GB1573176A

GB1573176A - Laminated mirror and a method of forming such mirror

Info

Publication number: GB1573176A
Application number: GB11775/77A
Authority: GB
Original assignee: MIRODAN ETS
Current assignee: MIRODAN ETS
Priority date: 1976-03-25
Filing date: 1977-03-21
Publication date: 1980-08-20
Also published as: FR2345737A1; FR2345737B1; DE2713351B2; SE7703036L; SE434211B; BE852501A; DK153987C; NL183934C; DK117277A; NL183934B; DE2713351C3; LU74641A1; IT1073478B; DE2713351A1; NO770961L; NL7703238A; DK153987B

Description

(54) A LAMINATED MIRROR AND A METHOD OF FORMING SUCH MIRROR (71) We, MIRODAN, ETS LOUIS D'HAENE & ZONEN naamloze vennootschap organised under the laws of Belgium, of Schutterstraat 3A, Kortrijk, Belgium, do hereby declare the invention. for which we pray that a patent may be granted to us, and the method by which is its to be performed, to be particularly described in and by the following statement: This invention relates to mirrors comprising a transparent front sheet the rear end of which bears a light reflecting coating, and a protective backing sheet which is secured to said coated front sheet.

Mirrors of this basic construction are known in which the backing sheet is glued to the coated side of the front sheet. Powerful adhesives of various compositions have been used in order to achieve as strong a bond as possible. The attainment of this bond sometimes necessitates subjection of the assembly to heating to high levels. The optical coating(s) must not be such as to be adversely affected by this treatment.

It is not easy to automate the bonding procedures. The handling properties of the most favoured adhesives do not lend them selves to such automation. In order to improve sealing, it has been proposed to apply a protective beading to the edge faces of the assembly to cover the glue joint between the front sheet and the backing sheets.

The sealing properties of the glue layer are in that case less critical. But of course the necessity to apply the beading makes the production process that much longer and costly.

It is an object of the present invention to achieve effective assembly of the mirror sheets by means of an adhesive which can be used conveniently, and without high cost, under industrial mass-production conditions.

According to the present invention there is provided a mirror comprising a transparent front sheet the rear face of which bears a light-reflecting coating, and a protective backing sheet which is secured to said coated front sheet, characterised in that the front and backing sheets are secured together at least at the margin of the assembly with or without the presence of intermediary sheet material, by means of a hotmelt adhesive.

Hot-metal adhesives, i.e. adhesives which acquire their bonding function by cooling in situ between the surfaces to be bonded together, are well known per se. They are in a category quite distinct from thermosetting adhesives and adhesives applied in a solvent medium and they have not hitherto been employed in the bonding of structural components of the kinds involved in the manufacture of mirrors. This is undoubtedly due to the fact that high-strength bonding has been regarded as a prerequisite, for which purpose solvent and thermosetting-type adhesives have a traditional longestablished place in industry.

The employment of hot-melt type adhesives affords a number of advantages.

Among these are the facility with which they can be handled and applied to form bonding layers of predetermined thickness and uniformity. Reproducible results can be achieved under rapid assembly conditions.

It is an easy matter to select the hot-melt adhesive formulation which will give the required combination of properties to the inter-sheet joints. Moreover the adhesive compositions can be selected so that these properties combine a very adequate bond strength with a high degree of impermeability by moisture. The use of a hot-melt type adhesive also contributes to lowering of production costs. This is due to the relatively low cost of the adhesive itself and the ease with which bonding can be achieved with very modest equipment and in a small working area.

The hot-melt adhesive is preferably one which is molten at a temperature of 150"C or lower, preferably between 60 and 1 20 C.

Hot-melt adhesive formulations include an elastomeric or thermoplastic material which melts easily to a low viscosity fluid. In order to achieve solidified bonding layers of adequate stength and cohesiveness such easily meltable ingredient is blended with a higher molecular weight polymeric material.

A very favourable balance of properties can be achieved by formulating the hot-melt adhesive to incorporate a combination of resins of different melt indices.

