WO1999043740A1

WO1999043740A1 - Films having improved scratch resistance

Info

Publication number: WO1999043740A1
Application number: PCT/US1999/002067
Authority: WO
Inventors: Wenbin Liang
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1998-02-27
Filing date: 1999-01-29
Publication date: 1999-09-02
Anticipated expiration: 2000-08-27
Also published as: AU2487999A

Abstract

Substantially clear polystyrene-based films contain predominantely polystyrene as well as a styrenic block copolymer, a nylon copolymer, and optionally inorganic particulates. Such films having improved scratch resistance are particularly useful as window envelope films.

Description

FILMS HAVING IMPROVED SCRATCH RESISTANCE

This invention relates to films having improved scratch resistance. The present invention discloses how to make substantially clear films consisting predominately of polystyrene that are more resistant to scratches and abrasions than films of the prior art. One particularly useful application of thin, substantially clear films is as window envelope film. A window envelope is an envelope with one or more openings of any shape, usually rectangular, which allows examination of any information, typically a name and an address, printed on a limited area of matter within. The opening or openings are sealed or closed by a window patch composed of a substantially clear film. Patching is the process in which window envelope film is cut into patches of a desired length and width, gummed around the edges, and applied to the window envelope. Alternatively, the edges of the window opening or openings may be gummed instead of the edges of the window patch.

The patching process requires the use of a vacuum drum to ensure proper positioning of the film. As the film passes over the surface of the vacuum drum there exists a disparity in the speed of the passing film and the angular velocity of the drum. This disparity of speed results in the abrasion and scratching of the film. If the abrasion and scratching is severe enough, particles are removed or flecked from the film surface resulting in film dust. Prior art methods for substantially reducing or eliminating film dusting incorporate a waxy agent or an anti-flecking agent into window envelope films comprised predominately of polystyrene; see, for example, US-A- 4,961 ,800; US-A- 5,009,953; and US- A- 5,190,814. Addition of these agents still results in the formation of scratches or grooves being formed, but the scratching does not cause material to be flecked from the film so dusting is substantially reduced or eliminated. Even though film dusting can be substantially reduced or eliminated by adding waxy agents or anti-flecking agents, the film is still scratched during the patching process. The resulting scratches are visible to the human eye and thus make the window envelope patches less aesthetically pleasing. It is an object of the present invention to produce a scratch-resistant polystyrene-based film that can be used as a window envelope film to produce a window envelope patch having few or no visible scratches.

According to the present invention, there is now provided a film that is dramatically more scratch resistant when used as window envelope film than window

1 envelope film of the prior art. Films of the present invention, when subjected to scratch testing, exhibit scratch resistance in two ways. First, upon visual inspection, films of the present invention have significantly fewer visible scratches than films of the prior art. Second, when films of the present invention are subjected to scratch testing, their haze value does not increase near as much as films of the prior art that are subjected to the same scratch testing.

For purposes of the present specification, all composition percentages represent weight percent unless expressly stated otherwise. Films of the present invention comprise predominately polystyrene. Along with the polystyrene, films of the present invention also contain a styrenic block copolymer, a nylon copolymer, and optionally inorganic particulates. More particularly, the present invention concerns a substantially clear film which comprises polystyrene and from 0.1 to 10 weight percent of styrene block copolymer and from 0.1 to 4 weight percent of nylon copolymer. The present invention further concerns window envelopes having substantially clear window patches made from such films. Films of the present invention are generally monolayer films and unless expressly stated otherwise the term "film" or "films" in the present specification will refer to monolayer films. However, it is within the scope of the present invention to produce a multilayer film comprising at least one scratch-resistant surface layer, wherein the scratch- resistant surface layer has the same composition as monolayer films of the present invention. Films of the present invention are substantially clear. By substantially clear it is meant that the films are sufficiently clear so that, when films of the present invention are utilized as window envelope films to make window envelope patches, printed information, such as a name and address, can be readily examined through the film.

Generally, films of the present invention useful as window envelope film are at least 0.5 mils (12.7 micrometers (μm)) thick, and preferably at least 0.9 mils (22.9 μm) thick. Films of the present invention useful as window envelope films are no more than 2 mils (50.8 μm) thick, and preferably no more than 1.4 mils (152.4 μm) thick. However, films of the present invention useful in other applications may be significantly thicker. For example, films of the present invention useful in certain lamination applications are up to 6 mils (152.4 μm) thick.

