WO2016102996A1

WO2016102996A1 - Method for limiting the propagation of cracks in an optical article

Info

Publication number: WO2016102996A1
Application number: PCT/IB2014/003128
Authority: WO
Inventors: Ker Chin Dave Ang; Jinhhao NG
Original assignee: Essilor International (Compagnie Générale d'Optique)
Priority date: 2014-12-23
Filing date: 2014-12-23
Publication date: 2016-06-30

Abstract

The present invention relates to the field of optical articles comprising a substrate at least coated with a hard coating. In particular, this invention relates to a method for limiting the propagation of cracks in an optical article such as a photochromic lens when it has to be exposed to a heat treatment, and to an optical article, especially a photochromic lens obtained by this method.

Description

METHOD FOR LIMITING THE PROPAGATION OF CRACKS IN AN

OPTICAL ARTICLE

PRIOR ART

It is a common practice in the art to coat at least one face of a lens with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. Thus, it is well-known to coat at least one face of a lens, with successively, starting from the face of the lens, an impact-resistant coating (i .e. impact resistant primer layer), a scratch-resistant coating (hardcoat layer or hard coating), an anti-reflecting coating, and optionally, a hydrophobic top coat. Other coatings such as a polarized coating, a photochromic or a dyeing coating may also be applied onto one or both faces of the lens. Numerous processes including a step of heat treatment have been proposed for coating a face of a lens. However, a problem which could arise with said processes is the propagation of cracks induced by said step of heat treatment.

Indeed, it is believed that some small defects on the edges of the lens serve as crack initiation sites. Said crack initiation sites could be a result of a casting process, a coating process, a rough handling or a combination thereof during the production of the desired lens. Since these crack initiation sites enhance stress concentration, they are more susceptible to initiate fractures when subjected to thermal stress (e.g. during a step of coating including a heat treatment), thus resulting in cracks.

It is also believed that some materials commonly used for the upper coatings of said lens may have a low resistance to heat treatments, involving crack initiation sites and propagation of cracks during said step of heat treatment.

Such a step of heat treatment can be for example a tinting step which allows suitable amount of dyes to be imbibed into the surface of the substrate to achieve a pair of tinted lenses.

Dyeing or tinting of lenses for aesthetic and sunscreening purposes is well known to those skilled in the art. Such dyeing processes include those which uniformly apply the dyes to the entire lens, when a uniform lens character is desired, or which apply the dyes having different concentrations to portions of the lens in order to provide special effects such as gradient lenses. It is also known to apply a first dye uniformly to a lens and a second dye in a non-uniform manner in order to vary the color characteristics of the lens in different portions thereof.

Tinted lenses are generally tinted by conventional wet tinting process which comprises at least one step of dipping the lenses in aqueous baths of dispersed dyes and various additives to promote the coloring process, the baths being maintained at a temperature of the order of 94°C. Lenses can also be tinted by a dry tinting process which comprises at least one step of thermal transfer printing. Said dry tinting process relies on the sublimation of a subliming dye previously prepared on a provisional inert support. The temperature required to perform said dry tinting process is generally of between about 50°C to about 150°C. Thus, all the conventional tinting processes to obtain a lens with the desired tint or gradient tint, include at least one step of heat treatment which can render the lens more susceptible to the propagation of cracks when the lens comprises a coating before implementing the tinting process.

In parallel, a popular trend that has emerged in recent years (since the 1960s) with regard to eyewear has been the prevalence of photochromic lenses. Thanks to the introduction of U.V. light-sensitive substances into the lens (e.g. photochromic coating), photochromic lenses are able to darken on exposure to specific types of light, most commonly ultraviolet (UV) radiation. Once the light source is removed, the lenses will gradually return to their clear state. Since the commercially available photochromic lenses only provide limited tints in whatsoever state (i.e. darken or clear state), a tinting step can be performed on the photochromic lenses to provide tints other than grey or brown usual tints in the darken state and/or a visible gradient tint in the clear state. However, again, cracking in the lens due to their exposure to such tinting step including a heat treatment is observed, damaging the hard coating thereby destroying the value of the lenses.

Efforts have been made in the past to limit the propagation of cracks more particularly when the optical article is exposed to an elevated temperature.

In particular, US2006/0172136 describes the use of flexibility imparting agents such as polysiloxane resins, acrylic resins, polyester resins, polyurethane resins or rubbers, or acrylonitrile rubbers within the hard coating of an optical article in order to increase its heat resistance, thus limiting cracking in said hard coating.

In parallel, FR 2 702 486 relates to a silane-based hard coating comprising colloidal silica and a specific aluminum catalyst, which does not crack and is easily tinted.

However, the compounds (e.g. flexibility imparting agents) or additional coatings used to limit the propagation of cracks in optical articles modify the chemical composition of the original coatings and/or of the optical articles, and thus can alter the other properties of said optical articles, such as their mechanical, photochromic, anti-reflection, anti-smudge or anti-dust properties. In addition, said compounds or additional coatings are required to be added in a specific part of the optical articles and/or in a specific type of coating of the optical articles, thus rendering their method of preparation not easily transposable to any type of lenses.