Examples of relatively easily meltable substances (hereafter referred to as "tack- ifier") which can be used in hot melt adhesive formulations to provide good surface tack are various natural and synthetic resins and waxes, e.g. terpene resins, hydrocarbon resins, polyterpenes, phenol-formaldehyde resins, alkyds, coumarone-indene resins, rosin and rosin derivatives and mineral, vegetable and petroleum waxes. Two or more tackifiers can be incorporated if desired in one and the same adhesive formulation.

In preferred embodiments of the present invention the hot-melt adhesive comprises one or more tackifiers selected from terpene and phenolic resins and nicrocrystalline waxes. Very good results are also attainable with styrenes and low-molecular homogues.

The proportion of tackifier(s) in the composition influences the melt viscosity of the adhesive.

Examples of higher molecular weight synthetic polymeric materials suitable as reinforcing or toughening ingredient of the hot melt adhesive composition, forming what is sometimes referred to as the adhesive backbone, are polyvinyl acetate, polyethylene, polyisobutylene (butyl rubber), polystyrene and styrene copolymers, ethyl cellulose, polyamides derived from dimerized fatty acids and diamines, and butyl methacrylates.

In preferred embodiments of the present invention the hot-melt adhesive comprises one or more elastomers or thermoplastics selected from butyl rubber and ethylene/vinyl acetate copolymers.

The hot melt adhesive may incorporate various other types of ingredients for conferring required properties. Examples of categories in which such supplementary ingredients fall are plasticizers, thermal stabilizers and fillers.

As plasticizers. use can be made of various resins which serve to improve adhesive wetting of the surfaces to be bonded, and the flexibility of the adhesive layers. Examples of plasticizers are phthalates. phosphates, chlorinated polyphenyls. rosin derivatives and polesters.

Stabilizers are used in hot-melt adhesives to improve the thermal stability of the composition. The stabilizers most usually employed at anti-oxidizing agents, e.g., steric phenols and phosphite derivatives.

Fillers are used for modifying the physical properties of the adhesive compositions.

They are substantially chemically inert under their conditions of use. Suitable fillers are e.g. zinc oxide, calcium carbonate.

titanium dioxide, barium sulphate and carbon black. Generally, it is preferable to employ inorganic fillers. These may have the effect of reducing the weight of the adhesive or colouring it or of reducing the viscosity or mechanically strengthening the product. However, organic fillers such as resins may also be used.

Of course, the various components used in the formulation of the hot melt adhesive must be mutually compatible.

The inter-sheet bond in a mirror according to the invention is preferably such that the mirror is highly resistant to the ingress of moisture between the front and backing sheets. The penetration of water, e.g.

atmospheric moisture, between the front and backing sheets may have harmful effects on the structure and/or the physical or chemical properties of the mirror sheets and particularly of the optical coating(s).

Moisture-tightness is therefore often of much importance. This is so for example in respect of mirrors to be used in shower rooms or bathrooms or which are to be exposed to the weather, e.g. mirrors used at road intersections and the external driving mirrors of vehicles.

In preferred embodiments, the waterresistance of the hot-melt adhesive is less than 0.5 and most preferably less than 0.1 g H20 per m2 of surface per 24 hrs per mm thickness per cm Hg of pressure.

Advantageously the hot-melt adhesive incorporates one or more saturated hydrocarbons. The presence of such hydrocarbon(s) increases the resistance of the adhesive to penetration by water. Examples of suitable saturated hydrocarbons are paraffin, chlorinated paraffins, polybutenes and polyisobutylenes.

It will be apparent that a particular given ingredient of the hot-melt adhesvie can fulful different important functions in the product. For example a saturated hydrocarbon can be selected to serve as an ingredient of the adhesive backbone. to contribute to surface tack, and to have plasticizing and stabilising functions. As another example. a tackifier may be selected which also confers excellent water-resisting properties on the adhesive. Waxes, e.g. fatty-acid ester/alcohol mixtures, are very effective tackifiers and also have good water-resisting properties. In addition they can be used to influence the viscosity of the hot-melt so as to obtain a low melting point.

Preferably the thickness of the or each layer of adhesive in the mirror is less than 1 50 microns. This condition is recommended because it exploits an important property of hot-melt adhesives, namely their ability to give very effective bonds. even as very thin layers. and because such thin layers leave a very small surface area of adhesive exposed to the environmental atmosphere.

