FR2645048A1 - Surface treatment process for polyamide substrates to improve the adherence of organic coatings - Google Patents

Abstract

To improve the adherence of organic coatings on polyamide substrates, a surface treatment of the latter is conducted with the aid of a solution, in an organic solvent, of at least one acid of oxidising nature and which does not form any water-soluble compounds either with the treatment mixture or with the substrate. A solution containing approximately 1 to 15 % by weight of the said acid is generally employed. The latter is especially nitric acid, a chromic acid or acetic acid, nitric acid being preferred. A treatment solution containing at least one nonbasic adhesion promoter which is compatible with the acid employed, in a proportion especially of 0.2 to 2 % by weight of the said promoter in relation to the weight of the treatment solution, may be advantageously employed. The adhesion promoter can also be incorporated in the material intended to form the primer or sizing or base coat, being applied onto the substrate treated with the acid of oxidising nature.

Description

 The present invention relates to a method of surface treatment of polyamide substrates for improving the adhesion of organic coatings.

The polyamides are polymers essentially obtained by polycondensation of a diacid and a diamine, or resulting from the polycondensation of an amino acid on itself, of an acid dichloride on a diamine, or of a polymerization by opening of cycle. The polyamides referred to here are just as well the conventional aliphatic polyamides, such as
polyamide-6: -CONH- (CH2) 5-,
polyamide-6,6: - (CH2) 4-CONH (CH2) 6-
polyamide-6, 10: -NHCO (CH 2) 8 CONH (CH 2) 6
- polyamide-11: - (CHZ) 1OCONH-, as polyarylamides or semi-aromatic polyamides, such as polyarylamide whose preparation involves adipic acid and metaxylylenediamine, and which is marketed under the name PAMXD6 IXEF over there
SOLVAY company.

 For practical applications, polyamides are most often used with additives such as: plasticizers, fillers and reinforcements, stabilizers, dyes; etc.

Polyamides and polyamide-based compositions are known to those skilled in the art: see for example Guides of Plastics Materials in mechanics by
C. EDGE, R. GIALLONARDO, PA SPORLI, G. VERCHERY.

 The substrates referred to herein are, in general, injection molded or extrusion molded parts.

 Polyamide articles or substrates are, for some applications, provided with coatings and / or paints. In the automotive industry, for example, where such articles are found: mirrors, wheel covers, door handles, and the like, polyamide articles are generally coated, firstly, with a primer or primer. for example, of the epoxy, epoxy-urethane, acrylic-urethane, acrylic-melamine, polyester, and / or metallized or non-metallized base type, for example of acrylic, acrylic-melamine, polyester-urethane, then by a varnish, for example, acrylic-urethane, acrylic-melamine, polyester-urethane. These multilayer coatings are applied in a known manner, for example using a spray gun (such as a pneumatic gun), a bowl or a high-speed disc.

 The problem that arises with these conventional coatings on polyamide substrates is that adhesion losses of the coatings are encountered.

It is known that polyamides can absorb up to 8% by weight of water and that their mechanical properties change depending on the water content. The most serious problems of adhesion of coatings arise with dehydrated supports, but the adhesion is not guaranteed on non-dehydrated common parts. In fact, the surface state is often heterogeneous and peelings are generally observed in the zones having the lowest surface tensions, notably lying around the zones of deformation of the support.

 In addition, aromatic polyamide articles have significant adhesion problems, regardless of their surface condition.

 Many systems and adhesion promoters have already been proposed (such as, for example, the use of silanes, such as 3-glycidyloxypropyltrimethoxysilane in paint), but they do not give good results. Moreover, certain additives (epoxy, x-phospophosphate, etc.) at the metallized base do not lead to the desired results either.

 To solve these adhesion problems, it is possible to flame the surface of the articles to be coated, in order to make this surface homogeneous. Flaming is a delicate operation, and it is difficult to obtain reliable results. Indeed, it is difficult to control parameters such as the length of the flame, its degree of oxidation, and the distance of the flame with respect to the substrate, so that one reserves this type of treatment only in case of necessity. Plasma treatment is also possible, but the technique is expensive.

 To solve the problem, one could consider immersing in water the polyamide articles to be coated. However, the very long immersion times that would be required to achieve the desired performance are incompatible with the requirements of mass production.

 The present invention provides a non-obvious solution to the problem. Indeed, according to the present invention, it is proposed to treat the surface of polyamide articles to be coated with a solution in an organic solvent of an acid having given characteristics.