Consequently, an alternative method which can limit the propagation of cracks when an optical article has to be exposed to a heat treatment during its preparation process, and in particular to a tinting process, without the above disadvantages is highly desirable.

A first aim of the present invention is to provide a simple and alternative method which is able to limit the propagation of cracks in an optical article, and especially in a photochromic lens, while keeping all the functional properties of coatings.

In addition, a second aim of the present invention is to provide an optical article which is resistant to the propagation of cracks when it has to be submitted to a heat treatment, and especially to a tinting process.

These objectives are achieved by the method and the optical article which are described below.

SUMMARY OF THE INVENTION

A first object of the present invention is a method for preparing an optical article for a heat treatment, the optical article having an optical center (C) and comprising a substrate and at least one hard coating, said substrate being coated with said hard coating, said hard coating having a thickness (e) and edges (E),

wherein said method comprises a step i₀) for limiting the propagation of cracks in said optical article when exposed to the heat treatment, said step i₀) including the on-purpose engraving of a continuous or discontinuous groove through at least said hard coating, the outline of the groove forming an open or closed shape which surrounds the optical center (C).

The method of the present invention is simple, economic and can be applied to any optical article comprising a substrate and at least one hard coating coated on said substrate and which is supposed to be submitted to a heat treatment. The method does not impair the optical and mechanical properties of the resulting optical article and does not modify the chemical composition of the original coatings and/or of the optical article. In addition, said method surprisingly leads to optical articles which have a central zone useable for making eyeglasses and protected from the propagation of cracks. Thanks to the use of step i₀) and thus, to the introduction of a continuous or discontinuous groove through at least said hard coating, the outline of which forming an open or closed shape which surrounds the center (C) of the optical article, the optical article is ready to be further heat treated without being damaged by the appearance and/or propagation of cracks. Indeed, the groove, the outline of which forming an open or closed shape which surrounds the optical center (C), delimits two distinct domains: a first domain (I) containing the optical center (C) of the optical article and containing points positioned between the groove and the optical center (C); and a second domain (II) containing points separated from the optical center (C) by the groove, i .e., for which a direct line cannot be formed toward the optical center (C) without overlapping the groove. Cracks initiation sites generally appear in the second domain (II) and the groove serves as a barrier to interrupt the propagation of cracks from said second domain to the first domain.

In some cases, a third domain (III) may appear for some points which are not positioned between the optical center (C) and the groove and are not separated from the optical center (C) by the groove. According to the invention, as long as this third domain (III) covers less than one third of the surface of the hard coating, it is considered that the groove serves as a barrier to interrupt the propagation of cracks toward the optical center (C).

A further object of the present invention is an optical article obtained by the above described method, said optical article having an optical center (C) and comprising a substrate coated with at least one hard coating, said hard coating having a thickness (e) and edges (E), wherein said optical article further comprises a continuous or discontinuous groove engraved through at least said hard coating, the outline of said groove forming an open or closed shape which surrounds the optical center (C).

DETAILED DESCRIPTION

The terms "comprise" (and any grammatical variation thereof, such as

"comprises" and "comprising"), "have" (and any grammatical variation thereof, such as "has" and "having"), contain (and any grammatical variation thereof, such as "contains" and "containing"), and include (and any grammatical variation thereof, such as "includes" and "including") are open-ended linking verbs. They are used to specify the presence of stated features, integers, steps or components groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps or components or groups thereof. As a result, a method, or a step in a method, that "comprises", "has", "contains", or "includes" one or more steps or elements possessing those one or more steps or elements, but is not limited to possessing only those one or more steps or elements.

Unless otherwise indicated, all numbers or expressions referring to quantities of ingredients, ranges, reaction conditions, etc. used herein are to be understood as modified in all instances by the term "about".

When an optical article comprises one or more surface coatings, the phrase "to deposit a coating onto the optical article" means that a coating or layer is deposited onto the outermost coating of the optical article, i.e. the coating which is the closest to the air.

A coating that is "on" a side of a lens is defined as a coating that (a) is positioned over that side, (b) needs not be in contact with that side, i .e., one or more intervening coatings may be disposed between that side and the coating in question, and (c) needs not cover that side completely.

As used herein, the phrase "outermost coating" or "outermost layer" means a coating or a layer which is the farthest from the substrate and conversely the phrase "innermost coating" of innermost layer" means a coating or a layer which is the closest to the substrate.

As used herein, the term "substrate" means a naked substrate or a naked substrate already coated with one or several functional coatings.

