VACCUM METALLIZATION OF SUBSTRATES

Abstract: The present invention provides an apparatus which utilises a drum or cylinder for "casting" an electron-curable premetallisation coating onto a paper substrate. The premetallisation process comprises the steps of (a) coating a drum (24) with a liquid coating, said coating being performed by said drum contacting at least one roller (21) to supply said coating; (b) passing a paper substrate (18) over the drum in contact with the coating; (c) electron curing in a chamber (11) of an electron curing apparatus (12) of the coating whilst in situ on the surface of the drum; (d) removing the paper substrate from the drum surface. In another aspect of the invention the paper substrate is first coated and then passed over the drum and electron cured.

Description: VACUUM METALLIZATION OF SUBSTRATES This invention relates to the vacuum metallization of substrates and more particularly to an apparatus for use in the vacuum metallization of substrates such as paper. Various processes have been employed for the metal cladding of web or sheet substrates. The traditional process involves adhering a thin metal foil to a paper or board substrate, aluminium foil being most commonly used. This process has proved to be uncertain from an economy viewpoint since the cost of metal foil is a fluctuating commodity-sensitive item and because a relatively thick foil was necessary. Another commonly used, and superior process is that known as vacuum metallization, in which a vaporized metal is condensed onto a substrate to be metallized. Other prior art processes have included so-called "transfer metallization" as described in U.S. Patent No. 4,215,170 (OLIVA), involving coating a substrate or a transfer agent with a varnish, laminating substrate and transfer agent, drying the varnish and stripping the transfer agent from the substrate. Curing time for the varnish may be form 24 to 26 hours, which period seriously interferes with inspection and subsequent treatment. This "time lag" problem was solved by radiation curing, that is to say, electron beam curing, of coatings and adhesives in the process of the metallization of paper substrates. Such a process including curing the varnish substantially instantaneously with electron radiation is disclosed in lapsed Australian Patent Application No. 23174/84. Two different paper metallization process systems are currently is use, viz, direct metallization and transfer metallization. This specification is concerned with direct metallization. The process of transfer vacuum metallization is practised by the deposition of metal, usually aluminium, (but not exclusively, as silver, gold, copper, tin, platinum etc. may be contemplated) in a vacuum chamber onto a plastic carrier web, and by the subsequent transfer of the metal layer from the carrier web to the chosen paper substrate by an adhesive laminating process. Finally, the carrier web is stripped by delamination of the plastic carrier web from the metal clad paper substrate. This process normally provides a metal clad paper surface which is a replica of the surface condition of the carrier web on which the metal was originally deposited. The stripping of the carrier web from the laminated paper structure is intended to occur without damage, thus enabling the carrier web to be eventually recycled. The economic viability of the above metallizing process for paper is critically dependant on the number of process cycles through which the carrier web can be re-used. When carried out in an effective manner, the visual quality of transfer metallized paper is superior to the appearance of direct metallized paper. However, because of the unpredictable cost factor of carrier web re-cycling, the cost predicability and the relative cost of the direct process are, respectively, significantly higher and consistently lower than the transfer process. Thus it will be appreciated that it is an object of the present invention to overcome at least one disadvantage by the provision of a metallization process which gives the finished product substrate a visual quality advantage; the direct process being of a lower, more predictable cost. According to one aspect of the present invention there is disclosed a premetallization process comprising the steps of: ( a) coating a drum with a liquid coating, said coating being performed by said drum contacting at least one roller to supply said coating; ( b ) passing a paper substrate over said drum, in contact with said coating; ( c ) electron curing said coating onto said substrate; and ( d ) removing the coated and cured substrate by peeling from the drum surface. This product can then be taken to the metallising process, after the application of which it is given a final top coating. According to another aspect of the present invention there is disclosed a metallization process wherein pressure is applied to said substrate to initiate contact said roller. According to another aspect of the present invention there is disclosed a metallized paper product produced by a process having after final top coating a specular reflectance value of with the range of 45 units to 100 units, said value being determined by illumination at an angle of 60° with the reflection being received by a Dr. Lange gloss meter model RB60M. In order that a better understanding of the invention may be gained, preferred embodiments