The basic idea of the present invention was to attack the surface of the polyamide substrate to make it more homogeneous. Various acids have been tested, and, surprisingly, the treatment with phosphoric acid, which is reputedly interesting for its aggressiveness with respect to polyamides, has not made it possible to increase the pull-out behavior (test according to standard NFT 30 062) on polyamide articles. Similarly, the immersion test
SIPON (according to RENAULT 1165-FL-3), carried out on various polyamide articles, treated with phosphoric acid, then coated with a primary system + metallised base + varnish, gave negative results. In accordance with the invention, the oxidizing and water-soluble compound-forming acids have been selected either from the treatment medium or from the substrate.

 the subject of the present invention is therefore firstly a method for surface treatment of polyamide substrates for improving the adhesion of organic coatings, characterized in that this treatment is carried out using a solution, in a organic solvent, at least one acid with an oxidizing character and forming water-soluble compounds neither with the treatment medium, nor with the substrate.

 According to the invention, such an acid should be used at a concentration sufficient to effect a surface etching of the substrate, but within the concentration limits to maintain the surface appearance of the substrate once coated by the organic coatings.

 Nitric acid appears as the preferred acid. Chromic acids, also effective, provide less stable solutions. Acetic acid is also usable.

 As mentioned above, the selected acid must not only have an oxidizing character, but also not form water-soluble compounds under the conditions of the treatment. Without wishing to be bound by any theory, it is believed that phosphoric acid, although having an oxidizing character, is not suitable for the purposes of the invention because it forms water-soluble phosphates, which cause a phenomenon of delamination during immersion tests.

 A solution of about 1 to 15% by weight of the oxidizing acid according to the invention is generally used. Practical ranges of this concentration are in particular the range of 1 to 5% by weight, and, for example, that of 2-3% by weight.

 The organic solvent must have a water-miscible character, total or partial and, preferably, a low surface tension, so as to promote the wettability of the substrate to be treated. Examples of the solvent include, inter alia, lower alcohols, such as ethanol, propanol, isopropanol, butanol, or ketones, such as acetone and methyl ethyl ketone.

 According to another characteristic of the process according to the invention, a treatment solution containing at least one non-basic adhesion promoter and also compatible with the acid used is used, in particular because of 0.2 to 2% by weight of said promoter relative to the weight of the treatment solution. Adhesion promoters that can be used are those of the non-basic silane type, those of basic silane type giving rise to undesirable phenomena, for example to precipitation.

 A preferred promoter, especially when the acid is nitric acid, is 3-glycidyloxypropyltrimethoxysilane.

 The treatment according to the present invention may be carried out, inter alia, by contacting, in particular by immersion or spraying, the polyamide substrates to be treated, for a period of 30 seconds to 2 minutes, in the solution of the chosen acid, which can be brought to temperatures generally between 200 and 800C, the substrates being, after being removed from said solution, preferably dried at about 30-600C, for about 1-2 minutes, for example in forced air, before receiving the conventional organic coatings.

 It has also been found that if good adhesion results are obtained when the treatment is carried out at a temperature close to room temperature, a moderate increase in temperature in the ranges indicated above can have a favorable influence, in particular on the resistance. to the impact of the coating.

 Another advantage of the process of the invention is that the treatment can be carried out directly on the polyamide parts, without these having needed to undergo a preliminary treatment, such as degreasing.

The present invention also relates to a process for coating polyamide substrates with one or more successive layers of organic material strongly adhering to the substrates, characterized in that
the method of treatment as defined is carried out
above;
- then apply a layer of primer or
primer, for example epoxy, epoxy-urethane,
acrylic-urethane, acrylic-melamine, polyester and / or
a metallic base or not, of acrylic type,
acrylic-urethane, acrylic-melamine or polyester
urethane, and finally,
a final layer of varnish is applied, for example,
of acrylic-urethane, acrylic-melamine or polyester-urethane type.

According to a characteristic of this process, in the material intended to constitute the primer layer or the base layer, which is applied to the acid-treated substrate according to the invention, at least an adhesion promoter, at 0.2 to 1.5% by weight, based on the weight of said material, said adhesion promoter to be compatible with the constituents of said primer or primer layer; As the adhesion promoter, silane promoters are also used, which can be as basic in this case, as well as adhesion promoters of the titanium type (see, for example, Abbreviated Reference Manual for KEN REACT Titanium). coupling agents - KENRICH
PETROCHEMICAL Titaniti per industria degli inchiostri da stampa - CARLO PARDUCCI>, such as titanium (dioctylpyrophosphate) oxyacetate. Among the suitable silane promoters, mention may be made of 3-glycidyloxypropyltrimethoxysilane (GTS) and 3-aminopropyltrimethoxysilane (the latter being, in view of its basic character, excluded as an adhesion promoter for incorporation into the solution. acid treatment).

