Non-Stick Coating Primer Compositions and Processes for the Preparation Thereof

Abstract

Provided are non-stick coating hybrid primer compositions based on polyimide resin and/or polyamide-imide resin. Also provided are processes for preparing such compositions. Finally, non-stick coatings incorporating such compositions and a process for manufacturing articles comprising such non-stick coatings are provided.

Claims

1. A non-stick coating hybrid primer composition comprising a matrix of polyimide resin and/or polyamide-imide resin in which silica domains of a size less than 200 nm are dispersed in a polar aprotic solvent.

2. The hybrid primer composition according to claim 1, wherein the relative dry weight content of the silica domains relative to the polyimide resin and/or polyamide-imide resin matrix is between 10 and 25% of said hybrid primer composition.

3. The hybrid primer composition according to claim 1, wherein the polar aprotic solvent is present at a concentration of 1 to 70% by weight relative to the total weight of the composition.

4. The hybrid primer composition according to claim 1, wherein the composition is free of fluorocarbon resin.

5. A non-stick coating primer composition comprising: a polyimide resin and/or a polyamide-imide resin, an alkoxysilane and/or metallic alkoxylate, a coupling agent able to link the polyimide resin and/or the polyamide-imide resin with the alkoxysilane and/or the metallic alkoxylate, and a polar aprotic solvent.

6. The composition according to claim 5, wherein the alkoxysilane is selected from methyltrimethoxysilane (MTMS), tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane, methyltriethoxysilane (MTES), dimethyldimethoxysilane, phenyltriethoxysilane and mixtures thereof.

7. The composition according to claim 5, wherein the total relative dry weight content of the coupling agent relative to the polyimide resin and/or the polyamide-imide resin is between 2 and 20% of said primer composition.

8. The composition according to claim 5, wherein the coupling agent is selected from the silazanes, silane derivatives and mixtures thereof.

9. The composition according to claim 5, wherein the polar aprotic solvent is present at a concentration of 1 to 70% by weight relative to the total weight of the composition.

10. The composition according to claim 5, wherein the composition is free of fluorocarbon resin.

11. A non-stick coating comprising at least one layer of the non-stick coating hybrid primer composition according to claim 1.

12. A method of preparing a non-stick coating hybrid primer composition according to claim 1, the method comprising the following steps: A) introducing a coupling agent into a solution comprising the polyimide resin and/or polyamide-imide resin and the polar aprotic solvent; and B) adding, in a basic medium, the alkoxysilane and/or the metallic alkoxylate, and water to the mixture resulting from step A).

13. A method of preparing a non-stick coating hybrid primer composition according to claim 1, the method comprising the following steps: A) introducing the coupling agent into a solution comprising the polyimide resin and/or the polyamide-imide resin and the polar aprotic solvent; and B) adding the alkoxysilane and/or the prehydrolyzed metallic alkoxylate to the mixture resulting from step A).

14. The method according to claim 13 for the preparation of a non-stick coating hybrid primer composition also comprising a step C) the hydrolysis of the alkoxysilane and/or the metallic alkoxylate to produce prehydrolyzed alkoxysilane and/or metallic alkoxylate, this hydrolysis step C) being performed prior to step B) and comprising: the hydrolysis, in a basic environment, of the alkoxysilane and/or the metallic alkoxylate in a mixture of water and alcohol, or the hydrolysis, in a basic environment, of the alkoxysilane and/or the metallic alkoxylate in a mixture of water and polar aprotic solvent.

15. A method of manufacturing an item comprising a non-stick coating according to claim 11, the method comprising the following steps: A) providing a substrate having two opposite surfaces; B) applying onto at least one of the surfaces of said substrate of at least one layer of the non-stick coating hybrid primer composition, and C) sintering the primer composition at a temperature between 390 C. and 430 C.

16. An item made by the method as defined according to claim 15.

Description

EXAMPLES

[0091] Tests

[0092] Determination of Dry Extract in a Composition

[0093] Principle

[0094] The dry extract of a composition is the solid residual portion that remains after the volatile matter which it contains has been evaporated. The temperature and duration of drying play an important role, as high boiling point solvents, fractions of monomers, reactive diluents and by-products of the reaction (depending on the degree of their retention), are slow to leave the film in formation. It is consequently very important to formally define the standardized drying conditions, in a manner that is as practice-oriented as possible.

