US-12625313-B2 - Polyester copolymers for use in optical films

US12625313B2US 12625313 B2US12625313 B2US 12625313B2US-12625313-B2

Abstract

Polyester copolymeric materials include 40 to 51 mol % substituted naphthalate units, such as dimethyl-2,6-naphthalene dicarboxylate units, 10 to 40 mol % ethylene units, and 10 to 40 mol % hexane units. The polyester copolymers can be used to prepare multi-layer optical films by coextrusion and/or co-stretching. The copolyester polymeric materials have desirable optical properties and permit thermal processing at lower temperatures.

Inventors

Stephen A. Johnson
Carl A. Stover
Kristopher J. Derks
Richard Yufeng Liu

Assignees

3M INNOVATIVE PROPERTIES COMPANY

Dates

Publication Date: 20260512
Application Date: 20201123

Claims (11)

1 . An optical film comprising a plurality of alternating polymeric first and second interference layers numbering greater than 25 in total and disposed between, and integrally formed with, first and second skin layers, each of the first and second interference layers having an average thickness less than about 250 nm, the first interference layers comprising a substituted naphthalate, hexanediol and ethylenediol copolymer, the first skin layer comprising polyester, at least one of the first skin and the first interference layers having a glass transition temperature between about 60°° C. and 110°° C., the second skin layer comprising at least 70% by weight of polyethylene terephthalate and having an average thickness greater than about 0.5 micrometers, for substantially normally incident light and for each wavelength in a predetermined wavelength range extending at least from about 430 nm to at least about 600 nm, the optical film having an optical reflectance of at least 40% for a first polarization state, adjacent first and second interference layers having respective in plane indices of refraction: n1x and n2x along the first polarization state, n1y and n2y along an orthogonal second polarization state, and n1z and n2z along a z-axis orthogonal to the first and second polarization states, such that for at least one wavelength in the predetermined wavelength range: n1x is greater than each of n1y and n1z by at least 0.18; a difference between n1y and n1z is less than about 0.10; a maximum difference between n2x, n2y and n2z is less than about 0.01; and a difference between n1x and n2x is greater than about 0.18.
2 . The optical film of claim 1 , wherein glass transition temperatures of the first skin and the first interference layers are within 5°° C. of each other, wherein the optical film is formed integrally and has an average thickness greater than about 50 micrometers, and wherein the plurality of alternating polymeric first and second interference layers and the first and second skin layers are co-extruded and further co-stretched at a temperature between about 190 and 220° F.
3 . An optical stack comprising: a structured film comprising a plurality of structures, each structure comprising opposing facets meeting at a peak; the optical film of claim 1 disposed on the peaks of the structures; and a first adhesive layer bonding the optical film to the peaks of the structures, the optical stack having a flexural modulus of greater than about 1500 MPa, as measured according to ASTM D790 standard.
4 . An optical film comprising a plurality of alternating polymeric first and second interference layers numbering greater than 25 in total and disposed between, and integrally formed with, first and second skin layers, each of the first and second interference layers having an average thickness less than about 250 nm, the first skin layer comprising polyester, the first interference layers comprising a substituted naphthalate, hexanediol and ethylenediol copolymer, wherein at least 30% by weight of diol moieties in the first interference layer is hexanediol, the second skin layer having an average thickness greater than about 100 micrometers and comprising at least 70% by weight of polyethylene terephthalate, and, for substantially normally incident light and for each wavelength in a predetermined wavelength range extending at least from about 400 nm to at least about 600 nm, the optical film having an optical reflectance of at least 50% for each of orthogonal first and second polarization states, adjacent first and second interference layers having respective in plane indices of refraction: n1x and n2x along the first polarization state, n1y and n2y along the second polarization state, and n1z and n2z along a z-axis orthogonal to the first and second polarization states, such that for at least one wavelength in the predetermined wavelength range: each of n1x and n1y is greater than n1z by at least 0.18; a maximum difference between n2x, n2y and n2z is less than about 0.01; and a difference between n1x and n2x is greater than about 0.18.
5 . The optical film of claim 4 , wherein the optical film has a flexural modulus of greater than about 1500 MPa, as measured according to ASTM D790 standard.
6 . The optical film of claim 4 , wherein the first and second skin layers, in combination, comprise at least 70% polyester by volume.
7 . The optical film of claim 4 , wherein the plurality of alternating polymeric first and second interference layers and the first and second skin layers are co-extruded and then co-stretched at one or more temperatures between about 190° F. to about 220° F., the first polymeric layers having in-plane birefringences greater than about 0.21.
