Diffuse Reflector

Abstract

The present invention relates to a diffuse reflector comprising a nonwoven sheet of fibers wherein in the nonwoven sheet has a reflectance and a delamination selected from the group consisting of a reflectance of at least about 97.0% and a delamination of at least about 0.18 N/cm, a reflectance of at least about 96.0% and a delamination of at least about 0.49 N/cm, and a reflectance of at least about 95.0% and a delamination of at least about 0.70 N/cm.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a diffuse reflector comprising a nonwoven sheet for use in products such as televisions and lighting requiring high reflectance while resisting sheet delamination.

[0003] 2. Description of the Related Art

[0004] Nonwoven sheets with high reflectance can be used in light reflective end uses. Nonwoven sheets must be bonded for structural integrity and to prevent delamination. However, the bonding process lowers the reflectance. Coatings can be applied to the bonded nonwoven to increase the reflectance.

[0005] It would be desirable to have a diffuse reflector made from a nonwoven sheet having high reflectance while resisting sheet delamination and without the need for a coating.

SUMMARY OF THE INVENTION

[0006] The present invention relates to a diffuse reflector comprising a nonwoven sheet of fibers wherein in the nonwoven sheet has a basis weight of from about 30 grams per square meter (gsm) to about 275 gsm and has a reflectance and a delamination selected from the group consisting of a reflectance of at least about 97.0% and a delamination of at least about 0.18 N/cm, a reflectance of at least about 96.0% and a delamination of at least about 0.49 N/cm, and a reflectance of at least about 95.0% and a delamination of at least about 0.70 N/cm.

DETAILED DESCRIPTION OF THE INVENTION

Definition of Terms

[0007] The term "nonwoven" or "web" as used herein means a structure of individual fibers or threads that are positioned in a random manner to form a planar material without an identifiable pattern, as in a knitted fabric.

[0008] The term "plexifilamentary fibers" as used herein means a three-dimensional integral network or web of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean film thickness of less than about 4 micrometers and a median fibril width of less than about 25 micrometers. The average film-fibril cross sectional area if mathematically converted to a circular area would yield an effective diameter between about 1 micrometer and 25 micrometers. In plexifilamentary structures, the film-fibril elements intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form a continuous three-dimensional network.

[0009] The term "polymer" as used herein, generally includes but is not limited to, homopolymers, copolymers (such as for example, block, graft, random and alternating copolymers), terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term "polymer" shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic, and random symmetries.

[0010] The term "polyolefin" as used herein, is intended to mean any of a series of largely saturated polymeric hydrocarbons composed only of carbon and hydrogen. Typical polyolefins include, but are not limited to, polyethylene, polypropylene, polymethylpentene, and various combinations of the monomers ethylene, propylene, and methylpentene.

[0011] The term "polyethylene" as used herein is intended to encompass not only homopolymers of ethylene, but also copolymers wherein at least 85% of the recurring units are ethylene units such as copolymers of ethylene and alpha-olefins. Preferred polyethylenes include low-density polyethylene, linear low-density polyethylene, and linear high-density polyethylene. A preferred linear high-density polyethylene has an upper limit melting range of about 130.degree. C. to 140.degree. C., a density in the range of about 0.941 to 0.980 gram per cubic centimeter, and a melt index (as defined by ASTM D-1238-57T Condition E) of between 0.1 and 100, and preferably less than 4.

Description

[0012] The present invention is directed to a diffuse reflector comprising a nonwoven sheet of fibers wherein in the nonwoven sheet has a combination of high reflectance and good delamination resistance.

[0013] The diffuse reflector of the present invention can be made by flash spinning a plexifilamentary fiber and the resulting nonwoven sheet generally by using the disclosures in U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,227,794 to Anderson et al. and U.S. Pat. No. 3,860,369 to Brethauer et al., which are hereby incorporated by reference, with the process exception as noted herein. The flash spinning process produces a flash spun nonwoven sheet of plexifilamentary fibers. However, it has been found that flash spinning at low spinning temperatures produces plexifilamentary fibers with unexpectedly high surface area and the resulting bonded nonwoven sheet has high reflectance with good delamination resistance.

[0014] A method of forming a flash spun plexifilamentary fiber comprising providing a solution of 12% to 24%, by weight of the solution, of polyethylene and a spin agent consisting of a mixture of 68 weight percent normal pentane and 32 weight percent cyclopentane; or 75 weight percent normal pentane and 25 weight percent cyclopentane and flash spinning the solution at a spinning temperature from about 165.degree. C. to about 175.degree. C. to form the flash spun plexifilamentary fiber.

[0015] The flash spun plexifilamentary fiber can be made with any suitable combination of polymer and spin agent for flash spinning. A useful polymer is a polyolefin polymer, preferably polyethylene.

[0016] The nonwoven sheet was made by collecting the flash spun plexifilamentary fibers to form a sheet followed by bonding.

[0017] The nonwoven sheet has a reflectance and a delamination selected from the group consisting of: a reflectance of at least about 97.0% and a delamination of at least about 0.18 N/cm, a reflectance of at least about 96% and a delamination of at least about 0.49 N/cm, and a reflectance of at least about 95.0% and a delamination of at least about 0.70 N/cm. It is preferred not to have an optical brightener, however a coating with an optical brightener may be added to the nonwoven sheet to improve the reflectance. For example, titanium dioxide with a binder can be coated onto the nonwoven sheet.