thereof will now be described, by way of example only and with reference to the accompanying drawings in which:- Figure 1 illustrates schematically, a "state of the art" transfer vacuum metallization apparatus; Figure 2 illustrates also schematically, a drum transfer or casting apparatus according to the present invention; and Figure 3 illustrates a drum transfer process according to a further embodiment of the present invention. It will be understood that the coating of films or coatings from cylindrical or drum-like surfaces, usually of polished or engraved metal, is a well-established art and has been commercially practised for both the coating of paper and the printing of textiles, to name but two examples. This invention involves the employment of a somewhat similar drum adapted to supply high velocity electrons from an electron generator and for the drum to receive an electron-curable coating applied either directly to the drum surface or to the paper web immediately prior to its contact with the drum surface, such coating being specifically formulated for later metallization. The prior art transfer vacuum apparatus shown in Figure 1 comprises a vacuum metallized carrier web 1 - usually a plastic film - feeds from a carrier storage reel 2 to a coating station comprising an assembly of coating rollers 3, 4 and 5, the lowermost roller 5 rotating in a tank 6 of a suitable laminating adhesive. From the coating station carrier web 1 with its adhesive coating 6 passes to a pair of laminating nip rollers 7 and 8 where it is "nip laminated" with a paper substrate 9 which is fed from a paper storage reel 10. The completed web, that is to say the carrier web 1 , adhesive coating 6 and paper substrate 9, then travel through a chamber 11 of an electron-curing apparatus 12 where the adhesive coating layer 6 is substantially instantaneously cured. The completed and cured web then pass between a pair of release nip rollers 13 and 14 where the carrier web is stripped from the cured web and fed to a carrier storage reel 15 for re-use. The finished product 16 is fed to another storage reel 17 to await usage. At this point it should be observed that such an electron-curing apparatus as 12 utilizes either a heated filament or a gas plasma/cold cathode system to generate a "curtain" of electrons which penetrate the adhesive coating on the substrate web, which travels normal to the longitudinal axis of the linear filament. The system is self- shielded and its accelerating voltage limited to, say, 350 kv. Turning now to Figure 2, an inventive drum transfer, or casting, apparatus is schematically illustrated. A paper substrate 18 feeds from a paper storage reel 19 into a chamber - referenced 11 as that chamber of Figure 1 - of an electron-curing apparatus again 5 referenced 12. Within chamber 11 , substrate 18 passes about an idler nip roller 20 and around the periphery of a drum 24. The drum 24 can be of an appropriate finish depending upon the end result desired. If a high gloss finish is required on the substrate after the metallization and top coating processes, then a highly polished drum surface is required in the premetallizaton process. If a matt finish is required on the substrate 1 0 after the metallization and top coating processes, then a matt finished drum surface is used in the premetallization process. Simultaneously, a pair of coating rollers 21 and 22, the lower roller 22 rotating in a tank of suitable coating 23 and being located outside chamber 1 1 , apply a coating to the periphery of the drum 24 which becomes itself "laminated" to a paper substrate 18, by virtue of nip contact with roller 20. A typical 1 5 formulation for a coating composition for use with the present invention is formulated from 70 parts by weight of polyester tetra-acrylate dispersed in 30 parts by weight of tripropylene glycol diacrylate. After this "lamination" process has been effected the "laminate" is carried, on the drum's surface, to the electron beam irradiation station 25 of curing apparatus 2 0 12 which is positioned close to the moving substrate. The coating is thus cured in intimate contact with the peripheral surface of drum 24 and so acquires the surface contour and profile of the drum's surface. Within that same revolution of drum 24, carrying the coated and cured paper substrate, the coated paper is then stripped by delamination, at release nip roller 26, 25 (located in chamber 11) of the substrate from the surface of drum 24. The coated and cured paper substrate is rewound onto a storage reel 28 and the now de-coated surface of drum 24 continues its rotation to the coating station 21 , 22 and 23 for continuation of the coating, curing and stripping process. The apparatus of Figure 3 embodies a form of the invention similar to that of 30 Figure 2. Like items in Figures 2 and 3 have been identified with like numbers. The process differs from that of Figure 2 in that the coating 23 and the coating rollers 21 , 21 A and 22 are located in a position prior to the substrate 18 contacting the drum 24. An additional roller 21 A is utilized to ensure proper contact and transfer of coating 23 from roller 21 onto substrate 18. To avoid contamination, cleanliness of the drum surface is achieved by the stripping of the coated and cured paper substrate from the surface. The stripping is brought about by:- ( i ) Correct formulation for non-adhesion of the