[0095] Technique

[0096] The procedure used to measure this dry extract is as follows: [0097] an aluminum measuring cup is weighed: m.sub.0=mass of the cup; [0098] 0.5 g-3 g of the composition to be studied is placed into the cup; [0099] the filled cup is weighed: m.sub.1=mass of the filled cup; [0100] the filled cup is placed in a drying oven at a temperature of 210 C. for two hours; [0101] after drying and after cooling, the cup is weighted again: m.sub.2=mass of the filled cup after drying and cooling; [0102] the dry extract is calculated with the following formula:

Dry extract=100*[(m.sub.2m.sub.0)/(m.sub.1m.sub.0)]

[0103] Measurement of Particle Size and Size Distribution Using Light Diffraction

[0104] The grain size of the colloidal solution obtained by hydrolyzing the TEOS (see Example 1 below) can be characterized using a laser diffraction particle size analyzer marketed by the Malvern company under the commercial name Nanosizer, which measures the Brownian motion of particles in suspension in relation to the size of the colloids (a small particle moves faster than Larger Particles) Via the Diffraction of Light.

[0105] Assessment of the adhesion of a coating on a smooth aluminum substrate

[0106] A cross-hatch adhesion test is carried out in accordance with the standard NF T 30-038 (standard NF EN ISO 2409), followed by the immersion of the coated substrate for 18 hours (consisting of three 3-hour cycles in boiling water and three 3-hour cycles in oil heated to 200 C.). Then, the non-stick coating is inspected for evidence of disbonding. The rating is determined according to the criteria in the table below.

TABLE-US-00001 Rating Description 0 The edges of the cuts are perfectly smooth: none of the squares in the grid are detached. 1 Detachment of small flakes of the coating at the intersections of the cuts, affecting an area of approximately 5% of the cross-hatched area. 2 The coating has flaked along the edges or at the intersections of the cuts, with the affected area representing significantly more than 5% and up to 15% of the cross-hatched area. 3 The coating has flaked along the edges of the cuts partly or wholly in large ribbons or has detached partly or wholly in various places within the cross- hatched area. The detached surface represents significantly more than 15% and up to 35% of the cross-hatched area. 4 The coating has flaked along the edges of the cuts in large ribbons or some squares have detached partly or wholly. The detached surface represents significantly more than 35% and up to 65% of the cross-hatched area. 5 Any degree of flaking that cannot be classified by Rating 4.

[0107] Assessment of Yellowing

[0108] Visual comparison of yellowing between coated substrates after firing.

[0109] Morphological Analysis

[0110] The morphological analysis of silica filler in a polymer resin matrix is carried out using TEM analysis.

[0111] TEM analysis is an efficient method used to visualize the morphology of silica charges within a polymer matrix, such as a PAI resin, whether the fillers were pre-formed (Ludox-type colloidals) or obtained in-situ using a sol-gel method.

Example 1

[0112] In a first step, a basic solution of hydrolyzed TEOS is prepared by mixing, under magnetic agitation and at controlled room temperature, the following components:

TABLE-US-00002 Pure TEOS 262.28 g NEP 142.43 g a mixture of water and a base: water 52.21 g ammonia solution at 10.25% by weight in water 42.85 g TOTAL 499.77 g

[0113] This solution is maintained under slow agitation for two hours at room temperature. The resulting solution is opalescent, stable for several months, and has the following characteristics:

[0114] Characteristics: [0115] pH: 9.6 [0116] viscosity: 280 mPas [0117] percentage of TEOS: 52.5%/total mass of wet solution [0118] appearance: translucent, iridescent [0119] particle size analyzed using laser diffraction particle size analysis: 20 to 40 nm [0120] pot life: more than several months

[0121] In a second step, a solvent-based PAI resin is introduced at 29% by weight into the NEP, in a reactor. After dilution of the PAI resin in the NEP, the APTES is added The neutralization reactions of the terminal acid groups leads to the chemical grafting of the silane. The reaction occurs at room temperature under slow agitation for two hours.

[0122] Finally the hydrolyzed TEOS solution is added according to the directions provided in the 1st step and the mixture is left under slow agitation at room temperature for 20 to 24 hours.

[0123] The proportions of the mixture are indicated below:

TABLE-US-00003 PAI resin at 29% by weight in the NEP 218.35 g NEP 98.31 g APTES 5.40 g basic solution of hydrolyzed TEOS 177.94 g TOTAL 500.00 g

[0124] The mass ratio (expressed as dry mass/dry mass) of PAI resin/SiO.sub.2 is 77/23.

