the basic concept of this invention that most of the foregoing difficulties of a phosphorus-trichloroethylene pretreatment which have been mentioned above can be eliminated or substantially reduced while retaining the benefits of the better adhesion of metal film to plastic substrates which that system provides. Instead of using a true solvent-solute system of phosphorus dissolved in trichloroethylene as the major constituent of the treatment bath, the present invention employs a stable, aqueous emulsion of the oil-inwater type wherein phosphorus dissolved in a limited amount of trichloroethylene is emulsified, using suitable surfactants, so that water comprises the major constituent of the bath. It is found that such emulsion compositions can be prepared in read-to-use form which can be shipped to a processing site and are ready for use without further preparation other than raising the temperature to increase the speed of the pretreatment. The optimum treatment temperature, moreover, is substantially lower than the heretofore required temperatures of l60 F. For example, the novel emulsion compositions operate quite effectively at temperatures on the order of 1l0l20 F. It is also found that lower phosphorus concentrations are quite as effective as the much higher phosphorus concentrations required in the prior organic solvent bath. A further substantial advantage of the new bath composition is that of greater ease in removing excess pretreatment solution from the surface of the treated substrates prior to transfer to the electroless bath. In the novel process here disclosed a single cold water rinse is all that is required, thus eliminating any organic solvent rinse. Other advantages of the present system over the earlier one include much lower evaporation losses, reduced toxicity and ventilation problems, substantially lower flammability hazards, lower concentrations, i.e. more economical operation and reduced corrosion of treatment tanks.
The invention is illustrated by the following example.
The noyel pretreatment emulsion is best prepared in two steps in accordance with the following procedure:
'Step A An appropriate amount (e.g. 100 ml. for a 15% by volume composition) of trichloroethylene is mixed with 95 ml. water and 2.2 grams of white phosphorus is added. The mixture is heated to 110 with stirring until all of phosphorus is dissolved.
Step B The appropriate volume of water to make up the desired final solution (in this case 400 ml.) is heated while adding suitable surfactants, as for example 100 ml. of Kamar-O and 5 ml. of Benax 2A1. Kamar-O is the trade name for a wetting agent produced by Finetex Inc., East Paterson, N.I., comprising an anionic blend of alkylaryl hydrotypes. Benax 2A1 is the trade name 7 ethylene, water and surfactants which comprises the steps of Dow Chemical Company for sodium dodecyl diphenyl ether disulfonate. The resulting solution of Step B is added to that of Step A gradually with stirring while maintaining the temperature at approximately 110 F. The resulting aqueous emulsion is an extremely fine dispersion and is highly stable so that it may be stored for long periods of time without break down. The emulsion is ready for immediate use in a pretreatment tank upon heating to about 110120 F.
Molded polyolefin parts to be chemically plated are immersed in the foregoing pretreatment emulsion and held in the bath for 15 to 20 minutes. No special precleaning of the parts is necessary unless the surface is greasy or otherwise badly soiled. In such case any of the conventional proprietary cleaning compositions known in the art can be used. Upon withdrawing the parts from the pretreatment emulsion, they are immersed for one minute in a cold water rinse, placed in a nickel sulfateammonium hydroxide bath as before. A suitable composition here comprises about 18 grams per liter of nickel sulfate (hexahydrate) dissolved in equal volumes of water and concentrated (50%) ammonium hydroxide. Suitable operating conditions are about 145 F. with an immersion period of about 15 minutes.
Following the phosphiding bath, the parts are again given a cold water rinse. They may then be transferred to a suitable electroless metal plating bath. A typical electroless nickel bath suitable for this purpose is described in US. Pat. No. 2,532,283, Example V, Table I. Thereafter the plated parts may be given an additional metal deposit in an electrolytic bath in conventional manner.
In place of the electroless nickel deposit, an electroless Copper p ate may b s bs ituted at the corresponding of first admixing milliliters of trichloroethylene and 2.2 g. of white phosphorus in 95 milliliters of Water while heating to 110 F. and stirring to dissolve the phosphorus, and subsequently adding to the solution of this first step a separately prepared solution comprising the surfactant and water, said second solution being added to the first with stirring while maintaining the admixture at about 110 F.
2. The method of claim 1 wherein in the second solution the amount of the surfactant is milliliters and the amount of the water is 400 milliliters.
3. The method of claim 1 wherein the surfactant employed is one having an alkylaryl terminal group.
4. The method of claim 1 wherein the surfactant utilized is an anionic surfactant.
5. The method of claim 1 wherein the surfactant employed is a mixture of:
(a) an anionic blend of alkylaryl hydrotopes and (b) sodium dodecyl diphenyl ether disulfonate.
6. The method of preparing a stable oil-in-water emulsion consisting essentially of white phosphorus, trichloroethylene, water and surfactants which comprises the steps of first admixing white phosphorus and approximately equal volumes of trichloroethylene and 'water while heating to F. and stirring to dissolve the phosphorus and subsequently adding to the solution of this first step a separately prepared solution comprising the surfactant and the balance of the water, said second solution being added to the first with stirring while maintaining the admixture at about 110 F, the phosphorus being present to the limit of its solubility in the emulsion and the trichloroethylene comprising about 15% by volume of the total emulsion.
References Cited UNITED STATES PATENTS 2,140,519 12/1938 Elston 252312 3,266,922 '8/1966 Cummings et al. 252312 Re. 24,062 9/ 1955 Horton 117-47 2,873,241 2/1959 Strzyzewski et al. 117-47 3,212,918 10/1965 Tsu et al. 117-47 3,394,023 7/1968 Wilhelm 11747 3,441,434 4/1969 Jezl 117138.8 3,489,585 1/1970 Miller 106-287 WILLIAM D. MARTIN, Primary Examiner M. SO FOCLEOUS, Assistant Examiner US. Cl. X.R.