As used herein, the term "optical article" means one of an ophthalmic lens, an ocular visor, magnifying lenses and protective lenses or visors such as found in spectacles, glasses, goggles and helmets and sight optical systems. Especially, ophthalmic lens is a lens which is designed to fit a spectacles frame so as to protect the eye and/or correct the sight and can be an uncorrective (also called piano or afocal lens) or a corrective ophthalmic lens. Corrective lens may be a unifocal, a bifocal, a trifocal or a progressive lens.

The optical article prepared according to the present invention is a transparent optical article, preferably a lens or lens blank, and more preferably an ophthalmic lens or lens blank. The optical article may be coated on its convex main side (front side (Cx)), concave (Cc) main side (back side), or both sides using the process of the invention.

Herein, the term "lens" means an organic or inorganic glass lens, comprising a lens substrate which may be coated with one or more coatings of various natures.

The lens naked substrate may be made of mineral glass or organic glass, preferably organic glass. The organic glasses can be either thermoplastic materials such as polycarbonates, thermoplastic polyurethanes, poly(methylmethacrylates), polyacrylates, polyethylene, cyclic olefin homopolymers or copolymers, or thermosetting (cross-linked) materials such as diethylene glycol bis(allylcarbonate) polymers and copolymers (in particular CR-39® from PPG Industries), thermosetting polyurethanes, polythiourethanes, polyepoxides, polyepisulfides, poly(meth)acrylates and copolymers based substrates, such as substrates comprising (meth)acrylic polymers and copolymers derived from bisphenol-A, polythio(meth)acrylates, as well as copolymers thereof and blends thereof.

The substrate usually has a refractive index of between about 1.5 and about 1.74.

Unless otherwise indicated, the refractive indexes to which reference is made in the present invention are expressed at 25°C for a wavelength of 550 nm.

Within the meaning of the present invention, the term "hard coating" means generally a "hardcoat", i.e. an abrasion or scratch-resistant coating, such as those disclosed in US 2005/0123771, EP 0 614 957, US 4 211 823 and US 5 015 523. However it may also be a layer of same performances or composition even if deposited with another aim than for abrasion resistance.

However, with a broader meaning, the term "hard coating" used therein means a coating that is brittle and susceptible to form cracks under stress conditions. According to this broader meaning, a hard coating may be an anti-shock coating, an anti-scratch coating, a primer coating or an adhesion coating, said coating having a thickness higher than about 1 μηι .

Within the meaning of the present invention, the expression "a shape which surrounds the optical center (C)" means that the barycenter of the shape is in proximity of the optical center (C) of the optical article.

The surface of the optical article onto which the hard coating is applied may optionally be subjected to a pre-treatment step intended to improve adhesion, for example a high-frequency discharge plasma treatment, a glow discharge plasma treatment, a corona treatment, an electron beam treatment, an ion beam treatment, an acid or base treatment.

Said hard coating can be any coating typically used for improving abrasion- and/or scratch-resistance of a finished optical article as compared to a same optical article but without the abrasion- and/or scratch-resistant coating. Preferred abrasion- and/or scratch-resistant coatings are

(meth)acrylate based coatings and silicon-containing coatings. The latter are disclosed, for example, in FR 2 702486 The composition for an abrasion-resistant coating disclosed in FR 2 702 486 comprises an epoxytrialkoxysilane and dialkyldialkoxysilane hydrolyzate, colloidal silica and a catalytic amount of aluminum-based curing catalyst, such as aluminum acetylacetonate, the remainder being essentially composed of solvents conventionally used for the formulation of such compositions. Preferentially, the hydrolyzate used is a [gamma]-glycidoxypropyltrimethoxysilane (GLYMO) and dimethyldiethoxysilane (DMDES) hydrolyzate or else a [gamma]-glycidoxypropyltrimethoxysilane (GLYMO) and triethyl orthosilicate (TEOS) hydrolyzate. In one particular example, the hard coating may also comprise at least one layer of an antimony-containing-siloxane-based hard coating.

The thickness of the hard coating may usually be comprised between 1 and 10 μιη, and is often comprised between 1 μηη and 5 μηι.

Within the meaning of the present invention, the term "edges (E) of the hard coating" means the line or curve determining the limits of the surface of the hard coating or the periphery (i .e. the perimeter or the boundary) of the hard coating. In most cases, the edges (E) coincide mostly with edges of the optical article and thus, the center of the hard coating coincides roughly with the optical center (C) of the optical article.

The outline of the groove formed in step i₀) by engraving a continuous or discontinuous groove through at least the hard coating, can have any shape providing that the shape surrounds the center (C) of the optical article.

The outline of the groove can have for example a square, a rectangular, a trapezoidal, a circular (i .e. round), an ellipse shape or a more complex shape such a free-form shape or a star shape; it can be an open shape or a closed shape. Accordingly, the shape formed by the outline of the groove can be a part or the whole of a square, with acute or rounded angles, a rectangle, with acute or rounded angles, a trapezoid, with acute or rounded angles, a circle, an ellipse or a more complex shape such a free-form shape or a star shape.