[0125] The resulting colloidal solution of the PAI resin/silica hybrid material in the NEP thus obtained has a dry extract of 16.5%, a pH of 10.2 and a viscosity (measured in accordance with the standard DIN EN ISO 2433/ASTM 05125 in a DIN 4 flow cup) of 80 seconds. This solution is stable for up to three weeks stored at room temperature.

[0126] According to one embodiment (application onto a pre-formed substrate), this hybrid material colloidal solution is used as is, and applied in a thin layer by spraying onto an aluminum substrate in the shape of a dome (preformed substrate) previously subjected to a surface treatment.

[0127] The performance characteristics obtained are as follows: [0128] The thin layer achieved above, analyzed using TEM, demonstrates that a hybrid organic/inorganic material with dispersed nanoscale silica domains has been obtained (co-continuous networks of silica and PAI resin). The TEM analysis reveals individual silica particles less than 100 nm in diameter with no agglomeration or aggregation. [0129] The silica filler is present across the entire thickness of the film, as depicted in FIG. 1, which is a TEM image of a cross section of the formed coating demonstrating a homogenous distribution of the nanoscale silica particles in the PAI resin matrix and their dimensions: the mean diameter of the silica particles is approximately 374 nm. [0130] The presence of a co-continuous network of silica is demonstrated using calcination. A film is obtained for which the chemical analysis using infrared spectroscopy reveals two bands characteristic of silica at =1075 cm.sup.1 and =804 cm.sup.1 (condensation of SiOSi siloxane bridges) [0131] The implementation of this hybrid primer layer in a three-layer coating system (hybrid organic/inorganic primer layer+first PTFE-based finishing layer without PAI resin applied wet on wet+second PTFE-based finishing layer) sintered under standard conditions (11 minutes at 430 C.) results in a ranking of 0 in three cycles of adhesion testing. [0132] The resulting hybrid coating is non-yellowing (35% yellowing compared to a standard PTFE-based primer system such as the system described in Comparative Example 1). [0133] A corrosion resistance test of the hybrid coating, conducted by exposing the coating to a boiling, 10% salt water solution for 24 hours, yielded no points of corrosion.

[0134] Furthermore, according to a second embodiment (application onto a substrate with post-forming of the substrate), the hybrid primer composition obtained in Step 2 is applied to an aluminum disc that had been previously subjected to a chemical stripping process, dried and then heated. A first, PTFE-based finishing layer without PAI resin is then applied wet on wet to this primer layer, then a second PTFE-based finishing layer is applied to the first finishing layer. The ensemble is then sintered under standard conditions (11 minutes at 430 C.). The resulting coated substrate is then stamped to form a dome that is coated on its interior surface.

[0135] This post-formed coated dome demonstrates similar performance characteristics as the preformed coated dome described above, illustrating the suitability of the hybrid coating for stamping.

Example 2

[0136] A solvent-based PAI resin is introduced at 29% by weight into the NEP in a reactor. It is diluted with the NEP, then the APTES is added. The neutralization reactions of the terminal acid groups by the aminosilane leads to the chemical grafting of the silane. The reaction occurs at room temperature under slow agitation for two hours.

[0137] Next the TEOS is added with demineralized water and a base, then the mixture is left under slow agitation at room temperature for 20 to 24 hours. The proportions of the various components are as follows:

TABLE-US-00004 PAI resin at 29% by weight in the NEP 218.35 g NEP 149.20 g APTES 5.40 g Pure TEOS 93.38 g demineralized water 18.41 g ammonia solution at 10.25% by weight in water 15.26 g TOTAL 500 g

[0138] The mass ratio (expressed as dry mass/dry mass) of PAI resin/SiO.sub.2 is 77/23.

[0139] The resulting colloidal solution of the PAI resin/silica hybrid material in the NEP has a dry extract of 16.5%, a pH of 10.2 and a viscosity (measured in accordance with the standard DIN EN ISO 2433/ASTM 05125) of 65 seconds. This solution is stable for up to three weeks stored at room temperature.

[0140] This solution is used as is and applied in a thin layer by spraying onto an aluminum substrate in the shape of a dome (preformed substrate) previously subjected to a surface treatment.