 The invention also relates to the polyamide substrates coated by the process as just defined.

 The following Examples are intended to illustrate the present invention. In these examples, percentages are by weight unless otherwise indicated.

The substrates used are made of the following polyamides:
a polyamide-6,6 containing mineral fillers,
marketed under the name of MINLON 13 T1 by
the company DUPONT DE NEMOURS (for embellishers
wheels)
a polyamide-6,6 marketed under the name
MINLON 11 C1 by this same company (for
wing mirror);
a polyamide-6 sold by the company EMSERWERKE
under the name GRILON PMV 4H (for mirrors);
a polyamide-6,6 marketed by the BAYER Company
DURETHAN AKV30 (for mirrors)
the aforementioned polyamide PAMXD6 Ixef (for handles of
door)
The primers, bases and varnishes used have the following compositions (indicated in% by weight)
Adret A
- Epikote 1009 ....................... 12
- Barium sulphate .................. 15
- Talc ................................ 12
- Titanium dioxide ................. 20
- Desmodur N75 ........................... 10
- Solvents ....................... 31
ADcrêt B
- Hydroxylated acrylic resin ............... 12
(based on hydroxy methacrylate
ethyl / methyl methacrylate /
styrene / acrylic acid) - Cellulose acetobutyrate ......... 5 - Titanium dioxide ...................... 34 - Desmodur N75. ..................... 6 - Solvents .......................... ..... 43
Primer C - URAFLEX 103BSx30 (solution of a
aliphatic polyurethane elastomer) .. 10 - Titanium dioxide - 9 - Baryta ........................... 10 - Solvents ..... ................... 71
Base A: - Hydroxylated acrylic resin
(see Primer B) .................... 8 - Cellulose Acetobutyrate ................ 9 - Pate d aluminum .................. 8 - Desmodur N75 ..................... 6 - Solvents .. ............................. 69
Base B: - Hydroxylated acrylic resin
(same as Base A) ....................... 8 - Cellulose Acetobutyrate ................ 8 - Aluminum paste .................. 6 - Colorants ........................ .. 2 - Desmodur N75 ........................... 6 - Solvents .............. ............. 70
Varnish A:: - Polycaprolactone diol .................. 14 - Hydroxylated acrylic resin
(same as Primer B) ........................ 38
- Desmodur N75 ......................... 29
- Solvents ............................ 19
Various tests are used to control the effectiveness of the treatment to make the coatings strongly adherent to the polyamide substrate
- grid test according to the NFT 30-038 standard: this
test consists of incising the coating until
substrate by means of a sharp instrument, in
making a grid and then looking at the appearance of
the incision and behavior of squares formed by the
grid, in order to express them by values
characteristics.This test is completed by a test
adhesion using adhesive tape
applied to the grid portion of the coating and
the results are compared to the results of a test
similarly done on a witness. We appreciate
by classes the percentage of areas taken off. The
class O (the edges of the incisions are perfectly
smooth and none of the squares of the grid
detached) represents the best result, and the class
5 (maximum flaking degree), the worst
result;
- the tearing test according to the NFT 30-062 standard,
giving a pulling force for a surface of
substrate SO - 2.8 cm2, for a tensile speed of
1 mm / minute;
- the SIPON immersion test (according to RENAULT 1165 standard
FL-3): this test involves immersing the coated support
in a solution & 10 g / liter of a detergent that is
sodium dodecylsulphonate in demineralized water,
for 72 hours at 55 OC. After we have withdrawn
the support of this solution and that it has been dried,
practice the adherence test according to the NFT 30-038 standard,
as indicated above;
- the test of the monoimpact resistance according to the standard
RENAULT D24 1699: on the support to be tested, we send,
under a pressure of 4 bars and has 20 impacts
per piece, a steel ball coated with alumina.

We observe, using a magnifying glass at 1 / 10th of
millimeter, if there has been a higher burst
millimeter or not. the result is positive
if we do not observe more than 50% bursting
greater than one millimeter. The test is conducted before and
after applying an adhesive tape t
cohesion test of the coating on the substrate: this
test, of an empirical nature, is often used to
judge the cohesion of a coating on a substrate.

