Patent Application
20100188751
Polymer co-extrusion is a common technology and is utilized in many polymer film applications, such as optical films for use in active display devices, static display devices such as graphic signs, solid state lighting, and the like. The co-extrusion process uses a structured roll in order to impart structure into one surface of the film during the co-extrusion process. However, it can be difficult to obtain desired replication fidelity, meaning that the structure on the film does not adequately correspond with the structure on the roll. Also, co-extrusion processes to make optical films typically use expensive polymer materials, increasing the cost of the resulting film.
Accordingly, a need exists for an improved co-extrusion process to make films and for improved replicated films, such as optical films.
A co-extrusion method for making an optical film, consistent with the present invention, includes the steps of providing at least two materials and co-extruding them between a nip roll and a structured roll. The optical film comprises a core layer material and a replicated layer material. The structured roll has a surface structure that is replicated onto the replicated layer, and the core layer is an internally conformable layer that conforms with the replicated layer. The film can optionally include a backside layer material adjacent the core layer on a side opposite the replicated layer. The backside layer can optionally possess a replicated surface structure.
The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings,
Embodiments of the present invention relate to a film article and an associated co-extrusion process to make the film in which the internal or core layer of the film conforms to the replicated structure on one or both outer surfaces of the film. The internal structure is automatically aligned with the external replicated structure and may affect the optical or other characteristics of the film compared to a film in which the internal layers are substantially parallel to the plane of the film. By varying the materials, processing parameters, and replicated structure, the co-extrusion process can be used to create tunable optical properties for a range of films and improve the performance of the films. For example, the index of refraction of the core polymer or the depth of penetration of the core layer into the external structure can be varied.
Depending upon the tooling structure, the backside, replicated, and core layers can contain a variety of replicated patterns or structure. For example, the layers can contain prisms, grooves, intersecting prisms or grooves, optical microlenses, or other discrete microstructures. Any of these exemplary features can form optical microstructures. These features can be ordered, random, or pseudo-random in nature. Any of the layers can have one or more additional coatings or additives such as the following: a UV absorber; a UV stabilizer; a static dissipative additive; or an optical enhancer. Also, the external surfaces of the film can have a matte surface created by subtractive, additive, or displacement processes applied to the tooling rolls. Fixed abrasive media processing, electro-deposition of surface topography, or loose media impact (bead blasting) are respective examples of these three processes.
The ratio of the thickness of the replicated layer to the height of the replicated structure determines the extent to which the internally conformable core layer conforms to the replicated layer. A replicated layer with a thickness such that the structured portion of the film consists almost entirely of the replicated layer will produce a film with the internally conformable layer being essentially planar. A thin replicated layer will create an internal core layer which extends extensively into the film structure and conforms more closely with the structure.
It is generally advantageous for the co-extruded film to be symmetrical about its mid-plane such that the backside layer and replicated layer are of the same material and approximately the same thickness. This symmetry balances the internal stresses, or reduces unbalanced stresses, in the final film thereby reducing curling, and it also aids in extrusion of the film from the die. A film having different materials for the backside and replicated layers may be advantageous when additives such as UV absorbers, antistatics, colorants and others are to be added, or when a subsequent process is to be applied such as adding an adhesive coating to the backside layer.
The various layers of the film 24 can be indexed matched. Tailored properties can be achieved by selecting to which layer performance enhancing additives can be added. Also, backside layer 30 can include a UV absorber, and replicated layer 26 can include an anti-static material or coating. Alternatively, other layers can include the UV absorber or anti-static material or coating. Other coatings can also be applied to the film. The backside layer can alternatively be designed to function as a matte diffuser. The backside layer can also be formed as a strippable skin layer. A protective premask can be added to either side or both exterior surfaces of the film. The materials for the various layers are preferably transparent or substantially transparent for use of the replicated film as an optical film for a display device. For example, the replicated films are particularly suitable for use as a gain diffuser.
Polymers that can be used as the replicated layer include the following: styrene acrylonitrile copolymers; styrene(meth)acrylate copolymers; polymethylmethacrylate; polycarbonate; styrene maleic anhydride copolymers; nucleated semi-crystalline polyesters; copolymers of polyethylenenaphthalate; polyimides; polyimide copolymers; polyetherimide; polystyrenes; syndiodactic polystyrene; polyphenylene oxides; cyclic olefin polymers; and copolymers of acrylonitrile, butadiene, and styrene. One preferable polymer is the Lustran SAN Sparkle material available from Ineos ABS (USA) Corporation.
Polymers for the core layer include but are not limited to polycarbonate, poly-methylmethacrylate, and poly-acrylonitrile-butadiene styrene. These polymers are chosen primarily for their high flexural modulus, thermal stability, and relative low cost compared to some polymers. One preferable polymer is the Makrolon polycarbonate material available from Bayer Corporation.
Polymers that can be used for the backside layer include the following: polycarbonates; polyesters; blends of polycarbonates and polyesters; copolymers of styrene; copolymers of acrylonitrile, butadiene, and styrene; block copolymers of styrene with alkene-polymerized midblocks; acid and anhydride functionalized polyolefins; and copolymers of polyethylene and polypropylene
A 10 inch wide three-manifold extrusion die (manufactured by Extrusion Dies, Inc) was used to extrude a three-layer film into a nip between a nip roll and a structured tooling roll. The structured tooling roll had as its structure linear grooves oriented around the circumference of the roll. These grooves had a 90° included angle and a pitch of approximately 356 microns for a groove depth of approximately 178 microns. Applying nip pressure between the nip roll and tooling roll created the structured film. The structured tooling toll was created using conventional diamond turning with the structure in only a single down web direction.
Table 1 provides the film construction and Table 2 provides co-extrusion process parameters for this example.
In this example, the core layer structure was shown to closely conform to the tooling structure. In particular, the film was shown to have sharp peaks of the internal core layer compared to more rounded external peaks of the replicated layer. The use of a strippable layer as the replicated layer, and its subsequent removal from such a three-layer construction, can enable sharp pointed features to be formed without the complete filling of the tooling structure.
A feedblock was used to feed three polymer layers to a 36 inch wide die. This co-extruded film was extruded directly into a nip between a structured pattern roll and a smooth metal nip roll and subsequently around a strip-off roll prior to winding. All three rolls were temperature controlled using water. Nip pressure applied to the extrudate between the nip roll and tooling roll creating the structured pattern in the film.
The channels in the tool were approximately triangular in cross-section with a depth of 60 microns and a pitch (groove to groove spacing) of approximately 114 microns. The cross-direction grooves were aligned at a 10° bias angle to the down web grooves. The tooling roll pattern was created as described in U.S. patent application Ser. No. 12/362,048, entitled “Method for Making an Optical Film Having a Variable Prismatic Structured Surface,” and filed on Jan. 29, 2009.
Table 3 provides the film construction and Table 4 provides co-extrusion process parameters for this example.
In this example, the core layer extended approximately one-third the height of the structure creating rounded peaks of the internal core layer.
The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/148,235 and filed Jan. 29, 2009, which is incorporated herein by reference as if fully set forth.
| Number | Date | Country | |
|---|---|---|---|
| 61148235 | Jan 2009 | US |