Films of the present invention are comprised predominately of polystyrene. Along with the polystyrene, films of the present invention also contain a styrenic block copolymer, a nylon copolymer, and optionally inorganic particulates. Films of the present invention may also contain small amounts of other additives, such as stabilizers or hazing agents. For example, in one particularly preferred embodiment of the present invention, 0.3 percent of stabilizer and 7 percent of high impact polystyrene is included. By being comprised predominately of polystyrene is meant that films of the present invention contain at least 90 percent polystyrene, and preferably at least 95 percent polystyrene. The type of polystyrene useful in the present invention is typically general purpose polystyrene ("GPPS").

The styrenic block copolymer effects the scratch-related surface failure of the polystyrene film when being scratched. The styrenic block copolymer substantially reduces the brittle fracture and yields partial ductile fracture during scratching. The addition of a styrenic block copolymer adds a rubbery component to the otherwise more brittle polystyrene surface. This rubbery component helps diminish the tendency for scratches to propagate in either the same direction or propagate away from the initial scratch in a direction approximately perpendicular to the initial scratch.

Preferred styrenic block copolymers include styrene-isoprene-styrene block copolymers ("SIS"), styrene-butadiene-styrene block copolymers ("SBS"), styrene-ethylene- propylene block copolymers ("SEP"), and hydrogenated styrenic block copolymer such as styrene-(ethylene butylene)-styrene block copolymers ("SEBS") (e.g., Kraton™ G1657). Preferably, films of the present invention utilize SEBS.

Films of the present invention have a sufficient amount of styrenic block copolymer present to produce the desired effects listed above. If too much styrenic block copolymer is present then the film becomes rubbery and difficult to cut. Generally, films of the present invention comprise at least 0.1 percent styrenic block copolymer, preferably at least 0.3 percent styrenic block copolymer, and more preferably at least 0.4 percent styrenic block copolymer. Generally, films of the present invention comprise no more than 10 percent styrenic block copolymer, preferably no more than 2 percent styrenic block copolymer, and more preferably no more than 1.4 percent styrenic block copolymer.

Films of the present invention also contain a nylon copolymer. Nylon copolymers useful in the present invention will have a substantial amorphous fraction and have a melt temperature less than the temperature at which the film will be extruded. Thus, nylon copolymers useful in the present invention have a melt temperature less than 200°C, and preferably a melt temperature less than 150°C. It is believed that the amorphous fraction of the nylon copolymer reduces flecking of scratched portions of the polystyrene matrix. Nylon copolymers having lower melt temperatures disperse more readily at normal polystyrene melt-extrusion process temperatures than nylon copolymers having higher melt temperatures. It is believed that the nylon particles have higher abrasion and scratch resistance than the general purpose polystyrene in the film. It is further believed that a more uniform dispersion of nylon copolymer in the polystyrene matrix promotes scratch resistance of the film. Nylon copolymers useful in the present invention include nylon 6/6,9 copolymer (e.g., Grilon™ CF62SE) and nylon 6/12 copolymer (e.g., Grilon™ CA6E). Preferably, films of the present invention utilize nylon 6/6,9 copolymers.

Generally, films of the present invention contain a sufficient amount of nylon to produce the desired benefits of scratch resistance. If too much nylon is added, the film will not have the desired degree of transparency. Films of the present invention contain at least 0.1 percent of a nylon copolymer, and preferably at least 0.5 percent of a nylon copolymer. Films of the present invention contain no more than 4 percent of a nylon copolymer, and preferably no more than 2 percent of a nylon copolymer.

Films of the present invention also may optionally contain inorganic particulates. The addition of very small amounts of inorganic particulates further improves the scratch resistance of the films of the present invention while at the same time dramatically reducing the amount of nylon necessary to promote scratch resistance in films of the present invention.

Inorganic particulates useful in the present invention will have a Mohs' hardness of 3 or higher, and preferably 6 or higher. Generally, particulates smaller in size are more useful in the present invention than particulates that are larger in size. It is believed that the smaller particle size helps reduce the tendency for scratches to propagate. Generally, inorganic particulates useful in the present invention will have an average particle size of no more than 5 μm, preferably no more than 2 μm, and more preferably no more than 1 μm.