The groove can be continuous or discontinuous. When it is discontinuous, it can be a discontinuous curved line made of a succession of dots or dashes or portions of curves or lines or a mix thereof, separated by spaces. Further, the groove can form one rank or two or more ranks of dots or dashes or portions of curves or lines. In case of multiple ranks, one rank may be composed of dots, dashes or portions of curves which are at least partially in front of the spaces of another rank of the groove.

The outline of the groove may form a closed shape or an open shape. An open shape may be for example illustrated by a shape which surrounds the optical center (C) on at least 2/3^rd of the directions or can even surround it completely.

In another embodiment, the shape may be an open shape still surrounding the optical center (C) but on all directions, it may thus be an open shape such a spiral with one end of the groove forming a second rank in front or behind the other end of the groove forming a first rank.

Said in another manner, the groove may have an apparent covered angle, as seen from the optical center (C), ranging from about 4/3 of Pi to about 2 Pi .

The apparent covered angle as used in the present description corresponds to the sum of the angular sectors formed, on the surface of the lens, by the directions which originate from the optical center (C) and head towards the edge of the lens, and which are interrupted by the groove before reaching the edge of the lens. For a continuous arc, it can be expressed as the center angle of the arc, or for a continuous free-form shape it can be expressed as the angular sector covered by the shape as seen from the center.

In case of a discontinuous groove, for example a groove made of multiple segments or portions of curve, the sum of the angular sectors covered by the multiple segments or portions of groove may range from 4/3 of Pi to 2 Pi.

When the groove comprises at least one part of curve made of a succession of dots or dashes, the overall covered angle of the groove is evaluated by counting as if the succession of dots or dashes was a continuous groove, as long as the distance between two dots or dashes is smaller than 3 times, preferably 2 times the width of the groove.

As for an arc, the length of the groove is defined along the outline formed by the groove. Accordingly, the width of the groove is defined at any point, as the direction locally perpendicular to the outline of the groove. In case of the groove being locally a dot, the width of the groove corresponds to a diameter of the dot. In general, the width of the groove, at any point, is the smallest dimension of the groove, along the surface of the lens, without taking into account the depth of the groove.

The introduction of the groove according to step i₀) should not introduce more sites for crack initiation or regions of high stress concentration. Hence, it is proposed that the groove has a substantially round or U-shaped cross-section.

In a preferred embodiment, the groove is positioned near to the periphery (i.e. the edges) of the optical article and/or to the edges (E) of the hard coating where the sites of cracks initiation are located; and far enough from the center (C) of the optical article which is used to make an eyeglasses lens.

Thanks to this configuration, the groove is able to stop early the propagation of the cracks. The method is not limited to the preparation of final optical articles with restricted dimensions.

In an embodiment, the groove is positioned in an area of the optical article which is a non-disturbing area for vision of a wearer. Preferably, the optical article is a lens which has to be edged to fit into a frame, and the groove is positioned in part of the lens which has to be removed. In particular, the groove is positioned out of an area corresponding to the part of the lens that has to fit into the frame.

In an example, the outline of the part of the lens that has to fit into the frame is known, called desired lens outline, and the outline of the groove may be within 3 mm from the desired lens outline, between said desired lens outline and the edges.

In another example, any part of the groove is farther than 1.8 mm from the optical center (C), preferably farther than 2 mm, or farther than 2.5 mm from the optical center (C).

The outline of the groove of the optical article can be at a mean distance from the edges (E) of the hard coating ranging from about 0.1 to about 20 mm, preferably from about 1 to about 10 mm, and for example of about 5 mm . Preferably the edges (E) of the hard coating correspond to the edges of the optical article.

The outline of the groove can be at a constant distance from the edges (E) of the hard coating. In this embodiment, the outline of the groove has thus a shape similar to the one of the optical article. For example, its shape is a part or the whole of an ellipse or of a circle.

Depending on the number and the nature of the possible various coatings between the hard coating and the substrate, the cracks can propagate into the hard coating but also into the other coatings under the hard coating.

Thus, it is preferable that the engraving step i₀) produces a continuous or a discontinuous groove with an engraving depth (d) at least equal to the thickness (e) of the hard coating, which may itself include different coatings that are brittle and susceptible to form cracks under stress conditions.

Thus, the hard coating is defined as comprising at least one coating susceptible to form cracks under stress conditions and possibly be composed of multiple such coatings in vicinity with the others.

The engraving depth (d) can be at least equal to the thickness (e) of the hard coating, preferably greater than e, more preferably above 2e, such as between 2e and 3e, or may even reaching the substrate.

In one preferred embodiment, the engraving depth (d) is comprised between about 2 μηη and about 70 μηι, and preferably between about 3 and about 30 pm .

When the engraving depth (d) is greater than the thickness (e) of the hard coating, the groove is engraved through the hard coating, and also through some of the coating(s) which are between the hard coating and the substrate. As an example, the groove may go through the whole of the hard coating and through the whole of a layer immediately below the hard coating.