It consists, using a piece of 0.20 FF, to attempt
to make a scratch on the substrate from a
cut in the coating to the support,
with an angle of attack of 450.

Example X (Comparative): Use of a phosphoric acid treatment solution
(a) On a wheel cover in MINLON 13T1, a treatment solution consisting of 3% phosphoric acid in isopropanol is applied with a brush, maintaining a contact time of about 1 mm then it is dried for about 5 minutes at 55 ° C. Then the metallic base A is applied, then the varnish A.

 other comparative tests were carried out by subjecting only the hubcaps to a degreasing treatment with isopropanol.

 While in the grid test + adhesion, and in that of cohesion, conducted on a hubcap after degreasing only, there are numerous detachments, these same tests, conducted after application of the treatment solution, allow to observe a gain appreciable adhesion. These tests are however not sufficient to judge the good quality of the result.

 Indeed, the SIPON test, for its part, gave unsatisfactory results, namely important problems of adhesion and appearance: it has formed many blistering related to the formation of water-soluble phosphates.

(b) For one minute, a plate is immersed in
MINLON llcl in the treatment solution of Example 1 (a), and then dried for about 5 minutes at 550 ° C. The metallic base A is applied, then the varnish A. The tearing forces (NFT standard 30-062) on a only degreased plate and on the treated plate are respectively 144 + 16 N and 135i6 N. The difference between these two values is not significant.

Example 2 (comparative): Use of a hydrochloric acid treatment solution
The procedure is as in Example 1 (a), but using a 3% solution of hydrochloric acid in isopropanol, to obtain a trim coated with the metallized base A and then varnish A.

 The grid test according to the standard NFT 30-038 conducted on this trim has given unsatisfactory results (class 2 & 3).

Example 3: Use of treatment solutions with chromic acid (H2Cr04) and with nitric acid (HNO3).

(a) The procedure is as in Example 1 (a), except that two plates are used which are respectively treated with (A) a solution of chromic acid at 3% in isopropanol and (B) a solution of 2.1% nitric acid in isopropanol. The pullout forces were calculated according to the NFT 30-062 standard:
- control plate (degreased plate only): 144 # 16N
- degreased plate, then treated with solution (A):
270 + 23 N
- degreased plate, then treated with solution (B):
231 # 15 N.

 The observation of the fracture facies makes it possible to note an excellent cohesion of the metallized base on the support in the case of the plate treated with the solution (A). A very good level of tearing force is obtained for the plates treated with the solutions (A) and (B) relative to the witness plate, reflecting a significant gain in adhesion.

 (b) The use of solutions (A) and (B) for treating MINLON 13 T1 hubcovers, which then received metallized base A and varnish A, confirms the above observations.

 Solution (B), being very stable, will be preferred.

Example 4: Use of 3-glycidyloxypropyl trimethoxysilane (GTS) as an adhesion promoter in the treatment solution
General procedure
A polyamide article, previously degreased with isopropanol, is immersed for 1 minute at 200 ° C. in a solution in isopropanol of a given percentage of an acid with an oxidizing characteristic, and which also contains 1% of GTS. Then, the part is removed, dried for about 5 minutes at 55 ° C., then it is coated with the base A and then with the varnish A.

(a) (Comparative)
As a support, a hubcap in MINLON 13 T1 and a treatment solution with 3% phosphoric acid is used.

The results are similar to those of the Example
Comparative l (a). This incomplete performance is attributable to the particularly oxidizing nature of chromic acid, which reacts in this case on silanes.

(b) (Comparative)
The same support as in 3 (a) and a 3% chromic acid treatment solution are used. There is a reaction between chromic acid and GTS.

(c) A MINLON IICl carrier and a 2.1% solution of nitric acid + GTS (solution C) were used. The test according to the standard NFT 30-062 provides the following results
- defatted support only: 144 + 16N
- Defatted support then treated with solution B
231 + 15N
- Defatted support then treated with solution C 294 + 32N.

 The influence of the GTS promoter incorporated in the solution C on the adhesion of the base system + varnish appears clearly.

Example 5: Use of different adhesion promoters in the solution (B)
General procedure
A MINLON 13 T1 support is used and various concentrations of promoters are incorporated in the solution (B). The support is immersed for 1 minute at 200C in the resulting solution and then dried for 5 minutes at 500C. Base A and varnish A.

