A Study of the Mechanism of ABS Plastic Surfaces Using Mixed SnC12/PdC12 Catalysts Prior to Electroless Plating

ABS plastic components are electroplated with metal coatings of up to one hundred micrometres thickness for a variety of decorative and functional applications. Since the plastic base is non-conducting, a chemically deposited thin metallic coating must first be applied. This is achieved by chemical reduction from aqueous solution (electroless plating). Noble metal catalysts are required in order to initiate electroless plating on the plastic surface. the catalysis process, known as activation, is often achieved by treating the plastic with mixed SnC12,/PdC12 solutions, followed by a treatment with a strong acid, base or fluoride ion (called acceleration).

Prior to the start of the research programme described in this thesis very little was known about the nature of the mixed SnC12,/PdC12 catalyst or the mechanics of activation, The catalyst was thought to be a colloidal solution of metallic palladium, the colloid being stablised by a protective sol of stannous and stannic salts.
The work described in the thesis can be divided into three parts:-
(i) She formation and nature of mixed SnC12,/PdC12 catalysts
(ii) The mechanism of activation by mixed SnC12,/PdC12 catalysts,
(iii) The effect of process variables on activations
Mixed SnC12,/PdC12 are prepared by reacting PdC12 with excess SnC12 in acid solutions The reaction is observed to go through several intermediate stages before forming the final catalyst solutions. The reaction between SnC12 and PdC12 was studied using spectrophotometry and was found to proceed via a SnPdc14 intermediate, the rate being dependent on the square of SnC12, concentration. Although 2.5 - 3 mole of SnC12 per mole of PdC12 were required to stabilise the catalyst, the active component was identified as SnPd7C116 grossly deficient in SnC12. SnPd7C116 is thought to contain a metal cluster compound Pd6C112 in which direct Pd « Pd bonding occurs. This compound when solubilised in excess stannous chloride is not colloidal but probably exists as a mixture of several complex chlorides of Sn(S%) and Pd(II).

In order to determine the mechanism of activation, the species present on the ABS surface at each step of the pretreatment cycle was identified using electron diffraction analysis. The proposed mechanism involved:
(1) Adsorption of a Sn(II)Pd(II) chloride complex within the surface cavities produced by etching ABS
(2) Hydrolysis of the complex during the post catalyst rinse to yield a surface precipitate consisting of stannous hydroxy chloride containing adsorbed palladium chloride.
(3) A surface redox reaction between the adsorbed species during the accelerator treatment:
vis Pd(II) + Sn(II) Accelerator Pd + Sn(IV)
All accelerators appear to encourage this redox reactions
(4) Removal of masking and unwanted tin salts by post accelerator rinsing.
The catalytic activity of the ABS surface was found to depend on the surface roughness, the amount of catalyst adsorbed, the degree of acceleration, the type of accelerator, the degree of rinsing and even the bath loading of the processing vats.