The engraving step i₀) can for example be performed by laser.

The power of the laser can be adjusted to obtain a certain engraving depth (d). As an example, the engraving depth (d) is approximately of 47 μιη when the laser power is set at 25%. The person skilled in the art of marking ophthalmic lenses using a conventional laser would know how to adjust the parameters of the laser to reach the adequate depth depending on the material of the hard coating.

Alternatively, the groove may be formed using other cutting means such as diamond point.

The heat treatment, for which step i₀) is a preparation, can be executed during a next step i). The heat treatment of step i) can be conducted at a temperature greater than 60°C, and can even be greater than about 80°C or even 90°C.

In one embodiment, the heat treatment of step i) is applied during a tinting step.

The tinting step i) can be a wet tinting step or a dry tinting step.

The wet tinting step generally comprises a sub-step of dipping the optical article in an aqueous bath comprising at least a dye, said aqueous bath being maintained at a temperature of about 94°C, a sub-step of removing the optical article from the bath after a time sufficient to tint said optical article, and two sub-step of imbibing and then drying the optical article, for example in an oven at about 100°C, in particular for about an hour. During this type of tinting step, both faces of the optical article are put into contact with the aqueous bath, and are submitted to the heating in the oven. Accordingly, any of the faces that comprises a hard coating which is subject to crack generating may form some cracks.

The susceptibility to form cracks concerns especially hard coatings deposited on a material with a different thermal dilatation coefficient. It often concerns hard coating separated from the substrate by a soft coating (primer coating in some cases, adhesive coatings, matrix for containing dyes, etc.). However, some hard coatings may have thermal dilatation coefficients very different than some substrates and may generate cracks during heat treatments. Thus, in one embodiment, the groove of the invention may be applied to any of the faces of the substrate, and may be applied on both faces of the substrate. This embodiment is not limited to lenses destined to be submitted to a wet tinting step. In this embodiment, the grooves on both sides may have a similar shape, and/or their outline may be superposed; however while this is esthetically pleasant, this is not necessary.

The dry tinting step can for example comprise a tinting process by sublimation, such as the one developed by the company Nidek in JP2004-121434. Such dry-tinting step generally comprises a sub-step of sublimating a dye from a carrier medium so as to deposit it by condensation on the lens at a temperature of about 100 to 250°C. Subsequently, an imbibing sub-step is generally applied to enable the dye to migrate from the surface of the lens into the substrate. This imbibing sub-step takes place in an oven and often reaches temperatures of about 50°C to 150°C, generally above 80°C or above 90°C.

The tinting step i) can provide a uniform tint or a gradient tint on the optical article.

It is to be noted that the heat treatment i), for which the step i₀) of the invention can be of use for preparing the lens, may be another step than a tinting step.

The hard coating according to the invention may be deposited onto the naked substrate or onto the outermost coating of the substrate if the substrate is already coated with at least one surface coating. Said at least one surface coating may be, without limitation, an impact- resistant coating, also called primer, an abrasion and/or scratch resistant coating which is different from said hard coating, a polarized coating, or a coating containing a dye, such as one or more photochromic dyes and/or one or more permanent dyes. In case the hard coating comprises multiple layers, some layers may be temporary coating(s) or layer(s).

The impact- resistant coating which may be used in the present invention can be any coating typically used for improving impact resistance of a finished optical article. This coating generally enhances adhesion of the abrasion and/or scratch-resistant coating on the substrate of the finished optical article. By definition, an impact-resistant primer coating is a coating which improves the impact resistance of the finished optical article as compared with the same optical article but without the impact-resistant primer coating.

Typical impact- resistant primer coatings are (meth)acrylic-based coatings and polyurethane-based coatings, in particular coatings made from a latex composition such as a poly(meth)acrylic latex, a polyurethane latex or a polyester latex.

Thus, the optical article used in step i₀) can further comprises an impact-resistant primer coating between the substrate and the hard coating, so as to obtain a substrate coated with the impact- resistant primer coating, said impact-resistant primer coating being further coated with the hard coating.

The coating containing a dye which may be used in the present invention, can be any coating typically used for providing a matrix adapted to contain and disperse adequately a dye. When at least a dye is a photochromic dye, the coating containing said photochromic dye is called hereafter "photochromic coating". A photochromic dye is defined as any dye enabling a reversible change in color, i .e. a change in the absorption spectrum in the visible range of light (380-780 nm) of a finished optical article under activation by light of a given wavelength, typically UV light.

Thus, the optical article used in step i₀) can further comprise a photochromic coating between the substrate and the hard coating, so as to obtain a substrate coated with the coating containing a photochromic dye (i .e. photochromic coating), said photochromic coating being further coated with the hard coating.