The selected promoters are:
3-aminopropyl-triethoxysilane (basic character)
(ATS)
3-methacryloxypropyltrimethoxysilane (MTS)
3-glycidyloxypropyltrimethoxysilane (GTS).

Results
Grid tests indicate good results with MTS at a concentration of 2% to 2%, and with GTS at a concentration of 1% to 2%. ATS causes the formation of a precipitate.

Example 6
As a carrier, a PA 66 mirror and, as a treatment solution at 200C, the solution (B) with a concentration of GTS between 0.5% and 15% is used.

The base system B + varnish A is applied. The grid test according to the NFT 30-a38 standard is very satisfactory (class o). In addition, it is noted that the gloss and the slipperiness of the support increase when the concentration of
GTS increases.

Example 7
a PA 66 mirror and a 200C treatment solution (S) having the following composition are used as support

<tb><SEP> S1 <SEP> S2 <SEP> S3
<tb> Isopropanol <SEP> 98 <SEP> 91.5 <SEP> 84
<tb> HNO3 <SEP> 1 <SEP> 7.5 <SEP> 15
<tb> GTS <SEP> 1 <SEP> 1 <SEP> 1
<Tb>
The grid test according to NFT30-038 gives very satisfactory results (Class 0). However, it is observed that at a concentration of 15%, the surface appearance begins to deteriorate as a result of a major attack of the PA 6-6 substrate.

Example 8
General surgical mode
As a support, a hubcap in MINLON 13T1 is used and, as a treatment solution & 200C, a 3% solution of a given acid in a given solvent, said solution also containing 1% GTS. The grid test is conducted according to NFT 30-038
(a) (Comparative)
A 3% solution of phosphoric acid is used

<tb><SEP> Solvent <SEP> Class
<tb> Methyl ethyl ketone
<tb> Isopropanol <SEP> 2 <SEP> to <SEP> 3 **
<Tb>
* 100% detachment ** 20% detachment.

 (b) A solution of 3% chromic acid or 3% nitric acid is used. Both in the case of the use of isopropanol and methyl ethyl ketone as the solvent, the grid test gives positive results: class 0, that is to say 0% of peeling.

 However, in the same way as in Example 2, there is a problem of stability of chromic acid in these two solvents.

Example 9: Use of GTS in the primer
General procedure
A MINLON 13 T1 support is used which is treated by immersion in a solution of 2.1% by weight of nitric acid at 20 ° C. in isopropanol, to which 1% by weight of GTS is added. After this preliminary treatment, primer A, B or C is applied and cooked at 80 ° C. for 25 minutes.

(a) Primer A containing a given amount of GTS is used. The adhesion test gives the following results:

<tb>% <SEP> of <SEP> GTS <SEP> in <SEP> Force <SEP> Breakout
<tb> the <SEP> primer in <SEP> N
<tb><SEP> 0.5% <SEP> 92 + 5
<tb><SEP> 2% <SEP> 83 + 8
<Tb>
(b) Primer B is used. ATS Adhesion Agent is suitable
(c) Primer C is used. A test conducted with 0.5% GTS in the primer shows satisfactory results in adhesion tests.

Example 10
A series of undehydrated PA 66 mirrors were treated by immersion in an isopropanol: 96 / HNO 3: 3 / GTS solution for 1 minute. After drying at 550C for 5 minutes, it is coated with the primer A, then with the base B, and finally with the varnish A under the same conditions as above.

 In the same way, a series of these PA66 mirrors is coated, without the prior acid treatment (control), successively with the same primer, the same base and the same varnish.

The results of the tests of resistance in mono impact and adhesion (norm NFT 30-038) are reported in the
Next table
Board

<tb><SEP> MONOIMPACT <SEP> MONOIMPACT <SEP> ADHERENCE
<tb><SEP> Before <SEP> ruben <SEP> After <SEP> ruben <SEP> intiate <SEP> After <SEP> SIPON
<tb> adhesive <SEP> adhesive <SEP> (% <SEP> of <SEP>) <SEP> (% <SEP> of <SEP>)
<tb><SEP> (% glare> 1mm) <SEP> (% glare> 1mm) <SEP> glue) <SEP> glue)
<tb><SEP> RND1 <SEP> control <SEP> 35 <SEP> 55 <SEP> 90 <SEP> 10
<tb><SEP> RND2 <SEP> control <SEP> 40 <SEP> 65 <SEP> 100 <SEP> 20
<tb><SEP> RND3 <SEP> control <SEP> 35 <SEP> 40 <SEP> 100 <SEP> 30
<tb><SEP> RND1 <SEP> Treated <SEP> 20 <SEP> 20 <SEP> 0 <SEP> 0
<tb><SEP> RND2 <SEP> Treated <SEP> 25 <SEP> 25 <SEP> 0 <SEP> 0
<Tb>
RND t Undesired mirror Example 1l
Example 10 was repeated but using dehydrated mirrors. The results of the adhesion test (NFT standard 30-038) are reported in the following table.