The hot-melt adhesive layer or layers may incorporate reinforcement, e.g., glass or plastics fibres or webs of such fibres, or metal wires or meshes.

The hot-melt adhesive is preferably present over the whole area of the mirror. The constituent sheets of the mirror. i.e., the front sheet. the backing sheet and (possibly) an intermediate sheet as hereafter referred to, are thus united as plies of a laminate.

This type of construction is particularly easy to manufacture. The invention however includes mirrors wherein the hot-melt adhesive is confined to a margin of the assembly.

The invention includes mirrors wherein the backing sheet is bonded to the coated front sheet indirectly. through the intermediary of further sheet material. Preferably the intermediary sheet material is a synthetic polymeric material e.g. a polyester. Such an intermediary sheet material can be used as a support for the layers of hotmelt adhesive prior to introduction of such sheet material and thus facilitate the mirror production process as will hereafter be more particularly described.

The intermediary sheet material, if used, must be selected with due regard to other properties which it must have if the mirror is to meet the required specifications. For example. if the mirror must have a very high resistance to ingress of moisture between the front and backing sheets, the composition of the intermediary sheet must itself have the required water-impermeability.

By way of example the intermediary sheet material may be chosen from among hydrophobic polymers. These usually have non-hydrophilic chains having high lateral symmetry. Suitable materials for the intermediary sheet (the selection being of course dependent on the required properties).

includes vinylidene chloride/isobutylene copolymers, vinylidene chloride/vinyl chloride copolymers. vinylidene chloride/vinyl chloride/acrylonitrile terpolymers, vinylidene chloride/acrylonitrile/vinyl actate terpolymers, polytetrafluoroethylene, polychlorotrifluoroethylene.

vinylidene fluoride/chlorotrifluoroethylene copolymers, propyelene/fluorinated ethylene copolymers. polyethylenes and copolymers thereof, and polypropylenes.

The intermediary sheet material, if used, preferably has a thickness between 10 and 150 microns. This range provides a satisfactory compromise between a minimum exposure of sheet material to the environmental atmosphere and a high degree of safety in the event of breakage of a glass sheet.

The intermediary sheet material preferably extends over the whole area of the mirror. If it is laminated to the front and backing sheets, then in the event of breakage of a glass sheet (if such is present), the intermediary sheet is capable of retaining pieces of the broken glass more effectively than the adhesive would do if it were used without such intermediary sheet.

However the invention also includes mirrors wherein intermediary sheet material is present only between the margins of the backing sheet and the coated front sheet, said sheets being united with such intermediate sheet material by means of hot-melt adhesive layers.

The front sheet of the mirror is preferably a glass sheet. In an alternative embodiment a transparent plastics sheet is employed.

The front sheet may of course be one or more coatings additional to the lightreflecting coating. The front sheet may be coated in accordance with any of the conventional practices in mirror production.

For example the front sheet may bear a silver layer, deposited by one of the wellknown methods, and one or more protective coatings on such silver layer. For example the silver layer may be successively covered by a layer of copper and a layer of protective paint, which layers may likewise be formed in well-known manner. A metal other than silver can be used, e.g. aluminium or copper.

The light-reflecting layer may reflect all incident visible light or it may be semitransparent, (if the backing sheet is transparent), giving a so-called semi-mirror.

The backing sheet is preferably a selfsupporting sheet. The backing sheet is preferably composed of vitreous material. This sheet may be a sheet of glass which may be thermally or chemically tempered glass as known per se. As an alternative the backing sheet may be composed of vitro crystalline or vitroceramic material.

The backing sheet may itself bear one or more coatings. In certain mirrors according to the invention, the backing sheet bears on its internal face one or more electrically conductive coatings adapted to be connected to a voltage source so that the mirror can be heated by Joule effect. Such coating(s) may be composed of a metal or metal compound or of particles of a metal or metal compound incorporated in a binder. For example the conductive layer(s) may be based on a vitreous enamel and contain a relatively large amount of metal. In other embodiments of the invention, electrically conductive elements e.g. wires or strips are used instead of an electrically conductive coating or coatings. The electrically conductive coating(s) or element(s) may be connected to distribution electrodes secured to terminals for connection to an electric current source. A high resistance to penetration by water is very important for avoiding impairment of the electrical heating system of such mirrors. Mirrors with a heating system as above described can be of de-misting or heating type, these different types requiring to be heated to different levels.