In an example, a polyurethane-based coating is used to serve as a matrix for containing the photochromic dye. In other words, the photochromic coating can be a polyurethane-based coating comprising a photochromic dye. The thickness of the photochromic coating may be comprised between about 1 pm and about 30 μηι.

The hard coating can be deposited on the preceding coating by any appropriate technique, for example by dipping, centrifuging, spraying, sprinkling or application with a brush or roller, preferably by dipping or centrifuging. It is subsequently cured by the appropriate route (preferably thermal or UV radiation).

The hard coating may in itself be a double or triple layer comprising two or three layers of different materials.

For example, the hard coating may comprise two layers, in particular an acrylic-based coating and a further coating such as a temporary coating or layer, said further coating being the outermost coating of the hard coating. The stack of both the acrylic-based coating and the further coating (e.g. a coating such as the antimony-containing-siloxane-based hard coating) may be understood as the hard coating of the invention.

Indeed, both coatings can be considered as susceptible to crack propagations; being in contact with each other, they are in the vicinity of each other, and without the method of the invention [i .e. step i₀)], cracks are propagated through both layers simultaneously during a heat treatment.

In this embodiment, the acrylic-based coating can be directly coated on a polyurethane-based coating containing a photochromic dye, used as a photochromic coating, and is thus protecting the photochromic coating.

The method can further comprises after step i), a step ii) of depositing at least one additional coating on said optical article and/or a step iii) of edging the optical article.

Indeed, the present method is flexible and allows incorporation of other functional coatings onto the substrate either before step i₀) or after (or during) step i).

For example, during step ii), an additional hard coating may be deposited, and eventually an antireflective stack may be deposited on the additional hard coating after step ii) by means known to the person skilled in the art. Further, a top coat, either an anti-smudge or an anti-fog coating, may be deposited on top of the a nti reflective stack.

In some cases, if the hard-coating comprises multiple layers, especially if said multiple layers include a temporary layer or coating as the outermost layer, the method of the invention can further comprise between steps i) and step ii), a step of stripping part of the hard coating, such as stripping the temporary layer.

It is noted that the groove engraved in step i₀) does not introduce additional defects during the following steps such as step ii) and/or step iii).

A second object of the present invention is an optical article obtained by the method as defined in the present invention, said optical article having an optical center (C) and comprising a substrate coated with at least one hard coating, said hard coating having a thickness (e) and edges (E), wherein said optical article further comprises a continuous or discontinuous groove engraved through at least said hard coating, the outline of said groove forming an open or closed shape which surrounds the optical center (C).

The optical article, the groove, and the outline and the shape of the outline are as defined in the first object of the present invention.

Thanks to the groove engraved through at least said hard coating, said groove having an outline which forms a shape which surrounds the optical center (C), the optical article can be submitted to any heat treatment such as a tinting step without being damaged by the appearance and/or the propagation of cracks. In particular, cracks initiating at the edges (E) of the hard coating during the heat treatment see their progression toward the optical center (C) stopped by the groove. Accordingly the optical article after the heat treatment, even when cracks are formed, keeps a surface around the optical center (C) which is crack-free and which is delimited by the outline of the groove.

A third object of the present invention is the use of a continuous or discontinuous groove in an optical article having an optical center (C) and comprising a substrate and at least one hard coating, said substrate being coated with said hard coating, said hard coating having a thickness (e) and edges (E), the groove being engraved through at least said hard coating, the outline of said groove forming an open or closed shape which surrounds the optical center (C), to limit the propagation of cracks in said hard coating when the optical article is exposed to a heat treatment.

The optical article, the groove, the outline of the groove and the shape of the outline are as defined in the first object of the present invention.

EXAMPLE 1

Photochromic lenses of the 6^th generation sold under the brand name of Transitions ® (Transitions VI) by Transitions Optical, Inc. were used as starting optical articles to perform the method of the present invention.

Each photochromic lens comprised a substrate having a refractive index ranging either of 1.60 (MR8) or 1.67 (MR7), a polyurethane-based coating (PU coating acting as a matrix for containing a photochromic dye; ie. acting as a photochromic coating) having a thickness of about 20 μιη and coated on the substrate, an acrylic-based coating, called first hard layer, having a thickness of about 8.5 μιτι coated on the polyurethane-based coating, and an antimony-containing-siloxane-based hard coating, called second hard layer, having a thickness of about 2 μηη coated on the acrylic-based coating.

Each photochromic lens was submitted to an engraving step i₀) according to the method of the present invention.

Step i₀) was performed by using an ILS-II laser cutting and engraving machine commercialized by Laser Tools & Technics Corp.

By controlling the power of the laser, a groove, the outline of which forming a shape surrounding the optical center (C) of the lens, was engraved through each layer of the hard coating with various engraving depths on each photochromic lens, at a distance of about 5 mm from the edges of each lens. Each photochromic lens was subsequently subjected to a heat treatment at a temperature of about 145°C for about 2 hours (cf. step i) of the method of the present invention).

