Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-01-10
2003-06-17
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S232000, C525S238000, C525S240000, C525S241000
Reexamination Certificate
active
06579946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low-gloss biaxially oriented polymer films comprising a vinyl aromatic polymer and substantially non-spherical rubber particles. “Low-gloss” films have a 60-degree (°) gloss of less than 80, as measured according to American Society for Testing and Materials (ASTM) method D-2457.
2. Description of Related Art
Low-gloss films are useful as window films for mailing envelopes. Window films are clear polymeric films that cover an opening in an envelope. Window films allow reading of addresses that are enclosed within an envelope. Optical character reading (OCR) devices are useful for reading addresses in automated mail sorting systems, but OCR accuracy is affected by film gloss. OCR device accuracy typically decreases when window film gloss increases. Therefore, low-gloss window films facilitate accurate sorting of mail when using OCR devices.
Altering, or roughening, the surface of a polymer film is one way to reduce its gloss. Unfortunately roughening the surface also typically increases film haze, decreasing optical clarity through the film.
Additives, such as rubber modifiers and inorganic fillers, can also reduce the gloss of a polymeric film. Additives are normally included at concentrations of 2 weight-percent (wt %) or higher, based on film weight. Additives tend to affect film properties other than gloss. For example, a film's haze typically increases with increasing concentration of polymeric modifiers in the film. Additives may also increase directional gloss differential (DGD) and surface gloss differential of a film. DGD is a difference in gloss in a machine direction (MD) relative to a transverse direction (TD) on a single film surface. MD is a direction parallel to a direction that a film travels during production. TD is a direction perpendicular to MD and parallel to a film surface. Surface gloss differential (SGD) is a difference in gloss between opposing film surfaces. Measure SGD either in MD or TD, as long as the direction is the same on both surfaces. Increasing SGD and DGD values are unattractive because they correspond to a less homogeneous film gloss. Increasing haze is unattractive for window films because it hinders visibility through the film.
A low-gloss film comprising less than 2 wt % additive, based on film weight, is desirable. Such a film that further has at least one of the following characteristics is even more desirable: a SGD of 25 or less; a DGD of 15 or less; a haze value of 30 or less when measured on a 1.15 mil (29.2 &mgr;m) thick film.
BRIEF SUMMARY OF THE INVENTION
In a first aspect, the present invention is a polymer film comprising a vinyl aromatic polymer and rubber particles, wherein said rubber particles: (a) have an aspect ratio greater than five; (b) have an unconstrained particle size of at least 2.5 micrometers; and (c) are present at a concentration of less than 2 weight-percent, based on film weight; and wherein said polymer film is biaxially oriented and has a 60-degree gloss of less than 80 in both the machine and transverse direction on both sides of the film, as measured by American Society for Testing and Materials method D-2457.
In a second aspect, the present invention is a polymer film comprising a blend of general purpose polystyrene and high impact polystyrene, wherein said film contains rubber particles that: (a) have an aspect ratio greater than five; (b) have an unconstrained particle size of at least 2.5 micrometers; and (c) are present at a concentration of less than 2 weight-percent, based on film weight; and wherein said polymer film is biaxially oriented and has a 60-degree gloss of less than 80 in both the machine and transverse direction on both sides of the film, as measured by American Society for Testing and Materials method D-2457.
Preferably, the films of the first and second aspects have as least one of the following additional characteristics: (d) a surface gloss differential less than 25; (e) a directional gloss differential less than 15; and (f) a haze value of 30 or less, when measured on a 1.15 mil (29.2 micrometer) thick film.
In a third aspect, the present invention is an envelope comprising at least two materials; wherein at least one of said materials is the film of the first or second aspect.
The present invention meets a need in the art by providing a low-gloss film comprising less than 2 weight-percent (wt %) additive, based on film weight.
DETAILED DESCRIPTION OF THE INVENTION
Herein, specified ranges include range endpoints unless otherwise indicated.
The present invention is a biaxially oriented low-gloss polymer film comprising a vinyl aromatic polymer (VAP) and substantially non-spherical rubber particles.
The films of the present invention have a 60° gloss in both machine and transverse direction on both sides of the film of less than 80, preferably 75 or less, more preferably 70 or less, most preferably 65 or less. Films having gloss values of less than 80 allow OCR devices to read through them more accurately than films having gloss values of 80 or higher.
Concomitantly, the films of the present invention preferably have a SGD of 25 or less, preferably 20 or less, more preferably 15 or less in both MD and TD. Films also preferably have a DGD of 15 or less, preferably 10 or less, more preferably 7 or less on each side. Films having SGD values greater than 25 and DGD values greater than 15 are less attractive because their gloss is highly dependent on film orientation. As a result, OCR accuracy when reading through such films would be dependent upon the orientation of the film.
Films of the present invention desirably have a haze value of 30 or less, preferably 27 or less, more preferably 24 or less when measured on a 1.15 mil (29.2 &mgr;m) thick film according to ASTM method D-1003.
Biaxially oriented films of the present invention include substantially non-spherical rubber particles. Substantially non-spherical rubber particles have a largest dimension (L), shortest dimension (T), and a third dimension (W). L, T, and W are mutually perpendicular and traverse the center of the particle. L and W are parallel, or essentially parallel to a film surface. “Essentially parallel” means within 45° of parallel. T is perpendicular, or essentially perpendicular to a film surface. “Essentially perpendicular” means within 45° of perpendicular. Substantially non-spherical rubber particles each have an aspect ratio and a major cross sectional area (MCSA). The aspect ratio is equal to L divided by T. The MCSA is &pgr;(L+W)
2
/16.
Substantially non-spherical rubber particles have an aspect ratio in both the machine direction and transverse direction (defined below) greater than five, preferably greater than 10, more preferably greater than 30, still more preferably greater than 50. Theoretically, there is no maximum for the aspect ratio. Higher aspect ratios are more desirable than lower aspect ratios for particles within a film because higher aspect ratios typically correspond to a lower film gloss.
Measure an aspect ratio of a rubber particle using a transmission electron microscope (TEM) image of a cross section of the rubber particle. Expose the cross section by cutting a film perpendicular to a film surface and parallel to either the machine direction (defined below) or transverse direction (defined below). Cutting the film parallel to the machine direction reveals rubber particle aspect ratios in the machine direction. Cutting a film parallel to the transverse direction reveals rubber particle aspect ratios in the transverse direction.
Film gloss for the films of the present invention is a function of the MCSA of rubber particles therein. Increasing the MCSA of rubber particles within a film typically decreases the film's gloss. Increasing the aspect ratio of the rubber particles also tends to increase the MCSA.
The MCSA and aspect ratio of rubber particles within a film typically increase by stretching a film in one direction, preferably two perpendicular directions parallel to the film'
Dow Global Technologies Inc.
Mark Steven W.
Nutter Nathan M.
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