Coating processes – With stretching or tensioning – Running lengths
Reexamination Certificate
2001-09-05
2003-11-18
Beck, Shrive P. (Department: 1762)
Coating processes
With stretching or tensioning
Running lengths
C427S171000, C427S172000, C427S393500, C427S508000, C427S558000, C427S595000, C427S256000
Reexamination Certificate
active
06649216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method of coating and curing UV-curable coatings in-line on polymer film, and the film produced thereby. More particularly, the present invention is directed to a commercially viable process for producing a coated polymer film having various beneficial properties through in-line coating and UV curing steps.
2. Description of Related Art
Polymer film can be modified in various ways to enhance its usefulness for specific applications. Coatings applied to one or both surfaces of the film are commonly used to achieve such modification. Various qualities such as adhesion, smoothness, oxygen permeability, printability, opacity, scratch resistance and the like can be altered through the judicious use of coating technology. Among such coatings, UV curable coatings are known. These coatings can be cured by the application of ultraviolet radiation to the coating. Known UV curable coatings are applied to film off-line, after the film has been oriented, set and cooled.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to a provide for the in-line application of UV curable coatings.
It is a further object of the present invention to provide an economical and efficient method for coating film with UV curable coatings.
It is another object of the present invention to provide a UV curable coating that provides a texturing treatment to a film surface.
It is a further object of the present invention to provide a UV curable binder or carrier for particulate matter suitable for application to the surface of film.
The present invention has accomplished these objectives by providing in a preferred embodiment a method of coating polymer film that includes the steps of extruding a cast sheet; stretching the sheet in a machine direction; applying a UV-curable coating to the sheet; stretching the sheet in a transverse direction in a tenter; and exposing the coated sheet to UV radiation.
DETAILED DESCRIPTION OF THE INVENTION
The present invention encompasses a method of creating UV cured coatings in-line in a biaxially oriented film production process. As discussed above, known methods of UV curing are limited to off-line application and curing of the coating. It has traditionally been considered not feasible to coat and cure UV-curable coatings in-line as part of the film manufacturing process.
Nonetheless, the present inventors have conducted an investigation into the possibility of such in-line coating and curing. While initially determining that it would be preferred to coat and cure the UV curable coating prior to the transverse direction stretch (in part because of the reduced width of the treatment area), such curing was found to result in a powdery, cracked coating that is unacceptable for many uses.
The inventors have discovered that when UV curing occurs in the neutral zone at the end of the stretch zone (or its equivalent), it prevents cracking of the cured coating in the transverse direction stretch and also allows curing before the photo-initiators can be driven off by the hot crystallizer zone temperatures. This allows films to be formed that have the properties of many known off-line UV cured films. However, this method eliminates the costly, labor-intensive off-line coating step. In addition, by applying and curing the UV curable coatings in-line, it is possible to create thinner coatings that can perform equivalently well to thicker, off-line coated UV curable coatings. UV cured products are typically expensive, so the reduction in the coating thickness can result in substantial cost savings. Moreover, when both coating application and curing occur after forward draw, the risk of cast sheet coatings coming off on and contaminating the rolls is eliminated. Furthermore, by coating the forward drawn sheet prior to the transverse direction stretch, a narrower area is coated. By such a method, it is possible to generate a coated width of approximately four times the width of the treated cast sheet, once the coated sheet is stretched approximately four times in the transverse direction. This contrasts with off-line coating, which requires coating equipment having the full width of the finished film.
The neutral zone effectively isolates the TD stretching step from the crystallizing step. The feasibility of UV curing in the neutral zone has been demonstrated by coating samples after the forward draw. Tentering then occurs, preferably with the crystallizer zones turned off or to a lower heat setting. This prevents the photoinitiator from being lost through evaporation in the hot crystallizer zones, allowing those samples to be captured and UV cured. Samples prepared via this method have shown good steel wool scratch resistance. Alternatively, coating could occur prior to forward draw (although it would be more difficult to keep coatings on the film without attrition via this method, and roll contamination is more likely).
Furthermore, higher temperature initiators which are expected to be available could be used which would not be driven off in the crystallizer zones. This would permit the use of post-tenter UV curing, in addition to neutral zone curing. In addition, electron beam curing can be used to minimize or eliminate photoinitiators. This electron beam curing could also take place in the neutral zone or post-tenter. Combinations of UV curing and electron beam curing can also be used. However, UV curing is preferred for use in many of the uses of the present invention. For example, e-beam curing typically generates x-rays. As a consequence, increased monitoring, regulatory involvement and other protective measures are required. In addition, such x-rays may have an effect on the finished film. Furthermore, e-beam curing is substantially more capital intensive than UV curing. UV curing also typically runs hotter than e-beam treatment. This can have a beneficial effect on the finished film, including its heat-set properties. However, for many applications, e-beam curing is viable. Most UV curable materials can be cured by e-beam, but no photoinitiators are needed. However, as discussed above, e-beam equipment is substantially more costly than UV curing equipment.
No significant subsequent heat cure occurs in the neutral zone. The UV unit generates only a few additional degrees of heat. Typical temperatures of about 90 to about 140, alternatively about 100 to about 110 degrees Celsius, are employed during the second stretch. Supplemental cooling equipment such as a conventional cool air supply with supply and return lines are optionally used to control the UV unit and/or film temperatures. IR blocking glass may also be used alone or with such cooling equipment to reduce heating of the PET film.
One preferred application of the present invention is for use in forming scratch-resistant lamination films. Many other UV-curable coating systems are commercially available. Of these, emulsion-type or 100% solids type coatings are preferred for use in the present invention. Various UV-curable coatings are set forth in commonly owned U.S. Pat. No. 4,822,828 to Swofford, the disclosure of which is incorporated herein by reference. A preferred hard coat coating is commercially available as Gafgard 300 from ISP.
UV curable coatings can be used to create a substantially clear, scratch-free film. While off-line coatings (including UV curable coatings) are typically applied very thickly to achieve reasonable scratch-resistance, the in-line coated UV curable coatings of the present invention have been found to provide excellent scratch-resistance at much reduced coating thicknesses. Following the machine direction stretching of a film, a UV curable coating is applied that is thick enough to fill minor imperfections created during prior processing (typically referred to as scuffs and scratches). The coating is preferably molten in both the preheating and stretching processes. It is then cured to form a layer on the surface of the film that is smooth and essentially free from ph
Gust Steven J.
Smith Patrick M.
Westermeier Jan C.
Beck Shrive P.
Jolley Kristen Crockford
Mitsubishi Polyester Film LLC
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