The invention relates to a method for manufacturing a polymer article having a thin silicon coating formed on at least one of its side by PECVD. The invention relates also to a polymer article manufactured by the method, this article being of any shape and obtained by injection, extrusion moulding, blow moulding, compression moulding, vacuum forming and the like . The invention relates more particularly to a method for manufacturing a polypropylene or polyethylene shaped article that is adapted to be used as a food container by having excellent surface properties such as reduced tendency of being stained, good resistance against chemicals, this container being washable in a dishwasher, and being also able to be placed in a refrigerator, a freezer or a microwave oven.

Plasma Enhanced Chemical Vapour Deposition (hereinafter referred to as PECVD) is used for depositing a variety of thin films at lower temperature than those utilized in CVD reactors.

PECVD uses electrical energy to generate a glow discharge in which the energy is transferred into a gas mixture. This transforms the gas mixture into reactive radicals, ions, neutral atoms and molecules and other exited species.

PECVD is largely used in electronics in depositing many films such as silicon nitride, diamond like carbon DLC, poly-silicon, amorphous silicon, silicon oxynitride,

silicon oxide, silicon dioxide.

Silicon oxide films deposited by plasma enhanced chemical vapour deposition are receiving considerable attention in the packaging industry due to their excellent gas barrier performance.

As it is well known, plastics used for containers permits low molecular gas, such as oxygen and carbon dioxide, to permeate there through, and furthermore, plastic sorbs inside therein low molecular inorganic compound. As a consequence, aroma component can be absorbed inside the plastic; oxygen can gradually oxidize the content of the container, deterioring flavour, quality and purity of said content.

US 5 691 007 disclose a PECVD process whereby a coating of inorganic material may be placed on 3-D articles in a closely spaced matrix. This inorganic material can be a metal oxide such as SiOx wherein x is from about 1.4 to about 2.5; or an aluminium oxide based composition. The silicon oxide based composition is substantially dense and vapour-impervious and is desirably derived from volatile organosilicon compounds and an oxidizer such as oxygen or nitrous oxide. Preferably, the thickness of the silicon oxide based material is about 50 to 400 run. A flow of 2.6 seem HMDSO (hexamethyldisiloxane) and 70 seem oxygen is established and pressure regulated to 120 mTorr by pump throttling and a 3 min SiOx deposition is produced with an 11.9 MHz 120 watt RF excitation on PET tube.

US 6 338 870 disclose the use of HMDSO or tetra methyl disiloxane TMDSO for deposition of SiOxCy on PET laminated product wherein x is within the range of 1.5-2.2 and y is within the range of 0.15-0.80.

US 2003/0215652 disclose a polymeric substrate having a barrier coating comprising

a polymeric substrate a first condensed plasma zone of SiOxCyHz, wherein x is from 1 to 2.4, y is from 0.2 to 2.4 and z is from zero to 4 on the polymeric substrate wherein the plasma is generated from an organosilane compound in an oxidizing atmosphere and a further condensed plasma zone of SiOx on the polymeric substrate wherein the plasma is generated from an organosilane in a oxidizing atmosphere sufficient to form the SiOx.

This substrate is used for polymer bottle, particularly the non refillable bottle used for carbonated beverages, the aim of the coating being to be a barrier to the permeation of odorants, flavorants, ingredients, gas and water vapour. It is contended that the condensed plasma coatings of this prior art document may be applied on any suitable substrate including polyolefin such as polypropylene or polyethylene. However, examples 1 to 7 in this prior art document are plasma coatings on PET, no information being given for examples 8a, 8b and 8c as regards the polymer used, a 150 microns HDPE film being mentioned in example 9, PET films being used in example 10, polycarbonate being used for the last examples 11 to 13.

Summary of the Invention

The present inventor have noticed that it remains particularly difficult to obtain SiOx or SiOxCyHz or SiOxCy layers with good adhesion properties on some polymer substrate, especially polypropylene and polyethylene, using the PECVD route.

The present inventor has also noticed that the major part of patent literature on PECVD deposits for polymer substrate relates on PET (see e.g. EP 469 926,

FR 2 812 568), the silicon oxide thin films on PP obtained by prior art PECVD techniques (see example 4 of US 5 378 510 or FR 2 814 382, FR 2 670 506, EP 787 828) not being able to produce washable containers having reduced tendency of being stained.

A first object of the present invention is a polymer article having a thin silicon coating on at least one of its sides, said coating comprising a first coating of SiOxCyHz and a second coating of SiOxCyHz, the first and second coatings having different chemical compositions, said polymer article having reduced tendency of being stained.

Another object of the present invention is a method for manufacturing a polymer article having a thin silicon coating formed on at least one of its side by plasma, this article being washable and having reduced tendency of being stained.

A further advantage of a container incorporating a coating according to the present invention is that it remains a good barrier to the permeation of odorants, flavourants, ingredients, gas and water vapour, the use of non stick coating spray being thus avoided.

The method according to the invention comprises deposition of a first coating of SiOxCyHz and a subsequent deposition of a second coating of SiOxCyHz, the first and second coatings having different chemical compositions.

In one embodiment, the method according to the invention comprises a plasma treatment made on said polymer article before deposition of said first coating of SiOxCyHz.