Films of the present invention that utilize inorganic particulates will contain at least 0.005 percent of inorganic particulates, and preferably at least 0.01 percent. Films of the present invention contain less than 1 percent of inorganic particulates, preferably less than 0.2 percent, and more preferably less than 0.1 percent. Inorganic particulates useful in the present invention include calcium carbonate, titanium dioxide ("Tiθ2")> and silicon dioxide ("Siθ2"). Preferably, inorganic particulates used in the present invention are Tiθ2 or Siθ2- More preferably, films of the present invention will utilize Tiθ2 as an inorganic particulate. It is especially surprising that the addition of small amounts of Tiθ2 is able to produce these scratch resistance benefits in substantially clear films because Tiθ2 is often used in the art as a pigment to color films. However, in the present invention the amount of Tiθ2 added can be sufficiently small so as to not give color to the film and noticeably affect the transparency of the film. Nonetheless, this small amount of Tiθ2 provides for significant scratch resistance benefits.

Films of the present invention may also include small amounts of other additives such as a stabilizer (e.g., Irganox™ Bl 171). For example, some preferred embodiments of the present invention contain 0.05 to 0.3 percent of Irganox™ Bl 171.

Films of the present invention may also contain a hazing agent. Hazing agents useful in the present invention are taught, for example, in US-A- 5,190,814 at column 3, line 58 to column 4, line 42.

Films of the present invention can be made by conventional film producing techniques and processes known in the art, such as cast tentering and blown film processes. The following examples illustrate the present invention further. As stated previously, unless otherwise indicated, all composition percentages are based upon weight. Examples

Two films were prepared on a conventional blown film process. Film #1 was comprised of 0.6 percent of a SEBS block copolymer (Kraton™ G1657X, available from Shell Chemical Company), 1.7 percent of a nylon 6/6,9 copolymer (Grilon™ CF62SE, available from EMS-American Grilon, Inc.), 0.03 percent Tiθ2, and 0.02 percent of high impact polystyrene ("HIPS"). The remaining composition of Film #1 was GPPS. Film #2 had the same composition except that the nylon composition was 0.9 percent and the Tiθ2 composition was 0.04 percent.

Both films were tested for scratch resistance. In the test for scratch resistance, the haze level of each film was measured and recorded. The films were then subjected to a relative motion against a LF Smithe 527 bronze drum under a vacuum of 10-in Hg (33.9

5 kilopascals (kPa)) to cause scratch on the film. Next, the haze level of each film was again measured and recorded. The extent of scratch on the film was then measured by calculating the change in the measured haze value of each film before and after scratching. A film was considered more scratch resistant when the change in film optical haze due to scratching is less than a film with more haze changes caused by scratching under the same condition. Film #1 exhibited a change in measured haze value of 50 percent and Film #2 exhibited a change in measured haze value of 23 percent.

A comparative film example representing the prior art was also produced. This comparative example contained 99.8 percent GPPS and 0.2 percent polytetrafluoroethylene ("PTFE") and was produced according to the teachings of US-A- 5, 190,814. The comparative example and Film #2 were tested for scratch resistance in the same manner as the two films discussed above except the bronze drum was under a vacuum of 12-in Hg (40.6 pKa) instead of 10-in Hg (33.9 pKa). The comparative example exhibited a change in measured haze value of 84 percent and Film #2 exhibited a change in measured haze value of 2.4 percent.

Claims

WHAT IS CLAIMED IS:

1. A substantially clear film which comprises polystyrene and from 0.1 to 10 weight percent of styrene block copolymer and from 0.1 to 4 weight percent of nylon copolymer.

2. The film of Claim 1 further comprising from 0.005 to 1 weight percent of inorganic particulate.

3. The film of Claim 1 or Claim 2 wherein the styrene block copolymer is styrene- isoprene-styrene (SIS), styrene-butadiene-styrene (SBS), styrene-ethylene-propylene (SEP) or styrene-(ethylene butylene)-styrene (SEBS) block copolymer.

4. The film of any one of the preceding claims wherein the styrene block copolymer is SEBS.

5. The film of any one of the preceding claims wherein the styrene block copolymer comprises from 0.3 to 2 weight percent of the film.

6. The film of any one of the preceding claims wherein the nylon copolymer has a melt temperature less than 150 ° C.

7. The film of any one of the preceding claims wherein the nylon copolymer is nylon 6/6,9 or nylon 6/12 copolymer.

8. The film of any one of the preceding claims wherein the nylon copolymer comprises from 0.5 to 2 weight percent of the film.

9. The film of any one of Claims 2 through 8 wherein the inorganic particulate is titanium dioxide or silicon dioxide.

10. A window envelope having substantially clear window patches formed of the film of any one of the preceding claims.

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