coating to the drum surface after cure is effected; ( i i ) correct formulation for 100% cohesion of the coating after cure; ( i i i ) surface treatment in the form of silicone based or other release type coatings of the drum surface to provide low wetting adhesion of cured coating to the drum surface; and ( i v ) an auxiliary drum surface cleaning system (not illustrated), which can comprise vacuum and cleaning brush assemblies or an oscillating brush and vacuum system, which is located between the delamination point and the coating station in the region of quadrant 29. When this process is performed effectively, the paper carries a cured coating eminently suitable for subsequent vacuum metallization in a vacuum paper metallizer, of appropriate design, in the usual manner. If the drum surface used in the pre-coating process is highly polished the paper surface after metallization will possess an identical mirror-like finish. On the other hand, if an embossed or uniformly satin-matt surface is required of the metallized substrate the drum surface can be exchanged for a replaceable drum or drums with the desired surface finish. After metallizing the surface should then be top-coated by application of a printable or protective lacquer by whatever coating method is appropriate. One option is to top-coat the metallized substrate on the casting drum machine and electron cure the coating in the same manner as the pre-coat was cured. Alternatively, a roll coating process may be used, also involving electron curing, or a conventional thermal cure coating could be applied. The process of the present invention will result in a vacuum metallized paper product, which after the final top coating process, will result in a specular reflectance value at 60° angle of illumination and reception by the Dr. Lange Glass Meter Model RB60M, in the range of 45 to 100 units, if a high finish drum surface is utilized. By comparison, by this test procedure, a mirror surface shows a specular reflectance value of 86 units, whilst a conventional paper metallizing process yields a value of 20 to 30 units. This invention therefore enables the manufacture of a mirror-like metallized paper or paperboard or alternatively an engraved or embossed surface finish is achievable or a combination of both could be applied by changing cylinders between pre- coating and post coating. This process eliminates the recycled carrier web of the transfer metallization process, with the obvious benefit of cost predictably of this new version of the direct process of metallization. Coatings of the free-radical polymerisation reactive type or of the cationic cure type, both involving electron irradiation for cure are essential in order to solidify the liquid coatings more or less instantly (within 500 milliseconds) prior to their peel removal from the drum surface. From the above it will be realised by those who are skilled in the art that numerous variations and modifications may be made to the invention without departing from the spirit and scope thereof as described and illustrated hereinbefore.

Claims: CLAIMS 1 . A premetallization process comprising the steps of: ( a ) coating a drum with a liquid coating, said coating being performed by said drum contacting at least one roller to supply said coating; ( b ) passing a paper substrate over said drum, in contact with said coating; ( c) electron curing said coating whilst in situ on the surface of said drum; and ( d ) removing said substrate from said drum surface. 2. A premetallization process as claimed in Claim 1 wherein said paper substrate is unwound from a roll and rewound onto another roll upon completion of the process. 3. A premetallization process as claimed in Claim 1 or 2 wherein pressure is applied to said substrate to initiate contactor or separation to or from said drum roller. 4. A metallization process comprising the steps of: (a) coating a paper substrate with a liquid coating; ( b ) passing said coated paper substrate over a drum, whereby said coating is located between said substrate and said drum; ( c) electron curing said coating whilst in situ on the surface of said drum; and ( d ) removing said substrate from said surface. 5. A premetallization process as claimed in Claim 4 wherein said paper substrate is unwound from a roll and rewound onto another roll upon completion of the process. 6. A premetallization process as claimed in Claim 4 or 5 wherein said substrate is coated by means of rollers. 7. A premetallization process as claimed in any one of Claims 4 to 6 wherein pressure is applied to said substrate to initiate contact or separation from said drum. 8. A high gloss metallized paper product produced by a process including a premetallization process as claimed in anyone of the preceding Claims having after metallizing and top coating a specular reflectance value in the range of 45 to 100 units, said value being determined by illumination at an angle of 60° with the reflection being received by a Dr. Lange gloss meter model RB60M. 9. A high gloss metallized paper product as claimed in Claim 8 wherein said specular reflectance value is greater than 75 units but less than 100 units.

Inventors: O'NEILL JOHN R, O'NEILL, JOHN, R.
Applicants: RALJOS INTERNATIONAL PTY LTD, RALJOS INTERNATIONAL PTY. LTD.
Application year: 1991
Main classification: D21H19
IPC codes: B05D1, B05D3, C23C14, D21H19, D21H23, D21H25