It is within the scope of the invention for the components herein referred to as the front sheet and the backing sheet, or either of them, to be itself composed of one or more plies. For example the front sheet may comprise a sheet of glass which bears the aforesaid coating(s) on its rear face and has a protective film on its front face. The backing sheet may e.g. comprise a sheet of vitreous material uniteil to another ply e.g. a sheet of plastics.

A mirror according to the invention can be of any shape, and may be flat or curved.

The invention includes a method of making a mirror by providing a transparent sheet having a light-reflecting coating on a face thereof, and a protective backing sheet, and sealing such backing sheet to said coated sheet, characterised in that said coated transparent sheet and said backing sheet are seamed together directly or via intervening sheet material, by means of one or more layers of a hot-melt adhesive applied between said sheets at least at the margin of the assembly.

The advantages of this production method are apparent form what has been said in the foregoing description of mirrors according to the invention. In particular the employment of a hot-melt type adhesive affords advantages due to the facility with which it can be handled and used and the fact that bonding can be achieved at low cost and without the necessity to allow prolonged drying times or to provide special heating plant for evaporation of solvents.

In carrying out this method, it is preferred to employ a hot-melt adhesive having one or more of the preferred features pertaining to such adhesives as hereinbefore described.

According to a particularly preferred feature. the hot-melt adhesive composition is such that the minimum temperature at which it is effectively molten is below 1 500C and preferably between 60 and 120"C and the sealing is effected by heating the assembly of sheets and one or more intervening layers of said adhesive to a temperature below 1500C to cause melting of the adhesive, whereafter the assembly is cooled.

Pressure may be applied before and/or during heating. Preferably the pressure does not exceed 1 kg/cm2.

It is particularly recommended to employ a hot-melt adhesive incorporating one or more saturated hydrocarbons and/or at least one wax.

Preferably the composition of the hotmelt adhesive in its cooled condition is such that its water-permeability is less than 0.5 and most preferably less than 0.1 g H20 per m2 of surface per 24 hours per mm thickness and per cm Hg pressure.

The hot-melt adhesive in its final condition preferably forms one or more layers which or each of which layers has a thickness of less than 150 microns. Such thin layers are adequate for the purpose of bonding the constituent sheets together and leave a very small surface area of adhesive exposed to the environmental atmosphere.

In certain embodiments of the invention, the said adhesive layer or layers is or are transferred as such from a temporary support to one or more of the sheets to be sealed together, before these sheets are assembled. This method exploits one of the very important potential advantages of hot-melt adhesives, namely their ability to be formed into layers. The formation of such layers on temporary supports from which the layers can be transferred to the work preparatory to the assembly operation and heating, is a very advantageous procedure which greatly simllifies mass production manufacture and can be applied with accurately reproducible results. The layers of adhesive for transfer can be prepared under accurate control with regard to layer thickness and area, completely independently of the mirror assembly line. For example, material comprising a temporary support bearing a coating of the hot-melt adhesive composition can be manufactured in sheet form or as a coil able web from which pieces of the required sizes for incorporation in the mirrors can be cut. A particularly suitable temporary support material is silicone paper.

Advantageously the hot-melt adhesive is applied so that the adhesive layer or layers cover(s) the whole or substantially the whole area of the front and backing sheets.

The invention also includes methods of manufacture wherein the backing sheet and the coated front sheet are secured together indirectly. via intermediary sheet material which extends over the whole or substantially the whole area of the front and backing sheets or which is confined to a marginal zone of the mirror assembly. The method can be performed without adhesive transfer operations. The hot-melt adhesive layers can be applied ab initio to the intermediary sheet material. Moreover this intermediary sheet Preferably a said intermediary sheet material is used which does not significantly reduce the resistance of the mirror to ingress of moisture between the front and backing sheet. Such intermediary sheet material is preferably a synthetic polymeric material, e.g. a polyester. This type of intermediary sheet material is advantageous from the safety point of view in mirrors incorporating one or more sheets of glass.

The thickness of the intermediary sheet material is preferably between 10 and 150 microns.