[0141] The performance characteristics obtained are as follows: [0142] The implementation of this hybrid primer layer in a three-layer coating system (hybrid organic/inorganic primer layer+first PTFE-based finishing layer without PAI resin applied wet on wet+second PTFE-based finishing layer) sintered under standard conditions (11 minutes at 430 C.) results in a ranking of 0 in three cycles of adhesion testing. [0143] The resulting hybrid coating is non-yellowing (30% yellowing compared to a standard PTFE-based primer system such as the system described in Comparative Example 1). [0144] A corrosion resistance test of the hybrid coating, conducted by exposing the coating to a boiling, 10% salt water solution for 24 hours, yielded no points of corrosion.

Comparative Example 1

[0145] A semi-finished aqueous composition is produced comprising the following components, the respective quantities of which are detailed in figure g below:

TABLE-US-00005 PAI resin at 29% by weight in the NEP 220.0 g NEP 79.0 g triethylamine 22.0 g demineralized water 350.0 g colloidal silica 63.5 g TOTAL 734.5 g

[0146] The colloidal silica used has no surface modifications, a specific surface of approximately 220 m.sup.2/g, and has the form of an aqueous dispersion of nanoparticles with a dry extract of 30% by weight in water.

[0147] The procedure to produce the semi-finished aqueous mixture is as follows: [0148] the PAI resin is placed into a Discontimill reactor with the solvent and the triethylamine; [0149] the resulting mixture is then ground in the Discontimill reactor at room temperature; then [0150] water is added gradually to achieve the aqueous phase and obtain a dispersed polyamide-amic acid; [0151] grinding is continued for two hours to obtain an intermediate mixture.

[0152] The properties of the semi-finished mixture thus obtained are as follows: [0153] theoretical dry extract: 9.5% [0154] measured dry extract: 9.0% [0155] the product is honey-colored, translucent and viscous [0156] viscosity (in accordance with the standard DIN EN ISO 2433/ASTM 05125): 140 sec

[0157] The dispersion of colloidal silica of 40 to 200 nm in size is introduced the intermediate mixture, at room temperature, to obtain the semi-finished aqueous mixture.

[0158] The properties of the semi-finished aqueous mixture thus obtained are as follows: [0159] theoretical dry extract: 11.3%, [0160] measured dry extract in the solution: 11.0%, [0161] the product is honey-colored, translucent and viscous [0162] viscosity (in accordance with the standard DIN EN ISO 2433/ASTM 05125): 150 seconds, [0163] the proportion of silica is 23% by weight with respect to the polyamide-amic acid.

[0164] Then, from the aqueous semi-finished mixture, an aqueous binder primer composition is produced with the following components:

TABLE-US-00006 dispersion of PTFE at 60% by weight in water 30.5 g dispersion of carbon black at 25% by weight in water 3.5 g the above partially finished aqueous mixture 50.0 g non-ionic tensoactive system at 12.5% dry extract 5.1 g NH.sub.4OH 1.4 g demineralized water 9.5 g Total 100.0 g

[0165] This aqueous binder primer composition presents the following characteristics: [0166] the fluorine-containing resin content in the dry primer composition is approximately 71.9% by weight relative to the total dry weight of the composition, [0167] the polyamide-amic acid content of the dry primer composition is approximately 17.1% by weight relative to the total dry weight of the composition, [0168] the silica content of the composition is 23/77 by weight relative to the dry weight of the polyamide-amic acid, [0169] the theoretical dry extract is 25.5%, and [0170] the viscosity (measured in accordance with the standard DIN EN ISO 2433/ASTM 05125) is 65 seconds.

[0171] This aqueous binder primer composition is applied in a thin layer by spraying onto an aluminum substrate in the shape of a dome (preformed substrate) previously subjected to a surface treatment.

[0172] The performance characteristics obtained are as follows: [0173] The implementation of this thin binder primer layer in a three-layer coating system (primer layer as described above+first PTFE-based finishing layer without PAI resin applied dry on dry+second PTFE-based finishing layer) sintered under standard conditions (11 minutes at 430 C.) results in rankings of 3 or 4 in three cycles of adhesion testing. [0174] A corrosion resistance test of the obtained coating, conducted by exposing the coating to a boiling, 10% salt water solution for 24 hours, yielded 5 points of corrosion. This result is inconsistent. [0175] The binder primer layer comprising a dispersion of colloidal silica is analyzed using TEM. FIG. 2, which is a TEM image of a cross section of the resulting primer layer, reveals agglomeration of the filler across the entire thickness of the film, in a circular or oval shape, and of equivalent dimension across the thickness of the film (length of the agglomerations ranging from 0.5 m to 3 m).