According to various embodiments, the method of the present invention exhibits one or some of the following features .

The x value is between 1.4 and 1.7, the y value being between 0.5 and 0.8, the z value being between 0.35 and 0.6 for the first SiOxCyHz coating; the x value being between 1.7 and 1.99, the y value being between 0.2 and 0.5, the z value being between 0.2 and 0.35 for the second SiOxCyHz coating; said method comprises generation of a plasma from an organosilane compound in an oxidizing atmosphere, the ration between oxiding atmosphere and organosilane compound being sufficient to form said first and said second SiOxCyHz layer; the polymer article is configured in the form of a container, its inner side being plasma treated and coated; - the polymer article is made in polypropylene, polyethylene (HDPE or LDPE) , polycarbonate, polyetherimide, polyethersulfone, polyethylenetherephtalate, polybutylene- therephtalate, or in a liquid crystal polymer; silicon coating is made using magnetic guidance, or a plasma generating electrode, or both magnetic guidance and a plasma generating electrode; plasma silicon coating is obtained by generating plasma from an organosilane compound selected from the group consisting of dimethoxydimethylsilane, methyltrimethoxysilane, tetramethoxysilane, methyltrietho- xysilane, diethoxydimethylsilane, methyltriethoxysilane,- triethoxyvinylsilane, tetraethoxysilane, dimethoxymethyl- phenylsilane, phenyltrimethoxysilane, 3-glycidoxypropyl- trimethoxysilane, 3-methacryloxypropyltrimethoxysilane, diethoxymethylphenylsilane, tris (2-methoxyethoxy) vinyl- silane, phenyltriethoxysilane, dimethoxydiphenylsilane, tetramethyldisiloxane, hexamethyldisiloxane, hexamethyl- silazane, and tetramethylsilazane;

plasma silicon coating is obtained by generating plasma from TMS tetramethylsilane in the presence of oxygen; power is loaded to the plasma using a frequency of 13,56 MHz; the ratio between oxygen and TMS is between around zero and three so as to obtain said first coating, said ratio being between around four and ten so as to obtain said second coating onto said first one; - the ratio between oxygen and TMS is maintained during a first step of around one to four seconds at its first value of around zero to three, said ratio being maintained during a second step of around ten to thirty seconds at its second value of around four to ten; - said polymer article is a 3D shaped one, this article being placed in a vacuum chamber and defining an internal volume and an external volume, the inner part of the article defining the internal volume as the .reacting chamber, pressure inside said reacting chamber being around 0.01 mbar, pressure inside the external volume surrounding the reacting chamber being between around 20 and 100 mbar.

Description of Preferred Embodiment

In one embodiment, a 3D polypropylene container of the type used for food is placed in a vacuum chamber, thus defining an internal volume and an external volume, the internal volume forming the reaction chamber for the plasma treatment.

Pressure is gradually reduced inside the vacuum chamber so that the external volume pressure goes down to a value of around 20 to 100 mbar.

Pressure inside the reaction chamber is then lowered to a value of around 0.01 mbar.

Argon plasma treatment is made on the inner surface of the 3D container.

Then a first plasma deposit is made on the plasma treated inner surface of the container, using tetramethylsilane and oxygen. Power is loaded to the plasma by RF, the frequency being of 13.56 MHz. The ratio between oxygen and TMS is between zero and three and the treatment time is between one to four seconds.

The first deposit is a SiOxCyHz layer of a few nanometers thick.

Using a ratio of around 2 between oxygen and TMS, the chemical composition of this first coating is the following (Esca, Ftir and Erd analyses) :

Si: 27.6% 0: 43.6%

C: 17.1%

. H: 11.7%

Formula SiOxCyHz x being 1.58, y being 0.62 and z being 0.42. A second plasma deposit is then made on the coated inner surface of the container, using TMS and oxygen again. Power is again loaded by RF, same frequency being used. The ration between oxygen and TMS is maintained between four and ten and the treatment time is between ten to thirteen seconds.

The second deposit is a SiOxCyHz layer of a few nanometers thick.

Using a ratio of around 4.5 between oxygen and TMS, the chemical composition of this first coating is the following (Esca, Ftir and Erd analyses) : Si: 28.5% 0: 50.55% C: 12.55%

H : 8 . 35%

Formula SiOxCyHz x being 1.77, y being 0.44 and z being 0.29

Using a ratio of around 8.5 between oxygen and TMS, the chemical composition of this first coating is the following (Esca, Ftir and Erd analyses) :

Si: 28.75%

0: 54.95%

C: 8.9% H: 7.4%

Formula SiOxCyHz x being 1.91, y being 0.31 and z being 0.257.

The present inventor has surprisingly discovered that the shaped article obtained has a very low tendency of being stained during its lifetime, this shaped article being washable in a dishwasher, and being also able to be placed in a refrigerator, a freezer or a microwave oven.

A number of 125 washings were made at 85°C using a detergent named Neodisher Alka 300 and a rinsing liquid agent named Neodisher TS, supplied by Dr Weigert Cie.

The tendency of being stained was characterized by putting the containers in which different kinds of aggressive alimentary sauces were placed, in a oven at 80 ⁰ C during 24 hours. The tendency of being stained was measured before and after dishwashing.

Very good results were obtained with the containers treated by the above described method.