Board

<tb><SEP> Adhesion <SEP>
<tb><SEP> Initial <SEP> After <SEP> SIPON
<tb><SEP> (% <SEP> of <SEP> decol- <SEP> (% <SEP> of <SEP> decol
<tb><SEP> lement) <SEP> lement)
<tb> RD * <SEP> 1 <SEP> control <SEP> 100 <SEP> 0
<tb> RD2 <SEP> control <SEP> 100 <SE> O <SEP>
<tb> RD1 <SEP> processed <SEP> 0 <SEP> 0
<tb> RD2 <SEP> processed <SEP> 0 <SEP> 0
<tb> * RD = dehydrated mirror.

 The adhesion test (NPT standard 30-038) gives particularly favorable results on both dehydrated and non-dehydrated parts.

 In all tests on polyamide substrates with chromic or nitric acid, the SIPON test has always given satisfactory results.

Claims (14)

 1 - Process for surface treatment of polyamide substrates for improving the adhesion of organic coatings, characterized in that this treatment is carried out with the aid of a solution, in an organic solvent, of at least one acid and oxidizing character forming water-soluble compounds neither with the treatment medium, nor with the substrate.  2 - Process according to claim 1, characterized in that said acid is used at a concentration sufficient to effect a surface attack of the substrate, but within the concentration limits to maintain the surface appearance of the substrate.  3 - Process according to one of claims 1 and 2, characterized in that one uses a solution of about 1 to 15% by weight of said acid.  4 - Process according to claim 3, characterized in that one uses a solution of about 1 to 5% by weight of said acid.  5 - Process according to one of claims 1 to 4, characterized in that a solution of nitric acid, chromic acid or acetic acid, including nitric acid.  6 - Process according to one of claims 1 to 5, characterized in that a solution of the oxidizing acid is used in a solvent totally or partially miscible with calf, said solvent being chosen in particular from lower alcohols and ketones .  7 - Method according to one of claims 1 to 6, characterized in that one uses a treatment solution containing at least one nonbasic adhesion promoter and otherwise compatible with the acid used, in particular 0.2% 2% by weight of said adhesion promoter, relative to the weight of the treatment solution.  8 - Process according to claim 7, characterized in that one uses, as adhesion promoters, those of the non-basic silane type.  9 - Process according to claim 8, characterized in that 3-glycidyloxypropyl-trimethoxysilane is used as adhesion promoter.  10 - Process according to one of claims 1 to 9, characterized in that the treatment is conducted by contacting the polyamide substrates to be treated for a period of 30 seconds to 2 minutes in the solution of said acid to oxidizing nature, the substrates being, after being removed from said solution, dried at about 300C-600C, for about 1-2 minutes.  11 - Process for coating a polyamide substrate with one or more layers of organic material strongly adhering to the substrate, characterized in that:  the method of treatment as defined in  one of claims 1 to 10:  - then apply a layer of primer or  primer, for example epoxy, epoxy-urethane,  acrylic-urethane, acrylic-melamine, polyester and / or  a metallized base or not, for example of the type  acrylic, acrylic urethane, acrylic-melamine,  polyester-urethane, and finally,  a final layer of varnish is applied, for example  acrylic-urethane type or acrylic melamine and / or  polyester-methane type.  12 - Process according to claim 11, characterized in that it incorporates, in the material intended to form the primer layer or primer, or base, coming to be applied on the substrate treated with acid to oxidizing character, at least one adhesion promoter compatible with the constituents of said layer, in a proportion of 0.2 to 1.5% by weight of said adhesion promoter relative to the weight of said material.  13 - Process according to claim 12, characterized in that a silane promoter, such as 3-glycidyloxypropyltrimethoxysilane and 3-aminopropyltrimethoxysilane, and / or a promoter of a silane promoter, such as 3-glycidyloxypropyltrimethoxysilane, is used as adherence promoter. Titanate type adhesion, such as (dioctylpyrophosphate) oxyacetate titanate.  14 - Polyamide substrates coated by the process as defined in one of claims li to 13.