Figure 1 represents the engraving depth (in μηι) as a function of the laser power (in %). The man skilled in the art, knowing that the ILS-II engraving machine was used, can easily reproduce this test, even using other engraving machines. In figure 1, it is also shown the occurrence or the absence of crack propagation.

It was observed that some crack propagation could persist on lenses which were engraved with laser power of below 10%, which corresponds to an engraving depth of less than 20 μηι. It is to be noted that at laser power of 5% and below, no engraving at all could be obtained. At laser power of 15% or more, corresponding to a minimum engraving depth of 25 μηι, crack propagation could be successful prevented. The depth of 25 μηη corresponds to about 238% of the cumulated thickness (e) of the second hard layer and the first hard layer, both of which being harder than the PU coating used as a matrix for containing the photochromic dye. In that case, the second hard layer and the first hard layer can be considered as forming the hard coating layer of the invention. Thus, in that example, the adequate engraving depth is of about 2.4 times "e", written "2.4e", or greater.

Figure 2a represents a microscopic analysis of a cross section of the photochromic lens obtained after step i) when the laser power was set to 15% and figure 2b is a schematic representation of the various layers (or coatings) within the photochromic lens with their accompanying thicknesses.

As it can be seen in figure 2, the cracks were mostly limited within the two external layers of the photochromic lens (~10 μηι). It is thus believed that the polyurethane-based coating is more ductile and as such is not comprised into the hard coating and hence, limits crack propagation on its own.

However, in order to prevent the propagation of cracks through some part of the polyurethane-based coating at other locations, it is preferred, as stated above, that an engraving depth greater than the simple thickness (e) of the hard coating layer(s) is applied.

Figure 3 is a schematic representation of the method of the present invention, in particular to obtain a gradient tint on a starting optical article such as a photochromic lens. The method of the present invention comprises a first step i₀) of engraving a groove through at least the hard coating (i .e. comprising both the first hard layer and second hard layer) of the photochromic lens described above, thus forming a shape surrounding the center (C) of the optical article. As it can be seen in figure 3, the groove has a circular shape, and it is at a constant distance from the edges (E) of the hard coating. Then, the obtained photochromic lens is submitted to a conventional tinting step i) and provides a gradient tint on the optical article, followed by a step ii) of depositing an anti reflective stack and an anti-smudge layer, and an edging step iii).

As seen in figure 3, the groove is positioned in an area of the optical article which is a non-disturbing area for vision of a wearer. In particular, the lens having a diameter of about 160 mm to 180 mm, any part of the groove is farther than 30 mm from the optical center (C), which is farther than 2.5 mm,

2 mm and 1.8 mm from the optical center (C). Further, if the optical article is edged into a lens which has to fit into a frame, the groove illustrated in figure

3 would be positioned in part of the lens which has to be removed during edging.

Figure 4 is a schematic representation of a lens obtained after an engraving step i₀) and a step i) of heat treatment according to the method of the present invention. Figure 4a represents the lens having crack initiation sites. After step i₀), a groove is engraved through at least the hard coating of said lens and then, said lens is submitted to a heat treatment i). Thanks to the use of said groove, the propagation of the cracks is completely stopped. In figure 4a, the groove is continuous and the outline of the groove forms a closed circular shape which surrounds the optical center (C) of the optical article. In figure 4a, a first domain (I) contains the optical center (C) of the optical article and points positioned between the groove and the optical center (C) and a second domain (II) contains points separated from the optical center (C) by the groove. Figure 4b is a cross section of figure 4a. Indeed, the groove serves as a crack propagation barrier, in particular by creating discontinuity in the starting lens used in step i₀).

Figure 5 is a schematic representation of a lens having crack initiation sites and obtained after an engraving step i₀) and a step i) of heat treatment according to the method of the present invention. In figure 5a, the groove is continuous and the outline of the groove forms a closed free-form shape which surrounds the optical center (C) of the optical article. In figure 5b, the groove is discontinuous and is made of dots and dashes, and the outline of the groove forms a closed star shape which surrounds the optical center (C) of the optical article. In figure 5c, the groove is discontinuous and is made of two ranks of dashes, and the outline of the groove forms a closed rectangular shape which surrounds the optical center (C) of the optical article. In figure 5d, the groove is continuous and the outline of the groove forms an open ellipse shape that surrounds the optical center (C) of the optical article. The apparent covered angle is of about 4/3 Pi (ττ) and ranges from 4/3 Pi to 2Pi. In figure 5d, a first domain (I) contains the optical center (C) and points positioned between the groove and the optical center (C); a second domain (II) contains points separated from the optical center (C) by the groove, a third domain (III) appears for some points which are not positioned between the optical center (C) and the groove and are not separated from the optical center (C) by the groove.