8 . An optical film comprising a plurality of alternating first and second polymeric layers numbering greater than 25 in total and disposed between, and co-extruded and then co-stretched at one or more temperatures between about 190° F. to about 220° F., with first and second skin layers, each of the first and second polymeric layers having an average thickness less than about 250 nm, the first and second polymeric layers and the first and second skin layers having glass transition temperatures less than about 95° C., the first polymeric layers having in-plane birefringences greater than about 0.21, the second skin layer comprising at least 70% by weight of polyethylene terephthalate, wherein for substantially normally incident light and for at least one wavelength between about 400 nm to about 1500 nm, the optical film reflects at least 20% of the incident light having a first polarization state, wherein the first polymeric layers comprise a copolyester polymeric material comprising: 40 to 51 mol % substituted naphthalate units; 10 to 40 mol % ethylene units; and 10 to 40 mol % hexane units.
9 . The optical film of claim 8 , wherein the substituted naphthalate units comprise dimethyl-2,6-naphthalene dicarboxylate.
10 . An optical film comprising a plurality of alternating first and second polymeric layers numbering greater than 25 in total and disposed between, and co-extruded and then co-stretched at one or more temperatures between about 190° F. to about 220° F., with first and second skin layers, each of the first and second polymeric layers having an average thickness less than about 250 nm, the first and second polymeric layers and the first and second skin layers having glass transition temperatures within about 5°° C. of each other, the first polymeric layers having in-plane birefringences greater than about 0.21, the second skin layer comprising at least 70% by weight of polyethylene terephthalate, wherein for substantially normally incident light and for at least one wavelength between about 400 nm to about 1500 nm, the optical film reflects at least 20% of the incident light having a first polarization state, wherein the first polymeric layers comprise a copolyester polymeric material comprising: 40 to 51 mol % substituted naphthalate units; 10 to 40 mol % ethylene units; and 10 to 40 mol % hexane units.
11 . The optical film of claim 10 , wherein the substituted naphthalate units comprise dimethyl-2,6-naphthalene dicarboxylate.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a national stage filing under 35 U.S.C. 371 of PCT/IB2020/061041, filed Nov. 23, 2020, which claims the benefit of Provisional Application No. 62/941,107, filed Nov. 27, 2019, the disclosures of which are incorporated by reference in their entireties herein. FIELD OF THE DISCLOSURE The current application relates to polyester copolymers for use in optical films. BACKGROUND Polymeric films are used in a wide variety of applications. One particular use of polymeric films is in optical films to control light. Examples of optical film that control light include mirrors and reflective polarizers that reflect light of a given polarization or wavelength range. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays to enhance brightness. A reflective polarizing film may be placed between the user and the backlight to recycle the polarization state that becomes an image, thereby increasing brightness. A mirror film may be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. Another use of polarizing films is in articles, such as sunglasses, to reduce light intensity and glare. One type of polymer that is useful in creating polarizer or mirror films is a polyester. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light. SUMMARY The current application relates to polyester copolymers for use in optical films. The copolyester polymeric material comprise 40 to 51 mol % substituted naphthalate units, 10 to 40 mol % ethylene units, and 10 to 40 mol % hexane units. In some embodiments the substituted naphthalate units comprise dimethyl-2,6-naphthalene dicarboxylate. The polyester copolymers can be used to prepare multi-layer optical films by coextrusion and/or co-stretching. BRIEF DESCRIPTION OF THE DRAWINGS The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which: FIG. 1 is a cross-sectional view of a multi-layer optical film of this disclosure. FIG. 2 is a cross-sectional view of an optical stack article of this disclosure. FIG. 3 is graph showing the correlation of Tg to mol % hexanediol in copolymers of this disclosure. FIG. 4 is graph showing the correlation of in plane birefringence to % hexanediol in copolymers of this disclosure. While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims. DETAILED DESCRIPTION Polymeric films are used in a wide variety of optical applications. Among uses of polymeric films is in mirrors and reflective polarizers that reflect light of a given polarization or wavelength range. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays to enhance brightness. A reflective polarizing film may be placed between the user and the backlight to recycle the polarization state that becomes an image, thereby increasing brightness. A mirror film may be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. Another use of polarizing films is in articles, such as sunglasses, to reduce light intensity and glare. One type of polymer that is useful in creating polarizer or mirror films is a polyester. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light. It is often desirable for the multi-layer reflective polarizer films to include relatively thick and rigid outer layers to serve to increase the rigidity and handleability of these films, especially since the individual layers of the multi-layer reflective polarizer films are so thin (typically, 55 to 130 nanometers) and in some instances there are relatively few layers. Typically, relatively thick and rigid outer layers are laminated to the multi-layer reflective polarizer films. There are a number of disadvantages to lamination of out