FR2930180A1 - METHOD FOR METALLIZING A NON-CONDUCTIVE SUBSTRATE OF PLASTIC MATERIAL - Google Patents

Abstract

This method, which comprises a step of pretreating the surface of said substrate, consisting in forming a porosity and reactive species on the surface of the substrate, is characterized in that said pretreatment is chosen from the group consisting of flaming, treatment with plasma, corona discharge treatment, laser treatment and electron beam processing.

Description

The present invention relates to a method of metallizing a non-conductive substrate made of plastic, such as a polyolefin substrate such as polypropylene, or styrenic (co) polymer such as ABS.

Metallization is widely used in the automotive industry and the cosmetics industry to impart metallic appearance to appearance parts such as doorknobs, hubcaps and perfume bottle stoppers. Metallization can take various forms such as vacuum metallization, paint metallization and electroplating metallization. All these techniques require the prior application of a first metal layer on the surface of the substrates to be metallized.

This first layer, on which will then be formed a succession of other metal layers up to the desired thickness, must imperatively be strongly anchored to the surface of the substrate for the good performance (adhesion) of both this first layer and metal layers subsequently formed on the latter. This requires a prior attack of the surface attack by strong concentrated acids and oxidizing sulfochromic acid type (containing chromium VI) creating chemical functions and surface porosity, likely to generate a subsequent chemical and mechanical anchoring of metal deposits subsequently made to the surface of the substrate. However, the current laws and regulations aim to restrict or even prohibit the use of chemicals containing chromium VI because of the harmful and polluting nature of the latter. The object of the present invention is therefore to propose an alternative to attacking the substrate with a substance containing chromium VI.

Thus, this invention relates to a metallization method as defined in the first paragraph of the present description, comprising a step of pretreatment of the surface of said substrate, consisting in forming a porosity and reactive species on the surface of the substrate, this method being characterized in that said pretreatment is selected from the group consisting of flaming, plasma treatment, corona discharge processing, laser processing and electron beam processing.

Due to the non-chemical nature of these pretreatments, the use of chromium VI is excluded, with all the advantages that this implies, namely in particular elimination of the risk presented to operators by sulfochromic acid baths and removal of the obligation wastewater depollution, that is to say respect for the environment and human health. Among the pretreatments referred to above, flaming is the technique of choice. It is indeed a gentle, safe and non-polluting method, which has no toxicological character for the operators and users of the final product; moreover, it allows a significant reduction in the volume of polluted water discharged by the global metallization operations, because the flaming is a dry treatment technique.

Furthermore, the flame is easy to implement, fast and economical and easily adaptable to the desired surface porosity. It will be recalled that flaming is a well-known technique which consists in exposing the surface of a plastic substrate to a flame generated by the combustion of a combustible gas-air mixture, with relative displacement of the substrate and of the flame. The combustible gases that can be used include propane and natural gas.

During combustion, porosity and various reactive species are formed on the surface of the substrate. Among these reactive species, mention will in particular be made of free radicals (for example OH), peroxy and hydroperoxy groups, ions, excited or non-excited molecules and carboxyl, hydroxyl and carbonyl functional groups. The aforementioned porosity and these reactive species have the effect of promoting, first, a mechanical attachment to the substrate of the metal layer to come and the second, a chemical bonding of this layer to this substrate by interaction between these species and the substrate. It will be specified that according to a preferred embodiment: the flame speed is 100-800 mm / s, the relative displacement of the flame with respect to the substrate can be achieved by a robotic device allowing a good reproducibility of the operations; rate of aeration of the flame (namely the ratio of the number of m3N / h of combustion air on the number of 20 m3N / h of gas) is of the order of 0.8-1.2, and - the distance between the base of the flame and the surface of the substrate to be treated is 20-80 mm. As for the duration of flaming, it can be easily determined by a few tests, the duration being deemed sufficient when the holding (adhesion) of the subsequent metal deposition to the substrate is satisfactory. As noted above, pretreatment of the substrate surface may be accomplished by other means, such as plasma processing, corona discharge treatment, laser treatment, and electron beam processing. These techniques are briefly discussed below: Plasma Treatment A plasma is a partially ionized gas containing electrons and ions in such proportions that the medium is generally neutral.

In addition to charged species, a plasma emits radiation in the ultraviolet, visible, infrared and has excited molecular fragments, molecules and atoms.

When exposed to a plasma, a polymer substrate undergoes physical and chemical transformations on the surface that are the direct or indirect consequence of the interactions of its surface with the excited species of the plasma. This results in particular the erosion on the surface of the substrate (promoting the mechanical anchoring of the subsequent metal layer) and the grafting of reactive chemical functions (promoting the chemical adhesion of said metal layer). Corona discharge treatment This treatment is carried out by means of a two-electrode generator allowing a 3D treatment of the substrate by means of a high-voltage and low-frequency electrical discharge. The operating parameters are the power (P), the electrode-sample distance (d), the running speed (V) and the number of passes.