After assembling the sheets and the hotmelt adhesive layer(s) the assembly may be subjected to heat and pressure (e.g. a pressure below 1 kg/cm2) either simultaneously or successively, and if the latter, then in either order. In one procedure, the pressure is exerted at ambient temperature and the heating is carried out subsequently at atmospheric pressure. This method is particularly simple. In an alternative very satisfactory method, the assembly is heated to a temperature appropriate to the hot-melt adhesive and the pressure is applied at the end of this heating stage. The assembly has usually to be heated, in all procedures to a temperature of at least from 60 to 1 200C.

The method according to the invention is very advantageous for use when the front and backing sheets are sheets of glass. However it is not essential for these sheets to be of glass. Alternative materials have been mentioned hereinbefore in connection with the manufactured mirrors.

Certain embodiments of the invention, selected only by way of example, will now be described with reference to the accompanying diagrammatic drawings, in which: Figs. 1 to 3 are cross-sectional views of three different mirrors according to the invention; and Fig. 4 is a plan view of a variant of one of the sub-assemblies employed in the mirror shown in Fig. 3.

In these figures the thickness of various coatings and layers have been greatly exaggerated for clarity.

Fig. 1 shows a semi-mirror of a kind used as "spy" mirrors through which a watch can be kept for thieves in business and other premises.

The semi-mirror comprises a first sheet of soda-lime glass 1 covered with a transparent layer 2 of silver. The formation of a layer of this kind is well known to persons skilled in the art and does not require further description.

The glass sheet 1 and its coating 2 forms a first sub-assembly 3 of the mirror. A second sub-assembly 4 comprises a backing sheet of ordinary transparent soda-lime glass 5. The glass sheet 5 protects layer 2 against mechanical damage e.g. by scratching. The two sub-assemblies are secured at their margins via a ban 6 consisting of a film of "Tedlar PVF", 100 microns thick, marketed by Dupont de Nemours International S.A.

Each surface of the band is covered by a layer 7 of "hot melt" adhesive. The "Tedlar" film has considerable resistance to penetration of water and gas. Each layer of "hot melt" is 100 microns thick. The "hot melt" used has the following formulation in part by weight: A butyl-rubber type elastomer 40 A plasticizer in the form of poly butene 15 A tackifier such as a polyterpene 20 A filler such as carbon black 2 A phenol-type stabilizer 1 A saturated hydrocarbon, e.g. a paraffin, for increasing resistance to penetration by water 22 The adhesive has a water-permeability which is less than 0.5 grams H20 per m2 of surface per 24 hours, per mm thickness and per cm Hg pressure.

The mirror is manufactured in the following manner.

The glass sheet 1 with its silver coating 2 is placed horizontally on a support. Bands of "Tedlar" 6 coated on both sides with "hot-melt" layers 7 are disposed along the four margins of the sheet, on the silver coating. The glass sheet 5 is then placed in position. The margins of the assembly are then subjected to slight pressure, e.g. by calen-dering. Next, the structure is placed in an oven and brought to a temperatore of 1 000C for 15 minutes, to bring the "hot-melt" to molten state. The assembly is then cooled.

The mirror is then ready for use. The "Tedlar" band 6 and the adhesive layers 7 provide effective protection agains the penetration of moisture and other atmospheric agents between the two sub-assemblies 3 and 4, thus preventing premature destruction of the silver layer 2 by chemical corrosion.

If required, the margin of sheet 5 can, before being placed in position, be treated with a coupling agent such as silane, compatible with the adhesive, in order to increase the strength of the adhesive joint and its resistance to moisture.

In an alternative embodiment, interference layers, e.g. based on oxides such as TiO2 and Six2. are used instead of a silver layer to form the semi-transparent mirror.

The sub-assembly 4, instead of being an ordinary glass sheet 5, can be a laminate comprising two sheets of glass joined to an intervening sheet of polyvinylbutyral.

In another embodiment, the sub-assembly 4 is formed by a sheet of plastics such as a polycarbonate.