EXAMPLE 2 A MR7 photochromic lens of the 6^th generation sold under the brand name of Transitions ® (Transitions VI) by Transitions Optical, Inc. was used as a starting optical article in the method of the present invention. It was first engraved with the ILS-II laser cutting and engraving machine described in example 1 using an engraving depth of about 46.5 μηι . The groove was positioned at about 5 to about 10 mm away from the edges of the photochromic lens. The groove can be however positioned even closer to the edges and its engraving depth can be controlled by adjusting the power of the laser until having about 25 μηι.

Then, the photochromic lens was subsequently tinted via a dry tinting step i), so as to obtain a photochromic lens U. Thus, U was prepared according to the method of the invention.

To provide a comparative example, another MR7 lens comprising a Transitions ® VI photochromic stack was submitted to the same dry tinting step i) described above, but without carrying out an engraving step i₀)before said step i), so as to obtain a photochromic lens L₂. Thus, L₂ was not prepared according to the method of the invention.

Figure 6 represents images of U (figure 6b) and L₂ (figure 6a). As shown in Figure 6b, the groove clearly stops the propagation of the cracks in Li, whereas as shown in figure 6a, L₂ which was not prepared according to the method of the present invention displays cracks.

EXAMPLE 3

A MR8 photochromic lens of the 6^th generation sold under the brand name of Transitions ® (Transition VI) by Transitions Optical, Inc. was used as a starting optical article in the method of the present invention .

As shown in figure 7, extensive cracks were observed on the convex side of the photochromic lens when it was subjected to a wet tinting step i) at about 94°C without carrying out an engraving step i₀) before said step i). Similar observations were made when a lens with the same configuration was tinted using a dry tinting step i) with a temperature ranging between about 125°C to about 145°C, without carrying out an engraving step i₀)before said step i).

Claims

1. A method for preparing an optical article for a heat treatment, the optical article having an optical center (C) and comprising a substrate and at least one hard coating, said substrate being coated with said hard coating, said hard coating having a thickness (e) and edges (E), wherein said method comprises a step i₀) for limiting the propagation of cracks in said optical article when exposed to the heat treatment, said step i₀) including the on-purpose engraving of a continuous or discontinuous groove through at least said hard coating, the outline of the groove forming an open or closed shape which surrounds the optical center (C).

2. The method as claimed in claim 1, the groove having an apparent covered angle, as seen from the optical center (C), ranging from 4/3 of Pi to 2 Pi.

3. The method as claimed in anyone of claims 1 or 2, the groove being a discontinuous curved line made of a succession of dots or dashes or portions of curves or lines or a mix thereof, separated by spaces.

4. The method as claimed in anyone of claims 1 to 3, the groove being positioned in an area of the optical article which is a non-disturbing area for vision of a wearer, preferably, the optical article being a lens which has to be edged to fit into a frame, and the groove being positioned in part of the lens which has to be removed.

5. The method as claimed in anyone of claims 1 to 4, comprising a next step i) of heat treatment for which step i₀) is a preparation.

6. The method as claimed in claim 5, the heat treatment of step i) being conducted at a temperature greater than 60°C, and being preferably applied during a tinting step.

7. The method as claimed in claim 1 to 6, step i₀) producing a continuous or discontinuous groove with an engraving depth (d) at least equal to the thickness (e) of the hard coating.

8. The method as claimed in anyone of claims 1 to 7, the outline of the groove being at a mean distance from the edges (E) of the hard coating ranging from 0.1 to 20 mm.

9. The method as claimed in anyone of claims 1 to 8, the outline of the groove being at a constant distance from the edges (E) of the hard coating.

10. The method as claimed in anyone of claims 1 to 9, the hard coating comprising an acrylic-based coating and a further coating, said further coating being the outermost coating of the hard coating.

11. The method as claimed in anyone of claims 1 to 10, the optical article used in step i₀) further comprising a photochromic coating between the substrate and the hard coating, so as to obtain a substrate coated with the coating containing a photochromic dye, said photochromic coating being further coated with the hard coating.

12. The method as claimed in claim 11, the photochromic coating being a polyurethane-based coating comprising a photochromic dye.

13. An optical article obtained by the method as defined in anyone of claims 1 to 12, said optical article having an optical center (C) and comprising a substrate coated with at least one hard coating, said hard coating having a thickness (e) and edges (E), wherein said optical article further comprises a continuous or discontinuous groove engraved through at least said hard coating, the outline of said groove forming an open or closed shape which surrounds the optical center (C).

14. The optical article of claimed in claim 13, the continuous or discontinuous groove having an apparent covered angle, as seen from the optical center (C), which ranges from 4/3 of Pi to 2 Pi.

15. Use of a continuous or discontinuous groove in an optical article having an optical center (C) and comprising a substrate and at least one hard coating, said substrate being coated with said hard coating, said hard coating having a thickness (e) and edges (E), the groove being engraved through at least said hard coating, the outline of said groove forming an open or closed shape which surrounds the optical center (C), to limit the propagation of cracks in said hard coating when the optical article is exposed to a heat treatment.