Corona discharges are atmospheric pressure plasmas obtained by applying a voltage between two electrodes to generate a homogeneous electric field. The latter excites the gas molecules (air) and dissociates some of them. The charged active species thus formed can react with the molecules of the substrate and thus form sites promoting a chemical anchoring of the substrate to the subsequent metal layer. Laser treatment This technique consists in exposing the surface of the substrate 30 to a laser beam, especially at an IR wavelength or a UV wavelength. The result obtained is the obtaining of a fine surface roughness of the substrate (mechanical anchoring) and the formation of polar functions on the surface of said substrate (chemical anchoring).

Electron Beam Treatment Electrons are generated by heating a tungsten or tantalum cathode. They then undergo an acceleration, either by a very high potential difference (Van de Graaff generator), or by a succession of accelerating voltages (linear accelerator). Their deflection is ensured by an electromagnetic system. The set of reaction mechanisms occurring in the treated materials is very complex. Electrons can pull out hydrogen atoms (creation of macroradicals), open double bonds (polymerization), provide energy to initiate oxidation reactions (if the treatment is performed in the presence of oxygen), penetrate thickness (deep modification). The surface treatment will depend more on the quantity of electrons received than on their energy. Advantageously, the metallization process according to the invention further comprises a step of stabilizing said reactive species.

During the pretreatment of the surface of the substrate, there is formation at this surface, in addition to relatively stable reactive chemical species (for example hydroxyl, carboxyl and carbonyl groups), reactive chemical species which are not very stable, such as the groups peroxy and hydroperoxy and free radicals (eg - OH.). For a good behavior in time of the subsequent metallic deposits, these species must according to the invention be stabilized and the stabilization step comprises, after the pretreatment, advantageously the treatment of the surface of the substrate with a reducing agent, preferably a mild reducing agent. This reducing agent may be chosen from the group consisting of hydrazine, peroxidases, reductases, formaldehyde, hypophosphite salts, alkyl phosphites, metal hydrides, thioesters, ascorbic acid, citric acid, sugars and reducing metal salts such as ferrous, cuprous or cerous salts. According to one embodiment, this stabilization step comprises immersing the pretreated substrate in a stabilizing bath, advantageously by bubbling therein a neutral gas such as nitrogen, the bath being agitated mechanically or by ultrasound. The method according to the invention further comprises, after the stabilization step, either at least one step of forming a metal layer by a chemical route, or a step of forming a metal layer by a chemical route, followed by at least one step of vacuum metallization, metallization by electroplating or metallization by painting.

The present invention is described in more detail below with reference to some illustrative and non-limiting examples. EXAMPLE 1 The following various successive treatments are carried out on a piece of plastic material: (a) cleaning the surfaces in an alcohol-based bath, in a tank equipped or not with an ultrasound source; this cleaning is intended to degrease the surfaces; (B) flaming under the following operating conditions: combustible gas: propane; theoretical stoichiometric air / propane ratio 23.5 / 1; air flow: 400 1N / min; propane flow rate: 15.6 1N / min; aeration rate of 400 / (15.6 x 23.5) = 1.091; distance between the base of the flame and the surface to be treated 55mm; flaming speed: 400mm / s; (c) repetition of treatment (a) above; (d) immersion in a tank containing a stabilization solution at ambient temperature, constituted by an aqueous alkaline solution of hydrazine hydrochloride composed of 0.5 or 5 moles of sodium hydroxide, respectively 0.1 or 1 mole of hydrochloride of hydrazine and respectively 7.5 or 75 ml of water; nitrogen is bubbled into this solution and the contents of the tank are stirred mechanically or by the action of ultrasound; the duration of treatment is 1 minute; (e) immersion for 2 minutes in a wetting bath (for example aqueous solution at R 0.6m1 / 1 of PM 2110 - product of the company ROHM and HAAS Electronic Materials);

(f) immersion for 30 seconds in a preactivation bath at room temperature, this bath containing 250 ml of concentrated hydrochloric acid, the balance to 1 liter being demineralised water;