In the embodiment in Fig. 2, the mirror comprises two sub-assemblies 10 and 11, the first being a silver-coated sheet of glass and the second being an uncoated sheet 12 of ordinary glass. The first sub-assembly comprises a glass sheet 13 which is coated in succession by well-known methods, with a layer 14 of metallic silver, a copper layer 15 and a layer 16 of protective paint. The two sub-assemblies 10 and 11 are joined by a hot-melt adhesive layer 17 resistant to penetrate by water and extending over the entire area of the mirror.

The formation of the hot-melt adhesvie in parts by weight is as follows: EVA 607 (ethylene/vinyl acetate copolymer manufactured by Union Carbide Corp.) 40 DYLT (ethylene homopolymer manufactured by Union Carbide Corp.) 5 CKM-2400 (tackifier based on phenolic resin. manufactured by Union Carbide Corp.) 15 KLYRVEL 90 (plasiticizer and tackifier containing hydrocarbons, manufactured by Velsicol Chemical Corp.) 7.5 PICCOLYTE (Trade Mark) A-1 15 (polyterpene-based tackifier manufactured by Pennsylvania Industrial Chemical Corp.) 12.5 BE SQUARE 190-195 (microcrystalline wax manufactured by Bareco Div.

of Petrolite Corp.) 20 ANTIOXYDANT 330 (supplied by Ethyl Corp.) 0.1 The water permeability of the adhesive when set is less than 0.5 grams H20 per m2 surface per 24 hours per mm thickness and per cm Hg pressure.

The mirror is manufactured in the following manner. The sub-assembly 10, i.e. the mirror component, is disposed on a horizontal surface. A piece of silicone paper covered on one surface with a layer of the hot-melt adhesive 20 ,u thick is cut from stock. the dimensions of the cut piece being slightly greater than the dimensions of the mirror. This piece of material is then placed on the paint layer 16 of the mirror component with the hot-melt adhesive layer in contact with that paint layer. Slight pressure is then applied to the silicone paper over the entire area of the mirror component so as to cause the adhesive layer to adhere to the mirror. Next. the margins of the adhesively covered silicone paper sheet projecting from the mirror edges are cut off with a knife or razor. The silicone paper is then peeled away leaving the adhesive layer 17 in place.

The glass sheet 12 is then placed in position.

The resulting assembly is calendered while cold to ensure uniform distribution of the adhesive and elimination of any air bubbles. After this operation, the assembly is brought to 110"C in an oven, at atmospheric pressure. for 20 minutes. The assembly is then cooled and is ready for immediate use.

The resulting mirror was tested in a salt fog. In this test. the mirror was placed in a chamber at a temperature of 40"C in a saturated saline atmosphere. Examination of the mirror after quite a large number of hours, revealed no deterioration in the silver layer of the mirror. whereas a conventional mirror was rendered useless by treatment in an identical manner.

Similar results were obtained when using a hot melt layer 40 ,u thick deposited on a silicone paper support marketed by Euro-M. the "hot-melt" containing inter alia butyl and wax. The water-permeability of such adhesive, expressed in the same units as before, was less than 0.1.

It is sometimes advantageous to pre-heat the mirror, e.g. to about 60"C.. before applying the hot melt adhesive thereto.

As a variant of the manufacturing method just described, a vitroceramics sheet can be used instead of the glass sheet 12.

In an alternative manufacturing method.

after placing the second glass sheet 12 in position the resulting assembly is heated to 1200C for 20 minutes and the hot assembly is then subjected to pressure, e.g. by calendering as above referred to. The assembly is then cooled and is ready for immediate use.

Fig. 3 shows a de-misting mirror. This type of mirror is being used to an increasing extent in shower-rooms.

The de-misting mirror is of laminated construction incorporating two subassemblies 20 and 21. Sub-assembly 20 is a mirror comprising a glass sheet 22 bearing a silver layer 23 and a copper layer 24. Subassembly 22 comprises a glass sheet 25.

which has been thermally or chemically tempered. bearing an electrically conductive layer 26 of SnO2 deposited by one of the methods well-known in this art. Su crystalline wax manufactured by Bareco Div. of Petrolite Corp.) 10 POLYWAX 2000 (wax manufactured by Bareco Div. Petrolite Corp.) 10 ANTIOXYDANT 330 (supplies by Ethyl Corp.) 0.1 The water-permeability, expressed in the same units as before. was less than 0.5.