[0018] The nonwoven sheet of the present invention has a preferred basis weight of from about 30 gsm to about 275 gsm. An increase in basis weight can give rise to an increase in reflectance.

TEST METHODS

[0019] In the non-limiting Example that follows, the following test methods were employed to determine various reported characteristics and properties. ASTM refers to the American Society of Testing Materials.

[0020] Reflectance was obtained from single nonwoven sheets. A Model SP64 meter available from X-Rite, Incorporated, 4300 44.sup.th St SE, Grand Rapids, Mich., 49512, USA is placed on the sample and the reading is obtained. The output is a percent reflectance at a wavelength of 550 nm.

[0021] Delamination Strength of a sheet sample is measured using a constant rate of extension tensile testing machine such as an Instron table model tester. A 2.54 cm by 20.32 cm sample is cut such that its long direction is parallel to the machine direction and is delaminated approximately 3.18 cm by inserting a pick into the cross-section of the sample to initiate a separation and delamination by hand. The delaminated sample faces are mounted in the clamps of the tester which are set 2.54 cm apart. The tester is started and run at a cross-head speed of 5.08 cm/min. The computer starts picking up force readings after the slack is removed in about 1.3 cm of crosshead travel. The sample is delaminated for about 14 cm during which force readings are taken and averaged. The average delamination strength is the average force divided by the sample width and is expressed in units of N/cm. The test generally follows the method of ASTM D 2724.

[0022] Basis Weight was determined according to ASTM D-3776 and reported in gsm.

EXAMPLE

[0023] Hereinafter the present invention will be described in more detail in the following example.

Example 1

[0024] Example 1 represents a process for making a plexifilamentary fiber and resulting nonwoven sheet of the present invention. Plexifilamentary fibers and resulting nonwoven sheet were made from flash spinning technology as generally disclosed in U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,227,794 to Anderson et al. and U.S. Pat. No. 3,860,369 to Brethauer et al. Plexifilamentary fibers were flash spun from a 19 weight percent concentration of high density polyethylene having a melt index of 0.7 g/10 min (measured according to ASTM D-1238 at 190.degree. C. and 2.16 kg load) and 3.5% TiO2 in a spin agent of 68 weight percent normal pentane and 32 weight percent cyclopentane at a solution temperature of 175.degree. C. A nonwoven sheet was made by collecting the plexifilamentary fibers into a sheet and whole surface bonding the sheet between two pre-heated rolls at 132.2.degree. C., two pairs of bond rolls at 133.3.degree. C., one roll for each side of the sheet, with backup rolls made by formulated rubber that meets Shore A durometer of 85-90, (with 34.5 bar hydraulic pressure) and two chill rolls to form the bonded nonwoven sheet. The nonwoven sheet (basis weight, 90.2 gsm) had a reflectance of 97.2% and a delamination of 0.49 N/cm.

Example 2

[0025] Example 2 was made the same as Example 1, except the nonwoven sheet was made by collecting the plexifilamentary fibers into a sheet and whole surface bonding the sheet between two pre-heated rolls at 126.7.degree. C. and 121.1 C, two pairs of bond rolls at 133.3.degree. C. and 137.8 C, one roll for each side of the sheet, with backup rolls made by formulated rubber that meets Shore A durometer of 85-90, (with 34.5 bar hydraulic pressure) and two chill rolls to form the bonded nonwoven sheet. The nonwoven sheet (basis weight, 89.2 gsm) had a reflectance of 97.1% and a delamination of 0.30 N/cm.

Example 3

[0026] Example 3 was made the same as Example 1, except the plexifilamentary fibers were flash spun from a 19 weight percent concentration of high density polyethylene having a melt index of 0.7 g/10 min (measured according to ASTM D-1238 at 190.degree. C. and 2.16 kg load) and 5.0% TiO2 in a spin agent of 75 weight percent normal pentane and 25 weight percent cyclopentane at a solution temperature of 175.degree. C. A nonwoven sheet was made by collecting the plexifilamentary fibers into a sheet and whole surface bonding the sheet between two pre-heated rolls at 126.7.degree. C. and 132.2 C, two pairs of bond rolls at 142.2.degree. C. and 137.8 C, one roll for each side of the sheet, with backup rolls made by formulated rubber that meets Shore A durometer of 85-90, (with 34.5 bar hydraulic pressure) and two chill rolls to form the bonded nonwoven sheet. The nonwoven sheet (basis weight, 119.4 gsm) had a reflectance of 97.2% and a delamination of 0.32 N/cm.

Claims

1. A diffuse reflector comprising at least one nonwoven sheet of fibers wherein the nonwoven sheet has no optical brightener coating, has a basis weight of from about 30 gsm to about 275 gsm and has a reflectance and a delamination selected from the group consisting of: a reflectance of at least about 97.0% and a delamination of at least about 0.18 N/cm, a reflectance of at least about 96.0% and a delamination of at least about 0.49 N/cm, and a reflectance of at least about 95.0% and a delamination of at least about 0.70 N/cm.

2. The diffuse reflector of claim 1, wherein the fibers comprises a polyolefin polymer.

3. The diffuse reflector of claim 2, wherein the polyolefin polymer is polyethylene.

4. The diffuse reflector of claim 1, wherein the fibers are flash spun plexifilamentary fibers.

5. The diffuse reflector of claim 1, wherein the at least one nonwoven sheet has an optical brightener coating.