(g) immersion for 3 minutes in a bath of

catalyst (29 ° C) consisting of 20m1 / 1 Cataposit 449 (catalyst consisting of a mixture of colloidal Pd and stannous chloride - product of the company ROHM and HAAS Electronic Materials), 6ml / 1 of Conductor activator (product of the company ROHM and HAAS Electronic Materials) and 250m1 / 1 of concentrated hydrochloric acid, the complement to 1 liter being demineralised water; this treatment leads to the formation in the pores generated by the flame, Pd coated with a thin layer of Sn; (h) successive rinsing with water; (I) immersion for 3 minutes in a bath

accelerator at 46 ° C, consisting of 45 g / l Accelerator PM 964 and 12 ml / 1 concentrated sulfuric acid, the balance to 1 liter being demineralized water; this treatment has the effect of removing the thin layer of Sn and thus exposing the metal Pd;

(j) successive rinsing with water at room temperature; (k) immersion for 10 minutes in a nickel plating bath chemically at 30 ° C., R consisting of 145 ml / l of NIPOSIT PM 980 (product of the company ROHM and HAAS Electronic Materials) and 22 ml / l of concentrated ammonia (to adjust the pH to about 9), the

complement to 1 liter being demineralized water;

(1) successive rinsing with water at room temperature.

The nickel formed by the treatment (k) interacts with the exposed Pd formed in step (i), which ensures an anchoring of the nickel layer made in step (k) to the substrate. This layer has sufficient adhesion to the surfaces of the treated part (level 0 in the tape resistance test on a grid according to ISO 2409) to support subsequently formed metallic deposits either chemically or electroplated.

Example 2

A plastic part is subjected to the same successive treatments as in Example 1 except that:

in step (b), the air flow rate is 1651N / min, the

gas flow rate is 6.41N / min and the aeration rate is

165 / (6.4 x 23.5) = 1.097, and in step (d), the stabilizing solution consists of a 37% aqueous solution of formaldehyde or such a solution diluted to 10% and the duration of treatment is 5 minutes.

Example 3 Pretreatment by Plasma of a Polypropylene or ABS Substrate Two tests were performed under the following conditions.

Test Time Potency of the gas used Generator exposure pressure (Pa) (W) (min) A 50 50 22 22 5 150 He 22 Example 4 Pretreatment by corona discharge of polypropylene or ABS plates. Test Number of Power Voltage (U) Speed of passageways (P) in watts in kV passage (V) electrode - in mm / s sample (d) in mm Al 1 to 2 100-120 28 15 to 20 1 B1 1 to 2 100-120 28 15 to 20 1 Example 5 Laser pretreatment of ABS or polypropylene (PP) plates. The characteristics of the lasers used are as follows: Laser 1:. type: UV YAG (355nm) VMc6. field of application: 90 x 90 mm. power consumption: 1 W 25 Laser 2:. type: IR CO2 5. field of application: 180 x 180 mm. power consumption: 100W

ABS plate tests Laser 1 Laser 2 Frequency Activation Focal length (kHz) (%) passage (Distance (mm / s) source - sample) in mm 0 X 40 KHz 32 12 104 X 0 10 KHz 95 300 237 0 : pretreatment not possible X: pretreatment possible

PP plate tests with laser 2 Frequency (KHz) Activation (~) Focal Speed (Passage distance (mm / s) source - sample) in mm 40 28 4 104 10

Claims (7)

Revendications1. A method of metallizing a non-conductive plastic substrate, such as a polyolefin substrate such as polypropylene, or a styrenic (co) polymer such as ABS, which comprises a step of pretreating the surface of said substrate, comprising forming a porosity and reactive species on the surface of the substrate, characterized in that said pretreatment is selected from the group consisting of flaming, plasma treatment, corona discharge treatment, laser treatment and beam treatment. electrons. 2. Method according to claim 1, wherein the pretreatment is constituted by the flame with relative displacement of the substrate and the flame, characterized in that the flaming speed is 100-800 mm / s, the aeration rate is 0.8-1.2 and the distance between the base of the flame and the surface of the substrate to be treated is 20-80 mm. 3. Method according to claim 1 or 2, characterized in that it further comprises a step of chemical stabilization of said reactive species. 4. Process according to claim 3, in which the said reactive species are free radicals, peroxy and hydroperoxy groups, ions and excited or non-excited molecules, characterized in that the chemical stabilization step comprises the treatment of the surface of the substrate by a reducer. 5. Method according to claim 4, characterized in that the reducing agent is selected from the group consisting of hydrazine, peroxidases, reductases, formaldehyde, hypophosphite salts, alkyl phosphites, metal hydrides, thioesters , ascorbic acid, citric acid, sugars and reducing metal salts. 6. Method according to any one of claims 3 to 5, characterized in that it further comprises, after the stabilization step, at least one step of forming a metal layer chemically. 7. Method according to any one of claims 3 to 5, characterized in that it further comprises, after the stabilization step, a step of forming a metal layer by a chemical route, followed by at least one step of vacuum metallization, electroplating metallization or paint metallization.