In the production of the mirror, the polyester film 27 bearing the adhesive layers 28 and 29 was disposed between the subassemblies 20 and 21. The assembly was calendered when cold and then brought to a temperature of 120"C in an oven at atmospheric pressure.

The resulting mirror showed good resistance to the saline fog in a test as hereinbefore described. Neither the silver layer 23 nor the electrically conductive layer 26 showed signs of damage.

In addition, the film 27 affords a measure of safety in the event of breakage of the glass.

In a modification, the polyester film 27 is replaced by polyvinylidene chloride or polytetrafluorethylene or polyethylene of polypropylene film, pretreated if required so as to receive the adhesive layers.

As an alternative production procedure.

the hot-melt adhesive layers can be initially deposited on the sub-assemblies 20, 21. e.g.

by transfer from a non-adhesive temporary support as described earlier. instead of using a film pre-coated with adhesive. The two methods may be used together if desired.

In a further variant, illustrated by Fig. 4.

the sub-assembly 21, instead of comprising a sheet of glass 25 bearing a conducting layer 26, comprises a glass sheet 30 bearing conductive strips 32 which are connected to current distributors 32. The conductive strips 31 can be made of conductive silverenamel which is baked during thermal tempering of sheet 30, by a method known to those skilled in the art.

A film such as 27, as used in the mirror described with reference to Fig. 3. may be used in the mirror which is otherwise as shown in Fig. 2.

WHAT WE CLAIMS IS: 1. A mirror comprising a transparent front sheet the rear face of which bears a light-reflecting coating, and a protective backing sheet which is secured to said coated front sheet. front and backing sheets are secured together at least at the margin of the assembly with or without the presence of intermediary sheet material, by means of a hot-melt adhesive.

2. A mirror according to claim 1, characterised in that the composition of said hot-melt adhesive is such that the minimum temperature at which it is molten is 150"C or lower.

3. A mirror according to claim 2, characterised in that the composition of said hot-melt adhesive is such that the minimum temperature at which it is molten-is between 600 and 1200C.

4. A mirror according to any preceding claim characterised in that said adhesive comprises one or more elastomers or thermoplastics selected from butyl rubber and ethylene/vinyl acetate copolymers.

5. A mirror according to any preceding claim, characterised in that said adhesive comprises one or more tackifiers selected from terpene and phenolic resins, and microcrystalline waxes.

6. A mirror according to any preceding claim, characterised in that the waterpermeability of the hot-melt adhesive is less than 0.5g H20 per m2 of surface per 24 hours per mm of thickness and per cm Hg of pressure.

7. A mirror according to any preceding claim, characterised in that the adhesive incorporates one or more saturated hydrocarbons.

8. A mirror according to any preceding claim, characterised in that the or each layer of said adhesive is less than 150 microns in thickness.

9. A mirror according to any preceding claim, characterised in that said adhesive is present over the whole area of the mirror, the said front and backing sheets being united as plies of a laminate.

10. A mirror according to any preceding claim, characterised in that the backing sheet is bonded to the front sheet through the intermediary of sheet material, composed of a polyester or other synthetic polymeric substance.

11. A mirror according to claim 10 characterised in that said intermediary sheet material has a thickness between 10 and 150 microns.

12. A mirror according to any preceding claim, characterised in that the front sheet is a glass sheet.

13. A mirror according to any preceding claim, characterised in that the lightreflecting coating on said front sheet is covered by one or more protective coating layers.

14. A mirror according to any preceding claim, characterised in that the backing sheet is a sheet of vitreous material.

15. A mirror according to any preceding claim, characterised in that the backing sheet bears on its internal face one or more electrically conductive coatings or elements adapted to be connected to a voltage source so that the mirror can be heated by Joule effect.

16. A method of making a mirror by providing a transparent sheet having a light-reflective coating on a face thereof,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. crystalline wax manufactured by Bareco Div. of Petrolite Corp.) 10 POLYWAX 2000 (wax manufactured by Bareco Div. Petrolite Corp.) 10 ANTIOXYDANT 330 (supplies by Ethyl Corp.) 0.1 The water-permeability, expressed in the same units as before. was less than 0.5. In the production of the mirror, the polyester film 27 bearing the adhesive layers 28 and 29 was disposed between the subassemblies 20 and 21. The assembly was calendered when cold and then brought to a temperature of 120"C in an oven at atmospheric pressure. The resulting mirror showed good resistance to the saline fog in a test as hereinbefore described. Neither the silver layer 23 nor the electrically conductive layer 26 showed signs of damage. In addition, the film 27 affords a measure of safety in the event of breakage of the glass. In a modification, the polyester film 27 is replaced by polyvinylidene chloride or polytetrafluorethylene or polyethylene of polypropylene film, pretreated if required so as to receive the adhesive layers. As an alternative production procedure. the hot-melt adhesive layers can be initially deposited on the sub-assemblies 20, 21. e.g. by transfer from a non-adhesive temporary support as described earlier. instead of using a film pre-coated with adhesive. The two methods may be used together if desired. In a further variant, illustrated by Fig. 4. the sub-assembly 21, instead of comprising a sheet of glass 25 bearing a conducting layer 26, comprises a glass sheet 30 bearing conductive strips 32 which are connected to current distributors 32. The conductive strips 31 can be made of conductive silverenamel which is baked during thermal tempering of sheet 30, by a method known to those skilled in the art. A film such as 27, as used in the mirror described with reference to Fig. 3. may be used in the mirror which is otherwise as shown in Fig. 2. WHAT WE CLAIMS IS:

1. A mirror comprising a transparent front sheet the rear face of which bears a light-reflecting coating, and a protective backing sheet which is secured to said coated front sheet. front and backing sheets are secured together at least at the margin of the assembly with or without the presence of intermediary sheet material, by means of a hot-melt adhesive.

and a protective backing sheet, and securing such backing sheet to said coated sheet, characterised in that said coated transparent sheet and said backing sheet are secured together directly or via intermediary sheet material. by means of one or more layers of hot-melt adhesive at least at the margin of the assembly between such sheets.

17. A method according to claim 18, characterised in that the composition of said hot-melt adhesive is such that the minimum temperature at which it is effectively molten is below 150"C and the sealing is effected by heating the assembly of sheets and of one or more intervening layers of said adhesive to a temperature below 150do to cause melting of the adhesive, whereafter the assembly is cooled.

18. A method according to claim 16 or 17, characterised in that the hot-melt adhesive incorporates one or more saturated hydrocarbons.

19. A method according to any of claims 16 to 18, characterised in that the hot-melt adhesive is such that in its cooled condition its water-permeability is less than 0.5g H20 per m2 of surface per 24 hours per mm thickness and per cm Hg pressure.

20. A method according to any of claims 16 to 19, characterised in that the hot-melt adhesive forms in its final condition one or more layers which or each of which has a thickness of less than 150 microns.

21. A method according to any of claims 16 to 20, characterised in that the said adhesive layer or layers is or are transferred as such from a temporary support to one or more of the sheets to be sealed together, before these sheets are assembled.

22. A method according to any of claims 16 to 21. characterised in that said hot-melt adhesive layer or layers cover(s) the whole or substantially the whole area of said front and backing sheets.

23. A method according to any of claims 16 to 22, characterised in that a said intermediary sheet material is present which covers the whole or substantially the whole area of said front and backing sheets.

24. A method according to any of claims 16 to 22 characterised in that a said intermediary sheet material is present and this sheet material and said adhesive layers are confined to a marginal zone of the mirror assembly.

25. A method according to claims 23 or 24, characterised in that said intermediary sheet material is a synthetic polymeric material.

26. A method according to any of claims 23 to 25, characterised in that said intermediary sheet material has a thickness between 10 and 150 microns.

27. A method according to any of claims 16 to 26, characterised in that said sheets and said intervening adhesive layer or layers are subjected to pressure at ambient temperature and the assembly is subsequently heated at atmospheric pressure to melt the adhesive.

28. A method according to any of claims 16 to 26, characterised in that the said sheets and said intervening adhesive layer or layers are heated at atmospheric pressure to melt the adhesive, and the assembly is then subjected to pressure while still hot.

29. A mirror substantially according to any of the Examples herein.

30. A mirror production method substantially according to any of the Examples herein.